Nonlinear hot image is one of the key elements that limit the output performance of high-power laser systems. In most hot-image researches, only one hot image peak is observed in the conjugate position for a single defect. Generally, multiple hot image peaks occur for multiple defects or cascaded nonlinear media. However, a new phenomenon is found by numerical simulation in our work: one defect can also afford two hot-image peaks near the conjugate position when considering the defect edge steepness. The super-Gaussian defect model is employed to mimic the defect edge steepness. When the super-Gaussian order is higher than one, there could be two hot image peaks under certain conditions. The formation of the double hot image peaks is primarily due to the co-effect of the hard-edge diffraction and the self-focusing effect. The influence of different factors, including the super-Gaussian order, defect size, modulation depth, and Kerr medium thickness, on the double hot image peaks intensity and location is systematically investigated. The results show that with the increase in the super-Gaussian order, the intensity of the double hot image peaks increases gradually. The defect size has a great influence on the position of the two hot image peaks. The modulation depth and thickness of the Kerr medium influence the intensity of the two hot image peaks; however, they have less impact on the peak location. Importantly, the defect edge steepness and size dependences of multiple nonlinear hot-image formation from a single-phase defect are further discussed in this paper. The two hot image peaks are fatal to optical components in high-power laser systems; in particular, the hot image peak behind the conjugate position is totally unexpected for a single defect. This research provides insights into basic physical images and hot-image formation laws. It also provides important guidance for optical defect specification evaluation and optical component layout design, as well as for beam quality control, in high-power laser systems.
Imaging for weak-phase objects is a challenging issue in the linear imaging process. Here, we demonstrate a high-contrast phase imaging method based on a nonlinear holographic hot image model. Due to the nonlinear Kerr effect, the holographic hot image can transform a weak phase into strong amplitude as a signal amplifier. The phase information is iteratively obtained from the light field distribution of the holographic hot image. The strong signal-to-noise ratio helps improve the imaging contrast. Using a tunable photorefractive crystal, we numerically and experimentally demonstrate the advantage of this method for imaging weak-phase objects. For the determined sample, our method doubles the imaging contrast. As far as we know, this is the first report using the nonlinear holographic hot image for imaging technology. This study can provide a potential strategy to achieve high-contrast imaging for various weak-phase objects applied in biomedical imaging, semiconductor metrology, and photolithography.
Phase defect detection with micrometer scale on large aperture optical elements is one of the challenges in precision optical systems. An efficient scheme is proposed to detect phase defects. First, the defects are positioned in a large aperture by dark-field imaging based on large aperture photon sieves to improve the detection efficiency with a relatively low cost. Second, static multiplanar coherent diffraction imaging is used to retrieve the phase of the positioned defects in a small field of view. Here, a spatial light modulator is used as a multifocal negative lens to eliminate the mechanical errors in multiplanar imaging. The use of a negative lens instead of a positive lens has the advantage of a larger imaging space for the system configuration. Compared to the traditional interferometry system, this diffraction detection system has a simpler optical path and doesn’t require sparse distribution of the defects. Experiment results demonstrate the success of the proposed scheme with a detection resolution better than 50 µm. We believe this work provides an effective method to rapidly detect phase defects on large aperture optics with high accuracy and high resolution.
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