OCT super-resolution film thickness measurement method based on trigonometric polynomial fitting

Measuring thin film parameters is crucial in industrial production inspection. Optical coherence tomography can obtain film thickness information ranging from a few microns to several millimeters. Combined with spectral domain phase detection, it can obtain submicron-level depth information of single-layer reflective surfaces. However, when the film thickness is less than the OCT resolution, existing OCT measurement methods cannot accurately measure thickness information. To address this problem, we propose an OCT super-resolution depth measurement method based on trigonometric polynomial fitting. For two reflectors separated by a distance less than the OCT resolution, the interference spectrum is iteratively fitted using trigonometric polynomials to obtain depth information of the upper and lower surfaces, thereby determining the film thickness. To verify the proposed method, a air wedge model with linearly decreasing axial thickness was constructed in simulation experiments. Depth information was obtained using both conventional OCT image processing methods and the proposed method. The error between the wedge thickness obtained using the proposed method and the actual thickness did not exceed ±0.05 μm, and the minimum resolvable distance was 0.54 μm which is 26 times the theoretical resolution. It obtains depth information that exceeds the resolution capability of traditional methods without increasing hardware costs.