Fiber-Optic Microphone Based On Aluminum-Polyimide Film And It's Applications

Along with the development of fiber optical sensor,fiber optical microphone has more widely application prospects in the fields such as micro nano detection,photoacoustic spectroscopy and so on.Interferometric fiber-optic microphone has been attracted many researchers since it appeared because of its advantages such as simple structure,high sensitivity,resistance to electromagnetic interference.In the fiber optical microphone system with FP interference structure,the performance and preparation process of the thin film are very important.The quantitative measurement of the vibration amplitude of the thin film can directly reflect the sensitivity of the sensor.This paper studied the fiber optical sensor with high sensitivity.To get the high precision of the sensitivity,we combined the sensor with low-coherence fiber differential interferometer which can realize the physical quantity of absolute measurement,and apply it into photoacoustic spectroscopy measurement and gas leakage detection.First of all,we present a new kind of aluminum-polyimide composite film with large area(3.1 mm diameter)which is prepared by MEMS plasma etching.This method could release the internal stress of the film in some extent and improve smoothness of the surface.The thickness of the film is lower than 500 nm by diluting the concentration of polyamic acid.The aluminum layer can protect the polyimide not to be damaged during the etching process,while it can improve the reflectivity of the film.The strength of polyimide is good,and it can ensure the overall strength of the film due to the characteristics of polyimide.Then we study the vibration characteristics of the film.Because low-coherence differential interferometric system can realize the non-contact absolute amplitude measurement of the vibration,we combine it with the aluminum-polyimide film probe into the low-coherence differential interferometric fiber-optic microphone system.Experimental results showed that the system has a nice linear response and the sensitivity of acoustic pressure is stable from 1KHz to 5KHz with an average of 120nm/Pa,the minimum detectable acoustic pressure is 53uPa.Due to the structural stability of aluminum-polyimide film,the properties of the sensor keep for four months.The film has good temperature stability and it has a weak negative influence on the sensitivity by the rise of temperature where the maximum rate is 9.3%.Based on fiber microphone with aluminum-polyimide film,the application of photoacoustic spectroscopy and gas leakage detection is studied.In this paper,the modular design of optical fiber photoacoustic spectroscopy measurement system,including tunable excitation light module,micro cavity open photoacoustic cell and low coherence interference differential microphone module,and carries out experiments on each module,after the experiment of the whole system.The experimental results show that the resonant frequency of micro cavity open photoacoustic cell is 3KHz,can produce maximum photoacoustic signal is 2.3mPa;compared with the traditional electrical microphone(B&K 2192),the signal intensity in the pulse frequency 6KHz and 7KHz respectively by 10.2dB and 9.9dB.In the aspect of gas leakage detection,by collecting the quadrupole acoustic signal of gas leakage,the characteristic spectrum of gas leakage is identified to determine the information of gas tank leakage.Experimental results show that the gas tank with internal pressure and leak size change,gas leakage characteristic spectrum only fluctuate overall no change on the spectrum.This research provides a new method for gas leak detection.