Research On High Speed Picosecond Pulsed Light Source

Quantum cryptography is a password system based on the theory of quantum mechanics,and it is a password system of unconditional security which is through the "one-time-pad"(OTP)method of encryption.Since the ideal single-photon source is difficult to prepare,a weak coherent light source is used in the practical quantum key distribution(QKD)system.As a multi-functional weak coherent light source,high speed picosecond pulsed light source can be applied to the experiment and test in the actual QKD system.In this dissertation,the working principle and characteristics of semiconductor lasers are analyzed,and the gain switch principle of generating ultra-narrow light pulses is described.The hardware driving circuit of DFB laser is designed,which is based on the operating characteristics of laser and the demand of high speed picosecond pulsed light source.It mainly includes DC bias circuit,narrow pulse drive circuit and temperature control circuit.In addition,the integrated driver chip is applied to design DC bias circuit and narrow pulse drive circuit.Among them,the DC bias circuit is essentially a voltage-controlled current source circuit,and deep negative feedback is used to ensure the stability of the output current.Narrow pulse drive circuit includes three parts which are input trigger circuit,narrow pulse generation and modulation drive.The temperature control circuit is a closed-loop feedback system which consist of a thermistor and a thermoelectric cooler(TEC)in the laser,and an external circuit.It is FPGA of Altera's Cyclone III series that is chosen as the master in the board of light source;The master achieve the chip drive control,position PI algorithm of temperature control and UART communication.Finally,the design of the electromagnetic compatibility for the PCB is carried out.For the two kinds of driving scheme of the light source,the optical pulse waveform and the spectral characteristic are tested under different operating modes.Experiments and tests show that the high-speed picosecond pulsed light source whose pulse width can reach 65 ps and frequency of trigger can reach 1.25 GHz,can meet the requirement of performance of light source for the experiment and test in the practical QKD system.