Study On Low-cost Fiber Grating Sensing Characteristics And System

In this thesis, the fabrication, characteristics and applications of the novel long-periodfiber gratings (LPFGs) induced by focused high-frequency CO₂ laser pulses are studied.The main work is summarized as follows:1.Phase-shifted and cascaded long-period fiber gratings are written byhigh-frequency CO₂ laser pulses in standard optical fiber. During the process offabrication, the computer is used to draw the structural graph of the gratings and tocontrol the power and time of laser pulses exposed on the optical fiber. In such a way,the gratings are made automatically. Because the instant power of laser pulses is highand the exposure time is short, the thermal shock effect on the fiber is obvious. As aresult, high quality phase-shifted and cascaded long-period fiber gratings are fabricatedeasily. It is proved by experiment that this method can be used for standardtelecommunication fiber, so the cost is much lower than that of the UV-induced fibergratings, and their stability is better too. The temperature and strain response of thesegratings are studied and the conclusion is that there can be good sensing components.2.The high-temperature characteristics of long period fiber gratings, which writtenby high-frequency CO₂ laser pulses in common fiber, is measured. When thetemperature is below 800℃,the relation between the wavelength shift of the grating andthe change of the temperature is approximately linear, however, when the temperature isover 800℃,the ratio of the wavelength shift to the change of the temperature increaserapidly, so the relation becomes nonlinear; Experiment shows that the strain response ofthe grating in high temperature is the same. So this grating can be high temperaturesensing component.3.Based on the effect of edge-filter, a novel long-period fiber grating is used assensor to measure static and dynamic strain. The input signal from a light source ofsingle wavelength is attenuated by the long-period fiber grating, and then received by aphotodetector. The loss band of the LPFG will shift in the direction to shorterwavelength when the grating is strained. Due to the effect of edge-filter, the lightintensity received by the photodetector will change in response. As a result, theelectrical output of the photodetector can be used to reflect the strain forced on thegrating. In the static measurement, the result shows that the relation between theelectrical output of the sensor and the strain forms a conic. The resolution of thissensing system is better than 10 με, and the error is about ±10 με. This system canresponse dynamic strain over 5kHz too. This sensor system is low-cost and fast.