Extended measurement range of optical profiler using chirped fiber Bragg grating-based chirped spectral-domain interferometry

Abstract

A conventional spectral-domain interferometer (SDI) has limitations in measurement range due to using broadband sources with low coherence lengths and spectrometers with limited pixel numbers. We propose chirped spectral-domain interferometry (CSDI) to extend the measurement range. The CSDI exploits chirped fiber Bragg gratings (CFBG) to induce dispersion in the reference arm and generates a chirped interference signal from an unbalanced interferometer. Convolution-based signal processing allows us to uniquely determine the surface position of the sample from the chirped interference signal. The sample surface geometry could then be reconstructed by analyzing the change in position of the convolution peaks. From the theoretical background, we know that the maximum measurable range of the CSDI is determined by the effective refractive index and length of the CFBG used. Experimentally, a maximum measuring range of 59 mm was measured. It is a significant improvement over conventional SDI, despite using a broadband light source and a limited number of pixels. We developed the surface profiler and demonstrated its performance in surface imaging on various samples with different reflectivity.