Optical Demodulation Method Of Microwave N-DPSK Signal

DPSK is one of the basic digital modulation methods.Traditional electrical demodulationDPSK signal depends on the multiplier,and its bandwidth is limited by the nonlinear response speed and efficiency of the diode and the transistor,which has approached the theoretical limit.Microwave photonic technology is used to process microwave signals in the optical domain.It has the advantages of wide band,low loss and no electromagnetic interference,and can overcome the"electronic bottleneck"problem.Therefore,the main work of demodulating microwave DPSK signal by optical method is as follows:(1)Firstly,from the background of microwave photonics,the advantages of microwave photonic technology compared with traditional microwave technology are explained.Then,the methods and existing problems of microwave DPSK signal demodulation are summarized.(2)In this paper,the microwave 2DPSK signal is transferred to the optical carrier phase by optical phase modulation,and an optical demodulation method of microwave2DPSK signal is proposed.The optical power after coherent superposition of the optical field has the feature of the product term of the optical field,so as to avoid using a multiplier.According to the structure of the system,the working principle is deduced and the feasibility of the system is verified by simulation.The influence of system error on demodulation signal,including delay deviation,laser frequency drift and optical carrier,is studied.Deviation?T/T₀=1/6 at that time delay,or laser frequency drift?f/B=1/6,3dB power to be compensated.The additional phase compensation can effectively reduce the optical carrier's influence on the demodulation signal.The simulation results show that the bit error rate of this scheme is basically the same before or after optical fiber transmission.(3)On the basis of the demodulation of 2DPSK signal,the optical demodulation methods of nDPSK signal are also proposed.Two structures are proposed to demodulate multibase DPSK signals.The first one uses an Asymmetric Mach-Zehnder interferometer(AMZI)structure,in which the output optical power of each differential phase value is different by offset of the additional phase value.Through calculation and simulation verification,DQPSK signal can be converted into four level intensity modulated signal with equal spacing when the deflection Angle is tan (??)=1/3.The relation between the bit error rate and the additional phase is calculated.For 8 DPSK cannot get strict evenly spaced intensity modulation,when??=10.32°can minimize the distance variance to 0.002.The effect of different additional phases on the received signal is verified by simulation.The second method utilizes the characteristic that the highest level and the lowest level correspond to the phase difference in the demodulation of 2DPSK signal,and a single AMZI link can detect two phase differences.Parallel n/2 AMZI links,each AMZI deflects at different angles to alternately reach the highest or lowest level of the two phase differences,and finally determine N phases jointly.After theoretical analysis,DQPSK signal needs two parallel channels,each additional phase is 0 and?/2.The 8DPSK signal requires four parallel channels,each with additional phases of 0,?/4,?/2,and 3?/4.The simulation results show that DQPSK signal and 8DPSK signal are demodulated successfully by this method.