Superimposing interferogram method using a multi-slit array to enhance sensitivity and interference definition of spatia

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Superimposing interferogram method using a multi‑slit array to enhance sensitivity and interference definition of spatial‑phase‑shift interferometers Natsumi Kawashima1   · Tomoya Kitazaki1 · Kosuke Nogo1 · Akira Nishiyama2 · Kenji Wada2 · Ichiro Ishimaru1 Received: 8 April 2020 / Accepted: 27 September 2020 © The Author(s) 2020

Abstract We propose a superimposing interferogram method to enhance the sensitivity and interference definition of spatial-phase-shift interferometers. The proposed method uses a multi-slit array as a redesigned field stop for the spatial-phase-shift interferometer. The widths of the apertural and blocked areas correspond to half the wavelength of the fringe pattern on the imaging plane and determine the wavelength that improves the interference definition while eliminating those wavelengths that worsen it, thus improving the interference definition of the interferogram. Additionally, to improve the interferometer’s sensitivity, several apertural areas were aligned to increase the amount of incident light. We performed spectroscopic measurements and confirmed improvements in both the sensitivity and the interference definition using a prototype of a bean-sized mid-infrared spectroscopic imager based on a one-shot Fourier spectroscope and a prototype multi-slit array. Although mid-infrared light is absorbed well by water and its intensity is thus weak for detection using the array sensors of cameras, mid-infrared spectroscopic imagers that incorporate our proposed method can be used for biomedical measurements of samples containing water. Keywords  Fourier spectroscopy · Sensitivity · Interference definition · Interferometers · Infrared light · Visible light · Field stop

1 Introduction We propose a method to improve the sensitivity and definition of a spatial-phase-shift interferometer using a field stop that has more than one aperture area running periodically along the horizontal axis. Our intention is to improve these parameters in our proposed spectrometer to enable the development of a small and low-cost mid-infrared spectroscopic imager that can be introduced into regular everyday applications as a non-invasive health-care sensor (e.g., for blood glucose monitoring). The mid-infrared region (wavelength 6–15 µm, wavenumber 1500–650 cm−1) is called the fingerprint region because components are easily identified by their spectroscopic absorbance in this region. However, * Ichiro Ishimaru [email protected]‑u.ac.jp 1



Faculty of Engineering and Design, Kagawa University, 2217‑20 Hayashi‑cho, Takamatsu, Kagawa 761‑0396, Japan



Faculty of Medicine, Kagawa University, 1750‑1 Ikedo Miki‑cho, Kita‑gun, Kagawa 761‑0701, Japan

2

it is difficult to obtain data that have a high signal-to-noise ratio using mid-infrared spectroscopic imagers because the photons of mid-infrared light are absorbed readily by water and the light intensities are thus very weak. There are two types of spectroscopy, wavelength dispersive [1–3] and Fourier, which have different optical systems. In

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