Directional Couplers for Integrated Optics

Abstract

In the field of integrated optics directional couplers will play an important role. These couplers are not only important for providing just a direction dependent means of coupling but they are also important components for the realisation of optical modulators or switches /1,2/. The usual design for such directional couplers is shown in Fig. 1. Two parallel dielectric waveguides are placed in such close proximity that the evanescent fields overlap. In this case energy will be exchanged periodically between the two waveguides. Because the penetration depths are typically of the order of some fraction of an optical wavelength and the coupling strength depends exponentially on the separation of the two guides, the separation of the two waveguides must be very small and the tolerance for it must be kept very tight. For typical values of relative refractive index differences Δn/n between the core and. the cladding of 1 % for n≈1.5 and a separation of 1 μm the required coupling length for complete energy transfer would be ≈ 4 mm and a tolerance of 10 % (i.e. 0.1 μm) in the separation of the coupled waveguides would already require a 20% change in the coupling length /3/. While the relative tolerances are typical also for microwave components the absolute values are extremely small due to the much smaller optical wavelength. In addition to these tolerance problems there is still the problem of phase matching. The above numerical results were calculated under the assumption of two identical i.e. phase matched waveguides.