I. Introduction
Semiconductor lasers with Fabry–Pérot (FP) cavities show multiple longitudinal modes, and the multimode nature of FP lasers has extensively been studied. The main-mode power increases as the injection current to the laser increases. However, the side-mode power saturates in the above-threshold regime because the carrier density is cramped at the threshold. According to conventional semiconductor laser theory [1], therefore, the side-mode suppression ratio (SMSR) increases as the injection current increases. Experimentally, however, a degradation in the SMSR and spectral broadening are observed in high-injection regimes. In particular, long-wavelength FP lasers show a broader spectrum compared with short-wavelength FP lasers. Anomalous behaviors of longitudinal modes such as spectral asymmetries and wavelength shifts have been reported [2]–[4]. Experimental setup. All measurements are done after coupling to an SMF through an optical isolator with an isolation of 60 dB.