Prism Coupler - Coupler Configuration

Coupler Configuration

A prism coupler is used to couple the power from an incident laser beam into a thin film. The film lies on a substrate such as a glass microscope slide and might have a thickness of the order of the wavelength of the incident light (0.550 μm for green light). The refractive index of the film is made greater than that of the glass slide, the film can serve as a dielectric planar waveguide for light via total internal reflection off the film–glass interface (and film–air interface). The prism coupler consists of a near cube of high–refractive-index glass and a second thin film at the bottom that contacts the waveguide film and serves the function of partially containing the guided wave over the coupling distance. The thin film at the bottom of the prism is referred to as the tunneling layer. The tunneling layer must have a lower refractive index than the waveguide film and may actually be implemented as a layer of air. The thickness of the tunneling layer will be on the order of a fraction of a wavelength (tens to hundreds of nanometers for visible light).

The prism and tunneling layer are pressed against the waveguide film. The beam enters the front face of the prism and strikes the tunneling layer somewhat more than half a beam width away from the face opposite the entry face of the prism. The ranking of refractive indices of the four regions of the combined coupler and waveguide structure must be as follows: the refractive index of the glass slide and the tunneling layer must be lowest, next is the refractive index of the guide film, and highest is the index of the prism.