A Semi-Transparent Mirror-Type Directional Coupler for Optical Fiber Applications (Letters)

A directional coupler for optical fiber applications is constructed of two pieces of optical fibers cut obliquely and a thin dielectric film. Coupling coefficient -20 dB to -10 dB depending on the refractive index of the dielectric film, insertion loss 1 dB, and directivity -20 dB are measured. They agree with the analytical results.