Delay induced high order locking effects in semiconductor lasers

Abstract : The semiconductor laser undergoing feedback is one of the most well-studied, non-linear laser configurations. It is of vital importance for applications given the ubiquity of lasers in modern technology and it is also the source of many fundamental non-linear phenomena. Semiconductor lasers have two intrinsic timescales: the cavity repetition rate (set primarily by the length of the laser) and the relaxation oscillation frequency (describing the rate at which energy is transferred between the electric field and the charge carriers). These two timescales are very different in typical devices. The relaxation oscillation frequency is typically on the order of a few GHz while the cavity repetition rate is often many tens of GHz. While nonlinear systems involving multiple time scales often lead to resonant effects, the vast separation in these two timescales means this is not typically observed with semiconductor lasers. By including delayed optical feedback, a third timescale is introduced— the external cavity repetition rate. This is often much lower than the solitary cavity repetition rate. We show here that for lasers with highly damped relaxation oscillations, resonances between the relaxation oscillations and the external cavity modes can and do occur and indeed, lead to spontaneous mode-locking. In fact, high order locking effects are possible with multipulse intensity trains.