1860-nm polarization-maintaining figure-of-9 mode-locked fiber laser and optimization of its output characteristics

A thulium-doped conventional-soliton mode-locked fiber laser was demonstrated by applying an all-polarization-maintaining (PM) figure-of-9 (Fo9) nonlinear amplified loop mirror (NALM) configuration. This fiber laser can generate stable 1860-nm mode-locked pulses with a pulse duration of 360 fs, a repetition frequency of 51.5 MHz, and an output power of 17 mW. By further adjusting the angles of the waveplates, the central wavelength of the laser pulses can be tuned from 1860nm to 1894nm with decreased pump power. The pulse duration of the 1894-nm mode-locked laser pulse is 306 fs. To further optimize laser output characteristics to meet practical application requirements, we employed violet phosphorene (VP) nanosheets, which exhibit excellent nonlinear optical (NLO) properties, as the saturable absorber (SA) to achieve hybrid mode-locked laser output. The results show that the saturable absorption effect introduced by VP SA effectively solves the problem of intensity reduction at the central wavelength of the laser spectrum, significantly reduces the mode-locking self-starting threshold, and improves the stability of ultrashort pulses. This work not only provides a stable 1860-nm ultrafast laser source but also offers new insights for optimizing its output characteristics.