Auto Bias Control For Mach-zehnder Modulator And Its Application In BOTDA System

Distributed optical fiber sensor has some common advantages with conventional fiber optic sensor, such as light weight, small size, corrosion resistance, electrical insulation, anti-electromagnetic interference ability, non-invasive and so on, this technique also can test hundreds of thousands of measurement points, Because of those advantages, distributed optical fiber sensor has broad application prospects.in structural health monitoring, early warning project, national defensing, energy exploring and so on. Laser modulation with electro-optic modulation is the core technology in distributed optical fiber sensing, which directly affects the system’s measured distance, spatial resolution and measurement accuracy. Due to the EOM is susceptible to environmental factors, its working point will drift, thus affect the performance of the fiber optic sensing system.To solve the problem, we proposed an algorithm based on step and expert PID. Introduce the theory of how EOM works, built auto bias control system and applied the system in differential pulse Brillouin optical time domain analysis(DPP-BOTDA) System, the detail as follows:1. Study the theory of EOM’ characteristics, then simulated the theory. Analysis the reason of EOM working point drifts, and the influence in modulated signal of working point drift. Research on the theory of pulse modulation and microwave modulation in BOTDA, and simulated.2. Design an auto bias control system, and experimental verification. The control system includes hardware and software components, hardware includes signal acquisition module, signal processing module, bias voltage output module and power supply module; software includes hardware control and data processing algorithms. Use the auto bias control system control one EOM, study the result of the system, including the lock-up time required, the accuracy and stability of the system.3. Applied the auto bias control system in DPP-BOTDA system. Use BOTDA system testing cantilever’s deformation, dispose the collected data, analyze Brillouin gain spectrum obtained a three-dimensional view and a center frequency of Brillouin curve.