Tunable spectrally-flat multi-wavelength cylindrical vector beam fiber laser by using a re-circulating frequency shifter loop with two external injections

Abstract

We experimentally demonstrate a tunable spectrally -flat multi-wavelength cylindrical vector beam (CVB) fiber laser based on a two-mode long-period fiber grating (TM-LPFG) and a re-circulating frequency shifter loop with two external injections. The TM-LPFG is utilized as a mode converter in the re-circulating frequency shifter loop to excite the LP11 mode in the two-mode fiber. By adjusting the wavelength interval of the two injected seed signals, the power of the injected seed signals, and the pump power can produce a flat intensity envelope between the two injected wavelengths. Multi-wavelength CVBs of 12, 18, or 30 channels with a peak-to-peak power variation lower than 2 dB between the two injected wavelengths are obtained with a wavelength spacing of approximately 0.08 nm. The wavelength spacing can be adjusted by changing the frequency of the radio frequency signal. Multi-wavelength CVBs of 14 channels with a peak-to-peak power variation lower than 2 dB between the two injected wavelengths are obtained with a wavelength spacing of approximately 0.2 nm. The purity of the generated multi-wavelength CVBs is higher than 98%. Due to the absence of interference-type filter elements, the multi-wavelength CVB fiber laser exhibits excellent stability. The maximum wavelength deviation of each channel is less than 6.5 pm, and the maximum intensity fluctuation is less than 0.7 dB over a period of one hour.