Distributed bonding interface characterization of composite laser crystal based on optical low-coherence reflectometry

The bonded composite laser crystals are common solid active media in the field of high-power lasers and amplifiers, used to improve thermal effect and laser output. There is a potential risk of affecting optical transmission due to the imperfect bonding interface. To characterize the interface for crystal fabrication and laser design, we propose a distributed interferometer-based interfacial loss characterizing method. Based on the stable broad-band light source and photodetector, our Optical Low-Coherence Reflectometry (OLCR) is easier to obtain the low noise background and realize the high-accuracy measurement, which is important for detecting the weak reflection of the interface. Using a fiber probe, any local area on the interface can be detected individually. With scanning the optical delay line, the surficial reflection and interfacial reflection of one light spot can be monitored separately to enable the lateral and vertical distributed measurement. Specifically, a Yb:YAG/YAG crystal was measured and demonstrated as an example. To avoid absorption and realize high spatial resolution(40μm), a source located at 1535nm with a bandwidth of 58nm is used. Meanwhile, the reflection coupling loss (RCL) of the fiber probe is self-calibrated and eliminated by the simulation and calculation model. Moving the fiber probe, the vertical and lateral interfaces are scanned. The point cloud distribution testing of vertical bonding interfaces is accomplished, and a total internal reflection method is employed to detect the lateral interface. At last, the resolution (~0.5μm) of the probe scanning is validated and discussed using a photomask.