Stabilizing Multimode Hopping Oscillations and Reducing Associated Noise in Long Wavelength Laser Diode Using External Optical Feedback

We report on converting the multimode hopping oscillation (MHO) in long-wavelength semiconductor laser into single-mode oscillation (SMO) by applying external optical feedback (OFB). We characterize and compare the noise performance of the laser when supporting SMO and multimode oscillations. 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 1.55um-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 (RIN). We show that when OFB synchronizes with the asymmetric gain suppression (AGS), it enhances the gain of one longer wavelength mode and supports SMO. In this case OFB improves the noise performance of the laser. On the other hand, when OFB works against AGS, it sustains hopping multimode oscillation (HMMO) and deteriorates the side-mode suppression ratio (SMSR) and the noise performance.