Plastic and nonmetallic article shaping or treating: processes – Optical article shaping or treating – Composite or multiple layer
Reexamination Certificate
2000-12-07
2003-06-17
Ball, Michael W. (Department: 1733)
Plastic and nonmetallic article shaping or treating: processes
Optical article shaping or treating
Composite or multiple layer
Reexamination Certificate
active
06579477
ABSTRACT:
DESCRIPTION
This invention relates to a method for fabricating optical components by replication using a matrix.
The invention also concerns the optical components obtained with this method.
These optical components are in particular diffractive optical components, such as light-diffraction networks which are used in optical devices in order to divert a light beam according to a preferential order of diffraction.
The diffraction components are in particular used in laser systems, in which the diffraction network can fulfil several functions such as deviation, chromatic separation and optionally focalisation, passing through a minimum amount of material; in this respect, reference may be made to French patent application n
o
96 11378 relating to a focalising diffraction network of very great efficiency.
FIG. 1
gives a cross-section diagram of a conventional diffraction network.
This network 1 has an etching profile formed of a succession of elementary etched patterns which are for example in crenate form.
The efficacy of said component relates essentially to the characteristics of the etching profile.
These characteristics are:
the depth h of etching of the material forming the network, this depth may be optimized in order to achieve maximum diffraction efficiency;
spacing a;
the step or period b;
the “filling rate” t, which is defined by the ratio
t
=
(
b
-
a
)
b
which is less than 1; the optimal depth to be given to etchings is also dependent upon this filling rate;
the “aspect ratio” r which is defined as the ratio
r
=
(
b
-
a
)
h
·
The greater the depth, the smaller the aspect ratio (r<1) but in this case the term profile with a high aspect ratio is used, since reasoning is made with reference to a unit profile width (b-a): for example, in the following, an aspect ratio of 1:1 is easy to replicate, while an aspect ratio of 1:5 is difficult to replicate.
Evidently, as a general rule an accurate replication method must be able to provide replicated objects or copies having the same characteristics as the master object or matrix.
This requirement has to be fulfilled for the fabrication by replication of optical components, and more particularly diffractive optical components.
Therefore, for a diffraction network, the method of fabricating such a network by replication must ensure exact reproduction of the copied etching profile, that is to say it must transpose with the highest fidelity the characteristics of the above-defined etching profile, namely, depth, spacing, period, filling rate, and aspect ratio.
The accuracy with which the characteristics of the copied profile are reproduced determines the optical properties, diffraction efficiency in particular, which must be similar to those of the master.
Also, the material used to produce the replica must have the required properties for the desired application, for example in a laser system.
The replication methods described in the literature may be classified into three main categories, namely hot embossing, injection moulding and casting.
The embossing method is a technology widely used in the hologram production industry, in particular as described in the document by Kluepfel et al., 1991, Holography Market Place, Berkeley, Calif. USA, Ross Books.
This technique is based on the principle consisting of pressing a thermoplastic material such as polyvinyl chloride or polycarbonate, that is heated, onto a planar carrier using a matrix.
This technique is particularly suitable for reproducing profiles having a depth of less than 1 &mgr;m, or deeper but with a low aspect ratio, that is to say r is greater than 1.
However, the document by Knope and Gale 1980, Surface-Relief Images for Colour Reproduction, London, Focal Press, describes the replication using a laboratory method of networks having an aspect ratio of 1/4.3.
The document by Becker et al., 1986, LIGA Process, Microelectronic Engineering 4, 35-36 describes a technology intended for the replication, by embossing, of microstructures having a high aspect profile.
The second major category of replication methods is the moulding category, which was mass developed for the production of Fresnel lenses as described in the document by Miller et al., 1951, JOSA, 41, 807-815, of condensers for overhead projectors, or audio or video compact disks as described in the document by Teyssier et al., 1990, Lasers & Optronics, December 1990, 50-53.
With this method it is possible, by injecting under pressure a softened plastic material such as a polycarbonate, polymethyl acrylate or other into a profiled mould, to accurately reproduce a microstructure.
Nonetheless, the industrial production of replicas of high resolution, that is to say much less than 1 &mgr;m, and of great depth, that is to say greater than 1 &mgr;m, has not been achieved simultaneously.
Finally, the casting replication method, as described for example in the document by Hutley 1982, Diffraction Gratings, 125-127, Academic Press, London, consists of the application of a material to the surface of a profiled matrix and its temperature-assisted forming (casting), or by photopolymerization as indicated in the documents by Coops, 1990, Philips J. Res, 44, 481-500 and by Shvartsman, 1993, SPIE Critical Review Proceedings, CR49, 117-137, SPIE, WA., USA.
This method particularly applies when high resolution profiles are needed and a high aspect ratio. However, cycle times are long.
Also, sol-gel, non-plastic, materials have been used to prepare diffractive optics or waveguides by the INM institute (Institut für neue Materialien) in Sarrebrück.
Therefore, the document by Krug et al., 1994, New J. Chem. 18, 1125-1134 describes experiments using embossing and UV photo-crosslinking of the hybrid material Zr(OR)
4
/CH
2
═CCH
3
COOH/(RO)
3
Si(CH
2
)
3
OCOC(CH
3
)═CH
2
to replicate transmission diffractive networks in the visible region or near-infrared.
None of the above-described processes meets simultaneously all the requirements mentioned above for a method to fabricate an optic component, such as a diffractive network, by replication, that is to say an accurate, exact reproduction of the etching profile defined by the above-mentioned characteristics and having optic and other properties required for the desired application such as transmission (clarity), plasticity . . .
In particular, none of the methods of the prior art provides a simple way to accurately replicate matrices of high density and with a high aspect profile.
Also, none of the methods of the prior art, other than the requirements already mentioned, is able to meet the additional constraints encountered in power lasers.
These lasers, whose future generation will for example have a power of 500 TW at 0.35 &mgr;m, require the use of large-size diffraction networks, for example 400 by 440 mm, having high efficiency and above all strong resistance to the laser flow in a region possibly ranging for example from the near-ultraviolet to the near-infrared.
A need therefore exists for a method to produce optical components by replication which simultaneously meets all the requirements and constraints described above.
A need particularly exists for a method providing a simple way to accurately replicate matrices having a high density and high aspect ratio.
A further need also exists for a method with which it is also possible to prepare optical components such as diffraction networks of large size.
Finally the need exists for a method with which it is possible to accurately replicate optical components, irrespective of the copy material and in particular with copy materials having all the optical and other (mechanical, thermal . . . ) properties required for the desired application and, among other properties, excellent resistance to laser flow.
Moreover, this method must be simple, reliable, easy to implement, and low cost.
The purpose of the invention is, among others, to meet these needs.
The purpose of the invention is also to provide a method for fabricating optical components by replication which does not have the disadvantages, defects and lim
Belleville Philippe
Gacoin Philippe
Kaladgew Sophie
Ball Michael W.
Commissariat a l'Energie Atomique
Hayes & Soloway P.C.
Musser Barbara J.
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