Probing optical microfiber nonuniformities at nanoscale

We demonstrate a novel, simple, and comprehensive method for probing optical microfiber surface and bulk distortions with subnanometer accuracy. The method employs a regular optical fiber as a probe that slides along a microfiber transmitting the fundamental mode. The fraction of radiation power absorbed in the probe depends on the local distribution of the mode propagating in the microfiber. From the measured variation of the absorbed power, we determine the variation of the effective microfiber radius, which takes into account both the microfiber radius and refractive index variations. Furthermore, we verify the cylindrical symmetry of the microfiber nonuniformities by probing the microfiber from different sides. These results explain observed transmission losses in silica microfibers and open broad opportunities for microfiber investigation.