Study of optical feedback effects in an extremely short external cavity configuration

We present an experimental and theoretical study of optical feedback in a semiconductor laser for the case of an extremely short external cavity (EC) configuration. When the length of the EC is changed both the output power and the voltage drop onto the laser are modulated with a period of half of the solitary laser wavelength. We also perform modulation experiments in which the EC length is modulated with amplitude corresponding to the half of the solitary laser wavelength and with different signal shapes. In this way we prove that by using optical feedback we are able to detect very small features. Such detection is of general interest from an application point of view, e.g. for optical data readouts, resulting in a reduced number of optical components. Optical feedback also affects the frequency of the laser light and results in a longitudinal mode hopping. With increasing the EC length we observe mode hops between neighboring solitary laser modes followed by large jumps at the EC frequency splitting. These large EC mode hops can be exploited for broad band frequency tuning of the emitted light. We also study the dependence of the amplitude and the period of the EC mode hops on the EC length. We reveal the existence of a cut-off EC length of a few micrometers for which the amplitude of the EC mode hops reaches a maximum and then strongly decreases. We give a theoretical explanation of our experimental findings based on Fabry-Perot resonant condition for coupled cavities.