Pseudo-Random Speckle Illumination Patterns Allow for High-Resolution Imaging

Researchers at the University of Tokyo have demonstrated the use of a multimode fiber (MMF) in combination with an integrated optical phased array (OPA) chip for single-pixel imaging in potential biomedical applications. The technology could allow for smaller devices with which to perform pseudo-random speckle pattern imaging applications such as ultra-thin endoscopy or in vivo neural imaging.

According to Taichiro Fukui, a Ph.D. student in the group, previous research indicated that illuminating a target using random speckles, rather than a focused spot, enhances the spatial resolution of an imaging process. “This is because unlike a focused spot, the random speckle illumination consists of interference patterns that contain higher spatial frequency elements,” he said.

By integrating an MMF output with an OPA chip, which splits the input light into a number of independent phase shifters, the group was able to generate different random speckle patterns to illuminate the target.

“Although we have previously demonstrated random-speckle-based imaging by just using an OPA without MMF, we could not resolve a large number of points dude to the limited number of phase shifters on the OPA,” Fukui said. “Surprisingly, in this work, we discovered that by transmitting through an MMF, the number of resolvable points can be increased drastically.”

With a simple matrix multiplication operation of the illumination pattern matrix and the transmitted optical power matrix, the image of the target can be quickly reconstructed.

In testing this method, the team was able to achieve 490 resolvable points using 128 phase shifters and 600 illumination patterns — a result comparable to other MMF techniques, but with a smaller, cheaper, and faster device.

“The most important finding in this work is that the resolvable number is essentially determined by the spatial capacity of the MMF and no longer by the OPA, as in conventional methods of using an OPA only,” Fukui said. “We have confirmed that the number of resolvable points can even exceed 1000 if we have stronger intermodal coupling inside the MMF. Our work is just the first step to demonstrate such possibilities.”

Pending several improvements — such as reducing the on-chip losses by integrating optical amplifiers on the OPA chip — the technique can be used to develop small, high-speed, low-cost, high-resolution imaging technologies.

“We believe that our scheme can be employed not only in endoscopic applications, but also in various optical sensing and imaging scenes, such as flow cytometry and optical coherence tomography,” Fukui said.

The results and additional research will be presented at OFC 2020.