The research team of High Power Fiber Laser Technology Lab in China Academy of Science Shanghai Optics precision mechanics institution recently proposed a nonlinear optical gain modulation technique that can convert the single-frequency CW laser into high coherent femtosecond pulse recently. It’s a new technical means to obtain ultrafast pulses with flexible wavelength, and the related results were published in Advance Photonics Research.
Ultrashort pulse lasers have a major impact in a wide range of fields like basic research, industrial processing, optical communication, etc. The main technology for obtaining ultrashort pulse lasers are mode-locking, electro-optic modulation, optical Kerr effect micro-resonator. Compared with the above methods, nonlinear optical gain modulation that generates femtosecond pulses this new technology is better in structure, stability, light-to-optical conversion efficiency, output pulse energy. The wavelength-flexible femtosecond laser source generated by the new technology would be widely used in basic research, biomedicine, optical precision metrology, etc industries.
1121nm single-frequency CW laser source and 1064nm picosecond laser source were coupled into a length of Raman gain fiber by WDM during the experiment. The picosecond Raman gain provided by the pump laser source will firstly shape the single-frequency CW laser source into ultrafast pulse in the time domain, then generates a new longitudinal mode component by nonlinearity in the frequency domain, and finally modulates the single longitudinal mode to wide broad-spectrum at the nanoscale in bandwidth. The nonlinear gain modulation device in the experiment can generate a stable and highly coherent 1120 nm laser with a spectral bandwidth of 9.5 nm, pulse energy of 25.7 nJ, a pulse width of 436 fs, and an optical-to-optical conversion efficiency as high as 69.4%.
The researchers revealed the evolution mechanisms between pump and Raman pulses in the time domain and frequency domain based on numerical simulation of the Generalized Nonlinear Schrodinger Equation. It shows that spatial walk-off has a great impact on nonlinear optical gain modulation’s optical-optical conversion efficiency and limit width of output compression. In addition, it can amplify output Raman pulse energy to μJ level by enriching the pulse energy and optimizing other system parameters, which confirmed that nonlinear optical gain modulation is the effective way to produce high pulse energy picosecond laser.
