GB2512323A

GB2512323A - Laser beam intensity profile modulator for top hat beams

Info

Publication number: GB2512323A
Application number: GB1305480.4A
Authority: GB
Inventors: Daniel Wellburn
Original assignee: Individual
Current assignee: Individual
Priority date: 2013-03-26
Filing date: 2013-03-26
Publication date: 2014-10-01
Anticipated expiration: 2033-03-26
Also published as: GB201305480D0; GB2512323B

Abstract

There is provided an optical radiation beam shaping apparatus consisting of a central portion which creates a central uniform intensity feature inside an annular ring intensity feature at an intended image plane from an impingent radiation beam, and an annular portion surrounding the central portion, the annular portion creates the annular ring intensity feature surrounding the central intensity feature at the intended image plane from the impingent radiation beam. The impingent radiation beam cross-section outline is circular. The beam is either collimated or convergent, and has a Top-Hat or uniform intensity profile. Modulation of the relative intensities of the annular ring and central intensity features is carried out by expanding or contracting the input diameter of the impingent radiation beam before entering the radiation beam shaping apparatus. The invention allows the optical axis of the apparatus to be offset relative to the axis of the beam.

Description

Laser Beam Intensity Profile Modulator for Top Hat Beams

Description

Field of the invention

This invention relates to laser optical apparatus in general, and to a laser beam intensity profile modulator in particular.

Background of the invention

Laser surface treatments allow modification of the surface properties of a bulk material or the addition of a layer of a new material to the surface of a component or assembly. This creates a material with separate properties for the bulk than the surface -such materials benefit a wide number of applications particularly in the aerospace, automotive and other manufacturing industries.

Laser surface heat treatments often rely upon a uniform heat treatment in order to produce consistent surface properties over the treated area. Where multidirectional processing is important, it is preferable for the outline of the impingent beam to be circular. However, where multidirectional processing is not important, other beam profiles may also be useful, such as an elliptical, square or line beam outline. For beams with a circular outline, traditional beam intensity profiles such as Gaussian or Top Hat (homogenised) beams are ineffective at producing a uniform maximum temperature rise on the surface of any material. Furthermore, any fixed intensity profile which has been optimised for any specific surface heating effect on any one material would become ineffective if the material type or material thermal properties were to change or if a change in the processing speed was required.

In laser conduction welding of dissimilar materials a modified beam intensity profile is required in order to accommodate differences in material properties between the adjoining materials.

Laser drilling applications may also benefit from an improved laser beam intensity profile.

Therefore, an improved laser beam intensity profile for use in laser surface heat treatments, laser conduction welding of dissimilar materials and laser drilling, and means for producing the improved laser beam intensity profiles are sought.

Summary of the invention

The present invention provides an optical apparatus comprising a first central portion operable to create a central uniform intensity feature inside an annular ring intensity feature at the intended image plane from the impingent radiation beam, and a second annular portion radially surrounding the first central portion, the second annular portion operable to create the annular ring intensity feature surrounding the central intensity feature at an intended image plane from an impingent radiation beam. -2-

Optionally, the impingent radiation beam may be a collimated radiation beam or converging radiation beam. The impingent radiation beam may have a circular cross-section outline and Top-Hat (uniform) intensity profile.

Optionally, the optical apparatus may further comprise an impingent radiation beam modulator, operable to modulate the diameter of the impingent radiation beam to vary the relative characteristic intensities of the annular ring and central intensity feature.

Optionally, the optical apparatus may further comprise a spatial offset device, operable to spatially offset an optical axis of the optical apparatus relative to the axis of the impingent radiation beam, to cause an asymmetry in the intensity profile at the image plane.

Optionally, the optical apparatus comprises field mapping optics operable to transform an incident intensity profile of the impingent radiation beam to the annular ring and central intensity feature at the intended image plane, and wherein the field mapping optics comprises any one of: refractive optics; reflective optics; or diffractive optics.

Optionally, the refractive or reflective optics comprises a spherical or aspherical central portion, and an aspheric orAxicon surrounding annular ring portion.

Optionally, the optical apparatus comprises beam integration optics comprising an array of micro elements operable to split the impingent radiation beam into a plurality of beam-lets arranged to overlap at the intended image plane, and wherein the individual micro-elements comprises and one of: a refractive micro-lens array; a reflective micro-lens array; or a diffractive micro-lens array.

Optionally, the aspheric surface surrounding annular ring portion for use in a specific implementation is determined by calculating a gradient of the aspherical surface operable to remap the rays at the first incident surface of the refractive lens or reflective surface to new positions at the image plane.

There is also provided an optical apparatus configuration operable to modulate an incident radiation beam into an output form having an annular ring portion and a central fill portion about a central axis of the annular ring portion, wherein there is a method by which to modulate the relative characteristic intensities of the annular ring and central intensity features.

Optionally, the central intensity feature comprises a homogenised plateau profile region, or a Gaussian profile region, or an inverse Gaussian profile region.

There is also provided a method of shaping radiation, comprising providing an optical apparatus as described herein.

Optionally, the method may further comprise providing an incident radiation beam and offsetting the incident radiation beam to form an annular ring having a greater intensity on one side in use than another side.

These and other aspects of the invention will be apparent from and elucidated with reference to the embodiments described hereinafter.

When a single lens is described herein, the incident surface' of the lens is the surface onto which the laser source impinges, and the transmissive surface' is the surface from which the laser light is transmitted, i.e. the incident surface' is nearer the laser source than the transmissive surface'. -3-

Brief description of the drawings

Further details, aspects and embodiments of the invention will be described, by way of example only, with reference to the drawings. In the drawings, like reference numbers are used to identify like or functionally similar elements. Elements in the figures are illustrated for simplicity and clarity and have not necessarily been drawn to scale.

Figure 1 schematically shows possible layout configurations for embodiments of the present invention; Figure 2 shows different example combinations of central and surrounding annular portions of a refractive and of a reflective beam shaping optic according to embodiments of the present invention; Figure 3 shows a specific refractive beam shaping optics arrangement according to an embodiment of the present invention; Figure 4 shows an example of how the output intensity profile at the image plane changes with decreased input beam diameter according to an embodiment of the present invention; Figure 5 shows an example of how the output intensity profile at the image plane changes with increased input beam diameter according to an embodiment of the present invention; Figure 6 shows parameters of the beam shaping optic according to an embodiment of the present invention; Figure 7 shows parameters of the output intensity profile at the image plane that are used in calculations of the beam shaping lens shape function, according to embodiments of the present invention; Figure 8 shows a plot of the calculated gradient of the incident surface of the annular portion of the beam shaping optic, according to an embodiment of the present invention; Figure 9 shows a plot of the calculated dimensionless surface profile of the incident surface of the annular portion of the beam shaping optic, according to an embodiment of the present invention;

Detailed description of the preferred embodiments

Because the illustrated embodiments of the present invention may for the most pad be implemented by those skilled in the art, details will not be explained in any greater extent than that considered necessary for the understanding and appreciation of the underlying concepts of the present invention and in order not to obfuscate or distract from the teachings of the present invention.

To improve the accommodation of different materials and processing speeds during laser surface treatments, the laser beam profile may be modulated by adjusting the intensity at the outer edge of the laser beam (cross-section) relative to the intensity throughout the middle of the laser beam. Some laser surface treatments such as those which require multiple treatment cycles may also benefit from modulation of the beam profile between cycles as the surface properties change.

Modulation of the laser beam profile using a single optical system may allow efficient creation of the optimum laser beam profile for any combination of material properties and processing speeds. -4-

This would allow industrial processes to use either a single beam shaping system for multiple uses or provide a ready solution to almost any surface treatment where uniformity in the finished surface properties is necessary.

It has been found that a focussed annular ring with a variable intensity fill is advantageous in the area of laser surface treatments, with a variable plateau form of variable intensity fill being most advantageous.

Accordingly, embodiments of the invention may comprise a beam shaping optic, i.e. lens, with a first central surface profile region surrounded by an outer annular region of a second profile shape. The resultant lens may be used with any kind of incident/impingent radiation beam, such as a laser.

Further to its uses in laser surface heat treatments, the steep sided intensity profile and variable intensity fill in the centre of the beam may be of significant benefit to laser drilling applications in which hole taper and thick recast layer on the hole entrance are major disadvantages.

In addition, the impingent radiation beam may be offset from the optical axis of the beam shaping optical configuration, to cause the annular ring to become more intense on one side than the other in a direction perpendicular or parallel to the movement axis of the beam. The advantages of such an output formation where the offset is perpendicular to the motion axis of the beam are that it enables the capability for conduction welding of dissimilar metals -this is particularly useful when the melting temperature differential between the materials is large. The resultant asymmetric intensity profile may be varied by changing the magnitude of the offset. The advantages of such an output formation where the offset is parallel to the motion axis of the beam are that it enables a uniform temperature field to be created under the footprint of the moving beam.

A uniform temperature field significantly reduces molten pool stirring in laser melting applications such as laser cladding, where heavy stirring can increase unwanted dilution of the substrate material into the clad layer.

The key consideration is the production of a suitable output intensity profile at the image plane. As will be appreciated, there are a number of different optical methods suitable doing this, according to different embodiments of the present invention. For example, beam integration optics or field mapping optics may be used in embodiments of the present invention.

Field Mapping Optics embodiments

Field mapping optics transform the incident intensity profile by mapping the incident rays (of the impingent radiation beam) from a position at the incident optical surface to new positions at the image plane. This can be achieved either by refraction, reflection or diffraction optics or any combinations thereof. Field mapping optics are most useful where the beam intensity distribution is known and can be sensitive to beam misalignment and beam size and mode fluctuations.

The field mapping may be carried out by shaping the surface or gradient of the refractive index of a lens (transmission), shaping the surface of a mirror (reflection) or organising the grating on a diffractive mirror (reflection) or diffractive lens (transmission). Depending on whether reflection, -5-refraction or diffraction is used, the beam shaping optic can therefore be either transmissive or reflective -Refractive Embodiments Refractive embodiments of the invention can take a number of forms, and examples of possible formations of the optics are shown in Figure 2. These include using spherical or aspherical central portions and aspherical or conical (Axicon) annular portions, The central portion of the optic forms the central intensity feature at the image plane and the surrounding annular portion forms the surrounding annular ring. As shown, there are several ways in which this can be achieved. For example, refractive embodiments are shown listed from Figure 2 and are described in Table I below: incident Surface (1) Transmissive Surface (2) ____________ ___________ _______________________ Additional Converging Embodiment Central Annular Central Annular Optic Portion Portion Portion Portion Spherical A A-Axicon Flat Required Concave Spherical B V-Axicon Flat Required Concave Spherical C Spherical Convex A-Axicon Not Required Concave Spherical D Spherical Convex V-Axicon Not Required Concave Spherical Aspherical E Flat Not Required Convex Convex TABLE 1: Refractive embodiments surface profiles Reflective Embodiments The reflective embodiments are fewer in number, since only one surface is involved per optic.

Once again the beam shaping can be completed using a single optic. The surface of each reflective embodiment in Figure 2 is described in Table 2 below: Surface Additional Converging Embodiment ______________________ ______________________ Central Portion Annular Portion Optic F Spherical Convex A-Axicon Required G Spherical Convex V-Axicon Required H Spherical Concave Aspherical Concave Not Required TABLE 2: Reflective embodiments surface profiles -6-As shown in Figure 2 the reflector embodiments may take the form of mirrored surfaces.

Just as in the case of the refractive embodiments, the optics can be designed to work with either a collimated or converging beam to transform a Top-Hat intensity profile impingent radiation beam into an annular ring with a central intensity feature at the image plane. The surfaces of reflector embodiments F and G are conical in the annular portion and convex spherical in the central portion.

The surface of reflector embodiment H is aspheric convex in the annular portion and spherical concave in the central portion.

Diffractive Embodiments Diffractive (or Holographic) optical elements may be either transmissive or reflective, and may take the form of either volume (multiple layer) or surface binary (single layer) diffraction gratings. Diffractive embodiments of the design may therefore possess layout configurations such as those in Figure 1. The central and annular portions of the optic may consist of diffraction gratings designed to replace the reflective or refractive surfaces described in either of the reflective or refractive embodiments shown in Figure 2.

Beam Integration Optics embodiments Some embodiments may utilise beam integration optics. These usually comprise an array of micro elements which split the input beam into a series of beam-lets which are made to overlap at the image plane to produce the desired intensity profile. The individual elements can be reflective, refractive or diffractive and are shaped to produce the desired effect (i.e. intensity profile) at the image plane. A converging optic may be used after the microelement array to cause the beam-lets to overlap at the image plane. Beam integration optics are particularly useful when the impingent radiation beam is unstable or spatially incoherent, and are less affected by beam misalignment and beam size and mode fluctuations. In beam integration embodiments of the present invention, the micro elements in the central portion of the beam may be shaped to produce the central intensity feature inside the annular ring at the image plane, with these microelements surrounded by an annular array of microelenients shaped to produce the annular ring itself.

Specific example of a dual element refractive embodiment.

There is now described an example of the design steps involved for a field mapping optics based exemplary embodiment utilising a refractive beam shaping optic of the type (B) shown in Figure 2 and a convergent impingent radiation beam. The overall optics arrangement is as shown in Figure 3.

The profile of the incident surface of the annular portion (12) of the lens (10) may be conical such that the annular portion of the input Top-Hat beam may be transformed into a focussed annular ring. The profile of the incident surface of the central section of the lens (10) may be concave spherical such that the central portion of the impingent Top-Hat beam may be transformed into a uniform intensity fill in the centre of the annular ring at the image plane as described in more detail below. -7-

A particular embodiment takes the form of a refractive beam shaping lens (10) having a Axicon profile annular portion (11) and a spherical concave central portion (12), and is designed for use with a converging beam which is achieved from a collimated impingent (i.e. input) radiation beam by using a converging (objective') lens (20) placed before the beam shaping lens (10). A means by which the beam diameter of the collimated beam entering the objective lens can be expanded or contracted may be included, for example beam modulator (30), in order to provide a mechanism for modulating the relative intensities of the beam intensity profile features at the image plane.

In particular, modulation of the relative intensity of the annular ring compared with the intensity throughout the rest of the beam can be controlled by expanding or contracting the impingent radiation beam with a zoom telescope (e.g. Donders telescope) or other similar device (30) before it enters the objective-beam shaping lens combination. This adjusts the relative proportion of the laser beam energy passing through the V-Axicon (11) and concave spherical (12) regions of the beam shaping lens (10), causing variation in the relative intensities of the annular ring and central fill intensity features at the image plane. Low central fill intensities are beneficial when processing at low Péclet number (that is, low laser traverse speed or rapid thermal diffusion in the material). Meanwhile, higher central fill intensities are beneficial at higher Féclet numbers.

Figures 4 and 5 show how the central intensity fill feature beam profile changes in magnitude relative to the annular ring intensity feature (i.e. has a variable fill level) according to the variation in impingent radiation beam diameter.

This example refractive embodiment is designed using Snell's Law and the Intensity Law to construct rays through the optical system and to map the electric field amplitude. It assumes the geometrical optics approximation which ignores diffraction effects. This is deemed to be a reasonable approximation in laser beam shaping systems so long as the dimensionless parameter /3 is greater than 32: -2IJWOi,ZTOUt p Where w01 is the beam radius entering the lens, r0is the beam radius of the shaped beam at the image plane, f is the focal length of the system and A is the wavelength of the laser radiation.

In order that the system may operate over the largest range, a beam modulator may be used.

For example, a variable beam expander may be used (such as a Donders telescope), and this should have the capacity to expand or contract the beam from its smallest size, being the diameter of the central portion of the beam shaping lens, to its largest, being the clear aperture of the beam shaping lens overall.

The focal length of the objective lens and its separation from the beam shaping lens/optic should be chosen appropriately. The focal length of the objective lens will determine the work piece stand-off and the annular ring spread as well as the depth of field at the focal region of the annular ring. The focal length of the lens should therefore be long enough to avoid an impracticable stand-off and shallow depth of field, yet short enough that the annular ring spread is sufficient to give the sharp drop-off in intensity at the edge of the ring preferable for achieving the -8-desired heating effect on the surface of the material. The beam shaping lens-objective lens separation should be chosen to be as small as is practicable.

The outer radius of the annular portion of the beam shaping lens r0 (as shown in Figure 6) must large enough to allow a maximum k value of 0.05 when the input beam is expended to its maximum allowable diameter. The quantity k is the ratio of the characteristic intensity of the central intensity fill OPL to the characteristic intensity of the annular ring feature 1OAR at the image plane as illustrated in Figure 7. The k value of 0.05 is a compromise in that it approximates an annular ring without any intensity well enough to serve any low F'ëclet number applications whilst ensuring that the overall diameter of the beam shaping optic is kept to a reasonable size. The maximum allowable radius of the beam rm entering the beam shaping lens should be no more than 70% of route,. Therefore: = Where: / (b + WDAR)2 -(b -wflAp)2 Tmar = ifl7teT1 k -Where TjnoerS the radius of the central portion of the beam shaping lens and: 2M2Af0 = -nncr) Where f0 is the objective or converging lens focal length, b is the distance from the peak of the annular ring intensity feature to the optical axis and WoA is the half width of the annular ring feature.

The two equations above are solved simultaneously to obtain a value for rmaxand hence Touter, the overall radius of the beam shaping optic.

The angle of the Axicon annular portion of the beam shaping lens eAX is determined by considering the intended size of the beam bat the image plane: b + WOAR °AX = (f0 -d -CT + h12)(n -1) Where d is the lens separation between the converging and beam shaping lenses, CT is the thickness of the beam shaping lens, h12 is the distance between the second vertex and the second principal plane of the converging lens and n is the refractive index of the beam shaping lens.

The central portion of the beam shaping lens must be piano-concave in order that the beam maintain its uniformity and be focussed to a point beyond the image plane causing the annular ring to have a uniform intensity fill in its centre. The focal length of the central portion of the beam shaping lens is governed by r10 and the choice of b: -9- -( flnner -b -I --d + h12) ft -d) The radius of curvature R0 of the central portion of the beam shaping lens is therefore: R=f2(n-1) rj,,1r, r0r,0, R, OAXand CT are dimensions of the beam shaping optic of this embodiment as shown in Figure 6. b and WOAR are parameters of the output beam intensity profile at the image plane as shown in Figure 7.

Specific example of a single element refractive embodiment.

To show that other embodiments of the design are also mathematically feasible, a second example -a single element embodiment as shown in (E) of Figure 2-is described in detail.

Output beam diameter and focal length of the annular ring are chosen. The size of the central portion is chosen such that the value of is greater than 32. The focal length of the central portion of the incident surface is then chosen such that the beam diameter at the intended focal plane of the annular ring is equal to the intended output beam diameter. The gradient of the annular portion of the incident surface is calculated by numerically solving the following equation: / z'(r) = -CT tan -z'(r) -sin' -z'(r) -BFD0tan [sun sun (stir' [_z'(r) -sun' )P11 An example of the surface gradient z'(r) vs. dimensionless distance r/roor for an example of this embodiment is shown in Figure 8.

The surface sag of the annular portion of the incident surface can be calculated by numerically integrating z'(r). This is completed by splitting the area under into trapeziums. The first value, z(r)1, is chosen according to the overall thickness of the lens. Subsequent values z(r)N+l are calculated from their preceding values z(r) using the following formula: = z(r)N +(rN+1 rN)(z@N) +z'(rN÷l)) A dimensionless plot of the actual lens surface excluding the central section is shown in Figure 9.

Other modifications, variations and alternatives are also possible, since there are multiple ways to produce the desired output beam intensity profile at the image plane, as exemplified above.

The specifications and drawings are, accordingly, to be regarded in an illustrative example sense, -10-rather than in a restrictive sense. For example, the Axicon region may be either A-or V-type Axicons, both being an alternative way to form the annular ring intensity feature. Moreover, due to the additive nature of optical arrangements, the beam shaping lens may be formed to have a single surface or dual surface, and be used as a single element, or dual element. This is to say, the convergence of the beam may be achieved by either a separate converging lens, or by incorporating the converging lens aspect into the main beam shaping lens itself, as shown in Figure 2 in particular.

In the claims, any reference signs placed between parentheses shall not be construed as limiting the claim. The word comprising' does not exclude the presence of other elements or steps then those listed in a claim. Furthermore, the terms "a" or "an," as used herein, are defined as one or more than one. Also, the use of introductory phrases such as "at least one" and "one or more" in the claims should not be construed to imply that the introduction of another claim element by the indefinite articles "a" or "an" limits any particular claim containing such introduced claim element to inventions containing only one such element, even when the same claim includes the introductory phrases "one or more" or "at least one" and indefinite articles such as "a" or "an." The same holds true for the use of definite articles. Unless stated otherwise, terms such as "first" and "second" are used to arbitrarily distinguish between the elements such terms describe. Thus, these terms are not necessarily intended to indicate temporal or other prioritization of such elements. The mere fact that certain measures are recited in mutually different claims does not indicate that a combination of these measures cannot be used to advantage.

Unless otherwise stated as incompatible or the physics or otherwise of the embodiments prevent such a combination, the features of the following claims may be integrated together in any suitable and beneficial arrangement. This is to say that the combination of features is not limited by the claim forms, particularly the form or numbering of the dependent claims. Moreover, portions of the clams may be used separately. For example, the annular ring portion of the beam shaping lens may be aspherical regardless of whether the central portion is an Axicon.

In the foregoing specification, the invention has been described with reference to specific examples of embodiments of the invention. It will, however, be evident that various modifications and changes may be made therein without departing from the broader scope of the invention as set forth in the appended claims.

Claims

Claims 1. An optical apparatus comprising: a first central portion operable to create a central uniform intensity feature inside an annular ring intensity feature at the intended image plane from the impingent radiation beam, and a second annular portion radially surrounding the first central portion, the second annular portion operable to create the annular ring intensity feature surrounding the central intensity feature at an intended image plane from an impingent radiation beam.
2. The optical apparatus of claim 1, wherein the impingent radiation beam cross-section outline is circular.
3. The optical apparatus of claim 1 or 2, wherein the impingent radiation beam is either a collimated radiation beam or converging radiation beam.
4. The optical apparatus of claim 1, 2 or 3, wherein the impingent radiation beam has a Top-Hat or uniform intensity profile.
5. The optical apparatus of any preceding claim, further comprising an impingent radiation beam modulator, operable to modulate the diameter of the impingent radiation beam in order to vary the relative intensities of the annular ring and central intensity feature.
6. The optical apparatus of any preceding claim, further comprising a spatial offset device, operable to spatially offset an optical axis of the optical apparatus relative to the axis of the impingent radiation beam, to cause an asymmetry in the intensity profile at the image plane.
7. The optical apparatus of any preceding claim, wherein the optical apparatus comprises field mapping optics operable to transform an incident intensity profile of the impingent radiation beam to the annular ring and central intensity feature at the intended image plane, and wherein the field mapping optics comprises any one of: refractive optics; reflective optics; or diffractive optics.
8. The optical apparatus of claim 7, wherein the refractive or reflective optics comprises either a spherical or an aspherical central portion, and an A-Axicon or V-Axicon surrounding annular portion.
9. The optical apparatus of any of claims 1 to 7, wherein the optical apparatus comprises beam integration optics comprising an array of micro elements operable to split the impingent radiation beam into a plurality of beam-lets arranged to overlap at the intended image plane, and wherein the individual micro-elements comprises and one of: a refractive micro-lens array; a reflective micro-lens array; or a diffractive micro-lens array.
10. The optical apparatus of any preceding claim, further comprising a converging lens operable to focus the impingent radiation beam.
11. An optical apparatus configuration operable to modulate an incident radiation beam into an output form having an annular ring intensity feature and a central fill intensity feature about a central axis of the annular ring portion wherein the relative maximum intensities of each feature can be adjusted.
12. The optical apparatus configuration of claim 11, wherein the central fill portion comprises a homogenised plateau profile region, a Gaussian profile region, or an inverse Gaussian profile region.
13. A method of shaping radiation, comprising: providing the optical apparatus of any of claims Ito 12.
14. The method of claim 13, further comprising: providing an incident radiation beam; and offsetting the incident radiation beam to form an annular ring having a greater intensity on one side in use than another side.Amendments to the claims have been filed as follows Claims 1, An optical apparatus arranged to receive an impingent radiation beam, the optical apparatus comprising: a central portion arranged to create a central uniform intensity field at an image plane from the impingent radiation beam; and an annular portion, radially surrounding the central portion, the annular portion arranged to create an annular ring intensity field surrounding the central uniform intensity field at the image plane from the impingent radiation beam, wherein the impingent radiation beam has a substantially Top-Hat or substantially uniform intensity profile.2. The optical apparatus of claim 1, whercin the impingent radiation beam cross-section outline is circular.3. The optical apparatus of claim 1 or 2, wherein the impingent radiation beam is either a collimated radiation beam or converging radiation beam.4. The optical apparatus of any preceding claim, further comprising an impingent (\J radiation beam modulator, arranged to modulate the diameter of the impingent radiation beam.5. The optical apparatus of any preceding claim, further comprising a spatial offset device, arranged to spatially offset an optical axis of the optical apparatus relative to the axis of the impingent radiation beam.6. The optical apparatus of any preceding claim, wherein the optical apparatus comprises field mapping optics comprisilig any one of: refractive optics; reflective optics; diffractive optics; or a combination thereof 7. The optical apparatus of claim 6, wherein the refractive or reflective optics comprises either a spherical or an aspherical central portion, and a spherical or Axicon surrounding annular portion.8. An optical apparatus as claimed in any preceding claim wherein the optical apparatus is refractive and the central portion and annular portion comprise an incident surface and a transrnissive surface and have a shape selected from a row ofthe following table:Incident surface Transmissive surface Central portion Annular portion Central portion Annular portion Spherical convex Spherical concave A-Axicon Spherical convex Spherical concave V-Axicon Spherical convex Aspherical convex Flat 9. An optical apparatus as claimed in any one of claims ito 8 wherein the optical apparatus is reflective and the central portion and annular portion have a shape LI) selected from a row of the following table: 0 Central portion Annular portion Q!) Spherical concave Aspherical concave 10. The optical apparatus of any preceding claim, further comprising a converging lens arranged to focus the impingent radiation beam at the image plane.ii. An optical apparatus as claimed in claim 10 wherein the optical apparatus is refractive and the central portion and annular portion comprise an incident surface and a transmissive surface and have a shape selected from a row of the following table: Incident surface Transmissive surface Central portion Annular portion Central portion Annular portion Spherical concave A-Axicon Flat Spherical concave V-Axicon Flat 12. An optical apparatus as claimed in claim 10 wherein the optical apparatus is reflective and the central portion and annular portion have a shape selected from a row of the following table: Central portion Annular portion Spherical convex A-Axicon Spherical convex V-Axicon 13, The optical apparatus of any of claims 1 to 5, wherein the optical apparatus comprises beam integration optics comprising an array of micro elements an'anged to split the impingent radiation beam into a plurality of beam-lets arranged to overlap at the image plane, and wherein the array of micro-elements comprises at least one of: a refractive micro-lens array; a reflective micro-lens array; or a diffractive micro-lens array.14. The optical apparatus configuration of any preceding claim wherein the central U') uniform intensity field comprises a homogenised plateau profile region. C)C\J
15. A method of shaping an impingent radiation beam using an optical apparatus having a central portion and an annular portion radially surrounding the central portion, the method comprising: using the central portion to create a central uniform intensity field at an image plane from the impingent radiation beam; and using the annular portion to create an annular ring intensity field surrounding the central uniform intensity field at the image plane from the impingent radiation beam, wherein the impingent radiation beam has a substantially Top-Hat or substantially uniform intensity profile.
16. An optical apparatus or method of shaping an impingent radiation bcam substantially as hereinbefore described with reference to the accompanying drawings.