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. 2022 Apr 25;22(9):3278.
doi: 10.3390/s22093278.

Design of a Linear Wavenumber Spectrometer for Line Scanning Optical Coherence Tomography with 50 mm Focal Length Cylindrical Optics

Sevin Samadi  1 , Masoud Mohazzab  2 , Javad Dargahi  1 , Sivakumar Narayanswamy  1
Affiliations

Design of a Linear Wavenumber Spectrometer for Line Scanning Optical Coherence Tomography with 50 mm Focal Length Cylindrical Optics

Sevin Samadi et al. Sensors (Basel). .

Abstract

Optical coherence tomography (OCT) has a wide range of uses in bioimaging and nondestructive testing. Larger bandwidth light sources have recently been implemented to enhance measurement resolution. Increased bandwidth has a negative impact on spectral nonlinearity in k space, notably in the case of spectral domain OCT (SD-OCT). This nonlinearity reduces the depth-dependent signal sensitivity of the spectrometers. A grating and prism combination is extensively used for linearizing. In an earlier study, we used a combination of the reflective grating and prism, as well as a cylindrical mirror with a radius of 180 mm, to achieve a high SR ratio with low nonlinearity. A creative design for a spectrometer with a cylindrical mirror of radius 50 mm, a light source with a center wavelength of 830 ± 100 nm (μm-1 - 6.756 μm-1 in k-space), and a grating of 1600 lines/mm is presented in this work. The design optimization is performed using MATLAB and ZEMAX. In the proposed design, the nonlinearity error reduced from 157∘× μm to 10.75∘× μm within the wavenumber range considered. The sensitivity research revealed that, with the new design, the SR ratio is extremely sensitive to the imaging optics' angles. To resolve this, a spectrometer based on Grism is introduced. We present a Grism-based spectrometer with an optimized SR ratio of 0.97 and nonlinearity of 0.792∘× μm (Δθ/Δk). According to the sensitivity study, the Grism-based spectrometer is more robust.

Keywords: Grism; SD-OCT; cylindrical mirror; line scanning; linear k space; spectral analysis.

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Conflict of interest statement

The authors declare no conflict of interest.

Figures

Figure 1
Figure 1
Exit angle of the light from the grating vs. wavenumber.
Figure 2
Figure 2
Schematic of the spectrometer using reflective grating and prism (two wavelengths, orange shorter and green longer).
Figure 3
Figure 3
The angular variation of the exit angle for reflective grating and prism combination.
Figure 4
Figure 4
Design of a spectrometer with a prism and reflective grating (a) side view; (b) top view.
Figure 4
Figure 4
Design of a spectrometer with a prism and reflective grating (a) side view; (b) top view.
Figure 5
Figure 5
(a) Spot diagram before optimization; (b) spot diagram after optimization.
Figure 6
Figure 6
Strehl ratio of the proposed spectrometer and previous works [15].
Figure 7
Figure 7
Strehl ratio of the proposed spectrometer for different angles of the optical components.
Figure 8
Figure 8
Strehl ratio of the proposed spectrometer for variation of the cylindrical mirror’s angle.
Figure 9
Figure 9
Design of a spectrometer with Grism [30].
Figure 10
Figure 10
Strehl ratio of different spectrometers designed with reflective grating and prism, transmission grating and prism and Grism.
Figure 11
Figure 11
Strehl ratio of the proposed spectrometer for variation of cylindrical mirror angle.
Figure 12
Figure 12
Strehl ratio of the proposed spectrometer for variation of cylindrical mirror’s radius.
Figure 13
Figure 13
Strehl ratio of the proposed spectrometer for variation of number of lines of grating.
Figure 14
Figure 14
Strehl ratio of different spectrometers.
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