Dissertation > Industrial Technology > Radio electronics, telecommunications technology > Photonics technology,laser technology > Optical waveguide and integrated optics > Fiber optic components

The Basic Research on Chemical Sensing of Fiber Bragg Gratings

Author QinYing
Tutor QianYing
School Jilin University
Course Microelectronics and Solid State Electronics
Keywords Optical fiber Bragg Grating Chemical corrosion Refractive index sensitive
Type Master's thesis
Year 2006
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Fiber grating is a new kind of all-fiber passive device thatresults from that the refractive index of optical fiber core isperiodicity microperturbation. Because of the excellent propertiessuch as compatible with optical fiber, easy fabrication, littleinsert loss, small volume and low price, fiber gratings have wideapplication in optical fiber communication and sensor field. Thus,since the successful fabrication of first fiber grating, thefabrication and application of fiber gratings is central issue ofinterest all along.Fiber Bragg grating (FBG) is one type of reflection gratingswhose reflective wavelength depends on the product of grating’seffective refractive index and grating’s period, and grating’sreflectivity and bandwidth depend on the change in refractive indexalong grating and grating’s length. Then, designing grating’speriod and modulating the change in index can fabricate Bragg fibergrating with different reflective wavelength and reflectivity;inaddition, Bragg fiber grating has especially sensitivity to thetemperature and strain. It is these unique properties that makeBragg fiber gratings have unique applications both in opticalcommunication and sensor field, such as filters and sensors.This article is mainly about chemical sensing characteristicsof fiber Bragg gratings.We have explained the reason of chemicalsensing characteristic of FBG in the theory, and we have done lotsof experiments to test our theory.At the same time, we haveintroduced some advanced research in the FBG’s chemical sensingcharacteristic in the world, and brought forward the realisticvalue of our research.Firstly, We deduced waveguide modes of single and multimodefibers by use of Maxwell equation.We deduced briefly fielddistribution of fiber gratings using couple mode theory.We strictlydeduced couple mode theory of FBGs using weak waveguideapproximation, and the relation of wavelength and grating periodwas obtained, which is the useful tool for analyzing optical spectraof FBGs.Secondly, based on all the theories described above, we designand fabricate some FBG with different period.Through a series ofexperimental fabrications, we summarized factors of affectingspectra of FBGs including the length of FBGs, grating period, orderof couple modes, refractive-index changes of fiber and the quittingof Hydrogen.FBG cannot be directly used as gas or liquids sensors, becauseit is not sensitive to external refractive index. By a wet chemicaletch-erosion procedure, the cladding layer of FBG can be reducedto several micrometers. The radiation field can reach the externalmedium. When the external refractive index changes, the effectiverefractive index of fiber changes and the center wavelength of theetch-eroded FBG shifts accordingly. Thus,the fiber Bragg gratingcan be used as the chemical sensor to measure solution concentrationor index.Then we focused on the experiments of chemical sensingcharacteristic of fiber Bragg gratings.According to the experimentresults, we drew pictures of wavelength and index.At the same timewe put the same fiber Bragg grating in different solution, andobserved the shift of resonance peaks. In the experiment ,the NaClsolution with different concent ration is used to change theexternal refractive index of FBG. The center wavelength ofetch-eroded FBG shifts when the concent ration of NaCl solutionchanges. The relationship between the off set of wavelength and theconcent ration of NaCl solution is approximately linear. Inaddition , the relationship between the transmit ting power and theconcent ration of NaCl solution is al so approximately linear.In industry, sometimes people need measure concentration ofsome dangerous solution.Fiber Bragg gratings sensor are verysuitable to this measurement.Along with development of study offiber gratings, the application field of fiber Bragg gratings willbe broader. The properties of fiber Bragg gratings will be usedbetter.

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