Relaxation-oscillation-free semiconductor laser with optical feedback

Semiconductor lasers often make use of external optical feedback, for various reasons, like single-mode selection, stabilization or linewidth narrowing. A draw-back of such method is that delayed feedback can easily lead to sustained relaxation oscillations (RO). The occurrence of the RO is sensitive to, among other things, the applied settings of the phase of the feedback light. It is known from early studies that the onset of ROs already occurs with weak feedback. However, the onset of ROs is shifted to higher feedback strengths under resonance conditions, i.e. when the product of RO-frequency and external delay time equals an integer. This finding was based upon certain numerical and analytical considerations, but no simple explanation was given. From a theoretical analysis based on the Lang and Kobayashi equations, the existence of RO-free bias-current intervals of substantial width will be demonstrated for realistic pumping values, irrespective of the feedback phase. It will be shown that for conventional semiconductor lasers with weak optical feedback under RO-resonance condition the laser with feedback behaves as if no feedback is present. Therefore, under these conditions the RO is damped, hence will be suppressed. This result is valid in the regime of sufficiently weak feedback, such that the RO frequency is not deviating significantly from its value in the solitary laser and is supported by calculations of the RIN spectrum.