Al-free InGaAsP/InGaP/GaAs-based diode lasers in the wavelength range 730 - 980 nm have been grown by metalorganic chemical vapor deposition (MOCVD). A large transverse spot size is obtained using a Broad Waveguide (BW) design, permitting record output powers to be obtained.

This paper presents the performance characteristics and reliability data of GaInAsP- and AlGaInP-based laser diodes emitting at the wavelengths from 650 to 1,300 nm. The lasers are grown by toxic-gas-free all-solid-source molecular beam epitaxy (SS-MBE).

The use of 980-nm laser diodes as pump sources in erbium-doped fiber amplifiers (EDFAs) offers better noise figures and larger useful optical bandwidth in comparison to the use of a 1480-nm pump sources. With many millions hours of operation in field deployment the reliability of the 980-nm pumped EDFAs has been proven to fulfill the requirements posed by the terrestrial telecommunication applications. The future WDM enhancements in telecommunications networking and the extension into the submarine systems will require higher operational power levels and will tighten the reliability budget of the 980-nm pumped EDFA systems. The 980-nm pump- laser reliability as available today will be reviewed and the status of the reliability engineering towards higher laser power levels and higher system confidence will be discussed.

This paper presents a technique for determining carrier lifetimes which does not require a fast detector or rely on an experimentally complex implementation. The technique is based both on a measurement and a parallel calculation: (1) A Hakki- Paoli measurement of modal gain versus current density, g(J), and (2) A theoretical determination of the modal gain versus carrier sheet density, g(N). Once the gain relationships have been determined, the carrier sheet density, N, can be functionally related to the current density, J, and the lifetime determined. We demonstrate this method on two InGaAs single quantum well lasers. This method may prove particularly useful for carrier lifetime estimations in long-wavelength semiconductor lasers.

Antiguided array lasers were fabricated in thin p-clad, InGaAs/GaAs single quantum well material. The required lateral refractive index variation was achieved by precisely modulating the thickness of the GaAs cap layer using a novel pulsed anodization/etching technique. Edge emitting arrays have 5 lasers on 9.5 micrometer centers with 6 micrometer wide gain regions and arrays having 20 lasers on 7 micrometer centers with 5 micrometer wide gain regions were characterized. At 1.2 times the pulsed current threshold (I_th), the central lobe of the lateral far field of the 5 element arrays contained about 55% of the beam power and were nearly diffraction limited (FWHM equals 1.3 degrees). At 1.2 I_th, the central lobe of the 20 element arrays contained about 75% of the beam power and were about twice the diffraction limit (FWHM) equals 0.8 degrees). At 10 I_th, the central lobe of the 20 element arrays contained about 60% of the beam power and were about 1.6 degrees wide.

Theoretical analysis of complex-coupled, surface-emitting distributed feedback (CC-SE-DFB) diode lasers with distributed Bragg reflectors (DBR) is presented. These devices are shown to produce symmetric-mode (single-lobe) surface emission with high efficiency (30%) and uniform near-field in the longitudinal direction. High efficiency and uniform near-field are both necessary for achieving high powers in a single-lobed beam. It is also shown that the device studied here may be combined with a Resonant Optical Waveguide array device which has a uniform near-field in the lateral direction to achieve a two-dimensional surface-emitting device capable of providing greater than 1W CW power in a stable, single-lobed beam.

Semiconductor lasers with tapered gain regions are well suited for applications requiring high output powers and good spatial mode quality. In this paper, the development of 1.5-micrometer InGaAsP/InP quantum well (QW) material suitable for this type of device will be discussed and initial results on high-power tapered lasers fabricated in this material presented. Several different 1.5-micrometer QW laser structures grown by metalorganic chemical vapor deposition are being evaluated. Structures containing three compressively strained QWs have shown transparency current densities J_T as low as 170 A/cm² and net gains of approximately equal 40 cm^-1 at less than 800 A/cm². With 5QWs, these parameters were J_T approximately equals 275 A/cm² and net gain approximately 40 cm^-1 at 600 A/cm², respectively. Self-focusing at high current densities and high intensity input into the taper section has been identified as a fundamental problem in these devices that has to be dealt with. Tapered devices with a 0.6-mm-long single-mode gain section coupled to a 1.4-mm-long tapered region fabricated in 5QW material have shown CW output powers of greater than 1.0 W at 3.8 A. Approximately 80% of the 1 W is in the near- diffraction-limited central lobe of the far field-pattern.

Filamentation currently limits the amount of diffraction- limited power that can be obtained from broad-area semiconductor amplifiers. This paper examines the filamentation tendencies of a wide input-aperture tapered amplifier. Experimental measurements of filament gain under varying duty-cycle are offered and compared to theory. A numerical simulation of the device operation, which addresses non-uniform current injection, is also presented.

In high-power, high-brightness laser diodes, beam filamentation is one of the main physical effects that limit the device performance. Due to the interaction between the optical power and the carrier density in the active region of broad area devices, spatial hole-burning leads to an inhomogeneous optical index that causes the degradation of the optical beam profile. We show, that epitaxial layer structures with low optical confinement are much more insensitive to beam filamentation because of their reduced differential gain. Experimentally we find, that the beam quality of tapered laser oscillators can be improved by an order of magnitude, when epitaxial layer structures with reduced modal gain are used for the device fabrication. Two mm long tapered devices with a 200 micrometer wide output facet show near diffraction limited farfield profiles up to output powers of more than 2 W cw.

Single crystal thin films with compositions from the AlN-InN- GaN system were grown via metal-organic chemical vapor deposition (MOCVD) on single crystal 6H-SiC substrates. Blue light emitting (LED) and laser diode (LD) structures were fabricated. A conducting buffer layer was developed which uses an AlGaN buffer layer which provides a conduction path between SiC and the active device region. This conducting buffer layer was utilized in both the LEDs and the LDs. The external quantum efficiency of the LEDs was 3% at 20 mA (3.6V) with a peak emission wavelength of 430 nm. Violet and blue LDs were fabricated which consisted of an 8-well InGaN/GaN multiple quantum well (MQW) active region in a separate confinement heterostructure (SCH) design. The devices lased at room temperature under pulsed and continuous wave operation with an emission wavelength of 404-435 nm. The lowest pulsed operation threshold current density obtained for lasing under was 10.4 kA/cm².

We report on the OMVPE (organometallic vapor phase epitaxial) growth and characterization of AlGaInN heterostructures and laser diodes, including measurements of electrical properties (Hall), structural characteristics (x-ray diffraction and TEM), and room temperature, pulsed laser operation of a 10- quantum well InGaN/AlGaN heterostructure.

Room temperature (RT) pulsed operation of blue (420 nm) nitride based multi-quantum well (MQW) laser diodes grown on a-plane and c-plane sapphire substrates has been demonstrated. A combination of atmospheric and low pressure metal organic chemical vapor deposition (MOCVD) using a modified two-flow horizontal reactor was employed. The emission is strongly TE polarized and has a sharp transition in the far field pattern above threshold. Threshold current densities as low as 12.6 kA/cm² were observed for 10 X 1200 micrometer lasers with uncoated reactive ion etched (RIE) facets on c-plane sapphire. Cleaved facet lasers were also demonstrated with similar performance on a-plane sapphire. Differential efficiencies as high as 7% and output powers up to 77 mW were observed. Laser diodes tested under pulsed conditions operated up to 6 hours at room temperature. Performance was limited by resistive heating during the electrical pulses. Lasing was achieved up to 95 degrees Celsius and up to a 150 ns pulse length (RT). Threshold current increased with temperature with a characteristic temperature, T₀, of 125 K.

Optical gain and spontaneous emission spectra are calculated for CdZnSe/ZnSSe single quantum well (QW) structures at room temperature with various many-body effects taken into account. It is found that Coulomb enhancement has a large effect on the gain-current relation derived from these spectra. When Coulomb enhancement is ignored, values of threshold current at various cavity lengths are overestimated by about 40 to 50% compared with the measured threshold currents. Good agreement with experiment is reached when Coulomb enhancement is included in the calculation.

We have developed a one-step-grown real-index-guided AlGaInP/GaInAsP red laser called a self-aligned stepped substrate (S³) laser. This S³-laser has a double- hetero structure formed on a stepped nonplanar substrate and an active layer inclined at an angle to the horizontal. The fabrication process of the laser utilizes the following two key techniques: (1) a lateral p-n junction formed during the growth of the nonplanar substrate by alternate doping with Zn and Se that have strong impurity incorporation dependencies on the substrate's orientation, and (2) a natural (411)A-like growth facet formation on the nonplanar substrate by metal organic vapor phase epitaxy (MOVPE). The real-index waveguide structure with an inclined active layer provided the laser with a stable lateral-mode and good beam characteristics (a small astigmatism and a small aspect ratio). Its optical-loss- free structure leads to a low threshold current of about 20 mA, a high quantum efficiency of about 1.2 W/A, and a high characteristic temperature of about 150K within 25 degrees Celsius to 50 degrees Celsius. A low operating current of the laser with the wavelength of 680 nm under 70 degrees Celsius, 35 mW conditions, would make it highly reliable for optical storage applications.

The superior performance of n-type modulation doping in 1.3- micrometer InP-based strained multi quantum-well (MQW) lasers is demonstrated. Experimental results, which is in good agreement with theoretical results, confirm that the threshold current density, carrier lifetime, and internal loss in n-type modulation-doped (MD) MQW lasers is lower than those in conventional undoped MQW lasers at room and high temperatures. In addition, 2.5-Gb/s modulation under zero-bias current is achieved with the modified n-type MD-MQW laser at 85 degrees Celsius. These results confirm the suitability of this type of laser as a light source for high-density parallel optical interconnections.

High quality strained InAsP was grown using gas-source molecular-beam-epitaxy (GSMBE) and the band structure was determined. The conduction band discontinuity ratio of InAsP/InP heterostructure was about 0.35, contrary to the reported value of 0.75. We proposed a new InAsP/InAlGaAs material system with type-I superlattice suitable for a high performance LD for 1.3-micrometer optical subscriber systems. The crystal quality was improved by introducing an InP spacer layer and the RTA process. High characteristic temperature of 143 K was achieved with this material system.

Data is reported on 1.3 micrometer uncooled MQW DFB lasers and OC-12(622Mbps) optical transmitter modules. The devices show stable single longitudinal mode operation up to a maximum output power of 100mW with a SMSR greater than 46dB and the characteristic temperature measured continuously between 20 and 80 degrees Celsius is 78K. An average optical power of -2dBm and an extinction ratio of 14 dB were obtained from the OC-12 transmitter module with the DFB laser.

The InGaAsP/InP material system has a large conduction band offset ((Delta) E_c equals 0.72 (Delta) E_g which provides strong electron confinement and prevents carrier overflow under high temperature operation. Therefore, AlGaInAs/InP lasers have better performance at high temperature operation.

We have obtained GaInNAs/GaAs quantum wells with emission at 1.3 micrometer at room temperature. We also show that another novel material InNAsP grown on InP is a viable material for long-wavelength lasers. The maximum temperature of operation for an InNAsP/GaInAsP microdisk laser is 70 degrees Celsius, which is about 120 degrees Celsius higher than that of a similar laser fabricated from GaInAs/GaInAsP. The characteristic temperature T_o of the former is 97 K, also higher than that of the latter.

In this paper, we fully review our recent progress in GaInNAs/GaAs long-wavelength lasers. An excellent characteristic temperature was confirmed for the GaInNAs laser diodes with a 1.2-micrometer wavelength. A record high value (T₀ equals 126 K) was obtained in the temperature range from 25 to 85 degrees Celsius. We have also succeeded in applying GaInNAs to long-wavelength laser didoes that lased under room-temperature continuous-wave operation in the 1.3- micrometer wavelength range suitable for optical fiber communications. The temperature dependence of the lasing wavelength was as low as 0.35 nm/degrees Celsius. Thus, we have experimentally demonstrated that the GaInNAs laser diodes are very promising for application in optical fiber communications to overcome the poor temperature behavior of the conventional InGaAsP-based long-wavelength lasers.

Quantum cascade lasers are new light sources in the mid- infrared (3.5 - 12 micrometer), based on resonant tunneling and optical transitions between quantised conduction band states. Quantum engineering of electronic energy levels and tailoring of the wavefunctions are used to obtain population inversion and optimize the overall laser performance. Advanced structures from a point of view of the quantum design, such as two wavelength emitters, are presented. New waveguide confinement based on surface plasmon are also discussed. In this configuration it is shown that optical confinement can be achieved without the growth of cladding layers. The paper concludes with a general comparative discussion between GaAs/AlGaAs and GaInAs/AlInAs material systems for the new generations of quantum cascade lasers.

A new type of semiconductor unipolar laser operating in the mid-infrared spectral region, the Quantum Fountain intersubband laser, is demonstrated. It is based on optical pumping of a three-bound-state coupled quantum well structure in the GaAs/AlGaAs material system. The lasing transition occurs between the two excited states. Population inversion can be achieved by benefitting from LO-phonon resonance between the two lower subbands. The optical pumping scheme enables a simpler design of the active region than electrically pumped intersubband lasers. Moreover, because doped layers and metallic contacts are not necessary for the operation of the Quantum Fountain laser, free-carrier and plasmon absorptions can be minimised, thus allowing long- wavelength operation. Large optical gains are measured using pump-probe experiments with a free-electron laser. Lasing action under optical pumping by a pulsed CO₂ laser has been achieved at a record long wavelength of 15.5 micrometer with an output peak power of the order of 0.6 W at low temperatures.

We report on the fabrication and characterization of buried heterostructure quantum cascade (BH-QC) lasers. The buried heterostructure is formed by regrowth of InP lateral on the side walls and on top of the InAlAs/InGaAs laser structure by molecular beam epitaxy (MBE) after in situ surface cleaning. Thermal Cl₂ etching is applied to the etched laser structure to remove the native oxides of the ternaries prior to regrowth of InP. Buried heterostructure QC lasers demonstrated excellent performances with lower threshold current densities (as low as 4.5kA/cm² at T equals 300K) and higher slope efficiencies that we attribute to lower waveguide losses and a better heat dissipation.

We review our recent progress in the design and operation of 2-micrometer InGaAsSb/AlGaAsSb quantum-well diode lasers. The devices have InGaAsSb quantum-well active regions and AlGaAsSb cladding layers, and all were grown lattice-matched to GaSb substrates using molecular-beam epitaxy. The broadened- waveguide (BW) design produces internal losses as low as 2 cm^-1, which leads to external quantum efficiencies as high as 53%. Single-quantum-well lasers with 200-micrometer apertures and 2-mm-long cavities exhibit output powers of 1.9 W CW and 4 W quasi-CW. The lowest threshold current densities are 115 A/cm². Small arrays of similar multi-quantum-well- diodes emit 10.6 W CW. The broadened-waveguide design should improve the performance of all mid-infrared diode lasers.

We report GaInAsSb/GaSb multiple quantum well lasers with type-II band alignment operating at room temperature. Basic properties of GaInAsSb/GaSb system in presence of strains are presented. Room temperature lasing has been achieved at wavelengths up to 2.65 micrometer. For the first time, stimulated emission has been obtained from a type-III quantum well structure at room temperature at 1.98 micrometer and 2.32 micrometer for the structures with 6- and 12-angstrom-thick InAs quantum wells, respectively. Modification of the band structure near interfaces of the type-II quantum wells due to carrier injection is shown to be a decisive factor allowing to obtain low threshold lasing in quantum well structures with indirect radiative recombination.

Compressively strained 2 micrometer GaInAsSb quantum well lasers with large valence band offsets and broadened waveguides display a record characteristic temperature, T₀ equals 140 degrees Kelvin for a 4-QW laser and a differential efficiency of 0.74 for a pulsed 2-QW device. The T₀ of these antimonide lasers is 65% more than that reported for phosphide-based lasers operating at 2 micrometer wavelength. A room-temperature threshold current density as low as 173 A/cm² has been observed for a 2-QW device and 225 A/cm² for the 4-QW laser.

Laser efficiency is an important issue for mid-IR Sb-based semiconductor lasers. It has been the key limiting factor in the efforts to develop high power lasers in recent years. This paper reviews the basic aspects of the problem and discusses some recent results. A number of factors affect the efficiency, one of which common to many materials was a high internal loss that increases rapidly versus temperature. A major contribution to this internal loss is the large intervalence band carrier absorption that occurs in all mid- to-low-gap III-V semiconductors. Recent studies of both types of Sb-based laser materials with InAs-like valence band (InAsSb) and GaSb-like valence band (InAs/GaInSb/AlSb type-II quantum wells) have showed such a strong absorption. Coupled with the thermal effects, the internal loss behavior results in an efficiency roll-off that limits the power performance. Issues for the improvement of the efficiency are discussed.

A 2.9 micrometer superlattice diode laser with an InAs/GaSb/Ga_0.75In_0.25Sb/GaSb active region displayed high temperature operation and low current injection thresholds. The maximum operating temperature was 260 K, and at 200 K the threshold current density was 1.1 kA/cm² and the quantum efficiency greater than 15%. The peak output power per facet for this laser exceeded 800 mW at 100 K and 200 mW at 200 K for a 0.05% duty cycle. For two similar lasers, internal losses were extracted from optical pumping experiments. The first laser was designed to minimize Auger recombination by avoiding resonances between the bandgap and intervalence transitions (at zone center), while the second was designed to maximize these resonances. Internal losses for the Auger minimized (maximized) laser diode were 14(10) cm^-1 at T equals 100 K, and rose rapidly to 51(120) cm^-1 at 200 K. However, Auger coefficients were suppressed (less than or equal to 1.6 X 10^-27 cm⁶/s at T equals 260 K) for both samples when compared to a type-I material with a similar bandgap. For optical pumping, peak output powers up to 6.5 W per facet at 100 K and 3.5 W per facet at 180 K were obtained for these samples at lambda approximately equals 3.1 micrometer.

Recently, we demonstrated a new type of quantum cascade lasers based on interband transitions in type-II heterostructures. It takes advantage of the broken-gap band alignment in the InAs/Ga(In)Sb heterostructure to recycle electrons from the valence band back to the conduction band, thus enabling sequential photon emission from active regions stacked in series. A peak optical output power of approximately 0.5 W/facet from a broad area gain-guided interband cascade laser with a threshold current density of 290 A/cm², and a slope of 211 mW/A per facet, corresponding to a differential external quantum efficiency of 131%, were obtained at 80 K and at a wavelength of approximately 3.9 micrometer. Differential quantum efficiencies exceeding 200% were also observed from mesa structure lasers. Comparable device performance was also achieved based on a 'W' configuration cascade laser at approximately 2.9 micrometer, which has been operated at temperatures up to 225 K. Another W interband cascade laser has displayed lasing at 3.6 micrometer and nearly to room temperature (286 K).

To investigate the cascade process, we study two sets of mid- infrared InAs/GaInSb/AlSb multiple quantum well electroluminescent devices. Each set has nominally the same device parameters, differing only by the number of periods. We find, as expected, that for the same driving current the larger the device period the more intense is the emission. We also find that the scaling is far from ideal. We correlate the deviation of the exact scaling with the variation of the wafer-to-wafer structural quality.

Stimulated emission in InAs/InGaSb/InAs/AlSb type-II quantum- well (QW) lasers was observed up to room temperature at 4.5 micrometer, optically pumped by a pulsed 2-micrometer Tm:YAG laser. The absorbed threshold peak pump intensity was only 1.1 kW/cm² at 300 K, with a characteristic temperature T₀ of 61.6 K for temperatures up to 300 K. We have also studied another type-II QW laser using 0.808-micrometer pumping sources with a much longer pulse length of 50 microseconds. The devices demonstrated a maximum output power of 1.6 W per facet at 83 K, with a corresponding differential external quantum efficiency of 24.8%.

We examine theoretically the influence of temperature and composition on the threshold current densities of mid-wave infrared lasers with active regions consisting of InAs/InGaSb superlattices. Temperature shifts of the bands may result in significant variations in intersubband absorption and Auger recombination rates, giving rise to a threshold current density that is not well parameterized by a characteristic temperature T₀. Superlattices that are optimized to have minimal threshold current densities are shown to require plus or minus 3.5 angstrom accuracy in InGaSb layer thicknesses, and plus or minus 0.25 angstrom accuracy in InAs layer thickness in order to retain optimum operating characteristics.

Gain in heterostructures with CdSe quantum dots (QDs) in ZnMgSSe matrix has been studied theoretically taking account of many-body effects. Three-dimensional strain distribution in the QD structure has been calculated employing Green's function method for anisotropic crystals of cubic symmetry. An analytical formula in form of the Fourier series has been obtained for the spatial dependence of the strain tensor in periodical array of disk-like planar QDs. The carrier spectrum and wave functions have been calculated taking account of actual 3D potential modified by strain effects. It is demonstrated that in wide range of structure parameters the carriers in QD-system are weakly localized. Gain spectrum is shown to be strongly modified by many-body effects. The calculated value of the carrier-induced enhancement of the refractive index is in a good agreement with available experimental data.

Laser action is demonstrated in InGaN/GaN double heterostructures with cleaved facets. Hydride vapor phase epitaxy is used to grow a 10-micrometer-thick buffer layer of GaN on (0001) sapphire, and metal-organic vapor phase epitaxy is used to subsequently grow a GaN/In_0.09Ga_0.91N/GaN double heterostructure. One-mm-long cavities are produced by cleaving the structure along the (1010) plane of the sapphire substrate. A pulsed Nitrogen laser is used for optical excitation. At room temperature, the laser threshold occurs at an incident power density of 1.3 MW/cm². Above threshold, the differential quantum efficiency increases by a factor of 34, the emission linewidth decreases to 13.5 meV, and the output becomes highly TE polarized.

Strained single-quantum-well GaInAsSb/AlGaAsSb diode lasers have exhibited room-temperature threshold current densities as low as 50 A/cm², one of the lowest values reported for diode lasers at room temperature. These lasers, grown by molecular beam epitaxy, have emission wavelengths of approximately 2.05 micrometers, characteristic temperature of 65 K, internal quantum efficiency of 95%, and internal loss coefficient of 7 cm^-1. Single-ended cw power of 1 W is obtained for a 100-micrometer-wide broad-stripe laser. Tapered lasers with a 140-micrometer aperture have exhibited diffraction-limited cw power up to 600 mW.

Continuous-wave (CW) room temperature operation of InGaN/GaN multi-quantum well (MQW) lasers is reported. Far-field beam divergence as narrow as 13 degrees and 20 degrees for parallel and perpendicular directions to epilayer planes were measured, respectively. The MQW lasers showed strong beam polarization anisotropy as consistent with QW laser gain theory. Dependencies of threshold current on cavity-length and temperature are also consistent with conventional laser theory. No significant degradation in laser characteristics was observed during lifetime testing for over 140 hours of CW room temperature operation.