E-mail Alert
RSS2021 Vol. 4, No. 12
Cover story: Liu YL, Chen YH, Wang F, Cai YJ, Liang CH et al. Robust far-field imaging by spatial coherence engineering. Opto-Electron Adv 4, 210027 (2021).
Optical coherence state, as one of the intrinsic characteristics of a light field, describes the electric field correlation at pairs of spatial locations and times. In the space-frequency domain, spatial coherence properties are characterized by the two-point spectral degree of coherence (DOC) function, being a normalized version of the cross-spectral density function. The pioneering works have revealed that the DOC function has the ability to provide a new degree of freedom for carrying information. However, the DOC function evolves as the beam propagates, because of optical diffraction. It implies it’s impossible to be directly used for optical information transfer and recovery. Recently, Prof. Yangjian Cai’s research group from the School of Physics and Electronics of Shandong Normal University reported on a method to achieve robust far-field imaging by spatial coherence engineering. They ferreted out when a Schell-modal beam carrying a cross phase (CP) propagates in free space, in a paraxial optical system or in a turbulent medium, the modulus of DOC function in the focal plane (far-field) acquires the same distribution as it has in the source plane. Hereto, they first encoded the image into the DOC function of a partially coherent beam via van Cittert-Zernike theorem. Such beam carrying with the CP, propagating through both free space and atmospheric turbulence, is received by a CCD camera in the focal plane of a thin lens. The modulus of DOC function can be recovered through the normalized fourth-order correlation function, and then was played with the Fienup’s phase retrieval algorithm to restore the image in free space and even in the turbulent atmosphere. The results well demonstrated their proposed approach is confident and robust. They provide a promising avenue to the optical imaging and remote sensing in natural environments exhibiting optical turbulence.
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Prof. Xiangang Luo
