Repetition Rate Adjustable Dual-wavelength Solitons Fiber Laser Based on Highly Nonlinear Fiber

A dual-wavelength soliton fiber ring laser with adjustable repetition rate was reported. Through adding highly nonlinear fiber (HNLF) as nonlinear medium in the fiber ring cavity, the dual-wavelength bright-dark soliton pair can be formed. The 1st to 4th, 5th to 8th and 9th to 11th harmonics bright-dark solitons pairs were obtained by rotating the Polarization controller (PC) when the power are 160 mW, 230 mW and 340 mW, respectively. In order to investigate the influence of loss on the laser system, a variable optical attenuator (VOA) was embedded in the ring laser cavity. Then the dual-wavelength with Kelly sidebands was obtained by properly adjusting the PC and VOA. The center wavelengths of the dual-wavelength laser were located at 1557.8 nm and 1568.5 nm and the 3 dB bandwidths of two wavelengths are 2.2 nm and 2.1 nm, respectively. The spacing between the two wavelengths was 10.7 nm. Due to the existence of nonlinearity, the 1st to 22th harmonics bright solitons sequence (corresponding to the repetition rate tunable from 196.8 kHz to 4.33 MHz) was obtained by increasing the pump power. The intervals of bright-dark solitons pair and bright solitons all were $5.08\ \mu\mathrm{s}$ , which corresponds to fundamental repetition rates are 196.8 kHz. The signal-to-noise ratio of the bright solitons was approximately 58 dB, which means the operation of laser was stable.