A simple and cost-effective method for precise length measurements of optical fibers

Accurately measuring optical fiber length is crucial across various fields, yet traditional methods like Optical Time-Domain Reflectometry (OTDR) and Optical Frequency-Domain Reflectometry (OFDR) have notable drawbacks, and emerging alternatives often require GHz modulation frequencies, adding to expense and complexity. We present a cost-effective method utilizing modulation frequencies up to 110 MHz, enabling the use of affordable, off-the-shelf components. Our technique involves intensity-modulating a laser and recording the phase difference between the fiber of interest and a reference path through a phase-locked loop. This relationship directly corresponds to the optical time delay, eliminating the need for extensive data processing. We also address considerations for conversion to the spatial domain, ensuring accurate measurements. This method achieves sub-millimeter precision, making it highly suitable for laboratory or clinical environments where fiber systems are too fragile or sensitive for physical length measurements. This includes fiber-based OCT systems and sterile fiber optic integrated endoscopic probes.