Manipulating the Spectral-Temporal Characteristics of Multi-GHz Ultrashort Pulses by Leveraging the Nonlinear Phase Modulation and CW-Seeded Four-Wave Mixing

Ultrashort pulse fiber lasers with multi-GHz repetition rates have recently attracted significant attention in various fields. However, the parameter tunability of multi-GHz ultrashort pulse fiber lasers is inherently limited by their miniaturized design, particularly the central wavelength and transform-limited pulsewidth. To this end, here we propose a method for externally manipulating the spectral-temporal characteristics of multi-GHz ultrashort pulses by exploring continuous-wave (CW)-seeded four-wave mixing (FWM) and nonlinear phase modulation (NPM) driven by miniaturized mode-locked fiber laser with a multi-GHz fundamental repetition rate. We first conduct a theoretical investigation on the picosecond-pulse-driven CW-seeded FWM, and then study the NPM for manipulating the spectral-temporal performance of multi-GHz ultrashort pulses, i.e., associated with the CW-seeded FWM for pulsewidth tunability. Then, we perform experiments to validate the theoretical findings, and a pulsewidth tuning range of 598 fs–5.3 ps is achieved. Furthermore, a continuous wavelength tuning range from 1520 nm to 1550 nm is obtained. The experimental results show good agreement with the numerical simulation results. These efforts can provide a promising route to realize high-quality multi-GHz ultrashort pulses with flexible tunability of wavelength and pulsewidth through manipulating the spectral-temporal characteristics.

I. Introduction

Ultrashort laser pulses at multi-GHz repetition rates have been widely applied for scientific researches and industrial applications, including high-speed nonlinear imaging [1], dual-comb spectroscopy [2], [3], coherent optical communication [4], [5], low-noise microwave generation [6], high throughput micromachining [7], [8], [9], [10], etc. High-repetition-rate (HRR) mode-locked fiber laser, as one of the promising technologies for generating multi-GHz ultrashort pulses, has been paid special attention [11], [12], [13], [14], [15], and fundamental repetition rates of up to >10 GHz have been reported at different wavelengths [16], [17], [18]. The ultrashort cavity design of HRR mode-locked fiber lasers, on the other hand, inherently imposes restrictions on manipulating the performance of mode-locking, particularly the tunability of central wavelength and transform-limited pulsewidth, which prevents their applications in the fields requiring wavelength sweeping and pulsewidth tuning [19], [20], [21], [22], [23].