GB2099176A

GB2099176A - Polarizing element for laser

Info

Publication number: GB2099176A
Application number: GB8212788A
Authority: GB
Original assignee: CONTROL LASER Ltd
Current assignee: CONTROL LASER Ltd
Priority date: 1981-05-05
Filing date: 1982-05-04
Publication date: 1982-12-01

Abstract

Means for effecting circular polarization of a plane polarized laser beam 4 comprises two coated reflective plates 2, 3 each producing right angular reflections of the input laser beam and each effecting approximately a 45 DEG phase shift. The plates are mounted within a block 5 having input and output ports, the reflective plates being cooled by a coolant medium passing via tubes 8 through internal ducts in the block. Rotation of the block 5 enables compensation for small manufacturing variations in the phase shift of each reflective plate. The means is compact and easily positioned within the output beam from the laser which is used to cut or otherwise work, eg a metal workpiece. <IMAGE>

Description

SPECIFICATION Lasers This invention relatesto lasers and is more particularly concerned with lasers used for industrial purposes and having medium to high power outputs up to several kilowatts. Such lasers are used for precision cutting or other processing operations on metallic or non-metallicworkpieces.

In metal cutting in particular, a variation in the quality of the cut is apparent according to the instantaneous angular relationship of the plane of polarization of the laser beam to the direction in which the cutting track is proceeding. When the plane of polarization is along the track a better quality is produced than when the plane is at an angle thereto. This becomes most evident when cutting a curved or circular profile. The aforementioned effect is noticeable with the use of lasers which produce a steady and defined direction of polarization in the beam.

It is an object of this invention to overcome the disadvantages referred to and to provide a practi cally convenient arrangement whereby the quality of the cutting or processing operation is independent of the track direction of the beam.

According to this invention there is provided in or for a laser apparatus having a fixed plane of polarization of the beam, means for affecting substantially circular polarization of the plane polarized laser beam and operative in two or more stages.

It has been found that the resulting circular polarization produces a consistent quality in the workpiece irrespective of the processing direction, without any apparent compromise.

The circular polarization of the beam is effected by resolving the plane polarized beam into two equal components and phase shifting one component 90" with respect to the other.

The phase shift is effected by reflection of the beam from a surface having thereon an appropriately deposited surface layer or layers, wherein the thickness and nature define the phase shift between the two components. The two components are at right angles, one of the components containing the axis about which the reflecting surface has been rotated.

According to the invention in the case of two operative phase shift stages one coated surface at which reflection occurs produces a phase shift, the beam then being reflected again from a second coated surface also producing a phase shift between the two components, giving a sum shift of 90". For a given coated surface the angle between the surface and the incident beam controls, the phase shift, and small phase shift changes can be made by slight adjustment ofthe angle of incidence. The total shift thus results in the required circular polarization.

More than two reflecting surfaces may be used providing the overall total phase shift sums to 90".

In practical terms the reflecting surface is turned about an axis through the plane of the surface by 45" from a position where the surface is normal to the incident laser beam, and then the polarization axis of the beam is made about 45" onto the reflecting surface. This then sets up the required condition for resolving the beam into two 90" components.

The advantage deriving from the use of two reflecting surfaces, being a preferred embodiment, are that the output beams can be made parallel to and in the same direction as the input beam by virtue of equal and opposite angles of reflection at each reflecting surface and thus the beam is only displaced laterally from the original path. Using two reflecting surfaces to achieve the 90" shift in phase overall results in greater accuracy of shift due to better accuracy in manufacture and adjustment of the assembly can be used to optimise the shift produced. A single 90" phase shift would, of course, require a plane mirrorto restore the beam direction.

This invention also provides a method of providing circular polarization of a laser beam having a fixed plane of polarization, in which the method beam is reflected from a first surface angularly positioned in relation to the beam to effect resolution of the beam into two components mutually at 90 , the surface having a coating thereon to produce a 45" phase shift between said components, the reflected beam being passed to a second surface to effect a further 450 phase shift.

An embodiment according to the invention is shown by way of example in the accompanying drawings, wherein: Figure 1 shows a forward looking view of a means for effecting circular polarization of a linearly polarized laser beam, Figure 2 is a side view of the apparatus, and Figure 3 is a view looking along axis A of Figure 1.

As shown in the drawings the means for affecting a 90" phase shift and producing circular polarization of the laser output beam 1 comprises two coated reflective plates 2 and 3, each producing right angular reflections of the input laser beam 4 and each effecting approximately a 45" phase shift. The plates are mounted within an aluminium block 5 having input and output ports 6 and 7 respectively. The reflective plates are cooled by a suitable coolant medium passing via tubes 8 through internal ducts.

For an input laser beam polarised in the plane PP, rotation of the aluminium block about the axis AA on the mounting block 9 enables compensation for small manufacturing variations in phase shift of each reflective plate.

The means is compact and easily positioned within the output beam from the laser. The requirements for cooling will depend on the laser power output.

1. Means in or for a laser apparatus having a fixed plane of polarization of the beam, said means effecting substantially circular polarization of the plane polarized laser beam and operative in two or more stages.

2. Means in accordance with Claim 1, wherein

**WARNING** end of DESC field may overlap start of CLMS **.

Claims

**WARNING** start of CLMS field may overlap end of DESC **. SPECIFICATION Lasers This invention relatesto lasers and is more particularly concerned with lasers used for industrial purposes and having medium to high power outputs up to several kilowatts. Such lasers are used for precision cutting or other processing operations on metallic or non-metallicworkpieces. In metal cutting in particular, a variation in the quality of the cut is apparent according to the instantaneous angular relationship of the plane of polarization of the laser beam to the direction in which the cutting track is proceeding. When the plane of polarization is along the track a better quality is produced than when the plane is at an angle thereto. This becomes most evident when cutting a curved or circular profile. The aforementioned effect is noticeable with the use of lasers which produce a steady and defined direction of polarization in the beam. It is an object of this invention to overcome the disadvantages referred to and to provide a practi cally convenient arrangement whereby the quality of the cutting or processing operation is independent of the track direction of the beam. According to this invention there is provided in or for a laser apparatus having a fixed plane of polarization of the beam, means for affecting substantially circular polarization of the plane polarized laser beam and operative in two or more stages. It has been found that the resulting circular polarization produces a consistent quality in the workpiece irrespective of the processing direction, without any apparent compromise. The circular polarization of the beam is effected by resolving the plane polarized beam into two equal components and phase shifting one component 90" with respect to the other. The phase shift is effected by reflection of the beam from a surface having thereon an appropriately deposited surface layer or layers, wherein the thickness and nature define the phase shift between the two components. The two components are at right angles, one of the components containing the axis about which the reflecting surface has been rotated. According to the invention in the case of two operative phase shift stages one coated surface at which reflection occurs produces a phase shift, the beam then being reflected again from a second coated surface also producing a phase shift between the two components, giving a sum shift of 90". For a given coated surface the angle between the surface and the incident beam controls, the phase shift, and small phase shift changes can be made by slight adjustment ofthe angle of incidence. The total shift thus results in the required circular polarization. More than two reflecting surfaces may be used providing the overall total phase shift sums to 90". In practical terms the reflecting surface is turned about an axis through the plane of the surface by 45" from a position where the surface is normal to the incident laser beam, and then the polarization axis of the beam is made about 45" onto the reflecting surface. This then sets up the required condition for resolving the beam into two 90" components. The advantage deriving from the use of two reflecting surfaces, being a preferred embodiment, are that the output beams can be made parallel to and in the same direction as the input beam by virtue of equal and opposite angles of reflection at each reflecting surface and thus the beam is only displaced laterally from the original path. Using two reflecting surfaces to achieve the 90" shift in phase overall results in greater accuracy of shift due to better accuracy in manufacture and adjustment of the assembly can be used to optimise the shift produced. A single 90" phase shift would, of course, require a plane mirrorto restore the beam direction. This invention also provides a method of providing circular polarization of a laser beam having a fixed plane of polarization, in which the method beam is reflected from a first surface angularly positioned in relation to the beam to effect resolution of the beam into two components mutually at 90 , the surface having a coating thereon to produce a 45" phase shift between said components, the reflected beam being passed to a second surface to effect a further 450 phase shift. An embodiment according to the invention is shown by way of example in the accompanying drawings, wherein: Figure 1 shows a forward looking view of a means for effecting circular polarization of a linearly polarized laser beam, Figure 2 is a side view of the apparatus, and Figure 3 is a view looking along axis A of Figure 1. As shown in the drawings the means for affecting a 90" phase shift and producing circular polarization of the laser output beam 1 comprises two coated reflective plates 2 and 3, each producing right angular reflections of the input laser beam 4 and each effecting approximately a 45" phase shift. The plates are mounted within an aluminium block 5 having input and output ports 6 and 7 respectively. The reflective plates are cooled by a suitable coolant medium passing via tubes 8 through internal ducts. For an input laser beam polarised in the plane PP, rotation of the aluminium block about the axis AA on the mounting block 9 enables compensation for small manufacturing variations in phase shift of each reflective plate. The means is compact and easily positioned within the output beam from the laser. The requirements for cooling will depend on the laser power output. CLAIMS

2. Means in accordance with Claim 1, wherein the circular polarization of the beam is eftectsd by resolving the plane polarized beam into tVJO equal components and phase shifting one component 90" with respect to the other.

3. Means in accordance with Claim 3, wherein the phase shift is effected by reflection of the beam from a surface having thereon an appropriately deposited surface layer or layers, wherein the thickness and nature define the phase shift between the two components.

4. Means in accordance with Claim 3, wherein the two components are at right angles, one of the components containing the axle about which the reflecting surface has been rotated.

5. Means in accordance with Claim 3 or4, wherein the phase shift is effected through two reflecting surfaces.

6. Means in accordance with any one of claims 3 to 5, wherein the reflecting surface is turned about an axis through the plane of the surface by 45 from a position where the surface is normal to the incident laser beam, and then the polarization axis of the beam is made about 45 onto the reflecting surface.

7. Means in accordance with Claim 5, wherein the output beam is parallel to and in the same direction as the input beam by virtue of equal and opposite angles of reflection at each reflecting surface and the beam is displaced laterally from the original path.

8. Method of providing circular polarization of a laser beam having a fixed plane of polarization, in which method the beam is reflected from a first surface angularly positioned in relation to the beam to effect resolution of the beam into two components mutually at 90 , the surface having a coating thereon to produce a 45Q phase shift between said compo nents, the reflected beam being passed to a second surface to effect a further 45" phase shift.

9. Means for effecting a 90 phase shift and producing circular polarization of a laser output beam comprising two coated reflective plates and each producing right angular reflections of the input laser beam and each effecting approximately a 45" phase shift, the plates being mounted within a block having input and output ports, the reflective plates being cooled by a coolant medium passing via tubes through internal ducts in the block.

10. Means for producing circular polarization of a laser beam as described herein and exemplified with reference to the accompanying drawings.