Stable system technique for measuring the refractive index profile of an optical fiber by modified fiber-type confocal microscope method

Using a fiber-type confocal scanning optical microscope system, we have obtained a stable refractive index profile measurement system of good performance. We could acquire excellent index precision and good spatial resolution by using a fiber-optic system for the majority part instead of a bulk-optic system and a single mode fiber at visible region with a 4 μm core diameter instead of a pinhole structure. Also, using a power detection system that is synchronous with fiber-coupled detector, we have improved system stability by reducing noise generated by the roughness of a sample surface because of using the optical fiber as a pinhole system. The light reflected by the sample surface was divided by a beam splitter; one ray passed back through the optical fiber in order to detect a confocal point and another ray entered the synchronous power detector in order to detect reflected power. The power detected by the synchronous power detector without a pinhole is less sensitive to the surface roughness than the power detected by the fiber-coupled detector. We could implement the simple and robust index measurement system by using a fiber-optic system and a synchronous detection system, and a single mode fiber was measured to demonstrate the effectiveness of our proposed measurement system.