This PDF file contains the front matter associated with SPIE Proceedings Volume 7230, including the Title Page, Copyright information, Table of Contents, and the Conference Committee listing.

We demonstrate GaAs-based metamorphic lasers in the 1.3-1.55 μm telecom range grown by molecular beam epitaxy. The introduction of dopants in a compositionally graded layer is shown to significantly influence material properties, as well as having impact on the laser device design. Investigating and understanding of strain relaxation and dislocation dynamics is useful for improving material quality, performance and robustness of metamorphic devices. We demonstrate pulsed lasing up to 1.58 μm and continuous wave lasing at 1.3 μm at room temperature with low threshold currents.

In this paper, we will discuss the necessary properties of the light source needed for high resolution and high penetration OCT. We examine the performance of a self pulsating edge emitting quantum dot laser diode light source emitting at ~1050nm and configured as a split contact device with separate gain and absorber sections. The device can be configured to operate with a reverse biased saturable absorber section or with the whole device forward biased into gain. With the device operated without a saturable absorber section the laser emits a number of discrete narrow modes, which merge to form a broad continuous lasing spectrum (width up to 10nm) on application of the saturable absorber. In the time domain we observe continuous emission that becomes self pulsation, with pulse widths of 200-300ps and frequency of 0.6 - 1.5GHz depending on drive current and reverse bias, on application of the saturable absorber.

This paper reports on the measurement and analysis of the coherence function for broadband emitters such as superluminescent diodes (SLDs) and novel broadband laser diodes (BLDs) from self-assembled InGaAs/GaAs quantumdot (QD) and InAs/InP quantum-dash (Qdash) structures that emit at center wavelengths of 1150nm and 1650nm, respectively. Using the fiber-based spectral interferometry system, coherence lengths in fiber of 23 μm and 48 μm have been measured from the QD and Qdash BLDs. Larger spectral bandwidth of 137 nm and 78 nm have been measured from the QD and Qdash SLDs that yield coherence lengths in fiber of 3 μm and 10 μm, respectively. The coherence function of both BLDs and SLD reveals negligible secondary coherence subpeaks and sidelobes indicating the possibility of using these broadband sources to produce low artifacts optical coherence tomography (OCT) images.

This paper details the development of broadband sources at 1050 nm for optical coherence tomography applications. The careful optimization of the current supplied to different sections of a multi-contact device allows both high power and broadband CW emission to be obtained.

The performance characteristics of InAlGaN multiple quantum well (MQW) lasers grown on sapphire and low defect density bulk AlN substrates has been compared. The group III-nitride laser heterostructures were grown on (0001) AlN and c-plane sapphire substrates by metalorganic vapor phase epitaxy (MOVPE). Lasing was observed for optically pumped AlGaInN heterostructures in the wavelength range between 333 nm and 310 nm. A comparison of laser thresholds shows reduced threshold power densities for lasers grown on bulk AlN with threshold densities as low as 175 kW/cm² for lasers emitting near 330 nm.

For broad ridge (Al,In)GaN laser diodes, which are inevitable for high output power applications in the near-UV to blue spectral region, filaments appear, which influence the far-field beam quality. We present an extensive study of the optical mode profile of conventional c-plane LD test structures with ridge widths from 1.5 to 10 micrometers. The broad ridge samples are optimized to reach several hundred milliwatt of cw output power. Spectral and spatial resolved near- and far-field measurements show, that the characteristic lateral multi-lobed far-field pattern can be interpreted as superposition of interfering phase-locked filaments in the ridge waveguide.

Staggered InGaN quantum wells (QWs) are analyzed as gain media for laser diodes to extend the lasing wavelength towards 500 nm. The calculation of band structure is based on a 6-band k•p method taking into account the valence band mixing, strain effect, and spontaneous and piezoelectric polarizations as well as the carrier screening effect. Staggered InGaN QWs with two-layer and three-layer step-function like In-content InGaN QWs structures are investigated to enhance the optical gain for laser diodes emitting in the green regime.

We review and compare recent results on the design and fabrication of semiconductor microring and microdisk lasers for applications in all-optical switching and signal processing. The design of the optical cavities will be analyzed in detail with a strong emphasis on the evaluation of the various loss mechanisms. Both lithographic and etching technologies were thoroughly optimized to ensure high-resolution patterning and deep and vertical waveguide sidewalls. The majority of the fabricated devices exhibited continuous wave operation down to 7 μm and strong unidirectional bistability down to device footprints as small as 30 μm. While both microdisks and microrings show similar behaviour for medium radii (30-60 μm), in smaller devices the microdisk geometry shows much lower losses and better performance.

The use of the non-selective, O₂-enhanced wet thermal oxidation of deep-etched sidewalls in GaAs-based heterostructures has enabled the fabrication of low-loss, high-index-contrast ridge waveguides suitable for ring resonator laser devices. In a self-aligned process, the grown native oxide simultaneously provides excellent electrical insulation, passivation of the etch-exposed bipolar active region, and smoothing of etch roughness. The resulting strong lateral optical confinement at the semiconductor/oxide interface has enabled half-racetrack ring resonator (R³) lasers with bend radii r as small as 6 μm. In this work we have experimentally characterized the loss due to the mode mismatch at the straight to curved waveguide transition from analysis of efficiency data of half-R³ lasers with multiple cavity lengths. Using an 808 nm InAlGaAs graded-index separate confinement heterostructure, the transition losses are extracted from an inverse efficiency 1/η_d vs. length L plot for half-R³ lasers with r=150, 100, 50, 25 and 10 μm and 3 different ridge widths, w. The round trip transition loss ranges from 11.5 to 37.0 dB (for w=7.3 μm), 6.7 to 27.0 dB (w=4.2 μm), and 1.8 to 16.2 dB (w=2.1 μm) with decreasing radii, showing a clear decrease with width and corresponding improved mode overlap in the transition region. Simulation results elucidate the role of mode mismatch vs. radiative bend loss in high-index-contrast racetrack ring resonator lasers. We demonstrate a full-ring laser having a tangential stripe output coupler guide fabricated via e-beam lithography and non-selective oxidation with a threshold current density of 719 A/cm² for an r=150 μm, w=6 μm ring.

High-channel-count WDM will eventually be used for short reach optical interconnects since it maximizes link bandwidth and efficiency. An impediment to adoption is the fact that each WDM wavelength currently requires its own DFB laser. The alternative is a single, multi-wavelength laser, but noise, size and/or expense make existing options impractical. In contrast, a new low-noise, diode comb laser based on InAs/GaAs quantum dots provides a practical and timely alternative, albeit in the O-band. Samples are being evaluated in short reach WDM development systems. Tests show this type of Fabry-Perot laser permits >10 Gb/s error-free modulation of 10 to over 50 separate channels, as well as potential for 1.25 Gb/s direct modulation. The paper describes comb laser requirements, noise measurements for external and direct modulation, O-band issues, transmitter photonic circuitry and components, future CMP applications, and optical couplers that may help drive down packaging costs to below a dollar.

The wavelength spectra of ridge waveguide Fabry Perot lasers can be modified by perturbing the effective refractive index of the guided mode along very small sections of the laser cavity. One way of locally perturbing the effective index of the lasing mode is by etching features into the ridge waveguide such that each feature has a small overlap with the transverse field profile of the unperturbed mode, consequently most of the light in the laser cavity is unaffected by these perturbations. A proportion of the propagating light is however reflected at the boundaries between the perturbed and the unperturbed sections. Suitable positioning of these interfaces allows the mirror loss spectrum of a Fabry Perot laser to be manipulated. In order to achieve single longitudinal mode emission, the mirror loss of a specified mode must be reduced below that of the other cavity modes. Here we review the latest results obtained from devices containing such features. These results clearly demonstrate that these devices exceed the specifications required for a number of FTTH and Datacomms applications, such as GEPON, LX4 and CWDM. As well as this we will also present initial results on the linewidth of these devices.

Using a combination of molecular beam epitaxy (MBE) and metal-organic, chemical-vapor deposition (MOCVD), highperformance, buried-heterostructure, distributed feedback (DFB), laser diodes are being manufactured for multiple, uncooled (-20 to 85 °C and -40 to 95 °C) product lines. MBE is used to grow the active regions and the p-type cladding layers, while MOCVD is used for the Fe-doped blocking layers. Multi-wafer growths are used to reduce device costs. Devices, employing the same basic active region design, have been fabricated operating at wavelengths from 1490 to 1610 nm for applications including coarse wavelength division multiplexing (CWDM) OC-48 digital, analog return path, and 2.2 GHz (3G) wireless code division multiple access (W-CDMA). These devices show good linearity (analog return path and wireless) and high-speed operation (digital). Accelerated lifetime testing of these devices shows excellent reliability with a median lifetime of 17 years at 90 °C.

Advances in the development of mid-IR antimonide type-II "W" interband cascade lasers (ICLs) have recently led to the first demonstration of continuous wave operation at room temperature. The 5-stage narrow-ridge Auelectroplated ICL emitted at λ= 3.75 μm produced over 10 mW of cw power at 300 K and operated to 319 K. The considerable increase in Tmax was realized by carefully optimizing both the design and the MBE growth of these complicated multilayer structures. The internal loss was decreased by reducing the doping in the claddings and separate-confinement regions, and then using fewer stages to take advantage of the lower dissipated power density while still having enough gain to reach threshold. We find that the improved properties are similarly available in devices spanning the spectral window of at least 3.2-4.2 μm.

Interband cascade lasers are efficient and compact semiconductor mid-infrared (3-6 μm) light sources with low-power consumptions. We report our recent progress in the development of interband cascade lasers with separate confinement layers. Broad-area (0.1mmx1mm) lasers have been operated in cw mode at temperatures up to 213K near 3.36 μm. For narrow ridge-waveguide (0.01mmx1.5mm) lasers, cw operation has been achieved at temperatures up to 266K near 3.43 μm, 260K near 3.7 μm, and 238K near 4.04 μm. The results on both broad-area and narrow-ridge IC lasers are discussed in comparison with previous regular IC lasers without separate confinement layers.

Quantum Cascade (QC) wafer quality testing requires intensive processing and characterization. Here, we demonstrate a minimally invasive technique that gives rapid feedback on wafer quality. A mesa is fabricated using only a single etch and metallization step. The device is electrically pumped and optically and electrically characterized. The peak wavelength position and the full width at half maximum (FWHM) as a function of applied electric field, turn-on voltage, maximum operating current density and threshold current density of the mesas are measured. Results of the mesa and lasers processed from the same wafer are compared and differed by less than 10 %.

Single wavelength hybrid silicon evanescent lasers are described based on wafer bonding III-V multiple quantum wells to gratings patterned on a silicon waveguide. Distributed Bragg feedback and distributed Bragg reflector lasers are demonstrated integrated with passive silicon waveguides showing thresholds as low as 25mA and output powers as high as 11mW around 1600nm wavelength.

The microscopic processes accompanying the catastrophic optical damage process in semiconductor lasers are discussed. For 808 and 650 nm edge-emitting broad-area devices relevant parameters such as surface recombination velocities, bulk and front facet temperatures are determined and discussed. Facet temperatures vs. laser output and temperature profiles across laser stripes reveal a strong correlation to near-field intensity and degradation signatures. Furthermore, the dynamics of the fast catastrophic optical damage process is analyzed by simultaneous high-speed infrared thermal and optical imaging of the emitter stripe. The process is revealed as fast and spatially confined. It is connected with a pronounced impulsive temperature flash detected by a thermocamera.

For novel devices such as quantum dot lasers, the usual thermal characterization using temperature induced wavelength shift is ineffective due to weak thermal shift of the inhomogeneously broadened gain-peak. This calls for new thermal characterization techniques for such devices. To this end we have analyzed bulk thermal properties of broad area quantum dot lasers theoretically, and have experimentally verified these calculations using the novel technique of microthermography. InGaAs/GaAs 950 nm emitting, 50 μm wide and 1.5 mm long, large optical cavity quantum dot lasers were used for the study. Our two-dimensional steady-state model self-consistently includes current spreading and distributed heat sources in the device and using finite element method reproduces high resolution temperature maps in the transverse cross section of the diode laser. A HgCdTe based thermocamera with detection spectral range 3.5-6.0 μm was employed for micro-thermography measurements. Its microscope with 6x magnification has a nominal spatial resolution of 4 μm/pixel for full frame images of 384×288 pixels. A ray tracing technique was used to model the propagation of thermal radiation inside the transparent laser die which in turn links calculated and experimentally derived temperature distributions. Excellent agreement was achieved which verifies the model-calculation and the thermal radiation propagation scheme inherent in the experimental approach. This result provides a novel means for determining reliable bulk temperature data from quantum dot lasers.

Quantum dots (QD) offer significant advantages over quantum wells (QW) as the active material in high power lasers. We have determined power density values at catastrophic optical mirror damage (COMD), a key factor limiting high power laser diode performance, for various QW and QD red and NIR emitting structures in the in the AlGaInP system. The devices used were 50 μm oxide stripe lasers mounted p-side up on copper heatsinks operated pulsed. The COMD power density limit decreases as pulse length increases. At short pulse lengths the limit is higher in QD (19.1±1.1 MW/cm²) than in QW devices (11.9±2.8 MW/cm² and 14.3±0.4 MW/cm² for two different spot sizes). We used the high energy Boltzmann tail of the spontaneous emission from the front facet to measure temperature rise to investigate the physical mechanisms (non-radiative recombination of injected carriers and reabsorption of laser light at the facet) leading to COMD and distinguish between the behaviour at COMD of QW and QD devices. Over the range 1x to 2x threshold current the temperature rise in the QW structures was higher. Scanning electron microscopy showed a difference between the QD and QW lasers in the appearance of the damage after COMD.

Recently, broad-area InGaAs-AlGaAs strained quantum well (QW) lasers have attracted much attention because of their unparalleled high optical output power characteristics that narrow stripe lasers or tapered lasers can not achieve. However, broad-area lasers suffer from poor beam quality and their high reliability operation has not been proven for communications applications. This paper concerns reliability and degradation aspects of broad-area lasers. Good facet passivation techniques along with optimized structural designs have led to successful demonstration of reliable 980nm single-mode lasers, and the dominant failure mode of both single-mode and broadarea lasers is catastrophic optical mirror damage (COMD), which limits maximum output powers and also determines operating output powers. Although broad-area lasers have shown characteristics unseen from singlemode lasers including filamentation, their effects on long-term reliability and degradation processes have not been fully investigated. Filamentation can lead to instantaneous increase in optical power density and thus temperature rise at localized areas through spatial-hole burning and thermal lensing which significantly reduces filament sizes under high power operation, enhancing the COMD process. We investigated degradation processes in commercial MOCVD-grown broad-area InGaAs-AlGaAs strained QW lasers at ~975nm with and without passivation layers by performing accelerated lifetests of these devices followed by failure mode analyses with various micro-analytical techniques. Since instantaneous fluctuations of filaments can lead to faster wear-out of passivation layer thus leading to facet degradation, both passivated and unpassivated broad-area lasers were studied that yielded catastrophic failures at the front facet and also in the bulk. Electron beam induced current technique was employed to study dark line defects (DLDs) generated in degraded lasers stressed under different test conditions and focused ion beam was employed to prepare TEM samples from the DLD areas for HR-TEM analysis. We report our in-depth failure mode analysis results.

The latest result at the Center for Quantum Devices about high power, high wall plug efficiency, mid-infrared quantum cascade lasers (QCLs) is presented. At an emitting wavelength of 4.8 μm, an output power of 3.4 W and a wall plug efficiency of 16.5% are demonstrated from a single device operating in continuous wave at room temperature. At a longer wavelength of 10.2 μm, average power as high as 2.2 W is demonstrated at room temperature. Gas-source molecular beam epitaxy is used to grow the QCL core in an InP/GaInAs/InAlAs material system. Fe-doped semiinsulating regrowth is performed by metal organic chemical vapor deposition for efficient heat removal and low waveguide loss. This accomplishment marks an important milestone in the development of high performance midinfrared QCLs.

The voltage tuning of gain spectra in three types of Quantum Cascade laser designs is investigated. The gain spectra of the luminescence device are tunable over the whole voltage operation range for all designs. The lasers are as tunable as the electroluminescence below threshold, while a reduced tunability is observed in all lasers above threshold. This is attributed to the decrease of resistance across the laser active region as the photon density increases. A resumed tunability high above threshold occurs in all lasers based on the anti-crossed designs. Lasers based on the anti-crossed diagonal transition are tunable above threshold, with a tuning range of about 40 cm^-1 (~4% of the laser emission wavenumber) at room temperature, i.e. a tuning rate of 800 cm-1 per volt per period of active region and injector.

In this work we present the characteristics of a novel type of quantum-cascade (QC) laser: the deep-well (DW) QC device, which, unlike conventional QC lasers, contains a superlattice of quantum wells and barriers of different composition, respectively. The fabrication of DW-QC devices is made possible by the use of metal-organic chemical vapor deposition (MOCVD), a crystal growth technique which allows one to easily vary the composition of wells and barriers within QC structures, thus providing significantly increased flexibility in optimizing the device design. We have designed such varying-composition QC structures to have deep quantum wells in and tall barriers in and around the active region. DW- QC laser structures have fabricated into 19 μm-wide ridges and 3 mm-long chips. Threshold-current densities as low as 1.5 kA/cm² are obtained at room temperature in the 4.6-4.8 μm wavelength region. In conventional QC lasers emitting in the 4.5-5.5μm range there is substantial thermionic carrier leakage from the upper laser level to the continuum, as evidenced by a significant decrease in the slope efficiency above 250 K, which is understandable given the relatively small (i.e., ~ 200 meV) energy differential, δE, between the upper lasing level and the top of the exit barrier. For the DW design carrier leakage is suppressed due to deep active wells and tall barriers, such that δE reaches values in excess of 400 meV. Preliminary results include a threshold-current characteristic temperature, T₀, value of 218 K over the temperature range: 250- 340 K.

Mid-infrared beam shaping concepts are presented, which rely on coherent emission from QCLs. Grating coupled surface emitting quantum cascade ring lasers allow for far-field tuning, ranging from highly symmetric spot- to ringshaped beam patterns, depending on the grating period. In single-mode operation, the devices exhibit low beam divergence, represented by a full width at half maximum of ~3°. Moreover, a tree shaped resonator is investigated, which enables coherent parallel coupling of six laser elements into a single waveguide by means of several Y-junctions. The lasers were investigated in terms of optical power, near and far field characterization. Phase-locking was observed and leads to in-phase emission on both sides of the devices. Both concepts demonstrate the feasibility of high-brightness midinfrared quantum cascade lasers with prospective applications in spectroscopy and high power laser arrays.

We report a high performance operation of quantum cascade lasers based on Single Phonon-Continuum depopulation (SPC) structures. The lasers exhibit low CW threshold current densities and high characteristic temperatures in the wide wavelength range between 4.5 and 10.8 μm. An 8.2 μm laser, despite a bare ridge structure which is extremely simple configuration without any intentional thermal dissipation equipments and any HR coatings, demonstrates the high device performance: a low threshold current density of 1.66 kA/cm² and a high maximum output power of 76 mW (from one facet) at room temperature in CW operation. Our shortest wavelength 4.5 μm laser with HR coating reveals a low threshold current density of 1.7 kA/cm² and maximum output power of 161 mW at room temperature in CW operation. For long wavelength, we present the first room temperature, CW operation of DFB QCL with top grating. The DFB laser emits ~9.6 μm single mode spectra at temperatures between -5 °C and 50 °C. The wide tuning range is obtained to be from 1031 to 1039 cm^-1.

We propose and theoretically study the performance of short-wavelength mid-infrared quantum cascade lasers integrated with resonant second-harmonic generation (SHG) cascade. We show the feasibility of SHG in the near infrared range 1.5-2 μm with nonlinear susceptibility of ~150 pm/V. Theoretical treatment takes into account the non-sphericity and the non-parabolicity of the conduction band within a modified energy-dependent effective mass approximation based on the 14 band kP model. This approximation is shown to coincide with the first conduction band calculated by the 30 band kP model up to 2.3 eV above the band edge. We demonstrate the possibility of modal phase matching in the InGaAs/AlAsSb/InP QC laser which yields the intracavity frequency conversion efficiency of the order of 0.2 mW/W².

We have implemented a self-consistent non-equilibrium Green's functions approach for vertical charge transport and optical gain in terahertz quantum cascade lasers (THz-QCL) in the stationary limit. We present theoretical results of the current-voltage characteristics and the temperature dependence of the optical gain for GaAs/Al.₁₅Ga.₈₅As THz-QCLs and compare our results with experimental data. We find excellent quantitative agreement for the current-voltage characteristics and the peak gain energy and identify non-radiative transitions between the laser levels as primary factor that limits the maximum operation temperature. Furthermore, we find significant coherent leakage of electrons in the upper laser level that increases the threshold current density. We propose a broadening of the collector well to efficiently suppress this coherent leakage and to reduce the threshold current.

The gain profile of a quantum cascade laser is strongly influenced by the lifetime of the carriers in the upper and lower laser state. The quantitative description of gain within the concept of nonequilibrium Green's functions allows for a detailed understanding of various features affecting the gain spectrum: Compensation effects between scattering processes in the upper and lower laser level, reduction of gain due to coherences between nearly degenerate upper laser states, and dispersive gain without inversion.

An external linear optical system (with no feedback to the laser) converts the laser's multimode emission into a multilongitudinal but spatially single-mode (diffraction-limited) spot. Recent progress in laser diode mode analysis demonstrating the high stability of the mode pattern of broad-area laser diodes and the development of the low-loss highspectral- resolution phase modulator are the key enablers of such a technique. These advancements led to the demonstration of 60% conversion efficiency of multimode 980 nm diode-laser emissions into a single-mode fiber.

High-brightness diode lasers at 1060 nm are useful in display applications (to provide green light by frequency doubling) and in free-space optical communications. On Al-free active region laser structures, we have obtained low optical losses of 0.9 cm^-1, a high internal quantum efficiency of 98% and a low transparency current density of 64 A/cm². On uncoated broad-area lasers (2 mm x 100 μm) at 20°C CW, we have obtained a high maximum wall-plug efficiency of 66%, and an optical power higher than 3W per facet. Based on these good results, we have realized 3.7 mm long gain-guided tapered lasers, delivering a high power of 3W at 10°C CW, together with a low M² of 3 at 1/e² and a high maximum wall-plug efficiency of 57%. We have also realized separate electrode lasers, in which the ridge and tapered sections are biased separately. In this configuration, the current through the ridge section is only a few tens mA while the current on the tapered section is several Amps. This allows to control a large output power with only a small change of the ridge current. By moving the ridge current from 0 to 50 mA, keeping a constant 4A current through the tapered section, we have obtained a large change of the output power from 0.09 W to 2.6 W, which corresponds to a high modulation efficiency of 50 W/A under static operation. In dynamic regime, the separate electrode laser can be operated at 700 Mbps, showing a high modulation efficiency of 19 W/A, optical modulation amplitude of 1.6 W and extinction ratio of 19dB [1]. These modulation efficiencies are, to our knowledge, record values.

High-brightness narrow line-width 1060 nm tapered lasers with an internal distributed Bragg reflector were realized. The devices reach a maximal output power of 12 W with a narrow spectral line-width below 40 pm (95% power). A nearly diffraction limited beam quality was measured up to a power of 10 W. The vertical structure is based on an InGaAs triple quantum well (TQW) active region embedded in a 4.8 μm broad AlGaAs super large optical cavity. This leads to a narrow vertical divergence of 15° (FWHM). Tapered devices were processed a total length of 6 mm consisting of 2 mm long ridge waveguide (including 1 mm DBR mirror) and 4 mm tapered sections. A full taper angle of 6° was manufactured. The input currents to both sections can be independently controlled. The devices had a conversion efficiency of about 50%. A first reliability test showed failure-free operation at 5 W without a deterioration of the beam quality and the spectral properties.

Spectral and output power data of distributed Bragg reflector lasers emitting in the technologically important wavelength range from 780 nm to 1083 nm are presented. These devices are fabricated in a single molecular beam epitaxy growth step, and the gratings are defined by holographic interferometry. Spectral dependencies on the grating and gain section lengths are systematically investigated. Experimental data for the side-mode suppression ratio, mode spacing, and thermal wavelength shift are given for devices emitting in the near infrared wavelength range between 780 nm and 1083 nm.

A long-standing challenge for semiconductor lasers is scaling the optical power and brightness of many diode lasers by coherent beam combination. Because single-mode semiconductor lasers have limited power available from a single element, there is a strong motivation to coherently combine the outputs of many elements for applications including industrial lasers for materials processing, free space optical communications, and defense. Despite the fact that such a coherently-combined source is potentially the most efficient laser, coherent combination of semiconductor lasers is generally considered to be difficult, since precise phase control is required between elements. We describe our approach to coherent combination of semiconductor lasers. The Slab-Coupled Optical Waveguide Laser (SCOWL), invented at Lincoln Laboratory, is used as the single-mode diode laser element for coherent combination. With a 10-element SCOWL array, coherently combined output power as high as 7 W in continuous wave using an external cavity has been demonstrated, which is the highest output level achieved using a coherent array of semiconductor lasers. We are currently working on a related approach to scale the coherent power up to 100 W.

Precise gyroscopes and atomic clocks are in high demand for positioning and flight navigation systems or measurement of fundamental constants. The development of techniques such as atom optical pumping (Cs or Rb) requires laser diodes with high power and excellent spectral (narrow linewidth) and beam qualities. For spatial applications a high reliability is required (mission lifetime is around 15 years). We have realized different studies of reliability on our Al-free DFB lasers: Catastrophically Optical Mirror Damage (COMD) evaluation, lifetest, optical and spectral measurements before and after ageing. We obtained high COMD densities (respectively 13MW/cm² in continuous wave CW and 19MW/cm² in pulsed mode. Furthermore, we have realized ageing test on these DFB laser diodes emitting at 852.12nm (D2 line of Cs). We used five different ageing conditions (power and temperature) to determine ageing properties. The extrapolated lifetimes of our DFB laser (for operating current variation equal to 100%) are higher than 140000 hours (about 15 years) for an ageing at T= 25°C and P= 40mW. This confirms the excellent potential of this Al-free technology for long life spatial mission. The Side Mode Suppression Ration (SMSR) of the aged D2 line DFB lasers remains very high with a measured change of -1.4dB ± 8dB. There are no significant drifts of the DFB laser wavelength after aging (average ~0.03 nm). We also measured the linewidth of our aged DFB lasers by the self-heterodyne technique and obtained narrow beating linewidths of around 900kHz.

Distributed feedback ridge-waveguide lasers have been developed. The distributed feedback is provided by a second order grating, formed into an InGaP/GaAsP/InGaP multilayer structure. A threshold current of 40 mA and a differential quantum efficiency of 0.8 W/A is achieved. The lasers emit up to 200 mW in a single lateral and longitudinal mode around 794.7 nm and 780.0 nm. These wavelength regions are of particular interest for applications in absorption spectroscopy of the rubidium D1 and D2 lines and atomic clocks. These applications require a stable lasing wavelength with small spectral line width and possibility of a fine tuning. By changing the output power with current and / or the heatsink temperature the wavelength can be tuned to reveal the hyperfine structure of the rubidium lines. This was verified by passing the laser emission through a 80 mm long rubidium cell and measuring the transmitted power versus current and temperature. It is shown, that a hyperfine structure measurement of the rubidium lines can be performed in a less than 9 μs. Due to the large side mode suppression ratio of >45 dB and the small spectral line width of ~ 200 kHz these lasers are ideally suited for absorption spectroscopy

An individually addressable visible semiconductor laser diode array with a 20 μm pitch is demonstrated that is highly suited for deployment in next-generation digital print systems. The array, operating at 660 nm, comprises 22 single mode lasers fabricated on a single GaInP/AlGaInP/GaAs substrate. The laser array is flip-chip bonded onto a patterned ceramic submount that enables the individual elements to be driven independently and is integrated into a 26-pin butterfly package. Arrays tested CW exhibit low threshold current (<20 mA per emitter), up to 50 mW output power per channel with a high slope efficiency (0.9 W/A) and a high characteristic temperature of over 100 K.

High-power diode lasers in the mid-infrared wavelength range between 1.8μm and 2.3μm have emerged new possibilities for application fields like materials processing, medical surgery and for military applications like infrared countermeasures. GaSb based diode lasers are naturally predestined for this wavelength range and offer clear advantages in comparison to InP based diode lasers in terms of output power and wall-plug efficiency. We will present results on different MBE grown (AlGaIn)(AsSb) quantum-well diode laser single emitters and linear laser arrays, the latter consisting of 19 emitters on a 1 cm long bar, emitting at different wavelengths between 1.8 and 2.3 μm. Each emitter has a resonator length of 1.0 mm or 1.5 mm and stripe widths of 90 μm or 150 μm. The distance from emitter to emitter is 500μm for both types, resulting in 20% and 30% fill factors. For single emitters the electrooptical and beam behaviour and the wavelength tunability by current and temperature have been carefully investigated in detail. For diode laser arrays mounted on actively cooled heat sinks, nearly 20W at 1.94μm in continuous-wave mode have been achieved at a heat sink temperature of 20 °C. Even at 2.2μm more than 15W with a wall plug efficiency of 23% have been measured, impressively demonstrating the potential of GaSb based diode lasers well beyond wavelengths of 2μm.

An increase in the output power of semiconductor waveguide lasers is commonly achieved through broadening the stripe width of the active waveguide region. However, the resulting amplification of high order modes may degrade the beam quality of the laser diode. Further, Filamentation and high peak power densities will limit the lifetime of the device by optical facet damage. We report an approach to control the slow axis mode behaviour by embedding diffractive phase structures directly into the waveguide layers of the active laser region. Using this technique it is possible to enhance the amplification by increasing the overlap with the gain region, whilst additional diffraction losses for higher order modes are generated. By shaping the zero order mode the output beam quality can be increased and a high efficiency of the device maintained. Finally we discuss manufacturing techniques of these monolithic waveguide lasers and show how to integrate phase structures through an additional lithographic step. In our experimental realisation we will demonstrate that micro structured broad area lasers show a smooth transversal mode shape with significantly reduced current dependency.

Daylight Solutions has integrated commercially available quantum cascade gain media with their advanced coating and die attach technologies, mid-IR micro-optics and telecom-style assembly and packaging to yield cutting edge performance. When orchestrated together into Daylight's external-cavity quantum cascade laser (ECqcL) platform, significant performance has been achieved. These ECqcL modules have now be

In this work, a novel self-aligned process utilizing non-selective, O₂-enhanced wet thermal oxidation is presented for fabricating InP-based, ridge waveguide mid-infrared (λ=5.4 μm) quantum cascade lasers (QCLs) with a straincompensated, 30-stage (1.53 μm thick) InGaAs/AlInAs active region, grown via metal organic chemical vapor deposition. This process, previously used in GaAs-based diode lasers containing low-Al content AlGaAs or even Alfree III-As alloys, forms a highly-insulating native oxide layer while simultaneously smoothing and passivating the etchexposed active region, resulting in low-loss, strongly-confining waveguides. Here we report the first application of this process for directly oxidizing the deeply-etched QCL InGaAs/AlInAs active region ridge waveguide sidewalls and field (outside the ridge), eliminating the need for a deposited dielectric for electrical isolation, thus allowing self-aligned device fabrication. An 8 hour, 500 °C wet oxidation with 7000 ppm added O₂ (relative to N2 carrier gas) yields a uniform oxide of ~350 nm in the field outside the ridge to ~500 nm on the ridge sidewall. Laser devices tested under room temperature, pulsed excitation exhibit a threshold current density of J_th~3.2 kA/cm² for a 19.5 μm wide x 3 mm long stripe width.

We report on a study to determine the effect of waveguide side-wall roughness on Quantum Cascade (QC) laser performance, such as threshold current density, slope efficiency, far-field beam pattern and group refractive index, using two two-wavelength heterogeneous cascade QC laser structures, one with emission wavelengths of 7.0 μm/11.2 μm, and the other with 8.7 μm /12.0 μm. For the range of roughness standard deviation values from about 0.4 μm to 1.0 μm for which all four QC lasers were operating, the threshold current density increases by 12%-15% and the slope efficiency decreases by 30%-70% with stronger performance degradation for the shorter wavelength lasers, which is in agreement with a model based on Rayleigh scattering. Moreover, no significant change in the far-field beam patterns for different σ_rough values was observed, and the group effective index values of the four wavelengths have several values for each rough waveguide indicative of multiple transverse modes in the waveguides.