Interrogation Technique For Diametric Load On Fiber Bragg Grating

Fiber grating is a novel passive photon device. A fiber Bragg grating (FBG) is a periodic modulation of the refractive index within the core of an optical fiber and acts to couple a forward propagation fiber core mode to a backward propagation core mode at a particular wavelength. Sensors using FBG as the sensing component have many advantages including light freight, small size and anti-electromagnetism interference etc. In particular, they have a self referencing capability and can easily be multiplexed. Multiple point detection of materials and structures can be achieved by connecting several fiber Bragg grating sensors into a net using variety of multiplex techniques. The so called distributed sensing is a unique technique of fiber grating sensors. Since their introduction in the late 1980, fiber Bragg grating sensors have proved to be particularly promising for applications in process and health monitoring in materials and structures, due to these advantages over other existing fiber optic sensors. Fiber Bragg grating have high sensitivity to temperature and strain with good linearity over quite a large measurement range. Applications of fiber Bragg grating sensors for axial strain and temperature sensing have been intensively investigated. At present, fiber Bragg gratings are mostly used as axial strain sensors. However, for structural monitoring applications, it is often desirable to measure strain components transverse to the optical fiber in addition to the axial strain. Thus, it is important to correctly evaluate the characterization of the response of fiber Bragg grating subjected to a diametric load. On the other hand, the diametric size of the fiber Bragg grating is much smaller than the axial size, so it is much more promising for the fiber Bragg gratings to be used as diametric load sensors.One of the key issues in designing an fiber Bragg grating sensing instrument is the selection of a suitable wavelength demodulation scheme in terms of its resolution, its reliability and both its initial and maintenance costs. In this thesis, the characterization of the response of fiber Bragg grating subjected to a diametric load was studied and the diametric load sensitivity of fiber Bragg grating was demonstrated. A demodulation system for measuring the Bragg wavelength splitting changes using a electronically tunable Fabry-Perot filter was also established, which could eliminate the deviations caused by the environmental influence on the initial length of the cavity of the Fabry-Perot filter, and hence a resolution of 0.02nm could be obtained. For the advantages of easily made and low-cost, it is promising for engineering applications.Analysis: A fiber Bragg grating is a periodic modulation of the refractive index within the core of an optical fiber and acts to couple a forward propagating fiber core mode to a backward propagating core mode at a particular wavelength given byλ_B = 2n_effΛ(1) Whereλ_B, n_eff=1.45 andΛare the reflected Bragg wavelength, effective refractive index and the grating period, respectively. A change in the effective refractive index and/or the grating period will cause a shift in the reflected Bragg wavelength.Bragg gratings are normally used for axial strain measurement, in which case the response is well known and can be expressed as whereΔλ_B is the Bragg wavelength shift due to strain,ν=0.19 is Poisson's ratio of the fiber,ε_Z is the applied axial strain, and P1 1=0.113 and P1 2=0.252 are the stain-optic coefficient.When a fiber Bragg grating is subjected to a diametric load, a stress state exits in the fiber Bragg grating,which is given byσ_X=2F/πhD andσ_Y=-6F/πhD (3)Here the diameter of the disk D is 125μm, and the h is the length of grating which is 10mm. Once the stress is known, the values of the strain state can be found from Hooke's law for plane strain.ε_X = (1+ν)/E[σ_X (1 _ν) _νσ_Y] andε_Y = (1+ν)/E[σ_Y (1 _ν) _νσ_X] (4)In this equation E=70GPa is Young's modulus. According to the spring effect, the two different diametric strainsεX andεY will make the effective refractive index changes and splits into two values. This appearance of birefringence leads to the separation of the single reflected Bragg peak into two distinct ones. The wavelength shifts are given byΔλ_B,X/λ_B =Δn_eff,X/n_eff = -n_eff²/2(P₁₁ε_X + P₁₂ε_Y) andΔλ_B,Y/λ_B =Δn_eff,Y/n_eff = -n_eff²/2(P₁₁ε_Y + P₁₂ε_X) (5)From these equations we find the peak separation in this case is proportional to the diametric load F: The theoretical change in wavelength separation per change in applied load is 0.00785nm.It is well know that the transmission of an ideal Fabry-Perot filter, is governed by the transfer function of the Fabry-Perot cavity, and is thus given by R is the reflectivity of the filter andΦ= 4πL/λ(8) whereλis the wavelength of the filter transmission spectrum. This can be written asλ=λ₀ +Δλ, whereλ₀ is the central wavelength of the filter transmission spectrum,Δλis the variation of the wavelength. L is the separation of the cavity, which itself can be written as L = L₀ +ΔL, where L₀ is the initial separation, andΔL is the scanned distance of the filter.The spectrum of the reflected light from a Bragg grating can be described by a Gaussian function, and this is given by: I (λ) = exp{-4 ln 2[(λ-λ_B) /δλ]² } (9) whereλis the wavelength of the reflected spectrum,λ_B is the central wavelength, andδλis the spectral bandwidth(FWHM).When the Fabry-Perot filter is tuned to scan over the spectrum of the light reflected from a Bragg grating, the transmitted optic power, S, at the output of the filter is a function of the scanning distance of the filter, and is given by when the wavelength of the filter transmission spectrum matches the central wavelength of the reflected spectrum of the fiber Bragg grating, the transmitted optic power gets its maximum intensity.In the computer simulations, the initial cavity separation of the filter is set to be 464.7μm, giving a wavelength of 1549nm at the center of the peak transmitted region of the filter, from which the separation is increased with a step size of 10nm over the region of 600nm, yielding a scanned spectral range of 2nm. The free spectrum range of the filter is 2.4nm. The spectral width and the central wavelength,λ₀ , of the reflected light form the grating are set to be 0.2nm and 1550nm respectively. The wavelength,λ, is integrated from 1549nm to 1551nm. The normalized transmitted optic power when the the reflectivity of the filter, R, is 0.8, 0.85, 0.9 and 0.95 respectively is shown in Fig.1. When a diametric load is applied to the fibre Bragg grating, and the scanning frequency and the scanning distance of the filter remains the same value, the wavelength separation of the reflected spectrum of the fiber Bragg grating, ?λB=?λB,y- ?λB,x ,could be obtained by calculating the elapsing time when the transmitted optic power getting its maximum intensity, ?T . This could eliminate the deviations caused by the environmental influence on the initial length of the cavity of the Fabry-Perot filter.Experiment: The schematic diagram of this experiment is shown in fig.2. Broadband light from a LED amplified by an Erbium doped fibre amplifier is coupled into the fibre Bragg grating through a single-mode coupler. The reflected light from a Bragg grating is sent into the Fabry-Perot filter through a collimator. The transmitted light is detected by a high-speed InGaAs photo diode and simultaneously sampled via a data acquisition (DAQ) card. The data is analyzed with the use of a PC.The arrangement for the application of diametric load to the fiber Bragg grating is shown in Fig.3. The fiber Bragg grating is placed between two glass plates. A second fiber of the same type is placed aside the first one to ensure that the two glass plates are parallel. The ball situated between the lever and the aluminium plate ensures the transmission of a uniform load from the suspended mass to both the fibers. Results: The Bragg wavelength splitting changes as a function of the applied diametric force are shown in Fig.4. The diametric load sensitivity of fiber Bragg grating is 0.0085nm/N,which is in good agreement with the theoretical result.