U.S. patent application number 09/767195 was filed with the patent office on 2002-02-07 for tunable frequency stabilized fiber grating laser.
This patent application is currently assigned to CYOPTICS (ISRAEL) LTD.. Invention is credited to Zimmermann, Micha.
Application Number | 20020015433 09/767195 |
Document ID | / |
Family ID | 22648469 |
Filed Date | 2002-02-07 |
United States Patent
Application |
20020015433 |
Kind Code |
A1 |
Zimmermann, Micha |
February 7, 2002 |
Tunable frequency stabilized fiber grating laser
Abstract
A laser, including a grating structure having two or more
gratings having a plurality of different wavelength peaks for
reflection of optical radiation therefrom. The laser further
includes a semiconductor device, having an active region which is
operative to amplify the optical radiation, and a reflective
region, which is adapted to reflect the optical radiation at a
tunable resonant wavelength of the reflective region, the device
being optically coupled to the grating structure so as to define a
laser cavity having a single cavity mode defined by tuning the
resonant wavelength of the reflective region to overlap with one of
the wavelength peaks of the grating structure.
Inventors: |
Zimmermann, Micha; (Haifa,
IL) |
Correspondence
Address: |
DARBY & DARBY P.C.
805 Third Avenue
New York
NY
10022
US
|
Assignee: |
CYOPTICS (ISRAEL) LTD.
|
Family ID: |
22648469 |
Appl. No.: |
09/767195 |
Filed: |
January 22, 2001 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
60177405 |
Jan 20, 2000 |
|
|
|
Current U.S.
Class: |
372/96 |
Current CPC
Class: |
H01S 5/0264 20130101;
H01S 5/146 20130101; H01S 5/0683 20130101; H01S 5/50 20130101; H01S
5/06256 20130101; H01S 5/0601 20130101; H01S 5/1218 20130101; H01S
5/1215 20130101; H01S 5/0687 20130101; H01S 5/0625 20130101; H01S
5/1209 20130101; H01S 5/1085 20130101; H01S 5/0657 20130101; H01S
5/026 20130101 |
Class at
Publication: |
372/96 |
International
Class: |
H01S 003/08 |
Claims
1. A laser, comprising: a grating structure, comprising a two or
more gratings having a plurality of different wavelength peaks for
reflection of optical radiation therefrom; and a semiconductor
device, comprising an active region, which is operative to amplify
the optical radiation, and a reflective region, which is adapted to
reflect the optical radiation at a turntable resonant wavelength of
the reflective region, the device being optically coupled to the
grating structure so as to define a laser cavity having a single
cavity mode defined by tuning the resonant wavelength of the
reflective region to overlap with one of the wavelength peaks of
the grating structure.
2. A laser according to claim 1, wherein the grating structure
comprises a super structure grating (SSG) written in a fiber
optic.
3. A laser according to claim 2, wherein the fiber optic comprises
a lens which focuses optical radiation from the semiconductor
device to the grating structure.
4. A laser according to claim 1, wherein the two or more gratings
are adapted to partially transmit optical radiation at the
different wavelengths, so as to provide output optical
radiation.
5. A laser according to claim 1, wherein the reflective region
comprises a Distributed Bragg Reflector (DBR) written onto the
semiconductor device, and wherein the resonant wavelength of the
reflective region is tuned by a current injected into the DBR.
6. A laser according to claim 1, wherein the plurality of different
wavelength peaks are substantially equidistantly spaced by a first
separation, wherein the reflective region comprises a Distributed
Bragg Reflector with a super structure grating (DBR-SSG) having a
plurality of different wavelength peaks substantially equidistantly
spaced by a second separation different from the first separation,
so that the first separation is related to the second separation in
a vernier-like manner and so that the single cavity mode is defined
when one of the grating structure wavelength peaks overlaps with
one of the DBR-SSG wavelength peaks
7. A laser according to claim 1, and comprising: an optical length
changer which varies an optical length of at least one of a group
of optical elements comprising the grating structure, the active
region, and the reflective region, so as to vary accordingly an
optical length of the laser cavity; a detector which is adapted to
monitor a level of the optical radiation responsive to the
variation in the optical length of the at least one of the group;
and a stabilizer which responsive to the measured output from the
detector supplies a control signal to the optical length changer to
control an optical length of at least one of the group, so that the
laser cavity resonates stably in the single cavity mode.
8. A laser according to claim 7, wherein the optical length changer
comprises at least one of a thermally active group comprising a
heater and a thermoelectric cooler, and wherein the at least one of
the thermally active group is adapted to alter a temperature of at
least one of the group of optical elements.
9. A method for generating a laser output, comprising: providing a
grating structure having a plurality of different wavelength peaks
for reflection of optical radiation therefrom; optically coupling a
semiconductor device to the structure so as to define a laser
cavity between the structure and a reflective region of the device,
which is adapted to reflect the optical radiation at a tunable
resonant wavelength of the reflective region; and tuning the
resonant wavelength of the reflective region to overlap with one of
the wavelength peaks of the grating structure so as to generate a
laser output in a single cavity mode defined by the overlap.
10. A method according to claim 9, wherein providing the grating
structure comprises writing a super structure grating (SSG) in a
fiber optic.
11. A method according to claim 9, wherein providing the grating
structure comprises providing two or more gratings which are
adapted to partially transmit optical radiation at the different
respective wavelength peaks, so as to provide output optical
radiation.
12. A method according to claim 9, wherein the reflective region
comprises a Distributed Bragg Reflector (DBR) written onto the
semiconductor device, and wherein tuning the resonant wavelength of
the reflective region comprises injecting a current into the
DBR.
13. A method according to claim 9, wherein the reflective region
comprises a Distributed Bragg Reflector with a super structure
grating (DBR-SSG) having a plurality of different DBR-SSG
wavelength peaks substantially equidistantly spaced by a first
separation, wherein providing the grating structure comprises
substantially equidistantly spacing the plurality of different
wavelength peaks by a second separation different from the first
separation and related to the first separation in a vernier-like
manner, and wherein tuning the resonant wavelength comprises
overlapping one of the grating structure wavelength peaks with one
of the DBR-SSG wavelength peaks
14. A method according to claim 9, and comprising: varying with an
optical length changer an optical length of at least one of a group
of optical elements comprising the grating structure, an active
region of the semiconductor device, and the reflective region, so
as to vary accordingly an optical length of the laser cavity;
monitoring a level of the optical radiation responsive to the
variation in the optical length of the at least one of the group;
and supplying a control signal to the optical length changer
responsive to the monitored level so as to control an optical
length of at least one of the group, so that the laser cavity
resonates stably in the single cavity mode.
15. A method according to claim 14, wherein varying the optical
length comprises altering a temperature of at least one of the
group of optical elements using at least one of a thermally active
group comprising a heater and a thermoelectric cooler.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] This application claims the benefit of U.S. Provisional
Application No. 60/177,405, filed Jan. 20, 2000, which is assigned
to the assignee of the present patent application and is
incorporated herein by reference.
FIELD OF THE INVENTION
[0002] The present invention relates generally to lasers, and
specifically to stabilization of lasers operating in multiple
modes.
BACKGROUND OF THE INVENTION
[0003] FIG. 1 is a schematic diagram showing operation of a lasing
system 18, as is known in the art. System 18 comprises two mirrors
20 and 22 separated by a distance L. In order for system 18 to lase
i.e., to resonate, at a wavelength .lambda., a medium 24 between
mirrors 20 and 22 must provide gain, and an effective optical path
length L.sub.eff between the mirrors must be an integral number of
half-wavelengths. Quantitatively,
L.sub.eff=nL (1a)
[0004] so that
m.multidot..lambda./2=nL (1b)
[0005] or
f=m.multidot.c/(2nL) (1c)
[0006] wherein m is a positive integer, n is a refractive index of
medium 24, f is the frequency corresponding to the wavelength
.lambda., and c is the speed of light.
[0007] From equation (1c), a separation .DELTA.f of lasing
frequencies is given by
.DELTA.f=c/(2nL) (2)
[0008] Each such lasing frequency corresponds to a longitudinal
cavity mode. Since f=c/.lambda.,
.DELTA.f.apprxeq.-c.multidot..DELTA..lambda./.l- ambda..sup.2 so
that equation (2) can be rewritten to give a separation
.DELTA..lambda. of lasing wavelengths:
.DELTA..lambda..apprxeq..lambda..sup.2/(2nL) (3)
[0009] FIG. 2 is a graph of intensity I vs. wavelength k
illustrating cavity modes for system 18, as is known in the art. A
curve 30 represents an overall gain of medium 24 in system 18.
Peaks 32A and 32B, with separation AX, show the cavity modes
present in system 18, each node corresponding to a different value
of m. As is evident from FIG. 2, there are many possible cavity
modes for system 18.
[0010] Optical communications within fiber optic links require that
the laser carrier have as small a frequency spread as possible,
particularly when multiple wavelengths are to be multiplexed on a
single fiber. Thus, for efficient communication only one cavity
mode should be used, and optimally the frequency spread within the
mode should be minimized. Typically, methods for stabilizing the
frequency of the laser include utilizing distributed feedback (DFB)
lasers and/or distributed Bragg reflector (DBR) lasers. DFB lasers
have a frequency-selection grating built into the laser chip, the
grating being physically congruent with the gain medium. The
grating in a DBR laser is external to the gain medium. The gratings
in DFB and DBR lasers are part of the semiconductor material, which
is unstable. DFB and DBR lasers are therefore typically externally
stabilized utilizing an external wavelength reference in order to
achieve good stability.
[0011] FIG. 3 shows the effect of adding a tuning element such as a
fiber grating to system 18, as is known in the art. A curve 34
shows the resonance curve of the fiber grating, which has a
bandwidth .DELTA..lambda..sub.G of the same order as
.DELTA..lambda., the separation between the longitudinal cavity
modes. If the grating is optically coupled to system 18, then mode
32A is present, and other modes such as mode 32B, are
suppressed.
[0012] FIG. 4 is a schematic diagram showing a gain medium 38
coupled to a fiber grating 50, as is known in the art. Gain medium
38 is formed from a semiconductor gain element 44 having a laser
gain region 42. Light from region 42 exits from a facet 56 of
region 42 to a medium 46, and traverses medium 46 so that a lens 48
collects the light into a fiber optic 52. Fiber grating 50 is
mounted in fiber optic 52, which grating reflects light
corresponding to curve 34 of FIG. 3 back to region 42. The mirrors
of the laser cavity comprise a rear mirror which in this example is
a back facet 57 of the semiconductor gain element, and an output
coupling mirror which in this example is fiber grating 50. The rear
and output coupling mirrors could also be reversed. In the reversed
configuration the rear mirror would be the fiber grating and the
output coupling mirror would be back facet 57 of the semiconductor
gain element.
[0013] It is desirable to eliminate parasitic reflections due to
surfaces and interfaces internal to the cavity. To eliminate
parasitic reflection from the facet of the semiconductor closest to
the fiber grating, in this case facet 56, that facet is usually
anti reflection coated. It is also useful to anti reflection coat a
tip 49 of the fiber closest to the semiconductor gain element to
again reduce parasitic reflections. Preferably, grating 50 is
written directly at the end of the fiber optic facing the laser.
Alternatively, a length L.sub.f of a fiber 63 is interposed between
lens 48 and fiber optic grating 50. Thus region 42, medium 46,
fiber optic 63 and grating 50 form a resonant system 60
corresponding to region 24 of FIG. 1. This architecture is
generally known in the art as an external cavity laser or more
specifically as a fiber grating laser (FGL).
[0014] System 60 has an effective optical path length L.sub.eff
given by:
L.sub.eff=n.sub.1.multidot.L.sub.1+n.sub.0.multidot.L.sub.0+n.sub.f.multid-
ot.L.sub.f+n.sub.g.multidot.L.sub.gef (4)
[0015] wherein
[0016] n.sub.1 is a refractive index of region 42;
[0017] L.sub.1 is a length of region 42;
[0018] n.sub.0 is a refractive index of medium 46;
[0019] L.sub.0 is a length of medium 46;
[0020] n.sub.f is a refractive index of fiber 63;
[0021] L.sub.f is the length of fiber 63.
[0022] n.sub.g is a refractive index of grating 50; and
[0023] L.sub.gef is an effective length of grating 50.
[0024] Replacing the optical path length nL of equation (1b) by
that given by equation (4) leads to the following equation giving
cavity modes for the system of FIG. 3:
m
.lambda./2=(n.sub.1.multidot.L.sub.1+n.sub.0.multidot.L.sub.0+n.sub.f.mu-
ltidot.L.sub.f+n.sub.g.multidot.L.sub.gef) (5)
[0025] In constructing system 60, it is necessary to adjust and
maintain the positions of curve 32A and 34 (FIG. 3) to have their
peaks at the same wavelength. Changes in temperature and/or changes
in injection current into region 42 and/or mechanical changes
affect one or more parameters of the optical path length given by
equation (4). Such changes can thus cause mode hopping, which
refers to the phenomena whereby mode 32A shifts underneath
resonance curve 34 of the fiber grating. When that shift is large
enough, an adjacent mode will at some point experience a larger
gain and start to lase. These mode hops occur underneath the
resonance curve of the fiber grating (curve 34 in FIG. 3) resulting
in wavelength shifts and intensity noise when the mode hops. For
example, referring to FIG. 3, mode 32B could shift within resonance
curve 34 of the fiber grating and resonate instead of mode 32A.
[0026] An article titled "1.5-1.6 .mu.m dynamic-single-mode (DSM)
lasers with distributed Bragg reflector," by Koyama et al., in the
Vol. 19 (1983) issue of IEEE Journal of Quantum Electronics, which
is incorporated herein by reference, describes a method for tuning
a DBR by injecting current.
[0027] Super structure grating (SSGs) are known in the art as
structures which comprise a plurality of gratings distributed along
a fiber in a manner such as to provide a spectral response with
several peaks. An article titled "Long periodic superstructure
Bragg gratings in optical fibers," by Eggleton et al., in the Vol.
30 (1994) issue of Electronics Letters, which is incorporated
herein by reference, describes one such SSG. An SSG in a fiber
provides a system having a plurality of relatively highly stable
fixed wavelengths.
[0028] SSG systems have been implemented in semiconductor devices,
to take advantage of the fact that when implemented therein the
cavities so formed are tunable. An article titled "Theory, design
and performance of extended tuning range semiconductor lasers with
sampled gratings," by Jayaraman et al., in the Vol. 29 (1993) issue
of IEEE Journal of Quantum Electronics, which is incorporated
herein by reference, describes two such systems which are tuned in
a vernier-like manner. Unfortunately, due to the inherent
characteristics of the semiconductor, SSG devices implemented in
semiconductors are relatively unstable, as is also true for single
wavelength DBRs implemented therein.
[0029] The information carrying capacity of a single lasing line
can be increased by increasing the frequency of modulation of the
laser. However such increased frequency widens the laser line width
through chirp, which in turn reduces the transmission range due to
dispersion. In order to circumvent the effects of dispersion, a
method known in the art is to use wavelength division multiplexing
(WDM) wherein a plurality of laser lines are each modulated at an
intermediate bandwidth. The total bandwidth is then the number of
laser lines multiplied by the intermediate bandwidth. For example,
instead of modulating one line at 10 Gbit/s, four lines can each be
modulated at 2.5 Gbit/s to provide the same information carrying
capacity.
[0030] Standard ITU-T G. 692 of the International
Telecommunications Union (ITU), Place des Nations CH-1211, Geneva
20, Switzerland, defines allowable wavelengths for WDM systems, so
that systems implemented by different manufacturers will be
compatible with each other. Systems known in the art for
implementing WDM use a plurality of lasers, each having a different
fixed wavelength corresponding to the ITU standard.
SUMMARY OF THE INVENTION
[0031] It is an object of some aspects of the present invention to
provide improved methods and apparatus for generating a plurality
of laser wavelengths.
[0032] In preferred embodiments of the present invention, a
semiconductor device comprises an active gain region and a
distributed Bragg reflector (DBR), which acts as a first,
highly-reflecting mirror at one end of a laser cavity that contains
the gain region. The DBR is tuned by, most preferably, varying a
current injected into the DBR. The active gain region is coupled at
the side opposite the DBR to a fiber optic comprising a super
structure grating (FO-SSG) having a plurality of relatively highly
stable resonant peaks, the peaks most preferably being separated
substantially equidistantly. The FO-SSG acts as a second,
partially-reflecting mirror at the other end of the laser cavity.
An output of the cavity is derived from light transmitted by the
FO-SSG into the fiber optic.
[0033] To operate the laser, the gain region is activated, and the
DBR is tuned so that a resonant peak of the DBR is aligned with one
of the resonant peaks of the FO-SSG. Thus, the laser resonates in a
single cavity mode defined by the two resonant peaks, and all other
modes are substantially suppressed. By scanning the tuning of the
DBR over the range of the FO-SSG, all the different resonant peaks
of the FO-SSG may be selected at will, producing corresponding
single cavity modes. Coupling a DBR with an FO-SSG combines the
advantages of tunability associated with the DBR and stability
associated with the FO-SSG for all modes of the cavity. Preferred
embodiments of the present invention thus enable a single laser to
be used as a generator in a WDM system.
[0034] In some preferred embodiments of the present invention, the
DBR is written as a super structure grating (DBR-SSG) within the
semiconductor device. The spacing of peaks of the DBR-SSG is
implemented to be slightly different from the spacing of the peaks
of the FO-SSG. The cavity produced by the combination of the
DBR-SSG with the FO-SSG is then tuned in a vernier-like manner, by
setting one of the peaks of the DBR-SSG to align with one of the
peaks of the FO-SSG. Because of the vernier-like spacing
relationship between the two SSGs, all other peaks, apart from the
aligned pair, are misaligned, so that only one mode defined by the
aligned pair resonates and all other modes are suppressed. All the
resonant peaks of the FO-SSG may be selected by scanning the
DBR-SSG over a range that is substantially the same as the spacing
of two peaks of the DBR-SSG.
[0035] In some preferred embodiments of the present invention, the
laser cavity is stabilized by thermally modulating one or more
optical elements, and/or parameters thereof, so as to vary an
effective length of the cavity. A method of thermal modulation is
described in detail in PCT patent application PCT/IL00/00401 which
is assigned to the assignee of the present invention and which is
incorporated herein by reference. The modulation generates an error
signal which is dependent on a relationship of the oscillating mode
with resonant frequency of the cavity, and the error signal is used
in a negative feedback loop to ensure that the mode and resonant
frequency substantially coincide.
[0036] There is therefore provided, according to a preferred
embodiment of the present invention, a laser, including:
[0037] a grating structure, including two or more gratings having a
plurality of different wavelength peaks for reflection of optical
radiation therefrom; and
[0038] a semiconductor device, including an active region, which is
operative to amplify the optical radiation, and a reflective
region, which is adapted to reflect the optical radiation at a
tunable resonant wavelength of the reflective region, the device
being optically coupled to the grating structure so as to define a
laser cavity having a single cavity mode defined by tuning the
resonant wavelength of the reflective region to overlap with one of
the wavelength peaks of the grating structure.
[0039] Preferably, the grating structure includes a super structure
grating (SSG) written in a fiber optic.
[0040] Preferably, fiber optic includes a lens which focuses
optical radiation from the semiconductor device to the grating
structure.
[0041] Preferably, the two or more gratings are adapted to
partially transmit optical radiation at the different wavelengths,
so as to provide output optical radiation.
[0042] Preferably, the reflective region includes a Distributed
Bragg Reflector (DBR) written onto the semiconductor device,
wherein the resonant wavelength of the reflective region is tuned
by a current injected into the DBR.
[0043] Preferably, the plurality of different wavelength peaks are
substantially equidistantly spaced by a first separation, wherein
the reflective region includes a Distributed Bragg Reflector with a
super structure grating (DBR-SSG) having a plurality of different
wavelength peaks substantially equidistantly spaced by a second
separation different from the first separation, so that the first
separation is related to the second separation in a vernier-like
manner and so that the single cavity mode is defined when one of
the grating structure wavelength peaks overlaps with one of the
DBR-SSG wavelength peaks
[0044] Preferably, the laser includes:
[0045] an optical length changer which varies an optical length of
at least one of a group of optical elements including the grating
structure the active region and the reflective region, so as to
vary accordingly an optical length of the laser cavity;
[0046] a detector which is adapted to monitor a level of the
optical radiation responsive to the variation in the optical length
of the at least one of the group; and
[0047] a stabilizer which responsive to the measured output from
the detector supplies a control signal to the optical length
changer to control an optical length of at least one of the group,
so that the laser cavity resonates stably in the single cavity
mode.
[0048] Further preferably, the optical length changer includes at
least one of a thermally active group comprising a heater and a
thermoelectric cooler, wherein the at least one of the thermally
active group is adapted to alter a temperature of at least one of
the group of optical elements.
[0049] There is further provided, according to a preferred
embodiment of the present invention, a method for generating a
laser output, including:
[0050] providing a grating structure having a plurality of
different wavelength peaks for reflection of optical radiation
therefrom;
[0051] optically coupling a semiconductor device to the structure
so as to define a laser cavity between the structure and a
reflective region of the device, which is adapted to reflect the
optical radiation at a tunable resonant wavelength of the
reflective region; and
[0052] tuning the resonant wavelength of the reflective region to
overlap with one of the wavelength peaks of the grating structure
so as to generate a laser output in a single cavity mode defined by
the overlap.
[0053] Preferably, providing the grating structure includes writing
a super structure grating (SSG) in a fiber optic.
[0054] Preferably, providing the grating structure includes
providing two or more gratings which are adapted to partially
transmit optical radiation at the different wavelength peaks, so as
to provide output optical radiation.
[0055] Preferably, the reflective region includes a Distributed
Bragg Reflector (DBR) written onto the semiconductor device,
wherein tuning the resonant wavelength of the reflective region
includes injecting a current into the DBR.
[0056] Preferably, the reflective region includes a Distributed
Bragg Reflector with a super structure grating (DER-SSG) having a
plurality of different DBR-SSG wavelength peaks substantially
equidistantly spaced by a first separation, wherein providing the
grating structure includes substantially equidistantly spacing the
plurality of different wavelength peaks by a second separation
different from the first separation and related to the first
separation in a vernier-like manner, and wherein tuning the
resonant wavelength includes overlapping one of the grating
structure wavelength peaks with one of the DBR-SSG wavelength
peaks
[0057] Preferably the method includes:
[0058] varying with an optical length changer an optical length of
at least one of a group of optical elements including the grating
structure, an active region of the semiconductor device, and the
reflective region, so as to vary accordingly an optical length of
the laser cavity;
[0059] monitoring a level of the optical radiation responsive to
the variation in the optical length of the at least one of the
group; and
[0060] supplying a control signal to the optical length changer
responsive to the monitored level so as to control an optical
length of at least one of the group, so that the laser cavity
resonates stably in the single cavity mode.
[0061] Further preferably, varying the optical length includes
altering a temperature of at least one of the group of optical
elements using at least one of a thermally active group comprising
a heater and a thermoelectric cooler.
[0062] The present invention will be more fully understood from the
following detailed description of the preferred embodiments
thereof, taken together with the drawings, in which:
BRIEF DESCRIPTION OF THE DRAW S
[0063] FIG. 1 is a schematic diagram showing operation of a laser
system, as is known in the art;
[0064] FIG. 2 is a graph of intensity vs. wavelength, illustrating
cavity modes for the system of FIG. 1, as is known in the art;
[0065] FIG. 3 shows the effect of adding a spectrally selective
element such as a fiber grating to the system of FIG. 1, as is
known in the art;
[0066] FIG. 4 is a schematic diagram showing a semiconductor gain
medium coupled to a fiber grating forming a fiber grating laser
(FGL), as is known in the art;
[0067] FIG. 5 is a schematic diagram of a laser system, according
to a preferred embodiment of the present invention;
[0068] FIG. 6 shows schematic graphs of intensity vs. wavelength
relationships for different elements of the system of FIG. 5,
according to a preferred embodiment of the present invention;
[0069] FIG. 7 is a schematic diagram of a laser system, according
to an alternative preferred embodiment of the present
invention;
[0070] FIG. 8 shows schematic graphs of intensity vs. wavelength
relationships for different elements of the system of FIG. 7,
according to a preferred embodiment of the present invention;
and
[0071] FIG. 9 is a schematic diagram of apparatus for locking a
longitudinal mode of the system of FIG. 5 to a super structure
grating peak, according to a preferred embodiment of the present
invention.
DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS
[0072] Reference is now made to FIG. 5, which is a schematic
diagram of a laser system 70, according to a preferred embodiment
of the present invention. A semiconductor device 72 comprises a
gain medium 74, within which is formed a laser gain region 76 which
acts as an active region amplifying optical radiation. Device 72 is
generally box-shaped, and has front and back facets 78 and 80 which
are most preferably anti-reflection coated in order to eliminate
parasitic reflections within region 76. Adjacent to facet 80,
region 76 comprises a Distributed Bragg Reflector (DBR) section 82,
which is written onto device 72 by a photolithographic technique,
as is known in the art. Distributed Bragg Reflector (DBR) section
82 has a relatively broad spectral response curve. The relationship
between the curve of DBR 82 and resonant curves of other elements
of system 70 is described below with reference to FIG. 6. During
operation of system 70, DBR section 82 acts as a reflective region
which is a substantially fully reflecting first mirror of a
resonant cavity 84.
[0073] Cavity 84 also comprises a fiber optic 86, the fiber optic
most preferably supporting single mode transmission. Fiber optic 86
comprises a super structure grating (SSG) 88 which is written in
the fiber optic. SSG 88 acts as a grating structure which is
implemented to have a plurality of gratings 88A, 88B, 88C, . . . ,
with dead zones in between. The gratings may be of different
lengths as may be the dead zones. The plurality of gratings 88A,
88B, 88C, . . . and dead zones of SSG 88 provide a device with
several spectral reflection features as will be described below. If
system 70 is to be used in a wavelength division multiplexing (WDM)
system, these spectral reflection features are most preferably
written to correspond to wavelengths of the WDM system. Fiber optic
86 is coupled to device 72 by methods known in the art, for
example, by using a lens 90 between the fiber optic and device 72
or by butting the fiber optic to facet 78. Preferably, if the fiber
optic is butted to device 72, the device comprises a mode converter
for better collection efficiency between the semiconductor and the
fiber. The mode converter is most preferably implemented in the
semiconductor, preferably by an architecture known in the art as a
taper. SSG 88 acts as a partially reflecting second mirror of
cavity 84, so that light transmitted through the SSG into fiber
optic 86 is output light of system 70.
[0074] Device 72 comprises a first upper electrode 96 and a second
upper electrode 97. Electrode 96 and lower electrode 94 are used to
inject current into region 74 in order to cause region 76 to lase.
Current injected via electrode 97 is varied so as to tune DBR
section 82, by methods known in the art.
[0075] FIG. 6 shows schematic graphs of intensity vs. wavelength
relationships for different elements of system 70, according to a
preferred embodiment of the present invention. A graph 100
corresponds to a fundamental gain curve of device 72, according to
the composition of the device. As described above in the Background
of the Invention with reference to equations (1b), (2), and (3),
cavity 84 has longitudinal cavity modes 102A, 102B, 102C, . . .
separated by .DELTA..lambda., with wavelengths which are a function
of an optical length between the mirrors of the cavity and the
number of half-wavelengths comprising the mode. A graph 104
corresponds to the overall spectral reflection features of SSG 88,
wherein each peak 104A, 104B, 104C, . . . of the graph is a
relatively narrow resonant curve. Most preferably, each resonant
curve is sufficiently narrow so that substantially only one
longitudinal mode 102A, 102B, 102C, . . . can resonate.
[0076] A graph 106 corresponds to a resonant curve of DBR section
82. DBR section 82 is preferably written in device 72 so that its
resonant curve substantially encloses only one of the peaks of
graph 104. Thus in FIG. 6, longitudinal cavity mode 102K will
resonate since it is within the resonant curve of the SSG, at
.lambda..sub.B, and section 82 is tuned to this wavelength. Modes
such as 102J, 102H, and 102L will be substantially suppressed since
they are on the wings of graph 106. As described above, DBR section
82 is tunable, so that for mode 102L to resonate the section is
tuned to lower wavelength .lambda..sub.A. Similarly, for modes
102H, 102F, and 102C to resonate, section 82 is respectively tuned
to higher wavelengths .lambda..sub.C, .lambda..sub.D,
.lambda..sub.E. Thus system 70 can be effectively scanned from
.lambda..sub.A to .lambda..sub.E by tuning DBR section 82 across
the same wavelength range.
[0077] FIG. 7 is a schematic diagram of a laser system 120,
according to a preferred embodiment of the present invention. Apart
from the differences described below, the operation of system 120
is generally similar to that of system 70 (FIG. 5), so that
elements indicated by the same reference numerals in both systems
70 and 120 are generally identical in construction and in
operation. Adjacent to facet 80 of device 72, region 76 comprises
an SSG 124 implemented in a DBR section 122. SSG 124 comprises a
plurality, preferably the same plurality as SSG 88, of gratings
124A, 124B, 124C, . . . Most preferably, the separation between
adjacent resonant peaks of SSG 124 is different from the separation
between adjacent resonant peaks of SSG 88, and the two separations
are related in a vernier-like manner, as is described in more
detail hereinbelow with reference to FIG. 8. Setting the
separations to be different enables system 120 to be scanned over
the whole range of wavelengths of SSG 88 using a reduced wavelength
range scan of DBR section 122.
[0078] FIG. 8 shows schematic graphs of intensity vs. wavelength
relationships for different elements of system 120, according to a
preferred embodiment of the present invention. A graph 136
corresponds to a resonant reflection curve of DBR section 124. For
clarity, graph 136 is shown separate, i.e., not overlaid, from
graphs 100 and 104. Peaks 136A, 136B, 136C, . . . of section 124
are assumed to be separated by a spacing .DELTA..sub.DBR, and peaks
104A, 104B, 104C, . . . of SSG 88 are assumed to be separated by a
different spacing .DELTA.80 .sub.F. In general, if SSG 88 comprises
j gratings each having a resonant wavelength separated by
.DELTA..lambda..sub.F, and DBR section 124 comprises j spectral
reflection peaks separated by .DELTA..lambda..sub.DBR, then the
separations should be implemented so as to satisfy: 1 DBR = ( j - 1
j ) F ( 6 )
[0079] Setting .DELTA..lambda..sub.F and .DELTA..lambda..sub.DBR to
be related according to equation (6) allows a vernier effect to be
used to accomplish the tuning, as explained hereinbelow.
[0080] In the situation represented by FIG. 8, wherein by way of
example j=5, mode 102L will resonate, and other modes of system 102
will be substantially suppressed, 25 since peak 136A is
substantially aligned with peak 104A, and no other peaks of graphs
104 and 136 align. To tune system 120 to mode 102K, corresponding
to peak 104B, curve 136 needs to move right by .delta..lambda.,
where
.delta..lambda.=.DELTA..lambda..sub.F-.DELTA..lambda..sub.DBR
(7)
[0081] so that peak 136B substantially aligns with peak 104B, and
the other peaks of graphs 104 and 136 do not align.
[0082] Similarly, to tune system 120 to modes corresponding with
peaks 104C, 104D, . . . , curve 136 needs to move right by
2.delta..lambda., 3.delta..lambda., . . . . The resonant peaks of
graph 136 should have narrow enough widths so that substantially
only one longitudinal mode of system 120 lases at each of the
alignments of curves 104 and 136. Thus, as illustrated by FIG. 8,
system 12C can be effectively scanned from .lambda..sub.A to
.lambda..sub.E by tuning section 124 by a total of
4.delta..lambda..
[0083] FIG. 9 is a schematic diagram of a system 150 for locking a
longitudinal mode to an SSG peak, according to a preferred
embodiment of the present invention. In implementing system 120 and
system 70, it is necessary to adjust respective DBR sections 124
and 82 so that all modes in the respective SSG are suppressed apart
from one. Most preferably, the position of the relevant cavity mode
should be adjusted to correspond with the aligned peaks of the SSG
and DBR section. Once adjusted, it is necessary to maintain the
mode in position. It will be appreciated that effects such as
temperature change, change in injected current, and mechanical
changes will tend to move the peak of the mode relative to the
peaks of the SSG and the DBR, causing mode hopping. System 150 is
implemented for system 70, but the principles of system 150 apply
also to system 120. The implementation of a system substantially
similar to system 150 is described in detail in the
above-referenced PCT patent application.
[0084] In system 150, system 70 is mounted on a substrate 154,
beneath which is coupled a thermoelectric cooler (TEC) 152. Above
electrode 96 of system 70 is mounted a heater 156. Heater 156 and
TEC 152 may be adjusted either separately or together to alter
temperatures of elements of cavity 84, and so change optical
lengths of elements of the cavity. System 150 further comprises a
detector 160 which measures a parameter of light output from cavity
84. Preferably, detector 160 measures the output from facet 80.
Alternatively, detector 160 is positioned at another point in
system 150 where it is able to measure the parameter without
substantially interfering with the operation of the system.
Detector 160 supplies an error signal, generated responsive to the
parameter output, to a wavelength stabilizer 158, which acts as a
controller of heater 156 and TEC 152.
[0085] Referring back to FIG. 5, an effective length of cavity 84
is given by an equation:
L.sub.eff=n.sub.DBR.multidot.L.sub.DBR+n.sub.g.multidot.L.sub.g+n.sub.SSG.-
multidot.L.sub.SSG (8)
[0086] wherein
[0087] n.sub.DBR is a refractive index of DBR region 82;
[0088] L.sub.DBR is a length of region 82;
[0089] n.sub.g is a refractive index of gain region 76;
[0090] L.sub.g is a length of region 76;
[0091] n.sub.SSG is a refractive index of SSG 88; and
[0092] L.sub.SSG is an effective length of SSG 88.
[0093] Stabilizer 158 preferably modulates the temperature of one
or more of DBR region 82, gain region 76, and SSG 88, so that their
lengths and/or refractive indices, and 25 thus the effective length
L.sub.eff of cavity 84, are modulated. The modulation is performed
by stabilizer 158 sending modulation signals to heater 156 and/or
TEC 152. The parameter measured by detector 160 is a function of
the modulation, such as a phase of the output compared to 30 the
modulating input, in which case the phase is used by detector 160
to generate the error signal. The error signal is used by
stabilizer 158 as a negative feedback control so as to shift, as
required, a mode of cavity 84 to an SSG 88 peak.
[0094] It will be appreciated that the preferred embodiments
described above are cited by way of example, and that the present
invention is not limited to what has been particularly shown and
described hereinabove. Rather, the scope of the present invention
includes both combinations and subcombinations of the various
features described hereinabove, as well as variations and
modifications thereof which would occur to persons skilled in the
art upon reading the foregoing description and which are not
disclosed in the prior art.
* * * * *