Recently, a research team from Shanghai Institute of Optics and Fine Mechanics (SIOM) of the Chinese Academy of Sciences (CAS) presented a new approach to transform continuous-wave (CW) laser into femtosecond-scale pulses by nonlinear optical gain modulation (NOGM). Despite much progress of generating ultrafast sources, it is still challenging to obtain high performance ultrafast pulses with wavelength versatility in a simple approach for instance. This method opens a new possibility to obtain highly-coherent pulses with flexible wavelength. The results were published in Advanced Photonics Research on December 22, 2021.

In the demonstration, an 1121 nm single frequency laser is injected into a Raman fiber amplifier, which is pumped by a 1064 nm pulsed laser. The picosecond pump laser provides an ultrafast varying Raman gain, which reshapes the CW laser into ultrafast pulses in the temporal domain and generates a broad spectrum in the frequency domain. Under 37 nJ, 14 ps gain modulation, the proposed setup can generate stable and highly-coherent laser at 1120 nm with a spectral bandwidth of 9.5 nm, a pulse energy of 25.7 nJ, a pulse duration of 436 fs, and an optical efficiency of 69.4%.

Numerical simulations based on generalized nonlinear Schrodinger equation (GNLSE) are carried out to reveal both temporal and spectral evolution mechanisms of the pump and Raman pulses in the experiments. Through the simulation, it is shown that group velocity mismatching between the pump and Raman pulses has a huge impact on the optical conversion efficiency and dechirped pulse duration of the NOGM system. Further scaling the NOGM pulse energy up to μJ-level is feasible by increasing the pump pulse energy and optimizing the system parameters correspondingly.

The proposed NOGM approach gains the advantages including simplicity, reliability, efficiency and high-energy accessibility.

Fig. 1.(a)Principle of nonlinear optical gain modulation; (b) Experimental setup; (c) Spectrum and (d) autocorrelation trace of the pump laser at a pulse energy of 37 nJ; (e) Spectrum of the 1121 nm single frequency seed laser.(Image by SIOM)

Fig. 2. (a) Spectrum (b) Pulse train (c) Fundamental radiofrequency (RF) spectrum (d) autocorrelation trace (e) spectrumand (f) pulse shape and chirp of the simulated output pulse. (Image by SIOM)