A comparative study of intensity pulsations in a double heterostructure AlxGa1?xAs laser with and without an external resonator present

Abstract

This paper describes the pulsing behaviour of AlxGa1−x As double heterostructure (DH) lasers with and without an external dispersive resonator present. We have investigated only those lasers which show self-sustained oscillations under dc excitation. For the solitary diode we have found pulses possessing a width at half-power points, Δτp, where Δτp=0.9 ns and a repetition frequency,fos, ranging from 176 to 775 MHz depending on injection currentJ. Introducing a correlation time,tc, as a measure of the stability of the pulse height we have obtained 5 <tc < 40 ns. The maximum value oftc occurs in the vicinity of the crossing point offos2 andτc−2, whereτc is the effective carrier lifetime. Output pulsations are accompanied by spectral shifts of the longitudinal diode cavity modes. The frequency halfwidth, ΔνHw, ranging from 30 to 90 GHz is closely related to the maximum pulse power,Pmax. Coupling the laser diode to an external resonator, pulse widths, oscillation frequencies and correlation times have been determined as functions ofJ/Jth (Jth is the threshold injection current), provided the photon roundtrip time is nearly equal toτc and the equivalent reflection factorR* of the external resonator isR* ∼ 0.87. By evaluating the experimental findings pulsing under dc excitation may be described, without a detailed knowledge of the reasons leading thereto, by using the concepts of a nonstationary quality factorQns and time-dependent resonator losses characterized by ΔνHw/δτp. Oscillations become self-sustained forQns=const., i.e. the minimum photon density with which the pulses start to grow has to be time and excitation independent, in this casefos2 is proportional toJ/Jth as well as Δτp to (J/Jth−1)−1/2.