Non-Destructive Direct Pericarp Thickness Measurement of Sorghum Kernels with Fiber-Based Extended Focus Optical Coherence Microscopy

Optical coherence tomography (OCT) [1] enabled non-invasive measurements of the 3-dimensional structure of tissues. OCT is most proliferated in ophthalmology, but has applications in many other medical, biological and technological studies. While the resolution in the propagation direction of the light is (approximately) constant across the tissue and can be very fine depending on the bandwidth of the light source used, the lateral resolution depends on the focusing conditions and deteriorates with increasing distance from the geometrical focus. Thus, it is challenging to obtain a high resolution in all three dimensions for a large volume. Several approaches have been demonstrated to tackle this issue, of which the most widespread is the use of Bessel beams [2]. An ideal Bessel beam is non-diffracting, and practical realizations of Bessel beams allow to achieve a considerable length of constant small beam size [3], at the cost of a significant portion of the energy contained in concentric rings around the sharp central peak. In practical terms, many OCT systems with Bessel beam illumination are implemented using axicon lenses, which make the system extremely alignment sensitive and cumbersome. Our approach replaces the axicon lens generating the Bessel beam by a higher-order-mode fiber (HOMF) [4]. The LP
02
mode is excited exclusively by transmission through a fiber long-period grating, which allows splicing the HOMF directly to a standard single mode fiber and achieves the LP
02
mode at the output. The LP
02
output from our higher-order-mode fiber allows us to achieve a sharper focus, that is maintained over a longer range, than the fundamental (Gaussian, LP
01
) output from the standard single mode fiber.