high-performance infrared narrow-bandpass filters for the

High-performance infrared narrow-bandpass filters forthe Indian National Satellite System meteorological

instrument (INSAT-3D)

G. J. Hawkins,1,* R. E. Sherwood,1 B. M. Barrett,1 M. Wallace,2

H. J. B. Orr,2 K. Matthews,3 and S. Bisht4

1The University of Reading, Infrared Multilayer Laboratory, School of Systems Engineering,Whiteknights, Reading, Berkshire, RG6 6AY, England, United Kingdom

2NDC Infrared Engineering Limited, Bates Road, Maldon, Essex, CM9 5FA, England, United Kingdom3Crystran Limited, 1 Broom Road Business Park, Poole, Dorset, BH12 4PA, England, United Kingdom

4Indian Space Research Organisation, Space Applications Centre, Department of Space,Government of India, Ahmedabad, 380015, Gujarat, India

*Corresponding author: [email protected]

Received 1 February 2008; accepted 4 March 2008;posted 20 March 2008 (Doc. ID 92345); published 28 April 2008

This paper describes the design and manufacture of a set of precision cooled (210K) narrow-bandpassfilters for the infrared imager and sounder on the Indian Space Research Organisation (ISRO) INSAT-3Dmeteorological satellite. We discuss the basis for the choice of multilayer coating designs and materialsfor 21 differing filter channels, together with their temperature-dependence, thin film deposition tech-nologies, substrate metrology, and environmental durability performance. © 2008 Optical Society ofAmerica

OCIS codes: 120.2440, 350.6090, 350.2460, 310.3840, 310.1860, 310.6188.

1. Introduction

The INSAT-3D instrument is an advanced infraredgeostationarymeteorological satellite [1] being devel-oped by the Indian Space Research Organisation(ISRO) Space Applications Centre (SAC) for high re-solution monitoring of temperature and trace chemi-cal species in the atmospheric regions between thetroposphere and stratosphere. INSAT-3D is one ofthree satellites under development by ISRO exclu-sively to improve domestic weather forecasting andtrack cyclones and monsoons originating from theBay of Bengal and Arabian Sea. The instrument com-prises a six channel imaging radiometer designed tomeasure radiant and solar reflected energy fromareas sampled on the Earth and a high resolution in-

frared sounder to measure vertical temperature pro-files, humidity, surface and cloud top temperatures,and ozone distribution. It is planned for launch ontheGSLVMk.2 launch vehicle in 2008 for deploymentin a 38; 500km equatorial plane geostationary orbit.This orbit will provide a continuous stationary viewover the Indian Ocean for regular observations ofcloud patterns and monitoring of the path of tropicalcyclone formations to predict the time and place oflandfall for disaster warning. The INSAT-3D sounderinstrument measures radiation using 19 precisionnarrow-bandpass interference filters to isolate anddiscriminate between spectral bands. The filters re-ported here are particularly notable for their demand-ing combination of large-shaped coating aperture,tolerance of coating uniformity, and positional accu-racy of spectral placement. The filters project, ledby the University of Reading together with NDC In-frared Engineering Ltd., has been responsible for the

0003-6935/08/132346-11$15.00/0© 2008 Optical Society of America

2346 APPLIED OPTICS / Vol. 47, No. 13 / 1 May 2008

design and manufacture of the narrow-bandpassfilters defining the spectral band definition of the in-strument. The payload instruments are required tooperate for a lifetime of 7 years after launch.

2. Instrument Design

A. Imager

The INSAT-3D Imager is a six channel imaging radio-meter consisting of one visible and five infrared chan-nels. The two imager interference filters reportedhere (IIF1 and IIF2) occupy a visible wavelength at0:65 μm with a full width half maximum (FWHM)bandwidth of 30.8%, and short wavelength infrared(SWIR) at 1:625 μm(9.2%FWHM). The spectral chan-nels of the two filters operate in a temperature rangeof 15–35 °C. The imager instrument comprises a scanmirror, a Cassegrain telescope with a 310mm dia-meter primary mirror that concentrates radiationonto a 50mm diameter secondary mirror, dichroicbeam splitters, focusing optics and filters. The visibleand SWIR detectors comprise 16 square-elementphotodiode arrays of silicon (Si) and indium galliumarsenide (InGaAs), respectively, at a controlled tem-perature of 15� 10 °C.

B. Sounder

The INSAT-3D Sounder, shown by the optical sche-matic in Fig. 1, is a 19-channel radiometer that willmeasure the atmospheric vertical temperature andmoisture profiles together with surface and cloudtop temperatures, and ozone distribution. Precisionnarrow-bandpass filters, between 1% and 6% fullwidth half max (FWHM), isolate the selected spectralbands over three infrared regions; seven long-wave(LWIR 12–15 μm), five midwave (MWIR 6:5–11 μm)and six short-wave (SWIR 3:7–4:6 μm), a further visi-ble narrowband filter for observations of daytimeclouds is also included at a wavelength of 0:695 μm(7.2% FWHM). The three sets of filters are con-structed on segmented ring shaped substrates andpo-sitioned at three concentric radii on a rotating filterwheel assembly, shown in Fig. 2, to be synchronized

with the motion of the instrument scan mirror. At aparticular mirror position, the filter wheel rotatesto sequentially illuminate all 18 of the bandpass fil-ters in the optical paths of the three infrared bands.The arc lengths and interfilter gaps of the filter posi-tioning are optimized to achieve maximum spatialperformancewith the size of thewheel. The total timefor one filter wheel revolution is 100ms (600 rpm),during which time it is possible to perform up to fourmeasurements before the scan mirror steps to thenext position. The complete filter wheel assemblyand its associated cooler are designed to operate ata temperature of 210K with �10K stability in an in-cident illuminated cone angle that approximates to acollimated f =10 parallel beam. The warmer (15 °C)visible filter is sampled independently of the positionof the filter wheel.

Radiation through the 19 sounder channels is ac-quired from four distinct detector head assemblies.The Si detector array for the visible channel is similarto the imager except comprises a larger element size,and each of the infrared channel arrays contain fourdetectors compared to eight in the imager. The LWIRand MWIR channels use cooled HgCdTe detectorsoperating in a photoconductive mode. The SWIRchannels used cooled InSb detectors operating inphotovoltaic mode. These detectors are optimizedusing selected stoichiometric composition for maxi-mum detectivity at the appropriate measurementwavelengths and mounted in a passive cooler for op-eratingbelow95K,with stability better than�0:25K.Thermal control of the spacecraft is crucial to main-tain the operating temperature of the detectors below95K. This is achieved by design of passive coolers andheaters to suppress any additional heat load thatmayoriginate in the cooler field of view. The spectral band-definition channels for the filters described in thispaper are shown in Table 1. Six deliverable flight-quality filters were required to be manufactured forFig. 1. Optical schematic of INSAT-3D Sounder.

Fig. 2. Filter wheel arrangement.

1 May 2008 / Vol. 47, No. 13 / APPLIED OPTICS 2347

each channel. The spectral channels that were se-lected for the INSAT-3D Imager and Sounder containsimilar wavebands to infrared channels currently de-ployedon theNOAAGOESgeostationary satellite [2].

3. Substrates

As one of the key priorities of the instrument was toachieve a high quality of image, the metrology andsurface quality of the filters required substrateand coatings to be precisely specified. Flatness andparallelism values required surfaces within onefringe over the full surface for both sides measuredin 632:8nm light and maximum wedge anglesof <20 arcsec.The substrates of monocrystalline optical grade

(5–40Ωcm) germanium for the SWIR, MWIR, andLWIR channels of the sounder possess transparencybetween 3 μm and 15 μm [3]. The material wassourced as rectangular pieces of f111g orientation.The linear and angular and radius tolerances of thefinal shapes necessitated the use of custom designedmechanical jigs. These took the form of circularwheels with recessed positions, one wheel for eachof the three sizes; SWIR,MWIR, and LWIR, such thatthe outer and inner radii of each size could be ma-chined in consecutive operations. Once finalized,the tooling defined completely the substrate shape.Amachine shop lathe was temporarily adapted by re-placing the tool post with an independently drivendiamond impregnated grinding wheel; the viabilityof the design was proven by machining a test batchof glass pieces of each design, which were subse-quently available for use as proof pieces for the coat-ing jigs. Chipping of the brittle germanium edgesduring machining was high risk and minimized bycareful control of cutting speeds and coolant.

The specified parallelism of the faces (<20 arcsec)defined double-sided polishing [4] as the only practi-cal method available for the relatively large numberof pieces required. The polishing machines used forthis produced the required half-wave flatness mea-sured at 633nm with their normal speed ratio set-tings. Because of the rectangular arc shape of thepieces, irregularity was sometimes difficult to con-trol, although this parameter was not separately spe-cified and was simply kept within the flatnessspecification. The surface roughness was measuredon a Talysurf CCI6000 Optical Profiling Systemand the specified requirement of <2nmrms was rou-tinely exceeded. Quality assurance measurements toverify compliance of parallelism and flatness wereperformed by an autocollimator of a Trioptics Prism-Master, and a Fisba Optik interferometer.

The dimensional metrology and surface flatnessproperties of the germanium substrates in Tables 2and 3 show the compliance achieved for center thick-ness, parallelism, and optical flatness. Generally, toachieve the simultaneous compliance of these threeparameters together makes considerable demandson the fabrication of substrates by conventional op-tical workshop techniques. On this occasion compli-ance was achieved by use of double-sided polishingmethods, from which interpretation of the averagestandard deviation (σ) on the dimensional metrologymeasurements in Table 2 indicate that less than 5%(≡2σ) of the statistical normal distribution possesseddeviations greater than approximately 15% of thetolerance available. As a result of the high value sub-strates with limited, or often no opportunity for op-tical reworking demanded, meticulous attention wasdevoted to the setup configuration and operatingparameters of the deposition plants [5] together with

Table 1. INSAT-3D Spectral Channel Requirements at Filter Operating Temperatures

Filter SW50% ðμmÞ Center λ0 ðμmÞ Δλ0 (nm) LW50% ðμmÞ FWHM (%) Function

VIS IIF1 0.550 0.650 5 0.750 30.77 Visible ImagingSi IIF2 1.550 1.625 10 1.700 9.23 SWIR Imaging

SIF1 0.670 0.695 3 0.720 7.19 Daytime Clouds

MWIR SIF2 10.726 11.03 20 11.334 5.51 Surface TemperatureHgCdTe SIF3 9.593 9.71 15 9.828 2.42 Total Ozone (O3)

SIF4 7.278 7.43 15 7.582 4.09 Low Level H2OSIF5 6.823 7.02 20 7.217 5.61 Mid Level H2OSIF6 6.383 6.51 15 6.638 3.92 Upper Level H2O

SWIR SIF7 4.546 4.57 4 4.594 1.05 N2O and Low Level TempInSb SIF8 4.497 4.52 4 4.544 1.04 CO2=N2O and Mid Level Temp

SIF9 4.427 4.45 4 4.473 1.02 CO2=N2O and Upper Level TempSIF10 4.096 4.13 5 4.164 1.65 CO2=N2O and Upper level TempSIF11 3.947 3.98 5 4.013 1.67 Surface TemperatureSIF12 3.670 3.74 10 3.810 3.74 H2O and Surface Temperature

LWIR SIF13 14.570 14.71 20 14.851 1.91 CO2 Stratospheric TemperatureSIF14 14.236 14.37 20 14.504 1.86 CO2 Tropopause Temperature

HgCdTe SIF15 13.932 14.06 20 14.188 1.82 CO2 Upper level Temp and CloudSIF16 13.811 13.96 20 14.109 2.13 CO2=H2O Mid level Temp CloudSIF17 13.227 13.37 20 13.513 2.14 CO2 Low level Temp and CloudSIF18 12.420 12.66 20 12.901 3.80 Total H2O and Surface TempSIF19 11.659 12.02 20 12.382 6.01 H2O and Surface Temperature


the substrate mounting methods, temperature regu-lation, and accurate measurement and control of op-tical thickness during deposition.The bandpass filters for the visible IIF1 (0:65 μm)

and SWIR IIF2 (1:625 μm) channels of the imager,and visible SIF1 (0:695 μm) channel of the sounderwere deposited on 15:0mm∅ × 6:0mm circularSchott glass substrates. The imager IIF1 filtercomprised a titania (TiO2) / silicon dioxide (SiO2)high-pass / low-pass multilayer, deposited by plasma-assisted electron-beam evaporation on Schott-GG495yellow color glass. This glass has a colloidal shortwavelength absorption edge with increasing opacityfrom ≤540nm, to provide continuous short wave-length blocking. The IIF2 filter, deposited on clearSchott-BK7 glass, comprised a silicon (Si) / silicon ni-tride (Si3N4) / silicon dioxide (SiO2) triple half-wave(THW) bandpass filter to isolate the desired pass-band, and a combined high-pass / low-pass blockingfilter to provide out-of-band rejection. The SIF1 visi-ble filter in the sounder instrument was deposited onred Schott-RG630 glass, possessing a colloidal ab-sorption edge at ≤660nm, and comprised a narrowSi3N4=SiO2 THW bandpass filter deposited bypulsed-DC sputtering on one surface and TiO2=SiO2 blocker by plasma-assisted electron-beam de-position, as described below. These filters were de-signed to operate at a temperature range between15–35 °C. The substrate flatness achieved from the

transmitted and reflected wave front error measure-ments are shown in Tables 4 and 5.

4. Spectral Requirements

The spectral design of the imager and sounder instru-ments placed the following specification [6] require-ments for the passband and blocking designs of thefilters; center wavelength placement accuracy de-manded that within the operating temperaturerange, the Δλ0=λ0 tolerance typically needed a place-mentaccuracywithin∼0:1–0:3%,dependingupon thebandwidth of the filter. This precision placed consid-erable demands in achieving repeatable complianceupon realization of many of the SWIR, MWIR, andLWIR bandpass structures. These originated fromsmall irreproducible variations within the parametercontrols of the deposition process and subtle devia-tions of material properties during film growth, de-pending on the repeatable deposition environment.Further tolerance margins of center wavelength pla-cement was occupied by uniformity of coating thick-ness across the aperture arc length, from whichdistributions typically ranged up to 20% of the avail-able tolerance for theMWIR filters. However, by com-parison, as the LWIR filters possessed an arc lengthapproximately twice this distance and typically dou-ble the thickness, variations of uniformity occupiedthe full center wavelength tolerance margin avail-able. These tolerances inevitably required many re-peat depositions of the LWIR bandpass structure to

Table 2. Measured Dimensional Properties of SWIR, MWIR, and LWIR Germanium Substrates

Parameter SWIR Spec MWIR Spec LWIR Spec

Thickness (mm) 4.997 5.0 5.049 5.0 5.066 5.0(σ) 3:8 × 10−2 (�0:1) 1:3 × 10−2 (�0:1) 1:7 × 10−2 (�0:1)Outer Radius (mm) 91.28 91.33 58.65 58.7 123.98 124.0(σ) 5:0 × 10−3 (þ0:0= − 0:1) 2:5 × 10−3 (þ0:0= − 0:1) 5:0 × 10−3 (þ0:0= − 0:1)Inner Radius (mm) 63.65 63.60 31.05 31.0 96.38 96.33(σ) 7:5 × 10−3 (þ0:1= − 0:0) 0.0 (þ0:1= − 0:0) 5:0 × 10−3 (þ0:1= − 0:0)Parallelism (˝) 2.08 <20 1.29 <20 3.19 <20(σ) 1.79 – 0.84 – 2.20 –

Arc Angle (°) 41.26 41.25 46.57 46.61 36.02 36.03(σ) 4:6 × 10−2 (�0:25) 5:7 × 10−2 (�0:25) 4:9 × 10−2 (�0:25)

Table 3. Measured Reflected Wave Front (RWF) Irregularity (λ at 632:8 nm)

Parameter a SWIR (Ge) MWIR (Ge) LWIR (Ge)

S1 Mean P–V 0.806 0.665 1.841S1 P-V (σ) 0.183 0.105 0.897S2 Mean P–V 0.821 0.630 1.511S2 PV (σ) 0.142 8:4 × 10−2 0.739S1 Mean RMS 0.101 6:9 × 10−2 0.136S1 RMS (σ) 1:5 × 10−2 1:5 × 10−2 5:1 × 10−2

S2 Mean RMS 0.105 6:9 × 10−2 0.136S2 RMS (σ) 2:3 × 10−2 1:2 × 10−2 3:8 × 10−2

S1 Mean Power −0:246 −0:360 −0:490S1 Power (σ) 0.735 0.283 0.481S2 Mean Power −0:580 −0:355 −0:146S2 Power (σ) 0.633 0.376 0.656

aS1, incident surface; S2, rear surface; P–V, peak-valley irregularity (waves); RMS, root mean squared irregularity; (σ), standarddeviation.


acquire close compliance from a wide distribution. Inorder to ensure that an acceptable spectral energygrasp of the filters was preserved upon realization,the bandpass shape needed to be as rectangular aspossible, consistent with the practical constraints ofmultilayer design margins. This was specified asthe spectral interval between bandpass points at in-crementing transmittance decades to which a band-width multiplying factor was applied.All of the infrared filters from SWIR to LWIR cover

a large spectral range from ∼3:0–15 μm make use ofsubstrate as well as the coatingmaterials which havegood transmission in the entire spectral range of in-terest. The detectors used in all the three channelsalso have measurable responsivity from ∼2:0–5:5μm in InSb (SWIR) and from <2 μm—higher cut-offdecided by the MCT composition (MWIR and LWIR).This imposed very stringent blocking requirements,which in turn reduce the overall transmission ofthe filters. To reduce the blocking requirement andthus improve the average transmission in the re-quired narrow bands, the exit and entrance windowsof the filter wheel assemblywere suitably coated withwide passband filters to restrict the blocking require-ment to 3:0–5:5 μm for SWIR, 6–12 μm forMWIR, and11–15:5 μm for LWIR channels. Figure 3 shows thespectral performance of the three SWIR, MWIR,and LWIR channels without the filter in the respec-tive channels. Interference blocking subsequently re-quired rejection levels of <10−4 to overlap with the

folded spectral response and detector limit describedby the optical train.

5. Deposition Technologies

A. Visible Filters

Deposition of the imager and sounder visible filter(IIF1 and SIF1) multilayers, containing titanium di-oxide (TiO2) and silicon dioxide (SiO2) materials toprovide continuous spectral blocking, were per-formed by the University of Paisley using a Satis-Vacuum MC380 box coater containing a high energyplasma source to assist with electron-beam deposi-tion. This equipment contains a patented plasma de-position source (PDS) with the capability to modifythe growing thin film microstructure to producedense, near-stoichiometric coatings that are imper-vious to temperature and humidity variations [7].Unlike ion-assisted processes, PDS enables film den-sification at higher deposition rates (0:6−1:0nm=s)and reduced process temperatures (max 70 °C) withcoverage over the entire work holder. The primarydesign feature of this source enables area coverageand uniformity by tuning spatial distribution ofion current density independent of ion energy andplasma neutralization. This advantage, comparedto an ion source is that the plasma fills the vacuum

Table 4. Measured Reflected Wave Front (RWF) Irregularity (λ at 632:8 nm)

Parametera IIF1 (GG495) IIF2 (BK7) SIF1 (RG630)

S1 Mean P–V 0.554 5:8 × 10−2 0.464S1 P–V (σ) 0.250 1:3 × 10−2 0.330S2 Mean P–V 0.385 6:8 × 10−2 0.576S2 PV (σ) 0.140 2:0 × 10−2 0.230S1 Mean RMS 0.137 1:2 × 10−2 0.132S1 RMS (σ) 0.069 5:0 × 10−3 0.050S2 Mean RMS 0.080 1:5 × 10−2 0.130S2 RMS (σ) 0.039 6:0 × 10−3 0.042S1 Mean Power −0:487 −4:1 × 10−2 −0:463S1 Power (σ) 0.250 1:6 × 10−2 0.180S2 Mean Power −0:266 −4:9 × 10−2 −0:448S2 Power (σ) 0.150 2:0 × 10−2 0.140

a S1, incident surface; S2, rear surface; P–V, peak-valley irregularity (waves); RMS, root mean squared irregularity; (σ), standarddeviation.

Table 5. Measured Transmitted Wave Front (TWF) Irregularity(λ at 632:8nm)

Parametera IIF1 (GG495) IIF2 (BK7) SIF1 (RG630)

Mean P–V 0.572 7:9 × 10−2 0.681P–V (σ) 0.220 8:0 × 10−3 0.110Mean RMS 0.143 1:8 × 10−2 0.159RMS (σ) 0.057 2:0 × 10−3 0.026Mean Power 0.517 6:1 × 10−2 0.584Power (σ) 0.230 5:0 × 10−3 0.100aP–V, peak–valley irregularity (waves); RMS, root mean squared

irregularity; (s), standard deviation. Fig. 3. Folded spectral and detector response of the optical train.


chamber and couples into the evaporant, inducingpartial ionization. The refractive indices obtainedare close to the bulk material properties. The deposi-tion system is equipped with two electron-beam eva-porators, reactive gas inlet mass-flow controllers, aplasma-assistance source for densification of oxidefilms, a precision quartz oscillator film thicknessmonitor, and a single-rotation substrate holder. Spec-tral measurements of the realized filters are shownin Fig. 4 at the operating temperature of 15 °C.

B. Short Wavelength Infrared Filters

Narrow-bandpass multilayers deposited for theshort-wave infrared (SWIR) filters by NDC InfraredEngineering Ltd., used a Satis-Vacuum SP100 DCsputter coater. Films are deposited with a high de-position rate (0:6–1:0nm= sec) from which the refrac-tive index is controlled by introduction of reactive orinert gas, as appropriate to the coating design. It isequipped with a high purity 600 silicon target, whichcan be used to deposit films of pure silicon (Si), silicondioxide (SiO2), and silicon nitride (Si3N4). The stoi-chiometry and refractive index of each film type isregulated by precision mass-flow control of the se-lected process gas.The SWIR substratesweremounted on a vertical 2-

axis planetary holder with target/substrate distanceof 100mm that provides high uniformity of coatingthickness (<0:3%) over a 100mm diameter substrateplane. Film thickness is controlled by the timed-duration of each layer, and calibrated from test de-positions for each filmmaterial. This ismade possibleby the high stability of the pulsed-DC power supplieswhich deliver a controlled amount of energy in eachpulse applied to the target. Each pulse therefore re-sults in a consistent incremental growth in film thick-ness. By regulating the voltage of the discharge, thefilms achieve high uniform bulk densities, and aregenerally found to be stronger andmore durable thanfilms deposited by other conventional methods.Blocking multilayers for the SWIR filters were de-

posited using electron-beam evaporation in a conven-tional Balzers BAK760 box coater. This system isequipped with two electron-beam evaporator sources

and two stationary thermal sources. Precision quartz-crystal oscillator film thickness monitors controldeposition thickness. Substrates are mounted on asingle-rotation domed calotte. This evaporator wasespecially used to deposit layers of germanium (Ge),silicon (Si), and silicon monoxide (SiO).

Narrow-bandpass filters and broadband blockingcoatings for the germanium SWIR channels were de-posited using various alternate layer combinations ofgermanium (Ge), silicon (Si), silicon nitride (Si3N4),and silicon monoxide (SiO) deposited by eitherpulsed-DC sputter, nonreactive electron-beam de-position or thermal evaporation. Bandpass multi-layers typically comprised 37 layer triple-half-wave(THW) designs, containing mixed combinations ofcavity-layer materials and thickness orders betweenλ=2 to 3=2 λ to achieve the desired bandwidths. Thelocal side-band rejection and reflector stack index-matching layers were performed using between 6 to10 intercavity layers, depending on the design re-quirement which were antireflected using 3-layerequivalent index Herpin simulations [8]. Blockingmultilayers were deposited on the rear surface thatcomprised a composite of overlapping long-wave passquarter-wave stacks and contained antireflectionlayers to index-match between the substrate andair interfaces. Typical layer count of the total blockingfilter structures comprised in excess of 40 layers, con-sisting of principal and subsidiary rejecting stacks.

As a result of the need for mixed materials and dif-fering thickness orders of the cavity layers to achievethedesiredFWHM%, theSWIR filters exhibited spec-tral coefficients of thermal expansion (dλ=dT), some ofwhich were nonlinear because of differing tempera-ture-dependant dispersion (dn=dT) properties be-tween cavities. Temperature coefficients measuredon cooling of theSWIR filters are shown inFig. 5 usinga relative wavelength scale, normalized for operatingtemperature at 210K (λT=λ210K). Mean temperatureshifts between 15 and 20nm were observed duringcooling from ambient RT to 210K, depending onmul-tilayer design type. Spectral measurements of the

Fig. 4. Measured imager visible and NIR filters at 15 °C.Fig. 5. Measured temperature coefficient (dλ=dT) of mixed cavitySWIR filters.


realized SWIR filters are shown in Fig. 6 at the oper-ating temperature of 210K.

C. Mid and Long-Wavelength Infrared Filters

Coatingsdeposited for theMWIRandLWIR filter setswas performed by conventional thermal evaporationusing a modified Balzers BA510 bell-jar evaporator.This deposition system is especially fitted with tool-ing for deposition of II–VI and group IV midinfraredmaterials containing a geometry of rotating evapora-tion sources and stationary substrates, which hasbeen reported extensively elsewhere [9,10]. This ar-rangement is special for the deposition of filters inthemidinfrared as the need for accurate temperaturecontrol of the substrates is essential to ensure gooddeposition uniformity, particularly as the stickingcoefficient of thematerials is temperature-dependant[11]. This deposition arrangement was used for de-position of lead telluride (PbTe), germanium (Ge)and zinc selenide (ZnSe) multilayers, with indicesof 5.5–5.7, 4.0, and 2.4, respectively, across this wave-length range at 210K. To isolate the desired spectralpassband of the MWIR and LWIR filters, traditional3-cavity (triple half-wave) bandpass were designed tocomply with the bandwidth requirements. This de-sign approach has been described by Jacobs [12] withthe advantage of using quarter-wavelength layersthroughout the intercavity reflector stacks.The number of cavities chosen provided a prag-

matic solution between the idealized rectangularspectral response required to maximize energy graspof the waveband, and the compromise of practical de-position thickness and accuracy control. As the num-ber of cavities increases, the spectral passband of thefilter becomes more rectangular with increased side-band rejection depth, however the total number oflayers and composite multilayer thickness also in-creases, impinging on the intrinsic strength andstress limit of the materials. Additionally, the nar-rower the bandwidth is desired, so it becomes neces-sary to use higher-order cavity-layer thicknesses (λ or3=2 λ) and increase in the number of intercavity re-flector layers, to whichmatching of equivalent admit-tances of the stacks results in further thickness

increases. Generally as the bandwidth narrows,transparency losses rise sharply. This is caused byincreased roughness of the deposited microstructureand intrinsic layer absorption properties resultingfrom the increase in multiple internal cavity reflec-tions. There is also increased sensitivity of the pass-band shape to layer thickness errors. All of thesefactors conspire to limit the thickness of bandpass fil-ters and the number of cavity and reflector layersthat can be used by a conventional multilayer ap-proach. Empirically, at the wavelengths desired forthe MWIR and LWIR filters, with the selection oftransparent materials and deposition parameters,there is a diminishing advantage in using 4-cavitydesigns or cavity-layer thicknesses in excess of3=2 λ, almost certainly as a result of reaching thelimiting accuracy of the infrared optical thicknessmonitoring technique. As a result of this rationale,3-cavity designs were considered the most suitableapproach for the MWIR and LWIR filters.

The choice of cavity type, using high- or low-indexlayermaterials also required consideration of the evo-lution of the bandpass filter designs. The use of PbTe(H-layer) with ZnSe (L-layer) yields a high effectiveindex (n�) of 2.70 in the low-index cavity case, and3.6withhigh-index cavities. In low-index cavity band-pass filters, although there is greater sensitivity to tiltangle (dλ=dθ), they posses smaller temperature coef-ficients (dλ=dT) than H-cavity designs, hence, the in-teraction between the large intercavity negative(dn=dT) coefficients of PbTe nearly cancel with posi-tive dn=dT coefficients of the L-index material. In H-cavity filters these coefficients do not cancel and cancause large temperature coefficients of the filter. Asthe INSAT-3D instrument filters required cooled op-eration (210K) at normal incidence and operating in anominally parallel beam, L-cavity filters were consid-ered the most appropriate design type. Other practi-cal considerations for not choosinghigh-indexPbTeasthe cavity material involves a thickness-dependentabsorptive losswhich becomes increasingly dominantin the thicknesses required for the cavity layers.Further, the thicker long-wavelength (LWIR) filterswould need to posses cavity thicknesses that areknown to exhibit mechanical failure due to tensilestress— the amount of stress being closely relatedto thickness.

The out-of-band blocking requirements wereachieved using combinations of long-wave and short-wave pass edge filters in conjunction with the localside-band rejection provided by the bandpass struc-ture itself. The bandpass filter and blocking multi-layers were deposited separately on opposingsurfaces of the same substrate following spectral ver-ification of the bandpass center wavelength compli-ance. The blocking multilayers contained variousoverlapping combinations of refined Tschebysheff[13] polynomial designsandquarter-wave stackswithappropriate index-matching layers between stacksand application of broadband antireflection layers.Deposited physical thicknesses of the bandpass coat-

Fig. 6. Measured SWIR narrowband filters at 210K operatingtemperature.


ings typically ranged between 13–27 μmforMWIR fil-ters and 25–43 μm for LWIR filters, depending onwavelength. The blocking coatings deposited on thereverse surface ranged between 10–17 μm for MWIRcoatings and 13–25 μm for the LWIR coatings. Spec-tral measurements of the realized MWIR and LWIRfilters are shown in Fig. 7 and 8 at the operating tem-perature of 210K.As a result of using high-thickness order cavities,

the thermal wavelength shift of the narrower filtersexhibited near-temperature invariance. Seeley [14]and Zheng et al. [15] modeled the sensitivity of nar-row band filters to temperature changes in theirlayers from which the cavities and next two adjacentlayers are dominant over other intercavity reflectorlayers from which temperature shift can be pre-dicted. When the negative temperature coefficient(−dn=dT) in PbTe is combined with the positive shift(dn=dT) in Group II–VI low-index dielectric, tem-perature invariant compensation of L-cavity is possi-ble, the degree of compensation being proportional tothe relative cavity thickness order and refractiveindices as shown in Fig. 9.

6. Spectral Measurements

The spectral measurements for the various filter setswere performed in three differing spectrophot-ometers; Visible and NIR filters (IIF1, IIF2, andSIF1) used a Perkin-Elmer (PE) Lambda nine dual-beamUV-VIS-NIR (190–3000nm) spectrometer. Thisis a standard dispersive grating instrument usingtungsten and deuterium source lamps, from whichmeasurements were obtained at 0:5nm resolution.SWIR measurements were measured by fouriertransform infrared (FTIR) spectroscopy using a PESpectrum GX Optica spectrometer. This instrumentis optimized for infrared wavelengths between2–8 μm with a CaF2 beam splitter and deuteriumdoped DTGS midinfrared detector. Measurementswere performed at 1 cm−1 resolution using a GrasebySpecac cryostat placed in the sample compartment.This cryostat uses liquid nitrogen cooling, rotary va-

cuum pumping and heated windows to stabilize thecooled measurement temperature at 210K.

Spectral measurements of theMWIR and LWIR fil-ter sets were performed using a Perkin–Elmer Spec-trum 2000 Optica FTIR spectrophotometer. Thisinstrument is the originating model of the GX Opticafamily. It has a high midinfrared energy grasp be-tween 1.5 and 50 μm using a DTGS detector and ce-sium iodide (CsI) beam splitter, and, being of theOptica design, has been specially adapted to preventartifacts from optical train aliasing and stray reflec-tions reaching the detector [16]. Optical setup para-meters of the J- and B-stops were configured toproduceanominalbeamsizeof7:0mmrepresentativeofauniformly illuminated f =8 coneangledistribution.A Filler [17] apodization function was chosen for at-tenuation of the interferogram sidelobes. This func-tion is characterized by high accuracy, highconvergence, and/or very low ripple of backgroundor sample spectra for regions with either low spectralenergyorwhere thepresenceof strongabrupt spectralfeatures are present—on this occasion being the side-band transition of narrow-bandpass filters. The at-tenuation rate of ripple for the filler apodization is

Fig. 7. Measured MWIR narrowband filters at 210K operatingtemperature.

Fig. 8. Measured LWIR narrowband filters at 210K operatingtemperature.

Fig. 9. Measured center wavelength shift (dλ=dT) of MWIR andLWIR filters versus temperature (K) for differing cavity thicknessorder (from substrate (ns), 2 ¼ λ=2, 4 ¼ λ, 6 ¼ 3=2λ).


1=ðΔυÞ3, where υ is the distance in cm−1 from the in-terferogram peak. Cooled temperature measure-ments on representative 25:0mm∅ witness filtersthat originated from the same deposition weremounted in an Air Products Displex DE 202 cryostatfitted with KRS-5 windows in the spectrometer sam-ple compartment. To ensure intimate thermal contactduring cooling, thewitness filterswere clamped to thecryostat cold finger using a copper fixture with Pb an-nular washers, spring tensioned screws and indiumshim. Temperature was measured using a reverse-biased diode at the cryohead and thermocouple at-tached to the mounting fixture. Average passbandmeasurements and distribution of center wavelengthplacement accuracy for the compliant flight-qualityfilters is shown in Table 6.

7. Environmental Testing

To assess long term environmental performanceand spectral aging stability, a verification thermal-vacuum screening test was performed on all of thespace-flight filters to the requirements of ESA-PSS-01-702. This required the coated optics to be subjectto aminimumof 10 cyclic thermal excursions betweentemperatures of 60 °C and −83 °C in a vacuum of10−5 Torr. Spectral measurements performed at am-bient RT before and after testing verified coating sta-bility. This required spectral shifts to comply within1% transmission and between�1nm and�5nm cen-ter wavelength position, depending on bandwidth.Testing was performed on each space-flight set usinga thermal cycling chamber facility in the clean assem-bly area of the Department of Atmospheric, Oceanicand Planetary Physics, Oxford University. Furtherenvironmental testing performed on the filters were

conducted to the requirements of the general provi-sions of military test specification MIL-F-48616.These tests included humidity at 49 °C (�2 °C) for24h in >95% relative humidity, moderate abrasiontesting, and adhesion resilience using 3M 810 scotchtape (∼0:175� 0:025 g=mm2 pull force). Comparisonsof measurements performed before and after testingverified compliance within these requirements.

Surface roughness measurements performed onrepresentative deposited coatings of each filter setwere required to comply within a roughness limitno greater than 50Å rms to minimize Mie scatteringlosses [18] resulting from either the background sur-face texture, or from point defects and microspatterwithin the coatings. This measurement was per-formed using a Taylor Hobson Talysurf CCI 3000Ånoncontact 3D surface profiler, which can measuremicroroughness and step heights using coherencecorrelation interfereometery [19] to a vertical resolu-tion of better than 0:1Å. Results from thesemeasure-ments showed typical variations of surface roughnessbetween 41 and 48Å for the LWIR filters. Highervalues of surface roughness were recorded fromlocations containing either isolated surface defectphenomena or coating inhomogeneities that requiredomission, these were considered as unrepresentativesampling compared to the background surfacetexture.

Further assessment of surface quality requiredscratch and dig measurements to comply beforeand after coating to better than 60∶40 (E:E) ofMIL-F-48616. Interpretation of this quality identifiesscratches with widths greater than 60 μm (disregard-ing scratch widths less than 10 μm) and digs, beingconsidered as point defect seed populations or coating

Table 6. Average Passband Measurements from Six Compliant Flight-quality Filters

Filter SW50% ðμmÞ Center λ0 ðμmÞ LW50% ðμmÞ FWHM (%) Ave T% (at 80%Δλ) Δλ0 a(nm) λ0σ b ð×10−2ÞIIF1 0.556 0.659 0.763 31.3 88.8 9 0.07IIF2 1.559 1.626 1.693 8.25 83.0 1 0.6SIF1 0.672 0.697 0.723 7.31 88.6 2 0.3SIF2 10.741 11.018 11.295 5.03 81.8 −12 0.8SIF3 9.599 9.710 9.821 2.29 85.5 0 0.7SIF4 7.291 7.430 7.570 3.76 88.1 0 0.7SIF5 6.823 7.015 7.208 5.49 81.8 −5 0.6SIF6 6.388 6.511 6.635 3.79 88.0 1 1.5SIF7 4.551 4.575 4.599 1.05 65.8 5 0.3SIF8 4.502 4.526 4.550 1.07 62.7 6 0.4SIF9 4.428 4.453 4.478 1.11 65.0 3 0.6SIF10 4.095 4.130 4.165 1.71 71.4 0 0.3SIF11 3.949 3.984 4.019 1.76 73.2 4 0.3SIF12 3.679 3.747 3.816 3.66 79.0 7 1.0SIF13 14.565 14.705 14.845 1.90 72.3 −5 2.1SIF14 14.243 14.375 14.508 1.84 75.1 5 1.4SIF15 13.932 14.061 14.191 1.84 77.9 1 1.7SIF16 13.816 13.962 14.108 2.09 77.4 2 0.6SIF17 13.234 13.376 13.518 2.12 77.7 6 1.7SIF18 12.434 12.659 12.884 3.55 80.0 −1 1.4SIF19 11.681 12.023 12.366 5.70 75.0 3 1.4aMean center wavelength deviation from specification.bStandard deviation of center wavelength spread.


spatter, with diameters greater than 400 μm (disre-garding diameters less than 100 μm). Although themajority of coated optics compliedwithin this require-ment, the inevitability of high substrate exposure tothick thermally deposited coatings on the LWIR fil-ters resulted in occasional spatter defects presenton the bandpass coated surface. This coating struc-ture was of greater susceptibility to the occurrenceof surface defects than the blocking coatings due tothe high composite physical layer thicknesses(∼35–45 μm) and lengthy exposure times necessaryfor the high layer thicknesses, particularly duringcavity growth (∼8:5 μm) . These high thicknesses in-crease the probability of spatter originating from theevaporation sources as they become depleted, andeventually shrink to particulate sizedmaterial whichis ejected from hot spots, while blocking coatings pos-sessing thinner fractional layers andmoremultilayerinterfaces are exposed to shorter exposure times andmore periodic protection beneath the substrate shut-ters. Typically the incidence of surface spatter com-prising populations of <4 sites were noted onapproximately 20% of flight deliverable LWIR band-pass coatings with dimensions between 400 and1000 μm. Improvements to obtain full defect-free coat-ings could possibly be postulated by use of ion-beamassisted deposition techniques [20–22] to increase themobility of the vapor and structural morphology offilmgrowth, however this technologywasunavailableduring this program. Inspection of coated surfaceswas performed by scanning the coating through astereo microscope at 40× zoom magnification witha reticule template calibrated through a CCD cameraand line scan image processing software.To maintain traceability and identification of indi-

vidual substrates, engraved substrate marking wasperformed along the side edge of each arc segmentand associated witness filters using a high speedHereaus Dynamo Plus electric tool fitted with a spe-cial 0:5mm superfine diamond bur. The substrateidentification lettering, of 3mm height, was appliedthrough a thin 100 μm stainless steel precision sten-cil. This equipment provided a high precision scrib-ing technique with constant high speed (35; 000 rpm)and electronic circuitry to ensure smooth operationregardless of load.

8. Conclusions

The fabrication of the narrow-bandpass filters re-quired for the INSAT-3D instrument has demandedthe development ofmany innovative and adaptive de-position techniques and procedures, from which thevarious specification challenges have been achieved.The visible and SWIR filter depositions have shownthat optically stable and robust sputtered films canbe accurately deposited by automatic process controlusing a timed-duration thicknessmonitoringmethod,with composite multilayer thicknesses not performedpreviously. The deposited films of the bandpass de-signs have also shown that high layer thickness con-trol andmatching ofmultiple cavities can be achieved

using this nonoptical monitoring procedure, fromwhich compensation of target erosion was accommo-dated. The MWIR and LWIR filters have shown thataccurate thickness deposition of the temperature-dependant dielectric films has been achieved withgood center wavelength placement and coating uni-formity across a wide aperture. Further refinementsof the optical monitoring techniques to provide im-proved accurate thickness control of thick multilayerbandpass structures has developed as a result of thisprogram. The filters are complete and in preparationfor integration and launch.

The authors thank colleagues at the Indian SpaceResearch Organisation Space Applications Centreand Bryka Electrosystems & Software LLC, withwhom discussions and support for this work is grate-fully acknowledged. Thanks also to colleagues at theUniversity of Oxford and the University of Paisleyfor their assistance with testing and deposition facil-ities and to Basil Barrington, Lawrence Patrick, andKarim Djotni in consultation.

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