Enhanced Raman Detection Based On The Metal-lined Hollow-core Fiber

Rapid detection and identification of chemical substances are of utmost importance in many fields,such as food safety,drug monitoring,industrial wastewater discharging,etc.Real-time on-site detection of chemical substances is an important guarantee for maintaining public safety.However,many equipments of the existing detection methods are bulky and expensive,and are only suitable in the laboratory,such as chromatography,mass spectrometry,infrared spectroscopy,etc.In addition,high requirement for environmental conditions is necessary.Therefore,we select the Raman spectroscopy method,which has been considered as the "fingerprint spectrum" of the matter.This method can realize the rapid detection of the material spectrum without pre-processing the sample and damaging the sample.Besides,the Raman signal of water is very weak,so it is very suitable for detecting liquid samples.However,the Raman signal itself is very weak.This shortcoming limits the application of Raman detection.Many efforts have been made to enhance Raman signal,such as Surface Enhanced Raman Spectroscopy,Tip Enhanced Raman Spectroscopy,etc.,but these methods are not suitable for fast detection.Moreover,multiple tests cannot be performed,which leads to cross-contamination of the sample,and any impurities can seriously affect the Raman scattering signal.The metal-lined hollow-core fiber can solve the above problems well.The diameter of the hollow core is very large,which can realize rapid sample exchange.In addition,the metal-lined hollow-core fiber can simultaneously transmit the excitation light and the sample to increase the volume of light-matter interaction and thus improve the signal collection efficiency.The hollow-core photonic crystal fiber has a relatively smaller hollow core diameter of only a few micrometers,which requires very high pressure to inject the sample into the hollow core.Besides,the refractive index of the sample is different from that of air,which affects the light transmission in the hollow core photonic crystal fiber.In this paper,the concept of volumetric enhanced Raman scattering is proposed based on hollow-core fiber.When the sample is with low absorption rate of light,the low-loss hollow-core fiber can transmit the excitation light to a long distance,and the optical path is also a liquid path.The volume of the sample excited by light is increased.The Raman signal intensity of each component of the sample is proportional to its volume ratio in the sample,so cross-contamination that may occur when exchanging samples does not seriously affect the detection results.We theoretically compare the direct detection of Raman,probe with the detection in the metal-lined hollow-core fiber and analyze the light guiding principle of the fiber.The reflectance of the 785-nm laser and its Raman light reflecting on the surface of gold and silver is very high.So we choose the silver-lined hollow-core fiber in the experiment,and also propose a method for producing the gold-lined hollow-core fiber.Secondly,we designed a Sagnac ring and gold film reflection Raman detection structure based on silver-lined hollow-core fiber.For the Sagnac ring structure,we insert one end of two large-core fibers into both ends of the hollow-core fiber,and the other end are parallel and couple with the Raman probe.The structure can simultaneously collect transmission and reflection Raman light,but the efficiency of coupling the Raman probe with two fibers is low.For the gold film reflection structure,a large-core fiber is inserted into one end of the hollow-core fiber and couples with the Raman probe.A glass fiber coated with gold film is inserted into another end.The gold film can simultaneously reflect the excitation light and the Raman light to enhance the volume of the light-matter interaction.Therefore the collection efficiency of Raman light is improved.In the experiment,we have set up a fiber-cutting and fiber-inserting platform,and the inserting structure is packaged in a three-way valve to achieve rapid sample exchanging and high-stability of the device.Moreover,we have designed a portable structure for coupling the fiber with Raman probe to satisfy the requirements of field testing.The experimental results fully prove the high efficiency of fiber-inserting coupling and the gold film reflection,which makes full preparation for the resonant cavity Raman detection.Finally,we designed a resonant cavity Raman detection device based on the metal-lined hollow-core fiber.The gold film and the filter film are respectively inserted into both ends of the hollow-core fiber,which serves as the high reflection mirror of the cavity.In the experiment,we have set up an observation platform of etching fibers to achieve precise corrosion.The excitation fiber has been etched to the outer diameter of 30?m,and the excitation light is transmitted into the cavity through the hole of the gold film.In addition,we have verified the high reflection of the filter film for the excitation light and high transmittance for Raman light.The excitation light is reflected back and forth within the cavity to further increase the volume of light-matter interaction,thereby increasing the collected Raman signal.The experimental results prove that the resonant cavity structure can increase the Raman signal by 4.83 times,and the Raman signal has a linear relationship with the sample concentration.The concept of volumetric enhanced Raman scattering has also been proved.In this paper,the enhanced Raman detection systems based on the metal-lined hollow-core fiber have been studied theoretically and experimentally,including Sagnac ring type,gold film reflection type and resonant cavity type.Besides,portable packaging method has been designed.Our device has the merits of exchanging samples quickly without residue,high detection speed,high sensitivity and high precision,so they have a promising prospect in the field of on-line fast Raman detection.