ITRM20130462A1 - OPTICAL DEVICE TO ELIMINATE THE LIGHT NOT INTENDED IN AN OPTICAL SYSTEM AND USING OPTICAL SYSTEM SUCH A DEVICE. - Google Patents

Description

Baizanò & Zanando

1

OPTICAL DEVICE TO ELIMINATE UNWANTED LIGHT IN AN OPTICAL SYSTEM AND OPTICAL SYSTEM USING SUCH

DEVICE

The present invention refers to an optical device for eliminating unwanted light (so-called straylight) in an optical system for scanning a laser radar for measuring the distance and / or reflectivity of a target.

More specifically, the invention relates to the structure of an anti-straylight optical device of the above type which allows to reduce or even eliminate the unwanted light contribution due to the diffuse and specular portion that is generated by a scanning element present in a system optical, typically a prism, when said prism is hit by a laser beam. The present invention further relates to an optical system comprising said anti-straylight optical device.

Currently, when a prism of an optical system is hit by a laser beam, the cathets of said prism transmit said laser beam by launching it on the target of interest whose distance and / or reflectivity is to be determined, thanks to the fact that the laser beam it is reflected internally by the surface of the hypotenuse.

The use of a prism, and in general of a transmitting device in an optical scanning system involves some disadvantages.

The main disadvantage is that the

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faces corresponding to the catheti of the rectangle isosceles prism (or other scanning element, even a non-rectangle prism) can cause stray light.

For this reason, the faces corresponding to said cathets are normally provided with an anti-reflective coating.

The face corresponding to the hypotenuse of the prism is equipped with a total reflection coating so that said face totally reflects the light beam that affects it.

These coatings, both the one applied on the faces of the prism corresponding to the cathets and the one applied on the face of the prism corresponding to the hypotenuse, wear out over time, especially inside fusion machines, where they undergo the effects due to a gamma irradiation, to a high neutron flux, as well as to the dust (s) deposited (e) that is deposited (no) on the prism itself.

These effects reduce the efficiency of the coatings by causing an undesired signal that must be vectorially added to the useful signal coming from the target that the electronic system of a typically amplitude-modulated optical radar must process. Consequently, the measurement of the distance and / or reflectivity of the target of interest is affected by this unwanted signal.

A first example of an optical device of a known type for reducing unwanted light in a polychromatic system consists of two monochromators from

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series connecting which cooperate to both select the same wavelength in the output light beam.

However, this known optical device has some drawbacks.

One disadvantage is that it has a complex structure.

A second disadvantage is the cost. The purpose of the present invention is to provide an anti-straylight optical device, structurally simple and at low cost, to reduce or eliminate the unwanted light contribution in an optical system, in such a way as to allow a correct measurement of the distance and / or of the reflectivity of a target.

A further object of the present invention is to provide an optical system comprising this anti-straylight optical device.

The object of the present invention is an anti-straylight optical device for intercepting an unwanted light produced by a light beam traveling along a first axis incident on a surface of a scanning element of an optical device for scanning the light beam on a target. , characterized by the fact of including:

- an annular element with a center, an internal diameter sized to allow the passage of said light beam, and an external diameter, said annular element being arranged on a first plane;

- a pair of fins such that:

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or they are arranged on a second plane parallel to said first plane and connected to said annular element in such a way as to be 180 ° from each other; And

or each fin of said pair of fins is substantially symmetrical with respect to a second axis lying on said second plane and passing through the projection of said center.

According to an embodiment of the invention, characterized in that said external diameter and said internal diameter are dimensioned in such a way as to intercept or limit the solid angle of the Lambertian component of the unwanted light.

According to an embodiment of the invention, said first floor coincides with said second floor.

According to an embodiment of the invention, each fin of said pair of fins is connected to said annular element by means of a respective arm.

According to an embodiment of the invention, each arm is perpendicular to said first plane and said second plane.

According to an embodiment of the invention, each fin of said pair of fins is rectangular.

A further specific object of the present invention is an optical device for scanning a light beam, in particular a laser beam, which travels along a first axis incident on a surface of a scanning element included in said optical scanning device, the element of

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scanning being rotatably arranged on a tilt plane orthogonal to a pan plane of the optical system for scanning the light beam on a target, the optical scanning device being characterized in that it comprises an anti-straylight optical device according to the invention, and in which said anti-straylight optical device is positioned so that:

- is rotatable about said first axis on said pan plane orthogonal to said first axis,

- said tilt plane always passes through said first axis and said second axis.

According to an embodiment of the invention, said external diameter is between 1/8 and 1/12 of the distance between said center and the point of said scanning element on which the light beam affects.

According to an embodiment of the invention, said anti-straylight optical device is arranged diametrically inside a circular opening on said pan plane.

According to an embodiment of the invention, said scanning element is a scanning prism, in particular an isosceles rectangle prism.

A further specific object of the present invention is an optical scanning system, in particular of a viewing and ranging device, comprising an active module and a passive module, characterized in that said passive module comprises the optical scanning device according to the invention .

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According to an embodiment of the invention: - the passive module comprises a focusing lens for focusing a light beam received by said active module on said target, and - the dimension of each fin transversely to said second axis is less than or equal to the diameter d of said focusing lens.

According to an embodiment of the invention, said dimension of each fin transversely to said second axis is comprised between 0.9d and d.

According to an embodiment of the invention, said passive module comprises:

- a focusing shifter which receives a control signal from said active module and moves said focusing lens;

- a beam splitter placed at the output of said focusing lens;

- an imaging fiber bundle which collects the part of the light beam transmitted by the beam splitter and transports it to said active module;

- a mirror which intercepts a part of the light beam deviated from said beam splitter and reflects it generating a reflected light beam, said mirror having a first reflecting face and a second face opposite to said first face; - said optical scanning device which scans said light beam reflected on said target;

- a receiving lens turned towards said second face of the mirror, which focuses the light in

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receiving from said target on a further bundle of optical fibers for imaging which carries the signal to said active module;

- a translator of the receiving lens,

and by the fact that said active module includes:

control electronics, in particular including radar signal processing electronics,

- a light beam source which generates a light beam carried by a single mode optical fiber to said focusing lens;

- a photodetector which receives said further bundle of optical fibers for imaging and is connected to said control electronics;

- a light beam profilometer which receives said bundle of optical fibers for imaging and sends to said control electronics a waist signal of the light beam transmitted by said beam-splitter; - a connection between said control electronics and said focusing translator adapted to carry a control signal.

According to an embodiment of the invention, when said waist is greater than the diameter of said bundle of optical fibers for imaging, a focusing lens with demagnification, in particular with greater demagnification, is placed between said beam splitter and said bundle of optical fibers for imaging. of 6.

In one embodiment, the design parameters of the passive module are chosen so that the diameter of the reflected light beam is less than 5mm.

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The present invention will now be described, for illustrative but not limitative purposes, according to an embodiment thereof, with particular reference to the attached figures, in which:

- figure 1 shows a laser beam which strikes on a real surface according to a scattering pattern of a known type;

- figure 2 shows an antistraylight optical device according to the invention arranged in an optical device for scanning a laser beam, which comprises a prism on which a laser beam is directed,

- figure 3 shows in detail the anti-straylight optical device used in the optical device of fig. 2;

- figure 4 shows an antistraylight optical device according to the invention arranged in an optical scanning device as in figure 2, in which the connection details with the prism moving means are illustrated;

figure 5 shows the effect of the annular region of the anti-straylight optical device according to the invention;

Figure 6 shows a diagram of an embodiment of the system according to the invention. Figure 1 shows what happens when a laser beam L hits a real surface S.

The laser beam - matter interaction generates a diffused light component (so-called component

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Lambertian) indicated with the LM reference, a reflected light component indicated with the SDM reference and a retro-reflected component indicated with the

"

RM reference.

In an optical system for scanning a laser radar, the Lambertian and specular components must be eliminated or reduced as these components cause the generation of an unwanted signal which represents an instrumental noise that prevents a correct measurement of the distance of a target and / or the reflectivity of the target itself, as it adds to the useful signal to be processed.

With particular reference to Figures 2 and 3, an anti-straylight optical device 1 is described for intercepting the unwanted light that is generated by a scanning element P belonging to an optical system. In the embodiment that is described, said scanning element P is a prism.

In particular, said prism P has a right-angled triangle section.

Said anti-straylight optical device 1 is arranged on the terminal part of a scanning head (probe) 3, and is positioned at a predetermined distance from said prism P, and comprises:

- an annular element 1A having a hole 10A to allow the passage of a laser beam L incident on said prism P,

- a first flap and a second flap 1B, arranged on the outer edge of said annular element 1A in such a way as to be at 180 ° a

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on the other, or along the same median axis B;

wherein said annular element and said fins are configured to intercept the unwanted light generated by the prism.

In particular, the antistraylight optical device 1 is arranged with respect to the source that emits the laser beam L in such a way that the latter passes through the probe 3, passes through the center of the hole 10A of the annular element 1A and goes to engrave on a face of the prism P (as indicated by the arrow in fig. 2, ie along a first axis A). Subsequently, the laser beam is reflected by the hypotenuse of the prism and transmitted on a target T. In the embodiment described, said anti-straylight optical device has a main extension direction and is symmetrical with respect to a median axis B passing through the center of the hole 10A of the annular element 1A (figs. 2 and 3).

Each fin 1B has a first end connected to the annular element 1A and a second end connected to movement means 4 to rotate said anti-straylight optical device in a clockwise or anticlockwise direction (as indicated by the double arrow in fig. 2). Said anti-straylight optical device rotates around the first axis A, along which the laser beam L travels, on a foreground or PAN plane, orthogonal to said first axis A.

Said anti-straylight optical device is arranged with respect to the prism P in such a way that a second plane

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or tilt plane, orthogonal to said first plane, passes through said first axis A and said median axis B of the anti-straylight optical device itself.

The prism P also rotates on said second plane and later (with reference to Fig. 4) it will be explained in detail how said prism rotates.

In particular, each fin 1B has a substantially rectangular section (but can have another symmetrical shape with respect to the axis B) to intercept also the beam of light reflected by the prism P, in particular due to the loss of efficiency of the coatings applied on the faces. of the prism itself corresponding to the legs of the right triangle.

As shown in figure 2, in fact, the prism, which is hit by the laser beam L, on one side transmits a beam of light LT on a target T whose distance and / or reflectivity you want to determine (as indicated by the arrow) , and on the other hand it reflects said laser beam L in such a way that a beam of light LR is directed towards the antistraylight optical device (as indicated by the arrow).

The annular element included in the anti-straylight optical device is arranged as close as possible to the prism 2 (and therefore all the anti-straylight optical device lying on a plane), and has dimensions such as to intercept or limit the solid angle of the diffused light or Lambertian component, in such a way as to avoid that the unwanted signal arrives on a focusing lens of an optical reception system. Figure 5 illustrates the effect of this

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distance on the light collected by a lens receiving the beam returning from the target.

Said annular element has dimensions such as to intercept or limit the solid angle of the Lambertian component of the straylight.

In particular, said annular element has an internal diameter 10A and an external diameter 10C, where said external diameter 10C is a function of the distance between the center 10B of said annular element 1A and the point of said prism P on which the laser L affects. case of approximation for a point source, the external diameter 10C can be determined between 1/8 and 1/12 of the distance mentioned above.

More generally, said external diameter 10C can be a function of the size of the region (scattering source) of the prism P which generates a Lambertian scattering and of the distance between said annular element 1A and said region. Furthermore, the external diameter 10C may also depend on the size and relative position of the system for receiving or collecting the light beam returning from the target. Finally, the same external diameter 10C can be a function of the signal ratio (receiving light beam) / noise (unwanted noise signal).

Furthermore, an excessive value of this external diameter would imply a geometric occlusion of the signal diffused by the target T.

In an example of embodiment, the external diameter 10C is equal to lem at a distance of 10cm from the prism P.

Each fin 1B is dimensioned in such a way that

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its dimensions are comparable with the dimensions of the light beam reflected LR from a face of the prism P corresponding to a cathetus.

In any case, each fin 1B must have a width (i.e. in the direction perpendicular to the axis B) less than or equal to the diameter of a focusing lens, preferably between 0.9 and 1.

Excessive lateral dimensions (i.e. transversal dimensions with respect to axis B) of each fin cause an occlusion of the light beam reflected by the target T and retransmitted by the same prism, which would result in insufficient collection of the light returning from the target.

As already mentioned, the prism P is able to rotate on said second plane or tilt plane which is orthogonal to the PAN plane and passing through said first axis A and said median axis B of the anti-straylight optical device itself.

As shown in figure 4, in order for the prism P to rotate on said second plane, respective movement means 6 are provided to rotate said prism P on the tilt plane also in continuous regime so that the laser beam can be transmitted only through the faces of the catheti.

The handling means 4 for rotating the anti-straylight optical device on the PAN plane are rigidly connected to said handling means 6 by means of two arms 5 so that a rotation of the anti-straylight optical device on the PAN plane corresponds one rotation of the prism P

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on the plane of tilt around a third axis C, perpendicular to the first axis A around which said anti-straylight optical device rotates, as well as a displacement of the prism itself along a circular trajectory on a plane parallel to said first plane during said rotation so that the light beam LR reflected by the prism P is always intercepted by at least one of the two fins 1B.

Although in figure 4 two arms 5 are shown for connecting the handling means 4 for rotating the anti-straylight optical device to the handling means 6 for rotating the prism P, it is possible to provide any number of arms, even a single arm, or other means of connection.

However, it is possible to foresee that a contribution of diffused light (so-called Lambertian component) is not intercepted by the fins 1B, but only by the annular element 1A, depending on the distance of the target T with respect to the anti-straylight optical device.

The laser beam reflected from the target contained in the solid angle subtended by the prism is collected by the receiving lens and focused on the detector surface.

The light contained in said solid corner is collected in the opening 4A on which the anti-straylight optical device is superimposed. Since the latter covers only a small portion of the aperture, the light returning from the target will be collected sufficiently for the scanning purposes of the general optical system.

Advantageously, as already mentioned, the

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anti-straylight optical device object of the invention has a simplified structure capable of intercepting the unwanted light that is generated inside an optical system by a scanning element, such as for example a prism having a surface intercepted by a laser beam .

Another advantage is given by the fact that said anti-straylight optical device can be inserted inside an optical system without making structural changes to the optical system in which it is inserted.

With reference to Figure 6, an embodiment of an optical system incorporating the optical scanning device according to the invention is described.

This system includes an active MA module and a passive MP module. The laser signal generated by the laser L in the active module MA is sent to a focusing lens Lfoc through a single-mode fiber FSM (the passage from the fiber to the focusing lens is carried out in a known way, see the aforementioned document IT1343719). The focusing lens is moved by a focusing shifter TRfoc which receives the control signal from the control electronics E of the active module MA.

A BS beam splitter is placed downstream of the Lfoc focusing lens and allows part of the laser beam to arrive on a bundle of BF imaging fibers. The signal thus obtained is transferred to a BP light beam profilometer (laser) and then the beamwidth Sbw signal is sent

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to the control electronics. This, in turn, as a function of the Sbw signal generates a STRfoc signal which is sent to the focusing shifter TRfoc which moves the focusing lens Lfoc {through a suitable CETR connection).

The BF fiber optic bundle measures the spot size of the laser beam exiting the BS beam splitter.

This is done for at least one bi-step focus position, because the other position can be calculated from the relative position. If this distance varies, then the procedure must be carried out for both positions (or for several positions in the case of multi-step focus).

In this way, the focus position will be kept "locked" in order to minimize the effects of defocusing during the measurement. Two equivalent optical solutions are proposed for sending the laser beam on the bundle, because the diameter of the spot is larger than that of the bundle. A lens or focusing system with demagnification greater than 6 can be used. In this way, the beam is focused entirely on the BF bundle and the spot measurement is immediate.

In the case of bundles with a small section (when, due to bending radius problems, it is necessary to use a bundle with a diameter smaller than the waist of the laser beam), an off-axis alignment with the fiber can be used, which allows to measure the radius of curvature of the part. of the spot that falls in the bundle.

In other words, either the whole radius of

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both steps, or a part and the diameter is calculated by extrapolation.

Returning to the optical system diagram, the beam deflected by the beam splitter BS is reflected by a mirror M and becomes the main beam MR, which will then pass through an anti-straylight tube TSL and an optical scanning device (not shown) comprising a prism P .

The return light from the target is collected in the opposite direction to that of the main beam MR behind the mirror M by an Lric receiving lens moved by a TRric translator, which focuses the light on another bundle BR which carries the signal on a photodetector APD connected to the control electronics E.

The present invention has been described for illustrative but not limitative purposes, according to a preferred embodiment thereof, but it is to be understood that variations and / or modifications may be made by those skilled in the art without thereby departing from the relative scope of protection, such as defined by the attached claims.

Barzanò & Zanardo Roma S.p.A.

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Claims (6)

Barzanò & Zannalo 18 CLAIMS L. Optical device for scanning a light beam (L), in particular a laser beam, traveling along a first axis (A) incident on a surface of a scanning element (P) included in said optical scanning device, the element being rotatably arranged on a tilt plane orthogonal to a pan plane of the optical system for scanning the light beam on a target (T), the optical scanning device being characterized in that it comprises an anti-straylight optical device to intercept an unwanted light produced by said light beam (L), which includes: - an annular element (1A) with a center (10B), an internal diameter (10A) sized to allow the passage of said light beam (L), and an external diameter (10C), said annular element (1A) being arranged on a first floor; - a pair of fins (1B) such that: or they are arranged on a second plane parallel to said first plane and connected to said annular element (1A) in such a way as to be at 180 ° from each other; And or each fin (1B) of said pair of fins is substantially symmetrical with respect to a second axis (B) lying on said second plane and passing through the projection of said center (10B). and wherein said anti-straylight optical device is PF-AP A / 17468 Bar / .anò & Zanardo - 19- positioned so that: - is rotatable about said first axis (A) on said pan plane orthogonal to said first axis (A), - said tilt plane always passes through said first axis (A) and said second axis (B). 2. Device according to claim 1, characterized in that said external diameter (10C) and said internal diameter (10A) are dimensioned in such a way as to intercept or limit the solid angle of the Lambertian component of the unwanted light. 3. Device according to claim 2, characterized in that said external diameter (10C) is between 1/8 and 1/12 of the distance between said center (10B) and the point of said scanning element (P) on which the light beam (L). 4. Device according to any one of claims 1 to 3, characterized in that said first plane coincides with said second plane. 5. Device according to any one of claims 1 to 4, characterized in that each fin of said pair of fins (1B) is connected to said annular element (1A) by means of a respective arm. 6. Device according to claim 5, characterized in that each arm is perpendicular to said first plane and to said second plane. 7. Device according to any one of claims 1 to 6, characterized in that PF-APA / 17468 Barzanò & Zanardo 20 each fin (1B) of said pair of fins is rectangular. 8. Device according to any one of claims 1 to 7, characterized in that said anti-straylight optical device is arranged diametrically inside a circular opening (4A) on said pan plane. Device according to any one of claims 1 to 8, characterized in that said scanning element (P) is a scanning prism, in particular an isosceles rectangle prism. Optical scanning system, comprising an active module (MA) and a passive module (MP), characterized in that said passive module (MP) comprises the optical scanning device according to any one of claims 1 to 9. 11. System according to claim 10, characterized in that: - the passive module comprises a focusing lens (Lfoc) for focusing a light beam received by said active module (MA) on said target (T), and - the dimension of each fin (1B) transversely to said second axis (B) is less than or equal to the diameter d of said focusing lens (Lfoc). 12. System according to claim 11, characterized in that said dimension of each fin (1B) transversely to said second PF-AP A / 17468 Barzanò & Zanardo - 21 axis (B) is between 0.9d and d. 13. System according to any one of claims 10 to 12, characterized in that said passive module comprises: - a focusing shifter (TRfoc) which receives a control signal from said active module (MA) and moves said focusing lens (Lfoc); a beam splitter (BS) placed at the output of said focusing lens (Lfoc); an imaging fiber bundle (BF) which collects the part of the light beam transmitted by the beam splitter (BS) and transports it to said active module (MA); - a mirror (M) which intercepts a part of the light beam deflected by said beam splitter (BS) and reflects it generating a reflected light beam (MR), said mirror having a first reflecting face and a second face opposite to said first face; - said optical scanning device which scans said reflected light beam (MR) on said target (T); - a receiving lens (Lric) turned towards said second face of the mirror (M), which focuses the light received by said target (T) on a further bundle (BR) of optical fibers for imaging which carries the signal reflected by the target (T) to said active module (MA); - a translator (TRric) of the receiving lens (Lric), PF-AP A / 17468 Barzanò & Zanardo 22 and by the fact that said active module (MA) includes: - a control electronics (E), - a light beam source (L) which generates a light beam carried by a single mode optical fiber (FSM) to said focusing lens (Lfoc); - a photodetector (APD) which receives said further bundle (BR) of optical fibers for imaging and is connected to said control electronics (E); - a light beam profilometer (BP) which receives said bundle of optical fibers for imaging (BF) and sends to said control electronics (E) a waist signal (Sbw) of the light beam transmitted by said beam-splitter (BS) ; - a connection (CETR) between said control electronics (E) and said focusing shifter (TRfoc) adapted to carry a control signal (STRfoc). Optical system according to claim 13, characterized in that, when said waist is greater than the diameter of said bundle of optical fibers for imaging (BF), between said bearti splitter (BS) and said bundle of optical fibers for imaging (BF) ) a focusing lens with demagnification is placed, in particular with demagnification greater than 6. Barzanò & Zanardo Roma S.p.A. PF-AP A / 17468