Distance measurement by delayed optical feedback in a ring laser

We numerically study the behavior of a semiconductor ring laser subject to bidirectional
delayed optical feedback, when the isolated laser is in the quasi-unidirectional regime.
The optical feedback, provided by two external refectors located in front of the ring output waveguides, can modify the laser regime produced by the cross-saturation between
the clockwise and the counter-clockwise mode. Thus, the system exhibits new diferent regimes, most of which are asymmetric and bidirectional, with alternating dominant
mode. Two of these regimes are of special interest in view of applications, because the
laser switching period, between the clockwise and the counter-clockwise mode, is linearly
related to the time of fight from the laser to one or both refectors. In these operating conditions, the laser is thus suitable to implement a telemeter. A convenient electrical output
signal is obtained by a photodiode located behind one (partially refecting) fxed mirror,
or by measuring the voltage drop across the laser junction. Simulations are performed by
mathematical models based on rate-equations, assuming typical literature parameters for a
1 mW ring laser.