Experimental Study On The Fabrication And Application Of The Bandpass Long-period Fiber Gratings

Three kinds of bandpass long-period fiber gratings (LPFGs) includinginterferometric bandpass LPFG, reflective bandpass LPFG and transmission bandpassLPFG are studied in this thesis. The torsion characteristic of the interferometricbandpass LPFG is first studied theoretically and experimentally. Then a method fornarrowing the linewidths of the LPFG based sensors by using fiber ring laser is studiedsymmetrically in order to realize the demodulation of the LPFGs with broadbandlinewidth and long distance sensing. And a LPFG based long distance sensing system isrealized based on the reflective LPFG and fiber ring laser. Finally the transmissionbandpass LPFG is fabricated based on the asymmetric cladding modes exciting andcoupling, which has the advantages of ease of fabrication and compactness, enabling itto be used widely in wavelength division multiplexing system. The contents of thisthesis are as followings:1. The torsion characteristic of the Mach-Zehnder interferometer (MZI, i.e., theinterferometric bandpass LPFG) formed by two cascaded LPFGs with rotary refractiveindex modulation along the fiber axis is studied. And the experimental results show thatthe direction to which the fringes shift is related to the torsion direction applied to theMZI. The fringes shift to the longer wavelength when the torsion with counterclockwisedirection is applied to the MZI. On the other hand, the fringes shift to the shorterwavelength when the MZI is twisted in the clockwise direction. The valley at1576nmhas a torsion sensitivity of0.10nm/(rad/m) within±100rad/m. Then the MZI asbandpass filter and torsion sensor simultaneously is incorporated into the cavity of afiber ring laser, by which the linewidth of the senor could be narrowed to less than0.06nm. The torsion resolution of the sensor is0.12rad/m when the wavelengthresolution of the optical spectrum analyzer is10pm. Moreover the torsion directioncould be determined directly through the shift direction of the lasing wavelength.2. A reflective bandpass LPFG based on the coupled-mode theory of LPFG andguidance mechanism of hollow core fiber is proposed, which can be fabricated byfusion-splicing a piece of hollow core fiber to the LPFG end. And the Fresnel reflectionoccurring in the core-air interface is eliminated by increasing arc discharge and pushingdistance of the single mode fiber. Then the bandpass LPFG is employed as the filter andsensor simultaneously by the sub ring of a fiber ring laser to measure the temperature at the spot that is1km away from the light source. The lasing wavelength shows red shiftwith a temperature sensitivity of~0.0254nm/℃within20-150℃. Moreover thereflective bandpass LPFG is compatible with the fiber Bragg grating (FBG) based longdistance quasi distributed sensing network, where the LPFG could be used to measurethe high temperature that the FBG could not endure or measure multi-parametersimultaneously by combing LPFG and FBG.3. The asymmetric mode exciting theory by offsetting two pieces of single modefiber laterally is studied and the band-rejection LPFG is changed into transmissionbandpass LPFG by use of mode coupling theory. The transmission bandpass LPFG canbe formed by writing an asymmetric LPFG with CO2laser in one of the two fibers withlateral offset of40μm, however the relative power of the central wavelength is less than-10dB. The simulation results show that asymmetric LPFG has higher mode couplingefficiency than that of symmetric LPFG with same refractive index modulation depth.The insert loss of the transmission bandpass LPFG formed by writing a LPFG in twofibers with lateral offset of40μm, respectively, is~6.811dB while the size is doubled.The relationship between the direction in which the asymmetric LPFG is written and thelateral offset direction has effect on these bandpass LPFGs. And the direction effect canbe eliminated in thebandpass LPFG consisting of the LPFG with rotary refractive indexmodulation along the fiber axis, which could be used as a symmetric LPFG to a certainextent.