A method includes transmitting light through at least one transmission optical fiber towards a target, receiving light reflected from the target through at least one signal optical fiber, filtering the received light at least two different wavelengths, and using the filtered light to detect a clearance variation. An optical fiber probe includes a plurality of optical fibers, a moisture-resistant enclosure enclosing the optical fibers, a hydrophobic layer situated over an end of the optical fiber probe for preventing moisture from reaching the optical fibers, and a broadband transmission layer between ends of the optical fibers and the hydrophobic layer.