WO2004099849A1

WO2004099849A1 - Optical unit and system for steering a light beam

Info

Publication number: WO2004099849A1
Application number: PCT/IL2004/000382
Authority: WO
Inventors: Yaakov Amitai
Original assignee: Elop Electro-Optics Industries Ltd.
Priority date: 2003-05-12
Filing date: 2004-05-06
Publication date: 2004-11-18
Also published as: IL155859A0

Abstract

There is provided an optical system for steering a light beam and/or for scanning a target, including an optical unit consisting of at least one pair of prism arrays located in alignment along a common optical axis, wherein for each array the wedge angle of the prisms is identical for all the prisms in the array, and each array is capable of being independently rotatable about the axis.

Description

OPTICAL UNIT AND SYSTEM FOR STEERING A LIGHT BEAM Field of the Invention

The present invention relates to an optical unit and system for steering a light beam and for scanning a target.

The present invention is capable of being implemented in a large member of applications. For example, in laser rangefmders, laser target designators, light direction and ranging (Lidar) systems and the like. Background of the Invention

The f ndamental objective of the scanning optical Lidar is to scan a certain field of view, one pixel at a time, and determine the range information for that pixel. The scanning optical Lidar may do this over very short and very long ranges and at a very high frame rates with high resolution in target position, both lateral position and range position. The Lidar scanning device must also be capable of satisfying certain design constraints while meeting these objectives. The design constraints include: small system size, i.e., the beam steering device must be compact enough to allow implementation in environments, especially for airborne systems, where space and weight are restricted; low system cost to allow wide use of the system; wide range of operation, namely, the scanning optical rangefmder should meet minimum performance standards independent of weather and atmospheric conditions; and data reliability, especially for collision avoidance systems.

The existing solutions suffer from various drawbacks: the scanning unit is relatively large and heavy, limiting the performance of systems in which compactness is a requirement; mass production is quite expensive; the scanning rate is severely limited by the mechanical system; the rotating module suffer from wobble which must be restrained in order to allow accurate scanning. These drawbacks are particularly crucial for airborne collision avoidance systems, where the compactness, the accuracy and the stability of the system are very important. Other solutions that employ microlens array, diffractive devices or holographic optical elements are two expansive and complicated for practical systems. It is important to emphasize that the beam steering system is complicated because it must be inserted in the common pass of the transmission and the receiving channels. On one hand the aperture of the transmitted beam is not very wide, usually a beam with a diameter of 20-30 mm can achieve the required high quality and the desired divergence of the transmission beam. However, the optical quality of this beam should be very high, any non-desired aberration or distortion can significantly degrade the performance and reduce the detection range of the system. On the other hand, the optical quality of the received beam deteriorated during the round trip through the atmosphere and high-quality receiving optics is usually not necessary. However, in order to optimize the sensitivity of the system and to minimize the extinction ratio, the aperture of the receiving channel should be as high as possible to detect the maximal reflected energy. This is particularly important for airborne collision avoidance systems. The transmitted power cannot be very high to avoid hazardous radiation in civilian regions. The system, however, should detect obstacles like high- voltage cables from large distances of a few hundreds of meters. These contradicted requirements can be achieved only by increasing the aperture of the receiving channel to more than 250 mm diameter. Therefore, the optical system should be very accurate and with high optical quality because of the transmission channel and with a very wide aperture because of the receiving channel. As a result, it is expected that a conventional solution will be very complicated, cumbersome and expensive.

In many optical systems, scanning of a plane wave over a wide field of view, or linear scanning of a focused beam on a plane, is required. A few examples are angular scanners for Laser-Radar, whereby the transmitted narrow beam is to cover a solid angle much wider than the angular divergence of the beam; aiming systems in which the central aiming point moves as a function of the target range and velocity; linear scanners for laser printers or plotters, and others. In the existing systems, beam steering is performed with various optical elements, such as a polygonal mirror or a pair of prisms. These systems suffer from various drawbacks: the scanning unit is relatively large and heavy, limiting the performance of systems in which compactness is a requirement; mass production is quite expensive; the scanning rate is severely limited by the mechanical system; rotating systems usually suffer from wobble which must be restrained in order to allow accurate scanning. These drawbacks are particularly crucial for military and/or airborne systems, where the compactness and the stability of the system are very important.

Several proposals have been made to perform beam steering by microlens array translation with either diffractive or refractive lenses. These approaches usually suffer from high aberrations at small f-numbers. In addition, they must rely on fairly complicated and costly equipment, which often limits the performance of the microlens arrays. Disclosure of the Invention

It is therefore a broad object of the present invention to provide a compact, relatively inexpensive, accurate and simple optical beam steering unit and system having a high scanning rate for various light transmitter/receiver systems.

It is a further object of the invention to provide a compact and accurate scanning device for light transmitter/receiver which operates separately on the two optical channels and which exhibits both large field of view and high range resolution.

In accordance with the present invention, there is therefore provided an optical system for steering a light beam and for scanning a target, comprising an optical unit consisting of at least one pair of prism arrays located in alignment along a common optical axis, wherein for each array the wedge angle of the prisms is identical for all the prisms in said array. Brief Description of the Drawings

The invention will now be described in connection with certain preferred embodiments with reference to the following illustrative figures so that it may be more fully understood.

With specific reference now to the figures in detail, it is stressed that the particulars shown are by way of example and for purposes of illustrative discussion of the preferred embodiments of the present invention only, and are presented in the cause of providing what is believed to be the most useful and readily understood description of the principles and conceptual aspects of the invention. In this regard, no attempt is made to show structural details of the invention in more detail than is necessary for a fundamental understanding of the invention, the description taken with the drawings making apparent to those skilled in the art how the several forms of the invention may be embodied in practice.

In the drawings: Fig. 1 schematically illustrates the arrangement of a light transmitter/receiver system having an optical beam steering unit; Figs. 2a and 2b schematically illustrate a Risley pair consisting of two sequential wedge prisms; Fig. 3 schematically illustrates the two parameters that describe the deviation of a scanned optical beam; Fig. 4 schematically depicts various angles of a deflected optical beam in relation to the four different surfaces of the Risley prism pair; Figs. 5a and 5b schematically illustrate a plurality of prism pairs arranged according to the invention; Fig. 6 schematically illustrates a side view of a beam steering unit consisting of a plurality of prism pairs according to the invention; Fig. 7 schematically illustrates a top view of another embodiment of a beam steering arrangement based on a plurality of prism pairs according to the invention, and Figs. 8a and 8b schematically illustrate another possible arrangement of a steering unit based on two different array pairs; Detailed Description

Fig. 1 illustrates an embodiment of a light transmitter/receiver system with a beam steering unit for scanning and/or alignment purposes. A light source transmitter 2 is expanded and collimated by a beam expander 4 into a high quality transmitted beam 6, wherein the expanded beam is optionally reflected by an optical element 8, e.g., a mirror into the required direction. The beam 6 is then passed through a beam-splitter 10, which could be either a polarizing or a partial beam-splitter, and transmitted onto a distant target. The beam 12, which is reflected from the target is reflected by the beam-splitter 10 and focused by a focusing lens 14 onto an optical receiver 16. By proper disposition of the optical element 8 and beam splitter 10, it is possible to ensure that the transmitted beam 6 and the reflected beam 12 will be aligned along the same optical axis. By positioning a beam steering unit 18 into the common path of said two beams, it is possible to deviate the beams into different directions. As shown in the figure, the transmitted beam 20 is deflected into a new direction 22, which is different by an angle α from the original direction of beam 6. Since the beam steering unit covers the entire aperture of the receiving channel, the receiver 16 sees the reflected beam 24 arriving from the new direction 22. A controller 26, which is connected to the beam steering unit 18 and the receiver 16 can calculate the reflectance of the target as a function of the new direction 22. The controller 26 and receiver 16 may be operationally linked so as to form a feedback loop.

One simple use of a transmitter/receiver laser is a rangefmder. Rangefmder laser devices emit a single pulse or series of pulses toward a target and a counter is activated when the pulse is emitted. When the light contacts the target, acting as a diffuse reflector, it is scattered in all directions. The receiver 16 receives the light reflected back to the rangefmder and deactivates the counter. Thus, the distance from the rangefmder to the target can be calculated from the time of travel between the laser and target and the speed of light using the formula

* = ^ . (1) where R represent the range in meters, c represents the speed of light (3 x 10⁸ m/sec), and t represents time in seconds for the pulse of light to travel the round trip, which is why it is necessary to divide by 2.

Another use of a transmitter/receiver laser is as a target designator. Laser systems accomplish tactical target designation by emitting a series of pulses toward a diffuse reflection target, which scatters the light. The main use for laser designators is for military applications, where programmed optical sensors respond to the particular code of pulses that the designator emits and direct munitions toward the target. In some designation systems the receiving channel is separated from the transmitting channel, for example, where an infantry soldier designates a target which is detected by an airborne system. However, there are self-directed designators where both transmission and detection are performed by the same system.

One possible utilization of the beam steering device is for an accurate alignment of the laser axis with another separated optical module such as a surveillance or a thermal imaging system. In this case, most of the alignment procedure is performed during the assembly process and only the fine-tuning is achieved using the beam steering device. Hence, the dynamic range of the scanning device is limited to a field of view (FOV) of a few degrees.

There are however, systems where a much larger FOV is required. For instance, light direction and ranging (Lidar) systems should provide both range and lateral (horizontal and vertical) position data on targets. One application of Lidar is as a warning system for cars, planes and helicopters which simultaneously provides range and lateral position data on targets for collision avoidance. In that case, a minimal FON of 30° in both axes is required for a practical operation. Another application is as a three-dimensional object detector, where by utilizing the scanning system the laser beam can be directed to various parts of the target. By determining the small differences in distance to the target, the surface contours can be determined.

One possible solution is to utilize an optical device 28 called Risley prisms. The Risley optical scanning system consists of two sequential wedge prisms 30, 32 (Fig. 2), which have wedge angles cti. and α₂, that are capable of rotating about the optical scan axis at angular speeds ω_\ and ω₂ in any direction. When a collimated laser beam is directed along the optical scan axis and through the prisms, the emergent beam is deviated in a direction according to the relative orientation of the prisms with respect to each other. A single wedge prism 30 deviates the beam according to its wedge angle and refractive index. If rotated in a circle about an axis perpendicular to its face, it will rotate the beam in a similar circle. As shown in Fig. 2a a second, identical prism 32 in series with it, can cancel the deviation of the first prism or alternatively double the angle of the beam rotation and generate a circle of twice the radius (Fig. 2b). If they rotate in opposite directions, one motion is canceled and a line is generated. In fact, all sorts of scanning patterns can be obtained using this scheme. There are two parameters that describe the deviated beam as shown in Fig. 3 : the off-axis angle β between the deviated beam and the main axis of the system which is set by the relative orientation between the two prisms, and the phase angle γ which is set by the rotation of the combined setup of the two prisms.

In order to calculate the achievable FOV, it is important to calculate the maximal deviation angle /J_max for a given prism pair with wedge angle α and a refractive index v. Figure 4 illustrates the various angles of the deflected beam in relation to the four different surfaces of the system. In the figure the superscript n=l,2,3,4 denote the respective surface.

Assuming that the incoming beam is normal to first surface, then,

where the subscripts z^',o denote the input (i) or the output (o) of the that surface. The output deviation angle from the second surface of the first prism is β = arcsin(v ■ sin β ) = arcsin(v • sin a ) , o\

Where the deviation angle from the original direction is

θ; = β_o ^l - β = arcsin(v • sin a) - a . (4)

The deviated beam impinges on the first surface of the second prism with the off-axis angle o

hence, the off-axis angle inside the prism is

β = arcsin(— • sin β ) = arcsin(v • sin ) . ^ v

The beam now impinges on the second surface of the second prism with an off-axis angle

Therefore, the output angle from the system is

^ = ^₀ ⁴ = arcsin(v sin^⁴) . (8)

It is easy to show that for small angles the maximal deviation is

where for optical material such as BK7 or B270 the refractive index is n= 1.51, hence the maximal deviation angle is

θ a . (10)

In any case, equation (10) is given only for general knowledge, and for practical implementation the exact deviation angles should be calculated according to Eqs. (2)-(8). For example, if an horizontal FOV of 40° is required, that is, maximal deviation angles of ±20° are needed, then the wedge angle of the prism pair is α=18°. Assuming that the receiving aperture is 250 mm and that the optical material is BK7 or similar, the total volume of s single prism is around 2,000 cm and its weight is around 5 kg! Apparently, this prism pair arrangement is not practical for the desired scanning system where a very fast, accurate and stabilized system is required.

It should be noted that and, as explained above, the complication of the scanning system is a result of the combination of the two optical channels: the very wide aperture of the receiving channel and the high quality requirements of the transmission channel. Consequentially, it is advisable to separate between the two channels. A possible approach is to totally separate between the two channels, that is, to insert a different scanning device for each one of the optical channel. According to this approach, a very accurate alignment device must be added to the system to ensure that the two separated channels are aligned along the same optical axis. This alignment device might be very complicated and expensive; hence this approach is unpractical for most of the applications. A different approach is to perform a "functional separation" rather than total physical separation between the channels. That is, the two channels are kept together along the same axis but a different scanning mechanism operates on each different channel.

Figs. 5a and 5b illustrate the basic concept that the invention is based upon. Instead of using a single prism pair, a plurality of prism pairs 30ι, 32ι; 30₂, 32₂ to 30_n, 32_n are arranged together to form a prism array 34 with a similar operation to that of a single pair device 28 (Figs. 2a, 2b). The different prism pairs should not necessarily be of the exact same size, but they should have the same wedge angle and they should be arrange in the same orientation.

There are two main differences between the arrangements of a single prism pair and a plurality of prism pairs as illustrated in Figs. 2a, 2b and 5a, 5b, respectively. On the one hand the optical quality of the prism array is inferior compared to that of a single pair. That is, assuming that there are n identical prism pairs, the diffraction- limited performance of the prism array is deteriorated by a factor of n compared to that of a single pair. In addition, there is some energy loss on the boundary area between the different pairs. On the other hand, the volume and the mass of the prism array can be reduced by a factor of n compared to that of a single pair with the same wedge angle.

Figure 6 illustrates a side view of a beam steering unit 18 based on a plurality of prism pairs. The transmitted beam 36, which is much narrower than the reflected beam 38, passes through the central prism pair, while the reflected beam 38 passes through the entire array.

Figure 7 illustrates a top view of another embodiment of the present invention. The optical aperture 40 of the received beam is as large as the aperture of the entire array while the optical aperture 42 of the transmitted array is inside the aperture of the central pair. In this particular example the optical aperture 42 of the central pair is a little bit wider than those of the other pairs. Hence, it is possible to reduce the total volume and weight of the system by a factor larger than the ratio between the diameter of the received and the transmitted beams. The advantages of this embodiment are apparent. The volume and the weight of the optical elements are reduced by an order of magnitude. Therefore, a much simpler, faster and more stabilized steering device can be implemented even for systems with a very wide FOV and a large receiving aperture. It is true that the performance of the receiving channel is decreased because of the array, but this decrease is practically negligible. Assuming an array with 10 different pairs, the diffraction limit of the receiving channel along the array axis 44 is increased from a few microns to a few tens of microns (the diffraction limit remains the same along the orthogonal axis 46). A typical size of the detector is, however, in the order of 100-200 microns, hence, this increase can be easily compensated and the overall performance is not really reduced. On the other hand, since the entire transmitted beam passes through a single pair, the optical performance of this beam is not affected at all. Moreover, since the thickness of the central beam is much smaller then a single large prism pair with the same aperture of the receiving channel, it is expected that the distortion and the absorption as a result of the optical material imperfections will be reduced. In the present invention, the facts that the required aperture of the transmitted beam is much narrower than that of the received beam, while the required optical performance of the received beam is much lower than that of the transmitted beam, are exploited.

As explained above in conjunction with Fig. 3, the beam steering in both horizontal and vertical axes can be performed by the rotation of the two prisms arrays: the off-axis angle β between the deviated beam and the central direction of the FOV is set by the relative orientation between the two arrays, and the phase angle γ is set by the rotation of the combined setup of the two arrays. A complete scanning process in the two required axes can be performed by two arrays of prisms. However, this scanning process involves two different rotations - one is the relative rotation between the arrays and the second is the rotation of the entire module. As a result, the rotating function of each array can be fairly complex and significantly different from that of the other array. This can be performed to produce the required scanning pattern, by using independent drive mechanism for each array and controlling the speed and direction of rotation of said mechanism by a motion-control computer. This computer is programmed with a specific scan pattern and can be periodically changed. The program is based on equations (2) to (8), thus being capable of calculating and controlling any speed and direction of rotation of each array.

An alternative steering approach is illustrated in Figs. 8a and 8b. As mentioned above, if the two prism arrays 34, rotate in opposite directions with the same angular velocity, one motion is canceled and a line is generated, that is, a one-dimensional scanning pattern is performed. The control system that is required for this scanning pattern is fairly simple and easy to perform. The beam steering in the orthogonal axis might be performed by a second array pair 34', with an orthogonal orientation. In that case, a different wedge angle may be chosen for each different array pair. For instance, if for a collision avoidance system a horizontal FOV of 40° on a vertical FOV of 20° is required, and if the optical material is BK7 or a similar one, then the required wedge angles are 18° and 9.6° for the horizontal and the vertical scanning respectively. It is true that two array pairs are required, but the scanning procedures of both array pairs are simple and with the alternative methods one will have to resort to complicated scanning procedures with a maximal deflected angle of 25° and a wedge angle of 22°. In general, for each particular laser the scanning method will be determined according to FOV, the complexity of the scanning pattern and the maximal allowed weight of the beam steering unit.

Even a more complexed scan pattern can be performed during a shorter time from the beginning to the end of a scan cycle, when the rotation speed and direction of each single prism array is controlled independently as hereinbefore described with respect to the control of two arrays.

It will be evident to those skilled in the art that the invention is not limited to the details of the foregoing illustrated embodiments and that the present invention may be embodied in other specific forms without departing from the spirit or essential attributes thereof. The present embodiments are therefore to be considered in all respects as illustrative and not restrictive, the scope of the invention being indicated by the appended claims rather than by the foregoing description, and all changes which come within the meaning and range of equivalency of the claims are therefore intended to be embraced therein.

Claims

WHAT IS CLAIMED IS:

1. An optical system for steering a light beam and/or for scanning a target, comprising an optical unit consisting of at least one pair of prism arrays located in alignment along a common optical axis, wherein for each array the wedge angle of the prisms is identical for all the prisms in said array, and each array is capable of being independently rotatable about said axis.

2. The system according to claim 1, wherein all the prism pairs in the said pair of prism arrays are identical to each other.

3. The system according to claim 1, wherein a central prism pair of said array of prisms has a larger optical aperture than other prism pairs in said pair of prism arrays.

4. The system according to claim 1, wherein the wedge angle of one pair of prism arrays is identical to the wedge angle of a second pair of prism arrays.

5. The system according to claim 1, wherein said wedge angle of one pair of prism arrays is different than the wedge angle of a second pair of prism array.

6. The system according to claim 1, further comprising a drive mechanism coupled to each prism array for rotating each of said prism array about said common optical axis.

7. The system according to claim 6, further comprising a controller for independently controlling the speed and direction of rotation of each prism array.

8. The system according to claim 2, wherein said pair of prism arrays rotate in opposite directions to each other.

9. The system according to claim 1, wherein the scanning direction of the target of each pair of said prism arrays is normal to that of the other.

10. The system according to claim 1, further comprising a light beam transmitter for transmitting a beam through a central prism pair of said pair of prism arrays.

11. The system according to claim 10, wherein said transmitter transmits a beam of a predetermined width.

12. The system according to claim 10, further comprising a receiver for receiving a reflected optical beam passing through said prism arrays.

13. The system according to claims 11 and 12, wherein the width of the beam received by said receiver is wider than the width of the beam transmitted by said transmitter.

14. The system according to claim 12, wherein said received beam passes through more than one prism pair of said pair of prism arrays.

15. The system according to claim 12, wherein said received beam passes through the entire optical aperture of said pair of prism array.