Device for the contactless recording of surface deviations of a specimen caused by ultrasound waves.

p0001The invention relates to a device for the contactless detection caused by ultrasonic waves surface deflection of a test specimen, having a monochromatic, coherent scanning beam generating for the test sample laser whose light spot on the sample surface by means of an imaging optical system on the one hand via a reference arm and on the other hand, via a little delay of an interferometer a photodetector is represented with a downstream evaluation.

p0002Such a device is described in DE-OS 24 57 253 and is used in particular for non-contact ultrasonic receiving highly heated and / or fast continuous running, for example, heavy plates. The light reflected by the rough surface of the test sample light is split with the aid of a partially transparent mirror in two different lengths interferometer and then brought to interference in order to be detected by the photodetector. Here, the light spot is imaged on the sample as possible with the same magnification over the two arms on the photodetector. In the interferometer used in the known device is a Laufzeitinterferometer, which is distinguished by a high light gathering power, so that the use of the known device also on rough surfaces is possible. But to interfere the light waves both arms with a certain phase relationship, the known device arise when operating under real conditions with considerable difficulties. Fluctuations in the path difference of both arms as they arise, for example, by building vibrations, thermal variations and other vibrations must be reduced to a fraction of the wavelength of light. In the known device, the length of Verzögerungsarms is to interferometrically measured and adjusted accordingly by adjusting or regulating element in order to compensate for variations of the mirror of the interferometer. The complex compensation process by automatically adjusting the mirror in the interferometer little delay but allows only partially to suppress the disturbances.

p0003Starting from this prior art, the invention aims to improve a device of the aforementioned type so that the influence of interference on the interferometer is particularly largely suppressed by vibration and simultaneously there is a sensitive and reproducible signal evaluation at a high light gathering power.

p0004This object is achieved in that in at least one of the two arms of the interferometer a frequency-shifting cell is arranged and that the evaluation includes a demodulation circuit, which can be supplied with the phase-modulated by the surface deflection of the specimen carrier signal at the photodetector output and the ultrasonic waves associated useful signal of lower frequency noise signals separated.

p0005In the frequency shifting cell may be a working according to the acousto-optic effect Bragg cell or a working according to the electro-optic effect cell. If the beam in question interacts in an A / O deflector connected to a high-frequency carrier generator with the generated acoustic wave there, there is a shift to tender the light frequency to the frequency of the carrier generator. for frequency offset when the electro-optic effect is utilized, an interaction of light with an externally applied high frequency electric field within a medium.

p0006When the light of one of the two arms, that is, either the main beam in the little delay or the reference beam is frequency offset in the reference with respect to the light of the other arm, provides the applied over both optical paths photodetector in addition to a direct current component, an alternating with the difference frequency of the two interferometer arms or light arms sinusoidal carrier signal to which all variations of the path difference of the two arms are modulated in a characteristic way the interferometer as jitter. The frequency of the carrier signal is, for example 200 MHz. In a photodetector the downstream electronic demodulation caused by faults low-frequency modulation signals are compensated for, while the radio-frequency, corresponding to the sample surface Ultraschallauslenkungen modulation signal is output for further evaluation.

p0007In an advantageous embodiment of the invention, the interferometer can be optimally adjusted by changing the length of Verzögerungsarms each sound frequency. In particular, in short delay times and high bandwidths can be achieved at high frequencies. The delay is preferably about half a period of the ultrasonic signal. An increase or decrease of Verzögerungsarms can be achieved in a simple manner, if the distances are changed by adjusting the deflection mirrors or prisms of Verzögerungsarms the interferometer.

p0008If the little delay as a light guide with a medium whose refractive index is totally or partially greater than 1 is realized, it is possible to construct the device particularly compact. In addition, it can be ensured in a simple manner in this way on both arms of the interferometer, a same magnification.

p0009The connected to the photodetector transmitter has a loaded with the carrier signal of the photo detector and the oscillator signal of a voltage controlled local oscillator mixer, whose output is one hand to a band-pass filter for the ultrasonic waves in the test object associated with the useful signal and the other hand to a control loop, by the frequency of the local oscillator to suppress low frequency noise can be controlled. In a preferred embodiment, the control loop is provided with a noise up to 200 kHz processing control voltage generator and amplifier.

p0010Embodiments of the invention will become apparent from the drawing. Show it:<ul><li>Fig. 1 shows an embodiment of a device according to the invention in a schematic representation,</li><li>FIG. 2 shows another embodiment of the invention with a an optical fiber having little delay and</li><li>Fig. 3 is a block diagram of a circuit for the electronic demodulation supplied by the photodetector output signal.</li></ul>

p0011The apparatus shown in Fig. 1 for the contactless detection of ultrasonic waves has a laser 1 which generates a coherent monochromatic scanning beam 2, the rough surface 4 of a test piece 5 loaded in the direction of an arrow 3 and produces a light spot 6 in the region of impact of the scanning beam. In the area of ​​the light spot 6, the laser light is reflected as diffusely Meßstrahlbündel 7.

p0012The the rough surface 4 having DUT 5 is subjected to ultrasonic waves 8, which are shown symbolically in the drawing. The device for the contactless detection of ultrasonic waves is used to measure the amplitude and the frequency spectrum of the ultrasonic waves 8 without contact. The force required to generate the ultrasonic waves 8 ultrasonic excitation is optically, electro-mechanically or acoustically. In particular, it is possible to generate the ultrasonic waves 8 by means of short laser pulses, which can be generated by a high power laser, not shown in the drawing. A sound generation by short laser pulses has the advantage, also be carried out without contact.

p0013By determining the frequency response with the aid of the device shown in FIG. 1, information can be obtained about the device under test to study its material properties more accurately.

p0014The diffusely reflected from the rough surface 4 Meßstrahlbündel 7 feeds an interferometer 9 with components and devices of a Laufzeitinterferometers a Heterodyneinterferometes, which are described in more detail below.

p0015The Meßstrahlbündel 7 applied on entry to the interferometer 9 comprises a first optical beam splitter 10, through which a portion of the Meßstrahlbündels 7 is coupled out transversely with respect to the embodiment illustrated by an arrow 11 the propagation direction of light reflected at the surface 4 Meßstrahlbündels 7 in the direction of an arrow 12 as a reference beam bundle 13 , The light of the reference beam 13 passes through a first lens system 14, with the aid of the light spot 6 is imaged on a photodetector 15th The reference beam 13 forms the reference arm of the interferometer 16 9 further has a little delay 17, which is described below.

p0016The transmitted through the optical beam splitter 10 part of the diffusely reflected Meßstrahlbündels 7 loaded in the direction of the arrow 11 as the main beam 18 the little delay 17 of the interferometer 9. The little delay 17 is designed so that the light spot 6 with the magnification of the reference arm 16 corresponding magnification on the photodetector is ready 15th

p0017The light of the main beam 18 is collected by a second lens system 19, the light spot 6 reflects on the surface 4 of the specimen 5 in a Bragg cell 20, which serves in the usual case Heterodyneinterferometern way to a frequency offset or wavelength offset of the main beam 18 cause. The Bragg cell 20 is an A / O-deflector utilizes the acousto-optic effect to achieve a frequency offset, whose magnitude is dependent on the frequency of a radio-frequency carrier generator 21, via a line 22 to the Bragg cell 20 connected is. Instead of the Bragg cell 20 also operates according to the electro-optic effect cell can be used. The frequency of the carrier generator 21 is for example 50 to 200 MHz.

p0018On the second lens system 19 opposite side of the Bragg cell 20, a third lens system 23 is provided through which is collected and collimated divergent 20 exiting frequency shifted light from the Bragg cell. The collimated main beam frequency-shifted beam 24 is deflected 26 by means of a first deflection mirror 25 as well as by means of a second deflection mirror twice and moved in parallel with a fourth lens system directed 27th Instead of deflecting mirrors 25, 26 of course, prisms or other optical components can be used which allow the construction of a Verzögerungsarms 17th The optical components 23 to 27 are used to image the image formed in the Bragg cell 20 of the light spot 6 on the photodetector 15th To this end a second beam splitter 28 is further also provided, by means of which the light of the frequency-shifted main beam 24 is coupled into the optical path of the reference beam thirteenth

p0019From the above description follows that an image of the light spot 6 on the photodetector 15 on the one hand via the reference arm 16 and the other, on the little delay 17 takes place with the same magnification. It goes without saying that the propagation time of the light in the little delay 17 is greater than the time required for propagation in the reference sixteenth Preferably, the delay is selected such little delay at 17, that its duration the duration of a half period of the ultrasound to be detected corresponds approximately.

p0020The output of the photodetector 15 is connected to a in Fig. 3 as a block diagram shown demodulation stage 29 which in turn, via a line 30 provides a useful signal for an evaluation 31st

p0021The photodetector 15 provides in addition to a DC component a with the difference frequency of the two interferometer arms 16, 17 alternating sinusoidal carrier signal to which all the variations of the path difference of the two arms 16, 17 are modulated in a the interferometer characteristic way as phase fluctuations. In the demodulation stage 29, the interference caused by the low frequency modulation signals are compensated for to output the high-frequency, the Ultraschallauslenkungen the surface 4 of the specimen corresponding modulation signal to the evaluation unit 5 31st

p0022If the DUT 5 is not acted upon by ultrasound, the roughness of the surface 4 overlap at the location of the light spot 6 corresponding phase fronts 34, 35, which are shown in Fig. 1 in the vicinity of the photodetector 15, constructively and the photodetector 15 provides a high-frequency carrier signal with the frequency of the carrier generator 21. When the test piece 5 is acted upon by ultrasound is that generated by the photodetector 15 and the demodulation stage 29 supplied carrier signal so phase-modulated that the phase modulation of the Auslenkungsdifferenz the surface 4 at the location of the light spot 6 between two different is proportional times. The time between the two points is equal to the time difference of the light in the interferometer arms 16 and 17. The detectable frequency components (bandwidth) of Auslenkungsdifferenz are also by the different lengths of maturity in the two interferometer 16 and 17 conditionally. Low frequency noise signals are already part due to the system weakened and further suppressed in the subsequent demodulation stage 29 again, as is clear from the description of FIG. 3.

p0023Fig. 2 shows a second embodiment for the apparatus for the contactless detection of ultrasonic waves, whereby in Fig. 2 components which correspond to those in Fig. 1 have the same reference numerals and will not again separately in connection with Fig. 2 will be described.

p0024Unlike the embodiment of FIG. 1 is a simplified lens system 40 is provided which receives a portion of the diffusely reflected Meßstrahlbündels 7 and the light spot 6 maps on the specimen 5 to the photo detector 15. The imaging is carried out in two different ways, namely, on the one hand the reference arm 16 and on the other hand little delay 17. The splitting of the light of the Meßstrahlbündels in the different ways is effected by means of the beam splitter 10, which is formed as a partially transparent mirror.

p0025While the figure via the reference arm 16 through the medium takes place through the air, the light path results in little delay 17 through a medium 41 having a refractive index which is greater than 1, so that the running time is increased, but at the same time an equal magnification is ensured. The medium 41 is suitably made of a light guide with a light input surface 42 and a light exit surface 43. The light input surface 42 is net so angeord that the 11 incoming light from the main beam 18 can be collected by the beam splitter 10 in the direction of the arrow. The light exit surface 43 is arranged so that the light of the frequency-shifted main beam applied to the second beam splitter 28th Within the medium 41 to continue to recognize two deflecting mirrors 44, 45 that the deflecting mirrors correspond functionally 25 and 26th

p0026As can be seen further in Fig. 2, is in the medium 41 which is connected to the radio-frequency carrier generator 21 frequency-shifting cell, in particular, the Bragg cell 20 is introduced. The operation of the apparatus shown in Fig. 2 is analogous to the operation described in connection with FIG. 1,. In particular, a superposition of the little delay 17 passing through the light with the fed via the reference arm 16 of light at the beam splitter is carried out 28 Analogously to the apparatus described in FIG. 1 provides the photodetector 15, a high frequency carrier signal on which variations in the path difference of the arms 16, 17 in form of phase fluctuations are modulated. One way of demodulation results from the block diagram of FIG. 3.

p0027The embodiment of FIG. 2 has the advantage that the image of the light spot 6 on the photodetector 15 by a relative to the outlined in Fig. 1 exemplary simplified lens system takes 40, allowing through the medium having a refractive index greater than 1 in little delay 17 becomes.

p0028In Fig. 3 is a block diagram of an advantageous embodiment of the demodulation stage 29 is shown within the electronics of the device. As seen in Fig. 3, the photodetector 15 is connected to the first mixing input 50 of a mixer stage 51. The second mixing input 52 of the mixer stage 51 is connected to the output of an adjustable on the carrier frequency of the voltage controlled local oscillator 53rd The input 54 for controlling the oscillator frequency of the oscillator 53 receives the control signal of a control loop 55, by means of which the oscillator frequency is adjusted for suppressing low-frequency interference in the intermediate frequency output signal of the mixer stage 51st The available at the output 55 of the mixer 51 intermediate frequency signal fed on the one hand to the input 56 of the control loop 55 and on the other hand a bandpass filter 57, of the desired the ultrasonic waves 8 assigned useful signal at its output 58 free of low-frequency through the loop 55 suppressed noise.

p0029The output of the photodetector 15 supplies to the input 50 of the mixer stage 51 supplied carrier signal of frequency omega of the carrier generator 21. This signal is phase-modulated with the useful signal Delta (t) and the interference signal Xi (t), so that the entire phase-modulated signal theta ( t) = delta (t) + X i (t). In the control loop 55, the low-frequency interference signals xi (t) are detected and so related to the frequency control of the oscillator 53 that the interference signals at the output 55 of the mixer stage, that is, are suppressed in their intermediate frequency signal. By the superposition of carrier signal and the signal of the oscillator, the intermediate frequency signal is generated and eventually detects the useful signal over the bandpass 57th