Laser beam refract traversely through a graded-index preform to determine refractive index ratio and gradient profile

A technique is described which permits the determination of geometric and refractive index characteristics of graded-index preforms from measurements of the refraction of rays traced through the preform perpendicular to the preform axis. A computer program was developed to trace rays through graded-index preforms to display the refracting effect of the index properties and relate the ray incidence angle to its deflection in traversing the preform. An experimental apparatus has been developed in which a narrow beam of laser radiation is directed at the preform and its deflection angle measured. Comparison between the experimental results and the ray trace calculations using a curve-fitting procedure gave nondestructive determinations of the refractive index ratio and the gradient profile parameter, as well as measurement of the core dimensions.