WO2018126173A1

WO2018126173A1 - Electro-wetting on dielectric (ewod) activated optical switch using capillary liquid control

Info

Publication number: WO2018126173A1
Application number: PCT/US2017/069015
Authority: WO
Inventors: Csaba ENDRÖDY
Original assignee: Commscope Technologies Llc; Technischen Universität Ilmenau
Priority date: 2016-12-30
Filing date: 2017-12-29
Publication date: 2018-07-05

Abstract

An optical switching device uses a liquid management system for switching optical signals, where two immiscible fluids are actuated by electro-wetting on dielectrics (EWOD). The liquid management system has respective reservoirs for different fluids and a capillary cell proximate the first waveguide light coupling region of an EWOD optical switch. One of the fluids is a polar liquid. The capillary cell is connected to the reservoirs to permit fluid flow between the cell and the reservoirs. In embodiments using multiple capillary cells, the liquid management system contains a first volume of the first fluid and a second volume of the second fluid, where the volumes are selected so that, when the first fluid fills one or more of the capillary cells, the second fluid fills the remaining capillary cells.

Description

ELECTRO-WETTING ON DIELECTRIC (EWOD) ACTIVATED OPTICAL SWITCH USING CAPILLARY LIQUID CONTROL

Cross-Reference to Related Application

This application is being filed on December 29, 2017 as a PCT International Patent Application and claims the benefit of U.S. Patent Application Serial No. 62/441,011, filed on December 30, 2016, the disclosure of which is incorporated herein by reference in its entirety.

Background of the Invention

The present invention is generally directed to optical communications, and more specifically to active optical switch systems that use electro-wetting on dielectric (EWOD) activated optical switches.

Optical fiber networks are becoming increasingly prevalent in part because service providers want to deliver high bandwidth communication and data transfer capabilities to customers. As optical networks become more complex, it has become increasingly important to manage optical signals in the network. Many optical signal management functions, such as redirecting signals to bypass faulty components, or opening new channels to facilitate the addition of more users of the network, can be accomplished using active optical switches, such as electro-wetting on dielectric (EWOD) activated optical switches. Such active optical switches are based on the principles of microfluidics: two fluids with different refractive indices, wherein at least one fluid is a liquid, are moved on the surface of an adiabatic waveguide coupler. Depending on the location of the fluids relative to the waveguide coupler, the coupler switches between two states, either facilitating or prohibiting the transition of a propagating optical signal from one waveguide to another.

Prior designs of EWOD activated optical switches have relied on the movement of droplets of one liquid within a second, ambient liquid. This approach requires a complex filling mechanism and requires the formation of droplets of the first liquid within the second liquid. There is, therefore, a need for a liquid management system that has a less complex filling mechanism and that does not rely on the complex process of forming droplets of one liquid within another liquid. Summary of the Invention

One embodiment of the invention is directed to an optical switching device that has a first electro-wetting on dielectric (EWOD) optical switch comprising a first waveguide coupling region and a liquid management system for controlling fluids activating the first EWOD optical switch. The liquid management system has a first reservoir for a first fluid, a second reservoir for a second fluid different from the first fluid, and a first capillary cell proximate the first waveguide light coupling region. The first capillary cell comprises a first neck permitting fluid flow between the first capillary cell and the first reservoir, and further comprises a second neck permitting fluid flow between the first capillary cell and the second reservoir. One of the first fluid and the second fluid is a polar liquid.

Another embodiment of the invention is directed to an optical switching device that has an optical switch array comprising a plurality of electro-wetting on dielectric (EWOD) optical switches and a liquid management system for controlling fluids activating the plurality of EWOD optical switches. The plurality of EWOD optical switches includes at least a first EWOD optical switch and a second EWOD optical switch. The liquid management system includes a first reservoir for a first fluid, a second reservoir for a second fluid different from the first fluid, and a first plurality of capillary cells positioned proximate respective waveguide coupling regions of a first set of EWOD optical switches of the plurality of EWOD optical switches. The capillary cells of the first plurality of capillary cells each couple between the first and second reservoirs. The liquid management system contains a first volume of the first fluid and a second volume of the second fluid. The first volume of the first fluid and the second volume of the second fluid are selected so that, when the first fluid fills one of the capillary cells, the second fluid fills the others of the plurality of capillary cells. One of the first fluid and the second fluid is a polar liquid.

The above summary of the present invention is not intended to describe each illustrated embodiment or every implementation of the present invention. The figures and the detailed description which follow more particularly exemplify these embodiments. Brief Description of the Drawings

The invention may be more completely understood in consideration of the following detailed description of various embodiments of the invention in connection with the accompanying drawings, in which:

FIG. 1 schematically illustrates an embodiment of an active optical switch system according to an embodiment of the present invention;

FIGS. 2A and 2B schematically illustrate a cross-sectional view through a portion of an active optical switch system according to an embodiment of the present invention;

FIG. 3 schematically illustrates an embodiment of a 4 x 4 EWOD optical switch array;

FIGS. 4A and 4B schematically illustrate embodiments of 1 x 2 EWOD optical switch arrays;

FIGS. 5A and 5B schematically illustrate liquid management systems for the EWOD optical switch arrays shown in FIGS. 4A and 4B according to embodiments of the present invention;

FIG. 6 schematically illustrates a liquid management system for a 1 x 2 EWOD optical switch array, according to an embodiment of the present invention;

FIGS. 7A-7D schematically illustrate operation of a liquid management system for a 1 x 2 EWOD optical switch array, according to an embodiment of the present invention;

FIGS. 8A-8C schematically illustrate operation of a liquid management system for a 1 x 2 EWOD optical switch array, according to an embodiment of the present invention;

FIGS. 9A-9C schematically illustrate steps take for filling a liquid management system for a 1 x 2 EWOD optical switch array, according to an embodiment of the present invention;

FIGS. 10A and 10B schematically illustrate an embodiment of a liquid

management system for a 1 x 2 EWOD optical switch array that incorporates a calibrating capillary, according to an embodiment of the present invention; and

FIG. 11 schematically illustrates a liquid management system for a 2 x 4 EWOD optical switch array according to an embodiment of the present invention.

While the invention is amenable to various modifications and alternative forms, specifics thereof have been shown by way of example in the drawings and will be described in detail. It should be understood, however, that the intention is not to limit the invention to the particular embodiments described. On the contrary, the intention is to cover all modifications, equivalents, and alternatives falling within the spirit and scope of the invention as defined by the appended claims.

Detailed Description

The present invention is directed to systems, devices, and methods that can provide benefits to optical communication networks.

An exemplary embodiment of an active optical switch 100 is schematically illustrated in FIG. 1. The active optical switch 100 incorporates a first waveguide 102 and a second waveguide 104. The first and second waveguides 102, 104 are situated physically closer to one another in a waveguide-light coupling region 106, a region where light propagating along one of the waveguides 102, 104 may couple to the other waveguide 104, 102. Whether light couples between the waveguides 102, 104 depends on the effective refractive index experienced by the light as it propagates along the waveguides 102, 104. The effective refractive index can be altered by positioning a fluid of greater or lesser refractive index close to the waveguide-light coupling region 106 and a waveguide-fluid coupling region 108, discussed further below.

In many embodiments, the active optical switch includes two fluids that are movable to change the state of the switch. The figure shows a first fluid 110 positioned over the waveguide-light coupling region 106 and the waveguide-fluid coupling region 108. A second fluid 112 is shown generally filling the remaining space of a fluid channel 114. The first fluid 110 has a first refractive index and the second fluid 112 has a second refractive index, different from the first refractive index. The first and second fluids 110, 112 may move within the fluid channel 114, so for example, the first fluid 110 may move away from waveguide-light coupling region 106 and waveguide-fluid coupling region 108 to the location shown as 110a, with the second fluid 112 generally filling the remaining space in the fluid channel 114. Thus, in this embodiment, the first fluid 110 is in the form of a droplet within the second fluid 112. One or more of the inner surfaces of the fluid channel 114 may be coated with anti-wetting coatings 116, 118 to assist in controlling the position of first and second fluids 110, 112 with respect to the waveguide-light coupling region 106 and waveguide-fluid coupling region 108. One of the first and second fluids is a polar liquid.

In the illustrated embodiment, an optical signal transmitted into the first waveguide 102 is coupled to the second waveguide 104 when the first fluid 110 is positioned close to the waveguide-light coupling region 106 and the waveguide-fluid coupling region 108. This is referred to as the switch's "cross state." An optical signal transmitted into the first waveguide 102 is maintained in the first waveguide 102 when the first fluid 110 is positioned away from the waveguide-light coupling region 106 and the waveguide-fluid coupling region 108, and instead the second fluid 112 is positioned near coupling regions 106, 108. This is referred to as the switch's "bar state." Microfluidic optical switches have previously been described, for example in WO 2016/107769, entitled "Integrated Optical Switching and Splitting for Optical Networks," published on July 7, 2016; in WO 2016/131825, entitled "Remote Control and Power Supply for Optical Networks," published on August 25, 2016; and in WO 2015/092064A1, entitled "Adiabatic Coupler," published on June 25, 2015, all of which are incorporated herein by reference.

A cross-sectional view through a portion of an exemplary embodiment of an active optical switch system 200 is schematically illustrated in FIG. 2A. In this embodiment, optical fluids are moved in a fluid channel relative to waveguides using the technique of electro-wetting. A first fluid 210 and a second fluid 212 are disposed within a fluid channel 202 formed between two structures 204, 206. In this embodiment, the first fluid 210 is in the form of a droplet within the ambient second fluid 212. In the illustrated embodiment, the first fluid 210 and the second fluid 212 are both liquids. The first fluid 210 has a first refractive index and the second fluid 212 has a second refractive index, different from the first refractive index. The first structure 204 is provided with a common electrode 208 that is insulated from the fluid channel 202 by a first dielectric layer 214, which provides at least partial electrical insulation between the common electrode 208 and the fluids 210, 212 and the fluid channel 202. A first anti-wetting layer 216 may be deposited on the first dielectric layer or substrate 214 to facilitate movement of the fluids 210, 212 in the fluid channel 202. Note that the elements of this drawing are not drawn to scale, but are intended to be illustrative only.

The second structure 206 is provided with multiple electrodes 218, 219, 220 that can be activated with an applied voltage independently of each other. A fluidic driving mechanism, generally 222, comprises the common electrode 208 and the independently addressable electrodes 218, 219, 220. Only three independently addressable electrodes 218, 219, 220 are shown in the illustrated embodiment, but it will be appreciated that other embodiments of the invention may include a larger number of independently addressable electrodes. In another embodiment, the multiple independently addressable electrodes 218, 219, 220 may be located in the first structure 204, while the common electrode may be located in the second structure 206. In alternative embodiments, it may not be necessary to insulate each electrode from the fluids in an EWOD activated optical switch, which may require only one electrode being insulated from the fluids. Other additional embodiments may have the independent addressable electrodes and a common electrode located in the same substrate, for example the first structure 204. Alternative

embodiments may include only independently addressable electrodes, without a common electrode incorporated into the active optical switch system, wherein the independently addressable electrodes are located, for example, in the first structure 204.

A second dielectric layer or substrate 224, having an upper surface 226, at least partially insulates electrodes 218, 220 from the fluids 210, 212 and the fluid channel 202. In the illustrated embodiment, the surface 226 is also the bottom surface of the fluid channel 202. A second anti-wetting layer 228 may be deposited on the second dielectric layer or substrate 224, for example on the shared surface 226, to facilitate movement of fluids 210, 212 in the fluid channel 202.

The second substrate 224 contains a first waveguide 230 and a second waveguide 232. An etched region 234 of the second substrate 224 above the second waveguide 232 exposes the second waveguide 232 at or close to the upper surface 226 of the second substrate 224, on which the second anti -wetting layer 228 may be deposited. The etched region 234 defines a waveguide-fluid coupling region 224a, approximately delineated by the dashed lines, of the second substrate 224, in which the refractive index of the fluid located above the second waveguide 232 can affect the propagation constant of light passing along the second waveguide 232. The first waveguide 230 is located away from the etched region 234 of the second substrate 224 and away from the waveguide-fluid coupling region 224a, remaining isolated within the second substrate 224 so that the refractive index of the fluid above the first waveguide 230 has substantially no impact on the propagation constant for light passing along the first waveguide 230.

In the illustrated embodiment, the first fluid 210 has a relatively higher refractive index than the second fluid 212. The first fluid 210 is located within the fluid channel 202 and in the etched region 234, so that the relatively higher refractive index of the first fluid 210 affects the effective refractive index experienced by light propagating along the second waveguide 232. According to the illustrated embodiment, light can couple between the first and second waveguides 230, 232 when the first fluid 210 is in the etched region 234. In other words, when the first fluid 210 is in the etched region 234, the switch 200 is in the cross state. In another switch state, schematically illustrated in FIG. 2B, when the first fluid 210 has been moved to a position outside the etched region 234, and the second fluid 212, having a relatively lower refractive index, is in the etched region 234, the effective refractive index experienced by light propagating along the second waveguide 232 is changed, preventing coupling of light between the waveguides 230, 232 and the switch is in the bar state. In other embodiments, the first fluid may have a lower refractive index than the second fluid, so that the first fluid may induce the switch to assume the cross state when the first fluid is in the etched region. Alternative embodiments may include a first fluid of relatively higher refractive index than the second fluid, which induces a bar state when in the etch region, and vice versa.

The electro-wetting (EW) effect arises when the contact angle of a liquid is changed due to an applied electrical potential difference. In the illustrated embodiment, when an electric field is generated between, for example, electrodes 208, 220, the surface energy of the liquid 210, 212 is altered so that the liquid 210 wets the surface it contacts. As in the embodiment illustrated in FIGS. 2A and 2B, because the EW effect is applied to the first liquid 210 separated from electrodes 208, 218, 220, by dielectric layers 214, 224, this configuration is referred to as electro- wetting on dielectric (EWOD). As discussed above, only one electrode need be insulated from the fluids of the switch to qualify as an EWOD optical switch.

In the illustrated embodiment, the fluidic driving mechanism 222 selectively applies electric potentials to the electrodes 208, 218, 219, 220 of optical switch 200 to move the fluids 210, 212 inside the fluid channel 202. For example, in the configuration illustrated in FIG. 2B where fluid 210 is positioned away from the waveguide-light coupling region, i.e., not in the etched region 234, voltages may be applied to the first electrode 218, together with common electrode 208, and then the second electrode 220. Such activation of electrodes 218, 220 may result in the first fluid 210 moving to the location shown in FIG. 2A, above the second waveguide 232 and in the etched region 234. The movement of the first fluid 210 causes corresponding movement of the second fluid 212 inside the fluid channel 202. In this way, the state of the optical switch system 200 can be selected to be in a bar or cross state. In other embodiments, electrodes may be provided only on one side of the fluid channel.

The use of the EW effect to move liquid droplets is well known, and the use of microfluidics in the control of optical waveguide devices has been described in the references discussed and incorporated by reference above. It will be appreciated, however, that other conformations and configurations of electrodes and liquids can be used to move fluids 210, 212. For example, in the embodiments illustrated in FIGS. 1, 2A and 2B, the liquids move in a direction transverse to the waveguides in order to change the effective refractive index of one of the waveguides. It should be understood that this is not intended to be a limitation and the liquids may move in any other direction relative to the waveguides, including in a longitudinal direction, parallel to the waveguides in the coupling region of the optical switch.

The first and second fluids may be any suitable fluids that provide suitable performance of the EWOD optical switches. Liquids that are more polar in nature are more susceptible to an electro-wetting force, while non-polar liquids are less susceptible to an applied electric field, if at all. Polar liquids that may be used in the present invention have surface energies that are sufficiently affected by an applied electric field that the liquid moves under the resulting electro-wetting force. Examples of suitable polar liquids that may be used include, but are not limited to, water; acetonitrile; short chain alcohols, such as ethanol or propanol; propylene carbonate, hydroxypropylene carbonate; ethylene glycol, propylene glycol dimethylformamide; and variations thereof. For example, the preceding liquids, other than water, may be hydroxylated to some degree. Examples of non-polar fluids that may be used include, but are not limited to, diphenyl sulfide, triphenyl sulfide, quinoline, aniline, thioanisole, chloraniline, methylaniline, toluidine, and various medium alkanes (e.g. C4-C10). The non-polar fluid may also be a gas, such as nitrogen, air, and the like.

EWOD optical switches may be arranged in many different configurations to form different optical circuits. One exemplary embodiment of an optical circuit that might employ EWOD optical switches is a 4x4 switch array 302, as shown in FIG. 3. A substrate 300 contains a switching network, a number of EWOD-activated coupler switches 304, input waveguides 306, connecting waveguides 308 and 310, and output waveguides 312 and 314. In some situations, output waveguides 312 may be used for test purposes with output waveguides 314 being used as device outputs. The switches 304 are coupled together using connecting waveguides 308, 310 to form a switching network configured as a cross-bar network. In this type of network, the switches 304 are arranged in rows and columns. There are two types of connecting waveguides, viz. the row connecting waveguides 308 that connect from the output of one switch 304 to the input of an adjacent switch 304 in the same row, but a different column, and the column connecting waveguides 310 that connect from the output of one switch 304 to the input of an adjacent switch in the same column, but different row.

For purposes of this description, the rows are designated with the upper case capital alphabetic characters, A, B, C, D, while the columns are designated with lower case alphabetic characters a, b, c, d. Accordingly, the switches 304, input waveguides 306, connecting waveguides 308, 310 and output waveguides 312, 314 may be designated according to their row and column in the network. For example the input waveguide 306 on the third row down, row C, is designated input waveguide 306C, and the switch on the third row down, row C, and the second column across, column b, is designated switch 304Cb. The row connecting waveguide 308 on the third row, row C, that connects from the second switch in the same row, switch 304Cb, to the third switch in the row, switch 304Cc, may be referred to as row connecting waveguide 308Cb. The column connecting waveguide 310 on the second column, column b, that connects from the third switch in the column, switch 304Cb, to the fourth switch in the same column, switch 304Db, may be referred to as column connecting waveguide 3 lOCb. The output waveguide 312 on the third row down, row C, may be designated as test waveguide 312C, while the output waveguide on the second column, column b, is designated as output waveguide 314b.

The illustrated embodiment of the cross-bar network is in a 4 x 4 arrangement, with four rows and four columns, but it will be understood that other sizes of network may also be used, such as an 8 x 8 or 16 x 16 network. In addition, the network need not be square, but may have more rows than columns or vice versa, for example 4 x 8 or 8 x 4. In addition, other arrangements of switches may be used to form a switch network. These types of switch networks have been described in greater detail in U.S. Patent Application No. 62/331,777, titled "Integrated Optical Switch Network with High Performance and Compact Configuration," filed on May 4, 2016, and incorporated herein by reference.

Prior approaches to controlling the position of the liquids that affect the switching mechanism of the EWOD optical switches have included the use of microfluidic channels. According to the present invention, capillary action can be used to control the movement and positioning of liquids relative to the optical switches. This approach can be used to avoid the use of liquid droplets in a microfluidic channel, which simplifies the

mechanisms used for initially filling the liquid channels with the liquids and allows for more compact and reliable switch arrays.

Capillary liquid control is explained initially in the context of a 1 x 2 switch array 400, as shown in FIG. 4A. The switch array 400 has optical switches 402, 404 that include respective waveguide-light coupling regions 406, 408. In this exemplary embodiment, an input waveguide 410 couples to the first switch 402. Depending on whether the first optical switch 402 is in its bar state or cross state, light entering the first optical switch 402 from the input waveguide is directed to a first output waveguide 412 or a connecting waveguide 414 that connects light output from the first optical switch 402 as input to the second switch 404. Light entering the second optical switch 404 along the connecting waveguide 414 can be directed to a second output waveguide 416 when the second optical switch 404 is in its bar state. Typically, either the first or second optical switch 402, 404 is in the bar state and directs light to a respective output waveguide 412, 416. In this configuration, the optical switches 402, 404 are generally oriented so that the waveguides within the switches 402, 404 are parallel to the direction in which light moves across a substrate 418 that supports the switch array elements, e.g., the x-direction.

Another configuration of a 1 x 2 optical switch array 450 is shown in FIG. 4B. This array 450 contains elements like those in the array 400, but the optical switches 402, 404 are oriented in a direction such that the waveguides within the switches 402, 404 are generally perpendicular to the direction in which light propagates across the substrate 418, the x-direction.

FIGS. 5A and 5B schematically illustrate capillary-based liquid management systems for the switch arrays 400 and 450. For clarity, the switch arrays 400 and 450 are shown in dashed line and the capillary -based liquid management systems are shown in solid line overlying the optical switches and waveguides. In FIG. 5 A, the capillary -based liquid management system 500 includes a first reservoir 502 for the first fluid and a second reservoir 504 for the second fluid, where one of the first and second fluids is a polar liquid. In some embodiments, the first fluid is a nonpolar liquid, having a contact angle, θ_γ, relative to a surface that is less than 90 degrees and the second liquid is a polar liquid whose electro-wetting properties enable actuation of the EWOD optical switch. Under zero-field conditions, the contact angle of the second liquid with a surface, θ_γ, is typically greater than 90 degrees. The first fluid may also be a gas. In other

embodiments, the first fluid is a polar liquid and the second fluid is nonpolar, either liquid or gas. The interface between the first and second fluids, which are immiscible, is curved, with a radius of curvature depending on the relative surface tensions of the two fluids. The contact angles of the interface to the sidewalls along the contact line are, under zero- field conditions, constant and equal. A capillary cell 506 is located between the first and second reservoirs 502, 504. The capillary cell is located over the waveguide-light coupling region of its associated optical switch 402 so that, by changing the refractive index of the medium in the capillary cell 506 by changing the liquid within the capillary cell 506, the optical properties of the waveguide-light coupling region of the switch 402 can be changed, resulting in a change of the switch state, from cross to bar or vice versa. Fluid flow from the first reservoir 502 to the first capillary cell 506 is provided through a first neck 508, which has a cross- sectional area that is smaller than the cross-sectional area of the first capillary cell.

Likewise, fluid flow from the second reservoir 504 to the first capillary cell 506 is provided through a second neck 510, whose cross-sectional area is reduced relative to that of the first capillary cell 506. In a similar manner, a second capillary cell 512 is associated with the second switch 404, and is connected to the first and second reservoirs 502, 504, via respective necks 514 and 516.

The capillary cells 506, 512 have dimensions that permit capillary flow to take place, and are typically 5 - 200 μιη in width and in height. Preferably the height of the capillary cell is in the range 15 μιη - 100 μιη. Typically, the length of the capillary cell will match the optical coupling length of the switch, for example in the range 600 μιη - 1300 μιη, although it may also lie outside this range in some embodiments.

FIG. 6 shows the first and second capillary cells 506, 512 in greater detail. In some embodiments, the first reservoir 502 is formed with conducting walls and is grounded. First and second control electrodes 602, 604, shown with hatching, respectively lie close to the first and second capillary cells 506, 512, and are used to selective apply an electric field to a capillary cell. Cross-hatching represents an electrode that is not activated, while single hatching represents an electrode that is activated. In FIG. 6, the second electrode 604 is activated.

Operating principles of the capillary liquid management system are described with reference to FIGS. 7A-7D. FIG. 7A shows the two capillary cells 506, 512 in an initial condition where both cells 506, 512 contain both the first fluid 702 (light shading) and the second fluid (dark shading) 704. In this embodiment, the second liquid 704 is a polar liquid. The interface 706 between the first and second fluids 702, 704 forms an angle θ_γ, relative to the sidewall of the cell, the angle being determined by the relative surface energies of the two fluids 702, 704. A tapered region 708, connecting between the neck 508 and the capillary cell 506 forms an angle, a, between the sidewall of the neck 506 and the tapered region 708.

FIG. 7B shows how the fluids react when an electric field is applied to the second capillary cell 512 via the second electrode 604. The surface energy of the second liquid 704 in the second capillary cell 512 is altered under the applied electric field, resulting in a reduction in the contact angle, θ_γ. In the illustrated embodiment, the contact angle has reduced to the extent that the liquid interface 706b in the second capillary cell 512 has a center of curvature that lies on the first liquid side of the interface 706b. This contrasts with the interface 706b shown in FIG. 7A under zero-field conditions, where the center of curvature of the interface 706b lies on the second liquid side of the interface. This decrease in the curvature of the interface 706b leads to an imbalance in the Laplace- pressure, which is a function of the interfacial tension and the mean curvature, because the curvature of the interface 706a remains unchanged. This imbalanced force results in the second liquid 704 in the second capillary cell 512 being forced towards the first reservoir 502, pushing the first fluid 702 into the first reservoir 502. Also, because of the imbalanced force, the interface 706a in the first capillary cell 506 moves towards the second reservoir 504, with the result that the first fluid 702 fills the first capillary cell 506, displacing the second liquid 704 from the first capillary cell 506.

The liquid motion stops when a new equilibrium of the Laplace-pressures at a local free surface energy minimum is found, for example as shown in FIG. 7C. The Laplace- pressure is equal when the mean curvatures of the interfaces (and the interfacial tension) are equal. The mean curvature can be equal if the contact angle at the three phase contact lines are equal (the three phases being the first liquid, the second liquid, and the wall material) and the cross sections of the channels are the same, assuming that the channel walls are parallel. The configuration shown in FIG. 7C corresponds to a minimum in surface energy, where the surface area of the interfaces 706a, 706b are minimized because they lie in the neck regions 510 and 514. This assures that the fluids remain in the configuration, with the first fluid filling the first capillary cell and the second liquid filling the second capillary cell, when the applied voltage is removed from all electrodes.

If, from the starting position shown in FIG. 7A, the electric field is applied via the first electrode 602 instead of the second electrode 604, the second liquid is forced into the first capillary cell 506, as shown in FIG. 7D. The following discussion of how the necked portions connecting a capillary cell to respective reservoirs are used to stabilize the positions of the two liquids, makes reference to FIGS. 8A-8C. The angle a of the capillary has a theoretical maximum at which the switching of the states is still possible. To calculate this maximum angle, the achievable contact angle change of the three-phase system in the specified electric field has to be known. FIGS. 8A-8C illustrate a simplified two-dimensional model of a two capillary cell system. In FIG. 8A, the capillary cell system is in a stable, equilibrium state with the second fluid filling the first capillary cell 506 and the first fluid filling the second capillary cell 512. No electric field is applied to the second fluid, so the interfaces 706a, 706b both have a positive radius of curvature, i.e., the center of curvature is to the side of the second fluid. In this case, the contact angle of both interfaces 706a, 706b is θ_γ > 90° and is determined by the properties of the first and second fluids.

In FIG. 8B, an electric field has been applied to the second capillary cell 512 such that the second interface 706b changes its radius of curvature. The contact angle under the electro-wetting conditions, 0_EW, is less than θγ. The resulting difference in the Laplace- pressure results in movement of the two interfaces 706a, 706b in the directions shown by the arrows. Therefore, the Laplace-pressure difference pushes the polar liquid to reach a state where both interfaces have the same mean curvature. If the angle is

as shown in FIG. 8C, then both interfaces 706a, 706b will have the same curvature after leaving the necked regions of the channels. Thus, the angle a should be less than this limit to permit activation. The condition θ_γ-θ_Ε\ν > 2α has to be fulfilled in order to switch states. Since the electro-wetting contact angle, 6EW, is dependent on the strength of the applied electric field (E), the condition for switch activation is that 6EW(E) < θ_γ - 2α. The three dimensional case is more complex and is not described here. However, the applied voltage needed to switch states decreases when the value of a becomes smaller and when the second liquid is more responsive to the applied electric field.

A method of filling the capillary-based liquid management system is described with reference to FIGS. 9A-9C. The first reservoir 502 is provided with an outlet port 902 and the second reservoir 504 is provided with an inlet port 904. The first fluid can be filled through the inlet port 904 so as to fill the second reservoir 504, the capillary cells 506, 512, and the first reservoir 502, as shown in FIG. 9A. Excess first fluid can escape via the outlet port 902. Next, the second fluid is filled into the second reservoir 504 via the inlet port 904, so as to fill the second reservoir, as shown in FIG. 9B. The first fluid displaced by the second fluid can escape via the liquid outlet port 902. These first two steps are carried out without any voltage applied to any of the capillary cells 506, 512. Next, a voltage is applied to the second capillary cell 512 and more of the second fluid is added via the inlet port 904 until the second cell 512 is filled with the second fluid, as shown in FIG. 9C. The inlet port 904 and outlet port 902 can then be sealed and the capillary-based liquid management system is ready for use. It will be appreciated that the ports used as the inlet 904 and outlet 902 may be used in reverse, where the liquid is initially filled into the system via port 902 and drained from the system through port 904.

When sealing off the inlet port 904, it is possible that more of the second fluid can enter the system than is required or than is optimal. Problems arising from this can be addressed through the use of a calibrating capillary, as is described with reference to FIGS. 10A and 10B. FIG. 10A shows a two-cell capillary system that includes a calibrating capillary 1002 that couples between the first and second reservoirs 502, 504. The calibrating capillary 1002 includes a number of narrow segments 1004 alternating with wider segments 1006 along the length of the calibrating capillary 1002. Each wider segment 1006 has an associated, individually addressable electrode 1008, whose purpose is explained below.

In FIG. 10A, the second fluid is shown to have been over filled in the capillary- based liquid management system, with the result that the interface 706a is beyond the necked region 510. Also, an interface 706c between the first fluid and second fluid in the calibrating capillary 1002 has moved beyond the first neck region 1010a from the second reservoir 504. The calibrating capillary 1002 can be used to reduce the volume of the second fluid available to the capillary cells in the following manner. By activating the first electrode 1008a of the calibrating capillary 1002, the second fluid fills the first wide segment 1006a, reducing the level of the second fluid in the first capillary cell 506. If this is insufficient to reduce the level of the second fluid in the first cell 506 to a required level, the calibrating capillary can sequentially fill up the second and third wider segments 1006b, 1006c, and so on, by sequential activation of their respective electrodes 1008b, 1008c. In this way, the calibrating capillary 1002 can be used to reduce the amount of the second fluid available to the capillary cells in a step wise manner, by sequentially filling wide segments 1006 until the fluid level in the capillary cells reaches a desired level. Thus, the calibrating capillary is used to adjust the positions of the interfaces between the first and second fluids in the capillary cells 506, 512, and performs no optical function. In the example shown in FIG. 10B, the first two wide segments 1006a and 1006b have been filled in order to place the interface 706a at the desired location of necked region 510.

The capillary-based liquid management system is not limited to use in a 1 x 2 optical switch array, but may be implemented in any size of array. In some embodiments, a 1 x 2 capillary cell array may be extended to cover a row of switches by simply adding additional capillary cells between the first and second reservoirs. Thus, for example, a 1 x 4 switch array may be managed using a capillary system having four capillary cells located between the first and second reservoirs. A second row in a switch array, for example in a 2 x 4 switch array may be accommodated using two capillary systems, each with four capillary cells.

In another approach, multiple rows of switches may be accommodated using a common reservoir between the rows. An embodiment of such an approach is

schematically illustrated in FIG, 1 1, which shows an embodiment of a capillary -based liquid management system 1100 for a 2 x 4 optical switch array. In this embodiment, an upper reservoir 1102 and a lower reservoir 1104 contain the first fluid, while the common reservoir 1106, located between the upper and lower reservoirs 1102, 1104, contains the second fluid. There are four upper capillary cells 1108 connecting between the common reservoir 1106 and the upper reservoir 1102, and four lower capillary cells 1110 connecting between the common reservoir 1106 and the lower reservoir 1104. The liquid management system 1100 may also be provided with an upper calibrating capillary 1112 between the common reservoir 1106 and the upper reservoir 1102, to permit the adjustment of the positions of the interfaces between the first and second fluids in the upper capillary cells 1108, and a lower calibrating capillary 1114 between the common reservoir 1106 and the lower reservoir 1104, to permit the adjustment of the positions of the interfaces between the first and second liquids in the lower capillary cells 1110.

Activation of a particular one of the upper capillary electrodes 1116 results in the second liquid flowing into the upper capillary cell 1108 associated with the activated electrode. Thus, selective activation of an upper electrode 1116 determines which of the associated optical switches in the upper row is activated. Likewise, activation of a particular one of the lower capillary electrodes 1118 results in the second liquid flowing into the lower capillary cell 1110 associated with the activated electrode. Thus, selective activation of a lower electrode 1118 determines which of the associated optical switches in the lower row is activated. During switching of one of the upper capillary cells 1108, an electric field may also be applied to the filled capillary cell of the lower capillary cells 1110 so as to avoid inadvertent switching of the lower capillary cells 1110 when it is desired to switch only one of the upper capillary cells 1008.

This liquid management system can be filled in a manner similar to that described above for the 1 x 2 array. The first liquid is injected into the system through an inlet 1120 so that all the reservoirs 1102, 1104, 1106 and the capillary cells 1108, 1110 are filled with the first fluid. Excess of the first liquid can escape through outlets 1122. The second fluid is then injected to fill the common reservoir 1106 and then an upper capillary cell 1108 and a lower capillary cell 1110 is activated to draw the second fluid into the respective capillary cells. The first fluid displaced by the second fluid escapes from the system via the outlets 1122. The inlet 1120 and outlets 1122 can then be sealed. The calibrating capillaries 1112, 1114 can then be adjusted to correct the volumes of the first and second fluids available for the capillary cells 1108, 1110.

It will be understood that the number of upper and lower capillary cells can be different from that shown. For example, there may be some other number of upper and lower capillary cells, such as 8 or 16. Furthermore, the number of upper capillary cells need not be equal to the number of lower capillary cells. For example, the liquid management system of FIG. 11 may include four upper capillary cells as shown, but may have one, two three, or more than four lower capillary cells.

Various modifications, equivalent processes, as well as numerous structures to which the present invention may be applicable will be readily apparent to those of skill in the art to which the present invention is directed upon review of the present specification. The claims are intended to cover such modifications and devices. For example, in the examples discussed above, it was assumed that the first liquid is a nonpolar liquid and the second liquid is a polar liquid. This need not be the case, and the first liquid can be a polar liquid while the second liquid is nonpolar. It can be understood that the capillary-based liquid management systems described herein may be adapted for use with any size of switch array, including 16 x 16, 32 x 32 and 64 x 64 arrays.

As noted above, the present invention is applicable to optical switching systems for communication and data transmission. Accordingly, the present invention should not be considered limited to the particular examples described above, but rather should be understood to cover all aspects of the invention as fairly set out in the attached claims.

Claims

Claims What is claimed is:

1. An optical switching device, comprising:

a first electro-wetting on dielectric (EWOD) optical switch comprising a first waveguide coupling region; and

a liquid management system for controlling fluids activating the first EWOD optical switch, the liquid management system comprising a first reservoir for a first fluid, a second reservoir for a second fluid different from the first fluid, and a first capillary cell proximate the first waveguide light coupling region;

wherein the first capillary cell comprises a first neck permitting fluid flow between the first capillary cell and the first reservoir, and further comprises a second neck permitting fluid flow between the first capillary cell and the second reservoir; and

wherein one of the first fluid and the second fluid is a polar liquid.

2. An optical switching device as recited in claim 1, further comprising a first activating electrode proximate the first capillary cell.

3. An optical switching device as recited in claim 1, further comprising a second EWOD optical switch having an input coupled to an output of the first EWOD optical switch, the second EWOD optical switch having a second waveguide coupling region, the liquid management system comprising a second capillary cell proximate the second waveguide coupling region, the second capillary cell comprising a third neck permitting fluid flow between the second capillary cell and the first reservoir, and a fourth neck permitting fluid flow between the second capillary cell and the second reservoir.

4. An optical switching device as recited in claim 3, further comprising a first activating electrode proximate the first capillary cell and a second activating electrode proximate the second capillary cell.

5. An optical switching device as recited in claim 1, further comprising a calibrating capillary connected between the first reservoir and the second reservoir, the calibrating capillary comprising a plurality of calibrating wide portions separated by respective neck portions.

6. An optical switching device as recited in claim 1, further comprising a first fluid port coupled to the first reservoir to permit fluid flow into and out of the first reservoir, and a second fluid port coupled to the second reservoir to permit fluid flow into and out of the second reservoir.

7. An optical switching device as recited in claim 1, further comprising a third reservoir and a third capillary cell, the third capillary cell comprising a fifth neck permitting fluid flow between the second reservoir and the third capillary cell, the third capillary cell further comprising a sixth neck permitting fluid flow between the third capillary cell and the third reservoir.

8. An optical switching device as recited in claim 7, wherein fluid in the third reservoir comprises the first fluid.

9. An optical switching device as recited in claim 1, wherein the other of the first fluid and the second fluid is a non-polar liquid.

10. An optical switching device, comprising:

an optical switch array comprising a plurality of electro-wetting on dielectric (EWOD) optical switches, the plurality of EWOD optical switches including at least a first EWOD optical switch and a second EWOD optical switch; and

a liquid management system for controlling fluids activating the plurality of EWOD optical switches, the liquid management system comprising a first reservoir for a first fluid, a second reservoir for a second fluid different from the first fluid, and a first plurality of capillary cells positioned proximate respective waveguide coupling regions of a first set of EWOD optical switches of the plurality of EWOD optical switches, the capillary cells of the first plurality of capillary cells each coupling between the first and second reservoirs;

wherein the liquid management system contains a first volume of the first fluid and a second volume of the second fluid, the first volume of the first fluid and the second volume of the second fluid being selected so that, when the first fluid fills one of the capillary cells, the second fluid fills the others of the plurality of capillary cells, and

wherein one of the first fluid and the second fluid is a polar liquid.

11. An optical switching device as recited in claim 10, wherein the liquid management system further comprises a calibrating capillary connected between the first reservoir and the second reservoir, the calibrating capillary comprising a plurality of wide portions separated by respective neck portions.

12. An optical switching device as recited in claim 10, wherein the first fluid is non-polar and the second fluid is a polar liquid.

13. An optical switching device as recited in claim 10, wherein the first fluid is a polar liquid and the second fluid is non-polar.

14. An optical switching device as recited in claim 10, wherein the liquid management system further comprises a plurality of electrodes disposed proximate respective capillary cells.

15. An optical switching device as recited in claim 10, wherein the liquid management system further comprises a third reservoir for the first fluid and a second plurality of capillary cells, positioned proximate respective waveguide coupling regions of a second set of EWOD optical switches of the plurality of EWOD optical switches, the capillary cells of the second plurality of capillary cells each coupling between the second and third reservoirs, wherein the first volume of the first fluid in the liquid management system and the second volume of the second fluid in the liquid management system are selected so that, when the first fluid fills one of the first plurality of capillary cells and one of the second plurality of capillary cells, the second fluid fills the others of the first plurality of capillary and the others of the second plurality of capillary cells.

16. An optical switching device as recited in claim 10, wherein the one of the first fluid and the second fluid is a non-polar liquid.