The Characteristics And Applications Of Fiber Sensors Based On Microfiber Couplers

Microfiber is always formed by heating and stretching the conventional optical fiber.The diameter of the microfiber is several microns or several hundred nanometers,which is analogous with the wavelength.It has a number of optical and mechanical properties,including large evanescent fields,strong confinement,great configurability,and low-loss connection.Microfiber has a waist region with a small diameter,two tapered transition regions and two pigtails.Due to the small diameter,the optical field is no more confined in the fiber core.It will form the evanescent field around the waist region and propagate forward.Utilizing this special property,the interaction between optical field and ambient is enhanced.So,microfiber will be widely studied and developed in the areas of sensors,lasers,biological chemistry,and so on.Microfiber devices are fabricated based on evanescent field of microfibers,which include microfiber couplers,microfiber resonators,Sagnac interferometers,gratings and so on and have attracted much attentions.These devices are easy-fabricated,low-cost and compact.Started from the coupled mode theory,two kinds of microfiber devices were fabricated,which were a microfiber coupler and a microfiber coil resonator.Using these two devices,combined with elasto-optical effect,thermo-optic effect and thermal-expansion effect,the measurement of electric current,magnetic field and microfluidic flow rate with a high sensitivity was realized.1.First,a microfiber was successfully fabricated with "flame-brushing method".The low insertion loss and large evanescent field were both realized.Besides,with the same methods,two conventional optical fibers were heated and stretched.Microfiber coupler was formed with the coupled fields between the two waveguides.The coupler had two input ports and two output ports.2.The mechanical response of the microfiber coupler based on the elasto-optical effect was studied theoretically and experimentally.When the tension was applied on the fiber longitudinally,the refractive index of the fiber and the length of coupling region were changed.The coupling efficiency and the power distribution between two output ports were changed.Based on this result,a differential twin receiving fiber-optic magnetic field and electric current sensor was fabricated.The electric current flowed through the Al wire which was placed in the perpendicular magnetic field.The Ampere force was generated and applied on the coupling region.Utilizing the differential data processing,the perturbation of the incident power could be ignored.With this device,the amplitude of magnetic field under a constant current or the amplitude of electric current under a constant magnetic field could be measured.3.The thermal response of the microfiber coupler based on the thermo-optic effect and thermal-expansion effect was studied.When the coupler was placed into an environment with a varying temperature,its refractive index and length of coupling region would change synchronously.The coupling coefficient between the two waveguides would change.And the output spectrum would shift.When the coupling region was coated with Teflon which had a negative thermo-optic coefficient,a fiber coupler with a good thermal stability was fabricated,which was useful in the future communication system.Based on these two thermal effects,a microfluidic flowmeter was fabricated.The liquid channel was coated with gold film,and the microfiber coupler was wrapped around the gold film with one turn.The evanescent field was absorbed and the "hot-wire" appeared.The liquid flowed through the hot zone,took away the energy and cooled down the temperature.Using this device,the measurement of the microfluidic flow rate with an order of magnitude increased sensitivity was realized.4.The coil resonator which was fabricated with a single microfiber was demonstrated.The microfiber was wrapped around the rod with two turns which was coated with a piece of monolayer graphene sheet in advance.And the spacing between adjacent coils was adjusted to realize the coupling.Based on this self-coupled device,the current measurement was realized and the sensitivity had two orders of magnitude increase compared with other current sensors based on thermal effect reported before.The monolayer graphene sheet had a large sheet resistance.When the current flowed through the sheet,heat generated,and it would change the refractive index and the diameter of the micro fiber and shift the output spectrum due to the thermo-optic effect and thermal-expansion effect.