Research Of Diffractive Optical Element Effect On Performance Of Compound Space Optical Communication System

Compared with traditional space microwave communication,space optical communication has the advantages of small divergence angle,strong antielectromagnetic interference ability and high data rate.Therefore,space optical communication technology has been widely concerned and investigated.It has been investigated for more than 60 years worldwide and some mature theoretical models of laser transmission characteristics,atmospheric effects,receiving intensity distribution etc.have been established to help the investigation of space optical communication.However,compared with fiber optical communication,long distance of free space channel greatly increases the risk of information being eavesdropped.With the development of quantum computing,the traditional software encryption technology that based on public secret key has faced the risk of being eavesdropped.Chaos encryption is a kind of hardware encryption technology.It uses the working parameters of key components in transmitter as the secret key,which can avoid the potential eavesdropping risk of software encryption technology.Therefore,the nonencrypted/chaotic encrypted compound space optical communication technology able to simultaneously realize the classification transmission of ordinary and high-level information has become a research hotspot in this field.Nowadays,the requirement for data rate is getting higher and higher.With the development of space optical communication technology,the design parameters have reached the limit in recent years.Therefore,the optimization of system design for both the non-encrypted and chaotic encrypted space optical communication systems has become an urgent demand in this field.The Cassergreen-RC reflective optical antenna has beem widely used in space opical communication systems for its advantages of short frame,large magnification,elimination of spherical aberration and transmitter-receiver sharing.However,the secondary mirror of Cassergreen-RC reflective optical antenna blocks a part of transmission beam and affects the emission efficiency of optical subsystem.Thus,we introduce diffractive optical element(DOE)into the optical subsystem to reshape the transmission beam to avoid the obscuration of the secondary mirror.So the emission efficiency of the optical subsystem can be enhanced and the design pressure of other system parameters can be reduced.In this paper,a non-encrypted/chaotic encrypted compound space optical communication system that can simultaneously realize the classification transmission of ordinary and high-level information is proposed based on practical engineering requirements.The chaotic encryption module uses a kind of photoelectric feedback chaotic oscillation structure.In the optical subsystem of this communication system,a couple of DOEs have been introduced to solve the obscursation problem of the secondary mirror in Cassegrain-RC reflective optical antenna.For the specific design of DOE,we compare and analyze two most common hollow beam shaping methods: the hollow flat-top shaping and hollow Gauss shaping.Numerical simulation results show that the hollow Gaussian spot has better far-field transmission characteristics.Then we further analyze the size of the hollow radius,and the results show that the radius of the secondary mirror is the optimal choice for the hollow Gaussian radius.Based on the results above,we take the hollow Gaussian shaping method for the DOE introduced into the compound space optical communication system and give the detailed phase design of the DOE.Then,we further investigate the influence of DOE on the performance of the nonencrypted/chaotic encrypted compound space optical communication system.For the non-encrypted subsystem,we derive the bit error rate(BER)model of the nonencrypted space optical communication subsystem with DOE firstly.Based on this BER model,we investigate the influence of DOE both on the uplink and downlink communication systems.The results indicate that the DOE can reduce the BER by more than 10-1 order of magnitude in both cases.Combined with the optimization design of basic system parameters,the enhancement of BER performance for the use of DOE will be better.For the chaotic encryption subsystem,we also derive the BER model of the chaotic encrypted subsystem with DOE.Considering that the chaotic mismatch and atmospheric condition will affect the performance of DOE in chaotic encryption subsystem,we conduct a detailed analysis of above problems based on the BER model.Simulation results show that DOE can also reduce the BER of chaotic subsystem by more than 10-1 order of magnitude if the mismatch level is controlled below 10-2.For the effects of atmospheric turbulence,under medium/weak turbulence conditions,DOE can significantly improve the performance of both uplink and downlink.While under strong turbulence conditions,the influence of DOE on BER improvement of uplink is much smaller than that of downlink.This paper investigates the influence of DOE on the performance enhancement and parameter design optimization in a non-encrypted/chaotic encrypted compound space optical communication system.The simulation results are an important reference for introducing DOE into practical engineering.The application of DOE is quite meaningful for decreasing the difficulty of system design,improving system performance and building the low-power and miniaturized satellite terminal.