分析联合变换相关器(JTC)光学加密系统解密图像噪音大、质量差的原因，提出一种非线性JTC 光学加密系统。将JTC 加密图像除以密钥功率谱作为新加密图像。一方面，新加密图像能够消除密钥傅里叶谱振幅分布不均匀引起的噪音，提高解密图像质量。仿真结果表明，Lena 图像的消噪音解密图像和原始图像的相关系数可由0.4104增加到0.7190，均方根(RMS)从0.8154减小到0.7089。二值文本图像的消噪音解密图像和原始图像的相关系数由0.8458 增加到0.9785，RMS 从0.6887 减小到0.4583；另一方面，新加密图像能抵御唯密文攻击(COA)算法的攻击，仿真结果表明，利用COA 算法能从JTC 加密图像恢复出高质量的原始图像信息，但不能从新加密图像恢复出任何原始图像信息，有效提高JTC加密系统的安全性。

By analyzing the reason of big noise and low quality of decryption image in the joint transform correlation (JTC) encryption system, a kind of nonlinear JTC image encryption system is proposed. A new encryption image is obtained when JTC encryption image is divided by key power spectrum. On the one hand, new encryption image can remove noise caused by amplitude non- uniformity of key Fourier spectrum and improve the quality of decryption image. Simulation results show that the correlation coefficient between new Lena decryption image and original image increases from 0.4104 to 0.7190 and root mean square (RMS) value decreases from 0.8154 to 0.7089, and that the correlation coefficient between new binary text decryption image and original image increases from 0.8458 to 0.9785 and RMS value decreases from 0.6887 to 0.4583. On the other hand, new encryption image can resist chiphertext only attack (COA) arithmetic attack. Simulation results show that using the COA arithmetic, high quality original image can be restored from JTC encryption image, but information of original image can not be obtained from new encryption image. This system improve the security of JTC encryption system effectively.