Femtosecond Laser-Written Small-Period Long-Period Fiber Grating for an L-Band Normal Dispersion Mode-Locked Fiber Laser

Abstract

We utilize a femtosecond laser-inscribed small-period long-period fiber grating (SP-LPFG) to induce a nonlinear polarization rotation (NPR) effect for mode-locking pulses in a normal dispersion erbium-doped fiber laser (EDFL). The SP-LPFG has a length of 2.5 mm and a period of 25 μm. At wavelengths of 1556 nm and 1561 nm, it exhibits polarization-dependent loss (PDL) values of 20 dB and 14.5 dB, respectively, sufficient to trigger the NPR mechanism. With the pump power increased to 500 mW, the laser achieves normal dispersion mode-locked pulses centered at 1575 nm in the L-band, with a 3 dB bandwidth of 1.35 nm and a pulse width of 1.61 ps. The radio frequency (RF) spectrum reveals an signal-to-noise ratio (SNR) of up to 63.6 dB, demonstrating the excellent stability of the laser operation. This SP-LPFG holds promising applications, paving the way for efficient, compact, and stable normal dispersion ultrafast fiber lasers.

Keywords

• fiber laser
• mode-locked pulses
• nonlinear polarization rotation
• polarization-dependent loss