Optimized linear wavenumber spectrometer based spectral-domain optical coherence tomography system

Abstract

In spectraldomain optical coherence tomography the sample is illuminated by a broadband light source, and the spectrum of the interference light between the light returned from the sample and a reference mirror is detected by a grating spectrometer. Conventionally, the grating spectrometer is comprised of a diffraction grating, a focusing lens, and a line-scan camera. According to the grating equation the diffraction angle from the grating is approximately linearly related to the optical wavelength. Thus the distribution function of the light spectrum at the line-scan camera is nonlinearly dependent on wavenumber. For the high-quality image reconstruction, the numerical resampling of the spectral interference data from wavelength-space to wavenumber-space is commonly required prior to the Fourier Transformation. The nonlinear detection of the spectral interferograms in wavenumber space also degrades the depth-dependent signal sensitivity in conventional linear-wavelength spectrometer based spectraldomain optical coherence tomography. Recently reported spectraldomain optical coherence tomography based on a linearwavenumber spectrometer does not need the resampling or interpolating of the nonlinearwavenumber interference spectral data, which greatly reduces the cost of computation and improves the imaging sensitivity. Various methods based on the different evaluation protocols for optimizing the design of the linear-wavenumber spectrometer have been reported. Here we report an effective optimization method for linear-wavenumber spectrometer used in a high-resolution spectral domain optical coherence tomography system. We take the reciprocal of the fullwidthhalfmaximum of the simulated point spread function as an evaluating criterion to optimize the structure parameters of the linearwavenumber spectrometer, including the refractive index and the vertex angle of the dispersive prism and the rotation angle between the diffraction grating and the dispersive prism. According to the optimization, an F2 equilateral dispersive prism is used to construct the optimized linearwavenumber spectrometer with a rotation angle of 21.8°. We construct an optimized linearwavenumber spectrometer and implement the spectrometer in a developed spectraldomain optical coherence tomography system as a detection unit. We evaluate the performances of the linear-wavenumber spectrometer both theoretically and experimentally. The experimentally measured axial resolution of the spectraldomain optical coherence tomography system based on the linear-wavenumber spectrometer is 8.52 μm, and the sensitivity is measured to be 91 dB with -6 dB sensitivity roll-off within a depth range of 1.2 mm. The experimentally measured sensitivity roll-off curve accords well with the theoretical sensitivity roll-off curve. Utilizing the general parallel computing capability of a GPU card, the highquality spectraldomain optical coherence tomography images of the human finger skin can be reconstructed in real time without any resampling or interpolating process.