The United States government and industry have a long standing interest in developing high power in-plane semiconductor lasers for a variety of applications. These include material processing, long range sensing, and long range communications. The wavelength depends on the application, and sometimes on eye safety considerations. Key development goals are high brightness and high efficiency. This paper will review some of the applications, key laser performance features desired, and some of the accomplishments in high power diode lasers from the High Power Semiconductor Laser Technology program.

Use of aluminum-free epitaxial structures for near-infrared laser diodes has been an active research area in the past decade. These edge-emitting laser diodes have demonstrated operational lifetimes exceeding 10,000 hours at high output powers and high efficiencies. Improvements in epitaxial structure, processing, and packaging have enabled these results. In this paper, we will review developments in aluminum-free laser diodes. We describe our recent work with these devices and conclude by discussing reliability issues.

This paper focuses on the laser diode performance. The devices of interest operate in the spectral range of 800 nm to 915 nm. Some fabrication aspects of highly efficient laser diodes are drawn. Brief comparison of two major material systems is presented.

Tensile-strained GaAsP quantum wells (QWs) embedded in AlGaAs waveguide and cladding layers are an alternative approach for the wavelength range 700 - 800 nm. We will present a detailed experimental and theoretical study of the dependence of the threshold current on the thickness and the strain of the QW for 800 nm. The optimum thickness of the GaAsP QW for a minimum threshold current density is about 14 nm and is thus much larger than for compressively strained QWs. Higher characteristic temperatures T₀ can be obtained with even thicker QWs. In order to achieve high optical output powers and good fiber coupling efficiencies, we used broad waveguides with weak optical confinement and small far field divergence. We prepared two structures with 1 micrometers thick Al_0.65Ga_0.35As (structure A) and 2 micrometers thick Al_0.45Ga_0.55As (structure B) waveguides, respectively. For structure B, the thickness of the Al_0.70Ga_0.30As cladding layers must be carefully optimized in order to suppress higher-order transverse modes. Whereas structure B yields a higher maximum cw output power of AR/HR coated broad-area devices, structure A shows a better high-temperature behavior. Aging tests performed at 2 W (100 micrometers stripe width) and 25 degree(s)C suggest a very good reliability of these devices.

Recent progress in tapered high-brightness lasers emitting in the near infrared region from 1.3 to 2.0 micrometers is reviewed. Improved power and beam quality are obtained for tapered lasers operating near 1.55 micrometers using Gaussian distributed lateral carrier injection profiles. Results for high-brightness 9-element arrays of tapered lasers emitting near 2.0 micrometers are included. Also included is a discussion of the use of mass-transported microlenses for collimating the output of the astigmatic tapered devices and coupling them into optical fibers.

The maximum useful optical power of laser bars is limited due to thermal and lifetime constraints to typical values of 50 W/cm cw or 120 W/cm qcw. A promising new approach is the so-called microstack laser in which several laseractive areas are integrated vertically in the same monolithic structure. In order to drive these structures in series with high efficiency low-resistance tunnel-junctions have to be realized. By optimizing the MOVPE growth process tunnel- junctions with a specific differential resistivity of 2.5 X 10^-4 (Omega) cm² could be obtained, which are suitable for the monolithic inter-connection of laser structures.

MOVPE grown InAlGaAs SQW lasers for approximately 730 nm have been optimized for low oxygen incorporation, where diethylether associated with trimethylindium (TMI) provides the dominant oxygen impurity. An ether free preparative route to TMI, together with a large indium fraction for the SQW has provided device lifetimes of approximately 2000 hours when operating at 1.2 W and 743 nm.

980 nm InGaAs/InGaAsP/AlGaAs strained quantum well lasers with novel large optical cavity and asymmetrical claddings was fabricated by MOCVD. Very high differential quantum efficiency of 90% (1.15 W/A) and low vertical divergence angle of 24 degree(s) at long cavity length were obtained for 100 micrometers stripe lasers. The differential quantum efficiency is up to 94% (1.20) at cavity length of 500 micrometers .

We have investigated dependence of the saturation output power on the wavelength and the cavity length of red laser diodes (LDs) with the wavelength range of 646 - 689 nm. In the 60 degree(s)C CW operation of 650 micrometers -long-cavity LDs, the saturation powers of the 659 nm-LD and the 687 nm-LD were 90 mW and 124 mW, respectively. As a result of extension of the cavity lengths from 650 micrometers to 900 micrometers , the saturation output powers of the 659-nm LD and the 687 nm-LD are increased to 121 mW and 165 mW, respectively. This improvement has led to the first realization of 1000-hour, 100 mW CW operation of the 659 nm-LDs. Also, the 900 micrometers - long 687 nm-LDs have shown the reliable 120 mW CW operation at 40 degree(s)C for 1800-hour, for the first time.

We implemented various laser structures in the InGaAlP material system for operation in the 635 - 680 nm wavelength range. By incorporating a grating reflector within the device we can stabilize the lasing frequency, or maintain single spatial mode operation in a broad area device. Such devices include DBRs, tunable DBRs, monolithic MOPAs, DFBs, and angled-grating DFBs. Such performance can also be obtained without a buried grating layer by using an external fiber grating to stabilize the frequency or with a flared structure for high power single mode operation.

The role of a robust impurity free quantum well intermixing process in fabricating high-power high-brightness AlGaInP semiconductor laser diodes is outlined. Characteristics of the process are discussed and its attributes summarized. Bandgap shifted lasers have been fabricated to demonstrate the integrity of the material after the quantum well intermixing process. Oxide stripe lasers with non-absorbing mirrors are shown to increase the catastrophic optical damage threshold of semiconductor laser devices. Finally high brightness extended cavity lasers are shown to significantly improve the beam quality, and the insignificant change in the threshold current and slight decrease of the external efficiency demonstrates that the process is low loss.

Recent progress towards the realization of high-power, non- cryogenic (quasi-)cw mid-IR lasers based on the `W' configuration of the active region is reported. Type-II diodes with AlGaAsSb broadened-waveguide separate confinement regions are the first III-V interband lasers to achieve room-temperature pulsed operation at a wavelength longer than 3 micrometers . For cw operation, T_max was 195 K and P_out equals 140 mW was measured at 77 K. Optically- pumped W lasers recently attained the highest cw operating temperatures (290 K) of any semiconductor laser emitting in the 3 - 6 micrometers range. For a (lambda) equals 3.2 micrometers device at 77 K, the maximum cw output power was 0.54 W per uncoated facet. In order to maximize the absorption of the pump in the active region, an optical pumping injection cavity structure was used to create an etalon cavity for the 2.1 micrometers pump beam. The pulsed incident pump intensity at threshold was only 8 kW/cm² at 300 K for this edge- emitting mid-IR laser. The differential power conversion efficiency was 9% at 77 K and 4% at 275 K, which indicates promising prospects for achieving high cw output powers at TE-cooler temperatures following further optimization.

IV/VI diode lasers are the workhorse of mid infrared gas analysis with high sensitivity and selectivity. These lasers are made commercially and their emission covers the wavelength range from 3 micrometers to 20 micrometers within the material family. Continuous emission is observed to above 200 K, and pulsed emission to above 80 degree(s)C. Typical emission properties like tuning rate, output power and line width are discussed.

We report the recent progress of interband cascade (IC) lasers based on InAs/Ga(In)Sb/AlSb type-II quantum wells. For the 4.5-micrometers IC lasers, the internal loss was 11.6 cm^-1 and the internal quantum efficiency was 460% at 90 K. When mounted epi-side down on diamond, cw operation was observed with an external quantum efficiency (EQE) of 193%, a cw output power over 500 mW, and a threshold current density as low as 35 A/cm² at 80 K. Dual-wavelength IC laser was also demonstrated. The device lased simultaneously at 4.482 and 4.568 micrometers . At 110 K, a peak output power of 150 mW per facet was achieved with 5-microsecond(s) pulses at 1-KHz repetition rate. The threshold current density, average EQE, and peak output power of a 0.4-mm long device were 119 A/cm², 278%, and 150 mW per facet, respectively.

We report our progress to date in the development of type-II interband cascade lasers emitting in the mid-IR (3.8-to-4.0 micrometers ) spectral region. We have demonstrated significant improvements over previously reported results in terms of differential external quantum efficiency (approximately 500%), peak power (>4 W/facet), peak power conversion efficiency (approximately 7%), maximum operating temperature (217 K), and continuous-wave (cw) operation. We briefly review some results for pulsed excitation and then present more detailed operating characteristics for the cw performance of our lasers, including the output power characteristics and the dependence of the output spectrum on current.

The first demonstration of an epi-down mounted type-II optically pumped (OP) mid-infrared (IR) laser grown on an InGaAs-GaAs bonded substrate is reported. The device consisted of 60 periods of InAs/InGaSb/InAs/AlSb quantum wells, and was mounted epi-side down on a Cu heat sink. The InGaAs-GaAs bonded substrate allowed the device to be pumped from the substrate side of the laser with a 980 nm diode laser array. The laser emitted at 4.6-micrometers at 80 K, and peak power output was 300-mW per (uncoated) facet for 10 microsecond(s) pulses, and 200-mW per facet for 500-microsecond(s) pulse duration, 500-Hz-repetition rate. For comparison, a low- filled-factor mid-IR OP laser grown on a GaSb substrate was also studied. This device ((lambda) equals 3.67 micrometers ) showed improved external quantum efficiency (approximately 26%) compared with previously type-II OP lasers, and high peak output power (> 0.45 W per facet 500-microsecond(s) pulse).

We have proposed a new physical approach to design mid-IR lasers based on type II heterostructures with strong asymmetric band offset confinement at the interface. It allows to create the high barriers for carriers and to reduce leakage current from an active region, that leads to increase the quantum efficiency of the emission due to the strong accumulation of recombining carriers. Here this approach was successfully used for fabrication high power lasers operating at (lambda) equals 3.26 micrometers . The laser structure containing narrow-gap active InGaAsEb (Eg equals 0.380 eV) layer and wide-gap confined InAsSbP (Eg equals 0.520 eV) and GaInAsSb (Eg equals 0.640 eV) layers lattice-matched to InAs substrate was grown by LPE. Such heterostructure has the band energy diagram with strong asymmetric band offsets and allows to provide high barriers for electrons at the InGaAsSb/GaInAsSb heterointerface ((Delta) E_c equals 0.60 eV) and for holes at the InGaAsSb/InAsSbP one ((Delta) E_v equals 0.15 eV). Maximum output power of 1.5 W was achieved in pulsed mode with pulse duration 1 microsecond(s) and repetition rate 100 Hz for 100 micrometers broad area laser with cavity length about of 1000 micrometers . Threshold current density was about 450 A/cm². Characteristic temperature T₀ equals 47 K was observed in the range of 77 - 140 K.

The longer lifetime is desired for high power AlGaInN based violet lasers. We found that lifetime is strongly dependent on both the initial operating consumption power and the dislocation densities in the laser stripe. Pd/Pt/Au as a metal and AlGaN/GaN superlattice as a p-type cladding layer were incorporated to reduce the operating voltage. The optimization of device parameters as well as the stripe width and the RIE etching device depth led to the lower threshold current of 3.4 kA/cm². We used the Pendeo epitaxy technique to get lower dislocation density approximately 10⁷ cm^-2. The LDs with these technologies showed an output power as high as 35 mW under room temperature CW condition without kink. The lifetime is more than 500 hours under CW operation with a constant power of 20 mW at 25 degree(s)C.

We report the results of an experimental study on near- threshold gain mechanism in optically pumped GaN epilayers and GaN/AlGaN separate confinement heterostructures (SCHs) over the temperature range of 10 to 300 K. We show that in GaN epilayers the near-threshold gain mechanism is inelastic exciton-exciton scattering for temperatures below approximately 150 K, whereas at elevated temperatures an electron-hole plasma is the dominant gain mechanism. An analysis of the relative shift between the spontaneous emission and lasing peaks in SCH samples, combined with the temperature dependence of the lasing threshold, reveals that exciton-exciton scattering is the dominant gain mechanism leading to low-threshold ultraviolet lasing in the GaN/AlGaN SCH over the entire temperature range studied. Strongly polarized (TE:TM > 300:1) lasing peaks were observed in a wavelength range of 358 - 367 nm. We found that high finesse lasing modes originated from self-formed microcavities in the AlGaN and GaN layers. The lasing threshold was measured to be as low as 15 kW/cm² at 10 K and 105 kW/cm² at room temperature. Based on our results we suggest ways for the realization of GaN-active-medium UV laser diodes.

Future high performance printing systems will require higher resolution, higher print speed, and better image quality. One way to enhance the bandwidth in raster-output printing systems is to reduce the wavelength of the laser diodes. Since the optical assembly in polygon scanner systems is diffraction limited, a reduction in wavelength of the scanning beam would significantly improve the optical resolution of the printing system compared to the currently used IR and red lasers. This paper discusses the design and performance characteristics of III-nitride based multi- quantum well laser diodes grown on sapphire substrates by metalorganic chemical vapor deposition. Room-temperature continuous-wave operation with threshold currents as low as 45 mW have been achieved with threshold voltages of 7.5 V. CW operation was observed up to 60 degree(s)C with emission wavelength around 410 nm. By using short-period AlGaN/GaN superlattice the cladding layer thickness could be significantly increased resulting in an improved transverse far-field pattern.

Following a review of ring resonator research in the past decade we shall report a novel bi-level etching technique that permits the use of standard photolithography for coupling to deeply-etched ring resonator structures. The technique is employed to demonstrate InGaAsP laterally- coupled racetrack ring resonators laser with record low threshold currents of 66 mA. The racetrack laser have curved sections of 150 micrometers radius with negligible bending loss. The lasers operate CW single mode up to nearly twice threshold with a 26 dB side-mode-suppression ratio. We shall also present a transfer matrix formalism for the analysis of ring resonator arrays and indicate application examples for flat band filter synthesis.

This paper describes work to develop a new class of micro- photonic components for use in future opto-electronic circuits. Recent advances aimed at creating practical devices include achieving continuous room-temperature operation in optically and electrically driven microdisk lasers. The improved performance is realized by the simultaneous optimization of thermal, optical, and electrical design. A novel post-processing technique to precisely control the lasing wavelength of optically pumped microdisk lasers will be discussed. Using this technique, the lasing wavelength can be tuned by more than 8 nm. Dependence of the wavelength shift on the radius of the microdisk lasers will be addressed. The ultimate performance of micro-photonic components is constrained by the physics governing device operation. As these micro-photonic devices approach the nanometer scale, quantum effects become important placing new constraints to practical device design. Noise characteristics of scaled devices with very small numbers of photons and carriers need to be understood.

Thin-p-clad, 0.97 micrometers -emitting lasers with 2^nd-order Au/air gratings and asymmetrically facet-coated, surface emit in a diffraction-limited (0.11 degree(s)) lobe, in agreement with theory. Then, symmetrically AR-coated surface-emitting devices with central phase shifts >= (lambda) /4 are found to lase in an orthonormal single lobe with good efficiency and near-field uniformity. The best solution is to use surface emitters with central phase shift and DBR end mirrors: external differential quantum efficiencies of 60 - 70% are obtained over wide ranges (120 - 160 degree(s)C) in central-phaseshift value.

Uncooled direct modulation of a 1.55 micrometers distributed feedback (DFB) laser is demonstrated at 10 Gbit/s is demonstrated. The small signal performance of the laser at 25 degree(s)C gives rise to a -3 dB bandwidth in excess of 9 GHz, reducing to 8 GHz at 55 degree(s)C. Under uncooled, direct modulation at 10 Gbit/s the back-to-back eye diagrams obtained at temperatures up to 55 degree(s)C are found to be open, with extinction ratios of 6 dB for a peak-to-peak current swing of 40 mA. Transmission at 10 Gbit/s over standard singlemode fiber, is demonstrated over 12 km with the uncooled device. Using this device, multi-level coding is demonstrated at 20 Gb/s, and transmission at 5 Gb/s over 700 m of multi-mode fiber at 60 degree(s)C.

For a novel type of asymmetric multiple-quantum-well heterostructure lasers it is shown that a flat modal gain spectrum is obtained in a wide spectral range. It occurs since the quantum wells varied in widths and chemical compositions give a definite contribution to the total gain in different intervals of the spectrum. A certain design of the laser structures (chemical composition, thickness, doping, and arrangement of active and barrier layers) provides the conditions of non-uniform excitation of the quantum wells that results in the broad-band flat gain spectrum. Output power characteristics of the tunable laser diodes with a grating external cavity are examined in detail. For the spectral interval near the wavelength of 820 nm, the GaAs-AlGaAs system is preferred. In this case, the width of the gain band reaches up 50 nm and the tuning curve is practically flat at the output power about 10 mW in a single-mode regime without mode hops. Use of the other ternary or quaternary semiconductor compounds transfers the tuning range to a necessary spectral region. The described quantum-well heterostructures are suitable to make effective tunable laser diodes for a wide variety of applications, such as WDM optical networks, coherent spectroscopy, chemical analysis, metrology, and environment monitoring.

All manufactured sampled grating structures are mainly used for `wavelength tunability'. In this paper we succeeded in generating two modes by using Sampled Grating DFB laser. The two modes, will be optically heterodyned on a photodetector for mm-wave (via mode beating) signal generation. The advantage of that design is its size compactness (a single section, single electrode fed with a DC current supply), stable mode spacing, and relatively lower cost compared with other means of mm-wave generation. The simulation results will be presented using the well established model. Two parameters are shown to be affecting on the dual mode spacing and amplitude: the coupling coefficient and the carrier density variation along the (SG) structure. The former (coupling coefficient) mainly influences the mode spacing which is translated into the mm-wave frequency at the photodetector level. The anomalous coupling coefficient behavior leading to a quasi-stable mode spacing independent on the DFB section length increase is studied, interpreted and optimized. The latter (carrier density) plays a dramatic effect on that structure, resulting on fading away one of the two modes. We, succeeded in confining these two chirped modes into the structure by detuning the gratings. Finally, detuned grating effect will be explained.