Researchers from the Shanghai Institute of Optics and Fine Mechanics (SIOM) of the Chinese Academy of Sciences have proposed and tested a new approach for further power scaling of frequency-doubled fiber lasers. In their experiment, resonant frequency doubling of a periodically phase modulated continuous-wave (CW) single-frequency fiber laser was used to achieve a near-30 W output at a 532 nm wavelength with a maximum conversion efficiency above 80%.
To obtain a stable and reliable high-power visible laser, one solution is to use the fiber laser and an external resonantly enhanced cavity with a lithium triborate crystal to achieve high-power second-harmonic generation (SHG). Single-frequency fundamental lasers are required in external-cavity SHG for efficient laser injection and resonant enhancement. However, the output power of single-frequency fiber lasers is limited by stimulated Brillouin scattering (SBS). Phase modulation is an elegant approach for SBS suppression, achieved by expanding the linewidth and reducing spectral power density in a highly controllable way. However, after phase modulation, the laser output is no longer single-frequency.
The researchers use a periodically phase-modulated fundamental laser and resonant-cavity SHG to solve this problem. In the experiment, simple sine-wave phase modulation is applied to a fiber laser with a 1064 nm wavelength, resulting in a few-frequency fundamental laser. By adjusting the modulation frequency to match the free spectral range of the frequency-doubling cavity, resonant enhancement was achieved after cavity locking. Sinusoidal modulation is applied in the experiments for simplicity, but any periodical phase modulation is feasible, say the researchers, including the widely used pseudorandom binary sequence (PRBS) modulation. As phase modulation is commonly used in high-power narrow-linewidth fiber amplifiers for SBS suppression, resonant frequency doubling of phase-modulation-generated few-frequency fiber lasers is an interesting approach for increasing the output power of visible fiber lasers. Reference: X. Zeng et al., Opt. Lett. (2020); https://doi.org/10.1364/ol.401348.