  We performed theoretical analysis of the phase randomness in a gain-switched
semiconductor laser in the context of its application as a quantum entropy
source. Numerical simulations demonstrate that phase diffusion r.m.s. exhibits
non-linear dependence on the bias current, which could be of significant
practical importance, particularly, in application to high-speed optical
quantum random number generators. It is shown that phase diffusion between
laser pulses cannot always be assumed to exhibit required efficiency,
particularly, at high pulse repetition rates. It was also revealed that the
gain saturation significantly affects the r.m.s. value of the phase diffusion
and, in essence, determines the degree of non-linearity of its dependence on
the pump current.
