Long Period Fiber Grating Based On Bending Fiber Characteristics Research

Prof. Rao, the supervisor invented the fabrication method of long-period fibergratings （LPFGs） in the single-side of fiber by high-frequency CO₂laser pulses in2000.In the past several years, the fabrication method, formation mechanism, basedcharacteristics and applications have been investigated and gained some prominentachievements. In recent years, with the demand for information explosive increased up,FTTH has made great progress, thanks to the invention of bend-insensitive fiber. Witha large number of bend-insensitive fiber to the end of the optical fiber communicationsystem, to study the sensing properties is particularly important and useful.A new kind of LPFG was written in the bend-insensitive fiber by high frequencyCO₂laser. We studied the sensing properties of the LPFG and have made a certainamount of innovative research results.The main content is as follows:1) We successfully written the LPFG to the bend-insensitive fiber with nano-ringstructure by high frequency CO₂laser. The temperature sensitivity compared withordinary LPFG has increased nearly triple. And we found different period of theresonant peak has differences strain sensitivity. With the difference temperature andstrain characteristics of the different period LPFG, the simultaneous measurement of atemperature and strain sensor structure was reported.2) We wrote the LPFG with specific size of tension by high frequency CO₂laser.This LPFG had two resonant peaks. And we found the temperature and straincharacteristics of the resonant peaks ware difference. Then we proposed a sensorstructure based on a single long period fiber grating for temperature and strainmeasurements at the same time, and applications for invention patents.3) We studied the bending and twisting characteristics of the new LPFG. Thebending sensitivities ware difference in the difference directions. It was very larger in aspecial direction such as CO₂laser write direction. And in the other direction it was low.The difference period LPFG had difference twisting characteristics. The low order modeof resonance peak was significantly sensitive with the direction of twist. The higher order mode of resonance peak was insensitive. Resonant wavelength distortions werecyclical ups and downs.4) Paper using a small number of cycles successfully written long period fibergrating, the resonant peak bandwidth greatly improved, to reach240nm, from thetransmission spectrum can be seen across the1310nm and1550nm communicationband. It was able to measurement temperature with the edge of the filter effect. Thissensor system is simple structure, low cost, fast response, large measuring rangesuitable for high temperature measurement.