GB2215075A - Optical reflection filters - Google Patents

Abstract

A reflection filter comprises a first, multi-layer system I containing a metallic absorbing layer 50 for absorbing radiation with wavelengths outside a predetermined bandwidth, and a second system II including a reflector 52 for reflecting radiation passing through the first system I. The first system includes a set of dielectric layers 55, 57 having at least two layers of dielectric materials with different refractive indices, positioned between the absorbing layer 50 and the reflector 52. Selection criteria for the absorbing layer and dielectric layer materials are defined. <IMAGE>

Description

REFLECTION FILTERS The invention relates to multi-layer reflection filters for reflecting optical radiation at wavelengths in a predetermined passband within a wider operating band of wavelengths.

Theoretical reflection filter structures including an absorption layer have been described by Zheng and Lit in "Design of a Narrow-band Reflection IR Multilayer", Can.

J. Phys. 61.361 (1983) pages 361-368. However, the filters described by Zheng and Lit suffer from the disadvantage of having a narrow spectral range. The wavelength rejection on either side of the reflected, filtered band would not be adequate, for example, to provide satisfactory filtering characteristics over the preferred spectral transmission band from around 1250-1600nm as used in optical fibre transmission systems.

It is an object of the present invention to provide a multi-layer reflection filter with improved spectral range and filtering characteristics.

In accordance with the present invention, a reflection filter comprises a first, multi-layer system including two dielectric systems and a metallic absorbing layer positioned therebetween for absorbing radiation with wavelengths outside a predetermined bandwidth, the metallic absorbing layer having a refractive index of the form n=n'+in" being selected to have a high product n'n" and a low dispersion over the predetermined bandwidth, and at least one of the dielectric systems including a set of at least two layers of dielectric materials with substantially different refractive indices, and a second system including a reflector arranged to reflect radiation passing through the first system back through the first system.

The applicants have found that with such a filter a significant reduction in the reflectance in regions outside the desired central peak reflection can be achieved compared to previous filter structures. In contrast with the arbitrary choices of absorbing layer in prior art filters the present invention is based on the application of clear selection criteria which enable the spectral range to be considerably increased. In particular, it has been found that where the product n'n" of real (n') and complex (n") components of refractive index n of a metallic absorbing layer is as large as practicable, and where the dispersion of the absorbing layer is low throughout the reflection peak, filter performance is enhanced.It has also been found that where the absorbing layer material is chosen such that variations in n" with wavelength throughout the passband region are small, it is possible to achieve lower reflectances in the rejection (high absorptance) bands, further improving filter performance.

A guide to the optical suitability of a given metal for use as an absorbing layer in a reflection filter according to the present invention may be obtained by consulting published tables of optical parameters. A list of optical properties for various metals, for example, has been published by Ordal et al, Appl. Optics, Vo122, No.7, 1 April 1983, pp1099 et seq., entitled "optical Properties of the Metals Al, Co, Cu, Au, Fe, Pb, Ni, Pd, Pt, Ag, Ti and W in the infrared and far-infrared. From such tables it is possible to determine the values of n, n" and dispersion at relevant passband wavelengths and to select an absorbing layer metal accordingly.However, since such tables generally report the results of experiments on bulk samples, actual values for thin metallic films as employed in the present reflection filters may be different.

Consequently, in some cases, some experimentation using known refractive index measuring techniques may be required to confirm the appropriate selection parameters.

Ideally, the metallic material used chosen has stable optical properties when exposed to the atmosphere as a thin film within a reflection filter according to the present invention. Layers of metals such as cobalt or platinum have been used by the present inventors to produce filters operating in the 1250-1600nm region.

Preferably the difference in refractive index between the at least two dielectric materials forming a set of dielectric layers is made as large as practically possible. It has been found that such a selection further enhances the free spectral range and selectivity of the filters according to the invention. It will be clear that the dielectric materials should be substantially non-absorbing at wavelengths within the desired peak reflection bandwidth.

For the avoidance of doubt, in this specification references to "optical'1 do not imply any restriction to specific wavelength ranges mentioned here but extend to include radiation at wavelengths in the UV and near and far infra-red regions which are transmissible via an optical waveguide. Filters may be constructed according to the present invention to operate at wavelengths within this broad optical spectrum.

Some examples of reflection filters in accordance with the present invention will now be described with reference to the accompanying drawings, in which: Figure 1 is a schematic diagram showing the constituent layers of the multilayer construction of an absorbing reflection filter according to the invention; Figure 2 is a schematic diagram of the absorption system of the Figure 1 filter in more detail; Figure 3 is a graph showing the variation of reflectance with wavelength for an example of a reflection filter using a cobalt absorbing layer, with the filter structure according to Table 3 in the Appendix to this specification; Figure 4 is a graph showing the variation of reflectance with wavelength for another example of a reflection filter, also using a cobalt absorbing layer, with the filter structure according to Table 4 in the Appendix;; Figure 5 is a graph showing the calculated variation in transmittance of a monitor signal through the various layers of the filter according to Table 4 illustrating the method of controlling layer thickness during fabrication of the filter; Figure 6 is a graph showing the variation of reflectance with wavelength for an example of a reflection filter using a platinum absorbing layer, with the filter structure according to Table 5 in the Appendix; Figure 7 is a graph showing the variation of reflectance with wavelength for another example of a reflection filter using a platinum absorbing layer, with the filter structure according to Table 6 in the Appendix; and Figures 8 and 9 are graphs showing the measured variation of reflectance with wavelength for two filters having the structure according to Table 6.

An absorbing reflection filter according to the present invention can be considered to comprise two (sub)systems, I and II, as illustrated in Figure 1.

System I is a multi-layer system in this case having just one absorbing layer 50, having potential transmittances g 11 and amplitude reflectances rl, rl in the directions indicated by the arrows in the Figure.

System II may be viewed simply as a high reflectance mirror. Typically the mirror system II will comprise a metallic layer 52 acting as a broadband reflector together with a dielectric layer 51. The mirror system II is arranged to provide an overall reflectance r2 close to unity and such that there is no overall phase change on reflection. An intermediate spacer layer 53 connecting the two (sub)systems is provided to appropriately position the mirror system II relative to the absorbing layer 50 according to the centre wavelength of the desired passband. The complete filter structure is itself conveniently mounted on a glass substrate 58 which provides suitable support and acts as the incident medium for input to the filter.

The system I will now be analysed in more detail to explain the structure. As shown in Figure 2, the absorbing layer 50 is sandwiched between a pair of dielectric spacer layers 54,55. These spacer layers 54,55 position the absorbing layer 50 with the requisite calculated separation from the pair of dielectric quarter-wave systems 56,57, in order to achieve the desired filter characteristics. The quarter-wave system 57 is itself positioned adjacent the system I-system II spacer layer 53.

The magnitude of the amplitude reflectance r2 for the mirror system II can be calculated for any wavelength independently of the parameters of the rest of the filter. From predetermined values of r2 and the absorbing layer thickness, d, and from specified values of fractional bandwidth (= 2nX/X) at half peak reflectance, and minimum reflectance, Rmin, the system I parameters rll, fr1,f 1 1 and ;1' can be evaluated as shown below.

The inventors have determined that these important parameters may be conveniently expressed in terms of filter bandwidth and rejection parameters (approximating r2 to unity for simplicity) as follows: fril = X + A (1 - X), rlll 1 - 2 and g l - #/2)/(1 + X), #1' = (1 - X)/2 where X = (Rmin)l/2 The parameters of the dielectric spacer layers 54,55 are determined according to the required filter characteristics.Values of |rA|, |rB|, IrI, , dA and dB required to define these layers and to provide the filter performance desired can be found by using an iterative minimising routine varying the values of the amplitude reflectances, r , |r'|, and |r'|, and corresponding reflection phase changes, p, p' within acceptable limits (values of |r| , p, for a given value of ;1 and of ir'i, p' for given ;1 define circles in the argand diagram) and using conventionally derived amplitude reflection equations relating all these quantities to each other and to rl and |r1'|. With the addition of the derived values of |rA|, |rB|, irAl, IrI, dA and dB the structure of the system I is essentially fully defined.

Since the total spectral domain over which the filter is to operate is large (eg. up to 350nm on either side of the central peak for a filter with a narrow bandpass in the desired 1250-1600nm range) the dielectric quarter-wave systems must also have a large reflection bandwidth. To achieve this it has been found that ideally the quarter-wave systems are composed of two dielectric materials, both substantially non-absorbing at the passband wavelength, with as large a difference of refractive index as possible.

It is useful to summarise in simple terms some of the priciples which govern the operation of a reflection filter incorporating a metal absorbing film. The properties of the filter depend crucially on those of the absorbing layer 50 and the amplitude reflectance r at the boundary of that layer away from the side of incidence.

The magnitude of the peak reflectance of the overall filter, for example, depends on the thickness of the absorbing layer 50, a smaller thickness giving a higher peak reflectance. The amplitude and phase components of reflectance r, namely |r| and p respectively can be made to vary rapidly with wavelength at a predetermined wavelength by the effective selection and design of the system of layers beyond the absorbing layer. This leads to rapid transitions between reflectance and absorption, ie. a narrow reflection band at the design wavelength, the latter being determined principally by the thickness dB of the dielectric layer 55 adjacent to the metal absorber and that of the spacer layer 53.The width of the free spectral range is also associated with the choice of materials in the dielectric quarter-wave system 57; as previously stated, it is advantageous to choose materials with as large a refractive index difference as possible.

In operation, each wavelength in an incident beam which is reflected will interfere so that a number of standing waves are set up in the filter. The spacer layer is chosen so that the absorbing layer 50 is at an electric field node of the standing wave produced by the wavelength to be filtered. It will therefore have little absorption at that wavelength.

It is useful to consider the construction of an example filter according to the present invention designed to operate with a passband centred on 1425nm and the relevant design selection steps. For convenient reference, the structure of this example filter is tabulated in the Appendix at Table 3. The filter structure is indicated in conventional layer terminology (eg. n=metal; H=high index; L=low index; etc). The layer thicknesses are shown in nm. The calculated performance for this filter is shown in the graph of Figure 3.

From consideration of the relevant parameters for a wavelength of 1425nm cobalt appears an appropriate medium for the absorbing layer, having a relatively large product n'n" and thus potentially high absorption at wavelengths outside the desired reflection band.

At 1425 nm, which is in the middle of the wavelength range 1250-1600nm for present optical transmission systems, silver (broadband reflector layer 52 of Figure 1) with a 236.3 nm layer 7 of NgF2 (dielectric layer 51 of Figure 1) gives a zero phase reflectance amplitude of Ir2i = 0.9987. A 15nm layer N-of Co as the absorber (50 of Figure 1) forms a continuous layer and gives a high peak reflectance. Specifying that Rmin=O.40/o and that the half maximum fractional bandwidth 3,14x10'2 it is found that several different values of |rA| rA and rug for the dielectric spacer layers (54,55) can potentially be used to fulfil the relevant requirements.

However, where |rA|= 0.29, leading to |rglt 0.94, the lowest reflectance values at a large spectral distance from the peak reflectance are achieved. Thus, fixing the value rA = 0.2870, which is conveniently the reflectance amplitude in Zns layer 2 from a NgF2 quarter-wave layer L (dielectric spacer layer 54 + quarter-wave system 56 of Figure 2) on a glass substrate, the desired values of tr1i, Irit 1 and g1 are all satisfied for rB = 0.9571. A reflectance amplitude 0.9647 is obtainable from three quarter-wave layers L H L of Si and NgF2 (quarter-wave system 57 of Figure 2).Using iterative minimising techniques it is then possible to determine that a ZnS layer 2 of thickness 106.437nm (dielectric spacer layer 54 Figure 2) and a Si layer 4 of thickness 15.588nm (dielectric spacer layer 55 of Figure 2) on the incident and emergent sides of the Co layer respectively will meet the requirements. Finally, the thickness ds of the spacer layer S (53 of Figure 1) joining sub-systems I and II together is calculated simply from the equation: ds = (1 - p1'/2n)X/2ns where ns is the refractive index of the spacer material at the centre wavelength X. In this case a layer S of 188.57nm of silicon is required.

The spectral performance of this filter is illustrated graphically in Figure 3 and it will be seen that a relatively narrow bandwidth is reflected and there is rejection of wavelengths over a wide range.

The performance of a second example of a filter according to the present invention, in this case with a more complex 15 layer + Ag reflector structure, including a cobalt absorbing layer is illustrated in Figure 4. The structure of this filter is likewise tabulated with the thicknesses and refractive indices of the respective layers set out in Table 4 of the Appendix. The peak reflectance is at around 1425nm and again there is a wide rejection band, which here extends down to around 1250nm before a lower broad reflection peak appears.

Filters according to the present invention may be fabricated using conventional thin film deposition techniques where possible, provided that particular care is taken to avoid contamination with impurities. Where actual filter performance deviates substantially from calculated expectations it has been found that the accidental introduction of contaminants during deposition and/or subsequent atmospheric deterioration of the layers are generally the causes. DC magnetron sputtering is a preferred technique, which appears to reduce the risk of accidental contamination, in particular for deposition of the absorbing metal layer.

It is important to control the thickness of each deposited layer of the filter in order that the performance of practical embodiments matches that calculated. Control is conveniently achieved by actively monitoring changes in reflectance/transmittance during deposition of each layer. In the control method used by the inventors, changes in transmittance are monitored. In this case a monitor beam is transmitted and detected at a wavelength where the required layer thickness is calculated to produce a turning point or a specified absolute value in the monitored transmittance. Dispersion is taken into account. A broad spectrum optical source in combination with a monochromator may be used to provide a monitor beam at a suitable wavelength for each layer.For most layers, deposition is continued until there is a turning point in the intensity of the beam transmitted through the layer (and thus the system transmittance) at which point deposition is terminated. For other layers, generally the thinner layers, the absolute intensity of the transmitted radiation is monitored and compared with predetermined values to control the deposition.

Figure 5 shows graphically the variation in monitor signal, in this case at the specific design frequency of 1425nm, for the multi-layer filter structure tabulated in Table 4, illustrating the kind of transmittance changes which may be expected. The actual monitoring wavelengths for each layer for which the corresponding layer thicknesses produce a turning point in transmittance are also indicated on Figure 5. A full list of the relevant monitoring parameters for the same filter is given in Table 1 of the Appendix.

As one alternative to cobalt for the metallic absorbing layer in a filter with 1425nm centre wavelength, platinum has desirable properties. Platinum has a similarly high product n'n" and, additionally, is relatively more inert in air under normal conditions and readily forms a fine grained film structure.

The calculated performance of a filter according to the present invention using platinum is shown graphically in Figure 6. As with the previous cobalt filters, the dielectric layers are agin provided here by ZnS (high index) and NgF2 (low index). Away from the passband, this filter shows an improvement in rejection over that of the filter of Figure 4. Monitoring parameters for the platinum filter structure of Figure 6 are listed in Table 2 of the Appendix.

The calculated performance of a further example filter using a platinum absorbing layer is shown in the graph of Figure 7. In this case cryolite replaces MgF2 as the low index dielectric. In the calculation of this performance an allowance has been included for stabilisation processes, due to penetration of atmospheric water, which are known to induce relative changes in refractive index of cryolite of the order of (1.7 + 0.08)x10-2. This is responsible for the shift in peak reflection from the nominal desired 1425nm to the 1434.4nm indicated on Figure 7.

Two examples of actual filters were fabricated to the specifications of Table 6. As recorded, four materials were used to make the filter layers, namely cryolite, zinc sulphide (ZnS), platinum (Pt) and silver (Ag). Cryolite was used in preference to NgF2 to permit deposition onto a cold substrate, which is not suitable for deposition of MgFZ. The layers were deposited onto a glass substrate, which was precleaned before mounting in a vacuum deposition system where it was initially bombarded with neutral Ar atoms to remove any residual surface contaminants before deposition. The film thickness was monitored using the procedures described above adapted as appropriate.The thickness of all but the thinnest two layer was achieved by terminating the deposition process at turning points in the transmittance at predetermined wavelengths. The deposition of the other layers was terminated at prescribed transmittance values with some allowance made for response lag in the monitoring and deposition systems.

The measured reflectances of the two example filters made by this method are shown graphically in Figures 8 and 9, and relevant results for each filter are shown in Tables 7 and 8 of the Appendix respectively. Comparing the actual performance of these filters with the calculated performance of Figure 7, there is generally close agreement. The slight offset of centre wavelength is considered to be due to minor deviations in actual layer thicknesses, the performance being relatively sensitive, for example, to the precise thickness of the absorbing layer. It is anticipated that with more experience and closer control of the deposition processes actual filter performance can be made to match even more closely to that predicted, so that full advantage can be taken of the improvement offered by filters according to the present invention.

The filters described are fixed bandwidth filters but it is possible to modify them so as to be tunable, for example, by varying the thickness of the layers laterally across the filter in a manner similar to that described in "Narrow-band position-tuned multi-layer interference filter for use in single-mode fibre systems", Electronics Letters Vol. 21, No. 18, pages 798-799.

APPENDIX (i) TABLE 1 Intensity transmittance and layer turning point wavelengths of cobalt reflection filter 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 Design: Glass/L 2 M 4 L H L H L H L S L H 7/Ag 1 2 3 4 5 6 Material MgF2 ZnS Co ZnS MgF ZnS Thickness 258.15 106.44 15.00 41.69 258.15 157.54 (nm) Initial trans at 95.7 98.7 69.1 21.8 25.5 34.2 1425nm Wavelength of trans 1425 1023.93 (1425) (1425) 1759.76 1538.33 turning pt Initial trans 95.7 97.7 (69.1) (21.8) 23.9 33.8 at turning pt wavelength Final trans at turning pt 98.7 64.7 (21.8) ( 25.5) 33.2 11.2 wavelength TABLE 1 CONTINUED Layer No. 7 8 9 10 11 Material NgF2 ZnS NgF2 Zns NgF2 Thickness 258.15 157.54 258.15 157.54 258.15 (nm) Initial trans at 12.3 20.4 5.0 9.1 1.9 1425nm Wavelength of trans 1480.50 1455.02 1442.29 1435.12 1431.07 turning pt Initial trans at turning pt 11.6 19.9 4.9 9.0 1.9 wavelength Final trans at turning pt 19.6 4.8 8.9 1.9 3.6 wavelength TABLE 1 CONTINUED Layer No. 12 13 14 15 16 Material ZnS MgF2 ZnS MgF2 Ag Thickness 314.06 258.15 157.54 236.29 70.0 (nm) Initial trans 3.6 3.6 1.9 9.2 5.1 at 1425nm Wavelength of trans 1425 1425 1425 1419.24 turning pt Initial trans at turning pt 3.6 3.6 1.95 9.0 wavelength Final trans at turning pt 3.6 1.95 9.2 5.0 wavelength APPENDIX (ii) TABLE 2 Intensity transmittance and layer turning point wavelengths of platinum reflection filter 1 2 3 4 5 6 7 8 9 10 11 12 13 14 Design:Glass/L 2 N 4 L H L H L S L H 7/ Ag 1 2 3 4 5 6 Material NgF2 ZnS Pt ZnS NgF2 ZnS Thickness 258.15 134.25 14.00 29.89 258.15 157.54 (nm) Initial trans at 1425 nm 95.7 98.7 64.5 17.9 19.6 28.9 Wavelength at turning pt 1425 1245.13 (1425) (1425) 1617.18 1501.48 Initial trans at turning pt 95.7 98.6 (64.5) (17.9) 18.7 28.9 wavelength Final trans at turning pt 98.7 63.3 (17.9) (19.6) 28.4 8.3 wavelength TABLE 2 CONTINUED Layer No. 7 8 9 10 11 Material MgF2 ZnS NgF2 ZnS MgF2 Thickness 258.15 157.54 258.15 313.09 258.15 (nm) Initial trans at 8.7 15.1 3.5 6.4 6.4 1425 nm Wavelength at turning pt 1463.85 1446.33 1437.37 1425 1425 Initial trans at turning pt 8.5 14.9 3.4 6.4 6.4 wavelength Final trans at turning pt 14.8 3.4 6.3 6.4 3.5 wavelength TABLE 2 CONTINUED Layer No. 12 13 14 Material ZnS MgF2 Ag Thickness 157.54 236.29 70.0 (nm) Initial trans 3.5 15.2 8.8 at 1425 nm Wavelength at turning pt 1425 1419.17 Initial trans at turning pt 3.5 15.0 wavelength Final trans at turning pt 15.2 8.7 wavelength l018P APPENDIX (iii) TABLE 3 re: Fig 3 9 LAYERED REFLECTION FILTER DESIGNED AT 1425 nm STRUCTURE:GLASS/ L 2 N 4 L H L S 7 / Ag THICKNESS(nm) REFRACTIVE INDEX GLASS 1.52 L 258.1822 1.38 NgF2 R max=90 09 / 2 106.4370 (2.2613) ZnS R1075=5.090/o M 15.0 (3.4586+6.8447i) Co R1775=7.520/o 4 15.5882 (3.7697) Si H 94.5035 (3.7697) Si S 188.5690 (3.7697) Si 7 236.29 1.38 NgF2 Ag --- (0.1352 + 10.3131i) Ag TABLE 4 re: Fig 4 ABSORBING REFLECTION FILTER OF MgF2j ZnS, Co. & Ag STRUCTURE: GLASS/ L 2 M 4 (L H)3 L S L H 7 / Ag THICKNESS ( nm) REFRACTIVE INDEX L 258.1522 1.38 NgF2 Rmax=91.18% 2 106.4370 (2.2613) ZnS M 15.0 (3.4586+6.8447i) Co 4 41.6949 (2.2613) ZnS H 157.5421 (2.2613) ZnS S 314.0563 (2.2613) ZnS 7 236.2946 1.38 MgF2 TABLE 5 re:Fig 6 ABSORBING REFLECTION FILTER OF ZnS NgF2, PE and Ag STRUCTURE: GLASS/ L 2 M 4 (L H)2 L S L H 7 / Ag THICKNESS(nm) REFRACTIVE INDEX L 258.1522 1.38 NgF2 rA =0.286952 2 134.2462 (2.2613) ZnS #1/2=5x10*-3 N 14.0 (4.9306+6.9603i) Pt Rmax=9376 /o 4 29.8923 (2.2613) ZnS R1775=4.060/o H 157.5421 (2.2613) ZnS Bandwidth at S 313.0917 (2.2613) ZnS Rmax = 6.5nm 7 236.2946 1.38 NgF2 2 Rol% after 1383 nm and falls R < 20/0 after 1452nm APPENDIX (iv) TABLE 6 re: Fig 7 THEORETICAL REFLECTANCE OF CRYOLITE ZnS, Pt and Ag ABSORBING REFLECTION FILTER, INCLUDING STABILIZATION OF CRYOLITE STRUCTURE:GLASS/ L 2 M 4 (L H)2 L S L H 7 / Ag THICKNESS(nm) REFRACTIVE INDEX L 263.8889 1.373 Cryolite Rmax-93.6 /o 2 134.0866 (2.2613) ZnS at 1434.5 nm M 14.5 (4.9306+6.9603i) Pt R1775=4.60/o 4 27.8271 (2.2613) R > lO/o after H 157.5421 (2.2613) 1377nm S 313.4271 (2.2613) and falls 7 242.0258 1.373 R < 20/o after Ag (0.1352+10.313li)Ag 1463nm TABLE 7 re: Fig 8 MEASURED REFLECTANCE OF Cryolite, ZnS, Pt and Ag ABSORBING REFLECTION FILTER 510/1 Monochromator bandwidth = 0.8nm Same design as fig 7 for peak reflectance at 1425 nm (nominal) Measured after 14 days: Rmax= 77.90/0 at 1440 nm Bandwidth at Rmax = 5.5 nm 2 R1780= 12.20/0 RN"30/o between 1160 and 1400 nm TABLE 8 re: Fig 9 MEASURED REFLECTANCE OF Cryolite, Zns, Pt and Ag ABSORBING REFLECTION FILTER 510/2 Monochromator bandwidth = 0.8nm Same design as Fig 7 for peak reflectance at 1425 nm (nominal) Measured after 7 days Rmax= 82.60/0 at 1435.6nm Bandwidth at Rmax = 6.1nm 2 R1780= 9.60/0 R X 20/0 between 1170 and 1410nm

Claims (7)

1. CLAIMS 1. A reflection filter comprising a first, multi-layer system including two dielectric systems and a metallic absorbing layer positioned therebetween for absorbing radiation with wavelengths outside a predetermined bandwidth, the metallic absorbing layer having a refractive index of the form n=n'+in" being selected to have a high product n'n" and a low dispersion over the predetermined bandwidth, and at least one of the dielectric systems including a set of at least two layers of dielectric materials with substantially different refractive indices, and a second system including a reflector arranged to reflect radiation passing through the first system back through the first system.
2. 2. A filter according to claim 1, wherein the absorbing layer is positioned such that in use it is located substantially at an electric field node of the standing wave set up in the filter at the centre wavelength to be filtered within the predetermined bandwidth.
3. 3. A filter according to claim 1 or claim 2, wherein the absorbing layer is of platinum.
4. 4. A filter according to any preceding claim, wherein a set of dielectric layers comprises a layer of one dielectric material sandwiched between two layers of a second dielectric material.
5. 5. A filter according to any preceding claim, wherein the materials of the two dielectric layers of a set comprise ZnS and MgF2.
6. 6. A filter according to any preceding claim, wherein the materials of the two dielectric layers of a set comprise ZnS and cryolite.
7. 7. A reflection filter substantially as hereinbefore described.