GB2250376A - Cooling laser beam transmitting components - Google Patents

Abstract

Laser apparatus 1 produces a laser beam predetermined spatial laser beam intensity profile (eg annular) and corresponding local heating in the beam transmitting component 10, the transmitting component mounting means 12 being provided with thermal isolation means 13 preventing edge cooling of transmitting component (eg point or knife-edge contact) and gas jet cooling means 16 forming a spatially variable cooling effect (15a) across the component 10 (eg fig. 3) generally proportional to the predetermined spatial laser beam intensity profile. Apparatus 1 substantially minimises thermal gradients and consequential refractive distortion in laser beam radiation transmitting components 10 (eg output windows of zinc pelenide or gallium arsenide and focussing lenses). A window mounting means is detailed (Fig. 2) including 'O'-ring seals (33, 34) and low thermal mass/high thermal impedance tubular support (36) (eg stainless steel). <IMAGE>

Description

LASER WINDOW The present invention relates to a cooling system suitable for use in cooling radiation transmitting components such as output windows and focusing lenses in lasers, especially long wavelength, high-power lasers with annular laser beams utilising low pressure laser gas mixtures such as may be used for welding, heat treatment, and other applications.

Annular beams, i.e. beams with a substantially circularly symmetrical profile, most of whose energy lies within an annulus, often arise from the use of a laser cavity configuration commonly referred to as "diffraction coupled" or "unstable". These configurations are often well-suited to long wavelength high power gas lasers.

The present choice of output window materials that are capable of use in containing the low pressure laser gas mixture used in such high power lasers and that also allow the laser beam to pass through is limited and the more commonly used materials include semi-conductor materials such as zinc selenide and gallium arsenide.

Unfortunately such materials have high refractive indices and also high coefficients for variation of refractive index with variations in temperature.

Further, degradation of the laser beam quality due to variations in the refractive index of the window material resulting in turn from variations in temperature thereof can arise. Such effects are well known to users of CO2 lasers and are referred to as "thermal lensing" or "thermal blooming".

It is known practice that in order to minimise the effects of "thermal lensing" or "thermal blooming" in laser beam radiation transmitting components, it is desirable to prevent or minimise the thermal gradients in such components caused by the localised heating effects of laser beams passing therethrough. Known methods of cooling transmitting components include the use of a cooling gas jet directed onto the transmitting component; or providing a heat sink at the edges of the transmitting components. This latter will inevitably always result in thermal gradients whilst in the former case significant temperature gradients can still occur due to the 'blanket' cooling effect.Such temperature gradients and their attendant thermal lensing effects are particularly serious in the case of systems employing high power annular laser beams since in these cases the "thermal lens" so produced will not approximate to a simple spherical lens, as can be the case with a simple beam profile with a central maximum, but will more likely correspond to a complex refracting element which will introduce a significant degradation of the beam quality in terms of its ability to focus to a spot of minimum size using conventional optical components.

It is an object of the present invention to avoid or minimise one or more of the above disadvantages.

The present invention seeks to provide a laser apparatus having laser beam generating means and a laser beam radiation transmitting component, wherein, in use of said laser apparatus, there is produced a predetermined spatial laser beam intensity profile in said transmitting component, said apparatus having a transmitting component mounting means for mounting said component, said transmitting components mounting means having thermal isolation means formed and arranged for substantially preventing transmitting component edge cooling, and transmitting component cooling means for the removal of a substantial part of the heat generated by the passage of said laser beam through said component, said cooling means comprising gas jet cooling means formed and arranged so as to provide, in use, of the apparatus, a predetermined gas flow profile producing a spatially variable cooling effect across said component generally proportional to said predetermined spatial laser beam intensity profile, thereby substantially to minimise thermal gradients and consequential refractive distortion in said transmitting component.

In a preferred apparatus of the invention wherein is produced in use, an annular laser beam, said gas jet cooling means is formed and arranged so as to provide, in use, a predetermssined annular gas flow profile producing an annular shaped cooling effect across the component generally proportional to the annularly shaped laser beam intensity profile.

The laser beam radiation transmitting component includes at least one of a focusing lens and an output window and conveniently in use of the apparatus said output window includes partially reflecting output windows. It will be understood that in most circumstances said window is distinct and separate from said focusing lens but on occasion said lens can perform the function of the window and vice versa.

The heat flow to the edge of the component is determined by two factors, firstly the impedance to heat flow from the edge of the component to the main body of the laser system and secondly the difference between the temperatures at these positions. If, for example, in the absence of heat flow from the edge of the component, the temperature at the edge, as determined by the heat input and the cooling effect of the cooling gas (which itself is partly also determined by the gas temperature) happened to coincide with that of the main body, the effect of making contact at the edge would not introduce any additional heat flow and the temperature distribution would remain unchanged.It can be seen from this, therefore, that, in principle, heat flow from the edge can be minimised either by arranging for a large thermal impedance between the edge of the component and the main body of the laser or by carefully arranging the temperatures as described, possibly by adjusting the temperature of the cooling gas or of the main body of the laser in the relevant region.

In practice, there are limitations in the method of arranging for a minimum temperature difference. The main problem arises from the fact that the adjustment has to take account of the amount of heat being absorbed by the component and has to be varied as this varies.

This difficulty could, in principle, be overcome by varying the flow of cooling gas to correspond with the laser power at any instant but this would be a complex solution and it could be difficult to maintain the require cooling profile at different heat flows. The method of providing a large thermal impedance at the edge means that there is no need to provide a particular temperature or flow of the cooling gas.

It is important also to consider not only the static behaviour of the thermal gradients in the component but also the transient behaviour immediately after changing the heat input due switching on or changing the laser power level. It is not possible in this case, to eliminate the formation of some temperature gradients since those parts of the transmitting component exposed to higher intensities will initially heat up more rapidly. The extent of such transient gradients will however be less, the more effective the gas cooling.

The use of the method of minimising temperature differences, rather than using that of using a high thermal impedance, would also have an effect on transient behaviour. If the temperatures were set to give a zero heat flow for a finite laser power then at zero power, when the laser was off, there would be a heat flow into the component and a corresponding temperature gradient.

Yet another consideration is that in order to minimise the delay in the rise of temperature at the edge, not only should the thermal impedance at the edge be high but the thermal mass of material in contact with the edge, which also has to be heated up, should be a minimum and thus the use of point or knife-edge contacts is preferred.

The method by which a high thermal impedance at the edge is achieved will depend on how the function of the component influences different possible methods of mounting. If mounting against a metal surface is required, possibly to maintain precise alignment of the component, the area of contact with the metal can be minimised and the impedance to heat flow thereafter can be maximised by arranging for a relatively long heat path through a minimal cross section area of metal.

This can be achieved, in effect, by supporting the component in contact with one end of a long thin-walled metal tube. Such an arrangement will also achieve the objective of minimising the thermal mass of metal in contact with the component which, as explained, will help to minimise transient effect.

Where the component is also employed as a gas seal, some form of 'O'-ring seal will be in contact with it. This will normally be made of a rubber type of material which will have a relatively low thermal conductivity but, if necessary, a high impedance heat path from the other side of the seal can also be provided. The essential requirement is that the design should be such that the majority of the heat is removed by the cooling gas, rather than from the edge.

The gas jet cooling means may comprise a single shaped nozzle formed and arranged to direct a jet of cooling gas onto the surface of the transmitting component in, a predetermined pattern but preferably comprises a plurality of individual gas jet nozzles formed and arranged together to produce a more or less accurately defined cooling g flow pattern which impinges onto the transmitting component surface to provide a predetermined spatially variable cooling effect generally proportional to the said spatial laser beam intensity profile therein.

Advantageously the cooling nozzles may be formed and arranged so as to produce a swirling cooling effect, which further facilitates the removal of heat away from the window.

Preferably the arrangement of gas jet cooling nozzles is tailored more or less closely to the laser beam intensity profile produced by particular laser beam used in the apparatus. In practice it is not essential for the gas cooling effect to correspond precisely to all local or small scale variations in the laser beam intensity profile since the conductive properties of the transmitting component material largely smooth out any corresponding variations in the transmitting component temperature profile. It will be appreciated that the gas jet cooling means may be used on either and/or both sides of a laser beam transmitting component.

Conveniently the gas jet cooling means may use a compressed air supply provided with filtration and other cleaning equipment to remove any impurities therein and thereby preventing contamination of the transmitting component. Other gases such as inert gases may also be used in special applications.

In another aspect the present invention seeks to provide a method of substantially eliminating thermal "blooming" or "lensing" in laser beam transmitting components which method comprises the steps of thermally isolating said component so as to minimise substantially heat generated therein from dissipating out of the periphery thereof and providing gas jet cooling means selectively to cool the area(s) of the transmitting component heated by the laser beam travelling therethrough, at a rate corresponding generally to the rate of heating of the transmitting component by the laser beam thereat.

Further preferred features and advantages of the present invention will appear from the following detailed description given by way of example of some preferred embodiments illustrated with reference to the accompanying drawings in which: Fig. 1 is a schematic sectional side elevation of a laser apparatus of the invention; Fig. 2 is a detailed sectional side elevation view of a modified form of the window mounting means of the apparatus of Fig. 1; Fig. 3 is a schematic front elevation of the laser output window and cooling means of the apparatus of Fig. l;and Fig. 4 is a graphical representation of the relationship between the spatial temperature profile of the window of and the laser beam intensity.

Reference is first made to Fig. 1 which shows a schematic view of a first laser apparatus of the invention and is generally indicated by reference number 1. The laser apparatus 1 comprises a body 2 to contain a laser beam generating means 4. The laser generating means 4 includes a laser discharge region 3 in a low pressure lasing gas environment. The body 2 further contains a more or less fully reflecting, concave, mirror 5a at one end 3a and a similar but smaller diameter reflecting, convex, mirror 5b at the other end 3b thereof. Electrodes 7a, 7b on either side of the discharge zone 3 provide the excitation for the laser by providing a discharge in the lasing gas. The laser beam density and shape thereof is determined by the arrangement and shape of the mirrors 5a, 5b, and in this case is generally annular.A laser output window 10 is mounted in the laser body 2 by point contact mounting means 12 and the output window 10 is provided with cooling means 14 comprising a series of gas jet cooling nozzles 16 mounted 19 around the periphery 18 of the window 10. The nozzles 16 are provided with a compressed air supply 21.

Fig. 2 is a side elevation of the window mounting means 12 of a similar apparatus of the invention. The mounting means 12 comprises a male sleeve 24 attached around an aperture 5 in the laser body 2 by four bolts 26. The male sleeve's outer wall 24a is screwthreaded 27 and is formed and arranged for threaded interconnection with a female sleeve 28. The male sleeve 24 has a stepped annular abutment 30 formed and arranged for engaging one end 36a of a thin-walled tubular support 36. A laser output window 10 is mounted on the other end 36b of said tubular support 36. The female sleeve 28 is provided with an 'O'-ring seal 33 formed and arranged to engage with the laser body 2 radially outwardly of the male sleeve 24. The female sleeve 28 has a further 'O'-ring seal 34 to engage the outer surface lob of the laser output window 10 at a radially outer edge portion lOc thereof. As the female sleeve 28 is screwed onto the male sleeve 24, the tubular support 36 presses the laser window 10 onto the 'O'-ring seal 34 thereby sealing the lasing gas G inside the laser body 2. The minimal contact between the mounting means 12 and the laser window 10 and the use of 'O'-ring seals 33, 34 made of thermally insulating materials such as synthetic rubber reduces edge cooling between the laser window 10 and the surrounding laser body 2. Furthermore the tubular support 36 has a low thermal mass and high thermal impedance e.g. is made of stainless steel, to reduce edge cooling. Gas cooling jets 16 direct cooling gas onto the laser window 10.

Fig. 3 is a schematic front elevation of the laser output window 10 of Fig. 1 showing the annular form of the laser beam 6, and further details of the window mounting means 12. As may be seen in Fig. 3 the laser output window 10 is mounted in the laser body 2 via a circumferentially extending seal 22 made of thermally insulating material 13. The seal 22 is formed and arranged to provide a gas-tight joint between the output window 10 and the laser body 2 to retain lasing gases contained therein.A predetermined gas flow profile producing an annular cooling zone 15a (Shaded area) corresponding to the annular laser beam intensity profile, is provided by eight individually directionable gas jet cooling nozzles 16 mounted around the periphery 18 of the window 10 so as not to interfere with the laser beam 6 exiting therefrom and to direct cooling gas, in the desired annular pattern onto the window surface 20 selectively to cool those portions of the window 10 that have been heated by the laser beam passing therethrough.

The arrangement of the nozzles 16 Ls formed and arranged to produce a swirling effect of the cooling gas on the window surface 20 further to facilitate the removal of heat away from the heated portions of the window 10.

Fig. 4 shows graphically the temperature profile of the window 10 in Figs. 1 to 3 from the centre to the periphery 18. Curve T indicates the substantially flatter temperature profile across the window using selective gas jet cooling in accordance with the invention in combination with substantially reduced edge cooling. The curve T shows how the temperature is generally the same at the edge of the window as it is in the centre which helps to prevent thermal refraction and distortion. The curve L indicates the laser beam intensities, L, across the window, for the embodiment of Figs 1 to 3. Curve L shows the maximum laser beam intensity of the annular laser beam 6 in the embodiment of Figs. 1 to 3, is a position radially spaced from the centre of the window.

Claims (10)

1. A laser apparatus having laser beam generating means and a laser beam radiation transmitting component, wherein, in use of said laser apparatus, there is produced a predetermined spatial laser beam intensity profile in said transmitting component, said apparatus having a transmitting component mounting means for mounting said component, by said transmitting component mounting means having thermal isolation means formed and arranged for preventing transmitting component edge cooling, and transmitting component cooling means for the removal of heat generated by the passage of said laser beam through said component, said cooling means comprising gas jet cooling means, formed and arranged so as to provide, in use, of the apparatus, a predetermined gas flow profile producing a spatially variable cooling effect across said component generally proportional to said predetermined spatial laser beam intensity profile, thereby substantially to minimise thermal gradients and consequential refractive distortion in said transmitting component.

2. A laser apparatus as claimed in claim 1 wherein is produced in use, an annular laser beam, said gas jet cooling means being formed and arranged so as to provide, in use, a predetermined annular gas flow profile producing an annular shaped cooling effect across the component generally proportional to the annularly shaped laser beam intensity profile.

3. A laser apparatus as claimed in claim 1 or claim 2 wherein said thermal isolation means comprise a zone substantially free of thermally conducting material and formed and arranged to minimise thermal conduction between said transmitting component and its surrounding.

4. A laser apparatus as claimed in any one of claims 1 to 3 wherein said mounting means are formed and arranged for restricted area contact with the transmitting component so as to minimise edge cooling.

5. A laser apparatus as claimed in any one of claims 1 to 4 wherein said laser beam radiation transmitting component comprises at least one of a focusing lens, and an output window.

6. A laser apparatus as claimed in any one of claims 1 to 5 wherein said gas jet cooling means has a plurality of individual gas jet nozzles formed and arranged so as to provide said predetermined gas flow profile.

7. A method of substantially eliminating thermal "blooming" or "lensing" in laser beam transmitting components which method comprises the steps of thermally isolating said component so as to minimise substantially heat generated therein from dissipating out of the periphery thereof and providing gas jet cooling means selectively to cool the area(s) of the transmitting component heated by the laser beam travelling therethrough, at a rate corresponding generally to the rate of heating of the transmitting component by the laser beam thereat.

8. A method of substantially eliminating thermal "blooming" or lensing, as claimed in claim 7, in a laser beam transmitting component wherein is produced, in use, an annular laser beam intensity profile, in which method is provided gas jet cooling means formed and arranged to produce an annular shaped cooling effect proportional to said annular laser beam intensity profile.

9. A laser apparatus substantially as described hereinbefore with particular reference to Fits. 1 to 4.

10. A method of substantially eliminating thermal "blooming" or "lensing" in laser beam transmitting components substantially as described hereinbefore with particular reference to Figs. 1 to 4.