Coherent light generators – Particular active media
Reexamination Certificate
1999-03-03
2003-06-10
Leung, Quyen (Department: 2828)
Coherent light generators
Particular active media
C372S096000
Reexamination Certificate
active
06577657
ABSTRACT:
The invention relates to a substrate for a solid-state laser based on organic and/or inorganic laser material and to a solid-state laser, in which the substrate consists of thermoplastic or thermoset and is structured on at least one surface, the structured surface of the substrate having a periodic surface profile.
BACKGROUND OF THE INVENTION
Solutions of fluorescent molecules in organic solvents have been used in dye lasers for many years. The fluorescent molecules used are distinguished by a high fluorescent efficiency, a wide tuning range, and the fact that they are very stable against oxidation and are easy to process. Dye lasers have become widely accepted and used worldwide. They are primarily employed when intense collimated monochromatic light is to be supplied with the possibility of tuning it in a wide wavelength range. Examples of dye lasers are described in Schäfer F. P. (ed.), Dye Lasers, 2nd Ed., Topics in Applied Physics, Vol. 1 (Springer, Berlin, Heidelberg, N.Y. 1978).
Disadvantages with dye lasers are that there is little possibility of scale reduction, they are expensive to produce, it is comparatively difficult to process the dye solutions used in the dye lasers and the problems with disposing of these solutions.
At the same time, construction and alignment of the optical resonator is intricate and requires experienced personnel. These disadvantages prevent the use of dye lasers on a mass-produced scale, as laser diodes made of inorganic material have in optoelectronic applications over recent years.
The discovery of electroluminescence in polymer layers (cf. e.g. U.S. Pat. No. 5,247,190) and molecular layers deposited using vacuum techniques (cf. e.g. Patents U.S. Pat. No. 4,720,432 and U.S. Pat. No. 4,539,507) and the work prompted by these have aroused great interest in organic light-emitting diodes intended to be used in the field of optoelectronics. The progress made in recent years with the development of organic light-emitting diodes is likely to yield a commercial application in the near future. The initial difficulties with rapid ageing of organic light-emitting diodes have been satisfactorily solved by optimizing the multilayer structure, refining the cathodes and improved encapsulation. One first broad field of application for organic light-emitting diodes will be the field of luminescent displays and background lighting panels.
The increase in efficiency achieved with organic light-emitting diodes has made it likely that organic laser diodes, which can be excited to emit light by applying an electric field, will be developed in the near future. Organic laser diodes differ from organic light-emitting diodes in that they involve amplified spontaneous emission. The consequence of this is that organic laser diodes can emit collimated polarized light. The emitted light furthermore has a narrow bandwidth compared with its wavelength, and the radiation density is higher than in organic light-emitting diodes.
In electrically or optically excited lasers, special substrates can be used to lower the lasing threshold and define the emission wavelength. With inorganic lasers (semiconductor lasers), it has been found particularly suitable to use substrates which form a DBR (Distributed Bragg Reflector) or a DFB (Distributed Feedback) laser. A factor common to both substrates is that they have a periodic structure, the period being of the order of the wavelength of visible light, i.e. about 200-2000 nm.
The periodic structure in DBR substrates consists of a multilayer system in which thin films having different refractive indices are applied alternately to a transparent plane-parallel substrate. This structure was first proposed by I. P. Katinov et al., Appt. Phys. Lett., 18, 497-499, (1971). With DBR substrates, the amplified light emission is perpendicular to the substrate surface. Such substrates are used to make VCSELs (Vertical Cavity Surface Emitting Lasers), as described in “Jewell et al.; Scientific American, November 1991”. In technical terms, DBRs are produced by successive vacuum deposition of individual inorganic metal oxide layers such as SiO
2
and TiO
2
. This process is elaborate and cost-intensive.
Conversely, DFB substrates have a periodically structured surface. In DFB lasers, the amplified light emission is parallel to the surface and perpendicular to the periodic structure, as described in Kogelnik, H. Shank, C. V. “Stimulated Emission in a Periodic Structure”, Appl. Phys. Lett. 18, 152-54 (1971). In technical terms, the periodic surface structure is formed by photolithographically etching inorganic substrates, so that an alternating sequence of ridges and furrows in formed in the substrate. This photolithographic etching process is elaborate and expensive.
There is therefore a requirement to develop a substrate for a solid-state laser, which can be produced straightforwardly and which, in particular, is suitable for use in lasers having organic laser material.
SUMMARY OF THE INVENTION
The object is achieved according to the invention by a substrate for a solid-state laser based on organic and/or inorganic laser material, which forms the subject-matter of the invention and is characterized in that the substrate consists of thermoplastic or thermoset and is structured on at least one surface, and in that the structured surface of the substrate has a periodic surface profile.
In particular, the substrate has periodically arranged ridges and troughs on its structured surface in at least one cross section through the substrate, the period being from 50 to 10,000 nm, preferably from 80 nm to 10,000 nm, particularly preferably from 80 nm to 5000 nm and quite particularly preferably between 100 nm and 5000 nm.
The substrate preferably has lateral periodicity in at least one direction in space, the number of periods being at least 5, preferably at least 10.
In a preferred embodiment, the depth of the surface profile is from 1 nm to 100 &mgr;m, preferably from 5 nm to 30 &mgr;m.
DETAILED DESCRIPTION
The profile of the periodic surface structure is arbitrary. Suitable examples are periodic geometrical profiles, in a cross section through the surface of the substrate, such as a sinusoidal, rectangular, trapezoidal or sawtoothed profile or a combination of these profile shapes. Pyramidal structures are also possible. Rectangular and trapezoidal profiles are particularly suitable.
The period of the ridges and troughs is not limited to a single period length (wavelength). Superposition or juxtaposition of up to 100 periodic profiles with different period lengths is also suitable. It is particularly suitable to superpose up to 10 periodic profiles with different period lengths.
The plastic for the substrate is preferably a plastic selected from the following list: polycarbonate, poly(methyl)acrylate, (meth)acrylate copolymers, polystyrene and styrene copolymer, poly-&agr;-methylstyrenes, acrylonitrile polymer, styrene/acrylonitrile copolymer, ABS, vinylpolymer, poly(cyclo)olefin, polysulphone, polyether sulphone, polyester, polyester carbonate, polyether carbonate, polyvinyl chloride and polyvinylcarbazole.
Further examples of suitable thermoplastics and/or thermosets are described in the “Encyclopaedia of Polymer Science and Engineering”, 2nd Edition, John Wiley & Sons, and in Hans Domininghaus, “Die Kunstoffe und ihre Eigenschaften” [Plastics and their Properties] 4th Edition 1992, VDI-Verlag GmbH, Düsseldorf. All transparent plastics can in general be used.
Suitable thermosets include, in particular, reactive resins in which a periodic surface profile can be formed by casting, compression moulding, injection moulding or reactive injection moulding and photopolymerization on at least one of the surfaces of the substrate. All the materials which can be processed using the methods described above are suitable as reactive resins, in particular those which exhibit little reduction in volume during the reaction.
The substrate can be produced using a variety of methods, in particular by injection moulding, hot press moulding, casting, compression
Bruder Friedrich-Karl
Elschner Andreas
Mahrt Rainer Friedbert
Scherf Ullrich
Wehrmann Rolf
Leung Quyen
Norris & McLaughlin & Marcus
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