image: Schematic of experimental setup for generation (part 1) and measurement (part 2) of the complex optical coherence structures of random light fields. view more
Credit: OEA
A new publication from Opto-Electronic Science; DOI 10.29026/oes.2023.220024 considers a precise measurement technology of optical coherent structure of random lights.
Optical manipulation and applications involve numerous fields, such as physics, information, materials and life sciences. It has been included in the national “14th Five-Year Plan” major engineering and projects. To better promote manipulation and application of the light beams, developing precision measurement technologies of optical parameters (such as intensity, phase, polarization and frequency, etc.) is vital, which is also an interesting topic. Compared with the fully coherent beam, partially coherent beams viewed as the dynamic random light fields are robust in complex environments. The amplitude and phase of random light fields fluctuate randomly over time, and its speckle intensity is shown in the following figure. Significantly, the valuable information is embedded in the statistical properties of random light fields. The optical coherent structure of a random electric field, as a second-order statistical parameter, can determine the beam evolution behavior, far-field intensity distribution and light-matter interaction etc. Up to now, the random lights with prescribed optical coherent structures have been found applications in coherence tomography, ghost imaging, super-resolution imaging and free-space optical communication. Recently, using the coherent structure of random lights as information carrier for high-security encryption and far-field robust imaging has also been proposed. The rapid development of optical coherence structures related research highly requires the precise measurement of optical coherent structures in return.
To fully recover optical coherent structure, as a complex function, we must precisely measure their real and imaginary parts (or amplitude and phase) simultaneously. The optical coherence structure has traditionally been measured using Young’s interference experiment, wherein its magnitude and phase can be predicted based on the visibility and position of the fringes, respectively. However, this experiment only considers two position points. Full characterization of the optical coherence structure requires that each point be scanned independently across the beam plane, which needs significant time and effort. Improvements to this experiment have been proposed, such as wavefront-folding interferometers, phase-space approach and self-reference holography, Hanbury Brown and Twiss (HBT) effect and generalized HBT effect have been developed to measure optical coherence structures. These methods involve a complicated, misalignment- and vibration-sensitive setup, or are limited to Gaussian optical statistic. Despite all efforts, measuring the complex optical coherence structure remains an open and great challenge.
Recently, the research group of Prof. Yangjian Cai from Shandong Normal University proposed a robust, convenient, and fast protocol for precise measurement of the optical coherence structures of random optical fields via generalized Arago (or Poisson) spot experiments. It had rigorous mathematical solutions. This method only required to capture the far-field intensity of the obstructed random light beams thrice, and was applicable to any optical coherence structures, regardless of their type and optical statistics. Fig. 1 shows the experimental setup for the generation (in Part 1) and measurement (in Part 2) of the complex optical coherence structures of random optical fields. In this setup, a spatial light modulator (SLM1) was used to reproduce random light beams with prescribed complex optical coherence structure and SLM2 was used to realize that the produced beam is obstructed by the obstacles.
The simulation and experimental results of the real and imaginary parts of the coherent structure were achieved by this method. Compared with theoretical results, the structural similarity SSIM of the simulated and measured results are both higher than 0.98, which strongly demonstrated the effectiveness and accuracy of the proposed protocol. In addition, they also conducted the measurements of the Schell mode sources and the non-Schell mode sources with non-Gaussian optical statistics. The relevant results were given in the manuscript, and proved this method was independent of the optical statistics and beam types of random light beams.
With aids of this protocol, they also achieved dynamic optical images encryption and decryption with random light fields, in which the coherent structure function of random light beams was used as the information carrier. A dynamic (shifting and rotating) optical image "OES" was encrypted into the coherent structure with a key. Figure 3(a)~(f) show the real and imaginary parts of the measured coherent structure functions at different time. They can precisely recover the dynamic optical images with the encryption key, displayed in Figure 3(g) ~(h). The speed of image recovery depends on the refresh rate of the SLM and frame rate of the CCD. In the lab, an ideal 20fps video can be decrypted. This technology has potential applications in the fields of optical information encryption, orbital angular momentum measurement, and optical communication etc.
Keywords: optical coherence / statistical optics / Arago spot / optical encryption / optical imaging
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Center of Light Manipulation and application of Shandong Normal University (www.gctk.sdnu.edu.cn) was established in May 2018 and was selected as the Shandong Provincial Engineering and Technical Center of Light Manipulation Research Center in the same year. Professor Yangjian Cai, winner of the National Science Fund for Distinguished Young Scholars、OSA fellow, serves as the center director. The center dedicated to the major demands of China and the priority development fields of the Shandong Provincial new and conventional energy conversion project. The center focuses on novel spatiotemporal lights manipulation and applications in complicated environments, material light processing, laser radar and optical imaging. Since 2018, the center has won more than 50 national-level projects such as major projects of the National Natural Science Foundation of China, national key research and development plan et. al., and published more than 200 SCI research papers on international authoritative journals including Physical Review Letters、Laser & Photonics Reviews、PhotoniX、Nano Letters et al.
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Opto-Electronic Science (OES) is a peer-reviewed, open access, interdisciplinary and international journal published by The Institute of Optics and Electronics, Chinese Academy of Sciences as a sister journal of Opto-Electronic Advances (OEA, IF=9.682). OES is dedicated to providing a professional platform to promote academic exchange and accelerate innovation. OES publishes articles, reviews, and letters of the fundamental breakthroughs in basic science of optics and optoelectronics.
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Liu X, Chen Q, Zeng J, Cai YJ, Liang CH. Measurement of optical coherence structures of random optical fields using generalized Arago spot experiment. Opto-Electron Sci 2, 220024 (2023). doi: 10.29026/oes.2023.220024
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