Asymmetric Separate-confinement Heterostructures Research Articles

Effect of laser cavity parameters on saturation of light – current characteristics of high-power pulsed lasers

We report an experimental study of power characteristics of semiconductor lasers based on MOVPE-grown asymmetric separate-confinement heterostructures with a broadened waveguide as functions of cavity length, stripe contact width and mirror reflectivities. It is shown that at high current pump levels, the variation of the cavity parameters of a semiconductor laser (width, length and mirror reflectivities) influences the light – current characteristic saturation and maximum optical power by affecting such laser characteristics, as the current density and the optical output loss. A model is elaborated and an optical power of semiconductor lasers is calculated by taking into account the dependence of the internal optical loss on pump current density and concentration distribution of charge carriers and photons along the cavity axis of the cavity. It is found that only introduction of the dependence of the internal optical loss on pump current density to the calculation model provides a good agreement between experimental and calculated characteristics for all scenarios of variations in the laser cavity parameters.

Saturation of light – current characteristics of high-power lasers in pulsed regime

Semiconductor lasers based on MOVPE-grown asymmetric separate-confinement heterostructures with a broadened waveguide and emitting in the wavelength range are studied. It is found that the intensity of spontaneous emission from the active region increases with increasing pump current above the lasing threshold and that this is caused by a growth in the concentration of charge carriers in the active region due to the modal gain enhancement needed to compensate for the growing internal optical loss at high pulsed pump currents. It is shown that the increase in the internal optical loss with increasing pulsed pump current is one of the main reasons for saturation of the light – current characteristics of high-power semiconductor lasers.

High-power InGaAs/GaAs quantum-well laser with enhanced broad spectrum of stimulated emission

We report the demonstration of an InGaAs/GaAs quantum well (QW) broadband stimulated emission laser with a structure that integrated a GaAs tunnel junction with two QW active regions. The laser exhibits ultrabroad lasing spectral coverage of ∼51 nm at a center wavelength of 1060 nm with a total emission power of 790 mW, corresponding to a high average spectral power density of 15.5 mW/nm, under pulsed current conditions. Compared to traditional lasers, this laser with an asymmetric separate-confinement heterostructure shows broader lasing bandwidth and higher spectral power density.

Spectral characteristics of multimode semiconductor lasers with a high-order surface diffraction grating

We have studied the spectral characteristics of multimode semiconductor lasers with high-order surface diffraction gratings based on asymmetric separate-confinement heterostructures grown by metalorganic vapour phase epitaxy . Experimental data demonstrate that, in the temperature range , the laser emission spectrum is in width and contains a fine structure of longitudinal and transverse modes. A high-order surface diffraction grating is shown to ensure a temperature stability of the lasing spectrum in this temperature range. From analysis of the fine structure of the lasing spectrum, we have evaluated the mode spacing and, thus, experimentally determined the effective length of the Bragg diffraction grating, which was in our samples.

On the temperature delocalization of carriers in GaAs/AlGaAs/InGaAs quantum-well heterostructures

The effect of temperature delocalization in semiconductor lasers (emission wavelength λ = 1060 nm) based on symmetric and asymmetric separate-confinement heterostructures fabricated by metal-organic vapor-phase epitaxy (MOVPE) is studied. Experimental and calculated estimates show that the carrier concentration in the waveguide increases by an order of magnitude when the temperature of a semiconductor laser is raised by ∼100°C. It is found that an increase in the temperature of the active zone leads to enhancement of the temperature delocalization of both electrons and holes. It is shown that the delocalization of holes begins at higher temperatures, compared with that of electrons. It is demonstrated experimentally that the onset of temperature delocalization depends on the threshold carrier concentration in the active region of a laser at room temperature. It is found that raising the energy depth of the active region by choosing the waveguide material makes it possible to fully suppress the temperature-delocalization process up to 175°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.

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.

Temperature dependence of the threshold current density and external differential quantum efficiency of semiconductor lasers (λ = 900–920 nm)

The temperature dependences of the emission characteristics of semiconductor lasers based on asymmetric separate-confinement heterostructures with a broadened waveguide fabricated by metalorganic chemical vapor deposition (radiation wavelength λ = 900–920 nm) have been investigated. It is found that an increase in the energy depth and number of quantum wells (QWs) in the active region makes it possible to increase the temperature stability of the threshold current density and the differential quantum efficiency of semiconductor lasers on the basis of these structures. A temperature stability of threshold current density with a characteristic parameter T0 = 290 K has been obtained in lasers based on a heterostructure with four QWs. It is experimentally shown that the laser parameters are stabilized due to the decrease in the threshold current density and threshold carrier density in the QWs of the active region. It is also demonstrated that, when the carrier concentration in these QWs reaches a certain value, the temperature stability of the threshold current density and differential quantum efficiency sharply decreases.

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.

Effect of the active region thickness on characteristics of semiconductor lasers based on asymmetric AlGaAs/GaAs/InGaAs heterostructures with broadened waveguide

The effect of the active region thickness on the basic characteristics of high-power semiconductor lasers based on AlGaAs/GaAs/InGaAs asymmetric separate-confinement heterostructures grown by MOCVD epitaxy has been studied. It is shown that the threshold current, temperature sensitivity of the threshold current density, internal quantum efficiency of stimulated emission, and differential quantum efficiency are improved as the active region thickness increases. It is demonstrated that the maximum attainable optical emission power of a semiconductor laser and the internal quantum efficiency of photoluminescence are the most sensitive to defect formation in the heterostructure and become lower as the critical thickness of the strained InxGa1–x As layer in the active region is exceeded.

Contribution of Auger recombination to saturation of the light-current characteristics in high-power laser diodes (λ = 1.0–1.9 m m)

Spectral and light-current characteristics of lasers based on the asymmetric separate-confinement heterostructures InGaAs/InGaAsAl/InP and InGaAs/GaAs/AlGaAs/GaAs were studied in the pulsed mode of lasing. It is shown that, at high levels of current pumping, the charge-carrier concentration in the active region of semiconductor lasers for the near-infrared optical region increases beyond the oscillation threshold; drastic saturation of the light-current characteristics is observed. Processes occurring in lasers as the charge-carrier concentration increases beyond the lasing threshold are studied theoretically. It is established that, at high pump levels, the rate of stimulated recombination decreases, the lifetime of charge carriers increases, and both the concentration of emitted photons and the quantum yield of stimulated radiation decrease. It is shown that variations in stimulated recombination, the decrease in the quantum efficiency, and saturation of the light-current characteristic in semiconductor lasers at high levels of current pumping are caused by the contribution of the nonradiative Auger recombination.

1.8-μm laser diodes based on quantum-size AlInGaAs/InP heterostructures

The concept of power semiconductor lasers based on asymmetric separate confinement heterostructures is applied to the AlInGaAs/InP heterostructures. The laser heterostructures emitting at the wavelength 1.75–1.8 μm are fabricated by metal-organic chemical vapor deposition. Based on the heterostructures grown, high-power multimode laser diodes of the mesa-strip design with the maximum emission power 2.0 W in the continuous mode at room temperature and 20 W in the pulsed operation mode are fabricated. The internal optical losses in the lasers were reduced to 2.2 cm−1.

High-power laser diodes (λ = 808–850 nm) based on asymmetric separate-confinement heterostructures

Symmetric and asymmetric separate-confinement AlGaAs/GaAs heterostructures have been grown by MOCVD in accordance with the concept of design of high-power semiconductor lasers. High-power laser diodes with a 100-µm aperture, emitting in the 808–850-nm range, were fabricated from these structures. The internal optical loss in asymmetric separate-confinement heterostructures with broadened waveguide is reduced to 0.5 cm−1. In the lasers with 1.7-µm-thick waveguide, the output optical power of 7.5 W in CW mode was achieved owing to reduction of the optical emission density at the cavity mirror to 4 mW/cm2.

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%.

Study on InGaAsP–InGaAs MQW-LD With Symmetric and Asymmetric Separate Confinement Heterostructure

We studied symmetric and asymmetric InGaAsP-InGaAs 1.55-/spl mu/m multiquantum-well (MQW) laser diodes (LDs) with highly p-doped layers in the two-step separate confinement heterostructure (SCH). The p-doping in p-SCH suppresses the electron overflow from the MQWs to p-SCH, but it is an origin of free carrier absorption loss. An additional InGaAsP layer inserted inside n-SCH makes asymmetric field distribution and, therefore, reduces the portion of optical field distribution in highly p-doped regions with high optical loss. Compared with symmetric structure, asymmetric SCH LD has low threshold current density, low internal loss, and high and flat slope efficiency with respect to temperature.