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Optical Shop Testing

Ptychography is a particularly powerful coherent diffraction imaging technique. In ptychography, a localized beam that illuminates the object is scanned in a step-wise fashion, resulting in an array of partially overlapping probing spots on the object. The intensity diffraction pattern from each spot is recorded separately. Then, a complex-valued image is computationally constructed from the set of measured diffraction patterns. Ptychography is based on scanning, hence it results with a long overall acquisition time, in the order of a second or more. Also, the scanning limited resolution, vibration stability, drift, and dynamic range weaken the performances of ptychographic microscopes. We propose and analyze single-shot ptychography, where tens or hundreds of quasi-localized, partially overlapping beams illuminate the object simultaneously. Various schemes for single-shot ptychography, in both the transmission and reflection modes, with coherent and partially coherent illumination and for single-shot Fourier ptychography are proposed. Experimentally, we demonstrate single-shot ptychography with a 180 ms exposure time (limited by the CCD minimal acquisition time) using a sub-milliwatt diode laser that simultaneously illuminates the object with 49 partially overlapping beams. Single-shot ptychography, which combines the celebrated power of ptychography with (ultra)fast imaging, will surely open up new opportunities in microscopy.

Single-shot ptychography

In this paper, we review the status of the multifunctional experimental platform at the National Laboratory of High Power Laser and Physics (NLHPLP). The platform, including the SG-II laser facility, SG-II 9th beam, SG-II upgrade (SG-II UP) facility, and SG-II 5 PW facility, is operational and available for interested scientists studying inertial confinement fusion (ICF) and a broad range of high-energy-density physics. These facilities can provide important experimental capabilities by combining different pulse widths of nanosecond, picosecond, and femtosecond scales. In addition, the SG-II UP facility, consisting of a single petawatt system and an eight-beam nanosecond system, is introduced including several laser technologies that have been developed to ensure the performance of the facility. Recent developments of the SG-II 5 PW facility are also presented.

https://www.cambridge.org/core/services/aop-cambridge-core/content/view/0112928A8FC4EE08F7C3E709BF1D2BB4/S2095471918000464a.pdf/div-class-title-status-and-development-of-high-power-laser-facilities-at-the-nlhplp-div.pdf

Status and development of high-power laser facilities at the NLHPLP

A novel optical encoding method based on single-shot ptychography is proposed for the application of optical watermarking. For the inherent properties of single-shot ptychography, the watermark is encoded into a series of tiny diffraction spots just in one exposure. Those tiny spots have high imperceptibility and compressibility, which are quite suitable for the optical watermarking application. The security of the proposed watermarking is mainly supported by the strong imperceptibility, as well as the introduction of compression encoding and scrambling encoding. In addition, the diversity of the multi-pinhole array and the structural parameters can also be served as security keys. Both numerical simulation and optical experiment demonstrate the high security and the easy implementation of the single-shot-ptychography-based optical watermarking. Further, the compression encoding can largely improve the embedding capacity that enables the multiple-watermarking for more transmitted information and higher security.

Optical watermarking based on single-shot-ptychography encoding.

Multiple distance phase retrieval methods hold great promise for imaging and measurement due to their less expensive and compact setup. As one of their implementations, the amplitude-phase retrieval algorithm (APR) can achieve stable and high-accuracy reconstruction. However, it suffers from the slow convergence and the stagnant issue. Here we propose an iterative modality named as weighted feedback to solve this problem. With the plug-ins of single and double feedback, two augmented approaches, i.e. the APRSF and APRDF algorithms, are demonstrated to increase the convergence speed with a factor of two and three in experiments. Furthermore, the APRDF algorithm can extend the multiple distance phase retrieval to the partially coherent illumination and enhance the imaging contrast of both amplitude and phase, which actually relaxes the light source requirement. Thus the weighted feedback enables a fast-converging and high-contrast imaging scheme for the iterative phase retrieval.

/pdf/a-fast-converging-iterative-method-based-on-weighted-ec8xrdh8hv.pdf

A fast-converging iterative method based on weighted feedback for multi-distance phase retrieval

An optical alignment and a corresponding reconstruction algorithm are proposed to realize a single shot Ptychographical Iterative Engine (PIE). Multiple light beams generated by a cross grating are used to illuminate the specimen, and the resulting diffraction patterns formed on the detector plane by each beam are recorded simultaneously. The modulus and phase images are properly reconstructed with standard PIE algorithm. The proposed single shot method omits the needs for a mechanical x-y scanning of standard PIE algorithm and hence dramatically reduces the data acquisition time.

Single shot ptychographical iterative engine based on multi-beam illumination

A single shot multi-wavelength phase retrieval method is proposed by combining common coherent modulation imaging (CMI) and a low rank mixed-state algorithm together. A radiation beam consisting of multi-wavelength is illuminated on the sample to be observed, and the exiting field is incident on a random phase plate to form speckle patterns, which is the incoherent superposition of diffraction patterns of each wavelength. The exiting complex amplitude of the sample including both the modulus and phase of each wavelength can be reconstructed simultaneously from the recorded diffraction intensity using a low rank mixed-state algorithm. The feasibility of this proposed method was verified with visible light experimentally. This proposed method not only makes CMI realizable with partially coherent illumination but also can extend its application to various traditionally unrelated fields, where several wavelengths should be considered simultaneously.

Single shot multi-wavelength phase retrieval with coherent modulation imaging.

Wavefront control is a significant parameter in inertial confinement fusion (ICF). The complex transmittance of large optical elements which are often used in ICF is obtained by computing the phase difference of the illuminating and transmitting fields using Ptychographical Iterative Engine (PIE). This can accurately and effectively measure the transmittance of large optical elements with irregular surface profiles, which are otherwise not measurable using commonly used interferometric techniques due to a lack of standard reference plate. Experiments are done with a Continue Phase Plate (CPP) to illustrate the feasibility of this method.

Measurement of the complex transmittance of large optical elements with Ptychographical Iterative Engine

The properties of a series of phase measurement techniques, including interferometry, the Hartmann–Shack wavefront sensor, the knife-edge technique, and coherent diffraction imaging, are summarized and their performance in high power laser applications is compared. The advantages, disadvantages, and application ranges of each technique are discussed.

/pdf/wavefront-measurement-techniques-used-in-high-power-lasers-4ogr94smta.pdf

Wavefront measurement techniques used in high power lasers

A single-shot phase retrieval algorithm based on a random aperture and partially saturated diffraction pattern is proposed. The diffraction pattern in the saturated area could be retrieved during the iterative process, which circumvents the problem of limited dynamic range of the detector. Besides, the random aperture is easier to be manufactured and if the accuracy of the random aperture is high enough, the design value could be used directly for iterations. It has the potential to be adapted for different wavelengths without additional transmission measurement of the wave modulator. The validity has been demonstrated by simulations and experiment.

Xingchen Pan

Papers

Single shot ptychographical iterative engine based on multi-beam illumination

Single shot multi-wavelength phase retrieval with coherent modulation imaging.

Measurement of the complex transmittance of large optical elements with Ptychographical Iterative Engine

Wavefront measurement techniques used in high power lasers

Coherent amplitude modulation imaging based on partially saturated diffraction pattern.