Method and device for the optical non-contact oscillation measurement of an oscillating object

#The invention relates to a device and method for optical, contactless vibration measurement of a vibrating object.

#Such a method utilizes and such a device comprises a laser Doppler vibrometer with a laser as light source for a laser beam. This laser beam is split into a measurement beam and a reference beam in a first beam splitter arrangement. Either the measurement beam or the reference beam experiences a frequency shift, for which an acousto-optic modulator, in particular a Bragg cell, is usually used. The measurement beam is directed to an object to be measured and scattered back or reflected from there. A second beam splitter arrangement combines the measurement beam scattered back from the vibrating object with the reference beam and superposes the two beams, resulting in an interference signal. The superposed measurement and reference beams are fed to a detector, which generates an electric measurement signal from the interference signal.

#Laser Doppler vibrometers are able to perform contactless measurements up to the megahertz range of vibrations of objects, in particular of object surfaces. This opens up applications when measuring vibrations of very small and light structures, such as in micro-systems technology. However, it is also possible to measure vibrations in the air and in fluids with the aid of a laser Doppler vibrometer. Here, the frequency of the measurement beam is modulated by the movement of the object surface to be measured as a result of the Doppler Effect during reflection. Since the laser emits coherent light, an interference signal emerges from the superposition of the measurement beam, which is frequency modulated by the object movement, and the reference beam, which remains unchanged, from which interference signal it is possible to derive the speed of the object. Hence the vibration speed of the surface of the object to be measured is acquired.

#An exemplary embodiment of the present invention will be described and explained in more detail below on the basis of the attached drawings. In detail:

#FIG. 1 shows two alternative block diagrams (FIGS. 1a and 1b) of a heterodyne laser Doppler vibrometer for use in a device according to the invention;

#FIG. 2 shows a schematic diagram of the amplification profile of a helium-neon laser, used in an exemplary fashion;

#Here, a device with a laser Doppler vibrometer, which in principle has a design as depicted in FIG. 1a, is described as exemplary embodiment for a device according to the invention. In the present exemplary embodiment, use is made of a helium-neon laser 1 with a resonator length of 20.4 cm, the coherent light of which is split by a first beam splitter S1 into a measurement beam 2 and a reference beam 3. The reference beam 3 is routed over a mirror Sp1 through a Bragg cell 4, which serves as an acousto-optic frequency shifter in this case, and reaches an optical detector 5 via a further beam splitter S3. Here, the Bragg cell 4 shifts the reference signal 3 in terms of its frequency by a frequency offset of typically 40 MHz.

#The measurement beam 2 is routed via a second beam splitter S2 and a lambda/4 plate L1 to a vibrating measurement object 6. The surface of the measurement object 6 scatters the measurement beam 2 back. In the second (polarization) beam splitter S2, the back-scattered measurement beam is reflected onto the third beam splitter S3 and there it is superposed on the reference beam 3. The superposed, time coherent measurement and reference beams form an interference signal 7, the intensity of which is received by the optical detector 5. In another design, the lambda/4 plate L1 can be omitted such that a normal beam splitter can be used as S2.

#As a result of the Doppler Effect, the light of the measurement beam 2 reflected at the vibrating measurement object 6 is frequency shifted in accordance with the current speed of the measurement object surface. This frequency shift is directly proportional to the speed of the scanned object surface. Since the measurement beam 2, which is frequency shifted thus, is not superposed on a reference beam 3 that remained unchanged, but rather superposed on the measurement beam 3, which was provided with a frequency offset by means of the Bragg cell 4, it is possible to determine not only the current vibration speed of the measurement object surface from the signal of the detector 5, but also the sign thereof. The vibration movement is therefore established uniquely.