Research On The Responsive Characteristics Of The Etched FBG Under Liquid Medium

Fiber Bragg grating (FBG) has many applications in optical fiber communication and opticalfiber sensing, and is a very important passive fiber optic components. This article studies thereflection spectra of the etched FBG under liquid medium by used MATLAB and COMSOLsimulation software.The main contents are as follows:1．This article describes the history of the development and classification of FBG and theresearch status of the etched FBG. Several methods of FBG theory is briefly described and use oneof the methods to analyze FBG with different structures when they under liquid medium.2. The reflection spectrum characteristic of three kind of etched FBG is be studied when theyunder isotropic liquid medium. It founds that with the decrease of the thickness of the claddinglayer etched FBG is more sensitive to changes in the external refractive index, the closer therefractive index of the liquid medium of the cladding dramatic changes in the Bragg wavelength.Single-ended FBG will form a reflection peak refractive index modulation by the liquid medium onthe basis of the original reflection spectrum of the grating, two reflectivity peaks proportional to thesize of the respective share of the grating area. Local etched FBG due to the introduction of thephase shift, so generating a narrowband transmission in the original reflection peak, this position ofthe narrowband transmission affected by the length of the etched area and effective refractive indexdifference. And the position of etched also affects narrowband transmission, when it in the middleof the grating region the maximum transmittance occur, when the etched area move to the gratingedge the transmittance decreases until narrowband transmission disappears.3. The reflection spectrum characteristic of the etched FBG is also be studied when they underanisotropic liquid medium. When substitution layer is etched completely it is equivalent to the cladanisotropic grating, in this case the refractive index of the liquid medium than the two principalaxes kclaffect grating reflectivity. The changes of the reflectivity more intense near kcl=1, afterfurther observed, under the conditions of the given parameters, the grating reflectivity is almost0when kcl=1.018, and then quickly recovered to1when kcl=1.024, this phenomenon can be used toachieve an optical switch technology. Finally it discusses the refractive index of two axis directionsof the liquid medium influence on the reflection spectrum of the grating，found that with theincrease of the refractive index of the two axis directions, the grating center wavelength moves to the long wavelength direction; with the decrease of the refractive index of the two axis directions,the center wavelength of the grating is moved towards shorter wavelengths. However, the centerwavelength of the grating by a lateral refractive index greater impact, the impact on the longitudinalrefractive index of the grating center wavelength is relatively small.