Study On Tilted Fiber Bragg Grating Based Sensors

Tilted fiber Bragg gratings (TFBGs) which belong to short-period fiberBragg grating (FBG) family have attracted a lot of research attentions. They can notonly couple the light from forward propagating core mode to backward propagatingcore mode as normal FBGs do, but also can couple the forward propagating coremode to backward propagating cladding modes. As the cladding modes of TFBG aresensitive to some ambient factors, TFBGs have been widely used in refractive index,bending, humidity and some other sensing measurement. In this paper, we proposedsome TFBG based sensors. The main contents are shown as following:1. Development of the TFBG based fiber sensors was briefly introduced. Afterinvestigating the TFBG sensing principle, some normal fiber grating fabricationmethod and the fabrication system in our lab were introduced in this part.2. Three fiber refractometers based on TFBG were proposed and experimentallydemonstrated. A FBG with broadband reflection spectrally locating at TFBGcladding modes was superimposed in the rear part of a TFBG. Through detectingoptical power reflected by the FBG, surrounding refractive index measurement canbe realized. To improve the sensitivity and measurement range, a chirped fiber Bragggratings (CFBG) with wide broadband reflection locates at TFBG cladding moderesonances was cascaded after the TFBG. Another refractometer was proposed byusing a waist-enlarge bi-taper to couple the backward propagating cladding modesexcited by TFBG from fiber cladding into core. And refractive index variation can bemonitored by demonstrating the optical power in reflection port.3. A glycerin level sensor based on waist-enlarged bi-taper and TFBG wasproposed. By measuring the optical power of cladding modes which decreases withthe level of glycerin increases, liquid level measurement from3to30mm wasrealized.4. Two fiber magnetic field sensors based on magnetic fluid (MF) coated TFBGwere proposed. The cladding mode resonances of TFBG are modulated by externalmagnetic field through the coated MF. Through measuring the extinction ratio ofcladding mode resonance and reflected optical power by a CFBG, two fiber magneticfield sensors can be realized respectively.