Imaging Quality Improvement Of Ghost Imaging And Its Applications In Optical Information Security

Quantum imaging,also known as “ghost imaging” or “correlated imaging”,is a new way of imaging by using correlations from quantum entanglement or from the second-order,even higher-order of classical light field’s fluctuations.Unlike traditional imaging methods,ghost imaging can recover the imaging of the object on the optical path without the object.This new imaging method has attracted widely attention and interest since it was proposed.However,the complexity and environmental factors have prevented its applications,so it is necessary to find a way to improve the imaging quality in the practical environment.Besides,with the further development of ghost imaging,the optical information security based on the ghost imaging have been widely developed.As a result,the dissertation mainly focuses on the imaging quality improvement of ghost imaging and its applications in optical information security.The main work is as follows.For the case where it is impossible to obtain the orthogonal measurement matrix in practical environment,the dissertation investigates the orthogonal-triangular(QR)decomposition of matrix theory,and presents a new ghost imaging scheme based on QR decomposition,named QR decomposition ghost imaging(QRGI).In the scheme,an orthogonal matrix for the ghost imaging as a measurement matrix is reconstructed by using QR decomposition.By using the orthogonal property,the imaging quality is greatly improved.The simulation and experimental results show that the proposed QRGI scheme could perfectly reconstruct the object comparing to the traditional ghost imaging(TGI)and differential ghost imaging(DGI).Besides,in comparison with the singular value decomposition ghost imaging(SVDGI),the imaging quality and the reconstruction time by using QRGI are almost the same,while the cost time(the time cost on the light patterns computation)is substantially reduced.As the time varying noise and the background noise greatly degrade the reconstructed images’ quality in practical environment,the dissertation investigates the spread spectrum technique of digital communications,and demonstrates a novel ghost imaging scheme based on the spread spectrum,named spread spectrum ghost imaging(SSGI),to overcome the time varying noise and improve the imaging quality in the practical environment.In the scheme,the ghost imaging system is considered as a communication system,where the light beam is regarded as a transmitter,the bucket detector is considered as a receiver,and the whole imaging process is taken as a communication channel.The light beam is first modulated by a designed spread spectrum code,then followed by a spatial modulator to obtain the temporal-spatial speckle patterns.These speckle patterns are then projected onto the unknown object successively.The bucket detector is used to collect the light transmitted through or reflected from the object.With the same spread spectrum code to despread the bucket value,the computed bucket value corresponding to the speckle pattern is then obtained.Finally,the object can be recovered by utilizing the speckle patterns and corresponding computed bucket values.The simulation and experimental results show that the proposed SSGI scheme has a stronger noise resistance capacity,it can suppress the time varying noise and imrove the imaging quality of the object.Besides,the longer the length of the spread spectrum code is,the stronger noise resistance capacity SSGI has.Moreover,based on the spread spectrum ghost imaging and secret sharing theory,a novel secret sharing scheme based on the spread spectrum ghost imaging is proposed.In the scheme,a dealer divides a secret key,which is obtained by a modulo 2 addition over n rows selected from a Hadamard matrix with high order,into n pieces to n participants,and then the dealer uses the secret key as a spread spectrum code to encrypt an image by using the SSGI method.The resultant bucket values,as the ciphertext,are then shared with n participants,and the modulated speckle patterns used in SSGI are also transmitted to n participants by private channels for preventing eavesdropping.Only if all participants are honest and cooperative,the secret key can be obtained,thus,the ciphertext can be decrypted into the original image with the help of the SSGI method.Otherwise,the image information cannot be extracted if one participant is unhonest.The simulation and experimental results show that the proposed scheme has enhanced the security of optical image transmission.In addition,the larger the number of the participants is,the higher security the scheme has.In order to widen the application field of spread spectrum ghost imaging in optical information security,the dissertation investigates an optical encryption(OE)scheme based on the spread spectrum ghost imaging.In the scheme,suppose that there are two authorized users,Alice and Bob,an unauthorized user,Eve.Alice firstly generates a Hadamard matrix of high order,and randomly selects M rows as M spread spectrum codes to encrypt an image with the help of the spread spectrum ghost imaging system.The resultant ciphertext and the modulated speckle patterns used in SSGI are both transmitted to Bob in a public channel.Besides,Alice regards the M rows’ numbers as the secure key and transmits to Bob in a private channel.The authorized user,Bob,could retrieve M correct rows of the Hadamard matrix(i.e.M spread spectrum codes)by the received key,and despread the ciphertext into valid bucket values,then reconstruct the image of the object with the theory of SSGI.The unauthorized user,Eve,could obtain M invalid rows(i.e.invalid spread spectrum codes)by the illegal key,and the resultant bucket values despreaded from the ciphertext have no correlations with the speckle patterns,so he cannot recover the original image.The simulation and experimental results show that the proposed scheme has effectively improved the security of optical encryption,and considerably reduced the number of bits sent from Alice to Bob.