  In second harmonic generation, the phase of the optical field is doubled
which has important implication. Here the phase doubling effect is utilized to
solve a long-standing challenge in power scaling of single frequency laser.
When a (-{\pi}/2, {\pi}/2) binary phase modulation is applied to a single
frequency seed laser to broaden the spectrum and suppress the stimulated
Brillouin scattering in high power fiber amplifier, the second harmonic of the
phase-modulated laser will return to single frequency, because the (-{\pi}/2,
{\pi}/2) modulation is doubled to (-{\pi}, {\pi}) for the second harmonic. A
compression rate as high as 95% is demonstrated in the experiment limited by
the electronic bandwidth of the setup, which can be improved with optimized
devices.
