Advanced chromatic confocal optical coherence tomography sensor with switchable acquisition modes

A fiber optic sensor combining the principle of chromatic confocal and spectral-domain optical coherence tomography (OCT) has been developed to simultaneously measure film thickness and refractive index. By capturing a spectrum that composed of the low-frequency confocal signal and the high-frequency interference signal, the sensor can initially estimate the confocal thickness and optical thickness of the tested film. Further, by combining the parameter of sensor system, the geometric thickness and refractive index of the film can be subsequently calculated. Additionally, an acquisition mode switching method and a covariance algorithm-based method were also presented to overcome the decrease in signal-to-noise ratio (SNR) induced by signal separation and the reduction in resolution caused by bandwidth narrowing. For validation, five PVC films with different thickness (0.1 mm, 0.2 mm, 0.3 mm, 0.4 mm, and 0.5 mm) and five 0.1 mm thickness films with different materials (PVC, PC, PMMA, PET, and FEP) were measured by the sensor. The results indicate that the absolute errors in thickness and refractive index measurements of the sensor are less than 2.5 % and 3.0 %, respectively.