Comparative Study On Sensing Mechanisms Of Cascaded Silica Capillary Based Device

Silica capillary has simple structure,low cost and natural hollow core circular symmetry structure,so it is easy to integrate with optical fiber to form various cascaded structures.Therefore,it is an important carrier to realize optical fiber sensor.Optical fiber sensors based on silica capillary mainly contain sensing mechanisms in the form of interferences such as Fabry-Perot(FP),Mach-Zehnder(MZ),Multimode Interference(MMI)or Antiresonant(AR).A three-section cascaded structure(SMF-Capillary-SMF)can be constructed by silica capillary and two single mode fibers.Its transmission and reflection characteristics are related to the device preparation process and size parameters.The FP,MZ,MMI and AR mechanisms of devices are investigated in this thesis based on cascaded silica capillary structure.Hybrid sensing mechanisms can be used for multi-parameters sensing,which is instructive for device research with similar hollow core structures.The main research work and achievements of this thesis are as follows:(1)The effect of the preparation process of cascaded silica capillary device on the sensing mechanism was studied.By adjusting the fusion parameters such as propulsion distance and discharge intensity,manual discharge fusion is performed for small inner diameter capillary with the single mode fiber.The greater the propulsion distance,the more silicon material into the hollow core at the fusion surface,so the easier it is to form the coupling zone at the fusion surface and then excite MMI.The stronger the discharge intensity,the easier it is for the capillary hollow core at the fusion surface to collapse to form the coupling zone for exciting MMI.Therefore,reasonable propulsion distance and discharge intensity can avoid the coupling zone in the fusion surface,thus inhibit MMI of small inner diameter cascaded silica capillary devices.We set the propulsion distance and discharge intensity to 20 ?m and-50 bits for small inner diameter silica capillary for device preparation of FP,MZ and AR mechanisms.(2)The sensing mechanism of cascaded silica capillary devices is compared by transmission and reflection spectra theoretically and experimentally,respectively.The total energy changes of capillary core and cladding modes in cascaded silica capillary structures with different inner diameters are monitored in the simulation.The greater the inner diameter of the capillary,the more uneven the distribution of energy in the hollow core and cladding,which makes the MZ mechanism disappear gradually.Furthermore,the existence of AR mechanism is proved by monitoring the capillary core mode,where the device of the capillary with an inner diameter of 15 ?m has two core modes.Experimentally,we fabricate cascaded capillary devices with different inner diameters and different lengths.Then,we measure and analyze their transmission and reflection spectra respectively.The transmission spectra show that MZ interference fringe contrast of devices with inner diameter of 10 and 15 ?m is 18.9 and 0.9 d B,respectively.The inner diameter of the capillary increases and the contrast of MZ mechanism decreases to disappear,which is consistent with the trend of simulation.It also proves that the cascaded structure with inner diameter of 15 ?m is the transition state of MZ interference from being to disappearing.In addition,the sensing mechanism obtained from the transmission and reflection spectra is consistent with the simulation results with the increase of the sample length when the inner diameter of the capillary is fixed.Experimental transmission spectra have a good agreement with numerical simulation transmission spectra of devices with an inner diameter of 20 ?m.The reflection spectra show that the appearance of the independent or mixing mechanism is related to the critical length of the silica capillary.The sensing mechanisms of the existence of three cascaded silica capillary devices with different inner diameters are summarized as following.The FP mechanism always exists in the devices and MZ mechanism is related to the inner diameter of the silica capillary,while AR mechanism related to the critical length.