OPTICAL FILTERS

Abstract: A method of manufacturing optical filters comprises the steps of depositing a plurality of monomolecular layers of at least one organic material on a substrate using Langmuir Blodgett techniques. The optical characteristics of the organic material or materials is known and, using the aforesaid technique, layers of predetermined optical thickness can be deposited. The organic material or one of the organic materials has non-linear optical characteristics so that a variable filter is formed.

Description: OPTICAL FILTERS This invention concerns optical filters, particularly, but not exclusively, optical rugate filters. Optical filters have generally been constructed using inorganic materials. Quarter wavelength filters are well known for the rejection of radiation of a specific wavelength. Multiple rejection may then be achieved by the incorporation of other quarter wavelength structures having respective λ rejec t ion at the further frequencies. High level rejection is possible at specific frequencies but the construction of such filters is difficult and costly. Rugate filters allow more generalised optical filter characteristics to be achieved more easily. A rugate structure is one in which the ι; refractive index varies continuously through the thickness of the filter. In its simplest form, this variation is sinusoidal, i.e., the refractive index n(t) varies as a function of film thickness, t, as: n(t) = riaverage - Δn COS foπ t) P where n av erage i the average refractive index Δ n is the maximum index excursion p is the thickness of one cycle and is given by: P = λ rejection ~ n average Multiple rejection may be achieved by superposition of the characteristics period of the different frequencies. Optical interference filters built up from multilayer stacks of high and low refractive index materials have conventionally been formed using inorganic dielectric materials because of their good mechanic and optical properties, and the availability of a wide range of refractive indices. However, with many of the deposition techniques employed, accurate control of the film thickness is difficult to achieve. The Lang uir-Blodgett (LB) deposition technique provides the capability to form high quality thin films using organic materials. The successive deposition of monomolecular LB layers enables films to be build up with extremely accurate control of thickness and uniformity, each monolayer being approximately 25_l thick. Additionally, the use of organic materials offers an increased range of refractive indices for the design of filter structures, and the capability for application-specific tailoring of the materials' mechanical, chemical or optical properties. The design of a multilayer optical stack filter requires an accurate knowledge of the refractive indices and monolayer thicknesses of the two materials employed. These properties are deteπαinable by a number of techniques including ellipsometry, reflectometry and optical waveguiding. The measured optical thicknesses may be used to design multi-cycle anti-reflection filters. It is an object of the present invention to provide an optical filter of novel construction using the LB technique. According to the present invention, there is provided an optical filter comprising a substrate having deposited thereon a plurality of mono-molecular layers, to provide a deposit of a predetermined thickness, of at least one organic material the organic material or one of the organic materials having non linear optical properties whereby its refractive index may be controlled such that the predetermined thickness constitutes a quarter wavelength rejection filter at a specific \ rejection. The plurality of mono-molecular layers may be of different organic materials having different refractive indices. The LB technique is particularly attractive for the formation of rugate type filters because of the fine resolution of film thickness it can yield. A rugate structure may thus achieve by depositing a sequence of LB layers that constitute a digital representation of the required sinusoidal refractive index profile - the smaller the digitisation step, the more accurate the representation of the desired refractive index profile. (Hence the attraction of LB films) . The LB process has application for the deposition of more coarse feature optical multilayer stacks such as quater-wavelength stacks or etalons. In this case, the fine resolution offered by LB deposition allows very accurate thickness control to be achieved. Variation of the refractive index may be achieved by applying an electric field across the structure. In a preferred embodiment, multiple rejection may provide for rejection of one or more frequencies over the visible and/or infrared wavebands. The invention also provides a method of manufacturing an optical filter comprising the steps of depositing a plurality of layers of at least one organic material, the organic material or one of the organic materials having non-linear optical properties whereby its refractive index may be controlled, on a substrate using LB techniques. Preferably, a plurality of layers of a first organic material are deposited followed by the deposition of a plurality of layers of a second organic material. The method may include the steps of forming a conductive film both beneath and above the layers whereby an electric field may be applied thereacross. The invention will be described further, by way of example, with reference to the accompanying drawings, in which:- Fig. 1 is a diagrammatic illustration of the manner of deposition of a mono-molecular layer on the surface of the substrate; Fig. 2 is a cross-section of an optical filter; Fig. 3 is a view, similar to Fig. 2 , of an electro-optically tunable filter according to the present invention. Fig. 4a and 4b are chemical formulae of two organic materials useful in forming optical filters in accordance with the present invention ; Fig. 5 is a graph illustrating the absorption spectrum of one of the organic materials of Fig. 4; Fig. 6 is a graph illustrating the response of a rugate filter made in accordance with the present invention; Fig. 7a . to 7f are graphs illustrating computer simulations of the response of a rugate filter according to the present invention; Fig. 8a . is a graph illustrating the response of an experimental rugate filter in accordance with the present i nvention; Fig. «Sb_ is a computer simu lation of the ex perimental ru_ . aιe filler of Fi«. 8a_, based on theoretical calculation; \ Fig. 9a and b are graphs illustrating an experimentally obtained and a computer simulation of a nine cycle quarter wave multilayer filter in accordance with the presents invention. Referring to the drawings, an optical filter, according to the present invention, comprises a substrate 10 having one or more layers of an organic material or organic materials deposited thereon. The substrate 10 has its surface cleaned and made hydrophobic or hydrophilic as required. A tank 12 contains a liquid vehicle 14 on the surface of which is formed a monomolecular layer 16 of an organic material. The substrate 10 is supported by a mechanism 18 for insertion through the surface layer 16 (or for extraction through the layer 16 depending upon the required orientation of the molecules of the layer 16 on the surface of the substrate, as is well known) . Repeated passage through the layer 16 deposits further monomolecular layers on the surface of the substrate. The same tank 12 or further tanks 12a, 12b and so on, as necessary, may be provided whereto the substrate 10 may be transported by the mechanism 18 for depositing further monomolecular layers of the same or different organic materials on the surface of the substrate 10. As shown in Fig. 2, an non-tunable optical filter comprises the substrate 10 e.g. glass whereon a quarter wave filter has been formed by the deposition of a plurality of mono-molecular layers 20 of the same or different organic chemicals to provide an optical thickness equal to a quarter of the wavelength of a radiation to be absorbed. Optical thickness is defined as being equal to physical thickness multiplied by refractive index. However, if according to the present invention, the ??? of the organic materials exhibits non-linear optical properties and an electric field is applied across the filter, the resultant filter is tunable. Fig.3 diagrammatically illustrates an electro-optically tunable filter according to the present invention. The filter has a plurality of multilayers 22 of organic materials on a substrate 10. Transparent conductive layers 24 are deposited on the substrate 10 below and above the plurality of multilayers 22. One or more of the organic materials forming the multilayers 22 exhibits non-linear optical properties when an electric field is imposed thereacross from a source 26 connected to electrodes 28 on the conductive layers 24. As is described more fully below, the multilayers comprise a plurality of mono-layers of a first organic material interspersed by a plurality of mono-layers of a second organic material. In the formation of a rugate filter, the plurality comprises a number of sets or cycles each composed of several multilayers of the first organic material interspersed by multilayers of the second organic material For the successful construction of LB optical filters, particularly rugate filters, several material properties must be considered. Two materials of different refractive index (n o ) are required. They must be capable of depositing as multilayer structures, and must be compatible with one another for LB deposition. In addition at least one of the materials is optically non-linear, so that the filter rejection wavelength(s) can be changed. Another important consideration is that of film stability. The materials should produce high quality amorphous films so that damage by incident irradiation and scatter are reduced. Degradation due to thermal, chemical, mechanical and ageing effects should not be significant. Materials found useful in this invention are PSA 50 and w-TA (see Fig. 4) . PSA 50 is a polysiloxane based material (described more fully in G.B. specification No. 2209169) that has a non-linear optical side-chain synthetically grafted onto the dimethyl polysiloxane backbone to produce approximately 50% backbone substitution. PSA 50 forms good LB multilayers which are known to provide relatively low loss waveguiding. It exhibits non-linear optical behaviour. As it is a polymetric material, it is believed to be more stable than conventional LB materials. w-TA also forms LB multilayers with ease. This material is commercially available. w-TA may have improved stability as the molecule contains a polymerisable function, and it is known that polymer resits for lithography can be formed. A further advantage of w-TA is that multilayers are deposited at a high rate - approximately 50 times that of conventional LB materials. For thin film filters such as rugate filters, the two material should have different refractive indices. A refractive index difference ( n) = 0.065 has been established for these two materials in the LB format. This difference is a direct result of the molecular structure of the materials and the manner in which they are deposited as LB films (orientation, packing, etc) . The refractive index of PSA 50 (n 0 = 1.555) is largely due to the conjugated side-group which is based on the azo benzine unit. To increase n 0 (increase .n) , it would be necessary to produce a system with a greater molecular conjugation. This would also increase the absorption of the material. Fig. 5 shows the absorption spectrum of PSA 50 which has a band-edge at approximately 400nm and tails into the he visible region. A similar material, but with more extended conjugation, would probably cause this band-edge to be shifted across the visible region towards the IR. This would limit the capability of the filter for operation in the visible/near IR region of the electromagnetic spectrum. Consequently, it is believed that it would be extremely difficult to develop a good non-linear optical LB material with a higher n 0 than PSA 50 for operation in the visible/near IR region. w-TA is a relatively simple, hydrocarbon based material with no regions of extended conjugation within the molecule. Reduction of its refractive index (n 0 = 1.490) would be difficult without considerable effort. As a result n would seem to represent a value that may only be increased by either extensive materials research or by compromising the performance of a rugate filter in accordance with the invention. Whilst the PSA 50 and w-TA combination represents, at the present time, the best material system known to the applicant, the invention is not limited to this combination or either of these materials. In the design of a rugate filter, it is necessary to know the refractive index (n 0 ) and thickness (t) of the components accurately. The values for PSA 50 and w-TA were obtained by measuring variations in reflectivity as a function of wavelength for LB multilayers of the two materials independently. Reflectivity maxima and minima are observed when the optical thickness of the film corresponds to an integer number (m) of quarter wavelengths ( ) : These values are the optical thickness of the material at a wavelength of 530nm. For many materials, n 0 varies with wavelength (dispersion). For ω-TA, it was established that there is virtually no dispersion over the wavelength range 300-800nm. However, for PSA 50, n 0 increases towards the absorption edge (400nm). The optical thickness of each material was therefore determined at the specific " wavelength for the filter application (530nm). Estimates were then made for the thickness of each material. A value for PSA 50 - 2.2nm per monolayer was used, as this value has previously been obtained from waveguiding studies. For ω-TA, a value of 3.0nm per monolayer was assumed, based upon a molecular model. From these values, and those of optical thickness, the refractive indices were calculated as 1.61 and 1.463 for PSA 50 and ω-TA, respectively. Using a computer program for calculating the response of a filter, a model was set-up based upon a cycle of six alternate multilayer stacks of ω-TA and PSA 50. The data for this single cycle is shown in Table 1. Figure 7 illustrates how the calculated response of the filter at 530nm develops as further cycles are added to the rugate for up to six cycles. Table 1 : Data For Single Cvcle Filter Based Upon PSA 50 and ω-TA An LB rugate filter corresponding to 3 cycles Fig. 7c. was prepared experimentally, and its response measured (Figure 6). The filter response can be seen to be qualitatively similar to the modelled result. However, the reflection depth was observed to be only _ 7% compared with the predicted value of 11 %. It was assumed that this was probably due to inaccuracy of the calculated layer thicknesses (which would produce a corresponding error in the n 0 values). By backmodelling, it was possible to obtain a much better fit between the computer simulation and the experimentally obtained rugate response. The values shown in Table 2 were used: Table 2: Monolayer Thickness and refractive Index Data Obtained by Backmodel ling Firstly, it was assumed that the optical thicknesses obtained by reflectivity measurements were accurate. Thereafter, various thicknesses and refractive indices (and refractive index differences were tried on the computer to produce a match between the experimentally obtained results and the simulations. The final t and n 0 values are within 4% of the original values, but the reduction in Δn to 0.065 markedly affects the filter behaviour. Using these values, the measured and the predicted rugate response are in good agreement (see Figure 8). To confirm these final values, a quarter-wave stack was prepared. Table 3 shows the composition of this stack which consists of nine quarter-wave multilayers of alternative ω-TA and PSA 50. Table 3: Data for Quarter-wave Structure Based Upon PSA 50 and ω-TA Figure 9 shows how the measured response (Fig. 9a) very closely matches the theoretical model (Fig. 9b), thereby confirming the accuracy of the t and n 0 values. The differences between the theoretical layer thickness and the value obtained by back-modelling can be explained as follows:- Studies of LB multilayers of ω-TA by other workers have determined tilt angle of the molecules in the layers. Although this has been found to vary with film thickness, surface pressure and substrate, for multilayers consisting of 10-20 monolayers, a tilt angle of typically 24-26° to the layer normal has been established. The quarter-wave structure prepared here consists of individual 30 monolayer stacks of ω-TA. The value of t = 2.95nm implies a tilt angle of 24.4° on the basis of a molecular length of 3.24nm. This agrees with the literature values and lends further support to the experimental results. A till angle cannot be calculated for PSA 50 as there is no sufficiently accurate value for the molecular (side-chain) length. However, the value of t = 2.28nm appears to be sufficiently high to indicate that the side-ch.ains are orientated normal, or near normal, to the layer planes. Two properties of considerable importance when assessing the performance of optical filters are scatter levels and optical damage thresholds. Measurement of the levels of diffuse scatter from individual multilayer samples of PSA 50 and ω-TA, and of the filter structures themselves (rugate and quarter-wave) have been made using a spectrometer system incorporating an integrating sphere. This arrangement allowed all the scattered light (excluding the specular reflection) to be collected and detected, and its variation with wavelength to be assessed. Measurements were performed in the 320-500nm wavelength range. Extremely low scatter levels were obtained, typically of order 0.1% for a 200nm thick film at 500nm wavelength. As expected, the thicker filter structures gave higher scatter levels, but were consistent with a scatter level of order 1% per micron film thickness, which is encouragingly low. Optical damage thresholds were obtained using high power pulses from a Q-swϊtched Nd : YAG laser operating at 1.06μm wavelength. These were focussed onto the film surface, and the % damage probability assessed as a function of optical energy density. A v alue for the optical damage threshold ( defined as the maximum energy densi ty of a 30nsec pulse for which no damage occurred ) for an 88 layer f ilm of v?-TA on BK 7 glass of _2 38 + 13Jcm was determined . This compares f avourably wi th the _2 value of 76 + 25Jem measured for an uncoated 3K7 substrate measured under the same conditions. Measurements of PSA 50 samples yielded slightly lower optical damage threshold values (6 ± 2Jcπr 2 ). However, the quality of these samples had been degraded during previous trials, so it is likely that higher damage thresholds are achievable. For high level rejection of radiation of an unwanted wavelength, it is necessary to have a high optical density (OD) LB film. For example, OD = 2 (99% reflection) is a likely minimum requirement for many applications. To achieve this level of rejection, it will be necessary to grow films of much greater thickness (somewhat greater than lOμm) than the currently demonstrated level (_0.5μm) which produces a reflection of _ 7%. This will simply be a case of depositing a greater number of rugate cycles. A notch filter of high OD may have a relatively narrow bandwidth. It may also be necessary to design a filter that produces notches at other specific wavelengths. Such multiple rejection will be achieved by superimposing individual rugate designs and forming an amalgamated rugate filter with multiple notches. To produce a filter that covers a range of wavelengths (as opposed to specific wavelength) , notch control is necessary. To produce a variable wavelength filter, at least one of the organic materials must exhibit non-linear optical behaviour (NLO). It is of pc'v ntaqe if second-order NLO behaviour can be utilised (as ve th E'SA..;)) as the effects are much larger than third-order NLO. Second-order NLO, for example, permits smaller operating voltages to be used. If the material exhibits third order NLO properties, it is possible to obtain wavelength control as a result of the Quadratic electro-optic effect. Electrodes are required (see Fig. 3) on each side of the filter to allow an electric field to be applied. The base electrode can be part of the substrate onto which the LB layers are deposited, so only the top electrode would need to be deposited after the rugate filter has been formed. This could be done by conventional techniques such as sputtering, evaporation, etc., and a transparent material such as Indium Tin Oxide (ITO) could be used if necessary. Either PSA 50 βr ω-TA exhibit may exhibit third order NLO behaviour. Second order NLO behaviour has been demonstrated for PSA 50. It should therefore be possible to produce a variable filter based upon the linear electro-optic effect. In this case, the layers of the active material need to be deposited in a non-centrosymmetric orientation (for third order NLO effects, there are no such symmetry restrictions). However, multilayers of PSA 50 (and ω-TA) deposit in a centrosymmetric manner. Nevertheless, by carefully designing the rugate structure, it is possible to deposit the active PSA 50 layers in a non-centrosymmetric orientation. This is achievable by using ω- TA as the low refractive index component (as before) and constructing the high index region from alternative monolayers of PSA 50 and ω- TA. Using this arrangement, the PSA 50 molecules (side-groups) will have their dipoles orientated in a unidirectional manner, as required for second order non-linear behaviour. Rugate filters produced in this way will have a reduced Δn value (projected Δn =0.033 ) , thereby requiring thicker structures for a given OD, but the notches will have a sharper response (reduced bandwidth). As before, it wi ll be necessary to apply electrodes to the rugate structure. If desired, conductive layers could also be interleaved between multilayer stacks to provide further control of one or more individual stacks of a filter structure. The invention is not confined to the precise details of the foregoing example and variations may be made thereto. For instance two organic materials capable of LB deposition have been specified. There are many known materials capable of such deposition and, depending on their refractive indices, may be suitable for forming optical filters as described herein. Optical filters according to the present invention may be used as "switches" in conjunction with optical fibre technology where the optical fibres carry for example, multiplexed multiple wavelength signals.

Claims: CLAIMS 1. An optical filter comprising a substrate having deposited thereon a plurality of mono-molecular layers, to provide a deposit of a predetermined thickness, of at least one organic material the organic material or one of the organic materials having non - linear optical properties whereby its refractive index may be controlled such that the predetermined thickness constitutes a quarter wavelength rejection filter at a specific rejection. 2. A filter as claimed in claim 1 wherein the plurality of mono molecular layers is of different organic materials having different refractive indices. 3. A filter as claimed in claim 1 or 2 having means for controlling the refractive index of the filter by applying an electric field across the structure. 4. A filter as claimed in any preceding claim having a plurality of multilayers, each comprising a plurality of mono molecular layers, for rejection of one or more frequencies over the visible and/or infrared wavebands. 5. An optical filter substantially as hereinbefore described with reference to Figure 2 or Figure 3 of the accompanying drawings. 6. A method of manufacturing an optical filter comprising the steps of depositing a plurality of layers of at least one organic material, the organic material or one of the organic materials having non-linear optical properties whereby its refractive index may be controlled, on a substrate using LB techniques. 7. A method as claimed in claim 6 wherein a plurality of layers of a first organic material are deposited followed by the deposition of a plurality of layers of a second organic material. 8. A method as claimed in claim 6 or 7 further including the steps of forming a conductive film both beneath and above the layers whereby an electric field may be applied thereacross. 9. A method of manufacturing an optical filter substantially as hereinbefore described. 10. An optical device incorporating an optical rejection filter as claimed in any of claims 1 to 6.

Inventors: CARR NEIL, GOODWIN MARTIN, HARRISON KENNETH JOHN, LEWIS KEITH LODER, CARR, NEIL, GOODWIN, MARTIN, HARRISON, KENNETH, JOHN, LEWIS, KEITH, LODER
Applicants: MARCONI GEC LTD, GEC-MARCONI LIMITED
Application year: 1990
Main classification: G02B5
IPC codes: G02B5, G02F1