Image restoration and enhancement based on phase conjugation of optical parametric amplification

Abstract

It is well known that the weak optical image can be amplified based on the optical parametric amplification (OPA), and the distorted wave-front can be recovered by the optical phase conjugation (OPC) method. In this paper, weak infrared images, which are barely recognizable after the propagation through the milk emulsion, are restored and optically amplified based on phase conjugation of OPA.The OPC property of OPA is demonstrated with a type-II phase matched nonlinear optical crystal KTiOPO4 (KTP). The near-infrared image at 1064 nm is the input of OPA as the signal beam, and a 10 Hz, mJ-level, 21 ps 532 nm laser is used as the pump beam. When the spatial and temporal overlap are achieved, the attenuated optical image is amplified. Due to the difference in polarization, the idler beam of the OPA is selected and detected with the CCD and the blurred image is restored by the re-entry of the turbid media.The resolution of restored image is 12 lines/mm, which has achieved a theoretical limit. Moreover, by combining the optical gain of the OPA process, over 17 dB image amplification is obtained, which is the highest for the OPC-based image restoration in turbid media to our knowledge. The significant improvement in image quality is also demonstrated by 160% increase of the peak signal-to-noise ratio. By taking advantage of tunability of the OPA, the operational wavelength of this technique can be extended to an optical therapeutic window, which is suitable for noninvasive image restoration, enhancement and detection.