The realization of high-performance single-frequency narrow-linewidth lasers in the L-band (1565–1625 nm) holds significant importance for several cutting-edge scientific and technological fields, particularly in the spectral region beyond 1.6 μm. Since this band not only resides within the low-loss window of optical fiber communications but also aligns well with the characteristic absorption lines of important gases such as methane and ethylene, offering unique advantages for high-sensitivity gas sensing and remote environmental monitoring. Furthermore, lasers in this band serve as ideal pump sources for Tm³⁺ singly-doped or Tm³⁺/Ho³⁺ co-doped gain media, demonstrating considerable application potential that has attracted widespread research interest. In areas such as coherent lidar, space-division multiplexing communications, distributed optical fiber sensing, and quantum information processing, 1.6 μm lasers with narrow-linewidth characteristics and superior coherence have become a key technological foundation for achieving high-precision velocity measurement, high-capacity data transmission, and long-distance detection. However, this band lies at the tail of the erbium-doped fiber gain spectrum, where the gain coefficient is significantly lower than that of the technologically mature C-band (1535–1565 nm). Consequently, achieving high-performance, narrow-linewidth single-frequency fiber laser output in the 1.6 μm band remains a considerable challenge. Although previous studies have reported single-longitudinal-mode operation in this band, linewidth compression capabilities remain relatively limited.

    In this work, 1.6 μm single-longitudinal-mode fiber laser scheme based on a hybrid filtering and feedback mechanism is proposed (as shown in Fig. 1). The scheme combines two mechanisms: a saturable absorber filter (SAF) and distributed Rayleigh feedback (DRF), effectively achieving linewidth compression while ensuring stable single-frequency output. The implementation proceeds as follows: The laser first utilizes an optimized 2.1 m length of un-pumped erbium-doped fiber as the saturable absorber, which, together with a fiber Bragg grating, forms a dynamic narrowband filtering system that effectively suppresses multi-mode competition. Subsequently, a 17.5 m long ultra-high numerical aperture (UHNA7) fiber is selected as the DRF medium. Through the coherent feedback effect of Rayleigh-scattered light, phase noise is significantly suppressed, thereby achieving effective linewidth narrowing (as shown in Fig. 2). This fiber addresses the limitation of traditional standard single-mode fibers, which have a low Rayleigh feedback coefficient and typically require tens of kilometers to provide effective feedback, thereby enhancing the compactness of the laser system. Experimental results demonstrate that after introducing the DRF mechanism, the laser linewidth is effectively narrowed from an initial 1050 Hz to 185 Hz. This linewidth performance surpasses that of previously reported 1.6 μm single-frequency fiber lasers (as shown in Fig. 3). The final laser output achieves an optical signal-to-noise ratio better than 83 dB, an output power of 6.9 mW at a wavelength of 1608.75 nm, a relative intensity noise below –143 dB/Hz, and exhibits good long-term power stability. This study provides a compact, reliable, and high-performance design and implementation reference for narrow-linewidth fiber lasers in the 1.6 μm band, holding positive significance for promoting application development in fields such as coherent optical communications, high-precision optical sensing, and next-generation lidar.

Fig. 1. Experimental setup of the proposed fiber laser.

Fig. 2. Operation principle of the proposed ultra-narrow linewidth 1.6 µm fiber laser.

Fig. 3. (a) Measured laser linewidths under three cavity configurations; (b) Measured laser linewidths for three cavity configurations versus pump powers. (c) Comparison of linewidth of the proposed laser with other sub-kHz SLM fiber lasers reported in the 1.6 µm band.