AlGaAs diode laser blue shift resulting from fast neutron irradiation

Six TJS (transverse junction stripe) AlGaAs diode lasers were exposed to fast neutrons with fluences ranging from 2×1012 to 3×1013 n/cm2, and their tuning, gain, and dispersion curves were measured. The tuning and gain curves of four lasers showed blue shifts of several meV at neutron fluences as low as 2×1012 n/cm2; the other two lasers never showed this blue shift. In addition, the lasers that displayed a blue shift also showed an increase in their threshold currents. None of the lasers exhibited any change in their dispersion curves. To explain the blue shift, it is hypothesized that neutron irradiation reduces the efficiency for lasing in the bandtail states of these devices, forcing lasing action to occur between states with greater energy separation. A model in which the reduced efficiency takes the form of a decrease in the transition matrix element is found to yield blue shifts of the correct order of magnitude. Though bandtail effects may also explain the difference in radiation sensitivity among the TJS lasers, at present the reason why only four of the six lasers blue shifted after neutron irradiation is not well understood.