Inverse spatially‐offset Raman spectroscopy using optical fibers: An axicon lens‐free approach

Abstract

Inverse spatially offset Raman spectroscopy (I-SORS) seeks to interrogate deep inside a Raman-active, layered, diffusely scattering sample. It makes a collimated laser beam incident onto the sample surface in the form of concentric illumination rings (of varying radii) from whose center the back-scattered Raman signal is collected for detection. Since formation of illumination rings of different sizes requires an axicon to be moved along the axis of the collimated laser beam and axicons below a certain minimum size (~1 inch) are not readily available, this classical configuration incorporating an axicon cannot be used for designing a compact I-SORS probe of narrower diameter. We report a novel scheme of implementing I-SORS which overcomes this limitation by implementing ring illumination and point collection using two multi-mode optical fibers. An important advantage of the proposed scheme is that unlike the previously reported inverse SORS configurations, it does not require physical movement of any of the optical components for generating spatial offsets needed for probing sub-surface depths.Another advantage is its fiber-optic configuration which is ideally suited for designing a compact and pencil-sized I-SORS probe, often desired in many practical situations for carrying out depth-sensitive Raman measurements in situ from a layered turbid sample.

Subsurface Spectroscopy in Heterogeneous Materials Using Self-Healing Laser Beams

“…Finally, as an aside, we note that while this is the first demonstration of using any type of structured light beams for subsurface spectroscopy (with other common beam types being Laguerre-Gaussian and Hermite-Gaussian beams) [22][23][24][25], there are two other "passive" approaches to subsurface spectroscopy: spatially offset Raman spectroscopy (SORS) and inverse-SORS [26][27][28][29][30][31][32][33]. In both techniques, light is directed onto the surface of a heterogeneous material, while a detector is focused onto a point spatially offset from the beam spot to collect Raman-scattered light.…”

Model-Based Optimization of Laser Excitation and Detection Improves Spectral Contrast in Noninvasive Diffuse Raman Spectroscopy

“…13 Multiple implementations of SORS techniques exist, including the classical approach of using fiber optic SORS probes based on central laser delivery and rings of detection fibers, use of axicon lenses or multimode optical fibers to create ring illumination for iSORS, using spatial light modulator to create patterns of detection and microscope defocusing techniques. [13][14][15][16][17][18][19][20] SORS and its variants have been used for a broad range of applications, including medical diagnosis, regenerative medicine, pharmaceuticals, cultural heritage, food industry and security. [21][22][23][24][25][26][27][28][29][30][31][32][33] One of the main challenges in SORS is the reduction in signal contrast and intensity caused by the photon diffusion process.…”