Design Of A Digital Light Source For Fiber Bragg Grating Sensor System

This thesis is focused on light source technology of fiber Bragg grating (FBG)sensor system. There are two major types of light sources of FBG sensing system,broadband light source system and narrowband tunable laser light source system.Broadband light source has some disadvantages, such as low output power and lowsignal-to-noise ratio. The demodulation accuracy of sensor signal is not enough. Narrowbandwidth light source does not have this deficiency, but there are high requirements forstability and accuracy of the laser output. This article designed a digital light source toachieve high precision tuning. The tuner precision is0.91pm. We can locate thetemperature changes by temperature drift Algorithm and high-precision voltagescanning. Compared with the conventional method of atomic absorption line, thismethord has prominent advantages in terms of cost and practicality. At the same time,temperature shift algorithm solved the sensing cross-sensitivity problem of sensingFBG.First, by analyzing the light source in the FBG sensor, we build a tunablenarrowband digital light source. Fiber Fabry-Perot filter (FFP-TF) is used in the lightsource system as wavelength tunable device. The voltage of Digital-to-AnalogConverter is regulated by digital signal processor (DSP) to control the output of thelaser signal. In order to solve the temperature drift, a voltage scanning method isdesigned. The relative wavelength drift of FFP-TF and reference FBG can be calculatedthrough the lock-in amplifier algorithm. By the above method we can identify changesin temperature, to achieve the calibration of the light source.Secondly, the paper designed a precision voltage control panel. The main chip ofcircuit board is a digital-to-analog conversion chip AD5791. Reference voltage of3.4Kilohms resistors is bufferred by external buffer amplifier. Through the proper selectionof the filter circuit, the output voltage accuracy reached0.228mV and the integralnonlinearity of DAC is12least significant bit (LSB) and the voltage differentialnonlinearity of DAC is2.5LSB high precision. The high precision reference voltagesource and the specifications of the printed circuit board is the key of DAC. Input andoutput interfaces used to drive the digital-to-analog converter. Triangular wave orsinusoidal signal is produced to control the laser output wavelength. A wide range of thelaser is pruduced by digitizing circuit.Finally, the stability and temperature drift of the tunable laser is measureddetailedly. FFP-TF of the temperature drift curve and the reference FBG temperaturedrift curve are measured in the experiment. The relative drift of FFP-TF and FBG wavelength is recorded. The temperature change can be calculated by temperature driftalgorithm.