WO2002006886A1

WO2002006886A1 - Switchable waveguide device

Info

Publication number: WO2002006886A1
Application number: PCT/GB2001/003169
Authority: WO
Inventors: Milan Momcilo Popovich; John James Storey; Giovanni Delrosso
Original assignee: Digilens, Inc.
Priority date: 2000-07-13
Filing date: 2001-07-13
Publication date: 2002-01-24
Also published as: AU2001270821A1

Abstract

A cylindrical core (10) is surrounded by a tubular cladding (11) composed of a polymer dispersed liquid crystal material. By applying an electric field (E) to the cladding (11), the refractive index of the material can be varied by causing the liquid crystal molecules to re-orient, thereby selectively matching and mis-matching the refractive index of the cladding (11) with that of the core (10). The field (E) is applied generally longitudinally of the cladding (11) by way of electrodes (12 and 13) which are spaced apart in the longitudinal direction of the cladding (11). In an alternative arrangement, the core (10) rather than the cladding (11) is made of polymer-dispersed liquid crystal material, and its refractive index can be selectively matched or mis-matched with that of the cladding.

Description

Title : Switchable Waveguide Device

Field of the Invention

This invention relates to a switchable waveguide device, such as that used in telecommunications systems.

Description of the Relevant Art

Waveguide devices typically comprise a core along which radiation can propagate, and a surrounding cladding. The respective refractive indices of the core and the cladding are usually mis-matched such that the radiation is totally internally reflected from the walls of the core. However, in certain circumstances (such as in in-and out-coupling) it is desired that the radiation is able to enter or escape the core in a lateral direction.

For this purpose, it has been proposed to construct the cladding of a material whose refractive index can be varied by the application of an external stimulus. In one such proposal, this variation is achieved by thermal control of the waveguide, but the response is too slow to be of practical value. Another proposal has involved constructing the cladding from bulk liquid crystal material and applying an electrical stimulus to vary its refractive index. However, the response time of the bulk liquid crystal material has again been found to be too slow for practical telecommunications applications.

It is an object of the present invention to obviate or mitigate these difficulties.

Summary of the Invention

According to the present invention, there is provided a switchable waveguide device comprising: a core portion along which radiation can propagate, and a cladding portion abutting said core portion, at least one of said core and cladding portions being composed of a polymer- dispersed liquid crystal material whose refractive index can be varied by the application of an electrical stimulus, said device being switchable by the application of said stimulus between first and second conditions in which the refractive indices of said core and cladding portions are respectively substantially matched and substantially unmatched.

Preferably, the cladding portion surrounds the core portion.

Advantageously, the polymer-dispersed liquid crystal material includes liquid crystal droplets whose molecules exhibit a preferred orientation with respect to a polar axis of each droplet, said material in a rest state thereof having said droplets arranged with the polar axes thereof in a random orientation with respect to the direction of propagation of said radiation, and when said electrical stimulus is applied the liquid crystal droplets become re-oriented such that their polar axes lie uniformly in a direction parallel to the direction of propagation of said radiation.

Alternatively, said material in its rest state can have said droplets arranged with their polar axes in a random orientation in three dimensions, and when said electric stimulus is applied the liquid crystal droplets became oriented such that their polar axes lie in a random orientation in a plane perpendicular to the direction of propagation of the radiation.

Desirably, said polymer-dispersed liquid crystal material comprises a mixture of liquid crystal material and pre-polymer which is subjected to phase separation, which can be polymerisation-, thermally- or solvent- induced. In the case where polymerisation-induced phase separation is used, this is preferably achieved by photopolymerisation using either visible or ultraviolet radiation. The mixture of liquid crystal material and pre-polymer can be subjected to flash illumination, or to illumination by radiation having a fine, random intensity distribution. The polymer-dispersed hquid crystal material can comprise an essentially random or uniform distribution of liquid crystal droplets within a polymer matrix. Alternatively, the material can contain holographic fringes formed by liquid crystal droplets, the separation of said fringes being substantially less than the wavelength of said radiation.

Preferably, the electrical stimulus is applied by means of a pair of electrodes which are spaced apart longitudinally of the waveguide device, thereby to produce an electric field which extends generally longitudinally of the waveguide device.

The electrodes can be of generally annular configuration and can extend around the waveguide device.

Each electrode can comprise portions which alternate with portions of the other electrode longimdinally of the waveguide device. In a particular example of this, each electrode can comprise a plurality of fingers extending transversely of the waveguide device with the fingers of one electrode alternating with the fingers of the other electrode longitudinally of the waveguide device. Alternatively, the electrodes can comprise a plurality of hoop portions which extend around the waveguide device and which alternate with one another longitudinally of the waveguide.

Brief Description of the Drawings

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

Figure 1 is a schematic sectional view of a first embodiment of a switchable waveguide device according to the present invention;

Figures 2 to 4 are diagrams showing different states of a polymer-dispersed liquid crystal material which forms part of the device shown in Figure 1 ; Figures 5 and 6 are diagrams showing the optical characteristics of a liquid crystal droplet which forms part of the material shown in Figures 2 to 4;

Figure 7 is a schematic plan view of a second embodiment of a switchable waveguide device according to the present invention;

Figures 8 to 13 show various stages in the manufacture of a switchable waveguide device according to the present invention, in the form of an evanescently coupled grating waveguide; and

Figures 14 to 19 show various stages in the manufacture of an alternative version of the switchable waveguide device, in the form of a ridge grating waveguide.

Detailed Description

Referring first to Figure 1, there is shown therein a first embodiment of a switchable waveguide device which comprises generally a cylindrical core 10 along which radiation (such as visible or infra-red light) can propagate in a direction X, and a tubular cladding 11 which surrounds the core 10. The core 10 may be optically homogeneous or its refractive index may vary along its length. The core may also embody holographic fringes in the form of a Bragg grating, these fringes forming a switchable reflection hologram. When activated, this hologram reflects light propagating along the waveguide device into a reverse direction.

The cladding 11 is composed of a polymer-dispersed liquid crystal (PDLC) material whose refractive index can be varied by the application of an electrical stimulus. This stimulus is applied by means of two electrodes 12 and 13 comprising a series of hoop portions which alternate along the length of the waveguide device. The application of an electrical signal to the electrodes 12 and 13 by means of a voltage source 14 causes an electrical field to be produced between the electrode portions 12 and 13, as indicated by broken lines E. Although this field is essentially toroidal in configuration, it effectively extends longitudinally of the waveguide device where it penetrates the cladding 11. The PDLC material essentially comprises a clear polymer material populated by a uniform yet random dispersion of liquid crystal micro-droplets. Iα its rest state (i.e. when the electric field is not applied) the PDLG material of the cladding 11 has the properties of bulk material, with an average refractive index determined by the combined indices of the liquid crystal material in its rest state and of the surrounding polymer matrix. When the electric field is applied, the molecules of liquid crystal in the droplets re-orient under the influence of this field, and thereby change the refractive index of the droplets. The PDLC material now has an average refractive index determined by the combined indices of the liquid crystal material in its re-orientated state and of the polymer matrix. By controlling the value of this refractive index relative to that of the core 10, it is possible to control the characteristics of the radiation propagating within the core. In particular, it is possible to control coupling of the radiation propagation between the core 10 and the cladding 11, for example for controlling evanescent coupling between the core and an output node (or vice versa), which has particular application to in- and out-couplers in the telecommunications sector. Since the average refractive index of the overall PDLC material will be dependent upon the magnitude of the applied electric field, it is possible to fine tune that index so that it matches exactly the refractive index of the core 10.

The PDLC material can be of a type in which the liquid crystal molecules are randomly oriented in its rest state, and in which the molecules become re-oriented parallel to the direction of the electric field when the latter is applied. Such a material is said to have positive amsotropy. Alternatively, the material can be of a type in which, in its rest state, the liquid crystal molecules are generally oriented at right-angles to the above, and in which the molecules become re-oriented perpendicularly to the applied field. Such a material is said to have negative anisotropy.

The PDLC material can be fabricated by dissolving liquid crystal material in a pre-polymer and then cooling the resultant mixture to cause thermally-induced phase separation. Alternatively, the liquid crystal material and the pre-polymer can be dissolved in a common solvent, and the solvent can be evaporated to cause so-called solvent-induced phase separation. A preferred method is, however, to induce phase separation by polymarisation of the pre-polymer, advantageously by using photopolyrnerisation techniques. In this latter case, either visible or ultraviolet radiation can be employed in the photopolyrnerisation process. The mixture of liquid crystal material and pre-polymer can be subjected to flash illumination, or to illumination by radiation having a fine, random intensity distribution.

In its most fundamental form, the PDLC material is formed from a monomer and a liquid crystal material: however, the material can also include a cross-linking monomer, a co- initiator and a photo-initiator dye. Photopolyrnerisation gives rise to liquid crystal droplets which are much less than one micron in diameter, and typically less than 200 nanometres in diameter. Such droplets exhibit a very fast switching rate (with a switching time of less than 150 microseconds, and possibly as low as a few microseconds), which means that they are ideally adapted for use in telecommunications applications.

In addition to a fast switching time, the waveguide device of the invention also has the advantage that it can readily be constructed so that it is insensitive to the polarisation state of the radiation propagating along the core 10. This will now be explained in detail with reference to Figures 2 to 6 of the accompanying drawings.

Figures 2 and 3 show typical alternative arrangements of the liquid crystal droplets (referenced 15) in the rest state of the PDLC material of the cladding 11. Within each droplet, there is a tendency for the liquid crystal molecule directors to exhibit a bipolar alignment pattern, as indicated schematically by lines 16, with the polar axis being indicated at P. One of the droplets 15 is shown in detail in Figures 5 and 6, with Figure 5 being a view perpendicular to the polar axis P and Figure 6 being a view parallel to that axis. Because of the way in which the molecule directors are oriented, the droplet will exhibit birefringence. More particularly, for light incident in a direction parallel to the polar axis P (i.e. as viewed in Figure 6) the droplet will have an ordinary refractive index n₀, whereas for light incident perpendicularly to the polar axis (i.e. as viewed in Figure 5) the droplet will have an extraordinary refractive index n., with n_β generally being greater than n₀.

In view of the above, it would be expected that the PDLC material in its rest state would exhibit polarisation sensitivity for light incident in a direction as viewed in Figures 2 and 3. In actual fact, however, this is not the case because the droplets 15 are randomly oriented throughout the volume of the material. This random orientation can be in three-dimensional space (as depicted in Figure 2) or alternatively the polar axis P of the droplets 15 can be confined to a plane perpendicular to the direction of light propagation, but oriented randomly within that plane, as depicted in Figure 3. Accordingly, the birefringence of the individual droplets is averaged out throughout the volume of the PDLC material, so that the material as a whole shows no net polarisation sensitivity.

Figure 4 shows a typical arrangement of the liquid crystal droplets 15 in the state where an electric field is applied in the direction of arrow E (i.e. into the plane of the drawing). The droplets 15 now become re-orieήted such that their polar axes P are aligned with the electric field vector (assuming that the PDLC material has positive anisotropy). As a result, light propagating in a direction parallel to the electric field vector will see only the ordinary refractive index n₀ , so there will be no polarisation sensitivity in this state either.

A droplet distribution of the type shown in Figure 2 can be achieved by subjecting the liquid crystal/pre-polyrner mixture to flash ilhmination during the photopolyrnerisation process. Alternatively, it is possible to produce a fine, random droplet distribution by recording in the PDLC material superimposed holographic fringes or the interference pattern produced by interaction of light waves that have propagated through a diffuser. In either event, the net effect is to generate a random distribution of fine droplets dispersed in a transparent polymer matrix.

As a further alternative, the liquid crystal droplets can take the form of holographic fringes that are densely populated by small liquid crystal droplets (in particular Bragg gratings) whose pitch or separation is significantly less than the wavelength of the light propagating along the waveguide device. Since droplet size is related to grating pitch, the. droplets in such an arrangement would be very fine. The grating pitch would be too small for the Bragg diffraction condition to be satisfied by any practical visible or near-infrared wavelengths, so that the PDLC material will act essentially as a variable refractive index medium. The droplet orientation in this case will be influenced by the grating geometry, and will typically conform to the droplet distribution shown in Figure 3.

A second embodiment of the switchable waveguide device is shown in Figure 7. This is generally similar to the first embodiment shown in Figure 1, and accordingly similar parts are designated by the same reference numerals. However, the waveguide itself is now of rectangular configuration, and the two electrodes 12 and 13 take the form of planar constructions deposited on one surface 20 of the waveguide. Each electrode 12, 13 includes a plurality of finger portions 21 extending in parallel spaced-apart relation and transversely to the longitudinal direction of the waveguide device. The finger portions of the electrode 12 are interdigitated with the finger portions of the electrode 13, such that these portions alternate longitudinally of the waveguide device. Such an arrangement of the electrodes gives rise to an electric field which extends generally parallel to the longitudinal direction of the waveguide device.

In the above description, it has been mentioned that the invention can be used to control evanescent coupling between the core of the waveguide and an output node. A typical waveguide for use in such an application is shown in Figures 8 and 9, wherein the cladding 11 takes the form of a glass block, and the core 10 is formed as a channel or trench 30 in a surface 31 thereof by diffusing dopants into the cladding using standard procedures based on thermal, ion exchange or other processes. There will now be described a method by means of which this can be incorporated into a waveguide device according to the present invention.

As depicted in Figures 10, 11 and 12, a slug 32 of PDLC material is deposited onto the surface 31 and is then compressed by means of a glass plate 33 into a thin layer 34 which is in direct contact with the core 10. The thickness of the layer 34 is controlled (typically to 3 to 5 microns) by means of spacers 35, such as rods or spheres. A Bragg grating is then recorded in the layer 34 by means of object and reference laser beams 36 (which operate ideally in the ultraviolet band), thereby forming holographic interference fringes inside the PDLC material. As can be seen to advantage in Figure 13, the glass plate 33 is pre-formed with interdigitated ITO electrodes 37 and 38, which are oriented such that (when a voltage is applied across them) they produce an electric field E which extends generally parallel to the longitudinal direction of the channel 30. The whole assembly is bonded using epoxy resin to a glass substrate 39, which is formed with grooves 40 for general packaging purposes.

In a rest state of the device (i.e. when the electrodes 37 and 38 are not energised), the net refractive index of the PDLC layer 34 is arranged to be significantly greater than that of the core 10. As a result, radiation propagating along the core continues to do so. However, when the electric field E is applied to the PDLC material by means of the electrodes 37 and 38, the Bragg fringes recorded in the layer 34 are partially or wholly erased, such that the net refractive index of the layer 34 becomes substantially matched with that of the core 10. Radiation propagating along the core 10 can thus now escape through the resultant optical coupling between the core 10 arid the layer 34. The above-described device thereby provides a means whereby radiation propagating within the core can be selectively evanescently decoupled from the waveguide. By an analagous method, the device can also allow radiation propagating in free space to be in-coupled into the core 10.

The above-described assembly can be incorporated into a package for ready integration into e.g. a fibre optics telecornmunications system. This process will typically involve: aligning input and output fibre optic cables with the core 10 using industry-standard fibre blocks fixing the input and output cables using bulk epoxy adhesive which is introduced using a standard industrial dispenser bonding connection wires to the electrodes 37 and 38 whereby a voltage source can be connected to the latter in use - the bonding process can make use of a standard industrial vacuum chuck and "flip-down" optical assembly incorporating the resultant assembly into a package case along with a thermistor, ground interconnections and a thermo-electric cooler, and bonding together the optical assembly and the thermo-electric cooler and welding a lid onto ^"the final package.

Figures 14 to 19 show a method of manufacturing an alternative form of device, in the form of a ridge grating waveguide device. More particularly, a layer 50 of PDLC material is first formed On a surface 51 of a glass substrate 52. Object and reference laser beams 53 (preferably operating in the ultraviolet region) are then used to record in the layer 50 holographic fringes in the form of a Bragg grating. Following on from this, a core ridge 54 is formed from the layer 50 by means of a standard industrial process, such as laser ablation using UN Excimer lasers (see Figures 15 and 16). The width and length of the ridge 54 are deterrnined by a photo mask 55, whilst its depth is determined by the thickness of the layer 50. Formation of the ridge 54 is complete when all of the PDLC material surrounding the latter has been removed. As can be seen to advantage in the inset to Figure 17 (which is a plan view of the waveguide), each end of the core ridge 54 terminates at a location displaced by e.g. 10 microns from the adjacent edge of the substrate 52. An additional polymer layer 56 is then deposited on the surface 51 of the substrate 52, thereby forming a cladding layer for the core ridge 54.

To the resultant assembly is attached a glass plate 57 which carries interdigitated electrodes 58 and 59. Unlike in the previously-described arrangement, the electrodes are not required to transmit light so they do not have to be fabricated from a transparent conductive material such as ITO. The whole assembly is then epoxy-bonded to a substrate 60 which has grooves 61 machined therein, which correspond to the grooves 40 shown in Figure 13. The resultant assembly can then be integrated into a package in the same manner as described previously. The electrodes 58 and 59 are oriented such that, when a voltage is applied across them, they produce an electric field E which extends generally in the longitudinal direction of the core energising and de-energising the electrodes, the net re;

match or a mis-match to that of the core 10.

Whereas the invention has been described in relation to what are presently considered to be the most practical and preferred embodiments, it is to be understood that the invention is not limited to the disclosed arrangements but rather is intended to cover various modifications and equivalent constructions included within the spirit and scope of the invention. For example, in the above description it has been assumed that the refractive index of the polymer in the PDLC material is matched to the refractive index of the hquid crystal material in the state where the electric field is applied. However, it is possible to formulate the PDLC material in such a manner that the polymer is not matched to the ordinary (lower) refractive index of the liquid crystal droplets, but rather to the average index in the rest state of the material. Moreover, although 'the invention has been described with reference to its application in telecommunications devices, it can be used in other areas of technology as well, such as in optical display systems.

Furthermore, the PDLC material utilised in the above embodiments has been described above as being of the type having positive anisotropy. However, it is possible to use a material having negative anisotropy instead. In such a case, the liquid crystal droplets can be orientated as indicated in Figure 2 in the rest state of the material (i.e. randomly orientated in three dimensions). When an electric field is applied to the material, the droplets can be arranged to re-orientsuch that their polar axes are still oriented randomly but are confined to a plane normal to the direction of the light propagation. In such a situation, the material will be insensitive to the polarisation state of the light in both of its aforesaid conditions, because the polarisation sensitivity of the individual droplets will be averaged-out throughout the volume of the material.

Claims

1. A switchable waveguide device comprising:

a core portion (10) along which radiation can propagate, and

a cladding portion (11) abutting said core portion,

at least one of said core and cladding portions (10, 11) being composed of a polymer-dispersed liquid crystal material whose refractive index can be varied by the application of an electrical stimulus,

said device being switchable by the application of said stimulus between first and second conditions in which the refractive indices of said core and cladding portions ( 10, 11 ) are respectively substantially matched and substantially unmatched.

2, . A switchable waveguide device as claimed in claim 1 , wherein the polymer-dispersed liquid crystal material includes liquid crystal droplets (15) whose molecules (16) exhibit a preferred orientation with respect to a polar axis (P) of each droplet (15), said material in a rest state thereof having said droplets (15) arranged with the polar axes (P) thereof in a random orientation with respect to the direction of propagation of said radiation, and when said electrical stimulus is applied the liquid crystal droplets (15) become re-oriented such that their polar axes (P) lie uniformly in a direction parallel to the direction of propagation of said radiation.

3. A switchable waveguide device as claimed in claim 1 , wherein the polymer-dispersed liquid crystal material includes liquid crystal droplets (15) whose molecules (16) exhibit a preferred orientation with respect to a polar axis (P) of each droρlet(l 5), said material in a rest state thereof having said droplets (15) arranged with the polar axes (P) thereof in a random orientation in three dimensions, and when said electrical stimulus is applied the liquid crystal droplets (15) became oriented such that their polar axes (P) lie in a random orientation in a plane perpendicular to the direction of propagation of the radiation.

A switchable waveguide device as claimed in claim 1, 2 or 3, wherein the cladding portion (11) surrounds the core portion (10).

5. A switchable waveguide device as claimed in any preceding claim, wherein the polymer-dispersed liquid crystal material comprises a mixture of liquid crystal material and pre-polymer which is subjected to phase separation.

6. A switchable waveguide device as claimed in claim 5, wherein said phase separation is polymerisation induced.

A switchable waveguide device as claimed in claim 6, wherein said phase separation is photopolyrnerisation induced, using either visible or ultraviolet radiation.

8. A switchable waveguide device as claimed in claim 7, wherein the mixture of liquid crystal material and pre-polymer is subj ected to flash illumination.

A switchable waveguide device as claimed in claim 7, wherein the mixture of liquid crystal material and pre-polymer is subjected to illumination by radiation having a fine, random intensity distribution.

10. A switchable waveguide device as claimed in claim 5, wherein the phase separation is thermally induced.

11. A switchable waveguide device as claimed in claim 5, wherein the phase separation is solvent induced.

12. A switchable waveguide device as claimed in any preceding claim, wherein the polymer-dispersed liquid crystal material comprises an essentially random distribution of liquid crystal droplets within a polymer matrix.

13. A switchable waveguide device as claimed in any one of claims 1 to 11, wherein the polymer-dispersed liquid crystal material comprises an essentially uniform distribution of liquid crystal droplets within a polymer matrix.

14. A switchable waveguide device as claimed in any one of claims 1 to 11, wherein the polymer-dispersed hquid crystal material contains holographic fringes formed by said liquid crystal droplets, the separation of said fringes being substantially less than the wavelength of said radiation.

15. A switchable waveguide device as claimed in any preceding claim, wherein the electrical stimulus is applied by means of a pair of electrodes (12, 13) which are spaced apart longitudinally of the waveguide device, thereby to produce an electric field (E) which extends generally longitudinally of the waveguide device.

16. A switchable waveguide device as claimed in claim 15, wherein the electrodes (12, 13) are of generally annular configuration and extend around the waveguide device.

17. A switchable waveguide device as claimed in claim 15, wherein each electrode (12, 13) comprises portions (21) which alternate with portions (21) of the other electrode longitudinally of the waveguide device.

18. A switchable waveguide device as claimed in claim 17, wherein each electrode (12, 13) comprises a plurality of fingers (21) extending transversely of the waveguide device with the fingers (21) of one electrode (12) alternating with the fingers (21) of the other electrode (13) longitudinally of the waveguide device.

19. A switchable waveguide device as claimed in claim 17, wherein the electrodes (12, 13) comprise a plurality of hoop portions which extend around the waveguide device and which alternate with one another longitudinally of the waveguide.

20. A switchable waveguide device as claimed in any preceding claim, in the form of an in- or out-coupler for use in an optical communications system.

21. A switchable waveguide device as claimed in any one of Claims 1 to 19, in the form of an evanescently coupled grating waveguide.

22. A switchable waveguide device as claimed in any one of claims 1 to 19, in the form of a ridge grating waveguide.