Generation of Ultra-High Signal to Noise Ratio Harmonics in an All-Polarization-Maintaining Fiber Laser

Harmonic mode locking (HML) was proved to be an effectively technique for generating high repetition rate ultrafast pulses. However, none of the various research methods currently used could improve the defects of HML's low signal to noise ratio (SNR), mostly less than 40 dB, limiting its further application. Using the self-made single walled carbon nanotubes (SWCNTs) as saturable absorber (SA) in an erbium-doped fiber laser to obtain HML pulses with an ultra-high SNR (>62 dB) was reported here. A series of harmonics from fundamental frequency to sixth orders could be observed by simply increasing the pump power, and the all-polarization-maintaining (PM) fiber path ensured time domain frequency stability. Moreover, by adjusting the polarization direction of the directional SA, a stable traditional soliton pulse was obtained. In this mode, the repetition frequency no longer split with the increase of pump power, and the SNR was as high as 83 dB. Such an environmentally stable, all-PM fiber, versatile operations laser could not only expand the industrial application prospects of high repetition frequency HML pulses, but also be an excellent seed source for ultra-high-power lasers research.