Recently, the high power fiber laser technology laboratory of Shanghai Institute of Optical Precision Machinery, Chinese Academy of Sciences has made new progress in the research of mode-locked Raman fiber laser. The high performance linearly polarized dissipative soliton Raman laser output is obtained by using a fully polarized non-linear optical ring mirror mode-locked. The time-domain stability of the laser pulse is greatly improved. The peak power clamping effect is introduced into the mode-locked Raman fiber laser to achieve high energy rectangular pulse output.
Raman fiber lasers use stimulated Raman scattering in the fiber as gain mechanism and have the advantage of flexible wavelength. Pulsed laser output in Raman fiber laser can effectively expand the application range of pulsed laser.
Ultrashort pulsed laser with special wavelength is often required to be linearly polarized in many basic scientific research and biomedical fields. In order to meet this demand, a fully polarized laser resonator is constructed by mode-locking with a non-linear optical ring mirror, and polarizing elements are added to the resonator to ensure the linear polarization of the output laser. At the same time, according to the fast response of Raman scattering in optical fibers, the researchers used a more stable amplified spontaneous emission source in time domain as the pump, and introduced a dissipation mechanism into the resonator to achieve the output of Raman dissipative soliton, which effectively improved the overall performance of Raman ultrafast laser. The pulse repetition rate of Raman ultrafast laser is 1.23 MHz, the maximum pulse energy and the minimum pulse width are 1.23 nJ and 63 PS respectively, and the signal-to-noise ratio of radio frequency spectrum is as high as 85 dB. Relevant research results have been published in Optics Express 27, 17905 (2019).
In order to improve the output pulse energy of mode-locked Raman fiber laser, the research group introduced the peak power clamping effect by optimizing the length of the ring mirror under the mode-locked mechanism of non-linear optical ring mirror, so that the pulse energy and pulse width increased with the increase of pump power without pulse splitting. Finally, a high-energy linearly polarized rectangular Raman pulse with a maximum energy of 64.1 nJ and a maximum pulse width of 25.1 PS was obtained. At the same time, it is observed that the time domain shape of Raman pulse gradually evolves from ordinary rectangular pulse to stepped rectangular pulse with the increase of pump power. Relevant research results have been published in Journal of Lightwave Technology 37, 1333 (2019).
The related research is supported by the National Natural Science Foundation of China and the Postdoctoral Science Foundation of China.