Greater than 20%-efficient frequency doubling of 1532-nm nanosecond pulsesin quasi-phase-matched germanosilicate optical fibers

We fabricated second-order nonlinear gratings in D-shaped germanosilicate f ibers, using thermal poling and periodic electrodes defined by standard pho tolithography. These gratings, which are up to 75 mm long, were used for ef ficient quasi-phase-matched frequency doubling of 1.532-mu m nanosecond pul ses from a high-power erbium-doped fiber amplifier. Average second-harmonic powers as high as 6.8 mW and peak powers greater than 1.2 kW at 766 nm wer e generated, with average and peak conversion efficiencies as high as 21% a nd 30%, respectively. (C) 1999 Optical Society of America OCIS codes: 190.0 190, 190.4370, 320.4240, 190.2620.