Single-frequency fiber laser at 1.5  µm with an 89  Hz linewidth and near shot-noise-limited intensity noise

In this paper, a distributed Bragg reflector single-frequency fiber laser (SFFL) operating at 1550 nm with ultra-narrow linewidth and near shot-noise-limited intensity noise is demonstrated experimentally. The low-noise fiber laser employs a short linear-cavity based on Er³⁺/Yb³⁺ co-doped phosphate fiber, achieving stable single-longitudinal-mode operation through a multi-technique noise-suppression scheme. By incorporating self-injection locking, pump noise optimization, optoelectronic feedback, and the gain saturation effect of a semiconductor optical amplifier to improve the noise performance, the SFFL exhibits a linewidth of less than 89 Hz. The relative intensity noise of the fiber laser is reduced to near the shot-noise limit (-156dB/Hz at 1 mW), with measured values of approximately -152.5dB/Hz in the 2-10 kHz frequency range and -155dB/Hz in the 1-10 MHz frequency range. Furthermore, the optical signal-to-noise ratio exceeds 70 dB, and the polarization extinction ratio is greater than 26 dB. These combined techniques enable the SFFL to simultaneously achieve an ultra-narrow linewidth (sub-100 Hz level) and low intensity noise, rendering it suitable for demanding applications such as coherent optical communications, precision metrology, and laser cooling.