英语翻译In this paper,we developed a stabilized F–P AE sensor sy

英语翻译
In this paper,we developed a stabilized F–P AE sensor system which was composed of an EDFA (erbium-doped fiber amplifier) source,a DWDM,two photodiodes and a data acquisition board.The generation of two quadrature phase-shifted signals from a DWDM was proposed in order to improve the practicability of sensor.By optimizing filtering wavelengths of DWDM,the quadrature precision of the two output signals can be improved.Tests for detecting simulated AE signals and the stability of sensor system demonstrate that this sensor system can be used in the NDT of structures.
The configuration of the sensor probe is shown in Fig.1.The sensor probe is an extrinsic fiber-optic F–P cavity,which is formed with two single-mode optical fiber endfacets aligned in the silica glass capillary and thermally fused together by CO2 laser beam.Fig.2 shows the fiber-optic F–P sensor system.The system consists of an amplified spontaneous emission (ASE) broadband light source,a photo detector and the signal processing subsystem.The spectral distribution of the ASE source is shown in Fig.3,which is measured with Optical Spectrum Analyzer AQ6317C.This ASE source has the wide wavelength range from 1530 to 1600 nm.The output of ASE source is coupled into a 1 2 singlemode fiber coupler and propagates to the F–P sensor probe.The light reflected from the F–P cavity is coupled into a DWDM.Through the DWDM,two quadrature phased-shifted signals are obtained.Then two signals from the DWDM are converted and processed by photodiodes and signal processing unit.
3.Operating principle
As the light reflected from the F–P cavity carries the information about the change of cavity length,AE signals can be extracted by the signal demodulation circuit when AE wave impacts on the F–P cavity.Since the reflectivity of the fiber end surfaces is about 4%,the multi-beam interferometer is approximate to a two-beam interferometer.
Thus,the output voltages V1 and V2 at two photodiodes can be expressed as:
Vj ¼ 2RZl
PðljÞM210 Pðbk=10ÞZjð1 cos yjÞ dl j ¼ 1; 2,
(1)
where P(l1,2) are the light power from two output channels of DWDM,M is the coupler split ratio,which can be considered as a constant,bk are loss coefficients in dB of various passive optical devices of sensor system,Z1,2 are responsivities of two photodiodes,y1,2 ¼ 4pnL/l are interferometric phases of the two output lights,l is the free space wavelength,L is the F–P cavity length,and n is the effective refractive index of the F–P cavity.
According to the test results,a Gaussian function is a good approximate expression for the intensity distribution of light passed by the DWDM to the two photodiodes: