Roger Hunneman

The temperature-dependent spectral properties of filter substrate materials in the far-infrared (6–40 μm)

Abstract This paper presents the experimental results on the low temperature absorption and dispersion properties for a variety of frequently used infrared filter substrate materials. Index of refraction ( n ) and transmission spectra are presented for a range of temperatures 300–50 K for the Group IV materials silicon (Si) and germanium (Ge), and Group II–VI materials zinc selenide (ZnSe), zinc sulphide (ZnS) and cadmium telluride (CdTe).

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.

An ultra-wide passband (5–30 μm) filter for FTIR studies of Photosystem II

This paper reports on the design and manufacture of an ultra-wide 5-30 mm infrared edge filter for use in FTIR studies of the low frequency vibrational modes of metallo-proteins. We present details of the spectral design and manufacture of such a filter which meets the demanding bandwidth and transparency requirements of the application, and spectra that present the new data possible with such a filter. A design model of the filter and the materials used in its construction has been developed capable of accurately predicting spectral performance at both 300 K and at the reduced operating temperature at 200 K. This design model is based on the optical and semiconductor properties of a multilayer filter . . containing PbTe IV-VI layer material in combination with the dielectric dispersion of ZnSe II-VI deposited on a CdTe . . II-VI substrate together with the use of BaF II-VII as an antireflection layer. Comparisons between the computed 2 spectral performance of the model and spectral measurements from manufactured coatings over a wavelength range of 4-30 mm and temperature range 300-200 K are presented. Finally, we present the results of the FTIR measurements of Photosystem II showing the improvement in signal to noise ratio of the measurement due to using the filter, together with a light induced FTIR difference spectrum of Photosystem II. q 1998 Elsevier Science B.V. All rights reserved.

Infrared filters and coatings for the High Resolution Dynamics Limb Sounder (6–18 µm)

We describe the spectral design and manufacture of the narrow-bandpass filters and 6-18-microm broadband antireflection coatings for the 21-channel High Resolution Dynamics Limb Sounder. A method of combining the measured spectral characteristics of each filter and antireflection coating, together with the spectral response of the other optical elements in the instrument, to obtain a predicted system throughput response is presented. The design methods that are used to define the filter and coating spectral requirements, choice of filter materials, multilayer designs, and deposition techniques are discussed.

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.

Spectral design and verification of HIRDLS filters and antireflection coatings using an integrated system performance approach

The HIRDLS instrument contains 21 spectral channels spanning a wavelength range from 6 to 18 micrometer. For each of these channels the spectral bandwidth and position are isolated by an interference bandpass filter at 301 K placed at an intermediate focal plane of the instrument. A second filter cooled to 65 K positioned at the same wavelength but designed with a wider bandwidth is placed directly in front of each cooled detector element to reduce stray radiation from internally reflected in-band signals, and to improve the out- of band blocking. This paper describes the process of determining the spectral requirements for the two bandpass filters and the antireflection coatings used on the lenses and dewar window of the instrument. This process uses a system throughput performance approach taking the instrument spectral specification as a target. It takes into account the spectral characteristics of the transmissive optical materials, the relative spectral response of the detectors, thermal emission from the instrument, and the predicted atmospheric signal to determine the radiance profile for each channel. Using this design approach an optimal design for the filters can be achieved, minimizing the number of layers to improve the in- and transmission and to aid manufacture. The use of this design method also permits the instrument spectral performance to be verified using the measured response from manufactured components. The spectral calculations for an example channel are discussed, together with the spreadsheet calculation method. All the contributions made by the spectrally active components to the resulting instrument channel throughput are identified and presented.