Ultra-high repetition rate pulsed laser

Ultra-high repetition rate pulsed laser

In the microscopic world of the interaction between light and matter, ultra-high repetition rate pulses (UHRPs) act as precise rulers of time – they oscillate at more than a billion times per second (1GHz), capturing the molecular fingerprints of cancer cells in spectral imaging, carrying massive amounts of data in optical fiber communication, and calibrating the wavelength coordinates of stars in telescopes. Especially in the leap of the detection dimension of lidar, terahertz ultra-high repetition rate pulsed lasers (100-300 GHz) are becoming powerful tools to penetrate the interference layer, reshaping the boundaries of three-dimensional perception with the spatiotemporal manipulation power at the photon level. At present, using artificial microstructures, such as micro-ring cavities that require nanoscale processing accuracy to generate four-wave mixing (FWM), is one of the main methods to obtain ultra-high repetition rate optical pulses. Scientists are focusing on solving the engineering problems in the processing of ultra-fine structures, the frequency tuning problem during pulse initiation, and the conversion efficiency problem after pulse generation. Another approach is to use highly nonlinear fibers and utilize the modulation instability effect or FWM effect within the laser cavity to excite UHRPs. So far, we still need a more dexterous “time shaper”.

The process of generating UHRP by injecting ultrafast pulses to excite the dissipative FWM effect is described as “ultrafast ignition”. Different from the above-mentioned artificial microring cavity scheme that requires continuous pumping, precise adjustment of detuning to control pulse generation, and use of highly nonlinear media to lower the FWM threshold, this “ignition” relies on the peak power characteristics of ultrafast pulses to directly excite FWM, and after “ignition off”, Achieve self-sustaining UHRP.

Figure 1 illustrates the core mechanism of achieving pulse self-organization based on ultrafast seed pulse excitation of dissipative fiber ring cavities. The externally injected ultrashort seed pulse (period T0, repetition frequency F) serves as the “ignition source” to excite a high-power pulse field within the dissipation cavity. The intracellular gain module works in synergy with the spectral shaper to convert the seed pulse energy into a comb-shaped spectral response through joint regulation in the time-frequency domain. This process breaks through the limitations of traditional continuous pumping: the seed pulse shuts off when it reaches the dissipation FWM threshold, and the dissipation cavity maintains the self-organizing state of the pulse through the dynamic balance of gain and loss, with the pulse repetition frequency being Fs (corresponding to the intrinsic frequency FF and period T of the cavity).

This study also conducted theoretical verification. Based on the parameters adopted in the experimental setup and with a 1ps ultrafast pulse laser as the initial field, numerical simulation was carried out on the evolution process of the pulse’s time domain and frequency within the laser cavity. It was found that the pulse went through three stages: pulse splitting, pulse periodic oscillation, and pulse uniform distribution throughout the entire laser cavity. This numerical result also fully verifies the self-organizing characteristics of the pulse laser.

By triggering the four-wave mixing effect within the dissipative fiber ring cavity through ultrafast seed pulse ignition, the self-organizing generation and maintenance of sub-THZ ultra-high repetition frequency pulses (stable output of 0.5W power after seed turning off) were successfully achieved, providing a new type of light source for the lidar field: Its sub-THZ level refrequency can enhance the point cloud resolution to the millimeter level. The pulse self-sustaining feature significantly reduces system energy consumption. The all-fiber structure ensures high stability operation in the 1.5 μm eye safety band. Looking to the future, this technology is expected to drive the evolution of vehicle-mounted lidar towards miniaturization (based on MZI micro-filters) and long-range detection (power expansion to > 1W), and further adapt to the perception requirements of complex environments through multi-wavelength coordinated ignition and intelligent regulation.