Spectral Characteristic Based on Fabry–Pérot Laser Diode with Two-Stage Optical Feedback

An optical device, consisting of a multi-mode Fabry–Pérot laser diode (MMFP-LD) with two-stage optical feedback, is proposed and experimentally demonstrated. The results show that the single-mode output with side-mode suppression ratio (SMSR) of ～21.7 dB is attained by using the first-stage feedback. By using the second-stage feedback, the SMSR of single-mode operation could be increased to ～28.5 dB while injection feedback power of ?29 dBm is introduced into the laser diode. In the case of up to ?29 dBm feedback power, the outcome SMSR is rapidly decayed to a very low level so that an obvious multi-mode operation in the output spectrum could be achieved at the feedback power level of ?15.5 dBm. Thus, a transition between single- and multi-mode operations could be flexibly controlled by adjusting the injected power in the second-stage feedback system. Additionally, in the case of injection locking, the outcome SMSR and output power at the locked wavelength are as high as ～50 dB and ～5.8 dBm, respectively.