New Non-Contact Measurement of Small inside-Diameter Using Tandem Low-Coherence Interferometer and Optical Fiber Devices

Recently, manufacturing techniques of small-scale products have been improved. As a result, precise measurement is required of small inside diameters, for example, of engine nozzles. However, an in situ measurement and measurement system for small-sized products has yet to be fully established. In this research, a contactless technique to measure small inside diameters is proposed. This new method uses tandem low-coherence interferometry and an optical fiber cut at an angle of 45°. This optical fiber is up to 30 μm in diameter and is used as a probe. Our objective is to measure holes as small as 50-μm inside diameter with an accuracy of 100 nm. In the present paper, we report on the measurement principle, calculate the measurement uncertainty and show that experimental measurements can be obtained of small-size holes up to 300-μm inside diameter with an accuracy of 100 nm.