Time-of-flight 3D imaging through multimode optical fibers

Abstract

Time-of-flight (ToF) 3D imaging has a wealth of applications, from industrial inspection to movement tracking and gesture recognition. Depth information is recovered by measuring the round-trip flight time of laser pulses, which usually requires projection and collection optics with diameters of several centimetres. In this work we shrink this requirement by two orders of magnitude, and demonstrate near video-rate 3D imaging through multimode optical fibres (MMFs) -the width of a strand of human hair. Unlike conventional imaging systems, MMFs exhibit exceptionally complex light transport resembling that of a highly scattering medium. To overcome this complication, we implement high-speed aberration correction using wavefront shaping synchronised with a pulsed laser source, enabling random-access scanning of the scene at a rate of ∼23,000 points per second. Using non-ballistic light we image moving objects several metres beyond the end of a ∼40 cm long MMF of 50 µm core diameter, with millimetric depth resolution, at frame-rates of ∼ 5Hz. Our work extends far-field depth resolving capabilities to ultra-thin micro-endoscopes, and will have a broad range of applications to clinical and remote inspection scenarios.

How to build the optical inverse of a multimode fibre

“…Averaging the power-ratio over all output locations yields the mean power-ratio p r . When our inverter is matched with an ideal fibre, the mean power-ratio of reimaged points p r = 0.74, a value comparable with the typical power-ratio for state-of-the-art serial wavefront shaping through MMFs [16,17].…”

“…A key characteristic of our proposed fibre-inverter imaging system is the spectral bandwidth over which light can be unscrambled. The bandwidth of the TM of an MMF, ∆λ fib , defines the range of wavelengths over which the same TM is valid [17]. Assuming that chromatic dispersion is negligible compared to spatial mode dispersion, ∆λ fib can be derived by considering the optical path length difference between a ray travelling at the critical angle of total internal reflection versus a ray travelling 'straight through', parallel to the central fibre axis, yielding…”

“…This method, known as wavefront shaping [10][11][12], has been used to look through MMFs centimetres deep inside living tissue [3,4] -a task difficult to accomplish in any other such minimally invasive way. A wide variety of imaging modalities have been demonstrated through these MMF-based holographic endoscopes, including fluorescence imaging [3,4,7], chemically and structurally sensitive microscopy [13][14][15], imaging beyond the distal fibre facet [16], and depth ranging using time-of-flight LiDAR [17].…”

Holographic Manipulation of Nanostructured Fiber Optics Enables Spatially‐Resolved, Reconfigurable Optical Control of Plasmonic Local Field Enhancement and SERS

“…In addition, recent reports have shown that the field of MMFs endoscopy can greatly benefit of Fourier plane-based approaches. [23,31,44] In this respect, the complex coupling between periodic plasmonic structures and the MMF's randomness can be harnessed by defining and holding a specific work point on the dispersion diagram (Figure 3E), setting the stage for endoscopic approaches based on EOT. This concept is shown in this work in a fiber optic featuring two orthogonal plasmonic nanogratings, and could be extended to ideally any type of photonic structures on the fiber facet, including dielectric metasurfaces.…”

Roadmap on wavefront shaping and deep imaging in complex media

“…In combination with time-of-flight detection, MMF-based endoscopes can nowadays also offer depth-perception in macroscopic three-dimensional scenes. Such far-field holographic endoscopes look set to further expand the range of applications within the biomedical and industrial inspection domains [183].…”