GB2256338A - Alignment means - Google Patents

Abstract

Bessel beams, typically derived from a laser source, have a central peak 18 surrounded by a number of concentric rings, the amplitude profile of the beam is proportional to the zero order Bessel function of the first kind. The use of this beam structure in co-operation with eg. a flurorescent annular target 46 having a pattern similar to this beam structure but differently scaled on its surface causes fringes to be produced due to misalignment of the beam and the target. Use of these fringes enables a high degree of alignment accuracy to be achieved. Various optical methods of producing the Bessel beams are described and uses include a target practice device and a space vehicle docking system, the space vehicle 44 approaching the 'mother ship' 42 by riding down one of the diverging laser beams 48 beyond the overlapping Bessel beam region, then aligning itself using the Bessel beam 50. An axis intensity chirping of the Bessel beam enables determining the speed and position of the vehicle. <IMAGE>

Description

ALIGNMENT MEANS The present invention relates primarily to a targetting system which has application in determining the misalignment of a Bessel beam source and a target. The form of the target can be varied to suit the application and the source can be adjusted to vary the accuracy which can be achieved.

According to the present invention there is provided a targetting system comprising a Bessel beam source and a target therefor, said target having a structure substantially in conformity with the cross-axis structure of the Bessel beam.

Preferably, the target structure is such as to produce a pattern when interacting with the Bessel beam in use.

Advantageously, the target structure is in the form of a fluorescent annular pattern on a planar substrate.

The present invention will now be described, by way of example, with reference to the accompanying drawings, in which: Figure 1 is a diagrammatic cross-sectional view illustrating the geometry of the creation of a Bessel beam; Figure 2 shows a typical axial cross-section of a Bessel beam; Figures 3(a) and 3(b) show diagrammatically two methods of generating a Bessel beam; Figure 4 shows an improved version of the method of generating a Bessel beam shown in Figure 3(b); Figure 5 shows an on-axis view of the image of the light intensity distribution in a Bessel beam; Figures 6(a), 6(b) and 6(c) show the image of a target when illuminated by a Bessel beam when in alignment and with increasing misalignment respectively; Figure 7 shows a view of a typical arrangement of a Bessel beam targetting system using the present invention;; Figure 8 shows a diagrammatic view of a vehicle docking system using the present invention; Figures 9(a) and 9(b) show the use of a zoom collimating lens to vary the range of a Bessel beam targetting system; Figure 10 shows the use of a series of detectors to monitor axial distortion.

The Gaussian intensity profile associated with the TEMoo mode of a laser resonator has, until recently, been the preferred beam shape in a majority of applications. The finite divergence and almost featureless profile of a Gaussian beam is, however, not always desirable. In some envisaged tasks where high directionality coupled with safe or covert operation are considered important, a new class of beam, characterised by a Bessel profile, offers distinct advantages. A Bessel beam, as it is commonly known, possesses a non-diverging central peak which after a predetermined distance dissipates rapidly, see Figure 1.

A Bessel beam may be of electromagnetic radiation in the optical, millimetre wave, microwave or other region of the spectrum.

For simplicity the following description is related to the use of light and generally to the use of laser sources, but is not intended to be limited to that region of the spectrum.

All fields of light, confined by an aperture or area of finite extent will undergo diffraction as they propagate. Generally, the larger the aperture and smoother the intensity profile, the smaller the divergence experienced. However, both these requirements tend to dilute the characteristics of a beam which should exhibit a high concentration of intensity along the axis of propagation.

There is a solution to the wave equation which is both beam-like and non-diffracting. The amplitude profile of such a field is proportional to the zero order Bessel function of the first kind, Jo(x) It propagates as a narrow axial beam surrounded by an infinite set of concentric rings whose intensity envelope decays proportionally with radial distance, x, see Figure 2. Each ring contains approximately the same amount of energy as that present in the central beam, hence such a Bessel beam would require an infinite amount of energy to create. In practice a Bessel-like beam would be apertured and contain a finite number of rings. This limitation does not effect the diffraction free property of the central peak (in terms of its radius) and in fact confers the potential advantage of a finite range to the beam.

Considering the generation of Bessel beams with reference to Figure 1, and considering an infinite set of equal amplitude plane waves 10, all travelling at the same angle, i, to the propagating axis but having different azimuthal angles ranging from 0 to 2 % radians. If the plane waves are apertured as shown, at an aperture 12 they will overlap for a certain distance, Zmax. The shaded region 14 beyond Zmax is known as the geometric shadow. If the plane waves are coherent i.e. of the same wavelength and phase, they will interfere in the overlap region 16. Since the angles of overlap remain constant with Z, the profile of the interference pattern generated also remains invariant as the beam propagates (apart from its radial extent which diminishes as Zmax is approached).The axial symmetry of the systems results in the interference pattern having an intensity profile which is proportional to the square of the zero order Bessel function and having a central peak 18 and the invariance of this pattern with Z leads to such a beam being given the description of diffraction-free.

Beyond ZmaxX the overlap and hence the interference ceases, and the on-axis intensity decays rapidly to zero. The light continues to propagate in the form of a diverging annular ring 20 with the energy associated with the beam rapidly dissipating over an increasingly large area.

The basic properties of a Bessel beam are determined by just three factors; the wavlength of the light ?\, , the radius of the limiting aperture Rb, and the angle which the plane waves subtend to the Z- axis, 8. From Figure 1, it can be seen that Zmax = Rb/tan G from which it can be, derived that Zmax = [8/3]Rbr/ ss where r is the radius of the central peak.

Thus Zmax can be increased by decreasing either or both the wavelength and overlap angle and increasing the aperture.

It was noted previously that a collection of uniform plane waves all propagating at the same angle to the Z axis is required to generate a Bessel beam. Such a set of conditions can be achieved by employing either a thin conical lens 22, known as an axicon or an annular mask/collimating lens combination, see Figures 3(a) and (b) respectively.

The axicon 22 shown in Figure 3(a) has a set of plane waves incident upon the plane face 24. The waves are refracted by the axicon and emerge from the conical face 26 as converging waves 28.

As described earlier with reference to Figure 1 a Bessel beam 30 is created in the space in which the beams overlap.

Alternatively, as shown in Figure 3(b), a plane wave 22 is incident upon a mask 32, having an annular aperture 34, which produces a diverging annular ring 36 of spherical waves. The ring is incident upon a lens 38 which produces the converging means 28 and the Bessel beam 30 as for the arrangement in Figure 3(a).

Due to the blocking of the majority of the incident light the system as shown in Figure 3(b) is extremely inefficient. A considerable improvement in the efficiency of the system by placing an annular lens 40 before the mask 32 as shown in Figure 4. This lens 40 focusses the wave on the aperture 34 minimising the blockage of light. The remaining features are as described with reference to Figure 3(b).

The use of an aperture introduces subtle Fresnel diffraction effects. In particular the on-axis intensity undergoes a chirped modulation along the Z-axis.

A Gaussian beam of the same wavelength and comparable size to the central lobe of a Bessel beam will spread to a size greater than the original aperture, Rb, over a distance of Zmax The foremost property of a Bessel beam is its unique combination over a defined range of profile invariance with high axial intensity. A Gaussian beam which has been expanded to the same size as the entire Bessel beam will not diffract significantly but its profile will contain little in the way of structural information compared to the Bessel beam and thus offer much less in terms of potential directional accuracy.

Finally, the finite range of the intense on-axis beam, with its abrupt transition into the geometric shadow, offers distinct advantages where safety and covert operation are important.

In aligning applications, the two main advantages of Bessel beams are their invariance with distance and their finite range.

The first results in an equivalent focussed laser beam of vast depth of field. For example, employing conventional optics a spot size ofw5vum (diameter) in the visible has a depth of field of only +13sum, (for a 5% increase in spot size). A Bessel beam of radius lmm would, however, possess a central lobe of similar dimensions whilst providing a range, (and an equivalent depth of field) of 17mm, a factor of 650 greater. In the mid-range of distances Bessel beams could be used to align such items as complex optical/microwave systems, or machine tools. Bessel beams could also be used to determine accuracy of fire in weapon target practice. To take full advantage of Bessel beams such tasks requires a detector which can utilise the fine structure of the beams.For example, a target screen consisting of a fluorescent ring pattern would exhibit fringes ('Moire' fringes) until exact alignment with the Bessel beam was achieved. These fringes would be highly sensitive to misalignment and in addition provide information as to the direction of the displacement. A further use is in microlithograpy or other precision engineering tasks.

In Figures 5, 6(a), 6(b), 6(c), the images of the light distribution are photographic negatives, i.e. a dark area represents light and a white area represents the absence of light.

Figure 5 shows a pattern representing a typical light intensity distribution of a Bessel beam in a plane perpendicular to the propagation direction.

Figure 6(a) shows the effect of superimposing the pattern of Figure 5 in register on a target having a similar pattern but scaled, so that the diameter of each ring is slightly smaller (10% in this example) than that of each corresponding ring, to produce a fringe pattern which is perceived as a set of broad light and dark rings. Such scaling can be achieved by the use of a zoom lens (not shown) within the optical system of a Bessel beam source. In Figure 6(b) the light pattern of Figure 5 has been displaced relative to the target pattern by a distance of 0.2 times the ring spacing of Figure 5. The perceived fringes shift parallel to the direction of displacement by an amount greater than the relative displacement of the patterns, in this case 10 times the actual shift for a 10% smaller target.The direction of misalignment can be deduced from the direction of movement of the fringes.

Figure 6(c) shows the effect for a larger displacement of 0.4 times the ring spacing of Figure 5.

To observe the effects described above under ambient lighting conditions one approach would be for the target to be formed as a pattern of rings with a Bessel distribution using a material for the rings which fluoresces when illuminated by light of the wavelength of the laser used to generate the Bessel beam. The size of the fluorescent pattern would be scaled relative to the Bessel light beam pattern (which is invariant with distance from the source) in order to produce the interference effect as described above. This combination of Bessel beam and interference patterns provides an alignment means which is highly sensitive to small displacements perpendicular to the beam axis which are immediately apparent to an observer.

The fluorescent material could be combined with or replaced by a retroreflective material. A further alternative would be to obscure the aperture of a television camera with the complement of a Bessel type ring pattern on a transparent substrate so that the interference pattern could be observed remotely e.g. close to the laser source.

The sensitivity of the system described is related to the number of rings present in the Bessel beam. If, for example, one full interference band is to be observed, then the ring number must equal the reciprocal of the scaling factor and hence equal the magnification factor required. Thus in the example described above, a 10 ring pattern would be required for one full interference band to be observed. The number of rings present in a Bessel beam, decreases approximately linearly with distance from the source, until at Zmax (Figure 1) only the central peak remains. Thus in a practical alignment system, the maximum operating range Z'max will be some fraction of Zmax at which distance the minimum number of rings remain in order to provide the necessary interference pattern.Conversely, for a given operating range Z', maximum sensitivity will be obtained when the number of rings present in the Bessel beam is also maximised. For a given operating wavelength, , and Bessel beam source radius, Rb, (Figure 1) it can be shown that in order to maximise the number of rings N' at Z', Zmax has to equal 2Z'.

Proof The number of rings N' at Z' is given by

where NB is number of rings at Z=0.

Now NB is given approximately by NB > ~ Rb (2) 1.3r where r is the radius of the central peak ; Using Zmax = 8 Rbr 3 > And from (2), And from (1),

The term in brackets is maximised when Zmax 2Z and hence for a given, Rb, )\ and Z' N' is maximised when Zmax = 2Z'.

The variable which needs to be adjusted to give this condition is the overlap angle, 4, i.e. for maximum sensitivity at Z', O = Rb/2Z'.

By way of example, a typical Bessel beam alignment system could have the following parameters Operating wavelength : 543 nm Operating radius, Rb : 3.3 cm Zm : 200 m Operating range Z' : 100 m Number of rings at Z': 10 Ring spacing : 1.6 mm Thus the clear indication of misalignment shown in Figure 6(b) corresponds to a displacement between the beam and target of only 0.32mm, which represents 0.66 seconds of arc (at 100 m).

In Figure 7 is shown a target arrangement for use in an alignment procedure. A target 46 is within the area of the converging waves 28 of a Bessel beam source, and intersecting the Bessel beam 30 movement of the target 46 across the axis of the beam 30 can be used to provide alignment of the target 46 as described with reference to Figures 5, 6(a), 6(b) and 6(c) above.

In docking manoeuvres, such as for a space vehicle on an automatically controlled transport vehicle, a Bessel beam source 42 could provide an informative beacon, as shown in Figure 8. An approaching vehicle 44, 44' would carry a target 46 such as described earlier. The vehicle 44 would first of all on the initial approach intersect the converging annular ring 48, and sensors (not shown) would guide it towards the source 42. When the predetermined range of the Bessel beam 50 was reached the guidance system would respond to the fringe pattern produced by interaction between the Bessel beam 50 and the target 46, aligning the vehicle 44' with the central peak of the beam.

The existence of the on-axis "chirp", referred to earlier, would provide information in conjunction with a sensor (not shown), enabling the position and speed of the vehicle 44' relative to the source 42 to be determined.

There are known laser resonator configurations which can directly generate Bessel type beams. These may be advantageous for use as sources within the invention.

A further use of a zoom lens, rather than to produce the "scaling', feature referred to above in respect of Figures 6(a) to 6(c) is a means of varying the range of a targetting system as shown in Figures 9(a) and 9(b). In Figure 9(a) the target 46 is within the geometric shadow area 52 of the Bessel beam system. By adjustment of the zoom lens 54 the range of the Bessel beam 30 can be increased as in Figure 9(b) until it reaches the target 46.

A Bessel beam also provides the ability to use a series of co-alignable targets or detectors with a central Bessel beam as shown in Figure 20, provided that the targets are within the range of the beam Zmax In Figure 10 as shown in part of a Bessel beam having a central beam 30. The beam strikes a first target 46 and produces a shadow 56. Behind the target 46 there is still the overlap region 16 wherein interference will again occur, regenerating the Bessel beam 30'. The beam 30' may then strike a second target 46' where the same effect occurs and the Bessel beam 30" is again regenerated. The occurrence of this effect is only limited by the size of the shadow created by the targets 46, 46' and the ultimate length of the Bessel beam being no greater than Zmax.

Claims (17)

1. A targetting system comprising a Bessel beam source and a target therefor, said target having a structure substantially in conformity with the cross-axis structure of the Bessel beam.

2. A targetting system as claimed in Claim 1, wherein the target structure is such as to produce a fringe pattern when interacting with the Bessel beam in use.

3. A targetting system as claimed in Claim 1 or 2, wherein the target structure is in the form of a fluorescent annular pattern on a planar substrate.

4. A targetting system as claimed in Claim 1, 2 or 3 wherein the target structure is in the form of an annular pattern on a transparent planar substrate.

5. A targetting system as claimed in any preceding claim, including means for adjusting the scale of the cross-axis structure of the Bessel beam.

6. A targetting system as claimed in Claim 5, wherein the means for adjusting the scale is a zoom lens.

7. A targetting system, as claimed in any preceding claim, comprising a plurality of co-alignable targets.

8. A targetting system, substantially as hereinbefore described, with reference to and as illustrated in Figures 1 to 8 and 10 of the accompanying drawings.

9. A surveying instrument including a targetting system as claimed in any preceding claim.

10. An alignment device including a targetting system as claimed in any one of Claims 1 to 7.

11. A target practice device including a targetting system as claimed in any one of Claims 1 to 7.

12. A vehicle docking system including a targetting system as claimed in any one of Claims 1 to 7.

13. A vehicle docking system as claimed in Claim 11, further including means for detecting on-axis intensity chirping of the Bessel beam and determining the position and speed of the vehicle relative to the Bessel beam source.

14. A range measuring device, comprising a Bessel beam source and a means of adjusting the range of the Bessel beam.

15. A range measuring device, as claimed in Claim 14, wherein the adjusting means comprises a zoom lens.

16. A range measuring device, substantially as hereinbefore described, with reference to and as illustrated in Figure 9 of the accompanying drawings.

17. A vehicle docking system, as claimed in Claim 12, including a range measuring device as claimed in Claim 14, 15 or 16.