Nondestructive Characterization of Internal Defects in Fiber Couplers Using OFDR-Based Thermal Excitation

Fiber-optic devices are widely used in communication, sensing, and other fiber-optic systems, and their quality directly affects system performance and reliability. However, internal defects in packaged devices remain difficult to characterize nondestructively, limiting their application in extreme environments. This work presents an active thermal-excitation method that uses the fiber embedded within the device as a distributed sensor. Controlled temperature changes are applied to excite the thermal response of the internal structure. Optical frequency-domain reflectometry (OFDR) then measures the distributed thermal strain along the fiber at millimeter-scale spatial resolution, enabling characterization of otherwise invisible internal defects. Using fused-fiber couplers as representative devices, we evaluated non-uniformity defects in the fused-taper and package-bonding regions. A finite-element model of an idealized coupler under thermal excitation was established to define the ideal thermal-response characteristics. The experimental results were broadly consistent with the simulated responses. Deviations from the predicted bonding-region symmetry and fused-taper uniformity indicated the possible presence of package-bonding and fused-taper defects, respectively. Dismantling representative couplers confirmed the locations and types of the defects. Applying the same criteria to the tested 2 × 2 and 1 × 3 couplers supported the applicability of the method across configurations within the present sample set. This method provides a transferable, high-spatial-resolution, and nondestructive means for quality assessment and reliability screening of packaged fiber-optic devices.