Calibration device for laser velocimeter

Abstract

The invention relates to a calibration device (10) intended to be coupled to a laser velocimeter (VL) comprising: a disk (D), a mirror (M) fixed to the disk, a first illumination / reception device (DIR1) configured to receive a first light signal (S1) at a first frequency, orient and focus the first light signal so as to form a first incident signal illuminating the surface of the mirror at an incidence normal to a position of the disk during a passage of the disk, and according to a determined diameter on said surface, recover a first reflected signal (SR1) by the mirror at a frequency shifted relative to the first frequency of a Doppler shift (Δf), said calibration device being further configured to provide at output, at least a fraction of said first reflected signal and a value of a first standard tangential velocity (vte1) determined from said angular velocity (ω) and said measurement radius (Rm),intended for calibrating the laser velocimeter (LV). Figure 1,

Description

Title of the invention: Calibration device for laser velocimeter FIELD OF INVENTION

[0001] The present invention relates to the field of metrology instruments, and more specifically to instruments based on laser velocimetry, called laser velocimeters. A laser velocimeter is a laser-based instrument that measures the speed of a moving object (translation, rotation, vibrational phenomena, etc.) illuminated by the laser, using the Doppler effect. The invention relates more particularly to devices for calibrating laser velocimeters for speed. STATE OF THE ART

[0002] When the speed of an object is moderate, up to about one hundred meters per second, and if the distance traveled by this object is easily accessible via detectors, a simple time measurement between two measurement points may be sufficient to determine with good accuracy the average speed of the object.

[0003] Therefore, there is no real technical problem in calculating the object's speed since the two basic quantities, the distance traveled and the flight time, are both measurable. Moreover, if both distance and time are measured accurately, the deduced average speed of the object can also be determined with high accuracy.

[0004] The two main means of measuring high object speeds are as follows: - High-speed cameras: Historically, the first devices were high-speed optical cameras. They can record a succession of around one hundred images at a very high frame rate (typically 4 x 10⁶ frames / s) for short periods (typically 20 ps). These techniques are complex to implement, as they require the moving object to be visible to the camera, which is not always the case. In practice, since the time between each image is known, the object's speed can be determined by observing the change in its position in each image. It should be noted that high-speed camera technologies are not widely used because they are particularly expensive (approximately €300,000 to €1 million) for a single measurement channel. - Laser Velocimetry: The second commonly used method of speed measurement is laser velocimetry, and in particular heterodyne velocimetry (HV). The latter is very attractive because it allows measurement under difficult conditions, for example when the object is inaccessible. Neither for timing instrumentation nor for remote observation. Indeed, optical velocity measurement probes are small (on the order of a millimeter in diameter and 10 mm in length), easily integrated into an experiment. Velocity information is transferred from the probe to the instrument's data acquisition bay via a very small cross-section single-mode optical fiber. Applications are numerous in industry and research thanks to the ease of implementation and moderate cost of the Heterodyne Laser Velocimetry measurement system (approximately €25,000 for one measurement channel).

[0005] A laser velocimeter typically comprises a transmitter / receiver configured to emit a light signal at a known frequency and to receive a signal reflected by the moving object, and a measurement module configured to superimpose a reference signal at the known frequency and the reflected signal, and to detect the beat signal resulting from the interference of the two signals, exhibiting a beat frequency due to the Doppler effect, referred to as the Doppler frequency. The velocimeter also comprises a processing unit configured to digitize the beat signal and extract velocity information from this Doppler frequency.

[0006] The speed is determined by applying the known formula:

[0007] vobj = X.fd / 2

[0008] with vobj the velocity of the object, / . the wavelength of laser illumination and fd the Doppler frequency.

[0009] One example is the calibration device for a laser Doppler velocimeter (LVC) described in the publication "Simple and accurate calibration system for laser Doppler" by Terre et al., International Journal of Light and Electron Optics 179 (2019), which uses a rotating disk of the optical chopper type for velocity calibration up to 30 m / s (1914 m / min). The disk simulates the velocity of an object with a reference velocity vref equal to rco, where r is the radius of the disk and co is the angular velocity, known precisely and measured using a dedicated laser beam that passes through the chopper at its periphery, and a detector. The velocimeter also illuminates the rotating disk edge-on and measures its velocity, which is compared to vref.

[0010] Generally, the speeds reached by rotating disks are limited to approximately 20-30 m / s. Currently, there is no simple calibration device available for setting an object in motion over a wide speed range up to 5000 m / s.

[0011] One object of the present invention is to remedy the aforementioned drawbacks by providing a calibration device for generating standard speeds stabilized in the range of 1 - 5000m / s, and which can be easily deployed, without specific constraints. DESCRIPTION OF THE INVENTION

[0012] According to a first aspect, the invention relates to a calibration device intended to be coupled to a laser velocimeter comprising: • a disk configured to rotate around an axis of rotation at a predetermined angular velocity, • a mirror (M) attached to the disk, presenting a surface, • a first lighting / reception device configured for: • receive a first light signal at a first frequency, said first light signal originating from the laser velocimeter, • orient and focus the first light signal so as to form a first incident signal illuminating the surface of the mirror at a normal incidence for a position of the disk during a passage of the disk, and according to a diameter on said surface determined, the center of said diameter being located at a measured distance from said axis of rotation, • to recover a first signal reflected by the mirror at a frequency shifted relative to the first frequency of a Doppler shift, said calibration device being further configured to provide at output, at least a fraction of said first reflected signal and a value of a first standard tangential velocity determined from said angular velocity and said measurement radius, intended for said laser velocimeter to be calibrated.

[0013] According to one embodiment, the calibration device further comprises a reinjection device configured to recover, amplify, and reinject, into the first illumination / reception device, the first reflected signal and a plurality of additional reflected signals formed by successive reflections on said mirror during said passage of the disk. The calibration device is further configured to provide at output a fraction of said plurality of additional reflected signals, intended for said laser velocimeter to be calibrated.

[0014] According to one embodiment the first illumination / reception device is fiber-connected and in which the reinjection device includes a first coupler, a first optical amplifier and a mirror optical fiber.

[0015] According to another embodiment the first illumination / reception device is fiber-connected and in which the reinjection device includes a second coupler and a second optical amplifier configured to loop back onto the second coupler.

[0016] According to one embodiment, the disk comprises regularly spaced, penetrating motifs arranged around the periphery of the disk on a determined radius, the device calibration system further including a second illumination / reception device configured for: • receive a second light signal at a second frequency, said second light signal originating from the laser velocimeter, • orient and focus the second light signal so as to form a second illuminating incident signal, with a non-zero angle of incidence (a) relative to the plane of the disk, either on the patterns or on the surface of the disk depending on the position of the disk during rotation, • to recover a second signal backscattered by the disk, exhibiting an alternation between a second signal of non-zero amplitude resulting from the backscattering of the disk surface, and a second signal of zero amplitude when the second incident signal passes through the disk via a pattern, said calibration device being further configured to provide said laser velocimeter, at least a fraction of said second backscattered signal.

[0017] According to one embodiment, the calibration device is intended to be in In addition to being coupled with a laser rangefinder, the disc then comprises patterns passing through it, arranged regularly around the periphery of the disc over a determined radius, the calibration device further comprising: • a backscattering object positioned under the disc at a specific distance from the disc, designed to backscatter light that has passed through the patterns, • a third lighting / reception device configured for: • receive a third light signal at a third frequency, said third light signal originating from the laser rangefinder, • orient and focus the third light signal so as to form a third illuminating incident signal, along an axis of incidence normal to the surface of the disc, either the patterns passing through or the surface of the disc depending on the position of the disc during rotation, • to retrieve a third backscattered signal (SR3') exhibiting an alternation between a third backscattered signal from the disk and a third backscattered signal from the object, said calibration device being further configured to provide said laser rangefinder (TL), at least a fraction of said third backscattered signal.

[0018] According to one embodiment the object is configured to be positioned at a plurality of determined distances along said axis of incidence of the third incident signal.

[0019] According to one embodiment, the calibration device further includes a balancing mass disposed on the disk and intended to balance a mass of the mirror.

[0020] According to one embodiment, the disk has, at least at the periphery, a thickness that decreases with a distance from the axis of rotation.

[0021] According to one embodiment, the traversing motifs are crenellations located on the periphery of the disk.

[0022] According to one embodiment the disc is bi-material, with a first material for a central part of the disc and a second material for a peripheral part.

[0023] According to one embodiment, the calibration device is configured so that a first maximum standard tangential speed reached by the disk is greater than or equal to 100 ms'.

[0024] According to another aspect, the invention relates to a calibrated speed measurement system comprising a laser velocimeter and a calibration device according to the first aspect of the invention, the laser velocimeter being configured to be coupled to said calibration device during calibration, the laser velocimeter comprising: • a transmitting / receiving device configured to emit the first light signal at said first frequency and to receive a signal reflected by a frequency-shifted target, • a measurement module configured to superimpose a reference signal and said signal reflected by the target, and to detect a beat signal at a beat frequency corresponding to the frequency shift, • a processing unit configured to digitize the beat signal and extract speed information from the beat frequency.

[0025] The laser velocimeter is further configured, during calibration, to receive said value of a first standard tangential velocity delivered by the calibration device and such that: • the transmitting / receiving device emits the first light signal at the first frequency towards the calibration device, • the transmitting / receiving device receives, from the calibration device, the fraction of said first reflected signal having a frequency shifted by the first frequency by a Doppler shift corresponding to said first standard tangential velocity, and the fraction of the plurality of additional reflected signals exhibiting respectively a frequency shifted from the first frequency by a plurality of shifts equal to i times the Doppler shift, i varying from 2 to n, • the processing unit extracts from said Doppler shift a first tangential measurement velocity to be compared with the first standard tangential velocity, and extracts, from said plurality of shifts equal to i times the Doppler shift, a plurality of additional associated tangential measurement velocities, to be compared respectively to a plurality of additional standard tangential velocities respectively equal to i times the first standard tangential velocity.

[0026] The following description presents several embodiments of the device of the invention: these examples are not limiting to the scope of the invention. These embodiments present both the essential features of the invention and additional features related to the embodiments considered.

[0027] The invention will be better understood and other features, objectives and advantages thereof will become apparent from the following detailed description and with reference to the accompanying drawings given by way of non-limiting examples and in which:

[0028] Fig. 1 illustrates a calibration device according to the invention.

[0029] Figure 2 illustrates an example of implementation of the calibration device according to the invention.

[0030] Figure 3 illustrates a variant of the calibration device according to the invention adapted for high-speed calibration and comprising a reinjection device.

[0031] Fig. 4 illustrates a first embodiment of the reinjection device variant in which the reinjection device comprises a first coupler, a first optical amplifier and a mirror optical fiber.

[0032] Figure 5 illustrates a time / velocity spectrogram of an example of velocity multiplication according to the first embodiment of the variant with reinjection device.

[0033] Figure 6 illustrates a second embodiment of the reinjection device variant in which the reinjection device comprises a second coupler and a second optical amplifier configured to loop back onto the second coupler.

[0034] Figure 7 illustrates a time / velocity spectrogram of an example of velocity multiplication according to the second embodiment of the variant with reinjection device, for the case of a velocity obtained without a loop.

[0035] Figure 8 illustrates a time / velocity spectrogram of an example of velocity multiplication according to the second embodiment of the variant with reinjection device, for the case of velocities obtained with the loop, but without the amplifier.

[0036] Fig. 9 illustrates a time / velocity spectrogram of an example of velocity multiplication according to the second embodiment of the variant with reinjection device, for the case of velocities obtained with the loop and amplifier in operation.

[0037] Figure 10 illustrates a variant of the calibration device according to the invention in which it is also configured to generate rapid, periodic velocity transitions and, for this purpose, includes traversing patterns. Figure 8 also illustrates a variant of the calibration device according to the invention in which it- This device is designed to be coupled to a TL laser rangefinder, enabling the latter to calibrate itself in terms of distance. For this purpose, the disk includes a backscattering object positioned beneath it. Figure 8 also illustrates a variant of the calibration device according to the invention, in which the rotating disk is arranged in a vacuum containment chamber.

[0038] Fig. 11 illustrates a rotating disk optimized for the calibration device according to the invention having an evolving thickness, thicker in the center than at the edge, which is bi-material and whose traversing patterns are slots located on the periphery of the disk.

[0039] Fig. 12 illustrates a speed measurement system calibrated according to a second aspect of the invention. DETAILED DESCRIPTION OF THE INVENTION

[0040] The basic idea of ​​the invention is to use a rotating metrology disk simulating the movement of an object as a calibration device. According to a first aspect of the invention, the calibration device 10 is thus a standard velocity generator for an object. It is intended to be coupled to an industrial measurement system of the laser velocimeter (LV) type, for example, a heterodyne laser velocimeter (HLV), in order to verify the accuracy of the velocities it provides. In other words, it is a matter of verifying whether the industrial LV measurement system is correctly calibrated across its entire measurement range.

[0041] The calibration device 10 according to the invention is illustrated [Fig. 1]. It is intended to be coupled to a laser velocimeter VL also shown in [Fig. 1] but not forming part of the invention according to a first aspect.

[0042] For the record, a laser velocimeter (VL) in a known manner comprises a transmitter / receiver (DER) configured to emit a light signal at a first frequency fl and to receive a signal reflected by an object to be characterized, shifted in frequency. It also comprises a measurement module (MM) configured to superimpose a reference signal and the reflected signal, and to detect a beat signal at a beat frequency corresponding to the frequency shift between the reference signal (at frequency fl) and the reflected signal, shifted in frequency by a shift resulting from the Doppler effect. Finally, the VL velocimeter comprises a processing unit (UT) configured to digitize the beat signal and extract velocity information from the beat frequency.

[0043] The calibration device 10 is made independently of the velocimeter VL, and is coupled to VL when it is desired to calibrate it.

[0044] The calibration device 10 according to the invention comprises a disk D configured to rotate about an axis of rotation AR at a co-determined angular velocity, that is to say, known, with precision. The precise measurement of the disk's rotational speed is obtained, for example, from an electronic reference (clock) derived from the control of the electric motor driving the rotating disk. According to another example, an optical revolution counting device measuring the disk's rotational frequency f is integrated into the calibration device according to the invention, and the angular velocity is determined by the relation co = 2.ir.f.

[0045] The calibration device also includes a mirror M attached to the disk, having a surface SM. The mirror is preferably placed at the periphery of the disk so that its tangential velocity is as high as possible given the rotational speed co.

[0046] The calibration device also includes a first illumination / reception device DIR1 configured to receive a first light signal SI at a first frequency, the first light signal SI originating from the laser velocimeter. The laser velocimeter typically has a fiber or free-space output. The device DIR1 is preferably (but not necessarily) fiber-optic. When both VL and DIR1 are fiber-optic, a coupler allows the two fibers to be joined.

[0047] The DIR1 device is also configured to orient and focus the first light signal so as to form a first incident signal illuminating the surface of the mirror SM at a normal incidence for a given position of the disk during a passage of the rotating disk. The incident beam presents a diameter Dsp (radius Rsp) on the surface SM, and the position of the center O of the diameter of the incident beam on the mirror relative to the axis of rotation is known precisely; this distance is denoted Rm. Typically, the diameter of the beam Dsp is on the order of 10–100 pm. For example, the distance Roe between point O and the end of the mirror on the axis of rotation and the distance Ram between the end of the mirror on the axis of rotation and the axis of rotation AR are known. The center O of the diameter of the incident beam on the mirror is thus located at the precisely known measurement distance Rm from the axis of rotation AR, equal, for example, to:

[0048] Rm = Ram+Rsp.

[0049] The tangential velocity of the mirror at point O, called the first standard tangential velocity vtel, is therefore known precisely with the formula:

[0050] vtel = co.Rm

[0051] Preferably, the surface of the mirror is parallel to a radius of the disk, but this is not mandatory. What is required is that the mirror be adjusted in autocollimation with respect to the incident beam, that is, so as to reflect the light back along the same path as it traveled. Given the rotational speed of the disk, the mirror remains in autocollimation for only a short time during a pass, typically on the order of a ps.

[0052] The DIR1 device is configured to recover a first SRI reflected signal by the mirror at a frequency shifted relative to the first frequency fl by a Doppler shift Af resulting from the tangential velocity of the mirror at point O. Thus, during the rotation of the disk and when the mirror is, for a brief instant, perpendicular to the incident beam of frequency fl, the return beam reflected by the mirror is frequency-shifted by the Doppler effect (frequency fl+Af) corresponding to the tangential velocity of the rotating mirror. The reflected beam containing the information [fl+Af] returns to the DIR device, legally referred to as the probe.

[0053] The calibration device 10 is further configured to provide, at the device output: • on the one hand the value of the first standard tangential velocity vtel, determined from the angular velocity co and the said measurement radius Rm, both known with precision; • on the other hand at least a fraction of the first reflected SRI signal at the frequency fl+Af, carrying information in frequency Af associated with the first standard tangential velocity vtel known from elsewhere.

[0054] These output data are intended to be provided to the VL laser velocimeter to be calibrated.

[0055] Thus the device according to the invention constitutes a generator of standard tangential velocities in the range of 1 to 500 m / s of an object (the rotating disk).

[0056] The first standard speed vtel can reach, with an optimized structure of the rotating disk at its maximum angular speed, a maximum value of approximately 500 m / s. Thanks to control of the angular speed of rotation of the disk co, the tangential speed at the periphery of the disk can vary in a wide range, typically [1; 500 m / s].

[0057] The calibration device according to the invention can be deployed in any location, without any special infrastructure or security constraints. Typically, the device 10 is fully enclosed.

[0058] Note that the first tangential velocity vtel of the disk generated is known with good accuracy, of approximately Avtel / vtel on the order of 10 4.

[0059] This is made possible thanks to precise knowledge of: • the angular velocity of the disk (œ), measured instantaneously at the time of each calibration, • the value of the radius Rm between the axis of rotation of the disk and the center O of the measurement point of the focused probe.

[0060] The angular velocity of the disk is preferably measured continuously in order to monitor its stability.

[0061] During calibration, the VL velocimeter is coupled to the device 10 according to the invention and therefore receives the SRI signal as input, mixes this signal with a reference signal at the frequency fl and determines the beat frequency related to Af, from which it extracts a first tangential measurement velocity vtml. This measured velocity vtml is to be compared to the first standard tangential velocity vtel to perform the calibration.

[0062] By way of non-limiting, an example of an implementation of the calibration device according to the invention is illustrated [Fig. 2]. The first illumination receiving device DIR1 is a fiber optic probe, i.e., comprising an optical fiber FOI. The use of an optical fiber is a preferred method, but the transmission of light signals is also possible in free space or in the form of integrated optics. The fiber optic probe is equipped with a focusing device enabling the generation of a focused incident beam 20 presenting on the mirror M a spot with a diameter Dsp on the order of 10 pm.The mirror M is fixed to the edge of the disk, on its periphery, and the device is configured so that the beam 20 impacts the mirror tangent to the edge of the disk (spot tangent to the edge of the disk). The first standard tangential velocity vtel is then maximum and equal to co.Rd, with Rd the radius of the disk D. The measurement radius Rm is in this case equal to: .

[0063] Rm = Rd + Rsp, Rsp radius of the spot of the beam on the mirror M.

[0064] The DIR1 probe is configured to recover the reflected beam during the brief instant of autocollimation; this reflected beam is then returned via the OF1 fiber. The OF1 optical fiber functions as an input / output device, enabling coupling with the VL velocimeter.

[0065] Optionally, a balancing mass ME is fixed on the disk to compensate for the mass of the mirror added to the edge of the disk.

[0066] The section along the AA direction at the top of [Fig.2] illustrates the particular configuration for which we have Rm = Rd + Rsp.

[0067] According to a variant called the multiplicative speed variant illustrated in [Fig. 3], which incorporates, without limitation, the implementation method of [Fig. 2], the calibration device according to the invention is adapted for calibrating high speeds exceeding vtel. For this purpose, the device 10 further comprises a reinjection device DRI configured to recover, amplify, and reinject, into the first illumination / reception device DIR1, the first reflected signal SRI and a plurality of additional reflected signals formed by successive reflections on the mirror M during the autocollimation instant as the rotating disk passes. These additional reflected signals are indexed SRi, with i varying from 2 to n, i = 1 corresponding to the first reflected signal SRI described above. The DRI device is arranged upstream of the device DIR1, between the latter and the output intended to be coupled to the velocimeter.

[0068] In this reinjection loop, the SRI signal, with frequency fl+Af, is recovered, amplified by the DRI device, and reinjected, again via this DRI device, into the first illumination / reception device DIR1. It then becomes an incident beam S2 on the mirror: S2 = SRI. This beam S2 is in turn reflected by the mirror to form SR2 and has a frequency shifted by Af relative to the frequency of S2 equal to fl+Af, i.e. a frequency fl + 2.Af, shifted by 2.Af relative to fl. This beam SR2 is in turn recovered, amplified and reinjected into DRU and becomes the incident beam S3= SR2 and so on until a reflected beam Sn of frequency fl + n.Af is obtained. The device 10 according to the invention thus generates a plurality of n reflected beams SRi having respectively a frequency fl+i.Af, i.e. a frequency shift of a value of i.Af relative to fl.

[0069] The limit on the number of possible iterations, i.e., the value of n, is limited by the beam attenuation not compensated by amplification and by the time during which autocollimation occurs with the measuring probe (DIR1). Indeed, very quickly the beam reflected by the mirror no longer follows the same path and is therefore not recovered by DIR1. Typically, n is between 10 and 25, depending on the rotation speed of the disk.

[0070] The calibration device 10 is further configured to provide as output, in addition to the SRI signal, a fraction of the plurality of additional reflected SRi signals. This additional output data is intended to be provided to the laser velocimeter VL to be calibrated.

[0071] During calibration, the VL velocimeter is coupled to the device 10 according to the invention and therefore receives as input, in addition to the SRI signal and through the same channel, the plurality of additional SRi signals, which are superimposed.

[0072] These signals are detected and processed by the velocimeter VL, and the processing unit UT of VL extracts from the plurality of shifts, equal to i. Af, a plurality of additional associated tangential measurement velocities vtmi. These measurement velocities vtmi are, for calibration purposes, to be compared respectively to a plurality of additional standard tangential velocities vtei respectively equal to i times the first standard tangential velocity vtel: vtei = i.vtel.

[0073] Thanks to the device according to the invention, it has been possible to overcome the constraints exerted on a rotating disk that limit its maximum tangential speed by generating standard pseudo-tangential velocities equal to multiples of the initial tangential speed vtei. Indeed, mechanical strength models of the disk systematically show failure of the disk in the central drive zone starting at a tangential speed of approximately 1000 m / s, even with the best metallic materials currently available. It is therefore impossible to directly obtain Doppler shifts (Af) corresponding to speeds exceeding approximately 1000 m / s.

[0074] To achieve Doppler shifts corresponding to standard tangential velocities of several hundred or even thousands of meters per second, the device according to the invention duplicates and accumulates the initial Doppler shift Af, corresponding typically at the maximum standard tangential speed actually attainable at the edge of the disk (for example 500 m / s).

[0075] In other words, it is a matter of artificially increasing this initial Doppler shift Af by multiplying it by a factor n allowing to generate a larger (and perfectly known) range of Doppler shifts.

[0076] Typically starting from a velocity vtel = 500 m / s, standard tangential velocities of 5000 to 10000 m / s can be reached.

[0077] The reinjection device constitutes a specific optical circuit in the form of an optical loop that multiplies the initial Doppler shift by n times. As an example, this multiplication of Doppler shifts (n x Af with n from 10 to 20) typically makes it possible to reach velocities of a pseudo-projectile of 5000 to 10000 m / s, starting from vte = 500 m / s.

[0078] In order to obtain good accuracy at these high speeds, the uncertainty on the base speed vtel must be as low as possible, typically Avtel / vtel on the order of 10⁴ to 10⁵.

[0079] The advantage of this speed-multiplicative variant of the device according to the invention is that it can perform speed calibration on a moving object over a very wide speed range, from a few meters to several thousand meters per second.

[0080] In the embodiment in which the device DIR1 comprises a first optical fiber OF1, the device DRI is coupled to the device DIR1 via this optical fiber OF1. According to another embodiment also shown [Fig. 3], the device DIR1 comprises a second optical fiber OF2 fulfilling the function of an input / output device enabling coupling with the velocimeter VL.

[0081] At the application level according to one embodiment the device 10 according to the invention is modular, with a basic mode MB allowing calibration up to vtel, comprising the rotating disk and the device DIR1 equipped with the fiber OF1, and a speed multiplication module MMV integrating the device DRI equipped with the optical fiber OF2, the output of the MMV module being coupled to OF1, which would be added to the basic module as needed.

[0082] The calibration device according to the invention can operate with a light signal incident at a chosen wavelength, the value of which is between the far-infrared and the UV, including the visible spectrum. A wavelength of 1550 nm is commonly used in fiber optic communication technologies, and therefore, fiber optic components suitable for implementing the device according to the invention are commercially available.

[0083] According to a first embodiment of the speed-multiplicative variant illustrated [Fig. 4], which reproduces, in a non-limiting manner, the embodiment of the fixed mirror On the edge of the disk, the first illumination / reception device DIR1 is fiber-connected, meaning it includes a first optical fiber OF1, and the reinjection device DIR includes a first COI coupler, a first optical amplifier OA1, and a mirror optical fiber OFM. Preferably, it also includes a second optical fiber OF2 as described previously. The first COI coupler is typically a 2(41, 42) x 1(40) coupler with three input / output ports 40, 41, 42. Port 40 is connected to DIR1 via the OF1 fiber, port 42 is connected to the optical amplifier OA1, and the other port 41 of the 2x1 coupler is connected to the OF2 fiber, through which the input signal at frequency fl (input) and the fraction of reflected signals SRi intended for calibration (output) pass.

[0084] Generally, the links between the various components of the calibration device according to the invention are typically established via optical fibers or in free space. Similarly, the components themselves are manufactured using free-space technologies.

[0085] The amplification of the optical signals and the multiplication of the reference Doppler shift Af is achieved by a series of round trips in the first amplifier OA1 using an OFM fiber mirror. This OFM fiber mirror functions to reflect the frequency-shifted signals back to the probe DIR1 in order to accumulate the Doppler shifts.

[0086] The optical paths of the first loop of duplication (or multiplication) of the Doppler shifts are detailed below with an example: • The initial incident beam SI at the frequency fl is recovered by port 41 of the COI coupler, for example a 10% (41) / 90% (42). • The SI beam passes through the COI two-way 10 / 90% coupler, meaning that 10% of the initial power at the frequency fl is found at the output on port 40 and then is injected into the optical probe DIR1. • During the rotation of disk D, at each passage of the mirror in front of the incident beam focused by DIR1, the beam reflected by the mirror is for a brief instant perfectly perpendicular to the incident beam (auto-collimation). A large part of the incident beam is therefore reflected by the mirror and returns to the DIR1 probe with the information of the first Doppler shift (fl+Af). This initial Doppler shift Af corresponds to the actual tangential velocity of the chosen disk, and represents the tangential velocity reference, called the first tangential velocity vtel, which is duplicated (multiplied) thereafter. • the SRI return beam containing the Doppler shift information (fl+Af) passes back through the COI coupler in the opposite direction. • The first output of coupler 41 at 10% returns the SRI signal shifted by (fl+Af) to the output of device 10, typically via OF2, and is injected into the input of the velocimeter VL for calibration. The processing of this signal by the processing unit UT of VL, with the shift (Af), provides the first tangential velocity measurement of the disk vtml as seen by VL. If the velocimeter VL is correctly calibrated, then vtml = vtel. • The second output of coupler 42 at 90% sends this same signal offset by (fl+Af) to the first optical amplifier OA1 in order to increase the beam power and compensate for losses due to the different components, in particular the losses of the return beam in the probe DIR1. At the output of amplifier OA1, a fiber with an OFM end mirror sends the amplified signal back into the amplifier. This assembly—amplifier OA1 and OFM mirror fiber—forms the optical beam return circuit to probe DIR1 for Doppler shift multiplication. At this stage, the beam frequency is still (fl+Af). • Then the SRI beam, offset by (fl+Af) passes back through the coupler at 90%. • Finally, the offset laser beam SRI, still of (fl+Af), is sent back a second time into the probe DIR1, deflects an incident beam S2 on the mirror, then is reflected again on the mirror M moving with its tangential velocity vtel. • The SR2 return beam in the DIR1 probe is therefore frequency-shifted by (fl+2xAf) and re-enters the COL coupler • The first output 41 of the 10% coupler returns the signal shifted by 2xAf to the output and the velocimeter VL. Processing this signal with the 2xAf shift gives twice the initial tangential velocity vtel of the disk. • The second output of the coupler at 90% returns the beam (fl+2xAf) to the beam amplification and return loop in the DIR1 probe.

[0087] The round trips via the multiplication circuit are typically performed between 10 and 20 times. The number of iterations depends on the quality of the signals, which degrade with each amplification, and the available auto-collimation time (dependent on the probe and the rotation speed of the disk).

[0088] Figure 5 illustrates a time / velocity spectrogram of an example of velocity multiplication according to the first embodiment (Fig. 4) of the velocity-multiplicative variant of the invention. The velocities on the ordinate are the vtmi velocities measured by a VL velocimeter (at the output of the processing unit) coupled to the calibration device generating the various SRi signals.

[0089] In the case of this example, the first actual reference standard tangential velocity vtel is 240 m / s ≈ 0.05 m / s. The tangential velocity of vtel = 240 m / s is known with high precision thanks to the precise knowledge of the product coRm.

[0090] The velocity obtained after a pass Pi is denoted vtmi. After 19 passes in the "multiplicative" loop, corresponding to n=20, the tangential velocity of the pseudoprojectile measured by the industrial VL system reached vtm20 = 4800 m / s = 20x240 m / s. Since vtml is known to correspond to vtel (240 m / s), vtmi corresponds to i times vtel, and vtm20 corresponds to 20 times vtel (i.e., 4800 m / s), the velocity scale in m / s can be accurately calibrated over the entire velocity range between 100 and 5000 m / s.

[0091] According to a second embodiment of the speed-multiplicative variant illustrated [Fig.6], which includes, but is not limited to, the embodiment of the mirror fixed to the edge of the disk, the first illumination / reception device DIR1 is fiber-linked, i.e. it includes a first optical fiber OF1 and the reinjection device DIR includes a second coupler CO2 and a second optical amplifier OA2 configured to loop back onto the second coupler CO2.

[0092] In this second embodiment, the amplification and duplication (multiplication) of the reference Doppler shift Af are performed by a fiber loop. This loop amplifies and sends the signals back to the probe DIR1 in order to accumulate the Doppler shifts.

[0093] In this embodiment there is no longer a fiber-reflecting mirror for the beam, but a true optical loop.

[0094] The optical paths of the first loop of duplication (or multiplication) of the Doppler shifts are as follows ([Fig.6]): • The initial incident beam SI at the frequency fl is recovered by port 61 of the CO2 coupler, for example a 4-port coupler (60, 61, 62, 63) 3x1 for example 33% (61) / 33% (62) / 33% (63). • The beam passes through the three-way CO2 coupler, for example 33 / 33 / 33: 33% of the initial power at the frequency fl is found at the output 60 of the coupler and is then injected into the optical probe DIR1. • During the rotation of the disk, at each passage of mirror M in front of the incident beam focused by probe DIR1, the beam reflected by the mirror is for a short instant perfectly perpendicular to the incident beam (auto-collimation) coming from probe DIR1. A large part of the incident beam is therefore reflected by the mirror and returns to probe DIR1 with the information of the first Doppler shift (fl+Af) relative to (once) the tangential velocity of the mirror. • After reflection on the rotating mirror M, the SRI return beam containing the Doppler shift information (fl+Af) passes back through the coupler in the opposite direction (via 60). • The first output 61 of the 33% coupler returns the shifted signal (fl+Af) to the output of the device, intended to be coupled to the velocimeter VL. Processing this SRI signal with the shift (Af) provides the initial tangential velocity of the disk vtml as seen by VL. If the VL lasers are well calibrated, then vtml = vtel. • The second output 62 of the coupler at 33% sends this same SRI signal offset by (fl+Af) to a second optical amplifier OA2 in order to increase the power of this beam and compensate for the losses due to the different components, in particular the losses of the return beam in the probe DIR1. • At the amplifier output, the beam is sent back to channel 63 at 33% of the coupler. Thus, the amplified beam, shifted by (fl+Af), is sent back to the DIR1 probe. • The beam from output 62 of the coupler, amplified and returned to output 63 of the coupler, represents the loop for performing the multiplication of Doppler shifts. At this stage, the beam frequency is still (fl+Af). • Finally, the offset laser beam, still of (fl+Af), is sent back a second time into the probeDIRl, becomes the incident beam S2 and is then reflected again on the mirror moving with its tangential velocity vtel. • The SR2 return beam in the DIR1 probe is therefore shifted in frequency by (fl+2xAf) and enters the coupler again via channel 60. • The first output of coupler 61 at 33% returns the signal, offset by 2xAf, to the output and the velocimeter VL. Processing this signal with the 2xAf offset gives a measured speed vtm2 corresponding to twice the initial tangential speed of the disk vte2=2.vtel if VL is correctly calibrated. • The second output of the 33% coupler 62 returns the beam (fl+2xAf) to the amplification loop. The beam returns to the DIR1 probe via the passage through the coupler from 63 to 60.

[0095] The round trips via the multiplication circuit are carried out between 10 and 20 times. The number of iterations depends on the quality of the signals, which degrade over the course of amplifications, and the available time for auto-collimation.

[0096] Figures 7, 8 and 9 illustrate a time / velocity spectrogram of an example of velocity multiplication according to the second embodiment ([Fig.6]) of the velocity multiplicative variant of the invention for 3 different cases.

[0097] As with [Fig.5], the velocities in the ordinate are the vtmi velocities measured by a velocimeter VL coupled to the calibration device generating the different SRi signals.

[0098] The spectrogram in [Fig.7] shows the velocity obtained without a loop. This is therefore the measurement vtml of the initial tangential velocity of the rotating mirror, which is known to be equal to the standard value of vtel of 240 m / s, corresponding to the initial Doppler shift Af.

[0099] The spectrogram in [Fig.8] shows the velocities obtained with the loop, but without the amplifier OA2. Without the amplifier, the various power losses in the optical circuit still allow the loop to be performed 3 times and the initial Doppler shift to be multiplied by 4, and a tangential velocity of 960 m / s to be reached.

[0100] The spectrogram in [Fig. 9] shows the velocities obtained with the loop and amplifier in operation. With the amplifier, the various power losses in the optical circuit are reduced and allow the loop to be completed 13 times, and for this example, to multiply the initial Doppler shift 14 x Af by 14 and to reach a tangential velocity of 3360 m / s.

[0101] Since we know that vtml corresponds to vtel (240m / s), vtmi corresponds to i times vtel, vtml4 corresponds to 14 times vtel (i.e. 3360 m / s), the speed scale in m / s can be calibrated accurately over the entire speed range between 100 m / s and 3500 m / s.

[0102] For the calibration procedure, the calibration device 10 is coupled to a velocimeter from which it acquires the signal at frequency λ. The device 10 stores the value of Rm. The disk is then rotated at a first tangential speed to be measured, vtel(setpoint). For this, it is necessary to precisely measure the angular velocity of the disk at the time of calibration.

[0103] We then aim with DIR1 at the rotating mirror and the calibration device sends back the useful reflected signal to the velocimeter, which processes it and extracts the speed measurements.

[0104] Starting from the standard value vtel provided by device 10, the velocimeter calculates the multiples i.vtel. The measurement scale in m / s of VL can be precisely calibrated thanks to the knowledge of vtel and all the velocity multiples accessible by the time / frequency measurement. The number of multiples for which the comparison can be made, necessarily less than or equal to n, is limited by the sensitivity of the velocimeter for recording weak signals and by the illumination area of ​​the mirror by autocollimation (this area decreases with the number of velocity jumps).

[0105] According to an illustrated embodiment [Fig. 10], the calibration device according to the invention is also configured to generate rapid periodic velocity transitions (typically less than 20 ns) between the velocity of the moving object (disk) and a A fixed reference is used at zero velocity. For this purpose, the disk includes regularly spaced MT patterns around its periphery along a defined radius Rmot. The calibration device 10 further includes a second illumination / reception device DIR2 configured to receive a second light signal S2' at a second frequency f2, this second light signal originating from the laser velocimeter. The frequency f2 may be the same as or different from f1. The illumination / reception device DIR2, or second probe, is also configured to orient and focus the second light signal so as to form a second incident illuminating signal, with a non-zero angle of incidence α relative to the plane of the disk, either on the MT patterns or on the disk surface, depending on the disk's position during rotation. Thus, periodically, the incident beam is intercepted by the upper surface of the disk or passes through the patterns.The DIR2 device is finally configured to recover a second SR2' signal backscattered by disk D, exhibiting an alternation between a second non-zero amplitude signal resulting from backscattering off the disk surface, and a second zero amplitude signal when the second incident signal passes through the disk via a pattern.

[0106] The calibration device 10 is further configured to provide the laser velocimeter VL with at least a fraction of the second backscattered signal SR2'.

[0107] For calibration the velocimeter processes the fraction of a second backscattered signal and thus generates a periodic speed signal, between a non-zero speed and a zero speed.

[0108] This aiming with the second probe thus generates standard transitions between the tangential velocity of the disk (measurement of the scattering of the incident beam at the disk surface) and zero velocity when the beam passes through one of the holes in the disk. In other words, a standard velocity transition generator is created. The controlled transition time (typically > 20 ns) depends on the diameter of the focused beam (for example, on the order of 10 pm) and the maximum tangential velocity of the disk (vtemax).

[0109] This standard velocity transition generator made with the second illumination / reception device DIR2 of the calibration device 10 according to the invention makes it possible to qualify the percussion response of the entire acquisition chain of the velocimeter (reception, digitization and signal processing for velocity extraction) to stimuli in the form of short and calibrated velocity pulses.

[0110] Note that, because of the angle a between the direction of the measured velocity vm by the second probe and the actual direction of the tangential velocity vtm of the disk, the relation to be applied is: vm = vtm.cos(a).

[0111] For the second probe, the measured velocity is therefore always less than the actual tangential velocity. The uncertainty in the angle α is not important since here it is the knowledge of the velocity transition front that is sought.

[0112] According to another embodiment, which can be combined with the preceding embodiments and is also illustrated [Fig. 10], the calibration device 10 according to the invention is also intended to be coupled to a laser rangefinder TL, in order to allow the latter to calibrate itself in distance. For this purpose, the disk comprises, as in the preceding embodiment, MT patterns passing through the disk arranged regularly around its periphery on a determined radius Rmot. The calibration device 10 further comprises a backscattering object Obj disposed under the disk at a determined distance do from the disk and so as to backscatter light that has passed through the patterns. The calibration device 10 also comprises a third illumination / reception device DIR3 or third probe, configured to:

[0113] receive a third light signal S3' at a third frequency f3, the third light signal being from the laser rangefinder TL,

[0114] orient and focus the third light signal so as to form a third illuminating incident signal, along an axis of incidence N normal to the surface of the disk, either the patterns passing through or the surface of the disk depending on the position of the disk during rotation,

[0115] recover a third backscattered signal SR3' exhibiting an alternation between a third backscattered signal by the disk and a third backscattered signal by the object.

[0116] The calibration device is further configured to provide the laser rangefinder TL with at least a fraction of the third backscattered signal. The backscattered signal thus generates rapid, periodic transitions (on the order of tens of nanoseconds) between two distances (sighting at the disk surface and sighting at the object when the incident measuring beam passes through the disk patterns). These rapid, calibrated transitions between two distances allow the rangefinder's response to be qualified. A non-contact, dynamic distance calibration has thus been achieved. Note that the distance do must be known precisely for the rangefinder calibration, and the disk probe distance is preferably known to facilitate diagnostic adjustment.

[0117] According to one embodiment of this variant, the object is configured to be positioned at a plurality of determined distances along said axis of incidence N of the third incident signal, to obtain several distance standards (for example, the distance do can be varied between 1 and 100 mm).

[0118] According to an embodiment compatible with all the aforementioned variants and also illustrated [Fig. 8], the rotating disk is disposed in a primary or secondary vacuum containment chamber EV so that the disk can rotate frictionless with a suitable motor. The enclosure is preferably shielded to protect the user from the risk of the rotating disc bursting.

[0119] The probes DIR1, and DIR2 and / or DIR3 as appropriate, can be placed inside the enclosure (internal probe) or outside (external probe). In the latter case, a window H must be inserted to allow the incident beam (propagating in free space) to reach the disk (see, for example, DIR3 in [Fig. 8]).

[0120] The velocimeter, and where applicable the rangefinder, are placed outside the enclosure.

[0121] According to an embodiment illustrated in figures 2 and 8, the calibration device according to the invention further comprises a balancing mass ME disposed on the disk and intended to balance the mass of the mirror fixed on the disk.

[0122] According to one embodiment, the calibration device according to the invention is configured so that the first maximum tangential velocity reached by the disk vtelmax is greater than or equal to 100 ms'. It is then possible to perform with the device according to the invention a velocity calibration over a wide range of velocities: between 1 and 100 m / s using the device in its version without reinjection, then up to 2000 m / s or even 5000 m / s using the version with reinjection, allowing access to multiples of the order of 20 of vtel.

[0123] Achieving a tangential speed of 100 m / s or more requires optimization of the rotating disk in several aspects. With such optimization, the inventors simulated rotating disks capable of reaching speeds of 700 to 800 m / s.

[0124] Placing the disc in a vacuum chamber is a first option.

[0125] A second option is to optimize the shape of the disk.

[0126] After various studies, the inventors established that a disk such as the one illustrated [Fig. 9], having a progressively increasing thickness, thicker at the center than at the edge, improves the mechanical strength at the center of the disk where the stresses are greatest. In other words, according to one embodiment, the disk D has, at least at the periphery, a thickness e(r) that decreases with a distance r from the axis of rotation.

[0127] In order to increase the mechanical strength at the center of the disk, and thus substantially increase the rotation speed of the disk before it is damaged, according to one embodiment the disk is bi-material, made with a first material Matl for the central part of the disk and a second material Mat2 for the peripheral part.

[0128] According to one embodiment, the through-patterns are CR slots located on the periphery of the disc. Machined patterns on the edge of the disc, in the form of slots, allow for sharper transitions in speeds and distances.

[0129] According to one embodiment, the disc comprises a shoulder EP located at the periphery of the disc. Preferably, its external diameter defines the The diameter of the disk Dd (radius Rd). This shoulder is designed to precisely locate the distance between the axis of rotation and the outer diameter of the shoulder. Preferably, the focused incident beam is tangent to the edge of the shoulder with radius Rd. Knowing the diameter of the focused beam (Dsp = 2Rsp) allows for the precise determination of the actual measurement radius.

[0130] Rm = Rd + Rsp).

[0131] In addition, the shoulder also has a stiffening function and improves the mechanical strength of the outer part of the disc.

[0132] Preferably the edge of the mirror is fixed to the shoulder and the mirror is perpendicular to the external diameter of the shoulder.

[0133] According to a second aspect, the invention relates to a calibrated speed measurement system 20 illustrated [Fig. 12]. The system 20 comprises a laser velocimeter VL and a calibration device according to the first aspect of the invention.

[0134] The VL velocimeter comprises, in a known manner: • a DER transmitting / receiving device configured to emit a light signal at the first frequency fl and to receive a signal reflected by a frequency-shifted target, • a MM measurement module configured to superimpose a reference signal and the signal reflected by the target and detect a beat signal at a beat frequency corresponding to the frequency shift, • a processing unit UT configured to digitize the beat signal and extract speed information from the beat frequency.

[0135] The VL laser velocimeter of system 20 is configured to be coupled to the calibration device 10 during calibration.

[0136] During calibration, the velocimeter VL is configured to receive, from the calibration device 10, the value of the first standard tangential velocity vtel.

[0137] The DER transmitting / receiving device is further configured to transmit said first light signal at said first frequency fl to the calibration device 10.

[0138] The DER device is also configured to receive from the calibration device 10 i) the fraction of the first reflected signal having a frequency shifted from the first frequency by a Doppler shift Af corresponding to said first standard tangential velocity, and ii) the fraction of the plurality of additional reflected signals having respectively a frequency shifted from the first frequency by a plurality of shifts equal to i times the Doppler shift (Af), i varying from 2 to n.

[0139] During calibration, the first reflected signal and the plurality of additional reflected signals, originating from the calibration device 10 and received by the transmitting device DER reception, are processed in the same way as the signal reflected by the target by the UT processing unit of the VL velocimeter.

[0140] The processing unit UT of the velocimeter VL is thus further configured to extract from the Doppler shift Af a first tangential measurement velocity vtml, to be compared with the first standard tangential velocity vtel received from the calibration device, and to extract, from the plurality of shifts equal to i times the Doppler shift Af, a plurality of additional associated tangential measurement velocities vtmi, to be compared respectively with a plurality of additional standard tangential velocities vtei respectively equal to i times the first standard tangential velocity vtel.

Claims

Demands

1. Calibration device (10) intended to be coupled to a laser velocimeter (VL) comprising: • a disk (D) configured to rotate about an axis of rotation (AR) at a predetermined angular velocity (œ), • a mirror (M) fixed to the disk, having a surface (SM), • a first illumination / reception device (DIR1) configured to: • receive a first light signal (SI) at a first frequency, said first light signal being from the laser velocimeter, • orient and focus the first light signal so as to form a first incident signal illuminating the surface of the mirror at an incidence normal to a position of the disk during a passage of the disk, and with a predetermined diameter on said surface, the center of said diameter being located at a predetermined measurement distance (Rm) from said axis of rotation (AR),• to recover a first reflected signal (SRI) by the mirror at a frequency shifted relative to the first frequency of a Doppler shift (Af), said calibration device being further configured to provide at output, at least a fraction of said first reflected signal and a value of a first standard tangential velocity (vtel) determined from said angular velocity (œ) and said measurement radius (Rm), intended for said laser velocimeter (VL) to be calibrated.

2. Calibration device according to the preceding claim further comprising a reinjection device (DRI) configured to recover, amplify and reinject, into the first illumination / reception device, the first reflected signal and a plurality of additional reflected signals formed by successive reflections on said mirror during said passage of the disk, said calibration device being further configured to provide at output a fraction of said plurality of additional reflected signals, intended for said laser velocimeter (LV) to be calibrated.

3. Calibration device according to the preceding claim in which the first illumination / reception device is fiber-connected and in which the reinjection device comprises a first coupler (COI), a first optical amplifier (OA1) and a mirror optical fiber (OFM).

4. Calibration device according to claim 2 in which the first illumination / reception device is fiber-connected and in which the reinjection device includes a second coupler (CO2) and a second optical amplifier (OA2) configured to loop back onto the second coupler (CO2).

5. A calibration device according to any one of the preceding claims, wherein the disk comprises regularly spaced passing patterns (MT) around the periphery of the disk over a determined radius (Rmot), the calibration device further comprising a second illumination / reception device (DIR2) configured to: • receive a second light signal (S2') at a second frequency, said second light signal being from the laser velocimeter, • orient and focus the second light signal so as to form a second incident illuminating signal, with a non-zero angle of incidence (a) with respect to the plane of the disk, either on the patterns or on the surface of the disk depending on the position of the disk during rotation, • recover a second backscattered signal (SR2) from the disk, exhibiting an alternation between a second non-zero amplitude signal from the backscattering of the surface of the disk,and a second signal of zero amplitude when the second incident signal passes through the disk via a pattern, said calibration device being further configured to provide said laser velocimeter (LV) with at least a fraction of said second backscattered signal.

6. A calibration device according to any one of the preceding claims and further intended to be coupled to a laser rangefinder (TL), in which the The disk comprises patterns (MT) passing through, arranged regularly around the periphery of the disk on a determined radius (Rmot). The calibration device further comprises: • a backscattering object (Obj) positioned under the disk at a determined distance (do) from the disk, so as to backscatter light that has passed through the patterns; • a third illumination / reception device (DIR3) configured to: • receive a third light signal (S3') at a third frequency, said third light signal originating from the laser rangefinder (TL); • orient and focus the third light signal so as to form a third incident illuminating signal, along an axis of incidence (N) normal to the surface of the disk, either through the patterns or the surface of the disk, depending on the position of the disk during rotation.• to recover a third backscattered signal (SR3') exhibiting an alternation between a third backscattered signal from the disk and a third backscattered signal from the object, said calibration device being further configured to provide said laser rangefinder (TL) with at least a fraction of said third backscattered signal.

7. Calibration device according to the preceding claim in which the object is configured to be positioned at a plurality of determined distances along said axis of incidence (N) of the third incident signal.

8. Calibration device according to any one of the preceding claims further comprising a balancing mass (ME) disposed on the disk and intended to balance a mass of the mirror.

9. Calibration device according to any one of the preceding claims in which the disc has, at least at the periphery, a thickness (e) decreasing with a distance (r) from the axis of rotation.

10. Calibration device according to any one of the preceding claims, wherein the traversing patterns are notches (CR) located at the periphery of the disk

11. Calibration device according to any one of the preceding claims wherein the disc is bi-material, with a first material for a central part of the disc and a second material for a peripheral part.

12. Calibration device according to any one of the preceding claims configured so that a first maximum standard tangential velocity reached by the disk (vtelmax) is greater than or equal to 100 m.s1.

13. A calibrated speed measurement system (20) comprising a laser velocimeter (LV) and a calibration device according to any one of claims 1 to 12, the laser velocimeter being configured to be coupled to said calibration device during calibration, the laser velocimeter comprising: • a transmitting / receiving device (TRD) configured to emit the first light signal at said first frequency (fl) and to receive a signal reflected by a frequency-shifted target, • a measuring module (MM) configured to superimpose a reference signal and said signal reflected by the target, and to detect a beat signal at a beat frequency corresponding to the frequency shift, • a processing unit configured to digitize the beat signal and extract speed information from the beat frequency, the laser velocimeter (LV) further being configured, during calibration,to receive said value of a first standard tangential velocity (vtel) delivered by the calibration device (10) and such that: • the transmitting / receiving device transmits said first light signal at said first frequency (fl) towards the calibration device, • the transmitting / receiving device receives, from the calibration device, the fraction of said first reflected signal having a frequency shifted by the first frequency of a Doppler shift (Af) corresponding to said first standard tangential velocity, and, the fraction of the plurality of additional reflected signals exhibiting respectively a frequency shifted from the first frequency by a plurality of shifts equal to i times the Doppler shift (Af), i varying from 2 to n, • the processing unit extracts from said Doppler shift a first tangential measurement velocity (vtml) to be compared with the first standard tangential velocity (vtel), and extracts, from said plurality of shifts equal to i times the Doppler shift (Af), a plurality of additional associated tangential measurement velocities (vtmi), to be compared respectively with a plurality of additional standard tangential velocities (vtei) respectively equal to i times the first standard tangential velocity (vtel).

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