Characteristics And Research On Sensing Of Local Microstructured Fiber Grating

Fiber grating is a new-type sensing element and has gained abroad attention from it’sappearance and achieved high-speed development in the1980s. In the recently years, with thedevelopment of fiber grating technology, study on the application of FBG sensor has got morerecognition in optical communications.Recently, the devices based on the structure of photonic band gap, such as localmicrostructured fiber Bragg grating (LMSFBG), attracts more and more attention. Because ithas a special structure which let lights in different frequencies pass in the reflection. The cladof FBG was etched partly which produced the structure of non-periodic distribution.Therefore, a passband in the stopband came into being similar to phase shift grating. So localmicro-structured grating has a unique advantage in sensing in microfluidics, biochemistry, andchemical research.This paper primarily researches on local microstructured fiber Bragg grating and localmicrostructured long period grating (LMSLPG), theoretically and experimentally analysestheir temperature and surrounding refractive index sensing principle and applications. Themain content is:1) In Chapter1, the history and research status of FBG and LMSFBG is depicted. A briefintroduction to the system of fiber grating writing method is shown. Describe FBG andLMSFBG sensing advantages and practical problems which must be faced.2) In the second chapter, several theory analysis methods including the coupled-modetheory, transfer matrix approach for properties of fiber gratings are presented. Their owncharacteristics and scope of application are analyzed. On this base, we use Comsol tocalculate the phase shift of LMSFBG and get the spectrum transfer matrix of Matlab.The asymmetrical structure, ladder-type structure, both sides etched structure ofLMSFBGs are simulated, and characteristics and application are analyzed.3) An alternative solution for simultaneous measurement of temperature and refractiveindex is presented. A local micro-structured fiber Bragg grating (LMSFBG) as thesensing head is formed that a standard grating is etched by hydrofluoric acid (HF).According to the phase shift theory, the main spectral change of LMSFBG is theformation of a narrow allowed band. Ues LMSFBG to measure the temperature andrefractive index of liquid. The temperature-sensitive coefficient and the SRI-sensitivecoefficient of two reflection peaks is about0.0097nm/°C and510nm/RIU. Thetemperature-sensitive coefficient and the SRI-sensitive coefficient of transmission peakis about0.0116nm/°C and528nm/RIU. So the reflection peaks and transmission peakof local micro-structured fiber grating have a good linear relationship with temperatureand refractive index. LMSFBG can also be used in the frequency selecting devices of dual-wavelength laser.4) The history of etched long period grating is depicted. We simulate the LMSLPG andanalyze the cladding thickness on the sensitivity. LMSLPG can show manycharacteristics by changing the size and location of phase thift so it can be used in manyspecial filters. In experiment, we make a LMSLPG to measure temperature andrefractive index of liquid. The sensitivities of temperature and refractive index are0.058nm/°C and2691nm/RIU respectively and are more accurate compared withFBG.