Femtosecond Laser Direct Inscribed Temperature Insensitive Polarizer for Ultrafast Pulse Generation

We demonstrated a simple approach to fabricate an all-fiber polarizer by directly inscribing a long-period grating in a two-mode fiber using femtosecond laser pulses. Femtosecond laser processing features high spatial resolution, enabling the inscription of gratings with local asymmetric refractive index modulation, which facilitates polarization high-order mode conversion with varying efficiency. By connecting the grating to a single-mode fiber pigtail, the component exhibited a maximum polarization-dependent loss of 32.5 dB and a 10 dB bandwidth of approximately 30 nm. Also, gratings integrated into two-mode fiber exhibit lower sensitivity to temperature and environmental refractive index, making them more resistant to external disturbances than single-mode fiber devices. The operational wavelength of the polarizer is flexibly adjustable by varying the grating period, enabling tuning across different wavebands for ultrafast laser applications. By employing the grating-based polarizer as a polarization-selective element in a nonlinear polarization rotation fiber laser, femtosecond-level fundamental frequency pulses and harmonic mode-locked pulses were achieved by adjusting the cavity polarization state and pump power. The two states of ultrafast pulses, with stable output, exhibited a signal-to-noise ratio exceeding 70 dB. The femtosecond laser inscribed all-fiber polarizers in this experiment are expected to play a key role in future integrated ultrafast laser systems due to their compact structure and the simplicity of direct fabrication.