Rapid optical fiber delta measurement by refractive index tuning

A technique is presented for rapidly measuring Delta of optical fibers with profiles closely approximating n (r) = n(o) [1 - Delta(r/a)(alpha)], the Delta << 1 limit of the two-parameter Gloge-Marcatili profile. Error analysis indicates an accuracy of about +/-9%. The method is as follows: Using a Michelson interferometer together with a microscope, the transverse optical path through the center of a fiber immersed in index matching fluid is set equal to the optical path through the fluid. This is accomplished by varying the temperature of the fluid. The value of Delta can then be determined from a knowledge of the refractive index of the fluid inferred from temperature measurements. Good agreement is shown between Delta values measured by this technique and those measured by-two other interferometric techniques. The possible use of this method to determine alpha, or to determine the actual profile if it does not follow the assumed shape, is discussed.