Abstract
Nonlinear sampling of the interferograms in wavenumber (k) space degrades the depth-dependent signal sensitivity in conventional spectral domain optical coherence tomography (SD-OCT). Here we report a linear-in-wavenumber (k-space) spectrometer for an ultra-broad bandwidth (760 nm–920 nm) SD-OCT, whereby a combination of a grating and a prism serves as the dispersion group. Quantitative ray tracing is applied to optimize the linearity and minimize the optical path differences for the dispersed wavenumbers. Zemax simulation is used to fit the point spread functions to the rectangular shape of the pixels of the line-scan camera and to improve the pixel sampling rates. An experimental SD-OCT is built to test and compare the performance of the k-space spectrometer with that of a conventional one. Design results demonstrate that this k-space spectrometer can reduce the nonlinearity error in k-space from 14.86% to 0.47% (by approximately 30 times) compared to the conventional spectrometer. The 95% confidence interval for RMS diameters is 5.48 ± 1.76 μm—significantly smaller than both the pixel size (14 μm × 28 μm) and the Airy disc (25.82 μm in diameter, calculated at the wavenumber of 7.548 μm−1). Test results demonstrate that the fall-off curve from the k-space spectrometer exhibits much less decay (maximum as −5.20 dB) than the conventional spectrometer (maximum as –16.84 dB) over the whole imaging depth (2.2 mm).
Highlights
Nonlinear sampling of the interferograms in wavenumber (k) space degrades the depth-dependent signal sensitivity in conventional spectral domain optical coherence tomography (SD-OCT)
In Spectral domain optical coherence tomography (SD-OCT), signal sensitivity tends to be weaker in deeper imaging regions
In SD-OCT, depth profiles are constructed by the inverse Fourier transform (FT−1) of the interferograms under the premise that the spectrum is linearly sampled in wavenumber (k) space
Summary
For the reference light with the wavenumber k6, its chief ray goes th ro→ugh the p →rism (s ho→wn as the tri→angle ABC) with the interaction points of D a→nd E. Given a pair of light rays with their wavenumbers of k6−j and k6 +j, which are centered at the reference wavenumber of k6 (j = 1, 2, ..., 5), the chief ray of k6−j goes through the prism with the interaction points of F and G, while the chief ray of k6 +j goes through the prism with the interaction points of H and I. To calculate the optical path difference (OPD) between k6−j and k6 +j, we set a reference line of GK, which is perpendicular to the output chief ray of k6 at point J and interacts with the output chief ray of k6 +j at point K.
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