A Compact Linear-in-Wavenumber Spectrometer With a 1800 Lines/mm Grating for 4.8 mm Imaging Depth in SD-OCT

In SD-OCT, linear-in-wavenumber spectrometers play a key role in direct k-domain sampling, which eliminates interpolation artifacts and reduces computational load. However, prevalent designs in the 850 nm band using 1200 lines/mm gratings face a fundamental trade-off between imaging depth, system size and cost: achieving a large imaging depth necessitates large-volume, costly optics with long focal lengths and large apertures. To overcome this limitation, we present the design and experimental validation of a linear-in-wavenumber spectrometer employing a 1800 lines/mm grating. The high dispersion of this grating enables a compact system that achieves an imaging depth of ∼4.8 mm with a 76 nm spectral detection range, compatible with single-SLD sources. Compared to the conventional 1200 lines/mm design, our spectrometer reduces the total optical length from ∼325 mm to ∼190 mm and the entrance pupil diameter from 11 mm to 7.2 mm, allowing for smaller and more cost-effective components. Furthermore, our optimization method incorporates wavenumber nonlinearity introduced by the focusing group, overcoming the limitations of traditional approaches that neglect lens aberrations. Experimental results demonstrate a considerable wavenumber linearity (R² = 0.9999995) and a system sensitivity of 103.4 dB. High-quality in vivo imaging of human skin and nailfold, revealing microvasculature and layered structures, validates the practical utility. This work provides a high-linearity, compact, and cost-effective spectrometer solution, particularly suited for SD-OCT applications requiring large imaging depths.