Suppression of Relaxation Oscillation Dynamics in Semiconductor Lasers with External Optical Feedback

Semiconductor lasers often use external optical feedback, or self-injection. The drawback of such method is that delayed feedback can easily lead to sustained relaxation oscillations (RO), i.e. an intrinsic resonance between laser intensity and population inversion, which for a solitary laser is a damped oscillation. The occurrence of the RO is sensitive, among other things, to the applied settings of the phase of the feedback light. This has been studied recently with the help of electrically addressable phase shifters. It is known from an early study that, surprisingly, the onset of RO is hampered under resonance conditions, i.e. when the product of RO frequency and external delay time equals an integer. This prediction was based upon certain numerical and analytical considerations, but no simple explanation was given. From a theoretical analysis based on Lang-Kobayashi delay equations, we will demonstrate the existence of RO-free bias-current intervals of substantial width and for realistic pumping values, irrespective of the feedback phase. We show that for conventional FP-type semiconductor lasers with weak optical feedback under RO-resonance condition, i.e. when the product of RO frequency ν and delay time τ equals an integer, i.e. ν τ ≈ 0,1,2, ..., the laser with feedback behaves with respect to the RO as if no feedback is present. Therefore, under the above-mentioned conditions the RO is damped and, in fact, will be suppressed. This result is valid in the regime of weak feedback, such that the RO frequency is not deviating substantially from its value in the solitary laser.