  Gain-switched lasers are in demand in numerous quantum applications,
particularly, in systems of quantum key distribution and in various optical
quantum random number generators. The reason for this popularity is natural
phase randomization between gain-switched laser pulses. The idea of such
randomization has become so familiar that most authors use it without regard to
the features of the laser operation mode they use. However, at high repetition
rates of laser pulses or when pulses are generated at a bias current close to
the threshold, the phase randomization condition may be violated. This paper
describes theoretical and experimental methods for estimating the degree of
phase randomization in a gain-switched laser. We consider in detail different
situations of laser pulse interference and show that the interference signal
remains quantum in nature even in the presence of classical phase drift in the
interferometer provided that the phase diffusion in a laser is efficient
enough. Moreover, we formulate the relationship between the previously
introduced quantum reduction factor and the leftover hash lemma. Using this
relationship, we develop a method to estimate the quantum noise contribution to
the interference signal in the presence of phase correlations. Finally, we
introduce a simple experimental method based on the analysis of statistical
interference fringes, providing more detailed information about the
probabilistic properties of laser pulse interference.
