Type-II interband cascade (IC) lasers take advantage of the broken-gap alignment in type-II quantum wells to reuse electrons for sequential photon emissions from serially connected active regions. Here, we review our recent progress in InAs/GaInSb type-II IC lasers at emission wavelengths of 3.6 - 4 micrometers . These semiconductor lasers have exhibited significantly higher differential quantum efficiencies and peak powers than previously reported. Low threshold current densities (e.g., approximately 56 A/cm² at 80 K) and power efficiency exceeding 14% were observed from mesa-stripe lasers when operated in cw mode. Also, these lasers were able to operate at temperatures up to approximately 252 K in pulsed mode and approximately 142 K in cw mode. We observed slope efficiencies exceeding 1 W/A/facet, corresponding to a differential external quantum efficiency exceeding 600%, from devices at temperatures above 80 K. A peak output power of approximately 6 W/facet was observed from an IC laser at 80 K.

Recent improvements in quantum cascade laser technology have led to a number of very impressive results. This paper is a brief summary of the technological development and state-of- the-art performance of quantum cascade lasers produced at the Center for Quantum Devices. Laser design will be discussed, as well as experimental details of device fabrication. Room temperature QCL operation has been reported for lasers emitting between 5 - 11 micrometers , with 9 - 11 micrometers lasers operating up to 425 K. We also demonstrate record room temperature peak output powers at 9 and 11 micrometers (2.5 W and 1 W respectively) as well as record low 80 K threshold current densities (250 A/cm²) for some laser designs. Finally, some of the current limitations to laser efficiency are mentioned, as well as a means to combat them.

We describe some key growth issues for Mid-Infrared electroluminescent devices based on a quantum-cascade design using InAs/AlSb heterostructures grown on GaSb substrates. Structural and optical properties of antimonide/arsenide interfaces are first investigated on InAs/AlSb multiple quantum well samples with different types of Sb-like interfaces and various InAs thicknesses. We show that X-ray reflectometry is a powerful complementary tool to High Resolution X-ray Diffraction (HRXRD) to extract both individual layer thicknesses and interface roughnesses using only electronic densities as input parameters. The good structural quality of samples is evidenced by the persistence of sharp high order satellite peaks on HRXRD spectra. The associated optical properties are studied by photo-induced intersubband absorption. Strong E₁₂ p- polarized intersubband absorptions are observed with a full- width-at-half-maximum (FWHM) around 12 meV at 77 K showing good material quality. Absorption peak positions are compared to theoretical simulations based on a 2 X 9-band k.p calculation. These results allow us to properly design and fabricate InAs/AlSb quantum cascade light emitting devices in the 3 - 5 micrometers wavelength window taking into account the growth constraints. Well-resolved Mid-Infrared (3.7 - 5.3 micrometers ) electroluminescence peaks are observed up to 300 K with FWHM to emission energy ratio ((Delta) E/E) around 8%.

We have studied the far-field characteristics of mid- infrared angled-grating distributed feedback ((alpha) -DFB) lasers with W active regions as a function of etch depth, stripe width, and optical pumping intensity. Whereas near- diffraction-limited output is obtained for 50 micrometers stripes at 10 times threshold, the beam quality degrades rapidly when either the stripe width or the pump intensity is increased. A key finding is that most of the degradation may be attributed to the onset of Fabry-Perot-like lasing modes that propagate along the direct path normal to the facets. We further show that these parasitic modes may be effectively eliminated by using ion bombardment to create angled virtual mesas surrounded by loss regions. The bombarded structures show substantial improvement of the beam quality for wide pump stripes and high pump intensities, with only a modest reduction in the efficiency.

Three kinds of substrates were used for violet InGaN multi- quantum-well/GaN/AlGaN separate-confinement-heterostructure laser diodes (LDs). One of substrates is epitaxially laterally overgrown GaN (ELOG) substrate. Another is `free- standing GaN' substrate. In order to obtain it, thick GaN was grown on `ELOG', and then, sapphire and `ELOG' were removed. Third one is `ELOG grown on thick GaN' substrate. The threading dislocation densities of `ELOG', `free- standing GaN' and `ELOG grown on thick GaN' were 1 X 10<sup>6</sup>/cm<sup>2</sup>, 5 X 10<sup>7</sup>/cm<sup>2</sup> and 7 X 10<sup>5</sup>/cm<sup>2</sup>, respectively. LDs were fabricated with the structure of epi side up. The estimated lifetime of LD grown on `ELOG grown on thick GaN' was 15000 h under condition of continuous-wave operation, case temperature of 60 degree(s)C and output power of 30 mW.

As light sources of CD-R/RW and DVD-RAM/RW, highly efficient high-power 785 nm (AlGaAs) and 660 nm (AlGaInP) lasers are demonstrated, respectively. A real-refractive-index waveguide with small internal loss is applied to both the lasers in order to reduce the operating current by improvement of the external differential quantum efficiency. The mirror degradation level is increased by reduction of the optical power density and/or a non-optical-absorbing effect of the window-mirror. As a result, the 785 nm window- mirror with AlGaAs current blocking layer has showed stable transverse mode operation up to 250 mW (kink level: over 300 mW at CW) with the high slope efficiency of 1.1 W/A. Reliable 140 mW-CW and 180 mW-pulse operation has been realized at 70 - 75 degree(s)C. As for a 660 nm laser with the window-mirror, the operation current at 70 mW is reduced by 40% due to the high slope efficiency (1.08 W/A) resulting from the low-loss ridge-waveguide. The lateral mode is well stabilized up to 70 mW by the effect of the narrow ridge stripe formed by a dry etching technique. Reliable 70 degree(s)C, 70 mW pulse (duty cycle: 50%) operation with a low operating current of around 120 - 140 mA has been achieved. In addition, the lasers have operated for over 1000 hours even at 70 degree(s)C, 80 mW.

Threshold reduction and enhanced mode selectivity are demonstrated in pulsed GaN-based lasers upon the introduction of 5(lambda) /4 air/nitride Bragg gratings defined by focused ion beam (FIB) etching. A 13% reduction in threshold current is obtained from a laser with a 5 micrometers wide ridge by introducing a deep-etch air/nitride mirror. The presence of a reduced-depth Bragg grating, etched across 4 micrometers wide ridge structure using a lower FIB dose, results in single-peak spectral characteristics for currents up to 1.14(DOT)I_Th. The introduction of the Bragg mirrors always results in a broadening of the near field parallel to the epitaxial planes.

To obtain high coherent powers from large aperture devices in stable beam patterns one needs to use active photonic- lattice (APL) structures of large built-in index step. Resonant phase-locked arrays of antiguides have provided 1.6 W CW coherent power from 200 micrometers -wide apertures. ARROW- type devices, simpler APL structures, hold the potential for emitting 1 W single-mode power reliably in stable beam patterns.

Continuous wave room-temperature output power of approximately 3 W for edge-emitters and of about 1 mW for vertical-cavity surface-emitting lasers is realized for GaAs-based devices using InAs quantum dots (QDs) operating at 1.3 micrometers . Long operation lifetimes are manifested. The breakthrough became possible due to development of self- organized growth and defect-reduction techniques in QD technology. We show that the basic parameters of QD lasers outperform the parameters of the devices fabricated using competing GaAs-based `quantum well' technologies.

We have investigated the degradation behavior of high power diode-laser bars at 80 nm with single quantum and double quantum well structures in continuous wave operation. The 1 cm bars have a fill factor of 50%. Laser diodes with different resonator lengths from 300 micrometers to 2000 micrometers have been investigated. Different bars were mounted on actively cooled submounts and operated at comparable current densities and heat load.

In this paper we report on Al-free InGaAs/InGaAsP/InGaP broad area laser diodes emitting at 950 nm and on 810 nm- laser diodes with Al-free GaAsP quantum wells in AlGaAs waveguides. 2 mm long diode lasers show a high wall plug efficiency above 50% at output powers of about 3 W. The beam characteristics of these diode lasers benefit from small confinement factors. Results depending on stripe width and resonator length are given.

Al-free active region broad stripe lasers, consisting of an InGaAsP quantum well, InGaP waveguide layers and AlGaAs clad layers, prepared by low-pressure MOVPE have shown excellent long-term reliability and relatively low temperature sensitivity for the wavelength range of 795 - 815 nm, which is suitable for solid-state laser excitation. High bandgap cladding is essential for reducing the electron leakage from the active region. We have measured the facet temperature and its distribution using modulation reflectance method.

Properties of different Large Optical Cavity based high- power diode laser structures with an a 1 micrometers wide Al_0.3Ga_0.7As step-index waveguide are discussed. One key parameter is the position of the double quantum well (DQW) being either located centered or off-centered. By employing Near-field Scanning Optical Microscopy (NSOM) in emission mode with excitation wavelengths close to the laser emission wavelength of 808 nm we visualize the effect of the waveguide design on (1) the number of guided modes and (2) the spatial profile of both fundamental and higher order modes. Detailed analysis shows that the data depend distinctly on the spatial position of the DQW and the respective changes in mode structure. By changing the excitation photon energies towards very high values of 2.8 eV (442 nm) surface excitation is realized where the waveguiding effect becomes less effective. We demonstrate the ability to map the DQW location within the waveguide by its specific absorption properties and to verify its off- centered position. Thus the NSOM technique provides a sensitive tool for nondestructive analysis of diode laser structures including its waveguide mode properties.

Requirements for directly modulated semiconductor lasers are investigated in terms of directly measurable laser and link parameters. Data rates comparable to the bandwidth imposed by the active layer design are considered, and performance is assessed in terms of laser resonance frequency and damping. Link critical variables such as contrast ratio, fall time and error rate are thereby related directly to parameters extracted from static measurements. To allow error free operation, a resonance frequency equal to the data rate is acceptable, but to ensure a fast fall time, resonance frequencies exceeding one and a half times the data rate are necessary.

A novel technical approach to build a multiwavelength laser source for DWDM applications is described. The basic idea of this system is to maintain simultaneous lasing operation in a gain medium at different wavelengths without mode competition. The system uses a novel dispersive cavity. By designing this cavity structure appropriately, the system creates its own microcavities-channels each lasing independently at different wavelengths across the complete gain spectrum of the laser active material. Multifrequency lasing on the basis of a single diode laser chip was analyzed theoretically and demonstrated experimentally.

A diode laser array consisting of three initially uncoupled stripes is injection-locked by a 50 mW single-mode frequency tunable diode laser. Phase-conjugation is achieved by pumping a photorefractive crystal by the beams emitted from the master oscillator and the slave laser array. The injected beam is phase-conjugated with respect to the beam emitted by the slave diode laser array. The pumping beam from the slave laser is spatially filtered by a slit in the far field in the slow axis using a collimating lens. The slit consists of two mirrors, which act also as the output couplers.

A simple tunable laser structure with quasi-continuous tuning range of 15 nm, more than 30 dB side mode suppression ratio, 400 kHz linewidth and low variation in output power over the tuning range is demonstrated. The device involved minor modifications to a standard Fabry Perot (FP) laser and measurements have confirmed modeled predictions. Prototype fabrication has used focussed ion beam etching to generate three reflection/scattering sites with power reflectivity of approximately 1%, at precise locations along the optical axis of a 1500 nm FP laser. The device contact was electrically split at these reflection/scattering sites, thereby generating weak FP cavities within a strong primary FP cavity. Tuning is achieved by precisely changing the currents to each section and hence altering the refractive indices.

Since we first proposed the use of GaInNAs active layers to improve the high-temperature performance of long wavelength lasers in 1995, this material has been intensively investigated by many research groups and several promising results have been reported. We have used gas-source molecular beam epitaxy with N-radicals as a N-source for GaInNAs growth, and improved the crystal quality by optimizing the growth condition. Our growth method has the advantages of relative low temperature growth and a high N- radical sticking coefficient compared to metal organic chemical vapor deposition with Dimethylhydrazine. The effects of thermal annealing on the optical properties of GaInNAs layers have also been investigated. In-situ thermal annealing at temperatures above 550 degree(s)C was found to greatly enhance the photoluminescence intensity of GaInNAs. By optimizing both the crystal growth and thermal annealing conditions, we made a 1.3-micrometers GaInNAs/GaAs single-quantum- well laser that has a high characteristic temperature over 200 K and offers life time over 1000 hours. Therefore, we expect GaInNAs lasers to be put into practical use in the near future.

GaInNAs is a novel III-V semiconductor material and is a very attractive material for long-wavelength-range lasers. Highly strained 1.3 micrometers range GaInNAs/GaAs double quantum- well lasers grown by metalorganic chemical vapor deposition are demonstrated. A high characteristic temperature of 205 K (22 - 80 degree(s)C) was obtained with a low threshold current density of 0.92 kA/cm² (22 degree(s)C) in a broad stripe laser. The highest lasing operation temperature of 170 degree(s)C, and continuous-wave operation with a low threshold current of 27 mA were also obtained in a 7.5- micrometers -wide ridge-stripe laser. The GaInNAs/GaAs material system is very promising for next-generation long-wavelength lasers without any cooling device because they have stable characteristics at ambient temperature.

In_xGa_1-xAs_1-yN_y quaternary alloys offer the promise of longer wavelength, >= 1.3 micrometers optical transceivers grown on GaAs substrates. To achieve acceptable radiative efficiencies at 1.3 micrometers , highly- strained InGaAsN quantum wells (x approximately equals 0.4, y approximately equals 0.005) are being developed as laser active regions. By introducing GaAsP layers into the active region for strain-compensation, gain can be increased using multiple InGaAsN quantum wells. In this work, we report the first strain-compensated, 1.3 micrometers InGaAsN MQW lasers. Our devices were grown by metal- organic chemical vapor deposition. Lasers with InGaAsN quantum well active regions are proving superior to lasers constructed with competing active region materials. Under pulsed operation, our 1.3 micrometers InGaAsN lasers displayed negligible blue-shift from the low-injection LED emission, and state-of-the-art characteristic temperature (159 K) was obtained for a 1.3 micrometers laser.

Temperature insensitive threshold current in strain- compensated-(In_xGa_1-xAs/GaAsP) SQW-lasers with x X 0.3 is investigated by changing x equals 0.2, 0.25 and 0.3 by the spectral measurement and the threshold carrier density determined by lasing delay. Large energy separation between the heavy and the light hole subbands due to the highly compressive strain makes it not to exist the light hole subbands in the quantum well, which is confirmed by the induced Raman scattering spectral above threshold. Injected holds are almost contained in the heavy hole subbands, and injected electrons are also contained in the electron subbands at threshold in these lasers at room temperature.

In this paper, we will discuss the growth, material characterization and device studies of the highly strained InGaAs-QW on GaAs, operating at (lambda) equals 1.17 micrometers . Variations in structure and MOCVD growth conditions will be discussed. High performance, (lambda) equals 1.165 micrometers laser emission is achieved from InGaAs-QW/GaAsP strain-compensated single quantum well laser structures, with threshold current densities of 65 A/cm² for 1500-micrometers -cavity devices and transparency current densities of 50 A/cm².

A high power semiconductor laser with a novel lateral design using angular filtering by total reflection for increased brightness is demonstrated. In this so called `Z-Laser' two inner surfaces guide the laser beam by total reflection in a Z-shaped path through the laser. Higher order laser modes with larger divergence angles are suppressed because of a smaller reflectivity. This results in a reduced far-field angle. Simulations based on a 2D steady state wave equation solved by using the Pade approximation, an 1D carrier diffusion equation and a logarithmic gain model have been performed to design the device.