Iron phthalocyanine-based saturable absorber for dual-wavelength passive Q-switching in a thulium-doped fiber laser

This paper reports on dual-wavelength Q-switched operations in a thulium-doped fiber laser (TDFL) using a saturable absorber (SA) based on iron phthalocyanine (FePc). The SA was fabricated by embedding FePc into a water-soluble polyvinyl alcohol (PVA) polymer matrix at two different concentrations. The performance of these FePc-based SAs in Q-switching operations was evaluated and compared across the spectral, temporal, and frequency domains. The SA with 3.5 mg FePc concentration achieved a maximum repetition rate of 65.47 kHz and a corresponding pulse width of 2.639 µs. The emission spectrum exhibited dual peaks at 1916.841 nm and 1927.097 nm. Notably, the lower FePc concentration results in a smaller non-saturable loss, which enables improved control over pulse formation and supports stable Q-switched operation over a wider pump-power range. In contrast, the higher FePc concentration yields a slightly higher effective modulation depth, reducing the Q-switching threshold pump power, but introduces greater cavity loss, thereby shifting the laser operation toward longer wavelengths where the optical gain is higher. These findings demonstrate that FePc is a promising SA material for generating dual-wavelength laser pulses in the 2 µm region, opening up broad application potential in pulsed laser systems.