Recently, researchers from Shanghai Institute of Optics and Fine Mechanics (SIOM) of the Chinese Academy of Sciences (CAS) have proposed a new approach for further power scaling of frequency doubled fiber lasers. Resonant frequency doubling of periodically phase modulated single-frequency fiber laser is used as a method to achieve high-power visible fiber lasers. The relevant result is published in Optics Letters 45 (17), 4844-4947 (2020).

High-power, continuous wave (CW), narrow linewidth visible lasers have a wide range of applications in the scientific and industrial fields. In order to obtain a stable and reliable high-power visible laser, one solution is to utilize the fiber laser and external resonantly-enhanced cavity with LBO crystal to achieve high-power SHG. Single frequency fundamental lasers were 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 by expanding the linewidth and reducing spectral power density in a highly controllable way. But after phase modulation, the laser output is not single-frequency anymore.

The researchers propose to use periodically phase modulated fundamental laser for resonant cavity SHG to solve the power scaling issue in visible fiber lasers. In the experiments, simple sine-wave phase modulation is applied to generate a few-frequency fundamental laser at 1064 nm. By adjusting the modulation frequency to match the FSR of the doubling cavity, the resonant enhancement condition is achieved after cavity locking. An up to 30 W 532 nm laser is generated with a maximum conversion efficiency above 80%, which is not distinguishable with the single-frequency case.

Sinusoidal modulation is applied in the experiments for simplicity of explanation, but any periodical phase modulation is feasible, including the widely used pseudo-random binary sequence modulation. Since 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 further power scaling of visible fiber lasers.

Related research was supported by the National Key R&D Program of China (2018YFB0504600, 2018YFB0504602); Shanghai Science and Technology Commission (19441909800).

Fig. 1. The schematic diagram of the experimental setup for resonant cavity SHG of phase modulated few-frequency fiber laser. (Image by SIOM)

Fig. 2. Output and conversion efficiency of SHG for the cases of single-frequency and few-frequency with RF driver power of 17.2 dB, 23.2dB and 25 dB, respectively. (Image by SIOM)

Article website:
https://doi.org/10.1364/OL.401348

Contact:
Mr. Wu Xiufeng
General Administrative Office
Shanghai Institute of Optics and Fine Mechanics, CAS
Email: xfwu@siom.ac.cn