WO2007048195A1 - Filtres rugates - Google Patents

Filtres rugates Download PDF

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Publication number
WO2007048195A1
WO2007048195A1 PCT/AU2006/001600 AU2006001600W WO2007048195A1 WO 2007048195 A1 WO2007048195 A1 WO 2007048195A1 AU 2006001600 W AU2006001600 W AU 2006001600W WO 2007048195 A1 WO2007048195 A1 WO 2007048195A1
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WO
WIPO (PCT)
Prior art keywords
rugate
rugate filter
refractive index
wavelength
optical device
Prior art date
Application number
PCT/AU2006/001600
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English (en)
Inventor
Anne Gerd Imenes
David Robert Mckenzie
Original Assignee
The University Of Sydney
Priority date (The priority date is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the date listed.)
Filing date
Publication date
Priority claimed from AU2005905986A external-priority patent/AU2005905986A0/en
Application filed by The University Of Sydney filed Critical The University Of Sydney
Publication of WO2007048195A1 publication Critical patent/WO2007048195A1/fr

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    • G—PHYSICS
    • G02—OPTICS
    • G02B—OPTICAL ELEMENTS, SYSTEMS OR APPARATUS
    • G02B5/00—Optical elements other than lenses
    • G02B5/20—Filters
    • G02B5/28—Interference filters
    • G02B5/289—Rugate filters

Definitions

  • the present invention relates broadly to a method of designing a rugate filter structure, and to a rugate filter structure.
  • Two main types of dielectric interference filters are the multilayer stack, consisting of discrete layers, and the graded-index filter, which has a continuous transition in refractive index between successive 'layers'.
  • Graded-index coatings are typically used for antireflection purposes where they can match the substrate index to the surrounding media with a smooth, continuous index profile.
  • Rugate filters are graded- index optical coatings characterised by a continuous sinusoidal or sine-like periodic index variation along the direction of the substrate normal. Bragg-like reflections occur from such periodical structures, producing a narrow-band reflection filter, and are also sometimes referred to as notch filters.
  • the rugate structures exhibit similar properties to a quarterwave stack but without the higher-order reflectance peaks, which have their origins in the interference between beams reflected at all the different interfaces in the quarterwave stack.
  • AR antireflection
  • the rugate filter offers extremely high optical performance by the reduction of optical losses due to light scattering at interfaces and by the suppression of higher-order harmonic reflectance bands.
  • the favorable properties of the rugate coating makes it attractive also for applications where a filter with a broadened spectral region of high reflectivity is desired, such as in an edge, bandstop, or bandpass filter.
  • a filter with a broadened spectral region of high reflectivity is desired, such as in an edge, bandstop, or bandpass filter.
  • the rugate coating is designed for high reflection over an extended spectral region, a large number of rugate cycles is required, especially when the periodic variation in refractive index has to be kept small to avoid the occurrence of sidelobes outside the primary reflectance band.
  • the second method places the filters in series, stacking different rugate filters on top of each other.
  • rugate filters of different periodicity are deposited one after the other, and the individual filters are centered at respective wavelengths chosen so that the correspondingly shifted rejection zones create a broadened rejection band.
  • the bandwidth of the high-reflectance band of each of the rugate filters used in the series is dependent on the peak variation n p of the rugate index- function. Increasing the value of n p will increase the bandwidth, which would mean that the overall thickness for a broadband rugate coating could be kept to a minimum.
  • the third method of achieving a broadened spectral region of high reflectance is "frequency chirping", i.e., modulation of the periodicity of the refractive index profile as a function of coating depth z.
  • the rugate period can be varied in either in discrete steps or continuously throughout the coating.
  • the chirped periods will reflect different regions of the spectrum and, depending on their spectral distribution, can create a continuous high-reflectance band over a broadened spectral region. As the region of high reflectance is extended by adding more rugate cycles of different periodicity, the total physical thickness of the structure is increased.
  • the continuous nature of the chirped filter is less sensitive to changes in angle of incidence and when the chirping is performed in an optimum manner, the resulting coating thickness may be kept at a minimum for a given broadband reflectance.
  • a broadband bandpass filter may be composed of two chirped rugate filters in series.
  • the wider the region or regions of high reflectance the larger the overall thickness of the resulting filter. This is critical for the practical implementation of a broadband rugate filter since, as the coating gets thicker, the requirements to materials and manufacturing techniques become extremely strict.
  • At least preferred embodiments of the present invention seek to provide a method of designing a rugate filter structure and a rugate filter structure, that address one or more of the above mentioned design issues for broadband rugate filters.
  • a method of designing a rugate filter structure comprising the step of deriving a refractive index variation as a function of rugate filter thickness such that substantially an equal number of rugate periods; or parts thereof, reflect an incident optical signal at each targeted wavelength in a selected wavelength interval of interest.
  • the wavelength interval of interest is generally a stopband in the reflectance and/or transmission profile of the rugate filter structure.
  • the refractive index variation may be formed by at least two rugate filter sections, each having a respective characteristic centre wavelength and spectral bandwidth, and may be derived in accordance with one or more of the following steps: selecting minimum and maximum values for the wavelength interval of interest; selecting the wavelength spacing between the characteristic centre wavelength of individual rugate filter sections; determining the number of and position of the individual rugate filter sections required to reflect an incident optical signal at each targeted wavelength or wavelength interval within a stopband region of the rugate filter structure, determining the number of and position of the rugate filter sections required to achieve a certain value of optical density over the wavelength region of interest; and determining the total number of rugate cycles in the refractive index variation.
  • the at least two rugate filter sections may be contiguous stacks.
  • a method of designing a rugate filter structure comprising the step of deriving a refractive index variation as a function of rugate filter thickness such that a plurality of individual reflectance peaks overlap contiguously so as to form a stop band of the rugate filter.
  • a rugate filter structure fabricated based on a design obtained from a method as defined in the first, second or third aspects.
  • the structure may be fabricated by a deposition process that provides a continuously varying index, or an approximation thereof, with very low stress and may be fabricated by a plasma impulse chemical vapour deposition process.
  • an optical device comprising: a plurality of rugate filter sections, each filter section comprising a plurality of periods of refractive index variation to form a respective feature in the reflectance and/or transmittance profile of the optical device, each feature having a characteristic centre wavelength and spectral width wherein the features overlap contiguously to form a single feature in the reflectance and/or transmittance profile of the optical device.
  • the single feature may have substantially uniform optical response characteristics over a broadened spectral region larger than the spectral width of each of the respective features.
  • the single feature may be a uniform flat-topped feature in the reflectance and/or transmittance profile of the optical device.
  • the single feature may be either a bandpass feature or a bandstop feature in the reflectance and/or transmittance profile of the optical device.
  • Each of the plurality of rugate filter sections may have a different K value, and each of the plurality of rugate filter sections has an equal number of periods or fractions of periods of refractive index variations to produce a broadened region of high reflectance.
  • the plurality of rugate filter sections comprises a large number of refractive index variation periods, each period having a small refractive index variation.
  • the spacing between the characteristic centre wavelength of rugate filter sections having adjacent characteristic centre wavelengths may increase with increasing wavelength and may increase linearly with wavelength.
  • the variation in the refractive index of the periods in each of the plurality of rugate filter sections may be uniform throughout the thickness of each respective section, or in parts thereof.
  • the variation in the refractive index of the periods in at least one of the plurality of rugate filter sections may be non-uniform throughout the thickness of the at least one section.
  • the refractive index of the periods in at least one of the plurality of rugate filter sections is apodised throughout the thickness of the at least one section.
  • the refractive index of the periods in each of the plurality of rugate filter sections is apodised throughout the thickness of each section.
  • Figures Ia) to c) are plots of reflective index n(z) versus physical thickness z, reflectance R( ⁇ ) versus wavelength ⁇ , and reflectance R(X) versus wavelength ⁇ respectively, illustrating a method of designing a rugate filter structure according to an example arrangement.
  • Figures 2a) to f) are comparison plots of the product K(z)*z versus physical thickness z, and transmittance T versus wavelength ⁇ for different rugate filter structures according to different arrangements.
  • Figure 3 shows plots of reflectance versus wavelength for a rugate filter structure according to an example arrangement.
  • Figure 4 shows plots of reflectance versus wavelength for a rugate filter structure according to an example arrangement.
  • Figure 5 shows plots of reflectance versus wavelength for a rugate filter structure according to an example arrangement.
  • Figure 6 shows plots of reflectance versus wavelength for a rugate filter structure according to an example arrangement.
  • Figures 7A) to C) are plots of the product K(z)*z versus physical thickness z, transmittance T versus wavelength ⁇ , and reflectance R versus wavelength ⁇ respectively of rugate filter structures according to different arrangements.
  • Figures 8A) to C) are plots of the product K(z)*z versus physical thickness z, transmittance T versus wavelength ⁇ , and reflectance R versus wavelength ⁇ of a rugate filter structure according to another arrangement.
  • the degree of uniformity across the high-reflectance band depends on the distribution of individual rugate periods with a given K-value within the spectral region of interest.
  • the K-value may be defined as the ratio 4 ⁇ m a / ⁇ for a rugate cycle of period thickness ⁇ /2n a , but when varied as a function of thickness z it determines the spatial frequency modulation of the periodic refractive index profile. It has been recognised that it is desirable to have an equal number of rugate periods reflecting at every targeted wavelength ⁇ within the stopband.
  • the bandwidth BW 0 of the stopband is given by:
  • Figure 1 illustrates a rugate filter constructed from a series of individual simple rugate sections e.g., 102, each producing narrowband reflectance peaks e.g. 100, centered at different wavelengths ⁇ j and with corresponding spectral widths of high reflectance ⁇ ;.
  • the individual rugate sections, e.g., 102 have different Kj values, illustrated in Figure l(a), and the physical thickness ⁇ z; of each section increases with increasing rugate period.
  • the bandwidth of the reflectance band ⁇ j preferably also increases in a proportional way to keep BWc (equation (I)) constant.
  • the spacing between individual reflectance bands within the spectral region of interest therefore preferably increases linearly with wavelength, in order to achieve a constant reflectance level across that region in an example arrangement. This can be expressed as follows:
  • Ki- value is _ 4 ⁇ rn ⁇ _ 4 ⁇ r ⁇ ⁇ - ⁇ ⁇ i ⁇ l . (6)
  • Z N A (A 1 + C ⁇ i + C 2 X 1 + • • • + C ⁇ - 1 A 1 )
  • the total thickness of the flat-topped rugate filter in example arrangements can be determined from X 1 and the number of cycles per section, S.
  • Eq. (11) may be used with the subscript i substituted for N, allowing the coating thickness to be determined at the position of any given section.
  • the coating depth will be defined in steps throughout the filter. Dividing each step into S x p equal intervals, where p is the number of points defining each rugate cycle, the rugate sinusoidal profile can be determined at each position z throughout the coating.
  • the designer enters the lower and upper wavelength limits of the bandstop region, Xi and X N .
  • the reference wavelength of the bandstop may be set as the average of these limits.
  • Eq. (13) determines the number of consecutive narrowband sections that must be positioned in series, to cover the full bandstop region with a given relative spacing distance between each section.
  • the spacing between each section was in the methodology above assumed to be fixed by the constant bandwidth BW 0 . It is useful to introduce a spacing parameter W so that the designer is free to position the individual rugate cycles either closer together or further apart in different arrangements.
  • the designer then has the choice of selecting a value for either S or G, which determines the total number of rugate cycles in the coating and, hence, the resulting optical density (OD) of the resulting broadband reflectance region. [ 0047 ] It is also possible to use the desired OD as a design parameter, instead of the number of rugate cycles in different arrangements.
  • Figure 2 shows the effect of varying the spacing parameter W on the broadband performance of the flat-topped rugate filter.
  • the filter consists of approximately 400 rugate cycles in total, and has a targeted reflectance band from 500 to 1000 nm. No apodisation or matching functions have in this case been applied to the rugate refractive index profile.
  • the results show the K(z)*z product (curves 200a, 202a, 204a) and the corresponding transmittance profile (curves 200b, 202b, 204b), the latter plotted on a semi-logarithmic scale, for three options of W. In these figures, a low value of transmittance corresponds to a high value of reflectance.
  • Example arrangements produce a highly uniform and flat-topped broadband reflectance across the desired spectral regions.
  • the quality of the resulting stopband is dependent on the choice of the spacing parameter W and the number of rugate cycles. Best results may be achieved for small values of W, i.e. a tight packing of the N individual rugate sections, and with only a few cycles within each section.
  • thick rugate coatings may require a deposition process that provides a continuously varying index with very low stress within the coating.
  • stringent requirements may be imposed on the deposition technique and the materials used.
  • Recent advances in manufacturing capabilities have made the fabrication of such thick coatings a feasible option.
  • rugates or multilayer stacks without internal stresses may be attained using techniques such as the plasma impulse chemical vapor deposition. Stacks of thousands of discrete quarter- wave layers or graded-index rugate periods, resulting in a total thickness of several 100 ⁇ m or even up to the mm-range, may then be feasible.
  • Example arrangements have applications in a multitude of industrial and research areas, including optics, communications, astronomy, meteorology, and military applications.
  • the rugate filters in the example arrangements were designed by specifying the upper and lower wavelengths of the stopband region(s) and the total number of rugate periods, which determines the magnitude of the reflectance within the stopband region(s).
  • the wavelength boundaries of the stopband regions are carefully chosen, so that an excessively thick coating can be avoided while at the same time accommodating for most of the energy in the incident spectrum.
  • the lower and upper boundaries chosen for the evaluation of the example rugate filter arrangements in Figures 3-6 were set to 300 nm and 2500 nm, respectively.
  • Non-dispersive materials were assumed, and the reflectance caused by the glass-air interface at the back of the substrate was not taken into account.
  • an antireflective coating may have to be added to the back of the substrate, to ensure high transmittance within the transmissive region(s) of the filter.
  • the rugate profiles in the example arrangements have been evaluated with the standard matrix method, by approximating each rugate cycle as a stack of thin discrete layers and assuming small incremental differences in refractive index.
  • the resulting reflectance characteristics were further smoothed over 13-nm intervals (for a rugate filter designed for a mean weighted angle of incidence of the light beam at 54 degrees) and 23- nm intervals (for a rugate filter designed for a mean weighted angle of incidence of the light beam at 14 degrees), to simulate the effects of a cone of incident light.
  • the distribution of the mean weighted angle was assumed to be Gaussian, and was determined as the annual average angular distribution using raytrace simulations for a hybrid PV/thermal central receiver system.
  • Figure 3 shows the resulting reflectance profiles for s-polarisation, p- polarisation, average polarisation, and a target profile 400, 402, 404, and 406 respectively for a rugate bandstop filter according to an example arrangement designed for, and at 54 degrees incidence.
  • Figure 4 shows a comparison of the same bandstop filter of Figure 3 when used at an incidence angle of 14 degrees (curve 502) and a different rugate bandstop filter in accordance with another example arrangement (curve 500), designed for the angle of incidence of 14 degrees.
  • the refractive index n(z) is varied more rapidly with thickness z (more rapid frequency chirping) so as to accommodate for the effect of reducing the angle from 54 to 14 degrees (and thereby also reducing the thickness of the filter at 14 degrees, compared with the thickness at 54 degrees).
  • the uniformity of the reflectance profile causes the rugate filter to be relatively insensitive to changes in the angle of incidence, compared with e.g. discrete filters.
  • Discrete filters typically show a larger variation in the polarisation splitting and the filter performance as the incidence angle changes.
  • the rugate filters in example arrangements were found to have a slightly better solar-to-electric energy conversion efficiency for the hybrid PV/thermal receiver system, compared with discrete bandpass and bandstop filters.
  • a high optical performance for the rugate filters in example arrangements is achieved by using a small refractive index variation (n p ) and a large number of cycles.
  • n p refractive index variation
  • the result is coating thicknesses up to the millimetre-range when the filters are designed to reflect the major part of the broadband solar spectrum.
  • the coating thickness can be significantly reduced. This may have to be balanced against an increase in the undesired sidelobes.
  • a deposition process that provides a continuously varying index with very low stress within the coating is preferable.
  • Low-scatter, defect-free optical thin film coatings may e.g. be produced by high temperature plasma enhanced chemical vapour deposition (PCVD) methods, which have shown high surface and bulk damage thresholds.
  • PCVD plasma enhanced chemical vapour deposition
  • the latter can be important in high-flux applications, such as in solar central receiver systems where the concentration ratio on the filter surface may reach several hundred suns, and where 'hot spots' may arise from an inhomogeneous distribution of the incident solar flux.
  • the methodology disclosed in example arrangement may be used in a large range of systems, such as large dishes or troughs used for light collection, that will have a broad distribution of angles incident on an optical filter or a spectral receiver positioned in the focal region of the system.
  • the rugate filter in example arrangements can be designed for an incident angle that on average gives high performance over a range of angles.
  • the rugate filter of example arrangements can be a desirable design approach due to the simplicity of the design process, where the required level of reflectance and the position of the band-edges can be specified, and due to the smaller sensitivity to changes in incidence angle.
  • the latter implies that, the rugate filter of example arrangements can perform reasonably well if designed for an angle of incidence that takes into account the spectral shift of the band-edges, as well as the angle at which most of the light is incident.
  • the disclosed methodology in example arrangements represents a framework which is easy to adapt to a given situation. Other solutions to the design problem are possible, and the final design may be chosen based on an evaluation of the practical considerations, such as manufacture limitations, cost, and coating durability, versus the useful optical throughput or energy output that can be expected from the system.
  • Laser protection eyewear is an important industry with applications within a range of research disciplines, as well as for military purposes.
  • One example of a high- power laser is the copper vapor laser, with two laser lines at 511 nm and 578 nm (green/yellow region) .
  • Commercial laser safety goggles are available with a peak OD in the range of 5-6, but typically characterised by a highly non-uniform reflectance for the different laser line regions.
  • a lower or higher OD-level may be desired.
  • the laser protection would also be more effective and predictable if the reflectance was kept uniform across the desired spectral range.
  • the resulting broadband protective laser coatings, shown in Figure 7 for normal incidence have been designed for a band-stop region of 470-580 nm, blocking both of the laser lines at all angles up to 45°.
  • a partial linear apodisation over 20 cycles and quintic matching over 3 cycles were applied to all the rugate designs shown in Figure 7.
  • the lower and upper wavelength limits during the design of these flat-topped rugate filters were set to 455 and 600 nm, respectively. If a larger value of n p is assumed, the coating thickness can be reduced but at the expense of the steepness of the stopband edges.
  • the electric conversion efficiency of a solar concentrating system can be improved by employing a spectrally selective filter (beam splitter) that divides the solar radiation into optimised components for two or more receivers.
  • a broadband spectral beam splitter was designed in another example arrangement for an application for the flat-topped rugate filter.
  • the filter was optimised for maximum annual energy output in a hybrid photovoltaic/thermal central receiver system.
  • two flat-topped stopbands designed for two different spectral regions, are placed in series. The first stopband covers the wavelength region 300-590 nm, and the second stopband covers the wavelength region 1090-2500 nm.
  • the results can be seen in Figure 8.

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  • Physics & Mathematics (AREA)
  • General Physics & Mathematics (AREA)
  • Optics & Photonics (AREA)
  • Optical Filters (AREA)

Abstract

La présente invention concerne un procédé de conception d’une structure de filtre rugate et une structure de filtre rugate. Le procédé comprend une étape consistant à dériver une variation d’index de réfraction comme fonction de l’épaisseur d’un filtre rugate de sorte qu’un nombre sensiblement égal de périodes rugates, ou une partie d'entre elles, réfléchisse un signal optique incident à chaque longueur d'onde ou intervalle de longueur d’onde ciblée dans une bande atténuée du filtre rugate.
PCT/AU2006/001600 2005-10-28 2006-10-27 Filtres rugates WO2007048195A1 (fr)

Applications Claiming Priority (2)

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AU2005905986A AU2005905986A0 (en) 2005-10-28 Rugate filters
AU2005905986 2005-10-28

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WO2007048195A1 true WO2007048195A1 (fr) 2007-05-03

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Cited By (2)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
JPWO2015137183A1 (ja) * 2014-03-12 2017-04-06 コニカミノルタ株式会社 光学フィルター及び撮像装置
CN107132604A (zh) * 2017-06-26 2017-09-05 中国工程物理研究院激光聚变研究中心 渐变折射率薄膜制备参数获取方法、制备方法及滤光片

Citations (4)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US4952025A (en) * 1989-05-31 1990-08-28 The United States Of America As Represented By The Secretary Of The Air Force Rugate filter incorporating parallel and series addition
US5181143A (en) * 1991-10-03 1993-01-19 Rockwell International Corporation Multiple line rugate filter with index clipping
US5475531A (en) * 1993-05-06 1995-12-12 Hughes Aircraft Company Broadband rugate filter
US5488511A (en) * 1992-04-03 1996-01-30 Hughes Aircraft Company Spatially tunable rugate narrow reflection band filter

Patent Citations (4)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US4952025A (en) * 1989-05-31 1990-08-28 The United States Of America As Represented By The Secretary Of The Air Force Rugate filter incorporating parallel and series addition
US5181143A (en) * 1991-10-03 1993-01-19 Rockwell International Corporation Multiple line rugate filter with index clipping
US5488511A (en) * 1992-04-03 1996-01-30 Hughes Aircraft Company Spatially tunable rugate narrow reflection band filter
US5475531A (en) * 1993-05-06 1995-12-12 Hughes Aircraft Company Broadband rugate filter

Non-Patent Citations (2)

* Cited by examiner, † Cited by third party
Title
LORENZO E. ET AL.: "Fabrication and optimization of rugate filters based on porous silicon", PHYS. STAT. SOL. (C), vol. 2, no. 9, 2005, pages 3227 - 3231, XP003012487 *
LORENZO E. ET AL.: "Porous silicon-based rugate filters", APPLIED OPTICS, vol. 44, no. 26, 10 September 2005 (2005-09-10), pages 5415 - 5420, XP003012488 *

Cited By (4)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
JPWO2015137183A1 (ja) * 2014-03-12 2017-04-06 コニカミノルタ株式会社 光学フィルター及び撮像装置
US10274657B2 (en) 2014-03-12 2019-04-30 Konica Minolta, Inc. Optical filter and imaging device
CN107132604A (zh) * 2017-06-26 2017-09-05 中国工程物理研究院激光聚变研究中心 渐变折射率薄膜制备参数获取方法、制备方法及滤光片
CN107132604B (zh) * 2017-06-26 2020-01-14 中国工程物理研究院激光聚变研究中心 渐变折射率薄膜制备参数获取方法、制备方法及滤光片

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