206 nm deep ultraviolet laser from fifth harmonic generation of femtosecond fiber laser

Deep ultraviolet (DUV) femtosecond laser combines the advantages of high single-photon energy of DUV laser and high peak power of femtosecond laser, which is widely used in scientific research, biomedicine, material processing and so on. However, there is a problem in the process of generating DUV femtosecond laser based on nonlinear frequency conversion which the group velocity mismatch caused by dispersion will make the temporal walk-off of the nonlinear frequency conversion larger than the pulse duration of the femtosecond laser, which makes the generation of the DUV femtosecond laser very difficult. In this paper,based on a Yb-doped fiber femtosecond laser, the delay line was optimized to precisely compensate the spatial and temporal walk-off, so DUV femtosecond laser with a center wavelength of 206 nm and a repetition rate of 1 MHz is obtained, whose maximum output power is 102 mW. The maximum conversion efficiency is 4.25% from near infrared to DUV. The RMS power stability is 0.88% within 3 hours, and the peak-to-peak power stability is 3.75%. The evolution of laser spectra and beam quality during the process of second harmonic generation (SHG), FHG and SFG has been systematically studied. The experiment results provide a basis for the generation of DUV femtosecond laser from femtosecond fiber lasers.