  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.
