Intermode beating mode-locking technique for O-band mixed-cascaded Raman fiber lasers

Abstract

A novel intermode beating mode-locking (IBML) technique combined with a cascaded Raman process is proposed to mode-lock an O-band two-cascaded Raman fiber laser. Using a 980-m-long phosphosilicate fiber pumped by a 1064 nm laser, the second-order Raman oscillation at 1319 nm is generated by the mixed-cascaded Raman shifts of P2O5 and SiO2. By precisely matching the intermode beating frequencies of the 1064 nm pump laser and the second-order Raman cavity frequency, harmonic mode-locking at 1319 nm is initiated. The dynamic process of the IBML operation in the cascaded Raman laser is experimentally investigated. The 131st-order harmonic mode-locking with a repetition rate of 27.247 MHz is very stable with the radio-frequency (RF) signal-to-noise ratio of >56 dB and the RF supermode-suppression ratio of >43 dB. The mode-locked pulses with the square profile are confirmed as the noise-like pulses by an autocorrelator. The IBML technique, in combination with the cascaded Raman process, could offer an exciting new prospect for obtaining simple, compact, and arbitrary-wavelength mode-locked laser sources.