S. O. Slipchenko

Specific Features of the Injection Processes Dynamics in High-Power Laser Thyristor

A dynamic model of a laser thyristor based on a semiconductor heterostructure has been developed. The model takes into consideration for the first time the optically activated impact ionization in the space-charge region (SCR) and the nonlinear photogeneration of excess carriers in the base region, with the latter accounting for the threshold nature of the optical emission. It is shown that the maximum turn-ON rate of the laser thyristor is determined by the impact ionization rate in the SCR of the collector p-n junction, and the maximum current, by the rate of photogeneration of excess carriers in the base region. A satisfactory agreement between the calculated and experimental voltage and current waveforms is demonstrated.

A study of nonlinear lasing dynamics of an InGaAs/AlGaAs/GaAs heterostructure power laser-thyristor emitting at 905 nm

The results obtained demonstrate that the lasing dynamics reflects the current dynamics formed as a result of complex nonlinear couplings within the laser-thyristor heterostructure. The observed specific features mainly result from the appearance of new channels for generation of excess carriers in the p-base. These channels enhance the main optical activation channel formed by the photogeneration due to the absorption of the spontaneous emission from the active region of the laser part of the heterostructure. The additional channels of excess carrier generation may have an optical nature in the case of scattered laser light upon appearance of new high-Q modes. For nearly critical blocked voltages, generation of carriers can be initiated by an avalanche multiplication of photogenerated carriers.

Optical feedback in 905 nm power laser-thyristors based on AlGaAs/GaAs heterostructures

An experimental study of factors determining the optical feedback efficiency in the structure of a laser-thyristor emitting at a wavelength of 905 nm has been carried out. It is shown that the spontaneous emission spectrum undergoes a significant change in the working range of currents due to the presence of GaAs-spacers in the structure of the active region of the laser part and to the absence of saturation of the spontaneous emission flux beyond the lasing threshold. It is demonstrated that the influence exerted by the reverse voltage across the collector p–n junction of the transistor part comes down to the following two effects: deformation of the edge of the absorption spectrum and turn-on of the impact ionization. Experimental dependences of the photogeneration rate on both current and voltage were obtained for the p-base of the transistor part. These dependences are an important tool to be used in subsequent studies aimed to simulate and examine the injection and generation processes in power laser-thyristors.

Spatial dynamics of high current turn-on in low-voltage AlGaAs/GaAs phototransistors

Dynamic characteristics of low-voltage AlGaAs/GaAs heterostructure N-p-N phototransistors operating in the high-current mode have been studied. It was found that, above critical voltages, the turn-on process of a phototransistor consists of two phases separated in space and time. It is shown that the power of an optical-activation source affects the turn-on delay of the phototransistor and has no effect on the maximum current. An analysis of the spatial current dynamics in the plane of the collector p-n junction demonstrated that the first phase of the turn-on process is localized in the optical activation (optical window) region. In this case, the region of current localization during the first phase may be smaller than the optical-activation region. It was found that the whole current during the second phase is localized at the boundary between the optical window and the ohmic contact. The turn-on delay of the second phase is associated with the carrier transport in the base layer from the optical-activation region to the boundary with the ohmic collector contact.

High-efficiency and compact semiconductor lasers with monolithically integrated switches for generation of high-power nanosecond pulses in time-of-flight (TOF) systems

We present a new approach based on the integration of the functions of a high-efficiency current switch and a laser emitter into a single heterostructure as elements of time-of-flight (TOF) systems. The approach being developed employs the effect of an electrical bistability, which occurs in the general case in thyristor structures. We report recent results obtained in a study of the dynamic electrical and optical characteristics of the pulsed sources we developed. An effective generation of 2- to 100-ns laser pulses at a wavelength of 905 nm is demonstrated. The possibility of generating laser pulses shorter than 1 ns is considered. The maximum peak power reached values of 7 and 50 W for 10- and 100-ns pulses, respectively.

Generation of nanosecond and subnanosecond laser pulses by AlGaAs/GaAs laser-thyristor with narrow mesa stripe contact.

Lasers-thyristors with a narrow (20 μm) mesa stripe contact have been studied. It was shown that the laser peak power reaches a value of 2.5 W in the long-pulse mode at a pulse width of 13 ns. It was demonstrated that generation of a controlled train of laser pulses with peak power of 1.6 W and width of 90 ps is possible in the short-pulse mode. The maximum value of the pulse repetition frequency was 470 kHz at the following working characteristics of the laser-thyristor: blocking voltage 5.8 V, control current pulse 25 mA. A number of specific features were observed in the short-pulse mode. It was found that the blocking voltage and amplitude of the control current pulse affect the lasing process. We observed that in short pulse mode the lasing spectra have a two-band structure and the lateral near field may degenerate into a single spot with size substantially smaller than the mesa stripe width. It was shown that the main reason for the observed specific features of lasing is the clearly pronounced effect of the spatial localization of the current.

Generation of Laser Pulses in the Megahertz Range of Repetition Frequencies by Low-Voltage AlGaAs/GaAs Laser-Thyristors

The process of generation of a high-frequency train of laser pulses in laser-thyristors (LTs) based on AlGaAs/GaAs heterostructures with various optical activation efficiencies has been studied. It was found that the main factors affecting the frequency and peak power of the laser pulses are the charging current of the barrier capacitance of the collector p-n junction and the holding current. The charging current of the barrier capacitance determines the condition for an LT to pass to the uncontrollable turn-on mode. The holding current determines the condition for the transition of an LT to the sticking mode in the turned-on state. It was shown that the holding current reached a value of 0.2 mA for the LT heterostructure with high optical activation efficiency, which limited the maximum repetition frequency to 470 kHz at a peak power of 4 W. A decrease in the optical activation efficiency made it possible to raise the holding current to 100 mA, with the maximum repetition frequency reaching as a result 12 MHz at a peak power of 0.7 W. Lowering the repetition frequency to 8 MHz made it possible to raise the peak power to 3.2 W at a pulse width of 17 ns.

Two-state operation of high-power semiconductor lasers with a thick quantum well

We have examined the two-state operation process of the high-power edge-emitting lasers based on AlGaAs/InGaAs/GaAs heterostructure with a thick (90 A) quantum well in the active region. It has been demonstrated that the laser emission spectrum can be switched between spectral lines corresponding to different optical transitions between the quantum states in the active region. The switching process is governed either by pump current or by temperature. The dynamic characteristics of the switching laser pumped with 100 ns current pulses have been investigated and explained. It has been shown that the current density dependence of internal optical loss can cause the current-initiated switching. The temperature impact is much more important and consists in the charge carrier redistribution in the quantum well. The registered optical power near the switching point was near 2 W in continuous wave and 15 W in pulse mode of operation.

Effect of the spatial current dynamics on radiative characteristics of high-power lasers-thyristors based on AlGaAs/GaAs heterostructures

The effect of a local current turn-on in the heterostructure plane has been observed for low-voltage lasers-thyristors. It was shown that the spatial dynamics of the current-turn-on region is determined by the blocking voltage and the control current amplitude. For the first mode (blocking voltages up to 15 V), the current nonuniformity in the heterostructure plane is determined by the flux distribution of the spontaneous emission from the active region in the laser part to the side of the p-base of the phototransistor part of the heterostructure. The transition to the second mode (blocking voltages exceeding 15 V) is due to the sharp rise in the generation rate of excess carriers in the p-base of the phototransistor part of the heterostructure. In this case, the size of the region in which the original current turn-on occurs decreases to 70 μm. It was found that the rate at which the current-turn-on region expands depends on the working conditions of the laser part of the laser-thyristor and is 50 and 20...

Dynamic model of laser-thyristor based on AlGaAs/GaAs heterostructure for subnanosecond optical pulse generation

A new approach to high power laser pulse generation based on current switch integrated in to laser heterostructure has been demonstrated. The modeling of various structure designs has been performed and the possibility of obtaining short (2ns) and high amplitude (16 A) current pulses and generating of high-power (46W) optical pulses in optimized structure has been shown.