Needle-shaped beam by a novel diffractive optical element for the extended depth-of-focus of optical coherence tomography

Optical coherence tomography can acquire the entire depth-resolved profile of the sample at a single shot (A-scan). However, the lateral resolution changes with depth, achieving the smallest focus at the focal plane and increasing significantly with the defocusing distance, especially for the high numerical aperture lens. In order to maintain a high resolution over a large depth range, a novel diffractive optical element is developed to construct a needle-shaped beam with a depth-of-focus of 10- to 150-fold Rayleigh lengths, a relatively constant diameter, a uniform axial intensity distribution, and negligible sidelobes. The lateral resolution improvement is experimentally demonstrated.