WO2004021073A1

WO2004021073A1 - Optical waveplate based on liquid crystals in a transverse electric field

Info

Publication number: WO2004021073A1
Application number: PCT/GB2003/003568
Authority: WO
Inventors: Ralph Alexander Betts; Terry Bricheno
Original assignee: Pi Photonics Limited
Priority date: 2002-09-02
Filing date: 2003-08-13
Publication date: 2004-03-11
Also published as: AU2003255794A1; GB0220381D0

Abstract

A design and a manufacturing method for a liquid crystal variable azimuth waveplate based upon a transversely driven, homeotropically aligned nematic liquid crystal cell employing electroformed driving electrode structures (203) formed on one of the plates (201) that define the cell cavity. The arrangement provides goods field uniformity, reliable operational conditions, and a simple fabrication process well suited to mass production.

Description

OPTICAL AVEPLATE BASED ON LIQUID CRYSTALS IN A TRANSVERSE ELECTRIC FIELD

Field of the Invention

The invention relates to liquid crystal cells such as those for the active control of state of polarisation of light finding application in optical fibre telecommunication systems and networks, methods of construction of such cells, and devices incorporating such cells.

Background to the Invention

In optical fibre transmission systems, the state of polarisation (SOP) of light fluctuates randomly on a time scale of typically 1 ms or more. In some circumstances, it is necessary to convert this fluctuating SOP into a prescribed state, for example in order to apply compensating time delays to mitigate the effects of polarisation mode dispersion (PMD) or to avoid fading effects in coherent detection schemes.

Many arrangements of variable birefringent elements have been considered for the implementation of a SOP controller, and these fall into two classes. The first class, usually based on fixed azimuth/variable retardation elements (Refs: 1 ,2,3) suffers from the effect of finite range of retardation adjustment in the constitutive active element, which requires the combination to be provided with extra active elements and subjected to an "unwinding" algorithm in order to adequately track and convert the incoming SOP. A second class, usually based on fixed retardation/variable azimuth elements (Refs: 4,5), is termed "endless", and allows full SOP control functionality with the minimum number of active elements (three for arbitrary-to-arbitrary SOP conversion or two for arbitrary-to-defined SOP conversion) and the simplest control algorithms. The present invention relates to a simple construction method for the basic active element used in one such "endless" embodiment, the principle of operation of which is fully described in reference 5.

The operation of a variable azimuth retardation plate based upon a homeotropically aligned liquid crystal (LC) cell driven by a rotating transverse electric field generated between a plurality of electrodes is well known (Ref 5 (four electrodes), ref 6 (eight electrodes), and ref 7 (six electrodes)). The typical cell construction is as shown in Figure 1 , taken from Wiltshire's original patent (ref 5), where the transverse field in the central operating aperture (13) of the device is established by the fringing fields from thin electrode structures (9-12, 14-17) deposited upon the internal surfaces of the confining plates (2,3). Separate spacers are provided for setting the separation of the plates. An alternative shown is to use thick electrodes in the form of strips of a conductive foil of substantially the same thickness as the LC layer. In practice, these thick or thin electrodes are hard to manufacture and align, and in the case of the thin electrodes, even if well aligned, do not give good field characteristics.

References

L de Lang. US Patent 3,558,214. (1967) 2. Noe. Electronics Letters 22, 15, p772. (1986) 3. Rumbaugh et al. Journal of Lightwave Technology, 8, 3, p459. (1990)

4. Heismann and Whalen. IEE Photonics Technology Letters, 4, 5, p503. (1992)

5. Wiltshire. US Patent 5,313,562. (1992)

6. Chiba et al. Journal of Lightwave Technology, 17, 5, p885. (1999)

7. Dupont et al. Optics Communications, 209, p101. (2002) 8. Davidson (ed), Handbook of Precision Engineering, Volume 4, Chapter 5. (Macmillan, 1971 )

Summary of the Invention

An object of the invention is to provide improved apparatus and methods of construction for cells for example for an electrically driven variable waveplate of substantially fixed retardation but continuously variable azimuth.

In particular:

(i). One aspect of the invention provides a liquid crystal cell driven with a transverse electric field having a direction substantially parallel to the surfaces of the plates that define the cell cavity produced by a plurality of metal electrodes that are situated substantially throughout the gap that separates the plates that define the cell cavity and that are formed on one of the plates. Some advantages of this arrangement are that the cell can be provided with good field uniformity through out the thickness of the cell, and field concentration effects at the extremities of the thick electrodes (as would occur at the extremities of thin electrodes) can be reduced. Furthermore, the need to provide electrical connection to the internal surfaces of both cell plates can be avoided (thus simplifying assembly) and the thick metallic electrode structure is easily configured to provide wirebonding pads at the edge of the structure without additional processing (thus simplifying a route to reliable electrical connection). Notably since the electrodes are formed on the plates, the electrode spacings can be fixed, and hence a more uniform field can be obtained and electrical isolation of the electrodes maintained. Also these properties can be maintained more reliably through the life of the device.

(ii). The cell is filled with a liquid crystal material that is caused to undergo homeotropic alignment within at least the working aperture of the cell, giving rise to an optical element that is substantially optically inert (ie non-birefringent) in the absence of a driving field (eg during fault conditions).

(iii). The liquid crystal is a nematic material, wherein the orientation of the molecular director is driven by the rms value of the applied ac voltage, thereby avoiding the possibility of degradation resulting from the material being subjected to extended exposure to a finite dc bias.

(iv). The metal electrodes are formed by electroforming (plating) onto a thin, patterned, conducting layer on one of plates that define the cell cavity. This process ensures the provision of smooth vertical edges to the metal electrodes (which in turn contributes to good transverse field uniformity within the cell), provides reliable adhesion of the electrode structure to the substrate plate (via the good adhesion of the thin film patterned deposition layer, formed, for example by Cr/Au), avoids the need for mutual alignment of electrode structures formed on both internal surfaces of the cell plates, and provides good control of the cell thickness (and plate parallelism) through the good control of plated metal thickness inherent in the electroforming process, since the deposited metal electrodes form the spacing elements for the cell.

(v). The patterned thin film layer on one of the plates that define the cell cavity is employed as a self-aligned mask for patterning a negative photoresist to provide the mould cavities for the electroforming process, thus providing perfect registration for the cavities with the thin film pattern and simplifying the fabrication process by avoiding the need for secondary mask alignments.

Another aspect provides a liquid crystal cell having a cavity for retaining liquid crystal material in a path of incident light, and electrodes for generating an electric field in the cavity transverse to the path, the electrodes extending along at least a substantial proportion of the cavity in a direction parallel to the path, and being attached to a structure of the cavity to maintain a fixed spacing between the electrodes.

Attaching the electrodes to maintain fixed spacings can be carried out during manufacture by forming the electrodes on some part of the cavity, or by forming the electrodes in some other way then subsequently attaching them directly or indirectly to the cavity for example.

Additional features for dependent claims include the electrodes being arranged in a single layer having a normal substantially parallel to the path. This can simplify manufacture. Another such feature is the electrodes forming a spacer between opposing faces of the cavity. Again this can simplify manufacture. Another such feature is the electrodes being attached by being formed on one or both faces of the cavity. This can help achieve good alignment of the electrodes.

Other aspects provide a variable birefringent element having at least one of the cells, and methods of manufacturing such cells. In summary, the embodiments of the invention can provide a design and a manufacturing method for a liquid crystal cell suitable for a continuously variable azimuth waveplate that can provide good field uniformity, reliable operational conditions, and a simple fabrication process well suited to mass production. Other advantages will be apparent to those skilled in the art. Any of the additional features can be combined together or combined with any of the aspects. Embodiments of the invention will now be described by way of example with reference to the figures.

Brief description of the drawings Figure 1 shows a 'rotating waveplate' cell in the prior art. Figure 2 shows a cell constructed according to an embodiment of the present invention. Figure 3 (a) to (k) shows the steps involved in constructing a cell according to an embodiment of the present invention.

Figure 4 (a) to (k) shows an alternative sequence of steps for constructing a cell according to an embodiment of the present invention.

Detailed description

The known thin electrode construction mentioned above suffers several disadvantages - the lateral field uniformity in both strength and orientation is poor (which in practice limits the useful aperture of the device), the matching electrode patterns on the two confining plates must be accurately mutually aligned (to avoid yet further degradation of the field uniformity in the operating aperture), the electric fields at the extremities of the thin electrodes are much greater than the operating field required in the aperture (which could lead to degradation of the LC material in these regions) and the two sets of electrodes require independent electrical connection (which leads to additional complexity in the assembly procedure). The known thick electrode construction using metal foil strips gives manufacturing difficulties because the relative positioning of the electrodes , particularly at the tips of the electrodes where the spacing needs to be precise, uniform and symmetrical to ensure a uniform field with complete electrical isolation. The prior art does not explain how this can be achieved and maintained through the life of the device for electrodes which may be a millimetre long but less than 10 μm thick and only 20-30 μm wide at the tips for example, and may define an aperture of 40-60 μm. The embodiments of the present invention address these disadvantages by applying techniques such as electroforming (ref 8) to provide a metallic electrode structure that extends throughout the cell cavity thickness, that is formed on only one of the confining plates and that can be fabricated with good wall verticality. The general layout of a typical embodiment is shown in Figure 2. The thick electroformed electrode pattern (203), which defines the working aperture (204), is formed on the lower cell plate (201 ). The electrode structure extends to the edge of the lower plate to provide bond pads (206) for ease of electrical connection. The uniformly thick electrode structure also serves as a spacer layer to set the position of the upper cell plate (205). Both upper and lower plates are provided with alignment coatings (202) to achieve homeotropic alignment of the LC material. The LC material can be nematic or other types.

The fabrication steps are outlined in Figure 3 (a) to (k). An optically flat transparent substrate (301 ) (for example borosilicate glass) is first provided with a thin film electrode pattern (302) (for example Cr/Au or Ti/Au) by conventional photolithographic techniques. Typically, many cell structures are formed on a single substrate and the patterning is chosen to provide electrical continuity between all elements of all cells. The patterned plate is then coated with a negative photoresist (303), which will later serve as the 'mould' for the electroforming process. This photoresist is then exposed to ultraviolet light (304) through the patterned substrate, so that the pre-existing thin film pattern serves as a mask and only those parts of the resist (305) not shielded by the thin film pattern are exposed. Upon development, the exposed parts of the resist (306) form a moulding pattern in perfect registration with the original thin film pattern. The plate is then transferred to a plating tank (307) and electrical connection made to the thin film pattern in order to plate metal (308) (for example Au) into the aligned cavities of the photoresist pattern (electroforming). When the electroforming process has provided a suitable thickness of metal (typically in the range 5 to 20 urn), the substrates are removed.

At this point, the substrates may be diced into smaller units (bearing, for example, single cells or linear arrays of cells). After stripping the photoresist (309) and cleaning, the substrates are provided with a homeotropic alignment layer (310) (for example by dip coating to form an organo-trimethyloxysilane monolayer). The mechanical assembly of the cell or cells is completed by the attachment of an optically flat top plate (311 ), which has been precoated with a homeotropic alignment layer, using epoxy resin or a u/v curable adhesive (not shown). The distribution of the attachment adhesive is controlled to preserve one or more open channels from the edge of the cell to the central aperture region, and these channels are then used for vacuum backfilling or for capillary filling of the cell with an appropriate LC material (312). After filling, these filling channels are sealed at the edge of the cell. At this point, a linear array of cells may be further diced into single cell elements. Later, upon installation of the cell into an optical component, the electroformed extension pads at the edge of the substrate may be used to form a robust electrical connection (313) (for example by means of wirebonding).

An alternative sequence of fabrication steps is outlined in Figure 4 (a) to (k). An optically flat transparent substrate (401 ) (for example borosilicate glass) is first provided with a uniform thin film layer (402) (for example Cr/Au or Ti/Au) by conventional deposition techniques. The plate is then coated with photoresist (403), which will later serve as the 'mould' for the electroforming process. This photoresist is then patterned by exposure to ultraviolet light through an appropriate mask. Upon development, the exposed parts of the resist (405) serve to define the moulding pattern for the subsequent electroforming. The plate is then transferred to a plating tank (407) and electrical connection made to the thin film pattern in order to plate metal (408) (for example Au) into the cavities of the photoresist pattern (electroforming). When the electroforming process has provided a suitable thickness of metal (typically in the range 5 to 20 urn), the substrates are removed. After stripping the photoresist, the unplated regions of the underlaying thin film metallisation are removed by chemical etching (409). At this point, the substrates may be diced into smaller units (bearing, for example, single cells or linear arrays of cells), and, after cleaning, the substrates are provided with a homeotropic alignment layer (410) (for example by dip coating to form an organo-trimethyloxysilane monolayer). The mechanical assembly of the cell or cells is completed by the attachment of an optically flat top plate (411 ), which has been precoated with a homeotropic alignment layer, using epoxy resin or a u/v curable adhesive (not shown). The distribution of the attachment adhesive is controlled to preserve one or more open channels from the edge of the cell to the central aperture region, and these channels are then used for vacuum backfilling or for capillary filling of the cell with an appropriate LC material (412). After filling, the filling channels are sealed at the edge of the cell. At this point, a linear array of cells may be further diced into single cell elements. Later, upon installation of the cell into an optical component, the electroformed extension pads at the edge of the substrate may be used to form a robust electrical connection (413) (for example by means of wirebonding).

It is to be understood that the described arrangement is merely illustrative of the many possible embodiments that can be devised to represent application of the principles of the invention. Various other arrangements can be devised in accordance with these principles by those skilled in the art without departing from the scope of the claims; for example, homeotropic alignment of the liquid crystal material may be achieved without recourse to the precoating of the plate surfaces by employing low concentrations of a cationic surfactant as an additive to the nematic LC material.

The cells can be applied not only for polarisation control purposes in adaptive elements for optical telecommunications systems, but also for providing a variety of different polarisation states in instrumentation used for testing the polarisation dependency of devices under test for example.

The electrodes need not be all formed on one plate, they can be formed on different plates, though this would mean that the plates need to be accurately aligned. The electrodes need not be thick enough to fill the entire gap between the plates. The cell electrodes can have continuously variable or stepped control of voltage. Variation in field can also be achieved by providing many electrodes and switching off selected electrodes. This enables field control with digital switching without needing variable voltage levels. This can simplify power supplies though at the expense of needing more electrodes for a given granularity of field control.

Described above are a design and a manufacturing method for a liquid crystal variable azimuth waveplate based upon a transversely driven, homeotropically aligned liquid crystal cell employing electroformed driving electrode structures (203) formed on one of the plates (201 ) that define the cell cavity. The arrangement provides good field uniformity, reliable operational conditions, and a simple fabrication process well suited to mass production.

Claims

1. A liquid crystal cell driven with a transverse electric field having a direction substantially parallel to the surfaces of the plates that define the cell cavity produced by a plurality of metal electrodes that are situated substantially throughout the gap that separates the plates that define the cell cavity and that are formed on one of the plates.

2. The cell of claim 1 where the liquid crystal material is caused to undergo homeotropic alignment within at least the working aperture of the cell.

3. The cell of claim 1 where the liquid crystal is a nematic material.

4. The cell of claim 1 where the metal electrodes are formed by electroforming (plating) onto a thin, patterned, conducting layer on one of plates that define the cell cavity.

5. The cell of claim 4, where the patterned thin film layer on one of the plates that define the cell cavity is employed as a self-aligned mask for patterning a negative photoresist that is subsequently employed as the mould cavity for the electroforming process.

6. A liquid crystal cell having a cavity for retaining liquid crystal material in a path of incident light, and electrodes for generating an electric field in the cavity transverse to the path, the electrodes extending along at least a substantial proportion of the cavity in a direction parallel to the path, and being attached to a structure of the cavity to maintain a fixed spacing between the electrodes.

7. The cell of any preceding claim, the electrodes being arranged in a single layer having a normal substantially parallel to a path of incident light.

8. The cell of any preceding claim, the electrodes forming a spacer between opposing faces of the cavity.

9. The cell of any preceding claim, the electrodes being formed on one or both plates defining opposing faces of the cavity.

10. A variable birefringent element having at least one of the cells of any preceding claim.

11. A method of making a liquid crystal cell having a cavity for retaining liquid crystal material in a path of incident light, the cell also having electrodes for generating an electric field in the cavity transverse to the path, the method comprising the step of forming the electrodes on a structure of the cavity to extend along at least a substantial proportion of the cavity in a direction parallel to the path.

12. A method of making a liquid crystal cell having a cavity for retaining liquid crystal material in a path of incident light, and electrodes for generating an electric field in the cavity transverse to the path, the method comprising the steps of electroforming the layer of electrodes on a plate, and placing another plate onto the electrodes to form the cavity.