Abstract

We present results on the design and fabrication of 3-dB single-mode cross-couplers in Ag⁺ – Na⁺ ion-exchanged BK7 glass. The normal mode propagation method was used to determine the characteristics of a graded-index slab waveguide structure which simulates the effective index profile of our channel waveguide devices. The results indicate that the adiabatic condition $\left(\Delta \beta \gg 0.43\theta \overline{\text{γ}}\right)$ originally stated by Burns and Milton¹ for step index slab waveguide devices is practically unchanged for graded-index devices such as ours. This criterion was used to design a cross-coupler incorporating an angle of ~0.1°. Several couplers were fabricated on each BK7 substrate using asymmetric-symmetric 2×2 couplers by performing ion exchange for 12 h at 340°C in an AgNO₃ + NaNO₃ melt. The measured results demonstrate a 50:50 power split ratio when either input waveguide is excited. Furthermore, the device is fiber compatible, polarization independent, and relatively wavelength insensitive. Losses as low as 2.5 dB in 50-mm long devices are easily achieved. We are currently investigating how to minimize the device length by incorporating bends. This consideration is necessary if these devices are to be incorporated in larger integrated optic devices such as the 4×4 coupler.

© 1989 Optical Society of America