Optical frequency comb generation based on spectral broadening of a self-injection locked directly modulated microcavity laser

A coherent optical frequency comb (OFC) with suppressed linewidth is demonstrated based on the spectral broadening of a directly modulated semiconductor microcavity laser with self-injection locking. Due to the high electro-optical response, the directly modulated microcavity laser provides a 10-GHz-spaced seeding OFC with 9 comb teeth in a 10 dB flatness window. Besides, an optical feedback fiber loop with two sub-loops is introduced to reduce the linewidth of the microlaser and suppress the undesired longitudinal modes. Self-injection locking is realized at the feedback power of 72 µW, and the laser linewidth is narrowed from 10 MHz to 14.5 kHz. Moreover, spectral broadening and pulse compression are used to broaden the optical spectrum and compress the pulse width, which results in an OFC with 36 comb lines in a 10 dB power deviation and a 1.8 ps optical pulse. Owing to the high coherence of the directly modulated OFC, all comb lines feature narrow linewidths in the range of 14.5∼18.2 kHz.