Measurement of fiber-cladding diameter uniformity by use of whispering-gallery modes: nanometer resolution in diameter variations along millimeter to centimeter lengths.

Variations in fiber-cladding diameter on a nanometer scale were measured along millimeter to centimeter lengths by use of whispering-gallery modes (WGMs) in the elastic scattering of the fiber. The fiber was side coupled with a wavelength-tunable Gaussian beam. The scattered light was imaged approximately 1:1 onto a multichannel photodiode array detector. Based on the WGM wavelength shifts along the fiber, the taper of the fiber cladding's diameter was measured on a nanometer/millimeter scale. The fiber's surface roughness amplitude (in nanometers) and granular size ( approximately 100 microm) along centimeter-length fibers could also be revealed by use of higher- Q resonances.