WO2007107684A1

WO2007107684A1 - Beam transformer

Info

Publication number: WO2007107684A1
Application number: PCT/GB2006/001035
Authority: WO
Inventors: William Andrew Clarkson; Jacob Isa Mackenzie
Original assignee: University Of Southampton
Priority date: 2005-03-22
Filing date: 2006-03-21
Publication date: 2007-09-27
Also published as: GB0505794D0

Abstract

A beam transformer (30) changes the aspect ratio of the transverse spatial intensity profile I(x,y) of a light beam, the beam transformer comprising first and second rotationally symmetrical curved reflecting surfaces (14, 18) so arranged that the beam is reflected from the first curved surface (14) and subsequently by the second curved surface (18), the first reflecting surface (14) being tilted, with respect to the local propagation direction z of the beam (12), substantially about the local transverse x axis of the beam, and the second reflecting surface (18) being tilted, with respect to the local propagation direction z of the beam (16), substantially about the orthogonal local transverse y axis of the beam (16). The beam transformer can be used to change a laser beam from one with a highly asymmetric transverse intensity distribution (eg a highly elliptical beam profile (10; 60)) into one with a substantially axially-symmetric transverse intensity distribution (eg a circular beam profile (22)) or vice versa. One important example of such an application is a solid-state laser or semiconductor laser with a highly asymmetric gain medium. The beam transformer may be used within a laser resonator or external to the laser.

Description

BEAM TRANSFORMER

Field of the Invention

This invention relates to beam transformers, and more particularly to beam transformers that utilise mirrors to change the aspect ratio of the transverse spatial intensity profile of light beams, such as laser beams or intra-cavity resonator modes, whilst substantially preserving their brightness.

Background to the Invention

There are many applications where it is necessary to change a laser beam from one with a highly asymmetric transverse intensity distribution (eg a highly elliptical beam profile) into one with a substantially axially-symrnetric transverse intensity distribution (eg a circular beam profile) or vice versa. One important example of such an application is a solid-state laser or semiconductor laser with a highly asymmetric gain medium. The latter may result from the use of a slab or planar waveguide geometry and/or because of the shape of the excitation region. This laser geometry is very attractive since it facilitates thermal management and offers the potential for scaling laser output power to very high levels whilst avoiding thermally-induced damage. However, it suffers from the disadvantage that the resulting laser beam (which may consist of a single or multiple beams) has a highly asymmetric transverse intensity profile and hence limited applicability. There are a number of standard optical arrangements for remedying or alleviating this problem. Most of these generally employ one or more anamorphic prisms, or combinations of cylindrical lenses and/or cylindrical mirrors to transform the highly-asymmetric beam into one with a substantially axially-symmetric transverse intensity profile. These methods rely on refraction at angled or curved surfaces to achieve the desired effect, and hence suffer from the problems of chromatic aberration and loss in power due to reflection at the surface. Moreover, in the case of optical arrangements using cylindrical optical components, there is generally a degradation in beam quality, and hence brightness, due to additional contributions to phase aberration which arise from imperfections in the lens shape and the difficulty in manufacturing cylindrical optics to the required accuracy. The above problems are further exacerbated by the need for many optical components to achieve a large change (>10:l) in the beam's aspect ratio. The overall result is that these techniques have a limited range of applications and are generally unsuitable for use inside laser resonators.

An alternative technique for reducing the aspect ratio of the beam from a slab laser (or planar waveguide laser) is to use a hybrid stable-unstable resonator configuration as described in J. R. Lee et al. "High-average-power Nd:YAG planar waveguide laser that is face pumped by 10 laser diode bars," Optics Letters, vol.27, p.524-526, (2002). In this scheme, a stable resonator is employed for the plane of propagation perpendicular to the plane of the slab (or planar waveguide) gain medium, and an unstable resonator is employed in the orthogonal plane of propagation. The latter can be achieved by, for example, using a feedback mirror with a smaller aperture than the beam size in the plane of the slab gain medium, so that the output beam exits the cavity by the side edge of the mirror. This approach can yield relatively good beam quality, but the output beam is typically astigmatic, with dissimilar wavefront curvatures in orthogonal planes, and hence further correction for the astigmatism is needed. A further disadvantage of this approach is that it requires a laser medium with relatively high gain due to the relatively high cavity loss, which limits flexibility in resonator design and mode of operation.

Summary Of The Invention

According to one aspect of the invention we provide a beam transformer for changing the aspect ratio of the transverse spatial intensity profile I(x,y) of a light beam, the beam transformer comprising first and second rotationally symmetrical curved reflecting surfaces so arranged that the beam is reflected from the first curved surface and subsequently by the second curved surface, the first reflecting surface being tilted, with respect to the local propagation direction z of the beam, substantially about the local transverse x axis of the beam, and the second reflecting surface being tilted, with respect to the local propagation direction z of the beam, substantially about the orthogonal local transverse y axis of the beam.

The invention employs a new approach for transforming the aspect ratio of the transverse spatial intensity profile I(x,y) of a laser beam (comprising one or more beams), which can enable the transformation to be made without a significant decrease in power and brightness. The beam transforming technique of the invention is extremely simple and can be of relatively low cost, since it preferably comprises only two curved mirrors (for example, but not exclusively, spherical and aspheric such as parabolic) with high reflectivity dielectric coatings, and hence can be applied to extra-cavity and intra-cavity laser beams where a change in the aspect ratio of the beam is required.

The beam transforming device of the invention (herein referred to as the beam transformer) exploits the astigmatism generated by a standard curved mirror when light is incident at non-normal incidence (ie at an angle less than 90° to the mirror surface). This is a well-known property of a curved mirror and results in the mirror having different focal lengths for the beam in the plane-of-incidence and perpendicular to the plane-of-incidence (see for example F. A. Jenkins and H. E. White, "Fundamentals of Optics", McGraw-Hill: New York (1976)).

The beam transformer comprises two curved high reflectivity mirrors (in its simplest configuration) tilted so that the beam is incident on each mirror at non- normal incidence (ie at an angle of <90° to the mirror surface) and so that the plane-of-incidence at the first mirror is substantially orthogonal to the plane-of- incidence at the second mirror, having regard to the local transverse xy axes of the beam with the result that the magnification factor (M_x) in a first direction is different from the magnification factor (M_y) in a second direction. The overall result is a change in the aspect ratio of the transverse intensity profile I(x,y) of the beam by an amount which depends on the magnification factors (M_x and M_y), which are determined by the mirror curvatures, their separation and by the angles of incidence of the beam at the mirrors. Although either or both of the curved surfaces may be convex surfaces, preferably both the first and the second curved surfaces are concave surfaces.

The first and second curved surfaces are preferably arranged to produce a change in the aspect ratio of at least 3.

However the beam transformer can be designed to yield a very large difference in the magnification factors and hence produce a very large change (>10:l) in the aspect ratio of the beam if required.

The approach for changing the aspect ratio of a light beam according to this invention has many advantages over the prior art techniques described above owing to its potentially very simple and low cost construction, low loss, compactness and versatility. Moreover, this invention can yield a very large change in the aspect ratio of a laser beam without a significant degradation in beam quality and brightness. Thus, the technique can be employed to condition the output of a laser, where an application requires a different intensity distribution than was generated; or, within a laser resonator to achieve a large change in the aspect ratio of the laser beam, as may be required for flexibility in mode of operation (eg Q-switching, mode-locking, intra-cavity nonlinear frequency generation), and to produce an output beam with transverse beam profile tailored to the requirements of the intended application without incurring a significant reduction in the laser efficiency.

Brief Description Of The Drawings

For a better understanding of the invention and to show how the same may be carried into effect, reference is now made by way of example to the accompanying drawings throughout which like parts are referred to by like references, and in _ which: Figure 1 is a schematic view of a beam transformer (in its simplest form) according to a preferred embodiment of the present invention. The reference coordinate system (x,y,z) for beam propagation is indicated at the entrance and exit of the beam transformer,

Figure 2(a) and 2(b) are schematic plan and side views respectively of the beam transformer of Figure 1,

Figure 3(a) and 3(b) are schematic views of an asymmetric transverse spatial intensity profile of an incident beam received by the beam transformer of Figure 1 and the transformed transverse spatial intensity profile of the exiting beam leaving the beam transformer,

Figure 4(a) and 4(b) are schematic views of a standing-wave and ring laser device employing a beam transformer respectively,

Figure 5 is a schematic view of a double-pass amplifier employing a beam transformer to transform the shape of the signal beam so that there is a good spatial overlap with the excitation region in the gain medium and to transform the reflected beam to its original shape,

Figures 6(a) and 6(b) show examples of different laser gain media geometries and/or excitation region geometries that may be employed in said laser or amplifier devices,

Figure 7 is a schematic view of the beam transformer and its application to the intra-cavity laser beam produced by a side-cooled laser gain medium with slab geometry,

Figure 8 is a schematic view of the beam transformer and its application to the intra-cavity laser beam produced by a face-cooled laser -gain-medium with an elongated excitation region, Figure 9 is a schematic view of a standing-wave laser device employing two beam transformers,

Figure 10 is a schematic view of the application of the beam transformer extra-cavity to induce a desired aspect ratio change to a laser beam for the intended application, and

Figure 11 represents a schematic view of the application of two extra- cavity beam transformers, the first inducing the desired aspect ratio change for an intermediate purpose, whilst the second provides additional changes to the beam's aspect ratio for the final application.

Detailed Description

A preferred embodiment of a beam transformer in accordance with the present invention will now be described, by way of example only.

With reference to Figures 1, 2 and 3, an incident beam 12, with an asymmetric transverse intensity profile 10 and initial beam dimensions, W_x in a first direction (x) and w_y in a second direction (y), propagating in a third direction (z), where x, y and z are mutually orthogonal and are defined relative to the local beam propagation properties, enters the beam transformer device 30. The beam transformer (in its simplest form) comprises two highly reflective curved surfaces 14 and 18 (for example but not exclusively mirrors with high reflectivity coatings). Beam 12 is incident on the first curved mirror 14 of radius of curvature RA at an angle of incidence ΘA, where the plane of incidence (defined by the incident beam 12 and reflected beam 16) is parallel to the plane defined by the second direction (y) and the beam propagation direction (z), the mirror 14 being tilted, relative to the beam axis or propagation direction z, about local axis x.

The reflected beam 16 propagates distance d and is then incident on the second curved mirror 18 of radius of curvature R_B at angle of incidence Θ_B arranged, by tilting the mirrors 18 about the local transverse axis y of the beam, so that the plane of incidence at mirror 18 (defined by the incident beam 16 and reflected beam 20) is parallel to the plane defined by the first direction (x) and the second beam propagation direction (z).

The reflected beam 20 exits the beam transformer 30 with modified transverse spatial intensity profile 22 having beam dimensions w_x" and w_y" in the first direction (x) and second direction (y) respectively.

The focal length of mirror 14 for incident beam 12 in the plane of incidence is fA_//= RACOS(ΘA)/2 and in the orthogonal plane is fA-L= RA/(2 COS(ΘA)).

The focal length of mirror 18 for incident beam 16 in the plane of incidence is fβ_//

is RB/(2COS(Θ_B)).

In a preferred design procedure mirrors 14 and 18 are selected and arranged with respect to each other so that R_Asin(θ_A)tan(θ_A)=RBsin(θB)tan(θB) and hence (fA//+fβ-i-)-(fA-^J-+fB//)5 and mirror separation d is selected so that

In this way the afocal condition is satisfied simultaneously in orthogonal planes, but with different magnification factors M_X=(RA/RB).COS(ΘA)COS(Θ_B) and My=RA/(RB.cos(θ_A)cos(θB)) in the first direction (x) and second direction (y) respectively.

The net result is that the beam (20) emerging from the beam transformer has modified beam dimensions, w_x"~M_xw_x and w_y"~M_yw_y in the first and second directions respectively, and hence the aspect ratio of the beam is modified by a factor M_x/M_y= cos²(θ_A)cos²(θ_B).

In practice, it will be found necessary for laser operation to depart slightly from this calculated design. The required change in aspect ratio can be achieved via selection of appropriate mirror radii of curvatures, RA and RB, in conjunction with the appropriate mirror spacing and alignment with respect to the beam. In this way a light beam may be transformed from a highly elliptical beam to a circular beam, or vice versa, without a significant loss in brightness. Moreover, a very large change in the aspect ratio of the beam (by more than 10:1) is readily achievable, by careful selection of the mirror radii of curvature, RA and R_B, and angles of incidence, ΘA and θβ, without a significant loss in power via this simple technique.

It will be appreciated that the exiting beam 20 from the beam transformer of Figures 1 to 3 will not be parallel to the incident beam 12, and in other embodiments of this invention, one or more plane mirrors may be employed before or between or after the curved mirrors to change the beam propagation direction, so that incident and exiting beams propagate in specific directions as required by the application.

In one embodiment of the invention, one or more plane mirrors may be employed before or between or after the curved mirrors so that the incident and exiting beams propagate parallel to each other.

In a preferred embodiment of the invention, the mirrors are coated with very high reflectivity dielectric coatings to minimise losses.

The beam transformer may also incorporate additional optical components (eg half- wave plates) for rotating the polarisation of the beam to further reduce the loss at the mirror surfaces.

In another embodiment of the invention the mirror radii of curvature, RA and R_B, the angles of incidence, ΘA and θβ, and the mirror spacing d are so selected that the beam transformer can be employed inside a laser resonator to provide a change in the aspect ratio of an intra-cavity laser beam without a significant loss in power or brightness. The intra-cavity beam may comprise one or more constituent beams and each constituent beam may comprise one or more transverse modes.

Figure 4(a) illustrates a schematic view of a laser device with a standing-wave resonator configuration in which a beam transformer 30 is used to convert an elliptical intra-cavity beam with good spatial overlap with the gain region in laser medium 42 to an axially-symmetric circular intra-cavity beam. Feedback for laser oscillation is provided by optical feedback system 40, which comprises at least one plane or curved mirror with high reflectivity at the lasing wavelength, and by optical feedback system 46, which may comprise one or more optical components (eg lenses, curved mirrors, apertures) to facilitate transverse mode control and selection as required. The latter may also include an output coupling device such as a curved or plane mirror with partial transmittance at the lasing wavelength. Likewise, optical feedback system 40 may also comprise additional optical elements for the same purpose.

A mode-locking device and/or Q-switching device 44 may also be included in the resonator to allow the laser to be operated in the pulsed regime to achieve high peak powers.

The modulator device 44 could be, for example, an acousto-optic modulator, electro-optic modulator, or a passive Q-switching device or a passive mode- locking device. The modulator device 44 may be positioned in any part of the laser cavity where it is suitable for its function.

The resonator may also include a nonlinear device for intra-cavity frequency conversion (not shown) and the optical means to produce the required beam size in said device.

In the foregoing cases, the beam transformer 30 can be configured in accordance with the invention so as to transform the intra-cavity beam from one with a highly asymmetric or elliptical beam profile (which has a good spatial overlap with the asymmetric excitation region in the gain medium 42) to a circular or nearly circular beam with a substantially axially-symmetric transverse intensity profile.

The intra-cavity beam may comprise one or more constituent beams or transverse modes.

Figure 4(b) illustrates a schematic view of a ring laser device in which two beam transformers 30 in accordance with the invention are used to convert an elliptical intra-cavity beam with good spatial overlap with the gain region in laser medium 42 to an axially-symmetric circular intra-cavity beam. Feedback for laser oscillation is provided by two or more optical feedback systems 40, which comprises at least one plane or curved mirror with high reflectivity at the lasing wavelength, and by optical feedback system 46. The latter system 46 comprises a curved or plane mirror with partial transmittance at the lasing wavelength to serve as the output coupler, and may, in addition comprise additional optical components (eg lenses, curved mirrors, apertures) to facilitate transverse mode control and selection as required. Likewise, optical feedback system 40 may also comprise additional optical elements for the same purpose.

The resonator may also include a nonlinear device (not shown) for intra-cavity frequency conversion and the optical means to produce the required beam size in said device.

The resonator may also contain a unidirectional optical system 48 comprising one or more components which act to increase the propagation loss for one beam direction relative to the counter-propagating beam direction, and thereby enforce unidirectional lasing. In each of the foregoing cases, the beam transformers 30 are configured so as to transform the intra-cavity beam from one with a highly asymmetric or elliptical beam profile (which has a good spatial overlap with the asymmetric excitation region in the gain medium 42) to a circular or nearly circular beam with a substantially axially-symmetric transverse intensity profile.

It should be appreciated that if it is required that the elements 40, 30, 42, 30, 46, in Figure 4(b) are to be positioned physically in line, then the beam transformers 30 will need to be configured, as hereinbefore described, to provide for parallel incident and exiting beams.

Figure 5 shows a schematic view of a double-pass amplifier employing a beam transformer 30 to transform the shape of the incident signal beam signal beam 50 from an axially-symmetric transverse intensity profile to a beam 52 with an asymmetric intensity profile matched to the shape of the excitation region in the gain medium 42. The beam is amplified by a double-passing of the gain medium 42 with the aid of reflecting means 40, and then transformed back to a beam with an axially-symmetric beam profile after a second pass through the beam transformer 30, and exits the amplifier. The amplified beam 56 is extracted using a suitable beam extracting means 54, which may, for example, be a Faraday isolator.

Figures 6(a) and 6(b) show by way of example only two different geometries of laser gain media and/or excitation region 41 that may be employed in the laser or amplifier devices described with reference to Figures 4 and 5.

Figure 6(a) is a schematic view of a slab gain medium with a highly asymmetric pumped region 60 of width d_x in a first direction (x) and height d_y in a second direction (y), where d_x is much larger than d_y. The gain medium or pumped region may be a single element or may consist of multiple elements arranged in a linear array in the first direction (x). The slab may be transversely cooled (as shown) to reduce the detrimental effects of heat generation. This geometry has the attraction that it allows very simple thermal management and hence very high pump power may be used. Excitation of the laser medium may be achieved by optical pumping in the transverse directions (x or y) or by longitudinal pumping in the z direction. The slab may also comprise a planar waveguide to guide laser or signal light in the y-z plane.

Figure 6(b) is a schematic view of a face-cooled laser gain medium with an elongated excitation region 60 of width d_x in a first direction (x) and width d_y in a second direction (y), where d_x is much larger than d_y The shape of the excitation region may be defined by the transverse intensity profile of the pump beam, which may, for example, be produced by a diode laser array or diode-stack. The thickness, t, of the gain medium may be chosen to be much smaller than d_x to achieve predominantly axial heat flow (ie in the z direction), and hence to reduce detrimental thermal effects. The cross-sectional shape of the laser medium may be rectangular (as shown), or circular or may be of arbitrary shape, provided that its dimensions exceed the dimensions of the pump beam.

The highly asymmetric laser beam generated by a highly asymmetric excitation region within such a laser medium as used in Figure 6 may be transformed to a substantially axially-symmetric beam without a significant loss in power or brightness using a beam transformer in accordance with the invention.

Figure 7 shows, by way of example, a schematic view of a beam transformer in accordance with the invention and its application to the intra-cavity laser beam produced by a side-cooled laser gain medium 42 with a slab geometry. Feedback for laser oscillation is provided by optical feedback systems indicated schematically at 40 and 46, and each of which may comprise a number of optical components as required to produce the necessary feedback at the lasing wavelength and to facilitate transverse mode control and selection. In this embodiment of the invention the beam transformer comprises only two mirrors 14 and 18, configured so that the planes of incidence for the intra-cavity laser beam at the respective mirrors are substantially orthogonal to each other. The mirrors are positioned relative to each other, and to the laser gain medium, and to optical feedback systems 40 and 46 so as to transform the elliptical beam profile in the slab to a circular (or nearly circular) beam at the output coupler contained in optical feedback system 46.

This allows the highly asymmetric and inconvenient beam profile that is necessary for efficient extraction of the energy stored in the laser gain medium to be converted efficiently into a more useful and more convenient circular laser beam within the laser resonator and without a significant loss in power or brightness to serve the needs of a wide range of applications.

Figure 8 shows, by way of example, a schematic view of the beam transformer and its application to the intra-cavity laser beam produced by a face-cooled laser gain medium 42 with a highly elongated excitation region. The latter may be produced by a pump beam from a high-power diode laser array or diode-stack with a highly elliptical beam profile. An optical arrangement for achieving multiple transits of the pump beam (not shown) may be employed to increase the absorption efficiency of the pump light in the laser gain medium, and thereby increase the overall efficiency. This geometry of laser gain medium with face-cooling has the attraction that the heat flow is predominantly axial and hence detrimental thermal effects such as thermal lensing may be dramatically reduced compared to some other laser configurations. As before, feedback for laser oscillation is provided by optical feedback systems 40 and 46, each of which may comprise a number of optical components as required to produce the necessary feedback at the lasing wavelength and to facilitate transverse mode control and selection. Optical feedback system 46 may also comprise a mirror with partial transmittance at the lasing wavelength to produce an output beam. In the embodiment of Figure 8 the beam transformer comprises only two mirrors 14 and 18, and is configured so that the planes of incidence for the intra-cavity laser beam at the respective mirrors are substantially orthogonal to each other.

The mirrors are positioned relative to each other, and to the laser gain medium, and to the optical feedback systems 40 and 46 so as to transform the elliptical beam profile in the slab to a circular (or nearly circular) beam at the optical feedback system 46. This allows the highly asymmetric beam necessary for efficient extraction of the energy stored in the laser gain medium to be converted into a substantially circular laser beam within the laser resonator and without a significant loss in power or brightness.

More complex resonator designs, than those discussed with reference to Figures 4 to 8, containing additional optical components to extend functionality may be employed, if desired, and more complicated designs of the beam transformer, than that shown in Figures 1 to 3, comprising additional elements (eg half-wave plates) for polarisation control and one or more additional plane mirrors to effect a change in the relative directions of the incident and exiting beams for greater convenience.

However, in general the beam transformer is configured with the two curved mirrors 14 and 18 aligned so that the plane of incidence on mirror 14 (defined by the directions of the incident and reflected beams) is parallel to the plane defined by the beam propagation direction and the direction which corresponds to the minor axis of the elliptical incident beam, and so that the plane of incidence on mirror 18 (defined by the directions of the incident and reflected beams) is substantially perpendicular to the plane defined by the beam propagation direction and the direction which corresponds to the minor axis of the elliptical incident beam, and with mirrors 14 and 18 positioned with respect to each other and other cavity components so as to produce the desired change in the aspect ratio of the beam without a significant loss in brightness. A beam transformer in accordance with the invention may also be used to convert the circular beam in a side-cooled laser slab into a highly elliptical beam, which may be required in certain types of optical component to extend the functionality of the laser device and allow scaling of a particular mode of operation to higher power levels.

One example of such an optical component is a nonlinear crystal (such as periodically-poled lithium niobate) with a planar geometry or a planar waveguide geometry for nonlinear frequency conversion, where the disadvantage of a limited aperture size in one dimension may be compensated for by expanding the beam size in the orthogonal direction to reduce the risk of damage. Such components are difficult to employ inside laser cavities using standard laser resonator designs.

Figure 9 shows a schematic view of a laser resonator employing a first beam transformer 30 to convert the circular beam in a side-cooled laser slab 70 into a highly elliptical beam, as may be necessary for avoiding damage in optical component 74 (which may, for example, be a Q-switch, mode-locker or a device for nonlinear frequency conversion). A second beam transformer 30 is than employed to convert the elliptical beam back into a circular beam from which the output beam is derived. In this way, optical components which require elliptical beams may be employed within a laser cavity to extend the functionality of the laser without a significant reduction in brightness and efficiency.

By analogy a beam transformer in accordance with the invention may also be used extra-cavity to convert a circular beam into a highly elliptical beam, which may be required in certain types of optical components to extend their functionality or particular applications. One example is Faraday rotator media utilised in optical isolators between a laser source and an application, where weak optical absorption in these media leads to thermal distortions that degrade the beam quality of the transmitted beam. By distributing the thermal load in the Faraday rotator medium by providing an incident beam with a highly elliptical profile, results in weaker thermal lensing and thus less overall beam degradation. Figure 10 shows a schematic view of an extra-cavity beam transformer 30, transforming the laser beam 50 from an output coupler 46, inducing the desired aspect ratio change on the exiting laser beam 52 to suit the requirements of an optical component 74 (or application).

Figure 11 , by way of example, illustrates a schematic view of two extra-cavity beam transformers 30, the first changing an axially (or nominally axially- symmetric) beam 50 into one that is highly elliptical 52 as required by an optical component 74, whilst the second recovers the nominally axially-symmetric profile of the original beam 50.

There are many other types of laser, amplifier and optical systems which may benefit from the use of this beam transformer for changing the aspect ratio of a beam due to its simplicity, low cost architecture, low-loss performance and versatility.

References

J. R. Lee, H. J. Baker, G. J. Friel, G. J. Hilton and D. R. Hall, "High-average- power Nd: YAG planar waveguide laser that is face pumped by 10 laser diode bars," Optics Letters, vol.27, p.524-526, (2002).

F. A. Jenkins and H. E. White, "Fundamentals of Optics", McGraw-Hill: New York (1976).

Claims

1. A beam transformer (30) for changing the aspect ratio of the transverse spatial intensity profile I(x,y) of a light beam, the beam transformer comprising first and second rotationally symmetrical curved reflecting surfaces (14, 18) so arranged that the beam is reflected from the first curved surface (14) and subsequently by the second curved surface (18), the first reflecting surface (14) being tilted, with respect to the local propagation direction z of the beam (12), substantially about the local transverse x axis of the beam, and the second reflecting surface (18) being tilted, with respect to the local propagation direction z of the beam (16), substantially about the orthogonal local transverse y axis of the beam (16).

2. A beam transformer as claimed in claim 1 in which at least one of the first and second curved reflecting surfaces (14, 18) is a spherical reflecting surface.

3. A beam transformer as claimed in claim 1 in which at least one of the first and second curved reflecting surfaces is an aspheric surface.

4. A beam transformer as claimed in any one of the preceding claims in which at least one (14) of the first and second curved surfaces is a concave surface.

5. A beam transformer as claimed in any one of the preceding claims in which at least one (14) of the reflecting surfaces is a high reflectivity mirror surface.

6. A beam transformer as claimed in claim 5 in which said one high reflectivity mirror surface is a surface provided with a high reflectivity dielectric coating.

7. A beam transformer as claimed in any one of the preceding claims in which the first and second mirrors are so configured and arranged as to produce a change in said aspect ratio of at least 3.

8. A beam transformer as claimed in claim 7 in which the change in aspect ratio is at least 10.

9. A beam transformer as claimed in claim 4, , in which both curved reflecting surfaces are spherical concave surfaces and in which the radii of curvature of the first and second surfaces are RA and RB respectively, and the path length of the central axis of the beam between leaving the first surface and reaching the second surface is d, the configuration having been designed utilising the following relationships as one step of the design procedure:

where f_M = R_Acos(θ_A)/2 , f_Ax = RA/(2 COS(Θ_A))

f_B//= R_Bcos(θ_B)/2,

R_B/(2cos(θ_B))

f_/v_/ being the focal length of the first surface in the plane of the incidence,

f_AJ- being the focal length of the first surface in the orthogonal plane,

f_ez_/ being the focal length of the second surface in the plane of incidence, and

fβ-L being the focal length of the second surface in the orthogonal plane.

10. A beam transformer as claimed in any one of the preceding claims comprising one or more plane mirrors positioned in the beam path between the first and second curved surfaces, whereby a light beam is deflected by the one or more mirrors in passing from the first curved surface to the second curved surface.

11. A beam transformer as claimed in claim 10 in which the one or more mirrors are so configured as to compensate for the changes in beam direction produced by the first and second curved surfaces, whereby the beam exiting the beam transformer is propagated in a direction parallel to the beam incident on the beam transformer.

12. A beam transformer as claimed in any of the preceding claims incorporating one or more additional optical components adapted to rotate the polarisation of the beam transmitted in use through the beam transformer.

13. A beam transformer (30) as claimed in any one of the preceding claims positioned in the beam path of a laser that emits in use a beam (12) having a highly elliptical beam profile (10), the beam path being directed at said first curved surface (14) and the arrangement being such that the major transverse axis (x) of said elliptical beam incident on the first curved surface is substantially orthogonal to the plane of incidence of said central axis (z) of the beam (12) on said first curved surface whereby, in use, the beam (20) exiting from the beam transformer (30) has a substantially reduced aspect ratio (W_x/W_y) of the transverse spatial intensity (22) compared with that of the elliptical beam.

14. A beam transformer as claimed in any of claims 1 to 6 positioned in the beam path of a laser that in use emits a circular beam, the beam transformer being arranged to convert the circular beam into an elliptical beam having an aspect ratio of at least 3.

15. A beam transformer as claimed in claim 14 so arranged as to convert the circular beam into an elliptical beam having an aspect ratio of at least 10.

16. A beam transformer as claimed in claim 14 or claim 15 arranged in conjunction with a Faraday rotator (74) for supplying an elliptical laser beam to the Faraday rotator.

17. A laser device having a standing- wave resonator configuration, wherein a beam transformer (30) in accordance with any one of claims 1 to 11 is located in the resonator (40, 46) and is so configured as to convert, in use of the laser, an elliptical beam within a laser gain medium (42; 70) to a substantially axially- symmetric circular intra-cavity beam, and vice versa.

18. A ring laser device, wherein two beam transformers (30), each in accordance with any one of claims 1 to 11, are so arranged one before and one after the laser gain medium (42) in the ring (40, 42, 46, 40, 48), so as to provide, in use of the laser, in the laser gain medium (42) an elliptical intra- cavity beam, whilst permitting a substantially axially-symmetric circular intra- cavity beam in other parts of the ring.

19. A double-pass amplifier device comprising a gain medium, a beam transformer in accordance with any one of claims 1 to 11 and positioned in the beam path between a light beam source and one end of the gain medium (42) and so arranged as to convert an incoming beam of substantially axially-symmetric beam profile into a beam (52) of elliptical transverse intensity profile to pass through the gain medium, a reflecting means (40) being positioned beyond the far end of the medium to return the beam emitted from the gain medium back through the gain medium, to be converted back to a substantially axially- symmetric beam in return passing through the beam transformer.

20. A laser device as claimed in any one of claims 17 to 19 in which the gain medium is in the form of a side-cooled laser slab.