PHOTOJ EFRACTIVE DEVICES
The present invention relates mainly to photorefractive devices and in particular to photorefractive polymer devices especially for use in optical communications networks.
In optical communications it is usual for light propagating along a waveguide or fibre to contain information, encoded normally via amplitude or frequency modulation. Often wavelength division multiplexing (WDM) techniques are employed to increase the data transmission rates along a waveguide or optical fibre. In wavelength division multiplexing many data steams are transmitted simultaneously along a fibre at different optical wavelengths. Different wavelengths do not interact with each other significantly under conditions of moderate to low optical intensity or over short transmission distances.
WDM systems require components for wavelength control. An important class of components are filters, which can reflect or block one wavelength whilst allowing others to pass almost unaffected. Tunable filters offer clear advantages for network reconfigurability, wavelength switching and routing and inventory reduction. The tunable filter lies at the heart of many more complex systems such as a reconfigurable optical add-drop multiplexer, variable optical attenuator, dynamic gain equaliser and reconfigurable tap coupler as well as a discrete component.
Within optical communications networks, the add-drop multiplexer is a device with this ability to filter out one specific wavelength channel whilst allowing the others to pass unaffected. Add-drop multiplexers are positioned at the nodes of optical communications networks and are used as switches directing data streams along desired pathways. To be particularly useful an add-drop multiplexer must be capable of being reconfigured to filter out of a data stream or add in to a data stream signals of different wavelengths. A favoured known add-drop multiplexer design is based around an optical wavelength filter and is often a Bragg grating written into an optical fibre. A Bragg grating is a periodic modulation of the propagation constants of the fibre and may be formed in the fibre by etching, printing, chemical process or other applicable technique. The grating vector is directed perpendicular to the modulation planes, along the direction in which the spatial period is minimised. A suitable fibre Bragg grating can reflect light strongly when the light incident upon it has a wavelength equal to twice the grating period. This spectral wavelength dependent reflection property is used as an optical wavelength filter to reflect signals of a particular wavelength into or out of a data stream [ 1 ] .
A state of the art method for tuning a Bragg grating filter of this type is controlled thermal expansion [2]. The expansion of the fibre when heated increases the period of the grating written in the fibre. Successful operation of such a device may require a lot of control electronics and thermal isolation of the fibre from the surroundings. The maximum tuning rate is limited by the thermal inertia of the optical fibre. It would therefore be desirable to provide an improved tunable filter in which these problems may be avoided.
Alternative methods have been described for producing tunable Bragg gratings. One of these methods is optical addressing of a photorefractive material.
A photorefractive material is a material in which the refractive index changes in response to optical illumination. The photorefractive change in refractive index can be caused by different mechanisms in different materials. In some materials an optical interference pattern causes a photochemical reaction (material specific in its exact nature) that changes the local refractive index. Bright regions experience a change in refractive index and a refractive index pattern is formed matching the interference pattern. The photochemical change in refractive index caused by this process is usually permanent and cannot be reconfigured. In other photorefractive materials an electro-optic response causes the change in refractive index. An optical interference pattern photogenerates free charges that move due to diffusion or, when an electric field is applied, drift. These eventually become trapped resulting in a spatial charge distribution pattern mimicking the optical interference pattern. The non-uniform charge distribution leads to an electric field pattern that modulates the refractive index. The refractive index pattern that is created in response forms a hologram distributed within the bulk of the photorefractive material. Subsequent erasure of the hologram may be performed by uniform optical illumination to redistribute charge uniformly. A new hologram may then be recorded. Alternatively, the material can be illuminated by a new optical interference pattern, which will progressively record a new hologram and overwrite the previously recorded hologram. It is also possible to control subsequent hologram recording so that the original hologram is not completely erased, thereby writing two holograms into the material, a compound hologram.
Optical addressing is a means of recording a suitable hologram corresponding to a desired pattern in a photorefractive material. A Bragg grating can be formed in a photorefractive material using a two-beam interference pattern. Relevant optical addressing means are thus those that provide for the intersection of two coherent light beams in a photorefractive material.
In order for a tunable Bragg grating filter to be formed in a photorefractive material by optical addressing it is beneficial if the photorefractive material is a reconfigurable photorefractive material as described above. The optical addressing means may then generate two-beam interference patterns with a variety of different periods thereby corresponding to Bragg gratings of different periods. A simple method of doing this is to vary the intersection angle of the beams.
A number of methods of optical addressing have been described previously [3,4,5,6,7]. These have been directed towards optically addressing bulk photorefractive crystals rather than addressing a photorefractive grating in a more economic (i.e. small) filter.
A conceptually simple method of optical addressing is to split an input laser beam by any convenient means such as a thin film beam splitter. Actuated mirrors are then employed to direct the subsidiary beams to intersect in a photorefractive material and to vary the path of the beams in order that their intersection angle in the photorefractive material is also varied [3,4,5].
A variation on this method [6] relies on a similar interference pattern being formed on a chosen face of a prism placed adjacent to a photorefractive material. Part of the input laser beam is refracted and then totally internally reflected before being in incident on the chosen prism face. Another part of the input beam is incident upon the chosen face after being refracted but not totally internally reflected. The two parts of the beams thereby intersect at an angle to each other forming an interference pattern. The pitch of the pattern can be varied by varying the input angle of the beam to the prism either by rotating the prism or by rotating the laser.
As these methods rely on various moving parts it is likely that its operational lifetime and reliability will be limited. Furthermore as tuning the grating period involves rotating one or more objects there is likely to be appreciable delay in retuning the grating particularly when the retaining is over a significant proportion of the achievable range. This method is also likely to prove difficult to miniaturise.
Another method of optical addressing [7] has been described which does not depend on moving parts. This method uses a light source (a laser) and a spatial light modulator (SLM). Light from the laser is collimated and is then incident on the SLM. The SLM is controllable such that any two selected pixels will transmit light and the other pixels will not. In this manner two independent beams are formed. Subsequent to the SLM the light is incident upon a converging lens and is thereby focussed upon a photorefractive material. The two incident beams thereby form an interference pattern. The angle at which the beams intersect can be adjusted by varying the pixels of the SLM allowing the transmission of light.
This method does not have any moving parts but it does require a careful lens arrangement. As this lens arrangement requires a diverging lens in order to collimate the input beam and a converging lens to cause the subsidiary beams to intersect it is difficult to miniaturise. Therefore it is desirable to provide an optical addressing arrangement which may be implemented compactly and without moving parts.
In order to produce a commercially successful tunable filter for use in an add/drop multiplexer, for example, it is desirable to form optical Bragg gratings in material that is cheaper and easier to process than are typical photorefractive crystals such as barium titanate and lithium niobate. In recent years the performance of photorefractive polymers has improved sufficiently to exceed the performance of photorefractive crystals in some respects such as diffraction efficiency. Additionally photorefractive polymers are cheaper and easier to process than inorganic photorefractive crystals.
There are a number of examples of devices that use photorefractive polymers which have been described in research papers. They generally provide a photorefractive polymer film formed on a suitable substrate [8,9,10]. In [8], no hologram is written by any optical addressing means and the device acts to couple the light into a film via two beam coupling with radiation scattered into the waveguide. In [9], no hologram written by any optical addressing means and second harmonic light is generated in the film and then is self-guided via a light intensity dependent local increase in refractive index. In [10] a simple Bragg hologram is formed in a planar film and light is coupled into the film and is thereby transmitted through the film until it interacts with the written hologram. The hologram is, however, formed by a photochemical reaction, not by an electro- optic response to a reconfigurable space-charge field, and is thus not reconfigurable.
A commercially useful device would confine the input light signals and the output light signals to predetermined paths. This would allow the device to be made in as compact a form as possible thereby saving on raw materials and space. Also keeping the signals concentrated makes the device more efficient and prevents stray signals reaching other nearby devices. Therefore it is desirable to provide a photorefractive polymer component (suitable for use as a tunable filter in an add/drop multiplexer) that confines input and output light signals to predetermined paths. A further difficulty is that photorefractive material are often difficult to process effectively without compromising their photorefractive properties. An exception to this is a particular composite material suitable for injection moulding which has been shown to exhibit modest photorefractive properties [15]. Therefore it is also desirable to be able to provide an improved material or method for manufacturing such devices.
Many of the modern high performance photorefractive polymers are amorphous rather than crystalline in form and they might be expected to have very weak electro-optic properties. These polymers however are doped heavily with non-linear optical chromophores and are often operated in the presence of an electric poling field so that these chromophores generate the required photorefractive properties. The poling field can be applied during device fabrication at elevated temperature to achieve a quasi-permanent poling of the molecules. Alternatively the shear viscosity of the materials may be sufficiently low to allow poling at room temperature removing the need for poling during fabrication. Such materials show an added advantage of allowing chromophore alignment to the space charge field, which gives an added refractive index pattern via linear anisotropic molecular hyperpolarisability in a process sometimes known as the reorientational photorefractive effect [11]. Photorefractive devices manufactured from these low viscosity polymers benefit from the application of a poling field. To have maximum effect the poling field should be applied in a specific direction relative to the propagation direction of light in the device. Particular gratings such as the type of Bragg grating required for an add/drop multiplexer in a photorefractive polymer require poling fields parallel to the direction of light propagation in a device in order to be at their most effective.
There are documents [12,13] which disclose a method of generating an electric field in a photorefractive film, but these documents do not disclose any method of applying this field parallel to the direction of light propagation. It is therefore desirable to provide a means of generating an electric field component parallel to the direction of light propagation in a photorefraptive device.
It is therefore an object of the present invention to provide an improved tunable grating filter suitable for incorporating into an optical add/drop multiplexer.
It is also an object of the present invention to provide an improved optical add/drop multiplexer incorporating an improved tunable grating.
Additionally it is an object of the present invention to provide an improved method of optical addressing for forming a tunable grating.
It is a further object of the present invention to provide to provide a photorefractive component wherein light is confined to a predetermined path.
Furthermore, it is yet another object of the present invention of a photorefractive polymer component wherein an electric field component is provided in a direction parallel to the direction of propagation of optical signals. According to a first aspect of the present invention, there is provided an optical device, particularly a tunable or reconfigurable optical grating, comprising;
A photorefractive material waveguide wherein the guided mode is confined in two dimensions and the photorefractive material is of the kind wherein reconfigurable holograms can be recorded; and
Optical addressing means provided to tune or reconfigure the device by recording different holograms corresponding to desired optical devices in said waveguide.
The photorefractive material may be an organic and particularly a polymeric material.
Such a grating can provide a rapidly retunable Bragg grating suitable for use in devices such as a reconfigurable optical add-drop multiplexer, variable optical attenuator, dynamic gain equaliser and reconfigurable tap coupler as well as a discrete component or a dispersion compensator. The grating avoids the previously discussed problems with thermal inertia.
Said waveguide may be a channel waveguide, ridge waveguide or any other suitable form of waveguide. In particular, said waveguide may be a single mode waveguide. Alternatively, the waveguide may be a conventional waveguide in contact with a photorefractive material such that the guided mode of the waveguide overlaps with the photorefractive material and is influenced by any holograms recorded therein.
Preferably, the photorefractive polymer is a photorefractive polymer of the type disclosed in European patent no. 897577
Preferably, said optical addressing means is not positioned along the waveguide propagation axis. Preferably, said optical addressing means is used to record Bragg gratings of a variety of periods in the photorefractive polymer waveguide. The Bragg gratings recorded may have grating vectors slanted with respect to the waveguide axis if desired.
Preferably, the optical addressing means comprises means for splitting a laser beam into two parts and means for causing the two parts to intersect at a desired location and an optical interference pattern with an intensity distribution corresponding to a Bragg grating being thereby formed at the desired location. Most preferably, the optical addressing means retunes the filter by adjusting the angle at which said beam parts intersect or by tuning the wavelength of the optical addressing light source thereby adjusting the pitch of the written grating.
Possible means for splitting a beam into two parts may include a beam splitter, a spatial light modulator, a prism or any other suitable device. Possible means for adjusting the angle at which said beam parts intersect may include a movably mounted laser for generating the initial beam, one or more movably mounted mirrors, a rotatable prism or any other suitable device.
Alternatively the optical addressing means comprises a method of addressing the device by using a single addressing beam along with a phase mask. Tuning or reconfiguring can then be achieved by tuning the wavelength of the addressing light, which tunes the pitch of the optical interference pattern, or by tuning the phase mask properties, or by tuning the wavelength of the addressing light and using a phase mask that contains multiple holograms, written for different optical wavelengths.
Preferably, means are provided for allowing the optical interference pattern to be formed by more than two beam parts thereby providing for the possibility of forming compound gratings. Preferably, there is provided means for generating an electric field component directed along the waveguide propagation axis.
According to a second aspect of the present invention there is provided an optical add/drop multiplexer containing a tunable filter of the kind wherein the tunable filter is a tunable Bragg grating, characterised in that the tunable Bragg grating is provided in a photorefractive polymer waveguide, wherein the photorefractive polymer is of the kind wherein reconfigurable holograms can be recorded, and the grating is generated by and tuned by recording holograms corresponding to Bragg gratings of various periods in the photorefractive polymer waveguide.
An add/drop multiplexer of this form is tunable without being susceptible to becoming inaccurate after large temperature variations and does not have a tuning rate limited by thermal inertia. Said tunable filter may conveniently be a tunable optical grating of the type described in relation to the first aspect of the present invention.
The add/drop multiplexer may be a multiplexer of the type comprising a Bragg grating disposed between two optical circulators or a multiplexer of the type comprising a Bragg grating disposed between a pair of 3-dB couplers or a multiplexer of the type containing a Bragg grating disposed in a Mach-Zehnder interferometer geometry or a multiplexer of the type based on a Bragg grating provided in an adiabatic null coupler or any other suitable multiplexer arrangement.
According to a third aspect of the present invention there is provided an optical addressing arrangement for forming an optical interference pattern in a photorefractive polymer waveguide provided on a substrate, comprising: One or more coherent light sources; and
An array of addressing waveguides positioned on the substrate in order that light from one or more of said light sources may be coupled into any two or more of the addressing waveguides when desired and thereby directed to intersect at a portion of the photorefractive polymer waveguide, the light transmitted along each of the addressing waveguides being incident on the photorefractive waveguide at a different angle to that transmitted by every other addressing waveguide.
This arrangement allows an optical addressing system for a tunable Bragg grating to be embodied on a single substrate thus saving space. The optical addressing system also omits moving parts and therefore should be highly reliable.
Preferably, said light source is a vertical cavity surface-emitting laser (VCSEL). Most preferably, the light source is an array of VCSELs each dedicated to a single addressing waveguide.
Preferably said addressing waveguides are channel or ridge waveguides, particularly single mode channel or ridge waveguides.
Preferably, the addressing waveguides are arranged in a number of individually selectable waveguide pairs with a common first portion coupled to a VCSEL and then subsequently being split in to separate second portions by a Y- splitter. Additionally and preferably, the second portions of each and every waveguide pair are arranged so that light transmitted from the second ends of each waveguide pair converges at substantially the same portion of the photorefractive polymer waveguide at different intersection angles. In this manner by selecting a waveguide pair that has a desired intersection angle the grating is tunable to a desired resonance.
Preferably, the convergence of light from the second portions of the waveguide pairs forms an optical interference pattern corresponding to a Bragg grating. Most preferably, more than one waveguide pair can be used to optically address the photorefractive polymer waveguide at any time. In this mariner compound Bragg gratings may be formed in the photorefractive polymer waveguide allowing a plurality of wavelengths to be filtered at one time.
According to a fourth aspect of the present invention there is provided a method for optically addressing a photorefractive polymer waveguide provided on a suitable substrate by directing a coherent light beam into two or more addressing waveguides selected from an array of addressing waveguides provided on said substrate wherein each addressing waveguide is adapted such that light transmitted along it is incident on the same portion of the photorefractive polymer waveguide but at a different angle to light transmitted along every other addressing waveguide in the array.
According to a fifth aspect of the present invention there is provided a photorefractive polymer waveguide upon a suitable substrate wherein the guided mode of said waveguide is confined in two dimensions and said photorefractive polymer is a photorefractive polymer of the Jkind wherein reconfigurable holograms can be recorded.
Such a waveguide allows devices such as an add/drop multiplexer incorporating a photorefractive tunable Bragg grating to be manufactured as an integrated optical circuit on a single substrate.
The waveguide may be a channel waveguide or may be a ridge waveguide; particularly preferably, the waveguide is a single mode channel or ridge waveguide. Most preferably said waveguide is surrounded by cladding material, said cladding having a lower refractive index than said waveguide material.
Preferably, holograms are recorded in the waveguide in order to influence optical signals transmitted along the waveguide. Most preferably, holograms recorded in the waveguide correspond to a Bragg grating, which may be a compound Bragg grating. Most preferably, the photorefractive polymer is a photorefractive polymer of the type disclosed in European patent no. 897577
Preferably, there is provided means for generating an electric field component directed along the waveguide propagation axis.
According to a sixth aspect of the present invention there is provided a waveguide upon a suitable substrate wherein the guided mode of said waveguide is confined in two dimensions, the waveguide being in contact with a layer of photorefractive material, the photorefractive layer overlapping with the guided mode of the waveguide so that any hologram recorded in the photorefractive polymer layer influences transmission of signals by the waveguide.
This type of waveguide can prove useful in creating integrated optical circuits for applications where a completely photorefractive waveguide is unnecessary or undesirable for other reasons.
Preferably, the photorefractive material is a photorefractive material of the kind wherein reconfigurable holograms can be recorded and most preferably, it is a photorefractive polymer of the kind disclosed in European patent no. 897577
The waveguide may be a channel waveguide or may be a ridge waveguide; particularly preferably, the waveguide is a single mode channel or ridge waveguide. Most preferably said waveguide is surrounded by cladding material, said cladding having a lower refractive index than said waveguide material.
Preferably, there is provided means for generating an electric field component directed along the waveguide propagation axis.
Preferably, holograms are recorded in the photorefractive layer in order to influence optical signals transmitted along the waveguide. One particular hologram suitable for being recorded in said photorefractive layer is a Bragg grating, which may be a compound Bragg grating.
According to a seventh aspect of the present invention there is provided a an electrode arrangement for producing an electric field component along the propagation axis of a waveguide wherein said waveguide either has electrodes disposed on one surface or alternatively has electrodes disposed on opposite surfaces, the electrodes each being formed in a comb pattern comprising a spine and a number of teeth extending therefrom, particularly substantially perpendicular to the spine, the electrodes being disposed relative to each other in such a manner that the teeth of one electrode face and interleave with the teeth of the other electrode without touching and relative to the waveguide such that spines lie in the plane of the waveguide and substantially parallel to the propagation axis of the waveguide and the teeth of each electrode lie in the plane of the waveguide transversely, in particular substantially perpendicular to the propagation axis of the waveguide.
This electrode arrangement provides a means of providing a component of electric field in a direction running parallel to the plane of the film. This is particularly important if a Bragg grating is to be written in such a waveguide, constructed from an amorphous photorefractive polymer, and it is desired that the grating vector should be aligned with a component along the waveguide propagation axis.
Preferably, said waveguide is a single mode channel or ridge waveguide.
Preferably, said waveguide is formed from a photorefractive polymer. Most preferably, said photorefractive polymer is a photorefractive polymer of the kind wherein reconfigurable holograms can be recorded. Preferably, said waveguide is formed on a suitable substrate. Most preferably said waveguide is surrounded by cladding material, said cladding having a lower refractive index than said waveguide material.
Preferably, the inter tooth spacing of each electrode is say, a few hundred microns or less however a greater tooth spacing may be used in order to generate birefringent properties.
In order to avoid strong reflection of light due to the electrodes the tooth spacing may be non-uniform. A particularly preferred non-uniform electrode tooth spacing pattern is one where tooth spacing is at a minimum value at the centre of the pattern and increases towards each end.
The electrodes may be formed from a metal, a conducting ceramic material, a conducting oxide such as indium tin oxide or a conducting organic material or from any other suitable material.
The electrodes may be formed by vapour deposition, etching, hot embossing, printing, or by any other suitable method.
According to an eighth aspect of the present invention there is provided a method for generating an electric field along the propagation axis of a waveguide comprising the steps of: providing electrodes disposed either on one surface or alternatively providing electrodes disposed on opposite surfaces, the electrodes each being shaped in the manner of a comb comprising a spine portion and a number of teeth, the electrodes being disposed relative to each other in such a manner that the teeth of one electrode face and interleave with the teeth of the other electrode without touching and the spine portions of each electrode extend in the plane of the waveguide substantially parallel to the propagation axis of the waveguide. In order that the invention is more clearly understood, it is described more fully below with reference to the following drawings in which:
Figure 1 is a block diagram showing the main features of a typical optical add/drop multiplexer;
Figure 2 shows a number of alternative embodiments of an optical add/drop multiplexer using a tunable optical grating filter according to the present invention;
Figure 3 shows a number of alternative embodiments of an optical addressing system for a tunable optical grating filter according to the present invention; Figure 4 shows a further alternative embodiment of an optical addressing system for a tunable optical grating according to the present invention; Figure 5 shows two alternative embodiments of a photorefractive polymer waveguide according to the present invention; Figure 6 shows a non-photorefractive waveguide with a photorefractive layer according to the present invention. Figure 7 shows two alternative embodiments of an arrangement for applying an electric field to a waveguide perpendicular to the propagation axis of the waveguide according to the present invention; Figure 8 shows a plan view of an arrangement for applying an electric field to a waveguide along the propagation axis according to the present invention;
Figure 9 shows a side view of an alternative arrangement for applying an electric field to a waveguide along the propagation axis according to the present invention;
Figure 10 shows optical reflection spectra of photorefractive polymer composite waveguide Bragg grating filters for both (A) TE polarisation and (B) TM polarisation according to the present invention; Figure 11 shows a microscopic image of the cross section of a waveguide according to the present invention.
One of the applications of the present invention is a typical add/drop multiplexer 2, shown in figure 1. The multiplexer comprises input, output, add and drop channels 4-10. The channels can be optical data encoded on different optical wavelengths and guided within waveguides, optical fibres or any other item suitable for guiding optical radiation. The input and drop channels 4,6 are both connected to one side of a Bragg grating 12. The add and output channels 8, 10 are both connected to the other side of the Bragg grating 12.
If the multiplexer 2 is a tunable multiplexer then the Bragg grating 12 is a tunable Bragg grating. The multiplexer may be tuned by adjusting the grating period. As described previously herein tunable gratings 12 for add/drop multiplexers 2 are conventionally provided by controlled thermal expansion of a permanent grating formed in an optical fibre [2]. In the present invention the tunable grating 12 is provided in a photorefractive polymer waveguide and is generated and tuned by means of optical addressing.
Figures 2a-2d show various possible arrangements which can if desired be used to embody an add/drop multiplexer [1]. The Bragg grating 12 is a tunable Bragg grating provided in a photorefractive polymer waveguide according to the present invention. The grating 12 is formed from a reconfigurable photorefractive polymer and is tuned by a suitable optical addressing means (not shown).
In the embodiment of figure 2a optical circulators 14a,b are used to separate the output channel 10 and the drop channel 6 from the input channels 4 and to supplement the output channels 10 with an add channel 8. An optical circulator is a commercially available device, which operates to direct light incident on one input to a designated output. Input channels are directed by optical circulator 14a to the Bragg grating 12. Light reflected from the grating 12 is similarly directed by circulator 14a to the drop channel's waveguide 6. Similarly optical circulator 14b directs light that has passed through grating 12 to the output channel waveguide 10 and light input into the add channel 8 is directed into grating 12 from where it is reflected into the output channel waveguide 10.
Figure 2b shows an alternative arrangement wherein the grating 12 is placed in a Mach-Zehnder interferometer arrangement. The grating 12 is provided in one of a pair of waveguides between a pair of 3dB couplers 8a,b. light from the input channel 4 is coupled into both waveguide 16a containing the grating 12, and 16b. Wavelengths equal to the grating period undergo a phase shift resulting in destructive interference at the Mach Zehnder interformeter output and are thus directed to the drop channel 6. Wavelengths not equal to the grating period are subject to constructive interference in the Mach Zehnder interferometer and continue along waveguide 16a and into the output channel 10. Light input into the add channel 8 with a wavelength equal to the grating period has destructive interference at the drop channel 6 and is thus coupled into the output channel 10 instead.
Figure 2c shows a similar arrangement to figure 2b except that the couplers 18a,b couple between the waveguide 16a containing the grating and two unconnected waveguides 16c,d. This arrangement is more stable than the arrangement of figure 12b but losses are increased.
Figure 2d shows a further arrangement of an add/drop multiplexer 2 based on an adiabatic null coupler. The grating 12 is formed in one branch of a coupler 18c made from a pair of waveguides 16e,f of different cross-sections. This arrangement means that coupling only occurs when the propagating mode is resonant with the grating 12. Light from the input channel 4 therefore continues along waveguide 16f and directly into the output channel 10 unless it contains signals at a wavelength equal to the grating period. These signals are coupled into the grating 12 in waveguide 16e and reflected into the drop channel 6. Similarly signals in the add channel 8 are reflected and coupled into waveguide 16f if their wavelength is equal to the grating period. The grating 12 shown in figure 2d is aligned at a slight angle to the waveguide as this arrangement improves the performance of the device.
The grating 12 in each of these embodiments is a tunable photorefractive grating wherein the tuning is achieved by optical addressing. Providing the grating 12 in, adjacent to or near a waveguide is necessary to confine the signals to the designated channels. A number of different embodiments are possible for providing a suitable optical addressing arrangement for a tunable grating 12 in such a photorefractive waveguide 16a-f suitable for use in an add/drop multiplexer 2 of the type described above. In the following discussion, a generalised waveguide is considered and is identified by reference numeral 20.
A number of optical addressing arrangements are described in figures 3 and 4 [3-7]. The embodiments of figures 3a and 3b rely on splitting an input beam 22 from a laser 24 into subsidiary beams 22a,b and then recombining the subsidiary beams 22a,b in the waveguide 20 to form a grating 12. Tuneability is provided by varying the angle at which the subsidiary beams intersect in the waveguide 20. To simplify comparisons of each arrangement the waveguide 20 is considered to be provided on a suitable substrate (as is conventional). A set of Cartesian axes are also provided in each figure, the substrate being taken as the x- z plane and the waveguide propagation direction taken to lie along the z axis.
In the arrangement of figure 3a a laser beam 22 is incident on a pair of rotatable mirrors 26 which are controlled by a suitable actuator 28 such as a piezo- electric actuator. The input beam 22 is thereby spilt into two subsidiary beams 22a,b. The subsidiary beams are then incident upon a pair of fixed mirrors 30a,b which direct the beams 22a,b to waveguide 20. The two beams 22a,b form an interference pattern in the photorefractive waveguide 20 which in turn generates a refractive index variation corresponding to a Bragg grating 12. The grating 12 is tuned by using actuator 28 to vary the angle at which mirrors 26a,b deflect the incoming beam 22. The subsidiary beams 22a,b will then intersect in the waveguide 20 at a different angle creating an interference pattern of a different pitch and hence a grating 12 of a different period.
In the arrangement [3,4,5] of figure 3b the input beam 22 from laser 24 is incident on a thin film beam splitter 32 which splits it into a pair of subsidiary beams 22a,b. The subsidiary beams 22a,b are incident upon a pair of fixed mirrors 34a, 34b which direct the subsidiary beams to the waveguide 20 wherein an interference pattern is formed and hence a grating 12 is formed. Tuneability is provided by rotating the laser 24 which thereby alters the angle the subsidiary beams 22a,b intersect at. In an alternative embodiment the beam splitter 32 can be rotated rather than the laser 24.
A further modification of this design which can be envisaged is the replacement of at least one of the mirrors in these embodiment with a compound mirror thereby allowing a beam to be split into a number of further subsidiary beams. A number of these beams could then be incident on the same area creating a combined interference pattern corresponding to a compound Bragg grating.
In the arrangement [6] of figure 3 c the input laser beam is focussed into a line image by a suitable lens (not shown). The line image is on the first face 42 of a prism 40. The beam is refracted at the first face 42 and propagates through the prism 40. One part of the line image 38 is then incident on a second surface 46 of the prism directly. The other part is totally internally reflected from a third surface 44 of the prism and is then incident on the second surface 46. A photorefractive waveguide 20 is provided on the second face 46 of the prism. The intersection of the two parts of the line image beam 38 generates an interference pattern in the waveguide 20 and hence a grating 12 is formed. The interference pattern may be tuned by rotating the prism 40.
These arrangements all involve moving parts and therefore will have a limited lifespan. Figure 3d shows an optical addressing arrangement [7] which does not have any moving parts. A diverging beam 22 is output from a laser 24 and is incident on a collimating lens 48a. This light is then incident upon a spatial light modulator (SLM) 50. Two pixels 52a,b of the SLM 50 are made transparent and reduce the input beam 22 to a pair of subsidiary beams 22a,b. The subsidiary beams 22a,b are then incident on a converging lens which focuses them on to a photorefractive waveguide 20 wherein an interference pattern and hence a grating 12 is formed. The grating 12 is tuned by varying the pixels 52a,b which allow incident light to pass and thereby varying the intersection angle of the subsidiary beams 22a,b.
This arrangement has no moving parts and so is in principle more reliable than the arrangements of figures 3a,b&c. It should also be faster in tuning between extremes of the frequency range. It does however, in common with these arrangements occupy a certain minimum amount of space.
A more compact and therefore more commercially valuable arrangement is shown in figure 4. The arrangement is formed as an integrated optical component on a suitable substrate 54. A photorefractive waveguide 20 is provided on the substrate 54 as are an array of waveguide pairs 56 each comprising a common first portion 58 and being split into a pair by a Y-splitter 60. Light is coupled into the common first portion 58 of each waveguide pair 56 by a dedicated vertical cavity surface emitting laser (VCSEL) 64 provided in an array of VCSELs 62.
The second portions of each waveguide pair 56 are arranged so that light propagating onwards from them intersects in the photorefractive waveguide 20. The waveguide pairs 56 are arranged such that the light from each waveguide pair 56 intersects at waveguide 20 at' a different angle to that from every other waveguide pair 56. In this manner gratings of a number of different periods may be formed.
Waveguide crossings, of which there may be several, do not detriment the performance of this arrangement if the crossings are perpendicular or substantially perpendicular because at a perpendicular crossing very little light is coupled into the wrong waveguide.
In normal operation, only a single VCSEL 64 is lit and when the grating is to be retuned a different VCSEL 64 is lit. It is however possible to have more than one VCSEL 64 lit at any one time and thereby provide a compound grating able to add or drop signals of more than one wavelength from the data stream.
This arrangement allows optical addressing to be carried out parallel to the plane of the substrate making the arrangement more compact than the previously described arrangements which are all required to address the grating from perpendicular to the plane of substrate 54. This means the grating also benefits from being less susceptible to interference caused by vibration or dust. In addition, the grating has no moving parts meaning that mechanical reliability issues should be less of a problem; this also means that tuning over a wide range is quick and reliable. A further advantage is the ability to form a compound grating.
In order to provide a compact tunable Bragg grating filter for possible use in an optical add/drop multiplexer using this arrangement or indeed to minimise the size of an optical add/drop multiplexer using any of the other optical addressing arrangements it is necessary to provide a photorefractive polymer waveguide wherein light is confined to a predetermined path. Two suitable embodiments of such waveguides are shown in figure 5.
Figure 5a shows a channel waveguide 20 comprising a channel of photorefractive polymer material 66. The photorefractive channel 66 is surrounded by a cladding of lower refractive index 68, the waveguide and cladding combination being provided upon a suitable substrate 54. In use, light is guided along the photorefractive channel 66 in the z-direction. It is particularly advantageous to define the dimensions of the channel such that a guided wave forms only one transverse electromagnetic mode, a single mode waveguide. The dimensions required to achieve this depend strongly upon the refractive index difference between the photorefractive channel 66 and the cladding 68, but are typically around 6 x 6 μm for a 1 % refractive index difference.
The cladding layer 68 may or may not be photorefractive, the only constraint upon it being that it should be of lower refractive index than the waveguide channel 66. There are, however, three advantages of having a photorefractive cladding layer 68, firstly it may be easier to find a cladding material 68 with a suitable refractive index if it is made from a similar material to the channel 66. Secondly, a recorded hologram will extend into the cladding 68, thus avoiding potentially problematic boundaries and thirdly, similar processing techniques may be used for the channel 66 and its cladding 68. The thickness of the cladding layer 68 depends only upon the nature of the fabrication procedures, but must be thicker than the channel.
The substrate 54 does affect the function of the waveguide and so the choice of substrate material depends upon issues relating to fabrication and compatibility with the application. The substrate 54 could typically be silica or silicon, or even a flexible acrylic material. Often a buffer layer 69 is used between the substrate and the cladding. This can be for the purposes of processability or to prevent guided light leaking into the substrate.
Referring to figure 5b a ridge type waveguide 20 is shown. A ridge 70 formed from a photorefractive polymer is provided on top of a layer of cladding material 68. The dimensions of the ridge are ideally defined such that light propagating along the ridge 70 has a single transverse electromagnetic mode. The main reason for choosing a ridge waveguide design over a channel waveguide is for ease of fabrication of certain devices. The cladding 68 and the substrate 54 can conveniently be of the same form as those in the embodiment of figure 5a.
Referring to figure 6, an embodiment of a non-photorefractive waveguide 20 which may be incorporated in the present invention is shown which uses a layer of photorefractive material containing recorded holograms in contact with the waveguide to modulate optical signals carried by the waveguide.
A channel 72 is fabricated from any suitable material, with any suitable cladding 68. The cladding 68 must have a lower refractive index than the channel 72. The assembly is supported on a suitable substrate 54. The substrate may conveniently be of the same form as the substrates in figures 5a&b. The waveguide 20 of figure 6, is shown in the form of a channel waveguide but could alternatively be provided in the form of a ridge waveguide if desired.
The channel 72 is in contact with a photorefractive layer 74 which has an overlap with the guided electromagnetic mode profile of the channel 72. This means that the portion of the electromagnetic propagating mode overlapping with the photorefractive layer 74 will be influenced by any hologram recorded in the photorefractive layer 74. This is obviously not as significant an influence as will be generated by a hologram interacting with the whole of the guided mode but this influence is sufficient for many applications.
It is particularly desirable to form the photorefractive waveguide 20 (or the photorefractive layer 74) from an amorphous polymer containing non-linear optical chromophores as described above. In order to use such materials to their full potential it is necessary to apply an electric poling field to the waveguide 20.
Figures 7a, 7b, 8&9 show electrode arrangements for applying poling fields to a photorefractive waveguide 20 of the embodiment of figure 5a although similar arrangements can be implemented for the waveguides 20 of figures 5b&6 if desired. In each of the drawings the waveguide is arranged in order that its propagation axis is in the z direction. Figure 7a shows a pair of electrodes 76 for applying a potential difference to the waveguide in the x direction. Figure 7b shows a pair of electrodes 78 arranged to apply a potential difference to the waveguide 20 in the y direction. Of particular importance is the ability to apply a poling field parallel to or substantially parallel to the propagation axis of waveguide 20 (in the z direction). A poling field of this orientation would allow holographic gratings to be written with a grating vector parallel to, or substantially parallel to the waveguide propagation (z) axis. A reconfigurable hologram corresponding to a Bragg grating of this type written in waveguide 20 may be used as the basis of a tunable filter, attenuator, coupler or dispersion compensator.
Figure 8 shows a plan view of a suitable arrangement for achieving this. Electrodes 80a,b are disposed underneath photorefractive channel 66 although the electrodes 80a,b may alternatively be provided above the channel 66 or to one side of the channel 66 if desired. The electrodes 80a,b are in a comb shaped form comprising a spine portion 82 and a number of teeth 84. The electrodes 80a,b are held at different potentials and thereby generate a poling field component along the propagation axis. In an alternative embodiment shown in figure 9 the electrodes 80a,b are disposed on opposite sides of the photorefractive channel 66. In either embodiment the electrodes 80a,b may be disposed directly upon the channel 66 or there may be a cladding layer 68 provided in-between the channel and the electrodes 80a,b.
Figures 8 and 9 show electrodes 80a,b where the comb-tooth 84 spacing is uniform. Another example of electrode pattern producing a poling field component along z, is a variation of figures 8&9, but with a non-uniform distribution of distances between comb teeth 84. This tooth 84 separation distance may be random or it may be a carefully designed distribution tailored specifically to meet an application requirement. For example a constant tooth 84 separation distance would effectively form a Bragg grating that would cause reflection of signals in the waveguide that have wavelengths resonant with the electrode tooth 84 spacing or its harmonic overtones. This could cause interference with the operation of the grating. A random separation distance distribution would reduce this effect. A linear separation distribution would induce an optical chirp. It is advantageous to include some form of apodisation to the optical interference pattern formed in the waveguide 20. Apodisation is a term used to refer to a grating contrast that is non-uniform. According to the apodisation function used, the grating contrast varies as a function of z and examples of apodisation function include a cosine-squared function, a gaussian function or a Blackmann function. One important purpose of apodisation is to reduce unwanted side bands in the reflectivity spectrum of the filter and to narrow the reflection bandwidth. The exact function used depends upon the specifics of the application.
Apodisation can be generated by varying the physical properties of the waveguide 20 or the physical properties of the region around the waveguide. A particular example is that a cosine-squared apodisation function may be generated by varying the electrode tooth 84 spacing in accordance with a cosine-squared function from a minimum value in the centre of the pattern to a maximum value at its extremity.
Another example of producing an apodisation function in the grating contrast is through an optical arrangement to produce a corresponding apodisation of the contrast in the addressing optical intensity. This can be achieved in one example by addressing with collinear beams with a mutually short coherence length and a mutual phase difference which ramps transversely across the device.
All conventional ridge or channel waveguide fabrication methods can be modified to fabricate photorefractive waveguides 20. One of these is embossing, which is most suitable for fabrication of channel waveguides. A substrate, which could be glass or silicon with a thick oxide coating, is used onto which the material to be embossed is applied. The substrate could be a flexible acrylic if roll-to-roll processing is required. A UV curable layer, such as a polymer doped with cross-linkable azo-dye, is applied that will eventually form the waveguide cladding layer 68. A photolithographically mastered stamp is pushed into the cladding layer to form an embossed pattern of grooves that will eventually become waveguides when back-filled with core material. A cover material, such as a sol-gel or organically modified silicate (ORMOSIL), is applied over the whole tructure with a refractive index the same as the cladding material 68. This example of a fabrication method results in a structure similar to that shown in figure 5a. Another, more simple route, is to deposit the core material directly onto the substrate and to emboss ridges that will eventually form the waveguides. A superstrate, with a lower refractive index, is then bonded over the waveguides with an epoxy, to provide mechanical robustness.
A method suitable for fabricating ridge waveguides is direct write photolithography. In this method the cladding material 68 is deposited on the substrate 54 and a layer of photocurable core material is deposited above the cladding 68. The waveguide pattern is photolithographically written into the core material and the unexposed regions are washed away leaving only the ridge waveguides remaining. In a method similar to this, direct- write electrolithography may also be employed using an electron beam rather that a photon beam. This has the advantage of finer lithographic resolution.
An alternative method of fabricating channel waveguides is photolithography and reactive ion etching. The cladding layer 68 is deposited on the substrate along with a layer of photoresist. The photoresist is photo-exposed with the waveguide pattern and developed to leave a pattern of exposed cladding material that is etched by a plasma beam, reactive gas or electron beam to form channels. The channels are filled with waveguide core material, by dip-coating, capillary filling, spin coating, doctor-blading or evaporation before they are capped with another cladding layer 68.
A further effective technique for producing planar waveguides using high performance photorefractive polymer is to form planar waveguides on a suitable substrate by spin coating from concentrated solution. Typically the substrate 54 is quartz although other suitable materials may of course be substituted if desired. The resulting waveguides 20 may then be diced to produce end facets capable of efficient end fibre coupling. One particular photorefractive polymer composite material which can be used in this process is poly(N-vinylcarbazole) (PVK) photosensitised with 1 wt.% 2,4,7-trinitro-9-fluorenone (TΝF) and doped heavily at 47.5 wt.% with an azo dye derivative, l-(2'-ethyihexyloxy)-2,5-dimethyl-4-(4"nitrophenylazo) benzene (EHDΝPB). This material has the particular advantage that it is not necessary to modify the final material composition in any way in order to obtain sub- 10 μm waveguide layers in place of the previous > 100 μm thick bulk material samples typical in conventional holographic data storage devices.
In order for the process to produce good quality spin coated films a solvent of low volatility is required, particularly if the desired film thickness exceeds 1 μm. A low volatility solvent prevents the formation of ripples on the surface of the film through the reduction of evaporation during spinning. When the solvent evaporates, the change in surface energy of the film can cause the remaining material to form into droplets, which become ripples when the substrate is spinning.
A further consideration is the ability of the solvent to dissolve the solute in sufficient concentration to deposit enough material on the substrate 54. It is necessary to increase the concentration of the solvent used for spinning when thicker films are wanted. The required concentration can be as high as 40 wt.% for films around 8 μm thick after drying.
A number of suitable solvents can be used to form ripple-free films of photorefractive material, each solvent having the ability to dissolve sufficient quantities of photo refractive material but which have a wide range of vapour pressures at room temperature. Both cyclopentanone and γ-butyrolactone are found to have suitable properties as solvents and have vapour pressures at room temperature of between 20 and 30 mmHg. The ends of the waveguide can then be formed into diced end facets for fibre coupling. In order to prevent damage during the formation of the end facets, the photorefractive layer provided on the quartz substrate was given mechanical strength by a quartz cover slip 88 bonded over it with an epoxy adhesive 86 as shown in figure 11. End facets can then be cut, without damaging to the polymer layer, using a dicing saw.
Using the above technique with suitable materials, an insertion loss of less than 1 dB can be obtained for fibre-coupled signal wavelengths of 785 nm, 1319 nm and 1550 nm and an optical throughput can also be sustained in excess of 20 dBm for 500 hours.
Multilayer structures can also be fabricated using a combination of any of these techniques, possibly applied several times on the same structure.
The electrodes 80a,b may be fabricated using all of the possible methods described above. The fabrication of the complete device could thus be a multi- step process, electrode fabrication followed by waveguide 20 fabrication, or visa versa if the electrodes are to be deposited on top of the waveguide 20. Two extra methods may be employed to fabricate the electrodes however, photoablation of an electrode film to write the desired pattern or directly writing an electrode using a jet-type printer to deposit a pattern of conducting material directly on the substrate 54.
The electrodes may be formed from a metal, a conducting oxide such as
Indium Tin Oxide or a conducting organic material or from any other suitable material as desired.
In the above description the identity of the photorefractive polymer used to form the photorefractive waveguides 20 has not been specified. A number of polymers are suitable for use in this manner but of particular use are the photorefractive polymer composites described in granted European patent no. 897577 and its equivalents.
As an example of this invention, a channel waveguide with a single planar Bragg grating hologram written therein has been considered for application as an add-drop multiplexer. The device was to be constructed as follows. A 10 μm thick layer of cladding material is deposited on a non-specific substrate. The cladding material consists of a UV curable polymer doped with a high concentration of l-(2'-ethyl hexyl oxy)-2,5-dimethyl-4-(4"nitro- phenylazo) benzene electro-optic chromophore to increase the refractive index to a value of 1.69, nearly as high as the core material. This layer is embossed to form channels that are then filled with indium tin oxide electrode material. The electrode channel pattern is a double interleaving comb pattern, as illustrated in figure 8, where the teeth were of a 4 μm square cross-section. Teeth of opposite combs are separated by 15 μm in the centre of the structure, but increasing towards the edge to reduce the applied field and create a cosine-squared apodisation profile.
On top of the electrode pattern, another 7.5 μm thick cladding layer is deposited that was again embossed to form the waveguide channel. The waveguide channel is filled with core material, a photorefractive polymer composite that consisted of 43 % poly(N-vinylcarbazole), 2 % 2,4,7-trinitro-9- fluorenone and 55 % l-(2'-ethyl hexyl oxy)-2,5-dimethyl-4-(4"nitro- phenylazo) benzene. The refractive index of the core material is 1.7, 0.6 % higher than the cladding material. A 2.5 μm thick capping layer, made of the same material as the cladding, is deposited on top of the channel waveguides to completely enclose the waveguide core. A single sinusoidal Bragg interference pattern was produced within the waveguide using a single pair of interfering laser beams. To calculate the performance of the resulting optical filter, it is necessary to calculate the electric field profile within the waveguide, centered 5 μm above the top of the electrode pattern. This was done numerically, using the Jacobi method, and it was found that the average electric field in the waveguide core, centered in z, was 80 V μm"1 for electrode voltages of +/- 1000 V. Using the measured properties of the waveguide core material [14], the refractive index contrast of the planar Bragg hologram can thus be calculated. This value is different for the transverse electric and transverse magnetic polarisation modes respectively and was found to have average to peak values of -1.3xl0"3 and 8.4X10"4. The waveguide parameters and the values of the refractive index contrasts of the planar Bragg holograms for each polarisation were used in a calculation of the reflectivity spectrum of the filter. A commercially available beam propagation method (BPM) software package, beamPROP (available from R-soft), was used for the calculation, and the resulting reflectivity spectra are shown in figure 10. It can be seen from the figure that strong reflection occurs, making this tunable filter device suitable for use as an add-drop multiplexer.
Other pertinent parameters for the device were also calculated using the R- soft beamPROP software package including optical loss, polarisation mode loss and dispersion, filter width and crosstalk.
The channel spacing is defined as the full width of the reflectivity peak where the reflectivity is -20 dB and the channel width is defined as the full width of the reflectivity peak where the transmission is -30 dB. Both of these can be obtained from figure 13 and turn out to be 125 GHz and 50 GHz respectively, respectable figures for telecommunications applications. The bandwidth utilisation figure of merit, defined as the channel width divided by the channel spacing was calculated to be 0.4, compared to 0.5, which is the industry recognised figure.
Insertion losses were assumed for a whole add-drop device containing circulators, according to figure 2a. The losses assumed for the external components are typical commercial figures. The insertion losses for the photorefractive polymer waveguide filter were calculated using beamPROP software and assumed an absorption coefficient of the photorefractive polymer of 0.3 cm"1, a lithographic fabrication roughness of 50 nm and an imaginary refractive index of indium tin oxide of 0.02. The total insertion loss was calculated as 2.995 dB.
Optical losses are dominated by excitation of C-H bond stretching modes. These have random orientation, so the polarisation dependent loss is negligible. The central position of the reflection bands for each polarisation and the polarisation mode dispersion both depend of the effective waveguide refractive indices for each polarisation. Because of the electric field pattern, the waveguide is birefringent, but the index difference can be corrected for by defining a rectangular waveguide rather than a square one if the birefringence of the cladding is different to the core. The precision to which this compensation can be applied, thus depends on the fabrication precision of the waveguide dimensions, typically +/-10 %. The polarisation dependent center reflection wavelength can thus be controlled to +/-0.3 nm and the polarisation mode dispersion can thus be controlled to +/-10 fs cm"1.
Even assuming a cosine-squared apodisation function, the out of band reflection was calculated as 0.1 dB. This is the value of the channel to channel uniformity and also sets the adjacent channel cross-talk to be -17 dB.
It is of course to be understood that the invention is not to be restricted to the above details of the current embodiments which have been described by way of example only. For instance although the description relates mainly to an add- drop multiplexer as a use for a tunable optical grating, a tunable optical grating of the type described herein can be used to form other devices such as a variable optical attenuator, dynamic gain equaliser, reconfigurable tap coupler or a discrete component. REFERENCES
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