A New Vertical Directional Coupler for Optimum Edge-Coupling to an Embedded Photodetector

Abstract

Hybrid photodetectors on glass and lithium niobate substrates have been fabricated by combining passive waveguides with overlayers of grafted semiconductor material (GaAs) for optical receiver applications1. To improve the overall coupling from an optical fiber to the detector, a vertical directional coupler (VDC) which transfers power from a buried waveguide (with a minimal insertion loss) to a surface waveguide was recently proposed and demonstrated using a field-assisted A'+-ion exchange in glass2. Though this scheme works well for vertical coupling of light to the detector, its efficiency is impaired by the presence of a thin layer of low index amorphous material at the semiconductor-waveguide bonding interface3. An effective way to avoid this problem is to use edge-coupling to the detector that can be achieved by cladding the waveguide-detector structure with a dielectric thin film having a refractive index higher than that of the waveguide4. Recently, we proposed a VDC which takes advantage of this edge-coupling scheme by cladding the structure with an AI2O3 thin film5. We found that the cladding caused the peak* of the guided field to move closer to the interface. However, further increase of cladding thickness created a phase mismatch between the dielectric-clad surface guide and the buried guide, thus destroying the coupling behaviour. In this paper, a new VDC structure is proposed and realized experimentally. This device is composed of two parallel slab waveguides, one being a buried guide in glass and the other a dielectric thin-film surface guide, as shown in Fig. 1. The cladding is made