Single-frequency fiber lasers based on metal thiophosphites with sub-kHz linewidths

Metal thiophosphites (CrPX₄) (X = S and Se), emerging layered metal chalcogenophosphate materials, have found extensive utility in the fields of advanced electronic devices, magnetic technologies, and electrochemical systems. However, their applications in single-frequency fiber lasers have been relatively limited. Here, liquid phase exfoliation technology is used to exfoliate the CrPX₄ into few-layered samples. Subsequently, we explored the application of CrPX₄-based saturable absorbers (CrPX₄-SA) by precisely depositing CrPX₄ nanosheets onto the surface of D-shaped fibers. This led to the successful creation of two stable single-frequency fiber lasers (SFFLs) based on CrPS₄ and CrPSe₄ with center wavelengths of 1550.068 nm and 1550.092 nm, respectively. The linewidth of CrPSe₄ was tested and determined to be roughly 375 Hz, which is the narrowest ever value attained for an SFFL based on its structure, and its signal-to-noise ratio (SNR) was 74.17 dB. The maximum output power of each oscillator was 0.796 mW and 0.963 mW, and their corresponding power fluctuations were 2.5% and 1.4%, respectively.