infrared filters and coatings for the high

8/9/2019 Infrared Filters and Coatings for the High

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Applied Optics / Vol. 39, No. 28 / 1 October 2000

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Infrared filters and coatings for the High

Resolution Dynamics Limb Sounder (6-18m)

Gary J. Hawkins,Roger Hunneman, Richard Sherwood, and Barbara M. Barrett

The University of Reading, Infrared Multilayer Laboratory, Department of Cybernetics,

Whiteknights, Reading, RG6 6AY, England

We describe the spectral design and manufacture of the narrow-bandpass filters and 6-18mbroadband antireflection coatings for the 21-channel High Resolution Dynamics Limb Sounder. Amethod 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 apredicted system throughput response is presented. The design methods that are used to define thefilter and coating spectral requirements, choice of filter materials, multilayer designs, and depositiontechniques are discussed. 2000 Optical Society of AmericaOCIS Codes : 120.2440, 310.1210, 310.3840, 310.1860, 300.6170, 010.1290

1. IntroductionThe High Resolution Dynamics Limb Sounder 1 (HIRDLS) is a 21-Channel limb-viewing infrared radiometerdesigned for high-resolution monitoring of stratospheric and mesospheric temperature, trace chemical species,and geopotential height gradients in the Earths atmosphere. It is scheduled for launch on the NASA EarthObservation System Chemistry Mission satellite in 2002 and will provide data at higher resolution than has beenmeasured previously. The HIRDLS instrument is a descendent from earlier infrared limb-sounding instruments,namely, the Improved Stratospheric and Mesospheric Sounder 2-6 and the Pressure Modulator InfraredRadiometer 7-8 on the ill-fated Mars Observerand Mars Climate Orbiter. The HIRDLS uses 42 precise narrow-band interference filters to define the 21 channels in the focal plane array to simultaneously span a wavelengthrange between 6 and 18m. The filters are particularly notable for their demanding combination of small size,high mechanical precision, and positional accuracy of spectral placement.

2. Optical Layout and Spectral Design of the High Resolution Dynamics Limb SounderThe optical system layout for the HIRDLS instrument, as illustrated in Fig. 1, is that of an off-axis Gregoriantelescope 9 that focuses infrared radiation from the atmosphere onto the cooled focal plane detector assembly.The secondary ellipsoidal mirror (M2) focuses the reflected ray bundles from the scan optics onto the array ofwarm (301 K) band-defining filters (WF1-21) in f/7 illumination. Radiation transmitted through the filters is thendirected through the concave surface of an antireflected germanium lens (L1), from which the divergent raybundles fall onto a fold mirror (M4). Reflected rays from the fold mirror are then collected by the convex surfaceof a second antireflected germanium lens (L2) and focused through the antireflected zinc selenide (ZnSe)window of the detector package Dewar. The light then passes through the cold filter assembly (CF1-21) in f/1.5illumination and onto the array of cooled detectors. The detector assembly is an array of 21 separate cadmiummercury telluride infrared detectors, 10 optimized for maximum response at the selected measurement wavelengthand cooled by a Stirling cycle cooler to a temperature of approximately 65 K.

In each channel the spectral bandwidth and position are isolated by a warm interference bandpass filter,thermostatically controlled at 301 K, and placed at an intermediate focal plane of the instrument in a slowlyconverging illumination (~f/5.5-f/7). A second cold filter, spectrally positioned at the same wavelength butdesigned with a wider bandwidth, is placed at the primary focal plane of the instrument 100 m in front of eachcooled element in the detector assembly at 65 K. The cold filters, operating in a highly converging illuminationoff/1.5 at that position, are of a wider bandwidth to ensure that the warm filters unambiguously define thespectral band. There is a significant advantage in this arrangement as the crucial band-defining filters operate ina f /7 cone with minimal degradation of shape because of this less converging illumination. Because the coldfilters are of significantly wider bandwidth, they are much less affected by the f/1.5 illumination. Furthermore,the ratio of in-band to out-of-band signal is improved when two filters are used compared with the performanceof a single filter because this avoids multistack interactions, which can lead to unwanted transmission spikes inthe stopband. The cooled filter provides reduction of stray radiation (i.e., optical crosstalk) in the detector arrayof the instrument which can result from internally reflected out-of-band signals from a filter in one channel

returning as an in-band signal in another channel, the cooled filters also reduce the thermal background reaching
http://www.cyber.rdg.ac.uk/people/G.Hawkins.htmhttp://www.cyber.rdg.ac.uk/people/R.Hunneman.htmhttp://www.cyber.rdg.ac.uk/people/R.Sherwood.htmhttp://www.cyber.rdg.ac.uk/people/B.Barrett.htmhttp://www.cyber.rdg.ac.uk/irfilters/http://www.cyber.rdg.ac.uk/irfilters/http://www.cyber.rdg.ac.uk/irfilters/http://www.cyber.rdg.ac.uk/people/B.Barrett.htmhttp://www.cyber.rdg.ac.uk/people/R.Sherwood.htmhttp://www.cyber.rdg.ac.uk/people/R.Hunneman.htmhttp://www.cyber.rdg.ac.uk/people/G.Hawkins.htm


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the detectors. This combination of the two filters in each channel improves the out-of-band blocking because theinstrument blocking is distributed between multiple components. The instrument specification required out-of-band transmission levels of around 10-7; although this could be achieved with a single filter, because of theuncertainty caused by the coating microstructure or defects, we considered the advantages of distributedblocking as a preferred option. We also wanted to measure the out-of-band blocking regions of each filter, whichwe performed with measured transmission levels down to 104. The filters are of bandwidths ranging from 1 to

8% with a spectral placement tolerance of 0.1% in some channels.

3. Filter DesignWe achieved the spectral passband requirements for the HIRDLS filters 11 using traditional three-cavitybandpass designs with the high-index contrasts available from lead telluride (PbTe, n=5.5) in combination withthe II-VI compounds, ZnSe (n=2.35) or cadmium telluride (CdTe, n=2.67), or, in some cases, germaniummonoselenide (GeSe, n3.3) as layer materials. Figure 2 illustrates the dispersion profiles for the real part of thecomplex refractive index at temperatures of 300 and 50K12. By judicious use of the low-index II-VI compounds13 in combination with the L-layer requirements of the bandpass design, it was possible to fine tune the full widthat half maximum (FWHM) of the filter passband to ensure compliance with the specification. In other casesgermanium (n4.15 in layer form) was used as a substitute for selected PbTe layers; however with the indexcontrast being less, it lacks the sensitivity of an outcome that is obtainable with the other materials, even thoughit is still of occasional use. This approach to a narrow-band filter design is described fully by Jacobs 14; it has theadvantage that quarter-wavelength thickness layers are used throughout the reflector stacks, which, in our case,maintains the thickness monitoring regime and relative thickness accuracy of the layers.

Use of PbTe as the high-index layer material in the filters confers particular advantages both in the design and inuse of the filters. We have reported these advantages extensively elsewhere.15-18To summarize, these advantagesare the following: (i) Its high refractive-index value means that a minimum number of layers are required toperform a given spectral function. The high index also enhances the stop-band width and provides a higheffective index multilayer, reducing the size of the spectral shift caused by inclined illumination. (ii) A short-wavelength absorption edge at 3.5 m at room temperature removes the need for subsidiary blocking stacks tolink up with the germanium electronic absorption edge at 1.5 m. The long-wavelength movement of this edge to5.5 m at 65 K then further improves this advantage beyond that achievable with germanium-based multilayers.

Both of these factors have been utilized to realize the HIRDLS filters; indeed, the only filters that had to use theequivalent germanium-based multilayers were channel 21 warm and cold filters and the channel 20 cold filter.This was necessary because the bandpass positions of these particular filters were positioned at wavelengthsbelow the electronic absorption edge of PbTe at their operating temperatures.

A. Bandpass Cavity SelectionAs the squareness of the passband shape of a multicavity filter improves with the number of cavities (spacerlayers), the total number of layers and thickness of the multilayer also increases. In addition, the narrower thatthe filter is required to be, the larger the number of layers is required between the cavities to provide thenecessary reflectivity. When we reduced the filter bandwidth, transmission losses due to absorption rise sharply,together with an increased sensitivity of the passband profile to layer thickness errors. All these factors mitigateagainst use of four-cavity designs for these filters. Attempts to make such filters in the development phase of thisresearch resulted in filters with unacceptable variations in bandpass profiles - almost certainly a result ofreaching the limiting accuracy of our infrared optical thickness monitoring techniques.

Cavity type, high or low index, also has to be considered when filters are used in highly converging illumination.Use of PbTe with ZnSe yields a high effective index (n*) of 2.70 in the low- (L) index cavity case with ZnSe and3.6 (3.75 at 65 K) in the high- (H) index case with PbTe cavities. The f/1.5 cone at the cold (detector) primaryfocal plane in the HIRDLS suggested that it would be better to use H-spaced designs, at least for the narrowerchannels. Figure 3 shows the comparative shift in centre wavenumber (%) with angle of incidence for equivalent

L and H-spaced triple half-wave bandpass designs; this clearly shows the advantage of our using H-spaceddesigns.

However, as PbTe has a large temperature coefficient of refractive index, dn/dT, filters made with it as a cavitymaterial suffer with a large temperature coefficient (d/dT). If PbTe is used in combination with ZnSe, with the


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ZnSe being used for the cavity layers, the resulting filter will have a small temperature coefficient of centerwavelength shift because of the interaction of the larger negative temperature coefficient of expansion of thePbTe with the smaller positive temperature coefficient of the ZnSe, which is compounded with the layersensitivities in the filter. 16 In the other case of the H-spaced family, these coefficients do not tend to cancel out,and there is a large temperature coefficient for the filter. Typical values of the temperature coefficient for 2%bandwidth filters with first-order cavities over the range of 300-70 K are < -2.8 x 10 -5/C forL-spaced designs

and -10 x 10-5/C for the equivalent H-spaced designs. Importantly, the lower temperature coefficient of the L-spaced case permitted more tolerance in the operating temperature of both the optical bench and the detectors.

There were further, more practical, reasons, for our not choosing to use PbTe as a cavity material; PbTe exhibitsa thickness-dependent absorptive loss, which becomes increasingly apparent in thicknesses typical of cavitylayers. Also, the thicker, long-wavelength filters would need to have cavity thicknesses that are known to sufferwith the possibility of stress-induced mechanical failure, the magnitude of the stress in these layers being closelyrelated to thickness.

The multilayer designs describing the bandpass filter profiles for each of the warm and cold filters are shown inTable 1. In most cases, use of integral quarter-wave layer thicknesses satisfied the bandwidth requirements foreach filter, with the exception of channels 3, 4, 8, and 20 where optimized fractional layer thicknesses ofcombined short-wave- and long-wave pass edge filters were required to satisfy the wide bandwidth demanded forthe filter.

B. Filter and Optical Train InteractionAs a result of the effects caused by multiphonon absorption inherent in the bulk material properties of thetransmissive optics, it was necessary to adjust both the spectral position and the bandwidth of some of the filtersfrom the nominal requirements of the instrument-level specification to ensure that the combined spectralpositioning response of the instrument was achieved. These corrections were made to the filters in the long-wavelength group of channels (1-9) between 10 and 17.4m. Further adjustments were made to allow for theinherent wavelength shifts of the filters when they were illuminated in nonparallel illumination; the cold filtersoperating inf/1.5 being especially affected.

To quantify the size of adjustments required for the filter profiles to allow for these effects in each channel, anominally correct warm and cold bandpass design pair was introduced into an instrument spectral design model,from which the end-to-end channel profile and determination of the resultant channel placement and bandwidthwere calculated. From this analysis, a ratio between the warm filter profile and the instrument channel end-to-end profile was calculated, representing the compensation required for the warm filters to negate the effects ofabsorption. To this ratio, the spectral effects of thef/7 warm filter ray count distribution are included to producea final compensation factor that was then applied to the end-to-end channel passband placement specification toderive the warm filter spectral position requirement. Compensation ratios were also applied to the cold filterpassband to ensure correct spectral placement in the f/1.5 illumination. The need to apply these adjustments tothe passband positioning is illustrated in Fig. 4, which shows the spectral displacement (and changes in spectralprofile) caused by the differing illumination conditions between those of the measurement and those in theDewar for the cold filters. The example shown is for the narrowest HIRDLS band, channel 19, which shows themaximum effect. Figure 5, by comparison, shows the minimal effects on the narrower warm filter calculated for

this channel when illuminated in a f /7 ray count distribution. The final information from this processsubsequently allowed us to spectrally place the filters correctly or on occasion to change the filter design to onethat gave the correct bandwidth. The complete set of HIRDLS warm and cold filters at their respective operatingtemperatures and cone angle ray count distributions are illustrated in Figs. 6 and 7. Table 2 illustrates thespectral placement, range of filter bandwidths, and peak transmission values achieved in the manufacture of thewarm and cold filters.

C. Blocking DesignThe optical design of the instrument placed the following three requirements on filter blocking: (i) The need tosuppress ghost imaging, resulting in an out-of-band blocking requirement of T


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warm filters it is met by the product of the filter transmission and the optical system response (not including thedetector). (ii) The requirement that the cold-blocking coatings should reduce the thermal background seen by thedetectors; in general this requirement is met by the ghost image suppression. (iii) The instrument blockingrequires that a specified in-band to out-of-band radiance requirement be achieved. 19 This ratio is evaluated byour taking the blocking provided by the filters and antireflection coatings combined with the limb absorption andPlanck function temperature for the various heights in the atmosphere.

The blocking requirements are met by our using combinations of long-wave- and short-wave-pass edge filters inconjunction with the more localized blocking provided by the bandpass filter itself. Both the bandpass filter andthe blocking stacks are placed on the same substrate. The structure of the multilayer blocking stacks arerefinements of extracted Tschebysheff equiripple polynomials 20 and Herpin quarter-wave stacks. TheTschebysheff filter designs are characterized by alternate high- and low-index layers (PbTe and ZnSe) of varyingthicknesses that increase toward the center of the multilayer where they become of an equal nonquarter-wavethickness, followed by a symmetrical decrease toward the outer layers. Herpin quarter-wave stacks are ofsymmetrical thickness throughout the multilayer and, in addition to the Tschebysheff edge filter, provide goodshort-wavelength rejection. By our using overlapping combinations of these designs, the equivalent refractiveindex approximates to the original substrate refractive index at its outer surfaces, matching well to the Gesubstrate interface and providing a suitable equivalent index at the outer interface for the application of anantireflection multilayer of index nSub. The underlying Tschebysheff and Herpin PbTe and ZnSe blockingfilters are refined by optimization to reduce the ripple amplitude and enhance the in-band transmission.

Spectral blocking outside of the HIRDLS passband range of 6-18 m, is provided, in addition to that resultingfrom the multilayer action and material content of the filters, by a combination of the long wavelengthmultiphonon absorption from the material in the germanium lenses and zinc selenide dewar window togetherwith the short-wavelength roll-off of the antireflection coatings. These coatings, whose primary function is tomaximize in-band throughput over the wavelength range covered by the set of bandpass filters (6-18 m) alsobehave as cutoff filters to the short wave. They also have a lesser but still useful roll-off beyond 18 m as aresult of absorption in the layer materials. Account is also taken in the spectral design of the instrument of thedetector wavelength response. The long-wavelength roll-off of this response is dependent on the composition ofthe cadmium mercury telluride material used for the individual channel detectors. Details of how the spectralrequirements of the instrument are met by the design of the coatings by use of an integrated system performance

approach are described elsewhere.19

4. Filter Manufacture

A. Filter SubstratesThe warm filters were deposited onto 25.4 mm diameter x 0.875 mm thick germanium substrates; the cold filterswere deposited onto 16 mm diameter substrates. The individual thicknesses of the cold filters, by channel, spanthe range 409-524 m. The filters were then later subject to post deposition processing in which the motherpieces were diced into their finished rectangular shape, the warm filters being 7.60 mm x 3.40 mm and the coldfilters being 1.390 mm x 0.630 mm. An individual identification code was applied by excimer laser writing toeach diced filter. Both sets of filters used low-resistivity (5-40 cm) monocrystalline material to ensure hightransparency. We performed confirmatory spectral measurements prior to deposition and cutting.

With the need for the best image quality on the detectors, a unique thickness of the filter substrates was specifiedfor each spectral channel by the optical designers to compensate for chromatic and field curvature aberrations(optical path-length differences), defocusing the telescope subsystem over the wide spectral range of theHIRDLS channels. In addition, the specified thickness of the substrate was further adjusted to compensate forthe thickness (in equivalent-germanium thickness) of the multilayers themselves, which are of differingthicknesses according to spectral placement and the physical thickness of the deposited multilayers needed. Thisdesire for the best image quality also led directly to a tight tolerance on this overall equivalent-germaniumthickness figure, which in turn necessitated a tighter tolerance on the thickness of the manufactured uncoatedsubstrate of5 m with a thickness and parallelism tolerance of2 m. The equivalent-germanium thicknesswas calculated as the ideal thickness of the filter for its central passband wavelength on the assumption that thefilter is equivalent to a single thickness of germanium with no thin-film coatings. In practice, these measuresonly needed to be implemented in full for the cold filter set because of their operation in thef/1.5 cone.


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B. Filter Deposition TechniqueWe deposited the filters using vacuum deposition techniques in a Balzers 510 bell jar deposition plant fitted withtooling of our own in-house design. This tooling is unique in that the substrates are stationary and the thermalevaporation sources rotate below them. By use of this geometry, a stable platform for the reflective opticalmonitoring is ensured, together with good thermal control of the filter substrates and good layer thickness

uniformity. The temperature control of the filter substrates was further improved by our thermally clamping thesubstrates to the jig by using Pb washers, backing pieces, and disk springs. 15 Deposition temperatures of 185 Cwere used for the filters with PbTe and the II-IV compounds that used for the germanium and zinc sulphidefilters was 120 C. Deposition rates were in the range of 10 15 /s, and residual pressure in the chamber wasapproximately 4 x10-6 Torr. Oxygen was added to 5 x 10-5 Torr during the deposition of filters that used PbTe.

The HIRDLS filters have the thickness of the growing layers determined and controlled by in situ optical thicknessmonitoring. This method is particularly suited to the production of bandpass filters in which the layers are exactquarter-wavelength and multiple quarter-wavelength thicknesses. We carried out monitoring in a straightforwardmanner by terminating just past the layer reflection minima. By this method, the crucial parameter that determinesthe eventual spectral performance of the finished filter, the optical thickness of the layers, is the measured quantityduring deposition and the principal parameter used to control the process. Alternative methods such as quartz-crystal mass monitoring suffer from having several intermediate steps in the process, needing extensive calibrationsand a well-characterized deposition process. In the infrared there is also a severe problem of crystal loading andfailure because of the high thicknesses of the multilayers that are being deposited. In addition, the implementation ofquartz-crystal thickness monitoring in a plant with rotating evaporation sources has significant difficulties. Bycontrast, optical thickness monitoring is simple, robust, and reliable.

We deposited layers of nonquarter-wave thicknesses by using a separate shutter that was able to cover the filterswhile still allowing the deposition and optical monitoring to proceed on the central monitor piece. An algorithmwas developed relating reflectance to coating thickness, and this, through the agency of a spreadsheetcalculation, determines the shutter opening and closing reflectance levels for each fractional layer.

C. Quality AssuranceThe cut filters were demounted, dewaxed, and ultrasonic cleaned prior to microscopic visual inspection anddimensional measurement by calibrated image processing software to verify compliance with the specification.Both sets of filters were designed and manufactured to conform to military specification MIL-F-48616, withrepresentative testing confirming compliance to this specification. These tests included humidity (95% relativehumidity at 49 C for 24 h), solubility and cleanability, and adhesion testing of the mother pieces. Thermalcycling of the warm cut filters of 1 cycle from 211 to 344 K and cold cut filters of 30 cycles from 77 to 373 Kwere also required by the project. No failures occurred during this testing. In the case of the cut filters, theconditions experienced during the diamond sawing were actually more demanding than the formal testingrequired, particularly as some failures occurred early in the program when the tested mother pieces were sawed.This led to design changes in the coatings, reducing both the individual and the total layer thicknesses whereappropriate to reduce the stress levels and remove the problem.

The HIRDLS filters and coatings were spectrally measured with a Perkin-Elmer 2000 Optica Fourier-transform

infrared (FTIR) spectrometer. This instrument is a specially modified PE Spectrum 2000 FTIR that byincorporating design changes, has addressed the concerns about ordinate accuracy and interreflections causingdouble modulation, which has been commonly expressed by the optics industry. This instrument replaced aPE580 dispersive instrument because FTIR measurements offered the capability of direct measurement oftransmission down to below 10-4 together with speed and convenience of measurement data handling.

All warm cut filters were spectrally measured at 298 K (3 K) with anf-number illumination simulating that seenby the filter in the instrument (f-number range off /5.4 -f/7.7). Because of the small size of the cut cold filters,spectral measurements were performed on the uncut mother pieces at 65 K (2 K), with the f-number set at f/8.As it was not possible to set the FTIR spectrometer to f /1.5 as used in the HIRDLS instrument, we appliedcalculations to the measured values to correct for the effects of the f/1.5 illumination using the known ray countdistribution incident on the filter. Confirmatory spectral measurements were performed on representative cut

cold filters at 298 K (3 K) to verify that filter performance was not affected by the cutting process.Unfortunately it was not practical to provide spectral measurements of cut cold filters at their operating


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temperature (65 K) because of the length of time taken to perform the scans together with the impracticability ofour mounting such small samples in the cooler.

4. Antireflection CoatingsThe broadband antireflection coatings deposited on the ZnSe DW and Ge lenses were required to have the

highest and flattest possible transmission performance over the HIRDLS passband range of 6-18 m, whilesimultaneously reducing the reflectivity to less than 2.7% per surface, a level that was set by the suppression ofthe ghost images requirement. The minimum acceptable transmission for the coating alone on both surfaces,ignoring substrate absorption, required an in-band performance for each channel of >92% with a minimumaverage transmission over all the HIRDLS bands of 94%.

The antireflection coating developed to satisfy this requirement, illustrated in Fig. 8, comprised a singlemultilayer stack with 10 layers of alternating high and low refractive-index layers of PbTe and ZnSe. The stackis overlaid with a three-layer antireflection system comprising layers of ZnSe, BaF2, and an outermostmechanical protection layer of ZnSe. The materials were selected for their low absorption in film thickness atwavelengths throughout the 6-18 m instrument passband range. It is essential to use a low-index material at theoutside of such a wideband coating to obtain the highest and widest transmission zone.

The bulk properties of fluoride materials have been well documented 21 possessing refractive-index values lessthan 2; they provide good index matching across a wide wavelength range between the multilayer stack and theincident medium. Barium fluoride (BaF2, n1.35) was selected after we investigated thorium fluoride (ThF4, n1.4) and lead fluoride (PbF2, n 1.75). These three materials were investigated because we believed that theirabsorption out to 18 m would be low enough to be useful. ThF4 was found to have more absorption than BaF2beyond 12 m and in the samples tested it suffered with stress-induced mechanical failure. It also becameunacceptable to the project to use radioactive materials. Use of the remaining material, PbF 2, because of itshigher refractive index, led to an unacceptable reduction in passband width and a loss of average transmission;furthermore, in some cases, deposited layers containing free Pb caused excess absorption. BaF 2 was thusselected for use as the outer-layer material for the antireflection systems. The layer thickness of the original 13-layer multilayer stack was refined to provide an optimal flat spectral response. 22 These thicknesses are tabulatedin Table 3; the spectral transmittance and reflectance performance of the measured coatings are shown in Fig. 8.

A few weak absorption bands were observed in the antireflection coatings, most notably around the 6.4 mregion (1570 cm-1) between the spectral positions of channels 20 and 21. These bands are commonly seen invacuum-deposited fluorides and are associated with the uptake of water. They tend not to be present inmeasurements of coatings made directly on removal from the deposition plant, but gradually get stronger over afew weeks under ambient (uncontrolled) laboratory conditions. Both the ZnSe and the Ge antireflection coatingshave shown this behavior with the rate of change reducing with time and appearing to reach equilibrium. Thetransmission of the coating at channels 20 and 21 remains substantially unimpaired, the absorption bands notaffecting the amount, or lack of, reflection from the coatings.

6. Instrument Throughput Verification

A. Instrument Channel Passband ProfileAs a result of the radiance emitted by the atmosphere possessing a structure that varies rapidly with wavelength, themeasured emission profile is highly sensitive to both spectral positional accuracy and the shape of the filterpassband profile. The channel placement accuracy of the HIRDLS instrument required the filter edge positions to belocated typically within 2.5 cm-1 of the nominal spectral location. For certain channels, however, where there areunwanted spectral features adjacent to the band of interest, we obtained a spectral placement accuracy of1.0 cm-1.

Details of the specific requirements of each channel have been reported previously. 11 The spectral end-to-endthroughput of the complete channel required the shape of the energy grasp profile to meet two specific requirements:(i) the integrated throughput between the 0.2% and the 50% relative transmission points is required to contribute nomore than 30% of the integrated transmission between the 50% relative transmission points, and (ii) the width of thespectral interval between the 5% relative transmission points is not to exceed 1.6 times the passband width. Byapplying these criteria we can define both the shape and the edge steepness of the throughput profile. Figure 9shows an overlay of the measured spectral performance achieved for each of the contributing elements in channel 1,

together with the calculated throughput profile. Figure 10 illustrates the predicted instrument end-to-end passband


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throughput profiles calculated from the individual component measurements for each of the HIRDLS channels withcorrections for their appropriate temperatures and cone angle ray count distributions.

B. Instrument Channel Blocking PerformanceThe measured wideband spectral blocking performance of the components in the channel 1 example are illustrated

as overlaid spectra in Fig. 11.Although each individual component contributes only a fraction of the total out-of-band blocking, the system as a whole, depending as it does on the product of the individual transmissions, has muchlower out-of-band transmissions than required by the specification. There are a number of advantages to our usingdistributed blocking over a number of components in this way, because individual blocking levels at the 10-4 levelcan be and were verified by measurement. Also, this approach reduces sensitivity to possible minor coating defectsand greater margin for minor deviations in blocking.

In addition, at wavelengths below the 5 m region, the electronic absorption edge of the PbTe content of thecoatings dominates and is primarily responsible for reducing the transmission still further. The amount of PbTeused in the coatings is consistent with the minimum that is necessary to obtain the best in-band performancetogether with the necessary component level blocking; even this amount gives rise to the low levels of short-wave transmission observed.

7. ConclusionsThe design, manufacture, and testing of the HIRDLS filters and antireflection coatings have been described. Bycreating a spectral model of the instrument, we have obtained a predicted response for each from spectralmeasurements of the individual components. This response has then been used to verify the spectral throughputand radiometric performance of the instrument. The development and successful fabrication of these tightlyspecified filters and antireflection coatings have demanded the application of a unique combination of difficultand innovative techniques. The two sets of filters have now been delivered to the project and are in the processof being integrated into the focal plane arrays.

The authors thank colleagues at the University of Oxford Department of Atmospheric, Oceanic and PlanetaryPhysics and the Rutherford Appleton Laboratory Space Science Department in the United Kingdom; and at the

National Center for Atmospheric Research and the University of Colorado at Boulder in the United States fortheir collaboration. This research was funded by the Natural Environment Research Council whose support isgratefully acknowledged. Thanks also to Andrew Clark for his assistance with many of the filter depositions.

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14. C. Jacobs : "Dielectric square bandpass design",Applied Optics, 20, 1039-1042 (1981)

15. C. S. Evans, R. Hunneman, J. S. Seeley, A. Whatley, Filters for2 band of CO2: monitoring and

control of layer deposition Applied Optics, 15, 2736-2745 (1976)16. J. S. Seeley, R. Hunneman, A. Whatley, Temperature-invariant and other narrow-band filterscontaining PbTe, 4-40m Proc. SPIE Vol. 246, Contemporary infrared sensors and instruments, 83-94 (1980)

17. G. J. Hawkins, J. S. Seeley, R. Hunneman, Spectral characterization of cooled filters for remotesensing Proc. SPIE Vol. 915,Recent developments in infrared components and subsystems, 71-78.(1988)

18. J. S. Seeley, G. J. Hawkins, R. Hunneman, Performance model for cooled IR filters J Phys. D: Appl.Phys., 21, 71-74 (1988)

19. G. J. Hawkins, R. Hunneman, J. J. Barnett, J. G. Whitney, Spectral design and verification ofHIRDLS filter and antireflection coatings using an integrated system performance approach , Proc.SPIE 3437,Infrared Spaceborne Remote Sensing VI, 102-112, (1998)

20. J. S. Seeley, H. M. Liddell, T.C. Chen, Extraction of Tschebysheff design data for the lowpass

dielectric multilayer Optica Acta, Vol.20, No.8, 641-661 (1973)21. J.A. Savage : "Infrared optical materials and their antireflection coatings", Adam Hilger Ltd, ISBN 0-

85274-790-X, (1985)22. C. Cole, J.W. Bowen : Synthesis of broadband antireflection coatings for spaceflight infrared optics,

Proc. SPIE 2210,Space Instrumentation and Spacecraft Optics, 506-510 (1994).


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Figure 1. HIRDLS Instrument Optical Layout

Primary Diffraction Baffle (PDB)

WarmFilters (WF1-21)

301K

Ge Lens (L2)

Ge Lens(L1)

Paraboloid

ZnSeDewar Window (DW)

(M1)

ColdFilters(CF1-21)

65K

Fold Mirror(M4)Stop

First Field Stop

(M0)Mirror

Scan

Chopper

(ILS)Lyot Stop Ellipsoid

(M2)


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Figure 2. Temperature-dependent refractive index dispersion characteristics ofPbTe, Ge, CdTe, ZnSe, ZnS and BaF2 at 300 and 50K.

1.0

1.5

2.0

2.5

3.0

3.5

4.0

4.5

5.0

5.5

6.0

6.5

4 6 8 10 12 14 16 18 20 22 24 26 28 30Wavelength (m)

Refract

ive

Index

(n)

PbTe-300K

CdTe-300K

ZnSe-300K

BaF2-300K

Ge-300K

ZnS-300K

PbTe-80K

Ge-50K

CdTe-50K

ZnS-50K

ZnSe-50K


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Figure 3. Shift in centre wavenumber (%) with angle of incidence for equivalentL and H-spaced three cavity bandpass filter designs.

0.00.5

1.0

1.5

2.0

2.5

3.0

3.54.0

0.0 5.0 10.0 15.0 20.0 25.0 30.0 35.0 40.0 45.0

Angle of Incidence (Degrees)

Cen

tre

Wavenum

ber

Shift(%)

L-Spaced

H-Spaced

4.4%

2.75%

4.3%

2.66%

Figure 4. Channel 19 (7.10m) cold filter measurement inf7.9 illumination at 65K [A],and corrected measurement profile calculated for the highly convergingf/1.5 ray countdistribution [B].

0.00

0.100.20

0.30

0.40

0.50

0.60

0.70

0.80

0.901.00

136013701380139014001410142014301440Wavenumber (1/cm)

Transmittance

[B] [A]


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Figure 5. Channel 19 (7.10m) warm filter calculation in parallel illumination at300K [A], and corrected calculation for the slowly convergingf/7 ray count distribution [B].

0.00

0.100.20

0.30

0.40

0.50

0.60

0.70

0.80

0.90

1.00

139013951400140514101415142014251430Wavenumber (1/cm)

Transm

ittance

[B] [A]

Figure 6. Overlay of measured HIRDLS Warm (301K) band-definition filters (5.9-20m).

0.00

0.10

0.20

0.30

0.40

0.50

0.60

0.70

0.80

0.90

1.00

50060070080090010001100120013001400150016001700Wavenumber (1/cm)

Transmittance

12

34567

8910

111213

141516

17181920

21


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Figure 7. Overlay of measured HIRDLS Cold(65K) ghost-suppression filters (5.9-20m).

0.00

0.10

0.20

0.30

0.40

0.50

0.60

0.700.80

0.90

1.00

50060070080090010001100120013001400150016001700Wavenumber (1/cm)

Transmittance

12345

67

8

9

1011121314

1516

171819

2021

Figure 8. Measured ZnSe dewar window and Ge lens antireflection coatings (5-40m).

0.00

0.10

0.20

0.30

0.40

0.50

0.60

0.700.80

0.90

1.00

25050075010001250150017502000Wavenumber (1/cm)

Transmittance/Reflectance ZnSe

DewarWindow

ZnSeDewar

Window

Ge Lens

Ge Lens

Ge Lens


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Figure 9. Overlay of the measured passband profile for each element at the N2O Channel 1 wavelengthat 17.47m, together with the calculated instrument end-to-end throughput. (Correct foroperating temperature and illumination conditions).

0.00

0.100.20

0.30

0.40

0.50

0.60

0.70

0.80

0.90

1.00

500510520530540550560570580590600610620630640650

Wavenumber (1/cm)

Throug

hpu

t

Mirrors

Detector

Ge L2

ZnSe DW

Ge L1

Cold Filter

Warm Filter

Throughput

Figure 10. Predicted instrument end-to-end passband throughput profiles (5.9-20m).

0.00

0.100.20

0.30

0.40

0.50

0.60

0.70

0.80

0.90

1.00

50060070080090010001100120013001400150016001700Wavenumber (1/cm)

Throug

hpu

t

1

23

456

7

8910

11

12

1314

1516

17

18

1920

21


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Table 1. HIRDLS Warm and Cold Bandpass Filter Designsa

Channel Warm Filter Design (substrate air) Cold Filter Design1 S | LH 2L HLHLH 2L HLHLH 2L 1H 2L | 1.0 S | LM 2L HLH 4L HLH 2L M 2L | 1.02 S | LH 4L HLHLH 4L HLHLH 4L HLHL | 1.0 S | LM 2L HLMLH 2L HLMLH 2L M 2L | 1.03 S | ZH 2L HLHLH 2L HLHLH 2L HLHL | 1.0 1.0 | 0.70L 1.35(HL)6 1.4L 1.46H 1.63L | S |

0.21L 0.42105(HL)6

0.52L 0.45H 0.67L 0.60H0.59L 0.68H 0.71L 0.64H .62L 0.72H 0.73L0.64H 0.62L 0.74H 0.77L 0.59H 0.53L 0.87H1.33L | 1.0

4 S | LH 2L HLHLH 2L HLHLH 2L HLHL | 1.0 1.0 | 0.69L 1.34H 1.30L 1.38H 1.35L 1.3H1.32L 1.33H 1.33L 1.32H 1.32L 1.35H 1.39L1.40H 1.66L | S | 0.22L .39H .44(LH)7 0.29L0.15H 0.4L 0.57H 0.65L 0.68H 0.7(LH)3 0.71L0.72H 0.71L 0.69H 0.68L 0.68H 0.7L 0.64H1.43L | 1.0

5 S | LH 2L HZHLH 2L HLHZH 2L HLHL | 1.0 S | LM 2L HLH 4L HLH 4L M 2L | 1.06 S | LMLM 2L HLHLHLH 2L HLHLHLH 2L HLMLML | 1.0 S | LH 2L HLHLH 4L HLHLH 2L H 2L | 1.07 S | LH 6L HLHLH 6L HLHLH 6L HL | 1.0 S | LH 2L HLHLH 2L HLHLH 2L HLHL | 1.08 S | LH 2L HLHLH 2L HLHLH 2L HLHL | 1.0 1.0 | 0.69L 1.34H 1.34L 1.39H 1.32L 1.32H

1.34L 1.33H 1.32L 1.33H 1.34L 1.36H 1.38L1.44H 0.4L 0.15H 0.7Z | S | 0.21L 0.421(HL)5.52L 0.45H 0.67L 0.6H 0.59L 0.68H 0.71L0.64H 0.62L 0.72H 0.73L 0.64H 0.62L 0.74H0.77L 0.59H 0.53L 0.87H 1.33L | 1.0

9 S | LMLH 2L HLHLHLH 2L HLHLHLH 2L HLMLML | 1.0 S | LH 2L HLHLH 4L HLHLH 2L HLHL | 1.010 S | LMLH 2L HLHLHLH 2L HLHLHLH 2L HLMLML | 1.0 S | LH 2L HLHLH 2L HLHLH 4L HLHL | 1.011 S | LH 4L HLHLH 4L HLHLH 2L HLHL | 1.0 S | LH 2L HLHGH 2L HGHLH 2L HGHL | 1.012 S | LH 6L HLHLH 6L HLHLH 8L H 2L | 1.0 S | LH 2L HLHLH 4L HLHLH 2L HLHL | 1.013 S | LH 6L HLHLH 8L HLHLH 8L H 2L | 1.0 S | LH 4L HLHLH 4L HLHLH 4L HLHL | 1.014 S | LH 4L HLHLH 6L HLHLH 4L HLHL | 1.0 S | LH 4L HLHLH 4L HLHLH 4L HLHL | 1.015 S | LHGH 2L HLHLHGH 4L HLHLHGH 2L HGHGHL | 1.0 S | LH 2L HLHLH 2L HLHLH 2L HLHL | 1.016 S | LH 8L HLHLH 8L HLHLH 8L HLHL | 1.0 S | LH 2L HLHLH 4L HLHLH 2L HLHL | 1.017 S | LH 2L HLHLH 4L HLHLH 2L HLHL | 1.0 S | 2L GLH 4L HLH 4L HLH 4L HLGL | 1.0

18 S | LH 2L HLHLH 4L HLHLH 2L HLHL | 1.0 S | LH 2L HCHCH 2L HCHCH 2L HCHL | 1.019 S | LHGH 4L HLHLHLH 6L HLHLHLH 4L HGHLHL | 1.0 S | LH 6L HLHLH 6L HLHLH 6L HLHL | 1.020 S | ZM 2Z MZMZM 2Z MZMZM 4Z MZMZ | 1.0 1.0 | 1.37Z 0.72M 0.76Z 0.77M 0.70Z 0.74M

0.73Z 0.79M 0.76Z 0.78M 0.73Z 0.76M 0.73Z0.78M 0.75Z 0.78M 0.74Z 0.76M 0.71Z 0.74M0.72Z 0.77M 0.71Z 0.63M 0.43Z | S | 1.70Z1.31M 1.35Z 1.35M 1.33Z 1.26M 1.32Z 1.29M1.32Z 1.27M 1.31Z 1.28M 1.32Z 1.26M 1.33Z1.29M 1.30Z 1.27M 1.38Z 1.34M 1.25Z 1.30M0.71Z | 1.0

21 S | ZM 4Z MLMLMLM 6Z MLMLMLM 4Z M 2Z | 1.0 S | ZM 4Z MZMZM 4Z MZMZM 4Z M 2Z | 1.0

aMaterials: L, ZnSe (n 2.35); H, PbTe (n 5.5 at 300K, n 5.8 at 65K); M, Ge (n 4.1); Z, ZnS (n 2.15);

C, CdTe (n 2.65); G, GeSe (n 3.1); S, substrate (Ge, n 4.0); air, 1.0 where L, H, M, Z, C, and G arequarter-wave optical thickness at 0.


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Table 2. Measured Warm and Cold Filter Passbands

Cold Filters Warm FiltersChannel Target

SpeciesCentre(m)

FWHM(%)

Peak(T%)

Centre(m)

FWHM(%)

Peak(T%)

1 N2O 17.40 6.81 82.1 17.45 3.83 76.02 CO2 16.49 4.15 82.7 16.45 2.58 74.13 CO2 16.01 7.21 84.7 15.99 4.35 87.04 CO2 15.71 5.98 83.4 15.55 4.27 88.35 CO2 14.96 6.06 87.0 14.97 3.68 81.96 Aerosol 12.06 3.23 80.8 12.08 1.94 84.37 CFCl3 11.92 4.12 80.6 11.86 1.97 80.08 HNO3 11.32 9.73 88.4 11.34 4.46 89.69 CF2Cl2 10.80 2.98 79.2 10.82 1.77 85.710 O3 10.00 3.80 89.2 10.00 1.76 79.711 O3 9.73 4.83 90.1 9.72 2.95 93.212 O3 8.83 2.86 83.9 8.86 1.83 86.013 Aerosol 8.25 2.42 82.8 8.25 1.62 80.214 N2O5 8.05 3.92 83.3 8.03 2.22 92.115 N2O 7.87 4.05 79.9 7.89 1.75 83.916 ClONO2 7.78 3.01 79.6 7.76 1.61 83.117 CH4 7.44 6.26 87.9 7.43 2.93 87.818 H2O 7.09 4.52 85.4 7.09 3.32 80.319 Aerosol 7.11 1.77 79.7 7.10 1.05 80.520 H2O 6.86 12.7 89.4 6.76 7.27 86.621 NO2 6.24 5.76 92.6 6.22 2.97 90.8

Table 3. Antireflection Coating Designsa

Coating Filter DesignGe lens design at 0 = 4.15 m 1.0 | 0.267L 1.672B 2.677L 0.799H 0.769L 1.531H

0.492L 1.5H 0.464L 0.7H | S | 0.79H 0.431L 1.625H0.487L 1.438H 0.837L 0.742H 2.718L 1.556B 0.234L| 1.0

ZnSe DW design at 0 = 4.15 m 1.0 | 0.205L 1.545B 2.627L 0.784H 0.787L 1.471H0.584L 1.331H 0.863L 0.741H 1.311L 0.255H | S |0.251H 1.316L 0.731H 0.873L 1.311H 0.595L1.455H 0.793L 0.780H 2.632L 1.534B 0.209L | 1.0

aMaterials: L, ZnSe (n 2.35); H, PbTe (n 5.5); B, BaF2 (n 1.35) in thin-film form; and S, substrate (ZnSe, n 2.4 and Ge, n 4.0).

infrared filters and coatings for the high

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