Sub-kHz Linewidth Fiber Laser via Weak Fiber Bragg Grating Enabled Self-Injection Locking

We proposed a compact self-injection locked fiber laser, employing a weak fiber Bragg grating (wFBG) to achieve external feedback linewidth compression. The simulated results indicated that the output laser linewidth decreased with the increase of the length of the wFBG. According to the experimental results, the output laser linewidth was compressed to 300 Hz with an 8-mm long wFBG. However, a further increase in the length of the wFBG led to linewidth broadening, which can be attributed to the enhanced interference between the main cavity and wFBG. Additionally, the corresponding phase noise was suppressed by approximately 20 dB within the frequency range of 0 Hz - 100 Hz, and the relative intensity noise decreased to -123 dB/Hz. Furthermore, the amplitude of beat frequency fluctuated within 20 MHz over 1000-second period, and the output power exhibited fluctuations of less than 1% over a 7-hour period under laboratory conditions. Ideally, the compact fiber laser design will facilitate the miniaturization of sub-kHz-linewidth fiber lasers.