Gary John Hawkins

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).

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.

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.

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.

Cooled infrared filters and dichroics for the James Webb Space Telescope Mid-Infrared Instrument

The cooled infrared filters and dichroic beam splitters manufactured for the Mid-Infrared Instrument are key optical components for the selection and isolation of wavelengths in the study of astrophysical properties of stars, galaxies, and other planetary objects. We describe the spectral design and manufacture of the precision cooled filter coatings for the spectrometer (7 K) and imager (9 K). Details of the design methods used to achieve the spectral requirements, selection of thin film materials, deposition technique, and testing are presented together with the optical layout of the instrument.

High-performance infrared narrow-bandpass filters for the Indian National Satellite System meteorological instrument (INSAT-3D)

This paper describes the design and manufacture of a set of precision cooled (210 K) narrow-bandpass filters for the infrared imager and sounder on the Indian Space Research Organisation (ISRO) INSAT-3D meteorological satellite. We discuss the basis for the choice of multilayer coating designs and materials for 21 differing filter channels, together with their temperature-dependence, thin film deposition technologies, substrate metrology, and environmental durability performance.

Mid-infrared filters for astronomical and remote sensing instrumentation

As improvements to the optical design of spectrometer and radiometer instruments evolve with advances in detector sensitivity, use of focal plane detector arrays and innovations in adaptive optics for large high altitude telescopes, interest in mid-infrared astronomy and remote sensing applications have been areas of progressive research in recent years. This research has promoted a number of developments in infrared coating performance, particularly by placing increased demands on the spectral imaging requirements of filters to precisely isolate radiation between discrete wavebands and improve photometric accuracy. The spectral design and construction of multilayer filters to accommodate these developments has subsequently been an area of challenging thin-film research, to achieve high spectral positioning accuracy, environmental durability and aging stability at cryogenic temperatures, whilst maximizing the far-infrared performance. In this paper we examine the design and fabrication of interference filters in instruments that utilize the mid-infrared N-band (6-15 μm) and Q-band (16-28 μm) atmospheric windows, together with a rationale for the selection of materials, deposition process, spectral measurements and assessment of environmental durability performance.

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.

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.