Design Of Transmitting And Receiving Circuit For Frequency Coded Optical Fiber Ring Sensor

The frequency coded optical fiber ring sensor based on intensity modulation technique is a novel distributed optical fiber sensor with simple structure, easy demodulation and stable performance. The sensors can be used to measure temperature, stress, strain and other engineering applications in multiple measurement points. This thesis focuses on the design of the core module of the frequency coded fiber ring sensor, the frequency coded transmitting and receiving circuit, including the laser diode(LD) light source encoding frequency modulation circuit, photoelectric weak signal detection circuit and frequency encoding demodulation circuit.This thesis is made up of two parts:The first part introduces the research status and development direction of optical fiber sensor and the principle of frequency coded optical fiber ring sensor. This thesis describes the time division multiplexing, wavelength division multiplexing and code division multiplexing technology in optical fiber sensor networks, at the same time,the frequency coded multi sensor quasi frequency division multiplexing technology is introduced. The theoretical models of the stress sensing and the temperature sensing of the frequency coded optical fiber ring sensors are analyzed. The structure principle diagram and the principle block diagram of the transmitting and receiving circuit of frequency coded optical fiber ring sensor are presented. The characteristics of the reflected signal of the sensor array are analyzed.In the second part, the design of the frequency code transmitting circuit and the receiving circuit is introduced. The frequency modulation circuit for semiconductor laser(LD) light source is designed. The LM339 voltage comparator is used as an automatic power control circuit to stabling output of frequency coded modulation laser power. A weak signal detection and demodulation circuit for optical fiber ring sensor is designed. Super heterodyne frequency tracking and demodulation technology is used for frequency demodulation. The frequency information of the envelope signal peak position is obtained which reflects the change of temperature and stress. In the circuit design, narrow band filter is an active circuit using LC parallel frequency selective network as load circuit. The Pearce quartz crystal oscillator circuit is used as the intermediate frequency oscillator. In addition, ADI’s single-chip integrated chip AD603, AD831, AD8318 are used in the design as mixer circuit, IF amplifier circuit and detection circuit. These chips simplify the external circuit, and improve the signal to noise ratio(SNR) and sensitivity.