2D spectrometer for spectral and time domain optical coherence spectroscopy (Conference Presentation)

Abstract

We propose a new detection scheme for Spectral Optical Coherence Tomography (SOCT) that allows for a single shot depth-dependent visualization of spectroscopic properties of imaged objects. Compared to commonly used methods based on short time Fourier transformation or recently proposed technique based on Spectral and Time domain Optical Coherence Tomography (STdOCT) [1] it offers increased sensitivity and resistance to motion artefacts. The proposed method, called single shot Spectral and Time domain Optical Coherence (ssSTdOCS), is based on spectroscopic version of STdOCT, but uses a 2D detector in the spectrometer and thus allows for registration of a complete data set in a single acquisition event. Originally we proposed STdOCS as an alternative for commonly used methods based on short time Fourier transformation. The method utilizes spectral OCT setup with a spectrometer equipped with linescan CCD or CMOS camera as the detector. During the measurement continuous change in optical path difference between the two arms of OCT interferometer is introduced by an optical delay line in the reference arm. Resultant sequence of spectra is subject to 2D Fourier transformation that provides the representation of the OCT signal in “Doppler frequency” – “depth” space. The signal along Doppler frequency axis has envelope corresponding to the spectrum of the light coming back from particular depth in the object. In this aspect it can be regarded as spatially resolved Fourier transform spectroscopy. The measurement scheme required by STdOCS is sensitive to the internal motion of the imaged object, since the resultant 2D data set is built from the interference spectra acquired in time. To avoid this problem we propose here a modification of the measurement instrument. To acquire the whole 2D data set used in STdOCS we propose a spectrometer equipped with 2D array sensor. The broadband light from the superluminescent diode is collimated and brought to the input of Mach-Zehnder interferometer. In the object arm the beam is focused by the objective lens in the object plane and the light scattered within the object is brought to interference with the reference beam in the output beamsplitter. The beams from reference and object arms of the interferometer are incident on the diffraction grating at relative angle in the plane determined by the grating lines and the optical axis. In the perpendicular plane the diffraction angles were the same for both beams. In the reference arm the pair of kinematic mirrors provides the possibility of precise adjustment of the angle between the beams. The first diffraction order is focused on the CCD camera with the use of cylindrical achromatic doublet. In the resulting 2D interferogram each horizontal row contains interferometric spectra with different optical path differences between the optical paths of the reference and object beams resulting from different angles of incidence at the CCD plane. This single-shot exposure event provides data sufficient for STdOCS analysis scheme. We show that the effective quantitative measurement of the depth-dependent absorption spectra of indocyanine green solution placed in glass capillary is possible with the use of our method. 1. Maciej Szkulmowski, Szymon Tamborski, Maciej Wojtkowski, Spectroscopy by joint spectral and time domain optical coherence tomography, Proc. SPIE 9312, Optical Coherence Tomography and Coherence Domain Optical Methods in Biomedicine XIX, 93122P (2 March 2015);