GB2180363A

GB2180363A - UV laser beam homogeniser comprising bent tubular waveguide

Info

Publication number: GB2180363A
Application number: GB08522723A
Authority: GB
Inventors: Wilbert Ridd George; Stephen Rolt
Original assignee: STC PLC
Current assignee: STC PLC
Priority date: 1985-09-13
Filing date: 1985-09-13
Publication date: 1987-03-25
Also published as: GB2180363B; GB8522723D0

Abstract

An homogeniser for a UV laser beam comprises a tubular waveguide (1) with a bend (3) serving to ensure homogenisation through multiple internal reflection. The tubular waveguide may be of glass internally coated with aluminium (2), which has a high reflectivity for short wavelength (<0.35 mu m) UV Laser radiation and forms a very thin oxide film which enhances specular reflectivity. <IMAGE>

Description

SPECIFICATION Beam Homogeniser This invention relates to a beam homogeniser particularly for homogenising short wavelength ( < 0.35,um) ultraviolet laser radiation.

According to the present invention there is provided a beam homogeniser comprising a tubular waveguide through which the beam is directed in use thereof, the waveguide including a bend serving to ensure homogenisation through multiple internal reflection.

Embodiments of the invention will now be described with reference to the accompanying drawing which shows a cross-section through an homogenising tubular waveguide.

The output of a UV laser is not uniform over its diameter. A known type of homogeniser comprising a bent silica rod waveguide, as described in G. B. Patent Application No.

2037000A for example, has a very high absorption for UV, it follows a logarithimic law, with possible inherent danger to the glass of the homogeniser. Whereas the absorption problem can be at least partly overcome by using very high purity grades of silica, this is an expensive solution, especially for large diameter waveguides.

When radiation is passed along a simple straight rod waveguide the emergent light tends to be banded. This is because the intensity of the beam from the source falls off rapidly on a square law basis. This effect may by decreased by introducing a diffuser at the source end. Bending the rod waveguide through 90" as in the above mentioned patent application breaks up the banded pattern. A tapered rod section near the output end serves to eliminate the penumbra effect.

We have found that a tubular waveguide may be employed for beam homogenisation but that simple glass tubing is not particularly appropriate. With a simple bent glass tube much light passes through the glass wall at the end and additionally some of the light entering the glass wall was trapped there by internal reflection. It was also observed that the number of bands increased as the tube exit was approached.

These problems are overcome if, however, the tube is of silver or aluminium, for example, or a glass or other material tube is internally silver or aluminium plated. Aluminium has the advantage that it retains high reflectivity (about 80%) for a wavelength of 0.3tom and forms a very thin oxide protective film which enhances specular reflectivity, whereas silver tends to form an absorbent tarnish film and also has poorer reflectivity around 0.32,us.

A particular embodiment of homogeniser may be formed by polishing a tubular glass waveguide 1 along its internal bore and coating it with an aluminium layer 2 applied for example by evaporation. The tube 1 is required to be bent through 90" at 3 to ensure that homogenisation takes place by way of multiple internal reflection. To ensure multiple reflection the ratio of homogeniser bend radius to tube internal diameter should be at least 4:1, a ratio of 2.4:1 can give a single reflection. In the case of an internally plated (coated) glass tube the coating is preferably applied following bending of the tube to avoid the possibility of damage to the coating caused by the bending operation. Whilst not illustrated as such the end of the tube may be tapered near its output end to eliminate the penumbra effect.

1. A beam homogeniser comprising a tubular waveguide through which the beam is directed in use thereof, the waveguide including a bend serving to ensure homogenisation through multiple internal reflection.

2. A beam homogeniser as claimed in claim 1 wherein the tube is of a substrate material which is coated internally with a material enhancing surface reflection therefrom.

3. A beam homogeniser as claimed in claim 2 wherein the substrate material is glass.

4. A beam homogeniser as claimed in claim 2 or claim 3 wherein the surface reflection enhancing material is aluminium or silver.

5. A beam homogeniser as claimed in claim 1 wherein the tube is of material which has suitable surface reflection properties.

6. A beam homogeniser as claimed in claim 5 wherein the tube is of aluminium or silver.

7. A beam homogeniser as claimed in any one of the preceding claims and for use with short wavelength, ultraviolet lasers.

8. A beam homogeniser as claimed in any one of the preceding claims wherein the ratio of the bend radius of the tubular waveguide to the internal diameter of the tubular waveguide is at least 4:1.

9. A UV laser beam homogeniser substantially as herein described with reference to the accompanying drawings.

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

Claims

**WARNING** start of CLMS field may overlap end of DESC **. SPECIFICATION Beam Homogeniser This invention relates to a beam homogeniser particularly for homogenising short wavelength ( < 0.35,um) ultraviolet laser radiation. According to the present invention there is provided a beam homogeniser comprising a tubular waveguide through which the beam is directed in use thereof, the waveguide including a bend serving to ensure homogenisation through multiple internal reflection. Embodiments of the invention will now be described with reference to the accompanying drawing which shows a cross-section through an homogenising tubular waveguide. The output of a UV laser is not uniform over its diameter. A known type of homogeniser comprising a bent silica rod waveguide, as described in G. B. Patent Application No. 2037000A for example, has a very high absorption for UV, it follows a logarithimic law, with possible inherent danger to the glass of the homogeniser. Whereas the absorption problem can be at least partly overcome by using very high purity grades of silica, this is an expensive solution, especially for large diameter waveguides. When radiation is passed along a simple straight rod waveguide the emergent light tends to be banded. This is because the intensity of the beam from the source falls off rapidly on a square law basis. This effect may by decreased by introducing a diffuser at the source end. Bending the rod waveguide through 90" as in the above mentioned patent application breaks up the banded pattern. A tapered rod section near the output end serves to eliminate the penumbra effect. We have found that a tubular waveguide may be employed for beam homogenisation but that simple glass tubing is not particularly appropriate. With a simple bent glass tube much light passes through the glass wall at the end and additionally some of the light entering the glass wall was trapped there by internal reflection. It was also observed that the number of bands increased as the tube exit was approached. These problems are overcome if, however, the tube is of silver or aluminium, for example, or a glass or other material tube is internally silver or aluminium plated. Aluminium has the advantage that it retains high reflectivity (about 80%) for a wavelength of 0.3tom and forms a very thin oxide protective film which enhances specular reflectivity, whereas silver tends to form an absorbent tarnish film and also has poorer reflectivity around 0.32,us. A particular embodiment of homogeniser may be formed by polishing a tubular glass waveguide 1 along its internal bore and coating it with an aluminium layer 2 applied for example by evaporation. The tube 1 is required to be bent through 90" at 3 to ensure that homogenisation takes place by way of multiple internal reflection. To ensure multiple reflection the ratio of homogeniser bend radius to tube internal diameter should be at least 4:1, a ratio of 2.4:1 can give a single reflection. In the case of an internally plated (coated) glass tube the coating is preferably applied following bending of the tube to avoid the possibility of damage to the coating caused by the bending operation. Whilst not illustrated as such the end of the tube may be tapered near its output end to eliminate the penumbra effect. CLAIMS