The Demodulation Method And Multiplexing Network Of FBG Sensor Based On The Autocorrelation Digital Demodulation Method Using CDMA

Compared with the traditional electromachanical sensors, Fiber Bragg Grating (FBG) sensor has many advantages, such as light weight, small size, wide frequency band, low signal attenuation, high anti-electromagnetic interference ability, excellent corrosion resistance properties, long service life, easy to form distributed measurement system, immune to humid environment, and so on. FBG sensors have received people's attention continuously since its appearance, and have developed rapidly over the past 20 years, its measurement fields includes mechanical parameters, electronic parameters and chemical parameters, almost all-encompassing.However, the FBG sensor's application scope is extremely limited.Because of its complex structure, expensive price and poor maintainability; the FBG sensor could only be used for very special situations and only occupies a very small market share in the sensor market, although it is generally considered an outstangding transducer and its performance is very perfect.Based on the review of its basic and applied researches, the reason that caused above questions was summed up in the shortcomings of the FBG sensor's package, its demodulation method and multiplexing network in this dissertation. In details, (1) the FBG sensor's package, which manufactured the sensitive grating and the conduction optic fibers on the same fiber, decease its reliability and maintainability; (2) the FBG sensor's demodulator, in which the measurement results are obtained by frequency domain analysis entirely dependent on expensive optical devices, reduce its resolution and increase its price; and (3) the FBG sensor's multiplexing network using series structure, which transmissit the lightsource signal and the measuring signal along opposite directions in same fiber, limits its capacity and makes it unextensible. Above reasons increased the FBG sensor's installation costs and use costs, as a result, its application in engeering has been limited.To solve these problems, this dissertation proposed the following innovations:(1) A new digital demodulation method based on signal autocorrelation principle is proposed to detect the wavelength drift of FBG sensors used in engineering with high resolution and to reduce its expensive price.By autocorrelation analysis of photodiode output signal sequence, the digital demodulation method developed from the matched filtering method transform the wayelength measurement of FBG sensor's reflect spectrum into the time measurement of photodiode output electronic signal, overcome the influence of noise while realize high precision detection of wavelength drift. And, this method also has quick scan speed and powerful multiplexing capability by recognize the reflection spectrum of FBG sensors.The numerical simulation results indicated that the digital demodulation method can detect the wavelength drift accurately when the FBG sensors have different range, and can be used when the FBG sensors have the same range. But the precision is low, and has time delay.(2)The CDMA (Code Division Mulitple Access) technology has been introduced into the autocorrelation numeral demodulation method, to reduce its sensitivity to the FBG sensor's initial parameters and simplify its solving process of the nonlinear equations.CDMA is another technology of multiplexing and multi-access besides TDM and WDM, which supports multiple simultaneous transmissions in the same timeslot and the same frequency by coding in the optical domain. Especially, by autocorrelation analysis and threshold judgement, it can restore some user's signal from multi-user's superposition signal received at the same time using its unique address.By coding with Bragg grating, the sensitivity to the FBG'sensor's initial parameters reduced significantly for the reflection spectrum from different measurement point has an obviously difference, and the precision improved significantly for the nonlinear equations have been converted to linear equations.The numerical simulation results indicated that the digital demodulation method realized by CDMA can detect the wavelength drift precisely when the FBG sensors have the same or different range, and has not time delay.(3)Based on the autocorrelation digital demodulating method, two kind of multiplying networks named ladder FBG sensing network and star FBG sensing network are proposed, which have a similar structure to field bus system and can increase its capacity and improve its reliability, maintainability and expansibility.In the ladder FBG sensing network, the light source and FBG sensors are parallel to two independent fibers, which decrease its fault rate and make it easy to expand. At the same time, sensors with same central wave length but different spectrum can be differentiated through the autocorrelation operation of photodiode output signal, which guarantee the measuring accuracy's not be influenced by the measuring point quantity of system.The theoretical analysis and experiment study indicated that the ladder FBG sensing network has a lower fault rate, and the system's measuring accuracy is not influenced by its scale, but lower than calculated value.In the star FBG sensing network using CDMA, every measurement point encoded by FBG has a unique binary code and connects to the autocorrelation digital demodulator by the star network. The lights emitted from a broadband source are reflected by encoding/sensing FBG, and generate a series of discrete spectrum films decided by the measurand.Through a star coupler, the spectrum film from different measurement points are superimposed and send to an autocorrelation digital demodulator, and obtained the measurands finally.The theoretical analysis indicated that the star FBG sensing network using CDMA has good reliability, maintainability and expansibility due to its easy access and flexible network structure, and is suitable for remote measurement because the SNR can be improved by an independent lightsource provided to the measurement point.In conclusion, either the ladder FBG sensing network or the star FBG sensing network can reduce the FBG sensor's installation costs and use costs, and give full play to the FBG sensor's multiplexing ability and its remote measurement ability.(4) Finally, a new kind of general prefabricated strain sensor called FBG strain gauge is designed in order to improve the FBG sensor's reliability, maintainability and meet the engineer's requirements for CDMA coding in engineering. The FBG strain gauge has a separated plug structure. In this structure, the sensitive grating and the transmit optical fiber are designed separately in independent matrixes, and can be plugged in each other to realize the mechanical connection.Theoretical analyses indicated that the FBG strain gauge has the same reflection spectrum with FBG grating, and successive experiments indicated that it has good linearity and high sensitivity, whereas the temperature error changed with the specimen's changing. When the specimen and the strain gauge's matrix are made by the same material, the temperature error can be ignored.