On-line Raman Liquid Detecting Methods Based On Hollow-core Fiber

Raman spectroscopy is a method for detecting substance based on the "fingerprint spectrum" of a molecular group,and has been widely used in fields such as chemical synthesis,medical diagnosis,and biological sample analysis.Since the intensity of Raman light is very weak,its power is about 0.001%of the pump light power,so a series of complicated Raman enhancement steps are required for spectral detection.Traditional enhancement methods(such as surface-enhanced Raman scattering,resonance Raman enhancement,and tip-enhanced Raman scattering,etc.)have problems such as complicated processes,slow detection speed,low repetition rate,and poor device stability.It is difficult for the complex sample detection and rapid detection environment.Therefore,in order to improve the sensitivity of detection,to achieve rapid online sample detection,to prolong the service life,to enhance the stability and portability of the equipment,we design an on-line liquid Raman detection device based on hollow core fibers.The device has a wide range of applications in fields such as biomedical diagnosis and food safety analysis which requires rapid detection.The Raman enhanced reaction chamber made of hollow core optical fiber can meet the requirements of high efficiency,low carbon,and portability to some extent.Hollow fiber is an efficient light guide element.The incident light is bound in the inner hole of the fiber because of the reflection of the inner wall or the effect of the crystal band gap.At this time,the liquid sample is injected into the area and the light path and the liquid path are coincident.The efficiency of the photo-liquid reaction is very high,so the required length of the hollow fiber is usually several meters or less.If an air-core fiber waveguide has a very low transmission loss,only a few centimeters of light-liquid interaction distance are required,and a lower concentration liquid can be achieved.Raman detection techniques for liquids have been developed in many related fields.For example,in chemical synthesis,the intensity of Raman spectra of a substance in a reaction solution can be used to monitor the progress of the synthesis.In addition,many gas Raman detection techniques do not directly detect the gas.Generally,the gas will be extracted by loop extraction and enriched in the liquid solution.Such hollow core fiber based Raman enhanced reaction chambers have lower costs,require smaller sample volumes,are easy to clean,can be reused,and can be used for high-speed switching with high-voltage equipment.This paper deals with hollow-core photonic crystal fibers and metal-plated capillary tubes.We design a reasonable experimental scheme for these two different fibers and make a theoretical analysis.Insertion-coupling Raman detection device for hollow-core photonic crystal fibers and single-mode fibers,Raman reception enhancement device for optical fiber Sagnac ring structures,Raman reception enhancement device based on metal-lined capillary and gold-plated capillary,F-P cavity Raman enhancement device are the focus of this study.After the etched single-mode fiber is inserted into the hollow hole of the hollow-core photonic crystal fiber,the liquid enters the photonic crystal fiber through the slit and coincides with the optical path.The Raman detection device has a high detection sensitivity,but the inner hole is small,which make the device unsuitable for rapid detection.The Raman detection device of the optical fiber Sagnac ring structure inserts two large-core optical fibers on both sides of the inner-plated metal capillary,and the other two sides are combined to couple with the Raman probe to collect the two kinds of Raman signals:reflection and transmission mode.The detection speed is faster,but the sensitivity is lower than the previous one,and the coupling loss of the fiber and Raman probe is too large.Inserting a gold-coated glass rod into the mental-lined capillary is the third structure we studied.Due to the reflection of the gold film,the distance between the excitation light and the sample doubles,and both the reflection and transmission Raman signal can be received by the spectrometer.The overall device is more portable,and the coupling efficiency between the optical fiber and the probe is higher,but the SiO2 Raman noise of the device is very high.One end of the metal-lined capillary was inserted by a large-diameter optical fiber coated with a 785 nm long pass film on the end face,and the other end was inserted by a hollow capillary plated with gold on the end face.The pumped light was input through the hollow capillary by an etch SMF.The pump light is trapped in the Raman reaction chamber,which greatly increases the light-liquid reaction distance.The Raman threshold is very low,the detection speed is fast,and the detection sensitivity is high,but the manufacturing process of the device is more complex and the yield is very low.For these four kinds of Raman detection devices,the overall design idea is to enhance the interaction between light and liquid,while collecting the generated Raman signal as much as possible into the spectrometer.The system tends to be integrated,all-fiber,and portable.The research direction of this article is devoted to the development of a Raman spectrometer which adapts to the biomedical diagnosis and food safety analysis market.The current research results can be provided to Raman spectrometer manufacturers or research institutes in the form of Raman detection accessories to enhance Raman detection efficiency.There is still a certain application prospect,which lays the foundation for subsequent research and development.