Second-harmonic generation and electro-optic modulation in thermally poled and unpoled twin-hole silica-glass optical fiber

Second-harmonic generation (SHG) and electro-optic (EO) modulation were studied on thermally poled twin-hole fiber. Metal electrode wires were inserted into the side holes. The typical poling condition was 2.5 kV, 300 degrees C, and 40 min. SHG was measured using a Q-switched Nd:YAG laser. The SH power did not depend on the applied forward or reverse voltages. SHG without poling was also measured, then the maximum power was about 1/18 that of the poled SHG. EO modulation was performed using a twin-hole fiber inserted to a fiber-optic Mach-Zehnder interferometer. An AC modulation voltage was applied to the electrodes together with a DC bias voltage. Without poling, the modulation output was obtained only when a DC bias voltage was applied simultaneously. After poling, a modulation output was obtained without any bias voltage, and for the forward DC bias the modulation output increased with the bias voltage. For the reverse DC bias the modulation output showed the minimum for a bias voltage. The origin of the second-order nonlinearities and the other effects in the above SHG and EO modulation are discussed considering charge layers.