Simple and cost-effective method to measure the length of optical fibers

Precise and accurate length measurements of optical fibers are increasingly needed across various applications. However, many commercially available devices are either too inaccurate or, conversely, too advanced and costly for basic fiber length determinations. In this Letter, we present a simple and relatively inexpensive method to measure the length of optical fibers with a precision of 1.06 ps (∼212 μm). This method exploits the relationship between the phase difference of a fiber under test (FUT) and a calibrated reference path as a function of frequency, measured by a phase-locked loop during a frequency sweep. We highlight key processing steps to achieve reproducible results and important considerations regarding environmental conditions.