Research On On-line Raman Detecting System Based On Hollow-core Fiber

Intelligent detection technologies,as the technology base of intelligent technology,is one of the driving force behind the development of life facilitation.Substance detection technology is one of them,and has been developed in many related areas.For example,in medical pathology test,a physician can analysis the body exhaled gas components to assist in the diagnosis of a disease;in the field of environmental protection,air quality testing have been widely used.The importance of such testings is self-evident.In the related research,Raman detection is a commonly used method of substance detection.The principle of Raman detection is based on that the Raman scattering signal is fingerprint-like:Raman scattering signals are produced based on the vibrational energy levels of molecular bonds,so this signal can be used as an accurate signal for material identification.And the concentration of the substancec can be determined according to the intensity of the signal.This paper investigates the existing gas detection technologies,and designs a set of on-line Raman detecting system based on hollow-core fiber.Using the hollow-core fiber as the detecting carrier can make full use of this hollow-core light guiding conponent,for the fomation of this substance-light area achieves the perfect coincidence of the optical path and the sample chamber.The characteristics of two kinds of hollow-core fiber(metal-coated capillary and hollow-core photonic crystal fiber)are analyzed in detail for reasonable system design.In the on-line coupling of sample injection and light guiding,two sets of sample-light coupling structures are designed respectively for the metal-coated capillary and the hollow-core photonic crystal fiber.And as the excitation light of laser is transmitted via ordinary single mode optical fiber,the coupling between two transmission medium with different guiding principle is also included in the design considerations.The two stuctures are free space coupling method and fiber inserting coupling method.The sample injection structure is designed in the form of an external micro-sample cavity,and this cavity can be designed specifically according to the needs of different application scenarios.In the insertion coupling method,when the hollow-core photonic crystal fiber is involved,the single-mode fiber needs to be etched to accommodate the relatively small central hollow core of the photonic crystal fiber.Theoretical analysis shows that this new etching-inserting single-mode fiber/hollow-core photonic crystal fiber coupling method can guarantee a relatively high coupling efficiency.Considering the photonic-bandgap mechanism of photonic crystal fiber,we introduce a new divergence solid angle-numerical aperture solid angle matching theory.In the ideal case,all the energy guided by single-mode fiber tip can be collected by the hollow-core photoniccrystal fiber,with only little loss.If the fiber end Fresnel reflection loss is taken into account,the transmission loss is about 0.15dB.In order to improve the Raman detection sensitivity and enhance the Raman scattering signal,two enhanced cavities based on hollow-core fiber are designed.They are fiber inner wall surface-enhanced Raman scattering(SERS)structure and FP resonant cavity enhanced structure.To obtain larger Raman signals and enhance the system performance,the former enhances the local excitation light intensity,while the latter enhances the whole light field through cavity resonant.From the point of sample-light contaction,the SERS gain is mainly restrained around the inner wall surface,which is quite smaller than the resonator one.And there is also materialpollution problem for the SERS cavity.However,in terms of gain effect,the SERS method has a higher gain coefficient in its local enhancement.In the experiment,we designed and experimented two micro-sample chambers,demonstrating the design flexibility of the sample injecting structure.In the coupled design,the metal-coated capillary performs well in the free space coupling experiment,while the hollow fiber photonic crystal has a better coupling efficiency in the inserting coupling experiment:with NKT HC19-1500-001 and SMF-28 respectively as the hollow-core photonic crystal fiber and single-mode fiber,the coupling efficiency can up to 84.5%,and the corresponding theoretical coupling efficiency is 85%.In the design of Raman detection system,we initially set up two simplified systems:transmission type Raman detection system and reflective type Raman detection system.They both can achieve Raman detection function with good performance.Then,we further designed a bidirectional excitation and reception Raman detection system.And during the experiment,its detection performance is excellent.In general,the whole system design idea is that,on the basis of the existing relevant substance detection and Raman detection,emphasizing on on-line integrating design,and achieving the goal of all-optical syste.Also,avoiding fragmented structure and enhancing performance reasonably to prepare for subsequent application scenarios.