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

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.

Design and Optimization of a Linear Wavenumber Spectrometer with Cylindrical Optics for Line Scanning Optical Coherence Tomography.

We report the design of a high-efficiency spectral-domain spectrometer with cylindrical optics for line scanning optical coherence tomography (OCT). The spectral nonlinearity in k space (wavenumber) lowers the depth-dependent signal sensitivity of the spectrometers. For linearizing, in this design, grating and prism have been introduced. For line scanning, a cylindrical mirror is utilized in the scanning part. Line scanning improves the speed of imaging compared to fly-spot scanning. Line scanning OCT requires a spectrometer that utilizes cylindrical optics. In this work, an optical design of a linear wavenumber spectrometer with cylindrical optics is introduced. While there are many works using grating and prism to linearize the K space spectrometer design, there is no work on linearizing the k-space spectrometer with cylindrical optics for line scanning that provides high sensitivity and high-speed imaging without the need for resampling. The design of the spectrometer was achieved through MATLAB and ZEMAX simulations. The spectrometer design is optimized for the broadband light source with a center wavelength of 830 ± 100 nm (8.607 µm-1- 6.756 µm-1 in k-space). The variation in the output angle with respect to the wavenumber can be mentioned as a nonlinearity error. From our design results, it is observed that the nonlinearity error reduced from 147.0115 to 0.0149 Δθ*µm within the wavenumber range considered. The use of the proposed reflective optics for focusing reduces the chromatic aberration and increases image quality (measured by the Strehl ratio (SR)). The complete system will provide clinicians a powerful tool for real-time diagnosis, treatment, and guidance in surgery with high image quality for in-vivo applications.