Optical assembly for contactless vibration measurement

0001The invention relates to a device for non-contact, optical measurement of an object, in particular for displacement and / or vibration measurement, by means of a laser interferometer with at least one measuring and at least one reference beam, wherein the device has a means for generating a frequency shift.

0002Laser interferometers provide ideal measuring arrangements for high-resolution path or Vibration measurement dar. The measurements take place purely optically, ie without contact. In this case, coherent light, which is preferably generated by a laser, split into a measuring and a reference beam. The measuring beam is guided by itself in a so-called measuring arm to a test object, reflected by this and then superimposed with the reference beam. This results in interference patterns that are extremely sensitive to the length of the measuring arm.

0003For sign-correct determination of vibrations, so-called heterodyne interferometers have generally prevailed. In these measuring beam and reference beam are shifted in frequency against each other. This shift can be generated either by an optoacoustic modulator, in particular a Bragg cell, or by tuning a laser diode in conjunction with a transit time difference between the measuring and reference beam.

0004In the described, usually used two-beam arrangements, such as Michelson or Mach Zehnder, from which the preamble of claim 1, however, a large number of optical components is necessary, so that adjust the interferometer with considerable effort in four or more degrees of freedom and to adjust.

0005In addition, the sensitivity of the known laser interferometer leaves something to be desired, since usually a light guide is used, which is part of the measuring arm and thus makes the arrangement sensitive to vibrations and interference in the light guide. This noise may be particularly disadvantageous when measuring on poorly reflective surfaces of untreated DUTs.

0006By RI Laming et al. <img file="EP0932050A2_D0001.tif" />A Practical All-Fiber Laser Vibrometer ", Experimental Techniques March / April 1990, therefore proposes an interferometer in which the light is split into the measuring and reference beams only at the end of the light guide. Cell and the use of a tunable semiconductor laser possible.

0007Disadvantage of this construction, however, is that the frequency shift by the modulation of the injection current in the semiconductor laser is possible only for relatively low frequencies and the range of the measurable vibration frequencies and fast speeds is severely limited. Furthermore, weak reflecting surfaces at an intensity fluctuation of the laser light caused by the injection current modulation often fall out of the measuring range due to disturbances in the amplitude modulation. Finally, the use of a semiconductor laser over a gas laser generally has some drawbacks, since the short coherence length of the light emitted by the semiconductor laser in the interferometer causes phase noise, which increases linearly with the path length difference between the measuring and reference arm.

0008On this basis, the object of the present invention is to eliminate the aforementioned advantages, in particular to provide a laser interferometer, which is characterized by an increased sensitivity, a lower noise and a significantly reduced adjustment effort. Furthermore, an interferometer operation should not only be possible with semiconductor lasers and, finally, the interferometer should be suitable for use in the medical field.

0009This object is achieved in that the measuring or reference beam passes through the means for generating the frequency shift at least twice, so that the frequency shift of the beam is increased.

0010With <img file="EP0932050A2_D0002.tif" />Going through "is meant here any kind of interaction with the means for generating the frequency shift, such as a reflection.

0011By passing through the means for frequency shifting twice, the optical complexity is reduced considerably, since the same optical path can be used in both passes, thereby making large parts of the previously required mirror and lens arrangements superfluous. Furthermore, when passing twice, there is the additional advantage that the laser beam is shifted twice in frequency. Thus, an overall greater frequency shift is achieved in terms of magnitude, which increases the measurable speed and extends the measuring range to even higher oscillation frequencies.

0012It is favorable if the beam shifted in frequency is the measuring beam, because this gives a very simple arrangement if the measuring beam can be hit perpendicular to the test object and this is reflected back into itself. Then the beam will pass through the means for generating the frequency shift for the first time on its way to the DUT and once again on its return.

0013As means for generating the frequency shift can be advantageously use a Bragg cell, as it has proven many times in practice.

0014Often, a DUT must be measured under tight space conditions. It is advantageous if the laser can be placed at some distance from the object to be measured and the light coming from the laser is guided via an optical fiber to a measuring head. From the measuring head, the light is then projected onto the DUT.

0015In such an arrangement, the most favorable place for the means for generating a frequency shift has been found to be the area between the light guide and the object under test, as a rule the measuring head.

0016A particularly advantageous embodiment of the invention is that the division of the light in the measuring beam and the reference beam after passing through at least a portion of the light guide takes place. Characterized in that the measuring and reference beam are performed as long as possible together through the light guide, both are influenced in exactly the same way, so that cancel the disturbances and the measurement result is not affected. Furthermore, the length of the remaining measuring arm is reduced by the construction described, which is advantageous in that, in particular, the light from laser diodes does not have a large coherence length and thus leads to increased phase noise in the interferometer. This phase noise has the stronger effect the greater the absolute difference in length between the measuring arm and the reference arm of the interferometer.

0017Advantageously, the distribution of measuring beam and reference beam at the outlet end of the optical waveguide takes place in that the measuring beam emerges from the optical waveguide, while the reference beam is reflected back from the optical waveguide end into the optical waveguide. In the simplest case, the fraction reflected anyway at the light guide-air transition is used as the reference beam. This eliminates all previously necessary measures to reduce this proportion, which accounts for about 4% of the total intensity and made previously disturbing, because this proportion was greater than that of the measurement signal.

0018However, it is also conceivable to increase the intensity of the reference beam, for example by additionally mirroring the exit end.

0019The feature of the distribution at the exit send allows a particularly simple installation, since otherwise necessary, complex adjustments omitted. The reference beam is returned by itself.

0020In addition to a splitting at the outlet end of course, a splitting behind it or a separation in measuring and reference beam in the light guide is conceivable, but less advantageous. In the light guide, this z. B. generated by a generated in the production of the fiber optic jump of the refractive index at a defined point of the light guide.

0021The measuring arm is particularly short and thus particularly insensitive to unwanted adverse effects from the outside, if the division into measuring and reference beam only takes place in the measuring head.

0022The desired insensitivity to disturbances can be further increased if the measuring and reference beam are guided together not only on the way, but also on the way back. For this purpose, they are advantageously passed from a common optical fiber to a detector of the interferometer, with which the changes of the interference pattern are measured. Conveniently, this light guide is identical to the light guide mentioned above, so that you do not need a second light guide, which would have to be coupled with the first light guide. Rather, measuring and reference beam respectively enter at the same ends of the light guide or off - also concerning the supply to the detector.

0023In order to obtain an identical, canceling influence by external disturbances of measuring and reference beam in the light guide, these advantageously pass through the light guide with the same polarization, in particular both are identically linearly polarized. This applies both to the way back and for the way back through the light guide or the.

0024To increase the measurement sensitivity, the device preferably includes an isolator system which prevents light from being fed back into the laser but provides all but some of the signal power to the detector. The insulator system has a Faraday rotator and a polarization-discriminating element, in particular a polarization beam splitter.

0025By using the light guide and its insensitivity to vibrations, the device according to the invention is particularly well suited for a multiplex operation. In this case, a plurality of optical fibers are used, each with a measuring head, wherein the optical fibers may well be longer than usual and can be laid to the most varied points of one or more objects to be measured. You then only need a single control device with laser and detector, which saves costs. This control device expediently has an association device for allocating and coupling the laser light into the different light guides.

0026The device described above is particularly suitable for a method for optical measurement of an object, which is characterized in that is coupled to optimize the position of an implant, in particular a hearing aid, on or in the human or animal body of the measuring beam in the beam path of a surgical microscope. This expressly refers to an in vivo positioning. That is positioning during surgery on an animal or human.

0027Here, the advantages of the above-mentioned device according to the invention make particularly noticeable, since the implants can not be extra mirrored in the rule and one is dependent on a high sensitivity of the measuring arrangement because of their naturally low reflectivity. In addition, modern surgical microscopes are often hung for maximum freedom of the doctor in the operation extremely movable on long guide arms. In order to be able to follow this mobility in the interferometric measurement, it is necessary to use very long optical fibers whose sensitivity to vibration is eliminated by the device according to the invention.

0028While the weighty part of the interferometer with precision laser and high-sensitivity detector can be conveniently located apart, the light is guided via the flexible light guide to the pivotable part of the microscope and coupled here in the beam path so that the doctor during surgery part of the reflected from the implant Detect laser light as a measuring point and thus can determine the measuring location.

0029It is not absolutely necessary that the laser light also passes through the lens system of the microscope, the light can be coupled for simplicity between the microscope and implant in the beam path of the microscope.

0030Alternatively to the mentioned device with precision gas laser and light guide z. B. also be a miniaturized interferometer with semiconductor laser completely attached to the surgical microscope to perform the inventive method.

0031The method is particularly suitable for the investigation of mechanically induced damage to hearing organs, for the analysis of same and for the determination of suitable positions of implants and active hearing aids and hearing organs. Their functional control has so far been made only very rudimentary. In a further development of the invention

0032Method, the implant is then preferably during operation oscillated at different positions. These vibrations are measured with the interferometer, so that the ideal installation position of the implant can be determined.

0033For the most realistic possible function control, the vibrations are generated by sound, which is transmitted via the still intact elements of the hearing apparatus of humans or animals as vibration to the implant.

0034The measuring beam of the interferometer is suitably introduced through an operation opening in the head or body of the human or animal.

0035In development of the device described above, this can also be used as a microphone. Microphones of very high quality have the problem that the vibration picked up correctly by the diaphragm is corrupted by the transmission system (magnetic or capacitive). However, a laser vibrometer has a flat frequency response over a very wide range. Since one achieves a very small construction of the measuring head with the device according to the invention, one can install it as a vibration sensor in a microphone, in particular in its housing, which can also be emaciated to a frame. The laser beam absorbs the vibrations of the diaphragm and you have a very clean signal at the output of the vibrometer electronics. The advantage is that acoustic signals can be optically measured on the membrane, so that the vibration characteristics of the membrane are not affected.

0036The membrane vibrations can be measured particularly accurately if the measuring beam is coupled into the beam path of a microscope.

0037Other inventive features and advantages will become apparent from the description of an embodiment with reference to the drawing; shows<dl id="dl0001" compact="compact"><dt>FIG. 1</dt><dd>the schematic structure of a device for non-contact vibration measurement; and</dd><dt>FIG. 2</dt><dd>a sketch of a surveying procedure.</dd></dl>

0038FIG. 1 shows a laser 1 whose collimated light beam first passes through a beam splitter 2. The beam splitter 2 is used for decoupling the returning measuring and reference beam for the detection and will be described in its function below. For an optimized, selective decoupling of the returning beams, the beam splitter is preferably a polarization beam splitter, behind which a Faraday rotator 3 and a polarizer 4 are arranged. In certain cases it is also possible to divide the returning rays in a 50:50 beam splitter.

0039The thus polarized laser light is coupled via a lens system 5 in an inlet end 6 of a light guide 7. The light guide 7 is preferably a single-mode optical fiber. Light reflected back from the entrance end 6 of the light guide 7 is lost or undesirably superimposed with the returning measuring and reference beam. Therefore, the entrance end 6 is preferably treated so that as little as possible light is reflected back.

0040The light passes through at least a substantial part of the light guide 7, before it, in the illustrated embodiment at the outlet end 8 of the light guide, is divided into measuring beam and reference beam.

0041For the division, the exit end 8 of the light guide 7 reflects an appropriate portion of the laser light back into the light guide 7. It is essential that the reflected portion acts as a reference beam.

0042As a rule, the intensity of the portion reflected back at the glass-air transition of the outlet end 8 is about 4% of the intensity of the incoming laser light. In certain cases, however, it is advisable to use an additional coating of the outlet end 8 in order to increase the intensity of the reference beam.

0043The light not reflected back or absorbed at the exit end 8 acts as a measuring beam. It exits the light guide 7 at its exit end 8 and is collimated or nearly collimated by means of a second lens system 9 and directed to a means 10 for producing a frequency shift performed as a Bragg cell.

0044From the means 10 for generating a frequency shift, the measuring beam is reflected to a third lens system 11, which focuses the collimated measuring beam at a point 12.

0045The point 12 is preferably located in the focal plane of a fourth lens system 13 to allow the use of interchangeable and focusable front optics, such as in the form of camera lenses.

0046From the fourth lens system 13, the measuring beam is finally imaged on the measurement object 14. Thus results from the exit end 8 to the test object 14 as short as possible measuring arm for the incoming measuring beam.

0047The measurement beam is reflected by the measurement object 14 and experiences a Doppler frequency shift which is proportional to the instantaneous velocity of the surface of the measurement object 14 in the beam direction. The reflected back measuring beam passes through the opposite path of the incoming measuring beam. It is thus coupled via the lens systems 13, 11, the means 10 for generating a frequency shift and the lens system 9 in the outlet end 8 of the light guide 7.

0048It is essential that he interacts a second time with the means for generating a frequency shift, wherein he experiences a second, rectified, additional frequency shift.

0049The measuring head contains in the illustrated embodiment, the outlet end 8, the second lens system 9, the means 10 for generating a frequency shift, the third lens system 11, and the fourth lens system 13. The division into measuring beam and reference beam thus takes place in the measuring head. Due to the division at the outlet end 8 of the optical waveguide 7, the otherwise necessary beam splitter and the measuring head is built correspondingly small. Adjustments in the measuring head are virtually eliminated.

0050If the working distance from the measuring head to the measuring object 14 is sufficiently small and is not changed, the second lens system 9 can also be used for focusing. Thus, the third and the fourth lens system 11, 13 and the costs caused thereby can be saved.

0051The measuring beam is now returned in the light guide 7, ie on the same path as the reference beam. As a result, the light guide 7 is also insensitive to vibration and vibration with respect to the return of measurement and reference beam. From the inlet end 6 of the light guide 7 measuring and reference beam are projected in the direction of the beam splitter 2. By Faraday rotator 3, the polarization plane of the measuring and reference beam has been rotated on the way back and forth that the returning rays are completely directed with appropriate adjustment of the polarization beam splitter 2 to the detector 15 and not fed back into the laser 1.

0052In the case of a high-intensity helium-neon laser, the isolator system consisting of expensive Faraday rotator 3 and polarizer 4 can be dispensed with. The beam splitter 2 is replaced in this case by a 50:50 beam splitter. In each case 50% of the transmitted and received light will be lost.

0053The interference pattern of the measuring and reference beam, which changes as a result of the vibrations of the test object 14, is expressed on the detector 15 as an intensity modulation of the incoming light. The frequency of the intensity modulation is given by the sum of the double frequency shift produced by the Bragg cell 10 and the Doppler shift by the speed of the DUT 14. This optical signal is converted by the detector 15 into an electrical signal and filtered and amplified by means of a pre-processing stage 16. The signal thus generated is then converted by means of a demodulation electronics 17 in a known manner into a signal which the vibration speed or characterized the amplitude of the measured object.

0054In FIG. 2, the method according to the invention for the optical measurement of an object is sketched. In this case, a viewer 21, usually the doctor during the operation, looks through a surgical microscope 22, which is very freely movable on a microscope suspension 23. Through this microscope, he can observe the course of the operation and precisely control even very small details. This is especially important for operations in the head area, for example at the ear 24. Sensitive parts of the body, such as the eardrum or other parts of the middle ear, can only be operated on successfully.

0055It is essential that the measuring beam of a laser interferometer is coupled into the beam path of the surgical microscope 22. As a result, a path and / or vibration measurement can be made on an object visible through the microscope, it being even possible to see the measuring point in the microscope and thus to position it exactly.

0056The object is either an implant or the eardrum or another body part to be measured. By Wegund / or vibration measurement, the optimal installation of the implant can be found. For this purpose, the ear is excited, for example by sound or direct mechanical excitation to vibrate. Then, the implant is positioned so that it in turn has a predetermined, desired oscillation amplitude. As an alternative to measuring the implant, it is also possible to measure the vibrations of the eardrum or of another part of the body and to deduce the change in the vibration behavior to the correct installation location of the implant.

0057The coupling of the measuring beam into the beam path of the surgical microscope 22 takes place in the illustrated embodiment via a beam splitter 26. The measuring beam is generated by a laser, not shown, and passed through a light guide 27 to a measuring head 28.

0058In the measuring head 28, the measuring beam exits from the optical fiber 27 and passes through a collimator lens 29 to a Bragg cell 30. At the Bragg cell 30 of the measuring beam is frequency shifted and preferably via two more lenses 31 and 32 under deflection by the beam splitter 26 or a mirror rod focused on the object to be measured.

0059The measuring beam is coupled between the surgical microscope 22 and the object 25 in the beam path of the surgical microscope 22. For this purpose, a holder 33 is attached to the surgical microscope 22, which carries the measuring head 28 and the beam splitter or mirror rod 26. The light guide 27 is laid along the microscope suspension 23.

0060Alternatively to the focusing of the measuring beam on the eardrum 25 shown in FIG. 2, it can be focused directly onto the implant, for example, through an operating opening in the head.

0061Because of the normally very low reflectivity of body parts and implants and the great length of the light guide 27 to be used, the method according to the invention can be carried out particularly well with the device according to the invention described above. The device and method thus enable new surgical techniques and bring great progress for medical technology for the benefit of the patient.