WO2001095024A1

WO2001095024A1 - Switchable waveguide devices

Info

Publication number: WO2001095024A1
Application number: PCT/GB2001/002491
Authority: WO
Inventors: Milan Momcilo Popovich; John James Storey
Original assignee: DigiLens Inc
Current assignee: DigiLens Inc
Priority date: 2000-06-09
Filing date: 2001-06-07
Publication date: 2001-12-13
Anticipated expiration: 2002-12-09
Also published as: AU2001262522A1

Abstract

Holographic fringes (23) are recorded in a PDLC composite which is configured as a waveguide (21) (such as the core of an optical fibre), and are composed of liquid crystal droplets (30) whose molecules exhibit a preferred orientation with respect to a polar axis (31) of each droplet. In a rest state of the material, the polar axes (31) are oriented randomly in three dimensions. Although each invidual droplet (30) exhibits birefringence and is therefore sensitive to the polarisation state of incident radiation (A), the random orientation causes this sensitivity to be averaged out over the volume of the material. The fringes (23) can be erased by the application of an electric field (E), which causes the liquid crystal molecules to become re-oriented in a uniform direction, namely parallel to the radiation propagation direction. As a result, the PDLC does not show any sensitivity to the polarisation state of the radiation in either of these conditions. In an alternative arrangement, the application of the electric field (E) causes the polar axes (31) to re-orient so that they are randomly oriented in a plane perpendicular to the radiation propagation direction. Again, this random orientation serves to average out the polarisation sensitivity of the individual droplets (30).

Description

Title Switchable Waveguide Devices

Field of the Invention

This invention relates to switchable waveguide devices, such as those used in telecommunications systems.

Description of the Relevant Art

Switchable waveguides are typically used in the telecommunications industry for wavelength divisional multiplexing, attenuation, line testing and dynamic gain equalisation. However, waveguides in general are subject to losses due to changes in polarisation arising from e.g. fluctuations in the laser sources and from thermal or mechanical disturbances in the fibre itself. It is necessary to make those losses as low as possible: for example, Bellcore have published industry standards which demand polarisation losses of less than 0.2 dB. Whereas it may be possible to compensate for rotation of the polarisation vector by using electro- optical devices, these are likely to be expensive cumbersome and generally impractical to implement within practical telecommunications devices.

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 compήsing: an electrically switchable holographic composite configured as an elongate waveguide and having holographic f inges recorded therein, said fringes being composed of liquid crystal droplets whose molecules exhibit a preferred orientation with respect to a polar axis of each droplet, said droplets in a rest state of said composite having their polar axes oriented randomly in three dimensions, and means operative to apply an electric field to said holographic composite thereby to re-orient said molecules such that the polar axes of the droplets lie uniformly in a direction parallel to said radiation propagation direction, or such that said polar axes are randomly oriented in a plane perpendicular to said radiation propagation direction, such re-orientation of the liquid crystal molecules causing said holographic fringes to be activated and deactivated.

Preferably, the said means operative to apply an elect ir of electrodes.

In one arrangement, the electrodes are spaced apart transversely of the waveguide, such that said electric field is orthogonal to the longitudinal direction of the waveguide. In an alternative arrangement, the electrodes are spaced apart longitudinally of the waveguide, such that said electrical field extends generally longitudinally of the waveguide.

Advantageously, the electrodes are positioned longitudinally beyond the ends of the area occupied by the fringes.

Conveniently, the electrodes are of generally annular configuration and extend around the waveguide.

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

Advantageously, the holographic fringes are absent from the waveguide in regions that are adjacent to said electrodes or electrode portions. Conveniently, the holographic fringes are formed as a reflection hologram, and when activated are operative to reflect an incident beam of radiation back along the waveguide.

Preferably, the liquid crystal droplets of said holographic composite are dispersed in a surrounding medium (such as a polymer), and in one arrangement the refractive index of said

^{• •} 5 surrounding medium is matched to the refractive index of the liquid crystal material when said molecules are re-oriented under the action of said electric field, such that the application of said electric field causes the holographic fringes to be de-activated. In an alternative arrangement, the refractive index of said surrounding to the refractive index of the liquid crystal material when in said rest s uc, sucu mai me application of said

10 electric field causes the holographic fringes to be activated.

Conveniently, the waveguide device comprises holographic fringes recorded in two or more separate regions of said composite which are spaced apart longitudinally of the waveguide, and a respective means operative to apply an electric field to each of said regions. The said respective means are preferably independently operable. 15

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:

Figures 1 and 2 are diagrams showing a holographic material in two different states;

20 Figures 3 A and 3B are diagrams showing the optical characteristics of a liquid crystal droplet which forms part of the material shown in Figures 1 and 2;

Figure 4 is a diagram showing another form of holographic material;

Figure 5 is a schematic sectional side view of a first embodiment of a switchable waveguide device according to the present invention; Figure 6 is a schematic perspective view of part of the device shown in Figure 5;

Figures 7 and 8 are diagrams showing a holographic material of the waveguide device respectively when in its rest and de-activated states;

Figure 9 is a schematic sectional side view of a modified version of the device shown in Figure 5;

Figure 10 is a sectional view showing the linldn; aveguide device according to the present invention with a conventional waveyuiue,

Figure 11 is a schematic sectional side view of a second embodiment of a switchable waveguide device according to the present invention;

Figure 12 is a perspective view of a third embodiment of a switchable waveguide device according to the present invention;

Figure 13 is an exploded view of the device shown in Figure 12;

Figure 14 is a detailed view of electrodes which from part of the device shown in Figures 12 and 13;

Figure 15 is a diagram showing the modulated refractive index distribution in a switchable waveguide device according to the present invention; and

Figure 16 is a diagram of computed spectral response for the modulated distribution indicated in Figure 15.

Detailed Description

The present invention is concerned with minimising polarisation losses by utilising a waveguide which comprises an optical medium in which holographic fringes (preferably of the Bragg type) are recorded, these fringes being switchable between active and inactive states by the application of an external electrical stimulus. The optical medium typically ■ ■ .. comprises a polymer-dispersed liquid crystal (PDLC) material which, in its most fundamental 5 form, includes a monomer and a liquid crystal material: it can however, also include a cross- linking monomer, a co-initiator and a photo initiator dye. The material can also be ultraviolet exposed. The material undergoes phase separation during the hologram recordal process, creating fringes comprising regions densely populated bv liquid crystal micro-droplets interspersed with regions of clear polymer. When an ;d to the material0 by way of electrodes, the natural orientation of the uquiα crystal molecules is changed, causing the refractive index modulation of the fringes to reduce and the hologram diffraction efficiency to drop to a very low level, thereby effectively erasing the hologram. The optical characteristics of the material thus result from an ability to control the refractive index mismatch between the liquid crystal droplets and the surrounding polymer. Using such a system,5. it is possible to achieve very fast switching rates, typically with a switching time of less than 150 microseconds, and perhaps as low as a few microseconds. '

Typically, the holographic medium has two optical states corresponding to the electrical stimulus being on or off, these being equivalent respectively to the hologram being disabled or activated. With one type of PDLC material, in its normal or rest state the liquid crystal0 droplets tend to be randomly aligned. When the external electrical stimulus is applied the droplets tend to re-orient such that the liquid crystal molecules become aligned with the direction of the applied electric field. Such a material is said to have positive anisotropy. With another type of material, the liquid crystal molecules in their rest state are generally oriented at right-angles to the above, and become re-oriented perpendicularly to the applied5 electric field: such a material is said to have negative anisotropy.

Figure 1 shows a typical arrangement of the liquid crystal droplets (referenced 10) in their rest state. Within each droplet, there is a tendency for the liquid crystal molecule directors to exhibit a bipolar alignment pattern, as indicated schematically by lines 11, with the polar axis being indicated at P. One of the droplets 10 is shown in detail in Figures 3A and 3B, with Figure 3A being a view perpendicular to the polar axis P and Figure 3B being a view parallel to that axis. Because of the way in which the molecule directors are orientated, the droplet will exhibit birefringence. More particularly, for radiation incident in a direction parallel to the polar axis P (i.e. as viewed in Figure 3B) the droplet will have an ordinary refractive index n₀, whereas for radiation incident perpendicularly to the polar axis (i.e. as viewed in Figure 3A) the droplet will have an extraordinary refractive index n_e, with n_e generally being greater than n₀.

In view of the above, it would be expected that the 1 rest state would exhibit polarisation sensitivity for light incident in a uuct uπ αa vicwcu in Figure 1. In actual fact, however, this is not the case because the droplets 10 are randomly orientated ■ throughout the volume of the material. As a result, the polar axes P of the droplets are also arranged randomly, sα the material does not show any net average preferred polarisation direction.

Figure 2 shows (for one particular type of PDLC material) a typical arrangement of the liquid crystal droplets 10 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 10 now become re-oriented 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.

In the above-described arrangement, PDLC material shows no dependence upon the polarisation state of the incident radiation, because in the rest state the polar axes of the liquid crystal droplets are randomly oriented with respect to a plane perpendicular to the radiation propagation direction, so that any polarisation sensitivity is averaged out.When the electric field is applied, the polar axes become uniformly aligned in a direction parallel to the radiation propagation direction, and therefore exhibit only their ordinary refractive index to the radiation. As depicted in Figure 4, in an alternative type of PDLC material, in the rest state the polar axes P of the liquid crystal droplets 10 are randomly oriented in three dimensions. For the same reasons as stated above, the material shows no net polarisation sensitivity in this condition because of the randomness of the orientation of the liquid crystal droplets. When an electric field is applied, the polar axes P become randomly oriented in a plane perpendicular to the radiation propagation direction. Again, the material shows no polarisation sensitivity in this condition, because the radiation sees only liquid crystal droplets whose polar axes are randomly oriented, so anv υolarisation effects are balanced out.

In the ensuing description, it will be explained how this general principle can be applied in order to create switchable waveguide devices which are not polarisation sensitive. It is to be understood that, in each of the embodiments described, the PDLC material can be of either of the above general types, i.e. switchable between a first condition in which the liquid crystal droplets are randomly oriented in three dimensions and a second condition in which the droplets are either uniformly aligned or are randomly oriented in a plane perpendicular to the direction of radiation propagation.

Referring now to Figure 5, there is shown therein a first embodiment of a switchable waveguide device which comprises a length of capillary tube 20 comprising an inner capillary or core 21 typically of five microns diameter and an outer tubular cladding 22 typically of 80 microns external diameter. The core 21 is formed of PDLC material and has recorded therein a series of holographic fringes (represented schematically by lines 23). In the illustrated arrangement, the fringes are recorded in two separate regions 24 and 25 of the core 21 which are spaced apart longitudinally of the latter, although more or fewer such regions can be provided, depending upon the desired application of the device. One of these regions is shown in more detail in Figure 6, it being understood that the other region is substantially identical.

Provided in each of the regions 24 and 25 are a respective pair of annular electrodes 26 and 27 in the form of electrically conductive hoops which encircle the exterior of the capillary tube 20. An electrical stimulus can be applied across the electrodes in each pair by means of a respective voltage source 28 (shown schematically in Figure 6) thereby producing an electric field between the electrodes. The field is of toroidal configuration as indicated by arrows 29, but the electrodes 24 and 25 themselves are positioned beyond the ends of the area occupied by the respective fringes such that in that area the electric field effectively extends longitudinally of the core 21, as indicated by arrow 29'.

Referring now also to Figure 7, this shows the fringes 23 in the rest state of the PDLC material, i.e. when the electric field is switched off. The fringes 23 are formed by liquid crystal droplets 30 in which the polar axes are orient ited by lines 31.

In this condition, the fringes 23 act as a reflection ho.ugiam anu lencui υack any incident radiation propagating along .the core 21, as indicated by arrow A. The droplets 30 present themselves to the incident radiation in the manner described previously with reference to Figure 1 so the PDLC material does not show any polarisation sensitivity in this condition.

Figure 8 shows the PDLC material when the electric field (as represented by arrow E) is turned on. Since in this arrangement the material is of a type having positive anisotropy, the polar axes 31 of the liquid crystal droplets 30 become aligned parallel to the electric field E, and this effectively erases the hologram. Incident radiation is therefore able to continue its propagation along the core 21, as indicated by arrow B. In this condition, the liquid crystal droplets present themselves to the incident radiation in the manner described previously with reference to Figure 2, so again the PDLC material does not show any polarisation sensitivity.

Although in this embodiment the fringes are shown as extending generally perpendicular to the core, they can instead be slightly slanted.

Figure 9 shows a modified arrangement in which the electrode pairs are interdigitated in the longitudinal direction of the capillary tube 20. More particularly, the electrode 26 comprises a plurality of hoop portions 26a, 26b etc whilst the electrode 27 similarly comprises a plurality of hoop portions 27a etc, with the former alternating with the latter in equally- spaced relationship along the capillary tube 20. This arrangement is useful in increasing the interaction length between the electrodes and the fringes.

In both of the above-described arrangements, the holographic fringes are recorded in the core

21 prior to deposition of the electrodes 26 and 27 on the exterior of the tube 20. Also, because the presence of the electrodes will affect radiation propagating along the waveguide, these are arranged to be offset from the core 21, e.g. as depicted schematically in Figure 9.

Figure 10 shows the connection of the above-describe D a conventional waveguide 40 which comprises an optical fibre core 4ι surrounαeα oy a snica sheath 42. An end of the capillary tube 20 is abutted against the waveguide 40 within a ferrule 43, such that the core 21 and the core 41 are co-axially aligned. The waveguide 40 and the ferrule 43 are preferably items manufactured by Vitrocom. A suitable gel can be used between the abutting ends of the waveguide 40 and the capillary tube 20 to match the refractive indices of the cores 21 and 41.

Figure 11 shows a second embodiment of a switchable waveguide device, which is generally similar to that described above with reference to Figure 5: accordingly, similar parts have been accorded the same reference numerals. In this embodiment, however, the electrodes 26 and 27 in each pair are generally planar and are disposed on opposite sides of the capillary tube 20. The external surface of the tube 20 can have flats machined thereon to accommodate these electrodes. When an electrical stimulus is applied to the electrodes 26 and 27, an electric field is generated which extends transversely of the core 21 of the capillary tube 20, as indicated by arrows 43.

In the embodiments shown in Figures 5 and 11, the waveguide device of the invention is interposed between two conventional waveguides 40. In one arrangement, the device can be regarded as comprising two separate switching arrangements (denoted as X and Y). Each of these switching arrangements is disposed adjacent to arespective end of the overall switching device, and is composed of a region of the PDLC material in which the fringes are recorded, plus the associated electrodes 26 and 27. When the left-hand switching arrangement X is activated (i.e. when the electrodes are not energised), the fringes 23 operate to reflect back any radiation propagating along the left-hand waveguide 40. Similarly, when the fringes 23 of the switching arrangement Y are activated, the arrangement operates to reflect back any radiation propagating along the right-hand waveguide 40. However, forward propagation of radiation can be enabled (either in the leftward or rightward direction) by energising the electrodes of both switching arrangements, thereby de-activating both sets of fringes.

In an alternative system, the arrangements X and Y can form part of a common switching device and can be operated in unison. It is possible 5 formed by the fringes of one arrangement to have a different spatial frequent_ly um mo nniges in the other arrangement, such that each of the arrangements X and Y acts upon a respectively different wavelength of radiation propagating along the waveguide.

Figures 12 and 13 show a third embodiment of a switchable waveguide device. The device comprises an optical fibre 50 having an outer cladding 50a and a central core 50b within which are recorded holographic fringes in the form of a Bragg grating 51. The fibre 50 is housed in a channel 52 formed in one surface 53 of a silica block 54, the channel 52 being of generally convex configuration so as to impart a convexity to the fibre. After initial assembly, the region of the fibre 50 which contains the grating 51 stands slightly proud of the surface 53. The fibre is machined flat in this area so that it becomes flush with the surface 53, thereby exposing the part of the core 50b which contains the grating 51 (as can be seen to advantage in Figure 13). The grating 51 can have a uniform fringe spacing or can be chirped.

The exposed grating is covered by a cladding 55 which is applied to the surface 53 of the silica block 54. On its face which confronts the block 54, the cladding 55 is composed of a layer 56 of PDLC material on which are deposited a series of electrode pairs 57 and 58. Each electrode in each pair comprises a plurality of fingers 59 which extends transversely of the fibre 50, the fingers of one electrode being interdigitated with fingers of the other. This arrangement is shown in more detail in Figure 14. The refractive index of the layer 56 is chosen to match that of the cladding 50a of the fibre 50. In a typical arrangement, fingers 59 are 5 microns wide and the spacing between adjacent fingers is approximately 20 microns. The length of the grating 51 and the span of each electrode pair 57, 58 will depend upon the required interaction length, which will in turn be determined by the desired application of the device.

When an electrical stimulus is applied across an electrode pair 57, 58, this produces an electric field which extends from each finger 59 to its neighbour, i.e. in a direction parallel to the extent of the fibre 50. The resultant effect is therefore generally similar to that described above with reference to the embodiment of

In the embodiments of Figures 5 to 8, 9 and 12 to 14, there will be non-uniformity in the electric field immediately adjacent to (i.e. longitudinally aligned with) each electrode or electrode finger, this being shown to particular advantage in Figure 9. This means that the electrodes will not be effective in switching the Bragg grating in those areas. Indeed, spurious effects may arise due to the electric field being applied in a non-optimal direction.

For this reason, it is preferred that the grating is made of discontinuous form, so that there are no fringes in the areas immediately adj acent to the electrodes. Careful design is required to ensure that spectral peaks resulting from the contribution of the electrode spatial frequency are minimised.

Figure 15 shows in diagrammatic form a typical example of the modulated refractive index of the Bragg grating that can be used in such circumstances. In this diagram, it is assumed that the electrodes are 5 microns in width, the electrode separation is 20 microns, the modulation of the refractive index is 0.0007, the average refractive index is 1.52, and the Bragg fringe separation is approximately 0.5 microns. Figure 16 shows a computed spectral response for such a grating. The resultant curve shows a peak 60 corresponding to the wavelength for which the grating is designed. There are also side lobes 61 corresponding to the modulation of the grating, but these are much smaller than the response at the Bragg wavelength.

The above-described arrangements are suited for use in telecommunications applications, such as in wavelengths division multiplexing, attenuation, line testing and dynamic gain equalisation. However, the invention can be applied to other areas of technology as well, such as in optical display systems.

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 assur re index of the polymer in the PDLC material is matched to the refractive IUUCA UJ.

crystal in the state where the electric field is applied, so that the fringes are de- activated by application of the field. 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 liquid crystal index in its rest state. Under these circumstances, the fringes would be de-activated in the rest state of the PDLC material, but could be activated by the application of an electric field.

In addition, in the above described embodiments, the Bragg gratings formed by the holographic fringes can be simple constant frequency gratings, or can be chirped (i.e. such that the grating frequency varies along its length). In the latter case, the waveguide device can be used to act upon radiation of differing wavelengths (to which the Bragg grating is tuned).

Furthermore, in the embodiments described above, the electrodes are spaced apart longitudinally of the waveguide, so that the electric field extends generally in the longitudinal direction of the waveguide. As an alternative, the electrodes can be spaced apart transversely of the waveguide, so that the electric field is orthogonal to the longitudinal direction of the waveguide. This will, however, require consequential re-design of the fringes so that they can still be activated and de-activated by the application of the electric field. In addition, the electrodes can take other forms apart from those described above.

Claims

1. A switchable waveguide device comprising:

an electrically switchable holographic composite configured as an elongate waveguide (21) and having holographic fringes (23) recorded therein, said fringes

(23) being composed of liquid crystal droplets (30) whose molecules exhibit a preferred orientation with respect to a polar a Let, said droplets

(30) in a rest state of said composite having then υitu SAC., ^J I oriented randomly in three dimensions, and

means operative to apply an electric field (E) to said holographic composite thereby to re-orient said molecules such that the polar axes (31) of the droplets (30) lie uniformly in a direction parallel to said radiation propagation direction (A), or such that said polar axes (31) are randomly oriented in a plane perpendicular to said radiation propagation direction (A), such re-orientation of the liquid crystal molecules causing said holographic fringes (23) to be activated and de-activated.

A switchable waveguide device as claimed in claim 1, wherein said means operative to apply an electric field includes a pair of electrodes (26, 27).

3. A switchable waveguide device as claimed in claim 2, wherein the electrodes are spaced apart transversely of the waveguide, such that said electric field is orthogonal to the longitudinal direction of the waveguide.

A switchable waveguide device as claimed in claim 2, wherein the electrodes (26, 27) are spaced apart longitudinally of the waveguide (21), such that said electric field extends generally longitudinally of the waveguide (21).

A switchable waveguide device as claimed in claim 4, wherein the electrodes (26, 27) are positioned longitudinally beyond the ends of the area occupied by the fringes.

A switchable waveguide device as claimed in claim 4 or 5, wherein the electrodes (26, 27) are of generally annular configuratior ιe waveguide.

A switchable waveguide device as claimed in claim 4, 5 or 6, wherein each electrode comprises portions (26a, 26b) which alternate with portions (27a) of the other electrode longitudinally of the waveguide (21).

A switchable waveguide device as claimed in claim 7, wherein each electrode (57, 58) comprises aplurality of fingers (59) extending transversely of the waveguide (50), and the fingers (59) of one electrode (57) alternate with the fingers (59) of the other electrode (58) longitudinally of the waveguide (50).

A switchable waveguide device as claimed in claim 7, wherein the electrodes (26, 27) comprise a plurality of hoop portions (26a, 26b, 27a) which extend around the waveguide (21) and which alternate with one another longitudinally of the waveguide.

A switchable waveguide device as claimed in any one of claims 4 to 9, wherein the holographic fringes (23) are absent from the waveguide (21) in regions that are adjacent to said electrodes (26, 27) or electrode portions (26a, 26b, 27a). A switchable waveguide device as claimed in any preceding claim, wherein the holographic fringes (23) are formed as a reflection hologram, and when activated are operative to reflect an incident beam of radiation back along the waveguide.

A switchable waveguide device as claimed in any preceding claim, wherein the liquid crystal droplets (30) of said holographic composite are dispersed in a surrounding medium (such as a polymer), and t said surrounding medium is matched to the refractive index of uic n um wystαi material when said molecules are re-oriented under the action of said electric field (E), such that the application of said electric field (E) causes the holographic fringes (23) to be de- activated.

A switchable waveguide device as claimed in any one of claims 1 to 11, wherein the liquid crystal droplets (30) of said holographic composite are dispersed in a surrounding medium (such as a polymer), and the refractive index of said surrounding medium is matched to the refractive index of the liquid crystal material when in said rest state, such that the application of said electric field (E) causes the holographic fringes (23) to be activated.

A switchable waveguide device as claimed in any preceding claim, comprising holographic fringes (23) recorded in two or more separate regions (X,Y) of said composite which are spaced apart longitudinally of the waveguide (21), and a respective means operative to apply an electric field to each of said regions. A switchable waveguide device as claimed in claim 16, wherein said respective means are independently operable.

A switchable waveguide device as claimed in any preceding claim, wherein the waveguide is in the form of an optical fibre (50) comprising a core (50b) and a surrounding tubular cladding (50a), at least one of which contains said holographic composite.

A switchable waveguide device as claimed in claim 17, wherein the holographic composite is provided in said core (50b) which is held in a convex configuration, the cladding (50a) having been removed at the convexity to expose the core (50b) in the region where the fringes (23) are formed, and said means operative to apply an electric field is applied to the exposed region of the core (50b).