Design Of Michelson Interference-Based Fiber Optic Dimethyl Methyl Phosphate Sensor And Its Performance Study

Sarin is a colorless,odorless,volatile,and highly toxic nerve agent,which is in liquid form at room temperature.It disrupts the transmission of neurotransmitters by blocking acetylcholinesterase in the human body,and can cause death in a few seconds.Therefore,there is an urgent need to prepare a high precision,remote monitoring and fast response sensor for its detection.Because sarin is extremely toxic and difficult to handle in the laboratory,dimethyl methyl phosphate（DMMP）is usually used as a mimetic agent instead of sarin for detection during experiments.Michelson interferometric fiber optic sensors,which have high sensitivity,compact structure and can be monitored at a long distance,have already been studied in the field of gas sensing.Therefore,this paper proposes three Michelson interferometric fiber optic sensors based on sensitive material cladding for the detection of sarin simulant DMMP.The main research in this paper is as follows:1.A dimethyl methyl phosphate（DMMP）sensor based on MnO₂/ZnO composite film integrated fiber-optic Michelson interference structure is proposed.The sensing structure is formed by a thick taper between a single-mode fiber（SMF）and a four-core fiber（FCF）,and then the no-core fiber（NCF）is spliced at the other end of the FCF.To enhance reflection,the silver film was coated on the end of the NCF,and the fiber optic Michelson interference structure is formed.MnO₂/ZnO composite sensing film was coated on the FCF surface,and the structure,morphology and properties of the sensing material were analyzed by X-ray diffraction,Scanning electron microscopy,Fourier-transform infrared spectroscopy,X-ray photoelectron spectroscopy,etc.The average sensitivity and the response time of the sensor are 0.3397 d B/ppm and 180 s,respectively.The sensor also has good selectivity and stability.2.To further enhance the sensor sensitivity,the experiments combined with MOFs metal-organic framework material integrated fiber sensing technology to fabricate a Michelson interferometric DMMP sensor with single-mode-misaligned single-mode-hollow core fiber structure.Three sensitive materials:ZIF-8/PDDA,ZIF-8@Ag and ZIF-8@Ag/PDDA were wrapped on the fiber respectively,and their respective responses to DMMP were tested to select the suitable sensitive materials.The experimental results showed that the sensor sensitivity was the highest when the sensitive material was ZIF-8@Ag/PDDA,with an average sensitivity of 42.83 d B/ppm and a linear fit coefficient of 0.9882,indicating a good linear relationship between the sensor response spectral intensity and DMMP concentration in the range of 0-60 ppb,as well as a response time of 14 s and a recovery time of 12 s.Meanwhile,the sensor also has good temperature,humidity and time stability.3.In consideration of the volatile physical properties of sarin,an all fiber Michaelson interference cascaded fiber Bragg grating（FBG）sensor for simultaneous measurement of trace dimethyl methyl phosphate and temperature is proposed.One end of the four-core fiber（FCF）is spliced with a multimode fiber（MMF）,the other end is flattened and evaporated with silver film to enhance reflection,and the Michelson interference structure is formed.The Bragg grating is cascaded with the Michelson interference structure to obtain a cascaded FBG-MMF-FCF fiber sensor.The sensing film,MnCo₂O₄ is coated on the surface of FCF,and the structure,elemental composition and morphology of MnCo₂O₄were analyzed by X-ray diffraction,X-ray photoelectron spectroscopy and scanning electron microscopy.The sensitivity and the detection limit of DMMP are 87.17 d B/ppm and 0.1507 ppb,respectively.The response/recovery time is about 14/10 s.the temperature sensitivity can be compensated and calculated as 0.069 nm/°C.