U.S. patent application number 10/814050 was filed with the patent office on 2006-02-09 for surface emitting laser with an integrated absorber.
Invention is credited to Ursula Keller, Rudiger Paschotta, Silke Schon, Heiko Unold, Ian A. Young.
Application Number | 20060029112 10/814050 |
Document ID | / |
Family ID | 34964379 |
Filed Date | 2006-02-09 |
United States Patent
Application |
20060029112 |
Kind Code |
A1 |
Young; Ian A. ; et
al. |
February 9, 2006 |
Surface emitting laser with an integrated absorber
Abstract
A surface emitting laser (SEL) with an integrated absorber. A
lower mirror and an output coupler define a laser cavity of the
SEL. A monolithic gain structure positioned in the laser cavity
includes a gain region and an absorber, wherein a saturation
fluence of the absorber is less than a saturation fluence of the
gain region.
Inventors: |
Young; Ian A.; (Portland,
OR) ; Keller; Ursula; (Uitikon-Waldegg, CH) ;
Unold; Heiko; (Daenikon-ZH, CH) ; Paschotta;
Rudiger; (Zurich, CH) ; Schon; Silke; (Zurich,
CH) |
Correspondence
Address: |
Anthony H. Azure;BLAKELY, SOKOLOFF, TAYLOR & ZAFMAN LLP
Seventh Floor
12400 Wilshire Boulevard
Los Angeles
CA
90025
US
|
Family ID: |
34964379 |
Appl. No.: |
10/814050 |
Filed: |
March 31, 2004 |
Current U.S.
Class: |
372/7 |
Current CPC
Class: |
G06F 1/105 20130101;
H01S 5/18308 20130101; H01S 3/0604 20130101; H01S 5/024 20130101;
H01S 5/041 20130101; H01S 5/0657 20130101; H01S 3/0627 20130101;
H01S 3/0092 20130101; H01S 3/094084 20130101; H01S 5/0609 20130101;
H01S 5/423 20130101; H01S 3/1118 20130101; H01S 5/141 20130101;
H01S 5/18302 20130101; H01S 3/0615 20130101; H01S 5/18341 20130101;
H01S 5/18388 20130101; H01S 3/1115 20130101; H01S 5/18358
20130101 |
Class at
Publication: |
372/007 |
International
Class: |
H01S 3/30 20060101
H01S003/30 |
Claims
1. An apparatus, comprising: a lower mirror and an output coupler
defining a laser cavity; a gain region in a monolithic gain
structure positioned in the laser cavity; and an absorber
integrated with the gain region in the monolithic gain structure,
wherein a saturation fluence of the absorber is less than a
saturation fluence of the gain region.
2. The apparatus of claim 1 wherein the apparatus is a vertical
cavity surface emitting laser (VCSEL).
3. The apparatus of claim 1 wherein the apparatus is a vertical
external cavity surface emitting laser (VECSEL).
4. The apparatus of claim 1 wherein the absorber is aligned with a
peak field intensity of a standing wave pattern generated during
excitation of the gain region.
5. The apparatus of claim 1, further comprising an intermediate
mirror positioned in the monolithic gain structure, the
intermediate mirror to align a peak field intensity of a standing
wave pattern generated during excitation of the gain region with
the absorber.
6. The apparatus of claim 1 wherein the absorber comprises a
quantum dot layer and the gain region comprises a quantum well
layer.
7. The apparatus of claim 1 wherein the absorber comprises a first
quantum well layer and the gain region comprises a second quantum
well layer.
8. The apparatus of claim 7 wherein the first quantum well layer
comprises Gallium Indium Nitride Arsenide (GaInNAs) and the second
quantum well layer comprises Indium Gallium Arsenide (InGaAs).
9. The apparatus of claim 1, further comprising a plurality of
electrical contacts electrically coupled to the absorber to receive
an electrical signal to adjust the saturation fluence of the
absorber.
10. The apparatus of claim 1 wherein the monolithic gain structure
comprises the lower mirror.
11. The apparatus of claim 10 wherein the monolithic gain structure
comprises the output coupler.
12. The apparatus of claim 1, further comprising a nonlinear
crystal optically coupled to the output coupler to change a
wavelength of a laser output emitted from the output coupler.
13. The apparatus of claim 1, further comprising a thermal lens
within the laser cavity.
14. The apparatus of claim 1, further comprising a heat sink
thermally coupled to the lower mirror.
15. The apparatus of claim 1, further comprising a second output
coupler positioned proximate to the lower mirror to define a second
laser cavity, the absorber and the gain region within the second
laser cavity, wherein the first laser cavity defines a first SEL
and the second laser cavity defines a second SEL.
16. The apparatus of claim 15 wherein the first SEL and the second
SEL are independently addressable.
17. A vertical cavity surface emitting laser (VCSEL), comprising: a
gain region positioned proximate to a lower mirror; an absorber
positioned proximate to the gain region, wherein a saturation
fluence of the absorber is less than a saturation fluence of the
gain region; and a spacer positioned proximate to the absorber, the
spacer including a microlens, wherein the lower mirror, the gain
region, the absorber, and the spacer are a monolithic structure
fabricated from a substrate.
18. The VCSEL of claim 17 wherein the absorber comprises at least
one quantum dot layer and the gain region comprises at least one
quantum well layer.
19. The VCSEL of claim 17 wherein the absorber comprises at least
one quantum well layer of Gallium Indium Nitride Arsenide
(GaInNAs).
20. The VCSEL of claim 17 wherein the absorber is aligned with a
peak field intensity of a standing wave pattern generated during
excitation of the gain region.
21. The VCSEL of claim 17, further comprising a first contact
coupled to the lower mirror and a second contact coupled to the
spacer, the first and second contacts to be used in electrical
pumping of the VCSEL.
22. A system, comprising: a surface emitting laser (SEL) array,
comprising: a first output coupler and a lower mirror defining a
first laser cavity of a first SEL; a second output coupler and the
lower mirror defining a second laser cavity of a second SEL; a gain
region positioned in the first and second laser cavities; and an
absorber positioned in the first and second laser cavities
integrated with the gain region, wherein a saturation fluence of
the absorber is less than a saturation fluence of the gain region;
and an optical fiber optically coupled to the SEL array to receive
a first passively mode locked laser output from the first output
coupler and to receive a second passively mode locked laser output
from the second output coupler.
23. The system of claim 22 wherein the lower mirror, the gain
region, the absorber, the first output coupler, and the second
output coupler are a monolithic structure fabricated from a
substrate.
24. The system of claim 22 wherein the first SEL and the second SEL
are independently addressable.
25. A computer system, comprising: a chipset; and a clock
operatively coupled to the chipset, the clock comprising: a lower
mirror and an output coupler defining a laser cavity, the output
coupler to emit a passively mode-locked laser output for generating
a clock signal; a gain region in a monolithic gain structure
positioned in the laser cavity; and an absorber in the monolithic
gain structure, wherein a saturation fluence of the absorber is
less than a saturation fluence of the gain region.
26. The computer system of claim 25 wherein the monolithic gain
structure comprises the lower mirror, the gain region, the
absorber, and the output coupler.
27. The computer system of claim 25 wherein the clock to output an
optical clocking signal.
28. An apparatus, comprising: a quantum dot semiconductor saturable
absorber mirror; an output coupler, the quantum dot saturable
mirror and the output coupler defining a laser cavity; and a laser
medium positioned within the laser cavity.
29. The apparatus of claim 28 wherein the quantum dot semiconductor
saturable absorber mirror is integrated with the laser medium.
30. The apparatus of claim 29 wherein the output coupler is a
curved reflector integrated with the laser medium.
Description
BACKGROUND
[0001] 1. Field of Invention
[0002] The field of invention relates generally to lasers and, more
specifically but not exclusively, relates to a surface emitting
laser with an integrated absorber.
[0003] 2. Background Information
[0004] Semiconductor lasers have a variety of applications
including communication systems and consumer electronics.
Generally, semiconductor lasers may be categorized as edge-emitting
lasers or surface emitting lasers (SELs). Edge-emitting lasers emit
radiation parallel to the semiconductor wafer surface while SELs
emit radiation perpendicular to the semiconductor wafer surface.
Excitation of the gain region of semiconductor lasers may be
through optical pumping or electrical pumping.
[0005] Two common types of SELs are vertical cavity surface
emitting laser (VCSEL) and vertical external cavity surface
emitting laser (VECSEL). Referring to FIG. 1A, a VCSEL 100 is
shown. A gain region 106 is sandwiched between mirror 104 and
mirror 108. Such mirrors include distributed Bragg reflector (DBR)
mirrors. Mirrors 104 and 108 define a laser cavity 112. Laser
output 110 is emitted from the mirror 108 perpendicular to the gain
region 106.
[0006] FIG. 1B shows a VECSEL 150. Mirror 158 is mounted externally
and positioned above gain region 156. Mirror 154 and 158 define a
laser cavity 162. Mirror 154 includes a DBR mirror. Laser output
160 is emitted from mirror 158.
[0007] Mode-locked lasers are used to generate narrow optical
pulses on a time scale of picoseconds or less. In general, mode
locking involves aligning the phases of longitudinal modes of the
laser resulting in a periodic train of short pulses in the laser
output. FIG. 1C shows a graph 165 of optical power versus time of a
mode-locked laser. The repetition rate of the laser output is based
on the period between pulses in graph 165. Mode locking may be
achieved through active mode-locking or passive mode-locking.
Active mode-locking uses frequency modulation or amplitude
modulation through externally controlled modulators. Passive
mode-locking is achieved through an absorber, which may include a
saturable absorber material. The saturable absorber material may be
fabricated from semiconductor material. The saturable absorber
material may be fixed to a mirror, which may include a DBR mirror,
to form a semiconductor saturable absorber mirror (SESAM).
[0008] In a passively mode-locked laser, the desired laser output
of short pulses is provided via the absorber. The effect of a
saturable absorber in a laser cavity is to favor parts of the
circulating radiation with higher intensity over those with lower
intensity. After many round-trips, this often leads to the
formation of a single short pulse circulating in the cavity. This
mechanism is called mode locking because in the frequency domain it
corresponds to the creation of a fixed phase relationship between
the longitudinal modes of the cavity. The circulating pulse in the
laser cavity generates one output pulse each time it hits the
output coupler. Thus, a regular pulse train is produced.
[0009] FIG. 1D shows a VECSEL 170 with a non-integrated absorber.
VECSEL 170 includes a gain region 174 layered on a mirror 172. An
output coupler 176 is positioned above the gain region 174. Mirror
172 and output coupler 176 define a laser cavity 178. An optical
pump 182 provides the pump energy for VECSEL 170. A semiconductor
saturable absorber mirror (SESAM) 184 provides passive mode-locking
of VECSEL 170 and is separate from the gain region 174.
[0010] Today's passively mode-locked lasers use gain region and
absorber materials that generally exhibit very similar saturation
properties, so that rather different mode areas on the gain medium
and the saturable absorber are required for mode locking. This is
currently not achievable in monolithic structures.
BRIEF DESCRIPTION OF THE DRAWINGS
[0011] Non-limiting and non-exhaustive embodiments of the present
invention are described with reference to the following figures,
wherein like reference numerals refer to like parts throughout the
various views unless otherwise specified.
[0012] FIG. 1A is a block diagram illustrating a prior art
VCSEL.
[0013] FIG. 1B is a block diagram illustrating a prior art
VECSEL.
[0014] FIG. 1C is a graph illustrating the output pulses of a prior
art mode-locked laser.
[0015] FIG. 1D is a block diagram illustrating a prior art VECSEL
with a non-integrated absorber.
[0016] FIG. 2A is a block diagram illustrating one embodiment of a
SEL with an integrated absorber in accordance with the teachings of
the present invention.
[0017] FIG. 2B is a block diagram illustrating one embodiment of a
SEL with an integrated absorber in accordance with the teachings of
the present invention.
[0018] FIG. 2C is a graph of absorber and gain layers relative to
field intensity of one embodiment of a SEL with an integrated
absorber in accordance with the teachings of the present
invention.
[0019] FIG. 2D is a graph of fluence of one embodiment of a SEL
with an integrated absorber in accordance with the teachings of the
present invention.
[0020] FIG. 2E is a block diagram illustrating one embodiment of a
layer of quantum dots of an integrated absorber in accordance with
the teachings of the present invention.
[0021] FIG. 2F is a block diagram illustrating one embodiment of a
SEL with an integrated absorber in accordance with the teachings of
the present invention.
[0022] FIG. 2G is a block diagram illustrating one embodiment of
quantum wells in accordance with the teachings of the present
invention.
[0023] FIG. 3 is a block diagram illustrating one embodiment of a
SEL with an integrated absorber in accordance with the teachings of
the present invention.
[0024] FIG. 4 is a block diagram illustrating one embodiment of a
SEL with an integrated absorber in accordance with the teachings of
the present invention.
[0025] FIG. 5 is a block diagram illustrating one embodiment of a
SEL with an integrated absorber in accordance with the teachings of
the present invention.
[0026] FIG. 6 is a block diagram illustrating one embodiment of a
SEL with an integrated absorber in accordance with the teachings of
the present invention.
[0027] FIG. 7 is a block diagram illustrating one embodiment of a
SEL with an integrated absorber in accordance with the teachings of
the present invention.
[0028] FIG. 8 is a block diagram illustrating one embodiment of a
SEL with an integrated absorber in accordance with the teachings of
the present invention.
[0029] FIG. 9A is a block diagram illustrating one embodiment of an
array of SELs with integrated absorbers in accordance with the
teachings of the present invention.
[0030] FIG. 9B is a perspective view diagram illustrating one
embodiment of an array of SELs with integrated absorbers in
accordance with the teachings of the present invention.
[0031] FIG. 10 is a block diagram illustrating one embodiment of a
communication system in accordance with the teachings of the
present invention.
[0032] FIG. 11 is a block diagram illustrating one embodiment of a
computer system in accordance with the teachings of the present
invention.
[0033] FIG. 12 is a block diagram illustrating one embodiment of a
solid-state laser with a quantum dot saturable absorber mirror in
accordance with the teachings of the present invention.
[0034] FIG. 13 is a block diagram illustrating one embodiment of a
solid-state laser with a quantum dot saturable absorber mirror in
accordance with the teachings of the present invention.
DETAILED DESCRIPTION
[0035] Embodiments of a surface emitting laser with an integrated
absorber are described herein. In the following description,
numerous specific details are set forth to provide a thorough
understanding of embodiments of the invention. One skilled in the
relevant art will recognize, however, that embodiments of the
invention can be practiced without one or more of the specific
details, or with other methods, components, materials, etc. In
other instances, well-known structures, materials, or operations
are not shown or described in detail to avoid obscuring aspects of
the invention.
[0036] Reference throughout this specification to "one embodiment"
or "an embodiment" means that a particular feature, structure, or
characteristic described in connection with the embodiment is
included in at least one embodiment of the present invention. Thus,
the appearances of the phrases "in one embodiment" or "in an
embodiment" in various places throughout this specification are not
necessarily all referring to the same embodiment. Furthermore, the
particular features, structures, or characteristics may be combined
in any suitable manner in one or more embodiments.
[0037] Embodiments of the present invention provide a SEL with an
absorber integrated with the gain region. Embodiments of the SEL
provide ultra-short pulses (tens of picoseconds or less) with high
repetition rates (tens to hundreds of Gigahertz), high optical
average output power (tens to hundreds of milliwatts when
electrically pumped or optically pumped), and good beam quality
(M.sup.2 below 2). In contrast to edge-emitting semiconductor
lasers, embodiments described herein allow a freely scalable mode
spot size for high power output in combination with the high beam
quality needed for mode-locking.
[0038] FIG. 2A illustrates one embodiment of a SEL 200. The
embodiment of FIG. 2A shows an optically pumped gain structure with
an integrated absorber where the absorber is placed below the gain
region and pump mirror. An absorber 206 is positioned on lower
mirror 204. Lower mirror 204 may include a semiconductor Bragg
stack. Lower mirror 204 is highly reflective (HR) as to the laser.
A pump mirror 208 is positioned on absorber 206. Pump mirror 208 is
highly reflective as to the pump and is partially reflective as to
the laser. Gain region 210 is positioned on pump mirror 208. An
anti-reflective (AR) layer 212 is positioned on the gain region
210. AR layer 212 is anti-reflective for the laser and the pump
energy. Lower mirror 204, absorber 206, pump mirror 208, gain
region 210, and AR layer 212 form a monolithic gain structure 224.
In one embodiment, monolithic gain structure 224 is fabricated from
a substrate in a single fabrication process (discussed further
below).
[0039] An output coupler 216 is positioned above the AR layer 212.
Output coupler 216 and the lower mirror 204 define a laser cavity
220. In one embodiment, output coupler 216 includes a curved output
mirror. In operation, an optical pump 214 is applied to SEL 200. A
passively mode-locked laser output 218 is emitted from the output
coupler 216.
[0040] Absorber 206 is integrated with the gain region 210.
Absorber 206 includes a semiconductor material that is compatible
with the fabrication process of lower mirror 204, pump mirror 208,
and gain region 210. The absorber is integrated with the gain
region in the same semiconductor wafer. To position the absorber
and the gain region in the same monolithic structure, the absorber
and the gain region should be operated with similar mode spot size.
To allow mode-locking with similar mode sizes in the gain region
210 and the absorber 206, the saturation fluence of the absorber
206 must be lower than the saturation fluence of the gain region
210. In other words, the gain region can handle much greater power
densities than the absorber before reaching saturation. It will be
understood that integration will generally result in very similar
mode sizes inside the gain region and the absorber, because the
gain region and the absorber are within the Rayleigh range of the
Gaussian laser mode. The laser mode is defined by the lower high
reflector (lower mirror 204 in FIG. 2) and the output coupler (216
in FIG. 2).
[0041] Fluence describes the light energy per area in a laser
cavity. As the wave passes through a medium, such as an absorber or
a gain region, some of the power of the wave is lost due to
absorption in the medium. In an absorption versus fluence curve
(for example, FIG. 2D), the absorption initially depends linearly
on the incident fluence. When the medium reaches saturation
fluence, the curve breaks from a linear form and begins to flatten
out.
[0042] In general, semiconductor lasers possess a small gain
saturation fluence. This is important for passive mode-locking at
high repetition rates especially in combination with high average
laser output powers. If the saturation energy of the gain material
is too high, Q-switching instabilities may occur, which are
difficult to suppress if a high repetition rate is required, and
particularly if high laser output power is desired at the same
time. With their small gain saturation energy, semiconductor lasers
are not limited by such Q-switching instabilities.
[0043] Repetition rates in excess of a few Gigahertz (GHz) require
very short laser cavities. When using separate devices for the gain
structure and the absorber, geometrical constraints may limit the
achievable repetition rate. This limitation becomes even more
severe when significantly different mode areas are required on the
gain structure and the absorber.
[0044] Embodiments described herein utilize a gain structure with
an integrated absorber. This configuration allows for easy
construction of very short linear laser cavities. No folding mirror
is needed. Using an integrated absorber effectively removes the
geometrical restraint on the pulse repetition rate.
[0045] Further, an integrated absorber with reduced saturation
fluence allows for higher mode-locked output power at high
repetition rates. If the mode size on the absorber has to be very
small to achieve sufficient saturation, the danger of thermally
damaging the absorber rises quickly with increasing power and
repetition rate. When the absorber is made from a different
material which exhibits a smaller saturation fluence, the mode size
can be kept large and thermal damage is avoided, allowing for
higher output power and repetition rate.
[0046] Moreover, integration of the absorber into the gain
structure may result in low phase noise. This may to lead to very
small timing jitter due to a compact and stable setup.
[0047] For high repetition rates, the recovery time of the absorber
medium is reduced by appropriate means known in the art. Such
methods include low-temperature growth or ion bombarding. This
introduces non-radiative recombination centers which allow fast
trapping and recombination of the carriers generated by
absorption.
[0048] Various embodiments to reduce the saturation fluence of the
integrated absorber are presented herein. In one embodiment, the
saturation fluence of absorber 206 may be reduced by adjusting the
standing wave field intensities of the gain region 210 and the
absorber 206 independently.
[0049] In one embodiment to adjust the standing wave field
intensities, the absorber and gain region layers are placed
appropriately in the standing wave pattern. Referring to FIG. 2B, a
standing wave pattern of the laser wavelength 226 is shown in
monolithic gain structure 224 of FIG. 2A. A graph 228 references
the position versus field intensity of wave 226. The physical
location of absorber 206 has been positioned in the monolithic gain
structure 224 so as to be aligned with peak field intensity
positions of wave 226. In an alternative embodiment, the lower
mirror 204 or the output coupler 216, or both, may be positioned to
change the form of wave 226 so that the peak field intensity is
aligned with the absorber 206.
[0050] In another embodiment to adjust the standing wave field
intensities, an intermediate mirror structure is used. The
intermediate mirror layer may contain a Bragg mirror with
reflectivities for laser and pump wavelengths. The reflectivity for
pump wavelength is chosen such that the amount of pump light in the
absorber section is appropriate. In the embodiment of FIG. 2A, pump
mirror 208 is such an intermediate mirror. The reflectivity for
laser wavelength is chosen such that a coupled cavity may be
obtained to adjust the field intensity in the absorber section
independently of that in the gain region. The field intensity ratio
between absorber and gain region is chosen such that the saturation
fluence ratio is appropriate.
[0051] In another embodiment, the saturation fluence of absorber
206 may be reduced by using quantum dots (QD) in the absorber 206,
while quantum wells (QW) are used in the gain region 210. The
absorber 206 may include one or more layers of quantum dots. The
gain region 210 may include one or more layers of quantum wells. In
embodiments having multiple layers, transparent spacer layers may
separate the layers of quantum dots or quantum wells. The
individual quantum dot or quantum well layers may be spaced
individually or in groups in different positions of the standing
wave pattern. In the embodiment of FIG. 2C, sections of the
absorber or gain region are shown. The arrows point to layers of
quantum dots (in the case of the absorber) or layers of quantum
wells (in the case of the gain region). The number and individual
positions of quantum dot and quantum well layers may be used to
adjust saturation fluence, modulation depth, and wavelength
dependence of gain and absorption.
[0052] Referring to FIG. 2D, a graph 230 of absorption versus
fluence is shown. Curve 231 shows the fluence of the gain region
having quantum wells (QW), while curve 232 shows the fluence of the
absorber having quantum dots (QD). As shown in graph 230, the
saturation fluence of the absorber (shown at 232a) is significantly
lower than the saturation fluence of the gain region (shown at
231a).
[0053] FIG. 2E shows an embodiment of a layer of quantum dots in an
absorber. Quantum dots 244 are positioned on substrate 242. The
quantum dots may be grown by Molecular-Beam Epitaxy (MBE) or
Metal-Organic Vapor Phase Epitaxy (MOVPE) in a self-assembled
manner. The material used to fabricate the quantum dots includes
indium arsenide (InAs), indium gallium arsenide (InGaAs), aluminum
gallium arsenide (AlGaAs), or the like. The material surrounding
the quantum dots may be gallium arsenide (GaAs), AlGaAs, or the
like, in order to influence transition energies in the quantum
dots. In one embodiment, the quantum dots have an approximately
10-50 nanometer (nm) base diameter and an approximately 2-10 nm
height. The density of the quantum dots may be used to influence
the amount of absorption, i.e. the modulation depth of the
saturable absorber section. The distribution of the quantum dot
sizes may be used to achieve a desired spectral width of the
absorption in the saturable absorber section.
[0054] In an embodiment of an absorber having multiple layers of
quantum dots, a transparent spacer layer is positioned on top of
the quantum dots 244. On top of the transparent spacer layer are
positioned further quantum dot and spacer layers.
[0055] In yet another embodiment, the saturation fluence of
absorber 206 may be reduced by using quantum wells in the absorber
206 such that the absorber's saturation fluence is below the
saturation fluence of quantum wells in the gain region 210. In one
embodiment, the absorber 206 includes one or more quantum well
layers of gallium indium nitride arsenide (GaInNAs) while the gain
region 210 includes one or more quantum well layers of indium
gallium arsenide (InGaAs). In embodiments having multiple layers of
quantum wells, transparent spacer layers may separate the layers of
quantum wells.
[0056] Referring to FIGS. 2F and 2G, an embodiment of a SEL having
a gain region with quantum wells and an absorber section with
quantum wells is shown. In FIG. 2F, gain region 210 includes three
quantum well (QW) layers 250 of InGaAs. Absorber 206 includes three
QW layers 252 of GaInNAs. A standing wave pattern of the laser
wavelength 254 is shown in monolithic gain structure 224. A graph
256 references the position versus field intensity of wave 254.
[0057] FIG. 2G illustrates the minimum energy states of electrons
residing within the conduction bands of the QW layers in FIG. 2F.
Energy levels at QWs 258 correspond to the gain region QW layers
250, and energy levels at QWs 260 correspond to the absorber QW
layers 252.
[0058] In one embodiment, the saturation fluence of the absorber
206 may be adjusted by applying an electrical current to the
absorber. In FIG. 6, discussed below, an embodiment of an SEL is
illustrated having separate electrical contacts to allow tuning of
the absorber. In one embodiment, electrically adjusting the
saturation fluence of the absorber may be conducted in conjunction
with other means of lowering the saturation fluence of the
absorber.
[0059] In one embodiment, lower mirror 204, absorber 206, and gain
region 210 are assembled in a single fabrication process to form
monolithic gain structure 224. It will be understood that
additional layers, such as pump mirror 208 and AR layer 212, may
also be fabricated in the monolithic gain structure 224 during this
process. In one embodiment, the monolithic gain structure 224 is
formed in an MBE or an MOVPE reactor. In this instance, structure
224 may also be referred to as an epitaxial stack. Since the gain
region and the absorber are compatible for combined epitaxial
growth, they can be integrated into the same monolithic
structure.
[0060] For optically pumped embodiments, the substrate is
completely removed including the etch-stop layers in order to
expose the top layers of the structure (such embodiments shown in
FIGS. 2-4). For electrically pumped embodiments, the substrate may
be used as an additional intracavity spacer (such embodiments shown
in FIGS. 7-9). Substrates that may be used in fabrication of the
monolithic gain structure include, but are not limited to, gallium
arsenide, indium phosphide, or the like. During fabrication,
individual material fluxes are applied as appropriate to achieve
the desired composition of the various layers.
[0061] In SEL 200, the output coupler 216 may be rigidly attached
to the semiconductor surface. In another embodiment discussed
below, the output coupler 216 is fabricated in the semiconductor
material itself to form a microlens. In such an embodiment, the SEL
provides for wafer-scale production and testing. No post-dicing
alignment is needed. Further, such wafer-scale production allows
fabrication of two-dimensional arrays of SELs (discussed below in
conjunction with FIGS. 9A and 9B). In yet another embodiment, the
necessary shaping of the beam within the laser cavity can be
realized by tailoring temperature distributions to yield
appropriate thermal lenses. Thermal lenses may arise due to the
change of refractive index with temperature and thermal expansion
of the material caused by temperature distributions induced e.g. by
pump profiles. In such a case, the curvature of the output coupler
may be reduced or omitted completely.
[0062] Referring again to FIG. 2A, a heat sink 202 is coupled to
SEL 200. In the embodiment of FIG. 2A, heat sink 202 is coupled
below the lower mirror 204 of SEL 200. In order to alleviate
heating problems associated with high output power, embodiments
described herein may use upside-down mounting. The monolithic gain
structure is grown in reverse order, starting with an etch-stop
layer. After cleaving individual pieces, the epitaxial side is
bonded directly to a heat sink. The substrate may be removed using
techniques well known in the art. This mounting technique ensures
very small thermal impedance by strongly reducing the thickness of
the semiconductor structure. The resulting one-dimensional heat
flow then allows for power scaling by further increasing the mode
area in proportion to the power level, while in the geometry, the
maximum temperature excursion is not significantly increased due to
the increased mode area.
[0063] Referring again to FIG. 2A, in one embodiment, a non-linear
crystal 222 is optically coupled to the output coupler 216. In
another embodiment, crystal 222 is positioned inside of laser
cavity 220. Crystal 222 may be used to change the wavelength of
laser output 218 by second harmonic generation or optical
parametric oscillation. Crystal 222 may include, but is not limited
to, potassium tytanil phosphate (KTP), potassium tytanil niobate
(KTN), potassium niobate (KNbO.sub.3), lithium niobate
(LiNbO.sub.3), periodically-poled materials, such as
periodically-poled LiNbO3, or the like.
[0064] Referring to FIG. 3, a SEL 300 in accordance with one
embodiment of the present invention is shown. The embodiment of
FIG. 3 shows an optically pumped gain structure with an integrated
absorber where the absorber transmits the pump light to the gain
region. A gain region 310 is positioned on lower mirror 304. Lower
mirror 304 also serves as a pump mirror for optical pump 314.
Absorber 306 is positioned on gain region 310. An AR layer 312 is
positioned on absorber 306. Lower mirror 304, gain region 310,
absorber 306, and AR layer 312 form a monolithic gain structure
324.
[0065] An output coupler 316 is positioned above the AR layer 312
and emits laser output 318. The output coupler 316 and the lower
mirror 304 define a laser cavity 320. A heat sink 302 is coupled to
SEL 300 below lower mirror 304.
[0066] FIG. 4 shows a SEL 400 in accordance with one embodiment of
the present invention. The embodiment of FIG. 4 shows an optically
pumped gain structure with an integrated absorber where the gain
region is pumped from the backside. A gain region 410 is positioned
on lower mirror 404. The lower mirror 404 is highly-reflective for
the laser energy, but anti-reflective as to the optical pump
energy. A pump mirror 408 is positioned on the gain region 410. An
absorber 406 is positioned on the pump mirror 408. An AR layer 412
is positioned on absorber 406. Lower mirror 404, gain region 410,
pump mirror 408, absorber 406, and AR layer 412 form a monolithic
gain structure 424.
[0067] An output coupler 416 is positioned above AR layer 416 and
emits laser output 418. The output coupler 416 and the lower mirror
define a laser cavity 420. A heat sink 402 is coupled to SEL 400
below lower mirror 414. The heat sink 414 includes an aperture to
allow pump light to enter the laser cavity 420 from the backside of
SEL 400.
[0068] FIG. 5 shows a SEL 500 in accordance with one embodiment of
the present invention. The embodiment of FIG. 5 shows an
electrically pumped gain structure with an integrated absorber.
Lower mirror 504 is positioned on an isolator 503. Isolator 503
electrically isolates the lower mirror 504 from heat sink 502.
Isolator 503 includes an opening to allow current to pass from
contact 514a to contacts 514b,c in a defined opening. Gain region
510 is positioned on lower mirror 504.
[0069] A spacer 508 is positioned on gain region 510. In one
embodiment, the size of spacer 508 is determined at fabrication in
order provide sufficient length for the current injected between
contacts 514a,b,c to spread to form a profile which favors
fundamental-mode operation in the gain section. It will also be
noted that the size of spacer 508 affects the length of laser
cavity 520 and thus the repetition rate of the laser output
518.
[0070] An absorber 506 is positioned on spacer 508. An AR layer 512
is positioned on absorber 506. Isolator 503, lower mirror 504, gain
region 510, spacer 508, absorber 506, and AR layer 512 form a
monolithic gain structure 524.
[0071] An output coupler 516 is positioned above AR layer 512 and
emits laser output 518. Output coupler 516 and lower mirror 504
define laser cavity 520. A heat sink 502 is coupled to SEL 500
below isolator 503. SEL 500 is electrically pumped via contact 514a
coupled to lower mirror 504 and contacts 514b and 514c coupled to
spacer 508.
[0072] FIG. 6 shows a SEL 600 in accordance with one embodiment of
the present invention. The embodiment of FIG. 6 shows an
electrically pumped gain structure with an integrated absorber and
separate electrical contacts for tuning of the absorber. Lower
mirror 604 is positioned on an isolator 603. Absorber 606 is
positioned on lower mirror 604.
[0073] A current aperture layer 613 is positioned on the absorber
606. Contacts 611a and 611b allow for changing of the saturation
fluence of the absorber using an electrical signal. The current
aperture layer 613 electrically isolates the absorber 606 from the
gain region 610 and includes an opening to allow current to pass
from contact 614a to contacts 611a,b in a defined opening. The
current aperture layer is transparent to the laser wavelength to
allow light to pass between the mirrors of the cavity 620.
[0074] Gain region 610 is positioned on current aperture 613. AR
layer 612 is positioned on gain region 610. Isolator 603, lower
mirror 604, absorber 606, current aperture 613, gain region 610,
and AR layer 612 form a monolithic gain structure 624.
[0075] An output coupler 616 is positioned above AR layer 612 and
emits laser output 618. Output coupler 616 and lower mirror 604
define laser cavity 620. A heat sink 602 is coupled to SEL 600
below isolator 603. SEL 600 is electrically pumped via contact 614a
coupled to lower mirror 604 and contacts 614b and 614c coupled to
AR layer 612. The absorber 606 may be electrically tuned via
contact 614a coupled to lower mirror 604 and contacts 611a and 611b
coupled to absorber layer 606.
[0076] FIG. 7 shows a SEL 700 in accordance with one embodiment of
the present invention. The embodiment of FIG. 7 shows an
electrically pumped gain structure with an integrated absorber
using a dielectrically coated integrated microlens as the output
coupler. Lower mirror 704 is positioned on an isolator 703. Gain
region 710 is positioned on lower mirror 704. Absorber 706 is
positioned on gain region 710. A spacer 709 is positioned on
absorber 706. Monolithic gain structure 724 includes isolator 703,
lower mirror 704, gain region 710, absorber 706, and spacer
709.
[0077] In one embodiment, the monolithic gain structure 724 is
grown onto spacer 709. Spacer 709 includes a substrate such as
gallium arsenide (GaAs). In reference to FIG. 7, the layers would
be grown down starting with spacer 709, then absorber 706, and so
on to isolator 703. After monolithic structure 724 is grown,
additional layers, such as spacer 715 may be added to complete the
production of SEL 700.
[0078] An index matching layer 711 may be positioned on spacer 709.
Index matching layer 711 avoids additional reflections of the laser
light inside the cavity. It also provides a rigid attachment of
spacer 715 to contacts 714b,c and spacer 709.
[0079] A spacer 715 is positioned on index matching layer 711. The
sizes of spacers 709 and 715 may be adjusted at fabrication in
order to achieve a desired cavity length of laser cavity 720. In an
alternative embodiment of SEL 700, spacer 715 is not attached such
that microlens 716 is positioned on index matching layer 711. In
another embodiment, spacer 709 is removed during fabrication such
that index matching layer 711 is positioned on absorber 706.
[0080] A microlens 716 is positioned on spacer 715. Laser output
718 is emitted from microlens 716. Microlens 716 may be coated to
provide adequate reflectivity at laser wavelength. Laser cavity 720
is defined by lower mirror 704 and microlens 716. A heat sink 702
is coupled to SEL 700 below isolator 703. Electrical pumping is
provided to SEL 700 via contact 714a coupled to the lower mirror
704 and contacts 714b and 714c coupled to the index matching layer
711.
[0081] FIG. 8 shows a SEL 800 in accordance with one embodiment of
the present invention. The embodiment of FIG. 8 shows an
electrically pumped gain structure with an integrated absorber
using a dielectrically coated integrated microlens fabricated from
the same substrate as the gain structure. Lower mirror 804 is
positioned on an isolator 803. Gain region 810 is positioned on
lower mirror 804. An absorber 806 is positioned on gain region
810.
[0082] Spacer 809, which includes microlens 816, is positioned on
absorber 806. In SEL 800, microlens 816 is formed from the same
piece of substrate used to grow the monolithic gain structure 824.
The top shape of microlens 816 is etched using processes well known
in the art. Microlens 816 may be coated to provide adequate
reflectivity at laser wavelength. As described above in conjunction
with FIG. 7, the size of the spacer 809 may be grown or etched to
achieve the desired cavity length.
[0083] Laser cavity 820 is defined by lower mirror 804 and
microlens 816. A heat sink 802 is coupled to SEL 800 below isolator
803. Electrical pumping is provided to SEL 800 via contact 814a
coupled to the lower mirror 804 and contacts 814b and 814c coupled
to spacer 809. In operation, passively mode-locked laser output 818
is emitted from microlens 816.
[0084] SEL 800 provides a simple linear cavity that may be
electrically pumped. SEL 800 may generate a passively mode-locked
laser output with a high repetition rate. In one embodiment, SEL
800 may produce a 50-100 GHz signal. SEL 800 is fully integrated
resulting in a small size and has the benefits of wafer level
high-volume manufacturing. In one embodiment, SEL 800 is fabricated
using GaAs MBE.
[0085] FIG. 9A illustrates one embodiment of an array 900 of
electrically pumped SELs having integrated absorbers. Array 900
includes a SEL 930 and a SEL 940. A heat sink 902 may be coupled to
the bottom of SELs 930 and 940. In other embodiments, array 900 may
include additional SELs.
[0086] SEL 930 and SEL 940 share the following layers. Lower mirror
904 is positioned on an isolator 903. Gain region 910 is positioned
on lower mirror 904. Absorber 906 is positioned on gain region 910.
A spacer 909 is positioned on absorber 906. In one embodiment, SEL
930 and SEL 940 may be electrically isolated from each other. In
one instance, such isolation may be achieved by etching between SEL
930 and SEL 940.
[0087] SEL 930 includes the following layers. An index matching
layer 911 is positioned on spacer 909. A spacer 915 is positioned
on index matching layer 911. A microlens 916 is positioned on
spacer 915. Laser output 918 is emitted from microlens 916.
Electrical pumping is provided to SEL 930 via contact 914a coupled
to the lower mirror 904 and contacts 914b and 914c coupled to the
index matching layer 911.
[0088] SEL 940 includes the following layers. An index matching
layer 922 is positioned on spacer 909. A spacer 925 is positioned
on index matching layer 922. A microlens 924 is positioned on
spacer 925. Laser output 926 is emitted from microlens 924.
Electrical pumping is provided to SEL 940 via contact 920a coupled
to the lower mirror 904 and contacts 920b and 920c coupled to the
index matching layer 922.
[0089] In one embodiment, SEL 930 and SEL 940 are individually
addressable. A controller (not shown) coupled to array 900 may
provide control of each SEL. In one embodiment, electrical pumping
is provided only to the addressed SEL. In another embodiment,
spacer 915 and 925 may be of different sizes. In this instance, the
cavity length of SEL 930 is different than the cavity length of SEL
940 so that SELs 930 and 940 produce output with different
repetition rates.
[0090] FIG. 9B shows an embodiment of a two-dimensional array 950.
The array 950 includes SELs 951-956, where at least one SEL has an
integrated absorber as described herein. In one embodiment, each
SEL 951-956 is individually addressable. In another embodiment,
each SEL 951-956 is configured to provide a passively mode-locked
output at a unique repetition rate. In yet another embodiment, each
SEL 951-956 is configured to provide a passively mode-locked output
at a unique power level.
[0091] FIG. 10 shows an embodiment of a communication system 1000.
Optical transmitter 1001 includes a SEL array 1002 optically
coupled to a multiplexer 1003. SEL array 1002 includes at least one
SEL having an integrated absorber as described herein. Multiplexer
1003 includes a select input and a data input. The select input is
used to select a SEL of SEL array 1002. The data input is used to
receive data for modulation of the output of optical transmitter
1001. Tunable transmitter 1001 outputs an optical signal to an
optical channel 1004 optically coupled to optical transmitter 1001.
In one embodiment, the optical signal includes an optical time
division multiplexing (OTDM) signal. The optical channel 1004 is
optically coupled to a network 1006. In one embodiment, network
1006 is a photonic packet-switched network. Network 1006 is
optically coupled to an optical channel 1008. An optical receiver
1010 is optically coupled to optical channel 1008 to receive the
optical signal. In one embodiment, the optical channels 1004 and
1008 include optical fiber.
[0092] FIG. 11 shows one embodiment of a computer system 1100.
Computer system 1100 includes a chipset 1102 coupled to a processor
1104 and a memory device 1105 via bus 1101. Computer system 1100
also includes a system clock 1106 to provide clocking signals to
chipset 1102, processor 1104, memory 1105, and bus 1101. In another
embodiment, system clock 1106 only provides clocking signals to
processor 1104. In one embodiment, system clock 1106 outputs
clocking signals as optical signals. In other embodiments, clocking
signals of system clock 1106 are outputted as electrical
signals.
[0093] The system clock 1106 includes a SEL 1108 having an
integrated absorber, as described herein, to serve as an oscillator
for system clock 1106. In one embodiment, the system clock 1106 may
operate at 10 GHz or faster. In another embodiment, system clock
1106 may include one or more frequency dividers or one or more
frequency multipliers to provide clocking signals to components
coupled to system clock 1106.
[0094] FIG. 12 illustrates a solid-state laser 1200 having a
quantum dot semiconductor saturable absorber mirror (SESAM) 1202 in
accordance with one embodiment of the present invention. Generally,
a SESAM modulates the gain in the laser cavity as a function of
intensity. This passively mode-locks the laser. Absorber mirror
1202 includes one or more layers of quantum dots, as described
above, that give the absorber mirror 1202 a low saturation
fluence.
[0095] Solid-state laser 1200 includes a laser medium 1204 and an
output coupler 1206. Laser medium 1204 includes Er:Yb (erbium doped
ytterbium), Er:glass (erbium doped glass), Nd:Vanadate (neodymium
doped vanadate), Nd:YAG (neodymium doped yttrium aluminum garnet
(Y.sub.3Al.sub.5O.sub.12)), Nd:glass (neodymium doped glass), or
the like. In the embodiment of FIG. 12, solid-state laser 1200 is
pumped by a diode pump (not shown).
[0096] Using a quantum dot absorber 1202 with a low saturation
fluence in a solid-state laser facilitates cavity design as the
requirements on mode size ratio between gain and absorber materials
are significantly relaxed. In addition, absorber heating can be
much reduced by increasing the mode size on the absorber,
especially at high repetition rates and for high average
powers.
[0097] FIG. 13 illustrates a solid-state laser 1300 having a
quantum dot semiconductor saturable absorber mirror 1302 integrated
into the solid-state laser medium 1304. Laser medium 1304 includes
similar media as described above in conjunction with laser medium
1204. Laser medium 1304 includes a curved reflector 1306. Quantum
dot semiconductor saturable absorber mirror 1302 includes a high
reflector, including a semiconductor Bragg stack, and one or more
quantum dot absorber layers, separated by transparent spacer
layers. Thickness of the spacer layers is chosen to adjust optical
thickness of the total structure for optical standing wave effects
(resonance or anti-resonance).
[0098] The above description of illustrated embodiments of the
invention, including what is described in the Abstract, is not
intended to be exhaustive or to limit the invention to the precise
forms disclosed. While specific embodiments of, and examples for,
the invention are described herein for illustrative purposes,
various equivalent modifications are possible within the scope of
the invention, as those skilled in the relevant art will
recognize.
[0099] These modifications can be made to embodiments of the
invention in light of the above detailed description. The terms
used in the following claims should not be construed to limit the
invention to the specific embodiments disclosed in the
specification and the claims. Rather, the scope of the invention is
to be determined by the following claims, which are to be construed
in accordance with established doctrines of claim
interpretation.
* * * * *