Orthogonally polarized switchable ultrafast fiber laser using long-period grating

Abstract

We demonstrate an orthogonally polarized switchable nonlinear polarization rotation (NPR) mode-locked fiber laser using a long-period grating (LPG). The LPG is inscribed in a single-mode fiber using a femtosecond laser, which can couple the fundamental core mode to an orthogonally polarized cladding mode, producing wavelength-dependent orthogonal polarization selectivity within a broadband spectral range. By integrating the LPG into an NPR mode-locked fiber laser, two laser outputs at 1556.1 and 1564.0 nm corresponding to the orthogonally polarized states can be switched by tuning the cavity polarization controllers. Furthermore, by varying the pump power while maintaining the cavity polarization state, the repetition frequencies of the orthogonally polarized lasers at the two wavelengths can be tuned from 211.67 to 680.36 MHz and from 105.84 to 555.64 MHz, respectively. The measured degrees of polarization of the two orthogonally polarized mode-locked outputs reach 98% and 87%, respectively. This work provides a novel approach for generating ultrashort pulses with switchable orthogonal polarization states, which is promising for linear and nonlinear applications.