Lensless Diffractive Imaging Research Articles

Spatial coherence measurement and partially coherent diffractive imaging using self-referencing holography.

The complete characterization of spatial coherence is extremely difficult because the mutual coherence function (MCF) is a complex-valued function of four independent Cartesian coordinates. This difficulty limits the ability to control and to optimize the spatial coherence in a broad range of key applications. Here we propose an efficient and robust scheme for measuring the complete MCF of an arbitrary partially coherent beam using self-referencing holography, which does not require any prior knowledge or making any assumptions about the MCF. We further apply our method to lensless diffractive imaging, and experimentally demonstrate the reconstruction of a phase object under spatially partially coherent illumination. This application is particularly useful for imaging at short wavelengths, where the illumination sources lack spatial coherence and no high-quality imaging optics are available.

Lensless diffractive imaging with ultra-broadband table-top sources: from infrared to extreme-ultraviolet wavelengths

Lensless imaging is an approach to microscopy in which a high-resolution image of an object is reconstructed from one or more measured diffraction patterns, providing a solution in situations where the use of imaging optics is not possible. However, current lensless imaging methods are typically limited by the need for a light source with a narrow, stable and accurately known spectrum. We have developed a general approach to lensless imaging without spectral bandwidth limitations or sample requirements. We use two time-delayed coherent light pulses and show that scanning the pulse-to-pulse time delay allows the reconstruction of diffraction-limited images for all the spectral components in the pulse. In addition, we introduce an iterative phase retrieval algorithm that uses these spectrally resolved Fresnel diffraction patterns to obtain high-resolution images of complex extended objects. We demonstrate this two-pulse imaging method with octave-spanning visible light sources, in both transmission and reflection geometries, and with broadband extreme-ultraviolet radiation from a high-harmonic generation source. Our approach enables effective use of low-flux ultra-broadband sources, such as table-top high-harmonic generation systems, for high-resolution imaging. Researchers in The Netherlands have overcome the restriction of monochromatic illumination when performing lensless imaging. Stefan Witte and co-workers from LaserLAB Amsterdam have developed a lensless scheme that employs two coherent time-delayed pulses and is compatible with broadband sources. Lensless imaging — whereby diffraction patterns are interpreted to reconstruct an image of a sample — is popular in the X-ray and extreme-ultraviolet regimes, where high-quality lenses for performing conventional imaging are not available. Because this approach has traditionally been limited to narrowband coherent radiation, scientists have been eager to make it compatible with broadband sources such as tabletop high-harmonic generation. This new broadband technique involves scanning the pulse-to-pulse time delay and then applying a phase-retrieval algorithm to produce high-resolution images of complex objects.

Information multiplexing in ptychography

We show for the first time that ptychography (a form of lensless diffractive imaging) can recover the spectral response of an object through simultaneous reconstruction of multiple images that represent the object's response to a particular mode present in the illumination. We solve the phase problem for each mode independently, even though the intensity arriving at every detector pixel is an incoherent superposition of several uncorrelated diffracted waves. Until recently, the addition of incoherent modes has been seen as a nuisance in diffractive imaging: here we show that not only can the difficulties they pose be removed, but that they can also be used to discover much more information about the object. If the illumination function is also mode-specific, we show that we can also solve simultaneously for a multiplicity of such illumination modes. The work opens exciting possibilities for information multiplexing in ptychography over all visible, X-ray and electron wavelengths.

Structured-illumination-based lensless diffractive imaging and its application to optical image encryption

In this paper, we propose a new method in the structured-illumination-based lensless diffractive imaging using variable grating pitches. When a phase grating pitch is sequentially changed, a series of diffraction patterns can be recorded by a charge-coupled device (CCD) camera. Subsequently, a phase retrieval algorithm with a rapid convergence rate is developed to recover a high-quality object from the recorded diffraction patterns. The proposed method is further applied to optical image encryption, and simulation results are presented to demonstrate validity of the proposed method.

Lensless coherent diffractive imaging with a Fresnel diffraction pattern

Coherent diffractive imaging is a new lensless imaging technique which has important applications in optical measurements, microscopic imaging and adaptive optics. We propose a method for coherent diffractive imaging from one single Fresnel diffraction intensity pattern. In this method, a Fresnel diffraction intensity pattern of the object wave passing through a specially designed sampling array is recorded and the complex amplitude of the object wave can be extracted through some digital processing such as inverse Fresnel transform and spatial filtering to the recorded intensity pattern; and then the image of the object can be reconstructed in computer. Some theoretical analyses and digital simulations about how the diffraction parameters affect the rebuilding image are given, such as sampling aperture, diffraction distance, image sensor size, etc. We find that there exists an optimal recording distance when the pinhole size and the recording aperture are given. Some serious noise will appear if the recording distance is longer than the optimal value, while shorter recording distance will result in a worse resolution of the reconstructed image. The influence of the pinhole size on the imaging resolution power of the system is also discussed. As this method requires only a single measurement of the diffraction intensity pattern and it does not need any iterative algorithm and lens systems, it provides a practically valuable approach to real-time wavefront measuring and lensless diffractive imaging of a complex-valued object in a wide rang of wavelengths.

Tri-arm multipinhole interferometer for wavefront measurement and diffractive imaging

We propose a tri-arm (or Y-shaped) multipinhole (MP) interferometer for wavefront measurement based on a specially designed tri-arm MP plate. We demonstrate that the complex amplitude of a wavefront sampled by the tri-arm MP plate inserted between the object and the detector plane can be extracted directly from the Fourier transform of a far-field diffraction intensity pattern without the need for any iterative algorithm. A form of coherent diffractive imaging based on a rotatable tri-arm MP plate is also demonstrated, which provides a feasible approach for lensless diffractive imaging in real time.

Diffractive imaging based on a multipinhole plate

We report on a noniterative method for coherent diffractive imaging based on a specially designed multipinhole (MP) plate. We demonstrated that the complex amplitude of the wavefront penetrating through an MP plate inserted between the specimen and the detector plane can be directly extracted from the Fourier transform of a single far-field diffraction intensity pattern without need of any iterative algorithm. A form of scanning diffractive imaging based on a rotatable MP plate is also demonstrated, which provides a feasible approach for lensless diffractive imaging of complex-valued objects.

Lensless Diffractive Imaging Using Tabletop Coherent High-Harmonic Soft-X-Ray Beams

We present the first experimental demonstration of lensless diffractive imaging using coherent soft x rays generated by a tabletop soft-x-ray source. A 29 nm high harmonic beam illuminates an object, and the subsequent diffraction is collected on an x-ray CCD camera. High dynamic range diffraction patterns are obtained by taking multiple exposures while blocking small-angle diffraction using beam blocks of varying size. These patterns reconstruct to images with 214 nm resolution. This work demonstrates a practical tabletop lensless microscope that promises to find applications in materials science, nanoscience, and biology.