| Quantum key distribution has always been the most important research direction in the field of quantum communication.Its emergence solves the security problem in the communication system based on the classical cryptosystem.After several years of research and development,quantum key distribution technology has made great progress in theory and experiment,and is gradually becoming practical and commercial.In the quantum key distribution system,relying solely on the quantum characteristics in physics can not strictly prove the stability of quantum key distribution.It also needs to be combined with the key distribution protocol.BB84 protocol is a relatively complete and widely used protocol in the current research,and its security has been mathematically proved.The BB84 protocol requires that the light source must be a single photon source,and the preparation process of the single photon source is quite troublesome,but it can be replaced by the coherent superimposed light source with strong attenuation.When the system loss of the weakly coherent superimposed light source reaches a certain threshold,it will be threatened by the single photon number splitting attack(PNS),At this time,a decoy states scheme is proposed to enhance the stability of the system.The BB84 quantum key distribution system using decoy state needs to modulate three kinds of quantum states: signal state,decoy states and vacuum states.In the quantum key distribution system,an electro-optic intensity modulator is used to externally modulate the quantum light source,and the light intensity ratio of the control signal states to the decoy states is 10:1.However,due to the influence of temperature,mechanical vibration and other factors,the working point of electro-optic intensity modulator is very unstable,which is easy to cause the slow drift of the working point.In the process of information transmission,quantum key distribution system can not scan the correct offset working point to ensure that the light intensity ratio of signal states to decoy states is 10:1,which leads to the instability of the system and potential security problems.Therefore,it is necessary to control the correct working point of the electro-optical intensity modulator,which has great academic value and engineering significance for the security and stability of the quantum secure communication system.In order to solve the problem of working point drift of electro-optic intensity modulator,a bias control scheme based on phase-locked amplifier is proposed to realize decoy state modulation with high precision and stability.Firstly,the modulation principle of electrooptic intensity modulator and the reason of bias working point drift are analyzed;Then,the hardware circuit and logic design of the bias control system are carried out respectively;Finally,the functional modules are verified by simulation,and the bias working point stability control platform of electro-optic intensity modulator is built,and the relevant experiments are carried out.Specific implementation of the system: firstly,the optical pulse is generated by the high-speed picosecond laser,and the driving signal generated by the highspeed Ser Des(serializer deserializer)of FPGA(field programmable gate array)drives the electro-optical intensity modulator to modulate the generated optical pulse,modulate the signal state optical signal and decoy state optical signal,and then use the high-speed and high-precision analog-to-digital converter(ADC)to collect the modulated signal,And draw the curve through data processing,and then judge the bias voltage value and RF(radio frequency)value corresponding to the light intensity ratio of the best signal state and decoy state through data analysis.At the same time,according to the data drawn curve collected by the ADC after the lock-in amplifier,the reference value of the lock-in amplifier can be determined,and the determined bias voltage value and RF value are given to FPGA.Finally,FPGA drives the electro-optical intensity modulator to randomly modulate a certain number of signal states For the optical pulse in decoy state and vacuum state,when the system is running in real time,the actual phase-locked amplifier value is compared with the reference phase-locked amplifier value,and the above obtained data is transmitted to FPGA.After FPGA calculates and loads the voltage to the bias voltage end of electro-optical intensity modulator,the automatic compensation of light intensity can be realized.The Verilog code is simulated and verified by vivado software to ensure the basic realization of the functions of each module.Through the operation of the whole control system,it is concluded that the bias control system can maintain the output optical power in a stable range and the extinction ratio is stable at 31 d B;The debugging of IM bias self compensation verification board shows that the im bias self compensation control scheme based on phaselocked amplifier can keep its DC working point constant at the set value in the long-term operation of IM;The high and low temperature test of IM bias self compensation shows that the im bias self compensation control scheme based on phase-locked amplifier can still ensure that the three state return ratio of the receiver remains constant when the im working temperature changes greatly. |
