Tunable Harmonics Generation from Low Average Power Mode-Locked Er-Fiber Laser Using Periodic Poling Nonlinear Crystals

Abstract

Sufficient second, third, and fourth harmonic generation of mode-locked Er-doped fiber laser (ML - EDFL) was experimentally demonstrated. This was achieved by using periodically poled KTiOPO₄ (PPKTP) and PP LiNbO₃ (PPLN) PPLN nonlinear crystals. Harmonic generation in both crystals depended on the direction of polarization. The highest conversion efficiency was obtained by using a half-wave plate to rotate the polarization before the crystals. When using the PPKTP nonlinear crystal, conversion efficiencies of 4.88%, 0.02%, and 0.002% were obtained for second, third, and fourth harmonics generation at wavelengths of 980, 520, and 390 nm, respectively. For the PPLN nonlinear crystal, temperature and polarization direction were optimized for each harmonic generation wavelength. As a result, conversion efficiencies of 8.6%, 0.1%, and 0.007% were obtained for second, third, and fourth harmonic generation at the same respective wavelengths. Tunable wavelength ranges and their SHGs, as well as the multi-wavelength output and their corresponding SHG wavelengths, were also reported.