Prism coupler configuration

The prism coupler was the first device used to couple a substantial fraction of the power contained in a laser beam into a thin film without the need for precision polishing of the edge of the film and sub-micron accuracy in the alignment of the beam and the edge of the film. Using a prism coupler, the beam coupled into a thin film can have a diameter hundreds of times the thickness of the film. Invention of the coupler contributed to the initiation of a field of study known as Integrated Optics. The physical concepts underlying operation of the coupler are described below.

Fig. 1 illustrates a prism coupler used to couple the power from an incident laser beam into a thin film. The film lies on a substrate like a glass microscope slide and might have a thickness of the order of the wavelength of the incident light. If the refractive index of the film is greater than that of the glass slide, the film can serve as a dielectric planar waveguide for light.

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. For reasons described below, we may refer to the thin film at the bottom of the prism 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 might be in the range of tens to hundreds of nanometers.

In operation, the prism, with tunneling layer down, is pressed against the waveguide film lying on the glass slide. 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 later must be lowest, next is the refractive index of the guide film, and highest is the index of the prism. Figures 2 and 3 are photographs showing early experiments involving, respectively, prism coupling into a thin film, and coupling in and out of a thin film. Light scattered from the guided wave, clamps holding the prisms, and a beam reflected from the front face of the prism are visible in the photographs.

November 5, 2011

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