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Rendering the opaque transparent: Untangling light with bespoke optical transforms to see through scattering environments

Periodic Reporting for period 3 - PhotUntangle (Rendering the opaque transparent: Untangling light with bespoke optical transforms to see through scattering environments)

Reporting period: 2021-11-01 to 2023-04-30

When light the emanating from an object propagates through an opaque material, such as living tissue or a hair-thin strand of optical fibre, it fragments and scatters multiple times. The emergent wavefront no longer forms an image of the object, because the spatial information it carries has been scrambled. Reversing this scattering offers the prospect of using visible light for high-resolution imaging of structures deep inside the human body in a safe, non-ionising way. It has recently been shown that this light scattering can be characterised and inverted. Yet arbitrary ‘spatial mode inverters’ that can unscramble hundreds of light modes simultaneously to efficiently reform an image do not currently exist. The aim of this project is to understand how to design and build them. To achieve this, we are investigating the use of direct laser writing into glass to create structures that scatter light in a pre-determined way, and multiple passes on spatial light modulators - that allow reconfigurable control of the phase delay imparted to incident light beams at millions of independently addressable pixels.

We hope that the devices we create will lead to new ways to see deep inside the body and watch dynamic processes unfold at sub-cellular resolution. A driving application is the development of real-time imaging at the tip of a needle - allowing a window into living tissue, which can help guide biopsy needles and help to identify diseased cells.

Key aims of this project are to:
Understand how to design a new class of optical elements that can perform efficient spatial mode transforms on demand.
Build both passive spatial mode transformers to manipulate hundreds of modes simultaneously, and active transformers that can perform dynamically reconfigurable transformations at video-rates.
Apply this technology to unscramble light that has propagated through a moving multi-mode optical fibre in real-time, pushing towards ultra-thin micro-endoscopy, and explore an array of applications to next generation imaging systems and beyond.
My team and I have focussed on understanding how to design spatial mode inverters, and so far have made good progress with this objective so far. We have also spent time investigating new methods of imaging inside the body using multimode optical fibres, and have recently published two papers in this area:

[1] Shuhui Li, Simon AR Horsley, Tomáš Tyc, Tomáš Čižmár, and David B. Phillips. "Memory effect assisted imaging through multimode optical fibres." Nature Communications 12, no. 1 (2021): 1-13.
[2] Shuhui Li, Charles Saunders, Daniel J. Lum, John Murray-Bruce, Vivek K. Goyal, Tomáš Čižmár, and David B. Phillips. "Compressively sampling the optical transmission matrix of a multimode fibre." Light: Science & Applications 10, no. 1 (2021): 1-15.

Looking inside the body with sub-cellular resolution using harmless non-ionising light is a grand challenge in biomedical optics. The difficulty is that inhomogeneous tissue scatters light and scrambles the spatial information needed for imaging. Rather than looking directly through tissue itself, another option is to guide light along narrow waveguides -hair-thin multimode fibres (MMFs)- which allow imaging at the tip of a needle. Fibres also scramble images projected through them, but in a simpler way that can be calibrated and ‘unscrambled’. Fibre calibration conventionally requires access to both ends, and is extremely sensitive to even minute levels of bending or temperature change. Therefore in practice the required calibration can easily change during use, causing image reconstruction to catastrophically fail. In [1] we discovered a new form of correlation between light at either end of a fibre - a ‘radial memory-effect’. We showed how this discovery, coupled with ideas originally developed in astronomy to see through atmospheric turbulence, enables a fibre to be calibrated with access to one end only - meaning if it bends, recalibration can be rapidly performed in-situ while the endoscope is in use. Our work paves the way towards imaging through flexible micro-endoscopes in future - helping translate them from an academic curiosity to a clinical imaging tool. In [2] we showed that these memory effects can also be used to vastly speed-up the scattering characterisation of MMFs, even if we do have access to both ends.
The papers mentioned above detail our work going beyond the state-of-the-art so far. We expect to demonstrate a series of new ‘spatial mode inverters’, that are capable of unscrambling light that has passed through opaque media, by the end of the project. Beyond deep tissue imaging, the ability to create devices that can unscramble and manipulate light will also have implications for a number of other fields, including optical communications, emerging optical computing architectures, and quantum optics.
Patterns depicting the intensity (brightness) and phase (colour) of light scramble by an MMF.
Experimental images recored through a pre-calibrated MMF