A new era of ever increasing optical output power, efficiency, and reliability has been achieved for GaAlAs quantum well lasers over the past several years. Beginning in 1982 quantum well laser diode arrays were demonstrated to be capable of achieving CW room temperature operation" Since that time higher output powers, higher efficiency, and greater reliability lasers have been demonstrated.

Stable diffraction limited high order array mode operation has been achieved to high drive current levels above threshold. Data is presented from two different array structures, both of which incorporate the wide-waveguide-interferometric array pattern together with strong transverse antiguiding between array elements in order to select L=8 array mode operation. Evidence supporting recent theoretical predictions that higher-order modes of evanescently coupled arrays are stable, against spatial hole burning is also presented.

Single stripe broad area GaAs/A1GaAs laser diodes were fabricated by Metal-Organic Chemical Vapor Deposition (MOCVD) epitaxial growth method. A CW output power of more than 1 watt was obtained with a single-lobed transverse-mode operation. The power conversion efficiency of the laser was as high as 33% with relatively low operating current. Both the transverse and longitudinal modes show basically single-lobed envelopes with fine components. The characteristics of these lasers can be explained by the mixed state of the fundamental and higher-order modes.

Two-dimensional laser diode arrays can be divided into two basic types: (1) "stack-and-rack" and (2) monolithic. The first approach involves stacking linear arrays of edge emitters, which emit light parallel to the epitaxial layers, into 2-D arrays. These devices have been used primarily to side pump (in single device form) or surface pump (in 2-D arrays) solid state lasers. Monolithic arrays are a collection of devices which emit light normal to the epitaxial layers. Three designs currently under development can be divided into two categories: 1) conventional laser diodes which use either etched turning mirrors, or a Bragg grating to couple the light out of the epitaxial layers and 2) laser diodes with their optical cavity perpendicular to the epitaxial layers. This paper reviews the current status of each of these devices and compares the characteristics of each approach. These parameters include: packing density, electrical contact techniques, pulsed optical power density, CW optical power density, and processing and assembly issues. The results of this comparison clearly shows that the technology for the stacked array design approach is more mature than it is for the monolithic surface emitters and is the dominant approach for achieving high optical fluences.

A simple model of semiconductor diode optical amplifiers is developed and applied to broad-area structures. Small signal gains as high as 20dB are shown to be feasible. Effects of physically tapering the amplifier to saturate the gain more efficiently are examined. A potential 10% increase in extraction efficiency is shown. Practical limits on the amplifier gain are discussed.

Phase-locked operation of an array of semiconductor lasers has been extensively studied in recent years [1-12] with the eventual goal of obtaining high power (several hundreds of milliwatts) into a single spatial lobe of radiation. Earlier, the studies were concentrated on the integration of two types of lasers, namely, gain guided [1,2,5,6,] and index guided [7] stripe geometry lasers into an array with different stripe widths and varied spacing between the stripes. Phase lock of these lasers can only be achieved with a proper combination of width, stripe spacing and driving current. A considerable amount of interest has since been focused on the study of conditions under which a single spatial mode with narrow divergence angle can be produced. At present there is no reproducible process that can be used to fabricate single lobe phase-locked semiconductor arrays. The main application of the semiconductor array now is to pump solid state lasers. In this case we do not require phase locking or single lobe far field. In this paper we report fabrication of a different kind of semiconductor laser array, i.e. incoherent laser array. The main features of the incoherent laser arrays are : 1) Low threshold index-guided laser elements, 2) Single lobe far-field pattern, 3) Low astigmatism, 4) Low current operation, 5) Densely packed and 6) Total electrical and optical isolation. We expect such laser arrays will have important applications in multi-head optical-disk reading and writing, multi-fiber optical communications and line of sight communications.

Laser diode pumped, solid state lasers provide an efficient, compact, reliable, and stable source for many applications. We have previously demonstrated a diode pumped Nd:YAG slab laser that achieved over 100 mJ Q-switched output at 20 Hz with greater than 4% efficiency. Recent advances at MDAC-STL in laser diode performance and efficiency predict that over 10% is now achievable. In addition, we have demonstrated a five-fold increase in laser diode array peak power and an order of magnitude increase in array average power capability. Last year, we demonstrated a laser diode bar side-pumped, Nd:YAG rod laser with 2W cw fundamental mode output at 3% efficiency. We have recently extended cw pumping to two-dimensional laser diode bar arrays. We achieved over 50 W/cm² cw operation from a 0.28 cm², 2-D bar array with good efficiency. Array cooling was conductive to a water-cooled heat exchanger. This achievement lays the foundation for higher power cw, laser diode pumping of slab geometry lasers.

TWo methods are describrd in this paper to obtain high power in a single lateral mode operation in a broad stripe semiconductor laser. (1) Antiguiding index profile-solution to the wave equation for a one-dimensional, quadratic variation in the complex refractive index profile are used to calculate the optical gain and the intensity profile of the lateral modes of broad-stripe semiconductor lasers. A large gain difference between the fundamental mode and higher order modes is obtained in profiles with strong real-index antiguiding and weak gain-guiding. A criterion for comparing the tendency for single mode operation is introduced and applied to quadratic, step, and array profiles. (2) Variable facet reflectance-the reflectance at the facets of a wide stripe laser is made to vary spatially across the width of the stripe. It is found that the gain differences between the fundamental mode and higher order modes can be greatly increased. Several reflectance profiles are examined and the results are campared using the same criterion as in method (1).

Channeled-Substrate-Planar (CSP) AlGaAs/GaAs semiconductor lasers are important, established, commercial products and have been extensively studied both experimentally and theoretically.^1-12 They have single spatial mode output powers as high as any single element semiconductor laser^4,5 and have demonstrated long life at very high power.⁶ They have been used as the elements in linear^13,14 and Y-guide^15,16 arrays . Originally grown by liquid phase epitaxy (LPE) on n-types and later p-type2 GaAs substrates, functionally equivalent structures are also grown by metalorganic chemical vapor deposition (MOCVD)^17,18 and molecular beam epitaxy (MBE).^19,20 In contrast to CSP lasers, Grating Surface Emitting (GSE) lasers are just now moving beyond the "laboratory curiosity" phase, having demonstrated the capability for high power and moderate efficiencies.²¹ GSE lasers offer the potential for very high power (> 1W) and high brightness when integrated into coherent one-dimensional²² and two-dimensional arrays.²³ Although an early GSE laser was based on a conventional CSP structure,²⁴ recent rapid improvements in GSE performance are a result of advances in 1) material growth technologies which provide quantum well capability and 2) processing technology. In this paper, we review the lateral guiding mechanism of CSP lasers and compare experimental and analytical results of CSP lasers with geometrical and compositional nonuniformities. Finally, we review the structures and performance of recent GSE lasers.

A model is presented to explain some of the optical properties of traveling-wave amplifiers made from gain-guided diode laser arrays. The model describes the amplifier as a complex volume diffraction grating with the grating period being the array period. The volume grating has an aperture limited by the width of the array. Experimental measurements of' the far-field pattern and the diffraction efficiency into the side-lobes as a function of the incident angle of the injected signal are compared to calculations based on the model. Good fits are obtained to both sets of experimental data with only gain and refractive index modulation as parameters. New data are presented for the diffraction efficiency of the volume grating as a function of the current to the amplifier. These data are fit to this same model using the same parameters and assuming that the gain and index modulation at each current are proportional to the measured gain at the same current less the value of the gain at zero current. The implications of this model to amplifiers and lasers will be discussed.

High power semiconductor laser diodes from the Matsushita Electronics Corporation have been selected by TRW for use in the NASA space laser communications experiments. Three 70 mW buried twin ridge substrate (BTRS) GaA lAs lasers, operating in a band of wavelengths near 870 nm, will be outfitted with an external cavity "third" mirror element and coaligned using narrow bandpass filters. Diode selection criteria, design features, and measured performance data will be discussed.

In practical uses, life time is an important parameter for high power diode lasers. In general, one can find out two components in degradation mode of the A1GaAs/GaAs lasers, that is, gradual and sudden increases of operation current to keep the output power constant. The gradual degradation might be due to increase of non-radiative recombination centers in an active region, or to some kinds of facet oxidation which results in slow reduction of reflec-tivity. In order to avoid or eliminate the gradual degradation, it has been required to reduce the mechanical stress and temperature rising and to passivate the facets. The sudden increase of operation current could be understood in terms of facet degradation, which is mainly caused by high optical power density. The most radical case is the so-called COD(catastrophic optical damage). The COD is one of thermal effects coming from local temperature rising attributed to self-absorption of light at the facet region where the population inversion is lost by fast surface recombination velocity. The COD level is increased by passivation of dielectric film such as Si₃N₄ or Al₂0₃ to 1.5-2.0 times higher than the bare facet. As absorption intensity at the facet is determined by not output power but inner power at the facet, the inner power is an essential parameter for high power diode lasers. Output power is varied by changing the facet reflectivity, even though the inner power is controlled to be same level. It is generally known that inner power density at passivated facet corresponds to approximately 2-3MW/cm² . The relationship between output power Po and inner power Pi is expressed as

Recent results of our work an InGaAsP/InP single element, diode lasers are presented. Two structures are compared: 1) the double channel planar buried heterostructure (DCPBH) and 2) the buried crescent on p-type substrates (p-BC), for power and optical-beam quality. Lifetest data for our DCPBH's is given, and compared to recently published lifetest data for p-BOs. It is clear that InGaAsP/InP lasers have high reliability at high output powers, and that buried crescent-type structures have superior optical quality in the output beam over DCPBH structures.

The fabrication and operating conditions for semiconductor laser arrays to be phase-locked are estimated in an analysis based on the van der Pol theory of the locking of coupled oscillators. Typical numbers for 10 pm spacing show tat the average aluminum concentration in adjacent stripes must be the same to within ≈10^-4, the average epilayer thickness must be the same to within 0.2%, and there is a few percent tolerance on the stripe width and contact resistance. These close tolerances support the fact that high quality fabrication processing is usually required for phase-locked laser arrays, and increases in coupling may be desirable.

A one-dimensional thermal model has been extensively modified for calculating the maximum output power from InGaAsP/InP (X = 1.3 μm) semiconductor lasers, whose output power is limited by thermal considerations. The effect of Auger recombination which plays a significant role'in these lasers at high temperatures is included. We have also incorporated the temperature dependence of efficiency from first prin-ciples using experimentally available data for Auger and radiative recombination coefficients. Calculations made on InGaAsP/InP lasers show that a maximum CW power of 57 mW/facet (Diamond Heat Sink) and a maximum operating temperature of up to 132° C for a geometry similar to the Double-Channel Buried Heterostructure (DC-PBH) laser can be achieved. In addition, the model has been used to determine the maximum achiev-able power as a function of device geometry (active layer thickness, width and length of the device). We find that by increasing the length of the laser from 300 microns to 700 microns we can increase the output power of the laser by 79%. The results obtained agree reasonably well with experiment.

The fabrication of high performance laser diodes and other optoelectronic devices requires high resolution patterning of the energy bandgap and resistivity of III-V heterostructures. Several forms of laser-assisted processing have been demonstrated and applied to the fabrication of optoelectronic devices such as solar cells, LED's, and lasers. In this paper, we describe in detail two of the most promising approaches to laser-assisted processing, namely laser-assisted disordering (LAD) and laser-controlled generation of defects. Laser-assisted disordering (LAD) is a two step process for patterning impurity induced layer disordering (IILD). The Si impurity is first incorpor-ated into a GaAs-AlGaAs heterostructure by laser doping. Then, a standard thermal anneal is used to drive the Si deeper into the heterostructure to locally intermix the layers by Si-IILD. LAD has been studied using secondary ion mass spectroscopy and transmission electron microscopy, and we have used LAD to fabricate low threshold (Ith<3 mA) buried heterostructure lasers. The second form of direct-write laser-assisted processing uses thermal generation of defects to locally increase the resistivity of the GaAs cap layer. We have used this form to fabricate high-power diode laser arrays. Laser-assisted processing techniques are versatile and effective, and will play a prominent role in the development of high-power solid state sources.

Surface emitting lasers are of interest for various applications such as monolithic two-dimensional arrays and optical interconnects for integrated optics. Moreover, surface emitting lasers offer the advantage of wafer processing and testing. Several approaches for achieving surface emission are described. In addition, TRW's fabrication of a large monolithic two dimensional array of GaAs/AlGaAs surface emitting lasers which contains a total of 100 lasers is reported.

Continuous wave surface emitting distributed feedback lasers without facet reflections were fabricated which produced up to 25 mW of cw power and 200 mW of pulsed power at room temperature. The devices operated in a single spatial and single spectral mode with very low divergence output beams.

The buried heterstructure laser, which possesses highly desirable characteristics such as low threshold current and single lobed output, but is limited to several mW of output by catastrophic optical mirror damage (COMD), has been combined with nonabsorbing facet technology to achieve CW outputs in the 100+ mW region. At high power levels kinkings and far field shifts may occur. An analysis of the laser is performed which indicates techniques for minimizing these effects. The analysis also indicates the possibility of circularizing the far field pattern.

Since 1970 FLAN, in cooperation with industry, has been developing "Laser CRTs" (LCRT) and studying their potentialities. The parameters (the brightness, monochromacity, the spectral regions from UV to IR) of the Laser CRT are much higher in the Laser CRT than in conventional phosphorous screen CRTs.

Recent advances made in our laboratory toward performance optimization of (A1)GaAs quantum well lasers are described. Topics to he covered include: laser reliability for broad-area devices emitting less than 300mW and its relation to the epitaxial structure and operating current density; parametric crystal growth studies and the implications for device efficiency; realization of 57% cw power conversion efficiency in an oxide-defined device; progress in dry-etching technology including array fabrication and development of device-quality laser facets suitable for integration. Finally, work in the high-power regime (>5 Watt) will be discussed. This includes broad-area single-emitter lasers emitting 6W cw.

Graded barrier quantum well (GBQW) heterostructure broad area lasers are capable of high power pulsed and cw operation. In this paper, we describe some of the design issues involving broad area graded barrier quantum well heterostructure lasers grown by metalorganic chemical vapor deposition including the effect of junction heating in cw devices and the effect of various buffer layer structures on laser characteristics. We also outline some high power laser diode results for uncoated broad area devices.