1.94 & 2.04 μm dual-band four-wavelength-switchable single-frequency narrow linewidth fiber laser using a novel fs-laser direct-written polarization-dependent four-channel filter

We present a dual-band four-wavelength-switchable single-frequency thulium-holmium co-doped fiber laser (THDFL) utilizing a novel band-switchable polarization-dependent four-channel filter (BS-PDFCF). The filter incorporates two high-birefringence fiber Bragg gratings (HB-FBGs) and an electrical-controlled optical switch (EC-OS). Specifically designed for operation in the 1.94 mu m and 2.04 mu m bands, two HB-FBGs are fabricated using femtosecond (fs) laser direct-writing technology in two segments of polarization-maintaining fibers via a point- by-point method. Each HB-FBG features two narrow-band reflection channels with perpendicular polarization directions. The EC-OS switches the operation band of the BS-PDFCF. A 1.2 m long dynamic ultra-narrow-band filter is employed in the THDFL to achieve single-longitudinal-mode oscillation. The THDFL achieves exceptional output performance, including four single-wavelength and two dual-wavelength operations, spectral wavelength and power fluctuations of <0.012 nm and <0.386 dB, respectively, OSNRs of >76 dB, RINs of <-142.48 dB/Hz@>5 MHz, linewidths of <3.59 kHz@1.94 mu m band and <5.46 kHz@2.04 mu m band, and output power fluctuations of <0.109 dB in a measurement time span of 30 min.