U.S. patent application number 09/950359 was filed with the patent office on 2002-02-14 for channel-switched tunable laser for dwdm communications.
Invention is credited to Deacon, David A.G..
Application Number | 20020018507 09/950359 |
Document ID | / |
Family ID | 23672380 |
Filed Date | 2002-02-14 |
United States Patent
Application |
20020018507 |
Kind Code |
A1 |
Deacon, David A.G. |
February 14, 2002 |
CHANNEL-SWITCHED TUNABLE LASER FOR DWDM COMMUNICATIONS
Abstract
Laser source including materials with negative index of
refraction dependence on temperature and with temperature
independent coincidence between cavity modes and a set of specified
frequencies such as DWDM channels in telecommunications
applications. The free spectral range may be adjusted to equal a
rational fraction of the specified frequency interval. The
operating frequency may be defined by a frequency selective
feedback element that is thermo-optically tuned by the application
of heat from an actuator without substantially tuning the cavity
modes. The operating frequency may be induced to hop digitally
between the specified frequencies. In a particular embodiment,
semiconductor amplifier and polymer waveguide segments form a
linear resonator with a thermo-optically tuned grating reflector.
In a further embodiment, an amplifier and two waveguides from a
tunable grating assisted coupler form a ring resonator. Tuning may
also be accomplished by means of applying an electric field across
a liquid crystal portion of the waveguide structure within the
grating.
Inventors: |
Deacon, David A.G.; (Los
Altos, CA) |
Correspondence
Address: |
LAW OFFICE OF THOMAS SCHNECK
P.O. BOX 2-E
SAN JOSE
CA
95109-0005
US
|
Family ID: |
23672380 |
Appl. No.: |
09/950359 |
Filed: |
September 10, 2001 |
Related U.S. Patent Documents
|
|
|
|
|
|
Application
Number |
Filing Date |
Patent Number |
|
|
09950359 |
Sep 10, 2001 |
|
|
|
09421866 |
Oct 19, 1999 |
|
|
|
6324204 |
|
|
|
|
Current U.S.
Class: |
372/96 ;
372/20 |
Current CPC
Class: |
H01S 5/141 20130101;
G02B 6/4202 20130101; H01S 5/0622 20130101; G02B 2006/12164
20130101; G02F 1/1326 20130101; G02B 6/4212 20130101; G02B 6/12007
20130101; G02B 2006/12097 20130101; G02F 2202/022 20130101; G02B
6/136 20130101; H01S 5/0612 20130101; G02F 1/0147 20130101; G02B
6/1228 20130101; G02B 2006/12107 20130101; H01S 5/50 20130101; G02B
2006/12119 20130101; G02B 2006/12109 20130101; G02F 2201/307
20130101; G02F 1/065 20130101 |
Class at
Publication: |
372/96 ;
372/20 |
International
Class: |
H01S 003/08; H01S
003/10 |
Claims
1. A laser apparatus comprising: an amplifying waveguide segment
capable of providing optical gain over a first optical frequency
band, said amplifying waveguide segment characterized by a first
temperature dependent effective refractive index with a positive
refractive index change with increases of temperature; a passive
intracavity waveguide segment optically coupled to said amplifying
waveguide segment for providing a path for optical energy therein,
said passive intracavity waveguide segment characterized by a
second temperature-dependent effective refractive index with a
negative refractive index change with increases of temperature; and
a frequency selective feedback structure coupling optical energy of
a selected second optical frequency band within said first optical
frequency band back into said amplifying waveguide segment forming
a resonant cavity, wherein said frequency selective feedback
structure comprises a thermo-optical feedback waveguide segment, a
grating formed in said feedback waveguide segment, and a thermal
actuator for heating said feedback waveguide segment to produce a
change in refractive index of said feedback waveguide segment for
tuning said selected second optical frequency band, and wherein a
round trip optical path traversed by said optical energy within
said resonant cavity between successive couplings into said
amplifying waveguide segment has a round trip optical length that
is substantially independent of ambient temperature over a
specified ambient temperature range.
2. The laser apparatus of claim 1 wherein said specified ambient
temperature range comprises 5.degree. C.
3. The laser apparatus of claim 1 wherein said round trip optical
length varies by less than ten microns over said specified ambient
temperature range.
4. The laser apparatus of claim 1 wherein said passive intracavity
waveguide segment comprises a core material having a core
refractive index and a cladding material having a cladding
refractive index lower than said core refractive index, said core
material characterized by a positive refractive index change with
temperature increases and said cladding material characterized by a
negative refractive index change with temperature increases, with
the passive intracavity waveguide segment having a negative net
effective refractive index change with temperature increases.
5. The laser apparatus of claim 4 wherein said cladding material
comprises a polymer.
6. A laser apparatus of claim 1 wherein said round trip optical
length defines a free spectral range of said resonant cavity that
is a rational fraction of a specified communication frequency
channel spacing over a portion of said first optical frequency
band.
7. The laser apparatus of claim 1 wherein said resonant cavity is a
ring resonator.
8. The laser apparatus of claim 1 wherein said frequency selective
structure is a grating coupler for coupling energy between two
waveguide segments.
9. The laser apparatus of claim 1 wherein said amplifying waveguide
segment, said passive intracavity waveguide segment, and said
frequency selective feedback structure are fabricated on a common
substrate.
10. A laser apparatus comprising: an amplifying waveguide segment
capable of providing optical gain over a first optical frequency
band, said amplifying waveguide segment characterized by a first
temperature dependent effective refractive index with a positive
refractive index change with increases of temperature; a passive
intracavity waveguide segment optically coupled to said amplifying
waveguide segment for providing a path for optical energy therein,
said passive intracavity waveguide segment characterized by a
second temperature-dependent effective refractive index with a
negative refractive index change with increases of temperature; and
a frequency selective feedback structure coupling optical energy of
a selected second optical frequency band within said first optical
frequency band from said amplifying waveguide segment back into
said amplifying waveguide segment forming a resonant cavity,
wherein a round trip optical path traversed by said optical energy
within said resonant cavity between successive couplings into said
amplifying waveguide segment has a round trip optical length that
is independent of ambient temperature over a specified ambient
temperature range, wherein said round trip optical length defines a
free spectral range of said resonant cavity that is a rational
fraction of a specified frequency channel spacing over a portion of
said first optical frequency band.
11. The laser apparatus of claim 10 wherein said passive
intracavity waveguide segment comprises a core material having a
core refractive index and a cladding material having a cladding
refractive index lower than said core refractive index, said core
material characterized by a positive refractive index change with
temperature increases and said cladding material characterized by a
negative refractive index change with temperature increases, with
the passive intracavity waveguide segment having a negative net
effective refractive index change with temperature increases.
12. The laser apparatus of claim 10 wherein said second frequency
band of said frequency selective feedback structure is
substantially independent of temperature.
13. The laser apparatus of claim 12 wherein said frequency
selective feedback structure comprises an electro-optical feedback
waveguide segment, a grating formed in said feedback waveguide
segment, and an electrode actuator for applying an electric field
in said feedback waveguide segment to produce a change in
refractive index of said feedback waveguide segment for tuning said
selected second optical frequency.
14. The laser apparatus of claim 10 wherein said frequency
selective feedback structure comprises a thermo-optical feedback
waveguide segment, a grating formed in said feedback waveguide
segment, and a thermal actuator for heating said feedback waveguide
segment to produce a change in refractive index of said feedback
waveguide segment for tuning said selected second optical
frequency.
15. The laser apparatus of claim 10 wherein said resonant cavity is
a ring resonator.
16. A laser communication device comprising: a light amplifying
medium capable of providing optical gain over a first optical
frequency band extending over an optical communications frequency
band; optical feedback means, including a frequency selective
structure capable of providing optical feedback to said light
amplifying medium of a selected second optical frequency within
said first optical frequency band, for defining a resonant cavity
including said light amplifying medium said resonant cavity having
an effective round trip optical length characterizing a round trip
optical path of optical energy within said resonant cavity between
successive couplings of said optical energy into said light
amplifying medium said effective round trip optical path length of
said resonant cavity establishing longitudinal mode frequencies, a
subset of said longitudinal mode frequencies coinciding with
specified communications channels within said optical
communications frequency band; and an intracavity medium optically
coupled to said light amplifying medium in said round trip optical
path of optical energy within said resonant cavity, said
intracavity medium characterized by a negative refractive index
change with increases in temperature and having a length chosen
such that said effective round trip optical length is substantially
independent of temperature over a specified temperature range.
17. The device of claim 16 wherein said frequency selective
structure comprises a grating structure.
18. The device of claim 17 wherein said frequency selective
structure comprises a thermo-optical feedback element, and a
thermal actuator for heating said feedback element to produce a
change in refractive index of said feedback element for tuning said
selected second optical frequency to a specified communication
channel frequency.
19. The device of claim 17 wherein said selected second optical
frequency of said frequency selective structure is substantially
independent of temperature.
20. The laser apparatus of claim 19 wherein said frequency
selective structure comprises an electro-optical feedback element,
and an electrode actuator for applying an electric field in said
feedback element to produce a change in refractive index of said
feedback element for tuning said selected second optical frequency
to a specified communication channel frequency.
21. The device of claim 17 wherein said grating structure is formed
on a second waveguide segment coupled to said optical path in said
cavity.
22. The device of claim 16 wherein said resonant cavity is a ring
resonator.
23. A laser apparatus comprising: an amplifying waveguide segment
capable of providing optical gain over a first optical frequency
band, said amplifying waveguide segment characterized by a first
temperature dependent effective refractive index with a positive
refractive index change with increases of temperature; first and
second intracavity waveguide segments optically coupled to opposite
ends of said amplifying waveguide segment for providing a path for
optical energy therein, at least one of said passive intracavity
waveguide segments characterized by a second temperature-dependent
effective refractive index with a negative refractive index change
with increases of temperature; and a frequency selective feedback
structure coupling optical energy of a selected second optical
frequency band within said first optical frequency band between
said first and second passive intracavity waveguide segments
forming a ring resonant cavity, wherein a round trip optical path
traversed by said optical energy within said ring resonant cavity
between successive couplings into said amplifying waveguide segment
has a round trip optical length that is substantially independent
of ambient temperature over a specified ambient temperature range,
wherein said round trip optical length defines a free spectral
range of said resonant cavity that is a rational fraction of a
specified communication frequency channel spacing over a portion of
said first optical frequency band.
24. The laser apparatus of claim 23 wherein said specified ambient
temperature range comprises 5.degree. C.
25. The laser apparatus of claim 23 wherein said first and second
passive intracavity waveguide segments comprises a core material
having a core refractive index and a cladding material having a
cladding refractive index lower than said core refractive index,
said core material characterized by a positive refractive index
change with temperature increases and said cladding material of at
least one of said waveguide segments is characterized by a negative
refractive index change with temperature increases, with said at
least one passive intracavity waveguide segment having a negative
net effective refractive index change with temperature
increases.
26. The laser apparatus of claim 25 wherein said cladding material
comprises a polymer.
27. The laser apparatus of claim 23 wherein said frequency
selective feedback structure comprises a pair of feedback waveguide
segments coupled to said first and second passive intracavity
waveguide segments, with at least one of said pair being a
thermo-optical feedback waveguide segment, a reflective grating
formed in said pair of feedback waveguide segment, and a thermal
actuator for heating said feedback waveguide segment to produce a
change in refractive index of said thermo-optic feedback waveguide
segment for tuning said selected second optical frequency.
28. The laser apparatus of claim 23 wherein said frequency
selective means comprises a forward coupling grating, having a
first waveguide coupled to said first waveguide segment at a first
and of said grating and a second waveguide coupled to said second
waveguide segment at a second end of said grating, said first and
second waveguides having different propagation constants and said
grating characterized by a wavenumber matched to the difference in
propagation constants between the two waveguides.
29. The laser apparatus of claim 23 wherein one of said first and
second waveguides is characterized by a negative refractive index
change with increases in temperature and has heating means
associated therewith for tuning said frequency selective means.
30. The laser apparatus of claim 29 wherein the other of said first
and second waveguides is characterized by a substantially equal and
opposite positive refractive index change with increases in
temperature and also has heating means associated therewith.
31. The laser apparatus of claim 28 wherein said laser gain medium
has heating means operated in coordination with said heating means
associated with said forward coupling grating.
Description
CROSS-REFERENCE TO RELATED APPLICATION
[0001] This is a divisional of patent application Ser. No.
09/421,866 filed Oct. 19, 1999.
TECHNICAL FIELD
[0002] This invention relates to laser devices that produce optical
energy of tightly controlled optical frequency, particularly for
use in telecommunications applications. More particularly, the
invention relates to devices that produce a specified optical
frequency independent of thermal variations, while possessing the
ability to be tuned or switched among alternative optical
frequencies by thermal, electric field, or other control means
without modifying the specified frequencies.
BACKGROUND ART
[0003] The growth of demand for subscriber bandwidth has led to
great pressure to expand the capacity of the telecommunications
networks. Dense wavelength division multiplexing (DWDM) allows high
bandwidth use of existing fiber, but low-cost cost components are
required to enable provision of high bandwidth to a broad range of
customers. Key components include the source, the detector, and
routing components, but these components should preferably be
addressable to any of the frequency channels. These channels are
currently defined by the ITU as
.upsilon..sub.n=.upsilon..sub.0.+-.n d.upsilon., where
.upsilon..sub.0 is the central optical frequency 193.1 THz and
d.upsilon. is the specified frequency channel spacing that may
equal a multiple of 100 GHz or 50 GHz. Systems have also been
demonstrated based on other fixed spacings, and based on nonuniform
frequency spacings.
[0004] Semiconductor lasers with built-in gratings such as DFB and
DBR lasers are currently used to produce the frequency-specific
lasers needed to transmit over optical fibers. However, current
fabrication techniques do not allow high yield production of a
given frequency channel because of index of refraction variations
in the InP-based materials. Because silica, polymer, and other
optical materials offer greater stability of index of refraction,
many types of hybrid lasers have been tested in which a
semiconductor gain medium is combined with a grating fabricated in
another material. Single frequency hybrid waveguide lasers have
been demonstrated with semiconductor waveguide amplifiers to obtain
the benefits of frequency selectivity and tunability. See for
example * J. M. Hammer et al., Appl. Phys. Lett. 47 183, (1985),
who used a grating in an external planar waveguide, by * E.
Brinkmeyer et al., Elect. Lett 22 134 (1986) and * E. I. Gordon,
U.S. Pat. 4,786,132, Nov. 22, 1988 and * R. C. Alferness, U.S. Pat.
No. 4,955,028, Sep. 4, 1990, who used a grating in a fiber
waveguide, by * D. M. Bird et al., Elect. Lett. 27 1116 (1991) who
used a UV-induced grating, by * W. Morey, U.S. Pat. No. 5,042,898,
Aug. 27, 1991 who used a fiber grating with thermally compensated
package, by * P. A. Morton et al., Appl. Phys. Lett. 64 2634 (1994)
who used a chirped grating, by * D. A. G. Deacon, U.S. Pat. No.
5,504,772, Apr. 2, 1996, who used multiple gratings with optical
switches, by * J. M. Chwalek, U.S. Pat. No. 5,418,802, May 23,
1995, who used an electro-optic waveguide grating, by * R. J.
Campbell et al., Elect. Lett. 32 119 (1996) who used an angled
semiconductor diode waveguide, by * T. Tanaka et al, Elect. Lett.
32 1202 (1996) who used flip-chip bonding, and by * J- M. Verdiell,
U.S. Pat. No. 5,870,417, Feb. 9, 1999, who adjust for single mode
operation. Single frequency hybrid waveguide lasers have also been
demonstrated with fiber waveguide amplifiers. See * D. Huber, U.S.
Pat. No. 5,134,620, Jul. 28, 1992 and * F. Leonberger, U.S. Pat.
No. 5,317,576, May 31, 1994.
[0005] Many robust thermo-optic materials are available today
including glass and polymer materials systems that can also be used
in fabricating waveguide optical components. See * M. Haruna et
al., IEE Proceedings 131H 322 (1984), and * N. B. J. Diemeer, et
al., J. Light. Technology, 7, 449-453 (1989). Recently, thermally
tunable gratings have been fabricated in polymer waveguides and
resonators. See * L. Eldada et al., Proceedings of the Optical
Fiber Communications Conference, Optical Society of America, p. 98
(1999), and * N. Bouadma, U.S. Pat. No. 5,732,102, Mar. 24,
1998.
[0006] Thermal compensation of laser resonators is a requirement in
components that must operate robustly within the narrow absolute
frequency bands of the DWDM specifications. Thermally compensated
resonators have has been shown using polymer materials. See * K.
Tada et al., Optical and Quantum Electronics 16, 463 (1984).
Thermally compensated packages for fiber grating based devices have
also been shown. See * W. Morey, U.S. Pat. No. 5,042,898, Aug. 27,
1991, * G. W. Yoffe et al, Appl. Opt. 34 6859 (1995), and * J- M.
Verdiell, U.S. Pat. No. 5,870,417, Feb. 9, 1999. Thermally
compensated waveguides using mixed silica-polymer materials have
also been shown to produce temperature independent characteristics.
See * Y. Kokubun et al., IEEE Photon. Techn. Lett. 5 1297 (1993),
and * D. Bosc, U.S. Pat. No. 5,857,039, Jan. 5, 1999.
Silica-polymer waveguides have also been used for interconnecting
laser devices. See * K. Furuya U.S. Pat. No. 4,582,390, Apr. 15,
1986.
[0007] The grating assisted coupler is a useful device for
frequency control. Grating assisted couplers as described in * R.
C. Alferness, U.S. Pat. No. 4,737,007, Apr. 12, 1988, are known in
many configurations including with mode lockers, amplifiers,
modulators, and switches. See * A. S. Kewitsch, U.S. Pat. No.
5,875,272, Feb. 23, 1999. Grating assisted couplers have been used
in resonators including lasers, mode lockers, etalons, add-drop
filters, frequency doublers, etc. See for example * E. Snitzer,
U.S. Pat. No. 5,459,801, Jan. 19, 1994, and * D. A. G. Deacon, U.S.
Pat. No. 5,581,642, Dec. 3, 1996.
[0008] What is needed is a laser that operates robustly at a
frequency specified for DWDM systems, with the operating frequency
independent of environmental variations such as temperature and
humidity. Ideally, this laser should also be tunable among many or
all of the DWDM channels, and it should be inexpensive and easy to
produce and test.
SUMMARY OF THE INVENTION
[0009] According to the invention, an amplifier device is combined
with a material with negative index of refraction dependence on
temperature to produce a laser device with cavity length and index
of refraction control to accomplish temperature independent
coincidence between cavity modes and a set of specified frequencies
such as the DWDM optical channels in telecommunications
applications. The free spectral range may be adjusted to equal a
rational fraction of a specified frequency interval. The operating
frequency may be defined by a frequency selective feedback element
that is thermo-optically tuned by the application of heat from an
actuator without substantially tuning the cavity modes. The
operating frequency may be unique and it may be induced to hop
digitally between the specified frequencies. In a particular
embodiment, semiconductor amplifier and polymer waveguide segments
form a linear resonator with a thermo-optically tuned grating
reflector. In a further embodiment, an amplifier and two waveguides
from a tunable grating assisted coupler form a ring resonator.
Tuning may also be accomplished by means of applying an electric
field across a liquid crystal portion of the waveguide structure
within the grating. Methods are described of bringing the free
spectral range of the cavity within tolerance, including
intracavity methods of ablating material, depositing material, and
exposing material to radiation.
[0010] The advantages of the invention include the fact that it
provides a robust, athermal set of operating frequencies tied to a
specified set of optical frequency channels. Digital tuning may be
provided among these channels by thermal or other means without
substantially modifying the specified frequency channels so that
error-free channel selection is enabled among the provided
channels. No wavelockers or other means are needed to specify the
channels of operation. A simple method of channel selection is
available, and direct modulation of the amplifier medium is
available for data transmission, avoiding the need for a
modulator.
BRIEF DESCRIPTION OF THE DRAWINGS
[0011] FIG. 1 is an embodiment of a hybrid grating stabilized laser
chip.
[0012] FIG. 2 is a packaged embodiment in cutaway view of a
thermally stabilized integrated optic chip.
[0013] FIG. 3 is an embodiment of a laser chip coupled to a tapered
waveguide.
[0014] FIG. 4 is an illustration of the z-variation of the
effective mode indices of two coupled waveguides in a tapered
waveguide chip.
[0015] FIG. 5 is a top-view schematic diagram of a curved laser
waveguide array with an angled interface to a tapered waveguide
array.
[0016] FIG. 6 is an embodiment of a coupling region between a
waveguide and a V-groove.
[0017] FIG. 7A is an embodiment of a waveguide-end lens.
[0018] FIG. 7B is an alternative embodiment of a waveguide-end
lens.
[0019] FIG. 8 is a segmented embodiment of a tapered waveguide
coupler
[0020] FIG. 9 is a codirectional grating assisted coupler
embodiment of a ring hybrid laser chip.
[0021] FIG. 10 is a reflective grating assisted coupler embodiment
of a ring resonator hybrid laser chip.
[0022] FIG. 11A is a lateral cross section of a waveguide
embodiment with single upper cladding.
[0023] FIG. 11B is a lateral cross section of a waveguide
embodiment with double upper cladding.
[0024] FIG. 12A shows a hybrid grating stabilized laser embodiment
seen in a longitudinal cross section being illuminated for
adjusting a frequency of operation.
[0025] FIG. 12B shows a hybrid grating stabilized laser embodiment
seen in a longitudinal cross section having material removed for
adjusting a frequency of operation.
[0026] FIG. 12C shows a hybrid grating stabilized laser embodiment
seen in a longitudinal cross section having material being
deposited for adjusting a frequency of operation.
[0027] FIG. 13 is an embodiment of a tunable frequency-adjusted
ring resonator cross connect device.
[0028] FIGS. 14A, 14B, and 14C show lateral cross sections of three
stages of fabrication of a waveguide pair embodiment with differing
waveguide thermal coefficients.
[0029] FIG. 15 is a lateral cross section of an alternative
waveguide pair embodiment with differing waveguide thermal
coefficients.
[0030] FIG. 16 is a flow chart of a method for adjusting the free
spectral range of a resonator.
[0031] FIG. 17 is a dual grating embodiment of a tunable
frequency-selective cross connect device.
[0032] FIG. 18 is an illustration of a vernier method of frequency
tuning dual grating resonant devices.
DESCRIPTION OF PREFERRED EMBODIMENTS
[0033] Thermo-optic materials are both convenient and robust, and
may be incorporated into optical devices that actuate or tune upon
the actuation of a heater. However, for applications such as
communications, thermally insensitive devices are required for
reliability. This invention describes a unique way of including a
highly thermally sensitive tuning component into a device that is,
in key aspects of its operation, insensitive to temperature. In one
embodiment of the invention, a frequency tunable laser is described
that emits light which may be channel-switched among a set of
prepared frequencies that are independent of temperature. In
another embodiment of the invention, a frequency tunable
cross-connect device is described that cross-connects a frequency
which may be channel-switched among a set of prepared frequencies
that are independent of temperature.
[0034] Tunable Laser Operation
[0035] As is known in the art, laser tuning involves two factors
that are typically interdependent: a) tuning a frequency selective
element such as a grating within whose interaction band operation
will occur; and b) tuning the longitudinal modes of the laser that
determine the exact optical frequency that will lase within the
band of the frequency selective element. In the DBR (distributed
Bragg reflector) laser, for example, changing the chip temperature
shifts both grating and mode frequencies, typically at different
rates, leading to mode hopping behavior.
[0036] In this invention, means are described for making the cavity
free spectral range (FSR) independent of temperature for high
reliability. Further, the cavity round trip optical length may be
adjusted according to methods described herein to adjust the FSR to
equal a specified frequency channel spacing. The cavity is thereby
prepared to lase at the desired optical frequencies. If the cavity
is designed to be athermal, it may also provide an absolute
frequency reference to the communications channel frequencies
independent of drifts in environmental temperature, and no external
reference devices such as a wavelocker need be provided. The free
spectral range of a cavity is given by
FSR=c/(nL).sub.eff (1)
[0037] where the round trip optical length (nL).sub.eff is
(nL).sub.eff=.SIGMA..sub.i(n.sub.i eff L.sub.i) (2)
[0038] and where n.sub.i eff is the effective index of refraction
of the mode in the i.sup.th segment traversed by the light in the
cavity, L.sub.i is the physical length of the i.sup.th segment, and
the sum is over all the path segments traversed in a round trip of
the cavity. The effective index of an optical mode in a waveguide
is the index of refraction of an equivalent uniform medium that
would give a plane wave the same propagation constant
.beta.=2.eta.n.sub.eff/.lambda. (3)
[0039] or wavenumber (where the electric field of the mode varies
as E .varies.e.sup.i.beta.z along the direction of
propagation).
[0040] In an aspect of the present invention, the free spectral
range of the cavity may be designed to equal a predetermined DWDM
channel spacing set by a system integrator or by the ITU
(International Telecommunications Union). In the preferred
embodiment, the FSR is 50 GHz so that the effective round trip
optical length of the cavity is 6000 microns. With the linear
cavity shown in FIG. 1 and a preferred InP semiconductor laser chip
length of 400 microns at an index of about 3.3, the optical length
of the waveguide path between the edge of the laser chip and the
start of the grating is about 1680 microns. If the effective index
of the waveguide is about 1.45, its physical length is about 1160
microns, ignoring the small butt-coupling gap between the two
waveguides.
[0041] In a variation of the preferred embodiment, the free
spectral range of the cavity is designed to equal a rational
fraction of a desired communications channel spacing
FSR=(communications channel spacing).multidot.n/m (4)
[0042] where n and m are integers. If the desired channel spacing
is 50 GHz, for example, FSR=50.multidot.n/m GHz. This may be useful
if n/m<1 to increase the physical length of the resonator by a
factor of m/n, simplifying design and fabrication issues. In one
example, if n=1 and m=2, the rational fraction is 1/2 and the
desired cavity round trip optical path becomes 12000 microns,
allowing more than double the space in the resonator for thermal
compensation material, taper, etc. In this situation, the grating
tuning requirements are still the same, but the laser frequency
will hop in 25 GHz increments if the grating is tuned continuously,
with every other hop bringing the optical frequency to a desired
communications channel. (Some communications systems can use 25 GHz
channel spacing, in which case m=1 for this cavity length.) Or it
may be useful if n>1 to interleave successively addressed
frequency channels. In another example, if n=2 and m=1, the
rational fraction is 2 and the laser will hop successively to every
second communications channel, which might be useful for
interleaving two devices in the frequency domain, or for increasing
device stability against perturbations, ageing, and drifts. These
approaches may also be combined, as in another example, with n=2
and m=3 where the round trip optical path becomes 9000 microns, and
the laser frequency during operation will hop first to +33.3 GHz,
then to +66.6 GHz, both frequencies in-between communications
channels, and then to +100 GHz, a communications frequency two 50
GHz intervals away from the starting channel.
[0043] Athermal Cavity
[0044] In another aspect of the invention, a region of
thermo-optical polymer may be incorporated within the laser
resonator where the negative thermo-optic coefficient is exploited
to produce an athermal free spectral range. For the FSR to be
athermal or independent of device temperature, the device
parameters may be chosen to satisfy the relation
d(nL).sub.eff/dT=.SIGMA..sub.i(dn.sub.i
eff/dT.multidot.L.sub.i+n.sub.i eff.multidot.dL.sub.i/dT)=0 (5)
[0045] within a tolerance, where the sum is taken over all the
different longitudinal segments of the cavity along the optical
path. See K. Tada et al., Optical and Quantum Electronics 16, 463
(1984). Since these quantities are all positive in the common
non-polymeric materials including silica, silicon, InP, GaAs,
glass, lithium niobate, lithium tantalate, etc., a hybrid or
multiple material approach is needed. The conventional approach to
achieving temperature compensation has been to make a large
negative dL/dT in one of the lengths L.sub.i in the above summation
(usually a path length in air) equal to the difference between two
large lengths (usually overhanging members in a supporting
structure, one pointing away from the cavity and the other pointing
towards the cavity). By selecting an inward-pointing member to have
a larger coefficient of thermal expansion than the corresponding
outward-pointing member, the support structure can be arranged to
reduce the path length L.sub.i as the temperature is increased. See
for example W. Morey, U.S. Pat. No. 5,042,898, Aug. 27, 1991,
"Incorporated Bragg filter temperature compensated optical
waveguide device" and J M. Verdiell, U.S. Pat. No. 5,870,417, Feb.
9, 1999, "Thermal compensators for waveguide DBR laser
sources".
[0046] In the preferred embodiment of the present invention, a
polymer material is used to provide the negative thermo-optic
coefficient in the cladding of the waveguide, and the waveguide
design is adjusted for a negative net change in index with
temperature. The length of the polymer intracavity segment may be
adjusted until equation (5) is essentially met, within a tolerance.
Note that the material used need not strictly be a polymer; all
that is necessary is the negative thermo-optic coefficient. This
material is placed intracavity in order to affect the summation in
equation (5). Since it is the effective index of refraction that
appears in equation (5), it is sufficient that some optical energy
propagating in the optical mode traverse the negative coefficient
material in one segment of the round trip optical path. The polymer
material may be used in the cladding or the core, or in both
portions of the optical waveguide. Since the thermo-optic
coefficient of polymers tend to be large, only a fraction of the
optical mode volume swept out by the optical mode in a transit of
the optical cavity need be occupied by polymer.
[0047] The cavity is preferably also made athermal without changing
its FSR. To accomplish this objective, the preferred approach is to
adjust the overlap factor and the thermal coefficient of the
polymer while keeping the optical lengths at the values required
for the desired FSR. With the above lengths, a round trip through
the diode laser contributes about 0.2 micron/.degree. C. to the
first term of equation (5). Thermal compensation is achieved by the
polymer waveguide if its net thermal coefficient (the change in the
effective index of refraction with increase in temperature) is
approximately -9.times.10.sup.-5.degree. C..sup.-1. (The second
term of equation (5) is small.) While the core material in the
preferred embodiment has a positive thermal coefficient and is
traversed by the most intense part of the beam, the polymer
cladding material has such a large negative coefficient that it can
be effective in compensating the entire cavity.
[0048] The fraction of the optical mode power in the waveguide that
propagates inside the polymer compensating material is given by the
overlap factor .GAMMA..sub.c
[0049] .GAMMA..sub.c=(mode power propagating in the polymer)/(total
mode power) (6)
[0050] which may lie in the range of a few tenths of a percent up
to 40% or more, depending on the design of the waveguide core, and
the placement of the optical polymer. For the preferred 2 micron
square high contrast 2% silica waveguide on silica cladding, the
exponential tails of the mode penetrate far out of the core into
the polymer cladding as described below in relation to FIG. 11A.
Assuming the index of refraction of the polymer has been adjusted
in the preferred embodiment (by e.g. halogenation and/or mixing) to
equal that of pure silica, the overlap factor is about .GAMMA.=40%
since the polymer material forms the cladding on three out of four
sides of the rectangular waveguide core. A polymer material with
dn/dT of about -23.times.10.sup.-5.degree. C..sup.-1 will achieve
thermal compensation of this resonator. Materials with larger
negative dn/dT may be used with a design that has proportionately
smaller overlap factor or smaller physical length through the
negative dn/dT material. For example, if a material with dn/dT of
34.times.10.sup.-5.degree. C..sup.-1 is used, the desired overlap
factor is reduced to 26% in the above structure.
[0051] The tolerance within which equation (5) is satisfied depends
on the application. In the case of the communications application,
a mode frequency shift of a fraction of the communications channel
spacing, say 5 GHz, may be tolerated over a temperature range of
operating temperatures, which might be only a fraction of a degree
for temperature regulated devices, or as much as 5.degree. C.,
50.degree. C., or even higher for unregulated packages. A
50.degree. C. range would imply a tolerance of about +/-0.001
microns/.degree. C. in equation (5). To achieve this tolerance in a
real device, the waveguide lengths and the dn/dT of the polymer are
preferably controlled to an accuracy of a fraction of a percent.
Depending on the parameter values, the tolerance on the cavity
length may rise to about 10 microns (for a wide 400 GHz channel
spacing, for instance), or it may fall below one micron (for a
narrow channel spacing).
[0052] When the grating is integrated with the thermally
compensated waveguide design described above, an advantageous wide
tuning range results. The tuning range of the polymer clad grating
130 or 132 is large both because of the large thermo-optic
coefficient and large mode overlap factor of the polymer. When the
temperature of the grating polymer is scanned over a 100.degree. C.
range, the grating wavelength tunes over approximately 9 nm for the
above case of polymer material with -23.times.10.sup.-5.degree.
C..sup.-1, and 40% overlap factor.
[0053] In operation, the device will settle to a given temperature
profile along the optical path of the resonator. The laser
amplifier generates heat, and will rise to a temperature above that
of the polymer waveguide. Heaters or coolers attached to the
device, such as a substrate heater or the TE cooler described in
reference to FIG. 2, may also change the temperatures of the gain
section and the intracavity waveguide. Once the device in operation
has reached equilibrium, the thermal profile will vary spatially
along the waveguide but it will be constant in time. Changes in
ambient temperature will change the entire profile approximately by
a constant amount. Particularly if the thermal conductivity of the
substrate is large, such as is the case for the preferred silicon
substrate, changes in ambient temperature will produce spatially
uniform changes in the thermal profile. Such changes in temperature
do not substantially change the mode positions or FSR in an
athermal cavity as described by equation (5).
[0054] Laser Embodiment
[0055] FIG. 1 shows a preferred embodiment of the hybrid tunable
laser chip 100. A semiconductor laser chip 110 is flip-chip bonded
to the substrate chip 120 producing a hybrid of two integrated
waveguide chips. The laser chip is preferably fabricated from InP
so that it emits in the 1550 nm region or the 1310 nm region. The
waveguides 112 and 114 provide optical amplification when excited
by sufficient injection current, over an operating band of optical
frequencies including a desired wavelength such as 1550 nm or 1310,
980, 860, 780, 630, or 500 nm, or another useful wavelength region.
For a 1550 nm laser, a typical gain bandwidth would be about 50 nm
(such as from 1520 to 1570 nm or from 1560 to 1610 nm), and would
overlap a portion of the amplifying bandwidth of the Er-doped fiber
amplifier either in the conventional band or one of the extended
operating bands. The gain bandwidth may be smaller for lower
injection current, or as large as 120 nm or more for high injection
current and proper quantum well design. The two waveguides 112 and
114 of the laser chip are aligned in the x-z plane to butt couple
to two passive (they provide no gain) waveguides 122 and 124
fabricated on the substrate chip. A substantial fraction of the
energy emitted from the laser waveguides 112 and 114 is coupled
into the planar integrated waveguides 122 and 124, where the
coupling loss is preferably less than 10 dB or even less than 4 dB.
Vertical alignment (in the y direction) of the laser chip 110 is
obtained by controlling the thicknesses of the process layers in
and on the laser chip and the substrate. Light emitted from the
laser waveguides 112 and 114 is coupled into the waveguides 122 and
124 at the aligned butt coupled coupling region. Alternative gain
regions include variations on the active region of semiconductor
diode lasers, and fiber lasers, dye lasers, color center lasers,
solid state lasers generally, or other amplifying media capable of
providing optical gain over a useful frequency band.
[0056] Tapered waveguide segments 126 and 128 may be used to
improve the coupling efficiency between the differently shaped
waveguides 112 & 114 and 122 & 124. See FIGS. 3, 4, 7A, 7B,
and 8.
[0057] The waveguides 122 and 124 may be integrated on the
substrate 120 by one of a variety of common fabrication techniques.
In the preferred approach, as is known in the art, silica
waveguides are fabricated with low loss and good reproductability
using the flame hydrolysis method. In flame hydrolysis, layers of
particles produced in a flame (silica soot) are deposited onto the
surface with a chemical composition determined by the inputs to the
flame. Compaction of the particles into a solid film is typically
accomplished during a subsequent high temperature consolidation
process. Such waveguides are commercially available in various
index contrasts using Ge doped core material, including 0.4%, 0.75%
and 2%. Ge doped material has the further advantage of being
sensitive to UV irradiation as is known in the art, allowing
patterned regions of increased index of refraction (such a grating)
to be fabricated by exposure to patterned beams of light. Other
dopants are also known to have light-sensitive index of refraction,
which may occur as a result of a change in valence state. Most
useful layer thicknesses are available, including core thicknesses
in the range of 1 to 10 microns and beyond, and cladding
thicknesses in the range of a few microns to hundreds of microns,
if desired. Channel waveguides can be commercially fabricated
according to customer design. Channel waveguides are typically
fabricated by reactive ion etching (RIE) after deposition of the
core material on the lower cladding material. The RIE step removes
the higher index core material outside masked regions where
retention is desired to establish light guiding. Subsequent to the
channel waveguide fabrication, a top cladding of silica may or may
not be applied according to the desires of the customer. If
applied, the top cladding material is typically identical to the
lower cladding material (pure silica) in index, surrounding the
core material on all sides with cladding. In the preferred
embodiment, we have selected high contrast, 2% waveguide core
material, with a 2.times.2 micron channel dimension.
[0058] As an alternative the waveguides may be fabricated from
spun-on polymer layers chemically selected with a raised index for
the core layer, and patterned by RIE. Alternative substrates
include InP, GaAs, glass, silica, lithium niobate, lithium
tantalate, etc. Alternative waveguide materials include oxides such
as Ta.sub.2O.sub.5, Nb.sub.2O.sub.5, TiO.sub.2, HfO.sub.2, and
SiO.sub.2, semiconductors such as silicon, GaAs, InP, polymers, and
doped or mixed versions of all of the above materials with various
dopants including phosphorus, hydrogen, titanium, boron, nitrogen,
and others. Alternative fabrication methods include indiffusion,
sputtering, evaporation, wet and dry etching, laser ablation,
bleaching, and others. Many different waveguide structures are also
available including planar, rectangular, elliptical, ridge, buried
ridge, inverted ridge, diffused, air clad, hollow, coated, cladding
stripped, 3-layer, 4-layer, 5-layer, etc. Combinations of the above
materials, methods, and structures may be used as long as the
process flows are compatible (i.e. do not result in decomposition,
delamination, or unacceptable chemical change or physical
modification of the materials of the semi-processed article), the
optical losses are reasonably low (i.e. below 10 dB/cm for very
short chips and below about 1 dB/cm for longer waveguides), and the
transverse index of refraction profile of the finished structure
has a locally higher index of refraction compared to adjacent
materials in at least one dimension, creating at least a planar
waveguide that guides light in one dimension or a channel waveguide
that guides light in two dimensions.
[0059] FIG. 11A shows the preferred cross sectional embodiment of
the waveguides 122 and 124 of FIG. 1. The waveguides have a polymer
structure, fabricated with a silica under cladding, a patterned
Ge-doped silica core, and an over cladding of a polymer material
whose index of refraction has been selected to approximately equal
(within a tolerance of less than about 1.5%) the index of the
silica under cladding. The preferred polymer material is a
deuterated and halogenated polysiloxane such as is described in M.
Amano, U.S. Pat. No. 5,672,672, Sep. 30, 1997, as Compositions (G),
(H), (13), (14), (18), (20), (23), or (24). Other polymer materials
can also be used, including polysiloxanes, acrylates, polyimides,
polycarbonates, etc., with optional deuteration or halogenation to
reduce optical losses in the infrared, adjust the index of
refraction, and adjust adhesion to other layers. Depending on the
layer thicknesses, waveguide stripe width, and refractive indices,
a substantial fraction of the mode energy propagates in the polymer
cladding. This fraction may vary from a very small fraction of a
percent up to many tens of percent. The cross section of the
waveguide 1100 shows the optical mode 1180 represented by intensity
contour levels propagating along a channel waveguide emerging from
the page. The channel waveguide is fabricated on a silicon
substrate 1170 with a silica lower cladding 1144, and an
initially-uniform Ge-doped layer has been etched into a square
ridge 1140 that forms the core of the waveguide. Spun on top of the
ridge layer is the polymer upper cladding 1142, which shows a small
bump above the ridge due to incomplete planarization in the spin
and cure process.
[0060] The structure 1120 is an electrode. Depending on the nature
of the materials used, its usage and properties are slightly
different. For the preferred thermo-optic polymer device, layer
1142 is a thermo optic polymer, and the electrode 1120 is a
resistive stripe for heating the structure of the waveguide in a
controllable way. If the material 1142 is electro-optic such as a
poled nonlinear polymer or for instance, the structure 1120 is an
electrode for applying an electric field across the material 1142
towards another electrode which may be remote such as on the rear
surface of the substrate or a package wall, or on the top surface
shown but laterally displaced from the electrode 1120.
[0061] FIG. 11B shows an alternative waveguide embodiment with
split top electrodes 1122 and 1124, a lower electrode 1126 provided
below the lower cladding, and additional upper cladding material
1141. With lower electrodes as shown, the actuating voltage of an
electro-optic device is lowered since the separation between
electrodes so disposed is small. If the material 1142 is a polymer
dispersed liquid crystal (PDLC) for example, a split electrode
structure may be used to enable application of vertically and
laterally oriented fields. This enables rotation of the applied
electric field direction as well as changes in its strength,
producing changes in the TE and TM indices of refraction of the
PDLC film 1142. In further variations, the material 1140, 1144, or
1141 may be the tunable material with thermo optic, electro-optic,
etc. properties.
[0062] The waveguides 112 and 114 are preferably curved near the
front facet 118 of the laser chip so that the butt coupled
interfaces lie at an angle to the direction of propagation of light
in the waveguides, reducing the feedback from the coupling region.
The front facet 118 may alternatively be antireflection coated,
index matched, etc. These measures diminish the feedback from the
front facet relative to the frequency selective feedback from the
gratings 130 and 132, increasing the stability of the system.
[0063] FIG. 5 shows a detailed top view of the coupling region
between the diode laser chip 110 and the waveguide 122. The laser
waveguide structure 112 meets the HR-coated rear facet 116 of the
diode laser at nearly normal incidence for good coupling of light
reflected from the facet 116 back into the waveguide. However, the
waveguide is preferably curved in the region 510, with a radius of
curvature R, so that it meets the front facet 118 (which is
preferably parallel to the rear facet 116) at an angle
.theta..sub.i. The performance of curved waveguide diodes is
described in some detail in C- F. Lin et al., IEEE Phot. Tech.
Lett. 8, 206, (1996). The angle .theta..sub.i is chosen to be large
enough (preferably about 8.degree.) so that the reflection from the
interface at the facet 118 does not re-enter the waveguide,
preventing feedback from this interface. The minimum desired angle
depends on the contrast and dimensions of the waveguide, but as a
rough rule of thumb it can be chosen larger than about 5.degree.
for a tight waveguide. Since the effective optical index of
refraction inside the laser chip is approximately n=3.3 and the
effective index of the tapered waveguide core 126 is about 1.49,
the angle .theta..sub.r of the output waveguide will be
approximately .theta..sub.r=18.degree..
[0064] As mentioned in reference to FIG. 3, an index matching
material is preferably applied in the gap between the laser chip
110 and the waveguide structure 126 & 122. The gap is more
clearly shown in reference to FIG. 3 between the waveguide 330 and
the waveguide 320. The interface region includes two interfaces,
one between the waveguide 320 and the material in the gap, and a
second between the material in the gap and the waveguide 330. The
gap may be filled with air, vacuum, or a material with index of
refraction close to but preferably somewhat above the geometric
mean of the effective indices of the waveguides 320 and 330. (If
the waveguide structure 122 and 126 is fabricated after the
attachment of the diode laser to the substrate there may be no gap
at this interface.)
[0065] As an alternative to the butt coupling arrangement between
the two waveguides shown in FIG. 1, many other coupling approaches
may be used, including lens coupling, grating coupling, and
parallel coupling (including vertical coupling), and grating
assisted coupling, as is known in the art. In the case of vertical
coupling, the waveguide 122 is disposed parallel to and vertically
separated from the diode laser waveguide 112, as would be obtained
if the waveguide 122 is fabricated directly on top of the diode
laser waveguide. The coupling between the two waveguides may be
that of a broadband directional coupler, or it may be that of a
narrower band grating assisted coupler.
[0066] Monitor photodiodes 140 and 142 may be placed to receive a
portion of the light generated from the laser chip, in this case in
proximity to the rear facet 116 of the laser which has preferably
been high reflection (HR) coated, but which still transmits a
portion of the light incident on the facet. As shown, the monitor
photodiodes 140 and 142 are preferably waveguide detectors butt
coupled to the laser waveguides 112 and 114. This butt coupling is
non critical since the laser power is high and high detector
efficiency is not critical. A large separation between the laser
and monitor chips on the order of 50 microns or more is acceptable,
making possible reduced positioning tolerance for this chip. If
desired, one of many known configurations for the disposition of
surface photodiodes may alternatively be used.
[0067] Grating regions 130 and 132 are tunable frequency selective
feedback structures that reflect a portion of the light traveling
in waveguides 122 and 124, providing feedback into the laser chips,
and determining the wavelength regions in which the lasers
oscillate (see E. I. Gordon, U.S. Pat. No. 4,786,132, Nov. 22,
1988, "Hybrid distributed Bragg reflector laser"). The gratings 130
& 132 and the rear facet 116 of the laser chip form the cavity
mirrors for the laser oscillator of a hybrid external cavity,
grating stabilized laser. The waveguides 112 and 114, the butt
coupling regions, and a segment of the waveguides 122 and 124,
respectively, including the tapers, form the intracavity optical
path for propagation of optical energy within the resonator. These
grating regions 130 and 132 are shown in a separate segment of the
waveguides 122 and 124 from the tapered coupling regions 126 and
128. These separate segments of the waveguide may be identical to
or may differ from the other segments of the waveguide either in
structures or in materials. The grating structure may be fabricated
in the core layer 1140, one of the cladding layers 1142, 1142, or
1144, or in multiple layers.
[0068] When the drive current through the laser waveguide 112
exceeds a threshold value, the gain provided exceeds the round trip
optical loss of the oscillator, and laser operation is obtained.
The FSR of the linear cavity of FIG. 1 is determined by the optical
length between the first grating element and the rear facet 116 of
the laser amplifier chip. The partial waves of the reflections from
the other grating elements add in phase to determine the spectral
characteristics of the grating reflection. Together, these partial
waves also establish the amplitude and phase of the grating
reflection coefficient at the location of the first grating
element. Changes in index within the grating affect its spectrum
but not the FSR of the cavity, whether those changes in index are
uniform or have a complicated profile due to heating, the
application of an electric field to electro-optic material, the
application of stress, etc.
[0069] For single frequency operation as is required for high
bandwidth communications, the width of the grating interaction band
is preferably much smaller than the gain bandwidth of the amplifier
but comparable to the FSR of the resonator containing the
amplifying waveguide. If the full width at half maximum of the main
grating reflection band equals the longitudinal mode spacing, only
one mode at a time will lase. Adjacent modes will have lower gain,
and will be clamped below threshold until the grating band is tuned
far enough to equalize the modal gain of two adjacent modes. When
gain equality is obtained for two modes during tuning, the
operating frequency of the laser will jump suddenly from the
oscillating mode to the adjacent mode the grating is tuning
towards. It may be sufficient for the grating band full width at
half maximum to be substantially larger than the FSR, but the laser
stability will begin to become compromised as the band width
becomes significantly larger. There are some advantages to having
the band width smaller than the longitudinal mode spacing, but the
laser might become power modulated or even extinguished as tuning
progresses.
[0070] By modulating the drive current, the laser intensity may be
modulated, thereby modulating the output power coupled out of the
waveguides 122 and 124 into the output fibers. The waveguides 122
and 124 may be modulated with separate data, providing multiple
independent output channels, or simultaneously with the same data
stream, providing a dual output device that can be independently
routed to the desired destination for e.g. data communication
protection purposes. Laser modulation may be accomplished by
modulating the drive current through the stripe waveguides 122 and
124, or externally as is known in the art. Direct modulation is
accomplished with low chirp in this configuration because changes
in the drive current do not modulate the index of refraction of the
gratings 130 or 132, and because the effective index of most of the
optical path of the cavity is not modulated. The maximum rate of
modulation is typically limited by the round trip time in the
cavity to a fraction of the FSR, so if the FSR is set at 50 GHz,
the modulation rate may be limited to 10 to 20 GHz.
[0071] Since the amplifier chip generates heat, changes in the
average drive current will also change the longitudinal mode
positions. It is therefore preferable to establish an average drive
current that is maintained during operation. A constant drive
current may be maintained during modulation using a
transition-keyed modulation scheme, for example, if necessary. As
the laser ages, its average drive current must rise to maintain
constant output power and good modulation characteristics. It may
be desirable to apply a compensating level of current to
intracavity electrodes such as 150 and 152, for example, to
maintain the channel frequencies despite the ageing-related rise in
laser temperature.
[0072] The grating regions 130 and 132 are preferably fabricated by
patterned exposure through a phase mask as is known in the art. The
grating may be fabricated in the Ge-doped silica core after
sensitization with hydrogen or deuterium, or it may be fabricated
in the polymer cladding prior to full cure so that the cladding
material is still subject to chemical change such as by
crosslinking. Alternative grating fabrication methods include
exposure with interfering beams, patterning and dry- or
wet-etching, or direct patterned etching, all of which are known in
the art and may be applied to either core material or cladding
material. For a purely periodic grating, the Bragg wavelength
.lambda..sub.B for peak reflection in the retroreflecting
configuration shown is given by
.lambda..sub.B=2n.sub.eff.LAMBDA./m, (7)
[0073] where .LAMBDA. is the grating period, n.sub.eff is the local
effective index of refraction of the mode, and m is the order of
reflection. The result in equation (7) follows from the requirement
that to accomplish phasematching, the wavenumber of the grating
2.eta./.LAMBDA. must equal the sum of the forward propagating
wavenumber in the waveguide and the reverse propagating wavenumber
in the waveguide. With an effective index of about 1.446, Bragg
wavelengths of 1552 nm and 1310 nm are obtained with grating
periods of 537 nm and 453 nm, respectively. The exact wavelength of
operation depends on all of the optical parameters of the
waveguide, including the grating periods, and the refractive
indices and thicknesses of the films traversed by the optical
energy of the optical mode.
[0074] In practical devices, gratings are rarely strictly periodic,
and the grating period, the grating index modulation, and the
waveguide effective index of refraction may be varied along the
length of the waveguide to achieve various effects such as
apodization as is known to reduce sidemode reflection, to create
multiple grating peaks as known for example in superstructure
gratings and sampled gratings etc., or in general to engineer the
shape of the reflection spectrum. In a laser resonator (or
oscillator or cavity), it is preferable to chirp the grating period
towards shorter period (in the direction of light propagation away
from the amplifying waveguide segment) both for enhanced oscillator
stability as shown by P. A. Morton et al., Appl. Phys. Lett. 64
2634 (1994), and for reduced sidelobe amplitude on the grating
reflection bands as shown by A. Gnazzo et al., Integrated Photonics
Research Conference, Optical Society of America, p. 410 (1996). The
waveguide parameters such as lateral guide width may also be
spatially varied, changing the effective index and the grating
frequency as is known in the art.
[0075] The optical frequency band over which reflection occurs may
take on very different forms according to the shape of the grating
spectrum which may have only a single narrow peak, a broad peak, or
a more complex multipeaked structure. The shape of the spectrum
depends on the detailed design of the optical phase advance along
the grating structure. A single peaked grating may be used to
select a single channel, and a broad band or multipeaked grating
may be used to select multiple channels or provide vernier tuning,
etc.
[0076] The grating may be tuned (shifting the wavelength range for
interaction with light) by changing either the grating period or
the effective index of refraction of the light propagating through
the grating. The grating period may be changed by expanding the
material by one or more of several means including mechanical
stretching or compression, heating or cooling, acoustic excitation,
etc. The effective index in the grating may be changed by one or
more of several means including the preferred thermo-optic effect,
the electro-optic effect, the piezoelectric effect, etc. Materials
are available that change their index of refraction in response to
thermal, electric field, compression, shear, and other applied
changes, including nonlinear optical materials, crystals, liquid
crystals, and other types of material known in the art. Any portion
of the material traversed by the optical energy of the light mode
along the grating may be changed to affect a change in the grating
spectrum. The thermo-optic effect is preferred for shifting the
reflection band in a near-term product due to the availability of
reliable polymer materials with large dn/dT.
[0077] The thermo-optic effect is the property of some materials of
changing their index of refraction with temperature. Heating a
segment of the polymer waveguide of FIG. 11A changes the effective
index of refraction predominantly through the thermo-optic effect.
The effect of thermal expansion is relatively small. A few
materials have large thermo-optic coefficient (dn/dT) such as the
active waveguide of the InP laser (dn/dT=25.times.10.sup.-5.degree.
C..sup.-1), and a few materials have a small rate of change of
index with temperature such as silica
(dn/dT=1.times.10.sup.-5.degree. C..sup.-1). Polymer materials are
unusual in that their thermo-optic coefficient is negative and
large (dn/dT in the range of -10 to -35.times.10.sup.-5.degree.
C..sup.-1), see for example R. S. Moshrefzadeh et al., J. Lightwave
Tech. 10 420 (1992). In this invention, we use polymer layers along
the optical path to tune grating interaction frequencies, to tune
resonant frequencies, and to render devices athermal by
compensating the positive thermal change in index of refraction of
other materials traversed by the optical energy. Useful devices are
produced including in particular the combination of both thermally
insensitive structures such as resonators, and strongly thermally
tunable structures such as polymer gratings.
[0078] Changes in the temperature of the grating do not affect the
FSR substantially if there is no substantial "leakage" of the
grating thermal spatial profile into the resonant cavity. The
cavity as a whole is in the preferred embodiment made athermal
(compensated to be insensitive to uniform temperature changes).
However, individual segments within the cavity may still have a
substantial thermal coefficient. In the preferred embodiment, the
structure of the waveguide 122 is the same in the grating 130 and
in the cavity between the taper 126 and the grating 130. To the
extent that a portion of the thermal spatial profile from the
heater 160 overlaps the laser cavity, tuning the grating with
heater 160 will still produce a residual change in the mode
frequencies. Ideally, the temperature tuning of the grating is
accomplished by an abrupt spatial thermal profile that changes the
temperature of the grating but that does not change the temperature
of the intracavity waveguide structure. By designing the heater
electrodes 160 and 162 for low heating of the intracavity waveguide
region outside the length of the grating, and by providing a high
thermal conductivity substrate 120 such as silicon, and by keeping
the thickness of the process layers thin between the grating
waveguide and the substrate (subject to other constraints), we can
minimize the effect of the thermal tuning of the grating on the
longitudinal modes, so that the full tuning range of the grating
can be realized while limiting the undesired mode tuning to a
tolerance such as an acceptably small fraction of one FSR. As an
alternative, a segment of the intracavity waveguide adjacent to the
grating could be designed athermal in the region of "leakage" of
the grating thermal profile.
[0079] Under such conditions, tuning the Bragg wavelengths of the
gratings 130 and 132 by means of the currents flowing through the
heater stripes 160 and 162 produces a series of discrete frequency
jumps in the laser output (mode hops) from one longitudinal mode to
the next, without changing the longitudinal mode frequencies. The
optical frequency of operation tunes in a discontinuous, digital
manner, without traversing the frequency range between the
longitudinal modes of the cavity. If longitudinal modes coincide
with communications frequency channels, the device changes
communications channels digitally even though the current in the
heater stripes may be changed continuously in an analog fashion.
With digital tuning, the channel accuracy depends not on the
accuracy of the tuning actuator (e.g. heater current), but on the
accuracy of the specification of the channel frequencies.
[0080] A pair of serpentine heater traces 150 & 152 may be
disposed about the waveguides 122 & 124, at a location between
the grating regions 130 & 132 and the output facet 118. The
heater traces 150 and 152 terminate in electrodes 154 and 156, and
158 and 159, respectively. Injecting a current through the
electrode pair 154 and 156 excites the heater trace 150, raising
the temperature of the waveguide 122 along a portion of its length
as determined by the pattern of the heater trace 150 and the
diffusion of the heat away from the trace and (ultimately) into the
substrate. Likewise, injecting a current through the electrode pair
158 and 159 excites the heater trace 152, raising the temperature
of a portion of the waveguide 124. The heater traces may be
fabricated from stripes of resistive material such as platinum,
nickel, Nichrome, conductive polymer, etc., and may be in the form
of a single layer or of multiple layers as may be necessary to
produce the desired properties of conduction or wirebonding or
adhesion to the lower layer, or to modify the electrode response to
subsequent process steps such as laser ablation, etching, etc. The
stripe may be patterned as known in the art by lithographic means
such as photo resist patterning followed by liquid or dry etch
(e.g. chemical or RIE etch) of the resistive material and stripping
of the resist. These electrodes and heater traces may be used to
adjust the optical length of the round trip optical path of the
hybrid external cavity grating stabilized laser, where the round
trip optical path is the path followed by the optical mode through
the resonator between successive passages through the same point in
phase space in the resonator (such as a reflection from the grating
or a coupling into an amplifying waveguide segment), and traversing
the amplifier waveguide segment, and where n.sub.i eff is the
effective index of refraction of the mode in the i.sup.th segment
traversed by the light in the cavity, L.sub.i is the physical
length of the i.sup.th segment, and the sum is over all the path
segments traversed. The heaters lower the effective index of the
waveguides through the thermo-optic effect in the polymer cladding
material in the region determined by the heat flow adjacent to the
heaters. This reduces the optical length of the resonator,
increases the FSR and tunes the longitudinal modes to higher
frequencies, all other factors being constant.
[0081] In an embodiment of the invention, the round trip optical
length may be adjusted by means of the heaters 150 and 152 to
adjust the optical length and the free spectral range so that some
of the resonator longitudinal mode frequencies coincide with a
desired set of communications frequency channels. Or, the heaters
150 and 152 may be used to tune the operating frequency of the
device in a continuous analog fashion.
[0082] Serpentine heater traces 160 & 162 may be disposed about
the waveguides 122 & 124, at a location within the grating
regions 130 & 132 and substantially traversing the entire
grating regions. The heater traces 160 and 162 terminate in
electrodes 164, 166, 168, and 169. Injecting a current through the
electrode pair 164 and 166 excites the heater trace 160, raising
the temperature of the waveguide 122 along grating region 130 as
determined by the pattern of the heater trace 160, and the
diffusion of the heat away from the trace and (ultimately) into the
substrate. Again, the heaters lower the effective index of the
waveguides through the thermo-optic effect in the polymer cladding
material in the region determined by the heat flow adjacent to the
heaters. The change in the effective index in the grating region
tunes the frequency response of the grating as in equation (7);
heating a grating segment increases its frequency of interaction.
The gratings may be tuned together or separately simply by
controlling the respective heater currents or powers.
[0083] The stripe pattern of the heater traces 160 and 162 is
preferably uniform along the grating to form a thermal change as a
function of heater current that is uniform along the length of the
grating, thereby largely maintaining the spectral shape of the
grating interaction. As shown, the stripe pattern traverses both
sides of the waveguide in the grating region so that the thermal
change is also more uniform across the lateral dimension of the
waveguide. Use of a single heater stripe along the waveguide is a
reasonable alternative that offers the advantage that all the
gratings may be grounded together at one end.
[0084] Although the heater stripes 150 and 152 are also shown as
serpentine, uniformity is not a requirement for tuning the round
trip optical length of the resonator. The electrodes or pads 154,
156, 158, 159, 164, 166, 168, and 169 are preferably made of gold
or other material that resists oxidation in order to enhance the
bonding of connection leads to the heater power supplies (not
shown). The locations of these electrodes are not critical, and may
be moved to other locations on the chip, provided that the
connections between the electrode locations and the heater traces
have low resistance to reduce unwanted power consumption. Many
other heater and electrode designs are available and useful for
accomplishing the purposes described above.
[0085] In a variation of the invention, the thermal profile along
the waveguide induced by the heater stripes may be made nonuniform
along the length of the gratings by various means including varying
the width of the stripes, varying the distance of the stripes from
the waveguide axis, etc., so that the spectral shape of the grating
interaction may be changed by a distributed thermally induced phase
shift as a function of the heater current.
[0086] For thermo-optically tunable gratings, while the cavity may
be made athermal, the grating itself cannot be athermal. For this
reason, it may be desirable to stabilize the absolute temperature
of the substrate, limiting the frequency sensitivity of the grating
to changes in ambient temperature. If the substrate is thermally
stabilized, the heater power provided to the grating may also be
used to determine the absolute operating frequency. Some drift in
the grating frequency is acceptable provided it does not cause a
mode hop, so the substrate stabilization requirement is not very
stringent. (In electro-optic, piezoelectric, etc. devices, the
gratings are preferably designed to be intrinsically athermal,
eliminating the need for substrate thermal stabilization.) To
stabilize the substrate temperature, a simple temperature sensor
may be attached at or near the substrate with an electronic control
feedback loop provided as is known in the art to actuate a heater
and/or cooler (such as the TE cooler 212) and regulate the
temperature within a desired range.
[0087] A curved waveguide region 178 may be provided in the
waveguides 122 and 124 on the chip 100 to bend the waveguides back
through the angle .theta..sub.r to provide output coupling to a set
of optical fibers that is parallel to the diode laser chip,
allowing easy scaling of the design to multiple lasers on the same
chip. By expanding the chip laterally (in the x-direction), a wider
laser chip with 3 or 4 or more waveguides can be provided and
coupled to additional waveguides laid out adjacent to the existing
waveguides, with taper, grating, and heater sections, as well as
output fiber V-grooves. The dual-bend configuration allows this to
be done with identical length segments for each separate waveguide.
The radius of the curved waveguides in the region 178 may be chosen
to optimize the bend loss; a good choice for our 2% contrast silica
waveguides is a radius of curvature larger than or equal to about 2
mm. Notice that the bends have been placed outside of the resonant
cavity to reduce the length of the resonant cavity (increasing its
modulation bandwidth) and to reduce its loss. An alternative design
(not shown) incorporating this bend before or after the taper but
before the grating (and therefore inside the cavity) has the
advantage of greater compactness since the grating regions will
also be parallel with the laser chip.
[0088] V-grooves 170 and 172 may be provided to aid in coupling a
pair of fibers (not shown) to the output ends of the waveguides 122
and 124. The V-grooves extend across the bonding slot 176 and
terminate in the alignment slot 174 whose vertical sidewall allows
the butt coupling of the output fibers and the waveguides 122 and
124. The depth and position of the V-grooves are adjusted to align
the core of the output fibers approximately coaxial with the
waveguides 122 and 124 at the alignment slot 174.
[0089] FIG. 6 shows a detailed top view of the coupling region
between the waveguide 122 and the V-groove 170. Alignment slot 174
is preferably fabricated with a nearly vertical sidewall 610 in
which the waveguide core 122 terminates. A fiber is placed in the
V-groove 170, gently pressed against the two angled sides of the
V-groove, and gently pressed forward against the sidewall 610.
Adhesive is placed in the region 179 (see FIG. 1) and cured, to
affix the fibers in position. The bonding slot 176 prevents
adhesive from wicking along the fibers or V-groove towards the
optical interface at the sidewall 610. The position and angle of
the V-groove in the x-z plane, and the depth of the V-groove are
preferably set so that the attached fibers are aligned coaxial with
the waveguide core 122. The fabrication of V-grooves in silicon,
silica, and other substrates is known in the art, as is the slot
design and the fiber attachment process, providing multiple
alternative realizations.
[0090] The electrical connections to the common connection to the
back surface of the laser, and for the laser diode stripes, are
preferably made via wirebond connections to intermediate electrodes
181, 182, and 183, . . . , respectively. Connections to the monitor
photodiodes 140 and 142 may also be made via wirebonds to
connection pads (or electrodes) such as 184, 185, . . . , possibly
also using 181 as a common connection. If common connections are
electrically undesirable, as may happen in some electronic circuits
sensitive to noise, separate electrodes may be used for each common
function. For example, the monitor diodes may have one or even two
separate common electrodes (not shown).
[0091] FIG. 2 shows a cutaway view of a package arrangement 200 for
the hybrid chip assembly 100. The chip assembly 100 is preferably
bonded at its lower surface 102 to two thermo electric coolers 212
and 214 bonded in series. Two coolers are preferably used to enable
a large temperature difference between the chip 100 and the ambient
temperature, making possible a wide ambient temperature range over
which operation can be obtained while maintaining the chip 100
within its desired narrow temperature range. For broader or
narrower operating ranges, more or fewer coolers may be used. If
the range of ambient temperature excursion is as small as 5.degree.
C. or so, no cooler may be necessary. The TE coolers are in turn
bonded to a heat sink 220 shown as part of the package made from
one of several appropriate thermally conductive materials including
copper, aluminum, Kovar, ceramic, etc. This heat sink may have fins
or be attached to fins (not shown) for improved heat conduction
into the ambient air. Electrical connections 230-233 are shown
between the chip and the leads of the package 234 which emerge
through isolating regions 236. The connections 230-233 may be
directly to electrodes on the chip such as electrodes 164-169, or
directly or indirectly (via electrodes 181-185 . . . ) to electrode
regions on hybrid integrated elements such as the monitor diodes
140 or 142 or the laser diodes 112 and 114. The fibers connected to
the chip assembly 100 emerge through the package either via
connectors or seals (not shown). The sealing plate 240 may be
soldered or welded to the rest of the package if a hermetic seal is
desired to exclude humidity for example, or it may sealed with
adhesive, or even replaced with a potting material if hermeticity
is not required.
[0092] The frequency selective feedback structure may alternately
be a grating assisted coupler in codirectional coupling or
reflective coupling, a bulk-optics grating, a resonator or etalon
either in bulk form or in a waveguide (as for instance fabricated
by etching two parallel facets or trenches across a waveguide to
form a waveguide Fresnel reflector), or other devices capable of
selecting a spectrum within the optical frequency range that is
then fed back into the amplifier medium by means of optical
structures including bulk optics, waveguides, or other integrated
optical components. Some of these structures are compatible with a
ring laser embodiment including simple rings, multiple rings with
interconnections, and more complex topologies in three dimensional
waveguide structures.
[0093] Ring Laser Embodiment
[0094] FIG. 9 shows a ring resonator structure embodiment 900 which
is an alternative to the linear resonator structure embodiment 100
of FIG. 1. The optical radiation emitted from the two facets 916
and 918 at either end of the amplifier waveguide 912 in the
amplifier chip 910 is preferably butt coupled to waveguides 922 and
924, respectively. The amplifier chip 910 is preferably flip chip
bonded to the substrate 970 on which the waveguides 922 and 924 are
integrated. The waveguides 922 and 924 curve into a loop, passing
each other closely in a parallel coupler region where their
transverse mode profiles overlap but their effective indices
n.sub.eff1 and n.sub.eff2 are preferably dissimilar enough to
produce negligible coupling. Antireflection means are provided at
the facets 916 and 918 from among the alternatives described above,
as before including preferably angled waveguides at the facets, to
reduce optical feedback and suppress lasing between the facets. An
optical grating 930 overlaps optical energy flowing through the two
waveguides, and provides the phase matching that allows coupling
between the optical modes of the waveguides, allowing laser
feedback to occur around the ring resonator formed by the two
waveguides, coupler, and amplifier waveguide.
[0095] The grating period .LAMBDA. of the grating 930 needed to
accomplish this coupling follows from the phasematching requirement
that the wavenumber 2.eta./.LAMBDA. of the grating must equal the
difference in wavenumbers of the two waveguides, or
.LAMBDA.=.lambda./.vertline.n.sub.eff1-n.sub.eff2.vertline.(codirectional
coupler) (8)
[0096] When .LAMBDA. approximately satisfies equation (8), the
light traveling in one waveguide is coupled across the parallel
coupler to the other waveguide, and the direction of propagation of
the light is maintained in the same sense of propagation around the
ring. For example, light emitted from the facet 918 is coupled into
the waveguide 924, then across into the waveguide 922 by the
grating 930, where it travels back towards the laser chip 910, is
coupled back into the amplifier waveguide 912 at the facet 916.
This light is amplified and emitted again at the facet 918, having
made a round trip of the ring resonator. Light emitted from the
facet 916 is coupled into waveguide 922 and travels in the opposite
direction around the loop, so the ring resonator lases
bidirectionally unless a unidirectional element (not shown) is
introduced. Some of the light traveling in the waveguides 922 and
924 may be transmitted through the grating region and remain in its
respective waveguide. This transmitted light is conducted by its
respective waveguide to an output surface such as 925, where it
emerges for use. The structures and usages described in respect of
the individual embodiments herein also apply to the other
embodiments. Therefore, for example, V-grooves may be used to align
output fibers to the output ends of the waveguides at the output
surface 925, adjustments may be applied to the optical length of
the resonator to bring the FSR to equal a rational fraction of a
communications channel spacing, a polymer material and heater
stripes may be used within the grating region to tune the grating
to couple different cavity modes between the waveguides 916 and
918, etc.
[0097] The ring resonator of FIG. 9 lases when the gain in the
amplifier is sufficient to overcome the losses in the waveguide
ring resonator. The loss will be lowest for the longitudinal mode
of the cavity with frequency closest to the peak frequency of the
grating interaction. Even if the suppression of the gain for the
adjacent modes is small compared to the favored mode, the laser
will oscillate on the favored mode. To obtain a good sidemode
suppression ratio such as 30 to 50 dB, however, a significant
difference in gain between adjacent modes is desired, so the
grating is preferably designed with a substantial additional loss
for all other modes. If the injection current of the laser chip is
modulated to transmit data, two almost identical outputs are
provided that are modulated with the same data. This characteristic
may be advantageous in systems requiring a backup transmission line
in case of network failure on one of the lines. The output from the
two waveguides 922 and 924 may be identical but for the additional
bending loss experienced by the waveguide 924.
[0098] The above is a specific implementation of the general
situation in which light from an amplifier waveguide follows a
first path to couple with a grating structure and then returns to
the amplifier following a second path, the optical path forming a
closed loop figure in two or three dimensions. The entire loop in
FIG. 9 is comprised of waveguide segments, and the grating
structure is a grating assisted parallel coupler within the loop.
The ring resonator of FIG. 9 is preferably made athermal and its
modes adjusted to coincide with specified frequency channels. For a
tunable grating device, it is preferable for the tuning mechanism
to leave unchanged the cavity FSR, while tuning the grating
interaction frequency. The accomplishment of these objectives is
more complicated in this case since the grating lies intracavity.
The preferred approach is to make the thermal coefficients equal
and opposite for the two adjacent waveguides in the grating region
930. An on resonance mode that couples across the grating assisted
parallel coupler will traverse the same path length on each side of
the coupler. The thermal dependence of the two waveguide segments
then compensate each other when the grating temperature is varied
symmetrically about the center of the grating. Since it is the
difference in propagation constants that tunes the grating in a
co-directional coupler, equal lengths of opposite thermo-optic
coefficients add to produce a net thermal tuning sensitivity.
[0099] Equal and opposite thermal coefficients in a pair of
waveguides may be accomplished with one of the structures described
in reference to FIGS. 14C and 15. By adjusting the thickness of the
layer 1542, or the depth of the etch shown in step 14B, the amount
of overlap of the left mode (e.g. 1482) in the polymer material
1442 or 1546 may be adjusted. A second adjustable design parameter
is provided in the case of FIG. 14C by the thickness of the layer
1441, and in both cases by the thermo-optic coefficient of the
material 1442 or 1546. Adjusting these linearly independent
parameters is sufficient to bring the thermo-optic coefficients of
the two waveguides to the desired values, whether equal and
opposite, or zero and negative, or some other useful
combination.
[0100] An alternative method of rendering the tuning of the device
900 athermal is to apply a simultaneous heating input to the
semiconductor diode region 910. The opposite thermal coefficient of
the semiconductor amplifier results in cancellation of the effects
of the grating waveguides on the FSR during tuning provided that
the relative amounts of heat input are adjusted to produce equal
and opposite optical length changes. A wide tuning range may
preferably be obtained in this case by making one of the waveguides
in the grating region athermal while the other has a maximum
negative dn/dT.
[0101] FIG. 10 shows an alternative implementation 1000 wherein the
grating in the parallel coupler is a reflective grating and the
reflected light is coupled over to the second waveguide, which then
spatially diverges from the first waveguide, bringing the reflected
light back into the amplifier waveguide segment via a different
optical path. Reflective gratings typically offer narrower
bandwidth which can be advantageous in producing high side mode
suppression ratio. In a similar way to FIG. 9, the optical
radiation emitted from the two facets 1016 and 1018 at either end
of the amplifier waveguide 1012 in the amplifier chip 1010 is
coupled to waveguides 1022 and 1024, respectively, with
antireflection means. The amplifier chip 1010 is flip chip bonded
to the substrate 1070. The waveguides 1022 and 1024 curve into a
loop, passing each other closely in a parallel coupler region where
their transverse mode profiles overlap but their effective indices
n.sub.eff1 and n.sub.eff2 are preferably dissimilar enough to
produce negligible coupling. An optical grating 1030 overlaps
optical energy flowing through the two waveguides, and provides the
phase matching that allows coupling between the optical modes of
the waveguides. The grating period .LAMBDA. of the grating 1030
needed to accomplish this coupling follows from the phasematching
requirement that the wavenumber of the grating 2.eta./.LAMBDA. must
equal the sum of the wavenumbers of the two waveguides, or
.LAMBDA.=.lambda./(n.sub.eff1+n.sub.eff2)(contradirectional
coupler) (9)
[0102] When .LAMBDA. approximately satisfies equation (9), the
light traveling in one waveguide is reflected and coupled across
the parallel coupler to the other waveguide. Again, the direction
of propagation of the light is maintained in the same sense of
propagation around the ring. For example, light emitted from the
facet 1018 is coupled into the waveguide 1024, then across into the
waveguide 1022 by the grating, where it travels back towards the
laser chip 1010, is coupled back into the amplifier waveguide 1012
at the facet 1016. This light is amplified and emitted again at the
facet 1018, having made a round trip of the ring resonator. Light
emitted from the facet 1016 is coupled into waveguide 1022 and
travels in the opposite direction around the loop, so the ring
resonator lases when pumped above threshold.
[0103] Some of the light traveling in the waveguides 1022 and 1024
is transmitted through the grating region and remains in its
respective waveguide. This transmitted light is conducted by its
respective waveguide to an output surface 1025, where it emerges
for use. The lasing characteristics of the device 1000 are similar
to those of the device 900. As always, the structures and usages
described in reference to the other figures may also be applied in
various alternative embodiments of the devices in FIGS. 9 and 10.
The thermal coefficients of the two adjacent waveguides in the
grating region 1030 are preferably equal to each other. Temperature
tuning the grating does not change the optical length of the
resonator if the thermal profile for tuning the grating ends
abruptly at the edge of the grating. Since it is the sum of
propagation constants that tunes the grating in a
contra-directional coupler, equal thermo-optic coefficients in
equal length waveguide arms combine to give a net tuning range.
[0104] Method of Adjusting a Resonant Cavity
[0105] Due to fabrication tolerances, the FSR of a batch of devices
will vary with a mean and standard deviation that may not allow a
high production yield. A method is needed that allows the FSR of
individual devices to be adjusted to the desired value. A further
method is needed that allows the absolute frequency of longitudinal
modes to be adjusted to coincide with (fall within the acceptance
band of) one of the assigned frequency channels.
[0106] FIGS. 12A, 12B, and 12C, in another aspect of the present
invention, show methods of adjusting the resonant cavity, including
exposing Ge-doped silica, crosslinking a polymer, ablating
intracavity material, and depositing additional material in a
region traversed by intracavity optical energy, and including a
method of measuring and applying these methods to produce the
desired communications FSR within one tolerance, and to overlap a
desired optical frequency within another tolerance.
[0107] FIG. 12A shows the preferred method of adjusting the FSR to
the desired value. The waveguide 1240 is illuminated through
optical system 1281 with electromagnetic radiation 1282. In
response to an exposure, the material of the waveguide 1240 changes
its index of refraction, changing the optical length between the
reflecting facet of laser chip 1210 and the grating 1230. Verdiell,
U.S. Pat. No. 5,870,417, Feb. 9, 1999, has shown UV illumination of
an intracavity silica fiber being used to shift the longitudinal
modes of a resonator. In this invention, the FSR must first be
brought within tolerance before the longitudinal modes can be tuned
to a desired location. Many materials respond to electromagnetic
radiation by changing their index of refraction, and the frequency
of the electromagnetic radiation may be low or high according to
the mechanism of the interaction (which may include heating with
any of the frequencies, and other effects such as structural change
or chemical change). It is known that polymer mixtures containing
photoinitiator molecules will cross-link at C=C double bonds,
therefore gradually changing index of refraction when exposed to
visible or UV light with sufficient photon energy to activate the
photoinitiator molecule. Other materials also change index with
exposure, including silica, for example, which in the presence of
the Ge dopant will gradually increase in index when exposed to an
energy beam of ultraviolet light, and dye doped polymers wherein
the exposure induces a chemical change such as a bleaching or
molecular configuration change in the dye. Whatever the mechanism
may be for the radiation-induced intracavity change in index of
refraction, these methods may be applied to change the FSR of the
resonator, either increasing or decreasing it according to the
nature of the material modification that is being induced.
[0108] The use of ultraviolet or visible light is advantageous in
that the projected energy beam may pass through several layers of
the device structure, enabling change in index of material that is
buried underneath other layers. For example, a suitably prepared
Ge:SiO.sub.2 core may have its index adjusted by UV radiation even
though it is buried underneath a polymer cladding or protective
layer, provided that sufficient UV radiation reaches the core
material. In another example, a crosslinkable polymer layer such as
a top cladding layer may be index adjusted by UV or visible light
exposure through an upper protective layer that could have a
different composition. Or, if there is no upper protective layer,
the lower part of the polymer cladding layer, through which a
portion of the optical radiation passes, may be index adjusted by
light that passes through the upper portions of the cladding layer
which are remote from the optical mode.
[0109] To achieve the desired FSR in the cavity by correcting it
with UV exposure of Ge-doped silica (that decreases the FSR with
progressive exposure), the fabrication parameters prior to
correction may be chosen to produce a target FSR that is slightly
larger than the desired value so that the main part of the
statistical distribution of FSR in fabricated but uncorrected
devices lies above the desired FSR. The direction of the
pre-correction deviation of the target FSR is preferably chosen
opposite from the sense of the correction technique, thereby
allowing a single technique to be used (in this case increasing the
intracavity index) to produce the desired FSR in this main part of
the distribution. Ideally, the target FSR differs from the desired
ending value by a mean amount comparable to or larger than the
standard deviation of the FSR so that most of the distribution lies
on the same side of the target value. The target FSR is also
preferably chosen to differ from the desired FSR by not more than
the amount of change in FSR that can conveniently be accomplished
by applying the exposure. Good control of FSR correction may be
accomplished by making successive measurements of the FSR of an
individual resonator while applying successive exposure steps, each
exposure being smaller than the total exposure needed for full
correction. The energy in the exposure beam may be adjusted on each
step according to the desired amount of optical length adjustment
for that step. The spatial region being exposed may also be changed
during the adjusting process, which has the advantage of allowing a
larger net change in optical length, and a greater precision. The
free spectral range may be measured by measuring the optical
spectrum of the laser for two or more longitudinal modes, and this
method may be repeated until the FSR of the cavity equals a
rational fraction of a communications channel spacing within a
tolerance such as a few percent of the channel spacing.
[0110] To achieve the desired overlap of a subset of longitudinal
modes with assigned frequency channels, after the FSR has been set
to the desired rational fraction, the frequency of one of the modes
is shifted to coincide with the desired assigned channel. If the
free spectral range is equal to the assigned communications channel
spacing, and one longitudinal mode has been aligned to coincide
with one of the channels, all longitudinal modes will coincide with
assigned channels within a frequency range of linearity.
[0111] FIG. 16 shows the method of adjusting the cavity which
consists of the steps shown in the flowchart 1600. The amplifier
section is preferably operated in step 1610, preferably above the
threshold of the resonator, in which case there is a narrowing of
the spectrum around the longitudinal modes of the resonator. There
is an optical output that can be used to determine the longitudinal
modes either above or below the threshold, but the laser is
preferably operated above threshold since both the output power and
the spectrum depend on the drive current. In a variation of this
method, step 1610 is modified to become the step of operating a
light source to illuminate the optical path of the resonator.
[0112] In step 1615 the optical output from the cavity is monitored
and in step 1620 its spectral features are determined.
Specifically, the center frequencies of at least two longitudinal
modes are identified in 1620 in order to determine the FSR. This
step can be accomplished either above or below threshold. In step
1640 the refractive index of the resonator is modified after
calculating the desired rate or duration of the exposure in step
1635. The rate and duration of exposure are preferably calculated
based on the information produced from steps 1615 and 1620. The
modifying step 1640 may be accomplished in parallel with step 1615
to ensure there will be no overshoot, and steps 1620 and 1635 may
also be accomplished simultaneously. In step 1635, the process is
adjusted (e.g. the rate and duration of the process) so that the
measured FSR approaches and eventually equals the desired FSR
within a high degree of accuracy. When it is determined that the
target FSR is being approached, the modification rate is preferably
decreased, and when it is determined in step 1625 that the target
FSR is achieved within a tolerance, the optical length modification
may be stopped in step 1630.
[0113] FIG. 12B shows an alternative method of adjusting the FSR by
removing material traversed by the mode in the laser cavity.
Methods that remove material include the preferred laser ablation,
but also ion beam etching and other techniques. It is known that
many materials that absorb UV light will ablate when exposed to a
sufficiently high short pulse exposure of millijoules or Joules per
square centimeter. The energy beam 1283 that is projected onto the
intracavity waveguide is preferably an excimer laser beam that may
be directed, focused, or imaged onto the surface after passing
through a mask in the optical system 1281. Each laser pulse
typically ablates a few tenths of a micron of material from the
air/material interface, over a spatial extent that may be defined
by the laser beam or by the mask. After ablation, the volume
previously occupied with solid material is usually occupied by air
or a clearing gas, which has a much lower index of refraction than
most solid materials. The result is a lowering of the effective
index of refraction of the portion of the waveguide where ablation
occurs. (Of course, a later deposition of material filling the
removed volume can result in a later increase in effective index of
refraction.) The laser ablation technique is applied to a portion
of the material of the cavity that is prepared to lie at a surface
accessible to the ablating beam, and that is traversed by optical
energy circulating in the cavity. In FIG. 1, the laser ablation may
be applied to the regions directly over the waveguides 122 and 124
between the heater electrodes 150 and 152, for instance, or between
the heater electrode region and the tapered waveguide region, or in
the region of the tapered waveguides 126 and 128, etc. When
material is removed at these surfaces, the effective index is
reduced for the mode that traverses the optical cavity underneath
that surface, increasing the FSR. (The ablation technique may also
be applied to the semiconductor diode laser in that GaAs is known
to ablate, but there may be no large surface on the diode laser
that is conveniently available for ablation without disrupting
electrical contacts, passivation, or some other function.)
[0114] While the originally prepared surface of the material may be
far from the optical mode, by proceeding with the ablation process,
the desired deeper surface may be revealed. If a large change in
effective index is required for a given resonator, the laser
ablation process may proceed to a relatively greater depth, closer
to the core of the waveguide where the mode intensity is highest.
For a small change in effective index, a shallow laser ablation
process is arranged.
[0115] Since a high accuracy may be required even when large
changes are needed, multiple regions of ablation may be used. A
deep ablation may be used to accomplish most of the correction, and
a separate, shallow, ablation can be performed at a different
location on the same resonator. The shallow ablation provides a
small change in the effective index for a large exposure pulse,
allowing high precision to be obtained in adjustments to the FSR. A
further method of increasing the precision of adjustment is to
reduce the extent of the spatial region exposed to the ablation
beam.
[0116] FIG. 12C shows a further alternative method of FSR
adjustment by adding material to the laser cavity. Methods that
deposit material include the preferred evaporation process, and
also laser induced chemical reaction sometimes called laser
pantography, sputtering, and others. In the sputtering or
evaporation approaches, the incoming beam 1284 that is directed
onto the surface is a beam of particles, and the directing
apparatus 1285 may be an electromagnetic director or a mask or
baffle system. These methods can use standard equipment to deposit
material uniformly at a controlled rate so that all of the free
spectral ranges of all of the resonators on an entire wafer may be
shifted simultaneously. These approaches proceed in a vacuum, and a
vacuum-compatible FSR measurement approach is needed such as fiber
coupling to external measurement apparatus. In the laser induced
selective deposition technique, the incoming beam 1284 is a laser
beam, and the directing apparatus is a focusing optical system that
may or may not include a mask. The laser beam excites material at
or near the deposition surface, inducing a chemical reaction near
the region where the device is illuminated by the laser, and
resulting in deposition.
[0117] Again, the spatial location of the portion of intracavity
waveguide where the index modification is applied may be changed
during the process. In the deposition method, the rate of change of
index decreases with time until the location is changed to a fresh
portion of the intracavity waveguide when the rate increases again.
If the method chosen is to expose the material to radiation, the
index change may saturate with time at a given location, and
changing the location increases the rate of change of index at a
constant exposure condition.
[0118] By illuminating the individual devices on a wafer separately
by one of these deposition techniques, individual correction of the
device FSRs may be achieved. Because of process variations, FSRs
are likely to vary across a given wafer, and a spatially selective
approach such as UV laser exposure may be preferable.
[0119] Frequency Selective Cross Connect
[0120] FIG. 13 shows a frequency selective cross connect device as
another embodiment of the present invention that uses an athermal
resonator configuration adjusted to coincide with predetermined
frequencies, coupled to a waveguide via a tunable grating. The
device 1300 may connect light from an input port to alternate
output ports, depending on the frequency of the light. It may be
used as a wavelength switch, as an add-drop device, as a detector,
etc.
[0121] Waveguides 1322, 1324, and 1326 are fabricated on substrate
1370 from under cladding material 1344, core material 1340, and
over cladding material 1342 as described elsewhere. Grating 1330 is
fabricated in the parallel coupler region 1392 to form preferably a
reflective coupler. As is well known, the propagation constants of
the waveguides 1324 and 1322 are preferably different, and the
wavenumber of the grating 1330 equals the sum of the propagation
constants of the waveguides in the reflective grating
configuration. In the alternative codirectional coupler grating
configuration, the difference in the propagation constants is used,
and the directions of the light flow as designated by the arrows
1325, 1306, and 1308 are reversed. The transverse profiles of the
optical modes propagating in the waveguides 1324 and 1322 overlap
each other in the region of the grating 1330, producing a coupling
k1(.lambda.) at the wavelength .lambda. which is the fraction (in
the absence of feedback around the waveguide 1324 which forms a
resonator) of the power 1302 input in waveguide 1322 that is
coupled into the waveguide 1324 in the sense shown by arrow
1325.
[0122] The waveguide 1324 comprises a closed loop, forming a
resonator. Power in waveguide 1324 flows into the parallel coupler
1394 where the propagation constants of the waveguides 1324 and
1326 are approximately equal within a tolerance, producing a
coupling k2(.lambda.) of the 1324 waveguide power into the
waveguide 1326. The wavelength dependence of the coupling k2 is
much less than that of the coupling k1 in the absence of a grating
in 1394. Alternatively, a second grating structure could be used in
1394 as described below in reference to FIG. 17. This coupled power
exits waveguide 1326 at 1306. The remainder of the power in
waveguide 1324 follows the closed path of waveguide 1324 back into
the coupling region 1392, is partially reflective coupled back into
the waveguide 1322. The power circulating in the waveguide 1324 is
due to the interferometric sum of all the round trip electric field
components and the input field of the input beam; the exit power
1304 is the interferometric sum of the transmitted remainder of the
field of 1302 and the out coupled field from the resonator. The
heater stripe 1350, connected to the two electrode pads 1354 and
1356, allows thermo-optic tuning of the grating 1330. A detector
may be placed to measure the power 1306, in which case 1300
functions as a detector or receiver.
[0123] The resonator 1324 is preferably designed for a FSR that is
a rational fraction of a communications channel spacing, it is
adjusted in manufacturing both to equal the desired FSR and to
adjust the longitudinal mode positions within tolerances, and it is
designed to be athermal so as to provide an absolute frequency
reference, independent of the heating of the grating 1330. The
regions 1328 and if necessary 1329 are regions of modification of
the index of refraction of the waveguide 1324 in order to achieve
the desired FSR and longitudinal mode frequencies.
[0124] In view of the fact that the interaction between the two
waveguides at the grating is preferably strongly dependent on
temperature, the athermal requirement on the resonator presents
some problems in its realization. In the preferred embodiment,
these two seemingly contradictory requirements are met by designing
the resonator waveguide 1324 to be athermal along every portion of
its length, and by using a unique waveguide design shown in FIGS.
41C or 15. To retain tunability of the grating, the closely spaced
waveguide 1322 is preferably made strongly temperature
dependent.
[0125] FIG. 11B shows the preferred athermal design for the
waveguide 1324. In contrast to the design shown in FIG. 11A where
the overlap factor .GAMMA..sub.c is large, in the cross section of
the device 1110 in FIG. 11B, the overlap factor .GAMMA..sub.c in
the polymer layer 1142 is just small enough so that all of the
layers together produce zero net change in optical length
n.sub.effL with temperature
d(L n.sub.eff)/dT=0=d(L
.SIGMA..sub.i.GAMMA..sub.in.sub.i)/dT=.SIGMA..sub.-
i.GAMMA..sub.i(Ldn.sub.i/dT+n.sub.idL/dT) (10)
[0126] where .GAMMA..sub.i is the overlap factor for the ith
transverse portion of the waveguide traversed by the optical energy
of the mode, and in this case the sum is taken at a given
longitudinal position, over all the transverse portions of the
waveguide. If for example, three materials are involved in the
waveguide: polymer, cladding silica, and core silica, the sum is
taken over these three regions. The waveguide index contrast and
dimensions and layer thicknesses and etch depths are designed so
that the polymer overlap factor .GAMMA..sub.c takes on the correct
value to produce a null in equation (10) within a tolerance. Any
one of the electrodes in FIG. 11B may be used, but for thermo optic
excitation a simple single electrode such as 1120 is preferred as a
heater trace. In the preferred embodiment of the grating region
1330, the waveguide cross section is shown in FIG. 14C with the
waveguide 1324 being the waveguide on the right with transverse
mode 1481.
[0127] The preferred fabrication process for the devices 1100 and
1110 start the same. In both cases, a lower cladding 1144 of silica
is fabricated on a silicon substrate 1170. A core layer is
deposited, and the layers consolidated, patterned, and etched to
form the ridge(s) 1140. In the case of FIG. 11A, layer 1142 is then
spun on top, and the waveguide is complete. Because the polymer
layer 1142 is close to the axis of propagation of the mode and the
mode 1180 substantially overlaps the polymer layer (.GAMMA..sub.p
is large), this structure is a temperature tunable one with a
relatively large change in n.sub.eff with temperature. The
structure of FIG. 11A is suitable for the waveguides 1322, 122,
922, 1022, 1024, 1726, and the right hand waveguide of FIG. 15.
[0128] In the case of FIG. 11B, however, another layer, preferably
of silica 1141 and of thickness comparable to the core layer
thickness, is deposited and consolidated after patterning the core
material 1140. To achieve a relatively small overlap factor, the
layer 1141 is deposited with thickness in the range of 40% to 200%
or more of the thickness of the core layer. Layer 1141 is then
etched back by a fraction of its depth in the range of 0% to about
90%. The cladding layer 1142 is spun on and cured. Depending on the
depth of the layer 1141 and the depth of the etch-back, more or
less of the mode 1181 is revealed for propagation in the clad layer
1142. By adjusting these parameters, the overlap factor
.GAMMA..sub.c may conveniently be adjusted below the value obtained
in FIG. 11A. If the deposited layer is thin, in the range from 5%
to 40% of the core layer thickness, when the upper cladding layer
1142 is spun on, the overlap factor will take on an intermediate
value.
[0129] To obtain an inherently athermal waveguide, since
(dn/dT).sub.cladding may be more than 20 times larger than
(dn/dT).sub.silica, the overlap factor can be reduced to the
neighborhood of 0.05 or lower, depending on the exact values of the
thermo-optic coefficients. Where the condition (10) is satisfied in
the waveguide, any longitudinal segment of it may be heated or
cooled without affecting the resonator FSR. Transverse thermal
gradients across the structures 11A or 11B are preferably avoided
because they will introduce residual changes in the FSR and mode
positions.
[0130] The waveguide 1322, is preferably designed according to FIG.
11A so as to obtain a large tuning rate with temperature. Exciting
the heating stripe 1350 heats the grating region 1330 including
segments of both waveguides 1322 and 1324. The effective index of
the guide 1322 changes rapidly, tuning the grating wavelength. The
effective index of the adjacent guide 1324 remains constant and
independent of temperature, so that the longitudinal modes of the
resonator are substantially unaffected. To maintain the coupling
ratio of the coupling region 1394 independent of temperature, the
waveguide 1326 is preferably designed the same way as the waveguide
1324.
[0131] The reflection band of the grating 1330 is preferably
narrower than the communications channel spacing. The power
transferred from the waveguide 1322 to the waveguide 1324 depends
on both the grating assisted coupling function k1(.lambda.) and the
resonator spectrum. The longitudinal mode of the resonator with the
largest coupling value is most strongly excited. For the structure
1300 to act as a frequency selective detector, the coupling
function should be strong at the selected frequency(ies) and small
at the adjacent (or all the other deselected) longitudinal modes. A
preferred suppression would be greater than about 30 dB. To
accomplish this suppression, the bandwidth of the grating coupler
is preferably narrow so that when the grating is tuned to reflect
at one of the longitudinal modes, the coupling function is very
weak at the other longitudinal modes. The grating is also
preferably apodized and/or chirped and otherwise engineered to
accomplish this objective.
[0132] The grating 1330 couples a frequency into the waveguide
1324, but if the resonator formed by 1324 is off resonance, the
multiple passes around the resonator add up out of phase and
suppress the coupling. As the grating 1330 is tuned through a
resonance of the cavity, the multiple passes add in phase and the
coupling is enhanced. There is significant power coupling with the
resonator 1324 only on the frequencies defined by the resonator,
despite analog tuning of the coupling grating. Power is coupled out
of the waveguide 1322 one selected frequency at a time, digitally,
with strong suppression at intermediate frequencies. A fraction of
the power circulating in the waveguide loop 1324 is coupled into
the waveguide 1326. With a detector coupled to the power 1306, the
device 1300 becomes a tuned detector that can be switched between
preselected channels by controlling the current flowing through the
heater 1350. The remainder of the power flows transparently through
the waveguide 1322 and into the output 1304. If the device 1300 is
an add/drop device, the output 1304 is connected to the continuing
fiber in the transmission system, with the non-selected frequencies
passing through with insertion loss but otherwise essentially
unchanged. If the device 1300 is a tuned detector, the output 1304
is preferably coupled to an absorber and/or an angled reflector
and/or diffractor to ensure that there is no return signal.
[0133] Impedance Matching
[0134] If the coupling coefficient from the waveguide 1322 into the
resonator formed by 1324 is properly impedance matched at a
specific frequency, all of the power 1302 at that frequency will
flow through the resonator and into the port 1306 with no power
leaking through to 1304. When the resonator is excited
on-resonance, the electric field amplitude coupled in 1392 out of
the waveguide 1324 into waveguide 1322 is exactly out of phase with
the remaining input power from 1302 that transmits directly through
the grating coupler 1330 in waveguide 1322 into the output 1304.
Perfect impedance matching is obtained when the resonator loss and
the coupling strength are adjusted so that the amplitudes of those
two waves are identical, resulting in perfect cancellation and zero
power flow into the output channel 1304. With perfect impedance
matching, all of the input power from 1302 flows into the resonator
and is partitioned between the output 1306 and the dissipative
losses of the resonator 1324. If the resonator is imperfectly
impedance matched or if it is excited off resonance, some of the
power input at 1302 at that frequency will be passed through to the
output 1304.
[0135] The desired situation is to impedance match the coupler 1392
so that at the predetermined frequency of the resonator that is
selected by the tunable grating coupler, all the input power flows
to the output 1306. At all other frequencies not selected by the
grating, the input power flows transparently to the output
1304.
[0136] Perfect impedance matching can be designed once the
dissipative loss of the resonator is known, by choosing the
resonant coupling constant in coupler 1392 and the coupling
constant of coupler 1394 to meet the stated criterion. For
efficient operation, the coupling in 1394 should preferably equal
the dissipative losses or exceed them by a factor of two or so.
This leaves the coupling in the coupler 1394 as the main free
variable that is used to achieve perfect impedance matching. In
manufacturing, as before, perfect impedance matching will not
always be obtained of a large enough fraction of the production. A
method for trimming the impedance matching is needed for increasing
the yield. It is possible to adjust the coupling constant of the
parallel couplers 1392 and/or 1394 and/or the loss of the resonator
waveguide 1324 to achieve perfect impedance matching.
[0137] The coupling constant in the couplers depends on both the
transverse mode overlap between the two waveguides in the coupler,
and on the strength of the grating. In trimming the couplers, it is
convenient to measure a crosstalk quantity during fabrication of
the grating and to adjust the grating strength to minimize that
crosstalk. For example, if the selected crosstalk quantity is the
power feedthrough on-resonance power feedthrough to 1304 from 1302,
the ratio of these powers may be measured and minimized to produce
good impedance matching. This step is preferably done after the
waveguide is fully fabricated, i.e. after the cladding layers are
deposited. If the grating is formed by exposure to an energetic
beam such as visible or ultraviolet light, the exposure may be
adjusted until the crosstalk quantity lies below the desired
threshold. For example, UV exposure of the Ge-doped silica core can
be performed by illuminating the core through a fully cured polymer
cladding layer using the interfering beams approach or possibly a
phase mask. Or, exposure of a sensitized polymer cladding layer can
be accomplished by the same means. With good impedance matching,
the power at the resonance frequency that leaks through to 1304 can
be reduced to a desirably low value such as -30 dB, for
instance.
[0138] For example, a test beam may be injected into the waveguide
1322 providing an input signal 1302, and a detector placed to be
sensitive to a portion of the light exiting the waveguide 1322 as
output signal 1304. The ratio of these two signals at the frequency
of resonance is a type of crosstalk called the extinction ratio.
This may be done before or after slicing and dicing of the wafer
into individual chips. By measuring the fraction of the input
signal 1302 that emerges as an output signal 1304 while the
exposure of the grating 1330 is proceeding, the total exposure
required can be predicted, and the exposure stopped before reaching
an over-exposed condition. By remeasuring the extinction ratio, a
small increment of exposure may be calculated and applied to nudge
the impedance match to the desired optimum. As an alternative, the
add-drop crosstalk may be measured at a resonant frequency as the
ratio between the output into channel 1306 when the input 1308 is
excited, or vice versa.
[0139] The transverse mode overlap between the two waveguides may
be adjusted by small amounts by changing the index of refraction of
the region between the waveguide cores. Again, UV exposure of
either a sensitized silica region or of an incompletely cured
polymer region between the cores can accomplish this objective.
This approach has the advantage that it may be applied independent
of the mechanism chosen to fabricate the grating.
[0140] The resonator loss may be increased by several methods.
Provided that the coupling into the resonator in the grating 1330
is designed to slightly exceed optimum, the resonator loss may be
increased to bring the device to optimal impedance matching. The
resonator loss may be increased by laser ablation or wet or dry
etching over a portion of the waveguide 1324, for instance,
gradually bringing a somewhat roughened surface closer to the core
of the mode in the resonator and increasing the optical loss due to
factors such as diffraction, scattering, and mode coupling loss.
Alternatively, an index of refraction discontinuity may be created
transverse to the waveguide by laser ablation or by UV irradiation
of sensitized silica or polymer, creating a reflection or
diffraction loss. A shallow angle interface can reflect a large
amount of light with even a small index change. Other approaches
are also possible such as by doping a photo chromic molecule into
the resonator to produce changes in absorption upon exposure. Or, a
surface absorption loss may be introduced for example by depositing
a metallic film near enough to the waveguide core to produce some
optical loss. The loss induced by this film may be controllably
trimmed away by e.g. laser ablating portions of the film, thereby
also adjusting the resonator loss.
[0141] Note the desirable fact that if the grating drifts slightly
in center wavelength, the transmission spectrum of the output beam
1306 is little changed. The effect of drift shows up as a change in
the side mode suppression, and as a reduction in efficiency of
power transfer to the output leg 1306 (reducing the detection
efficiency of a detector mounted to receive the power 1306).
[0142] When the cavity is athermal, the grating coupler is tunable,
the FSR and one of the modes has been frequency adjusted, and the
impedance matching is complete, the device 1300 may be used as a
digital wavelength selective cross connect. A multiple frequency
(DWDM) input may be connected as 1302, and selected ones of the
frequency channels may be redirected from 1304 to 1306 with good
efficiency and low crosstalk. The device 1300 is essentially
transparent at the other frequencies.
[0143] An additional input port allows power to enter the system at
1308. By symmetry and time reversal invariance, if the coupler 1392
is impedance matched at a given frequency from the input 1302 into
the output 1306, then the coupler 1394 will also be impedance
matched at the same frequency from the input 1308 into the output
1304. This means that when the device is tuned to efficiently drop
a channel into 1306, it is also tuned to efficiently add a channel
from 1308 to 1304. Data at the optical frequency of coupling
determined by the grating 1330 and its actuator 1350 may therefore
be dropped out of 1302 and added into 1304, substantially
independently if the crosstalk is small, and without affecting the
other frequency channels of the system.
[0144] The waveguide 1324 forms a resonator. Light makes multiple
round trips before decaying away due to losses and coupling out of
the resonator. The multiple round trips interfere with each other,
producing the characteristic FSR and longitudinal mode spectrum
already referred to, along with the phenomena of impedance
matching. This situation is in marked contrast with the situation
in which a totally reflective grating is placed in the coupler
region 1392. In the high reflector grating case, light 1302 enters
the waveguide 1324 after one reflection from the grating 1330,
makes a single pass around the loop 1324, and exits as light 1304,
without substantial interference from multiple round trips in the
loop 1324. The functioning of such a high reflective grating device
would be essentially the same whether or not the waveguide 1324
forms a resonator. This high reflector grating case is described in
A. S. Kewitsch, U.S. Pat. No. 5,875,272, Feb. 23, 1999. Without
interference, the useful longitudinal mode resonances
disappear.
[0145] The waveguides 1322 or 1326 may be coupled to other devices,
including fibers, amplifiers, switches, reflectors, filters,
modulators, sources, and detectors. In particular, 1326 may also be
a communications waveguide carrying multiple optical frequency
channels that enter at 1308. In the configuration of FIG. 13, the
resonator 1324 is coupled simultaneously to substantially all these
wavelengths since the coupler 1394 is a broadband coupler. Light
input into 1308 with the frequency selected by the grating 1330
will be transferred into the output 1304. In this configuration,
the device 1300 acts as a channel-selective cross-connect switch,
with only the selected frequency (or frequencies, depending on the
grating design) being switched from 1302 to 1306 and from 1308 to
1304, and all the other frequencies passing through. However, as
shown, the waveguide 1326 is not as transparent as the waveguide
1322 because the coupler 1394 is broadband, and couples many
frequencies into the resonator 1324, whereas the coupler 1392 is
narrowband and may couple only one frequency into the resonator.
Although the frequencies that are coupled from waveguide 1326 into
the resonator may not be coupled into the waveguide 1322 if the
grating is off resonance, there will still be distortion in 1326 at
the coupling frequencies, such as pulse distortion and additional
power loss.
[0146] FIG. 17 shows another embodiment 1700 of the invention in
which the coupler 1794 is also fabricated as a narrowband coupler
comprising elements similar to those of coupler 1392. A
thermo-optic grating 1730 and heater stripe 1750 are provided as
described. The waveguides 1324 and 1726 have dissimilar mode
propagation constants, and preferably dissimilar thermal
coefficients as described in relation to FIGS. 14C or 15. Waveguide
1726 is fabricated in the same way as waveguide 1322. The input
1708 and output 1706 have reversed position from FIG. 13 since the
grating coupler in 1794 is reflective. The functioning of the
device 1700 may be almost identical to that of the device 1300
except that the waveguide 1726 is now transparent at the
frequencies away from the grating resonance(s). The inputs 1708 and
1302 may be symmetric, as for the outputs 1706 and 1304. To couple
a specific frequency across the resonator from 1302 to 1706 or from
1708 to 1304, both gratings 1330 and 1730 are typically tuned to
the desired frequency. Multiple optical frequency channels may be
present on both 1302 and 1708, and the data on a specified channel
is interchanged when both gratings are tuned to that frequency. The
device 1700 therefore may act as a frequency selective cross
connect or optical switch, including the subset functions of
add-drop and tuned detector. The two gratings 1730 and 1330 may be
tuned simultaneously by the same heater current, but there are
advantages to being able to tune them separately.
[0147] By cascading multiple devices such as 1700 or 1300 along a
single input channel 1302 (connecting 1304[i] to 1302[i+1])
multiple optical channels may be independently dropped into
multiple output channels 1706[i] or added from multiple input
channels 1708[i]. By cascading multiple devices such as 1700 or
1300 along two input channels (connecting 1304[i] to 1302[i+1] and
connecting 1708[i] to 1706[i+1]), multiple optical channels may be
independently cross connected between the two fibers each carrying
multiple data channels.
[0148] The devices 1300 and 1700 are reconfigurable since they may
be changed from acting on (i.e. cross-connecting) channel i to
acting on channel j. It may be desirable, when tuning the device
1700 from channel i to channel j, to avoid acting on the
intermediate channels that lie between channels i and j. If
channels i and j are separated by active communications channels
carrying data, it is essential to avoid significantly perturbing or
"hitting" the data transmission in those active channels when
tuning a device from cross-connecting channel i to cross-connecting
channel j. The device 1700 offers a unique way to avoid hitting the
intermediate channels. The two gratings 1330 and 1730 may be tuned
at different times or different rates so that their frequencies do
not overlap at the intermediate channels but only at the desired
end channels. If the two grating responses do not overlap,
impedance matching is not obtained across the resonator, and the
interaction with an intermediate channel while tuning across it is
greatly diminished: no light will be added, dropped, or switched.
(There will be a perturbation due to the coupling into the
resonator 1324, but if the tuning is accomplished slowly, this
perturbation may be negligible.) To tune from channel i to channel
j, for instance, one grating may be tuned first, and then the
other. The action on channel i will cease as soon as the first
grating is detuned, and the action on channel j will not begin
until both the first and the second gratings are tuned to channel
j. Or in another example, the two gratings may be detuned relative
to one another by a desired amount to stop the action, after which
the detuned pair is tuned across to the desired channel where they
are retuned into coincidence with each other.
[0149] FIG. 18 illustrates a vernier method of extending the tuning
range by using multiple grating peaks. The longitudinal modes of
the resonator 1324 are shown as a function of frequency by the
multipeaked curve 1810 where the width of the resonances has been
shown to be very small for simplicity. We assume that any
birefringence has been compensated so that the curves for the two
polarizations overlap. The resonator FSR 1814 is the separation
between adjacent peaks of the curve 1810. The two gratings 1330 and
1730 each have multiple peaks, as shown by the curves 1820 and
1830, respectively, which each have five peaks. The peaks of the
gratings are essentially equally spaced, but the peak spacing in
one grating may be larger than that of the other grating by the
frequency 1812, which in FIG. 18 is equal to twice the FSR. For the
tuning shown in FIG. 18, a pair of grating peaks 1823 and 1833
coincide in frequency. By tuning the pair of gratings together over
the range 1840 (7 FSR), eight adjacent longitudinal modes 1852 may
be selected. If the FSR equals one communications channel
separation and the modes 1810 overlap the channels as described,
eight successive communications channels may be selected by the
device 1700.
[0150] When one spectrum e.g. 1830 is detuned relative to the other
by a certain amount, in this case by one FSR, none of the modes
overlap, and no mode is active in the device 1700. The frequencies
of the two spectra may be tuned together in this condition without
dropping or switching any of the channels. By tuning the spectrum
1830 of the grating 1730 by two FSR increments to lower frequency,
the second pair of grating peaks 1824 and 1834 may be brought into
coincidence. The next eight successive channels 1854 may be
selected by again tuning the two gratings together over the range
1840, relative to the starting point where the grating peak 1834
has been adjusted to overlap 1824. It apparent clear that by tuning
the spectrum 1830 by either 0, .+-.2 FSR, or .+-.4 FSR, and by
tuning the pair of spectra 1820 and 1830 together over the range
1840, a total of at least 5.times.8=40 sequential channels 1850 may
be addressed. The required tuning is only about 16 FSR, including
the initial tuning required to superpose the grating peaks 1823 and
1833 on the desired channel. The number of channels that can be
addressed for a given tuning range has been increased by the use of
this vernier method at the expense of the additional channel
perturbation caused by the additional grating peaks, which is
preferably made small.
[0151] If it is desired to jump between noncontiguous tuning
ranges, such as from the range 1852 to the range 1856, the
intermediate modes such as 1824 and 1834 must be made to overlap
somewhere. If intermediate channels are in use for data
transmission for example, so that they cannot be hit, the tuning
may be adjusted so that the modes 1824 and 1834 overlap at
intermediate frequencies such as in between longitudinal modes
1810. Provided that the grating spectra of the peaks 1824 and 1834
are narrow, the coupling by the gratings into or out of the
resonator 1324 can be made small.
[0152] Many variations of this approach may be used. If the grating
peaks are narrow enough, the frequency interval 1812 may be smaller
than two FSRs, allowing use of additional grating peaks. If the
tuning range covers more (or less) than eight channels, the tuning
range may also be increased (or decreased). Unequally spaced peaks
may be used, etc. As a further alternative, the device 1300 or 1700
may be combined with a pair of optical switches on either side and
a bypass waveguide that route an optical input either through the
device or around it along the bypass waveguide to an optical output
in an arrangement known in the art as a hitless switch, so that the
optical transmission may be switched to bypass the device during
the tuning of the grating 1330.
[0153] Differential Waveguide Thermal Response
[0154] FIGS. 14A and 14B are two intermediate stages in the
fabrication of the embodiment of FIG. 14C which contains a pair of
adjacent waveguides fabricated with different (preferably
thermo-optic) coefficients. The first steps of fabrication of such
a structure preferably involve the deposition of a cladding layer
1444 followed by a core layer with higher index of refraction on a
substrate 1470. As described previously, this structure is
preferably fabricated from silica on a silicon wafer, with Ge
doping for the core, but many variations are available. The core
layer may then be patterned, producing the twin parallel ridge
structures 1440, seen in cross section end-on. The top cladding
1441 is then applied and an etch mask layer 1443 is deposited and
patterned to reveal only one of the two waveguides along a desired
portion of its length. FIG. 14A shows the semi-processed article at
this stage in its fabrication. An etching step such as RIE is now
applied to remove some of the silica cladding in the regions not
covered by the mask, resulting in the structure of FIG. 4B. After
removing the mask material of layer 1443, a thick polymer cladding
layer 1442 is spun on, and an electrically conductive layer may be
deposited and patterned to form the electrode structure 1445,
resulting in the device of FIG. 14C. Additional layers of
electrodes or additional waveguide cores, gratings, and other
components may optionally be fabricated either on top of, in, or
below this structure.
[0155] In the case of the grating coupler 1392 or 1794 for
instance, it is desirable to fabricate a grating such as grating
1330 in the layer 1441 between the two core regions 1440 (or in
another portion of the structure traversed by optical modes 1482
and 1481). This grating may be fabricated at several alternative
times during an overall process sequence, including before
depositing the layer 1443, and after applying the layer 1442. It is
apparent from FIG. 14C that the mode 1482 has a much larger overlap
factor in the layer 1442 than the mode 1481. If desired, the
waveguide design and the layer characteristics including thickness
may be adjusted so that the mode 1481 has no net temperature
dependence of its effective index of refraction (all such
conditions are met within a tolerance). Then the mode 1482 of the
adjacent waveguide has a net negative thermal tunability while its
partner is athermal, as called for in the preferred design of the
devices 1300 and 1700. If desired the temperature dependence of the
two waveguides may also be realized as equal and opposite by
further reducing the overlap factor of the modes in the negative
thermo-optic coefficient material 1442. Other variations are also
possible including making the mode 1482 athermal, in which case the
mode 1481 has a positive thermal coefficient.
[0156] In a different embodiment, the layer 1442 is an
electro-optic, and the element 1445 is an electric field applying
electrode. In this case the change in effective index with applied
field is different for the two waveguides 1440 and for the two
modes 1481 and 1482. As before, an electrode configuration similar
to that of FIG. 11B may be preferred for electro-optic devices.
[0157] FIG. 15 shows an alternative embodiment of an adjacent pair
of waveguides with different coefficients. The fabrication
technique involves a variation of the fabrication technique
described in reference to FIGS. 11A and 11B, but with the addition
of a masking step, an etch step, and another deposition step. An
adjacent pair of waveguide cores are fabricated as described e.g.
with reference to FIG. 11A. A cladding layer 1544 and a core layer
1540 are deposited on a substrate 1570. The core layer is patterned
to form the twin waveguide cores shown in FIG. 15, and cladding
layer 1542 is deposited. A mask layer may be applied above the
layer 1542 and patterned in the same way as described in reference
to the mask 1443 in FIG. 14A, to reveal only one of the two
waveguide cores along a portion of its length. An etch process is
applied to remove the layer 1542 where it is exposed by the mask as
shown on the right side of FIG. 15. This step is preferably a
selective etch so that all of the layer 1542 is removed where
exposed, without substantial removal of the silica layers 1544 or
1540. A final layer 1546 is applied, producing the structure of
FIG. 15. Optionally, the surface may then be planarized. In the
case of FIG. 15, the modes that propagate in the two different
cores may have different thermal coefficients because of different
thermal characteristics of materials used for the layers 1542 and
1546. If, for example, a heavily cross linked polymer is used for
layer 1542, and a lightly cross linked layer of a similar polymer
is used for layer 1546, all other things being equal, the mode with
the larger overlap factor in the more lightly cross linked material
1546 will have the stronger thermal coefficient (assuming the
thermal coefficient is dominated by the layers 1542 and 1546). Or,
if the operating temperature is well below the glass transition
temperature Tg of the polymer 1542, but well above the Tg of the
polymer 1546, the mode with the larger overlap factor in the
above-Tg material 1546 will have the larger (negative) thermal
coefficient. As an alternative in any of the above, if a layer such
as 1542 is itself photosensitive, an additional masking layer may
be omitted, and the layer such as 1542 may be patterned directly by
exposure and development to form the desired patterned
structure.
[0158] In the implementations of the invention described herein, a
liquid crystal material may be used to change the index of
refraction of a waveguide. For instance, layer 1546, or 1442, or
1142, or 842, or 742, or 342 or other layers traversed by the
optical mode within the grating region may be a liquid crystal
material of one of the many types known in the art. A particularly
convenient case is that of the polymer dispersed liquid crystal
(PDLC), which may be considered an electro-optic material since the
index of refraction changes with applied field (although the
response time is relatively slow compared to electro-optic
crystals, for example). Take for example the case of FIG. 11B where
layer 1142 may be a PDLC. Electrodes 1122, 1124, and 1126 are
provided to produce electric fields with controllable vertical and
horizontal components. An upper cladding (not shown) may also be
used to increase the voltage breakdown threshold between electrodes
on the same surface such as 1122 and 1124. When electrodes 1122 and
1124 are excited to the same polarity different from 1126, a
substantially vertical field is produced in the vicinity of the
waveguide, lining up the liquid crystal material in the PDLC in the
vertical direction. When electrodes 1122 and 1124 are excited to
opposite potentials and 1126 is at ground potential, a
substantially horizontal field is produced in the vicinity of the
waveguide core, lining up the liquid crystal material in the PDLC
in the horizontal direction. The index of refraction of the PDLC
and the effective index of the waveguide is substantially different
in the two cases for any component of polarized light (either TE or
TM) in the waveguide. The index of refraction may be varied
continuously by changing the potentials to the electrodes
appropriately between the two extreme situations described above.
If a grating has been fabricated along the section of the waveguide
with PDLC (such as in the core material 1140, or in one of the
cladding materials 1144, 1141, or 1142 within the optical mode
1181), the interaction of that grating will be tuned by applying
fields as described. Note that the waveguide structure of FIG. 11A
is preferably used to obtain a large tuning range, but the
electrode structure of FIG. 11B is preferably used to tune the
PDLC. The polymer matrix of the PDLC is also preferably selected to
approximately index match with the liquid crystal materials near an
operating condition, to reduce losses.
[0159] A PDLC is a convenient type of liquid crystal material to
use in this application because it does not require confinement,
the confinement of the liquid crystal droplets being by the polymer
of the PDLC. However, PDLC has the disadvantage of relatively large
optical scattering, depending on the indices of refraction of the
aligned liquid crystal droplets and the polymer matrix. More
standard liquid crystal materials may also be used. With these
latter materials, a confinement structure is preferably provided.
The electrodes 1120, or if needed electrodes 1122 and 1124 may be
provided on the top plate or on the additional layers, or otherwise
disposed spatially to create the desired electric fields. Alignment
layers for the liquid crystal materials and electrodes can be
provided to actuate the liquid crystal material as is known in the
art. Electrodes may alternatively be provided longitudinally
arranged along the waveguide with the electric field parallel to
the guide. Transparent electrodes as known in the art may be used
to diminish optical losses if a significant portion of the optical
mode traverses the electrode material. Excitation of the electrodes
change the index of refraction of the liquid crystal material,
either to increase or decrease the index of refraction, depending
on the arrangement of the initial voltage and/or the alignment
layer that establishes molecular alignment in the absence of an
applied field.
[0160] The athermal resonators described herein provide a useful
function in themselves, even without any tuning function. For such
devices, the heater stripes may be omitted, and the grating
elements may be fabricated in athermal waveguides to accomplish
temperature independent frequency selective devices. For example,
the devices of FIGS. 1, 9, and 10, may be used as fixed frequency
laser sources, and the devices of FIGS. 13 and 17 may be used as
fixed-frequency cross connects or as fixed frequency detectors or
add-drop devices.
[0161] In another alternative embodiment, all of the elements of
the structure are integrated onto a single substrate. If the
substrate is InP, the laser structure can operate in either the
1310 nm region or the 1550 nm region; other substrate materials
enable other wavelengths. In all cases, a polymer material may be
integrated into a portion of the optical cavity along the
waveguide, and a frequency selective structure may be used to
determine the operating frequency. The polymer structure may be
tuned to produce an athermal free spectral range, the FSR may be
adjusted to equal a rational fraction of a communications channel
spacing, and the grating may be tuned by a polymer section.
[0162] Tapered Coupling
[0163] FIG. 3 shows the preferred embodiment of a hybrid integrated
coupling apparatus including a taper, but ignoring other
complexities such as the waveguide bends of FIG. 5. A simple taper
described for example in * H G. Unger, U.S. Pat. No. 4,415,227,
Nov. 15, 1983 is often insufficient for coupling single mode
waveguides with largely different mode dimensions. The normalized
propagation constant V of a waveguide helps determine its modal
behavior:
V=k.sub.oT{square root}(n.sub.f.sup.2-n.sub.s.sup.2), (11)
[0164] where ko=2.eta./.lambda., T is approximately equal to the
smallest transverse dimension of the waveguide, n.sub.f is the
index of refraction of the core layer of that waveguide, and
n.sub.s is approximately the index of refraction of the underlying
layer adjacent to the waveguide. If for example one of the
waveguides operates in the middle of the single mode region with a
normalized propagation constant V=2.5, a factor of two up-taper
will result in multimode behavior. On the other hand, a down taper
of such a waveguide is not effective in reducing the mode
dimensions. Our problem is to match a small, high-contrast single
mode waveguide in a semiconductor diode laser to a larger, low
contrast single mode waveguide in a planar waveguide chip. We wish
to fabricate the taper on the more easily manufactured passive
waveguide substrate. We have designed a three-waveguide approach
where we use a third material of index of refraction intermediate
between the indices of either the small or the larger waveguides,
and we use an extreme taper that drives the third waveguide below
cutoff for the operating wavelength.
[0165] The cutoff of a waveguide is defined as the parameter value
of interest (e.g. wavelength, width, effective index, etc.) past
which the transverse confinement of the mode is lost and energy
propagates away from the core transverse of the propagation axis of
the guide. The cutoff situation is to be distinguished from the
coupling between well defined waveguides where energy is exchanged
between well defined (otherwise below cutoff) waveguides.
[0166] Diode laser chip 310 contains a waveguide section 320
fabricated on a substrate 314. The laser chip 310 provides net
amplification around a range of wavelengths such as 1552 nm. The
diode laser chip 310 has dimensions that are selected according to
the wavelength and material system and might be 500 microns thick,
400 microns long in the direction of the waveguide, and 400 microns
wide, for a 1550 nm laser. The rear facet 312 forms one end of the
laser cavity. The laser lases along the optical axis 324. The other
end of the laser cavity may be inside the waveguide 340 at the
location of a retroreflecting grating (see 130 or 132 of FIG. 1).
The dimensions of the waveguide section 320 are variable but may
typically be about 0.7 microns in the y direction, and about 2.1
microns in the x direction, with an optical mode size of perhaps
0.55 microns and 1.65 microns, respectively. We typically quote the
mode size as the 1/e.sup.2 half width of the intensity. The full
width of the mode is about twice the mode size, and the full width
at half maximum of the beam is about 1.18 times the mode size. The
laser propagation axis 324 along the waveguide 320 is fixed at a
vertical distance 325 above the laser substrate 314, in preparation
for bonding and alignment onto the substrate 370.
[0167] The laser is preferably attached to the substrate with
solder as shown in FIG. 3. The laser lower surface is prepared with
an adhesion layer 352, a barrier layer 354, and a cap layer 356.
Note that the laser chip is upside-down in FIG. 3 since the
substrate 314 is to the top of the figure. The adhesion layer is
preferably 0.1 microns of Ti, the barrier layer 0.4 microns of Pt,
and the cap layer 0.1 microns of Au. The bonding surface of the
substrate 370 may be prepared in a similar way with an adhesion
layer 358, a barrier layer 360, and a cap 362. The solder layer 350
is prepared on the cap layer of the substrate for convenience, and
may be fabricated from a few microns of AuSn solder with a melting
point of about 280.degree. C. The laser waveguide may be aligned in
the x-z plane, pressed down into place, and the solder heated for a
short time in an appropriate gas such as formic acid to prevent the
formation of oxides and to allow the laser chip to bond. Notice
that the thicknesses of the solder layer 350 and the other layers
are controlled so that the laser substrate 314 after bonding has a
desired vertical separation 326 from the waveguide substrate 370,
within a tolerance for good optical coupling. Since the laser
waveguide 320 lies a fixed distance above the laser substrate 314
according to the process sequence in the fabrication of the laser,
and the waveguide 330 lies a fixed distance 335 above the waveguide
substrate 370 according to the process sequence in the fabrication
of the waveguide 330, it follows that by controlling the bonding
layers 350, 352, 354, 356, 358, 360, and 362 and the bonding
process, we can adjust and control the relative vertical positions
of the waveguides 320 and 330 by adjusting and controlling the
separation between the two substrates 314 and 370. Within the
vertical tolerance, the vertical separation 326 may be adjusted to
equal the sum of the vertical height 325 of the waveguide 320 and
the vertical height 335 of the waveguide 330.
[0168] The waveguide 340 fabricated has dimensions chosen to
optimize other factors including the efficiency of coupling to
standard optical fiber, the fraction of the mode that propagates in
the cladding, and the propagation loss through structures including
bends. While again the dimensions of this waveguide may vary by a
factor of three to ten or so, depending on these factors and
fabrication factors such as index difference, we have chosen
commercial silica technology for waveguide 340 with a preference
for a waveguide dimension of 2 microns by 2 microns with a 2% index
difference core-to-clad.
[0169] The disparity in the dimensions of the two waveguides 320
and 340 would result in a high coupling loss if waveguide 320 were
to be butt coupled directly to waveguide 340 (close to 4 dB), even
with perfect alignment. In the hybrid grating laser, this loss is
intracavity and will increase the threshold and reduce the output
power. The tapered waveguide section 330 is preferably provided to
improve the coupling efficiency to better than 3 dB and preferably
less than 1 dB or even 0.5 dB. Waveguide 330 is preferably butt
coupled to waveguide 320, and parallel-coupled to waveguide 340
with a special taper that drops the effective index of the tapered
waveguide 330 below cutoff. Ideally, the input end 332 of the
waveguide 330 is designed so that the transverse dimensions of the
lowest order optical mode propagating in the input segment of 330
are equal to or near the transverse dimensions of the lowest order
optical mode emerging from the diode laser waveguide 320. By
providing an index matching material in the gap between the two
waveguides, the Fresnel reflections and optical scatter from the
two waveguide ends can also be minimized. FIG. 3 shows that the
waveguide 330 is preferably fabricated directly on top of the
waveguide 340, but in an alternate design may be below it or
separated by additional layers.
[0170] The tapered portion of the waveguide 330 brings the
propagation constant of waveguide 330 close to that of the
waveguide 340 allowing energy to couple between the waveguides. The
taper continues until the waveguide 330 goes below cutoff. Light
propagating out of the laser waveguide 320 is butt coupled into the
end of the tapered waveguide 330; in the down-tapered region it is
in turn transversely coupled into the parallel waveguide 340. Light
returning from the opposite direction in waveguide 340 is
transversely coupled into the same portion of the waveguide 330
which is now up-tapered for light propagating towards the laser,
whereupon it is butt coupled into the waveguide 320 of the diode
laser chip. The waveguide 340 is fabricated from a material with
index of refraction n.sub.2. The surrounding materials have lower
indices close to and possibly identical to that of silica n.sub.1.
The strength .DELTA. of the waveguide 340 depends on the local
transverse dimensions, the index of the cladding 342 if different
from that of silica, and the index difference
.DELTA..sub.340=(n.sub.2-n.sub.1)/n.sub.1. (12)
[0171] Depending on these parameters, the waveguide 340 will guide
a lowest order optical mode that has an effective index of
refraction n.sub.eff that lies approximately in the range
n.sub.1<n.sub.eff<n.- sub.2.
[0172] FIG. 4 illustrates the changes in index of refraction
produced by the operation of the taper. For the waveguide 330 to
function, its index of refraction n.sub.3 is larger than n.sub.2.
As shown in the region 460 in FIG. 4, the n.sub.eff 430 of the
first segment of the waveguide 330 is larger than the n.sub.eff 440
of the first segment of the waveguide 340. A taper reduces the
strength of the waveguide over the region 470, lowering the
n.sub.eff 430 and changing the mode shape. The taper can be
accomplished by varying the local transverse dimensions and the
index difference, but it is easier to vary the lateral width of the
waveguide by changing the mask used to determine the waveguide
pattern during fabrication. While the waveguide 330 is still
relatively strong, the taper can be rapid, as shown in the region
along the z axis of extent T.sub.1 in FIGS. 3 and 4. However, as
the taper progresses, the mode of the waveguide 330 begins to
couple with the modes of the waveguide 340 in the coupling region
450.
[0173] Coupling will be strongest with the lowest order mode of the
waveguide 340 because their effective indices are closest. Energy
exchange occurs over a limited region 450 in z where the two mode
indices approach each other, and where the respective indices
acquire an imaginary part. Looking at the propagation direction of
increasing z, as the taper progresses, the two modes perturb each
other more strongly, and the coupling becomes stronger. Eventually
the mode of the waveguide 330 disappears (below cutoff), the
imaginary part of the index returns to zero, energy exchange
ceases, and the perturbation of the lowest order mode of waveguide
340 disappears. To accomplish an efficient transfer of energy from
waveguide 330 into waveguide 340, it is desirable to accomplish the
taper slowly while the modes couple, allowing sufficient
interaction time for completion of energy exchange. The region
T.sub.2 is typically longer than the region T.sub.1 for this
reason.
[0174] There are many choices to be made in selecting a particular
waveguide and taper design. Materials for the upper and lower
waveguides are preferably stable, easily processable, and
compatible. Even within a given choice of materials set, there
remain many options and several degrees of freedom. The preferred
taper design for this application uses a silica (SiO.sub.2)
materials system for the waveguide 340 and lower cladding 344, and
a tantala (Ta.sub.2O.sub.5) core for the tapered waveguide 330,
with a polymer top cladding 342.
[0175] For the upper waveguide 330 we choose tantala because of its
stability, low loss, and high index n.sub.3=2.03 at 1.55 microns.
Note that the exact index of refraction depends on the process used
to deposit the film. Since the taper characteristics depend
sensitively on the index, all of the numbers quoted here are
calculated for n.sub.3=2.03, but are preferably adjusted
experimentally to obtain the desired performance for a given
manufacturing process. The vertical dimension of the input end 332
of the waveguide 330 is preferably chosen to be about 0.08 micron
(in the y dimension), producing a very weak vertical guide with a
vertical mode size of about 0.8 microns that approximately matches
the vertical dimension of the guided mode in the diode laser. The
lateral dimension is preferably about 2 microns (in the x
direction), producing a lateral mode size of about 1.3 microns,
close enough to the diode laser mode size of about 1.65 microns.
The effective index of this guide is preferably just slightly above
cutoff, at approximately n.sub.eff=1.49, so that the mode full
width is substantially larger than the core dimensions,
particularly in the vertical direction. The length of the initial
taper may be short, in the region of T.sub.1=100 microns or so, and
the majority of the taper may be accomplished here, reducing the
width of the waveguide by a factor that may be 2 or even 10 or
more, depending on the specific design. The exact length and amount
of taper in this region can be adjusted experimentally to optimize
the amount of mode size reduction while minimizing the optical loss
(the length T.sub.1 can be increased to reduce the optical loss)
Most of the taper but essentially no coupling is preferably
accomplished in this rapid taper section. The second taper may be
longer such as T.sub.2=500 microns or so, and sufficient taper is
provided to initiate the coupling between the modes, to carry the
upper waveguide mode below cutoff, and to decouple and
substantially reduce the perturbation of the lower waveguide. It
may be sufficient to taper this portion of the guide by 50%. Again,
the exact length and amount of taper in this region may be adjusted
experimentally to optimize the amount of energy transfer between
modes (the length T.sub.2 can be increased to reduce the optical
loss). The initial untapered portion may be very short or even of
zero length, or multiple tapered sections may be provided.
[0176] The preferred taper design uses a 0.08 micron tantala
thickness and a single taper (i.e. T.sub.1=0) in which the lateral
waveguide width is tapered down from 2 microns to 0.7 microns over
a distance T.sub.2=600 microns. As the mask width becomes smaller
than about 1 micron, factors such as lithographic resolution and
mask undercutting make it increasingly difficult to reproduce small
features. Although the mask provides taper in the lateral
dimension, the process of reproducing the mask and defining the
waveguide width will provide both somewhat narrower lateral
features than 0.7 microns, and a natural vertical taper that is
more pronounced at the small end of the waveguide 330. For this
reason, the effects of the fabrication equipment on the function of
the taper should preferably be taken into account, and the
dimensions adjusted accordingly. Indeed, this fact may be made use
of in the design to realize a lower effective index at the thin end
of the waveguide than is called for in the mask design. At the end
of the taper, the waveguide 330 may be terminated abruptly because
the waveguide 330 is below cutoff and very little optical energy
remains in the region of the core. Alternatively, the mask taper
may be continued to zero width, allowing the lithographic process
to terminate the waveguide naturally.
[0177] FIG. 8 shows an alternative taper design in which
segmentation is included in the design of the waveguide. A
substrate 870 is prepared with a waveguide core 840 of index
n.sub.2, upper cladding 842 of index approximately n.sub.1, and
lower cladding 844 of index n.sub.1. The waveguide core 840 has
higher index of refraction than the cladding layers so that it
guides at least one optical mode. Segmented waveguide 830 is
fabricated on the waveguide 840 with a layer of a desired thickness
and index n.sub.3, it is patterned by removing undesired material
into a tapered width that is preferably wider laterally at the
interface 832, and into a segmented pattern by removing material in
the regions 838. The removal of material is shown as complete, but
the removal may extend down to some degree into the core material
of waveguide 840, or the removal may be incomplete, leaving some
material behind, bridging the segments 836 (not shown). Notice that
the optical propagation axis 834 is a determined distance 835 above
the substrate 870, in preparation for a second chip to be attached
on the empty portion 872 of the substrate as discussed elsewhere
and in relation to FIG. 3. As a variation, the waveguide 840 may be
fabricated on waveguide 830, in which case, the segments 836 will
be embedded into the core material of 840 instead of into the
cladding material of 842. The device 800 may be a portion of the
devices 300 or 100 or other devices.
[0178] As shown, in FIG. 8, the high index waveguide 830 is
fabricated from regions 836 which are interspersed with regions 838
(segments) where the high index material has been removed. Provided
that the segmentation period (the sum of the widths of a region 836
and an adjacent region 838 along the direction of optical
propagation) is comparable or less than the vertical and lateral
Rayleigh ranges
z.sub.o=.eta.w.sub.o.sup.2/.lambda. (13)
[0179] where z.sub.o is the Rayleigh range, w.sub.o is the
1/e.sup.2 optical beam intensity half width, and .lambda. is the
vacuum optical wavelength of the mode, the optical mode will
propagate as if the waveguide core were continuous (non-segmented)
but had a lower index than that of the regions 836. The effective
index contrast with the cladding 842 is reduced by the local duty
factor
DF.sub.seg=(local length of segment)/(local segmentation period)
(14)
[0180] of the segmentation. A duty factor of 50%, obtained when the
segments and the removed regions are of equal length, will reduce
the effective index of the waveguide by a factor of approximately
two. By adjusting the parameters of the waveguide appropriately,
the desired mode sizes can be obtained with a segmented guide, but
additional degrees of freedom are now available to the designer:
the duty factor and the period of the segmentation. For example, if
the duty factor is 50% in the region near the input of the
waveguide, and the thickness of the tantala film used to fabricate
the segments 836 is increased by a factor of 1.414 compared to the
previous description to 0.11 microns, the waveguide strength and
mode sizes in the vertical and transverse dimensions will be
approximately the same as described above.
[0181] The duty factor may be reduced along the propagation axis
834 of the waveguide 830 by changing the lithographic mask pattern
used in fabricating the segmented waveguide 830, aiding in the
accomplishment of the taper. The taper required for the
non-segmented waveguide described above has a waveguide width at
the small end of the waveguide of 0.7 microns. Such a small
dimension may be a challenge to fabricate reproducibly. Larger
minimum dimensions are preferred. With the use of segmentation, we
choose a minimum dimension of 1.0 micron to obtain the same optical
effect of the taper, in terms of the variation of the effective
index of the optical mode. The waveguide 830 now tapers laterally
from a 2 micron width to a 1 micron width at the small end, the
length of the segments 836 is kept constant at 1 micron, and the
duty factor is varied from 50% to 25% by increasing the length of
the removal regions 838 gradually from 1 micron to 4 microns at the
small end. Note that many variations of the functional form of the
taper of the segmentation are possible, and many others can be
useful, including a linear taper of the duty factor, exponential,
hyperbolic, sinusoidal, and all the other mathematical forms. Note
also that the general segmentation taper includes the possibility
of a taper in the period with alternative functional forms,
provided that the period continues to obey the constraint of being
comparable or less than the Rayleigh ranges. Many variations are
also available in the geometry of segmentation, such as the
indented geometry of R. Adar, U.S. Pat. No. 5,577,141, Nov. 19,
1996, "Two-dimensional segmentation mode tapering for integrated
optic waveguides", multiple superposed layers of variously
segmented materials, etc.
[0182] The tantala waveguides shown in FIGS. 1, 3, 5, and 8 are
preferably fabricated by deposition of a tantala film of the
desired thickness after the fabrication of the germania/silica
core. The tantala film may then be patterned by RIE after a masking
step to define the desired boundaries of the tantala waveguide
segments. Several variations are available, but not shown in the
figures, including depositing the tantala waveguide after the top
cladding has been deposited. In the latter case, the top cladding
is patterned and removed down close to the top surface of the
germania/silica core wherever the tantala waveguide structure is
desired; a uniform deposition of the desired thickness of tantala
is then sufficient to create the desired waveguide.
[0183] Lensed Waveguide End
[0184] FIG. 7 shows an alternative preferred taper embodiment that
can provide good coupling efficiency between two dissimilar
waveguides. This approach is based on lensing the waveguide end.
FIG. 7A shows a substrate 770 prepared with a waveguide core 740
upper cladding 742 and lower cladding 744. The waveguide core 740
has higher index of refraction than the cladding layers so that it
guides at least one optical mode. As with the waveguides described
elsewhere herein, this waveguide may be called a single mode
waveguide if it guides predominantly a single mode. The higher
order modes may be cut off, in which case the guide is strictly
single mode, or a few poorly confined and higher loss modes may be
weakly guided, in which case the guide is effectively single mode
which may still be useful for many applications including coupling
to single mode optical fibers. A region 752 of the core 740
projects slightly from the surface 732. Although the projection is
shown with sharp edges in FIG. 7A, these edges may be rounded.
[0185] This projection may be fabricated by applying a two step
selective etch. First, the waveguide may be etched vertically by a
non selective etching process that etches both the cladding and
core layers at similar rates, exposing upper and lower cladding in
the region where the core 740 emerges at the surface. In this step
an etch barrier such as a metal coating of Au or Cr is deposited
onto the surface of the semi-processed article. A patterned layer
of photo resist is applied on top of the etch barrier in the
desired pattern of the removal region (and other patterns if
desired) and etched to transfer the photo resist pattern to the
etch barrier. A reactive ion etching process may be used to etch
down into the silica layers left exposed by the patterned etch
barrier layer. The etch time is preferably controlled to allow an
etch depth large enough to etch through the core 740 and into the
lower cladding 744. The etch proceeds approximately vertically down
towards the substrate 770. Many process alternatives exist to
accomplish this etch; the preferred method is to use CHF.sub.3 as
the reactive gas at a pressure of 20 mTorr. At this stage, the
exposed surface of the semi-processed device is flat.
[0186] Second, a selective etch may be performed on the surface,
that preferentially etches the cladding layers 742 and 744 compared
to the core 740. The preferred way to perform this etch is to
perform a chemical etch with a buffered HF solution (BHF: a mixture
of ammonium fluoride and hydrofluoric acid). As is known in the
art, BHF etches silica rapidly, but does not etch GeO.sub.2 at all,
so that the GeO.sub.2-rich core layer 740 etches more slowly than
the pure silica cladding layers. The exact profile of the
protrusion created by this process follows the concentration
profile of Ge. Other means of selective etching may also be used,
including dry etching. The distance by which the center of the core
projects from the surface 732 depends on the etching parameters
(materials, densities, time, temperature, etc.). If there is no
upper cladding used for the waveguide 740, i.e. layer 742 is absent
(not shown), the shape of the resultant lenticular structure will
be different, and asymmetric vertically. As was the case for FIG.
3, a diode laser chip may be aligned and attached to the substrate
770 (preferably by flip-chip bonding) so that the axis of the laser
waveguide 320 or 112 is coaxial with the axis 754 of the waveguide
740.
[0187] Although for simplicity FIG. 7 shows the core shape being
unchanged by the etching process, in reality, the shape of the
protrusion 753 in the region of the core 740 at the surface 732 has
no sharp corners and can be described by smooth curves as in FIG.
7B, where surface grid lines along the x-z planes and the y-z
planes are shown to give an impression of the smoothly varying
surface shape. FIG. 7B shows the protrusion 753 forming a
lenticular structure with two different curvatures in the x-z and
in the y-z planes, since the height (y-dimension) and width
(x-dimension) of the core 740 at the surface 732 are different.
Different curvatures are desired because the divergences of the
diode laser mode are quite different in the two planes due to the
different typical mode sizes, as described above. The small
vertical mode size leads to strong vertical divergence, so a strong
curvature is desired in the y-z plane. Only a weak (or no)
horizontal curvature is needed to compensate the horizontal beam
divergence. To accommodate this difference, the approximately
square cross-section waveguide 740 may be widened as shown in the
regions 750 for FIG. 7A and 751 for FIG. 7B. The curvature in the
y-z plane may be adjusted through the selective etching conditions,
with generally larger etching time producing a larger curvature,
all other things being equal. The curvature is preferably adjusted
until the vertical divergence of the diode laser beam may be
compensated. The curvatures of the protrusion 753 will vary
inversely as the widths of the waveguide in the surface 732, so the
desired ratio of curvatures (vertical to horizontal) can be
obtained by adjusting the ratio of the widths (vertical to
horizontal). Since a hyperbolic lens surface has no spherical
aberration, the fabrication conditions are preferably adjusted to
obtain a near-hyperbolic profile for the protrusion 753 in the
region near the axis 754 where the mode profile crosses the surface
732.
[0188] In the case of a planar waveguide, the region 740 is very
wide laterally compared to its vertical width, it supports a
plurality of different axes of propagation in the x-z plane, and
the lenticular structure will be translationally invariant along
the x axis, providing focusing mainly in the vertical y-z plane.
Note also that the surface 732 (ignoring the protrusion) is shown
as being locally normal to the axis of propagation 754 of the mode
of the waveguide 740. In this case, by the symmetry of the
situation, the local surface of the protrusion at the axis 754 is
normal to that axis. However, this surface may be inclined at an
angle to deviate the beam, or curved to provide lateral focusing,
or take on another shape for a different purpose.
[0189] In FIG. 7A, the rectangular portion 750 (or 751 of FIG. 7B)
of the waveguide is preferably kept smaller in length (along the
direction of propagation) than the lateral Rayleigh range so that
the optical mode does not have a significant opportunity to expand
in the horizontal dimension between the surface 732 and the
beginning of the roughly square region 760 of the waveguide 740. If
this condition is obeyed, no taper may be needed between the two
sections of waveguide 750 and 760. In use of a lensed waveguide
coupling section, it may be desirable not to use index matching
material. Use of an index matching material has the advantage of
reducing the Fresnel reflections, but it has the undesired effect
of requiring an increased curvature of the surface 732 which
undesirably increases the fabrication time and tightens the
manufacturing tolerances.
[0190] In an optional step of the fabrication of the lensed surface
753, the region may be heated to a temperature near the softening
temperature of the materials 740, 744, and 742. Above the softening
temperature but below the melting point, the surface tension of the
silica can change the surface profile. This can be called thermal
slumping of the surface. By applying a controlled thermal ramp to
the wafer, or to an individual part, the temperature may be raised
above the softening temperature for a time sufficient to allow a
reduction in the curvature of the protrusion to a desired value.
This step is of interest if the spatial profile of the protrusion
produced by the selective etch step described above is too sharp
(as shown in FIG. 7A). Another desirable effect of thermal slumping
is the smoothing of the surface, reducing optical scatter. The
heating may be accomplished of a single chip, the entire wafer, or
of individually selected regions. While an oven can be used for
wafer-scale processing, a laser can advantageously be used to heat
the region around the protrusion 752 or 753 if it is desired to
slump individual regions. Preferably, a CO.sub.2 laser may be used
to provide an energetic pulse of 10 micron optical radiation that
is directed onto the surface 732 and is partly absorbed in a volume
near the surface. If an optical pulse is applied so that between
about 0.5 to 1 J/cm.sup.2 is absorbed within the top few microns of
the predominantly silica waveguide material during a period of a
few microseconds, thermal slumping will be observed. For longer
pulses, more energy will be required, but the thermal diffusion
depth varies approximately as the square root of the time, so the
required energy increases as the square root of the pulse length
above a pulse duration of a few microseconds. By controlling the
laser pulse length and energy, (and wavelength), and the number of
pulses, the desired degree of slumping can be controlled to
approach the desired curvature.
[0191] The protrusion 753 may be used to refocus optical radiation.
An optical beam is propagated along the waveguide 740 towards the
protrusion 753. Provided that the material outside the waveguide
and across the interface 732 has a lower index of refraction than
the core 740, upon traversing the interface 732, the beam is
focussed by the curvature of the interface 732, and acquires a
converging characteristic. The higher index central portion of the
protrusion retards the phase fronts of the mode as it traverses the
interface, causing phase front curvature related to the curvature
of the protrusion, and focussing the mode. As the beam continues to
propagate towards a longitudinal position of minimum beam size, at
least one beam dimension continues to shrink or focus. Another
waveguide may be aligned in proximity to this position so that the
refocussed beam can enter the second waveguide with good coupling
efficiency. If the second waveguide is the active waveguide of a
semiconductor laser, the arrangement described may be a part of an
injection locking apparatus, an external cavity resonator
apparatus, an amplifier apparatus, or other structures.
[0192] Alternatively, the protrusion 753 may be used in effectively
coupling a second waveguide such as in a semiconductor laser to a
waveguide 740. In this case, the optical beam is originated inside
the diode laser, propagates to the protrusion where it is
refocussed from a diverging beam, and propagates along the axis 754
of the waveguide 740. Again the end of the second waveguide may be
aligned relative to the interface 732 such that the emission facet
is collocated with the minimum focus, within a tolerance to achieve
the desired coupling efficiency. If the second waveguide is
fabricated on the same substrate as the waveguide 740, the
alignment of the end of the second waveguide is accomplished
lithographically. If the second waveguide is a semiconductor diode
laser, the alignment of the second waveguide end is accomplished
during an attach process between the substrate of the diode laser
and the substrate 770.
[0193] In a further alternative preferred embodiment, an
indentation may be fabricated instead of a protrusion 753. To
fabricate the indentation, a selective etch process may be
performed that preferentially etches the core layer 740 compared to
the cladding layers 742 and 744. The preferred way to perform this
etch is to perform a chemical etch with an aqueous solution of
H.sub.2SO.sub.4. Since the etching rate of SiO.sub.2 in this
etchant is nil, while the etching rate of GeO.sub.2 is medium, a
process temperature above room temperature is preferred, such as
30.degree. C. or 50.degree. C. The GeO.sub.2-rich core layer 740
etches more rapidly than the pure silica cladding layers. Again,
the profile of the indentation created by this process follows the
concentration profile of Ge. Now, with air in the removal region,
the indentation defocusses the mode passing through the interface,
which may be useful for some applications. With an index matching
fluid in the removal region that has a higher index of refraction
than the waveguide core 740, focusing is again obtained at the
indentation. The general description of the structures
incorporating the protrusion and the usage of the protrusion also
apply to the indentation provided that attention is paid to the
reversal of the focusing properties according to the index of
refraction of the removal region.
* * * * *