WO2003038482A2

WO2003038482A2 - Tuneable filter

Info

Publication number: WO2003038482A2
Application number: PCT/GB2002/004831
Authority: WO
Inventors: Gilbert William Smith
Original assignee: Qinetiq Limited
Priority date: 2001-10-30
Filing date: 2002-10-23
Publication date: 2003-05-08
Also published as: AU2002341140A1; WO2003038482A3; GB0125996D0

Abstract

The invention relates to a tuneable filter comprising: at least two coupled Fabry-Perot filters each Fabry-Perot filter having an FP transmission spectrum comprising a plurality of peaks; characterised in that the tuneable filter further comprises a tuning means for altering at least one of the FP spectra until substantial coincidence is achieved between at least one peak from each FP spectrum and wherein the filters each comprise a cavity of differing optical thickness.

Description

Tuneable Filter

This invention relates to a new filter for filtering electromagnetic radiation. More specifically this invention relates to a new tuneable filter. Yet more specifically the invention relates to an optical multiplexor. Even more specifically the invention relates to an optical communications system comprising said tuneable filter.

Optical filters are used for channel separation within optical communications networks. Such filters operate by allowing a single channel to be separated from those transmitted through an optical fibre.

For many optical communication applications the filter should be a narrow band-pass filter, operating within the wavelength range 1520 nm to 1620 nm (which is the range typically used for optical communication), have a rectangular transmission versus wavelength profile, and have low out of band transmission. Ideally filters used for optical communications should be tuneable, but most prior art filters used are of the fixed wavelength type.

Filters used for optical communication typically comprise multiple cavity Fabry-Perot filters. Figure 1 (a) shows a schematic diagram of a single cavity Fabry-Perot (FP) filter generally indicated by 1 1. The FP filter comprises a cavity 13, and two dielectric mirrors 12. A typical transmission T versus wavelength λ spectrum is shown, for the FP filter, in figure 1 (b). As can be seen from figure 1 (b) the figure 1 (a) filter is not of the narrow band-pass type.

The situation may be improved by using multiple cavity FP filters as shown in figure 2(a). Each cavity 21 is separated from the adjacent cavity 21 or cavities 21 by a dielectric mirror 22. Such a FP filter yielding the transmission T versus wavelength λ spectrum shown in figure 2(b). The figure 2(a) filter is acting as a narrow band pass filter, suitable for optical communication systems. A problem with the figure 2(a) arrangement is that the spacing between the dielectric mirrors of each cavity has to be equal to a multiple of the half wavelength of the radiation to be transmitted. This restriction makes the fabrication of figure 2(b) devices an extremely stringent process. In addition to this problem the filter for each of the wavelength channels has to be produced separately, adding to the cost and complexity of the process.

PCT/GB97/00980 discloses a tuneable filter which comprises the use of a nano phase polymer dispersed liquid crystal (PDLC). The operation of the devices disclosed in PCT/GB97/00980 relies on the accurate thickness matching of the cavities. The cavities are separated by a single layer of quarter wave optical thickness which effectively produces a " Bragg" type configuration - this requires a significant number of structural units to obtain a significant optical depth. Further, the device described in PCT/GB97/00980 produces a transmission spectrum characterised by a large number of spectrally narrow reflection bands and a high overall transmittance.

EP0903615 describes a device where it is suggested, eg in embodiment 5 that individual wavelengths may be selected from the pass bands of two stacked non-identical Fabry-Perot filters whilst at the same time providing for a so-called top hat type pass band shape as would be required for telecommunications applications for the selected band. There are essentially two situations, to consider here - i.e. when the cavities are coherently coupled and when the cavities are not coherently coupled. Assuming that the cavities are not coherently coupled then the resulting band shape would not be top hat in form. For the case when the cavities are coherently coupled then the "thick" glass substrate (see Fig 21 a of EP0903615) between the cavities acts as a further thick cavity. This extra cavity produces a large number of extra transmission peaks in addition to those produced by the two tuneable cavities. These extra transmissions have the effect of modulating the transmission peaks from the tuneable cavities which means that the production of a top hat band will not be achieved.

It is an objective of this invention to address at least some of the above mentioned problems.

According to a first aspect the invention provides a tuneable filter comprising: at least two coupled Fabry-Perot filters each Fabry-Perot filter having an FP transmission spectrum comprising a plurality of peaks; characterised in that the at least two coupled Fabry-Perot filters each have a cavity of differing optical thickness and that the tuneable filter further comprises a tuning means for altering at least one of the FP spectra until substantial coincidence is achieved between at^' least one peak from each FP spectrum.

The tuning means may further comprise a means for altering the wavelength or wavelengths at which coincidence occurs.

Under one type of conditions one set of peaks may be coincident at a particular wavelength, while under another type of conditions another set of peaks may be coincident at a different wavelength. For the tuneable filter to transmit radiation at least one peak from each of the FP transmission spectra must be substantially coincident. If there is a single wavelength at which one peak from each FP transmission spectrum is substantially coincident, then the tuneable filter will transmit radiation at this wavelength. In this way the wavelength at which the tuneable filter transmits radiation may be switched between discrete values.

The tuning means - may further comprise a means for altering FP transmission peaks that are coincident. The spacing between the peaks in at least one of the FP spectra may differ from that of the other FP spectrum or each of the other FP spectra.

Each Fabry-Perot (FP) filter comprises two dielectric mirrors, which are adjacent to, spaced from, and substantially parallel to, each other. Each FP filter comprises an FP cavity, located between the two dielectric mirrors, that may contain a cavity material. Any two coupled FP filters may share a dielectric mirror, so that a single dielectric mirror may partly bound two adjacent cavities.

The chemical composition of a cavity material contained in one cavity may differ from the cavity material contained in the other cavity or each of the other cavities. A cavity material contained in a cavity may comprise more than one chemical compound.

For the purposes of this specification, if the FP cavity contains a cavity material then the optical thickness of a Fabry-Perot filter is equal to the product of the spacing between the two adjacent dielectric mirrors and the refractive index of the cavity material.

For each FP cavity to act as a narrow band pass filter, the optical thickness of each FP cavity must be equal to a one half the wavelength of the radiation to be transmitted, or a multiple of one half the wavelength. Therefore, by altering the optical thickness, the transmission spectrum of a FP filter can be altered.

The tuning means may comprise an optical thickness control means for altering the optical thickness of at least one of the Fabry-Perot filters.

The tuning means may alter at least one of the FP transmission spectra, by altering the optical thickness of at least one of the FP filters, to ensure coincidence between at least one peak from each transmission spectrum. The tuning means may change which of the FP transmission peaks are coincident by altering the optical thickness of one or more of the FP filters.

The optical thickness control means may comprise a spacing control means for altering the spacing between at least two adjacent dielectric mirrors. The spacing control means may comprise a means for mechanically altering the spacing between at least two adjacent dielectric mirrors.

The optical thickness control means may comprise a refractive index control means for altering the refractive index of FP cavity material contained in at least one of FP cavities.

Preferably the cavity material, contained in at least one of the FP cavities, comprises a material, the refractive index of which is alterable by the application of an electric field to the material. More preferably the cavity material, contained in at least one of the cavities, comprises a liquid crystal. Yet more preferably the liquid crystal comprises a nano phase polymer dispersed liquid crystal. Even more preferably FP cavity material, contained in at least one of the cavities, comprises Norland Optical Adhesive (RTM) NOA-65 and (Merck) RTM BL-036. FP cavity material, contained in at least one of the FP cavities, may consist of 60% Norland Optical Adhesive (RTM) NOA-65 and 40% Merck (RTM) BL-036.

The means for altering the refractive index control means may comprise a means for applying an electric field across at least part of at least one of the cavities.

FP cavity material, contained in at least one of the FP cavities, may comprise more than one liquid crystal compound. FP cavity material, contained in at least one of the FP cavities, may comprise more than one nano phase dispersed liquid crystal compound.

The width of each FP cavity may differ from those of the other cavities. For the tuneable filter to transmit electromagnetic radiation at a particular wavelength, the optical thickness of each of the coupled FP filters must be equal to a multiple of the half wavelength. The tuning means, that operates by altering the refractive index of the cavity material, may allow this criterion to be met .without the need to accurately position each of the dielectric mirrors.

Preferably the optical thickness of each FP cavity is greater than 500 nm. More preferably the optical thickness of each FP cavity is greater than 10 microns. Yet more preferably the optical thickness of each FP cavity is greater than 50 microns.

Advantageously cavity material contained in at least one of the FP cavities has a refractive index between 1 and 4. More advantageously cavity material, contained in at least one of the FP cavities, has a refractive index between 1 and 2.5. Yet more advantageously cavity material, contained in at least one of the FP cavities, has a refractive index between 1 and 2.3.

Preferably the width of each FP cavity, from which the tuneable filter is formed, is between 1 micron and 200 microns.

According to a second aspect, the invention provides a method oftuneably filtering electromagnetic radiation, the method comprising the steps:

(a) placing a tuneable filter, comprising at least two coupled Fabry-Perot filters, wherein each of the at least two coupled Fabry-Perot filters has a different optical thickness and each FP filter has a transmission spectrum possessing a plurality of peaks, in the path of the radiation;

characterised in that the method comprises the further step of:

(b) tuning the wavelength at which the tuneable filter transmits the radiation by altering the optical transmission spectrum of at least one of the Fabry- Perot filters until there is substantial coincidence between at least one peak from each of the Fabry-Perot transmission spectra.

Step (b) may comprise the further step (c) of altering the wavelength or wavelengths at which coincidence occurs.

Step (c) may comprise the step of altering FP transmission peaks that are coincident.

Each coupled Fabry-Perot (FP) filter comprises two dielectric mirrors, which are adjacent to, spaced from, and substantially parallel to, each other. Each FP filter also comprises an FP cavity, located between the two dielectric mirrors, that may contain a cavity material. Any two coupled FP filters may share a dielectric mirror, so that a single dielectric mirror may partly bound two adjacent cavities.

For the purposes of this specification, if the FP cavity contains a cavity material then the optical thickness of a Fabry-Perot filter is equal to the product of the spacing between the two adjacent dielectric mirrors and the refractive index of the cavity material. Step (b) and/or step (c) may comprise the step of (d) altering the optical thickness of at least one of the FP filters.

Preferably the spacing between the peaks of each FP transmission spectrum is substantially uniform. More preferably the spacing between the peaks of each FP transmission spectrum is between 1 nm and 100 nm. Yet more preferably the spacing between the peaks of each FP transmission spectrum is between 5 nm and 25 nm.

At least some of the peaks in each of the FP transmission spectra may lie between 1520 nm and 1620 nm.

Step (d) may comprise the step of altering the spacing between at least two adjacent dielectric mirrors.

Preferably the cavity material, contained in at least one of the FP cavities, comprises a material, the refractive index of which is alterable by the application of an electric field to the material. More preferably the cavity material, contained in at least one of the cavities, comprises a liquid crystal. Yet more preferably the liquid crystal comprises a nano phase polymer dispersed liquid crystal. Even more preferably FP cavity material, contained in at least_, one of the cavities, comprises Norland Optical Adhesive (RTM) NOA-65 and (Merck) RTM BL-036. FP cavity material, contained in at least one of the FP cavities, may consist of 60% Norland Optical Adhesive (RTM) NOA-65 and 40% Merck (RTM) BL-036.

The step (d) may comprise the step of altering the refractive index of a cavity material contained in at least one of FP cavities. Advantageously step (b) and/or step (c) comprises the step of applying an electric field to at least one of the FP cavity materials.

According to a third aspect the invention provides an optical multiplexor comprising at least one input optical fibre, at least one drop optical fibre and a tuneable filter, the tuneable filter comprising: at least two coupled Fabry-Perot filters each Fabry-Perot filter having an FP transmission spectrum comprising a plurality of peaks; characterised in that the at least two coupled Fabry-Perot filters have cavities of a different optical thickness and that the tuneable filter further comprises a tuning means for altering at least one of the FP spectra until substantial coincidence is achieved between at least one peak from each FP spectrum; the or at least one of the drop optical fibres, the or at least one of the input optical fibres, and the tuneable filter being arranged such that, when in use, radiation to be filtered is transmitted by the or at least one of the input optical fibres to the tuneable filter and radiation at the coincidence wavelength is transmitted by the or at least one of the drop optical fibres from the tuneable filter.

Preferably the tuning means further comprises a means for altering the wavelength or wavelengths at which coincidence occurs.

According to a fourth aspect the invention provides an optical communication system comprising an optical and at least one optical multiplexor and at least one light source, the optical multiplexor comprising at least one input optical fibre, at least one drop optical fibre and a tuneable filter, the tuneable filter comprising: at least two coupled Fabry- Perot filters each Fabry-Perot filter having an FP transmission spectrum comprising a plurality of peaks; characterised in that the at least two coupled Fabry-Perot filters have cavities of a different optical thickness and that the tuneable filter further comprises a tuning means for altering^' at least one of the FP spectra until substantial coincidence is achieved between at least one peak from each FP spectrum; the or at least one of the input optical fibres, the or at least one of the drop optical fibres, and the tuneable filter being arranged such that, when in^' use, radiation to be filtered is transmitted by the or at least one of the input optical fibres to the tuneable filter and radiation at the coincidence wavelength is transmitted by the or at least one of the drop optical fibres from the tuneable filter; the or at least one of the multiplexors and the or at least one of the light sources being arranged such that radiation from or at least one of the light source is transmitted into the or at least one of the input optical fibres.

By coupled Fabry-Perot filters according to the various statements of invention is preferably taken to mean that the coherence length of the transmitted light is greater than the separation of the cavities. Typically the cavities are separated by less then ten quarter wavelengths.

The invention will now be described by way of example only with reference to the following drawings, in which:

figure 1 (a) shows a schematic diagram of a single cavity Fabry-Perot (FP) filter; figure 1 (b) shows a schematic diagram of a typical transmission spectrum for a single cavity FP filter; figure 2(a) shows a schematic diagram for a multiple cavity FP filter; figure 2(b) shows a schematic diagram of a typical transmission spectrum for a multiple cavity FP filter; and figure 3 shows a schematic diagram of a tuneable filter, comprising two cavities, according to the invention; figure 4(a) shows a schematic diagram of the transmission spectra for each of the two cavities that form part of the figure 3 filter, none of the peaks in the two spectra being coincident; figure 4(b) shows a schematic diagram of transmission spectra for each of the two cavities that form part of the figure 3 filter, one peak from each of the two spectra being substantially coincident; figure 5 shows a schematic diagram of an optical multiplexor according to the invention.

Figure 3 shows a tuneable filter, generally indicated by 31 , according to the invention. The tuneable filter 31 comprises a first Fabry-Perot filter 37, a second Fabry-Perot (FP) filter 38, and a tuning means 39. The two coupled FP filters 37, 38 comprise first and second FP cavities 32 and 33. Dielectric mirrors 34 are disposed on either side of each of the cavities 32, 33. The spacing w-, between the two dielectric mirrors 34 that partly bound the first cavity 32, differs from the spacing w₂ between the two dielectric mirrors 34 that partly bound the second cavity 33. The tuneable filter 31 also comprises a first cavity material 35 which is disposed in the first and second cavities cavity 32, 33. The cavity material 35 comprises a mixture of 60% Norland Optical Adhesive (RTM) NOA-65 and 40% Merck (RTM) BL-036. The tuning means 39 comprises means for applying a potential difference 40, 41 across the two cavities 32, 33. Since one of the components of the cavity material 35 is a liquid crystal (Merck (RTM) BL-036), the refractive index of each of the cavity material 35 may be altered by applying such a potential difference.

Figure 4a shows a first transmission T versus wavelength λ spectrum, having peaks 42a, 42b, 42c, associated with the first cavity 32, shown in figure 3. Figure 4 also shows a second spectrum, having peaks 43a, 43b, 43c, associated with the second figure 3 cavity 33. Figure 4 corresponds to the situation in which no potential difference is applied across the cavities 32, 33.

Because both cavities contain a cavity material 35 having the same refractive index, and because the cavity material is at earth potential, the optical thickness of the first cavity also differs from the optical thickness of the second cavity. The result is that the spacing between the peaks 42 a to c differs from that between the peaks 43 a to c and none of the peaks from either group coincide. The result is that the filter is not capable of transmitting radiation over the range of wavelengths shown in the figure 4a spectra.

In this example the tuning means operates by applying a potential difference across at least one of the two cavities 32, 33. The change in refractive index caused by the tuning means results in a change in the optical thickness of at least one of the cavities 32, 33 and a change in the spacing of the two groups of peaks 42 a-c and 43 a-c. Therefore the tuning means can be used to alter the relative positions of the two groups of peaks 42 a-c and 43 a-c until at least one peak from each group is coincident with that from the other group. By further altering the potential difference applied to at least one of the cavities 32, 33, the positions of the peaks 42 a-c and 43 a-c may be altered until coincidence is achieved at a different wavelength. For example under one set of conditions peaks 42a and 43a may coincide at a first wavelength (a situation shown in figure 4b), while under a different set of conditions peaks 42b and 43b may coincide at a second wavelength. In this way the wavelength at which the tuneable filter transmits radiation may be changed between discrete values.

In a different embodiment, the dielectric mirrors shown in figure 3 may be mounted on or formed from one or more substrates. The arrangement of the dielectric mirrors is substantially identical to that shown in figure 3 save that the tuning means comprises a means for altering the length of one or more of the substrates and/or for altering the length of part of the or at least one of the substrates. By altering the dimensions of the substrate or substrates in this way, the optical thickness of at least one of the cavities may be modified, until coincidence is achieved. The position at which coincidence is achieved may further be altered by alteration of the separation between the adjacent dielectric mirrors.

Figure 5 shows an optical multiplexor, generally indicated by 51 , according to the invention. The multiplexor is of the add/drop type and comprises an input fibre 52, an output fibre 53, a drop fibre 54, a connection fibre 55, an add fibre 56, a first tuneable filter 57, and a second tuneable filter 58, each tuneable filters 57 and 58 being a tuneable filter according to the present invention. The input fibre 52 may carry electromagnetic radiation comprising a number of channels λ., , λ₂, λ₃, .... , λ_n and is associated with the first tuneable filter 57. The first tuneable filter 57 transmits one of the channels (λ₃) to the drop fibre 54 and reflects the other channels through the connection fibre 55 to the second tuneable filter 58, where they are reflected through an output optical fibre 53. The add fibre 56 carries electromagnetic radiation at the same wavelength (λ₃) as was transmitted by the first tuneable filter 57, and the second tuneable filter 58 transmits this radiation from the add fibre 56 to the output fibre 53.

Claims

.1 . A tuneable filter comprising: at least two coupled Fabry-Perot filters each Fabry-Perot filter having an FP transmission spectrum comprising a ^j plurality of peaks; characterised in that each of the at least two coupled Fabry-Perot filters has a cavity of differing optical thickness and that the tuneable filter further comprises a tuning means for altering at least one of the FP spectra until substantial coincidence is achieved between at least one peak from each FP spectrum.

2. A tuneable filter according to claim 1 wherein the tuning means comprises a means for altering the wavelength or wavelengths at which coincidence occurs.

3. A tuneable filter according to claim. 1 wherein the tuning means comprises a means for altering FP transmission peaks that are coincident.

4. A tuneable filter according to claim 1 wherein each Fabry-Perot filter cavity has disposed therein a cavity material.

5. A tuneable filter according to any of the preceeding claims wherein the tuning means comprises an ^optical thickness control means for altering the optical thickness of at least one of the Fabry-Perot filters.

6. A tuneable filter according to claim 5 wherein the optical thickness control means comprises a refractive index control means for altering the refractive index of FP cavity material contained in at least one of the FP cavities.

7. A tuneable filter according to claim 6 wherein the cavity material, comprises a liquid crystal.

8. A tuneable filter according to claim 7 wherein the liquid crystal forms part of a Polymer Dispersed Liquid Crystal device.

9. A tuneable filter according to claim 6 where in the refractive index control means comprises a means for applying an electric field across at least part of at least one of the cavities.

10. A method of tuneably filtering electromagnetic radiation, the method comprising the steps:

(a) placing a tuneable filter, comprising at least two coupled Fabry-Perot filters wherein each of the at least two coupled Fabry-Perot filters has a cavity of differing optical thickness and each FP filter having a transmission spectrum possessing a plurality of peaks, in the path of the radiation;

characterised in that the method comprises the further step of:

1 1 . A method according to claim 10 wherein step (b) comprises the step (c) of altering the wavelength or wavelengths at which coincidence occurs.

12. A method according to claim 1 1 wherein step (c) comprises^' the step of altering FP transmission peaks that are coincident.

13. A method according to claim 10 wherein each Fabry-Perot filter comprises a FP cavity with material disposed therein, located between two dielectric mirrors.

14. A method according to claim 13 wherein step (b) comprises the step of (d) altering the optical thickness of at least one of the FP filters.

15. A method according to claim 14 wherein the cavity material comprises a liquid crystal.

16. A method according to claim 15 wherein the liquid crystal is a part of a Polymer dispersed Liquid Crystal Device.

17. A method according to claim 15 or 16 wherein the step (d) comprises the step of altering the refractive index of the liquid crystal.

18. A method according to claim 13 wherein step (b) comprises the step of applying an electric field to at least part of at least one of the FP cavities.

19. An optical multiplexor comprising at least one input optical fibre, at least one drop optical fibre and a tuneable filter according to claim 1 , the or at least one of the drop optical fibres, the or at least one of the input optical fibres, and the tuneable filter being arranged such that, when in use, radiation to be filtered is transmitted by the or at least one of the input optical fibres to the tuneable filter and radiation at the coincidence wavelength is transmitted by the or at least one of the drop optical fibres from the tuneable filter.

20. An optical communication system comprising at least one optical multiplexor according to claim 19 and at least one light source; the or at least one of the multiplexors and the or at least one of the light sources being arranged such that radiation from the or at least one of the light sources is transmitted into the or at least one of the input optical fibres.

21. A tuneable filter comprising: at least two coupled Fabry-Perot filters each Fabry-Perot filter having an FP transmission spectrum comprising a plurality of peaks; characterised in that the tuneable filter further comprises a tuning means for altering at least one of the FP spectra until substantial coincidence is achieved between at least one peak from each FP spectrum.

22. A tuneable filter according to claim 21 wherein the tuning means comprises an optical thickness control means for altering the optical thickness of at least one of the Fabry-Perot filters.