However this technique has not been used yet because the

However, this technique has not been used yet because the mechanical rotating polarizer moves slowly. Therefore, we developed a real-time McFT spectrometer that can achieve high optical throughput and high-frequency measurement. In recent years, polychromators have been developed to realize high-throughput spectroscopy [10] and enable the measurement of blood in vitro[1]; however, they are inferior to McFT spectrometers in terms of optical throughput [4]. In McFT spectroscopy, an interferogram can be obtained from twice-polarized measurements. Interferograms can be categorized as in-phase or anti-phase. In-phase interferograms are formed when the phases of two interfering waves are the same, while anti-phase interferograms are formed when the interfering waves are exactly out of phase. The type of interferogram that is obtained can be switched by rotating the polarizer by 90°. These two types of interferograms have opposite signals but equivalent noise levels, which result from the incoherent light from multiple reflections and detector noise. Therefore, by subtracting an anti-phase interferogram from an in-phase interferogram, a new interferogram with twice the signal level and drastically reduced noise can be obtained. This procedure results in slower performance but compensates for noise. Simultaneous detection of both phase interferograms was proposed using a Wollaston interferometer for astronomical spectroscopy [2]. However, for a biological sample, the light source throughput should be increased, which is only possible by using a lateral shearing interferometer. Therefore, we proposed a biologically compatible method based on a lateral shearing interferometer. In this method, the in-phase and anti-phase interferograms are detected in the upper and lower halves of the area sensor, respectively. This procedure enables real-time measurement without affecting the optical throughput, which is necessary for in vivo biomedical measurement.
Methods In order to realize the real-time spectrometer, we built a parallelized system using an area Anisomycin instead of the often-used linear sensor. The optical setup is shown in Fig. 1. In this system, the polarization shearing interferometer has been modified by adding an area sensor and a combination polarizer. White light from a halogen lamp with an irradiation power density of 226mW/cm2 was guided through a 13-mm-diameter optical bundle fiber to illuminate a sample. The transmitted light was guided into the McFT spectrometer, which contains a lens, polarizer, Savart plate (Leysop, calcite, 10mm thick), Fourier lens, and combination polarizer. After passing through the first polarizer, the linearly polarized beam is split into two equal-intensity parallel beams with orthogonal linear polarizations by the Savart plate. The Fourier lens reunites the two beams on the focal plane array of the detector. The upper half of the combination polarizer is parallel to the first polarizer and produces an in-phase interferogram, while the lower half is perpendicular to it and produces an anti-phase interferogram. The phase interferograms are detected simultaneously using an InGaAs area sensor with 640pixels per line and 512 lines (NIRvana 640ST, Princeton Instruments, New Jersey, US). In this study, we used 190 lines out of the total 512 lines for each of the interferograms. However, the diffracted light from the joint region of the combination polarizer was detected several tens of lines around the center lines. Therefore, a boundary area of about 100 lines between in-phase and anti-phase interferograms was removed from the signal. The top 16 and bottom 16 lines of the area sensor were also removed because of the non-homogeneous light distribution. This interferometer is known for having a pincushion-like distortion primarily due to the oblique ray incident upon the Savart plate [12]. Although it is not of much concern when using a line sensor, this distortion has a slightly negative effect on the interferogram images acquired by an area sensor. We corrected this issue using software that warps interferogram images to obtain patterns with parallel lines.