D. N. Nikolaev

High-Power Laser Diodes Based on Asymmetric Separate-Confinement Heterostructures

Asymmetric separate-confinement laser heterostructures with ultrawide waveguides based on AlGaAs/GaAs/InGaAs solid solutions, with an emission wavelength of ∼1080 nm, are grown by MOCVD. The optical and electrical properties of mesa-stripe lasers with a stripe width of ∼100 μm are studied. Lasers based on asymmetric heterostructures with ultrawide (>1 μm) waveguides demonstrate lasing in the fundamental transverse mode with an internal optical loss of as low as 0.34 cm−1. In laser diodes with a cavity length of more than 3 mm, the thermal resistance is reduced to 2°C/W, and the characteristic temperature T0= 10°C is obtained in the range 0–100°C. A record-breaking wallplug efficiency of 74% and an output optical power of 16 W are reached in CW mode. Mean-time-between-failures testing for 1000 h at 65°C with an operation power of 3–4 W results in the power decreasing by 3–7%.

850-nm diode lasers based on AlGaAsP/GaAs heterostructures

Laser heterostructures with waveguides and emitter layers made of AlGaAs and AlGaAsP alloys are grown by hydride metal-organic vapor-phase epitaxy on a GaAs substrate and studied. It was shown that the heterostructure containing AlGaAsP layers has a large curvature radius in comparison with the structure consisting of AlGaAs layers. Ridge waveguide lasers with an aperture of 100 μm are fabricated based on the AlGaAsP/GaAs laser heterostructure and studied. The internal optical loss of the lasers is 0.75 cm−1; the characteristic parameter T0 = 140 K in the temperature range of 20–70°C. The maximum optical output power per mirror reached 4.1 W; cw lasing was retained at a heat sink temperature of 120°C.

Thermal delocalization of carriers in semiconductor lasers (λ = 1010–1070 nm)

The temperature dependences of the emission characteristics of semiconductor lasers based on MOVPE-grown asymmetric separate-confinement heterostructures (wavelengths λ = 1010–1070 nm) have been studied. It was found that, in the continuous-wave mode, the main mechanism of “saturation” of the light-current characteristic with increasing temperature of the active region is carrier delocalization into the waveguide layer. It was experimentally demonstrated that the thermal delocalization of carriers depends on the energy depth of the quantum well (QW) in the active region. It is shown that the minimum internal optical loss at 140°C is obtained in laser structures with the largest energy depth of the QW of the active region.

High-power laser diodes of wavelength 808 nm based on various types of asymmetric heterostructures with an ultrawide waveguide

The parameters of high-power multimode laser diodes featuring a radiation wavelength of 808 nm obtained on the basis of asymmetric heterostructures with an ultrawide waveguide in the systems of AlGaAs/GaAs and (Al)GaInP/GaInAsP/GaAs are compared. In the lasers based on an AlGaAs/GaAs system, the maximum optical power was limited by optical degradation of the SiO2/Si mirrors and amounted to 4.7 W. In the lasers based on an (Al)GaInP/GaInAsP/GaAs system, the maximum optical power was limited by thermal saturation and equaled 7 W. The obtained results show that the (Al)GaInP/GaInAsP/GaAs system is more reliable from the standpoint of an increase in both the maximum optical power and operation life of lasers.

Studying the characteristics of pulse-pumped semiconductor 1060-nm lasers based on asymmetric heterostructures with ultrathick waveguides

High-power semiconductor lasers based on asymmetric quantum-dimensional separate confinement InGaAs/GaAs heterostructures with ultrathick waveguides were fabricated by means of metalorganic hydride vapor phase epitaxy technology. The laser characteristics were studied in a pulsed pumping regime, in which the emission was excited by current pulses of 100 ns duration at a repetition frequency of 10 kHz and an amplitude of up to 200 A. The passage to a pulsed lasing regime allowed the active region heating to be reduced and the output power to be increased to 145 W for a laser diode with a 100-μm exit aperture. The results obtained for the pulsed lasing regime show that saturation of the output power-current characteristic observed in the continuous-wave regime is fully determined by overheating of the active region of a semiconductor laser.

850-nm diode lasers with different ways of compensating for internal mechanical stresses in an AlGaAs:P/GaAs heterostructure

MOS hydride epitaxy is used to grow AlGaAs/GaAs heterostructures both without compensation and with compensation for internal mechanical stresses by the introduction of phosphorus into different layers of the heterostructure; this compensation affects the lattice parameter of the layer and, thus, influences the value of internal stresses in the entire laser hetetrostructure. Multimode mesastripe laser diodes (with an aperture of 100 μm) emitting at a wavelength of 850 nm are fabricated on the basis of the above structures; the properties of these diodes are studied. It is shown that the structures with compensated internal mechanical stresses exhibit a linear power-current characteristic if the pump currents do not exceed those corresponding to the maximum of the output power. Such structures with compensated internal mechanical stresses exhibit larger values for the characteristic temperatures T0 and T1 in comparison with structures with uncompensated internal mechanical stresses.

Semiconductor lasers with internal wavelength selection

Laser diodes with a deep built-in trapezoidal large-period diffraction grating have been developed on the basis of InGaAs/GaAs/AlGaAs heterostructures. The electroluminescence and stimulated emission spectra and light-current characteristics of diffraction-grating laser diodes have been studied. A substantial narrowing of both the luminescence and stimulated emission spectra was achieved due to the spectral selectivity of the grating. The maximum output optical power was 1 W at a drive current of 4 A. At the highest power, the lasing spectrum had a FWHM of ∼2 Å. A narrowing of the lasing spectrum of the diffraction-grating laser by tens of times was demonstrated, compared with that of a laser with a Fabry-Perot cavity.

Temperature dependence of the threshold current density in semiconductor lasers (λ = 1050–1070 nm)

The temperature dependences of the threshold current density and threshold concentration in semiconductor lasers based on MOVPE-grown asymmetric separate-confinement heterostructures with an extended waveguide have been studied (wavelengths λ = 1050–1070). It is shown that the temperature dependence of the threshold current density in semiconductor lasers becomes markedly stronger at above-room temperatures, which is due to temperature-induced carrier delocalization into the waveguide layers of a laser heterostructure. It was found that the sharp decrease in the thermal stability of the threshold current density with increasing temperature correlates with the coincidence of the Fermi level with the conduction-band bottom of the waveguide layer in the laser heterostructure. It is experimentally demonstrated that an increase in the energy depth and number of quantum wells in the active region of a semiconductor laser improves the thermal stability of the threshold current density. It is demonstrated that the characteristic parameter T0 attains a value of 220 K in the temperature range from −20 to +70°C.

Selection of modes in transverse-mode waveguides for semiconductor lasers based on asymmetric heterostructures

Asymmetric Al0.3Ga0.7As/GaAs/InGaAs heterostructures with a broadened waveguide produced by the method of MOCVD epitaxy are studied. It is established that the precision shift of the active region to one of the cladding layers ensures the generation of the chosen mode of high order in the transverse broadened waveguide. It is experimentally established that this shift brings about an increase in internal optical losses and a decrease in the internal quantum efficiency of stimulated emission. It is shown experimentally that the shift of the active region to the n-type cladding layer governs the sublinear form of the power-current characteristic for semiconductor lasers; in the case of a shift of the active region towards the p-type cladding layer, the laser diodes demonstrated a linear dependence of optical power on the pump current in the entire range of pump currents.

Laser-diode bars based on AlGaAsP/GaAs heterostructures emitting at a wavelength of 850 nm

Two types of laser heterostructures, i.e., those without internal mechanical stress compensation, with AlGaAs-alloy emitter and waveguide layers (type 1), and laser heterostructures with stress compensation, with AlGaAsP emitter and waveguide layers (type 2) are grown by metal-organic chemical vapor deposition (MOCVD). Laser-diode bars 5 mm wide with a fill factor of 24%, emitting at a wavelength of 850 nm are fabricated. Their power parameters in the continuous-wave (cw) and pulsed lasing modes are studied. It is shown that type-2 laser diode bars exhibit better linearity of the light-current characteristics in the cw and pulsed lasing modes in comparison with laser diode bars based on the type-1 structure.