J. S. Seeley

Far infrared filters for the GALILEO-Jupiter and other missions

Extrapolation of PbTe/II-VI multilayer interference-filter technique from 20 to beyond 40 microm is described and PbTe transparency reviewed; improvements below 20 microm are reported. A composite filter cutting on steeply at 40 microm is described that uses absorptive films of ZnS and As(2)S(3), thin quartz, and supplementary multilayer interference. Absorptive filters are described containing the II-VI compounds since these are found transparent at wavelengths shorter and longer than their reststrahl.

New algorithms for synthesized design of optical filters

Chebyshev optical-filter algorithms for low-cost microcomputers have been improved. An offset ripple is now used for better transmission/matching in low-pass stacks. A prototype for narrowband filters is now more general and nearer practicability.

Filters for ν 2 band of CO 2 : monitoring and control of layer deposition

Two types of multilayer interference filter are required for temperature sounding the earths atmosphere. In relation to the 668-cm(-1) nu(2) band of CO(2) these are narrowband (4 cm(-1) wide for the Q branch, 10 cm(-1) for the R branch) or isolation (~60 cm(-1) wide for the complete center of the band). Difficulty in manufacturing the filters for NIMBUS-SCR and -PMR spaceflight has indicated a need for improvement in the monitoring and control of the deposition of layers, and the paper describes a realization of this. The consequent effect on filter performance (utilizing a particular combination of layer materials) is described.

Optical And Semiconductor Properties Of Lead Telluride Coatings

The optical and semiconductor properties of lead telluride coatings are dependent on various factors contributing to its performance. In this paper, we will present the temperature dependent effects of single layer lead telluride coatings on the dispersion and absorption characteristics, absorption edge, and carrier concentration from 15 K to 436 K using both experimental and theoretical analysis.

Investigation of dielectric spaced resonant mesh filter designs for PMIRR

The fabrication of the filters for the two longest wave channels of the Pressure Modulator Infra-red Radiometer (PMIRR), launched in September 1992 on Mars Observer, was not possible using conventional multilayer dielectric techniques. Thus the extension of far-infrared/microwave mesh filter designs was investigated using a new dielectric spacing material. Test filters were designed and fabricated and, using their measured transmission spectra, a new model of the behaviour of resonant meshes in both air and dielectrics was developed. The low transmission of these filters in the end made them unsuitable for PMIRR but the model provides a useful design tool for future dielectric spaced filters providing a new, less absorbing dielectric can be found.

High-performance multilayer interference filters for the region 12-50μm

The realization of filters containing PbTe and various II-VI materials to an optimum performance is described. In the pass-band, transmission is optimum due to the use of the Tschebysheff equal-ripple design concept; in the stopping bands, performance is optimum due to PbTe photoconductive absorption (low-pass) and II-VI reststrahl absorption (high-pass). Two example filters are discussed in detail: a cut-on/cut-off filter for the 668 cm-1 v2 band of CO2 useful in space research and a long-wave blocking filter cutting on a 490 cm-1 useful in infrared grating spectrophotometers.

Spectral Characterization Of Cooled Filters For Remote Sensing

The improvements obtained on cooling atmospheric remote-sensing instruments for space flight applications has promoted research in the characterization of optical filters necessary for spectral selection. By modelling the effects of temperature on the dispersive spectrum of some constituent thin film materials, the cooled performance can be simulated and compared. Two actual filters are discussed for the 7 pm region, one a composite cut-on/cut-off design of 13% HBW and the other an integral narrowband design of 4% HBW.

Performance model for cooled IR filters

A low-temperature model is described for infrared multilayer filters containing PbTe (or other semiconductor) and ZnSe (or other II/VI). The model is based on dielectric dispersion with semiconductor carrier dispersion added. It predicts an improved performance on cooling such as would be useful to avoid erroneous signals from the optics in spaceflight radiometers. Agreement with measurement is obtained over the initial temperature range 70-400 K and wavelength range 2.5-20 mu m.

Temperature-Invariant And Other Narrow-Band IR Filters Containing PbTe, 4-20μm

PbTe is unique amongst coating materials because of its negative temperature coefficient of refractive index. In combination with a conventional II-VII) material it can confer immunity to temperature variation from at least -23oC to +110oC for a narrowband filter. It also has an absolutely large refractive index so that filters are attainable with minimal degradation when tilted, and a minimum number of layers overall particularly towards 20 µm. Excellent transmission can be obtained.

Increments at the Interface between Layers during Infra-red Filter Manufacture

The effects on accuracy of incremental change during filter manufacture is discussed, in the situation of an anti-reflected receptor surface and layers which are otherwise absentee. A formula is presented by means of which corrections can be applied to some layers as compensation for ‘growth’ increments occurring in other layers.