Phase of second-harmonic light self-generated in a glass fiber

Focusing an intense laser beam and its second harmonic into a glass fiber transforms the fiber into a frequency doubler. We measure the temporal evolution of both the amplitude and the phase of the second-harmonic light produced by a germanium-doped fiber and so determine the initial phase of the second-harmonic light to be Deltatheta= -71 degrees +/- 3 degrees . We demonstrate that the fiber-produced green light can exceed the seeding green light even if these two beams are 90 degrees out of phase. We also show that cross-phase modulation in the fiber can limit the maximum useful interaction length and consequently the ultimate efficiency of second-harmonic generation in fibers.