Research On Measurement Of Non-uniform Distribution Of Refractive Index Based On A Fiber Gratings

The refractive index (RI) senor based on a Fiber Bragg Grating(FBG) is a sensor thatis used to measure the RI outside distributions of grating, whose sensing element is FBG.Because of its tiny size of the senor and the higher measuring precision, it becomesincreasingly an important research aspect in RI measuring. With the fast development ofmany fields such as biomedicine and biochemistry, the demand for measurement scale andprecision of physical parameters such as RI is enhanced. It has been an important issue byobtaining biological information of tiny scale in related research fields.For now, the research based on RI sensing of FBG is heavily focused on thedistributions of homogeneous RI, whose sensing depends on monitoring the drift of thecentral wavelength of grating resonant peak.When RI distribution of external medium isheterogeneous, the resonant peak of fiber grating shifts, and other response characteristicmay occur. However, these response characteristics which could show the outsidedistributions of RI haven’t attracted abundant attention and analysis in previous studies.In this context, the paper studies thoroughly the response characteristics of thetransmission spectrum of a novel long period fiber grating when distribution of externalmedium is heterogeneous. The paper also summarizes the change rules and the relativemechanism. Then a more complete response characteristic of long-period fiber gratings(LPFG) in liquid medium of RI is got.The main content of this paper is as follows:(1) To the RI senor of LPFG by stripping the cladding totally, two-layer fiber model isbuilt, and transmission spectrum of this model of the distribution in different RI gradient iscalculated. The change rule of the damping peak of transmission spectrum has beenanalyzed. Then the transmission spectrum of response characteristic of LPFG by strippingthe cladding is got.(2) The transfer matrix method and mode coupling theory is used to make a study onthe theories of two-layer fiber model. The reasons for all kinds of response characteristic oftwo-layer model are analyzed. And the paper emphasizes on mechanism of the full width athalf maximum (FWHM) of the damping peak of transmission spectrum has a good linearrelationship with RI gradient of external media.(3) To improve practical application performance based on the RI senor of LPFG, the mechanical strength and production processes are factored in. And the three-layer fibermodel has been built. Transmission spectrum of this model of the distribution in differentRI of external media is calculated. And the change rule of the damping peak has beenanalyzed. Then the transmission spectrum of response characteristic of LPFG is got, whoseconstruction is well.(4) According to the relative principle of optical, the transfer matrix method and modecoupling theory is used to make a study on three-layer fiber model. The producemechanism of the model in all kinds of response characteristic is elaborated. Besides, sidelobes of fiber gratings and what RI gradient of external media is like when it is increasingare analyzed emphatically.(5) It summarizes the advantages and disadvantages of two different structures of RIsensing models based on LPFG. Two sensors may be met in a practical application fromdifferent angles is also analyzed. Then the directions for the further designing and makingthe sensors of inhomogeneous reflective index based on LPFG are suggestedThe measuring method of non-uniform distribution about liquid medium of RI, whichis studied in this paper, can provide an effective measurement in many study fields such asbiomedicine and biochemistry. It lies down a significant theoretic foundation for furtherstudy on the sensor of inhomogeneous reflective index based on LFPG, and there is someguidance in realizing the measure of the RI under micro scale.