One has a center wavelength 1270 nm with a bandwidth of 70 nm, corresponding to an axial resolution of 10.17 μm in air (7.37 μm in tissue) and the other has a center wavelength 830 nm with a bandwidth of 40 nm, corresponding to an axial resolution of 7.60 μm in air (5.51 μm in tissue). 6, the center wavelength is 1310 nm and the bandwidth is 68 nm, corresponding to an axial resolution of 11.14 μm in air (8.07 μm in tissue, assuming an average refractive index 1.38) In ref. However, these k-space spectrometers were designed for SD-OCT systems with limited axial resolution. By utilizing the k-space spectrometer in SD-OCT, previous literatures 6, 7 have demonstrated obvious improvement in sensitivity as well as reduction in computing time. This combination can be either cemented as a “grism” 11 or separated to provide more degrees of freedom in optimization 6, 7. One of the possibilities to design such a spectrometer is to use a combination of a diffractive grating and a prism, where the nonlinearity in wavenumber caused by one can be offset by the other 11. In addition, the linear camera with more pixels tends to be more expensive, and both the time needed to capture an OCT frame and the time to process it is increased with more pixels.Īn alternative method to reduce the sensitivity fall-off generated by the unequal sampling is to use the k-space spectrometer which disperses the spectrum optically with the necessary degree of equidistance in wavenumber. If the point spread functions (PSFs) for the dispersed spectrum are much larger than the pixel pitches (for example, more than two pixels are required to sample one PSF of a particular wavenumber), the optical performance of the spectrometer rather than the number of the pixels becomes the limit for the effective spectral sampling. As a consequence, smaller pixel pitch often requires higher optics performance 10. In practice, there is often a trade-off between the pixel number and the pixel size under a limited pixel array dimension. Ideally, increasing the number of pixels of the detector can improve the spectral-sampling frequency and reduce the depth-dependent sensitivity loss 8, 9. Digital rescaling of the spectrum from λ-space to k-space is required prior to FT −1, resulting in the fact that the spectral bands integrated by the camera pixels are unequal and leading to the fact that the signal sensitivity is decreased in depth 5, 6, 7. However, the diffractive grating used in a conventional spectrometer disperses the light spectrum at the angles evenly spread versus wavelength ( λ) rather than wavenumber ( k). 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. Reducing sensitivity fall-off is a primary concern for the design of the spectrometer in a SD-OCT system. This depth-dependent loss in signal sensitivity is called “fall-off” 4. In SD-OCT, signal sensitivity tends to be weaker in deeper imaging regions. Spectral domain optical coherence tomography (SD-OCT) enables high-speed volumetric biomedical imaging with micrometric resolution and millimetric depth for scientific research and clinical study 1, 2, 3. 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). 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). 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. An experimental SD-OCT is built to test and compare the performance of the k-space spectrometer with that of a conventional one. 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. Quantitative ray tracing is applied to optimize the linearity and minimize the optical path differences for the dispersed wavenumbers. 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. Nonlinear sampling of the interferograms in wavenumber ( k) space degrades the depth-dependent signal sensitivity in conventional spectral domain optical coherence tomography (SD-OCT).