Method and apparatus for nondestructive determination of core size of hollow-core photonic bandgap fiber using Fabry-Perot interference

An apparatus for nondestructive determination of core size of a hollow-core photonic bandgap fiber (HC-PBF) using Fabry-Perot (FP) interference, comprising:

• a tunable laser source (TLS), a 1×2 single-mode (SM) coupler, an SM collimator, a six-axis translation stage, an optical detector, an oscilloscope, and a container holding an index-matching oil;
• wherein the TLS and the input end of the optical detector are connected to the 1×2 SM coupler, the output end of the optical detector is connected to the oscillator, the SM collimator is connected to the 1×2 SM coupler, the SM collimator is fixed by a support fixture connected to the six-axis translation stage for adjusting the position of the SM collimator, wherein when the SM collimator and an HC-PBF under test (FUT) are immersed in the index-matching oil in the container, an FP cavity is generated in the FUT by adjusting the position of the SM collimator with the six-axis translation stage and by rotating the FUT;
• wherein when the TLS is continuously tuned from one wavelength to another within a preset wavelength range, a light emitted from the TLS passes through the 1×2 SM coupler and the SM collimator in sequence and turns into a spatially collimated light, which enters the FUT and is reflected by the FP cavity in the FUT, and the reflected light passes through the SM collimator and the 1×2 SM coupler in sequence to reach the optical detector, so that the size of the core of the FUT may be calculated from the reflected light's spectrum.