Noniterative spatially partially coherent diffractive imaging using pinhole array mask
- May. 31, 2019
Reconstructing the phase of a field from intensity measurements is an old and ubiquitous challenge, known as the phase retrieval problem. However, optical detection devices that rely on converting photons to electrons (current), such as charge coupled device, cannot measure the phase of a light wave. Coherent diffractive imaging (CDI) provides one of the solutions. It has found wide application in material sciences and biology. CDI reconstruction techniques can be divided into two categories: iterative reconstruction algorithms and non-iterative methods (i.e. single-step methods).
Most reconstruction methods assume that the illumination is perfectly coherent. However, in some cases, like when the source is a free-electron laser or a third-generation synchrotron, or when the experimental environment is unstable (e.g. due to fluctuation of the ambient medium, like atmospheric turbulence), the influence of spatial partial coherence cannot be neglected. It may be relevant to consider how the algorithms perform or how they must be adapted when the illumination is spatially partially coherent. For the iterative algorithms, the solution usually involves mode decomposition and the lower the degree of coherence is, the more modes are required.
Recently, researchers at Soochow University (SCU) and Delft University of Technology (TUD) demonstrated the reconstruction of a phase object under spatially partially coherent illumination using a pinhole array mask (PAM). This PAM is specially designed such that one can retrieve the correlation function of the incident light by inverse Fourier transforming the measured diffraction pattern. Compared to the traditional diffractive imaging methods that use spatially partially coherent illumination (which are iterative), the newly proposed method is noniterative and robust to the degradation of the spatial coherence. In addition to diffractive imaging, the proposed method can also be applied to spatial coherence property characterization, e.g., free-space optical communication and optical coherence singularity measurement.
Schematic plot of two different applications of the experimental setup and the concept of the non-iterative diffractive imaging method. In the top branch, it is demonstrated how the method is able to reconstruct the object, even if the illumination has a non-trivial correlation structure. In the bottom branch, it is demonstrated how the object can be reconstructed with a large field of view, even if the coherence of the illumination is low.
This noniterative spatially partially coherent diffractive imaging method overcomes several challenges of conventional iterative CDI algorithms and holographic methods. In particular, it does not depend on the mode decomposition of the mutual coherence function (MCF) of the spatially partially coherent light and is particularly beneficial for achieving a large field of view when using a low degree of spatial coherence illumination. Moreover, they also demonstrate that the method can be used to calibrate the MCF of an arbitrary spatially partially coherent beam. The researchers also expect it can be applied to a wide range of wavelengths, from x-rays to infrared light due to its wavelength independence.