Fabrication and development of carbon dioxide laser-written long period fiber grating and its application to vibration measurement

Development of fiber gratings in has lead to its application as a narrowband filter in dense wavelength division multiplexing (DWDM) optical networks in optical communication, as well as sensors capable of detecting temperature, strain, displacement and other parameters. In this dissertation, a class of fiber grating called long period grating (LPG) fabricated by periodic perturbation via CO2 laser is introduced. The fabrication process and characterization of such device is discussed. By cascading the CO2-written LPG within an acoustic-optic tunable fiber (AOTF), a tunable band-pass filter, as well as a vibration sensor is built and demonstrated. The operating principle of both devices, which is based on energy mode coupling, is reviewed.;Thru computer automation, a highly compact (1.2 cm) long-period fiber gratings (LPGs) exhibiting strong grating effect (>30dB loss) and low insertion loss (<0.25dB) inscribed by a continuously scanned CO2 laser. The LPG resonances are also probed using acousto-optics tunable fiber (AOTF). The results show that these CO2 laser side-exposed LPGs predominantly couple to anti-symmetrical cladding modes.;By utilizing the cascading effect of the LPG-AOTF device described earlier, a compact, all-fiber frequency shifter is shown by cascading a CO2-written long-period grating with a flexural acoustic wave induced micro-bending grating on a single-mode fiber. Carrier and image sideband suppression of 30.0 and 28.1dB, respectively, are measured. An all-fiber vibrometer based on this frequency shifter capable of nanometer resolution at frequencies up to MHz is also demonstrated.;In addition, an efficient thermally controlled loss-tunable LPG is fabricated by bonding an etched fiber onto a periodic-corrugated metal fixture. The attenuation is thermally tuned using a thermo-electric cooler. At a fiber diameter of 52mum, a 25dB change in transmission is achieved with a change of temperature of 18°C. Also by fusion splicing a 3.25mm long two-mode fiber sandwiched between two single mode fibers to form an all-fiber Mach-Zehnder interferometer, a fiber sensor is developed capable of detecting temperature change from ambient to 425°C with temperature sensitivity of 58pm/°C, and 0.45muW/°C with 360muW of input power, respectively.