Dependence of the maximum time duration of the output pulse and adiabatic heating in GaAs junction lasers on laser length

Abstract

The time duration of the output pulse from a diode laser operated by a single current pulse is dependent on the rate of heating of the active region adjacent to the junction (Engeler and Garfinkel 1965). The effects of adiabatic heating of the junction and its immediate surrounding on the shape and duration of the light output pulsein GaAs junction lasers operated with asinglerectangnlar current pulse at 300'~ have recently been studied elsewhere (Broom 1968) both theoretically and experimentally. It has been observed (Broom 1968) that under adiabatic conditions, when the effects of heat diffusion away from tlle junction are neglected, the junction becomes excessively heated and tl~ertiial quenching of stimulated emission occurs in times of less than 1 psec. At room temperature, the junction temperature rise and the duration of the light output pulse in GaAs junction lasers have both been found (Broom 1968) to be functions of the drive and threshold currents. Thus it is felt by the present authors that these features of pulsed operation of GaAs junction lasers would depend also on other laser parameters which are yet to be investigated. The purpose of the present communication is to study the reflectivity dependence of the junction temperature rise and the maximum time duration of the output pulse in GaAs junction lasers operated with a single rectangular current pulse at 300% under adiabatic conditions of junction heating. The heating in jnnction lasers mostly results from the absorption of spontaneous and stimulated ,emission, the energy dissipittion by non-radiative transitions and the ohmic losses. . The relative magnitude of these sources depends upon the diode construction, the ambient temperature and the operating current density. Incidentally, it has been discussed by Broom (1.968) that under large operating currents at room temperature, the resistive heating and the heating due to non-radiative transitions are the most dominant. These dominant sources of heat dissipation lead to adiabatic heating of the junction when the thermal diffusion length I; is less than the thickness d of the active region. In GaAs diodes at 300°~, the thermal. conductivity K,? 0.5 Joules (cm degsec)-I for the n andp sides of the diode (Carlson et al. 1965),,the specific heat C = 0.32 Joules (g deg)-I (Piesbergen 1963) a!lditIle density p = 5.37 g ~m-~ , (Broom 1968). Therefore, the thermal diffusivity k, = K/Cp = 0.29 cm2 sec-I and L= d(k,t) is of the order of few microns'for a time t equal to few hundred , nanoseconds. Thus the diffusion of heat away from the junction may be ' neglected for current pulse lengths up to 200 ns or so since the thickness d of the