N. V. Fetisova

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%.

High power single-mode (λ=1.3–1.6 μm) laser diodes based on quantum well InGaAsP/InP heterostructures

The possibility of achieving maximal optical output power in the single-mode lasing for mesa-stripe laser diodes fabricated on the basis of InGaAsP/InP quantum-well heterostructures with separate confinement have been studied both experimentally and theoretically. The basic condition for the single-mode lasing of laser diodes in a wide range of driving currents is shown to be the precise choice of the effective refractive index ΔnL discontinuity in the plane parallel to the p-n junction. A InGaAsP/InP separate confinement heterostructure with a step waveguide, with a threshold current density of 180 A/cm2 and an internal quantum efficiency of stimulated emission of 93–99%, has been manufactured via the MOCVD method. The optimization of the mesa-stripe diode design for the developed InGaAsP/InP heterostructure is carried out with the aim of achieving maximal optical output power in the case of single-mode lasing. An output power of 185 mW is attained in the laser diode with the mesa-stripe width W=4.5 μm (λ=1480 nm). The maximal continuous output power was as high as 300 mW. The full width at half-maximum (FWHM) of the lateral far-field pattern increased by 1° relative to the threshold value.

Temperature delocalization of charge carriers in semiconductor lasers

The temperature dependences of emission characteristics are investigated for laser diodes based on asymmetric separate-confinement heterostructures with a broadened waveguide. It is established that an increase in the charge-carrier concentration in the waveguide layer is the basic mechanism of saturation in the light-current characteristic with increasing temperature in the CW mode. It is experimentally shown that the temperature delocalization of charge carriers leads to increasing internal optical losses and decreasing external differential quantum efficiency. It is shown that the degree of delocalization of charge carriers depends on the charge-carrier temperature distribution, the threshold concentration, and the quantum-well depth. The effect of thickness and energy depth of the quantum well on the temperature sensitivity of the threshold current and output optical power is considered.

A study of epitaxially stacked tunnel-junction semiconductor lasers grown by MOCVD

Design parameters of epitaxially stacked tunnel-junction asymmetric separate-confinement laser heterostructures are chosen. Technological modes for fabrication of heterostructures of this kind by metal-organic chemical vapor deposition in the system of AlGaAs/GaAs/InGaAs solid solutions are found. It is demonstrated that high-efficiency GaAs:Si/GaAs:C tunnel stru ctures and asymmetric AlGaAs/GaAs/InGaAs laser heterostructures with low internal optical loss can be fabricated in a single technological process. Conditions are chosen in which a deep mesa can be formed for fabrication of mesa-stripe diode lasers based on epitaxially stacked tunnel-junction laser heterostructures. Mesa-stripe diode lasers with a 150 × 12-μm aperture have been manufactured on the basis of these structures. These samples have a threshold current density Jth of 96 A cm-2, internal optical loss αi of 0.82 cm-1, and differential resistance R = 280 mΩ. Samples containing three laser structures have a slope efficiency of 3 W A-1 and a maximum peak output power of 250 W in the pulsed operation mode (100 ns, 1 kHz).

Laser diodes (λ=0.98 μm) with a narrow radiation pattern and low internal optical losses

Quantum-confined InGaAs/AlGaAs/GaAs heterostructures for laser diodes emitting at λ=0.98 μm, optimized to provide for reduced radiation divergence in the vertical plane and decreased internal optical losses, have been synthesized by metalorganic-hydride vapor-phase epitaxy. For the obtained laser diodes the far field pattern full width at half-maximum in the vertical plane falls within 16°–19°, the internal optical losses are about 0.7 cm−1, the internal quantum yield amounts to 97%, and the maximum continuous emission power reaches 8.6 W.

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.

A study of GaAs: Si/GaAs: C tunnel diodes grown by MOCVD

Technological modes in which high-efficiency GaAs: Si/GaAs: C tunneling structures can be fabricated by MOS-hydride epitaxy have been determined. It was demonstrated that use of C and Si dopants makes it possible to obtain a p-n junction with low diffusion spreading of dopant profiles. It was shown that fabrication of high-efficiency tunnel diodes requires that GaAs layers should be doped with acceptor and donor impurities to a level of ∼9 × 1019 cm−3. Tunnel diodes were fabricated using the tunnel structures and their current-voltage characteristics were studied. Peak current densities Jp ≈ 1.53 kA cm−2 and a differential resistance R ≈ 30 mΩ under a reverse bias were obtained in the tunnel diodes.

Quenching of lasing in high power semiconductor laser

Emission characteristics of high-power semiconductor lasers with a mesa-stripe design based on heterostructures with separate confinement are studied. It is shown that, in high-power semiconductor lasers with a mesa-stripe design, divergence of emission increases with the pump current as a result of generation of high order lateral optical modes. It is demonstrated that scattered radiation of high order lateral optical modes affects the transparency of passive regions outside the mesa stripe that forms a waveguide of the Fabry-Perot resonator. It is established that the transparency of passive regions results in fulfillment of threshold conditions for the closed optical mode in the semiconductor laser’s chip. As a result of generation of the closed optical mode in a cavity formed by four cleaved faces quenching of lasing modes in the mesa-stripe waveguide of the Fabry-Perot resonator occurs.

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.