Measurement Applications of Prism Couplers
Prism couplers are instruments used to measure the refractive index/birefringence and thickness of dielectric and polymer films. Since refractive indices of a material depend upon the wavelength of the electromagnetic radiation transmitted, a monochromatic laser is used in conjunction with a prism of known refractive index. The laser beam is directed through a side of the prism, bent, and is normally reflected back out the opposite side into a photo detector. However, at certain values of the incident angle theta, the beam does not reflect back out, but instead is transmitted through the base into the film sample. These angles are called mode angles. A computer-driven rotary table varies the incident angle of the laser. The first mode angle found determines the refractive index, and the angle difference from one mode to the next determines the sample thickness.
Prism couplers also allow for coupling light in and out of a waveguide without exposing the cross-section of the waveguide (edge coupling). To achieve this a phase matching condition is required between the propagation constant of the mth mode in the waveguide and the incident light at an angle normal from the waveguide surface.
where is the index of refraction of the prism.
where is the index of air (~1) and is the propagation constant of the waveguide. In order to have a guided mode, . This would imply that, which is not possible.
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