Second-harmonic generation from thermally-poled twin-hole silica-glass optical fiber and enhancement by quasi phase matching

The second-harmonic generation from optical fibers was studied using a twin-hole fiber. Metal wire electrodes were inserted into the holes of 35 μmϕ, and the poling voltage was applied to a 20-μm-thick layer including the germanosilicate core. Q-switched Nd:YAG laser pulses were coupled to the core of the fiber, and the second-harmonic wave was measured with a photomultiplier. The quasi phase matching was performed by periodic erasure of nonlinearity. The poled fiber was irradiated with a KrF excimer laser through a silica mask patterned with a 40-μm pitch chromium stripe. By changing the angle of the fiber with respect to the mask pattern the optimum period was found to be 41.1 μm, then the second-harmonic power increased by a factor of up to 15. Mechanisms of thermal poling and ultraviolet depoling were discussed.