파장 영역 광단층영상기의 특성 개선을 위한 분광기 개발에 관한 연구

Abstract

Spectral-domain optical coherence tomography(SD-OCT) is a technique for realizing tomographic images by measuring wavelength domain interference fringes in parallel. In the SD-OCT system, a spectrometer simultaneously maps interference signals dispersed by wavelengths through a diffraction grating to a planar detector array using an imaging lens and measures light intensity for each wavelength component. At this time, the wavefront of the electromagnetic wave mapped to the spectrometer's pixels proceeds to the spherical wave, not the plane wave, depending on the characteristics of the imaging lens used in the spectrometer. The plain camera records distorted interference signals that are different from the information contained in spherical waves. Thus, the visibility of the measured interfering signal is reduced and ultimately the depth information of the tomographic image is distorted. To solve these problems, we have developed a spectrometer that minimizes the effect due to the mismatch in the wavefront by designing an imaging lens with planar wavefront. Prior to the development of the spectrometer, the process for optimally designing the spectrometer to be used in the SD-OCT system was summarized in stages. The characteristics of the core optical elements at each step were identified and the required design factors were determined and calculated. The development of the spectrometer was performed using the calculated optical design factors. Two types of imaging lenses were used in the development of the spectrometer, one is the double-let lens widely used in the SD-OCT system and the other is the newly developed tube lens with improved wavefront characteristics. To understand the characteristics of each lens, simulation using Zemax was performed and confirmed that the effect of wavefront flattening was improved in the developed tube lens. After analyzing the characteristics of each imaging lens, two spectrometers were developed using respective imaging lenses. The performance of each spectrometer was compared and analyzed by measuring the wavelength domain interference signal and optical tomography image for the same sample using the same interferometer. Theoretically calculated depth direction resolution was about 15 μm. The resolutions measured with the doublet lens-based spectrometer and the developed tube lens-based spectrophotometer were 15 μm and 17 μm, respectively, and the results were similar to the theoretical values. However, the fall-off characteristics representing the maximum depth of the optical tomographic images showed differences in the performance of the two spectrometers. From the 3-dB signal drop point, the spectrometer using the double lens was expected to have an imaging depth of less than 1 mm and the spectrometer using the developed tube lens was predicted to have the imaging depth of about 1.5 mm. This feature can be clearly distinguished from the depth of the optical tomographic image acquired for the actual sample. From these results, it can be confirmed that the spectrometer developed using the newly designed tube lens is useful for realizing deeper tomographic images. By implementing the SD-OCT system using the developed spectrometer, we can extend the usability of tomographic images and improve the efficiency of optical imaging in various applications.