By depositing thin films on the end of an optical fiber, it is possible to make simple, in situ measurements of film growth, refractive index, and surface roughness. Agreement with witness samples is excellent.
T. F. Morse, T. F. Morse,
"A new technique for the measurement of thin film growth", Proc. SPIE 3746, 13th International Conference on Optical Fiber Sensors, 374626 (1 September 1999); doi: 10.1117/12.2302068; https://doi.org/10.1117/12.2302068