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My Publications

Temperature invariant infrared filter

T Jones, N Lawrence, G J Hawkins, R E Sherwood, K Djotni

British Patent No. GB 2530099 A (2016)

A temperature invariant narrow bandpass filter 5 to transmit mid-infrared radiation that may be used in a mid-infrared sensor for monitoring a species, which may be a component of a fluid or a solid material. The filter has a substrate 14, a first stack deposited on the substrate, a cavity 16 deposited on the first stack and a second stack deposited on the cavity. The first and second stacks are made from alternating high 10 and low 12 refractive index materials, and the cavity is a low refractive index material. By providing a high ratio of low refractive index material in the filter with respective to high refractive index material, the filter is configured so that wavelength transmission remains constant with varying temperature. The high refractive index material has a refractive index that decreases with temperature and the cavity is at least three full wavelengths or three half wavelengths of the transmission wavelength or the narrow band of transmission wavelengths.

UK Intellectual Property Office Patent Document

Cooled optical filters for Q-band infrared astronomy (15-40µm)

G J Hawkins, R E Sherwood, K Djotni, T M Threadgold

SPIE Astronomical Telescopes & Instrumentation, Advances in Optical and Mechanical Technologies for Telescopes and Instrumentation, Edinburgh, United Kingdom, 26 June – 1 July (2016)

With a growing interest in mid- and far-infrared astronomy using cooled imaging and spectrometer instruments in high-altitude observatories and spaceflight telescopes, it is becoming increasingly important to characterise and assess the spectral performance of cooled multilayer filters across the Q-band atmospheric window. This region contains spectral features emitted by many astrophysical phenomena and objects fundamental to circumstellar and planetary formation theories. However extending interference filtering to isolate radiation at progressively longer wavelengths and improve photometric accuracy is an area of ongoing and challenging thin-film research. We have successfully fabricated cooled bandpass and edge filters with high durability for operation across the 15-30µm Q-band region. In this paper we describe the rationale for selection of optical materials and properties of fabricated thin-film coatings for this region, together with FTIR spectral measurements and assessment of environmental durability.

DOI: 10.1117/12.2231669

Infrared optical coatings for the EarthCARE Multispectral Imager

G J Hawkins, D Woods, R E Sherwood, K Djotni

Applied Optics, Vol. 53, Issue 27, pp. 6983-6992 (2014)

The Earth Cloud, Aerosol and Radiation Explorer mission (EarthCARE) Multispectral Imager (MSI) is a radiometric instrument designed to provide the imaging of the atmospheric cloud cover and the cloud top surface temperature from a sun-synchronous low Earth orbit. The MSI forms part of a suite of four instruments destined to support the European Space Agency Living Planet mission on-board the EarthCARE satellite payload to be launched in 2016, whose synergy will be used to construct three-dimensional scenes, textures and temperatures of atmospheric clouds and aerosols. The MSI instrument contains seven channels: four solar channels to measure visible and short-wave infrared wavelengths, and three channels to measure infrared thermal emission. In this paper, we describe the optical layout of the infrared instrument channels, thin-film multilayer designs, the coating deposition method and the spectral system throughput for the bandpass interference filters, dichroic beam splitters, lenses and mirror coatings to discriminate wavelengths at 8.8, 10.8, & 12.0 µm. The rationale for the selection of thin-film materials, spectral measurement technique, and environmental testing performance are also presented.

DOI: 10.1364/AO.53.006983 | Download the full version

Cooled infrared filters and dichroics for the Sea and Land Surface Temperature Radiometer

G J Hawkins, R E Sherwood, K Djotni, P M Coppo, H Höhnemann, F Belli

Applied Optics, Vol. 52, Issue 10, pp. 2125-2135 (2013)

The Sea and Land Surface Temperature Radiometer (SLSTR) is a nine channel visible and infrared high precision radiometer designed to provide climate data of global sea and land surface temperatures. The SLSTR payload is destined to fly on the Ocean and Medium-Resolution Land Mission for the ESA/EU Global Monitoring for Environment and Security (GMES) Programme Sentinel-3 mission to measure the sea and land temperature and topography for near real-time environmental and atmospheric climate monitoring of the Earth. In this paper we describe the optical layout of infrared optics in the instrument, spectral thin-film multilayer design, and system channel throughput analysis for the combined interference filter and dichroic beamsplitter coatings to discriminate wavelengths at 3.74, 10.85 & 12.0 µm. The rationale for selection of thin-film materials, deposition technique, and environmental testing, inclusive of humidity, thermal cycling and ionizing radiation testing are also described.

DOI: 10.1364/AO.52.002125 | Citations by Google Scholar

Mid-infrared filters for astronomical and remote sensing instrumentation

G J Hawkins, R E Sherwood, K Djotni

Proc. SPIE 7101-42, SPIE Europe Optical Systems Design Conference, Advances in Optical Thin Films III, Glasgow – Invited Paper (2008)

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.

DOI: 10.1117/12.796635 | Citations by Google Scholar

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

G J Hawkins, R E Sherwood

Applied Optics, Vol. 47, No. 13, pp. C25-C34 (2008)

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.

DOI: 10.1364/AO.47.000C25 | Citations by Google Scholar

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

G J Hawkins, R E Sherwood, B M Barrett, M Wallace, H J B Orr, K Matthews, S Bisht

Applied Optics, Vol. 47, No. 14 (2008)

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.

DOI: 10.1364/AO.47.002346 | Citations by Google Scholar

Cooled Infrared Dichroic Beamsplitters and Filters for the MIRI Spectrometer and Imager (5-29µm)

G J Hawkins, R E Sherwood, B M Barrett

Proc. OSA Technical Digest Optical Interference Coatings, WDPDP1, ISBN: 1-55752-841-1 (2007)

The spectral design and fabrication of cooled (7K) mid-infrared dichroic beamsplitters and bandpass filter coatings for the MIRI spectrometer and imager are described. Design methods to achieve the spectral performance and coating materials are discussed.

DOI: 10.1364/OIC.2007.WDPDP1

Mid-Infrared Filters for the Diviner Radiometer Experiment on the Lunar Reconnaissance Orbiter (7-40µm)

G J Hawkins, R E Sherwood, B M Barrett, S J Wakeham

Proc. OSA Technical Digest Optical Interference Coatings, TuDPDP1, ISBN: 1-55752-841-1 (2007)

The spectral design and manufacture of precision mid-infrared bandpass filters for the JPL Diviner experiment focal plane array is described. Design methods to achieve the spectral requirements, selection of materials and physical construction are discussed.

DOI: 10.1364/OIC.2007.TuDPDP1 | Citations by Google Scholar

Interference Filters and Coatings for Mid-Infrared Astronomy (8-30µm)

G J Hawkins, R Hunneman, R E Sherwood, B M Barrett

Proc. SPIE Astronomical Telescopes and Instrumentation, Specialized Optical Developments in Astronomy, 4842-06, pp 43-55 (2002)

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.

DOI: 10.1117/12.458056 | Citations by Google Scholar

Infrared Filters and Coatings for the High Resolution Dynamics Limb Sounder (6-18µm)

G J Hawkins, R Hunneman, R E Sherwood, B M Barrett

Applied Optics, Vol 39, No 28, pp 5221-5230 (2000)

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.

DOI: 10.1364/AO.39.005221 | Citations by Google Scholar

Achieving Accurate FTIR Measurements On High Performance Bandpass Filters

R Hunneman, R E Sherwood, C Deeley, R Spragg

CP430, Pages 435-438, ICOFTS-11, the Eleventh International Conference on Fourier Transform Spectroscopy, USA (1997)

The optical industry has been slow to adopt Fourier Transform instrumentation for critical transmission measurements because of concerns about the ordinate accuracy. In this poster we look at the problems in characterizing a set of bandpass filters required for a satellite-borne instrument. Particular emphasis is placed on the measurement of weak features in the blocking regions of bandpass filters. The procedures we have adopted to identify any residual effects due to double modulation or nonlinearities are reported. Out of band features can be reliably measured at levels well below 0.01%T.

DOI: 10.1063/1.55774 | Citations by Google Scholar