Spectral compression by phase doubling in second harmonic generation.

In second harmonic generation, the phase of the optical field is doubled, which has important implications. Here, the phase-doubling effect is utilized to solve a long-standing challenge in the power scaling of single-frequency laser. (-π/2, π/2) binary phase modulation is applied to a single-frequency seed laser to broaden the spectrum and suppress the stimulated Brillouin scattering in a high-power fiber amplifier. The second harmonic of the phase-modulated laser returns to single frequency because the (-π/2, π/2) modulation is doubled to (-π, π) for the second harmonic. A compression-to-single-frequency rate as high as 95% is demonstrated in experiments limited by the electronic bandwidth of the setup. Such phase manipulation in wave-mixing processes may open up a new field of development in nonlinear optics and laser technology.