Controlling and understanding light propagation through a multimode fiber (MMF) requires knowledge of an optical Transmission Matrix (TM). Holography is essential for extracting phase information from intensity measurements, enabling the TM measurement. Usually, complex optical fields are retrieved through its interferences with a plane wavefront. Here we demonstrate and study TM measurements of an MMF using a self-reference approach, emphasizing its strengths and limitations. We focus on compensating for phase fluctuations to enhance image quality. The efficiency of this approach in precise TM measurements is experimentally confirmed by demonstrating high-quality light focusing, as well as complex patterns transmission through an MMF. This work enhances the understanding of self-reference holography in complex scattering media and its practical applications, particularly in studying and controlling light within MMFs.
Advances in imaging tools have always been a pivotal driver for new discoveries in neuroscience. An ability to visualize neurons and subcellular structures deep within the brain of a freely behaving animal is integral to our understanding of the relationship between neural activity and higher cognitive functions. However, fast high-resolution imaging is limited to sub-surface brain regions and generally requires head fixation of the animal under the microscope. Developing new approaches to address these challenges is critical. The last decades have seen rapid progress in minimally invasive endo-microscopy techniques based on bare optical fibers. A single multimode fiber can be used to penetrate deep into the brain without causing significant damage to the overlying structures and provide high-resolution imaging. Here, we discuss how the full potential of high-speed super-resolution fiber endoscopy can be realized by a holistic approach that combines fiber optics, light shaping, and advanced computational algorithms. The recent progress opens up new avenues for minimally invasive deep brain studies in freely behaving mice.
The transmission matrix (TM) measurement is a powerful and well-known tool for characterizing scattering media such as multimode fibers (MMF). Access to the phase of the optical field based on the results of intensity measurements is a long-standing problem known as the phase retrieval problem. To obtain the complex optical field and therefore phase information, it is necessary to use interferences with a known wavefront. In this work, we compare two interferometric methods: on-axis holography with a speckle reference beam propagating through the same optical path and off-axis holography with a plane wave reference beam propagating via an additional arm. These methods, combined with a DMD, provide a fast and accurate way to measure the TM.
We present results and a detailed comparison of the two methods of TM measurements of the same system.
The use of a single multimode fiber (MMF) as a high-resolution endoscopic imaging tool is
demonstrated. We show that the scrambled output of a MMF can be used for auto-fluorescence
compressive imaging. By scanning a light spot across the proximal side of the fiber we can create
uncorrelated speckle patterns at the output. Those patterns successively illuminate the biological
sample and for each pattern the integrated intensity is recorded in epi-direction. An image of the brain
tissue was computationally reconstructed using a regularization algorithm. Furthermore, the
presented technique has potential in enhanced acquisition speed and in improving the resolution limit.
Fiber-based endoscopic compressive imaging is a promising field for minimal invasive, in vivo imaging below the diffraction limit and enhanced acquisition time. Here, we analyze and present the theoretical limits of speckle-based compressive imaging.
Far-field optical microscopy typically suffers from limited resolution, speed and imaging depth. Endoscopic imaging via a multimode fiber combined with wavefront shaping and computational reconstruction offers imaging beyond the Abbe and Nyquist limits.
Endoscopy is a key technology for minimally-invasive optical access to deep tissues in humans and living animals. However, modern endoscopes, such as fiber bundles, still suffer from low spatial resolution. Multimode fiber is a very promising tool for high-resolution endo-microscopy. We use advanced wavefront shaping technique and experimentally demonstrate high-resolution fluorescent and label-free imaging through a multimode fiber. We also present an ultra-thin Raman imaging probe with an excellent ratio between field of view and probe diameter. However, state-of-the-art multimode fiber endo-microscopy still has several problems limiting its broad applications: slow speed, as well as requirements of complex wavefront shaping procedure and expensive spatial light modulators. Here we show the solution to all these problems. We propose and experimentally demonstrate a new method of high-resolution endoscopy: compressive multimode fiber imaging. The key idea is to integrate the compressive sensing technique with a multimode fiber probe, which produces a random basis of speckle patterns, collects the optical response and provides optical sectioning. This new approach allows high-speed diffraction-limited imaging at the full field of view of a probe and does not require complex elements, such as spatial light modulators or knowledge of the transfer matrix of the multimode fiber. We demonstrate high-resolution imaging through a fiber probe with the total number of measurements 20 times less than required for the standard raster scanning approach. Compressive multimode fiber imaging offers a unique tool for in vivo high-speed high-resolution endoscopy.
Scattering of light limits the depth at which a focus can be formed in turbid media. However, light can be focused through and inside thick and strongly scattering samples by spatially modulating the incident light using wavefront shaping techniques [1]. Wavefront shaping requires feedback from a localized reporter, for example, a point detector or a fluorescent ‘guide star’ inside the sample.
In some cases, a localized reporter is not available as a source of feedback. For instance in multiphoton fluorescence microscopy, the only available feedback signal is the total fluorescent signal coming from inside the sample. Even with this non-localized form of feedback, Katz et al. [2] were capable of forming a single diffraction-limited focus behind a strongly scattering layer. However, the statistics behind this nonlinear optimization procedure are poorly understood, and the location at which this blind focusing method will form a focus could not be predicted or controlled.
We developed an analytical model to predict the outcome of the blind focusing method. Our model allows us to determine under which conditions the optimization algorithm converges to a single diffraction-limited focus, and how the location of this optimized focus can be controlled. Furthermore, we can find the parameters that determine the convergence rate of this blind focusing procedure. The model is validated with experiments through strongly scattering samples, and an excellent agreement was found.
[1] I.M. Vellekoop, Optics Express 23, 1-18 (2015)
[2] O. Katz, E. Small, Y. Guan, Y. Silberberg, Optica 1, 170-174 (2014)
Light microscopy has been a key tool for biological and medical research for centuries, but the limited penetration depth due to light scattering has restricted its in vivo imaging ability to superficial regions. Nowadays, adaptive optics and active wavefront shaping techniques are increasingly used to compensate sample-induced aberrations in nonlinear optical microscopy. However, in most cases, the wavefront control element, such as deformable mirror, is imaged onto the pupil plane of the microscope objective. This configuration limits the field of view over which spatially irregular aberrations can be corrected. A better choice is to place the wavefront control element, in a plane conjugate to the primary source of aberrations.
Here we demonstrate a novel design of a variable-conjugation plane adaptive optics two-photon microscope for deep-tissue bioimaging and systematically investigate all the trade-offs in the design. We use a liquid crystals spatial light modulator for precise control of the initial wavefront. The design of the microscope allows not only to extend the corrected field of view but also to easily adjust the position of the conjugate plane for different imaging depths in a three-dimensional scattering sample. We demonstrate the feasibility of the microscope and the efficiency of aberration cancellation at different depths of up to more than 1 mm. The enhancement of the intensity in the focal spot over the whole volume has been carefully investigated for variety of samples.
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