Abstract

Here we report fundamental characteristics of directional couplers made by a two- step purely thermal ion-exchange. A large number of directional couplers with 11-20 µm axial separations (gap widths are 0-10µm) and 0-10 mm coupling-waveguide region lengths were fabricated. The ion-exchange process used T1 and K ions. The coupling lengths at 1300 nm and at 1550 nm wavelengths were estimated to be 2-55 mm and 2-18 mm, respectively. The relation between the coupling length and the axial separation was fitted well to an exponential function. The excess losses, including one coupling loss with a single-mode fiber for > 14µm axial separation, were all within 0.7 dB. The polarization dependency of the branching ratio of 3-dB couplers with half the coupling length was evaluated to be < ±2%. For the temperature change from −40°C to 85°C, the variation of the branching ratio measured with unpolarized light was as low as < ±1%.

© 1990 Optical Society of America