激光超声多通道接收器在无损检测应用的最新进展.doc

Page 1 of 6 RECENT PROGRESS IN LASER ULTRASONIC MULTI DECTECTORS RECEIVERS FOR NON DESTRUCTING EVALUATION A Wartelle1 B Pouet1 S Breugnot1 Bai Xiaobao2 1Bossa Nova Technologies 2Matrix U E technology Inc ABSTRACT New laser interferometric schemes were recently introduced in order to fully take advantage of possibilities offered by laser based ultrasonic LBU inspection and to broaden their integration into industrial inspection systems and laboratories setups First we will present the recent advances on a fiberized LBU receiver optimized for industrial applications We will show an example of results with thickness monitoring on moving sample using zero group velocity Lamb wave resonance Secondly we will present the latest progress in a LBU laboratory receiver with the development of an adaptive interferometer able to measure simultaneously both in plane and out of plane displacements Advantages of this receiver for the study of ultrasound propagation will be demonstrated with mapping of in plane and out of plane displacements generated by a thermoelastic laser source Keywords Speckle ultrasound quadrature two wave mixing interferometer laser 激光超声多通道接收器在无损检测应用的最新进展激光超声多通道接收器在无损检测应用的最新进展 A Wartelle1 B Pouet1 S Breugnot1 白小宝 2 1Bossa Nova Technologies 2 矩阵科技有限公司 摘要 摘要 该激光干涉技术充分利用了激光超声检测该激光干涉技术充分利用了激光超声检测 LBU 的优越性 将其应用到工业检测系统和实验的优越性 将其应用到工业检测系统和实验 室研究 首先介绍了为工业应用而优化的光纤激光超声接收器的进展 将演示一个使用群速度为零室研究 首先介绍了为工业应用而优化的光纤激光超声接收器的进展 将演示一个使用群速度为零 的兰姆波检测移动工件厚度的例子 然后介绍了的兰姆波检测移动工件厚度的例子 然后介绍了 LBU 实验室级接收器的最新进展 最新研发的自实验室级接收器的最新进展 最新研发的自 适应干涉仪能同时测量面内和面外的位移 通过绘制一个由热弹性激光源所产生的面内和面外的位适应干涉仪能同时测量面内和面外的位移 通过绘制一个由热弹性激光源所产生的面内和面外的位 移来说明这种接收器在超声传播方面的应用 移来说明这种接收器在超声传播方面的应用 关键词 光斑 超声 正交 二波混频 干涉仪 激光关键词 光斑 超声 正交 二波混频 干涉仪 激光 1INTRODUCTION Laser based ultrasonic LBU is now becoming a more mature technology making the transition from research laboratory equipment to industrial on line measurement system However among the various optical techniques available only few have been successfully implemented in industrial environment To date LBU industrial inspection systems are still mostly based on the confocal Fabry P rot interferometer 1 2 The main well known advantages of LBU are its ability to perform remote non contact measurements as well as its point like and broadband detection New type of interferometric receiver based on the use of multi channel detection specifically designed for measurements in laboratories or industrial environments were recently introduced 3 4 A first receiver for industrial application is based on multi channel random quadrature interferometer 5 and a second one for high sensitivity laboratory measurements is based on a two components two wave mixing interferometer 6 We shortly describe here the design and principle of operation of both interferometers their improvements being based on detectors array combined with parallel processing 2RECEIVER FOR THE INDUSTRY 2 1Multi Channel Random quadrature interferometer QUARTET This interferometer is based on a variation of the quadrature interferometer using multi element detectors instead of two single element ones 3 Replacing the single element detectors by multi element detectors allows collecting more light more speckles and thus increases the Page 2 of 6 interferometer sensitivity by also taking advantages of the random nature of speckles A 25x2 channel random quadrature interferometer such as the Quartet 5 is then equivalent to 50 simple quadrature interferometers working in parallel Initial developments were based on a free space setup but for industrial application a fiberized architecture is more desirable 2 2Compact fiberized architecture The use of a fiberized detection head can be very advantageous for inspection of hard to reach area The multi channel random quadrature interferometer described 3 7 is well suited for the design of a compact and light weight fiberized interferometer The corresponding optical setup is shown in Figure 1 A In this fiberized setup we take advantage of the partial reflection at the fiber end for further simplifying the optical setup Indeed the partial reflection at the fiber end is used for generating the reference beam needed for interferences The multimode fiber is used to deliver the laser beam onto the workpiece to collect the scattered object beam and to combine the reference beam and object beam for generating the optical interferences signal The laser light is coupled into the multi mode fiber after reflection by the polarizing beam splitter PBS and focused by the Lens L1 The laser beam exiting the fiber end is focused by L2 onto the sample surface and the scattered object beam is then focused back into the multi mode fiber A fraction of the laser beam is also back reflected by the partially reflective fiber end generating the reference beam The reference beam and the object beam are combined together during the back propagation in the multi mode optical fiber The optical interference exiting back the optical fiber is then focused onto the array of detectors The signals are then processed by the parallel processing circuitry described in References 3 7 This fiberized interferometer is very advantageous because very long length of fiber can be used without degrading its performances Laser with long coherence length is not required as the path difference between the two interfering beams is only the round trip between the fiber end and the sample It is independent of the fiber length The system is very rugged because there is no critical optical alignment and high efficiency coupling into a multi mode fiber is easy to achieve A top view of the demonstration prototype is shown on Figure 1 B Here a 200mW CW laser was used The partial reflection at the multi mode fiber end was around 4 5 from the glass air interface The only alignment requirement remains the coupling of the light into the 50 m diameter multi mode fiber which is not critical FIGURE 1 A Optical setup of the fiberized multi channel random quadrature Interferometer B Top view of the corresponding prototype 2 3Experimental results We present here results for measurement on a moving sample Simulation of on the fly measurements is done by measuring on a rotating aluminum wheel The experiment has been performed at different rotation speeds between 0m s and 3m s The ultrasound generation is done by an Nd YAG pulsed laser in the thermoelastic regime Generation is unfocused and superimposed to the detection The sample surface has been sanded to diffuse light uniformly Zero group velocity Lamb mode resonance can be used for very accurate thickness measurement 6 Fig 2 3 show results at zero speed and 3m s On the Fourier spectrum of Fig 2 the resonance peak at 2 88MHz corresponds to the lamb mode S1 The first peak at 1 3MHz A B Page 3 of 6 corresponds to the low frequency mode of the lamb waves A0 As the plate is thin 2 mm the compression waves echoes are not visible Fig 3 and only the resonances are visible At 3m s the low frequency oscillations are due to rotation of the wheel introducing a Doppler effect From the S1 lamb mode resonance frequency we can get accurate thickness measurement FIGURE 2 Results showing the power spectrum of a signal acquired on a rough surface wheel rotating at 0m s and 3m s Generation Laser Thermoelastic regime Stand off distance 47cm Sample aluminum 2mm thick Sanded surface diffusing Power spectrum 30 Averages FIGURE 3 Results showing the single shot time signal corresponding to both measurements shown in Fig 2 The ability to carry out measurement on moving surfaces also allows performing fast scanning Other example of application includes the measurement of Ultrasonic Emission UE during high speed laser welding for in process detection of weld defect 9 The remote non destructive evaluation nature of the interferometer allows it to measure ultrasound on black plastic and composite materials as well as on high temperature ceramics 10 3RECEIVER FOR THE LABORATORY 3 1Two components Two Wave Mixing interferometer TEMPO 2D We took advantages on our experience on multi channel detection and parallel processing to develop a two wave mixing TWM interferometer capable to measure simultaneously the two components of the surface displacement out of plane and in plane In order to achieve high sensitivity the interferometer must be able to collect as much as possible of the back scattered light and to process efficiently the large number of collected speckles One well suited interferometer is the adaptive interferometer based on TWM in photorefractive crystal PRC 6 Because adaptive interferometers are optimized to collect many speckles they are not limited with the single speckle requirement of classical interferometer The in plane information is carried out by the speckled beams that are scattered away from the specular reflection and an interferometer with a large collecting aperture collects speckle beam corresponding to different angle of incidence It is thus potentially well suited for in plane detection An important feature of an adaptive interferometer using TWM in PRC is that multiple beams can be independently processed inside the same crystal without cross talk issue This feature is used to realize a multiplexed interferometer for simultaneous detection of multiple beams corresponding to the observation at different viewing angles of the same illuminated point We used Page 4 of 6 the same layout than our commercial interferometer TEMPO 6 The optical setup Figure 4 was adjusted in order to ensure that the entrance pupil the collecting optic is imaged on the detector and the single element detector was replaced with a detector array Each element of the detector array corresponds to a small area of the entrance pupil and thus corresponds to light backscattered along well defined incidence angles Processing of the interference signals as a function of the back scattered angle provides simultaneously in plane and out of plane displacements 4 The multi detector is a linear array with 16 elements The collected light is focused on the linear array using a cylindrical lens In this configuration we detect the in plane component along the orientation of the linear array The orthogonal in plane component is detected by rotating of 90o both the linear detector array and the cylindrical lens FIGURE 4 Two components Two wave mixing interferometer for in plane and out of plane detection A Optical Set and B Principle of the electronic demodulation circuitry The light backscattered by the sample may not be uniformly distributed and each channel must be normalized before calculating the in plane and out of plane Normalization is achieved using automatic gain controlled AGC amplifiers monitoring the low frequency signal generated by an internal piezo mirror in the path of the reference beam After amplitude normalization the signals are processed by pairs having same incidence angles For each pair the two normalized signals are added to generate the elementary out of plane component and their subtraction gives the elementary in plane component 4 3 2Experimental results An example of measurement is shown Fig 5 corresponding to a through transmission experiment The generation was achieved in thermoelastic regime with the laser beam focused along a line and detection was carried out on the other side The sample was a thick aluminum plate 12 7 mm thickness In this experiment we use a sample free of defects The detection was scanned along a 50 mm line A B Page 5 of 6 FIGURE 5 B scan results for through transmission In plane and Out of plane measurement on a 12 7mm thick aluminum plate no defect with laser line generation The B scan results for both in plane and out of plane displacements are shown Figure 5 The strong shear wave is clearly visible Many reflected and converted wave arrivals are visible After 12 s some reflections from the sample edges are visible For these measurements the calibration coefficient is 100mV nm for both in plane and out of plane output Moreover the in plane signal is clearly identifiable as it shows sign difference on both sides of the epicenter which the out of plane signal doesn t 4CONCLUSION On the industrial side the fibered multi channel random quadrature interferometer has been developed for industrial non destructive evaluation measurements Results enhance its robust design taking advantage of speckle statistic and carrying out very sensitive measurements on moving sample with rough surfaces The integration of a fibered inspection head is very advantageous for industrial applications and furthermore led to a more compact setup by taking advantage of the light reflection at the fiber end On the laboratory side we have presented a compact optical arrangement which takes advantage of the large optical tendue of TWM interferometer for measuring simultaneously the out of plane and in plane ultrasonic displacements This two component ultrasonic receiver opens new possible subjects for further researches to better understand ultrasonic waves in various materials 5REFERENCES 1 Fiedler C J in Review of Progress in QNDE 20 eds D O Thompson and D E Chimenti Plenum New York 2001 p 308 2 Monchalin J P et al in Review of Progress in Q