GB2181265A

GB2181265A - Reflector for electromagnetic radiation

Info

Publication number: GB2181265A
Application number: GB8524767A
Authority: GB
Inventors: Stanley Young
Original assignee: Ferranti PLC
Current assignee: Ferranti International PLC
Priority date: 1985-10-08
Filing date: 1985-10-08
Publication date: 1987-04-15
Also published as: GB8524767D0; GB2181265B

Abstract

A reflector is provided for focussing electromagnetic radiation from a source (11) on to a target (10). The reflector includes a first elliptical surface (12) having the source (11) and target (10) at its foci and extending over such an area that radiation emitted by the source (11) and incident upon the surface (12) is reflected directly on to the target (10). Second and third elliptical surfaces (21, 22 and 23, 24) are provided adjacent to each of the source (11) and the target (10) contiguous with the first surface (12) and arranged such that radiation emitted by the source (11) and incident upon the second or third surfaces (23, 24) adjacent to the source (11) is reflected on to the target (10) by successive further reflections from the first surface (12) at the points of intersection (B, B') of that surface (12) with the minor axis thereof and then from the second or third surfaces (21, 22) adjacent to the target (10). The reflector has particular application to optically pumped lasers- the source being constituted by e.g. a flash tube and the target by a rod of solid, laser-active medium. <IMAGE>

Description

SPECIFICATION Reflector for electromagnetic radiation This invention is concerned with a reflectorforelectromagnetic radiation and in particular to such a re flectorforconcentrating radiation emitted buy a source on to a n an adjacent target.

The particular application ofthe invention isto optically-pumped lasers, and it is with such application thatthefollowing description will be primarily concerned.

Optically-pumped lasers usually comprise a rodshaped body of a solid laser active medium and at least one flash tube able to emit intense pulses of lightwhich are concentrated on to the rod. The rod and the flash tube are arranged parallel to one an otheratthefoci of a tubular reflector of elliptical cross-section. The object is to concentrate as much as possible ofthe radiation emitted by the flash tube on to the laser rod. However, if a complete elliptical section is used there is an area of the reflector behind the flash tube from which radiation cannot be reflected on to the laser rod because it is cut-off by the body ofthe flash tube itself. Similarlythere is an area ofthe laser rod which radiation from the flash tube cannot reach because of the body of the laser rod itself.Hence some of the available energy from the flash tube is wasted.

It is an object ofthe present invention to provide a reflectorforelectromagneticradiation in which the problem of cut-off described above is significantly reduced.

According to the present invention there is provided a reflectorforfocussing electromagnetic radiation from a source on to a target, which includes a first elliptical surface having the source and the target located at its foci and extending over such an area that radiation emitted by the source and incident upon the first surface is reflected directly on to the target, and second and third elliptical surfaces loc atedadjacenttoeach of the source and the target contiguous with the first elliptical surface and arran ged such that radiation emitted by the source and in cident upon the second and third surfaces adjacent to the source is reflected on to the target by suc cessive further reflection from the first surface at the intersection ofthat surface and the minor axis thereof and from the second and third surfaces adja centtothetarget.

The invention will now be described with reference totheaccompanying drawings,inwhich:- Figure lisa schematic end view of a known reflectorforan optically-pumped laser; Figure2 is a similarviewofa reflector according to afirstembodimentoftheinvention; Figure3 is an enlarged view of part of the reflector of Figure 2; and Figure 4 is a view of a second embodiment.

Referring nowto Figure 1, this shows a schematic end view of an optically-pumped laser having an el ongated laser rod 10 and aflashtube 11 ofsimilar length arranged parallel to it. A reflector 12 com prises a tubular member having an elliptical cross section with the rod 10 and flash tube 11 located at the two foci 13 and 14ofthe ellipse. The major axis of the ellipse is shown intersecting the ellipse at points A and A', and the minor axis intersects the ellipse at B and B'. It is a property of an elliptical reflectorthat radiation emitted atonefocal point will be reflected on to the other focal point. In practice, however, the presence of the bodies ofthe laser rod and flash tube impose limitations on this.Considering Figure 1, light emitted by flash tube 11 towards the laser rod 10 in directions between the lines 15 and 15' will impinge directly on the laser rod without reflection.

Outside the fairly small angle defined by lines 15and 15' light is reflected from reflector 13, as shown,for example, by line 16. However, lines 17 and 17' indicatethe limitofthis reflection, as lightdirectedtowards the point Awill be reflected but will be intercepted and deviated by the body ofthe flash tube 11.

Hence light emitted within the cut-offangleashown will not be reflected directly towards the laser rod 10.

Similarly, an area ofthe laser rod within the cut-off angle ss will not be illuminated by direct reflection, as the body of the rod itself prevents this.

Depending upon the dimensions oftheflash tube 11 and laser rod 10, and ofthe reflector 12, the angles a and ss may be significant, and the efficiency ofthe laserwill therefore be lowerthan the theoretical maximum.

Figure 2 illustrates how the efficiency of the laser may be increased. Referring to Figure 2 it will be seen thatforthe most part the elliptical shape of the reflector 12 is retained. However within the cut-off angles a and p the shape is varied to provide at each end, two subsidiary elliptical surfaces 21 to 24 which together are symmetrical aboutthe majoraxisAA'.Attheend adjacent to the laser rod 10 each elliptical surface 21 or 22 is part of an ellipse having as its foci either point 13 and point B or point 13 and point B', as is shown in greater detail in Figure 3forone ofthe surfaces only.

Similarly at the end adjacent to the flash tube 11 each elliptical surface 23 or 24 is part of an ellipse having as its foci either points 14 and B or 14and B'. At each endthe subsidiary elliptical surfaces are contiguous with the main body of the reflector 12 joining it at points C, C' and DD' respectively, and meet on the line AA' to form a cusp.

The effect of this construction is that all light emitted by the flash tube 11 which strikes the reflector within the angle a is reflected on to the main body of the reflector at either point B or point B'. From here the light is reflected onto the laser rod from thesub- sidiarysurfaceswithin the angle ss. This is shown, for example, by the line 25. The only exception to this is that light from the flash tube 11 which strikes the point atwhich thetwo subsidiary elliptical surfaces join will not be reflected in this manner. Howeverthis area is very small and the resulting loss is slight.

An improvement in reflective area ofthe reflector of up to 25% may result, leading to an increase in laseroutputenergyofupto 20%.

It will be seen thatthe diameters ofthe laser rod 10 and flash tube 11 are important. If these are larger than shown in Figures 2 or 3 then there will still be a cut-off effect caused by the body of the flash tube or rod. Whilst some adjustment ofthe shape ofthesub- sidiary elliptical surfaces is possible, this will notal- ways solvethe problem. In such a situation afurther subsidiary reflecting surface is positioned between the two elliptical surfaces 21 and 22 or23 and 24. As shown in Figure 4 this further surface is arcuate having as its centre the centre of the laser rod orflash tube as the case may be.Thus an arcuate surface 41, extending over an angle y, is formed between the elliptical surfaces 21 and 22 and having the centre 13 of laser rod 10 as its centre. The angley is the cut-off angle resulting from the diameter of the laser rod 10.

Similarly, adjacentto the flash tube 11, an arcuate surface 42, having the centre 14 oftube 11 as its centre, extends over an angle 8 between the two elliptical surfaces 23 and 24.

The arcuate surface 42 reflects light incident upon it back to the centre oftheflash tube, increasing its apparent intensity. At the other end the arcuate surface 41 reflects back on to the laser rod 10 any light from the flash tube which passes directly through the laser rod on to the arcuate surface.

If, as is possible, the laser rod and the flash tube are of different diameters, then the arcuate portion of the reflector may be present at one end but omitted atthe other.

The effect of reflection from the points B and B', which are the points of intersection ofthe minor axis ofthe main elliptical reflector 12andthe reflector itself, may be improved by forming a small flat on the reflector atthose regions.

As already stated, both the laser rod 10 and the flash tube 11 are elongated bodies and not point sources. Hence the reflector is a tubula r mem ber hav- ing agenerallyelliptical cross-section. Thusthesub sidiarysurfaces 21 to 24 are elliptical in one plane only,whilstthe arcuate surfaces 41 and 42 are in fact portions of a cylindrical surface.

In use, the space within the relfector is usuallyfitted with a circulating fluid coolant two remove heat from theflash tube andthe laser rod. The removal of the cusp by arcuate surfaces 41 and 42 can be used to provide a flow path forthis coolant along the surfaces ofthe laser rod 10 and flash tube 11, respectively, where such surfaces would have been unacceptably close to the cusps.

Claims

1. A reflectorforfocussing electromagnetic radiation from a source on to a target, which includes a first elliptical surface having the source and the target located at its foci and extending oversuch an area that radiation emitted by the source and incident upon the first surface is reflected directly on to the target, and second and third elliptical surfaces located adjacent to each ofthe source and the target contiguous with the first elliptical surface and arranged such that radiation emitted by the source and incident upon the second and third surfaces adjacent to the source is reflected on to the target by suc cessive further reflection from the first surface at the intersection of that surface and the minor axis thereof and from the second and third surfaces adja centto the target.

2. Areflectorasclaimed in Claim 1 whichinclu- des, between the second and third surfaces adjacent to each ofthe source and the target, an arcuate reflecting surface having as its centre the centre ofthe source orthetarget as the case maybe.

3. A reflectorforfocussing electromagnetic radiation from an elongated source on to an elongated targetarranged parallel thereto, which includes a tubular member having a cross-section as claimed in either of Claims 1 or 2.

4. A reflector as claimed in Claim 3 in which the source and the target are the flash tube and the active medium of a laser.

5. A reflector as claimed in any one of Claims 1 to 4 in which the electromagnetic radiation is in thevisible partofthespectrum.

6. A reflectorforfocussing electromagnetic radiation substantially as herein described with referpence to the accompanying drawings.