Q-switched and nanosecond mode-locked erbium-doped fiber laser based on bis(8-hydroxyquinolinato)(1,10-phenanthroline)tin(II) complex saturable absorber

In this work, a passively Q-switched and nanosecond mode-locked erbium-doped fiber laser (EDFL) is demonstrated using a bis(8-hydroxyquinolinato) (1,10-phenanthroline) tin (II) complex, denoted as Sn(phen)q, as a saturable absorber (SA). The Sn(phen)q–polyvinyl alcohol (PVA) composite thin film was fabricated via a simple solution-based method and subsequently integrated into the laser cavity using a ferrule-sandwich configuration. Stable Q-switched operation was achieved at a central wavelength of 1562.8 nm over a pump power range of 113–158 mW. At the maximum pump power of 158 mW, the radiofrequency (RF) spectrum exhibits a fundamental frequency of 62.9 kHz with a signal-to-noise ratio (SNR) of 55.4 dB, confirming stable pulse-train generation. Under the same conditions, the pulse duration, average output power, and pulse energy were measured to be 7.2 μs, 1.13 mW, and 18.0 nJ, respectively. By extending the cavity length, the laser transitioned into a mode-locked regime. In this configuration, nanosecond pulses with a duration of 285 ns were generated at a repetition rate of 1.8 MHz. The mode-locked laser operated at a central wavelength of 1571.2 nm and delivered a maximum output power of 2.86 mW with a corresponding pulse energy of 1.59 nJ at a pump power of 136.8 mW. The system exhibited excellent stability, as evidenced by a high SNR of 69.6 dB at the fundamental frequency. These results confirm the effective nonlinear loss modulation provided by the Sn(phen)q SA and highlight its strong potential as a complex-based thin-film saturable absorber for both Q-switching and mode-locking pulse generation in EDFLs.