LU86107A1 - Maintaining cleanliness of gas swept optical element - e.g. laser lens, by neutralising triboelectric charges - Google Patents

Abstract

The cleanness of an optical assembly, comprising at least one element, such as a lens, cooled by a high speed circulating gas stream, is maintained by neutralising the electrical charge formed at the surface of the element due to friction of the gas stream. Neutralisation may be effected by pre-ionisation of the gas, e.g. by alpha radiation or high ltage discharge, before contact with the surface or by exposing the surface to alpha radiation.

Description

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ISSSÔFSi'Îr & ltrçlgïiifS ë · bi-38oi / em / eg

Belgium: 1) October 18, 1984 under No 6 / 48.018 2) October 22, 1984 under No 6 / 48.020

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Invention patent C 2321E / 8510.

METALLURGICAL RESEARCH CENTER -CENTRUM VOOR RESEARCH IN DE METALLURGIE,

Non-profit making association -Vereniging zonder winstoogmerk in BRUXELLES, (Belgium).

Method of cooling an optical assembly.

The present invention relates to a method of cooling an optical assembly comprising au. minus an element such as a transparent lens or blade. This process applies preferably, but not exclusively, to the cooling of lenses exposed to a high power energy beam, such as a laser beam.

It is known that, in certain applications, for example for the surface marking of rolling mill rolls in order to give them a controlled roughness, lasers with a power of up to several kilcwatts are used. In this case, optical assemblies are generally used, cons- /

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2.- lens components, to focus the laser beam to the desired dimensions.

The lenses in question are therefore subjected to the action of a powerful energy beam, which causes them to heat up. This overheating if it is significant, leads to a deformation of the lenses, which can give rise to various drawbacks, such as an irregularity of transmission of the beam, a dispersion of this beam or a loss of focus.

The present invention relates to a method making it possible to avoid these drawbacks.

The process which is the subject of the present invention, for cooling an optical assembly comprising at least one lens exposed to an energy beam, is essentially characterized in that a current of gas is circulated over at least one face of said lens.

According to a particular implementation, said gas stream is made to flow in a direction substantially perpendicular to the optical axis of said assembly.

• According to another particularly advantageous implementation of the method of the invention, there is, perpendicular to the optical axis of said assembly and at a short distance from at least one face of said lens, a transparent blade n ' not affecting the optical quality of the assembly, and circulating said gas stream between the lens and said transparent blade.

In the context of this implementation, the distance between said transparent strip and the face of the lens is advantageously less than 1 mm, and preferably between 0.01 mm and 0, 5 mm.

It has indeed appeared that the cooling efficiency increases when the said distance decreases, that is to say when the gas film becomes thinner.

Also according to the invention, the flow rate of the gas stream is advantageously between 75 and 125 l / min. per side to cool. This flow rate can vary, within the limits indicated, in. as a function of the dimension of said face as well as of the power of the energy beam in question.

According to the present invention, the gas stream can consist of any gas which does not disturb the energy beam and does not contain substances liable to contaminate the surfaces of the optical assembly. For this purpose, it is possible, for example, to use filtered and dry air. However, it has proved advantageous to use helium, because of its inert nature and its excellent thermal conductivity.

However, it can happen that, despite the almost total cleanliness of the cooling gas used, a deposit of dust appears on the surfaces which is not entirely removed by the gas stream.

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This deposit of dust seems to be due to the appearance of weak triboelectric charges caused by the friction of the stream of cooling gas.

To maintain the cleanliness of these surfaces cooled by a current of gas, by preventing the deposition and retention of dust on these surfaces, it is proposed * still within the framework of the present invention, to neutralize the electrical charge appearing II on the said surfaces.

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According to a first variant, the cooling gas can be ionized before bringing it into contact with the surfaces to be cooled.

This ionization can be carried out by any means known per se, in particular by subjecting the gas to a radiation or to aromas (high voltage).

It is thus possible to neutralize said electrical charges by means of the gas itself, the mechanical friction action of which makes them appear.

According to another variant, said electrical charges are compensated as and when they appear.

To this end, the surfaces to be kept clean are subjected to the direct action of an appropriate means, such as a radiation.

In this variant, the cooling gas does not undergo any ionization: the electric charges can appear on the surfaces, but they are instantly eliminated by the radiation acting on them.

The method of the invention makes it possible to cool and, if necessary, to keep the surfaces of the constituent elements of said optical assembly clean and to ensure, therefore, correct transmission of the energy beam.

The appended figure schematically illustrates a preferred embodiment of the method of the invention.

In an operation for treating a surface S, a laser beam 1 is focused on the surface by means of a lens 2. This lens has a diameter between 25 mm and 50 mm, for example 35 mm; it preferably consists of 'Γ J) D.

5.- * * t * âe zinc selenide or gallium arsenide. The laser beam 1 transmits a power of 2 kW, a fraction of which is absorbed by the lens 2. To prevent the latter from heating up and risking deformation, the lens is interposed, before and after. 2, two transparent blades 3 and 4, optically inactive, held by suitable supports. The distance between a transparent blade and the point closest to the corresponding face of the lens is equal to 0.20 mm and the minimum gas passage section is 5 mm2.

The cooling gas, which in this case is helium, is blown in through the channels 5,6 provided in the supports, with a total flow rate of approximately 200 l / min. Before its insufflation at 5.6, the gas passes through an ionization station 7 by a radiation a; it is put into circulation by means of a compressor 8 which, on the one hand recycles the gas returning by a return circuit 9 and on the other hand injects fresh gas supplied by a pipe 10, from a source not shown.

This arrangement makes it possible to cool and keep the surfaces of the lenses and the blades which face them clean, and therefore to ensure an impeccable transmission of the laser beam to the treated surface.

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Claims (7)

1. A method of cooling an optical assembly comprising% at least one element, such as a lens, exposed to an energy beam, such as a laser beam, characterized in that a current of gas is circulated over at least one side of said element. 2. Method according to claim 1, characterized in that said gas stream is made to flow in a direction substantially perpendicular to the optical axis of said assembly. 3. Method according to either of claims 1 and 2, characterized in that one has, perpendicular to 1 '. optical axis of said assembly and at a short distance from at least one face of said element, a transparent blade does not affect the optical quality of the assembly, and in that the gas current is circulated. element and said transparent blade. 4. Method according to claim 3, characterized in that "" the distance between said transparent blade and the face of the element is less than 1 mm, and is preferably between 0.01 mm and 0.5 mm. 5. Method according to either of claims 1 to 4, characterized in that the flow rate of the gas stream is between 75 1 / min. and 125 1 / min. by face of the element. 6. Method according to either of claims 1 to 5, characterized in that said gas is ionized before bringing it into contact with the face of said element. 7. Method according to one or other of claims 1 to 5, characterized in that the facp ^ 3u said element is subjected to a Jrayonnage a. <~ A '