Recently, the researchers from Shanghai Institute of Optics and Fine Mechanics (SIOM) of Chinese Academy of Sciences (CAS) have made progress in nonlinear spectral manipulation of single frequency lasers. The phase modulated laser returns to the single frequency laser again after frequency doubling by the phase doubling in second harmonic generation (SHG). The approach avoids the limitation of SBS on power scaling of single frequency fiber laser. The relevant result is published on Optics Letters on January 3, 2022.

High-power single frequency lasers are of vital importance for various applications in scientific research and technology development, such as quantum information, laser remote sensing, and so on. High-power continuous-wave single-frequency lasers are mainly obtained by fiber amplifiers, but the output power is limited by stimulated Brillouin scattering. Phase modulation is an elegant technique and is widely used to suppress SBS by broadening the spectrum in a highly controllable way.

Nevertheless, the resulting amplifier output is not a single frequency. To meet the requirements of applications, a method of transforming a phase-modulated laser back to a single frequency is desirable.

Here, a passive spectral compression method is introduced by utilizing the phase-doubling effect in SHG. When a discrete phase modulation of difference π is applied to a single-frequency laser, lead a high threshold of SBS, the frequency doubled laser will return to single frequency output because the phase modulation is doubled in SHG. Typical binary phase modulation of square wave and pseudo-random binary sequence (PRBS) is investigated both theoretically and experimentally. High power single frequency SHG can be easily obtained by this method, and such kinds of laser sources are required in a wide range of applications.

Fig. 1. Illustration of the concept. (Image by SIOM)

Fig. 2. Spectral evolution in the case of square-wave phase modulation. (Image by SIOM)

Article website
https://doi.org/10.1364/OL.441282