Barr & Stroud Limited

Brian C Monachan

Ewan M Waddell

James S Orr

Christopher J Kelly

Michael A Biagi

Richard T Corbett

Alexander J Hope

An optical filter comprises a multilayer coating adherent to a substrate, the coating comprising at least four superimposed layer collections. Each layer collection has ordered first, second and third layers of which the first and third layers are made of a first material and are 0.6 quarter wavelengths thick whereas the second layer is made of a second material and is 6.8 quarter wavelengths thick. One of these materials has a high refractive index in the range 1.9 to 2.5 whilst the other of these materials has a low refractive index in the range 1.3 to 1.7. The preferred high index material is zinc sulphide and the preferred low index material is thorium fluoride. The optical filter is generally transmissive except in a number of narrow waveband regions which include respective known laser wavelengths.

Optical filters with coatings transmissive in narrow waveband regions

Optical components (16), particularly germanium components, are provided with a coating on exposed surface (15) within a vacuum chamber (17) by production of a glow discharge plasma containing carbon and another depositable element from feedstock gases which are fed to the chamber (17) via mass-glow rate controllers (9, 10, 11). The mass flow rate of the feedstock gases is maintained substantially constant at predetermined levels during respective time intervals to provide a multilayer coating of predetermined characteristics. Typically the coating has at least one first layer which is amorphous hydrogenated germanium carbide, at least one second layer which is amorphous hydrogenated germanium and at least one third layer which is amorphous hydrogenated carbon, these layers being ordered such that each second layer is bounded on each side by a first layer and the third layer is bounded on one side by a first layer, the other side of the third layer forming the exposed surface of the coating.

Optical coating

A process for coating the surface of a material with a carbonaceous coating by vacuum deposition derives the carbon in substantially pure form from a hydrocarbon gas plasma, the surface to be coated being coupled to an A.C. power source operating at a frequency below 500 kHz at which the electrical impedance of the plasma is substantially non-varying during the deposition process. Because the plasma impedance is substantially constant the process is safe and reproducible and can be carried out without dynamic monitoring and adjustment procedures and there is no requirement for an impedance matching network between the A.C. power supply and the vacuum chamber. Frequencies in the range 300-400 kHz with applied voltages of the order of 1KV or greater have been found to give good results.

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Brian C Monachan