Demonstration of interference and laser generation instabilities in semiconductor laser beam reflected from glass plate

A simple scheme of two-beam interference is presented in the work. It includes glass plate of thickness 1-2 mm and laser beam focused on it (focusing by a collecting lens with focal distance of about 10 cm). Observations of interference in reflected light were conducted at a remote screen placed at distance of several meters away. The formula for interference pattern period has been deduced for such a case. It has been shown that the scheme is good for educational demonstrations of interference. Besides, it has been shown that, provided the light source is a semiconductor laser with unstable generation spectrum, generation instability is revealed through interference pattern observations. Thus a simple but high-quality method of detecting laser generation instabilities is proposed.