Al-free active-region (ALFA) diode lasers have come of age. Record-high CW and quasi-CW powers have been obtained at (lambda) approximately equals 0.97 micrometer. For wavelengths at and below 0.81 micrometer, ALFA-type lasers are definitely superior to Al-containing -- active lasers. Al-free lasers both open a whole new wavelength range: 0.7 - 0.78 micrometer; for commercial use as well as make possible the easy fabrication of watt-range, coherent diode lasers with stable- beam behavior.

In this paper, we present results on diode lasers in the wavelength range between 715 nm and 840 nm with Al-free QWs which are embedded in a high-quality AlGaAs LOC broadened waveguide structure with low optical loss and a small vertical far field divergence. The laser structures were grown by LP- MOVPE. We studied tensile-strained GaAsP-QWs as well as compressively strained InGaAsP-QWs with strain compensating barriers. For lasers with GaAsP QWs, the lowest transparency current densities of about 130 A/cm² were obtained in the wavelength range between 750 nm and 800 nm. Very low transparency current densities were achieved with InGaAsP-QWs at wavelengths above 800 nm. At 810 nm, high output powers (100 micrometer aperture) of about 7 W was achieved with both types of QWs from devices mounted epi up. However, with respect to high temperature operation and reliability tensile- strained GaAsP QWs seem to be the better choice, especially for the wavelength range below 760 nm.

Fifty-micrometer wide Al-free active region SCH-SQW lasers ((lambda) equals 808 nm, 32%/97% coated) grown by low-pressure MOVPE have shown stable operation at 700 mW and 50 degrees Celsius over 2000 hours without using a broad waveguide (d/(Gamma) equals 0.45 micrometer). We have made systematic study on the effects of the waveguide width Wg for 50- and 200-micrometer wide stripe lasers in the range of Wg equals 0.22-1.2 micrometer while the well width d is kept constant at 10 nm. The internal loss is reduced from 2 to less than 1.5 cm^-1 when Wg is increased from 0.22 to 1.2 micrometer. For 200-micrometer wide lasers, the slope efficiency monotonically increases with Wg for pulsed operation, and however, it is maximum at Wg equals 0.8 micrometer (d/(Gamma) equals 0.6 micrometer) for CW operation. When 200-micrometer wide devices (20%/97% coated) were operated at 2 W and 30 degrees Celsius, the median degradation rate showed a minimal value of 3 X 10^-6 h^-1 at Wg equals 0.8 micrometer, which is 7 times smaller than that at Wg equals 0.22 micrometer. The facet temperature measured by the modulation reflectance is also minimized at Wg equals 0.8 micrometer. For broad area lasers, thermal effects compete with the reduction of the optical confinement in the active quantum well, which determines the optimal Wg.

High power InGaAs/AlGaAs laser diodes with decoupled confinement heterostructure (DCH) have been developed. Almost Al-free waveguide and cladding layers were realized in 980 nm DCH laser diodes without degrading temperature characteristics. The extremely low electrical and thermal resistances allowed high power and efficient operation. The maximum CW output power as high as 9.5 W was obtained with 100-micrometer-aperture broad area. DCH laser diode. The maximum efficiency was 55% at 2.5 W. The series resistance of 1.8-mm long cavity was 0.04(Omega) and internal loss was 1.5 cm^-1. The characteristic temperature (T₀) was 155 K. The substantially Al-free DCH structure enables easy fabrication of various index guided laser diodes. We have developed two types of real index guided laser diodes, buried- ridge and self-aligned structure. Buried-ridge laser diode presented 1.3 W maximum CW output power and 500 mW single mode operation. Self-aligned structure laser diodes showed 1.4 W CW output power and 700 mW single mode operation with better reproducibility.

State-of-the-art pump lasers for fiber amplifiers in optical telecommunication systems can deliver up to 250 mW optical power in single mode emission. Power approaching 500 mW is seen as the upper limit for single mode lasers without sacrificing reliability. For yet higher power, broad area laser diodes coupled into the cladding of ytterbium-erbium doped fibers have a great potential to achieve more gain. Here, Diode lasers with a ridge width of 30 micrometer mounted junction-side-up using a hard-tin solder are presented as an alternative over EDFA pumping by single-mode lasers. Reliable broad area lasers are obtained by adopting standard narrow-stripe single-mode laser technology with proven 110 FIT at 150 mW and 25 degrees Celsius varying only the ridge width. Reliability and packaging problems commonly associated with p- side down mounting are avoided. Due to the high power conversion efficiency greater than 55% of the laser chip, a cw maximum output power as high as 1.1 W can be obtained. Submicron fiber alignment tolerances are no longer required, as compared to single mode lasers. Due to low heat dissipation these 30 micrometer stripe lasers can be operated in standard butterfly packages and high coupling efficiencies are achieved above 80% into standard 50 micrometer multimode fibers. YEDFA's with 23 dBm output power using these lasers have been demonstrated.

High-power diode lasers have reached output power and reliability to meet requirements for industrial applications. Stacking of laser elements to modules increases the output power, beam shaping techniques allow to focus the radiation of a module to a single spot. An integrated diode laser systems with totally 50 laser bars is shown, that includes cooling, power supply and control unit. The laser radiation is transmitted by an optical fiber and an objective focuses the radiation onto the workpiece with a round spot of less than 1 mm diameter and cw power of more than 1 kW.

The fundamental question whether aluminum-free semiconductor lasers in the 808 nm band are significantly more reliable than Al-containing lasers is still open. We have fabricated and tested high-power InGaAlAs/GaAs-lasers which show excellent reliability data at and above 40 W cw. The laser structure consists of an InGaAlAs-double-quantum well (DQW) as active layer embedded in a large optical cavity (LOC) waveguide structure. The layers were grown in a low pressure MOVPE (LP- MOVPE) reactor using high quality precursors. Asymmetrically coated bars with a width of 1 cm containing 25 groups of 200 micrometer wide emitters were mounted junction down on actively cooled heatsinks. At a heatsink temperature of 18 degrees Celsius the slope-efficiency is 1.1 - 1.2 W/A. Due to the low series-resistance of 2.2 m(Omega) and the low internal losses in the range of 1.7 cm^-1 the overall efficiency at 40 W cw reaches 50%. Lifetime studies over 33 0000 h accumulated device hours show that the laser bars with a resonator length of 900 micrometer can be operated at 40 W with high reliability. The mean degradation rate is -0.11%/kh. This result emphasizes that Al-containing lasers can also have a very high reliability usually claimed for Al-free lasers. As a consequence of these encouraging results we will start further lifetime tests at 50 to 60 W.

High power diode laser arrays have been demonstrated using relatively simple manufactured water-cooled packages. The layer structure of the graded-index separate confinement heterostructure (GRIN-SCH) using AlGaAs material systems were employed for the 0.8 micrometer-emission. The laser bars have broad-area stripes (70 micrometer) with a pitch of 150 micrometer and a cavity length of 500 micrometer. A maximum power of 40 W per 1-cm-laser-bar was obtained for the layer structure with 10-nm-thick quantum well. We stack 15 laser bar modules (laser block) and fabricated two-dimensional laser arrays with 45 laser bars, consisting of 3 laserblocks. The total output power is about 680 W, which corresponds to an intensity of about 110 W/cm². The reliability data of 200- W laser blocks are shown to be about 5000 hours extrapolated by 2000 hours' data.

A variety of applications continue to demand semiconductor lasers with higher brightness. These applications include printing, marking, frequency-doubling, sensing, and free-space communication, as well as pumps for fiber-amplifiers and solid-state laser crystals. The traditional narrow-stripe single-mode laser is limited in power to 200 - 400 mW due to the small mode volume and high optical intensity at the mirrors. The traditional broad-area laser can deliver higher power but with a significantly larger aperture-divergence product. The key to very high-brightness lasers is to increase the output power without increasing the aperture-divergence product. Numerous concepts have been proposed, of which several have been demonstrated. As of today, only two types of broad-area single-spatial-mode lasers are commercially available; flared-amplifier lasers and angled-grating distributed-feedback ((alpha) -DFB) lasers. Near-diffraction- limited powers of 5 W and 1.6 W have been reported for these lasers, respectively. This paper will review properties of flared-amplifier lasers in several different configurations.

Near-resonant antiguided arrays with large effective-index step between element and interelement regions have been developed. 2.2 X diffraction-limited beamwidth (0.67 degrees) at 1.6 W and 9 X threshold in CW operation has been achieved. 1 W power resides in the central lobe. The external differential quantum efficiency and threshold current are 40% and 0.4 A, respectively, for 1 mm-long devices of 191 micrometer emitting aperture. The power conversion efficiency at 1.6 W is 23%. The thermal lensing effect in high-index-step near-resonant antiguided array hardly affects high-order modes and therefore, about 2 X diffraction-limited beam can be maintained to high powers.

The modal performance of focusing circular grating-coupled surface-emitting lasers has been investigated. Both the effects of reducing the resonator diameter and introducing an unpumped indentation in the center of the resonator have been studied. We have found that inclusion of such an unpumped central indentation can greatly improve the spatial mode behavior, resulting in a very uniform near field intensity, single wavelength emission, and a focused spot size only 1.5 times the diffraction limit for a laser oscillating in the fundamental circular mode. Our measurements at different drive currents show that the modal behavior of the laser is stable up to at least three times the threshold current, and based on the near and far-field intensity distributions, the laser is possibly lasing simultaneously in two orthogonal first order circular modes. Reducing the resonator diameter of lasers without an unpumped central indentation does not offer similar improvement to the modal behavior.

Strained Al-free InGaAs/GaAs quantum well (QW) structures are now under intensive investigation due to the potential application in high power laser diodes. During their MOCVD growth the position of the pn junction is shifted towards the p-doped barriers due to the unintentional n-doping of the nominally intrinsic region. Although this shift deteriorates the electro-optical properties it is difficult to detect this effect. Systematic optical investigations performed on coupled double quantum well (CDQW) structures as a function of applied dc bias voltage demonstrate that a combination of photocurrent (PC) and electroreflectance (ER) measurements is not only capable of revealing this effect but also allows the determination of many details of the CDQW subband structure. A comparison of these experiments with calculated transition matrix elements makes a further optimization of both the layer structure and the growth process possible.

We examined the temperature characteristics of light emission spectra and threshold currents for columnar-shaped self- assembled InGaAs quantum dot lasers which show lasing from the ground-state quantized level. While lasing occurred with one line including a series of longitudinal modes at room temperature, spectra at 80 K showed broad lasing emission over a range of 50 - 60 meV. We explain this result by a model that dots with different energies start lasing independently at low temperatures due to their spatial localization, while at room temperature the dots contribute to one-line lasing collectively via homogeneous broadening of optical gain. We attribute the origin of homogeneous broadening to scattering processes to exchange carriers between the ground state and excited states. The characteristic temperature of threshold currents was T₀ equals 60 - 90 K between 287 K and 353 K. By evaluating the current dependence of the spontaneous emission spectra up to lasing thresholds at various temperatures, we found that this low T₀ is due to the increase in the nonradiative current component, presumably caused by nonradiative carrier recombination through defects in the wetting layer. These results show the importance of controlling carrier distribution in the excited states including the wetting layer to achieve high-performance quantum-dot lasers.

Impurity-free vacancy disordering (IFVD) was used to elaborate AlGaAs quantum-well (QW) laser diodes with non-absorbing mirror (NAM) facets to rise the catastrophic optical mirror degradation (COMD) threshold. Prior to laser processing, blueshifted emission windows were formed by rapid thermal annealing at 930 degrees Celsius below SiO₂ cap layers. The photoluminescence (PL) energy blueshift measured between active and window regions is around 30 meV, with no degradation on the PL signal. Active region, free of SiO₂ encapsulation, exhibits however a non-negligible PL blueshift which is attributed to an internal interdiffusion process. This effect seems to be related to the silicon doping of the n-type AlGaAs cladding layer. 120-micrometer-aperture broad area lasers were fabricated both with and without 25 micrometer NAM sections on both sides. COMD was measured under pulsed operation (50 microsecond(s) - 100 Hz) using an epi-side-up mounting configuration. Uncoated NAM facets exhibit, on the average, a COMD threshold 1.5 times higher than that of standard ones: we achieved by this way a maximum peak power of 2 W from a 782-nm emitting laser.

Impurity free intermixing of In_0.22Ga_0.78As/GaAs quantum wells in a semiconductor laser has been carried out by rapid thermal annealing various width ridges capped with SiN_x film. We have observed greater blue-shifting of the lasing wavelength for wider ridge lasers, caused by greater degree of quantum well intermixing. This dependence of quantum well intermixing on the ridge width has been explained by the redistribution of stress fields created by the SiN_x capping film on the patterned structure.

A new method of additional transverse mode selection in high- power broad-waveguide separate confinement heterostructure quantum-well lasers by means of facet inclination is proposed. Theoretical investigation of mode selection and numerical calculations for GaAs-AlGaAs BW-SCH-QW laser with inclined facet with wavelength 860 nm and wide range of waveguide width has been carried out. The possibility of reducing power density on the mirror keeping single-mode generation and thus increasing fundamental mode output power 1.2 times is shown.

Long lifetime violet InGaN-multi-quantum-well (MQW)/GaN/AlGaN separate-confinement heterostructure laser diodes (LDs) were successfully fabricated using an epitaxially laterally overgrown GaN (ELOG) by reducing a large number of threading dislocations originated from an interface between GaN and sapphire substrate. The threading dislocations shorten the lifetime of the LDs through an increase of the threshold current density. The LDs with cleaved mirror facets showed an output power as high as 30 mW under RT-CW operation with a stable fundamental transverse mode. The lifetime of the LDs at a constant output power of 5 mW was estimated to be approximately 3,000 hours under CW operation at an ambient temperature of 50 degrees Celsius. These violet InGaN-based LDs are already commercially available in a market.

Laser diode structures were fabricated by metal-organic chemical vapor deposition (MOCVD) from the AlN-InN-GaN system on single crystal 6H-SiC substrates. An AlGaN conducting buffer layer was developed for these devices, which provides a vertical conduction path between SiC substrate and the active device region. Violet and blue multiple quantum well (MQW) separate confinement heterojunction (SCH) LDs were fabricated having InGaN wells and GaN barriers. The lowest pulsed operation room temperature threshold current density obtained for lasing was 7.1 kA/cm² in a 4-well structure. Lasing has also been obtained in these same devices at duty cycles up to 75%.

We have demonstrated a number of high power and single- frequency lasers at 635 - 680 nm by incorporating a grating reflector within the device, including DBRs, tunable DBRs, monolithic MOPAs, DFBs, and angled-grating DFBs. The DBR laser, with an unpumped grating as the rear reflector, is the simplest single-frequency structure, with about 20 mW output power. The device can be tuned about 3 nm by injecting current in the rear grating. Higher output power can be obtained by combining the DBR with a flared amplifier to form a monolithic MOPA with over 250 mW CW output power. Unlike DBR structures, the DFBs have a grating throughout their gain region, and therefore show no mode hops. Wavelengths as short as 634 nm and output powers as high as 90 mW have been obtained with DFBs. An angle-grating DFB is a broad area device where the angled grating forces lasing in a single spatial and longitudinal mode. More than 400 mW in single-frequency power has been obtained at 660 nm from such a structure. In general, grating-based red lasers are useful for interferometry, spectroscopy, and fiber-coupling applications.

High-power (greater than 50 mW) and high-temperature (greater than 70 degrees Celsius) operation of 650 nm-band AlGaInP lasers are required as light sources for high-density and recordable optical disk memory systems. However, there remains the problems such as small band-offset in the conduction band and facet degradation in AlGaInP material system which limit the high-power and high-temperature operation. This paper presents new techniques in order to achieve such high performance lasers: Window-mirror structure and real index guided structure.

We have developed a planar laser array that emits at a wavelength of 630 nm. The fabrication of this device relies on the diffusion of Zn into a conventional AlGaInP quantum-well heterostructure. Direct zinc diffusion into the AlGaInP layer forms a region with anomalously low carrier density of less than 10¹⁶ cm^-3. This provides current confinement and isolation between the elements. Zn diffusion into GaAs produces a region with a high carrier concentration that reduces the contact resistance between the GaAs cap layer and the electrode. Therefore, both a low resistive contact area and a high resistive confinement area can be formed by a single self-aligned diffusion process. This device structure requires only planar processes and a single crystal growth step. Therefore it is convenient for fabricating highly functional devices such as multiple-element laser arrays. The threshold current of each array element with a cavity length of 300 micrometer was 50 to 70 mA. Even without intentional isolation, no leakage current was observed from the current voltage characteristics. The cavity loss and intrinsic threshold current density determined from a series of devices with various cavity lengths, were 6 cm^-1 and 1.6 kA/cm², respectively. The threshold current density was close to that of a conventional buried-ridge structure and the cavity loss was one-third that of the conventional structure.

This paper presents the performance characteristics and reliability data of AlGaInP-based VISIBLE laser diodes emitting at the wavelengths from 630 to 670 nm. The lasers are grown by toxic gas free solid source molecular beam epitaxy.

In this work, we demonstrate fusion of GaAs-based laser structures to GaN-based light-emitting diode (LED) heterostructures. Successful operation of red and infrared lasers fused to functioning GaN LEDs is achieved. A single heterostructure consisting of AlGaInAs/AlGaAs quantum well (QW) and GaInP/AlGaInP QW laser diode structures was grown by low-pressure organometallic vapor phase epitaxy (OMVPE) on GaAs substrates. The GaN LED structure was grown by OMVPE on an A-face sapphire substrate. The heterostructures were fused at 650 degrees Celsius in an H₂ ambient, while under uniaxial pressure. To fabricate the lasers, the GaAs substrate was selectively etched, leaving the red and infrared QW laser stack structure on GaN. Ridge waveguide QW lasers and GaN LEDs were fabricated with the fused epilayers. Infrared, AlGaInAs QW lasers (4 X 500 micrometer), operated with a threshold current (I_th) of 40 mA and external differential quantum efficiency ((eta) _d) of 11.5%/facet at about 821 nm. Red, GaInP QW lasers (4 X 500 micrometer), operated with a I_th of 118 mA and (eta) _d of 18.7%/facet at about 660 nm. The adjacent InGaN/GaN LED emitted at 446 nm.

The general status of the intersubband quantum cascade laser technology is reviewed, highlighting the major advances. Results are then reported on the electrical tuning of a QC laser close to room temperature and on a new QC laser based on a N-I-P-I superlattice.

Novel approaches to the design of inter-miniband quantum cascade lasers with undoped active regions are presented. The applied electric field is compensated across the superlattices, either by exploiting the field generated by extrinsic charges in special modulation-doped injection regions, or with an effective 'quasi-electric field' obtained by varying the superlattice period and duty-cycle so that a flat miniband condition is reached. Broadening mechanisms and optical losses introduced by the otherwise needed dopants in the superlattices are thus reduced, dramatically lowering the threshold current densities and improving the temperature performance of the lasers. At the same time the appealing properties of inter-miniband based devices, i.e. automatic population inversion and high current carrying capabilities, are preserved. Room temperature record-high peak (approximately 0.5 W) and average (14 mW) powers are observed for a (lambda) equals 7.6 micrometer laser. The lowest threshold current densities reported up to now in quantum cascade lasers at 300 K (J_th equals 4.5 kA/cm²) are also obtained, which translates in very large maximum temperatures for continuous wave operation (160 K).Inter- miniband superlattice structures prove particularly useful in extending laser operation towards longer wavelengths, where the increased optical absorption losses and the reduced radiative efficiency require larger driving currents. This is shown by the realization of a laser with chirped superlattice active regions operating at (lambda) equals 17 micrometer, well beyond the longest wavelength ((lambda) equals 13 micrometer) so far demonstrated for conventional quantum cascade lasers based on inter-subband transitions.

A new type of cascade laser, the so-called interband cascade laser based on type-II quantum well staircase structures, has been demonstrated recently. Here, we will discuss the design and modeling of the interband cascade lasers in connection with device performance. Normalized excess voltage and current will be introduced to evaluate how efficiently the input power is converted into the optical output power in the sense of the utilization of both injection current and applied voltage. Our modeling is based on Kane's eight-band k(DOT)p theory with envelope wave-function approximation and strain effects included. The calculated threshold current density, which includes contributions of radiative current, direct tunneling and Auger recombination, is significantly smaller than the experimentally observed value. This indicates that there are additional current leaking channels, which may be related to interface scattering and material defects, suggesting significant room for improvement. Our modeling also found that the device performance could vary substantially with temperature. Implications of our analyses and the modeling results will be interpreted in terms of carrier transport, optical gain and loss, as well as power efficiency.

Lead salt tunable diode lasers (TDL) are recognized as key components for high sensitivity gas analysis in the mid- infrared. Commercially available device types include double heterostructures, available from 3.0 to 10.8 microns with cw operating temperatures in the range 80 - 180 K, as well as homostructure lasers (4.2 to 25 microns) with lower operating temperatures. Recent developments at Laser Components include the introduction of guaranteed single-mode devices, the extension of overall wavelength coverage, and a series of quality control procedures. These include the now well- established temperature cycling technique, which ensures wavelength stability; an artificial aging process which simulates long-term storage and a burn-in procedure which has beneficial effects on long-term laser power stability. Finally, a comprehensive tests and characterization procedure ensures that each laser is optimized for the chosen application. Ongoing studies of long-term laser behavior and the recent availability of compact, maintenance-free Stirling coolers promise to make lead salt TDLs more attractive for industrial applications in the near future. Pulsed mode operation of lead salt lasers at temperatures of up to 333 K was recently demonstrated.

In the work we continue our studies of broadened waveguide separate confinement InGaAsSb/AlGaAsSb quantum well diode lasers grown by MBE on n-GaSb substrates. To avoid the structure degradation associated with the miscibility gap in the 2.3 - 2.7 micrometer wavelength range, we used highly strained, 'quasi-ternary' In_xGa_1-xSb_1-yAs_y compounds with 0.25 < X < 0.38 and y approximately equals 0.02 as the material for QWs. From spontaneous emission measurements we have identified that the Auger process determines the rate of recombination in quantum well active region over the entire temperature range studied (15 - 110 degrees Celsius) for 2.6 micrometer lasers and at temperatures higher than 65 degrees Celsius for 2.3 micrometer lasers. Under these conditions, strong temperature dependence of Auger coefficient leads to the rapid increase of threshold current density with temperature (T₀ approximately 40 degrees Celsius). In the range of 15 - 65 degrees Celsius for 2.3 micrometer devices we believe monomolecular non-radiative mechanism dominates and T₀ is about 110 degrees Celsius. In addition, single-mode CW room temperature ridge-waveguide lasers with wavelength of 2.3 - 2.55 micrometer have been fabricated for the first time. The lasers display threshold currents around 50 mA with CW output powers of several milliwatts. Switching of the peak lasing position has been observed for both CW and pulsed operation and is related to second sub-band transitions. These results show that excess carrier energy distribution and their concentration are current dependent above threshold.

Optically-pumped type-II W lasers have exhibited improved high-temperature performance throughout the wavelength range of 2.7 micrometer to 7.3 micrometer. Low duty cycle pumping at 2.1 micrometer yielded maximum operating temperatures as high as 360 K at (lambda) less than or equal to 4 micrometer for 3 devices, with peak output powers exceeding 1.5 W at ambient temperature. Internal losses of 90 cm^-1 at 300 K were seen for one device and suppressed Auger recombination coefficients were observed for all three. Pulsed operation at wavelengths as long as 7.3 micrometer was seen in another device which had a maximum operating temperature of 220 K. For 1.064 micrometer optical pumping, the same laser was able to operate in continuous-wave (cw) mode to 130 K. Cw operation was also observed at temperatures as high as 290 K for lambda approximately equals 3.0 micrometer. Maximum cw output powers (per uncoated facet) of 260 mW at (lambda) equals 3.1 micrometer and 50 mW at (lambda) equals 5.4 micrometer were observed at T equals 77 K. With further improvements in the design and growth quality of these W laser structures, it is projected the cw output powers of 0.5 W or more should be achievable at thermoelectric cooler temperatures.

High-power mid-infrared (MIR) lasers are highly desirable for a variety of applications such as remote chemical sensing, infrared countermeasures, environmental monitoring, etc. We will report the recent progress in the development of optically pumped Mid-IR lasers based on InAs/InGaSb/InAs/AlSb type-II quantum wells (QWs) for high power applications. The potential of achieving Watts of quasi-cw output power by improving the material quality and device design will be discussed. The performance of our previous devices was mainly limited by the rising of internal loss versus temperature, even though the internal quantum efficiency was greater than 70% at temperatures up to 150 K. The internal loss rose from 10 cm^-1 at 80 K to 19.2 cm^-1 at 140 K, which was mainly due to inter-valence subband absorption. With such a high internal loss, power scaling by increasing device length is limited. At high temperatures, while lasing was possible under intensive and short pulses, the lasing efficiency and average output power were not sufficient for many high-power applications. For this device, the maximum peak output power was 0.98 W per facet with a pulse length of 0.05 ms. The net external quantum efficiency before thermal roll off was as high as 23.5% with an estimated pump power absorption of 87% in the 0.82-micrometer active region.

The development of mid-infrared interband diode lasers has been hindered by factors such as Auger recombination and intervalence band absorption, which become increasingly important at longer wavelengths. A number of structures have been proposed in which the effects of these processes are reduced. The maximum gain per unit volumetric current density can be used as a figure of merit for comparing different active region materials. Using this figure of merit, we compare a series of structures with band gaps near 0.3 eV (i.e., wavelengths near 4 microns). The figure of merit is obtained from gain spectra calculated using superlattice K(DOT)p theory and a combination of calculated and measured recombination rates. We show that devices based on active regions incorporating type-I InAsSb/AlInAsSb or InAsSb/InAsP quantum wells should have room temperature threshold currents 7 - 13 times smaller than those of devices based on bulk InAs. However, devices using type-II superlattice active regions should have room temperature threshold currents that are a factor of 3 - 4 times smaller than those of the type-I quantum wells. The figure of merit can also be used to determine the optimal thickness of the active region as a function of waveguide loss and optical mode width.

Within the last few years, high power laser diodes with remarkable improvements concerning output power, efficiency, and reliability have been investigated in the wavelength range between 780 nm and 1064 nm. The discussion, whether laser diodes fabricated from Al-free material systems can surpass the performance of devices made from the conventional InAlGaAs-material system is still ongoing. In our contribution to this discussion we present 980 nm high-power InAlGaAs-laser diodes and laser diode bars with high conversion efficiencies grown by MBE. Broad area laser diodes with 100 micrometer aperture show an output power as high as 9.2 W cw at room temperature corresponding to a COMD level of 17 MW/cm². Up to this output power the conversion efficiency remains above 46%. The highest efficiency of nearly 60% is reached at 2.5 W of output power. Reliability tests are ongoing and predict a lifetime of at least 20.000 h at a power level of 1.5 W cw. Laser diode bars of 1 cm width comprising 25 of these oscillators have been fabricated. Similar to single emitters these devices achieve a conversion efficiency of 58% at 62 W of cw output power. In terms of conversion efficiency and output power these results are among the best reported for both, Al-containing and Al-free laser diodes and laser diode bars. They can be attributed to the material quality, the facet coating technology, and the design of our devices. Clearly, they show the competitiveness of the material system used here.