Ultra-narrow linewidth 1.6 µm single-longitudinal-mode fiber laser enabled by a hybrid saturable absorber filter and distributed Rayleigh feedback

We demonstrate a 1.6 µm single-longitudinal-mode (SLM) fiber laser achieving an ultra-narrow linewidth of 185 Hz, enabled by a hybrid approach combining a saturable absorber filter (SAF) and distributed Rayleigh feedback (DRF). An optimized length of 2.1-m unpumped erbium-doped fiber acts as an SAF to enforce SLM operation, while a 17.5-m ultra-high numerical aperture (NA = 0.41) fiber provides effective DRF for linewidth compression, replacing tens of kilometers of standard single-mode fiber. Under a maximum pump power of 775 mW, the laser operates at 1608.75 nm, achieving an output power of 6.9 mW with a corresponding slope efficiency of 2.3% and an optical signal-to-noise ratio greater than 83 dB. The relative intensity noise of the laser is below −143 dB/Hz above 10 MHz. The peak wavelength and output power stability are also monitored over two and a half hours. To the best of our knowledge, this is the first implementation of linewidth compression utilizing the novel hybrid integration of SAF and DRF based on ultra-high NA fibers in the 1.6 µm band. This study offers a compact and efficient solution for high-coherence laser sources in L-band applications such as coherent communications, precision sensing, and next-generation LiDAR.