Analysis of multimode fiber bundles for endoscopic spectral-domain optical coherence tomography

Abstract

A theoretical analysis of the use of a fiber bundle in spectral-domain optical coherence tomography (OCT) systems is presented. The fiber bundle enables a flexible endoscopic design and provides fast, parallelized acquisition of the OCT data. However, the multimode characteristic of the fibers in the fiber bundle affects the depth sensitivity of the imaging system. A description of light interference in a multimode fiber is presented along with numerical simulations and experimental studies to illustrate the theoretical analysis.

Endoscopic optical coherence tomography with a flexible fiber bundle

“…16 As expected, according to the analysis following Eq. (1), OCT imaging performed with FB1 suffers significantly from multimoding artifacts.…”

“…Cross coupling, therefore, typically arises when the NA is small and the cores are closely packed. 13 As a consequence of both, multimoding and cross coupling, the OCT detection sensitivity is reduced by a loss in modulation contrast across the detected interference pattern as described by Risi et al 16 Other image quality degrading effects specific to FB imaging (but independent of its NA) are the pixelation artifact as well as reflections at the front and back surface of the FB. The latter can be avoided by angle polishing at both ends and, therefore, preventing backreflected light from the distal end to be coupled back into the core or from the proximal end to obscure the detected signal.…”

“…Associated effects, such as multimoding within each core, cross coupling (or cross talk) of the light among multiple cores, or coupling and transmission losses, might severely degrade image quality, 13,14 especially of the OCT tomogram. 11,12,[15][16][17][18][19] Multimoding becomes mostly prominent in FBs of high NA (i.e., large difference in refractive index between core and cladding) and/or large core size and may result in axial ghost artifacts within the OCT tomograms. 12,15 FBs exhibiting multimodal behavior are, however, less likely to cross couple the light since they are able to better confine light to their respective cores due to the mentioned refractive index difference.…”

“…In this so-called common-path OCT configuration, the backreflection of light at the distal surface of the endoscope serves as the reference plane or an additional reference surface is added distally. 11,16 However, such a configuration restricts the use of means for detection with optimal sensitivity by excluding balanced detection (BD). The latter is indispensable in current swept-source OCT to maintain sensitivity close to the shot noise limit at high imaging speeds.…”

“…Previous OCT investigations using flexible FBs reported imaging of nonorganic phantoms. 16,19 So far, none of the published works on rigid or flexible FBs reported in vivo results.…”