Surface emitting laser with an integrated absorber

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.

CROSS REFERENCE TO RELATED APPLICATIONS

This application claims priority to U.S. application Ser. No. 10/814,050 filed on Mar. 31, 2004.

BACKGROUND

1. Field of Invention

The field of invention relates generally to lasers and, more specifically but not exclusively, relates to a surface emitting laser with an integrated absorber.

2. Background Information

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.

Two common types of SELs are vertical cavity surface emitting laser (VCSEL) and vertical external cavity surface emitting laser (VECSEL). Referring toFIG. 1A, a VCSEL100is shown. A gain region106is sandwiched between mirror104and mirror108. Such mirrors include distributed Bragg reflector (DBR) mirrors. Mirrors104and108define a laser cavity112. Laser output110is emitted from the mirror108perpendicular to the gain region106.

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. 1Cshows a graph165of optical power versus time of a mode-locked laser. The repetition rate of the laser output is based on the period between pulses in graph165. 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).

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.

FIG. 1Dshows a VECSEL170with a non-integrated absorber. VECSEL170includes a gain region174layered on a mirror172. An output coupler176is positioned above the gain region174. Mirror172and output coupler176define a laser cavity178. An optical pump182provides the pump energy for VECSEL170. A semiconductor saturable absorber mirror (SESAM)184provides passive mode-locking of VECSEL170and is separate from the gain region174.

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.

DETAILED DESCRIPTION

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 (M2below 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.

FIG. 2Aillustrates one embodiment of a SEL200. The embodiment ofFIG. 2Ashows an optically pumped gain structure with an integrated absorber where the absorber is placed below the gain region and pump mirror. An absorber206is positioned on lower mirror204. Lower mirror204may include a semiconductor Bragg stack. Lower mirror204is highly reflective (HR) as to the laser. A pump mirror208is positioned on absorber206. Pump mirror208is highly reflective as to the pump and is partially reflective as to the laser. Gain region210is positioned on pump mirror208. An anti-reflective (AR) layer212is positioned on the gain region210. AR layer212is anti-reflective for the laser and the pump energy. Lower mirror204, absorber206, pump mirror208, gain region210, and AR layer212form a monolithic gain structure224. In one embodiment, monolithic gain structure224is fabricated from a substrate in a single fabrication process (discussed further below).

An output coupler216is positioned above the AR layer212. Output coupler216and the lower mirror204define a laser cavity220. In one embodiment, output coupler216includes a curved output mirror. In operation, an optical pump214is applied to SEL200. A passively mode-locked laser output218is emitted from the output coupler216.

Absorber206is integrated with the gain region210. Absorber206includes a semiconductor material that is compatible with the fabrication process of lower mirror204, pump mirror208, and gain region210. 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 region210and the absorber206, the saturation fluence of the absorber206must be lower than the saturation fluence of the gain region210. 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 mirror204inFIG. 2) and the output coupler (216inFIG. 2).

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.

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.

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.

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.

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.

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.

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.

Various embodiments to reduce the saturation fluence of the integrated absorber are presented herein. In one embodiment, the saturation fluence of absorber206may be reduced by adjusting the standing wave field intensities of the gain region210and the absorber206independently.

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 toFIG. 2B, a standing wave pattern of the laser wavelength226is shown in monolithic gain structure224ofFIG. 2A. A graph228references the position versus field intensity of wave226. The physical location of absorber206has been positioned in the monolithic gain structure224so as to be aligned with peak field intensity positions of wave226. In an alternative embodiment, the lower mirror204or the output coupler216, or both, may be positioned to change the form of wave226so that the peak field intensity is aligned with the absorber206.

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 ofFIG. 2A, pump mirror208is 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.

In another embodiment, the saturation fluence of absorber206may be reduced by using quantum dots (QD) in the absorber206, while quantum wells (QW) are used in the gain region210. The absorber206may include one or more layers of quantum dots. The gain region210may 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 ofFIG. 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.

Referring toFIG. 2D, a graph230of absorption versus fluence is shown. Curve231shows the fluence of the gain region having quantum wells (QW), while curve232shows the fluence of the absorber having quantum dots (QD). As shown in graph230, the saturation fluence of the absorber (shown at232a) is significantly lower than the saturation fluence of the gain region (shown at231a).

FIG. 2Eshows an embodiment of a layer of quantum dots in an absorber. Quantum dots244are positioned on substrate242. 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.

In an embodiment of an absorber having multiple layers of quantum dots, a transparent spacer layer is positioned on top of the quantum dots244. On top of the transparent spacer layer are positioned further quantum dot and spacer layers.

In yet another embodiment, the saturation fluence of absorber206may be reduced by using quantum wells in the absorber206such that the absorber's saturation fluence is below the saturation fluence of quantum wells in the gain region210. In one embodiment, the absorber206includes one or more quantum well layers of gallium indium nitride arsenide (GaInNAs) while the gain region210includes 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.

Referring toFIGS. 2F and 2G, an embodiment of a SEL having a gain region with quantum wells and an absorber section with quantum wells is shown. InFIG. 2F, gain region210includes three quantum well (QW) layers250of InGaAs. Absorber206includes three QW layers252of GaInNAs. A standing wave pattern of the laser wavelength254is shown in monolithic gain structure224. A graph256references the position versus field intensity of wave254.

FIG. 2Gillustrates the minimum energy states of electrons residing within the conduction bands of the QW layers inFIG. 2F. Energy levels at QWs258correspond to the gain region QW layers250, and energy levels at QWs260correspond to the absorber QW layers252.

In one embodiment, the saturation fluence of the absorber206may be adjusted by applying an electrical current to the absorber. InFIG. 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.

In one embodiment, lower mirror204, absorber206, and gain region210are assembled in a single fabrication process to form monolithic gain structure224. It will be understood that additional layers, such as pump mirror208and AR layer212, may also be fabricated in the monolithic gain structure224during this process. In one embodiment, the monolithic gain structure224is formed in an MBE or an MOVPE reactor. In this instance, structure224may 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.

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 inFIGS. 2-4). For electrically pumped embodiments, the substrate may be used as an additional intracavity spacer (such embodiments shown inFIGS. 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.

In SEL200, the output coupler216may be rigidly attached to the semiconductor surface. In another embodiment discussed below, the output coupler216is 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 withFIGS. 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.

Referring again toFIG. 2A, a heat sink202is coupled to SEL200. In the embodiment ofFIG. 2A, heat sink202is coupled below the lower mirror204of SEL200. 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.

Referring again toFIG. 2A, in one embodiment, a non-linear crystal222is optically coupled to the output coupler216. In another embodiment, crystal222is positioned inside of laser cavity220. Crystal222may be used to change the wavelength of laser output218by second harmonic generation or optical parametric oscillation. Crystal222may include, but is not limited to, potassium tytanil phosphate (KTP), potassium tytanil niobate (KTN), potassium niobate (KNbO3), lithium niobate (LiNbO3), periodically-poled materials, such as periodically-poled LiNbO3, or the like.

Referring toFIG. 3, a SEL300in accordance with one embodiment of the present invention is shown. The embodiment ofFIG. 3shows an optically pumped gain structure with an integrated absorber where the absorber transmits the pump light to the gain region. A gain region310is positioned on lower mirror304. Lower mirror304also serves as a pump mirror for optical pump314. Absorber306is positioned on gain region310. An AR layer312is positioned on absorber306. Lower mirror304, gain region310, absorber306, and AR layer312form a monolithic gain structure324.

An output coupler316is positioned above the AR layer312and emits laser output318. The output coupler316and the lower mirror304define a laser cavity320. A heat sink302is coupled to SEL300below lower mirror304.

FIG. 4shows a SEL400in accordance with one embodiment of the present invention. The embodiment ofFIG. 4shows an optically pumped gain structure with an integrated absorber where the gain region is pumped from the backside. A gain region410is positioned on lower mirror404. The lower mirror404is highly-reflective for the laser energy, but anti-reflective as to the optical pump energy. A pump mirror408is positioned on the gain region410. An absorber406is positioned on the pump mirror408. An AR layer412is positioned on absorber406. Lower mirror404, gain region410, pump mirror408, absorber406, and AR layer412form a monolithic gain structure424.

An output coupler416is positioned above AR layer416and emits laser output418. The output coupler416and the lower mirror define a laser cavity420. A heat sink402is coupled to SEL400below lower mirror414. The heat sink414includes an aperture to allow pump light to enter the laser cavity420from the backside of SEL400.

FIG. 5shows a SEL500in accordance with one embodiment of the present invention. The embodiment ofFIG. 5shows an electrically pumped gain structure with an integrated absorber. Lower mirror504is positioned on an isolator503. Isolator503electrically isolates the lower mirror504from heat sink502. Isolator503includes an opening to allow current to pass from contact514ato contacts514b,cin a defined opening. Gain region510is positioned on lower mirror504.

A spacer508is positioned on gain region510. In one embodiment, the size of spacer508is determined at fabrication in order provide sufficient length for the current injected between contacts514a,b,cto spread to form a profile which favors fundamental-mode operation in the gain section. It will also be noted that the size of spacer508affects the length of laser cavity520and thus the repetition rate of the laser output518.

An absorber506is positioned on spacer508. An AR layer512is positioned on absorber506. Isolator503, lower mirror504, gain region510, spacer508, absorber506, and AR layer512form a monolithic gain structure524.

FIG. 6shows a SEL600in accordance with one embodiment of the present invention. The embodiment ofFIG. 6shows an electrically pumped gain structure with an integrated absorber and separate electrical contacts for tuning of the absorber. Lower mirror604is positioned on an isolator603. Absorber606is positioned on lower mirror604.

A current aperture layer613is positioned on the absorber606. Contacts611aand611ballow for changing of the saturation fluence of the absorber using an electrical signal. The current aperture layer613electrically isolates the absorber606from the gain region610and includes an opening to allow current to pass from contact614ato contacts611a,bin a defined opening. The current aperture layer is transparent to the laser wavelength to allow light to pass between the mirrors of the cavity620.

Gain region610is positioned on current aperture613. AR layer612is positioned on gain region610. Isolator603, lower mirror604, absorber606, current aperture613, gain region610, and AR layer612form a monolithic gain structure624.

An output coupler616is positioned above AR layer612and emits laser output618. Output coupler616and lower mirror604define laser cavity620. A heat sink602is coupled to SEL600below isolator603. SEL600is electrically pumped via contact614acoupled to lower mirror604and contacts614band614ccoupled to AR layer612. The absorber606may be electrically tuned via contact614acoupled to lower mirror604and contacts611aand611bcoupled to absorber layer606.

FIG. 7shows a SEL700in accordance with one embodiment of the present invention. The embodiment ofFIG. 7shows an electrically pumped gain structure with an integrated absorber using a dielectrically coated integrated microlens as the output coupler. Lower mirror704is positioned on an isolator703. Gain region710is positioned on lower mirror704. Absorber706is positioned on gain region710. A spacer709is positioned on absorber706. Monolithic gain structure724includes isolator703, lower mirror704, gain region710, absorber706, and spacer709.

In one embodiment, the monolithic gain structure724is grown onto spacer709. Spacer709includes a substrate such as gallium arsenide (GaAs). In reference toFIG. 7, the layers would be grown down starting with spacer709, then absorber706, and so on to isolator703. After monolithic structure724is grown, additional layers, such as spacer715may be added to complete the production of SEL700.

An index matching layer711may be positioned on spacer709. Index matching layer711avoids additional reflections of the laser light inside the cavity. It also provides a rigid attachment of spacer715to contacts714b,cand spacer709.

A spacer715is positioned on index matching layer711. The sizes of spacers709and715may be adjusted at fabrication in order to achieve a desired cavity length of laser cavity720. In an alternative embodiment of SEL700, spacer715is not attached such that microlens716is positioned on index matching layer711. In another embodiment, spacer709is removed during fabrication such that index matching layer711is positioned on absorber706.

A microlens716is positioned on spacer715. Laser output718is emitted from microlens716. Microlens716may be coated to provide adequate reflectivity at laser wavelength. Laser cavity720is defined by lower mirror704and microlens716. A heat sink702is coupled to SEL700below isolator703. Electrical pumping is provided to SEL700via contact714acoupled to the lower mirror704and contacts714band714ccoupled to the index matching layer711.

FIG. 8shows a SEL800in accordance with one embodiment of the present invention. The embodiment ofFIG. 8shows 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 mirror804is positioned on an isolator803. Gain region810is positioned on lower mirror804. An absorber806is positioned on gain region810.

Spacer809, which includes microlens816, is positioned on absorber806. In SEL800, microlens816is formed from the same piece of substrate used to grow the monolithic gain structure824. The top shape of microlens816is etched using processes well known in the art. Microlens816may be coated to provide adequate reflectivity at laser wavelength. As described above in conjunction withFIG. 7, the size of the spacer809may be grown or etched to achieve the desired cavity length.

Laser cavity820is defined by lower mirror804and microlens816. A heat sink802is coupled to SEL800below isolator803. Electrical pumping is provided to SEL800via contact814acoupled to the lower mirror804and contacts814band814ccoupled to spacer809. In operation, passively mode-locked laser output818is emitted from microlens816.

SEL800provides a simple linear cavity that may be electrically pumped. SEL800may generate a passively mode-locked laser output with a high repetition rate. In one embodiment, SEL800may produce a 50-100 GHz signal. SEL800is fully integrated resulting in a small size and has the benefits of wafer level high-volume manufacturing. In one embodiment, SEL800is fabricated using GaAs MBE.

FIG. 9Aillustrates one embodiment of an array900of electrically pumped SELs having integrated absorbers. Array900includes a SEL930and a SEL940. A heat sink902may be coupled to the bottom of SELs930and940. In other embodiments, array900may include additional SELs.

SEL930and SEL940share the following layers. Lower mirror904is positioned on an isolator903. Gain region910is positioned on lower mirror904. Absorber906is positioned on gain region910. A spacer909is positioned on absorber906. In one embodiment, SEL930and SEL940may be electrically isolated from each other. In one instance, such isolation may be achieved by etching between SEL930and SEL940.

SEL930includes the following layers. An index matching layer911is positioned on spacer909. A spacer915is positioned on index matching layer911. A microlens916is positioned on spacer915. Laser output918is emitted from microlens916. Electrical pumping is provided to SEL930via contact914acoupled to the lower mirror904and contacts914band914ccoupled to the index matching layer911.

SEL940includes the following layers. An index matching layer922is positioned on spacer909. A spacer925is positioned on index matching layer922. A microlens924is positioned on spacer925. Laser output926is emitted from microlens924. Electrical pumping is provided to SEL940via contact920acoupled to the lower mirror904and contacts920band920ccoupled to the index matching layer922.

In one embodiment, SEL930and SEL940are individually addressable. A controller (not shown) coupled to array900may provide control of each SEL. In one embodiment, electrical pumping is provided only to the addressed SEL. In another embodiment, spacer915and925may be of different sizes. In this instance, the cavity length of SEL930is different than the cavity length of SEL940so that SELs930and940produce output with different repetition rates.

FIG. 9Bshows an embodiment of a two-dimensional array950. The array950includes SELs951-956, where at least one SEL has an integrated absorber as described herein. In one embodiment, each SEL951-956is individually addressable. In another embodiment, each SEL951-956is configured to provide a passively mode-locked output at a unique repetition rate. In yet another embodiment, each SEL951-956is configured to provide a passively mode-locked output at a unique power level.

FIG. 10shows an embodiment of a communication system1000. Optical transmitter1001includes a SEL array1002optically coupled to a multiplexer1003. SEL array1002includes at least one SEL having an integrated absorber as described herein. Multiplexer1003includes a select input and a data input. The select input is used to select a SEL of SEL array1002. The data input is used to receive data for modulation of the output of optical transmitter1001. Tunable transmitter1001outputs an optical signal to an optical channel1004optically coupled to optical transmitter1001. In one embodiment, the optical signal includes an optical time division multiplexing (OTDM) signal. The optical channel1004is optically coupled to a network1006. In one embodiment, network1006is a photonic packet-switched network. Network1006is optically coupled to an optical channel1008. An optical receiver1010is optically coupled to optical channel1008to receive the optical signal. In one embodiment, the optical channels1004and1008include optical fiber.

FIG. 11shows one embodiment of a computer system1100. Computer system1100includes a chipset1102coupled to a processor1104and a memory device1105via bus1101. Computer system1100also includes a system clock1106to provide clocking signals to chipset1102, processor1104, memory1105, and bus1101. In another embodiment, system clock1106only provides clocking signals to processor1104. In one embodiment, system clock1106outputs clocking signals as optical signals. In other embodiments, clocking signals of system clock1106are outputted as electrical signals.

The system clock1106includes a SEL1108having an integrated absorber, as described herein, to serve as an oscillator for system clock1106. In one embodiment, the system clock1106may operate at 10 GHz or faster. In another embodiment, system clock1106may include one or more frequency dividers or one or more frequency multipliers to provide clocking signals to components coupled to system clock1106.

FIG. 12illustrates a solid-state laser1200having a quantum dot semiconductor saturable absorber mirror (SESAM)1202in 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 mirror1202includes one or more layers of quantum dots, as described above, that give the absorber mirror1202a low saturation fluence.

Using a quantum dot absorber1202with 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.

FIG. 13illustrates a solid-state laser1300having a quantum dot semiconductor saturable absorber mirror1302integrated into the solid-state laser medium1304. Laser medium1304includes similar media as described above in conjunction with laser medium1204. Laser medium1304includes a curved reflector1306. Quantum dot semiconductor saturable absorber mirror1302includes 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).