Extended focal depth Fourier domain optical coherence microscopy with a Bessel-beam – LP₀₂ mode – from a higher order mode fiber

Abstract

We present a robust fiber-based setup for Bessel-like beam extended depth-of-focus Fourier-domain optical coherence microscopy, where the Bessel-like beam is generated in a higher order mode fiber module. In this module a stable guided LP02 core mode is selectively excited by a long period grating written in the higher order mode fiber. Imaging performance of this system in terms of lateral resolution and depth of focus was analyzed using samples of suspended microbeads and compared to the case where illumination is provided by the fundamental LP01 mode of a single mode fiber. Illumination with the LP02 mode allowed for a lateral resolution down to 2.5 µm as compared to 4.5 µm achieved with the LP01 mode of the single mode fiber. A three-fold enhancement of the depth of focus compared to a Gaussian beam with equally tight focus is achieved with the LP02 mode. Analysis of the theoretical lateral point spread functions for the case of LP01 and LP02 illumination agrees well with the experimental data. As the design space of waveguides and long-period gratings allows for further optimization of the beam parameters of the generated Bessel-like beams in an all-fiber module, this approach offers a robust and yet flexible alternative to free-space optics approaches or the use of conical fiber tips.

Non-Destructive Direct Pericarp Thickness Measurement of Sorghum Kernels Using Extended-Focus Optical Coherence Microscopy

“…The method we selected to analyze the lateral profile data disregards that the thickness can be slightly overestimated as, in fact, the true profile is convoluted with the instrument response (resolution). With our measurements, this instrument response function becomes wider as a function of distance from the geometric focus of the OCT scan lens [ 30 ], but much less so than for an OCM system using Gaussian illumination. With most of our measured lateral profiles, the lateral instrument response width only merits an insignificant correction to the measured width; hence, we have not included this correction.…”

“…The optical setup (see Figure 1 ) used to image the sorghum kernels is similar to the setup reported by Sen et al [ 30 ] but was modified to increase the measurement speed more than 1000-fold. Illumination is provided by an all-polarization-maintaining femtosecond Yb-doped fiber oscillator [ 33 ] emitting at a central wavelength of 1030 nm.…”

“…In addition to allowing the generation of narrow pencil-like beams with a long depth of focus, another important advantage of Bessel beams is their robustness to tissue scattering [ 27 , 28 , 29 ]. Low-order Bessel beams from specialty optical fiber allow for a moderate improvement of the DOF, but also have the advantage that they can be focused to a tighter spot than a Gaussian beam from a standard optical fiber [ 30 ]. Additionally, for beams guided through the same optical system and of comparable size, the central lobe of a low-order Bessel beam is always smaller than that of a Gaussian beam [ 30 ].…”

“…Low-order Bessel beams from specialty optical fiber allow for a moderate improvement of the DOF, but also have the advantage that they can be focused to a tighter spot than a Gaussian beam from a standard optical fiber [ 30 ]. Additionally, for beams guided through the same optical system and of comparable size, the central lobe of a low-order Bessel beam is always smaller than that of a Gaussian beam [ 30 ]. As such, replacing a Gaussian beam with a low-order Bessel beam will improve the transverse resolution.…”

Comparative study of optical coherence tomograph and histological images of eustachian tube nasopharyngeal region and adjacent structures in vivo and ex-vivo miniature pigs

“…Therefore, accurate and noninvasive evaluation of pathologic changes in the ET and nasopharynx is crucial to guide treatment. Optical Coherence Tomography (OCT) provides a promising solution, with higher resolution (2–10 um) and deeper tissue penetration (2–3 mm) than other imaging modalities [ 13 , 16 , 17 ]. In addition, it can obtain noninvasive histologic images in real time, allowing for a more accurate evaluation of these structures.…”