Study On Key Issues Of Gas Sensing Technology Based On Hollow Core Photonic Crystal Fiber

With the development of optical fiber sensing technology,the optical fiber gas sensor based on the principle of infrared spectrum absorption has been developed drastically.Hollow core photonic crystal fiber(HC-PCF)as a special of optical fiber,its cladding is consisted of photonic crystal structures formed by a series of periodically arranged air holes.Due to the bandgap properties of photonic crystals,the light with certain frequency would be limited to transmit in the fiber core.In addition,the fiber core is a structure of air hole,which could be convenient for the gas filling.When HC-PCF was used as chamber of the optical fiber gas sensor,it would not need to use optical collimator or high reflector optical devices.So it broke disadvantages that the effective absorption length of traditional fiber optic chamber was limited.At the same time,it has some significant advantages of small volume,light weight,easy to wind and so on,which provide a new way for designing a small,high sensitive and long-ranged gas sensor.But considering the practical application,gas sensor based on HC-PCF still has many problems to be solved.On the basis of a large number of domestic and foreign literatures,the thesis had completed the following contents:(1)HC-PCF with high power core,low transmission loss and wide bandwidth was designed.The thesis presented the HC-PCF by using rounded hexagon to increase the duty cycle,while introduced the silica core ring structure.By using COMSOL software,the thesis respectively analyzed the impact of the number of cladding layer under this structure,the core radius and the thickness of its core silica ring on the optical power of fiber core,limit loss and the effective mode field area.And the thesis got the relationship between core optical power,limit loss,the effective mode field area and wavelength.Finally,it was concluded that when the number of layers in cladding was 7,the best values of the core radius and the relative thickness of silica ring should be controlled respectively in the range of 1.4??1.8? and 3.5?5.5.(2)Sealed gas chamber,fixtures and mechanical coupling devices were designed.On the basis of the designing of the sealed chamber,the experiment has added V groove,a fixed trough and threaded holes to increase the stability of the system.Then the mechanical coupling device was constructed by using the bare fiber adapters and open casing as the core components.Using mechanical coupling device,the thesis identified the best distances between HC-PCF and single mode fiber,multimode mode fiber were respectively 10?m and 40?m.By using the sealed chamber and fixtures,the thesis studied the impact of pressure on gas filling time.Filling 2%acetylene gas of 0.05MPa and 0.10MPa into HC-PCF,which is 60cm length,produced by NKT and a model of HC-1550-02,required respectively 23.0min and 10.5min.Experiment results had showed that increasing the pressure difference of the inlet and outlet could significantly shorten the filling time.(3)Gas sensing system based on HC-PCF was designed.Taking acetylene gas as an example,the thesis has obtained the relationships between the output power and acetylene gas concentration under different light sources.When a tunable laser was used as the light source,through direct spectral absorption method,the thesis got that the maximum resolution and sensitivity were respectively 0.121 ppm and 8.259nW/ppm,maximum absolute error and relative error were 0.0432%and 8.450%respectively,maximum percentage difference of stability and repeatability were 2.470%and 9.419%respectively.When using the broadband light source to do the differential measurement,the stability and repeatability were improved at the same time,the corresponding values were 0.284%and 1.424%.This system had showed good performances.In the process of designing gas sensor based on HC-PCF,the thesis had analyzed and solved problems that existed in the building process,achieved the desired requirements and realized the optical fiber gas sensor with compact structure,high sensitivity and small volume.In next research,the further optimizations of sealed gas chamber,fixtures and mechanical coupling devices should be considered.Detection methods should be further improved in order to promote its further development in the practical use.