Study On Optical-acoustic Characteristics Of Anti-resonant Bragg Hollow Core Fiber

At present,most of the study on hollow core fibers(HCF)focus on photonic crystal fibers(PCF)and negative curvature anti-resonant hollow core fibers(NC-ARHCF).The key points are the optical characteristics of optical fiber,such as transmission loss and transmission window,while there is little study on the acoustic characteristics of HCF.In particular,the relative study on anti-resonant Bragg hollow core fiber(AR-BHCF)has not been reported yet.The research on the optical and acoustic characteristics of AR-BHCF is of great significance for its application in distributed optical fiber sensing,photoacoustic imaging,trace gas detection and other fields.Therefore,we introduced high refractive index doped layers into the cladding of ordinary hollow core fiber and proposed anti-resonant Bragg hollow fiber,whose cladding has a Bragg-like layered structure.Moreover,we have conducted study on the optical and acoustic properties of AR-BHCF.The specific work of this thesis are as follows:(1)An anti-resonant Bragg hollow core fiber(AR-BHCF)model was constructed.A new type of hollow core fiber,AR-BHCF,was proposed by introducing the doped layers with high refractive index into the cladding structure of ordinary HCF.We also analyzed the principle of anti-resonance in detail and put forward the theoretical model of AR-BHCF.(2)The spectral properties and field distribution of AR-BHCF were studied.In the form of simulation,we conducted study on optical characteristics of AR-BHCF and analyzed the differences between it and AR-HCF through the beam propagation method.It is found that there are resonance and anti-resonance effects in the cladding of AR-BHCF.There are also periodic loss peaks in the transmission spectrum.The difference of spectral characteristics and field distribution between them comes from the introduced doped layers.At the same time,the influence of structural parameters of AR-BHCF on its optical characteristics was analyzed.The thickness of the doped layers,the number of layers and the radius of the core would directly change the resonant wavelength of the fiber and affect the transmission spectrum of the fiber,but there was nothing to do with the length of the fiber.By adjusting and optimizing the parameters of the doped layers,the confinement ability of light of AR-BHCF could improve about 8% compared with AR-HCF under the same conditions.(3)The acoustic properties of AR-BHCF were investigated by means of finite element modeling.In the frequency domain,it was found that with the increase of frequency,the acoustic wave attenuates faster along AR-BHCF.In the time domain,we analyzed the characteristics of acoustic wave propagation in AR-BHCF with different core radius and fiber length.We found that the cavity formed by the core of AR-BHCF has a specific natural frequency and the standing wave frequency points of AR-BHCF with a certain length could be obtained according to the theory of standing wave tubes.However,standing wave frequency points are close to the natural frequency of the core cavity numerically.The results show that there are beat frequency and standing wave phenomena in the core of AR-BHCF and the beat length is equal to the difference between the natural frequency and the acoustic source frequency when the frequency is close to the natural frequency and the standing wave frequency in the range of 1Hz to 1MHz.In this thesis,we have carried out research on AR-BHCF from both of optics and acoustics,it lays a theoretical foundation for the combination of AR-BHCF and photoacoustic spectroscopy technology.