Abstract
The ability to retrieve image data through hair-thin optical fibers promises to open up new applications in a range of fields, from biomedical imaging to industrial inspection. Unfortunately, small changes in mechanical deformation and temperature can completely scramble optical information, distorting any resulting images. Correction of these dynamic changes requires measurement of the fiber transmission matrix (TM) in situ immediately before imaging, which typically requires access to both the proximal and distal facets of the fiber simultaneously. As a result, TM calibration is not feasible during most realistic usage scenarios without compromising the thin form factor with bulky distal optics. Here, we introduce a new approach to determine the TM of multimode or multicore optical fibers in a reflection-mode configuration, without requiring access to the distal facet. We propose introducing a thin stack of structured metasurface reflectors at the distal facet of the fiber, to introduce wavelength-dependent, spatially heterogeneous reflectance profiles. We derive a first-order fiber model that compensates these wavelength-dependent changes in the fiber TM and show that, consequently, the reflected data at three wavelengths can be used to unambiguously reconstruct the full TM by an iterative optimization algorithm. Unlike previous approaches, our method does not require the fiber matrix to be unitary, making it applicable to physically realistic fiber systems that have non-negligible power loss. We demonstrate TM reconstruction and imaging first using simulated nonunitary fibers and noisy reflection matrices, then using larger experimentally measured TMs of a densely packed multicore fiber (MCF), and finally using experimentally measured multiwavelength TMs recorded from a step-index multimode fiber (MMF). Parallelization of multiwavelength in situ measurements could enable experimental characterization times comparable with state-of-the-art transmission-mode fiber TM experiments. Our findings pave the way for online TM calibration in situ in hair-thin optical fibers.
- Received 30 April 2019
- Revised 29 August 2019
DOI:https://doi.org/10.1103/PhysRevX.9.041050
Published by the American Physical Society under the terms of the Creative Commons Attribution 4.0 International license. Further distribution of this work must maintain attribution to the author(s) and the published article’s title, journal citation, and DOI.
Published by the American Physical Society
Physics Subject Headings (PhySH)
Popular Summary
Being able to see individual cells deep inside the human body using advanced microscopy techniques could enable new discoveries in biology and improve understanding of diseases such as cancer. Such imaging can be achieved by transporting light into and out of the body via hair-thin pieces of glass called optical fibers. However, the bending of these fibers upon insertion causes distortion of images, limiting their use. We present a new way of reversing distortion during use, potentially enabling microscopic imaging deep within the body.
We exploit a bespoke reflector structure that reflects light differently at three closely spaced wavelengths and scrambles optical polarization. This behavior is achieved using wavelength-selective optical filters and nanostructured optical surfaces that alter the polarization of light. The reflector is characterized in advance and then affixed to one end of the fiber. During use, we send light at each of the three selected wavelengths and in several distinct polarization states into the opposite end of the fiber (i.e., outside the body) and record the spatial profile of the light reflected back. Our method fits a mathematical model of the fiber to the resultant data, enabling estimation and reversal of bending-induced distortion, producing clear images. We demonstrate a software implementation using real data recorded from two types of optical fiber used in imaging.
This method will pave the way for advanced microscopy techniques to be applied deep inside living specimens, which could enable observation of neurons deep within the brain to improve understanding of Alzheimer’s or of cellular processes involved in early-stage cancer.
