High-resolution measurement of the fiber diameter variations using whispering gallery modes and no optical alignment

Light is coupled from the guided mode along one tapered optical fiber into the whispering gallery modes around the circumference of another. Small diameter variations in the second fiber are measured by tracking, at different points along it, the wavelengths at which these modes exist. The measurement does not require any optical alignment, and its resolution can be 1 part in 10 000 or better.