Unknown Speckle Pattern Research Articles

Aberration correction in epi-fluorescence microscopy using unknown speckle illumination.

Diffraction-limited imaging in epi-fluorescence microscopy remains a challenge when sample aberrations are present or when the region of interest rests deep within an inhomogeneous medium. Adaptive optics is an attractive solution, albeit with limited field of view and requiring relatively complicated systems. Alternatively, reconstruction algorithms have been developed over the years to correct for aberrations. Unfortunately, purely post-processing techniques tend to be ill-posed and provide only incremental improvements in image quality. Here, we report a computational optical approach using unknown speckle illumination and a matched reconstruction algorithm to correct for aberrations and reach or surpass diffraction limited resolution. The data acquisition is performed by shifting an unknown speckle pattern with respect to a fluorescent object. A key advantage is that the speckle statistics are preserved upon propagation through the aberrations, which avoids the double pass of information through the aberrating medium typical of epi-fluorescence microscopy. The method recovers simultaneously a high-resolution image, the point spread function of the system that contains the aberrations, the speckle illumination pattern, and the shift positions.

TV-based phase retrieval for speckle coded diffraction imaging

Speckle coded diffraction imaging (SCDI) has emerged as a new and developing lensless imaging method, which provides an effective imaging method for dense and connected objects. The conventional alternating-projection-based phase retrieval algorithm for SCDI relies on phase constraints between multiple diffraction images, and its disadvantage is that it often necessitates a large number of measurements to ensure high-quality reconstruction. In order to reduce the measurements, we propose to regularize the non-convex phase retrieval with total variation (TV)–based prior, and thus construct the TV-based cost function for SCDI. The alternating direction method is used to solve the nonlinear cost function, and recover the object and unknown speckle illumination. As both simulations and experiments demonstrate, the proposed TV-based reconstruction algorithm can recover the complex object and the unknown speckle pattern with high fidelity using 16 measurements, compared with hundreds of measurements used for AP-based algorithm.

Resolution enhancement in random illumination microscopy using photon correlations.

Random illumination microscopy (RIM) could surpass the diffraction barrier in fluorescence microscopy by illuminating an object with unknown speckle patterns. It has been demonstrated that the resolution in RIM using second-order statistics is as good as that of conventional structured illumination microscopy (SIM) from the asymptotic point of view. Compared to classical SIM, RIM is more robust to optical aberrations and scattering introduced by thick samples. In this work, I show that the quantum correlations could further improve the resolution in random illumination microscopy due to the photon antibunching property of fluorophore emitters. In theory, the super-resolution capacity of this quantum-enhanced version of RIM corresponds to the fourth power of the point spread function under the general epi-illumination geometry.

Near-field Fourier ptychography: super-resolution phase retrieval via speckle illumination.

High spatial resolution is the goal of many imaging systems. While designing a high-resolution lens with diffraction-limited performance over a large field of view remains a difficult task, creating a complex speckle pattern with wavelength-limited spatial features is easily accomplished with a simple random diffuser. With this observation and the concept of near-field ptychography, we report a new imaging modality, termed near-field Fourier ptychography, to address high-resolution imaging challenges in both microscopic and macroscopic imaging settings. 'Near-field' refers to placing the object at a short defocus distance with a large Fresnel number. We project a speckle pattern with fine spatial features on the object instead of directly resolving the spatial features via a high-resolution lens. We then translate the object (or speckle) to different positions and acquire the corresponding images by using a low-resolution lens. A ptychographic phase retrieval process is used to recover the complex object, the unknown speckle pattern, and the coherent transfer function at the same time. In a microscopic imaging setup, we use a 0.12 numerical aperture (NA) lens to achieve an NA of 0.85 in the reconstruction process. In a macroscale photographic imaging setup, we achieve ~7-fold resolution gain by using a photographic lens. The collection optics do not determine the final achievable resolution; rather, the speckle pattern's feature size does. This is similar to our recent demonstration in fluorescence imaging settings (Guo et al., Biomed. Opt. Express, 9(1), 2018). The reported imaging modality can be employed in light, coherent X-ray, and transmission electron imaging systems to increase resolution and provide quantitative absorption and object phase contrast.

13-fold resolution gain through turbid layer via translated unknown speckle illumination.

Fluorescence imaging through a turbid layer holds great promise for various biophotonics applications. Conventional wavefront shaping techniques aim to create and scan a focus spot through the turbid layer. Finding the correct input wavefront without direct access to the target plane remains a critical challenge. In this paper, we explore a new strategy for imaging through turbid layer with a large field of view. In our setup, a fluorescence sample is sandwiched between two turbid layers. Instead of generating one focus spot via wavefront shaping, we use an unshaped beam to illuminate the turbid layer and generate an unknown speckle pattern at the target plane over a wide field of view. By tilting the input wavefront, we raster scan the unknown speckle pattern via the memory effect and capture the corresponding low-resolution fluorescence images through the turbid layer. Different from the wavefront-shaping-based single-spot scanning, the proposed approach employs many spots (i.e., speckles) in parallel for extending the field of view. Based on all captured images, we jointly recover the fluorescence object, the unknown optical transfer function of the turbid layer, the translated step size, and the unknown speckle pattern. Without direct access to the object plane or knowledge of the turbid layer, we demonstrate a 13-fold resolution gain through the turbid layer using the reported strategy. We also demonstrate the use of this technique to improve the resolution of a low numerical aperture objective lens allowing to obtain both large field of view and high resolution at the same time. The reported method provides insight for developing new fluorescence imaging platforms and may find applications in deep-tissue imaging.

Optical-resolution photoacoustic imaging with speckle illumination

Conventional approaches for optical-resolution photoacoustic microscopy generally involves raster scanning a focused spot over the sample. Here, we show that a full-field illumination approach with multiple speckle illumination can also provide diffraction-limited optical-resolution photoacoustic images. Two different proof-of-concepts are demonstrated with micro-structured test samples. The first approach follows the principle of ghost imaging [1], and is based here on solving a linear inverse problem under sparsity assumptions: the object is reconstructed through a pseudo-inverse computation of a reference matrix made of speckle patterns measured during a calibration step. The second approach is a speckle scanning microscopy technique, which adapts the technique proposed in fluorescence microscopy by Bertolotti et al. [2]: in our work, spatially unresolved photoacoustic measurements are performed for various translations of unknown speckle patterns. Because speckle patterns naturally appear in many vari...

Super-resolution photoacoustic microscopy using blind structured illumination

We present a method for enhancing the spatial resolution of optical-absorption-based photoacoustic imaging through or within highly scattering media. The optical speckle pattern that emerges as light propagates through diffuse media provides structured illumination to an object placed behind a scattering wall. The photoacoustic signal produced by such illumination is detected using a focused ultrasound transducer. We demonstrate through both simulation and experiment that, by acquiring multiple photoacoustic images, each produced by a different random and unknown speckle pattern, an image of an absorbing object can be reconstructed with a spatial resolution far exceeding that of the ultrasound transducer. The imaging approach and reconstruction method exploiting joint sparsity will be valuable for biomedical applications and other fields where high-frequency structural information is scrambled by diffusive processes.

Superresolved imaging based on wavelength multiplexing of projected unknown speckle patterns

We propose a method for resolution enhancement of a diffraction limited optical system based on the capture of a set of low resolution images. These images are obtained after projection of an ensemble of unknown speckle patterns on top of the high resolution object that is to be imaged. Each speckle pattern is generated by the same thin (and unknown) diffuser, but illuminated with a slightly different wavelength. From the ensemble of low resolution images, we obtain a system of equations that can be solved in an iterative manner that enables reconstruction of the high resolution object. As a result, we also achieve the projected high resolution speckle patterns used for the encoding.

Structured illumination microscopy using unknown speckle patterns

By illuminating a sample with several uncontrolled random speckles and implementing a blind structured illumination microscopy algorithm, researchers demonstrate that image reconstruction can be achieved without knowing the original illumination pattern, at a resolution two times better than that of conventional wide-field microscopy.