U.S. patent application number 11/058816 was filed with the patent office on 2008-10-23 for widely tunable laser.
This patent application is currently assigned to IOLON, INC.. Invention is credited to John F. Heanue, John H. Jerman, Jeffrey P. Wilde.
Application Number | 20080259972 11/058816 |
Document ID | / |
Family ID | 27496165 |
Filed Date | 2008-10-23 |
United States Patent
Application |
20080259972 |
Kind Code |
A1 |
Heanue; John F. ; et
al. |
October 23, 2008 |
WIDELY TUNABLE LASER
Abstract
A Fabry-Perot laser and a micro-actuator are utilized to provide
continuous tuning over a range of wavelengths.
Inventors: |
Heanue; John F.; (San Jose,
CA) ; Jerman; John H.; (Palo Alto, CA) ;
Wilde; Jeffrey P.; (Los Gatos, CA) |
Correspondence
Address: |
DORSEY & WHITNEY LLP
555 CALIFORNIA STREET, SUITE 1000, SUITE 1000
SAN FRANCISCO
CA
94104
US
|
Assignee: |
IOLON, INC.
San Jose
CA
|
Family ID: |
27496165 |
Appl. No.: |
11/058816 |
Filed: |
February 15, 2005 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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09491429 |
Jan 26, 2000 |
6856632 |
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11058816 |
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60167951 |
Nov 29, 1999 |
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60154899 |
Sep 20, 1999 |
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60167937 |
Nov 29, 1999 |
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Current U.S.
Class: |
372/20 |
Current CPC
Class: |
H01S 5/143 20130101;
H01S 5/142 20130101; H01S 5/0687 20130101; H01S 5/02325 20210101;
H01S 3/105 20130101; H01S 5/141 20130101 |
Class at
Publication: |
372/20 |
International
Class: |
H01S 3/10 20060101
H01S003/10 |
Claims
1-20. (canceled)
21. A tunable laser comprising a wavelength filter having an
optical path length and a plurality of external cavity modes and,
an external cavity optical path length adjustment mechanism coupled
to the wavelength filter for varying the optical path length of the
wavelength filter so as to select a single external cavity mode
from the plurality of external cavity modes, the optical path
length adjustment mechanism being substantially independently
operable with respect to the wavelength filter, and an
electromechanical actuator coupled to each of the wavelength filter
and the cavity length adjustment mechanism and control means
coupled to each electromechanical actuator for permitting
substantially independent adjustment of the wavelength filter and
the cavity length adjustment mechanism.
22. The tunable laser of claim 21 further comprising wherein at
least one of the electromechanical actuators is a micro-dimensioned
actuator coupled to at least one of the wavelength filter and the
optical path length adjustment mechanism.
23. The tunable laser of claim 21 further comprising wherein at
least one of the electromechanical actuator is a micro-machined
actuator coupled to at least one of the wavelength filter and the
optical path length adjustment mechanism.
24. (canceled)
25. A tunable laser apparatus comprising an optical source means
for providing light along an optical path, wavelength tuning means
in the optical path and having a tuning range and being configured
to feed light of a selected wavelength back to the optical source
means, microelectromechanical actuation means coupled to the
wavelength tuning means for positionally adjusting the wavelength
tuning means so as to select the wavelength of the light being fed
back to the optical source means and cavity length adjustment means
coupled to the wavelength tuning means for controlling the phase of
the light being fed back to the optical source means, the
wavelength tuning means and the cavity length adjustment means
being configured to allow independent adjustment of the wavelength
tuning means and the cavity length adjustment mechanism during the
tuning.
26. A tunable laser apparatus comprising an optical source means
for providing light along an optical path, wavelength tuning means
in the optical path having a tuning range and being configured to
feed light of a selected wavelength back to the optical source
means, microelectromechanical actuation means coupled to the
wavelength tuning means for positionally adjusting the wavelength
tuning means so as to select the wavelength of light being fed back
to the optical source means and to maintain approximately constant
phase of the light being fed back to the optical source means over
the tuning range of the wavelength tuning means, cavity length
adjustment means coupled to the wavelength tuning means for
providing additional control of the phase of the light being fed
back to the optical source means, and control means coupled to the
microelectromechanical actuation means and the cavity length
adjustment means for allowing substantially independent adjustment
of the microelectromechanical actuation means and the cavity length
adjustment mechanism during tuning.
27. A tunable optical element comprising a mirror rotatable about
an axis of rotation for directing an optical beam over a range of
angles to select a wavelength of light from a range of wavelengths,
a micromechanical actuator coupled to the mirror for rotating the
mirror, means for providing an optical reference beam directed at
the mirror, means for measuring the angle of the optical reference
beam reflected from the mirror and control means coupled to the
micromechanical actuator for adjusting the angle of the mirror as a
function of the angle of the optical reference beam reflected from
the mirror.
28. A tunable laser comprising a wavelength filter having an
optical path length and a plurality of external cavity modes, an
external cavity optical path length adjustment mechanism coupled to
the wavelength filter for varying the optical path length of the
wavelength filter so as to select a single external cavity mode
from the plurality of external cavity modes, the optical path
length adjustment mechanism being substantially independently
operable with respect to the wavelength filter, and a
micro-dimensioned actuator coupled to at least one of the
wavelength filter and the optical path length adjustment
mechanism.
29. A tunable laser comprising a wavelength filter having an
optical path length and a plurality of external cavity modes, an
external cavity optical path length adjustment mechanism coupled to
the wavelength filter for varying the optical path length of the
wavelength filter so as to select a single external cavity mode
from the plurality of external cavity modes, the optical path
length adjustment mechanism being substantially independently
operable with respect to the wavelength filter, and a
micro-machined actuator coupled to at least one of the wavelength
filter and the optical path length adjustment mechanism.
Description
RELATED APPLICATIONS
[0001] The present invention is related to, and claims priority
from, Provisional Applications Ser. No. 60/154,899 filed on Sep.
20, 1999; and Ser. No. 60/167,951 filed on Nov. 29, 1999; and Ser.
No. 60/167,937 filed on 29 Nov. 1999, which are incorporated herein
by reference.
FIELD OF THE INVENTION
[0002] The present invention is applicable to the field of tunable
lasers and is more specifically applicable to a tunable laser for
use in telecommunications.
INTRODUCTION
[0003] In telecommunications networks that utilize wavelength
division multiplexing (WDM), widely tunable lasers enable
transmission of information at different wavelengths. Many proposed
network configurations require transmitters that can be tuned to
transmit at any of N distinct wavelengths. Even in networks where
the individual transmitter wavelengths are held fixed, tunable
sources are desirable for maintaining stability of the wavelength.
Also, because the same part can be used for any channel, a tunable
transmitter is useful from an inventory control perspective.
[0004] One prior art tunable laser design uses an external optical
cavity, which is illustrated in U.S. Pat. No. 5,771,252. A basic
configuration from U.S. Pat. No. 5,771,252 is shown in FIG. 1 of
the present application. FIG. 1 shows a laser diode used in
combination with a diffraction grating and rotating mirror to form
an external optical cavity. In this configuration the grating is
fixed. As the mirror is rotated, the light propagating within the
optical cavity is fed back to the laser diode. The feedback causes
the laser diode to "lase" with a changeable frequency that is a
function of the rotation angle of the mirror. Unless accounted for,
the frequency of the laser may "mode hop" due to the distinct,
spatial longitudinal modes of the optical cavity. It is desirable
that the longitudinal mode spectrum of the output beam of the laser
diode change without discontinuities. This condition may be
satisfied by careful selection of the pivot point about which the
mirror is rotated, whereby both the optical cavity length and the
grating feedback angle can be scanned such that the single pass
optical path length of the external optical cavity is equal to the
same number of half-wavelengths available across the tuning range
of the laser cavity. If this condition is satisfied, rotation of
the mirror will cause the frequency of the output beam to change
without discontinuities and at a rate corresponding to the rotation
of the mirror. U.S. Pat. No. 5,319,668 also describes a tunable
laser. Both U.S. Pat. No. 5,771,252 and U.S. Pat. No. 5,319,668
disclose an expression for an optical cavity phase error, which
represents the deviation in the number of wavelengths in the cavity
from the desired constant value as a function of wavelength. The
expression for optical cavity phase error includes terms related to
the dispersion of the laser and other optical elements. U.S. Pat.
No. 5,771,252 teaches a pivot point whereby the cavity phase error
and its first and second derivatives with respect to the wavelength
all go to zero at the center wavelength. For all practical
purposes, the two methods describe the same pivot point.
[0005] The grating-based external cavity tunable laser (ECLs) of
U.S. Pat. No. 5,771,252 is a relatively large, expensive device
that is not suitable for use as a transmitter in a large-scale WDM
network. Because of the size and distance between components,
assembly and alignment of the prior art ECL above is difficult to
achieve. Known prior art ECLs use stepper motors for coarse
positioning and piezoelectric actuators for fine positioning of
wavelength selective components. Because piezoelectric actuators
exhibit hysteresis, precise temperature control is needed. In
addition, prior art ECL lasers are not robust in the presence of
shock and vibration.
[0006] Another prior art tunable laser design utilizes a
Vertical-Cavity Surface-Emitting Laser (VCSEL). In one embodiment
of this device, a MEMS (micro-electro-mechanical-system) mirror
device is incorporated into the structure of the VCSEL and is used
to tune the wavelength of the laser. Wide tuning range has been
demonstrated in such devices for operation around 830 nm, but so
far, the development of a reliable, high performance VCSEL at 1550
nm has proved elusive. This device is very difficult to build
because the MEMS device must be physically incorporated into the
structure of the VCSEL. Furthermore, development of the MEMS
actuators in InP-based materials is a formidable challenge.
[0007] In other prior art, angular motors have been used in angular
gyroscopes and as fine tracking servo actuators for magnetic heads
for disk drives. In "Angular Micropositioner for Disk Drives," D.
A. Horsley, A. Singh, A. P. Pisano, and R P Horowitz, Proceedings
of the 10.sup.th Int. Workshop on Micro Electro Mechanical Systems,
1997, p. 454-458, a deep polysilicon device is described with
radial flexures extending from a central fixed column, and radial,
parallel plate electrodes that effect rotation of less than 0.5
degree. Batch Fabricated Area Efficient Milli-Actuators, L.-S. Fan,
et. al., Proceedings 1994 Solid State Sensor and Actuator Workshop,
Hilton Head, p. 38-42 shows a rotary flexural actuator with what
appears to be 2 central flexures from central supports; the
rotational range is not given but appears to be small. Dual Axis
Operation of a Micromachined Rate Gyroscope, T. Juneau, A. P.
Pisano, and J. H. Smith, Proceedings 1997 Int. Conf. On Solid State
Sensors and Actuators, V.2, pp. 883-890 describes a polysilicon,
surface micromachined gyro, which has 4 radial springs supporting a
central circular mass. The springs are supported on the outside,
and have a small strain relief feature. The angular drive range is
not specified, but appears to be small. All of these prior art
devices provide limited angular range. These prior art devices
completely fill a circular area in a plan view, thus making it
difficult or impossible to arrange such an actuator to provide a
remote pivot location, as is required by ECLs.
[0008] Tunable Distributed Bragg Reflector (DBR) lasers are
currently commercially available, however, these lasers have a
limited tuning range. Total tuning of about 15 nm and continuous
tuning without mode hops over about 5 nm range is typical.
[0009] A tunable laser based on sampled grating DBR technology is
presently available. The DBR device is tunable over about 50 nm,
but the fabrication is difficult and the control electronics are
complex, requiring four different control currents.
[0010] Another prior art approach to making a tunable laser is to
fabricate multiple Distributed Feedback (DFB) lasers on a single
chip and couple them together with an arrayed waveguide structure.
Each DFB is fabricated with a slightly different grating pitch so
that each lases at a slightly different wavelength. Wavelength
tuning is accomplished by activating the laser that matches the
particular wavelength of interest. The main problems with this
approach are cost and insertion loss. Furthermore, fabrication of
multiple lasers on the same chip with different operating
wavelengths may require direct e-beam writing of the gratings.
Also, if one wants to cover a very wide tuning range, the number of
lasers required is prohibitively large. Additionally, the multiple
laser approach is lossy because coupling N lasers together into one
output waveguide results in an efficiency proportional to 1/N.
[0011] What is needed, therefore, is a tunable laser that provides
advantages over the prior art.
SUMMARY OF THE INVENTION
[0012] The present invention comprises a tunable laser assembly.
Advantages derived from the present invention include: the ability
to use commonly available inexpensive Fabry-Perot (FP) laser
diodes; high operating frequencies; reduced size and mass, thermal
and mechanical stability; precise alignment of optical components
made simple by use of photolithographically-defined features in
silicon, high production yields; and simple output frequency
control schemes.
[0013] The present invention may comprise a tunable laser,
including: a source means for providing a light along an optical
path with any wavelength selected from a continuous bandwidth of
wavelengths; a diffractive element positioned in the optical path
and from the source by a first distance to redirect the light; a
reflective element positioned in the optical path and from the
diffractive element by a second distance to receive the redirected
light from the diffractive element, and the reflective element
positioned in the optical path and from the diffractive element by
the second distance to redirect the light towards the diffractive
element; the diffractive element positioned in the optical path and
from the source by the first distance to re-direct the light
towards the source; and a micro-actuator means for selecting the
wavelength from the continuous range of wavelengths by altering the
optical path of the light.
[0014] The present invention may comprise a laser assembly that
includes a source for providing a light along an optical path with
any wavelength from a continuous range of wavelengths; a
diffractive element positioned in the optical path and from the
source by a first distance to redirect the light; a reflective
element positioned in the optical path and from the diffractive
element by a second distance to receive the redirected light from
the diffractive element, and the reflective element positioned in
the optical path and from the diffractive element by the second
distance to redirect the light towards the diffractive element; the
diffractive element positioned in the optical path and from the
source by the first distance to re-direct the light towards the
source; and a micro-actuator for selecting the wavelength from the
continuous range of wavelengths by altering the optical path of the
light.
[0015] The first distance and the second distance may define an
optical path length between the source and the reflective element
measured in wavelengths, and wherein the optical path length
remains constant over the continuous range of wavelengths.
[0016] The micro-actuator may be coupled to the reflective element
to displace the reflective element. The displacement may comprise
an angular displacement. The angular displacement may occur about a
virtual pivot point. The displacement may comprise a translation
and a rotation. The micro-actuator may comprise a micro-machined
actuator. The micro-machined actuator may be coupled to the
reflective element. The reflective element may comprise a
retro-reflector. The continuous range of wavelengths may comprise
from about 1520 nm to about 1560 nm. The wavelength may be about
1540 nm. The source may comprise a Fabry-Perot laser.
[0017] The present invention may also comprise a tunable laser,
including: a source means for providing a light along an optical
path with any wavelength selected from a continuous bandwidth of
wavelengths; a diffractive element positioned in the optical path
and from the source by a first distance to redirect the light; a
reflective element positioned in the optical path and from the
diffractive element by a second distance to receive the redirected
light from the diffractive element, and the reflective element
positioned in the optical path and from the diffractive element by
the second distance to redirect the light towards the diffractive
element; the diffractive element positioned in the optical path and
from the source by the first distance to re-direct the light
towards the source; and a micro-actuator means for selecting the
wavelength from the continuous range of wavelengths by altering the
optical path of the light.
[0018] The present invention may also comprise a method for
providing light with any wavelength selected from a continuous
range of wavelengths, including the following steps: providing the
light along an optical path; providing a diffractive element in
optical path to diffract the light; providing reflective element in
the optical path to reflect the light; and selecting a particular
wavelength of light from the continuous range of wavelengths by
altering the optical path through displacement of a
micro-actuator.
[0019] The method may also include the step of displacing the
reflective element with the micro-actuator to alter the optical
path.
[0020] The method may also include the step of displacing the
reflective element by a translation and a rotation.
[0021] The method may also include the step of displacing the
micro-actuator about a virtual pivot point.
[0022] The method may also include the step of selecting the
particular wavelength from a continuous range of wavelengths
comprising the range from about 1520 nm to 1560 nm.
BRIEF DESCRIPTION OF THE FIGURES
[0023] FIG. 1 shows a prior art tunable laser design;
[0024] FIG. 2 shows a MEMS based widely tunable external cavity
laser of the present invention;
[0025] FIG. 3 shows an alternative embodiment of the present
invention;
[0026] FIG. 4 shows the use of an electro-absorptive modulator in
an embodiment of the present invention;
[0027] FIG. 5 shows an embodiment in which a PSD is used;
[0028] FIG. 6 shows a second embodiment in which a PSD is used;
[0029] FIG. 7 shows a third embodiment in which a PSD is used;
[0030] FIG. 8 shows an embodiment in which a wavelength locker is
used;
[0031] FIG. 9 shows a second embodiment in which a wavelength
locker is used;
[0032] FIG. 10 shows a third embodiment in which a wavelength
locker is used;
[0033] FIG. 11 shows a mask layout of the actuator of the present
invention;
[0034] FIG. 12 shows another mask layout of the actuator of the
present invention;
[0035] FIG. 13 shows a 3d view of the present invention; and
[0036] FIG. 14 shows a module incorporating the present
invention.
DESCRIPTION OF THE INVENTION
[0037] Referring now to FIG. 2, there is seen a preferred
embodiment of a micro-electro-mechanical-system (MEMS) based
widely-tunable external cavity laser (ECL) of the present
invention. Advantages of the present invention over that of the
prior art that will be apparent from the description provided below
include: the ability to use commonly available inexpensive
Fabry-Perot (FP) laser diodes; high operating frequencies; wide
operating bandwidth; reduced size and mass, thermal and mechanical
stability; precise alignment of optical components made simple by
use of photolithographically-defined features in a silicon
substrate, high production yields; and simple output frequency
control schemes. Other advantages will become apparent from a
reading of the following description of the present invention.
[0038] In the preferred embodiment, a widely-tunable laser (ECL)
100 of the present invention includes a laser 101, a collimating
lens 102, a diffraction grating 103, a reflector 104, and a MEMS
based actuator 105. In the preferred embodiment, the actuator 105
is a rotary actuator, the laser 101 is a Fabry-Perot laser diode,
and the reflector 104 is a retro-reflector. The reflector 104
utilizes a high reflectivity coating on its surface, and the laser
101 utilizes a high-reflectivity coating on its rear facet and an
anti-reflection coating on its front facet. In the preferred
embodiment, the grating 103 is replicated in glass. The present
invention utilizes because the provide several advantages compared
to traditional polymer gratings, including: thermal stability;
replication and stability using thin substrates; and the ability to
be handled, diced, cleaned, etc.
[0039] In the present invention, light from the laser 101 is
directed through the lens 102 towards the grating 103, by the
grating 103 towards the reflector 104, by the reflector 104 back
towards the grating 103, and by the grating 103 back towards the
laser 101. The optical path traversed by the light from the laser
101 forms an external cavity, which causes an output beam 150 of
the laser 101 to lase at a particular wavelength that is a function
of the rotation angle of the reflector 104. In the exemplary
embodiment, the ECL 100 can be tuned over +/-26 nm with +/-2
degrees of motion of the actuator 105. For optimum performance of
the ECL 100, it is desired that the wavelength of the output beam
150 be continuously tunable (i.e., no mode hops occur as the laser
101 is tuned over a range of wavelengths). This condition can be
satisfied by selecting a virtual pivot point 108 about which the
reflector 104 rotates and/or translates, such that an optical path
length of the cavity formed between a rear facet of the laser 101
and the reflector 104 measured in wavelengths remains constant over
the desired tuning range. U.S. Pat. Nos. 5,319,668 and 5,771,252
disclose methods for calculating a pivot point and are incorporated
herein by reference. The two calculations made in these two prior
art patents result in pivot point locations that differ by 40 nm.
The calculations used in both of these patents are applicable to
the present invention because the component and manufacturing
tolerances of the present invention are greater than 40 nm. In
fact, adequate performance of the present invention may be obtained
by choosing a pivot point such that the cavity phase error and only
the first derivative go to zero at the center wavelength. This
condition gives an approximate location for the pivot point. During
assembly, the tuning performance of the present invention can be
measured, and the pivot point 108 adjusted in a manner described in
further detail below. The virtual pivot point of the present
invention allows for a compact geometry and results in a lower-cost
device with better optical performance than if a real pivot point
was used. Better optical performance is achieved because the
compact geometry results in greater spacing of the external optical
cavity modes and greater side-mode suppression.
[0040] In an exemplary embodiment, the optical path length of the
external cavity (a sum of the optical distance between the front
facet of the laser 101, the grating 103, and the front of the
reflector 104) is approximately 5 mm; and the center wavelength,
grating pitch, angle of incidence, and diffraction angle of the
grating 103 are 1540 nm, 1050 lines/mm, 85 degrees, and 38 degrees,
respectively. Although the overall tuning range of the ECL 100 is a
function of the width of the gain curve of the laser 101, which in
the preferred embodiment of the present invention can be tuned over
a range on the order of 40 nm, it is understood that a much broader
gain profile may be achievable using, for example, a Fabry-Perot
strongly-pumped quantum-well laser design, referenced in
Electronics Letters, Vol. 26, No. 11, pp. 742-743, "External
Grating Laser With Wide Tuning Range of 240 nm," by Epler et al. In
the present invention, single-mode operation occurs when the
spacing of the external cavity modes are greater than the linewidth
of the grating 103. The linewidth of the grating 103 is determined
by the angle of incidence and by the beam size. In an exemplary
embodiment, the grating 103 linewidth is about 21 GHz and the
external cavity modes are spaced by about 30 GHz. The ultimate
linewidth is determined by the external cavity mode spacing and by
the quality of the external cavity. In the exemplary embodiment,
with high reflectivity coatings on the reflector 104 and on the
rear facet of the laser 101, the linewidth is less than 1 Mhz.
[0041] Referring now to FIG. 3, and preceding figures and
descriptions as needed, there is seen one alternative of the
present invention. The present invention identifies that for high
data rate telecommunications applications, the output beam 150 of
the ECL 100 of FIG. 2 could be modulated directly by varying the
laser 101 current in accordance with the data stream to be
transferred. The present invention identifies that long external
optical cavity lengths make it more difficult to modulate the ECL
100 at very high frequencies and that it is, therefore, desirable
to keep the external optical cavity length of the ECL as short as
possible. As illustrated in FIG. 3, it is envisioned that the
present invention could be implemented in an alternative embodiment
in which the actuator 104 is used to displace the grating 103. In
the alternative embodiment of FIG. 3, it is understood that because
the grating 103 provides the reflective function of the reflector
103, the reflector need not be used and the optical cavity length
can be reduced over that of the preferred embodiment of FIG. 2.
However, it is identified that in the alternative embodiment of
FIG. 3, single-mode operation of the laser 101 is more difficult to
achieve than in the preferred embodiment because the there is only
a single-pass reflection of the output beam 150 from the
grating.
[0042] In another alternative embodiment, a Fabry-Perot laser 101
with as high a relaxation oscillation frequency as possible could
be used to achieve high data transfer rates. In this embodiment,
the laser should preferably maximize the differential gain,
maximize the internal photon density, and minimize the photon
lifetime. Multiple-Quantum-Well (MQW) lasers provide these
characteristics and have been demonstrated to operate with
modulation bandwidths well in excess of 10 GHz. See for example
IEEE Photonics Technology Letters, Vol. 9, No. 3, pp. 306-308,
"24-GHz Modulation Bandwidth and Passive Alignment of Flip-Chip
Mounted DFB Laser Diodes", by Lindgren, et al. With this approach,
direct modulation of the ECL 100 as high as 2.5 Gb/sec should be
possible.
[0043] In yet another alternative embodiment, the ECL 100 could be
designed to operate at frequencies corresponding to multiples of
longitudinal mode spacing (i.e., multiples grater than the
relaxation oscillation frequency). This approach would have the
drawback of decreasing the mode spacing and increasing the overall
size of the ECL 100.
[0044] Referring now to FIG. 4, and preceding figures and
descriptions as needed, there is seen an integrated
electroabsorptive modulator as used in a preferred embodiment of
the present invention. In an alternative embodiment, the present
invention identifies that an electroabsorptive (EA) modulator could
also be used to achieve high data transfer rates. At high data
rates, however, a decrease in laser modulation response occurs.
This decrease can be understood by considering the characteristic
lifetimes of photons. Photon lifetime for the laser 101 is given by
1/(ca), where .alpha. is the total loss distributed over the
equivalent free-space cavity. In a solitary laser, a photon spends
all its time in a highly absorbing medium so that the photon
lifetime is short. In the ECL 100, the photon spends a large
fraction of the time in loss-less free-space, so the lifetime is
proportionally longer. When modulating the ECL 100 at high
frequency, it is desirable that the photons disappear when the
current is turned off, but, this does not happen fast enough when
the photon lifetime is long. The present invention identifies that
if short photon lifetime is desired, the EA modulator could be
positioned in the external optical cavity as shown in FIG. 4. An
advantage with this approach is that the EA modulator can be
fabricated on the same chip as the laser 101. Because the EA
modulator absorbs photons at a speed corresponding to its
modulation frequency, it can be used to overcome the problems
associated with long photon lifetime. In an exemplary embodiment,
the EA modulator may be used to modulate the output beam 150 at up
to 10 Gbits/sec.
[0045] Referring now to FIG. 5 and preceding figures and
descriptions as needed, there is seen an embodiment in which a
position sensing detector (PSD) is used for servo-control of the
actuator. In the embodiment of FIG. 5, a PSD is used to measure the
angle of a reference beam of light that is reflected from the
reflector 104. The signal from the PSD is used in a servo loop to
set the voltage on the actuator 105. An advantage of this
embodiment is that the wavelength of the reference beam can be
matched to the sensitivity of commercially available PSDs.
[0046] Referring now to FIG. 6 and preceding figures and
descriptions as needed, there is seen a third embodiment in which a
PSD is used for servo control. In the embodiment of FIG. 6, the
grating 103 comprises wide enough grooves such that both first and
second order diffracted output beams are produced from the beam
150. Either the first order or the second order beam can be
directed to the PSD to find the angle of the reflector 104.
[0047] Referring now to FIG. 7, and preceding figures and
descriptions as needed, there is seen a second embodiment in which
a PSD is used for servo control. In the embodiment of FIG. 7, the
first order diffracted beam is reflected from the grating 103 after
reflection by the mirror 104 and is measured by a PSD to measure
the wavelength of the output beam 150. The signal from the PSD is
used in a servo loop to set the voltage of the actuator 105. It is
understood that in the embodiments of FIGS. 5-7, the signal from
the PSD can also be used for servo control of the power of the
laser 101.
[0048] In an alternative embodiment to those of FIGS. 5-7, a
capacitance measurement of the actuator 105 can be used as an
indication of the position of the attached reflector 104. As
discussed previously, movement of the reflector 104 determines the
output wavelength of the ECL 100. The present invention identifies
that the movement can be measured as a capacitance change in the
actuator 105. In this embodiment, the output wavelength vs. the
capacitance of the actuator 105 may be measured, and capacitance
sensing electronics comprising a servo-loop may be used to maintain
the position of the actuator 105 (and therefore the laser
wavelength) fixed over time. This method of servo control can be
implemented at low cost and does not require extra optical
components. Because the capacitance of the actuator 105 and
performance of the capacitance-sensing electronics are temperature
dependent, a thermo-electric cooler (TEC) may need to be used to
stabilize the temperature of the ECL 100.
[0049] In yet another alternative embodiment to those of FIGS. 5-7,
the wavelength vs. capacitance behavior of the actuator 105 may
measured at a number of different temperatures. In this embodiment,
a thermistor could be used to measure temperature, which in turn
could be used to determine which values to use for servo control.
In an exemplary embodiment, a stability of better than 1 part in
1000 is achievable with capacitance sensing.
[0050] Referring now to FIG. 8 and preceding figures and
descriptions as needed, there is seen an embodiment of a wavelength
locker as used with the present invention. The present invention
identifies that in an alternative approach to that of FIGS. 5-7, a
wavelength locker may be used to stabilize the wavelength of the
ECL 100. For a discussion of wavelength locking techniques, see
"Wavelength lockers keep lasers in line," Photonics Spectra,
February 1999, pp. 104-110 by Ed Miskovic. Similar techniques can
be used to stabilize the wavelength of the present invention. The
error signal from the wavelength locker may be used in a servo loop
to set the voltage applied to the actuator 105. In the embodiment
of FIG. 8, the wavelength locker is external to the ECL 100 and a
monitor signal is split off from the output beam 150 by an optical
beam splitter. The disadvantage of this approach is that the output
beam 150 intensity is reduced.
[0051] Referring now to FIG. 9, and preceding figures and
descriptions as needed, there is seen another embodiment of a
wavelength locker as used with the present invention. In the
embodiment of FIG. 9, light from the rear facet of the laser 101 is
directed to the wavelength locker, which may or may not be located
within the ECL 100 itself. In the embodiments of FIGS. 8 and 9, the
present invention identifies that the wavelength locker can also be
used to servo control the power of the laser 101.
[0052] Referring now to FIG. 10, and preceding figures and
descriptions as needed, there is seen another embodiment of a
wavelength locker as used with the present invention. In an
embodiment in which the wavelength of the output beam 150 of at
least one ECL 100 needs to be checked for stability only
intermittently, the present invention identifies that a single
wavelength calibrator/locker 108 can be shared to maintain a
particular wavelength of a particular ECL 100. In the embodiment of
FIG. 10, a 1.times.N switch is used to direct a monitor signal from
a ECL 100 to the locker 108. Elimination of N-1 wavelength
calibrators/lockers 108 represents a significant cost saving.
[0053] Referring now to FIG. 11, and preceding figures and
descriptions as needed, there is seen a detailed view of a mask
layout for the MEMS based actuator 105 of the present invention. In
the preferred embodiment, rotation of the actuator 105 about the
virtual pivot point 108 acts to rotate and translate the mirror 104
such that the external optical cavity is maintained with a constant
length over the entire rotation angle of the actuator. The present
invention identifies that changes in the geometrical relationship
between the components comprising the ECL 100 may change due to
temperature and/or mechanical effects and that, in doing so, the
optical path length of the external optical cavity and thus the
wavelength of the output beam 150 may change. As is discussed
below, the actuator 105 is designed to provide a mechanism which
compensates for these changes.
[0054] In the preferred embodiment, the actuator 105 is
manufactured from the mask shown in FIG. 11 using well known
micro-machining process steps. The actuator 105 comprises: a
silicon substrate 121, two sets of comb drive elements 111, bars
128, suspended trusses 125, suspension beams 110, a suspended frame
126, flexural couplers 123, and a suspended lever 122. The silicon
substrate 121 comprises etched features for receiving the laser
101, the lens 102, and the diffraction grating 103. Each of the
comb drive elements 111 comprises two sets of interlocking teeth
127. The interlocking teeth 127 comprise a plurality of fixed teeth
that are coupled by a respective bar 128 to the silicon substrate
121, and a plurality of movable teeth that are coupled to a
respective movable truss 125. The bars 128 are coupled through
respective electrical connections to respective bond pads 129-133.
Although it is preferred that the individual teeth 127 comprising
the comb drive elements 111 lie on circumferential arcs centered
about the pivot point 108, it is not necessary for the ends of the
teeth 127 to lie along radial lines extending from the center of
rotation. The ends of some of the teeth 127 may be arranged to lie
along a line that does not pass through the center of rotation,
which would allow the bars 128 to be made with added thickness
along the ends that point towards the pivot point 108 and yet
sufficient electrical isolation air-gap therebetween. Each of the
trusses 125 is suspended by respective suspension beams 110. The
suspension beams 110 are coupled to the suspended frame 126, which
is attached at its ends to the substrate 121 by two sets of
flexural couplers 123. One of the flexural couplers 123 serves as
an electrical ground connection to the upper bond pad 129. The
other flexural coupler is attached to the suspended lever 122. The
trusses 125, the suspensions 110, the frame 126, and the lever 122
are all suspended above the substrate 121. The reflector 104 is
attached to a slot in one of the trusses 125 by a mating post,
springs, adhesive, solder, or similar attachment means. In the
exemplary embodiment, the reflector 104 is about 2 mm long by 400
um high. A reflective surface of the reflector 104 is perpendicular
to the horizontal plane of the actuator 105. The mass and size of
the reflector 104 is taken into account by the design of the
actuator 105, which is designed to maintain mechanical
stability.
[0055] In the preferred embodiment, a potential applied to bond
pads 131 and 133 causes an electro-static potential to be created
between the respective fixed and movable teeth of the comb drive
elements 111, which causes the trusses 125 to rotate clockwise
about the virtual pivot point 108. A potential applied to bond pads
130 and 132 causes the trusses 125 to rotate counter-clockwise. In
the preferred embodiment, when the lever 122 is moved (for example,
manually or by other movement means such as micro-machined actuator
or the like) the coupler 123 that is attached to the lever 122
rotates around a point near its center. The opposite coupler 123
that is not connected to the lever 122 causes the small rotation of
the first coupler to be converted into a translational motion along
an axis extending through the two couplers. By arranging the
couplers 123 to be generally parallel to the optical axis of the
external optical cavity, motion of the lever 122 can be used to
adjust the external optical cavity length independent of the
rotation of the actuator 105. The adjustments can be made as
required to compensate for changes in temperature or variations in
the optical cavity length, or to compensate for small offsets in
the virtual pivot point 108.
[0056] The present invention takes into consideration that the comb
drive elements 111 may become unstable and "snap-over" in the
radial direction if the radial stiffness of the suspension beams
110 falls below a value equal to the derivative of the
electrostatic force between the comb drive elements 111 with
respect to radial motion, and that this instability becomes more
severe with large, static angular deflection. Although folded beam
suspension designs are known by those skilled in the art to provide
large rotational range, they do so with a penalty of reduced
out-of-plane and radial stiffness, which would work against the
desired goal of maintaining mechanical stability. The present
invention identifies a novel and new design that takes into
consideration the limitations of folded beam designs and instead
utilizes the "straight-beam" suspension beams 110 described above.
As described above, the basic structure for the actuator 105 is to
use 2 or more suspensions 110 that are radially disposed around the
axis of rotation of the actuator 105. In the preferred embodiment,
2 or 3 beams are used and are spaced 20-30.degree. apart with
respect to the rotation axis. It is understood that if larger
angles of rotation are desired, the size of the actuator 105 would
be increased. In the preferred embodiment, the rotary comb drive
elements 111 are arranged around the suspension beams 110, and can
either be contained between the suspension beams, or connected
outside the beams. If the comb drive elements 111 are arranged over
an arc of about 120.degree., it may be advantageous to have three
suspension beams 110 arranged at 60.degree. spacing.
[0057] In the preferred embodiment, the actuator 105 is fabricated
from a high aspect ratio process, which can also include plated
metal processes, for example, Lithographie, Gavanometrie and
Abformung (LIGA) process well known in the art. LIGA processing
techniques result in structures that comprise vertical dimensions
substantially greater than the horizontal width of the smallest
features of the actuator 105. With these processes, the resulting
stiffness of the actuator 105, the motion of the actuator may be
constrained to be substantially in the plane of the actuator.
[0058] Referring now to FIG. 12, and preceding figures and
descriptions as needed, there is seen a second mask layout for an
actuator of the present invention. The layout of FIG. 12 is similar
to the layout of FIG. 11, except that the virtual pivot point 108
location is changed and some aspects of the grating 103 and the
angle of the reflector 104 with respect to the grating are slightly
different.
[0059] Referring now to FIG. 13, and preceding figures and
descriptions as needed, a 3D view of the present invention
including: laser diode 101, lens 102, grating 103, reflector 104,
and output beam 150, is shown. As seen in FIG. 13, the output beam
150 is quite narrow along one axis, but the small incident angle of
the beam on the grating 103 causes the diffracted beam to be
extended along a perpendicular axis.
[0060] Referring now to FIG. 14, and preceding figures and
descriptions as needed, there is seen a module 106 incorporating
the ECL 100. The ECL 100 comprises a very small size and mass,
which enables the use of simple closed-loop methods to control the
components to accurately set and hold the wavelength of the output
beam 150. In contrast to the prior art, which may require novel
laser structures, such as, for example, a long-wavelength
vertical-cavity surface-emitting laser (VCSEL), the present
invention can be implemented using a inexpensive Fabry-Perot laser
as the laser 101, which is readily available in large quantities at
low prices. Use of a Fabry-Perot laser in the present invention is
further beneficial because, unlike VCSELs, they can operate at long
operating wavelengths, for example, up to and over 1700 nm, and in
particular 1540 nm, which is one wavelength currently used by
telecommunications equipment.
[0061] Because the laser 101 and actuator 105 of the present
invention can be made separately, the wafer fabrication processes
for their manufacture can be made simpler, which can provide higher
manufacturing yields than the prior art.
[0062] The present invention identifies that, other than in the
embodiment described above in which capacitance sensing is used for
servo control, the ECL 100 exhibits sufficient thermal stability
such that a thermo-electric cooler need not be used. This is an
advantage because TE coolers can be relatively unreliable and are
prone to fail.
[0063] Because the rotation angle of the MEMS actuator 105, and
hence the reflector 104, can be held steady under simple closed
loop control, the wavelength of the output beam 150 may also be
held steady. Furthermore, unlike prior art tunable VCSELs, in which
wavelength vs. actuator voltage must be re-calibrated as the laser
ages, the stable dispersive properties of the diffraction grating
103 of the present invention do not change with age, such that
after an initial calibration step, further calibration of the
module 106 is not necessarily required. Even if in some embodiments
the wavelength of the output beam 150 can not be held stable over
the lifetime of the module 106, the wavelength stability of the
present invention is good enough such that only intermittent
re-calibration is envisioned.
[0064] Although, the foregoing discussion has presented particular
embodiments of the present invention, it is to be understood that
the above description is not to be limited to only the described
telecommunications application and embodiments. For example, other
applications include: remote sensing or spectroscopy applications.
It will also be appreciated by those skilled in the art that it
would be possible to modify the size, shape, appearance and methods
of manufacture of various elements of the invention, or to include
or exclude various elements and stay within the scope and spirit of
the present invention. Thus, the invention should be limited only
by the scope of the claims as set forth below.
* * * * *