Fabrication And Application Of Chirped Fiber Grating Based On Strain Gradient

Chirped fiber grating is one type of fiber gratings whose optical period(product of grating's effective refractive index and refractive indexmicro-perturbation period). Light with different wavelength is reflected at axialdifferent point of chirped fiber grating. So chirped fiber gratings have broadreflection bandwidth. The group delay in chirped fiber gratings reflectbandwidth changes gradually. These properties are what other type fiber gratingsdo not have. It is these unique properties that make chirped fiber gratings haveunique applications both in optical communication and sensor field. So fabricationand application of Chirped fiber gratings is central issue of interest all along.Usual method to fabricate chirped fiber grating is using chirp phase mask. Butfor the price of chirp phase mask is high, and one phase mask can only fabricatechirped fiber gratings with certain reflect wavelength and bandwidth.In order to fabricate chirped fiber gratings at low cost, we have proposed asimple method for fabricating chirped fiber gratings by using fiber Bragg gratings(FBG): fiber Bragg grating strain gradient method. The method is simple and facile.Using this method we able fabricate chirp tuned chirped fiber grating withadvantage of a fiber Bragg grating. In this paper we adopted structure based oncantilever,the strain gradient along the length of grating is obtained when we applysome tension to the fiber Bragg grating. Thus a chirped fiber grating is formed.,In this paper we first introduced the progress of fiber grating and itsapplications in optical communication field and sensor field. Then we introducedthe basic structure and function of fiber grating. We emphasize principle andexperiments of fabrication tuned chirped fiber grating by using the advantage ofstrain gradient along the length of grating. At the same time we introduced it'sapplication in sensor. The main achievements of this thesis are shown as following:1. In respect of theory, we strictly deduced couple mode theory that describedthe principle of fiber gratings from Maxwell equation. By analysis theproperties of chirped fiber grating obtain the relation between chirped fibergrating's parameters such as change of refractive index, length and chirp aswell and the properties such as reflectivity, bandwidth and dispersion. Allof these prepare for the design and fabrication of chirped fiber gratings.2. In experiment, High quality fiber Bragg gratings were fabricated by themethod of UV writing using phase mask, before which we process thefiber on special-purpose to increase the photosensitive. The UV light wassent out by KrF (248nm) excimer laser. All of these prepare for the designand fabrication of chirped fiber gratings.3. And then, we proposed the method to fabricate chirp tuned chirped fibergrating based on structure of cantilever beam. Experiment on fabricatechirp tuned chirped fiber, a 2-cm-long FBG is glued in a slanted directiononto the lateral side of the triangle cantilever beam with length 12cm,width at the fixed end 3cm ,and thickness 1cm.The angle between the axisof the FBG and the natural layer of the beam is 20.When the cantileverbeam is bent by applying a vertical force or displacement at the free end,half of the grating is under a varying tension. Whereas the other half isunder a varying compression. The strain on the neutral layer of the beam,the center wavelength of the grating may keep approximately fixed duringthe tuning process since the strain at the two halves of the grating aresymmetrical. The chirp tuned chirped fiber grating is formed.When the free end varying in range of 0 to 6 mm, we can obtainedmaximum tuned bandwidth of 7.041nm, bandwidth tuned sensitivityK f=1.089nm/mm. In compare with other menthods such as phase mask,temperature gradient, this way has a larger advantage. The first is that itcan obtain larger reflection bandwidth than fiber Bragg grating. Thesecond center wavelength almost is fixed in tuned process. The others arethat the tuned menthod controled easier, and low cost and simple structureand tuned chirp.4. Finally, a fiber Bragg grating is slantingly bonded to the side of therectangle section cantilever beam and the chirp effect of the grating wouldappear by adjusting the displacement of the beam's free end .Thedisplacement sensing using the chirp effect of fiber Bragg grating understrain gradient is demonstrated in this paper. Because the bandwidth of thegrating is temperature –insensitive, the cross talk between displacementand temperature is avoided. The good linear response has been obtained.Then we can achieve goal of measurement only using a fiber grating, themethod is simple and tuned easier.When bandwidth varying in range of 0 to 7 nm, The experiment resultdemonstrates,the free end of cantilever beam is varying in range from 0 to5.7nm.In measurement range of displacement, which present good linearrelation with transformation of bandwidth. f =0.919 Δλchirp–0.789,Linerdegree is up to R2=0.993。Not only the sensing method is insensitive to temperature but also thereliability is good, the precision is high, a linear responsiveness is high, andoperation is easer. Slightly transforms, this installment also may use asstrength ,electricity ,magnetism sensor。Therefore, this project has the very highpractical application value.Besides, tunable bandwidth chirped fiber grating also apply in fields such asdispersion compensation, gain-flatten equalizer in EDFA.In a word, chirped fiber gratings have many applications both in opticalcommunication and sensor field. Along with develop of study of fiber gratings, theproperties of chirped fiber gratings will be more excellent, the application field ofchirped fiber gratings will be broader.