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United States Patent |
6,069,906
|
Hong
,   et al.
|
May 30, 2000
|
Generation of short optical pulses using strongly complex coupled DFB
lasers
Abstract
A compact source capable of generating continuously tunable high frequency
microwave radiation and short optical pulses in the
picosecond/sub-picosecond range is invented. It includes a laser structure
having two lasers formed on the same substrate which simultaneously
operate at different longitudinal modes. Each laser has a complex coupled
(gain-coupled or loss-coupled) grating which is formed by deep etching
through a multi-quantum well structure, either of the active medium or of
the additional lossy quantum-well layers, thus ensuring no substantial
interaction between lasers. The lasers have a common active medium and
shared optical path and provide mutual light injection into each other
which results in generation of a beat signal at a difference frequency of
two lasers. The beat frequency is defined by spacing between the laser
modes and may be continuously tuned by current injection and/or
temperature variation. Thus, the beat signal provides a continuously
tunable microwave radiation. To form a train of short optical pulses, the
beat signal is either further sent to a saturable absorber followed by a
semiconductor optical amplifier, or sent directly into an optical
compressor which includes a dispersion fiber. As a result, a duration of
each impulse is compressed, and a train of short optical pulses is formed.
Inventors:
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Hong; Jin (Nepean, CA);
Hui; Rongqing (Lawrence, KS);
O'Sullivan; Maurice S. (Ottawa, CA)
|
Assignee:
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Nortel Networks Corporation (Montreal, CA)
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Appl. No.:
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213088 |
Filed:
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December 15, 1998 |
Intern'l Class: |
H01S 003/19; H01S 003/10; H01S 003/08 |
Field of Search: |
372/22,50,96,102,25
|
References Cited
U.S. Patent Documents
5020153 | May., 1991 | Choa et al. | 455/606.
|
5936994 | Aug., 1999 | Hong et al. | 372/96.
|
Other References
Mamyshev, Dec. 15, 1994, Optics Letters, 2074-2076.
Weber et al, Feb. 1992, Journal of Quantum Electronics, 441-446.
Mamyshev et al, Oct. 1991, IEEE Journal of Quantum Electronics, 2347-2355.
Derickson et al, Oct. 1992, IEEE Journal of Quantum Electronics, 2186-2202.
|
Primary Examiner: Font; Frank G.
Assistant Examiner: Rodriguez; Armando
Attorney, Agent or Firm: Foley & Lardner
Claims
What is claimed is:
1. An optical pulse source, comprising:
a first single mode DFB semiconductor laser having a first grating for
generating light at a first frequency;
a second single mode DFB semiconductor laser having a second grating for
generating light at a second frequency;
the lasers having a common active medium and shared optical path, the
lasers providing mutual light injection imposed on each other resulting in
generation of a beat signal at a difference frequency of two lasers;
an optical compressor disposed for receiving the beat signal and
compressing the pulse duration of the signal, thus forming a train of
short optical pulses having a pre-determined duration and a repetition
rate.
2. A source as defined in claim 1 further comprising a saturable absorber
disposed to receive the beat signal before it is received by the
compressor, the absorber providing an initial time compression of the
signal, thus transforming the beat signal into an initial train of optical
pulses.
3. A source as defined in claim 2 further comprising an optical amplifier
for amplification the initial train of pulses.
4. A source as defined in claim 1 wherein the optical compressor comprises
a dispersion decreasing fiber.
5. A source as defined in claim 1 wherein the optical compressor comprises
a dispersion shifted fiber.
6. A source as defined in claim 1 wherein the optical compressor comprises
an erbium doped fiber amplifier.
7. A source as defined in claim 3 wherein the optical amplifier comprises
an erbium doped fiber amplifier.
8. A source as defined in claim 1 further comprising means for data
encoding into the train of short pulses.
9. A source as defined in claim 8 wherein the means for data encoding
comprises an optical modulator operating at a speed determined by the
repetition rate.
10. A source as defined in claim 1 wherein the repetition rate of the pulse
train is from approximately several tens GHz to approximately several
hundred GHz.
11. A source as defined in claim 1 wherein the repetition rate of the pulse
train is from approximately 20 GHz to approximately 80 GHz.
12. A source as defined in claim 1 wherein the duration of pulses in the
pulse train is within a range from sub picoseconds to picoseconds.
13. A source as defined in claim 1 wherein first and second DFB lasers
comprise one of the gain coupled and loss coupled DFB lasers.
14. A source as defined in claim 13 wherein the active medium includes a
multiple quantum well structure.
15. A source as defined in claim 14 wherein the first and second gratings
are formed by etching grooves directly through the multiple quantum well
structure.
16. A source as defined in claim 15 wherein each grating has a period
comprising a first section and a second section with substantially all
quantum wells being etched away from the second section, thus providing no
substantial photon emission in the second section and ensuring no
substantial interaction between the lasers.
17. A source as defined in claim 1 further comprising means for stabilizing
the frequency of one of the first and second lasers.
18. A source radiation as defined in claim 11 further comprising means for
stabilizing the frequencies of both lasers.
19. A source as defined in claim 1 further comprising means for tuning
frequencies of the first and second lasers.
20. A source as defined in claim 1 further comprising means for modulating
light generated by one of the first and second lasers.
21. A source as defined in claim 20 wherein the modulation is provided at a
frequency which is subharmonic to the beat frequency.
22. A source as defined in claim 1 further comprising means for modulating
light generated by the lasers simultaneously.
23. A source defined in claim 1 wherein pumping of the active medium is
provided by current injection into the active medium.
24. A source defined in claim 1 wherein pumping of the active medium is
provided by an external optical pumping source.
25. A source as defined in claim 1 wherein the first and second gratings
have same periods.
26. A source as defined in claim 25 wherein the first and second lasers
generate light at the same side of stopband.
27. A source as defined in claim 26 wherein the difference between the
first and second frequencies is provided by different current injection
into the first and second lasers.
28. A source as defined in claim 26 wherein the difference between the
first and second frequencies is provided by different width of the active
medium in the first and second lasers.
29. A source as defined in claim 26 wherein the difference between the
first and second frequencies is provided by difference in temperatures at
which the first and second lasers are maintained.
30. A source as defined in claim 25 wherein the first and second lasers
generate light at different sides of stopband.
31. A source as defined in claim 1 wherein the first and second gratings
have different periods.
32. A source as defined in claim 31 wherein the frequency of one of the
lasers which is remote from an output facet does not fall within a
stopband of the other laser which is closer to the output facet so that
light emitted by the remote laser can pass through the shared optical path
to the output facet.
33. A source as defined in claim 1 wherein the first and second gratings
comprise one of the uniform and chirped gratings.
34. A source as defined in claim 1 wherein the first and second gratings
are first order gratings.
35. A source as defined in claim 1 wherein the first and second gratings
are formed by one of the holographic writing and electron beam writing
onto the active medium.
36. An optical pulse source, comprising:
a first single mode DFB semiconductor laser having a first grating for
generating light at a first frequency;
a second single mode DFB semiconductor laser having a second grating for
generating light at a second frequency;
the lasers having a common active medium and shared optical path, the
lasers providing light injection of light imposed on each other resulting
in generation of a beat signal at a difference frequency of two lasers;
a saturable absorber disposed for receiving the beat signal and providing
an initial time compression thus transforming the beat signal into an
initial train of optical pulses;
an optical amplifier disposed to receive the initial pulse train after the
absorber;
an optical compressor disposed to receive the pulse train after the optical
amplifier and compressing duration of pulses of the train, thus forming a
train of short optical pulses having a pre-determined duration and a
repetition rate,
the first and second lasers, the saturable absorber and the optical
amplifier being formed on the same chip.
37. A source as defined in claim 36 wherein the first and second lasers,
the saturable absorber and the optical amplifier are integrated within a
package.
38. A source of radiation, comprising:
a first single mode DFB semiconductor laser having a first grating for
generating light at a first frequency;
a second single mode DFB semiconductor laser having a second grating for
generating light at a second frequency;
the lasers having a common active medium and shared optical path and
providing mutual light injection imposed on each other resulting in
generation of radiation at a beat frequency of two lasers.
39. A source of radiation as defined in claim 38 wherein first and second
DFB lasers comprise one of the gain coupled and loss coupled DFB lasers.
40. A source of radiation as defined in claim 39 wherein the active medium
includes a multiple quantum well structure.
41. A source of radiation as defined in claim 40 wherein the first and
second gratings are formed by etching grooves directly through the
multiple quantum well structure.
42. A source of radiation as defined in claim 41 wherein each grating has a
period comprising a first section and a second section with substantially
all quantum wells being etched away from the second section, thus
providing no substantial photon emission in the second section and
ensuring no substantial interaction between the lasers.
43. A source of radiation as defined in claim 38 wherein the beat frequency
corresponds to a wavelength of approximately microwave wavelength range.
44. A source as defined in claim 38 wherein the first and second gratings
have same periods.
45. A source as defined in claim 44 wherein the first and second lasers
generate light at the same side of stopband.
46. A source as defined in claim 45 wherein the difference between the
first and second frequencies is provided by different current injection
into the first and second lasers.
47. A source as defined in claim 45 wherein the difference between the
first and second frequencies is provided by different width of the active
medium in the first and second lasers.
48. A source as defined in claim 45 wherein the difference between the
first and second frequencies is provided by difference in temperature
control of the first and second lasers.
49. A source as defined in claim 44 wherein the first and second lasers
generate light at different sides of stopband.
50. A source as defined in claim 38 wherein the first and second gratings
have different periods.
51. A source of radiation as defined in claim 50 wherein the frequency of
one of the lasers which is remote from an output facet does not fall
within a stopband of the other laser which is closer to the output facet
so that light emitted by the remote laser can pass through the shared
optical path to the output facet.
52. A source of radiation as defined in claim 38 further comprising means
for modulation of light generated by one of the first and second lasers at
a frequency subharmonic to the beat frequency.
53. A source of radiation as defined in claim 38 wherein the source is
formed on a chip and integrated within a package.
Description
FIELD OF INVENTION
The invention relates to generation of short optical pulses with particular
application to transmission of data.
BACKGROUND OF THE INVENTION
Ultra high speed time domain multiplexing (TDM) optical transmission
systems in optical fibers require compact light emitting sources capable
of generating optical short pulse trains in a picosecond/sub-picosecond
range. General requirements for short pulse sources such as soliton
sources are narrow pulse width, low time jitter and a continuously tunable
repetition rate. For practical fiber optic systems, there are additional
requirements of long-term reliability, small size and easy data encoding
in the system application.
There are several known methods to generate short optical pulse trains.
Complicated passive or active mode locking techniques are available for
high speed optical pulse generation where pulses are generated at a fixed
repetition rate determined by the roundtrip time of the laser resonator,
e.g. D. J. Derickson et al., "Short pulse generation using multisegment
mode-locked semiconductor lasers," IEEE J. Quantum Electron., Vol. 28, pp.
2186-2202, 1992. These techniques are sensitive to phase matching
conditions and therefore difficult to build and maintain. Another method
is gain switching of lasers which suffer from high time jitter. Pulse
generation at repetition rates over 50 GHz is extremely difficult to
achieve in this method because of limitations of the device modulation
bandwidth and radio frequency supply as described, e.g. in publication by
A. G. Weber, W. Ronghan, E. H. Bottcher, M. Schell and D. Bimberg,
"Measurement and simulation of the turn-on delay time jitter in
gain-switched semiconductor lasers," IEEE J. Quantum Electron., Vol. 28,
pp. 441-445, 1992.
High repetition rate optical pulses can also be generated using a dual
wavelength light source as described, e.g. in publication P. V. Mamyshev,
S. V. Chernikov and E. M. Dianov, "Generation of Fundamental soliton
trains for high-bit-rate optical fiber communication lines," IEEE J.
Quantum Electron., vol. 27, pp. 2347-2355, 1991. Two wavelengths emitted
by two lasers are mixed to form a high frequency sinusoidal signal which
is sent though an optical combiner and optical amplifier followed by a
nonlinear fiber. As a result the sinusoidal signal is compressed into a
train of optical pulses. The common approach of dual wavelength light
sources is to use two discrete lasers, which is complex and suffers from
the long term stability issue. Dual wavelength operation can also be
accomplished by selecting the appropriate phase modulation sidebands from
an externally phase modulated light source, e.g. P. V. Mamyshev,
"Dual-wavelength source of high-repetition-rate, transform-limited optical
pulses for soliton transmission" Opt. Lett., Vol. 19, pp. 2074-2076, 1994.
This method requires high frequency modulation and optical filters. In
order to make this method more practical two solitary laser diodes are
usually used to generate the sinusoidal beat signal. Unfortunately,
frequency variations of each laser are subject to both thermal and
mechanical fluctuations which result in beat signal frequency
fluctuations. Phase noise of both lasers also contribute to the jitter of
the beat signal significantly. The control of the polarization from each
laser output and the effort to align and maintain them is also a practical
issue that decreases the system performance. Therefore, the resulting
signal performance is not satisfactory and practical use of such a
configuration in commercial ultra high speed applications is in question.
Accordingly, there is a need in the industry for a practical, compact and
reliable optical source of continuously tuning high repetition rate short
optical pulses which is suitable for optical transmission systems and high
speed optical signal processing.
SUMMARY OF THE INVENTION
It is an object of the present invention to provide an optical pulse source
which avoids the afore-mentioned problems.
Thus, according to one aspect of the present invention there is provided an
optical pulse source, comprising:
a first single mode DFB semiconductor laser having a first grating for
generating light at a first frequency;
a second single mode DFB semiconductor laser having a second grating for
generating light at a second frequency;
the lasers having a common active medium and shared optical path, the
lasers providing mutual light injection into each other resulting in
generation of a beat signal at a difference frequency of two lasers;
an optical compressor disposed to receive the beat signal and compressing
the pulse duration of the signal, thus forming a train of short optical
pulses having a pre-determined duration and a repetition rate.
The source may further include a saturable absorber disposed to receive the
beat signal before it is sent to the optical compressor. The absorber
provides an initial time compression of the signal, thus transforming the
beat signal into an initial train of optical pulses. Additionally, an
optical amplifier may be used for amplification the beat signal or the
initial train of pulses, e.g. including an erbium doped fiber. The optical
compressor may include a dispersion decreasing fiber, a dispersion shifted
fiber and/or an external erbium doped fiber amplifier. The source may
further include means for data encoding into the train of short pulses,
e.g. an optical modulator operating at a speed determined by the
repetition rate. A typical range of the repetition rates is from about
several tens GHz to about several hundred GHz with a sub-range from about
25 GHz to about 80 GHz being of special importance for data transmission.
A typical duration of pulses in the pulse train is within a
picosecond/sub-picosecond range.
A source includes gain coupled DFB lasers, or alternatively it may include
loss coupled DFB lasers. Preferably, the active medium of the lasers
includes a multiple quantum well structure. Advantageously, the first and
second gratings in the first and second lasers are formed by etching
grooves directly through the multiple quantum well structure.
Beneficially, each grating has a period comprising a first section and a
second section with substantially all quantum wells being etched away from
the second section, thus providing no substantial photon emission in the
second section and ensuring no substantial interaction between the lasers.
A source may further have means for stabilizing the frequency of one of
the first and second lasers, or means for stabilizing frequencies of both
lasers. Means for tuning frequencies of the first and second lasers may be
provided additionally. To ensure reliable and accurate mode, low frequency
modulation of light generated by one of the first and second lasers, may
be provided. Alternatively, light generated by the lasers may be modulated
simultaneously. Beneficially, the modulation is provided at a frequency
which is subharmonic to the beat frequency.
In the first embodiment a source includes first and second gratings which
have same periods, and it is arranged that lasers generate light at the
different sides of stopband.
In the second embodiment a source includes first and second gratings which
have same periods, and it is arranged that lasers generate light at the
same side of stopband. The difference between the first and second
frequencies is provided by different current injection into the first and
second lasers, or by difference in temperature control of the first and
second lasers. Alternatively it may be provided by different width of the
active medium in the first and second lasers.
In the third embodiment a source includes the first and second gratings
which have different periods. It is arranged that the frequency of one of
the lasers which is remote from an output facet does not fall within a
stopband of the other laser which is closer to the output facet so that
light emitted by the remote laser can pass through the shared optical path
to the output facet.
In the fourth embodiment, instead of pumping the active medium of the
source by current injection in to the active medium, the pumping is
provided by an external optical pumping source.
In modifications to the embodiments described above, a source may include
the first and second gratings which are either uniform or chirped, the
gratings preferably being first order gratings which are formed by
holographic techniques or electron beam writing onto the active medium.
According to another aspect of the invention there is provided an optical
pulse source, comprising:
a first single mode DFB semiconductor laser having a first grating for
generating light at a first frequency;
a second single mode DFB semiconductor laser having a second grating for
generating light at a second frequency;
the lasers having a common active medium and shared optical path, the
lasers providing light injection of light into each other resulting in
generation of a beat signal at a difference frequency of two lasers;
a saturable absorber disposed to receive the beat signal and providing an
initial time compression thus transforming the beat signal into an initial
train of optical pulses;
an optical amplifier disposed to receive the initial pulse train after the
absorber;
an optical compressor disposed to receive the pulse train after the optical
amplifier and compressing duration of pulses of the train, thus forming a
train of short optical pulses having a pre-determined duration and a
repetition rate,
the first and second lasers, the saturable absorber and the optical
amplifier being formed on the same chip.
Conveniently, the lasers, the saturable absorber and the optical amplifier
are integrated within a package. According to yet another aspect of the
invention there is provided a source of radiation, comprising:
a first single mode DFB semiconductor laser having a first grating for
generating light at a first frequency;
a second single mode DFB semiconductor laser having a second grating for
generating light at a second frequency;
the lasers having a common active medium and shared optical path and
providing mutual light injection into each other resulting in generation
of radiation at a beat frequency of two lasers.
Preferably, the source of radiation includes either gain coupled DFB lasers
or loss coupled DFB lasers with the active medium comprising a multiple
quantum well structure. Advantageously, the first and second gratings are
formed by etching grooves directly through the multiple quantum well
structure. To provide no substantial interaction between two lasers, it is
arranged that each grating has a period comprising a first section and a
second section with substantially all quantum wells being etched away from
the second section, thus providing no substantial photon emission in the
second section. A typical wavelength range of the radiation generated by
the source corresponds to microwave to millimeter wavelength range.
Conveniently, the first and second gratings have same periods, and it is
arranged that the first and second lasers generate light at the same side
of stopband. The difference between the first and second laser frequencies
may be provided by different current injection into the first and second
lasers, by different width of the active medium in the first and second
lasers and/or by difference in temperature control of the first and second
lasers.
Alternatively, a source of radiation may include gratings having same
periods while the first and second lasers generate light at different
sides of stopband. In yet another alternative a source may include the
first and second gratings having different periods. It is also provided
that the frequency of one of the lasers which is remote from an output
facet does not fall within a stopband of the other laser which is closer
to the output facet so that light emitted by the remote laser can pass
through the shared optical path to the output facet. Conveniently, the
source may further include means for modulation of light generated by one
of the first and second lasers at a frequency which is subharmonic to the
beat frequency. Advantageously, the source is formed on a chip and
integrated within a package.
BRIEF DESCRIPTION OF THE DRAWINGS
The invention will now be described in greater detail regarding the
attached drawings in which:
FIG. 1 shows a schematic view of a prior art optical pulse source;
FIG. 2 shows a schematic cross sectional view of an optical pulse source
according to a first embodiment of the invention;
FIG. 3 shows a detailed cross sectional view of the laser structure of the
source of FIG. 2;
FIG. 4 shows a schematic perspective view of the laser structure of the
source of FIG. 2;
FIG. 5 shows a dual-wavelength operation of the laser structure of the
source of FIG. 2;
FIG. 6 shows an autocorrelation trace of a beat signal obtained from the
dual-wavelength operation illustrated in FIG. 5;
FIG. 7 is a schematic view of an optical pulse source according to a second
embodiment of the invention;
FIG. 8 shows a continuously tunable optical spectrum of the laser structure
of the source of dual wavelength operation in FIG. 7 where both the
absorber and the SOA are not present in the demonstration device;
FIG. 9 shows a dual-wavelength operation of the laser structure of the
source of FIG. 7 corresponding to a beat frequency 50 GHz;
FIG. 10 shows an autocorrelation trace of a beat signal obtained from the
dual-wavelength operation illustrated in FIG. 9;
FIG. 11 shows an output optical spectrum after pulse compression for an
input signal of FIG. 9;
FIG. 12 shows an autocorrelation trace of an optical pulse train after
pulse compression for the input spectrum of FIG. 9 and the output spectrum
of FIG. 11;
FIG. 13 shows an autocorrelation trace of an optical pulse train after
pulse compression corresponding to a beat frequency of 25 GHz;
FIG. 14 shows an autocorrelation trace of an optical pulse train after
pulse compression corresponding to a beat frequency of 70 GHz;
FIG. 15 is a schematic view of an optical pulse source according to a third
embodiment of the invention;
FIG. 16 shows a dual-wavelength operation of the laser structure of the
source of FIG. 15; and
FIG. 17 is a schematic view of an optical pulse source according to a
fourth embodiment of the invention.
DESCRIPTION OF PREFERRED EMBODIMENTS
Structure and Operation
The prior optical pulse source 1 is schematically shown in FIG. 1. Light
generated at first and second wavelengths .lambda..sub.1 and
.lambda..sub.2 by first and second lasers 2 and 3 is mixed to form a high
frequency sinusoidal signal 4 at difference beat frequency. The beat
signal is further sent though an optical combiner 5 and optical amplifier
7 followed by a nonlinear fiber 8 where the initial pulse is amplified,
chirped and then compressed by the nonlinear Kerr effect. As a result, the
sinusoidal signal 4 is compressed into a train of optical pulses 9 as
illustrated in FIG. 1.
A schematic cross section through a source of short optical pulses 10
according to a first embodiment of the present invention is shown in FIG.
2. The source 10 comprises a first gain coupled DFB laser 11 and a second
gain coupled DFB laser 13 having their cavities arranged coaxially along
same optical axis, designated by a numeral I--I in FIG. 1. Lasers 11 and
13 have outputs arranged along the line I--I and in the same direction
shown by an arrow on the line I--I, the first laser 11 being closer to an
output facet 27. The source 10 is formed on a substrate 12 providing a
first confinement region, an active medium 14 comprising a multiple
quantum well structure 16 and a first and second uniform gratings 17 and
19 defined therein, and an overlying confinement region 20 . Means for
excitation the first and second lasers are formed thereon, and include a
contact to the substrate 26 , first and second current confining ridges 22
and 24 , first and second contact electrodes 30 and 32 being defined on
each ridge respectively for current injection into the laser structure.
The first and second gratings 17 and 19 positionally correspond to the
first and second lasers 11 and 13 respectively. The gratings have same
grating periods, and lasers 11 and 13 have different length so that they
will behave differently in lasing operation. For example, in the device
shown in FIG. 2, the first section is 150 micrometers long and the second
section is 550 micrometers long. The lasers provide a dual wavelength
operation, i.e. each laser provides a stable generation at its own
frequency/wavelength in the presence of another laser. In the first
embodiment it is arranged that lasers 11 and 13 generate at different
sides of a DFB stopband, e.g. the laser 11 generates at the left Bragg
mode of the stopband, while the laser 13 generates at the right Bragg mode
of the stopband. Therefore the wavelength/frequency spacing of the lasers
is determined by the stopband width. The lasers 11 and 13 have a common
active medium 14 and shared optical path to the output facet 27 and
provide mutual injection of light into each other. Due to mutual injection
of light into each other, a sinusoidal optical beat signal is generated by
the lasers, with the frequency spacing defined by the wavelength spacing
between the two stable operated modes. The frequency spacing is fine tuned
to the specific frequency spacing required in the system by current
injection into the lasers 11 and 13. The beat signal is a source of
tunable radiation having a typical wavelength within a microwave to
millimeter range.
Both gratings 17 and 19 are made by periodic etching grooves through the
active medium 14, the depth of etching being defined so as to provide a
substantial insensitivity of each laser to the external feedback and
random facet variations and thereby ensuring no substantial interaction
between lasers in the series as will be also described in detail below. An
additional short section of a reversely biased saturable absorber 15 and a
semiconductor optical amplifier (SOA) 23, both with no grating written
onto the active medium, are added on the same chip. By doing so, all the
sections are naturally aligned and the polarization state is maintained
through the device. Means for current injection into the absorber 15 and
amplifier 23 sections are formed similar to that of the laser excitation
means described above and include current confining ridges 25 and 29
respectively, with corresponding contact electrodes 31 and 33 being
defined on each ridge as shown in FIG. 2. As illustrated in FIG. 2, the
beat signal 4 is received by the saturable absorber 15 which provides a
partial initial compression of pulse duration and forms an initial optical
pulse train 6 which is amplified by SOA 23. Then the initial optical pulse
train 6 is further compressed by an optical compressor 21 including
dispersion fiber in such a way that the initial amplified train is
transformed into a train of short optical pulses 9 having a pre-determined
duration and a repetition rate. The resulting short optical pulse train 9
is further data encoded with a high speed external optical modulator (not
shown), which operates at a speed of the frequency spacing determined by
the wavelength spacing in the lasers 11 and 13. Optionally, light
generated by one or two lasers is modulated at a frequency which is
subharmonic to the beat frequency, and/or frequencies of one or both
lasers are stabilized by a wavelength locking device.
The beat signal 4 may be sent directly to the optical compressor 21, where
the initial pulse is amplified, chirped and then compressed by the
nonlinear Kerr effect. Therefore an internal (e.g. SOA 23 ) or additional
optical amplifier is required to amplify the signal before it is
compressed. The saturable absorber 15 is used to provide an initial pulse
compression and thus to reduce requirements to the optical compressor,
e.g. to use shorter lengths of dispersion shifted fiber or dispersion
decreasing fiber.
The structure of the lasers 11 and 13 forming the source 10 is shown in
more detail in FIG. 3, which illustrates an oblique cross-sectional view
through the laser part of the source structure 10, and FIG. 4 which shows
a perspective view of the source 10. The DFB semiconductor laser source 10
is fabricated from Group III-V semiconductor materials, and comprises a
heavily N-doped InP substrate 12, on which an N-doped InP buffer layer 34
of 1.5 .mu.m thickness is defined. The first separate confinement region
35, consisting of four confinement layers 36, 38, 40 and 42 of N-doped
InGaAsP with energy band gaps corresponding to wavelengths of 1.0 .mu.m,
1.1 .mu.m, 1.15 .mu.m and 1.20 .mu.m respectively, is provided over the
buffer layer 34. The thickness of each confinement layer is 20 nm, and the
confinement layer 36 corresponding to the 1.0 .mu.m wavelength is adjacent
to the buffer layer 34. The active region 14 overlies the confinement
region 35 and comprises a multiple quantum well (MQW) structure 16 which
includes four to eight 1% compressively strained N-doped, or undoped
InGaAsP quantum wells 44, each being 5 nm thick, separated by several
undoped or P-doped InGaAsP unstrained barriers 46 with a band gap
corresponding to wavelength of 1.20 .mu.m, each barrier being 10 nm thick.
The alloy composition and layer thickness of the MQW structure 16 are
tailored to have specific band gap energies to provide for lasing at a
required wavelength. Increasing the number of quantum wells provides
higher gain per unit length of the laser cavity. The band gap of the
quantum well structure described above provides a lasing wavelength of the
device at about 1.55 .mu.m. A second separate confinement region 47,
consisting of two P-doped InGaAsP confinement layers 48 and 50, having
energy band gaps corresponding to 1.1 .mu.m and 1.20 .mu.m wavelengths
respectively, is grown on top of the MQW active region 14 , each layer
being 20 nm thick.
As mentioned above, gratings 17 and 19 are defined by periodically etched
grooves through the active medium 14 . The pitch of the groove of each
grating is selected so as to define a first order grating for the lasing
Bragg wavelength. Grating 17 has a period comprising a first section 66
and a second section 68 as shown in FIGS. 2 and 4 . Grating 19 has
corresponding first and second sections 70 and 72 . Second sections 68 and
72 in the respective gratings 17 and 19 are V-shaped and characterized by
substantial etching away almost all quantum wells, namely, seven out of
eight quantum wells in this embodiment. The more quantum wells are etched
away from the section the less is the photon generation in the section.
Thus, a deep etching through the second sections 68 and 72 provides no
substantial photon emission in these sections. Usually deep etching is
avoided for a single laser because of the accompanied strong index
coupling. For the series of DFB lasers, deep etching provides
substantially independent generation of each laser in the series and no
substantial interaction between lasers as will be described below.
A P-doped InP layer 52, fills the grooves of the gratings. A 3 nm thick
etch stop layer 54 of P-doped InGaAsP, surrounded by P-doped InP buffer
layer 56 at the bottom and P-doped InP buffer layer 58 at the top is
formed next, the buffer layers being 100 nm and 200 nm thick
correspondingly. An upper cladding layer 60 of P-type InP, followed by a
highly doped P-type capping layer 62 of InGaAs for contact enhancement,
having thickness 1600 nm and 200 nm correspondingly, complete the
structure. The separation between the adjacent electrodes 30 and 32 is in
the range of 5 to 15 .mu.m to ensure both a sufficient electrical
isolation between the adjacent electrodes and a limited material
absorption loss. A bottom electrical N-contact 26 is provided at the
bottom of the substrate 12. Corresponding means (not shown) for
simultaneous two-wavelength generation of the lasers 11 and 13, means for
controllably varying current injection and changing temperature of each
laser for switching between lasing modes, preferably within a time
interval of several nanoseconds, and tuning of laser wavelength of each
laser around a corresponding lasing mode are provided.
Thus, a source 10 of short optical pulses and micrometer/millimeter
wavelength radiation is provided.
While the source described above is fabricated on a N-type substrate wafer,
alternatively, a complimentary structure may be fabricated on a P-type
wafer.
The substrate 12 on which the source 10 described above is fabricated is
made of InP material which results in generating light by lasers 11 and 13
within a range of 1.3-1.56 .mu.m, corresponding to a transparency window
of this material. In modifications of this embodiment, the substrate may
be made of GaAs material, having a window of transparency in a shorter
wavelength range of 0.8-0.9 .mu.m, which results in generating light in
this wavelength range.
In modifications of this embodiment, a source 10 may comprise a first 11
and a second 13 strongly loss coupled DFB lasers, each laser comprising a
loss coupling grating 17 and 19 correspondingly, and Bragg modes of the
lasers being at different sides of the stopband. Deep etching through the
quantum wells of the active regions provides strong loss coupling and
independent generation of each laser. Other arrangements of laser
wavelengths, providing simultaneous operation of two lasers and ensuring
generation of the beat signal, are also possible.
A height and a shape of the first and second sections of gratings periods
may also vary to define photon emission in the sections, which, for
example, have rectangular or trapezoidal shape. The grating may be formed
by either direct electron beam writing, phase mask printing, or double
exposures.
An optical amplifier 23 which is built on the same chip as the other
sections of the optical pulse source, may be substituted with an
additional amplifier, e.g. an external EDFA. Alternatively, both types of
amplifiers, may be used simultaneously.
The optical compressor 21 may include a dispersion decreasing fiber as
described above, or alternatively, a combination of dispersion shifted
fiber and normal single mode fibers.
Principles of operation, demonstrated on the source of DFB lasers 10 of the
first embodiment of the invention, are as follows.
It is known that complex coupled lasers provide an additional advantage
over index coupled and quarter-wavelength shifted DFB lasers in
suppressing one of the two originally degenerated Bragg modes. Both theory
and experiment have confirmed that in-phase gain coupled DFB laser will
predominantly lase on a longer wavelength side of the stop band (right
Bragg mode), while the anti-phase loss coupled DFB laser will
predominantly lase on a shorter wavelength side of the stop band (left
Bragg mode).
For a gain coupled laser 11 with a grating 17 formed by direct etching
through the active region, the second section of the grating period 68
where a portion of quantum wells is etched away, has smaller effective
refractive index than the first section of the grating period 66 where the
quantum wells are not etched at all. From a standing wave point of view,
the first section 66 having higher refractive index, will support a photon
emission at the longer wavelength, while the second section 68 having
smaller refractive index, will support a photon emission at the shorter
wavelength. Since there are more quantum wells in the first section 66
with a higher refractive index section, the emission at the longer
wavelength will dominate. Nevertheless, if only a small portion of the
quantum wells is etched away from the second section 68 of the grating
period, a noticeable photon emission will be still generated in this
section. In this case, there will be a chance that laser 11 will generate
at the short wavelength (left Bragg mode) as a dominant mode on certain
occasions, when a combination of external facet phases or external
feedback phases is in favour to the short wavelength, the situation being
typical for index coupled lasers. In order to eliminate such an
unpredictable combination of phases, in the laser of the embodiment
substantially all quantum wells from the second section 68 of the grating
period are removed, to ensure that no substantial emission is originally
generated in this section. This results in the photon emission in the
first section 66 predominantly, and hence, in lasing at the longer
wavelength side of the stop band (right Bragg mode) only. The lasing mode
of such a laser is therefore determined by an internal built-in and
distributed mode selection means, for example, by the grating defined by
deep etching rather than by the external facet phase and coating
asymmetry. When arranged in a series, each of such lasers, being phase
insensitive and providing stable single mode operation under almost all
phase combinations, ensures almost independent operation of each laser and
no substantial interaction between the adjacent lasers. We have called
such a laser a "strongly gain coupled DFB laser" and utilized it as a
building block for the source of short optical pulses and microwave
radiation 10.
Thus, the lasers 11 and 13 generate a single wavelength light
simultaneously, each at its own side of the stopband, and thus produce the
beat signal 4 in the microwave to millimeter wave range. As briefly
described above, the beat signal 4 is sent to the saturable absorber 15
followed by SOA 23. The length of the absorber 15 is short enough
(typically in the range from several tens of micrometers) in order to set
a necessary threshold for optical power level. Because of to the
difficulty of cleaving it may be technically difficult to create a short
section with the precise length at the end of the device. Thus is
partially compensated by forming the optical amplifier section 23. The
signal after SOA 23 is additionally amplified by an EDFA to 23 dBm and
passed through 4.4 km of dispersion shifted fiber followed by 1 km normal
single mode fiber (not shown). The reason for using dispersion shifted
fiber is to generate nonlinear phase modulation. The normal single mode
fiber is used to provide an adequate amount of chromatic dispersion to
compress the pulse. Alternatively, a 1 to 2 km of dispersion decreasing
fiber can be used for pulse compression.
Lengths of different sections are selected so that to make each of the two
DFB lasers reasonably long, typically in the range of 150 .mu.m to 500
.mu.m depending on coupling grating strength. It ensures that each laser
can be viewed as an independent DFB laser with a sufficient immunity to
possible external optical feedback and random facet phase variation. The
kL value for each laser is preferred to be within 2 to 8 but not limited
to this range.
In order to ensure long term stability of one of the lasing wavelengths,
e.g. the wavelength generated by the laser 13, a compact and external
wavelength locking device 76 is placed in the close proximity of a rear
facet 28 of the laser 13 to be locked. A wavelength locking device using
Fabry-Perot etalon and described in U.S. Pat. No. 5,825,792 to Villeneuve
(which is incorporated herein by reference) is used. Other known
wavelength locking techniques are also applicable.
To ensure reliable and accurate wavelength locking, a different low
frequency dithering current, other than the subharmonic modulation signal
is applied to the laser 13 on top of the CW bias current in order to
dither the output wavelength of the laser. As a result the external
wavelength locking device distinguishes the lasing wavelength from the
laser 13 and then locks it to greater accuracy without being disturbed by
wavelength fluctuations produced by the other laser. In order to further
lock two wavelengths with a stable channel spacing and to reduce phase
noise and linewidth substantially, a sub-harmonic modulation current i(t)
shown in FIG. 2 is applied to the electrode 30 at a frequency f.sub.m
=(f.sub.2 -f.sub.1)/N. Here f.sub.m is a frequency of the modulation
current i(t), f.sub.1 and f.sub.2 are frequencies of the two lasing modes
whose difference is assume to be already tuned to the required beat
frequency spacing, and N is an integer.
FIG. 5 shows a dual wavelength operation gain coupled DFB lasers 11 and 13
with uniform gratings 17 and 19 across the active medium 14 according to
the first embodiment of the invention. The dual wavelength operation shown
in FIG. 5 exhibits a beat frequency of about 538 GHz. It is further
illustrated by FIG. 6, where an autocorrelation trace of the beat signal
of FIG. 5 is recorded through the second harmonic generation device (an
optical autocorrelator). As shown in FIG. 6, a resulting CW signal with a
period between maxima/minima roughly equal to 1.9 ps is obtained which
corresponds to the beat frequency of about 538 GHz. Correspondingly, the
laser structure including lasers 11 and 13 can be used as a simple and
flexible high frequency (microwave to millimeter wave) generator. A
subharmonic injection locking is required to further reduce the linewidth
of the high frequency signal and to stabilize the frequency. After passing
through the SOA 23, the signal is additionally amplified by an EDFA to 23
dBm and sent through 4.4 km of dispersion shifted fiber followed by 1 km
normal single mode fiber (not shown). The resulted compression forms short
pulses having a duration of about 1.2 picosecond.
A fine tuning of laser frequencies by current injection and/or temperature
variation providing a continuously varying repetition rate for the pulses
in the optical train is also provided.
It is worth noting that the dual-mode operation of the lasers mentioned
above is fundamentally different from the conventional multi-mode lasers,
where modes are competing between each other which is commonly referred to
as mode partitioning. In order to verify that there is no competition
between the two modes in the lasers, a tunable optical filter was used to
select only one mode and sent it into a photodiode (DC--6 GHz). Using an
oscilloscope, the optical power fluctuation, if there was any, of the
selected mode was observed. The result of the measurement shows that power
fluctuations are less than 3%, which might be induced by current or
temperature fluctuations. It proves that mode competition does not exist
in the laser structure used in the source 10.
A source 100 of short optical pulses according to a second embodiment of
the invention is similar to that of the first embodiment described above
except for the lasers 11 and 13 generating at the same side of the
stopband and having laser cavities of approximately the same length. As
schematically shown in FIG. 7, it includes a first laser 111 generating at
a first wavelength .lambda..sub.1, and a second laser 113 generating at a
second wavelength .lambda..sub.2 followed by a saturable absorber 115, SOA
123 and an optional optical compressor 121 formed on the same chip. The
wavelength difference between two lasers is provided by different current
injections into the lasers trough corresponding electrodes 130 and 132. It
was chosen that each laser generated at the right Bragg mode (the longer
wavelength side of the stopband), and the frequency spacing between lasers
was fine tuned by changing injection currents. The source 100 of the
second embodiment is especially suitable for a relatively low frequency
generation in the range of several tens of GHz, where data encoding can
still be handled by existing electro-optic modulators. In practice, there
is no limitation for a maximum wavelength separation. However, a minimum
wavelength separation is limited by the effect of an optical injection
locking as discussed, e.g. in publication by R. Hui, A. D'Ottavi, A.
Mecozzi and P. Spano, "Injection locking in distributed-feedback
semiconductor lasers." IEEE J. Quantum Electron., Vol.QE-27, pp. 1688,
1991. In order to ensure stable dual wavelength operation and keep away
from injection locking between the two wavelengths, the lower limitation
of wavelength separation for this embodiment is typically in the order of
20 GHz.
A continuously tunable optical spectrum of the laser structure comprising
lasers 111 and 113 of the source 100 is demonstrated in FIG. 8. Since the
wavelength spacing can be tuned from 25 GHz to 80 GHz continuously by
current injection alone, a flexibility of generating a CW microwave signal
with a tunable frequency can be easily obtained. FIG. 9 illustrates a dual
wavelength operation of lasers 111 and 113 of the source 100 resulting in
generation of microwave radiation with at 50 GHz. It is also demonstrated
by an optical autocorrelation trace of the 50 GHZ beat signal shown in
FIG. 10 which corresponds to a period between peaks of about 20 ps. The
two small peaks at each side of the main modes are four-wave mixing side
bands. They are created by population pulsation at the beat frequency
between the two major stable modes.
FIG. 11 shows an optical spectrum after the beat signal has traveled
through the optical compressor including two fiber sections. It is seen
that dual wavelength light shown in FIG. 9 becomes a comb of wavelengths
through nonlinear phase modulation process of the nonlinear optical fiber.
The corresponding autocorrelation trace (time domain waveform) of the
optical train after pulse compression shown in FIG. 12 is measured by an
autocorrelator based on second harmonic generation. In this case, the
pulse repetition rate is about 50 GHz (about 20 ps separation between
pulses), and the pulse duration is approximately 5 ps (FWHM). As
illustrated by FIG. 8, the pulse repetition rate can be varied
continuously by adjusting the injection current of two lasers. By way of
example, time domain waveforms for pulse repetition rates of 25 GHz and 70
GHz are shown in FIGS. 13 and 14 respectively. Narrower optical pulses can
be obtained by further optimization of the optical compressor parameters,
e.g. by selecting precise fiber lengths and using dispersion decreasing
fibers.
In modifications to this embodiment the difference between laser
frequencies may be provided by difference in temperature control of the
lasers. Alternatively, it may be done by fabricating the active medium
which has a different width for the first and second lasers, thus
providing an effective difference in grating periods. For example, a
different width of the laser stripe in ridge waveguide will change an
effective modal index and as result will change an effective grating
period. Other known techniques providing an effective difference in
grating periods are also applicable.
A source 200 of short optical pulses according to a third embodiment of the
invention is schematically shown in FIG. 15. This source 200 is similar to
that shown in FIG. 7, and like elements are referred to by the same
reference numeral incremented by 100. For example, a first laser 211 and a
second laser 213 provide generation at two different wavelengths, the beat
signal from which is sent to a saturable absorber 215 and SOA 223 followed
by an optical compressor 221. The source 200 of the second embodiment
differs from that of the second embodiment in that, instead of
gain-coupled gratings 117 and 119 having same periods as shown in FIG. 7,
the gratings 217 and 219 have different grating periods. A center Bragg
wavelength separation of the gratings 217 and 219 is equal to 4.8 nm, the
first grating 217 having a shorter Bragg wavelength. To provide a path to
the output facet 227 for light generated by lasers 211 and 213, it is
necessary to satisfy certain requirements on laser wavelengths and pumping
conditions. It is known that when an active region is pumped just above a
transparency level (and below a threshold level), a DFB laser becomes
transparent for a light passing through if a wavelength of the passing
light is outside of the stop band. The same laser becomes lossy and not
transparent for the light passing through if the wavelength of the passing
light is within the stop band, regardless of the fact that the laser is
pumped above the transparency level. Moreover, when the wavelength of the
passing light is far enough from the stop band of the laser, it passes
through the laser without substantial interaction even if the laser is
pumped above a threshold level and generates a stable lasing mode by
itself. Thus, it is arranged that each laser in the source 200 generates
at the same side of its stop band, namely at the right Bragg mode, and the
Bragg modes of the lasers are arranged in such a way that the lasing
wavelength (including intended current and temperature tuning) of the
laser 211, which is closer to the output facet 227 of the source, does not
fall within a stop band of the adjacent laser 213, which is further away
from the output facet 227. It ensures that light generated by more distant
laser 213 will pass through the laser 211 which is closer to the series
output facet 227. Lasers 211 and 213 have a common active medium 214 and
shared optical path to the output facet 227 and provide mutual injection
of light into each other.
FIG. 16 shows a dual wavelength operation of the laser structure of the
source 200. There are clearly seen two stable Bragg modes separated by 4.8
nm from each other in the optical spectrum.
In modifications to this embodiment, the source 200 may comprise a first
211 and a second 213 strongly loss coupled DFB lasers, each laser
comprising a loss coupling grating 217 and 219 correspondingly. The
gratings will then have periods, defining corresponding laser stop bands
and a center Bragg wavelength separation, the first grating 17 having
longer Bragg wavelength. It is also arranged that each laser generates at
the left Bragg mode around its stop band, and the lasing wavelength of the
laser 211, which is closer to the output facet 227 of the series, does not
fall within the stop band of the adjacent laser 213, which is further away
from the output facet 227. Other arrangements of laser wavelengths
providing stable dual wavelength operation of the laser structure of the
source 200 are also applicable.
The wavelength spacing between the two modes in this embodiment is
determined by a pre-set difference in effective Bragg wavelengths of the
two lasers as shown in FIG. 15. When a large wavelength separation is
required, e.g. in the case of high repetition rates, a pre-defined
wavelength spacing can be set by using gratings with substantially
different periods formed through either two different holographic
exposures or an electron beam writing in such a way that the two gratings
are joined together along the cavity direction. The frequency separation
between the two modes, in general, can be designed to meet a large range
in the order of several tens of GHz to several thousand GHz. According to
the design, the separation can also be fine tuned by current injection
and/or temperature variation depending on the system requirement.
In yet other modifications to this embodiment, gratings associated with
particular lasers in the source may be either uniform or chirped gratings,
and periods of the gratings may vary to provide a predetermined center
Bragg wavelength separation (usually within a range of several nanometers
to several tens of nanometers) to ensure continuous laser tuning within a
certain wavelength range.
The optical pulse sources of the embodiments described above comprise
semiconductor diode lasers, i.e. lasers having contacts for electrical
excitation of the active region by current injection. It is also
contemplated that a source 300 of a fourth embodiment shown in FIG. 17 may
be provided with optical pumping means 330, 332, 331, 333, replacing
corresponding electrical contacts 30, 32, 31 and 33 of the first
embodiment (or corresponding electrical contact of the second or third
embodiments), e.g. by providing population inversion with suitable optical
coupling to another light source on the substrate. The structure and
operation of the source 300 is similar to that shown in FIG. 7, and
similar elements are referred to by the same reference numeral incremented
by 200 respectively.
In modifications to this embodiment, the source 300 may comprise a
combination of electrical and optical pumping means, e.g. the lasers 311
and 313 may be pumped optically while the absorber 331 and SOA 323 are
pumped electrically by corresponding current injections. Various
modifications described above with regards to other embodiments are also
applicable to the fourth embodiment, e.g. the source 300 may include
gratings having the same periods and provide generation of two wavelengths
at different sides of the stopband, or alternatively, it may include
gratings having different periods and providing a pre-set wavelength
separation. Optionally it may further include a wavelength locking device
for stabilizing one or two of generated wavelengths and/or sub-harmonic
modulation of one or two lasers.
It is also contemplated that a source of alternative embodiments may
comprise buried heterostructure lasers in contrast to the ridge waveguide
lasers described in the above embodiments.
The source of optical pulses and microwave radiation described above have
advantages over other similar structures which can be summarized as
follows.
The source has a compact design due to manufacturing all the components on
the same chip. It is capable of generating high frequency microwave and
millimeter range radiation and short optical pulses of sub-picosecond to
picosecond range with high and yet tunable repetition rate without the use
of corresponding high speed electronics. Since wavelength separation of
the lasers can be varied within a large range, the optical system,
employing such a short pulse generation and data encoding scheme, becomes
flexible and versatile. It can also be made easily upgradable to a higher
bit-rate system without the need to modify the system architecture. Since
the wavelength separation of the dual wavelength lasers is determined by
the DFB grating design, there is no practical limitation for the maximum
wavelength separation and thus high optical pulse repetition up to
tera-hertz can be achieved.
Fabrication
Fabrication of the source of short optical pulses 10 according to the first
embodiment shown in FIG. 2 proceeds in four stages as follows:
1. first epitaxial growth of substrate and multiple quantum well structure;
2. patterning of the grating structure;
3. second epitaxial growth of the overlying layers;
4. completion of the laser fabricating (e.g. ridge formation, contacts).
The prepared substrate 12 is loaded promptly into a commercially available
CVD growth chamber, and a buffer layer 34 of InP followed by the first
confinement region 35, including four layers of InGaAsP, is grown. The
active medium 14, comprising eight 1% compressively strained P-doped
InGaAsP quantum wells 44, separated by seven P-doped InGaAsP unstrained
barriers 46, is grown next.
The wafer is then removed from the growth chamber and processed so as to
form photo-lithographic gratings 17 and 19 by periodically etched grooves
through the active medium 14. First, a dielectric such a SiO.sub.2 (not
shown) is grown on the surface of the wafer, and the groove pattern is
created in the dielectric layer. The grooves are etched using reactive ion
etching or wet chemical etching process. The residual dielectric is then
removed. Using known crystal growth techniques, for example, a metal oxide
chemical vapor deposition, an InP layer 52 is grown in the grooves. Etch
stop layer 54 of InGaAsP grown between two buffer layers 56 and 58 of InP,
followed by cladding layer 60 of InP and capping layer 62 of InGaAs
complete the structure. Source fabrication is then completed using a
standard process. For example, to form rectangular ridge waveguides 22 and
24 perpendicular to the grooves of the gratings 17 and 19, a ridge mask is
provided on the substrate, and the ridges are formed by etching through
the capping layer 62 and top cladding layer 60, the ridges being 2 .mu.m
nominal width. The split top electrodes 30 and 32 are defined by the mask
used in the metalization step and created in the lift-off process. The
output facet 27 of the series is AR-coated (anti-reflection coated). The
back facet may be AR-coated, as-cleaved or HR-coated (high-reflection
coated). Alternatively, after the second regrowth, when a current
confining region is formed on the active region, a buried heterostructure
may also be grown. A phase mask generated by Electron Beam (EB)
lithography or a direct EB writing on wafer may be used as an alternative
to a holographic grating printing process for grating formation. The
saturable absorber 15 and SOA 23 are formed on the chip simultaneously
with the laser structure. The lasers 11 and 13, the absorber 15 and SOA 23
are integrated within same package.
Thus, it will be appreciated that, while specific embodiments of the
invention are described in detail above, numerous variations,
modifications and combinations of these embodiments fall within the scope
of the invention as defined in the following claims.
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