基于光参量放大相位共轭特性的图像修复与增强

非线性光学相位共轭技术可将经过散射介质后产生畸变的光学波前进行修复.本文基于光参量放大(OPA)过程的光学相位共轭(OPC)特性,进行了光学相位共轭图像修复和增强的实验研究.基于大能量532 nm皮秒抽运激光和大口径非线性光学晶体KTiOPO_4(KTP)(Ⅱ类相位匹配),对经过牛奶乳浊液后已无法识别的1064 nm近红外光学图像,进行相位共轭修复,修复后的图像分辨率达12线/mm,此外,结合OPA过程的光学增益特性,实现了超过17 dB的光学图像增强,为现有三波混频光学相位共轭修复畸变所获图像增益的最大值.在此基础上,峰值信噪比较修复之前有160%的提升.考虑到光参量过程所具有的波长可调谐特性,在实际应用中,可根据需要,选择与生物组织的光学治疗窗口相匹配的成像波长,从而保证更长的穿透深度,提升生物组织成像和医学无损检测的效果.

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

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.