Edmund Optics(cid:15)(cid:3)(cid:44)(cid:81)(cid:70)(cid:17)(cid:3)
`(cid:40)(cid:91)(cid:75)(cid:76)(cid:69)(cid:76)(cid:87)(cid:3)(cid:20)(cid:19)(cid:19)5(cid:3)
`(cid:3)
`
`0001
`
`
`(cid:40)(cid:91)(cid:75)(cid:76)(cid:69)(cid:76)(cid:87)(cid:3)(cid:20)(cid:19)(cid:19)5(cid:3)
`(cid:3)
`
`0001
`
`
`
`Optical Filter Solutions
`
`a Prof Angus Macleod
`University ofA rizona
`
`Energetic Thin Film
`Deposition Processes
`
`Dr. Bertrand Bovard
`
`Barr Associates, Inc.
`
`Filter Durability
`
`Ali Smajkiewicz
`Barr Associates, Inc.
`
`Practical Issues and
`
`Tradeoffs
`
`Ali Smqjkiewicz
`.453-'oci0zte;“; Inc.
`
`Barr
`
`1’l8475BX
`
`WiIsunJones®
`
`©1990 Wilson Jones Company
`
`0002
`
`
`
`‘
`,, /‘M A I 2‘
`171/ :7” /?9‘/3 .
`I:
`/ I"/VF 1./S
`\;<f9/u
`
`1951 \.~'-msur: Jana: Comanv
`
`0003
`
`
`
`Optical Filter Solutions
`
`Angus Macleod
`Thin Film Center Inc
`
`2745 E Via Rotonda
`
`Tucson, AZ 85716-5227
`
`The following is the text and figures from the viewgraphs.
`
`We consider linear spectral filters only, i.e. filters with properties independent of
`irradiance but dependent on wavelength, and emphasize filters that depend in some
`way on the properties of thin layers.
`
`We exclude tuned receivers, wavelength shifting filters, tuned amplifiers and the
`like.
`
`Optical filters are devices that are used with the deliberate intention of altering
`the quality of an incident beam of light.
`
`In this context quality means the spectral distribution of a parameter of the light
`and the filters are characterized by their spectral response, the ratio of the output and
`input parameter of the light, both measured in the same units, expressed as a
`function of the wavelength.
`
`Usually the parameter willbe spectral irradiance, measured in watt] (metre)2, and
`the spectral response will be spectral transmittance (T) or reflectance (R) both
`functions of wavelength 1.
`t
`
`The filters we deal with all operate by removing light from the beam according to
`wavelength and the response is a number between 0 and 1 or between 0 and 100%.
`
`-
`
`Response
`
`1
`
`Wavelength
`
`The light may be removed by redirection or by conversion or by both.
`
`A process of absorption is really one of conversion. The light energy is converted
`into kinetic and potential energy of molecules, atoms and electrons. Some of this
`energy may then be reemitted in a changed form or dissipated as heat.
`
`Optical Filter Solutions 1
`
`0004
`
`
`
`An important question that should always be asked about a filter is "where does
`the unwanted energy go?"
`
`Scattered
`
`Redirected
`
`/’
`
`aperture
`
`Most filters exhibit the various modifications of the input in the diagram. For
`example an absorption filter to eliminate short wavelengths will reflect residual
`light (redirection), will convert some light (fluorescence), will scatter some light and
`will emit thermal radiation - all unwanted. If this light is accepted back into the
`system, performance will suffer
`
`Aperture
`stop
`1
`
`I
`
`Receiver
`
`Sketch of a simple instrument showing the stops. The image of the aperture stop
`in source space is the entrance pupil and in receiver space the exit pupil.
`
`Note that the filter performance cannot be separated from the details of stops and
`pupils. These are part of the overall system design.
`
`Where does the unwanted light go? Can it be scattered back into the acceptance
`zone of the system?
`
`Baffles can help to prevent return of unwanted energy
`
`Optical Filter Solutions 2
`
`0005
`
`
`
`Since performance is system dependent an optical filter is usually specified with
`regard to standard (and that usually means ideal) conditions. Entrance and exit pupil
`are at infinity (light ideally collimated) and scattered, redirected, converted and
`emitted light components are assumed lost to the system.
`
`A real system will rarely have this arrangement. Therefore the performance may
`not correspond to the standard specified performance of the filter.
`
`Some filter types
`
`The operation of different filter types is often a mixture of several mechanisms
`some of which include
`
`Absorption
`
`Refraction
`
`Reflection
`
`Scattering
`
`Polarization
`
`Interference
`
`Absorption filters
`
`Semiconductors are intrinsically longwave pass absorption filters. Photons of
`energy greater than the gap between valence and conduction bands of the electrons
`are absorbed by transferring energy to electrons in the valence band to move them
`into the conduction band. High transmittance implies intrinsic semiconductors of
`high resistivity. Gallium arsenide, silicon, germanium, indium arsenide, indium
`antimonide are all useful. Note that these semiconductors have high refractive
`index so must be antireflected in the pass region. They are rarely antireflected in the
`absorbing region and so a large amount of the rejected light is actually reflected.
`
`Some colored filter glasses of the longwave pass type contain colloidal
`semiconductors.
`
`Optical Filter Solutions 3
`
`0006
`
`
`
`TNi&|ITTWfi (I)
`
`W K!!!
`
`Reflectance of the same filter.
`
`Other colored glasses have metallic ions dispersed in them. In general colored
`glass filters make excellent longwave pass filters but it is not possible to find
`shortwave pass filters with the same excellent edge steepness. In fact, shortwave pass
`filters present an almost universal problem.
`
`Prism and grating monochromators are variable filters of a very inexpensive
`nature. The prisms operate due to refraction that varies because of the dispersion of
`the index of the prism material. Diffraction gratings are essentially multiple beam
`interference devices that use diffraction to broaden the beams to give reasonable
`efficiency over slightly more than an octave. Disadvantages are the low throughput
`because of the narrow entrance and exit slits and the need for mechanical stability of
`a high degree.
`
`Strong absorbers are also strong reflectors. In the far infrared the reststrahlen
`bands associated with very strong lattice resonances show strong reflectance and are
`sometimes used as filters. Beryllium oxide reflects strongly in the 8-12um
`atmospheric window and can help to keep electrical insulators frost free.
`
`Optical Filter Solutions 4
`
`0007
`
`
`
`EMITTANCE
`
`.
`.
`..........,.............,...........
`
`.
`
`i
`;
`............!.............._,.._......
`
`.
`
`..;,............§..
`
`WAVELENGTH (pm)
`
`TRANSMHTANCE
`
`From: C G Ribbing. Reststrahlen material bilayers. An option for tailoring in the
`infrared. Optical Interference Coatings Technical Digest 1992 (Optical Society of
`America, Washington, DC, 1992), Vol 15, pp 89-91, 1992. The paper proposes the use
`of boron nitride to fill the small gap at the short wave side of the window.
`
`Scattering
`
`Christiansen filters consist of dispersed fragments or powder in a matrix. The
`dispersion curves of the two materials differ but cross at one wavelength at which
`the scattering disappears
`
`At very long wavelengths, >20um, Christiansen filters become Yoshinaga filters
`and are made of powders dispersed in flat polymer sheets.
`
`Wavelength
`
`Polarization
`
`Retardation is given by
`
`q) = 27l7(Yl1i- 712”
`
`so that a half wave retarder made of birefringent material is correct for only one
`wavelength. Let there be a polarizer, a retarder at 45° and an orthogonal polarizer in
`series. All the light transmitted by the first polarizer will be transmitted without loss
`through the remainder of the system provided the retardation is equivalent to an
`odd number of half wavelengths half wave plate. For any other value of retardation
`
`Optical Filter Solutions 5
`
`0008
`
`
`
`the light will be stopped by the second polarizer either totally, if the retardation is an
`integral number of wavelengths, or partially, if not. The irradiance is proportional
`to
`
`sin2 g
`
`This effect can be used in filters either in a single element or in a series of such
`elements. The Lyot filter consists of a series each member having an increased
`halfwave plate order and so a narrower, more rapidly changing response. The
`responses of the various elements combine to give a very narrow band width.
`
`X, Retard\ers
`
`Lyot filter
`
`The Solc filter has only two polarizers and a set of identical retarder plates in
`between with the axes arranged in a fan.
`
`In acousto-optic filters the periodic strain caused by an acoustic wave alters the
`refractive index in step with the wave. This impresses a thick Bragg phase grating on
`the material. The light interacts with this grating which effectively becomes a
`narrow band filter. In the collinear case the light is polarized and a narrow band is
`scattered into the orthogonal plane of polarization where it is selected by a suitably
`oriented polarizer. Variation of the frequency of the RF drive for the acoustic
`transducer varies the wavelength of the filter.
`
`Collinear acousto-optic filter
`
`Acoustic wav
`
`Liht scattered
`into orthogonal
`polarization
`
`In the non collinear type of filter the Bragg grating deflects the light with high
`efficiency. The light need not be polarized. Unwanted light is obscured from the
`receiver.
`
`Optical Filter Solutions 6
`
`0009
`
`
`
`Noncollinear acousto-optic filter
`
`Required
`
`beam;
`
`Bandwidths of a few nm with an aperture of a few degrees with area almost 1 cm2
`and with tuning over the visible region are possible. The collinear type has given
`bandwidths of 0.15nm with tuning range over the visible and near ultraviolet.
`
`Optical Coatings
`
`Thin film optical coatings operate by a mixture of interference and material
`properties
`
`Dielectrics
`
`n real
`
`y = "7!
`
`n independentof A
`271' n d
`1
`o<—
`A
`
`5:
`
`it
`
`y = constant
`
`.1/=-ik?
`km.
`
`=
`
`B
`
`27rkd
`1
`
`=
`
`constan
`y °= 1
`
`1;
`
`Semiconductors
`
`These are usually classified as either metal or dielectric depending on the spectral
`region
`
`Optical Filter Solutions 7
`
`0010
`
`
`
`With increasing wavelength
`
`Dielectrics
`
`Metals
`
`Become weaker
`
`Become stronger
`
`T increases
`
`R increases
`
`Dielectrics have lower losses
`
`Metals have higher losses
`
`2
`
`and T = 4-1--—0Re3:
`|yo+y|
`
`Metal layers
`
`evanescentwave
`
`ge-azeim t
`
`Dielectric layers
`
`progressive wave
`se 1(0) f-K‘ Z)
`
`For interference coatings we require presence of dielectric layers
`
`Optical Filter Solutions 8
`
`0011
`
`
`
`. Multilayer coatings
`
`All-dielectric or metal-dielectric
`
`Transmittance
`
`Dielectric laqer
`(lower loss)
`
`Reflectance
`
`Reflectance
`
`Metal laqer
`(higher loss)
`
`Transmittance
`
`Transmittance
`
`All-dielectric
`
`(lower loss)
`
`Reflectance
`
`Reflectance
`
`Metal-dielectric
`
`(higher loss)
`
`Transmittance
`
`Optical Filter Solutions 9
`
`0012
`
`
`
`Transparent conductors like ITO or ZnO:Al appear dielectric at shorter
`wavelengths and metallic at longer wavelengths.
`
`0
`
`0 IX!) XIXJSGIJ
`
`lflllfllfldlllfllh SGBRXDIIXX)
`WAVH_BK7TH'[l'l'I|J
`
`A metal film can be antireflected on either side by a dielectric system consisting of
`a phase matching layer and a reflector.
`
`Thicker metals need more powerful reflectors.
`
`Phase
`matching
`
`Phase
`'“°t’=h“19
`
`Reflector
`
`Metal
`
`Reflector
`
`For a very thin metal the mismatch between high-admittance phase matching
`layers and the surrounding media can give a sufficiently high reflectance.
`
`Incident
`medium
`
`Phase
`matcher
`
`Glass
`
`Massive
`
`TiO2
`
`28nm
`
`Metal
`
`Ag
`
`Phase
`matcher
`
`TiO2
`
`28nm
`
`Emergent
`medium
`
`Glass
`
`Massive
`
`This leads to a three—layer coating
`
`Optical Filter Solutions 10
`
`0013
`
`
`
`The three—layer coating consisting of a thin layer of silver with a phase matching
`layer of titania on either side and sandwiched in glass.
`(Wavelength units nm and reflectance and transmittance in %)
`
`nzoo man 1600 I800 zooo
`
`wA\/B.ENGTH
`
`How to make a higher reflector?
`
`A quarterwave layer acts as an admittance transformer.
`
`If the admittance of the emergent medium is ysuy, and that of the film yfthen the
`transformed admittance is yfi/ysub.
`
`An additional quarterwave of magnesium fluoride on either side, making a five-
`layer coating, permits the use of a thicker silver layer.
`
`1200 moo I600 1800 .2000
`
`WAVELENGTH
`
`Optical Filter Solutions 11
`
`0014
`
`
`
`Increasing the outer reflectance still further, and therefore the silver thickness, by
`adding a titania quarterwave narrows the transmittance zone and increases the
`reflectance.
`
`IJL
`
`1200 woo woo I800 zooo
`me 500 son 1000
`WAVHENQTH
`
`With a reflecting system consisting of four quarterwaves of titania and
`magnesium fluoride the coating is now a narrowband filter.
`
`Glassl HLHLH’AgH’LHLH Glass
`
`WAVELENGTH
`
`A coating to transmit the infrared and reflect a band in the visible would best be
`constructed from dielectric materials.
`
`In the quarterwave stack repeated use of the quarterwave transformer achieves
`high admittance mismatch and hence high reflectance.
`
`Air
`
`HLHLHLHLHLH Glass
`
`Optical Filter Solutions 12
`
`0015
`
`
`
`40
`
`60
`
`800 I000
`
`I200 I400 I600 1800 2000
`
`WAVHENGTH
`
`Ripple in the long-wave pass band can be reduced by changing the thicknesses of
`the outermost layers to eighth waves.
`
`.
`
`A11’
`
`H
`
`2
`
`-LHLHLHLHLHL *“
`
`H
`
`2
`
`I Glass
`
`1200 I400 I600 I800 2000
`
`WAVELENGTH
`
`We can use the quarterwave stack structure to replace the central metal layer in
`our narrowband filter and reduce losses. This allows us to make still narrower
`filters.
`
`1200 1400 1560 1860 zooo
`o 500 soo moo
`WAVELENGTH
`
`This has design
`
`Air
`
`I HLHLHHLHLHLHLHLHHLHLHI Glass
`
`Optical Filter Solutions 13
`
`0016
`
`
`
`Because the losses are so much lower, it is possible to achieve exceedingly narrow
`filters.
`
`T
`
`Design: (59 layers)
`
`697
`
`699
`
`701
`
`703
`
`WAVELENGTH
`
`Glass! HLHLHLHLH LL
`
`HLHLHLHLHLHLHLHLHLH LL
`
`HLHLHLHLHLHLHLHLHLHLL
`
`HLHLHLHLHI Glass
`
`Antireflection coatings
`
`Antireflection coatings with low loss imply dielectrics.
`
`Quarter wave matching
`
`Incident medium
`
`quarterwave film
`
`Substrate
`
`I/sub
`
`Optical Filter Solutions 14
`
`0017
`
`
`
`Magnesium fluoride is traditionally used for single layer antireflection coatings.
`. The performance is considerably better than the 4.25% reflectance of an uncoated
`glass surface.
`
`400
`
`450
`
`500
`
`550
`
`500
`
`550
`
`WAVEENGTH (nm)
`
`The inhomogeneous layer - the ultimate antireflection coating
`
`Incident
`medium
`
`Film
`
`Substrate
`
`Admittance profile
`
`REFLECTANCE [%J
`
`For air incident medium the inhomogeneous layer can be achieved only by
`microstructural variations. Processes involving etching, leaching, sol-gel deposition,
`photolithography have all been used.
`
`Optical Filter Solutions 15
`
`0018
`
`
`
`Because of the inherent weakness of the film, this solution is limited to very
`special applications
`
`Most rugged coatings use discrete layers with high durability.
`
`with L’ and H’ rather less than a quarterwave in thickness
`
`Aiv1LHHL’H’ |c1ass
`
`400
`
`450
`
`'
`50
`
`._-__-1‘
`600
`650
`700
`
`550
`
`WAVELENGTH (hm)
`
`Angle of Incidence Effects
`
`Characteristics of coatings shift towards shorter wavelengths with increasing
`angle of incidence and gradually exhibit polarization effects. For small tilts of a few
`degrees the shift is proportional to the square of the angle of incidence.
`
`M 1.5 ><10’4
`———0
`A
`,1;
`
`2
`
`where rte is the effective index of the coating (a value in between the highest and
`lowest indices in the coating) and where 29 is the angle of incidence in degrees.
`
`Optical Filter Solutions 16
`
`0019
`
`
`
`Another problem
`
`Multiple reflections between two coatings can alter the expected transmittance.
`
`TT
`T=.__.£..b_..
`(1"RaRb)
`
`and if the reflectances are not zero then the net transmittance is higher than the
`product of the individual transmittances.
`
`May 1995
`
`Angus Macleod
`Thin Film Center
`2745 E Via Rotonda
`
`Tucson, AZ 85716-5227
`Tel: (520) 322 6171
`Fax: (520) 325 8721
`
`Optical Filter Solutions 17
`
`0020
`
`
`
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`Hr//ck Hefere/Ice Index .S’y3Ie/n
`
`© 1991 Wilson James Eumpanv
`
`0064
`
`
`
`Filter Durability
`Effect of Temperature, Humidity, Radiation and Time
`
`Ali Smajkiewicz
`President
`
`Barr Associates, Inc.
`
`BarrAssoclates, inc.
`
`1. Background
`
`There is no general statement that can be made regarding all optical
`filters because there is a large range of materials and processes used
`to produce them. Generally, trade-offs are made between cost,
`performance, durability and dimensional requirements to build the
`greatest value filter for a given application.
`
`This discussion will focus on precision interference filters with primary
`emphasis on thin film coatings. Even though the coatings are a critical
`element, other components such as substrates, cements, housings can
`have an impact on a filter's reliability.
`
`0065
`
`
`
`Evironmental factors which can effect a
`
`fi|ter’s durabiiity
`
`I Humidity
`n Temperature
`1 Radiation
`u Time
`
`Barr Associatm_ Inc.
`
`0066
`
`
`
`ilter elements which influence the
`effects of environmental factors
`
`I Materials
`0 Deposition
`6 Substrate
`O Housings
`0 Cements
`1 Process
`or Deposition
`0 Assembly
`in Dimensions
`Q Design
`6 coating
`9 Filter
`
`Barr Associates. Inc.
`
`The challenge is to select among the above variables and develop a
`design to achieve the appropriate durability within the constraints of
`spectral performance, dimensions and budgetary requirements.
`
`0067
`
`
`
`hysical properties which influence
`filter durability
`
`Chemical stability
`Stress
`Hardness
`
`Density
`Flexibility
`Porosity
`Expansion coefficient
`
`Barmssociates, Inc.
`
`Fig. 4 lists the physical properties either inherent in the materials or
`produced by the process which influence the longevity of a filter.
`
`0068
`
`
`
`Typical construction for filters made with
`hygroscopic materials
`
`Barr Associales. lnc.
`
`- Filters made with coating materials which are hygroscopic and need to
`have the thin films sealed against moisture. Fig. 6 shows the typical
`method of sealing. Here, epoxy is used to provide a barrier for moisture.
`The effectiveness of the seal is dependent on the transparency to
`moisture of the epoxy, the adhesion and the path length the moisture
`must travel to reach the delicate films.)
`
`0069
`
`
`
`Typical construction of filters made with
`non-hygroscopic materials
`
`substrate
`
`Barr Associates, Inc
`
`- Filters made with non-hygroscopic materials. Fig. 7 shows typical
`constructions.
`
`0070
`
`
`
`Pp. of water in air as a function of
`temperature and RH
`
`BarrAssociatos. Inc.
`
`1. Humidity
`
`In discussing humidity, the focus generally is on the relative humidity
`(%RH) which is the % of the total amount of water air can hold at a
`given temperature. However, the temperature component is generally
`underestimated, if not totally ignored. The number of water particles in
`air is directly proportional to the partial pressure of water.
`
`Fig. 8 shows the Pp. of water as a function of temperature and relative
`humidity. One can readily see that increasing the relative humidity from
`50% to 95% at 25°C has less of an effect on the moisture content of the
`
`air than maintaining the relative humidity at 50% and increasing the
`temperature to 40°C. At an increased temperature, not only are there
`more particles of water, but they are more energetic.
`
`If one looks at climatalogical date for an area over a short period of time
`one sees that there generally is an increase in the RH at night and a
`corresponding drop in temperature. In fact, the amount of moisture in
`the air has not changed. At the lower night-time temperature the
`amount of water the air can hold is smaller. Therefore, the same
`
`amount of water represents a larger percentage. Over some period of
`time the moisture content stays about the same until another air mass
`arrives.
`
`0071
`
`
`
`Unprotected hygroscopic filters exposed
`to water and humidity chamber
`
`exposure
`
`I 20 minutes
`at 65°C.
`95% RH
`
`Barr Associates. inc.
`
`I 16 hours
`in water
`
`I 30 minutes
`at 65°C.
`95% RH
`
`Fig. 9 further demonstrates this. Here are depicted two filters made with
`hygroscopic materials where there is no protection provided to the
`coating. It extends. to the edge and is in direct contact with the
`environment.
`
`.
`
`0072
`
`
`
`
`
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`
`0073
`
`
`
`onstruction without a moisture seal, for
`example, in Figure 9
`
`I Sample construction
`
`hygroscopic
`
`color filter gass
`
`Barr Associates, Inc
`
`0074
`
`
`
`ypical humidity failure for filters made
`with hygroscopic materials
`
`Barr Associates. mo.
`
`1. a. Filters made with hygroscopic materials
`
`Humidity is the primary cause of failure for most filters made with
`hygroscopic materials like cryolite (Na3AlF5). There is a permanent
`change caused by a chemical reaction which causes a catastrophic
`failure which appears as a cloudiness and darkening starting at the
`edge of the filter and over time working its way toward the center.
`Generally there is an observable effect at the edges before any
`significant spectral changes occur in the visually clear area.
`
`0075
`
`
`
`0076
`
`
`
`ctors influencing the longevity of filters
`made with hygroscopic materials
`
`I Environment
`I Operating
`I Storage
`I Effectiveness of seal
`
`I Coating materials
`I Coating design
`I Coating process
`
`Barr Associates, Inc.
`
`The amount of time it takes for enough moisture to cause damage to
`films is dependent on a large number of factors. The overriding one for
`most filters is the quality of the seal provided. Epoxy is the most
`common sealant used in the industry today. All epoxies we have
`experimented with are transparent to moisture. There can be orders of
`magnitude differences from system to system, but in time, given
`exposure to moisture, enough water penetrates the epoxy to react with
`the films. When all the conditions in Fig. 12 are optimized for durability,
`filters can survive and function for very many years. There are filters
`made with hygroscopic materials in use today which were fabricated
`more than 25 years ago.
`
`0077
`
`
`
`actors influencing the effectiveness of
`the sea!
`
`I Epoxy system
`I Dimensions
`
`I Assembly process
`1 Properties of materials
`
`BarrAssocial. Inc.
`
`0078
`
`
`
`lflfififiilllflfiliéifilll
`
`R Days to failure with the same clear
`aperture but different diameters
`
`
`
`DaystoFailureat65°C,95%RH
`
`340
`
`380
`
`405
`
`460
`
`Center Wavelength
`
`BarrAssociates. Inc.
`
`‘ Group A: Minimum
`sealing area
`
`Group B: Sealing area
`.5mm larger
`
`.
`
`A factor in the hands ofthe engineers specifying the filter which can
`influence the quality of the seal is the amount of area available for
`sealing. This is primarily influenced by the difference in the outside
`dimension and the clear aperture. The thickness also influences the
`path length of epoxy available for sealing (see Fig. 6).
`‘
`
`Fig. 14 shows the days to failure (failure defined as when slightest signs
`of deterioration visually evident at the edges) of four different filter types
`held at 65°C and 95°/o'FtH. Group B has the same coa
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