Research Of Optical Image Encryption Based On Digital Holography

In the field of information security,traditional encryption methods such as computer or digital methods are slow to process massive information.The optical system can process data in parallel,and the information of the image can be hidden in multiple degrees of freedom of the light.Therefore,optical encryption techniques have been widely used in the field of information security,especially in the field of image encryption.The optical image encryption techniques based on digital holography has gradually become the research focus due to it can store the amplitude and phase information of the image in intensity,which facilitates the recording and transmission of ciphertext.In this paper,we have systematically analyzed the research status at home and abroad of optical image encryption based on digital holography.To solve the problems of the image encryption based on phase-shift holography and off-axis holography,which require conjugate the key and poor spatial bandwidth utilization,the corresponding methods are proposed.The main research contents of this paper are as follows:(1)The conventional optical image encryption method based on in-line holography can eliminate the conjugate and zero-order image by introducing phaseshift technology,which has achieved high-quality decryption.Most the currently existing encryption methods must conjugate the principal keys in the decryption process.Furthermore,the generated ciphertexts are more than one,which does not facilitate the transmission.To address these problems,an optical image encryption method based on phase-shifting holography and digital optical phase conjugation technology is proposed.By generating the conjugate of the ciphertext image,the key used during encryption can be directly used as the decryption key,thereby avoiding the need to use the conjugate key.The plaintext is transformed into four noise-like holograms with the help of the four-step phase-shifting digital holography.Lowfrequency components of the four holograms are extracted and combined them into a new image by the lifting wavelet transform.Then the combined image is scrambled by the nonlinear 3D chaos-based cyclic shift.In decryption,the digital optical phase conjugation technique is utilized to extract the information of plaintext.(2)The conventional optical image encryption method based on off-axis holography separates the zero-order and conjugate order image in the frequency domain,by introducing an angle between the sample and reference waves.Thus,the information of the image can be decrypted by the corresponding filter with correct keys.In the filtering process,to completely filtered the object term,the object spectrum is required to be separated from each other in the frequency domain with other orders.The image sensor discrete samples the light waves,and the limited pixel size determines the limited bandwidth of the hologram.To compensate for the loss of bandwidth,the Kramers-Kronig analytic relational is introduced into the off-axis holographic multiplexing encryption system.In the encryption process,multiple images go through a Fresnel diffraction process and modulated by a random phase key and then interfere with the reference wave to obtain an encrypted hologram.The single ciphertext hologram hides the information and number of the original images,so it has high security and confusion in the process of storage and transmission.The decryption process takes advantage of the analytic of the signal and can directly extract the signal from the positive frequency of the spectrum.Compared with the optical image encryption based on conventional off-axis digital holography,the proposed method achieves the decryption with the spectrum overlap.In summary,we have deeply concluded the research of optical image encryption based on digital holography.Aiming at the problems of image encryption based on phase-shifting holography and off-axis holography,the corresponding methods are proposed.Numerical simulation and some optical experiments demonstrate the feasibility of the proposed methods.The above mentioned results provide a new way for the field of digital holography image encryption.