WO2010000283A1

WO2010000283A1 - System for laser measurement of the target motion

Info

Publication number: WO2010000283A1
Application number: PCT/EP2008/005422
Authority: WO
Inventors: Simona Ottonelli; Franceso De Lucia; Michela Di Vietro; Maurizio Dabbicco; Gaetano Scamarcio; Francesco Paolo Mezzapesa; Maria Cristina Plantamura; Salvatore Polito; Costantino Florio; Fabio Ottonelli
Original assignee: Sintesi ScpA
Current assignee: Sintesi ScpA
Priority date: 2008-07-03
Filing date: 2008-07-03
Publication date: 2010-01-07
Anticipated expiration: 2011-01-03

Abstract

A system is described for the measurement of the motion of a target. The measurement system includes a self-mixing laser source for generating an output radiation and for receiving a feedback radiation, includes a movable target for receiving at least part of a direct radiation and for reflecting a corresponding reflected radiation and includes a lens interposed between the laser source and the target for receiving said output radiation and at least part of said reflected radiation and for transmitting therefrom said direct radiation and said feedback radiation, respectively. The lens is placed in such a way that a diameter of the reflected radiation evaluated at the target is greater than a diameter of the direct radiation evaluated at the lens.

Description

Title : "System for laser measurement of the target motion"

FIELD OF THE INVENTION

The present invention relates to a system for the measurement of the target motion. Still more in particular, the invention concerns a remote measurement performed using a laser source.

BACKGROUND OF THE INVENTION

It is known that the measurement of the motion of a target (like the distance of the target from a pre-defined point, the speed of the target, ...) can be performed remotely (that is, without any physical contact between a sensor and the target) using a laser source. Referring more specifically to figure 1, it shows a known measurement system 50 for the measurement of the motion of the target 3. The system 50 includes a laser source 1 for generating an output radiation which is a beam comprised between forward optical rays 10, 12 and includes a lens 2 for receiving the output radiation and focusing on the target 3 a direct radiation which is a beam comprised between forward optical rays 11, 13. The target 3 reflects the direct radiation to a reflected radiation which is a beam comprised between backward optical rays 14, 16 over the same optical path of the forward rays 11, 13. The lens 2 receives the reflected radiation and transmits to the laser source 1 a feedback radiation which is a beam comprised between the backward optical rays 15, 17.

Therefore at least part of the beam of the feedback radiation comprised between the backwards optical rays 15, 17 enters the cavity of the laser source 1, wherein it occurs the interference between the output radiation generated in the laser cavity

(and transmitted to the lens 2) and the feedback radiation which enters the laser cavity (this is also known as "self-mixing effect") . The interference causes a modulation of the amplitude or frequency of the oscillating field in the laser cavity and thus a variation of the optical properties of the output radiation generated by the laser source 1, like a variation of the optical power: this variation depends on the motion

(distance from the laser source, speed) of the target 3 and thus the variation of a laser property provides an indication of the measurement of the motion of the target 3. Moreover, depending on the intensity of the optical power of the radiation which enters the laser cavity, a different interference can occur; specifically, the measurement of the motion is performed in condition of a moderated feedback, which is defined by a feedback parameter comprised between 1 and 4.6.

It is also known to detect the variation of the optical power of at least part of the modified laser beam generated by the laser source using a photodetector 4 (for example, a photodiode) , which generates an electrical signal (for example, a current) having fluctations depending on the motion of the target 3. In case of moderated feedback, the current signal generated by the photodetector 4 has a periodic amplitude modulation and the number of peaks of the current signal provides a measurement of the motion of the target 3 with a resolution of λ/2; moreover, the condition of moderated feedback allows the measurement of the direction of the motion of the target 3 (that is, if the target 3 is moving towards the laser source 1 or if it's moving away) . Finally, the electric signal generated by the photodetector is processed electronically and an estimation of the measurement of the motion of the target 3 is provided. Therefore it is advantageous to provide a measurement system for the laser measurement of the motion of a target in condition of moderated feedback.

The moderated feedback is achieved in the prior art by placing the target 3 in the focus F of the lens 2 for maximising the intensity of the radiation received by the target 3 and by placing an optical device in the forward optical path (that is, in the optical path from the laser source 1 to the target 3) and/or in the backward optical path (that is, in the optical path from the target 3 to the laser source 1) , in order to control the intensity of the radiation which enters the laser cavity after the reflection on the target 3; the optical device can be a lens, a mirror, a beam-splitter, an optical attenuator, an optical amplifier.

A first prior art document for achieving a moderated feedback is the US patent number 6233045, which discloses to add an optical attenuator (see block 111 in Fig.9) in order to decrease the intensity of the reflected radiation reflected by the target, thus controlling the amount of the feedback radiation which enters the laser cavity.

A second prior art document for achieving a moderated feedback is the US patent number 5135307, which discloses a mask (see 30 in Fig.l) placed between the lens and the target for blocking part of the direct radiation transmitted from the lens to the target .

Therefore the known solutions have the disadvantage to require an optical device in the forward optical path and/or in the backward optical path, thus increasing the cost and the complexity of the system. Another disadvantage to place an optical device in the forward/ backward optical path is that the optical device can cause additional reflections and thus additional radiation is received in the laser cavity, affecting the interference: this increases the noise in the electric signal required for the estimation of the motion of the target, thus degrading the accuracy of the measurement of the motion of the target.

SUMMARY OF THE INVENTION

In view of the drawbacks of the known solutions, the main object of the present invention is to provide a measurement system and method for the laser measurement of the motion of a target. This object is achieved by a measurement system according to claim 1 and by a method according to claim 14.

Advantages of the invention are: - not to require an additional optical device in the forward/ backward optical path; - to reduce the noise in the electrical signal used for the estimation of the target motion;

- to increase the dynamic range and the accuracy of the estimation of the target motion; - to allow the control of the self-mixing effect by changing the position of the lens;

- to decrease the cost and the complexity of the system.

BRIEF DESCRIPTION OF THE DRAWINGS

Figure 1 schematically shows a measurement system for the laser measurement of the motion of a target according to the prior art .

Figure 2 schematically shows a measurement system for the laser measurement of the motion of a target according to a first embodiment of the invention.

Figure 3 schematically shows a measurement system for the laser measurement of the motion of a target according to a second embodiment of the invention.

Figure 4 schematically shows an example wherein the measurement system according to the first embodiment can be used. Figures 5a and 5b schematically show two examples wherein the measurement system according to the second embodiment can be used.

Figure 6 schematically shows a physical implementation of the laser source, of the lens and of the photodetector according to the invention.

BEST MODE FOR CARRYING OUT THE INVENTION

Referring to figure 2, it shows a measurement system 150 for the laser measurement of the motion of a movable target 103 according to a first embodiment of the invention. The measurement system 150 includes a laser source 101, a laser driver 105, a lens 102, a target 103, a photo-detector 104, a processing circuit 106 and a micro-controller 107 for the motion measurement. Figure 2 also shows a laser transceiver 140 including the laser source 101, the lens 102 and the photo-detector 104. The laser source 101, the lens 102 and the target 103 are placed on a common system optical axis 109; anyway, the laser source 101 can be placed on a first optical axis and the lens 102 on a second optical axis, and an angular tolerance between the first and the second optical axis can be accepted. The laser source 101 is a self-mixing laser source which is adapted to generate an output radiation and to receive a feedback radiation; for example, the laser source 101 is a Distributed FeedBack (DFB) InGaAsP laser diode adapted to generate the output radiation having a wavelength of 1310 nm.

The laser driver 105 is adapted to drive the laser source 101. Preferably, the laser driver 105 is adapted to supply the laser source 101 with a current which is substantially constant (for example, a value comprised between 25 mA and 150 mA) and which is substantially independent on the variation of the temperature and on the supply voltage of the system 150 (for example, the stability of the supply current is ±50 μA) : this allows to improve the stability of the wavelength generated by the laser source 101.

The target 103 is adapted to receive at least part of a direct radiation and to reflect a corresponding reflected radiation. The target 103 is movable along the direction of the system optical axis 109 (like indicated schematically by the arrow 120) and thus the system 150 is adapted to measure a linear displacement of the target 103 along the direction of the system optical axis 109. The target 103 can be a part of an object for which the motion (specifically, a linear displacement) has to be measured; alternatevely, the target 103 can be separate from the object but attached to the object, such that the measurement of the motion of the target 103 is equivalent to the measurement of the motion (specifically, a linear displacement) of the object .

The lens 102 is interposed between the laser source 101 and the target 103 and it's adapted to receive from the laser source 101 the output radiation and to transmit therefrom the direct radiation to the target 103, and it's adapted to receive at least part of the reflected radiation and to transmit therefrom a feedback radiation to the laser source 101. The lens 102 has a first focal point Fl placed on the system optical axis 109 on the side of the lens 102 towards the laser source (figure 2 shows that the first focal point Fl is on the left of the lens 102) and the position of the laser source 101 (that is, the position of the front facet of the laser source 101) is different from the position of the first focal point Fl; specifically, the position of the laser source 101 is comprised between the first focal point Fl and the lens 102.

The means including the photo-detector 104, the processing circuit 106 and the micro-controller 107 have the functionality to estimate the measurement of the motion of the target 103 from the variation of an optical property (for example, the optical power) of the output radiation generated by the laser source. The photo-detector 104 is adapted to generate an electrical signal from the variation of the detected optical power of the output radiation.

The processing circuit 106 is adapted to process the electrical signal . The micro-controller 107 is adapted to provide the measurement of the motion of the target 103.

It will be described hereinafter the functionality of the first embodiment of the measurement system 150, with reference to figure 2. The basic idea is to place the lens 102 in such a way that the diameter D2 of the reflected radiation evaluated at the target 103 is greater than the diameter Dl of the direct radiation evaluated at the lens 102 : this allows to control the optical power which enters the cavity of the laser source 101, thus allowing to achieve the moderated feedback, without using additional optical devices in the forward optical path (that is, in the optical path from the laser source 101 to the target 103) and/or in the backward optical path (that is, in the optical path from the target 103 to the laser source 101) .

The laser source 101 generates an output radiation which is a diverging beam comprised between forward rays 110 and 112. The lens 102 receives from the laser source 101 the output radiation (or at least part of the output radiation) and transmits therefrom to the target 103 a direct radiation which is a beam comprised between the forward optical rays 111, 113.

The position of the laser source 101 is comprised between the first focal point Fl and the lens 102, thus the direct radiation transmitted by the lens 102 is diverging from the lens 102 and it's a beam comprised between the forward optical rays 111 and 113 (figure 2 shows that the optical rays 111 and 113 are diverging from the lens 102) .

The diameter of the direct radiation evaluated at the lens 102 (that is, the diameter of the beam of the direct radiation evaluated on the surface of the lens 102 towards the target 103) is indicated in figure 2 with Dl: the diameter of the direct radiation evaluated at different positions of the optical path from the lens 102 to the target 103 is increasing, because the direct radiation is diverging from the lens 102 to the target 103. The target 103 receives at least part of the direct radiation and reflects a reflected radiation, wherein the reflected radiation carries at least part of the direct radiation. In order to explain the invention, it is supposed that the target 103 includes a surface which is substantially flat and substantially perpendicular to the system optical axis 109, but the flat surface and the perpenpendicularity are not essential for performing the invention: other shapes can be used which are adapted to reflect at least part of the direct radiation over a reflected radiation directed towards the lens 102 and an angular tolerance between the system optical axis 109 and the flat surface can be accepted. If the surface of the target 103 is substantially flat and the surface is substantially perpendicular to the system optical axis 109, the optical rays 111 and 113 are reflected to the optical rays 114 and 116 respectively according to known optical laws (angle of incidence equal to angle of reflection) , thus the reflected

, radiation reflected by the target 103 is a beam comprised between the backward optical rays 114 and

116, which are also diverging (like indicated schematically in figure 2 with the optical rays 114 and 116) . The diameter of the reflected radiation evaluated at the target 103 (that is, the diameter of the beam of the reflected radiation evaluated on the surface of the target 103 towards the lens 102) is indicated in figure 2 with D2 : the diameter of the reflected radiation evaluated at different positions of the optical path from the target 103 to the lens 102 is increasing, because the reflected radiation is diverging from the target 103 to the lens 202 (like indicated schematically in figure 2 with the optical rays 114 and 116) .

According to the first embodiment of the invention, the diameter D2 of the reflected radiation evaluated at the target 103 is greater than the diameter Dl of the direct radiation evaluated at the lens 102.

Preferably, the ratio between the diameter D2 of the reflected radiation and the diameter Dl of the direct radiation is greater than 1 and smaller than 20; more in general, the ratio between the diameter D2 of the reflected radiation and the diameter Dl of the direct radiation is such that the feedback in the cavity of the laser source 103 is moderated.

It is worth noting that the direct radiation transmitted by the lens 102 can be larger than the optical rays 111 and 113 shown in figure 2, that is figure 2 does not show the optical rays which are more diverging than the optical rays 111 and 113 and which are not received (thus not reflected) by the target 103.

The lens 102 receives at least part of the reflected radiation reflected by the target 103 and transmits therefrom a feedback radiation to the laser source 101. It is worth noting that the reflected radiation reflected by the target 103 can be larger than the optical rays 114 and 116 shown in figure 2, that is figure 2 does not show the optical rays which are reflected by the target 103 and which are more diverging than the optical rays 114 and 116 (and thus they are not received by the lens 102) .

The laser source 101 receives the feedback radiation (or at least part of the feedback radiation) , which enters the laser cavity and generates the self-mixing effect.

The photo-detector 104 detects the variation of the optical power of the output radiation generated by the laser source 101 and generates therefrom a current signal having fluctations depending on the target motion.

The processing circuit 106 includes an amplification stage receiving the current signal and converting it into a voltage signal, includes a low- pass filter receiving the voltage signal and providing a filtered voltage signal, includes an amplifier having an automatic gain control receving the filtered voltage signal and generating an amplified voltage signal having a controlled amplitude which is substantially constant over the motion of the target 103, includes a generator receiving the amplified voltage signal and generating positive and negative pulses from the rising and falling edges of the amplified voltage signal, and includes a comparator receiving the positive or negative pulses and generating positive or negative squared pulses.

The micro-controller 107 receives the positive and negative squared pulses, performs the calculation of the number of the positive and negative squared pulses, performs the control of the gain of the amplifier of the processing circuit 106 and provides an estimation of the measurement of the motion of the target 103. Preferably, the control of the amplifier of the processing circuit 106 is achieved detecting the amplitude of the positive and negative pulses of the generator of the processing circuit 106.

Advantageously, the first embodiment of the invention is used for the measurement of the motion of the target 103 having a dynamic range comprised between 0,1 m and 3 m, with a resolution of about 0,7 μm (that is, it's possible to detect a variation of the target distance greater or equal than 0,7 μm) . Figure 4 shows an example wherein the first embodiment of the invention can be used. A trolley 160 is movable along a direction and it's driven by a motor 161 through a rotatable threaded shaft 162 coupled to the motor 161 and to the trolley 160. The laser transceiver 140 (including the laser source 101, the lens 102 and the photo-detector 104) is mounted at one end of the path wherein the trolley 160 is movable and the target 103 is mounted on the trolley 160, thus the target 103 is movable along the direction which corresponds to the system optical axis 109. The linear displacement of the trolley 160 in the direction indicated by the arrow 163 is measured by the measurement system 150 according to the first embodiment. The considerations about the first embodiment of figure 2 are in part applicable to the second embodiment of figure 3, wherein the blocks having the same functionalities are indicated with the same reference numbers, while different blocks are indicated with different reference numbers and will be described hereinafter. The target 103 is movable in the direction indicated schematically by the arrow 220 and thus the system 250 is adapted to measure a linear displacement of the target 103 along the direction of the system optical axis 109.

The lens 202 has a first focal point Fl' placed on the system optical axis 109 on the side of the lens 202 towards the laser source 101 (figure 3 shows that the first focal point Fl' is on the left of the lens 202) and the laser source 101 has an image point II' placed on the system optical axis 109 on the side of the lens 202 towards the target (figure 3 shows that the image point is on the right of the lens 202) . The image point II' is the position of the image corresponding to the front facet of the laser source 101 and in figure 3 is indicated schematically with the image point II', although it's not a point but it's a region having a diameter of some millimetres. The position of the laser source 101 (that is, the position of the front facet of the laser source 101) is different from the position of the first focal point Fl' : specifically, the position of the laser source 101 is on the left of the first focal point Fl' , thus the direct radiation transmitted by the lens 202 is converging into the image point II' and it's a beam comprised between the forward optical rays 211 and 213 (figure 3 shows that the optical rays 211 and 213 are converging into II' ).

The target 103 is placed on the right of the image point II', thus the direct radiation transmitted by the lens 202 is diverging from the image point II' and it's a beam comprised between the forward optical rays 218 and 219 (figure 3 shows that the optical rays 218 and 219 are diverging from the image point II') .

The diameter of the direct radiation evaluated at the lens 202 (that is, the diameter of the beam of the direct radiation evaluated on the surface of the lens 202 towards the target 103) is indicated in figure 3 with Dl' : the diameter of the direct radiation evaluated at different positions of the optical path from the image point II' to the target 103 is increasing, because the direct radiation is diverging from the image point II' to the target 103.

The diameter of the reflected radiation evaluated at the target 103 (that is, the diameter of the beam of the reflected radiation at the reflecting side of the target 103) is indicated in figure 3 with D2 ' : the diameter of the reflected radiation evaluated at different positions of the optical path from the target 103 to the lens 202 is increasing, because the reflected radiation is diverging from the target 103 to the lens 202 (like indicated schematically in figure 3 with the optical rays 214 and 216) .

According to the second embodiment of the invention, the diameter D2 ' of the reflected radiation evaluated at the target 103 is greater than the diameter Dl' of the direct radiation evaluated at the lens 202.

Preferably, the position of the image point II' is comprised between 1/4 and 3/4 of the distance from the lens 202 to the reflecting surface of the target 103, such that the ratio between the diameter D2' of the reflected radiation evaluated at the target 103 and the diameter Dl' of the direct radiation evaluated at the lens 202 is comprised between 1 and 10; more in general, the ratio between D2' and Dl' is such that the feedback in the cavity of the laser source is moderated.

Advantageously, the second embodiment of the invention is used for the measurement of the motion of the target 103 having a dynamic range not greater than 100 mm and wherein the distance between the laser source 101 and the target 103 is equal to a value comprised between 100 mm and 3 m, with a resolution of about 0,7 μm.

Figures 5a and 5b shows two examples wherein the second embodiment of the invention can be used. An axel can become deformed in case of variation of the temperature; the axel can be the truss 171 of a bridge 170 like shown in figure 5a or a part 181 of an industrial machine 180 (a milling machine, a lathe, ...) like shown in figure 5b. The transceiver 140 is mounted on one position of the axel 171 or 181 and the target 103 is mounted on another position of the axel 171 or 181, wherein the target 103 is movable along the direction which corresponds to the system optical axis 109. The linear displacement of the target 103, which is caused by the deformation of the axel 171 or 181 in the direction indicated by the arrows 172 or 182, is measured by the measurement system 250 according to the second embodiment . Preferably, the lens 102 is aspherical, that is the lens 102 is defined by two surfaces which are not spherical (in other words, each surface is not defined by a single curvature) . This has the advantage to reduce the spherical aberrations. The above considerations about the aspherical lens 102 are also applicable to the lens 202. Preferably, the aspherical lens 102 is planoconvex, that is one of the two surfaces of the lens 102 is substantially flat and the other surface is convex (like shown schematically in figure 6) ; advantageously, the flat surface is placed towards the laser source 101 and the convex surface is placed towards the target 103, in order to minimize the aberrations. The focal length of the lens 102 is, for example, equal to 11 mm and the numerical aperture equal to 0,25. The plano-convex lens 102 allows to further control the optical power which enters the cavity of the laser source 101, in order to achieve the moderated feedback. A lens 102 suitable for the first embodiment of the invention is the lens C220TME-C sold by Thorlabs Inc. (www.thorlabs.com). The above considerations about the plano-convex lens 102 are also applicable to the lens 202.

Preferably, the target 103 includes a partially reflective surface for partially reflecting at least part of the direct radiation over the reflected radiation; for example, the reflectivity of the surface is comprised between 2% and 90%. This allows to further control the optical power of the reflected radiation and thus to control the optical power which enters the cavity of the laser source 101, in order to achieve the moderated feedback. In this case the moderated feedback is achieved controlling both the ratio between the diameters D2 and Dl (or between D2 ' and Dl') and the reflectivity of the target within the range 2%-90%: this has the advantage to improve the flexibility of the measurement system 150 (or 250) .

Preferably, the target 103 includes a surface which is substantially flat and which is substantially perpendicular to the system optical axis 109.

Preferably, the measurement system 150 (or 250) includes a sensor (not shown in the drawings) for detecting the temperature of the laser source 101 and includes a thermo-electric cooler (not shown in the drawings) thermally connected to the laser source 101. The micro-controller 107 is further adapted to receive from the sensor a signal indicating the temperature of the laser source 101 and to control therefrom the thermo-electric cooler, in order to keep the temperature of the laser source

101 to a substantially constant value.

Preferably, the thermo-electric cooler is a Peltier cell. Preferably, the laser source 101 and the lens

102 (or 202) are mounted into a first opening and into a second opening respectively of a single housing 190 (like shown in figure 6) made of a metal having an high thermal conductivity: some examples of the metal are alluminium, copper, silver, gold, magnesium, tungsten. The housing 190 has an external surface which is mainly closed, except for the first opening including the lens 102 and for the second opening including the laser source 101.

In this case, the sensor is adapted to detect the temperature of the housing 190 and the thermoelectric cooler (like the Peltier cell) is thermally connected to the housing 190 for keeping the temperature of the housing 190 to a substantially constant value, depending on the temperature detected by the sensor. This has the advantage to improve the stabilization of the temperature of the laser source 101 to the constant value, because the heat generated by the laser source 101 is transmitted to the housing 190 where it's quickly spread over the whole surface of the housing 190.

Preferably, the housing 190 is a parallelepiped, wherein the first opening is a first surface and the second opening is a second surface opposite to the first surface, wherein the laser source 101 is placed on the first surface and the lens 102 is placed on the second surface, like shown in figure 6. In this case the Peltier cell is thermally- connected to a third surface of the parallelepiped, wherein the third surface is substantially perpendicular to the first and second surfaces. Preferably, the housing 190 is a cube, wherein the length of each side of each surface of the cube is about 20 mm.

Preferably, the photodetector 104 is also mounted into the metal housing 190; in this case, the housing 190 corresponds to the laser transceiver 140 shown in figure 2, that is the laser transceiver 140 is implemented into the metal housing 190. Advantageously, the photodetector 104 is integrated into the laser source 101. Preferably, the position of the lens 102 (or 202) can be changed along the system optical axis 109 (for example in a range of 2 mm) , allowing to control the moderated feedback. This improves the flexibility of the measurement system 150 (or 250) , because the moderated feedback can also be controlled by changing the position of the lens 102 (or 202) when configuring the system 150 (or 250) for a specific application.

It is worth noting that figure 2 and figure 3 show the following positions (from the left to the right) : laser source 101, lens 102 (or 202) and target 103. Anyway, the invention is also applicable to the following opposite positions (from the left to the right) : target 103, lens 102 (or 202) and laser source 101. Therefore the considerations about the first and the second embodiments are applicable to the opposite positions.

According to another aspect, the present invention provides a method for performing a laser measurement. The method includes the step of providing a laser source, a lens and a movable target, the step of interposing the lens between the laser source and the movable target, the step of generating, from the laser source, an output radiation and receiving a feedback radiation, the step of receiving, at the movable target, at least part of a direct radiation and reflecting a corresponding reflected radiation and the step of receiving at the lens said output radiation and at least part of said reflected radiation and transmitting therefrom said direct radiation and said feedback radiation, respectively. The method further includes the step of evaluating a diameter of the reflected radiation at the target, the step of evaluating a diameter of the direct radiation at the lens and the step of placing the lens such that the diameter of the reflected radiation evaluated at the target is greater than the diameter of the direct radiation evaluated at the lens.

Preferably, the lens has a first focal point placed on a system optical axis on the side of the lens towards the laser source. The method further includes the step of placing the laser source to a position comprised between the first focal point and the lens, such that the direct radiation transmitted by the lens is diverging. Preferably, the lens has a first focal point placed on a system optical axis on the side of the lens towards the laser source and the laser source has an image point placed on the system optical axis on the side of the lens towards the target. The method further includes the step of placing the target to a position on the right of the image point and the step of placing the laser source to a position on the left of the first focal point, such that the direct radiation transmitted by the lens is converging into the image point .

Claims

1. A measurement system (150; 250) including: a self-mixing laser source (101) for generating an output radiation (110, 112; 210, 212) and for receiving a feedback radiation (115, 117; 215, 217); - a movable target (103) for receiving at least part of a direct radiation (111, 113; 212, 213) and for reflecting a corresponding reflected radiation (114, 116; 214, 216) ; a lens (102; 202) interposed between the laser source (101) and the target (103) for receiving said output radiation and at least part of said reflected radiation and for transmitting therefrom said direct radiation (111, 113; 212, 213) and said feedback radiation (115, 117; 215, 217), respectively; characterized in that the lens (102; 202) is placed in such a way that a diameter (D2; D2 ' ) of the reflected radiation evaluated at the target (103) is greater than a diameter (Dl; Dl') of the direct radiation evaluated at the lens (102; 202).

2. System according to claim 1, wherein the lens (102) has a first focal point (Fl) placed on a system optical axis (109) on the side of the lens

(102) towards the laser source, and wherein the position of the laser source is comprised between the first focal point (Fl) and the lens, such that the direct radiation transmitted by the lens is diverging.

3. System according to claim 2, wherein the ratio between the diameter of the reflected radiation evaluated at the target (103) and the diameter of the direct radiation evaluated at the lens (102) is greater than 1 and smaller than 20.

4. System according to claim 1, wherein the lens has a first focal point (Fl') placed on a system optical axis (109) on the side of the lens (202) towards the laser source (101) and the laser source (101) has an image point (H') placed on the system optical axis on the side of the lens towards the target, wherein the target (203) is placed on the right of the image point (H'), and wherein the position of the laser source is on the left of the first focal point (Fl'), such that the direct radiation transmitted by the lens is converging into the image point (H') .

5. System according to claim 4, wherein the ratio between the diameter of the reflected radiation evaluated at the target (103) and the diameter of the direct radiation evaluated at the lens (102) is comprised between 1 and 10.

6. System according to at least one of the previous claims, wherein the ratio between the diameter of the reflected radiation and the diameter of the direct radiation is such that feedback in the cavity of the laser source is moderated.

7. System according to at least one of the previous claims, wherein the target includes a partially reflecting surface for partially reflecting at least part of the direct radiation over the reflected radiation, the reflectivity of the surface being comprised between 2% and 90%.

8. System according to claim 7, wherein the surface is substantially flat and substantially perpendicular to the system optical axis.

9. System according to at least one of the previous claims, the system further including a housing made of alluminium, the housing including the laser source and the lens.

10. System according to claim 9, the system further including a sensor for detecting the temperature of the housing and including a Peltier cell thermally connected to the housing for keeping the temperature of the housing to a substantially constant value, depending on the temperature detected by the sensor.

11. System according to at least one of the previous claims, wherein the lens (102; 202) is aspherical and it's defined by a first surface substantially flat towards the laser source (101) and by a second convex surface towards the target (103) .

12. System according to at least one of the previous claims, the system further including means

(104, 106, 107) for estimating the measurement of the motion of the target from the variation of an optical property of the output radiation generated by the laser source.

13. System according to claim 12, wherein the estimating means include a photodetector (104) for detecting the variation of the optical property of the output radiation.

14. Method for performing a laser measurement, including the steps of : providing a laser source (101), a lens (102; 202) and a movable target (103) ; interposing the lens (102; 202) between the laser source (101) and the movable target (103) ; - generating, from the laser source (101) , an output radiation (110, 112; 210, 212) and receiving a feedback radiation (115, 117; 215, 217) ; receiving, at the movable target (103), at least part of a direct radiation (111, 113; 212, 213) and reflecting a corresponding reflected radiation (114, 116; 214, 216) ; receiving at the lens said output radiation and at least part of said reflected radiation and transmitting therefrom said direct radiation (111, 113; 212, 213) and said feedback radiation (115, 117; 215, 217), respectively; characterized by further including the steps of:

• evaluating a diameter of the reflected radiation at the target (103) ;

• evaluating a diameter (Dl; Dl') of the direct radiation at the lens (102; 202);

• placing the lens (102; 202) such that the diameter (D2; D2 ' ) of the reflected radiation evaluated at the target (103) is greater than the diameter (Dl; Dl') of the direct radiation evaluated at the lens (102; 202).

15. Method according to claim 14, wherein the lens (102) has a first focal point (Fl) placed on a system optical axis (109) on the side of the lens (102) towards the laser source, the method further including the step of placing the laser source to a position comprised between the first focal point (Fl) and the lens, such that the direct radiation transmitted by the lens is diverging.

16. Method according to claim 14, wherein the lens has a first focal point (Fl') placed on a system optical axis (109) on the side of the lens (202) towards the laser source and the laser source

(101) has an image point (H') placed on the system optical axis on the side of the lens towards the target, the method further including the steps of placing the target (203) to a position on the right of the image point (H') and of placing the laser source (101) to a position on the left of the first focal point (Fl'), such that the direct radiation transmitted by the lens is converging into the image point (H' ) .