Low-Noise Single-Frequency 1.5- $\mu$ m Fiber Laser With a Complex Optical-Feedback Loop

A low-noise narrow-linewidth distributed Bragg reflector (DBR) single frequency 1.5-μm fiber laser with a complex optical-feedback loop (COFL) is demonstrated. The COFL is used to reduce the relative intensity noise (RIN) and narrow the linewidth, which is composed of an erbium-doped fiber amplifier (EDFA), an unpumped Er80 fiber and a sagnac loop mirror. The COFL combines the nonlinear amplification effect of saturated optical amplifiers, the standing-wave saturation effects of saturable absorber (SA), and the self-injection locking technology at the same time. The effects of self-injection locking give a sufficient proof, which the RIN is reduced and the resonance peak shifts to lower frequency. The RIN of low frequency is greatly reduced by the EDFA. In addition, the SA narrows the linewidth. After the COFL suppression, the RIN of relaxation oscillation is reduced by 24.5 dB from 80.3 dB/Hz at 219 kHz to 104.8 dB/Hz at 87 kHz. Furthermore, the linewidth narrows from 7.47 kHz to 720 Hz. Even after two-stage fiber amplification, the RIN does not increase significantly, and a spectral linewidth well below 1.8 kHz is also achieved at the output power of 11.1 W.