FR2775778A1 - Device for measurement of surface vibration caused by laser light source for testing materials properties - Google Patents

Abstract

The device comprises a beam splitter (3) for separating off a part of the light beam before the test piece (6), with the separated light beam then being analyzed using a reference interferometer (10)

Description

 The invention relates to the measurement of vibrations and more particularly the remote measurement of surface vibrations of very small amplitude of a rough workpiece. This type of measurement is frequently carried out to control the quality of the parts (shape, homogeneity, bonding, cracks, characterization of materials and products ...) in a non-destructive manner.

 Indeed, it appeared interesting to control the quality of the parts, possibly during production, by subjecting them to ultrasonic waves, for example generated using a laser pulse, and by detecting the vibratory response of the part. so excited.

 An effective application of such a control requires a good reliability of the measurement of the vibratory response which most often occupies a large frequency domain depending on the material crossed. These vibration measurements are generally considered to be difficult to perform and it is possible to limit this analysis to the ultrasonic waves contained in the range [50 kHz; , 20 MHz]. Various proposals have already been made to carry out this type of measurement.

 J.P. Monchalin, for example in US Pat. No. 4,659,224, proposed to analyze a light beam reflected on the target by an interferometer of the confocal Fabry-Perot type. This solution proves to be well suited to the detection of ultrasonic frequencies higher than MHz, but is not very precise at lower frequencies and it is also subject to the frequency stability of 1n light source.

In addition, it was proposed by Scruby and Le Drain in (<Laser ultrasonics - 1990 - Techniques and applications to carry out such measurements of vibrations at a distance by the analysis of a laser beam, of wavelength Ro, backscattered by the surface to be analyzed using a two-wave interferometer producing an optical path difference L between these two waves. One way of presenting this type of measurement is that the emission frequency of the laser is modified, by effect Doppler, during its backscattering on the surface of the object to be analyzed, as a function of its speed u; the interferometer then behaves like a spectrum analyzer measuring this variation in frequency.
L = B0 / 4 modulo ho / 2, and c = 3 x 108m / s allow to write that the recombination of the waves of the two arms of the interferometer will give rise to the signal:
S = So (1 + Csin [47rLu / (cko)]}
This general formulation of the signal defines the main parameters of the measurement. So is proportional to the light flux entering the interferometer. C is a digital function (O <C <1) which translates the quality of the recombination of the waves and which depends on the vibration frequency of the target observed (in fact, C changes little as long as the vibration frequencies of the target remain low compared to c / L). L is the average value of the difference in optical paths in the arms of the interferometer, c is the speed of light. The variations of the signal S collected are therefore simply proportional to the speed u as a first approximation. However, this method has not so far been able to receive significant developments because of the defects it supports due to the instabilities of the laser sources (disturbing the quantities
S0 and A0).

 The object of the present invention is to remedy such instabilities. To this end, these have been analyzed and broken down into instabilities of the transmission power on the one hand, and instabilities of the transmission frequency on the other hand, and means have been proposed in order to s' free from such instabilities.

 It is also an object of the invention to provide such a vibration measurement device which is simple to manufacture and stable in use.

 It is yet another aim to propose such a device which can be manufactured under the best economic conditions.

 The invention therefore relates to a device for measuring the vibration of a part in which a source light flux directed on the part produces a backscattered flux which is analyzed by a measurement interferometer.

 According to the invention, this device comprises means for sampling part of the source light flux before the part and the sampled flux is itself analyzed by a reference interferometer compensating for the faults in the measurement system.

 It was indicated above that in such a device, the measurement interferometer accesses the frequency and the intensity of the backscattered light flux.

 The reference interferometer used according to the invention makes it possible to measure the frequency and the power of the source light flux, the comparison of these parameters with those measured by the measuring interferometer therefore allows direct characterization of the effect generated by the vibrations of the part.

 More precisely, the extraction of the vibration of the target, from the four parameters that we have just recalled, is summarized below. A calibration procedure, developed later, is useful for specifying the C factor of each interferometer. Knowledge of the instantaneous light power entering each interferometer (proportional to So) makes it possible to determine a normalized signal of the form [(S / So) -1] / C (according to the general formulation of the signal S), which represents the frequency information of each interferometer. It then suffices to subtract the normalized signals from these two interferometers to access the vibrations of the target.

According to different preferred embodiments, each presenting their particular advantages, the device has the following additional characteristics:
- at least one or the other of the measurement and reference interferometers is an interferometer with two arms of different lengths, provided with a first photometric sensor receiving recombined light waves after having each circulated in one of the arms the interferometer and underwent a phase difference, and a second sensor providing a signal making it possible to measure the intensity of the analyzed flux;
- a phase modulator is placed on one of the arms of each of the interferometers. This presents four interests ranked in descending order of priority
First of all, it is useful to control the value of L in the vicinity of Xo / 4 modulo h / 2, so that the measurement device is reliable.

The prior excursion of the possible values of S (performed by applying a ramp to the phase modulator) is a first possible method to then enslave L with the reference S - SMOY = (SMAX + +) / 2, OÙ
SMOY is the average value of S, SMAX is the maximum value of S and SMS is the minimum value of S.

A second more precise control method of L consists in generating with this phase modulator, a sinusoidal oscillation of L at a frequency distinct from the ambient mechanical vibrations and from the vibrational domain explored. In this case, the enslavement of L consists in minimizing the harmonics of this modulation frequency in the signal
S collected. This instruction has the advantage of automatically adapting to 5MOU drifts.

Knowing the amplitude of this modulation of L even makes it possible to determine in real time the value of the parameter C of each interferometer, in order to obtain a calibrated measurement of the movements of the target. It is however possible to carry out a calibration without modulating
L, but in two stages. The first step consists in measuring the two average values of the fluctuations of the S / SMOY signal of the two interferometers with a stationary target (. It should be remembered that these fluctuations are not zero since the variations of k0 can be formally interpreted as an apparent movement of apparent speed collected by the two interferometers). The relationship between these two measurements is then worth
A / B, where A = ckol (4nLC) for the measurement interferometer and where
B = ckol / (47cLC) for the reference interferometer. The second step consists in re-recording the 5 / 5MOU signal when the target describes a known movement, in order to determine A and to deduce B.

 Finally, it may be advantageous to also operate the high-frequency phase modulator, to modulate the useful signal, in order to shift this signal in frequency, to then operate a synchronous filtering and demodulation of the result.

- the second sensor of each interferometer receives part of the flow passing through one of the arms of the interferometer;
the second sensor of each interferometer receives recombined light waves after having circulated in each of one of the arms of the interferometer, the fluxes received by these two sensors having useful components in phase opposition;
- in certain embodiments, processing means receive the signals coming respectively from the two sensors of the measurement interferometer (Si, S2) and from the two sensors of the reference interferometer (S3, S4), and provide a measurement of vibration by combining these signals according to the formula
A (Sl - S2) / (Sl + S2) - B (S3 - S4) / (S3 + S4) where A and B are coefficients approximately constant in a given frequency domain;
- A and B are determined by calibration;
- Said calibration is carried out by applying to the phase modulators signals of the same intensity producing a known phase difference and by exploiting the signals thus produced by the sensors;
- the interferometers are fibered. That is to say that the optical phase shift between the two arms of each interferometer is carried out with optical fibers and couplers. This embodiment is particularly simple and stable. In addition, the optical path difference between the two arms can be significantly lengthened (L 10 to 100 m) compared to that of the non-fiber interferometers, thus increasing the sensitivity of the device to slow vibrations; this elongation of L being only limited by the coherence length of the laser and the absorption of the fibers;
- the same coupler produces the light flux taken from the source light flux and ensures the coupling of the backscattered flux and the measurement interferometer;
- two identical fibers are directed towards the target, to increase the sensitivity of the device when observing surfaces that are not very diffusing;
the same material elements constitute the measurement interferometers on the one hand and the reference interferometers, the waves passing from the laser to the target constituting the reference interference; and those passing in the other direction constituting the measurement interference.

The invention will be described below in detail with reference to the accompanying drawings in which:
- Figure 1 is a schematic representation of the device of the invention;
- Figure 2 is a schematic representation of one of the interferometers involved in the invention, in a first embodiment
- Figure 3 is the representation of an interferometer used in the invention, in a second embodiment;
- Figure 4 is a schematic representation of a device according to the invention produced in fiber form, in a first embodiment;
- Figure 5 is a schematic representation of a device according to the invention produced in fiber form, in a second embodiment
- Figure 6 a schematic representation of a device according to the invention produced in fiber form, in a third simplified embodiment.

 Figure 1 is a simplified representation of the device according to the invention. A laser 1 emits a source light flux 2 of wavelength.

 This light flux is divided by an optical element 3, for example constituted by a semi-transparent plate into a measurement flux 4 and a reference flux 5. The measurement flux 4 is diffused by the part 6 and addressed by the optical device 7 on the two-arm measurement interferometer 8.

 The reference flow 5 is sent by the optical system 9 to the reference interferometer 10.

 The two pairs of sensors present in the interferometers 8 and 10 'generate four signals (Si, S2, S3 and S4), which pass respectively through the wires 11, 12, 13 and 14 to the processing unit 15, where these signals are combined in order to provide the vibration parameter P of the part 6 (P is for example its position or its instantaneous speed).

 Figure 2 is a particularly advantageous embodiment, in fiber form, of the reference interferometer 10 and the measurement interferometer 8 which has the same structure.

The flow entering by the input fiber 20 is divided by the coupler 21 into two flows of approximately equal intensity coupled respectively to the fibers 22 and 23. A coupler 24 ensures the recombination and therefore the interference of the waves circulating in the fibers 22 and 23, and produces two outgoing flows in the fibers 25 and 26, which are measured by two photometric sensors 27 and 28. Here, the signals of these two sensors are in phase opposition as regards the measurement of the useful signal. This means that we will collect by the measurement interferometer, two signals S1 and S2 (with coefficients C1 = C2 in principle) such as:
S1 = S01 {1 + C1 sin [4 # L (u + uapparent) / (c # 0)]}
S2 = S02 (1 - C1 sin [4 it L (u + uapparent) / (c k0)]}
Likewise, the reference interferometer will give rise to the signals:
S3 = S03 {1 + C3 sin [4 # L uapparent / (c # 0)]}
S4 = S04 {1 - C3 sin [4 # L uapparent / (c # 0)]}
In this particular case, if the measured speeds are low compared to the meter per second and if the gains of the measurement channels are adjusted so that "S02 and S03 = S04 (this is easily done for each interferometer by equalizing the amplitudes of the oscillations generated by their respective phase modulator), we then obtain:
u = A (Sl - (S1 + S2) 'B (S3 S4) / (S3 + S4)
The vibration measurement is thus obtained by a particularly simple combination of signals S1 to S4, which completely compensates for the fluctuations of the laser source. This vibration measurement remains valid in the event of a drift in parameters A, B, L, Ground, S02, S03 and S04 which are measurable (for A and B) or even adjustable in real time, using the phase modulators of the interferometers .

 Figure 3 shows another embodiment of the interferometers 8 and 10 in fiber form for which the elements identical to those of the previous embodiment have been assigned the same reference numerals.

 After division by the coupler 21, the waves circulating in the fibers 22 and 23 are joined by the coupler 30 and their interference state is measured by the sensor 27. The signal supplied by this sensor 27 is therefore analogous to that supplied by the sensor with the same reference in the previous embodiment of Figure 2.

 An asymmetric coupler 31 takes a small part of the light flux circulating in the fiber 23 and sends it to a photoreceptor 32.

The flux thus measured is proportional to the flux entering the interferometer through the fiber 20 and therefore provides a signal S2, S4 capable of replacing the combined signals Si + S2, S3 + S4 of the previous embodiment described with reference to FIG. 2.

 Figure 4 illustrates a first embodiment of the measuring device of the invention fully fiber-optic from the output of the laser 1 to the interferometers 8 and 10. A coupler 40 receives the source light flux by a fiber 41 connected to the laser 1 by through an insulator 42 avoiding any disturbing effect of a light flux capable of being emitted back by the fiber 41 and reinjected into the laser 1.

 This coupler 40 divides the source flux into two equal parts, in the fibers 43 and 44. The fiber 43 conducts the light towards the part 6, via the optical device 7. The fiber 44 directs the light towards the interferometer of reference 10. The light scattered by the part 6 is then reinjected into the fiber 43 by means of the same optical device 7. Then, it returns to the coupler 40, which directs half of it in the fiber 45, towards the interferometer of measure 8 which can thus operate.

 FIG. 5 is an embodiment allowing the observation of weakly diffusing surfaces, since the light losses between the laser and the measurement interferometer 8 are notably reduced by the use of two identical fibers directed towards the target.

In fact, almost all of the light from laser 1 is sent to the target by fibers 43 and 44 (instead of half). A phase modulator 50 is optionally inserted in one of the two arms 43 or 44, in order to equalize the lengths of these two arms as best as possible, so that the waves collected back in the coupler 40 recombine as best as possible in fiber 45.
it is then necessary to add a coupler 51 on the path of the fiber 41, to direct a small portion of the laser light, through the fiber 52, to the reference interferometer 10.

 The fourth channel of this coupler 51 can also be directed, by the fiber 53, to a photometric sensor 54, verifying the equality of length of the two arms 43 and 44 (since this sensor will then receive the minimum of light).

 FIG. 6 represents a particularly simple embodiment of the invention in which the same hardware elements are used to produce both the reference interferometer 10 and the measurement interferometer 8.

 Starting from the laser 1, this device ensures the coupling of the source light flux in the fiber 41 which is isolated from the laser by an isolator 42. The coupler 60 ensures the division of the source beam, coupling each of the resulting fluxes, one to the fiber 61, the other to the fiber 62, these two fibers 61, 62 being connected to the coupler 63 which produces the interference of the two waves and ensures by the photometric sensor 64, the measurement of the state of reference interference (before any effect produced by part 6).

 As previously described with reference to Figure 3, the coupler 31 taking part of the reference flux on one of the arms of the interferometer, the measurement of the photometric sensor 65 provides the intensity of the reference flux.

 A second output of the coupler 63, constituted by the fiber 43, interacts with the target as in the other embodiments. The light returning from the target by this fiber 43 is separated into two waves, respectively coupled to the fibers 61 and 62. These two waves are recombined by the coupler 60 which addresses them, after interference, on the receiver 66. The receiver 66 therefore provides information representative of the state of interference of the backscattered waves. The coupler 31 takes part of the light flux passing through the fiber 62, and sends it to the photometric sensor 67 which thus supplies an electrical signal which is a function of the intensity of this backscattered flux.

 The signals Si, S2, S3, S4, respectively supplied by the photometric sensors 66, 67, 64, 65, are sent to the processing unit 15 which supplies the parameter P representative of the vibrations of the part 6.

 This arrangement, shown in Figure 6, therefore allows the realization of the vibration measuring device of the invention with a minimum of components, while allowing to overcome the instabilities of the laser. A high precision measurement is thus obtained with a stable and relatively inexpensive device.

The laser 1 used in the various embodiments described above is for example a gas laser, a solid-state laser, a laser diode (such as that known under the reference SDL-5722-H1 produced according to the Distributed Bragg technology Reflector and manufactured by the
SDL Company) or a continuous microlaser such as that developed by the SFIM ODS Company.

 The insulator 42 advantageously disposed at the output of the laser 1 is advantageously a Faraday insulator such as that known under the reference I-80-T5 from the company ISOWAVE.

The optical fibers used are advantageously monomode fibers with polarization maintenance such as those produced under the name FIBERCORE HB800G
The couplers are advantageously those known under the name IOM
Module of the German company SFIM GmbH. it should be noted that in its use for the control of parts subjected to ultrasonic waves excited by laser, it is preferable that the wavelength of the excitation laser is significantly different from that of the measurement laser.

Claims (12)

 1. Device for measuring the vibration of a part (6), in which a source light flux (2) directed onto the part (6) produces a backscattered flux which is analyzed by a measurement interferometer (8),  characterized in that it comprises means for taking (3) part of the source light flux before the part (6) and that the sampled flux is itself analyzed by a reference interferometer (10) compensating for the defects of the measurement system.  2. Device for measuring the vibration of a workpiece according to claim 1, characterized in that at least one or the other of the measurement and reference interferometers is an interferometer with two arms of different lengths, provided with a first photometric sensor (27) receiving recombined light waves after having each circulated in one of the arms of the interferometer and undergone a phase difference, and a second sensor (28) providing a signal making it possible to measure the intensity of the flow analyzed.  3. Device for measuring the vibration of a part according to claim 1 or 2, characterized in that a phase modulator (29) is placed on one of the arms of each of the interferometers (8, 10).  4. Device for measuring the vibration of a workpiece according to any one of claims 1 to 3, characterized in that the second sensor (32) of each interferometer (8, 10) receives part of the flux passing through the one of the arms of the interferometer.  5. Device for measuring the vibration of a workpiece according to any one of claims 1 to 3 characterized in that the second sensor (28) of each interferometer (8, 10) receives recombined light waves after having each circulated in one of the arms of the interferometer, the fluxes received by said sensors having useful components in phase opposition.  6. Device for measuring the vibration of a workpiece according to claim 5, characterized in that processing means (15) receive the signals coming respectively from the two sensors of the measurement interferometer (8) (Sî, S2) and two sensors of the reference interferometer (10) (S3, S4), and provide a measurement of the vibration by combining these signals according to the formula  A (Sl - S2) / (Sl + S2) - B (53 - S4) / (S3 + S4) where A and B are coefficients approximately constant in a given frequency domain.  7. Device for measuring the vibration of a workpiece according to claim 6, characterized in that A and B are determined by calibration  8. Device for measuring the vibration of a part according to claim 7, characterized in that said calibration is carried out by applying to the phase modulators (29) signals of the same intensity producing a known phase difference and by exploiting the signals thus produced by the sensors.  9. Device for measuring the vibration of a part according to one of claims 1 to 8, characterized in that the interferometers (8, 10) are fiber-reinforced.  10. Device for measuring the vibration of a part according to claim 9, characterized in that the same coupler (40) produces the light flux taken from the source light flux then ensures the coupling between the flux diffused by the target (6) and the measurement interferometer (8).  11. Device for measuring the vibration of a part according to claim 9, characterized in that a small portion of the laser light (1) is directed towards the reference interferometer (10), and that two fibers ( 43, 44) from near ends direct most of the laser light onto the target, then return the scattered light to the measurement interferometer (8).  12. Device for measuring the vibration of a workpiece according to claim 9, characterized in that the same material elements (60, 61, 62, 63) constitute the measurement interferometers (8) on the one hand and of reference ( 10) on the other hand, the waves passing from the laser to the target constituting the reference interference; and those passing in the other direction constituting the measurement interference.