Non-harmonic analysis applied to optical coherence tomography imaging

A new processing technique called non-harmonic analysis (NHA) is proposed for optical coherence tomography (OCT) imaging. Conventional Fourier-domain OCT employs the discrete Fourier transform (DFT), which depends on the window function and length. The axial resolution of the OCT image, calculated by using DFT, is inversely proportional to the full width at half maximum (FWHM) of the wavelength range. The FWHM of wavelength range is limited by the sweeping range of the source in swept-source OCT and it is limited by the number of CCD pixels in spectraldomain OCT. However, the NHA process does not have such constraints; NHA can resolve high frequencies irrespective of the window function and the frame length of the sampled data. In this study, the NHA process is described and it is applied to OCT imaging. It is compared with OCT images based on the DFT. To demonstrate the benefits of using NHA for OCT, we perform OCT imaging with NHA of an onion skin. The results reveal that NHA can achieve an image resolution equivalent that of a 100-nm sweep range using a significantly reduced wavelength range. They also reveal the potential of using this technique to achieve high-resolution imaging without using a broadband source. However, the long calculation times required for NHA must be addressed if it is to be used in clinical applications.