Use of external optical feedback to stabilize multimode hopping and reduce intensity noise in long-wavelength semiconductor laser

We report on the possibility of applying optical feedback (OFB) from a short external cavity to change the multimode hopping oscillation in long-wavelength semiconductor laser to single-mode oscillation. We characterize the influence of OFB on the modal oscillations and the associated noise spectrum of the multimode laser. The study is based on a modified time-delay multimode rate-equation model of the laser that includes mechanisms of spectral gain suppression along with OFB induced due to multiple reflections by an external reflector. The study is applied to 1.55 μm-InGaAsP laser that exhibits multimode hopping in its solitary version and supports wide bandwidth. The noise is evaluated in terms of the relative intensity noise. We show that strong OFB changes impact of the asymmetric gain suppression on the mode structure of the laser so that it changes the side mode suppression ratio (SMSR) and induces jumping of the strongest modes to longer or shorter wavelengths in random-like fashion. The change of SMSR of the laser occurs mainly when strong OFB induces multiperiodic and chaos oscillations. We specify the ranges of OFB over which SMO is induced. The low-frequency noise is lowest when the laser maintains CW operation or exhibits period-1 oscillations, while it is maximized when the laser dynamics are chaotic.