Real-time and high-performance calibration method for high-speed swept-source optical coherence tomography

Abstract

Optical coherence tomography (OCT) is a minimally invasive imaging technique that uses low-coherence gating to obtain up to submicron-level resolution, cross-sectional images of tissue microstructures.^{1, 2, 3, 4} With early embodiments, the OCT technology emerged as a time-domain (TD) interferometry in terms of TD-OCT. Recently, spectral-domain OCT (SD-OCT) techniques have attracted a considerable amount of attention.^{1, 5, 6, 7, 8, 9, 10, 11, 12} For the SD-OCT, which generally does not need mechanical movement in the reference arm of the interferometer, the spectral interferogram is measured, and the depth/amplitude information from the sample is obtained via the inverse Fourier transform of the interferogram in the wave number space. Fourier domain OCT (FD-OCT) and swept-source OCT (SS-OCT) are two subtypes of the SD-OCT depending on the methods of spectral interferogram recording. (The nomenclature is consistent with the literature. However, the authors believe that the term “Fourier domain” is better than “spectral domain” to generalize the technique, since both the spectrometer and swept-source systems are based on Fourier transformation for image reconstruction. In a spectrometer-based system, spectral (wavelength) interference is detected directly with a dispersive element. Hence, it is suitably named “spectral domain.” However, in a swept-source system, the spectral interference is not measured directly, since the interference is recorded as a function of time instead of spectrum (and this is the motivation of this paper). It is imprecise to use “spectral domain” to generalize both systems. However, we have kept consistent with the literature to avoid confusion.) The former still uses a wideband light source similar to the one in the TD-OCT, while the spectral interferogram is recorded by a spectrometer at the exit of the interferometer. The latter uses a sweeping/tunable laser source, and the spectral interferogram is measured sequentially by time. Most recently, researchers has shown substantial interest in the SS-OCT because of its potential on ultra-high scanning rate.^{8, 10, 12, 13, 14, 15, 16} We recently demonstrated that at least with current embodiments of the SD-OCT, performances of the SD-OCT in terms of dynamic range and penetration could be inferior to that of the TD-OCT.^{17, 18} As one of our serial studies for characterizing the SD-OCT technique, the work presented here will focus on the issue of real-time spectral calibration for the SD-OCT, specifically for high-speed SS-OCT.