High performance blocking filters for the region 1µm to 20µm

A set of filters based on the sequence of semiconductor edges is described which offers continuity of short-wave infrared blocking. The rejection throughout the stop region is greater than 103 for each filter and the transmission better than 70% through one octave with a square cutoff. The cutoff points are located at intervals of about two-thirds of an octave. Filters at 2.6 ,µm, 5.5 µm, and 12 µm which use a low-passing multilayer in combination with a semiconductor absorption edge are described in detail. The design of multilayers for optimum performance is discussed by analogy with the synthesis of electric circuit filters.

Interference filters and coatings for mid-infrared astronomy (8-30µm)

The VISIR instrument for the European Southern Observatory (ESO) Very Large Telescope (VLT) is a thermal-infrared imager and spectrometer currently being developed by the French Service d'Astrophysique of CEA Saclay, and Dutch NFRA ASTRON Dwingeloo consortium. This cryogenic instrument will employ precision infrared bandpass filters in the N-( =7.5-14µm) and Q-( =16-28µm) band mid-IR atmospheric windows to study interstellar and circumstellar environments crucial for star and planetary formation theories. As the filters in these mid-IR wavelength ranges are of interest to many astronomical cryogenic instruments, a worldwide astronomical filter consortium was set up with participation from 12 differing institutes, each requiring instrument specific filter operating environments and optical metrology. This paper describes the design and fabrication methods used to manufacture these astronomical consortium filters, including the rationale for the selection of multilayer coating designs, temperature-dependant optical properties of the filter materials and FTIR spectral measurements showing the changes in passband and blocking performance on cooling to <50K. We also describe the development of a 7-14µm broadband antireflection coating deposited on Ge lenses and KRS-5 grisms for cryogenic operation at 40K

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. (C) 2008 Optical Society of America.

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 (210K) 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. (C) 2008 Optical Society of America.

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

This paper describes the spectral design and manufacture of the narrow bandpass filters and 6-18µm broadband antireflection coatings for the 21-channel NASA EOS-AURA High Resolution Dynamics Limb Sounder (HIRDLS). 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 used to define the filter and coating spectral requirements, choice of filter materials, multilayer designs and deposition techniques are discussed.

Synthesis of infrared filters for use in spaceflight systems

Progress is reported in the development of a new synthesis method for the design of filters and coatings for use in spaceborne infrared optics. This method uses the Golden Section optimization routine to make a search, using designated dielectric thin film combinations, for the coating design which fulfills the required spectral requirements. The final design is that which uses the least number of layers for the given thin film materials in the starting design. This synthesis method has successfully been used to design broadband anti-reflection coatings on infrared substrates. The 6 micrometers to 18 micrometers anti-reflection coating for the germanium optics of the HIRDLS instrument, to be flown on the NASA EOS-Chem satellite, is given as an example. By correctly defining the target function to describe any specific type of filter in the optimization part of the method, this synthesis method may be used to design general filters for use in spaceborne infrared optics.

Design and fabrication of infrared filters for remote sounding instrumentation

Cooled infrared filters have been used in pressure modulation and filter radiometry to measure the dynamics, temperature distribution and concentrations of atmospheric elements in various satellite radiometers. Invariably such instruments use precision infrared bandpass filters and coatings for spectral selction, often operating at cryogenic temperatures. More recent developments in the use of spectrally-selective cooled detectors in focal plane arrays have simplified the optical layout and reduced the component count of radiometers but have placed additional demands on both the spectral and physical performance requirements of the filters. This paper describes and contrasts the more traditional radiometers using discrete detectors with those which use focal plane detector array technology, with particular emphasis on the function of the filters and coatings in the two cases. Additionally we discuss the spectral techniques and materials used to fabricate infrared coatings and filters for use in space optics, and give examples of their application in the fabrication of some demanding long wavelength dichroics and filters. We also discuss the effects of the space environment on the stability and durability of high performance infrared filters and materials exposed to low Earth orbit for 69 months on the NASA Long Duration Exposure Facility (LDEF).

Infrared filters for space-flight focal plane array applications

Infrared filters and coatings have been employed on many sensing radiometer instruments to measure the thermal emission profiles and concentrations of certian chemical constituents found in planetary atmospheres. The High Resolution Dynamics Limb Sounder ( HIRDLS) is an example of the most recent developments in limb-viewing radiometry by employing a cooled focal plane detector array to provide simultaneous multi-channel monitoring of emission from gas and aerosols over an altitude range between 8 - 70 km. The use of spectrally selective cooled detectors in focal plane arrays has simplified the optical layout of radiometers, greatly reducing the number of components in the optical train. this has inevitably led to increased demands for the enviromnetal durability of the focal plane filters because of the need to cut sub-millimeter sizes, whilst maintaining an optimal spectral performance. Additionally the remaining refractive optical elements require antireflection coatings which must cover the entire spectral range of the focal plane array channels, in this case 6 to 18µm, with a minimum of reflection and absorption. This paper describes the optical layout and spectral design requirements for filteriong in the HIRDLS instrument, and reports progress on the manufacturing and testing of the sub-millimetre sized cooled filters. We also report on the spectral and environmental performance of prototype wideband antireflection coatings which satisfy the requirements above.

High-performance infrared filters for the HIRDLS 21-channel focal plane detector array

The High Resolution Dynamics Limb Sounder is described, with particular reference to the atmospheric measurements to be made and the rationale behind the measurement strategy. The demands this strategy places on the filters to be used in the instrument and the designs to which this leads to are described. A second set of filters at an intermediate image plane to reduce "Ghost Imaging" is discussed together with their required spectral properties. A method of combining the spectral characteristics of the primary and secondary filters in each channel are combined together with the spectral response of the detectors and other optical elements to obtain the system spectral response weighted appropriately for the Planck function and atmospheric limb absorption. This method is used to demonstrate whether the out-of-band spectral blocking requirement for a channel is being met and an example calculation is demonstrated showing how the blocking is built up for a representative channel. Finally, the techniques used to produce filters of the necessary sub-millimetre sizes together with the testing methods and procedures used to assess the environmental durability and establish space flight quality are discussed.

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 18mm. For each of these channels the spectral bandwidth and position are isolated by an interference bandpass filter at 301K placed at an intermediate focal plane of the instrument. A second filter cooled to 65K 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, minimising the number of layers to improve the in-band 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.

Infrared filters and dichroics for the advanced along track scanning radiometer

The design and manufacture of the band-defining filters and their associated dichroic beam splitter for the 11- and the 12-µm infrared channels of the advanced along-track scanning radiometer are described. The filter requirements that have led to the choice of coating designs, coating materials, disposition of coatings, and effects of polarization are discussed. Overall spectral throughputs of the filter and dichroic interaction for the two channels are also presented.

Ultra narrow filters with good performance when tilted and cooled

Our latest research indicates that narrow bandpass filters of ~0.6% bandwidth (or any larger chosen width) and with good performance ar low temperature and in tilted and focused illumination can be realized by using multicavities and multimaterials.

Exposure to space radiation of high performance infrared multilayer filters and materials technology experiment

Infrared optical-multilayer filters and materials were exposed to the space environment of low Earth orbit on LDEF. This paper summarizes the effects of that environment on the physical and optical properties of the filters and materials flown.

Space exposure of infrared filters and materials on the NASA Long Duration Exposure Facility (LDEF)

Infrared optical-multilayer filters and materials were exposed to the space environment of low Earth for a period of nearly six years on the NASA Long Duration Exposure Facility (LDEF) mission. This report describes the effects of that environment on the physical and optical properties of filters and materials used in an experiment designed by the University of reading Infrared multilayer Laboratory. Results of the experiment comprise IR processed spectra both before (1983), and after (1990) exposure, in conjunction with unexposed control samples.