U.S. patent application number 10/770784 was filed with the patent office on 2004-08-26 for optical transmitter comprising a stepwise tunable laser.
Invention is credited to Garnache, Arnaud, Katchanov, Alexandre, Knippels, Guido, Levenson, Marc, Lodenkamper, Robert, Paldus, Barbara, Rella, Christopher, Richman, Bruce, Romanini, Daniele, Stoeckel, Frederic.
Application Number | 20040165641 10/770784 |
Document ID | / |
Family ID | 31982375 |
Filed Date | 2004-08-26 |
United States Patent
Application |
20040165641 |
Kind Code |
A1 |
Garnache, Arnaud ; et
al. |
August 26, 2004 |
Optical transmitter comprising a stepwise tunable laser
Abstract
An optical fiber transmitter for emitting an
information-carrying laser beam comprises an optically or
electrically pumped single mode MQW (multi-quantum well)
semiconductor amplifying mirror as a gain medium and a separate
external reflector to form a cavity. The external cavity length
defines a comb of optical modes, all or a subset of which
correspond to channel wavelengths of an optical telecommunications
system having plural optical channels. The semiconductor gain
element has a homogeneously broadened gain region; a tuning
arrangement tunes the laser from mode to mode across the gain
region, thereby selecting each one of the plural optical channels.
When the maximum gain bandwidth is less than mode-to-mode spacing
defined by the cavity, the tuning arrangement includes a means of
altering the temperature of the amplifying mirror, thereby
translating the frequency of the gain peak from one mode to
another. An optical modulator adds modulation to provide an
information-carrying laser beam, and a coupler couples the
information-carrying laser beam into an optical fiber of the
optical telecommunications system. A calibration method based on
detecting inter-mode optical transitions (mode hopping) is
described. The parameters of the amplifying mirror and tuning
arrangement can be adjusted during operation to switch quickly from
one cavity mode to another in stepwise fashion.
Inventors: |
Garnache, Arnaud;
(Montpillier cedex, FR) ; Romanini, Daniele;
(Grenoble, FR) ; Levenson, Marc; (Campbell,
CA) ; Lodenkamper, Robert; (Sunnyvale, CA) ;
Stoeckel, Frederic; (Vaulnaveys Le Haut, FR) ;
Katchanov, Alexandre; (Sunnyvale, CA) ; Knippels,
Guido; (Sunnyvale, CA) ; Paldus, Barbara;
(Sunnyvale, CA) ; Rella, Christopher; (Sunnyvale,
CA) ; Richman, Bruce; (Sunnyvale, CA) |
Correspondence
Address: |
Law Office of John Schipper
John F. Schipper
Suite 808
111 N. Market Street
San Jose
CA
95113
US
|
Family ID: |
31982375 |
Appl. No.: |
10/770784 |
Filed: |
February 2, 2004 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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10770784 |
Feb 2, 2004 |
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10104347 |
Mar 22, 2002 |
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6711203 |
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10770784 |
Feb 2, 2004 |
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09738277 |
Dec 13, 2000 |
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6658034 |
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10104347 |
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09930841 |
Aug 15, 2001 |
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6611546 |
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09930841 |
Aug 15, 2001 |
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09668905 |
Sep 22, 2000 |
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6741629 |
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Current U.S.
Class: |
372/97 ;
372/96 |
Current CPC
Class: |
H01S 5/041 20130101;
H01S 5/0222 20130101; H01S 5/0687 20130101; H01S 5/02253 20210101;
H01S 5/18355 20130101; H01S 3/094084 20130101; H01S 3/105 20130101;
H01S 5/141 20130101; H01S 5/0217 20130101; H01S 5/18377 20130101;
H01S 5/183 20130101; H01S 5/18375 20130101; H01S 2301/14
20130101 |
Class at
Publication: |
372/097 ;
372/096 |
International
Class: |
H01S 003/08; H01S
003/082 |
Claims
What is claimed is:
1. A vertical external cavity surface emitting laser formed by an
optical resonator, the laser comprising: a) an amplifying mirror
comprising: a multi-layer Bragg reflector having at least 90
percent reflectance, and a homogeneously broadened active gain
wavelength region comprising multiple quantum wells positioned on
top of the Bragg reflector; b) an external mirror confronting the
gain region along an optical axis of the resonator; and c) spacer
means for positioning and supporting the external mirror relative
to the amplifying mirror at a distance selected to provide discrete
tunability by setting an optical path length for the optical
resonator.
2. The laser of claim 1, wherein said external mirror has a concave
face confronting said amplifying mirror.
3. The laser of claim 1, wherein said Bragg reflector comprises at
least two epitaxially deposited semiconductor layers.
4. The laser of claim 1, wherein said Bragg reflector comprises at
least two dielectric layers.
5. The laser of claim 1 wherein said amplifying mirror further
comprises an antireflection region positioned on top of said gain
region.
6. The laser of claim 1, wherein said quantum wells are positioned
as plural pairs of quantum wells, each pair having an associated
gain wavelength located at or near a standing wave peak of a laser
longitudinal cavity mode present within said gain wavelength
region.
7. The laser of claim 1, wherein said gain region is optically
pumped.
8. The laser of claim 6, wherein a beam emitted from the optical
pump impinges on said amplifying mirror at an angle corresponding
to Brewster's angle, whereby an amplitude of a reflection of said
beam from said amplifying mirror is minimized.
9. The laser of claim 8, wherein said pump radiation absorption
means is positioned to absorb substantially all of said portion of
said beam which is reflected from said amplifying mirror.
10. The laser of claim 1, wherein said Bragg reflector is bonded to
a heat sink.
11. The laser of claim 10, wherein a metal layer is positioned
between said heat sink and said Bragg reflector, whereby a combined
reflectivity of said Bragg reflector and said metal layer is
increased.
12. The laser of claim 10, wherein a thermo-electric
heating/cooling element is in thermal contact with said heat sink,
said thermo-electric element having a control input to alter a heat
sink temperature, whereby the emission wavelength of said laser is
tunable.
13. The laser of claim 10, wherein said heat sink is bonded to a
piezo-electric element such that said heat sink is between the
piezo-electic element and said Bragg reflector, the piezo-electric
element having a control input to alter said optical path length of
said optical resonator, thereby tuning a laser emission
wavelength.
14. A stepwise tunable external cavity laser formed by an optical
resonator having a gain wavelength band, the laser comprising: a)
an amplifying mirror forming a first end of said optical resonator;
b) an external mirror forming a second end of said optical
resonator positioned such that an optical path length of said
optical resonator provides discrete tunability; and c) an
electronically-actuated, frequency selective element within said
optical resonator, wherein the frequency-selective element has a
control input to select an emission wavelength of a beam emitted by
the laser.
15. The laser of claim 14, wherein said amplifying mirror is an
epitaxially grown monolithic semiconductor structure having a
surface emitting gain-portion, the gain portion including a
plurality of active layers spaced apart by spacer layers arranged
to provide optical gain and overlying a reflector portion
comprising multiple layers of materials having at least two
different indices of refraction sufficient to produce reflectivity
at said gain wavelength band greater than 95 percent.
16. The laser of claim 14, wherein said amplifying mirror is
excited to produce gain by means of radiation focused onto a region
of said amplifying mirror.
17. The laser of claim 14, wherein said amplifying mirror is
excited to produce gain by an electrical current.
18. The laser of claim 14, wherein said amplifying mirror is
attached to heat-sink means so that a temperature of said laser can
be controlled.
19. The laser of claim 14, wherein said external mirror has a
concave surface, with a radius of curvature that maintains a single
transverse mode of oscillation over a range of optical pumping
power, and is coated with a plurality of layers of dielectric
materials having at least twodifferent indices of refraction
sufficient to produce a reflectivity greater than 95 percent over
said entire gain wavelength band of said amplifying mirror.
20. The laser of claim 14, wherein said external mirror is planar,
further comprising a focusing element positioned inside said
optical resonator so that said optical resonator is centered within
a resonator stability range.
21. The laser of claim 14, wherein said optical resonator
additionally comprises beam forming optics to reduce angular
divergence of said beam as said beam enters said frequency
selective element.
22. The laser of claim 21, wherein said beam forming optics
comprises a lens.
23. The laser of claim 21, wherein said beam forming optics
comprises a concave mirror with a multi-layer dielectric
coating.
24. The laser of claim 21, wherein said beam forming optics
comprises an off-axis parabolic mirror.
25. The laser of claim 14, wherein said frequency-selective element
comprises an intracavity etalon with electronically-actuated
control means to alter an angle the etalon makes with respect to an
intracavity laser beam, whereby an emission wavelength of said
laser is tunable.
26. The laser of claim 14, wherein said frequency-selective element
comprises a monolithic planar air-spaced etalon with at least one
free-standing dielectric film as a mirror and an etalon length that
allows at most one transmission maximum frequency to lie within
said gain wavelength band of said amplifying mirror, said
transmission maximum frequency being tunable by application of a
variable voltage to said mirror to alter a length of the etalon
length.
27. The laser of claim 14, wherein said frequency-selective element
comprises a planar etalon including a spacer medium comprising an
electro-optically active material and an etalon length which allows
at most one transmission maximum frequency to lie within said gain
wavelength band of said amplifying mirror, where the transmission
maximum frequency is tunable by the application of a variable
voltage to said spacer medium.
28. The laser of claim 27, wherein said spacer medium comprises a
nematic or smectic liquid crystal.
29. The laser of claim 14, wherein said frequency selective element
comprises: a) a polarization selective element; b) a birefringent
electro-optical medium with ordinary and extraordinary axes each
oriented at substantially .+-.45.degree. with respect to the high
transmission axis of said polarization selective element; and c)
means for applying a selection voltage to the electro-optical
medium, and for varying said selection voltage over a range of
values that accesses said entire gain wavelength band of said
amplifying mirror.
30. The laser of claim 14, wherein said frequency selective element
comprises: a) a polarization selective element; b) a first
birefringent electro-optical medium with ordinary and extraordinary
axes oriented at substantially .+-.45.degree. with respect to the
high transmission axis of thepolarization selective element; c) a
second birefringent electro-optical medium with ordinary and
extraordinary axes oriented so that the extraordinary axis of the
second electro-optic medium is substantially aligned with the
ordinary axis of the first electro-optic medium, and the ordinary
axis of the second electro-optic medium is substantially aligned
with the extraordinary axis of the first electro-optic medium; and
d) means for applying a first selection voltage to the first
electro-optical medium and for applying a second selection voltage
to the second electro-optical medium, and for varying at least one
of the first and second selection voltages over a range of values
that spans said entire gain wavelength band said amplifying mirror,
whereby a total length of electro-optic media in said laser can be
increased without reducing a laser tuning range, thereby allowing
use of at least one of a reduced first selection voltage and a
reduced second selection voltage.
31. The laser of claim 14, wherein said frequency selective element
comprises: a) a polarization selective element; b) a first
birefringent electro-optical medium with ordinary and extraordinary
axes oriented at substantially .+-.45.degree. with respect to a
high transmission axis of the polarization selective element; c) a
second birefringent electro-optical medium with ordinary and
extraordinary axes oriented such that the extraordinary axis of the
second electro-optic medium is substantially aligned with the
extraordinary axis of the first electro-optic medium, and the
ordinary axis of the second electro-optic medium is substantially
aligned with the ordinary axis of the first electro-optic medium;
d) a half wave plate positioned between the first electro-optic
medium and the second electro-optic medium, oriented such that
light polarized along an ordinary axis of the electro-optical media
is substantially converted to light polarized along an
extraordinary axis of the electro-optical media, and light
polarized along an extraordinary axis of the electro-optical media
is substantially converted to light polarized along an ordinary
axis of electro-optical media, in transmission through the half
wave plate; and e) means for applying a first selection voltage to
the first electro-optical medium and for applying a second
selection voltage to the second electro-optical medium, and for
varying at least one of the first and second selection voltages
over a range of values that spans said entire gain wavelength band
of said amplifying mirror, whereby an angular bandwidth of said
frequency selective element is enhanced, and a total length of the
electro-optic media in said laser can be increased without reducing
a laser tuning range, thereby allowing use of at least one of a
reduced first selection voltage and a reduced second selection
voltage.
32. The laser of claim 14, wherein said frequency selective element
comprises: a) a polarization selective element, b) a birefringent
medium with ordinary and extraordinary axes oriented at
substantially .+-.45.degree. with respect to a high transmission
axis of said polarization selective element, c) a birefringent
electro-optical medium with ordinary and extraordinary axes
oriented at substantially .+-.45.degree. with respect to the high
transmission axis of said polarization selective element, and d)
means for applying a selection voltage to said electro-optical
medium and for varying the selection voltage over a range of values
that spans said entire gain wavelength band of said amplifying
mirror,
33. The laser of claim 32, wherein said electro-optic medium is a
nematic or smectic liquid crystal.
34. A method for generating a discretely tunable laser beam
comprising the steps of: a) pumping a gain medium positioned within
an optical resonator; b) positioning a tuning element within the
optical resonator, where said tuning element has a control input to
select an emission wavelength of the laser; and c) providing an
optical path length for the optical resonator to provide discrete
tunability of the emission wavelength, so that varying the control
input to the tuning element causes the emission to assume a value
which lies on a predetermined frequency grid.
35. The method set forth in claim 34, wherein said predetermined
frequency grid comprises a predetermined optical telecommunications
frequency grid having substantially equally spaced optical
channels.
36. A method for generating a discretely tunable laser beam
comprising the steps of: a) pumping a gain medium positioned within
an optical resonator; b) positioning a tuning element within the
optical resonator, where the tuning element has a control input to
select an emission wavelength of the laser; and c) positioning a
grid-fixing etalon within the optical resonator to provide discrete
tunability of the emission wavelength, so that varying the control
input to the tuning element causes the emission wavelength to
assume a value which lies on a predetermined frequency grid.
37. The method set forth in claim 36, wherein said predetermined
frequency grid comprises a predetermined optical telecommunications
frequency grid having substantially equally spaced optical
channels.
Description
REFERENCE TO RELATED APPLICATIONS
[0001] The present application is a continuation-in-part of U.S.
patent application Ser. No. 09/738,277 filed Dec. 13, 2000, and is
a continuation-in-part of U.S. patent application Ser. No.
09/930,841, filed on Aug. 15, 2001, which is a continuation-in-part
of U.S. patent application Ser. No. 09/668,905, filed on Sep. 22,
2000.
FIELD OF THE INVENTION
[0002] The present invention relates to lasers emitting single
longitudinal and traverse mode radiation at selected wavelengths
defined by a frequency comb, in particular stepwise tunable
external cavity surface emitting semiconductor lasers pumped
optically or electronically for use in spectroscopy, process
control and optical communications. Particularly disclosed are
laser designs, and manufacturing and assembly processes for
frequency stable and rapidly tunable lasers for optical
communications.
BACKGROUND OF THE INVENTION
[0003] The use of lasers as part of a system for optical channel
switching in a fiber optic transport network is known. However,
existing systems utilize a multiplicity of individual lasers, each
of which emits at a single frequency. A further problem is that
current switching systems using single frequency lasers require
extremely complex circuitry to transform a set of input signals to
a set of output signals of different frequencies.
[0004] A significant enhancement of such laser based optical
channel switching systems would be achieved by use of a laser
having the following operating characteristics:
[0005] 1) the laser is rapidly tunable to specific desired output
frequencies, e.g. the frequencies of the ITU grid.
[0006] 2) The laser provides random access to any particular output
frequency (i.e. transmission channel).
[0007] 3) The laser is reliable and consistent in the output
frequency to which it is tunable over a long service life without
requiring extensive servicing or a carefully controlled operating
environment.
[0008] 4) The laser is consistently receptive to an input signal
since it must always tune to the correct output frequency (i.e.
channel number).
[0009] 5) The laser provides substantially uniform output power
independent of the particular output frequency selected.
[0010] We have discovered a laser system which fulfills the above
indicated performance requirements.
[0011] Typical lasers oscillate at a number of frequencies (i.e.
wavelengths) that correspond to modes of their optical cavities. In
certain applications, it is known to select a single such mode and
to adjust its frequency by varying the cavity length or some other
laser parameter. Methods of doing this are described in text books
such as A. Yariv, Quantum Electronics, John Wiley and Sons, New
York, 2.sup.nd edition, 1975 and M. D. Levenson and S. S. Kano,
Introduction to Nonlinear Laser Spectroscopy Revised Edition,
Academic Press, San Diego, 1988 and Demtroder, Laser Spectroscopy,
Springer, Berlin, 1996.
[0012] However, the task of quickly switching the laser output
frequency from one externally selected value to another externally
selected frequency--without undergoing laser action at intermediate
frequencies--poses significant technical difficulties. Known means
of doing this alter some other significant laser parameter, such as
output power, create an instability in the output frequency either
before or after the frequency switch, and/or switch to an unwanted
value which must then be homed-in on (actually adjusted to) the
desired frequency, which requires time. In many optical
communications and spectroscopy applications, delay in obtaining
stable operation after a frequency switch is certainly undesirable
and is frequently unacceptable.
[0013] Most semiconductor lasers generally follow two basic
architectures. The first laser type has an in-plane cavity, and the
second laser type has a vertical cavity, a so-called
vertical-cavity surface-emitting laser or "VCSEL". If the optical
resonance cavity is formed externally of the semiconductor
structure (active region) i.e. one of the reflecting surfaces is
physically separated from the active region, the laser is known as
a vertical external cavity surface-emitting laser or "VECSEL".
[0014] Electrically pumped diode lasers are most frequently of the
in-plane cavity type. Necessary optical feedback with the in-plane
type is most frequently provided by simple cleaved-facet mirrors at
each end of the optical cavity. The reflectance of such cleaved
mirrors, while generally sufficient is not very high, and laser
energy is thus emitted through the cleaved mirrors to the external
ambient at opposed edges of the structure, giving rise to
"edge-emitting" diode lasers. Such relatively simple structures are
sometimes referred to as Fabry-Perot diode lasers. Epitaxial
patterning of a grating pattern along a top surface of an
edge-emitting diode laser can be provided to set a design
wavelength, resulting in a so-called distributed feedback diode
("DFB") laser.
[0015] In-plane electrically pumped lasers, such as DFB lasers, are
typically single mode, and are also typically tunable continuously
across some wavelength band from near-infrared into the visible
light spectrum. Rapid tuning may be carried out by controlling the
electrical pumping current, while slower tuning may be carried out
by controlling the temperature of the laser via a heat sink and a
thermal cooler/heater arrangement. Such in-plane lasers have known
uses including optical wavelength absorption spectroscopy, storage,
printing and telecommunications. In-plane lasers are frequently
employed within telecommunications systems using optical fiber as
the information transfer medium. Conventionally, multiple channels
are carried through a single optical fiber, and it is therefore
necessary when using a Fabry-Perot laser or DFB laser as the
illuminating source to regulate the wavelength of the transmitting
laser in order to stay on a selected channel.
[0016] In order to keep an in-plane diode laser tuned to a desired
wavelength, current and thermal control loops must be provided to
stabilize the laser at a desired wavelength, particularly as the
laser ages during usage. Also, since there is no absolute
wavelength stabilization within these in-plane lasers, the emission
wavelength may drift, absent careful feedback control, during usage
and over the lifetime of the laser. This tendency to drift or
change emission characteristics with temperature and over time puts
stringent conditions on the materials and control systems used to
make the laser.
[0017] One known drawback of in-plane diode lasers, and most
particularly the Fabry-Perot type, is that they manifest a tendency
to mode-hop. Mode-hopping basically means that for a given pumping
current, the laser can unexpectedly hop to a completely different
mode (wavelength). As the current is increased, there are
wavelengths at which the mode hopping (wavelength jumping) becomes
uncontrollable. Moreover, diode lasers may manifest a hysteresis,
in that mode hopping may occur at different wavelengths as the
control current is increased as compared to when the control
current is decreased. Another drawback of in-plane diode lasers is
that output power is inextricably intertwined with active region
temperature and pumping current. Another issue with in-plane diode
lasers is that the transverse optical beam profile is typically
elliptical rather than circular and has high divergence, increasing
the complexity of coupling the laser energy into an optical fiber,
or coupling the laser waveguide mode to an external cavity
mode.
[0018] Dense wavelength division multiplexing (DWDM) for optical
fiber telecommunications applications requires optical transmitters
that can be tuned to any frequency in the standard ITU grid
(wavelength comb) with a relative frequency error not greater than
ten percent of the ITU channel spacing. This requirement implies
that an optical transmitter laser has excellent frequency stability
as well as broad tunability. For a 12.5 GHz channel spacing, the
transmitter must have 1.25 GHz of absolute accuracy and frequency
(wavelength) stability. Such control of the laser frequency cannot
be achieved with existing DFB lasers without complex electronic
control and frequently carried out diagnostics. Furthermore,
compensation algorithms must be developed in the laser control
system to handle the DFB's known aging processes, which is often
unpredictable.
[0019] A desirable characteristic of an ideal DWDM optical
transmitter is that a single laser can cover all of the DWDM
channels, and that it can be reliably and reproducibly set to any
desired one of the standard channel frequencies. Current laser
sources have only a limited tuning range, which covers only a
fraction of the full ITU grid. Known DFB lasers have limited
tunability; and the temperature tuning coefficient of telecom DFB
lasers is typically 0.09 nm/.degree. C. For a DFB laser thermal
operating range of .+-.20.degree. C., or 40.degree. C. total
temperature differential, one DFB laser could only be expected to
cover a wavelength range of 3.6 nm (or about 460 GHz, representing
only four channel coverage with 100 GHz channel spacing or 36
channel coverage with 25 GHz spacing) even if the necessary
accuracy in wavelength could be achieved.
[0020] In addition, DFB lasers only have about 35 to 45 dB of side
mode suppression. If the side modes are not sufficiently
controlled, the laser may excite two or three adjacent
communications channels, resulting in unwanted interference.
Because of these drawbacks, the telecommunications industry has
recently turned to VCSELs.
[0021] Micro-cavity VCSELs include semiconductor structures which
have multiple layers epitaxially grown upon a semiconductor
wafer/substrate, typically Gallium Arsenide or Indium Phosphide.
The layers comprise semiconductor or dielectric Bragg mirrors which
sandwich layers comprising quantum well active regions. Within the
VCSEL, photons emitted by the quantum wells bounce between the
mirrors and are then emitted vertically from the wafer surface. The
VCSEL type laser naturally has a circular dot geometry with lateral
dimensions of a few microns. The emitting aperture of a few microns
facilitates direct-coupling to optical fibers or other simple
optics, since the narrow aperture typically supports only a single
lateral mode (TEM.sub.00) of the resulting optical waveguide, but
is sufficiently wide to provide an emerging optical beam with a
relatively small diffraction angle. A 1.3 micron VCSEL is said to
have been developed by Sandia National Laboratories in conjunction
with Cielo Communications, Inc. According to a news report, "This
new VCSEL is made mostly from stacks of layers of semiconductor
materials common in shorter wavelength lasers--aluminum gallium
arsenide and gallium arsenide. The Sandia team added to this
structure a small amount of a new material, indium gallium arsenide
nitride (InGaAsN), which was initially developed by Hitachi of
Japan in the mid 1990s. The InGaAsN causes the VCSEL's operating
wavelength to fall into a range that makes it useable in high-speed
Internet connections." ("`First ever` 1.3 micron VCSEL on GaAs",
Optics.Org Industry News, posted 16 Jun. 2000). One of the
characteristics of micro-cavity VCSELs is that the laser cavity is
formed entirely within the semiconductor structure. A drawback of
such VCSELs is that they do not generate much power, on the order
of 3 mW for a small aperture of 5 .mu.m, for example. Also, there
is transverse spatial hole burning between the transverse modes
above about 3 mW.
[0022] As mentioned above, if a cavity is formed which is external
to the VCSEL semiconductor structure having a quantum well active
region, it is known as an external (extended) cavity laser (ECL).
The semiconductor structures used in ECLs have been typically
Fabry-Perot lasers having one facet antireflection coated so as not
to interfere with external cavity operation. However, the gain
medium can also be achieved using a SOA, (i.e. a semiconductor
optical amplifier which is typically an edge-emitting semiconductor
gain structure which however does not include the mirrors needed to
convert it into a laser oscillator) or half of a VCSEL.
[0023] A semiconductor optical amplifier (SOA) is a device that
amplifies an input signal of optical origin. The amplification
factor is typically high (>20 dB). A SOA amplifies light as it
propagates through a waveguide made of semiconductor material. A
SOA is typically less than 1 mm in length. It amplifies light
through stimulated emission (just as a laser produces radiation).
In essence, a SOA is a Fabry-Perot laser without feedback, having
optical gain when the amplifier is pumped (optically or
electrically) to create a population inversion leading to
stimulated emission. The optical gain of wavelength and signal
intensity depends on the SOA design and medium. However, a SOA can
form the gain medium of a laser.
[0024] Typically a SOA has two AR (anti-reflection) coated facets.
In some cases, the waveguide is defined by a ridge that has
non-normal incidence to the facets to further reduce the effective
facet reflectivity. The SOA design can lead to facet reflectivities
that are 10 to 100 times smaller than in a Fabry-Perot laser. Were
a SOA designed to have only one AR coated facet with non-normal
incidence and normal incidence of the waveguide at the other facet,
then it would form an active mirror. The non-normal incidence of
the facet could either be cleaved (30% reflectivity) or HR (high
reflectivity) coated. SOAs can also be designed to have a square
waveguide structure which lends itslf to a circular Gausian
beam.
[0025] In both VCSEL and SOA cases, one creates a so-called "active
mirror" (also referred to as an "amplifying mirror") that defines
one of the laser cavity reflectors but also provides the gain
medium for the laser. The gain in this active mirror results from
either electrical or optical excitation (pumping) of carriers that
recombine in the quantum wells to create photons. The external
cavity defines the coherence properties (wavelength) of these
photons. If the gain medium is half a VCSEL, the external cavity
version is referred to as a VECSEL. If the gain medium is a SOA the
laser is called ECSAL. In general, we will refer to both such types
of laser as a STECAM, a stepwise tunable, external cavity, active
mirror laser.
[0026] A VECSEL based active mirror is an epitaxially grown
semiconductor body, typically a few microns thick, which comprises
a multiple quantum well active gain region sandwiched between a
Bragg mirror grown on a semiconductor substrate and a capping
layer. The active mirror may also have an antireflection coating
that is either epitaxially grown or dielectrically deposited. An
external cavity is then formed by a second, passive mirror that
forms a stable resonator with the active mirror. Such an external
cavity can either be a high reflectivity dielectric concave mirror
or a plano/plano mirror with an intracavity refocusing element such
as a lens. One example of an optically-pumped VECSEL is described
in Published International Patent Application WO 00/10234, entitled
"Optically-Pumped External-Mirror Vertical-Cavity
Semiconductor-Laser", the disclosure thereof being incorporated
herein by reference. The disclosed VECSEL includes an
epitaxially-grown semiconductor structure having a multiple-layer
mirror structure integrated with a multiple-layer quantum-well
structure which provides a gain medium, and an external mirror
forming a resonant cavity with the integrated semiconductor
multilayer mirror. Optical pumping radiation is directed at the
quantum-well structure via an outermost (top layer) and is absorbed
by the quantum-well and pump-absorbing layers. The quantum-well
layers release photons in response to the pumping energy, and the
external cavity is dimensioned to result in laser energy output at
an approximate 976 nm wavelength in response to pumping energy at a
wavelength of approximately 808 nm. Because this VECSEL operates at
wavelengths below 1.1.mu. in the near infra-red spectrum, the
active gain medium is made to be aluminum-free, since aluminum ions
tend to diffuse in GaAs/AlGaAs lasers. Accordingly, the
quantum-well and pump-radiation absorbing layers are aluminum-free
layers of alloys of gallium arsenide and indium gallium arsenide
phosphide (GaAs/InGaAsP). However, a drawback of the VECSEL
described in this published International Patent Application is the
absence of any wavelength tuning mechanism enabling adjustment of
the laser emission wavelength.
[0027] Other VECSELs are described, inter alia, in a paper by
Sandusky and Brueck, entitled: "A CW External-Cavity
Surface-Emitting Laser", IEEE Photonics Tech. Ltrs., Vol. 8, No. 3,
March 1996, pp. 313-315; and, in a paper by Kuznetsov, Hamimi,
Sprague, and Mooradian, entitled: "High Power (>0.5-W CW)
Diode-Pumped Vertical-External-Cavity Surface-Emitting
Semiconductor Lasers with Circular TEM.sub.00 Beams", IEEE
Photonics Tech. Ltrs., Vol. 9, No. 8, Aug. 1997, pp. 1063-1065.
[0028] Co-inventors Garnache and Kachanov of the present invention
have previously reported that an optically pumped
multiple-quantum-well ("MQW") VECSEL is an excellent candidate for
use in high sensitivity intracavity laser absorption spectroscopy
("ICLAS") in "High-sensitivity intracavity laser absorption
spectroscopy with vertical-external-cavity surface-emitting
semiconductor lasers", Optics Letters, Vol. 24, No. 12, Jun. 15,
1999, pp. 826-828. In the ICLAS method an absorbent analyte is
placed inside an external cavity of a broadband laser with
homogeneously broadened gain. An L-shaped cavity was formed by the
integrated Bragg mirror, an external folding mirror having a 150 mm
radius of curvature, and a flat output coupler having 0.5 percent
transmission placed at the end of a one meter arm of the cavity.
The angle between the two arms was reduced to approximately 7
degrees to reduce astigmatism. A 500 mm long intra-cavity
absorption cell with Brewster-angle windows and containing an
analyte material was placed in the long arm. Generation time was
controlled by an optical chopper that interrupts or starts the pump
radiation beam and by an acousto-optic modulator that is triggered
after an adjustable generation delay time. Further work by these
authors with VECSELs in the field of spectroscopy is reported in a
paper by Garnache, Kachanov, Stoeckel and Houdre entitled:
"Diode-Pumped Broadband Vertical-External-Cavity Surface-Emitting
Semiconductor Laser: Application to High Sensitivity Intracavity
Laser Absorption Spectroscopy", JOSA-B-B, Vol. 17, No. 9, September
2000, pp. 1589-1598. The disclosures of these two articles are
incorporated herein in their respective entireties by this
reference thereto.
[0029] An intra-cavity etalon and a Lyot filter were said by Holm
et al. to stabilize VECSEL radiation at a single wavelength in
"Actively Stabilized Single-Frequency Vertical-External-Cavity
AlGaAs Laser", IEEE Photonics Technology Letters, Vol. 11, No. 12,
December 1999.
[0030] One approach for tuning a VECSEL is described in a note by
D. Vakhshoori, P. Tayebati, Chih-Cheng Lu, M. Azimi, P. Wang,
Jiang-Huai Zhou and E. Canoglu entitled, "2 mW CW single mode
operation of a tunable 1550 nm vertical cavity surface emitting
laser with 50 nm tuning range", published in Electronics Letters,
Vol. 35, No. 11, May 27, 1999, pp. 900-901, the disclosure thereof
being incorporated herein by reference. The described laser was
grown epitaxially upon an indium phosphide substrate and has a
cavity formed by a distributed Bragg reflector (DBR), a multiple
quantum well (MQW) active gain region, and an external dielectric
membrane mirror at a relatively short (.about.7.mu.) distance from
the active gain region. Because the VECSEL laser cavity is so
short, only one cavity mode can fit into the bandwidth of the MQW
gain structure. Cavity length can be changed by applying a
potential difference between the dielectric membrane and the
ambient supporting structure, thereby applying an electrostatic
force to the membrane mirror and causing its curvature (and hence
the cavity length) to change. Changing the cavity length shifts the
cavity resonance frequency which thereby results in laser frequency
tuning. The VECSEL is optically pumped by a 980 nm diode laser
which can be epitaxially grown below the DWDM laser. The authors
and an associated company, Coretek, have reported continuous tuning
of this VECSEL over a range of about 50 nm, which is more than 10
times the tuning range of a typical DFB laser. This Coretek VECSEL
is said to have a high quality TEM.sub.00 transverse mode and more
than 50 dB of side mode suppression.
[0031] However, the Coretek VECSEL does not appear to meet the DWDM
telecom requirements. The micro-machined membrane mirror must be
flexible in order to move the required tuning distance, and is
therefore necessarily sensitive to external perturbations or
vibrations and also can become self-excited into undesirable
vibrational modes by actuation. This system is also complex to
produce, with the evident difficulties of a multilayer epitaxial
structure being compounded by the need to form, align and attach a
precision micro-machined membrane external mirror. Thus, Coretek
type VECSELs would be challenging to manufacture at a reasonable
cost and yield in mass production. Furthermore, a complex feedback
control system would be required to maintain membrane mirror
position, thereby limiting absolute frequency set point stability
and reproducibility in laser tuning.
[0032] Caprara et. al. (e.g. U.S. Pat. Nos. 5,991,318; 6,097,742
and 6,167,068) have described a very large, high-power VECSEL with
intra-cavity harmonic generation crystals producing output
radiation at 488 nm, (well below current telecommunication
wavelengths). Since such a harmonic generation crystal creates loss
for the laser mode being converted to a shorter wavelength, an
additional intra-cavity wavelength control element is described.
The fixed element described is a Brewster-angle birefringent plate.
Such a tuning element requires mechanical rotation for adjustment
and thus cannot provide rapid tuning. Moreover, mechanical
adjustment causes energy to build up in the successive modes
traversed by the filter transmission maximum.
[0033] Telle and Tang (Applied Physics Letters 24, 85-87 (1974)
have described an electro-optic frequency selective filter for dye
lasers that might be capable of rapid tuning if sufficiently high
voltage can be sufficiently rapidly applied. However, the
multi-kilovolt potentials required by that filter are too high for
practical telecommunication use, and the beam collimation required
is not compatible with VECSEL type lasers. Other previously known
tunable filter technologies have too much loss for use with surface
emitting semiconductor gain media and/or transmit extra
unacceptable frequencies. However, when used in conjunction with a
higher gain medium such as a SOA, these filters (e.g. liquid
crystals) can provide a suitable lower voltage tuning
alternative.
[0034] From the foregoing description of the state of the art, it
is apparent that a hitherto unsolved need has remained for a
simplified, reproducible and widely tunable single mode MQW VECSEL
or SOA based laser for optical fiber telecommunications which
overcomes the limitations and drawbacks of the prior art
approaches. Especially, there remains a need for a compact near
infra-red laser system capable of switching quickly (<10 .mu.s)
among cavity modes spaced at .about.25-50 GHz from one another
without producing unwanted frequencies.
OBJECTS OF THE INVENTION
[0035] One object of the present invention is to realize an optical
fiber transmitter module including a single mode MQW VECSEL having
a semiconductor structure with a homogeneously broadened active
gain region and an external mirror spaced from the semiconductor
structure by a spacer such that a cavity length is in a range of
0.5 mm and 50 mm and is chosen to create a laser frequency comb
corresponding to a predetermined optical channel spacing
arrangement.
[0036] Another object of the present invention is to realize a MQW
VECSEL semiconductor structure formed by molecular beam epitaxy or
metal organic chemical vapor deposition in a manner enabling
removal of the semiconductor substrate, thereby overcoming
limitations and drawbacks of prior approaches in which the
substrate contributed to the presence of a Fabry-Perot etalon or
other unwanted optical element.
[0037] Another object is to realize an optical fiber transmitter
module including a SOA with homogeneously broadened and unpolarized
active gain and an external mirror.
[0038] Yet another object of the present invention is to realize a
laser with reproducible absolute emission wavelengths that
correspond to standardized wavelength division multiplex (WDM)
channel wavelengths, as used in optical fiber telecommunications
networks, such that the laser steps from channel to channel and
such that by design the emission wavelengths of this laser are
ensured to hit any desired channel wavelength accurately and have
channel separation with an accuracy better than ten percent of
channel spacing.
[0039] One more object of the present invention is to realize a
fiber optic transmitter having sidemode suppression in excess of 40
dB.
[0040] One further object of the present invention is to provide a
VECSEL or ECSAL for use as abaser source within a wide variety of
applications and environments including telecommunications test
equipment.
[0041] Another object of the present invention is to provide a
compact VECSEL or ECSAL device with axial cavity modes (i.e. axial
mode frequencies) which correspond to pre-determined communications
or spectroscopic channels, and which is capable of randomly
switching among such channels in 1 millisecond or less.
[0042] Another object of the invention is to provide a single ECSAL
able to access the entire C or L optical communications band.
[0043] Another object of invention is to provide a method to access
the entire C or L optical communications band with a single laser
device.
[0044] Another object of this invention is to provide a frequency
agile laser module meeting all current requirements for DWDM
optical fiber communications.
[0045] An object of the present invention is to provide an external
cavity type laser having a fixed cavity length selected so that
permitted laser modes match desired emission frequencies (e.g. the
frequencies of the ITU grid).
[0046] One further object of the present invention is to realize a
compact multi-quantum well (MQW) based optical transmitter with
cavity modes which correspond to pre-determined communications or
spectroscopic channels.
[0047] Another object of the invention is to realize an optical
transmitter that emits a stable TEM.sub.00 beam at the frequency of
a specific channel and which can be switched to another such
channel by changing some convenient control parameter.
[0048] Another object of the invention is to provide an optical
transmitter that is capable of randomly switching among such
channels in 0.1 millisecond or less.
[0049] A preferred laser includes an intra-cavity, fast
electro-optic tuning element providing minimum optical loss only at
selected frequencies.
[0050] A preferred laser also includes a gain medium that is
homogeneously broadened, coupled to a circular (Gaussian) external
laser mode.
[0051] An optical fiber transmitter comprises an active mirror for
emitting an information-carrying laser beam at a design wavelength
and has an external cavity length defining a plurality of optical
modes, each mode corresponding to a channel wavelength of an
optical telecommunications system having plural optical channels.
The active mirror (the mirror which is part of the gain medium)
such as the semiconductor structure of an optical-pump-excited
VECSEL or a SOA, has a homogeneously broadened multiple quantum
well (MQW) active region wherein the gain curve exceeds cavity
losses over a band which is less than mode-to-mode spacing, the
gain region being tunable to step from a first mode to an adjacent
second mode and to remain stably at the adjacent second mode. A
tuning arrangement tunes the laser from mode to mode thereby to
select each one of the plural optical channels. A conventional
external optical amplitude modulator adds user traffic to a beam
emitting from the laser to provide the information-carrying laser
beam, and a coupler couples the traffic-carrying laser beam into an
optical fiber of the optical telecommunications system. In the case
of a SOA, the structure of the gain medium may lend itself to the
integration of a modulator (e.g. electro absorptive or
Mach-Zender)
[0052] The external cavity length is determined in accordance with
the following parameters:
[0053] 1) The frequency spacing of the ITU grid (or whole number
fraction thereof) to be achieved.
[0054] 2) The optical and/or temporal dispersion produced by
intracavity optical elements.
[0055] 3) The effective optical and/or temporal dispersion caused
by the act of tuning.
[0056] In either a VECSEL or SOA based laser the external mirror is
normally spherical and positioned relative to the semiconductor
structure by a spacer stricture mounted to the heat sink at a
distance in a range of 0.5 mm and 50 mm to form the external cavity
and in the present invention this distance is chosen to create a
laser frequency comb corresponding to a predetermined optical
channel spacing arrangement. Equivalently to the spherical mirror,
a focusing intra cavity lens and plano plano mirror can be used to
form the external cavity. The key to the external cavity design is
that it be a stable resonator as defined in Siegman "Lasers"
University Science Books 1986 where the external cavity serves
primarily as a feedback stabilization mechanism.
[0057] In accordance with some embodiments of the present
invention, the VECSEL or ECSAL includes a heat sink structure and a
semiconductor structure grown by molecular beam epitaxy upon a
substrate and attached to the heat sink. As completed, the
semiconductor structure comprises a multi-layer semiconductor or
dielectic mirror region, a homogeneously broadened multiple quantum
well gain region having a thickness equal to at least one design
wavelength and having a plurality of quantum wells, each quantum
well being optimally positioned with respect to a standing wave in
the active gain region at the design wavelength, and an
antireflection coating region having a low reflectance at the
design wavelength. In the case of a MQW VECSEL the mirror can
advantageously be a semiconductor Bragg reflector achieving at
least 99% reflectance and the active region can be as short as one
design wavelength. In the case of a SOA as the gain region, one
mirror of the cavity can be a cleaved facet that may or may not
have a dielectric coating and its active region can be hundreds of
times the design wavelength, also in the case of a SOA, the active
mirror is normally electrically pumped. However, careful
consideration must be given to the waveguide design of the SOA to
ensure that a good overlap between the external cavity mode and the
fundamental waveguide mode is achieved and that higher order
waveguide modes are not excited.
[0058] In the case of an optically pumped MQW VECSEL, an in-plane
laser (e.g. Fabry Perot or DFB diode laser) providing pump
radiation is aligned relative to an external surface of the
semiconductor quantum well at Brewster's angle relative to the axis
of pump laser emission. In this embodiment the diode laser pump is
a sub-assembly which is aligned and secured in a sidewall of the
spacer structure and is thereby made integral therewith. A pump
radiation absorption element or aperture is preferably formed in
the spacer structure at Brewster's angle opposite an angle of
incidence of the pump radiation for absorbing any pump radiation
residually reflected from the external surface of the active
region. Note that by pumping the structure at Brewster's angle,
over 90% of the pump radiation is typically absorbed. An optimized
pumping arrangement can deliver as much as 99% of the pump
radiation to the VECSEL structure. Brewster's angle is about 74 for
InP based materials, which corresponds to a tan (74)=3:1 ratio for
the incoming pump beam axes to produce a circular spot on the
semiconductor. Because the geometry of most edge-emitting devices
is 3:1, pumping at Brewster's angle eliminates the need to
circularize the pump laser beam, reducing the complexity of the
pump optics. In fact, only focusing optics such as one or two
lenses are required to image the pump beam to the correct spot size
on the active region. A simple .lambda./2 plate provides the
correct polarization for incidence at Brewster's angle and hence
maximum absorption. As is known to those skilled skilled in the
art, the term Brewster's angle refers to the angle of incidence of
light reflected from the surface of dielectric material at which
the reflectivity for light whose electrical vector is in the plane
of incidence becomes zero. This is sometimes referred to as the
polarizing angle. In the context of the present invention where the
dielectric material has a chemically composite multilayer structure
and has in effect a composite refractive index, Brewster's angle
(sometimes referred to as an angle analogous to Brewster's angle)
is the angle corresponding to that composite refractive index. The
size of the pump beam on the VECSEL is advantageously matched to
the size of the external cavity transverse mode and accounts for
thermal effects (such as thermal lensing) in the VECSEL itself.
[0059] A method for calibrating a stepwise tunable, external cavity
amplifying mirror (STECAM) laser is also provided. The STECAM laser
includes an external cavity formed by an external mirror at a
length fixed by a spacer. The length defines a plurality of optical
modes, each mode corresponding to a channel wavelength of an
optical telecommunications system having plural optical channels.
The semiconductor structure of the active mirror has a
homogeneously broadened multiple quantum well (MQW) active region
wherein the gain curve exceeds cavity losses over a band which is
less than the mode-to-mode spacing, the laser output being tunable
to step from a first mode to an adjacent second mode and to remain
stably at the adjacent second mode. A tuning mechanism such as a
thermoelectric cooler for cooling and heating the active gain
region, or a frequency-selective element such as an intra-cavity
etalon, is provided for tuning the STECAM laser from mode to mode.
A digital controller including a wavelength-selective optical
sensor responsive to laser output radiation tunes the laser from
mode to mode and maintains the laser at each mode thereby to select
each one of the plural optical channels. The wavelength-selective
optical sensor generates pulses responsive to inter-mode optical
transitions. The method includes steps of:
[0060] sweeping the tuning mechanism between a longest and a
shortest wavelength mode capable of being generated by the laser in
accordance with a control parameter generated by the digital
controller,
[0061] recording in the memory of the digital controller a
transition control parameter presently being put out by the
wavelength-selective optical sensing means upon detection of pulses
responsive to inter-mode optical transitions, and
[0062] determining and recording single mode set values as
approximately half increment magnitudes between magnitudes of
adjacent recorded transition control parameters.
[0063] Thus, the tuning mechanism precision required is reduced
from having to land exactly on an ITU channel wavelength to simply
creating a profile such that the laser snaps itself to the
appropriate channel, whose wavelength has already been
predetermined by the external cavity. Such a laser does not require
expensive wavelength control circuitry.
[0064] These and other objects, advantages, aspects and features of
the present invention will be more fully understood and appreciated
by those skilled in the art upon consideration of the following
detailed description of various embodiments, presented in
conjunction with the accompanying drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
[0065] The accompanying drawings, which constitute part of the
specification, illustrate embodiments of the present invention, and
together with the detailed description given below, serve to
explain the invention. In the Drawings:
[0066] FIG. 1 is a pair of graphs of gain intensity as a function
of VECSEL optical mode, showing an initial higher gain and a
residual steady state lower gain suitable for exciting a single
mode in continuous wave (CW) operation, rendering the VECSEL output
single mode and thereby useful for mode switching in optical fiber
telecommunications.
[0067] FIG. 2 shows thermally induced mode hopping over extended
time intervals of a VECSEL useful for mode switching in optical
fiber telecommunications.
[0068] FIG. 3 is a schematic diagram of an optical fiber
transmitter unit including an optically pumped MQW VECSEL in
accordance with principles of the present invention.
[0069] FIG. 4 is a band gap energy diagram superimposed upon a
diagrammatic cross section of an epitaxially-grown semiconductor
structure preparatory to inverse structure processing into the FIG.
3 semiconductor VECSEL.
[0070] FIG. 5 is a graph of reflectivity (upper trace) and
photo-luminescence (PL) (lower trace) intensity measurements made
of a sample of the FIG. 3 VECSEL having an indium phosphide
semiconductor structure along a wavelength baseline. As is
customarily done, the PL intensity is given in arbitrary units, and
is plotted together with the reflectivity for convenience.
[0071] FIG. 6 is a schematic diagram of another optical fiber
transmitter unit including an optically pumped MQW VECSEL in
accordance with the principles of the present invention.
[0072] FIG. 7 is a schematic diagram of a MQW VECSEL similar to the
FIG. 3 MQW VECSEL with the addition of an annular piezoelectric
element for providing micro adjustment of the external cavity
length, thereby adding a further wavelength tuning mechanism having
a shorter time constant than is achievable with thermal tuning
alone.
[0073] FIG. 8 is a greatly enlarged schematic diagram of a MQW
VECSEL similar to the FIG. 3 MQW VECSEL in which a 150 mW diode
laser is mounted and aligned to pump the VECSEL semiconductor
structure at Brewster's angle, in accordance with the principles of
the present invention.
[0074] FIG. 9 is a greatly enlarged schematic diagram of a MQW
VECSEL similar to the FIG. 3 MQW VECSEL illustrating one preferred
arrangement for precisely aligning and securing the external mirror
spacer to the heat sink to achieve the cavity precision design
length.
[0075] FIG. 10 is a greatly enlarged schematic diagram of a MQW
VECSEL incorporating principles of the present invention and
further including an intra-cavity etalon enabling rapid mode
selection.
[0076] The relationships of various elements and components are
again illustrated schematically in FIGS. 11-22 in accordance with
the current invention. In these drawings, like components are
designated by like reference numbers.
[0077] FIG. 11 schematically illustrates an embodiment of a VECSEL
in accordance with the present invention having a resonator
incorporating an amplifying mirror and an electronically adjustable
tuning element within the laser cavity.
[0078] FIG. 12 is a graphical representation of the spectra of the
resonator 70, amplifying mirror gain 71 and tunable element 72
wherein the gain is flatter than that shown in FIG. 1.
[0079] FIG. 13 is a representation of an embodiment of the
amplifying mirror incorporating a Bragg reflector region and a gain
region combined within a monolithic structure.
[0080] FIG. 14 illustrates an embodiment of the present invention
in which the amplifying mirror is excited optically by a laser beam
originating outside the cavity.
[0081] FIG. 15 illustrates an embodiment of an electronically
adjustable tuning element in which a tilted plane etalon with an
electro-optically active spacer and a polarization selective
element constitute the tuning device.
[0082] FIG. 16 is another embodiment of an electronically
adjustable tuning element which incorporates a Brewster-angled
window as a polarization selective element and a birefringent
electro-optical medium to select the desired laser mode.
[0083] FIG. 17 illustrates the effect of a tilted birefringent
tuning element upon the polarization of the light in the laser
cavity.
[0084] FIG. 18 illustrates another embodiment of an electronically
adjustable tuning element in which there are a plurality of
electro-optically active birefringent media.
[0085] FIG. 19 illustrates another embodiment of an electronically
adjustable tuning element in which there are a plurality of
electro-optically active birefringent media and additional
birefringent media without electro-optical control.
[0086] FIG. 20 illustrates the polarization altering role of the
birefringent, but non-electro-optically control media in the
embodiment of the device shown in FIG. 9.
[0087] FIG. 21 illustrates another embodiment of an electronically
tunable frequency selective element wherein it incorporates a lens
to create a beam waist within the tuning device in addition to the
beam waist present in the amplifying mirror.
[0088] FIG. 22 illustrates another embodiment of an electronically
tunable frequency selective element wherein it incorporates an
off-axis concave mirror to create a beam waist within the tuning
device.
DETAILED DESCRIPTION OF THE INVENTION
[0089] The present invention provides a compact, tunable laser
source which can be tuned over a range of several tens of
nanometers within an optical telecommunications multi-channel band
plan and which will operate reliably and extendedly on selected
ones of the multiple channels thereof (as specified by the
International Telecommunications Union (ITU) DWDM grid plan, for
example). The laser source is an active mirror (MQW VECSEL or SOA)
having an epitaxially grown MQW gain structure with a reflector
(e.g. an incorporated distributed Bragg reflector (DBR)) or a
dielectric coating and an external dielectric coated spherical
mirror. An intra cavity tuning element can be used to achieve rapid
frequency selection. The external mirror is positioned to give an
optical path length L from the MQW structure such that
c/2L=.DELTA..nu..sub.DWDM, where c is the velocity of light and
.DELTA..nu..sub.DWDM is a required telecom optical channel spacing,
such as 12.5 GHz or 25 GHz, for example. For a channel spacing of
25 GHz cavity length L should be approximately 0.6 cm. Note that
.lambda. will typically include thickness and dispersion
compensation for any intra cavity elements.
[0090] Separation between the MQW structure and the external mirror
can be maintained by a spacer made of a material having a low index
of thermal expansion, .alpha., such as fused silica
(.alpha..about.10.sup.-6) or Zerodur.TM., a silica-like material
made by Heraeus-Amersil and which can have a thermal expansion
coefficient equal to essentially zero over a temperature range of
several tens of degrees C. Alternatively, if the cavity components
can be mounted on a miniature optical bench having a low (or
controllable) index of thermal expansion, then a free space version
of the laser is possible. The resultant optical cavity provides a
comb of fixed laser frequencies.
[0091] An absolute frequency reproducibility
.DELTA..nu./.nu.=.DELTA.L/L=.- alpha..DELTA.T of
6.3.multidot.10.sup.-6 (for T=6.degree. C.) can be realized with a
fused silica spacer. In this laser, the external cavity itself
serves as an absolute frequency standard (etalon). The spacer
length tolerance .delta.L should be
L.multidot.6.3.multidot.10.sup.-6=0.0- 4.mu., or 0.025 of the
working wavelength. Such accuracy is well within the existing
capability of the optical industry which can make retardation
plates to adjust the accuracy of a polarization vector with better
accuracy than one degree of retardation.
[0092] The radius of curvature of the external mirror is derived
from the cavity mode diameter at the MQW structure for a cavity
length equal to L. For example, with external mirror spacing L=0.6
cm a cavity mode diameter of 50.mu. will be achieved if the mirror
radius of curvature is equal to 0.63 cm. Such mirrors can be
manufactured by standard methods known in the optical industry,
such as molding the mirrors against a diamond-turned metal preform.
Note that in all laser designs, the cavity must be a stable
resonator. This entails that other embodiments are also useable,
such as plano/plano mirrors used in conjunction with an intra
cavity lens.
[0093] In a preferred embodiment of the present invention, single
frequency operation of the laser on a particular telecom channel is
achieved: 1) by the homogeneous broadening properties of the MQW
gain structure which results in a spectral narrowing of laser
output radiation after the initial intensity buildup of laser
radiation within the cavity, and 2) by laser cavity design in which
the active gain structure is positioned so as to minimize spatial
hole burning effects and favor single frequency operation, even if
no additional frequency selection mechanisms (such as intra-cavity
etalons or Lyot filters) are provided.
[0094] The transient behavior of a VECSEL made of InGaAs strained
quantum wells having GaAs barriers has been studied and reported in
the articles by co-inventors Garnache, et.al. cited in the
Background of the Invention. For a VECSEL having a homogeneously
broadened gain medium with gain bandwidth .GAMMA. (half-width at
half-maximum (HWHM)) and broadband mirrors, the intensity
M.sub.q(t.sub.g) of a mode q at a time t.sub.g (generation time)
which is measured from the instant when pumping started can be
described by the following equation (1): 1 M q ( t g ) = M t g / 2
L exp [ - ( q - q 0 2 L ) 2 t g ] , ( 1 )
[0095] where q.sub.0 is a central mode number, and .gamma. is the
cavity loss rate. Cavity loss rate .gamma. can be described by
equation (2):
.gamma.=-c ln.left brkt-bot.R.sub.oc(1-l.sub.i).sup.2.right
brkt-bot./2L, (2)
[0096] where c is the velocity of light, and the cavity has an
output coupler having reflectivity R.sub.oc, and internal loss
l.sub.i.
[0097] From equation (1) it follows that after a VECSEL begins
transitioning, its spectrum will be multimode with the total width
close to the gain bandwidth 2.GAMMA., but the intensities of the
side modes will decrease exponentially over time so that the
spectral width
.DELTA..nu.=.GAMMA.{square root}{square root over
(ln(2)/.gamma..sup.t.sub- .g)}. (3)
[0098] .DELTA..nu. (HWHM) will decrease in time inversely
proportional to the generation time t.sub.g, in accordance with
equation (3):
[0099] It has been experimentally confirmed that a requirement for
validity of equation (1) is that the gain medium of the active
region of the VECSEL is homogeneously broadened and that any
non-linear interactions between the modes are negligible for a
given generation time. If, for a certain laser, the spectral width
becomes smaller than cavity mode spacing, and equation (1) remains
valid, the laser will collapse to single frequency operation. In
the September 2000 JOSA-B paper of co-inventors Garnache, Kachanov
and Romanini with co-author Houdre, it was shown that for a VECSEL
active region comprising a strained InGaAs MQW in GaAs, this
equation is valid for a generation time t.sub.g at least as large
as one second. If one assumes a reasonable value of gain bandwidth
.GAMMA. as equal to 100 cm.sup.-1 or 3000 GHz, external coupler
reflectivity, Roc=0.99 percent, and cavity internal loss l.sub.i of
0.001, then for a cavity length L of 0.6 cm, the spectrum must
collapse to a bandwidth smaller than intermode spacing equal to 25
GHz at a time t.sub.g.about.0.03 ms. This means that the VECSEL
will then be operating single frequency at a mode closest to the
gain maximum. This time is significantly shorter than the time for
which equation (1) was experimentally confirmed as reported in the
above-referenced article. Note that for a higher gain active mirror
such as a SOA, the cavity can support higher losses, such as 30%
output coupling which corresponds to an even faster spectral
collapse time of 3 .mu.s. In principle, actual laser wavelength
switching should occur in <1 .mu.s.
[0100] FIG. 1 presents two plots of VECSEL active gain as a
function of wavelength and pumping intensity, and marks a series of
VECSEL cavity modes (resonances) across the abscissa of the graph.
The dotted line shows initial intensity buildup in the cavity which
takes place during the initial startup (and which is of particular
interest and importance in the case of ICLAS). After about one
microsecond the VECSEL active gain becomes clamped to the average
cavity losses, and from the solid-line curve of FIG. 1 it is seen
that only the mode closest to the gain maximum will be operating by
adjusting pumping radiation to an appropriate level, and by
thermally decoupling the VECSEL external cavity mirror, FIG. 2
shows that, following an initial mode-switch interval, it is
practical, by thermal control, to relatively slowly move the VECSEL
laser output radiation wavelength stably from mode to mode over
time. Faster mode selection (<1 .mu.s) may be achieved by
providing a controllable intra-cavity element such as an etalon or
Lyot filter.
[0101] Thermally controlled VECSEL mode hopping and stability at
each selected mode (following startup phase) over an extended time
period (hours) is shown in the FIG. 2 graph. This graph represents
data obtained from a VECSEL having a semiconductor structure grown
by molecular beam epitaxy on a 0.5 mm GaAs substrate. The bottom
stack of the VECSEL is a standard Bragg mirror and consists of 30.5
pairs of AlAs-Al.sub.0.27Ga.sub.0.93As quarter-wave layers with a
measured reflectance of 99.96 percent at a 1030 nm laser design
wavelength. The active region (MQW) consists of two sets of three
strained 8-n In.sub.0.2Ga.sub.0.8As quantum wells separated by
10-nm GaAs barriers. Each set of quantum wells is placed at the
maximum of the intracavity standing wave. 830-nm optical pump
radiation is focused into the GaAs absorbing layers within the gain
region (which has an optical thickness of 3.lambda./2. An AlAs
quarter-wave layer followed by an Al.sub.0.07Ga.sub.0.23As
half-wave layer was grown on top of the active region to prevent
carriers from diffusing to the semiconductor surface and to have an
Al-poor surface. A Ti:sapphire pump laser emitted 830 nm pump
radiation focused into the gallium arsenide absorbing layers within
the gain region. The semiconductor chip was soldered onto a copper
heat sink and cooled by a Peltier element and a heat radiator
operably connected to a temperature controller. The optical pump
source, the VECSEL structure and the external cavity mirror were
mounted onto an aluminum base plate. Tunability was achieved over a
range of 1012 to 1086 nm with less than 500 mW of pump power.
Further information concerning this particular VECSEL can be found
in the paper entitled: "High-sensitivity intracavity laser
absorption spectroscopy with vertical-external-cavity
surface-emitting semiconductor lasers", by co-inventors Garnache,
Kachanov, Stoeckel, and co-author Planel, in Optics Letters, Vol.
24, No. 12, Jun. 15, 1999, pp 826-828, cited and incorporated by
reference in the Background of the Invention, above.
[0102] In accordance with the principles of the present invention,
and as shown in FIG. 1, after a very short generation time
(t.sub.g.about.0.03 ms, for example) the telecommunications VECSEL
will operate single frequency on a mode closest to the MQW gain
peak. The gain (solid bell-shaped line) will be clamped to the
cavity loss value (horizontal dashed line) at this operating mode
frequency (clamp is denoted by small full circle). If the
temperature of the MQW active region is changed, for example by
changing the temperature set point value of a thermoelectric cooler
(element 112 in FIG. 3) of the VECSEL semiconductor structure, the
gain maximum will move to a higher or lower frequency,
corresponding to an adjacent optical telecom channel. The VECSEL
will remain in the same mode until the gain maximum reaches a
half-distance between the two adjacent modes. At this point due to
ever present perturbations (either spontaneous emissions or
mechanical perturbations, for example) the laser will manifest
intermittant (mode hopping) behavior. As the gain curve is
thermally moved further, the laser will start operating single mode
on arrival at a mode adjacent to the departure mode. This
experimental behavior of a non-stabilized VECSEL having a cavity
length of 2.5 cm and with an InGaAs active region is shown in FIG.
2. FIG. 2 shows a very stable single mode operation with very low
frequency drift approximating 6 MHz per minute, even though the
laser case temperature was not stabilized, and the external mirror
spacer was provided by the aluminum base plate. FIG. 2 shows zones
of intermittent behavior when the gain maximum is located
equidistantly between two adjacent modes. Since the cavity design
of the VECSEL of the present invention enables only operation on a
particular telecom channel frequency, temperature tuning of this
VECSEL provides a very efficient and stable way to tune from one
telecom channel to another. Tuning range of this VECSEL depends on
the band gap energy dependence of the active zone materials.
Temperature tuning coefficient of the laser described in the
September 2000 JOSA-B paper referred to herein above was 0.45
nm/.degree. C. By changing the temperature set point to the range
of .+-.25.degree. C. with respect to room temperature, a tuning
range of approximately 23 nm can be realized.
[0103] The spectrum of the laser will reach a stationary state at
some time .tau..sub.sp, which is determined by the spontaneous
emission rate into the cavity modes, and it can be found from
equation (4): 2 sp M 0 st , ( 4 )
[0104] where M.sub.0.sup.st is the intensity (photon number) of the
central mode, and .xi. is the ratio of the spontaneous emision rate
into one laser mode to the stimulated emision rate per photon,
(which is close to unity). The spectrum of the laser in this
stationary state will have a Gaussian shape with a width which can
be evaluated from equation (1). If it is assumed that the power
output of the VECSEL is 10 mW, then the photon number in the
central mode will be 3.multidot.10.sup.8 photons. .tau..sub.sp vill
then be 1.7 seconds, the width of the Gaussian spectral
distribution will be 0.17 GHz, and relative intensity of the
nearest neighbor to the laser mode will be 4.7.multidot.10.sup.-5,
or 31 43 dB.
[0105] A fiber optic telecommunications transmitter 100 is
diagrammatically illustrated in FIG. 3 for putting modulated
optical power emitted by a VECSEL 104 into an optical fiber 106 of
a communications network operating in the near infrared spectrum.
e.g. 1000 nm to 1700 nm and having a multiplicity of spaced apart
channels therein. e.g. 12.5 GHz or 25 GHz adjacent channel spacing.
A suitable amplitude lo modulator 105, such as a lithium niobate or
lithium tantalate crystal, or other electro-optical element or
grating having a current-modulated index of refraction or
electroabsorption modulator, for example, is preferably included in
the radiation path of the VECSEL 104 in order to impart the
necessary information signal to the VECSEL laser beam before
passing into the optical fiber 106.
[0106] A laser diode unit 102 puts out optical pumping radiation at
a desired wavelength. e.g. 980-1000 nm (1.24 eV at 300 K), and
power level, e.g. a minimum of 150 mw and typically 250 mW to 1 W.
When excited by e.g. 150 mW of pump power, the VECSEL 104 puts out
5 mW in the near infrared spectrum, e.g. 1560 nm, and output power
scales up as a function of pump power. A folding mirror 103 may be
provided to direct the pump radiation toward the VECSEL 104 which
has a relatively short cavity, e.g. on the order of 6 mm, between
an intrinsic DBR mirror 126 of a semiconductor structure 114 and an
external cavity mirror 116.
[0107] Other elements of the VECSEL 104 include a heat sink 110, a
thermoelectric cooler 112 for wavelength control (mode selection),
and an epitaxially grown inverse semiconductor structure 114
including the antireflection layer 122, the MQW active gain region
124, the DBR mirror layers 126 and metal film mirror layer 128
adjacent heat sink (base plate) 110.
[0108] A spacer 115 supports an external mirror 116 in place over
the top surface of the semiconductor 114 at a precise distance
establishing the VECSEL cavity spacing in a range between 0.5 mm
and 50 mm, (approximately 6 mm in the present example). While the
spacer shown in FIG. 3 is cylindrical, it may have a variety of
geometric shapes and be comprised of a single integral element, or
several integral or discrete elements such as posts, pillars, a
trough, or any other shape desired for or dictated by a particular
application. Thus, the term "spacer" as used herein includes
multiple structural elements as well as single integrated
structures.
[0109] The spacer 115 precisely fixes a VECSEL cavity mode
separation to be equal to dense wavelength division multiplex
("DWDM") channel spacing of the optical telecommunications network.
The ITU established DWDM telecommunications band at 190 THz, with
25 GHz channel spacing, requires an accuracy of absolute mode
positions equal to 2.5 GHz. In order to provide absolute frequency
control of each mode equal to ten percent of the nominal channel
spacing, a VECSEL cavity length precision on the order of
1.25.times.10.sup.-5 (.DELTA.L/L) is required.
[0110] The spacer 115 is most preferably formed of a material, such
as a molded glass component, (e.g. ULE glass, quartz, Zerodur.TM.
or other glass or metal such as Invar.TM. having a low coefficient
of thermal expansion) which thermally decouples the external mirror
116 from the semiconductor structure 114, so that temperature
regulation of the active region 124 with the thermoelectric unit
112 and heat sink 110 does not change the length of the VECSEL
cavity. The spacer material can be selected to compensate for
changes in the length of the semiconductor structure 114 when the
structure is heated. For example, if the semiconductor structure
elongates with temperature, the spacer material is chosen to expand
with temperature by an appropriate amount to offset any change in
wavelength which would otherwise result. With a spacer made of
fused silica having a thermal expansion coefficient .alpha.0 of
10.sup.-6, channel separation will be maintained within a
temperature change of plus or minus ten degrees C. Spacer materials
such as Zerodur ensure mode positions within temperature-changes of
plus or minus 100 degrees C. The VECSEL absolute cavity length
during manufacture should be reproducible within 0.4 micron.
Contemporary optical manufacturing technology can provide
thicknesses of optical materials within 0.5 micron. In order to
assure that the absolute cavity length is within specification,
trimming of the cavity length under optical feedback control is
used to meet the required cavity length tolerance.
[0111] The external mirror 116 may be a separate element bonded
onto the end of the spacer 115, or it may be formed integrally with
the spacer. The mirror 116 may be molded to the radius R1 with the
aid of a diamond turned metal preform or shaped to a desired
spherical contour by any other known method. The mirror surface of
structure 116 is of a very high reflectance, and it has a desired
spherical radius of curvature R1 relative to the MQW active gain
region to define a first highly reflective concave surface. The
mirror 116 and the semiconductor structure 104 form a VECSEL cavity
having sufficiently high finesse to realize effective single mode
laser operation when excited by an appropriate level of optical
pumping energy. The interior ambient environment of the VECSEL 104
may be dry air, helium, nitrogen, vacuum, or another medium,
depending upon an acceptable scattering/absorption tolerance as may
be required by a particular application or embodiment.
[0112] The external mirror 116 may have an outer curvature of
radius R2 forming a coupling lens for focusing the pumping energy
into the active region 124 and/or for focusing the VECSEL laser
emission beam into the optical fiber 106. Losses in the laser
cavity of VECSEL 104 need to be low enough so that the gain of the
MQW active region is sufficient to overcome those losses. An
external mechanical/optical coupler 108 may be provided to position
a fiber end and couple the VECSEL laser beam into the fiber 106.
Other laser/fiber coupling arrangements known in the art may also
be employed to position and stabilize the components and to couple
effectively the VECSEL laser beam into the fiber.
[0113] The optical transmitter also preferably includes a beam
splitter 117 in the output laser beam path which directs a
component of output radiation into a photodetector structure 119.
The photodetector provides optical feedback information in the form
of electrical signals to a controller 121, most preferably a
programmed digital controller. The digital controller 121 generates
thermoelectric cooler control signals which are suitably converted
into driving currents by an amplifier 123 and applied to control
the thermoelectric cooler 121. In some embodiments it may improve
performance of the transmitter 100 to include a temperature sensor
125 within the body of thermoelectric cooler 121 and feed sensed
temperature values back to the controller 121. The digital
controller can also feed back pump laser current control values to
the pump laser to control pump radiation to maintain a constant
output power out of the VECSEL via higher pump power levels at the
edges of the tuning range where VECSEL gain is not as high. The
digital controller 121 typically includes analog to digital and
digital to analog conversion circuits well known to those skilled
in the art and not included within the FIG. 3 illustration.
[0114] We have previously indicated that even though the
transmission of the DBR mirror 126 of the semiconductor structure
is very low, a small amount of the transmitted light reflected from
the back side of the substrate can reenter the cavity. This small
amount of light can introduce a spectral perturbation which has the
form of fringes with the spectral period defined by the optical
thickness of the substrate. Such modulation is unwanted and may
impede smooth tuning from one mode to another. We have found that
this effect can be significantly reduced by providing a wedge-shape
to the substrate of several degrees. However, for telecom
applications, such wedge shape may not be sufficient to remove this
effect because the VECSEL will operate in a stationary state and
the influence of the light scattered from the substrate may prove
to be strong enough to perturb smooth tuning from mode to mode. In
order to exclude completely any optical perturbation from the back
side of the substrate, we deposit a non-transparent metal film
layer between the DBR and the substrate. In order to make this
approach practical, we employ a reverse order epitaxial process in
which the epitaxial structure is grown in reverse order. As shown
in FIG. 4, for the exemplary telecommunications transmitter 100,
the semiconductor structure 114 is grown in reverse order on buffer
layer 120 most preferably by molecular beam epitaxy: AR layer 122,
then active region 124, then DBR mirror 126 and finally metal film
layer 128.
[0115] The VECSEL semiconductor structure 114 is most preferably
grown on a substrate 118 comprising InP having an etch stop layer
120 of In.sub.xGa.sub.1-xAs, where x and 1-x represent chemical
mole fractions of the respective elements of the crystalline buffer
layer material. As grown in reverse order, the first or bottom
layer comprises the antireflection layer 122 of an .lambda./4 thick
indium phosphide capping layer and an indium gallium aluminum
arsenide layer of a thickness 5.lambda./4 where lambda represents
the VECSEL nominal mid-band output wavelength, 1560 nm, for
example. The function of the antireflection layer 122 is to prevent
reflection within the semiconductor structure at the VECSEL laser
wavelength, as opposed to the pumping wavelength. Since the VECSEL
104 is pumped through the external mirror 116 or through some other
lens or opening through spacer 115, the antireflection layer 122
should be of a material selected for minimal absorption of energy
at the pump wavelength, so that a maximum pump power will enter the
active region, excite the quantum well carriers and yield efficient
laser action. For telecommunications it is necessary that the
antireflection coating 122 be effective across the entire optical
communications band, and not just a single channel.
[0116] A positive gain, active region 124, in one example having a
length 7.times..lambda./4, is then formed and has, for example,
three pairs of quantum wells 130 of indium gallium arsenide. While
FIG. 4 illustrates an arrangement of pairs of quantum wells 130
with each pair arranged at a peak of the optical standing wave,
other arrangements can be employed when optical pumping radiation
enters the active gain region 124 via the antireflection coating
122, such as three-two-one. In this alternative arrangement three
quantum wells are at an optical peak nearest antireflection coating
122, two are at a middle peak, and one is at a peak distant from
the antireflection coating. In the near infrared spectrum, quantum
wells typically have less gain than in the visible spectrum, and a
sufficient number of quantum wells must be provided to yield the
needed gain for reliable operation at the desired output power.
Each quantum well 130 has a thickness designed in relation to the
desired output wavelength at operating temperature (which, because
of absorption of the pump energy, will be higher than room
temperature). When the active region 124 is pumped, it heats up.
When a semiconductor is heated, it changes effective thickness and
index of refraction. Accordingly, the emitting wavelength of the
quantum wells 130 must be shifted to a higher energy level compared
to a room-temperature design wavelength of the structure:
.lambda..sub.QW.apprxeq..lambda..sub.DESIGN-20/30 nm approximately
(at T.sub.o=300K. at low excitation), so that the gain and the
design wavelength match when the VECSEL 104 is operating.
[0117] As shown in the FIG. 4 diagram, each pair of quantum wells
130 is located at a maximum of the active region standing wave.
Major separation or barrier layers between the quantum well pairs
have a length optimized for an absorption coefficient at the pump
radiation wavelength, which in this example is 980 nm. FIG. 4 not
only shows a diagrammatic cross section of the layers of the
semiconductor structure 114, it also plots relative band gap
energies of the various semiconductor layers 120, 122, 124, and
126.
[0118] A distributed Bragg reflector (DBR) layer stack 126 is then
formed on top of the active region. The DBR 126 comprises an odd
number of quarter-wavelength interleaved layers, preferably greater
than twenty pairs, plus one layer, to achieve an odd number of
quarter wavelengths. The DBR layers comprise alternating indium
gallium aluminum arsenide, and indium phosphide quarter wave
layers, so that total reflectance within the DBR at the design
wavelength is greater than 99 percent. Finally, a metal film mirror
layer 128, e.g. gold or gold alloy, is sputter-deposited onto the
DBR structure 126 to complete the fabrication of the semiconductor
structure by the epitaxial process. The metal mirror increases the
reflectance from 99 percent to approximately 99.5 percent. Index
matching and phase discontinuity issues are essentially avoided by
using both the DBR mirror structure 126 and the metal mirror
128.
[0119] The effective length between the DBR 126 and the external
surface of the antireflection coating 122 is set to an odd number
of nominal output quarter wavelengths, so the sub-cavity formed by
the semiconductor structure 114 operates in anti-resonance. The
pumping energy passes through an antireflection coating 122 to
reach an active region 124 of quantum wells.
[0120] For a fundamental transverse electromagnetic mode
(TEM.sub.00) operating wavelength of 1560 nm in the near infrared
spectrum and at the design operating temperature (300K), the
various layers of the semiconductor structure 114 from the gold
layer 128 to the external surface formed at the outer end of the
antireflection layer 122 are given in the following table:
1 Reference Layer No. Layer Material Thickness 126 InP (Phase
matched to metal 901 Angstroms 126 InGaAlAs (22 layers) 1117
Angstroms 126 InP (22 layers) 1240 Angstroms 124 InGaAlAs 1681
Angstroms 124 InGaAs (QW) 53 Angstroms 124 InGaAlAs 150 Angstroms
124 InGaAs (QW) 53 Angstroms 124 InGaAlAs 150 Angstroms 124 InGaAs
(QW) 53 Angstroms 124 InGaAlAs 150 Angstroms 124 InGaAs (QW) 53
Angstroms 124 InGaAlAs 150 Angstroms 124 InGaAs (QW) 53 Angstroms
124 InGaAlAs 150 Angstroms 124 InGaAs (QW) 53 Angstroms 122
InGaAlAs 1231 Angstroms 122 InP (capping layer) 1117 Angstroms 120
In.sub.0.53Ga.sub.0.47As 3000 Angstroms 120 InP (buffer layer) 5000
Angstroms 120 InP Substrate
[0121] FIG. 5 presents a graph superimposing an upper sinuous line
representing measured reflectivity of a sample semiconductor
structure 114 in accordance with the above table, and a louver
sinuous line representing measured photoluminescence of the same
sample.
[0122] After the epitaxial deposition processes are complete, the
indium phosphide substrate 118 is removed by an abrasion step, such
as ion beam milling. The indium gallium arsenide etch stop layer
120 is then removed by a second step of wet chemical etching with
an etching agent which favors removal of the gallium arsenide
substrate 120 (rather than the indium phosphide capping layer
within the antireflection layer 122). As indicated in the table
above and in FIG. 4, a dielectric layer of appropriate thickness is
then vacuum deposited onto the InP capping layer in order to
complete the e.g. 5.lambda.4 thick antireflection layer 122 and
made to have proper P polarization as with .lambda./2 plate, the
ellipticity turns into a circular pattern at the surface. A wafer
including multiple ones of semiconductor structures 114 is then
diced to yield individual semiconductor dies or "chips".
[0123] The metal mirror layer of a chip 114 is then bonded to a
silicon substrate or soldered to a copper heat sink 310. By
removing the substrate 320 in this preferred reverse order
formation process, thermal control of the active region 324 via the
thermoelectric element 312 and the heat sink 310 is much more
direct and positive, than if the heat had to be conducted through
the substrate, as is the case with conventional VECSEL and VCSEL
designs.
[0124] For example, within a telecom application, a thermal control
loop comprising elements 117, 119, 121, 123, 112 and 125 shown in
the FIG. 3 example is most preferably employed to establish and
maintain a central or reference wavelength within a multi-channel
optical band. As shown in the FIG. 1 graph, the VECSEL 100 will
lase on a mode closest to gain maximum. When the temperature of the
quantum well active region 124 is changed, the maximum of the gain
shifts with a turning of approximately 30 GHz/degree C. This tuning
range suggests that if the temperature of the gain structure is
kept stable within 0.1 degree C., such thermal regulation will
ensure that the VECSEL will be operating on a single selected mode.
This approach avoids the drawbacks of absolute temperature control
required for existing DFB lasers designed for use in the optical
fiber telecommunications industry.
[0125] As shown in FIG. 2, when the gain maximum is thermally tuned
to a position corresponding to the middle between two adjacent
modes, the VECSEL 104 will intermittently lase on one or the other
mode. If the laser is alternating between two modes, such
alternation can be detected by providing the photodiode assembly
119 with frequency selective-filtering characteristics. By
thermally tuning the telecommunications VECSEL 104 over the entire
frequency range, it is practical to detect all set points
corresponding to gain positions exactly between the modes. These
positions are then stored in the digital control unit 121 enabling
an exact match between temperature set points and the DWDM channel
to be computed and presented to the active gain region 124 via the
thermoelectric cooling unit 112. This particular method enables
thermal compensation over time for changes in the semiconductor
structure materials of the VECSEL 104 due to material aging, for
example. The digital control unit 121 also controls a startup
sequence of the pump laser 102 and regulates pump power during
steady state operation, so that the VECSEL 104 is started single
mode and remains single mode during operation.
[0126] The intermittent, mode hopping behavior shown in FIG. 2 is
most preferably used to determine automatically the relationship
between temperature set points and telecom channel numbers. The
temperature set points should correspond to the situation where the
gain curve maximum coincides exactly with the corresponding mode,
as illustrated in FIG. 1. As seen in FIG. 2, the intermittent laser
behavior takes place when the gain maximum is located exactly
between two modes, and the temperature range corresponding to such
intermittent behavior is significantly smaller than the temperature
increment necessary to move the laser from one mode to its nearest
neighbor mode. Thus, these temperature set points can be determined
with a very high precision by observing the laser output with
monitor photodiode 117 which has, in front of it, a filter having
an optical transmission gradient which varies from a low of a few
percent, value to a high, near 100 percent, value across the
telecom wavelength range. With such a detector, which is AC coupled
to the controller 121, the jumping of the laser from mode to mode
will result in intense random spikes (mode transitions) which
produce digitized values and intervals readily detected by the
control unit 121.
[0127] A presently preferred method for laser self-calibration is
as follows: The control unit 121 sets the laser temperature set
point to its lowest negative value causing, the thermoelectric
cooler 112 to reach its lowest nominal operating temperature. Then,
the control unit 121 causes the temperature to increase over a
calibration time interval. The control unit 121 continuously
monitors laser output to detect mode transitions, and it records
the temperature control parameter for that particular transition in
memory, most preferable a non-volatile electrically rewriteable
memory within controller 121, such as a "flash" memory element or
array. This operation of detecting mode transitions and recording
thermal control parameters continues until a maximum positive
temperature set point is reached. The control value for the first
mode is then given the value of one half the first thermal control
parameter, the control value for the next mode is then determined
as a value of the median between the second and third recorded
control parameters, and so forth, until all control values within
the thermal control range are determined. This dependence can be
expressed as a low order polynomial such as Ut(n), where Ut is, for
example, the control voltage input to the thermoelectric cooler
112. Then, the control value of the temperature control
(corresponding e.g. to voltage or current) for the first telecom
channel accessible by this particular laser will be found as Ut(1),
the control value for the second telecom channel will be Ut(2),
etc., and finally the control value for the last telecom channel
accessible by this particular laser will be found as Ut(n).
Obviously, Ut(0.5), Ut( 1.5), etc., are the set points
corresponding to the gain curve equidistant between the adjacent
modes. Finally, absolute wavelength of the first operating channel
for the particular laser is determined and recorded in the
controller memory (and externally as part of documentation
accompanying this particular laser) at the factory by use of an
optical spectrum analyzer, such as a Burleigh WA-7100. Once
calibrated, the laser will keep the values for a long time. The
distance in wavelength units between two telecom channels with 25
GHz channel spacing is 0.195 nm. Provided that the laser
temperature tuning coefficient is about 0.45 nm/.degree. C. the
tuning from one telecom channel to another will require about
0.4.degree. C. temperature increment. In order to keep the laser
operating at the chosen channel frequency, a readily obtainable
temperature tolerance of 0.1.degree. C. is sufficient.
[0128] After the laser leaves the factory, the automatic
calibration of the laser can be repeated in the field or operating
environment at any time, and any correction to temperature control
values (polynomial coefficients) can be refreshed by the control
unit 121 and replaced in memory. No measurement by a spectrum
analyzer would be required unless initial laser calibration data is
completely lost. This method enables each laser to be maintained in
a calibrated state for an arbitrarily long time, which can be
expected to approach closely the full useful life of the laser
device.
[0129] While the VECSEL 104 shown in FIG. 3 is presently preferred
because of its relative simplicity of manufacture, other VECSEL
arrangements and configurations are within the scope of the present
invention. In FIG. 6, a VECSEL-based telecommunications transmitter
200 includes a laser diode pump 202, a pump beam focus lens 203,
and a VECSEL 204. In this configuration, the pump energy enters one
side of the VECSEL 204, and the laser energy exits another side of
the VECSEL 204. A base 210, made of thermally conductive material
such as copper, defines a pump aperture 211. A thermoelectric
cooler such as cooler 112 (not shown in FIG. 6) automatically
controls temperature of the base 210. A semiconductor structure,
formed in reverse order in the same manner as the FIG. 3 VECSEL
104, has a pump-transparent dielectric mirror layer 214, an active
gain region 216 and an antireflection coating 218. An external
surface of the dielectric mirror layer 214 is polished very smooth
and is bonded to a transparent substrate layer 212, such as
diamond, by a suitable bonding method or agent. Vacuum bonding, or
peripheral soldering with a solder material, such as indium, is
preferred. Since the external lens 220 and spacer 222 are
equivalent to the lens and spacer of the VECSEL 304, the
explanations given above for those elements apply to the elements
220 and 222. With suitable modifications made clear by the
foregoing explanations of structure and function of respective
elements, the pump laser radiation could enter the active region of
VECSEL 204 via the external mirror 220, and the laser radiation
could exit via the aperture 211.
[0130] In cases when single wavelength operation and discrete
step-tuning is not necessary, such as in spectroscopy for example,
the spacer, instead of being formed of low thermal expansion
materials may be made of a material that changes its dimension
under external stimulation, such as a piezo-electric transducer, or
as a result of changes in external ambient pressure. FIG. 7 shows a
MWQ VECSEL 300 in accordance with these principles. VECSEL 300 is
similar in structure to VECSEL 104 shown in, and described in
conjunction with, FIG. 3, so that common elements bear the same
reference numerals and the previous descriptions of those elements
apply to the FIG. 7 structure. However, in VECSEL 300 an annular
piezoelectric element 302 is sandwiched between the base
110-heatsink 112 and the spacer 115. In this case the VECSEL cavity
length can be made even shorter, e.g. 1 mm or even a few hundred
microns, and the lase, will provide single-frequency mode-hop
tuning over several wave numbers (cm.sup.-1). In this regard it is
important that the piezoelectric element 302 apply a uniform force
around its circumference, so that the external mirror 116 remains
on the optical axis of VECSEL 300 as the cavity is lengthened or
shortened.
[0131] Alternatively, if the external mirror is coupled to a
piezoelectric transducer, the VECSEL will work as a tunable single
frequency source. The mode hop free tuning range will be close to
the cavity mode spacing, 25 GHz for a cavity length of 0.6 cm. If
the cavity length is reduced to about 1 mm for example, the mode
hop free tuning range (without synchronous MQW structure
temperature tuning) will be 150 GHz, or 5 cm.sup.-1, which makes
the resultant VECSEL a very good source for spectroscopic and gas
analysis applications.
[0132] A cost-saving improvement can be realized by use of a
conventional laser diode pump for directing optical pump radiation
at the surface of the semiconductor active region at an angle of
incidence equal to Brewster's angle (.theta.) and with P
polarization which maximizes absorption of pump radiation into the
semiconductor structure 114 and minimizes reflection at the surface
thereto. This arrangement is illustrated in FIG. 8 which shows a
MWQ VECSEL 400 in accordance with these principles. VECSEL 400 is
similar in structure to VECSEL 104 shown in, and described in
conjunction with, FIG. 3, so that common elements bear the same
reference numerals and the previous descriptions of those elements
apply to the FIG. 8 structure. In the FIG. 8 arrangement 400, a
miniature diode laser assembly 402 is aligned and secured within an
opening of the spacer 115 of VECSEL 400 so that pump radiation is
directed with P polarization at the external surface of the
semiconductor structure 104 at an angle analogous to Brewster's
angle (.theta.) i.e., 73.6 degrees. A micro-lens 404 may be
included within the DFB laser assembly 402 to collimate the pump
beam and limit spot size to between 50 and 70 microns, for example.
It is obvious that directing a pump beam from a diode laser having
a three to one ellipticity at an angle of incidence analogous to
Brewster's angle and made to have proper P polarization as with a
.lambda./2 plate, the ellipticity turns into a circular pattern at
the surface and mode mismatches between the pump laser beam and the
semiconductor are thereby minimized. In this manner the diode laser
assembly 402 need only emit at a power level of 150 mW to obtain
the equivalent pump efficiency requiring a much higher power laser
pump having a beam not incident at Brewster's angle. 150 mW diode
lasers emitting at 980 nm wavelengths are readily available, from
sources such as Nortel, JDS and Coset. The pump assembly 402 may
include its own integral heat sink and thermoelectric cooler in
order to facilitate thermal control of wavelength in addition to
direct current control, if desired for a particular
application.
[0133] The polarization of a conventional 1.times.3.mu. high
brightness laser diode is oriented parallel to its long emitter
side, so as to be opposite to what is wanted in order to pump at
large angles of incidence, and thus get expansion of the short
diameter due to the large angle. In this case a .lambda./2-plate
will be required in order to rotate the polarization. Such diodes
have some astigmatism, that is to say, the virtual point sources
which represent the diode radiation have different positions along
the diode's Z axis for fast (x) and slow(y) laser diode convergence
planes. When imaged with a lens, the diode surface image therefore
has different locations for x and y planes. The diode can therefore
be oriented so that its polarization is a right P polarization (3
.mu.m side in the incidence plane) and some intermediate plane used
between the two astigmatic image planes in order to get the spot on
the VECSEL semiconductor structure surface to be circular.
[0134] The arrangement shown in FIG. 8 is presently preferred as it
enables the diode pump laser 402 to be aligned and fixed in place
in the factory as part of the VECSEL assembly process and then
checked out before delivery to a user. In the event that a small
portion of pump energy is reflected by the outer surface of the
semiconductor 104, an aperture or other pump-energy absorbing means
406 may be defined in an opposite position in the spacer 115 to
prevent or impede further reflection of pump energy within the
external cavity of the VECSEL.
[0135] As pointed out above, the spacer can be manufactured within
a 1 .mu.m tolerance. During manufacturing it is necessary to make a
final adjustment of spacer length so that it meets the required
0.04.mu. tolerance in order for the laser mode spacing to equal the
telecom channel spacing. This fine adjustment is also required in
order to assure that the absolute frequency of any given mode is
within the required tolerance equal to the absolute frequency of
the telecom channel closest to this mode. It does not matter which
mode is tuned to the nearest telecom channel providing the mode
spacing corresponds correctly with the telecom channel comb. The
channel number can be adjusted in the processor unit 121.
[0136] To perform the fine adjustment of the spacer, the VECSEL
should be switched on so that it transitions at a single frequency
at some mode. Its output radiation is sent to a spectrum analyzer,
such as a Burleigh WA-7100 spectrum analyzer, or equivalent, which
provides a wavelength measurement accuracy of .+-.1.5 ppm or -0.19
GHz. The spectrum analyzer will display the actual laser
frequency.
[0137] With reference to FIG. 9, the mirror 116 is fixed to the
Spacer 115 in such a way that the border of its spherical surface
is sitting on the polished flat surface of the cylindrical spacer.
The mirror 116 in a suitable fixture, such as a spring loaded
mount, so that by adjustment of the spring, the spacer can be
axially displaced relative to a base plate 502 or given a small
elastic deformation. The base plate 502 may be provided with a
recessed or flanged region 504 sized to present a small
interference with the spacer 115 so that the spacer is initially
maintained at a starting position. Adjustment of 1.mu.along the
longitudinal axis of the VECSEL 500 while monitoring the spectrum
analyzer can readily be made in this manner. Once the spacer is
precisely adjusted to provide the external cavity with its
precisely correct length, it is securely bonded to the base plate
in this position by a bonding agent 506, such as low temperature
glass, solder, UV-curable resin system, or the like, (which may be
heated and flowed or reflowed ed incident to this adjustment).
Fixing a final position may be achieved by post-tensioning the
spacer 115 with three or more tensile members or posts 510 which
are automatically adjustably tensioned by computer control in the
factory between a flange 508 of the spacer and fine-pitched
threaded openings formed in the base plate 112. Other arrangements,
such as maintaining a spring bias force on the spacer may be
employed in the completed VECSEL 500.
[0138] With some added structural and fabrication complexity, an
additional electro-optical control element, such as a thin
dielectric tilted etalon having dielectric partial reflective
coatings, for example, can be included within the VECSEL cavity.
Such an etalon will work as a bandpass filter, and will reduce
effective gain bandwidth, thus reducing the time necessary to reach
single frequency operation and increasing side mode suppression.
The wavelength of the etalon transmission peak should be close to
the gain maximum. The tuning of the etalon may be achieved, for
example, by changing its temperature using a separate
thermoelectric cooler element or by changing its tilt angle or by
changing the index of its spacer material electro-optically.
Correspondence between etalon temperature/tilt angle and a selected
channel can be established using a procedure similar to the
calibration procedure set out in the preceding. Such an additional
element or controllable wavelength filter enables more rapid
selection/control of VECSEL emission wavelength than may be
realized by thermal control only of the VECSEL active region 114.
In addition, the intra-cavity element may be provided to speed up
single mode operation and further reduce the possibility that the
laser will operate multimode during startup. The closest cavity
mode to the transmission peak of this element becomes the chosen
operational mode. As the DWDM channel separation is 10.sup.-4 of
the telecom channel frequency, positioning of the intra-cavity
element's optical peak has to be made with an accuracy of a few
percent of the free spectral range. This is readily reproducible
with contemporary optical manufacturing techniques. Rapid tuning to
a particular channel wavelength can then be carried out by the
intra-cavity electro-optical element which effectively changes the
cavity length in a controlled manner correlated to the selected
channel wavelength. In this manner, from 500 to 1000 separate
channels can be realized over a 200 nm VECSEL laser radiation
wavelength tuning range centered at e.g. a 1560 nm wavelength.
[0139] FIG. 10 sets forth a greatly enlarged schematic diagram of a
VECSEL 600 including an intra-cavity element 602 forming a mirror
for reflecting optical energy emitted by the semiconductor
structure 114 to a spherical mirror 604. A two-pert spacer includes
a generally cylindrical body 606 which defines the mirror 604, and
further includes a plate 608 which aligns and secures the
intra-cavity element 602. The etalon 602 may be partially
transmissive, and the monitor photodiode 119 then can be mounted to
the plate 608 behind the etalon 602. The controller 121 includes an
etalon driver 610 for driving either a thermoelectric cooler or a
piezoelectric transducer which controls the etalon 602 in the
manner described above.
[0140] Suitable embodiments of the stepwise tunable, extended
cavity active mirror laser (STECAM) comprise an active mirror with
a gain bandwidth sufficient to overlap several modes of the overall
cavity formed between that mirror and another external mirror and
means for selecting the cavity mode that oscillates and of altering
that selection. In one preferred embodiment, that selecting means
is a tuning element placed inside the cavity. In certain of the
preferred embodiments described below, the amplifying mirror can be
fabricated as a surface emitting laser gain structure and the
STECAM becomes a vertical extended cavity surface emitting laser
(VECSEL) in previous parlance. In another preferred embodiment, the
amplifying mirror is fabricated as an edge-emitting semiconductor
optical amplifier (SOA).
[0141] In connection with the Figures, we have described various
implementations of a method of providing frequency-switched
radiation for optical communications or spectroscopy. Said method
comprises shifting the transmission band of a wavelength dependent
intra-cavity mode selector in a STECAM cavity by means of an
externally applied electrical signal. The STECAM cavity is designed
to have fundamental axial modes at pre-specified wavelengths of use
in communications and spectroscopy, and the action of the mode
selector is to choose which of them operates at a given time, not
to tune a wavelength about its pre-specified value. The use of
electro-optic media such as lithium niobate, lithium tantalate and
smectic and nematic liquid crystals in the tuning element allows
the radiation wavelength to switch on a millisecond and faster time
scale, much more rapidly than mechanical tuning means. The dynamics
of the STECAM cavity is such that no radiation is produced at
unwanted wavelengths or frequencies.
[0142] Further details of embodiments of the intra-cavity tuning
element of the present invention are delineated schematically in
FIGS. 11-20. For these embodiments, the gain spectrum of the
amplifying mirror is assumed to be less sharply peaked than in FIG.
1. The layer design of the required MQW and DBR structures are only
slightly different than those illustrated in FIGS. 4 and 5. When
the gain spectrum is relatively flat, there is a need for an
intra-cavity element to select one cavity mode for oscillation.
However, once the power in that mode increases, saturation
suppresses the gain of the other modes as shown in FIG. 1. The
intra-cavity element is intended to select one of several cavity
modes for oscillation, but not to alter the frequencies of those
modes, which are determined by the overall length of the optical
cavity.
[0143] In the drawings, wherein like components are designated by
like reference numerals: FIG. 11 schematically illustrates a
vertical external cavity surface emitting laser (VECSEL) in
accordance with the current invention. The amplifying mirror 20
along with the concave mirror 25 constitute the laser cavity. The
output beam is shown schematically as 60. A heat-sink device 11 is
outside the cavity but forms part of a mechanical support stricture
(not shown). The electronically adjustable frequency selective
element 30 responds to externally-originated electrical signals
sent on cable 40 by switching the laser output 60 from one
frequency to another.
[0144] FIG. 12 illustrates the spectral relationships of the
components in FIG. 11. The cavity formed by the mirrors 20 and 25
produces a spectrum of modes 70 equally spaced in frequency. The
frequencies of the modes are chosen so that a subset of them or all
of them correspond to desired communications channels or desired
molecular absorption signatures. The gain spectrum 71 of the
amplifying mirror 20 is substantially uniform across a considerable
portion of the mode spectrum, which would normally Live rise to
multi-mode oscillation and/or unstable competition among the modes.
The frequency selective device 30 has a transmission spectrum 72
that has a maximum for only one mode 73 within the gain band of the
amplifying mirror 20. The combination of the gain of the amplifying
mirror and the transmission of the frequency selective element
causes the selected mode 73 to rise in power until it depletes all
the inversion available in the homogeneously broadened gain medium.
Thus only mode 73 oscillates, and does so stably, without
competition from near-threshold modes. It is not necessary that the
frequency selective element 30 act as a high contrast or narrowband
filter. It is sufficient that the net unsaturated gain of one
preferred mode exceed the net unsaturated gain of each of the other
modes. An electronic signal applied to the mode selective element
30 via cable 40 causes the peak of the transmission of the mode
selective element to rapidly switch from the oscillating mode 73 to
any other mode within the gain spectrum 71. That switch
extinguishes oscillation in mode 73, but laser radiation is not
produced in any of the other modes until three requirements are
met:
[0145] 1.) Sufficient pump power is absorbed to create an inversion
sufficient to produce non-zero net gain in any of the
previously-sub-threshold modes.
[0146] 2.) The peak transmission of the mode selective element
arrives at the frequency of another mode.
[0147] 3.) Photons spontaneously emitted at the new mode frequency
make sufficient cavity round trips under low-loss, high-gain
conditions to build up a significant intra-cavity laser power.
[0148] Since the VECSEL cavity has a high Q (low loss) and the
amplifying mirror has low gain, this 3.sup.rd requirement is
relatively time consuming compared to requirement 2. In particular,
there is insufficient time for cavity power build up to occur in
the modes between the initial cavity mode extinguished and the
final one designated as the mode selective element peak traverses
their frequencies.
[0149] In an alternative embodiment of this invention, the energy
input (e.g. from a pump laser) to the amplifying mirror is briefly
reduced whenever the peak transmission of the mode selective
element traverses the frequency of an unwanted mode to ensure that
it does not reach threshold.
[0150] Returning again to the drawings, FIG. 13 illustrates one
embodiment of the amplifying mirror 20, wherein it contains a gain
region 21 and a Bragg reflector region 22, both fabricated out of
semiconductor materials such as InP, InGaP and InGaAsP. In this
embodiment, the amplifying gain region 21 comprises layers of
quantum-well material separated by layers of material transparent
over the gain bandwidth 71 of FIG. 2 although not necessarily
transparent at a pumping wavelength- said quantum well layers being
disposed at the anti-nodes of the cavity axial mode created by
reflections from the Bragg reflector region 22 and the concave
mirror 25 shown in FIG. 11. The reflectivity of the reflector
region 22 is greater than 95%, but it is necessary to suppress
variations in net reflectivity as a function of wave length due to
reflections from surfaces behind the reflector region 22 with
respect to the gain region 21 (i.e. the wafer substrate) by a
factor of at least 10.sup.4. In the amplifying mirror embodiment
shown in FIG. 13, the back surface 23 of the mirror 20 is tilted to
prevent such reflections propagating back into the optical cavity.
It will be recognized that other means of suppressing such
wavelength dependent reflectivity variations are possible,
including depositing a completely opaque film on back surface 23.
It is also desirable to suppress any phase variation of the
reflection from that medium. i.e. 22 plus 23 such as depositing an
opaque layer close to the outermost layer of the reflector region.
The surface of the amplifying mirror inside the resonator 24 is
also positioned or treated (e.g. coated) to suppress wavelength
dependent reflectivity over the gain band of the amplifying
mirror.
[0151] FIG. 14 illustrates an embodiment of this invention in which
the amplifying mirror 20 is excited by a pump laser beam 13 focused
on the amplifying mirror 20 by a lens 12 outside the cavity formed
by mirrors 20 and 25. The pumped spot on the mirror 20 is chosen to
ensure single transverse mode operation, in this embodiment. This
can be achieved for example by using a pump beam which has the
following characteristics:
[0152] 1) a transverse electromagnetic zero/zero
mode(TEM.sub.00);
[0153] 2) a circular projection spot onto the amplifying mirror 20;
and
[0154] 3) the projection spot is of such a size as to permit laser
action only in a TEM zero/zero (TEM.sub.00) transverse mode but not
in any higher order transverse modes.
[0155] One skilled in the art will recognize that other optical
pumping geometries are possible, including some with multiple beams
and others that would access the gain region 21 through the
reflector region 22 shown in FIG. 13. Such geometries are other
possible embodiments of this invention. The pumping laser beam 13
has a wavelength that is absorbed in the gain region thereby
causing population inversion in the quantum structures, thereby
producing gain.
[0156] In another embodiment, the amplifying mirror is pumped
electrically in the same manner used for a vertical cavity surface
emitting laser (VCSEL) without an external (extended) cavity. The
heterostructure diode structures necessary for electrical pumping,
carrier confinement and optical field confinement are known art in
the VCSEL field.
[0157] FIG. 15 illustrates an embodiment for the mode selective
element 30 wherein a thin plane Fabry Perot tilted etalon 31
transmits light at the mode desired for oscillation but reflects
sufficient light at other frequencies to discourage oscillation.
This element can be used to provide a signal showing the degree of
mutual detuning of tuning element and cavity modes for any given
control voltage without perturbing the operation of the laser. This
will allow keeping the tuning element transmission maximum always
in coincidence with the cavity modes and at the same time
compensate for any thermally induced change of the optical element.
This property is achieved due to 3 n T and n T
[0158] being equal where n is the index of refraction. The spacer
medium of the etalon is an electro-optical material bounded by
conductive, partly transmitting mirror surfaces onto which a
voltage can be applied by means of cable 40. The index of
refraction n of the spacer medium varies with applied voltage,
according to:
n(V)=n.sub.o+.lambda..sub.0/2d V/V.sub..pi.
or n(V)=n.sub.o-n.sub.o.sup.3rV/2t,
[0159] where n.sub.o is the index of refraction of the spacer
medium with no applied voltage, V is the applied voltage,
V.sub..pi. is the voltage necessary to achieve a .pi. radian change
in optical phase, t is the etalon thickness, .lambda..sub.o is the
center optical wavelength of the gain band, and r is the
electro-optic coefficient of the spacer medium. Thus the shift in
peak wavelength of the transmission function for the tilted etalon
is:
.DELTA..lambda.(V,
.theta.)=-.lambda..sub.on.sub.o.sup.4rV/2d(n.sub.o.sup.-
2-sin.sup.2.theta.).sup.-1/2.
[0160] Since electro-optic media are typically birefringent, it may
be necessary for the light within such a frequency selective
element to have a single polarization. This can be achieved by
means of a polarization selective element 32. Adequate low-loss
polarizers are known and include prisms with Brewster angled
surfaces, thin films with polarization sensitive reflection
coefficients and Glan-Thompson birefringent polarizing prisms. In
order for the etalon 31 to have only a single transmission maximum
within the gain band of the amplifying mirror (FIG. 13), the spacer
thickness t must satisfy
t<.lambda..sub.o.sup.2/(2n.sub.o.DELTA..lambda..sub.G)
[0161] where .DELTA..lambda..sub.G is the wavelength band producing
gain (71 in FIG. 12). The spacer thickness t is small, and the
voltages required by conventional electro-optic crystals to tune by
changing refractive index over .DELTA..lambda..sub.G(-V.sub..pi.)
may be unduly high. However. nematic and smectic liquid crystal
media capable of adequate change in refractive index at low voltage
are known and used, for example, in variable waveplates, choppers
and displays. In such devices, an applied AC or DC voltage changes
the orientation of the rod-shaped molecules of the liquid crystal
medium in a plane defined by the incident light polarization and
direction of propagation, thus varying the index of refraction.
While the scattering loss of such media may be too high for some
applications in high finesse, high-contrast Fabry Perot filters
requiring high reflection (R.about.95%) mirrors, the frequency
selective element of this example has low finesse and low contrast
and only requires relatively low reflectivity mirrors. We have
found that some loss due to scattering in the spacer medium is
acceptable. The use of such liquid crystal media in step tunable
external cavity semiconductor lasers is novel. While current
nematic liquid crystal materials require milliseconds to change
index of refraction, smectic-C materials are known that change
orientation (i.e. index of refraction) in a few microseconds, thus
adequate switching speed is obtainable. Alternatively, one can make
use of the Vernier effect in which the modes of a thick etalon are
spaced somewhat differently from a multiple of the spacing of the
modes of the overall cavity, but laser action occurs only on the
cavity mode most nearly centered on an etalon peak. The thickness
of such an etalon may be greater than stated above, with a
corresponding reduction in the necessary applied voltage.
[0162] In a further embodiment the etalon 31 contains air
(n.sub.o.about.1) between the conducting mirror surfaces, rather
than a solid or liquid medium. Air-spaced etalons with dielectric
mirrors deposited on thick transparent substrates are known, but
previous embodiments have had fixed spacing between the mirrors or
have been actuated by piezoelectric transducers. Such etalons
cannot switch frequencies rapidly enough (e.g. <1 millisecond)
for use in telecommunication applications. In a preferred
embodiment of the present invention, the mirrors are free-standing
semiconductor film stacks supported on flexure mounts and
fabricated by deposition and etching in a planar MEMS
(microelectro-mechanical systems) process. A voltage difference
applied to the mirrors causes them to attract one another, changing
the spacing between them, thereby varying the peak transmission
frequency of the frequency selective element 31. Individual high
reflectivity free-standing films with flexure support have been
fabricated as laser mirrors for VCSELs. However, the use of such
films in an intra-cavity etalon of a STECAM is novel.
[0163] FIG. 16 illustrates yet another embodiment of the
intra-cavity mode selector 30, wherein an electro-optically tunable
Lyot filter determines the oscillating mode. In this embodiment,
the polarization selective component is a Brewster-angle window 33
which transmits essentially 100% of light linearly polarized in the
plane of the page, but less of the light polarized in the
perpendicular plane. The Lyot filter comprises a birefringent
crystal 35 with ordinary 36 and extraordinary 34 axes oriented at
.+-.45.degree. to the high transmission direction of the
polarization selecting element 33. Electrodes are deposited on the
transverse faces of the crystal so that an electric field can be
created within the crystal along the extra-ordinary axis 34 by
applying a voltage V to the attached wires 40. Such an applied
voltage modifies the indices of refraction of the crystal
approximately according to:
n.sub.o(V)=n.sub.o-n.sub.o.sup.3r.sub.13V/2t and
n.sub.e(V)=n.sub.e-n.sub.- e.sup.3r.sub.33V/2t
[0164] where t is the thickness of the crystal along the axis 34
(between the electrodes). More generally, the voltage may be
applied along any crystal axis which exhibits electro-optic
activity (i.e. an axis with non-zero electro-optic coefficient).
The crystal 35 with length d along the light propagation axis acts
as a high-order wave plate with retardation (in waves) of,
N(V)=[n.sub.e(V)-n.sub.o(V)]d/.lambda..
[0165] When N(V)<.lambda..sub.o/(2.DELTA..lambda..sub.G), there
is only one transmission maximum for light of wavelength within the
gain bandwidth making a full round-trip through the filter. This
defines the maximum crystal length d in this embodiment. Those
skilled in the art will recognize that the faces of crystal 35 are
necessarily substantially perpendicular to the propagation
direction of the light in the cavity and require anti-reflection
coating to prevent their affecting the cavity resonances. Coatings
with reflectivities <0.1% are know n in the art.
[0166] FIG. 17 illustrates the polarization of light in a plane
perpendicular to the cavity axis in FIGS. 11 and 14 when using the
Lyot filter in FIG. 16. In FIG. 17, the high transmission direction
of the polarization selective element 33 of FIG. 16 is labeled "H"
and shown as item 33a. The orthogonal low transmission axis is
shown as "L" and 33b. Light of the high transmission polarization
"H" having propagated from right to left through 33 in FIG. 16
encounters crystal 35. The light of the "H" polarization is
resolved into components on "e" the extra-ordinary (34) and "o" on
the ordinary axis (36) of crystal 35. In propagating through the
crystal, these two polarization components accumulate a phase-shift
with respect to one another. Light propagating out of the crystal
to the left then encounters the amplifying mirror of the laser
cavity (20 in FIGS. 11 and 14), which does not alter the
accumulated phase shift, but reflects the light back through the
crystal 35 from left to right. A unique property of the VECSEL
amplifying mirror (unlike edge emitter mirrors) is that it does not
significantly affect the polarization of the amplified beam. In an
ECSAL the gain medium can also be polarization insensitive. The net
effect is that the two polarizations of light with wavelength
.lambda. accumulate a phase shift of 2N(V) (in waves) with respect
to one another as the result of a round trip through crystal 35. If
2N(V) is an integer, the light is returned to the polarization
selective element in the polarization "H" with which it began. This
condition represents the maximum transmission of the filter and the
axial mode with the wavelength .lambda. that corresponds most
closely to it will be the mode that would be caused to lase. In
general 2N(V) will not be an integer and the light will return to
polarization selector 33 in an elliptical state of polarization
shown graphically as 39 in FIG. 17. Such a polarization is not
transmitted without loss through the polarizer 33 in FIG. 16.
Additional loss occurs when that light is reflected by cavity
mirror 25 in FIGS. 1 and 4 back through polarizer 33. When the
voltage V is changed rapidly to V', the selected wavelength
.lambda. also changes rapidly. The oscillating mode (73 in FIG. 2)
is extinguished and the sub-threshold mode with wavelength
.lambda.' that makes 2N(V') most nearly integral begins to build
up.
[0167] FIG. 18 illustrates another embodiment of the mode-selective
element 30 wherein the birefringent electro-optical media are
constituted as two crystals (35a & 35b). In the embodiment
shown, the extraordinary axis of one crystal (34a) is parallel to
the ordinary axis of the other crystal (36b) and vice versa (34b
and 36a). The net effect of having these crystals rotated by
90.degree. with respect to one another is that the birefringences
of the two crystals act in opposition, producing a net phase shift
for round trip light of:
2N.sub.x(V.sub.a,
V.sub.b)=2{[n.sub.e(V.sub.a)-n.sub.o(V.sub.a)d.sub.a]/.l-
ambda.-[n.sub.e(V.sub.b)-n.sub.o(V.sub.b)]d.sub.b/.lambda.}.
[0168] where d.sub.a and d.sub.b are the axial lengths of crystals
35a and 35b, respectively, and V.sub.a and V.sub.b are the voltages
applied between electrodes on opposing faces perpendicular to the
extraordinary axes on crystals 35a and 35b, respectively. Said
voltages are applied by means of wires 41a and 41b attached to the
+electro-optic faces of the two crystals and wires 42a and 42b
attached to the -electro-optic faces. Because of the cancellation
in the phase shifts produced by the two crystals, considerably
longer crystal lengths d.sub.a and d.sub.b or larger birefringences
n.sub.e-n.sub.o can be employed in this design without violating
the "single transmission maximum" condition: N.sub.x(V.sub.a,
V.sub.b)<.lambda..sub.o/(2.DELTA..lambda..sub.G). When
V=V.sub.a=-V.sub.b the electro-optic phase variations of the two
crystals add, even though the static birefringences subtract. Such
a voltage arrangement can be obtained by connecting wires 41a and
42b to one side of the electro-optic driver circuit producing
mode-selection voltage V while connecting 41b and 42a to the other
side. Changing V to V.sup.1 then has the same effect on the modes
of oscillation as in the implementation illustrated and described
for FIG. 16. In particular the voltage-induced polarization changes
illustrated in FIG. 17 again apply. One skilled in the art will
recognize that the two crystals in FIG. 18 will require an
anti-reflection coating to prevent their faces, which are
substantially perpendicular to the light propagation direction,
from altering the performance of the resonator. Reflections from
the surfaces between crystal 35a and 35b in FIG. 18 can be
suppressed using an index thatching material or by optical
contacting using methods known to those skilled in the art.
[0169] FIG. 19 illustrates another embodiment of the mode-selective
element 30 of the present invention in which a birefringent optical
element (37) without electro-optic function is utilized. In this
embodiment, the birefringent element 37 is a zero-order half wave
plate with extra-ordinary axis 38 oriented at 45.degree. to the
crystal axes. Such a device allows the ordinary axes (36) of the
two crystals to be oriented parallel to one another at
.+-.45.degree. to the high-transmission axis of 33 and the
extra-ordinary axes (34) to be oriented either parallel or
anti-parallel to one another. In the illustrated case, the relative
phase shifts due to the static birefringences of the two
electro-optic crystals subtract as in the previous case (FIG. 18)
and proper connection of the leads attached to the electrodes on
the electro-optic crystals allows the voltage-dependent phase
shifts (and thus the tuning effects of the two crystals) to add.
Adding non-electro-optic birefringent elements to the electrically
controllable Lyot filter as illustrated in FIG. 19 can improve
performance by, for example, broadening the angular acceptance of
element 30 and reducing the required assembly precision. Those
skilled in the art will recognize that other functionally
equivalent layouts to those illustrated are possible.
[0170] FIG. 20 illustrates the effect of the half wave plate 37 in
FIG. 19 with extra-ordinary axis 38 and ordinary axis 39. The half
wave plate has the effect of reflecting an arbitrary polarization
43 in one crystal through the plane defined by the extra-ordinary
axis 38 and the propagation direction (out of the plane of the
paper in FIG. 20) leading to an output polarization 44 which
propagates into the second crystal. If the axes 38 and 39 of the
waveplate 37 are oriented at approximately 45.degree. to the axes
34 and 36 of the crystals 35a and 35b, then the projections of the
arbitrary initial polarization 43 on the ordinary axis 36 and the
extra-ordinary axis 34 of one crystal are exchanged by the
wave-plate induced transformation of polarization 43 into 44. Thus
the projection of 43 on the ordinary axis 36 in the first crystal
becomes the projection of 44 on the extra-ordinary axis 34 in the
second crystal. Thus even though the axes of the two crystals in
FIG. 9 are parallel, the polarizations behave as they do in FIG. 8
where the crystals are rotated 90.degree. with respect to one
another and the previous formula for N.sub.x(V) applies.
[0171] In a further implementation, the bulk of the Lyot filter
action of the mode selective element may be supplied by a uniaxial
material without electro-optic properties while a variable
waveplate with a range of 0-1 waves of retardation provides the
tuning. In such a device one crystal in FIG. 18 (e.g,. 35a) would
be made of calcite or a similar uniaxial substance, but would lack
wires and electrodes (41a and 42a) and would have length d
sufficient to give rise to a phase shift (in waves) between light
polarized along the ordinary and extraordinary axes of
N.sub.a=(n.sub.e-n.sub.o)d/.lambda..sub.min<.lambda..sub.min/(2.DELTA.-
.lambda..sub.G), where .lambda..sub.min is the minimum wavelength
of the desired tuning band. The second crystal modulator would be
replaced by a thin variable waveplate having a variable retardance
0<N.sub.b(V)<1 and oriented to add its retardance to N.sub.a.
Alternatively N.sub.a may be any N wherein .lambda. in the above
quotation is any .lambda. within the desired tuning range. Under
the circumstances N.sub.b must span a range of 1 which includes the
value 0 but the range need not be symmetrical around 0. The total
retardance of the combination would be
N.sub.t(V)=N.sub.a+N.sub.b(V)<.lambda..sub.min/(2.DELTA..lambda..sub.G-
) thereby assuring a single value transmission peak as previously
described. At V.sub.min, the retardance N.sub.t(V.sub.min) is a
half integer for the minimum wavelength .lambda..sub.min.
Increasing the retardance of the electro-optic modulator by
increasing the voltage then requires an increased value of .lambda.
to maintain a half intregal value of
N.sub.t(V)=(n.sub.e-n.sub.o)d/.lambda.+N.sub.b(V), thus tuning the
transmission maximum. Suitable variable waveplates are known and
have been fabricated using liquid crystal media with longitudinal
AC and DC electric fields applied through transparent electrodes.
Such a system can be compact and have a larger aperture than the
previously described electro-optic wave plate. While most variable
waveplates currently sold are generally too slow for
sub-millisecond wavelength switching, improved media e.g. smectic-C
liquid crystals, produce analog switching times below 1
millisecond.
[0172] Given that the electro-optic materials and coating
technologies are not perfect, it is desirable to minimize the
amplitude of the applied voltage integrated over time, in order to
avoid possibly damaging either the electro-optic material or
coating. Since the birefringence of electro-optic crystals can be
tuned with temperature as sell as with voltage, once the
electro-optic element is switched quickly (i.e., in <1
millisecond) to a desired wavelength by changing the applied
voltage, then the temperature of the crystal can be adjusted so
that the desired wavelength requires zero applied voltage. The
temperature will change slowly (over >>1 millisecond), and
the applied voltage should be reduced gradually as the temperature
changes so that the tuning wavelength of the filter remains
unchanged during this process. Switching quickly to a new desired
wavelength at any time during the temperature change, or after, is
still possible by a change in the applied voltage, where the change
in the voltage corresponds to the change in wavelength. After each
switch of the wavelength, the temperature should be adjusted toward
a new target corresponding to the current desired wavelength.
[0173] The voltage requirements of the crystals 35 in FIGS. 16, 18
and 19 in which the voltage is applied transversely across the
crystals can be reduced by reducing the separation between the
electrodes connected to the leads 40 and 41, necessarily reducing
the clear aperture of the device 30. In order to reduce loss due to
scattering of light by the edges of the crystal, it may be
desirable to create a secondary beam waist within or near the
crystals 35. The first beam waist is inherently formed within the
VECSEL cavity.
[0174] FIG. 21 illustrates an embodiment of this invention in which
an anti-reflective coated lens 61 has been added to the mode
selective element 30 within the cavity formed by the amplifying
mirror 20 and the outer mirror 25. Said lens 61 does not otherwise
alter the behavior of the device, merely facilitating lower cavity
loss. FIG. 22 shows another implementation in which the secondary
beam waist, which minimizes the cavity beam within the
electro-optic crystal 35, has been created by a mirror 62. Since
the light incident on the mirror is reflected at an angle, the
dielectric coatings of this mirror can be designed to have less
reflectivity for one polarization than the other. Thus mirror 62
can combine the roles of providing a secondary beam waist and a
polarization selective element (33 in FIG. 21). In a preferred
implementation, the reflecting surface of mirror 62 has the shape
of an off-axis paraboloid or ellipsoid, thus minimizing wavefront
distortions for the TEM zero/zero transverse mode of the cavity
formed by mirrors 20, 62 and 25. Although mirror 25 is illustrated
in FIGS. 21 and 22 as being concave it may be alternatively be of
planar configuration in which case the second beam waist is at the
nearby mirror 25 rather than within the crystal 35.
[0175] Amplifying mirrors of the sort illustrated in FIG. 13 can
also have polarization dependence. In a further implementation of
this invention the roles of the polarization selecting element 33
in FIG. 14 and the amplifying mirror 20 can be combined in a single
element. Alternatively, the polarization dependent gain of the
amplifying mirror 20 can act as a second polarization-selective
element in a cavity containing a mode selector 30 with its own
polarization selector (32 in FIG. 15, for example). A cavity with
multiple polarization selective elements requires care in aligning
polarization axes to provide minimum loss and adequate tuning.
[0176] Those skilled in the art will recognize that the specific
geometries described heretofore are for purposes of illustration
only and substantially the same principles apply for different
configurations. In particular, the extra-ordinary axis of a
uniaxial electro-optic crystal need not be oriented perpendicularly
with respect to both the ordinary axis and propagation direction in
order for the mode selective device to have the rapid electro-optic
switching behavior described. In some implementations, it may be
sufficient that the extra-ordinary axis lie at some angle in a
plane orthogonal to the ordinary axis, with that plane also
containing the axis of propagation. Additionally, electrical fields
may be applied in different directions to switch the electro-optic
media according to this invention. Frequency selective elements
combining reflection (as in the etalon case of FIG. 15) and Lyot
filter action (as in FIG. 16) are also useful implementations of
the present invention.
[0177] We have herein disclosed an advantageous tunable laser
structure where the wavelength of the emitted laser beam is
constrained to take on a value which falls on a predetermined
frequency grid. Those skilled in the art will recognize that the
advantages obtained in this manner do not depend on the nature of
the laser gain medium, or on the nature of the laser tuning
element(s), or on the optical resonator configuration of the laser.
It is convenient to refer to a tunable laser whose output
wavelength is constrained to fall on a predetermined frequency grid
as featuring "discrete tunability".
[0178] Discrete tunability provides advantages for applications
where the laser is required to tune to channels that lie on a
fixed, evenly spaced, frequency grid. The dense wavelength division
multiplexing (DWDM) application is a primary example of such an
application, where the ITU grid is the relevant fixed frequency
grid. In some DWDM applications, such as sources for test
equipment, the open loop wavelength accuracy provided by discrete
tunability can provide the required accuracy. In these cases,
implementation of discrete tunability allows the wavelength control
loop that is typically necessary in such sources to be eliminated,
which is clearly advantageous. For other DWDM applications, such as
sources for optical communication, implementation of discrete
tunability may not permit elimination of the wavelength control
loop. In such cases, discrete tunability still provides advantages,
such as reduced total size and part count (relative to a
laser+external locker configuration), and greater flexibility in
overall laser control.
[0179] The advantages of discrete tunability can be realized by
appropriate design of the optical resonator, such that the
longitudinal modes formed by the optical resonator are aligned to
the desired predetermined frequency grid. The spacing of the
longitudinal modes of the optical resonator is determined by the
round trip optical path length of the optical resonator. The
advantages of discrete tunability can also be realized by the
insertion of a fixed, passive etalon within the optical resonator
of a tunable laser, since such an etalon will ensure that laser
emission can only occur at etalon transmission peaks, which are at
very nearly equally spaced frequencies, and can be aligned to the
desired predetermined frequency grid. We refer to such an etalon as
a "grid-fixing etalon". A grid-fixing etalon can be a separate
component inserted into the optical resonator, or it can be created
by appropriate engineering of a parasitic etalon already present
within the optical resonator. A typical example of such a parasitic
etalon is the etalon formed by the two endfaces of an SOA.
[0180] In order to perform its intended function, a grid fixing
etalon is preferably inserted into an optical resonator such that
the etalon surface normals make a small angle with respect to the
intracavity beam, to thereby ensure that the beams reflected from
the etalon surfaces do not efficiently couple into the optical
resonator. The etalon finesse is chosen to provide low loss in
transmission through the tilted etalon, and the desired level of
spectral selectivity. Since the etalon serves as an absolute
wavelength reference for the laser, the etalon is preferably
fabricated using materials, such as fused silica, that are
mechanically stable and temperature insensitive.
[0181] Those skilled in the art will appreciate that many changes
and modifications will become readily apparent from consideration
of the foregoing descriptions of preferred and other embodiments
without departure from the spirit of the present invention, the
scope thereof being more particularly pointed out by the following
claims. The descriptions herein and the disclosures hereof are by
way of illustration only and should not be construed as limiting
the scope of the present invention.
* * * * *