Synopsis: Making Optical Fibers Immune to Bending

Bending an optical fiber can distort light traveling through it, but such a distortion can be mitigated in fibers with a smoothly varying refractive-index profile.  
Synopsis figure
D. E. Boonzajer Flaes et al., Phys. Rev. Lett. (2018)

Endoscopy allows doctors to see inside a patient’s body using a thin probe containing optical fibers. Endoscopes typically contain a bundle of optical fibers, but some new devices use just one single multimode fiber (MMF). Such MMF endoscopes are thinner and have higher spatial resolution than conventional ones. However, these endoscopes are highly sensitive to bending, leading to severe image distortions when the fiber is bent. This means that either the probe needs to be held fixed throughout the procedure, or the image needs to be corrected using complicated computations based on the exact shape of the probe. These complications currently prohibit the use of MMF endoscopes in a clinic.

A new study shows that these image distortions can be more easily managed using fibers with a special refractive-index profile. Dirk Boonzajer Flaes at VU Amsterdam and his colleagues performed calculations on a MMF with a parabolic index profile. They showed that such a fiber is almost immune to bending. What’s more, residual image distortions could be related to just two parameters that can be derived from the overall fiber curvature and the overall fiber twist. The implication is that one could correct for distortions without knowing every turn and wiggle of the probe. The team tested their predictions with fibers having a graded-index profile that approximates a parabolic profile. While worse than the idealized parabolic case, the bending sensitivity of graded-index fibers was significantly better than that of the more commonly used step-index fibers whose profiles vary abruptly.

This research is published in Physical Review Letters.

–Michael Schirber

Michael Schirber is a Corresponding Editor for Physics based in Lyon, France.


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