Optical coherence tomography with a structure of the spatial heterodyne spectrometer

In this study, we describe a simple method to produce signals which can reveal the cross-sectional information of samples in an optical coherence tomography (OCT) system. Instead of using the spectrometer and the Fourier transformation calculation in the conventional spectrum domain (SD) OCT system, we use a Mach-Zehnder interferometer structure of the spatial heterodyne spectrometer. In a spatial heterodyne spectrometer, because each position on the photodetector array could be mapped to a specific optical path difference, the spectral density distribution could be retrieved with Fourier transformation. And in an SD-OCT system, cross-section signals are obtained by conducting Fourier transformation to the spectrum signals. Therefore, in our OCT system, the spatial signals captured by the photodetector array is related to the cross-sectional signals obtained in an SD-OCT system. The theoretical study and the numerical simulation demonstrate that by applying our method in an OCT system, the heterodyne spectrometer structure could generate a symmetrical pattern composed of fringes with high spatial frequency. Then the photodetector array captures the pattern to generate a spatial signal. The spatial ordinate of this signal is linearly related to the optical depth in sample, while the amplitude of the signal intensity variation is linearly related to the intensity of coherent backscattered light in the sample. The imaging depth is theoretically unlimited. Also, because of the high spatial frequency of the signal, we further adjust the inclination angle in the heterodyne spectrometer structure to visualize the signal.