The scattering of light was long thought to prevent imaging through opaque materials. However, scattering from static objects is deterministic, and in the last 15 years, a series of pioneering studies have shown us that it is possible to use a technique called wavefront shaping to characterise and subsequently cancel out complicated scattering effects. Light that has undergone multiple scattering can be ‘untangled’ to see through opaque media, such as frosted glass, biological tissue, or multimode optical fibres.
We present a proof of concept for microscopy through a multimode fiber using single pixel imaging. We present two implementations, one using galvo scanners and a diffuser plate and one using a digital micromirror device (DMD). Using these setups we can generate thousands of distinct speckle patterns at the distal end of a 50 micron core fiber as illumination patterns for single pixel imaging. We show that the correlation between speckle patterns can be made as low as 30% and the repeatability as high as 98% for a sample of 3200 patterns, and show example single pixel images using a distal detector.
Single-pixel imaging based on structured illumination and compressed sensing has opened a new way to compress massive imaging data volume and significantly reduce the cost of image sensor without sacrificing imaging quality. However, conventional structured illumination methods based on digital micro-mirror device (DMD) or a liquid crystal based spatial light modulator (SLM) fall short in fresh rate, making it a real challenge for high-speed imaging applications, which are however of paramount importance in studying dynamic phenomena in living cells, neural activity, and microfluidics, and capturing important rare events.
In this work, we propose and demonstrate a new approach for ultrafast (20 Mfps) structured illumination single-pixel imaging using light beam speckles out of a multimode fiber due to multimode interference. Our experimental results show that the excited high-order modes, and hence the multimode interference, are strongly wavelength-dependent. Update of the random speckle patterns can be easily obtained by sweeping the incident wavelength. Ultrafast wavelength sweeping is achieved by stretching ultrafast optical pulses from a mode-locked laser using chromatic dispersion. Extremely broad bandwidth and small wavelength step guarantee a good number of illumination patterns. By measuring multiple dot products of a sparse image with a set of known speckle based random, the image can be reconstructed using an L1 minimization algorithm.
The most significance of this completely new design is that multiple (up to thousands) structured illumination measurements can be carried out within a single pulse period, enabling ultrafast pulse-by-pulse imaging. Moreover, thanks to structured illumination and compressed sensing, the proposed structured illumination single-pixel imaging system offers much higher imaging resolution than existing ultrafast photonic time stretch imaging systems for the same captured data size.
Photonic time stretch (PTS) has enabled real time spectral domain optical coherence tomography (OCT). However, this method generates a torrent of massive data at GHz stream rate, which requires capturing as per Nyquist principle. If the OCT interferogram signal is sparse in Fourier domain, which is always true for samples with limited number of layers, it can be captured at lower (sub-Nyquist) acquisition rate as per compressive sensing method. In this work we report a data compressed PTS-OCT system based on photonic compressive sensing with 66% compression with low acquisition rate of 50MHz and measurement speed of 1.51MHz per depth profile. A new method has also been proposed to improve the system with all-optical random pattern generation, which completely avoids electronic bottleneck in traditional binary pseudorandom binary sequence (PRBS) generators.
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