Design And Realization Of Double-Sideband Optical Signal Source Used In BOTDA

With the continuous development of the world economy,the sensors are becoming more and more important.Optical fiber sensors are widely used because of their high sensitivity,anti-electromagnetic interference and other excellent characteristics.The technology of BOTDA is one of the research hotspots in the field of optical fiber sensing.The double-sideband optical signal source which is responsible for generating continuous detection light is quite important to the overall performance of the system.Based on the requirement of BOTDA system,the thesis completes the software design of the double-side band signal source control system.The specific research results are as follows:1.Based on the main control chip and RF chip,the microwave source module is completed.The main chip establishes communication with the RF chip through SPI protocol.It completes the operations of initialization and VCO calibration by configuring relevant registers through flow process,realizing the functions of phase lock,frequency sweep and so on.The RF signal generated by the RF chip passes through the two-grade filter and amplification circuit,then becomes the modulation signal required by the electro-optic modulator.2.Based on the main control chip and auxiliary circuit,the electro-optic modulation module is completed,and the simulation of harmonic calculation in the module is accomplished.The lithium niobate electro-optic modulator used in the signal source system is quite sensitive to the external temperature,current and other environmental factors.The zero drift caused by environmental changes will seriously influence the modulation effect.In order to solve this problem,the low-frequency disturbance signal control method is adopted in the system.The jitter signal is introduced into the modulator continuously,and at the same time the fundamental and second harmonic components of the output signal are collected by AD module and calculated based on DFT algorithm by the main control chip.The system adjusts the input of the bias voltage according to the calculation result and control strategy,realizing the throughout monitor and real-time adjustment of the voltage bias point.This closed-loop control mechanism with feedback ensures that the modulator is always in the state of carrier suppression.3.Based on I2C protocal,the communication module which is mainly used for information exchange between boards is completed.After the main board sends out relevant instructions,the receiving thread in the software system starts the process of decoding and verification.If no error is detected in the verification,the receiving thread sends the corresponding semaphore to realize specific functions.The network management protocol of the module establishes the mapping relationship between specific instructions and codes,which greatly improves the system efficiency.4.With regard to the system response delay caused by the external materials of the optical fiber in the optical fiber sensing system such as BOTDA,the thesis conducts the study with G.652 bare fiber and the same type of optical fiber with protective layer.A rapid measurement system of ROTDR(Raman optical time-domain reflection technology),which adopts anti Stokes optical demodulation method to get the sensing temperature,is used and the real-time measurement temperature of the thermometer is taken as a reference.The system response time corresponding to bare fiber and optical fiber with protective layer under different temperature changes is studied,and the temperature response time delay caused by the outer layer of optical fiber to optical fiber sensing system is shown visually.