Second-order nonlinearity and phase matching in thermally poled twin-hole fiber

The second-order nonlinearity in poled optical fiber is promising for application to electro-optic switching and modulation, second-harmonic generation (SHG), and frequency conversion. In this paper, we poled a twin-hole fiber, which is similar to PANDA fiber but the strain applying part is vacant. Electrode wires were inserted into the side holes, and the fiber was poled with a voltage of 2.5 kV at 300°C for 40 min. We measured the SHG using a linearly polarized Q-switched Nd3+:YAG laser. The SH power was highest for polarization parallel with the direction of two holes. The SH power had a maximum for the fiber length of 5 cm and decreased for longer fiber lengths. We analyzed this phase matching considering cladding modes. We calculated numerically the propagation constants of the cladding modes. We showed that the ~40th-order cladding mode of the SH wave and the fundamental core mode of the fundamental wave are in phase matching. We also performed an SHG of poled twin-hole fiber using a 260-fs passively mode-locked Er3+-doped fiber laser as a fundamental-wave source. The SH signal from the poled fiber was proportional to the 1.82-th power of the fundamental power, and the polarization dependence agreed with that measured with an Nd3+:YAG laser. We discussed an application of the poled twin-hole fiber.