Interference filter

An interference filter, also referred to as a notch filter, of the type which reflects a certain wavelength range while letting pass over wavelengths bands of a range is disclosed. The filter is designed for operation in the visual and infra-red regions of the electromagnetic spectrum and in the disclosure the uniform layer pairs of material are such that the optical thickness of each layer of the or each layer pair deposited on a plane light-transmitting substrate is a non quarter-wave unequal thickness with respect to its complementary layer. Each layer pair has an overall optical thickness adding up to in total a half-wave thickness. The technique of degrading a standard quarter wavelength layer is also employed on a curved surface substrate with non-uniform layers, where one layer of a layer pair increases from the center to the periphery of the substrate and the other layer of the pair decreases from the center to the periphery of the substrate; a half-wave optical thickness being maintained throughout the layer pair.

BACKGROUND OF THE INVENTION 
This invention relates to an interference filter of the type reflecting a 
certain wavelength band within a given wavelength range while letting pass 
other wavelength bands of the range. 
Conventional interference filters of the above type are generally comprised 
of a plane surfaced substrate, such as glass, upon which has been 
deposited by e.g. vacuum deposition a plurality of alternate high and low 
refractive index thin uniform layers where each layer is arranged in a 
quarter-wave thickness or an odd multiple of a quarter-wave, at the chosen 
wavelength which is required to be reflected. Reflection efficiency and 
band width of the reflection zone are controlled by the ratio of the index 
of refraction of each high-low index layer pair. The efficiency of 
reflection can be increased by increasing the number of layer pairs. 
Such conventional quarter-wave designs are in theory capable of providing 
narrow band reflection at specified points anywhere within a wide spectral 
range but in practice there is a limit to the choice of materials 
available for any particular wavelength band which it is required to 
reflect. Whilst in some parts of the electro-magnetic spectrum there is a 
wide choice of materials available, in other parts, such as in the 
infra-red region, there are only a very limited number of materials 
available. This can make the construction of narrow band reflection 
filters difficult and in some cases impractical. 
It is an object of the present invention to provide an interference filter 
for narrow band reflection in which the aforementioned disadvantages are 
obviated. 
SUMMARY OF THE INVENTION 
The principle of the design technique used in accordance with the 
invention, as hereafter defined, is to degrade the quarter-wave optical 
thicknesses of each layer pair in the interference filter, but in such a 
way that the total thickness remains equal to a half-wave optical 
thickness or multiple thereof. This ensures that the maximum peak of 
reflection is maintained at the desired reference wavelength. Although it 
would be expected in such circumstances that small changes to the 
thicknesses of each layer pair would not seriously degrade the overall 
optical characteristic of the filter, surprisingly it has also been found 
that very substantial changes (which would be necessary to produce 
extremely narrow band reflection spikes of the order of 1% of the 
reference wavelength) do not either seriously degrade the overall optical 
characteristic of the filter. In fact, it has been found that as the 
thickness of one layer is increased and the other layer decreased from a 
quarter-wave optical thickness, the band width of the reflection zone and 
magnitude of the reflectivity at the peak reflection wavelength are 
simultaneously reduced, but the essential reflection spike characteristic 
is still maintained. 
If a filter is required having a band width of reflectivity of the order of 
1% this accordingly requires the thickness of one of the layer pairs to be 
just less than a half-wave optical thickness, and for the other layer of 
the layer pair to be extremely thin so as to maintain the total period 
thickness, i.e. the thickness of the or each layer pair, equal to a 
half-wave optical thickness or a multiple thereof. In order to increase 
the peak reflectivity, additional periods (layer pairs) can be introduced 
until the desired peak reflectivity is achieved. While this technique is 
discussed in relation to the application of uniform layers on to plane 
surfaced substrates, the technique is similarly applicable to curved 
surface substrates. 
A potential application of this technique is in the design of multilayer 
interference filters employed to provide protection from exposure to 
harmful laser radiation. If the laser operates within the visible 
wavelength spectrum, or more generally within the sensitivity waveband of 
the detector being employed, then a notch reflection filter can be used to 
attenuate the laser radiation whilst maintaining maximum transmission of 
other useful wavelengths. Very frequently, and particularly in the case of 
visual applications (e.g. spectacle lenses), the surfaces to which the 
laser filter must be applied is curved in shape. In addition, it is often 
found that the angular range over which protection is required is a 
variable function of position on the curved surface. 
In certain applications, it has been found that the angular protection 
range required is greater towards the periphery of the curved surface than 
it is at the centre of the lens. In order to cover a wide angular range, a 
notch filter must of necessity possess a correspondingly wide reflection 
waveband, since the entire optical characteristic shifts towards shorter 
wavelengths with increasing incidence angle. This means that if a uniform 
coating is deposited on the curved surface, the angular protection 
achieved at the centre of the component will be greater than necessary, in 
order that adequate protection can be conferred at locations towards the 
edge of the component. Consequently, the integrated transmission of useful 
energy through the centre of the lens will be reduced. This problem can be 
largely overcome by using the design techniques described herein with an 
arrangement whereby the layer thicknesses of vacuum deposited coatings 
applied to a curved surface are adjusted from the centre to the periphery 
of the surface to give the optimum bandwidth corresponding to the required 
angular coverage in the centre of the component and to give a wider 
angular coverage towards the periphery of the component. Accordingly, the 
invention provides an interference filter comprising at least one pair of 
high and low index of refraction light transmitting layers on a light 
transmitting substrate, the or each layer pair reflecting at least part of 
a certain wavelength band within a given wavelength range while 
transmitting other wavelength bands of the said range, each layer of the 
or each layer pair being of a non quarter-wave unequal thickness with 
respect to its complementary layer, and each layer pair having an optical 
thickness adding up in total to a half-wave thickness. 
The design technique employed in accordance with the teaching of the 
invention thus employs a filter construction whereby the total thickness 
of the or each layer pair is the same as that of a conventional 
quarter-wave layer-pair design, but where one layer is of a thickness 
greater than a quarter-wave, and less than a half-wave optical thickness 
and the thickness of said other layer is equal to an optical thickness 
which is required to complete a half-wave when added to the optical 
thickness of said one layer. Such a design is substantially different from 
a quarter-wave multi-layer filter design of conventional form and offers 
several distinct advantages. 
Such design techniques may be conveniently employed to an interference 
filter where one layer of the or each layer pair progressively increases 
in thickness from the centre to the periphery of the surface of the 
substrate, whereas the other layer of the or each layer pair progressively 
decreases in thickness from the centre to the periphery of the surface of 
the substrate. 
Preferably the thickness of said one layer and said other layer at the 
periphery and the centre respectively of the surface of the substrate is 
greater than a quarter-wave, and less than a half-wave optical thickness, 
whereas the thickness of said other layer and said one layer at the 
periphery and centre respectively of the surface of the substrate is equal 
to an optical thickness which is required to complete a half-wave when 
added to the optical thickness of said one layer and said other layer 
respectively at the periphery and the centre respectively of the surface 
of the substrate. 
Thus, the invention provides for the first time the possibility of making 
an interference filter, such as a notch filter, which is effective in 
parts of the electromagnetic spectrum such as the infra-red region which 
are otherwise difficult to cover in view of the scarcity of suitable 
materials for use in designs of conventional type. Conveniently, zinc 
selenide and zinc sulphide can be used if the filter is to be used to 
achieve narrow band reflection at 10.6 microns. Narrow band reflection at 
this wavelength is particularly difficult to achieve satisfactorily using 
conventional techniques in view of the scarcity of materials having the 
required refractive index in this region of the spectrum. 
The invention provides additional increased scope in all spectral regions 
for matching the exact bandwidth requirement with that which can be 
theoretically designed i.e. it is not limited to quantum changes in 
bandwidth achieved by using different refractive indices of available 
materials or by changing the order of the reflection harmonic. 
Preferably, an anti-reflection layer or layers is added to the interference 
filter according to the invention, which layer or layers may conveniently 
comprise or include thorium fluoride. 
For an understanding of the priniples of the invention, reference is made 
to the following examples and drawings in which:

DESCRIPTION OF THE PREFERRED EMBODIMENT 
Example 1 
The example shown in the drawing has the construction: 
EQU Air/0.336A,0.2,0.336A,(0.32C 1.68B).times.56, 0.32C/zinc selenide substrate 
where it is understood that A, B and C represent, respectively, 
quarter-wavelength thickness layers, at a reference wavelength of 10.6 
microns, of thorium fluoride, zinc selenide and zinc sulphide 
respectively. Thus, the first layer next to Air is a layer of thorium 
fluoride of thickness 
##EQU1## 
since the refractive index of thorium fluoride at 10.6 microns is 1.35, 
and so on. 
The designation "x 56" indicates a multilayer stack of 112 layers (or 56 
layer pairs) each layer being constructed in the manner indicated between 
the brackets. The first three layers in this construction are used for 
anti-reflection purposes. 
In the drawing there is shown the spectral characteristics of a filter 
constructed in accordance with the above example. It will be seen that a 
sharp dip or notch is present in transmission of the filter around the 
10.6 micron region. This in turn means that in this region of the spectrum 
the filter effectively reflects most of the electromagnetic radiation and 
achieves narrow band reflection as required, but without using a 
conventional quarter-wave design. Indeed, the design of the filter is 
anything but quarter-wave in design, although each layer pair still adds 
up to a total half-wave thickness or multiples thereof. The design of the 
filter made in accordance with Example 1 thus achieves a theoretical 
reflection efficiency in excess of 90% within a band width of less than 
about 2.3% of the reference wavelength of 10.6 microns. Additional layers 
can be used to increase the peak reflectivity above 90%, and the layer 
thicknesses can be adjusted to reduce the bandwidth, if desired. 
Example 2 
A second example in accordance with the invention has the construction: 
EQU Air/A,0.42B,0.164A,(1.948B,0.052A).times.41/zinc selenide substrate 
where it is understood that A and B represent, respectively, 
quarter-wavelength thickness layers at a reference wavelength of 10.6 
microns, of thorium fluoride and zinc selenide. A filter made in 
accordance with this example theoretically possesses similar optical 
performance characteristics to that of the first example and, once again, 
the first three layers in the design construction are also anti-reflection 
layers. 
Example 3 
A third example has the construction: 
EQU Air/A,0.424B,0.164A(1.92B,0.08C).times.47/zinc selecnide substrate, 
where it is understood that A, B and C represent, respectively, 
quarter-wavelength thickness layers at a reference wavelength of 10.6 
microns of thorium fluoride, zinc selenide and lead fluoride. This filter 
also possesses similar theoretical optical performance characterists to 
that of the first and second examples and, once again, the first three 
layers in the design construction are anti-reflection layers. 
In the application of the technique to the design of multi-layer 
interference filters to glass lenses, for instance, which have a curved 
surface, the angular range of protection required at the centre of the 
lens, in relative close proximity to the eye, is often less than that 
towards the periphery. Accordingly this means that the bandwidth of the 
filter in the centre of the lens need not be as great as that at the 
periphery. Therefore, by controlling the bandwidth of the filter to 
correspond closely to that required at any point on the surface of the 
lens ensures that the integrated visible transmission is maximised, 
particularly in the centre of the lens where high transmission is most 
important while maintaining full angular protection at the lens periphery 
where transmission is less critical. 
By employing deposition of multi-layers by vacuum evaporation across the 
curved surface of a lens to a prescribed thickness profile it is possible 
to provide a reflection notch filter whose bandwidth varies as a function 
of position on the lens surface. 
The bandwidth at any particular point on the surface of the lens will be 
the minimum required to achieve the specified angular protection thereby 
ensuring the maximum integrated visible transmission. 
In construction, one layer of a layer pair is arranged to progressively 
increase in thickness and the other layer of the layer pair is arranged to 
progressively decrease in thickness from the centre to the periphery of 
the lens. At the thicker portion of any layer, either at the centre or 
periphery of the lens, the thickness is greater than a quarter-wave and 
less than a half-wave optical thickness. At the thinner portion of any 
layer, at the centre or periphery of the lens, the thickness is equal to 
an optical thickness which is required to complete a half-wave when added 
to optical thickness of the other layer of a pair at an adjacent position 
thereof. Consequently the optical thickness of a layer pair at all points 
across the lens surface is always equal to a half-wave although their 
relative thicknesses vary between each point. 
Thus the bandwidth of the reflection notch is made to match the 
requirements of different angular coverages at different points on the 
surface of the component, ensuring the maximum possible integrated 
transmission. For ease of comparison with FIG. 1, FIG. 2 shows the 
transmission characteristic of a 10.6 micron reflection notch at two 
different points on the surface of an optical component. It should be 
understood, that while this illustrates the performance of a filter at 
10.6 microns, the same interference notch principles apply to other 
regions of the electromagnetic spectrum, and in particular, for example, 
the visible wavelength region. The narrower notch F1 gives an angular 
coverage of 0.degree. to 8.degree. at the 90% reflection level, and the 
wider notch F2, a corresponding coverage of 0.degree. to 21.degree.. This 
is illustrated in FIG. 3 where the reflectivity for p-polarised light has 
been computed as a function of incidence angle at a wavelength of 10.6 
microns for F1 and F2. It should be noted that in FIG. 1 and FIG. 2 the 
co-ordinates are, fractional transmission and wavelength (microns). In 
FIG. 3 the co-ordinates are fractional reflectance and angle of incidence 
(degrees). 
Examples of filters in accordance with the invention are illustrated in 
FIGS. 4 and 5. Referring to FIG. 4, a typical multiple layer coating 
comprises a first 3-layer group 10 constituting an anti-reflection 
coating, and a plurality of half-wave layer pairs generating a 
transmission characteristic of the type shown in FIG. 1, of which only one 
layer pair 12 is shown in FIG. 4. Each layer pair 12 comprises two layers 
12A, 12B, each representing degraded quarter-wave layers, one having an 
optical thickness greater than the quarter wavelength and the other having 
an optical thickness than one quarter wavelength. The substrate is shown 
by reference numeral 14. 
A curved substrate 16 is shown diagrammatically in FIG. 5, having a single 
layer pair 18, of which outer layer 18A decreases in thickness from the 
center of the substrate to the periphery, which the inner layer 18B 
increases in thickness in the same direction, the total optical thickness 
of the layer pair being maintained at a half wavelength. 
Other filters which use different materials operable in the infra-red 
region of the spectrum, such as germanium, lead telluride and cadmium 
sulphide can be designed using the techniques in accordance with the 
teaching of the invention. Indeed the same techniques can be used for 
other wavelength regions such as the visible portion of the 
electro-magnetic spectrum, using materials such as titanium dioxide, 
zirconium oxide, silicon dioxide, and aluminium oxide or other materials 
suitable for this spectral region. 
Using this design technique in accordance with the teaching of the 
invention has the advantage of greatly extending design flexibility, since 
a range of bandwidths can be covered which are not limited by the 
refractive index ratio. The total thickness of the filter, as compared to 
a conventional quarter-wave design, may also be reduced by about 25% which 
in turn means that the time taken to deposit the layers is reduced. 
Furthermore, since one material can be made significantly thinner than the 
other in each layer pair it follows that the most absorbing or scattering 
layer material in the design can be chosen as the thin layer. In addition, 
when it is required to achieve extremely narrow band reflection, say of 
the order of 1%, the use of higher order stacks is often precluded because 
of the consequential appearance of other reflection harmonics in regions 
close to the desired reflection spike, where high transmission is 
required. Such harmonics can be suppressed by known design techniques, but 
this adds greatly to the complexity of the filter. Using the design 
technique in accordance with the invention, no such undesirable harmonics 
appear in regions close to the desired reflection spike. 
The invention is particularly useful in the infra-red region of the 
spectrum, where materials of suitable refractive index are scarce anyway, 
and especially if the filter is to be used to reflect incoming laser light 
such as in the 10.6 micron region of the spectrum. Normally, quarter-wave 
thickness designs can be prone to thermal damage as a result of 
discontinuities in the standing wave electric field intensity profile when 
exposed to intense laser radiation. However, the design in accordance with 
the invention can, by the use of relatively thin layers and layers with 
refractive indices close to each other, reduce the standing wave electric 
field intensity profile discontinuities and hence increase the laser 
damage threshold. 
In addition to the foregoing, it is possible to evolve a design in 
accordance with the teaching of the invention whereby the problems 
associated with optically "matching" the multi-layer filter to the 
surrounding media are minimised. With conventional design techniques an 
intense reflection ripple pattern inevitably occurs in wavelength regions 
close to and either side of the main reflection spike of a conventional 
quarter-wave stack. Such a reflection ripple detracts from the 
transmission performance of a conventional filter and can be difficult to 
remove by heretofore known design techniques. However, the present 
invention has the further advantage in that if a material is chosen for 
the layer with thickness just less than a half wave optical thickness such 
that the layer possesses a refractive index close to or equal that of the 
substrate refractive index, then the reflection ripple will be negligible 
in amplitude and transmission can be increased simply using a 
conventional, simple anti-reflection coating of the same design as would 
normally be used for the substrate material without the interference 
filter being present. 
It will be apparent that the invention provides a significant step forward 
in the design and construction of interference filters and although the 
invention has been exemplified above it will also be apparent that other 
forms of filter can be constructed as required without departing from the 
spirit or scope of the present invention.