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Design and Manufacture of Infra-Red Bandpass Filter

S Wu

PhD Thesis - The University of Reading, Department of Cybernetics (1986)


The method of synthesis and analysis of Tschebysheff narrow bandpass filters is investigated in a general way. By combining the idea of the index simulation impractical (unusual high or low) index obtained from the Tschebysheff prototype filter is realised. The index simulation method considered is including of the Herpin tri-layer, Herrmann tri-layer, equivalent quarterwave stack and equivalent nested Herpin layers. The theory and algorithm for each type of simulation is tested by characteristic matrix representation. Its range of application and optical property is analysed. In this new approach bandpass filter with bandwidth from 10% to 20% can be designed easily, and the off-band blocking below 10-4 in transmission as also achieved. Design procedures and examples of this kind of narrow bandpass filter is given. A manufactured example of this type of filter is also included.

Other interested property of general thin-film multilayer is the relation between measured optical property and elements of the characteristic matrix in the inverse problem. The new derived relations is given after a general review of the thin-film multilayer theory. This has been applied to analyse the symmetrical dielectric multilayer.

In some Design Examples, more approximation of the index is made to reduce the number of layers. The deteriorated performance can be refined by the optimisation method. The overall feature of optimisation method used in thin-film multilayer design is discussed. Non-linear simplex method is implemented on the Apple II plus microcomputer.

A number of computer programs are described which are useful in the design work in the laboratory. All these programs are implemented on Apple II plus computer. A listing of the programs is given in the Appendix.

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