4.37 GHz L-Band Passively Harmonic Mode-Locked Fiber Laser Based on Nonlinear Polarization Rotation

L-band passively harmonic mode-locked (PHML) fiber lasers are important because various practical applications, such as high-speed optical sampling, dense wavelength division multiplexed (DWDM) systems and material processing, etc. Therefore, L-band PHML fiber lasers have been widely concerned in recent years. In 2012, J. Du et al experimentally achieved L-band PHML pulses by utilizing graphene as a saturable absorber (SA). Under a certain pumping power, 2
^nd
to 4
^th
harmonic orders can be tuned by adjusting polarization controllers (PCs). In 2014, Y. Meng et al obtained 5.88 GHz L-band PHML pulses with 19 dB super-mode suppression ratio (SMSR) in an Er-Yb co-doped double-clad fiber laser using a graphene SA. In 2018, Q. Huang et al built up a 550 MHz L-band PHML fiber laser based on carbon nanotubes film for the first time. It is worth noting that all of these results are focused on the generation of L-band PHML by utilizing a physical SA. However, the fabrication of physical SA can be complicated with high cost and its thermal damage threshold is not enough to support high pump power which have great impact on the repetition rate of harmonic pulses. Meanwhile, nonlinear polarization rotation as an effective SA have many advantages including high thermal damage threshold, low fabrication cost, high modulation depth and compactness. Therefore, it is desirable to build an L-band PHML fiber laser based on NPR.