U.S. patent application number 09/774904 was filed with the patent office on 2002-08-01 for wavelength-tunable semiconductor laser diode.
This patent application is currently assigned to Nova Crystals, Inc.. Invention is credited to Hummel, Steven Gregg, Kuo, Chau-Hong, Lin, Chenting, Lo, Yu-Hwa, Shek-Stefan, Mei-Ling, Zaytsev, Sergey V..
Application Number | 20020101898 09/774904 |
Document ID | / |
Family ID | 25102638 |
Filed Date | 2002-08-01 |
United States Patent
Application |
20020101898 |
Kind Code |
A1 |
Lo, Yu-Hwa ; et al. |
August 1, 2002 |
Wavelength-tunable semiconductor laser diode
Abstract
A wavelength-tunable distributed feedback (DFB) laser is
disclosed where the lasing wavelength can be adjusted by adjusting
the bias current of the laser diode. Since the output power of the
laser diode also changes with the bias current, a one-to-one
correspondence between the lasing wavelength and the output power
of the laser can be established. Consequently, the lasing
wavelength can be measured directly from the photocurrent of a
power monitoring detector facing the back-end of the laser diode.
This provides an extremely simple method for wavelength
monitoring.
Inventors: |
Lo, Yu-Hwa; (San Diego,
CA) ; Hummel, Steven Gregg; (Los Altos, CA) ;
Lin, Chenting; (Poughkeepsi, NY) ; Kuo,
Chau-Hong; (Sunnyvale, CA) ; Shek-Stefan,
Mei-Ling; (Sunnyvale, CA) ; Zaytsev, Sergey V.;
(Cupertino, CA) |
Correspondence
Address: |
FISH & NEAVE
1251 AVENUE OF THE AMERICAS
50TH FLOOR
NEW YORK
NY
10020-1105
US
|
Assignee: |
Nova Crystals, Inc.
|
Family ID: |
25102638 |
Appl. No.: |
09/774904 |
Filed: |
January 31, 2001 |
Current U.S.
Class: |
372/46.01 |
Current CPC
Class: |
H01S 5/0622 20130101;
H01S 5/3415 20130101; H01S 5/3407 20130101; H01S 5/3416 20130101;
H01S 5/1032 20130101; B82Y 20/00 20130101 |
Class at
Publication: |
372/46 |
International
Class: |
H01S 005/00 |
Claims
We claim:
1. An electrically pumped semiconductor laser device having a
waveguide region, the waveguide region comprising an active layer
and a carrier reservoir, wherein electric carriers are injected
into the active layer by tunneling through a reversed biased p-n
junction disposed between the active layer and the carrier
reservoir to produce lasing radiation by recombination in the
active layer, and wherein a lasing wavelength of the laser device
is determined by a carrier concentration in the carrier reservoir,
with said carrier concentration depending on the pump current.
2. The laser device of claim 1, wherein the laser device is a
distributed feedback (DFB) laser.
3. The laser device of claim 1, wherein at least one of the active
layer and the carrier reservoir comprise a quantum well.
4. The laser device of claim 1, wherein a bandgap of the active
layer is smaller than a bandgap of the carrier reservoir.
5. The laser device of claim 1, wherein the waveguide comprises
InGaAlAs.
6. An electrically pumped wavelength-tunable semiconductor
distributed feedback (DFB) laser comprising: a first cladding layer
of a first conductivity type, a second cladding layer of a second
conductivity type, and an optical waveguide region disposed between
the first cladding layer and the second cladding layer, the optical
waveguide region comprising an active layer and a carrier
reservoir, wherein the active layer is electrically isolated from
the carrier reservoir by a reverse-biased p-n junction disposed
between the active layer and the carrier reservoir so as to retain
a concentration of electric carriers in the carrier reservoir
substantially independent of a laser drive current, with the
concentration of the electric carriers in the carrier reservoir
determining a refractive index of the optical waveguide region, and
a grating disposed proximate to the waveguide region and
determining in conjunction with the refractive index a lasing
wavelength, wherein the wavelength of the DFB laser can be
controllably tuned by adjusting a laser drive current.
7. The laser of claim 6, wherein the first and second cladding
layers comprise Inp and the waveguide region comprises at least one
of AlGaInAs and InGaAsP.
8. A wavelength-tunable laser system comprising: an electrically
pumped semiconductor distributed feedback (DFB) laser producing a
laser beam and including a first cladding layer of a first
conductivity type, a second cladding layer of a second conductivity
type, an optical waveguide region disposed between the first
cladding layer and the second cladding layer, the optical waveguide
region comprising an active layer and a carrier reservoir, wherein
the active layer is electrically isolated from the carrier
reservoir by a reverse-biased p-n junction disposed between the
active layer and the carrier reservoir so as to retain a
concentration of electric carriers in the carrier reservoir
substantially independent of a laser drive current, with the
concentration of the electric carriers in the carrier reservoir
determining a refractive index of the optical waveguide region, and
a grating disposed proximate to the waveguide region and
determining in conjunction with the refractive index a lasing
wavelength, an optical amplifier receiving the laser beam and
producing an amplified laser beam, a first detector that measures
an output power of the laser beam, wherein the lasing wavelength is
determined by the measured output power, and a second detector that
measures an output power of the amplified laser beam, with the
output power of the amplified laser beam capable of being adjusted
independent of the lasing wavelength.
9. The laser system of claim 8, further comprising a modulator
which modulates the amplified laser beam in response to a
modulation signal applied to the modulator.
10. A method of producing wavelength-tunable laser radiation from a
DFB laser structure using a single pump current, comprising:
providing in a waveguide region of the DFB laser structure a
carrier reservoir that is electrically isolated from an active
layer by a reverse-biased p-n junction, adjusting the pump current
to change an index of refraction of the waveguide region through a
change in a carrier concentration in the carrier reservoir and an
output power of the DFB laser structure, and determining the
wavelength of the laser radiation from the output power.
11. The method of claim 10, wherein the wavelength of the laser
radiation is approximately 1.5 .mu.m.
12. The method of claim 10, wherein the DFB laser structure is made
of a material selected from the group consisting of InP, InGaAsP,
and GaAlInAs.
13. The method of claim 10, further including amplifying the laser
radiation so that an optical power of the amplified laser radiation
can be selected independent of the wavelength.
14. The method of claim 10, further including modulating the laser
radiation so that an optical power of the modulated laser radiation
can be selected independent of the wavelength.
Description
FIELD OF THE INVENTION
[0001] This invention relates to the field of wavelength-tunable
semiconductor lasers, and in particular to controllably tuning the
lasing wavelength by controlling the optical output power of the
laser.
BACKGROUND OF THE INVENTION
[0002] Wavelength-tunable lasers have found important applications
in optical communication and sensing. Wavelength-tunable lasers
play a central role in particular for dense wavelength division
multiplexing (DWDM) systems that form the backbone of today's
optical communication network. The term "wavelength-tunable laser"
is typically applied to a laser diode whose wavelength can be
varied in a controlled manner while operating at a fixed heat sink
temperature. At the 1550 nm wavelength regime on which most DWDM
systems operate, a wavelength shift of 0.1 nm corresponds to a
frequency shift of about 12.6 GHz. At a given heat sink
temperature, the central wavelength of a conventional distributed
feedback (DFB) laser diode may be red-shifted by as much as 0.3 nm
or about 40 GHz due to the rise in the temperature of the junction
by Ohmic losses. In contrast, at a given heat sink temperature, the
wavelength of a tunable laser may vary by several nanometers,
corresponding to hundreds or even thousands of GHz, covering
several wavelength channels on the International Telecommunication
Union (ITU) grid. Depending on the physical mechanisms of
wavelength tuning, the lasing wavelength can be tuned in either
positive (red) or negative (blue) direction. Controlled wavelength
tunability offers many advantages over conventional fixed
wavelength DFB lasers for DWDM operation. It enables advanced
all-optical communication networks as opposed to today's network
where optics is mainly used for transmission and the network
intelligence is performed in the electronic domain. All-optical
networks can eliminate unnecessary E/O and O/E transitions and
electronic speed bottlenecks to potentially achieve very
significant performance and cost benefits. In addition, a less
extensive inventory of wavelength-tunable lasers than of laser with
a fixed wavelength is required. Keeping a large inventory of lasers
for each and every wavelength channel can become a major cost
issue. For advanced DWDM systems, the channel spacing can be as
narrow as 50 GHz (or about 0.4 nm in wavelength), with as many as
200 optical channels occupying a wavelength range of about 80 nm.
For the reasons stated above, wavelength-tunable lasers have
attracted considerable interest in optoelectronic device
research.
[0003] There exist different design principles for tunable lasers.
Almost all wavelength-tunable laser designs make use of either the
change of refractive indices of semiconductor or the change of
laser cavity length to achieve wavelength tuning. For the former,
common mechanisms for index change include thermal tuning, carrier
density tuning (a combination of plasma effect, band-filling
effect, and bandgap shrinkage effect), electro-optic tuning (linear
or quadratic effect), and electrorefractive tuning (Franz-Keldysh
or quantum confined Stark effect). For DFB lasers, the wavelength
of the laser light propagating in the waveguide is basically
determined by the grating period A. The free-space lasing
wavelength .lambda. is given by .lambda.=2 n.sub.eff .LAMBDA.,
where n.sub.eff is the effective index of refraction of the
waveguide and .LAMBDA. is the period for first-order gratings.
Accordingly, the change .LAMBDA..lambda. in the lasing wavelength
.lambda. is directly proportional to the change An of the index of
refraction n.sub.eff.
[0004] Referring to FIG. 1, a prior art three-section DBR tunable
laser 100 includes an optical gain section 101, a phase control
section 102, and a tunable DBR section 103. A first current source
104 pumps the gain section 102 to generate optical gain; a second
current source 105 injects carriers to adjust the phase condition
of the phase control section 102 so that the resonant frequency
matches approximately the peak of the DBR reflectivity; and a third
current source 106 controls the reflectivity peak by changing the
effective index n.sub.eff of the Bragg waveguide section 103. With
proper selection of the currents in the DBR region 103 and in the
phase control region 102, quasi-continuous wavelength tuning can be
achieved. All three sections 101, 102, 103 are optically connected
to minimize residue reflections and coupling loss; however, the
sections 101, 102, 103 have to be electrically isolated from one
another, for example, by layers 107 disposed between the respective
sections 101, 102, 103. Three currents, responsible for the gain
region, DBR region, and phase control region, have to be supplied;
and the lasing wavelength depends on all three currents and is
particularly sensitive to the currents in the DBR and phase control
region. A continuous wavelength tuning range of about 10 nm can be
achieved using this design.
[0005] Modifications of the three-section DBR lasers include
sampled grated four-section DBR lasers and vernier-tuning sampled
grating DBR lasers (not shown). The last device requires four
separately controlled current sources to achieve the full tuning
range (about 80 nm quasi-continuous tuning).
[0006] Alternatively, the lasing wavelength can also be changed by
changing the physical length of laser cavity in the surface normal
direction. This mechanism has been applied, for example, to
vertical-cavity surface-emitting lasers (VCSELs) where typically
due to the short cavity length only one or at most very few lasing
modes fall within the gain peak. Referring to FIG. 2, a prior art
wavelength-tunable VCSEL structure 200 is based on surface
micromachining technology. The laser device 200 includes a bottom
dielectric DBR mirror 202, a top dielectric DBR mirror an
electrostatically controlled membrane 203, and an active region
204. Electrically pumped micro-electro-mechanically tuned VCSEL in
the 1550 nm wavelength regime have not yet been demonstrated.
However, the laser device 200 can be optically pumped by an
incoming pump beam 205 (e.g. a beam from a 980 nm wavelength pump
laser) through the bottom mirror 202, with the laser output 206
being emitted from the top mirror 201 disposed on the membrane 203.
Wavelength-tuning is obtained by changing the cavity length of the
VCSEL through the movement of the membrane 203. With a surface
micromachined tunable mirror, a continuous tuning range of 40 nm
has been demonstrated with an output power of up to 7 mW coupled to
a single mode fiber.
[0007] Multiple-section DFB lasers in general have a smaller tuning
range than multiple-section DBR lasers, except for the tunable
twin-guide (TTG) DFB lasers where relatively wide (about 6 nm) and
continuous tuning can be achieved.
[0008] In DWDM systems, the wavelength of the channel has to be
stabilized within a few gigahertz from the ITU grid, typically less
than 10% of the channel spacing. A change of the junction
temperature and/or device degradation can cause wavelength drift
beyond its acceptable range. Achieving wavelength stability
requires monitoring the wavelength in real time using a
sophisticated feedback mechanism. Several commercially available
devices and their operation for accurately monitoring the laser
emission wavelength are shown in FIGS. 3, 4, and 5. Common to these
devices is an optical interference device such as a Fabry-Perot
etalon placed between the laser and a photodetector. Critical for
the device performance are the mechanical stability and angular
precision of the etalon and the collimation of the laser beam
impinging on the etalon.
[0009] Referring now to FIG. 3, a wavelength-monitoring system 300
includes an optical beam splitter 301, a Fabry-Perot (F-P) etalon
302 connected to a first output of the beam splitter 301, a first
photodetector (PD) 303 following the F-P etalon 302, with a second
PD 304 connected to the second output of the beam splitter 301 as a
reference detector. Once the system is calibrated, the lasing
wavelength can be determined from the ratio of the photocurrents of
the PDs 303, 304, as illustrated in FIG. 4. In the illustrated
example showing exemplary target wavelengths .lambda..sub.n,
.lambda..sub.n+1, a rise in the ratio I.sub.1/I.sub.2 of the
measured photocurrents would indicate a decreasing lasing
wavelength, while a decrease in the ratio I.sub.1/I.sub.2 of the
measured photocurrents would indicate an increasing lasing
wavelength. Note that there exist multiple wavelengths
.lambda..sub.n, .lambda..sub.n+1, that can yield the correct
photocurrent ratio, and each wavelength may correspond to an ITU
wavelength channel. This design has problems with generating a
proper error signal when wavelength hopping occurs.
[0010] FIG. 5 shows a more detailed design of a commercial
wavelength monitoring system 400, excluding electronic circuitry. A
small fraction, typically a few percent, of the received laser
light is coupled into the monitoring system by an optical power
splitter 401. The beam is collimated in collimator 402 and then
split into two approximately equal signals by a beam splitter 403.
The photocurrent of PD 404 provides the reference signal
proportional to the power of the received laser light. The
photocurrent of PD 406 is related to the received power being
transmitted through the Fabry-Perot etalon 405. The ratio of these
two photocurrents does not depend on the output power of the
received laser light.
[0011] The manufacturing and operating complexity of the
wavelength-tunable lasers and the wavelength monitoring system
represent barriers for the production of low cost
wavelength-tunable laser modules for low-cost DWDM systems suitable
for metropolitan area networks (MAN). It would therefore be
desirable to provide a new design for a wavelength-tunable laser
where the lasing wavelength can be tuned by a single current source
and the lasing wavelength can be measured without requiring
interferometric devices.
SUMMARY OF THE INVENTION
[0012] According to one aspect of the invention, a
wavelength-tunable distributed feedback (DFB) laser structure is
disclosed where the lasing wavelength can be adjusted by adjusting
a single bias current of the laser diode. Since the output power of
the laser diode also increases with the bias current, one can
establish a straightforward, one-to-one correspondence between the
lasing wavelength and the output power of the laser. Consequently,
the lasing wavelength can be measured directly by a power
monitoring detector facing, for example, the back-end of the laser
diode.
[0013] To provide wavelength-tuning, the DFB laser structure
includes a second set of quantum wells or a waveguide layer next to
the lasing quantum wells as "carrier reservoir". The second set of
quantum wells or the waveguide layer has to meet several
requirements in order to function effectively as a carrier
reservoir without adversely affecting the laser performance. First
of all, the carrier reservoir has to have a higher bandgap than the
lasing quantum wells to minimize the optical loss. Secondly, a
carrier propagation barrier needs to be present between the lasing
quantum wells and the reservoir to avoid carriers falling into the
lasing quantum wells, which have the lowest bandgap of all
materials and the strongest tendency of attracting carriers.
Thirdly, the carrier reservoir has to be located in a region where
the intensity of optical field is significant so the carrier
induced index change can contribute to the change of lasing
wavelength of a DFB laser. Finally, the presence of the carrier
reservoir should not trigger high-order transverse modes. In other
words, the laser should operate in a single longitudinal mode and
single spatial mode. A structure meeting the above requirements
includes a reverse-biased tunnel junction made of heavily doped
p.sup.+- and n.sup.+-layers disposed between two sets of quantum
wells to prevent carrier leakage from the reservoir back to the
active quantum wells. Because of the carrier tunneling effect,
holes can tunnel through the n+/p+junction and reach the carrier
reservoir to meet the electrons. The carrier concentration in the
reservoir is then determined by the spontaneous emission rate and
Auger recombination rate. This follows approximately the empirical
equation I=BN.sup.2+CN.sup.3 where I is the current, N the carrier
concentration in the reservoir, and B and C the rates for
spontaneous and Auger recombination, respectively. Contributions
from defect-related recombination, which is linearly proportional
to the carrier concentration, are neglected.
[0014] Embodiments of the invention may include one or more of the
following features. The laser can be grown on an n-InP substrate
and have a p-InP as the upper cladding layer. A thin layer of
material having a higher refractive index than InP can be
introduced to form gratings for index-coupled DFB lasers. Two
unintentionally doped graded-index (GRIN) confinement regions can
be located on either side of the quantum well active layers to
provide carrier and optical confinement. Between the two GRIN
layers, the quantum wells forming the active layer and responsible
for lasing are positioned closer to the p-InP cladding layer and
additional quantum wells having a higher ground state energy than
the active layer and forming the carrier reservoir are located near
the n-InP lower cladding layer.
[0015] According to another aspect of the invention, the laser
output may be coupled to a semiconductor laser amplifier (SLA) to
allow an independent adjustment of the lasing wavelength and the
optical output power of the device. Optionally, the laser output
may be coupled to an optical modulator, such as an
electro-absorption modulator, to externally modulate the laser
light to reduce chirping. The modulator may be used with or without
the SLA. Detectors may be provided to measure an output power of
the laser beam and/or the amplified laser beam and/or the modulated
laser beam.
[0016] Further features and advantages of the present invention
will be apparent from the following description of preferred
embodiments and from the claims.
BRIEF DESCRIPTION OF THE DRAWINGS
[0017] The following figures depict certain illustrative
embodiments of the invention in which like reference numerals refer
to like elements. These depicted embodiments are to be understood
as illustrative of the invention and not as limiting in any
way.
[0018] FIG. 1 is a schematic diagram of a conventional 3-section
DBR tunable laser,
[0019] FIG. 2 is a schematic diagram of an optically pumped MEMS
VCSEL tunable laser,
[0020] FIG. 3 is a simplified block diagram of a conventional
wavelength monitor,
[0021] FIG. 4 shows schematically the operation of the wavelength
monitor of FIG. 3,
[0022] FIG. 5 shows the wavelength monitor of FIG. 3 in greater
detail,
[0023] FIG. 6 is a schematic diagram of a simplified epitaxial
layer structure for a wavelength-tunable distributed feedback (DFB)
laser of the invention,
[0024] FIG. 7 shows an energy band diagram of the laser structure
of FIG. 6 under an applied forward bias Va,
[0025] FIGS. 8a-c shows the laser characteristics for (a)
wavelength versus drive current, (b) optical output power versus
drive current, and (c) wavelength versus optical output power,
[0026] FIG. 9 shows a wavelength-tunable laser integrated with an
optical amplifier for simplified wavelength monitoring and an
optional external optical modulator.
DETAILED DESCRIPTION OF CERTAIN ILLUSTRATED EMBODIMENTS
[0027] One aspect of the present invention relates to a
semiconductor laser with a novel epitaxial layer structure, wherein
the laser can be wavelength-tuned by varying the pump power of the
laser using a single current source. Another aspect of the present
invention relates to a simplified arrangement for measuring the
lasing wavelength without employing interferometric devices.
[0028] A major challenge of using a single current source to tune
the wavelength is carrier pinning effect above threshold. In
semiconductor diode lasers having a conventional active layer
consisting of a single waveguide or single/multiple quantum wells,
the carrier concentration in the waveguide/quantum wells increases
with the injected current below the lasing threshold, with the
carrier concentration becoming essentially pinned at a fixed value
once the threshold current is reached. Beyond threshold, the laser
output power increases with the current linearly, but the carrier
concentration, hence the effective index of refraction, remains
more or less constant. Because of the carrier pinning effect, the
contribution of carrier-induced index change becomes negligible. A
solution provides by the present invention incorporates a second
set of quantum wells or a second waveguide layer next to the lasing
quantum wells as "carrier reservoir". The second set of quantum
wells or a second waveguide layer is electrically isolated from the
active quantum wells by a tunnel junction made of heavily doped
p.sup.+- and n.sup.+-layers. When one applies forward current to
the laser structure, the laser active junction is forward biased as
regular laser p/n junction and electron-hole recombination occurs
in the quantum wells. However, the n.sup.+/p.sup.+ tunnel junction
that separates two groups of quantum wells is reverse-biased.
[0029] FIG. 6 shows an exemplary epitaxial layer structure 600
according to the invention that provides the tunability. In all
other aspects, the device is similar to a conventional distributed
feedback (DFB) laser, with a grating similar to that shown for the
DBR section 103 of FIG. 1.
[0030] The layer structure 600 is fabricated on a semiconductor
substrate 601 and includes, in that order and starting from the
substrate 600, a lower cladding layer 602, a lower graded-index
(GRIN) layer 603, a carrier reservoir layer 604, a tunnel junction
605, an active layer 606, an upper GRIN layer 607, and an upper
cladding layer 608. Other typical laser device layers, such as
buffer layers and/or contact layers, are not shown. Also not shown
is the grating layer of the DFB laser structure 600 which can be
placed either below or above the active layer. The cladding layers
602, 608 and the GRIN layers 603, 607 have a higher bandgap and a
lower refractive index than the active layer 606 to assist in the
confinement of carriers and optical field in the active region. The
active layer 606 produces optical gain and roughly determines the
lasing wavelength within the gain profile. The gain profile is
typically about 100 nanometers wide. The active layer may be a
"bulk" semiconductor layer or one or more quantum wells.
[0031] Unlike a conventional epitaxial semiconductor laser device
structure which typically consists of layers 602, 603, 606, 607,
608 grown on substrate 601, the invented structure includes in
addition the carrier reservoir layer 604 and the tunnel junction
layer 605. The function of the latter two additional layers, the
carrier reservoir layer 604 and the tunnel junction layer 605, will
now be described.
[0032] Referring now to FIG. 7, from right to left is shown a
schematic energy band diagram 700 of the laser structure 600, with
the reference numerals referring to the respective layers of laser
structure 600. The quasi Fermi level 708 shows that the voltage
drop occurs at the laser active p-n junction and the carrier
reservoir p-n junction. These two junctions are stacked together
and separately by a tunnel junction having a negligible voltage
drop, as shown in the energy and diagram. The band diagram 700 can
apply to any material system known in the art for fabricating
semiconductor quantum-well heterojunction laser structures, such as
GaAlAs, InP/GaInAs, GaAlInN and the like. In an exemplary structure
for a 1550 nm wavelength laser emission, the following composition
and doping could be employed: substrate 601: n-InP; lower cladding
layer 602: n-InP; lower graded-index (GRIN) layer 603: n-InGaAlAs;
carrier reservoir layer 604: nominally undoped InGaAlAs MQW; tunnel
junction 605: n.sup.+/p.sup.+ InGaAsP or InGaAlAs having a bandgap
wavelength of around 1.4 .lambda.m; active layer 606: nominally
undoped InGaAlAs MQW; upper GRIN layer 607: p-InGaAlAs; and upper
cladding layer 608: p-InP. The InGaAsP and InGaAlAs layers are
preferably lattice-matched to InP and are understood to have the
composition In.sub.x(Ga.sub.yAl.sub.1-y).sub.1-xAs, wherein x
determines the lattice-match to the InP substrate and the bandgap
can be varied for a constant x by varying y. The effective bandgap
of the carrier reservoir 604 is selected to be lower than the
bandgap of the surrounding layers 603, 605 to form a "valley" to
collect carriers (both electrons and holes). However, the effective
bandgap of the carrier reservoir 604 is selected to be a slightly
higher than the active region to minimize the optical loss in the
laser waveguide 610 which is formed by all the layers 603, 604,
605, 606 and 607. The refractive index of the reservoir layer 604
decreases with the number of carriers in layer 604. This causes a
reduction .DELTA.n.sub.eff of the effective index n.sub.eff of the
laser waveguide 610, leading to a decrease .DELTA..lambda. in the
lasing wavelength .lambda. according to the relation
.DELTA..lambda.=2 .LAMBDA.*.lambda.n.sub.eff. .LAMBDA. is the
grating period of the DFB laser, as described above.
[0033] However, for the carrier reservoir 604 to function properly,
several conditions have to be fulfilled: (1) the carrier reservoir
604 has to retain a portion of the injected carriers, with the
portion being related to, for example, proportional to the total
number of injected carriers, i.e., the operating current of the
laser; (2) the carrier reservoir 604 should be electrically
isolated from the active region 606, while optically being a part
of the active region 606; and (3) the carrier reservoir itself
should not contribute to lasing, i.e., the gain peak of the
reservoir 604 should be outside the operating wavelength of the DFB
laser.
[0034] When a laser is operated above threshold, the quasi-Fermi
level and the carrier concentration in the active region 606 are
approximately "fixed". An increase in the injection current
converts more electron-hole pairs into photons to generate higher
optical power without significantly changing the carrier
concentration in the active region 606 or lasing wavelength, except
for band filling. If the carrier reservoir 604 is not electrically
separated from the active region 606, the carrier concentration in
the reservoir 604 will also be roughly fixed, independent of the
injection current. The carrier concentrations in both regions 604,
606 can be decoupled by introducing a tunnel junction between the
active region 606 and the carrier reservoir 604. In this case,
above threshold, the carrier concentration in the reservoir 604
increases with increasing current, while the carrier concentration
in the active region 606 remains fixed. Since, as mentioned above,
n.sub.eff decreases with increasing carrier concentration in the
reservoir 604, the lasing wavelength of a DFB laser can be tuned by
adjusting the carrier density in the reservoir 604, i.e., the laser
drive current. In other words, the lasing wavelength controllably
decreases with increasing injection current and hence also with
increasing optical power. Because a unique relation exists between
the output power and the lasing wavelength, the wavelength of the
laser can be measured and adjusted simply by measuring the output
power of the laser without requiring sophisticated wavelength
monitoring devices.
[0035] The epitaxial layer structure is fabricated in a
conventional manner, for example, by MOCVD or MBE, so that the gain
profile covers the intended operation wavelength range and the
layer and device structure favor operation in the fundamental
transverse mode.
[0036] The coupling coefficient .kappa. of the DFB grating on one
of the layers close to the active region is selected so that the
coupling is in the range of 30 to 300 cm.sup.-1 and the product of
.kappa.L is between 1 and 10, where L is the laser cavity
length.
[0037] For an operating wavelength of 1.5 .mu.m, the bandgap of the
carrier reservoir should be approximately 0.1 eV greater than the
bandgap of the active region. For this difference in bandgap, the
change in the refractive index with carrier concentration in the
reservoir is approximately dn/dN=-1.8.times.10.sup.20 cm.sup.-3,
and approximately dn/dN=-2.4.times.10.sup.2.degree. cm.sup.-3 when
the difference in bandgap is reduced to 0.05 eV. The approximate
wavelength tuning range is given by
.DELTA..lambda.=.eta.(dn/dN*.DELTA.N)*.lambda./n, wherein .eta. is
the confinement factor for the reservoir layer, .DELTA.N is the
increase in the carrier concentration variation with current above
the lasing threshold, and n is the effective index. Using typical
parameters of .eta.=0.2, dn/dN=-2.4.times.10.sup.20 cm.sup.-3,
.DELTA.N=3.times.10.sup.18 cm-3, n=3.3, and .lambda.=1550 nm, a
continuous wavelength tuning range .DELTA..lambda. of about 7 nm is
obtained. This is similar to the 6 nm tuning range of twin-guide
(TTG) DFB lasers also using carrier induced index change for
wavelength tuning.
[0038] The expected wavelength tuning range is eventually limited
by junction heating, carrier-induced optical losses, and carrier
recombination in the reservoir.
[0039] FIGS. 8a-c show schematically characteristic curves for
wavelength versus current (FIG. 8a), optical power versus current
(FIG. 8b), and wavelength versus optical power (FIG. 8c) of the
tunable DFB laser of the invention. As evident from FIG. 8c, the
wavelength can be tuned by adjusting a single current and
monitoring by the laser output power without requiring a
Fabry-Perot etalon. For DWDM systems of 50 GHz channel spacing, the
wavelength control has to be within .+-.5 GHz (or .+-.0.4 .ANG.).
If the tuning range of a laser diode is 7 nm, one needs to measure
the photocurrent to an accuracy of 0.4/70, which requires an 8 bit
resolution A/D converter. This requirement can be easily met with
low cost commercial A/D converters having 14-bit resolution. The
shot noise of the detector is not expected to be an issue either
since the wavelength monitoring detector operates at a very low
bandwidth.
[0040] According to another embodiment depicted in FIG. 9, the
wavelength of a laser system 900 can be adjusted independent of the
output power produced by the system 900. The system 900 includes
the wavelength-tunable DFB laser 901 of the type described above in
combination with an optical amplifier 902. The laser/amplifier
combination 900 may be, for example, a tunable laser monolithically
integrated with semiconductor optical amplifier or a hybrid
integration of the tunable laser with a fiber amplifier. The laser
system 900 further includes a back-end detector 903 for monitoring
the laser power, which in this case corresponds to the lasing
wavelength; focusing optics 904; a power splitter 905 receiving
light from the front end of the optical amplifier 902, with a
predetermined fraction of the received light split off and entering
an output power detector 906 to monitor the power coupled, for
example, into an optical fiber 907. Also shown in FIG. 9 is an
optional external modulator 908. Again, the wavelength of the laser
can be controlled by controlling only the laser drive current with
a single current source, while both the wavelength and the final
optical power into the fiber are monitored only by photodetectors
without interferometric components that are sensitive to
misalignment.
[0041] To modulate the light, the laser may be modulated either
directly by controlling the drive current or externally using an
external modulator. The preferred method of modulation depends on
applications. For directly detected (non-coherent) DWDM systems,
low chirping (wavelength/frequency variation with power) is
important to minimize dispersion penalty, so external modulation is
desirable. The external modulator 908 can be an electro-absorption
(EA) modulator or an interference-type electro-optic (EO)
modulator. The position of the modulator 908 and the optical
amplifier 902 can be interchanged although the arrangement shown in
FIG. 9 is more convenient from device fabrication and signal
isolation point of view. On the other hand, if coherent detection
systems such as homodyne and heterodyne systems are used, direct
modulation of tunable lasers might be preferred. The laser of the
invention has an optical FM efficiency that is approximately 100
times that of conventional DFB lasers (about 30 GHz/mA compared to
about 300 MHz/mA for conventional DFB lasers). This means that the
modulation current can be as much as 30 times smaller in an optical
frequency division multiplexing (OFDM) or optical
frequency-shift-keying (FSK) system, making the laser of the
invention more efficient than the conventional DFB laser.
[0042] While the invention has been disclosed in connection with
the preferred embodiments shown and described in detail, various
modifications and improvements thereon will become readily apparent
to those skilled in the art. Accordingly, the spirit and scope of
the present invention is to be limited only by the following
claims.
* * * * *