GB2251953A

GB2251953A - Reflectors

Info

Publication number: GB2251953A
Application number: GB8319467A
Authority: GB
Inventors: Jurgen Wisotzki
Original assignee: Individual
Current assignee: Individual
Priority date: 1982-08-10
Filing date: 1983-07-19
Publication date: 1992-07-22
Anticipated expiration: 2003-07-19
Also published as: GB2251953B; FR2671641A1

Abstract

A reflector made of copper, silver or gold, is formed by the surface of a single crystal. This surface can be polished. The surface can either be the (100)-orientation surface or, alternatively, the (111)-orientation surface of a copper single crystal, which has been etch-textured. In this condition, etch-pits 12 are exhibited by the crystallites forming the surface, each of these etch-pits individually forming a small concave mirror. The reflectors with etched surfaces scatter incident radiation and tolerate higher laser outputs than reflectors with polished surfaces. In order to protect comparatively large areas on satellites and rockets from laser beams, it is possible to position several single-crystal reflectors side-by-side, in contact with one another. The reflector surface can be plane or curved. <IMAGE>

Description

HIGH-PERFORMANCE MIRROR FOR LASER TECHNOLOGY The invention relates to high-performance mirrors for laser technology, made of copper, silver or gold.

Up to the present time, mirrors made of electrolytic copper, or of OFHC-copper, have been used in laser technology, these mirrors being conditioned to reflect the incident light with the greatest possible efficiency as a result of the polish which is imparted to their surfaces. The material forming these known mirrors lacks an ordered structure. The reflecting surface is accordingly intersected by grain boundaries, which contain impurities and give rise to absorption and light-scattering. Moreover, the grain boundaries form surfaces which can be attacked by oxidation. During the polishing process, they become partially smeared-out, and are covered by vapordeposited films.

The object on which the invention is based is to provide a more efficient mirror for laser technology, namely a mirror which is capable of tclerating more severe conditions, and, in order to achieve this object, it is proposed that the mirror be formed by the surface of a single crystal.

Of the single crystals which can be used, preference attaches to single crystals of copper, silver or gold, with a purity of 99.999%.

The following facts have been established in the course of various experiments with single-crystal mirrors of various diameters and shapes.

At the present time, lasers attain outputs of several gigawatt/cm2. The resulting laser beams have very small cross-sections, amounting, for example, to 0.6 x 0.4 mm. These high-energy, strongly focused beams destroy all known mirrors made of OFHC-copper and other metals. The reason why higher outputs can be obtained with single-crystal mirrors is seen to involve the interaction of several advantageous factors. In particular, it appears to be significant that the uniform structure of a single crystal, undisturbed by grain boundaries, results in a surface which can be polished to a smoother finish, and which therefore reflects better.The residual amount of radiation energy which is absorbed, rather than being reflected, is only small and can, moreover, be conducted away more rapidly, and be distributed over the surroundings, as a result of better thermal conductivity, for which the absence of grain boundaries in the single crystal is once again responsible.

The tests in which electrolytic copper mirrors and OFHC-copper mirrors were compared with single-crystal copper mirrors according to the invention showed an improvement in the reflection amounting to approximately liy..

Mirrors made of silver single crystals, or of gold single crystals, yielded an improvement in the reflection amounting to approximately 1.9%.

Since, in laser technology, the improvement in the reflection resulting from the coating of electrolytic copper mirrors and OFHC-copper mirrors amounts to only approximately 1/10%, the improvement in the reflection, obtained by utilizing the invention, is considerable.

In addition, the highly expensive coating process can be abandoned.

An additional advantage is the increased service life of the single-crystal mirrors, which results from their reduced susceptibility to oxidation.

It is also possible, by altering the crystal structure at the surface, to cause the incident light to be polarized, this alteration of the surface structure being accomplished through the selection of different orienta tons.

Overall, it has been found that, retaining the same polishing technique, improved reflection from the surface is obtained as a result of the single-crystal structure.

If it is desired to increase the efficiency of the mirrors still further, it would thus seem possible, from the experience gained in the development step described above, from the OFHC-copper mirror to the singlecrystal mirror, that it would really be appropriate to polish the reflecting surface even more highly, in order to achieve a further increase in the reflecting effect, and hence to minimize the absorption of radiation energy.

Surprisingly, it has been found, however, that measures of virtually the converse nature can also bring about the desired success. It emerged, in fact, that a mirror possessing a reflecting surface in the form of the (100)- orientation surface or the (111)-orientation surface of a copper single crystal, which had been etch-textured, exhibited the greatest capacity to withstand laser beams which has been attained to date.

To the eye, the etched surface appears rough, and anything but "mirror-bright". Up to the present, understanding the invention has been a matter of conjecture.

Among the effects which are believed to play a part, consideration should, once again, first be directed towards the conditions which prevail in the case of polished mirrors.

High-power C02 lasers destroy copper mirrors, due to the surface roughness which remains even after the polishing process. When an incident laser beam, in the gigawatt range, s reflected, a harmful part is played, above all, by the asperities which remain on the surface of the mirror after polishing. These polishing asperities possess the lowest thermal conductivity and, on being bombarded by high-power laser radiation, begin to evaporate away. The vaporized material, partly in the form of plasma, then deposits on the surface of the mirror, and this deposit dulls the polish, so that the reflection of the incident laser beam is reduced and instead the absorption is increased and, as a result of the latter, the material becomes unusable as a protective shield and, on being bombarded again by laser rays, is destroyed.

With regard to the effects of laser beams, the surface roughness produced by etching is not, apparently, comparable with the surface roughness which remains after polishing. Rather, a first effect of the etching process appears to manifest itself in the fact that polishing asperities are etched away.

In addition, it is suspected that an important pa-t is played by the enlargement of the reflecting surface which is brought about by the etching process.

The etched texture, produced in this way, of the (100)orientation surface of a copper single crystal exhibits etch-pits in all the cubic crystallites which form the surface, these etch-pits being virtually square, while the etched texture of the (111)-orientation surface exhibits etch-pits which are virtually triangular. The sum of the areas of the etch-pits is considerably greater than the area of the polished surface, prior to the etching treatment, and the radiation-energy density is correspondingly lower.

In addition, the etch-pits present themselves, to the incident laser light, as individual reflectors which in their plurality and irregularity, scatter the incident laser beam and deflect it in various directions, that is to say they disperse it.

Finally, significance could also attach to the fact that, as in the case of a concave mirror, the incident laser beam strikes only a small proportion of the reflecting area of the etch-pits precisely perpendicularly, and to the fact that the surfaces of the etch-pits may possibly possess a better reflectivity than a surface which has become furrowed in the course of mechanical polishing.

Only a very small proportion of the output of the laser devolves onto the reflector formed by a single etchpit. If the beam has a cross-section of 0.24 mm2, and if an etch-pit has, on average, a projected area of approximately 11.5 square Xngstrom, the beam strikes roughly 2 x 1012 etch-pits, or small reflectors. In the case of radiation at a power of, for example, 1 gigawatt/m2, approximately one millionth of a watt is directed on to each of the small reflectors which are formed by the etch-pits.This small proportion of the total power of the laser beam is essentially reflected by the surface of one etch-pit, and the remainder, which is minimal, can be conducted away without damage to the material, due to the fact that the copper single crystal has a high thermal conductivity, conduction taking place from the domed surface of the etch-pit, the area of which surface is, under certain circumstances, twice that of the projected area of the etch-pit.

The following picture emerged in the case of a test in which polished copper single-crystal mirrors were compared with copper single-crystal surfaces containing etchpits, these surfaces being oriented in the (100) or (111) direction.

A pulsed laser with a peak output of 1 gigawatts mm2 was used, the pulse duration being 40 nsec. After as few as 5 exposures to the laser beam, the polished copper mirror began to display signs of destruction of the reflecting surface, and these effects became more pronounced as the number of exposures to laser bombardment increased. The reflection decreased, from the value measured initially, as the destruction of the surface of the mirror increased.

The etched (100) and (111) copper single-crystal surfaces, bombarded by the same laser, exhibited no destruction effects of any kind. No extrinsic reduction in the reflectivity was noticeable and, moreover, this property could not even be measured, due to the scattering of the laser beam.

An illustrative embodiment of a high-performance mirror, according to the invention, is illustrated in the drawing, in which: Figure 1 shows a side view of a mirror, and Figure 2 shows, on an enlarged scale, a partial cross section in the region of the etched reflecting surface.

It is possible to produce copper single crystals, of large area, in shapes which are very thin, and which may be plane or domed, with the (100)-orientation surface or the (111)-orientation surface as the mirror surface.

The dimension D representing the thickness as drawn in Figure 1, can lie, for example, within the range from 0.1 to 10 mm. In cases involving still thinner mirrors, attention must be paid to ensuring that the heat can be conducted away adequately, taking account of the ambient temperature and of the adjoining parts, which may, for example be components which are covered with copper foil. The reflecting areas of several copper single crystals can be assembled together, side-by-side without joints, in order to present in their entirety a protective shield against laser bombardment. In this way, it is possible, for example, to provide effective protection for sensitive components and control elements on satellites and rockets, as well as to protect other devices which can be damaged by laser bombardment.

In the manufacture of a high-performance mirror according to Figure 1, care must be taken, first of all, to ensure that the reflecting surface, which is marked 10 on the drawing, is the (100)-orientation surface or the (111)-orientation surface of the copper single crystal.

This surface is expediently polished, in a first step, by the conventional method which is usual in the case of mirrors of this type. During this polishing process, the comparatively large surface irregularities are eliminated and a certain amount of work-hardening takes place in the material in the surface region. The surface 10 is then etched away, likewise by a conventional method, for example by means of nitric acid, so that the etched textures which are known for the abovementioned orientation surfaces of the single crystal are rendered visible. For example, Figure 2 shows, diagrammatically, the etch-pits in the indi visual cube-shaped crystal lites 14 forming the (100)-orien- tation surface, which forms the surface 10 of the mirror, these etch-pits being marked 12. As can be seen, the sum of the areas of the curved surfaces of the etch-pits 12 considerably exceeds the area of a smooth mirror-surface 10, and each individual etch-pit 12 forms a small reflector which reflects the incident portion of a laser beam in a defined, individual direction, irrespective of the directions in which the adjacent reflectors are pointing.

In addition to being suitable for use in laser optics, and for radiation sensors, mirrors of the type which has been described above are also suitable for use as shields for providing protection from laser beams.

Claims

PATENT CLAIMS

1. High-performance mirror for laser technology, made of copper, silver, or gold, which is formed by the surface of a single crystal.

2. Mirror as claimed in claim 1, wherein the reflecting surface is polished, by a technique which is known per se.

3. Mirror as claimed in claim 1, wherein the reflecting surface (10) is the (100)-orientation surface or the (111)-orientation surface of the single cyrstal, which has been etch-textured.

4. Mirror as claimed in claim 1 or 3, wherein the reflecting surface (10) has, overall, a curved shape, for example that of a concave mirror.

5. Mirror as claimed in claim 3 or 4, which is assembled from several etched single crystal surfaces.

6. Process for manufacturing a high-performance mirror as claimed in claim 3 or 4, wherein the (100)orientation surface (10) or the (111)-orientation surface (10) of a copper single crystal is etched in a manner such that the etched structure of the crystal is revealed, the surface (10) of the single crystal being either plane or curved.

7. Process as claimed in claim 6, wherein the surface (10) of the single crystal is polished prior to the etching treatment.

8. Process as claimed in claim 6 or 7, wherein the surface (10) of the single crystal ifs etched away by means of nitric acid.

Amendments to the claims have been filed as follows 1. A laser reflector for high energy laser beams, made of copper, silver or gold, which is formed by the surface of a single crystal.

2. The reflector of Claim 1, and being a mirror ref lector.

3. The reflector of Claim 1 or 2, wherein the surface has been polished by a mechanical technique.

4. The reflector of any one of the preceding Claims, wherein the surface has been etch textured.

5. The reflector of Claim 4, wherein the surface was polished prior to etching.

6. The reflector of Claim 4 or 5, wherein the single crystal is of copper and the surface was etched by nitric acid.

7. The reflector of any of the preceding Claims, wherein the surface is the (100) orientation surface of the single crystal.

8. The reflector.of any of Claims 1 to 6, wherein the surface is the (111) orientation surface of the single crystal.

9. The reflector of any of the preceding Claims, wherein the surface has overall a curved shape.

10. The reflector of Claim 9, wherein the surface is that of a concave mirror.

11. A laser reflector for high energy laser beams, assembled from several single crystal surfaces of the reflectors of any one of the preceding Claims.

12. A laser reflector for high energy laser beams, substantially as herein described with reference to, and as shown in, the accompanying drawing.

13. The reflector of any of the preceding Claims, when mounted in position so as to act as a protective shield.

14. The reflector of any of Claims 1 to 12, when mounted in position in a laser optical system or radiation sensing system.

15. The reflector of any of Claims 1 to 12, when used to reflect a high energy laser beam.

16. A process for manufacturing a laser reflector for high energy laser beams, substantially as herein described with reference to the accompanying drawing.