Objective U-band (1625-1675 nm) high repetition rate mode-locked fiber lasers have broad applications in expanding communication bands, methane and gas detection, photon microscopy, coherent Doppler lidar, and ocular surgery. At present, actively mode-locked fiber lasers based on SSFS can achieve 10 GHz U-band pulse output, but the high repetition rate U-band based on nonlinear polarization rotation (NPR) passively mode-locked has not been broken through. Passively harmonic mode-locking (HML) based on the soliton self-frequency shift (SSFS) is an effective approach for achieving high repetition rate pulses in the U-band. It is of great significance for practical applications.

Methods A passively HML fiber laser system based on SSFS has been constructed (Fig.1). Initially, a passively HML fiber laser based on NPR is constructed as the seed source. Through a cascaded two-stage amplification system, the output power is amplified up to 1.7 W. Subsequently, the amplified pulses are input into a section of dispersion-shifted fiber (DSF) with anomalous dispersion, where the SSFS effect is stimulated to shift the central wavelength to the U-band. The residual part of the seed source is filtered out by a long pass filter (LPF), thereby achieving U-band passively HML pulse output. For the first time, a GHz pulse output with a wavelength that can cover the entire U-band range has been achieved using an NPR passively HML fiber laser as an SSFS effect seed source.

Results and Discussions Repetition rate tuning of the seed source is achieved by adjusting the pump power and the polarization controller (PC). As the pump power increases from 31 mW to 862 mW, the laser source operates at harmonics spanning from 1^st to 39^th, corresponding to repetition rate ranging from 31.39 MHz to 1224.2 MHz (Fig.2). Three different repetition rate pulses (722.0 MHz, 941.7 MHz, 1224.2 MHz) are selected to be shifted to the U-band via SSFS, and the influence of pulse repetition rate on SSFS is investigated (Fig.3, Fig.4, Fig.7). Finally, a tunable high repetition rate U-band passively HML fiber laser is realized, covering the entire U-band wavelength range. When comparing the pump power required for the main amplifier to shift to the U-band (~1630 nm) at different repetition rates, it is observed that the required pump power varies. The required pump power is 4.2 W, 4.4 W and 4.0 W at repetition rate of 722.0 MHz, 941.7 MHz and 1224.2 MHz, respectively. At 722 MHz, the required pump power is 4.2 W, at 941.7 MHz it is 4.4 W, and at 1224.2 MHz it is 4.0 W. Notably, when the repetition rate is 1224.2 MHz, the required pump power decreases. This is due to the relatively weak Kelly sideband strength in the seed source. As a result, higher amplification efficiency is achieved, leading to a reduced pump power requirement for frequency shifting to the same wavelength. And since the states with repetition rates of 722 MHz and 1224.2 MHz accumulate excessive nonlinearities during the amplification process, the second-order Raman soliton thresholds are lowered, and the optical spectrum after SSFS shows double peaks in the U-band.

Conclusions A U-band passively HML fiber laser based on NPR mode-locking technology has been realized with a repetition rate tuned up to 1 GHz and a wavelength tuning range covering the entire U-band. The conditions for achieving SSFS at different repetition pulse rates (722.0 MHz, 941.7 MHz, 1224.2 MHz) are systematically discussed, along with the influence of the Kelly sidebands from the seed source under different states. Under unchanged conditions, higher repetition rates require higher pump power for the main amplifier to achieve frequency shifting into the U-band. However, excessively strong Kelly sidebands from the seed source would reduce the amplification efficiency.