WO2019113107A1

WO2019113107A1 - System and method for capacitive droplet location in microfluidically-controlled optical switches

Info

Publication number: WO2019113107A1
Application number: PCT/US2018/063904
Authority: WO
Inventors: Ian Miles Standish; Jan Jozef Julia Maria ERREYGERS; David James MATHER
Original assignee: Commscope Technologies Llc
Priority date: 2017-12-04
Filing date: 2018-12-04
Publication date: 2019-06-13

Abstract

A microfluidically controlled optical switch system includes a fluid channel proximate one or more waveguides and a droplet of a first liquid within the fluid channel A set of electrodes is located proximate fluid channel. An activating electrical circuit is connected to the electrodes to selectively apply a voltage signal to one or more of the electrodes to change a position of the droplet of first liquid within the fluid channel, and thus change the state of the optical switch. A droplet position detection unit comprises a circuit couplable to electrodes of the set of electrodes for capacitive detection of the position of the droplet of first liquid relative to the set of electrodes. The invention also includes a method of capacitively determining the position of the liquid droplet.

Description

SYSTEM AND METHOD FOR CAPACITIVE DROPLET LOCATION IN MICROFLUIDIC ALLY-CONTROLLED OPTICAL SWITCHES

Cross-Reference To Related Application

This application is being filed on December 4, 2018 as a PCT International Patent Application and claims the benefit of U.S. Patent Application Serial No. 62/594,315, filed on December 4, 2017, the disclosure of which is incorporated herein by reference in its entirety.

Field of the Invention

This invention is related to optical switch systems used in optical fiber data communication systems, and particularly to the control of optical switches in such systems.

Background of the Invention

Optical fiber networks are becoming prevalent in part because service providers want to deliver high bandwidth communication capabilities to customers. Such networks are a desirable choice for delivering high-speed communication data because they can avoid the use of active electronic devices, such as amplifiers and repeaters, between a central office and a subscriber termination. The absence of active electronic devices may decrease network complexity and/or cost and may increase network reliability.

As networks become increasingly complex, however, the need for management of the optical signals becomes increasingly important. Many optical signal management functions, such as redirecting signals to bypass a faulty component, or opening new channels as more users are added to the network, can be accomplished using active optical switches that use microfluidic control of liquid droplets, for example electro-wetting on dielectric (EWOD)-activated optical switches and totally internally reflecting waveguide (TIRW) switches. Such switches typically use the movement of a liquid droplet to alter the optical properties of an optical element. For example, the movement of a liquid droplet can affect the effective refractive index of an adiabatic coupler in an EWOD- activated optical switch or may alter whether light is totally internally reflected at a waveguide interface in a TIRW optical switch. Some approaches to implementing optical circuits rely on the integration of many such optical switches onto a single chip: for example, an 8 x 8 switch circuit will use 64 switches. As the number of switches on a chip increases, the microfluidic system used to control the positions of the liquid droplets, and hence the states of the different optical switches on the chip, becomes more complex. One important aspect of controlling the position of the liquid droplets used in such systems is to know where the liquid droplets are in relation to the optical switches.

There is, therefore, a need to develop a system and method for detecting the location of microfluidic droplets in microfluidically-controlled optical switch networks, in order to allow better management of microfluidic systems used in optical switch circuits.

Summary of the Invention

One embodiment of the invention is directed to a method that includes providing an optical switch that has at least a droplet of a first liquid within a fluid channel. A state of the optical switch is changeable by movement of the droplet of first liquid within the fluid channel. The optical switch further includes a plurality of electrodes disposed proximate the fluid channel. The capacitance of one or more electrodes of the plurality of electrodes is measured, the one or more electrodes being proximate the droplet of first liquid. The position of the droplet of first liquid is determined based on the capacitance measurements.

Another embodiment of the invention is directed to an optical switching system, that includes an optical switch having at least one waveguide, a fluid channel proximate the at least one waveguide and a droplet of a first liquid within the fluid channel. A set of electrodes is located proximate fluid channel. A droplet position detection unit comprises a circuit couplable to electrodes of the set of electrodes for capacitive detection of the position of the droplet of first liquid relative to the set of electrodes.

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 a plan view of an embodiment of an electro- wetting on dielectric (EWOD) optical switch according to an embodiment of the present invention;

FIG. 2 schematically illustrates a cross-sectional view of an embodiment of an EWOD optical switch according to an embodiment of the present invention;

FIGs. 3 A-3D schematically illustrate plan and cross-sectional views of a totally internally reflecting waveguide (TIRW) optical switch, according to an embodiment of the present invention;

FIG. 4 schematically illustrates a 4 x 4 optical switch matrix on a substrate, according to an embodiment of the present invention;

FIG. 5 schematically illustrates a microfluidic channel liquid handling system that may be used to control the 4 x 4 optical switch matrix of FIG. 4;

FIG. 6 schematically illustrates the 4 x 4 optical switch matrix of FIG. 4 overlaid by the microfluidic channel liquid handling system of FIG. 5;

FIG. 7 schematically illustrates an embodiment of control circuitry for selectively applying a voltage to electrodes in a microfluidically controlled optical switch and for capacitively detecting the position of the liquid droplet in such a switch, according to an embodiment of the present invention;

FIG. 8 schematically illustrates a cross-section through a portion of a

microfluidically controlled optical waveguide switch, according to an embodiment of the present invention;

FIG. 9 is a schematic representation of cross-section illustrated in FIG. 7, showing various capacitive features associated with a liquid droplet in a microchannel environment;

FIG. 10A shows a circuit that represents the capacitive features shown in FIG. 9;

FIG. 10B presents a plan view of the schematic representation shown in FIG. 8;

FIG. 11 (A and B) presents a schematic of a first circuit that may be used to measure capacitance of a liquid droplet in an optical switch’s microfluidic channel, according to an embodiment of the present invention; FIG. 12 (A and B) presents a schematic of a second circuit that may be used to measure capacitance of a liquid droplet in an optical switch’s microfluidic channel, according to another embodiment of the present invention;

FIG. 13 shows an embodiment of a probe head circuit used for measuring the value of an electrode’s capacitance, Cx, according to an embodiment of the present invention;

FIG. 14 presents a schematic circuit showing various capacitances, including parasitic capacitance, of the measuring circuit, according to an embodiment of the present invention; and

FIG. 15 presents a calibration graph showing experimental values of output voltage obtained using various test capacitors, obtained using the circuit illustrated in FIG. 12.

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 various optical devices and systems that can provide benefit in optical networks. In particular, the invention is directed to integrated optical switching devices that are controlled microfluidically and, more particularly, to capacitive methods of detecting the locations of liquid droplets used in such switch networks.

An exemplary embodiment of an active optical switch 100, such as an adiabatic optical switch, according to one embodiment of the present invention is schematically illustrated in FIG. 1. The optical switch 100 incorporates a first waveguide 102, a second waveguide 104 and a third waveguide 106. In this embodiment, the second waveguide 104 is a continuation of the first waveguide 102 so that light propagating along the first waveguide 102 passes to the second waveguide 104 if there is nothing to cause light to be coupled to third waveguide 106. The first and second waveguides 102, 104 are attached at the end of the waveguide switching region 108, where the first waveguide 102 is situated physically closer to the third waveguide 106. The waveguide switching region 108 is a region where light propagating along the first waveguide 102 may, under certain conditions, couple to the third waveguide 106. Under other conditions, the light propagating along the first waveguide 102 continues to the second waveguide 104.

Whether light couples from the first waveguide 102 to the third waveguide 106 depends on the effective refractive index experienced by the light as it propagates through the waveguide switching region 108. The effective refractive index can be altered by positioning a liquid of greater or lesser refractive index close to the waveguide switching region 108 and the waveguide-fluid coupling region 110 discussed further below.

In some embodiments, the active optical switch may include two fluids that are moveable to change the state of the switch. The embodiment illustrated in FIG. 1 is shown with a first liquid droplet 112 positioned over the waveguide switching region 108 and the waveguide-fluid coupling region 110. An optional second fluid 114, which may be present on either side of the droplet 112, is shown generally filling the remaining space of a fluid channel 116. In some embodiments, the second fluid 114 may be air, or some other gas, such as nitrogen. In other embodiments, the second fluid 114 may be a second liquid that does not mix with the first liquid. The first liquid droplet 112 has a first refractive index and the second fluid 114 has a second refractive index, different from the first refractive index. The first liquid droplet 112 and the second fluid 114 may move within fluid channel 116 so, for example, the first liquid droplet 112 may move away from the waveguide switching region 108 and waveguide-fluid coupling region 110 to the location shown as 112’, with the second fluid 114 generally filling the remaining space in the fluid channel 116. One or more of the inner surfaces of the fluid channel 116 may be coated with a material that has a photosensitive surface energy, such as coatings shown as 118, 120 to assist in controlling the position of first liquid droplet 112 and the second fluid 114 with respect to the waveguide switching region 108 and the waveguide-fluid coupling region 110.

In the illustrated embodiment, an optical signal transmitted into the first waveguide

102 is coupled to the third waveguide 106 when the first liquid droplet 112 is positioned over the waveguide switching region 108 and the waveguide-fluid coupling region 110. This is referred to as the switch’s“cross state.” An optical signal transmitted into the first waveguide 102 propagates to the second waveguide 104 when the first liquid droplet 112’ is positioned away from the waveguide switching region 108 and the waveguide-fluid coupling region 110, and instead the second fluid 114 is positioned near the waveguide switching region 108 and the waveguide-fluid coupling region 110. This is referred to as the switch’s“bar state.” In other embodiments, depending on the coupling constant between the first and third waveguide 102, 106, the switch 100 may in the bar state when the liquid droplet 112 is positioned over the waveguide switching region 108 and the waveguide-fluid coupling region 110, and in the cross state when the liquid droplet 112’ is positioned away from the waveguide switching region 108 and the waveguide-fluid coupling region 110.

A cross-sectional view through a portion of an exemplary embodiment of an elementary active optical switch system 200 is schematically illustrated in FIG. 2. In this embodiment, optical fluids are moved in a fluid channel relative to waveguides using the technique of electro-wetting. A first fluid 202 and a second fluid 204 are disposed within a fluid channel 206 formed between two structures 208, 210. Either of the first and second fluids 202, 204 may be in a liquid or gaseous phase, though in the illustrated embodiment, at least the first fluid 202 is liquid and, in a preferred embodiment, the second fluid 204 is also a liquid. The first fluid 202 has a first refractive index and the second fluid 204 has a second refractive index, different from the first refractive index. The first structure 208 is provided with a common electrode 226, insulated from the channel 206 by a first dielectric layer 212, which provides at least partial electrical insulation between the common electrode 226 and the fluids 202, 204 and the fluid channel 206. A first anti-wetting layer 216 may be deposited on the first dielectric layer or substrate 212 to facilitate movement of fluids 202, 204 in the fluid channel 206. The second structure 210 is provided with multiple electrodes 228, 230 that can be activated with an applied voltage independently of each other. A fluidic driving mechanism, generally 224, includes the common electrode 226 and the independently addressable electrodes 228, 230. In the illustrated embodiment, only two independently addressable electrodes 228, 230 are shown, but it will be appreciated that other embodiments of the invention may include a larger number of independently addressable electrodes. It will further be appreciated that the multiple independently addressable electrodes 228, 230 may be located in the first structure 208, while the common electrode 226 can be located in the second structure 210. It will also be appreciated that, in alternative embodiments, it may not be necessary to insulate each electrode from the fluids in an EWOD-type switch, which may require only one electrode to be insulated from the fluids of the switch. Alternative embodiments also may have independently addressable electrodes and a common electrode located in the same substrate, for example, structure 208.

In the illustrated embodiment, a second dielectric layer or substrate 214, having an upper surface 234, at least partially insulates electrodes 228, 230 from the fluids 202, 204 and the fluid channel 206. In the illustrated embodiment, the surface 234 also forms bottom surface of the fluid channel 206. A second anti -wetting layer 218 may be deposited on the second dielectric layer or substrate 214, for example on the shared surface 234, to facilitate movement of fluids 202, 204 in the fluid channel 206.

The second substrate 214 contains a first waveguide 220 and a second waveguide 222. An etched region 232 of the second substrate 214 above the second waveguide 222 exposes the second waveguide 222 at or close to the upper surface 234 of the second substrate 214, on which the second anti -wetting layer 218 may be deposited. The etched region 232 defines a waveguide-fluid coupling region 2l4a of the second substrate 214, in which the refractive index of the fluid located above the second waveguide 222 can affect the propagation constant of light passing along the second waveguide 222. The first waveguide 220 is located away from the etched region 232 of the second substrate 214 and away from the waveguide-fluid coupling region 2l4a, remaining isolated within the second substrate 214 so that the refractive index of the fluid above the first waveguide 220 has substantially no impact on the propagation constant for light passing along the first waveguide 220.

In the illustrated embodiment, the first fluid 202 has a relatively higher refractive index than the second fluid 204. The first fluid 202 is located within the fluid channel 206 and in the etched region 232, so that the relatively higher refractive index of the first fluid 202 affects the effective refractive index experienced by light propagating along the second waveguide 222. According to the illustrated embodiment, light can couple between the second and first waveguides 222, 220 when the first fluid 202 is in the etched region 232. In other words, when the first fluid 202 is in the etched region 232, the switch is in the cross state. In another switch state, when the first fluid 202 is outside of the etched region 232, and the second fluid 204 with a relatively lower refractive index is in the etched region 232, the effective refractive index experienced by light propagating along the second waveguide 222 is changed, preventing coupling of light between waveguides 222, 220, and the switch is in the bar state. It will be appreciated that in alternative embodiments, the first fluid may have a lower refractive index than the second fluid, so that the first fluid could induce the switch to assume the cross state when the first fluid is in the etched region. Alternative embodiments may also employ a first fluid of relatively higher refractive index than the second fluid, and which induces a bar state when in the etched region, and vice versa.

The electro-wetting (EW) effect occurs when an applied potential difference induces a change in the contact angle of a liquid at a surface. In the illustrated embodiment, when an electric field is generated between, for example, electrodes 228 and 230, the surface tension of liquid 202 lying between the electrodes 226 and 230, can be reduced, allowing it to“wet” the surface it contacts. As in the embodiment illustrated in FIG. 2, because the EW effect is applied to liquid 202 separated from electrodes 226, 228, 230 by dielectric layers 214, 212, this configuration is referred to as electro-wetting on dielectric (EWOD).

In the illustrated embodiment, the fluidic driving mechanism 224 selectively applies electric potentials to the electrodes 226, 228, 230 of optical switch 200 to move fluids 202, 204 inside fluid channel 206. For example, in a configuration (not shown) where fluid 202 is above the first waveguide 220, i.e. not in the etched region 232, voltages may be applied to the second electrode 230, together with common electrode 226. Such activation of electrodes 226, 230 may result in fluid 202 moving from a location above the first waveguide 220 to the location shown in FIG. 2, above the second waveguide 222 and in the etched region 232. The movement of fluid 202 causes corresponding movement of fluid 204 inside fluid channel 206. In this way the bar state and cross state of optical switch system 200 can be set.

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

W02015/092064A1,“Adiabatic Coupler,” filed on December 21, 2014, in ET.S.

Provisional Patent Application NO. 62/094,506,“Integrated Optical Switching and Splitting for Optical Networks,” filed on December 19, 2014, and in ET.S. Provisional Patent Application No. 62/116,784, entitled“Remote Control and Power Supply for Optical Networks,” filed on February 16, 2015, all of which are incorporated herein by reference.

It will be appreciated that other conformations and configurations of electrode and fluid or liquid can be used to move fluids 202, 204. It will further be appreciated that such approaches can be used to move two or more liquids. For example, if a channel contains two immiscible liquids, separated at an inter-liquid interface, movement of one of the liquids via the EW effect can result in both liquids being moved in the channel. The second liquid can be moved along the channel by the EW forces acting on the first liquid, even though the second liquid does not itself exhibit EW behavior. For example, liquids that respond well to EW typically are polar in nature, but the second liquid may be non polar, yet still be moved because an EW force applied via a polar liquid. The EW technique can also be used to move liquid droplets around a network of microchannels, so long as electrodes are suitably positioned along the different channels.

Another type of microfluidically-controlled optical switch that may be integrated on an optical chip is a totally internally reflecting waveguide (TIRW) switch), an embodiment of which is discussed with reference to FIGs. 3 A-3D. FIG. 3 A shows a plan view of an embodiment of a TIRW optical switch 300 in a first, reflective switch state.

The switch 300 includes a first input waveguide 304, a first output waveguide 306 and a second output waveguide 308 on a substrate 302. A channel 310 crosses the input waveguide 304 at a crosspoint 312. The channel 310 may be formed in the substrate 302 using any suitable technique, e.g. photolithography and reactive ion etching (RIE). In many embodiments the channel 310 is mostly filled with air or another gas, such as nitrogen or the like. The first output waveguide 306 is located across the channel 310 from the input waveguide. In the illustrated embodiment, the channel 310 is empty at the crosspoint 312, so light 314 in the input waveguide 304 is total internally reflected at the wall of the channel 310 into the second output waveguide 308.

The illustrated substrate 302 also includes a second TIRW optical switch 320. In the illustrated embodiment the second TIR optical switch 320 is in a second, transmissive state. The second TIR optical switch 320 is formed with an input waveguide 324 terminating at the channel 310, a first output waveguide 326 across the channel 310 from the input waveguide 324, and a second output waveguide 328 that terminates at the channel 310 at a second crosspoint 332. A droplet 330 of liquid material is located within the channel 310 at the crosspoint 332. The refractive index of the liquid material is selected so that light 334 propagating along the first waveguide is incident on the wall of the channel 310 at an angle that does not result in total internal reflection at the wall of the channel 310. Instead, the light 334 propagates through the droplet 330 of liquid material and into the first output waveguide 326. Thus, a TIRW optical switch can be in either of two states, a reflective state or a transmissive state, depending on whether the liquid material is present at the crosspoint between the input waveguide and the fluid channel.

The droplet 330 of liquid material may be moved along the channel 310 using an applied electro-wetting force, which results from the application of an electric field asymmetrically across the droplet 330. A cross-sectional view through the substrate 302, along line AA’, is shown in FIG. 3B. The cross-sectional view shows the ends of waveguides 304 and 324 terminating at the wall of the channel 310. The figure also shows the droplet 330 of liquid at the end of waveguide 324. The upper and lower surfaces 352, 354 of the channel 310 are provided with a dielectric coating, preferably a low surface energy, anti-wetting dielectric coating, such as PTFE or an alkyl silane, as described in U.S. Provisional Application No. 62/393,473, incorporated herein by reference. A ground electrode 356 is provided below the lower surface 354 and above a lower substrate layer 358. A number of individually addressable electrodes 360a, 360b, 360c, 360d, 360e may be positioned above the channel 310, in a cover layer 362.

The droplet 330 of liquid may be made to move via an electro-wetting force applied via the electrodes. The application of an electric field to an electro-wetting liquid reduces its surface energy. If the electric field is applied asymmetrically to only one side of a droplet of the liquid, the surface energy of that part of the droplet exposed to the electric field is reduced, resulting in the liquid droplet flowing to the side of the droplet of the applied electric field. Thus, the liquid droplet can be moved via sequential application of an electric field to electrode 360c, then electrode 360d and then electrode 360e. FIG.

3D shows a cross-sectional view of the resulting switch configuration, where the liquid droplet 330 has been moved from a first position at waveguide 324 to a second position at waveguide 304. A plan view of the switches 300, 320 in this second configuration is shown in FIG. 3C. The liquid droplet 330 is at the crosspoint of the first switch so the light 314 passes from the input waveguide 304 through the droplet 330 into the first output waveguide 306, while the light 334 is totally internally reflected at the wall of the channel 310 and along the second output waveguide 328.

It should be understood that the angle a between the input and second output waveguide may be selected to be any suitable angle, depending on various factors including, but not restricted to, the refractive indices of the waveguides, the waveguide numerical aperture, the refractive index of the liquid material and manufacturing tolerances. The value of a is 90° in the illustrated embodiment, but values smaller or greater than this value may also be selected that result in total internal reflection when the TIR optical switch is in the reflective state and transmission through the liquid material when it is present at the crosspoint.

Different liquids may be used as index-matching liquids in the microfluidic channels of the optical switches discussed above with respect to FIGs. 1-3. For example, when a droplet 330 of liquid material is moved via the application of an electro-wetting force, a liquid such as hydroxypropylene carbonate or propylene carbonate may be used. Additional liquids that may be used include preferably polar organic compounds such as methanol, ethanol, and other alcohols, ethylene glycol and propylene glycol, methyl formamide, or formamate, as discussed in U.S. Provisional Patent Application No.

62/393,463, titled“Liquids For Use With Electro-Wetting On Dielectric Active Optical Switch,” filed on September 12, 2016, and incorporated herein by reference.

Optical chips may contain multiple optical switches, such as the EWOD switch or TIRW switches discussed above. For example, an optical chip may contain a 16 x 16 or a 8 x 8 matrix of switches. An embodiment of optical chip that uses a 4 x 4 matrix of switches on a substrate 400 is schematically illustrated in FIG. 4. The substrate 400 contains a switching network 402 formed using a number of EWOD-activated coupler switches 404, input waveguides 406, interconnecting waveguides 408 and 410 and output waveguides 412 and 414. In some situations, output waveguides 412 may be used for test purposes with output waveguides 414 being used as device outputs. The switches 404 are coupled together using interconnecting waveguides 408 and 410 to form a switching network 402 configured as a cross-bar network. In this type of network, the switches 404 are arranged in rows and columns. There are two types of interconnecting waveguides, viz. the row interconnecting waveguides 408 that connect from the output of one switch 404 to the input of an adjacent switch 404 in the same row, but a different column, and the column interconnecting waveguides 410 that connect from the output of one switch 404 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 404, input waveguides 406, interconnecting waveguides 408 and 410 and output waveguides 414 may be designated according to their row and column in the network. For example the input waveguide 406 on the third row down, row C, is designated input waveguide 406C. The switch on the third row down, row C, and the second column across, column b, is designated switch 404Cb. The row interconnecting waveguide 408 on the third row, row C, that connects from the second switch in the row, switch 404Cb, to the third switch in the row, switch 404Cc, may be referred to as row interconnecting waveguide 408Cb. The column interconnecting waveguide 410 on the second column, column b, that connects from the third switch in the column, switch 404Cb, to the fourth switch in the column, switch 404Db, may be referred to as column interconnecting waveguide 4lOCb. The output waveguide 412 on the third row down, row C, may be designated as test waveguide 412C, while the output waveguide on the second column, column b, is designated as output waveguide 414b. The illustrated embodiment of 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 of vice versa, for example 4 x 8 or 8 x 4.

In other embodiments, each optical switch 404 is in the cross state unless the liquid droplet activates it into the bar state. In these embodiments, when there is only one droplet per column, the position droplet determines which output waveguide 406 is connected to which output waveguide 414. For example, if a droplet is located at optical switch 404Ac, resulting in optical switch 404Ac being in the bar state, then the signal input at waveguide 406A is connected to output waveguide 4l4c. Likewise, if there is a droplet located at optical switch 404Dd, then optical switch 404Dd is in the bar state, and the signal input at waveguide 406D is connected to output waveguide 4l4d.

One embodiment of a microfluidic network 500 that might be used to activate a switch network 402 is schematically illustrated in FIG. 5. The microchannel network 500 is formed over the substrate 400. A reservoir 502 is provided as a store for the electro- wetting liquid. A main microchannel 504 leads from the reservoir 502 to row

microchannels 506, where each microchannel 506 is associated with a respective row of EWOD -activated optical switches 404. Each row microchannel 506 is provided with a number of access channels 508 that permit the droplet of electro-wetting liquid access to the region above respective optical switches 404. In some embodiments, the EW liquid is used along with a second liquid, and an escape channel (not shown) may be provided between an access channel 508 and a row microchannel 506 to permit the second liquid to flow out of the access channel 508 when the EW liquid droplet enters the access channel 508. A system of electrodes, not shown, may be used to bud a droplet of the electro- wetting liquid from the reservoir 502 and deliver it via the main microchannel 504 and a selected row microchannel 506 and access microchannel 508 to a selected optical switch 404. When a droplet 510 has been delivered via an access channel 508 to a selected optical switch 404, in a specific row, the droplet may be backed out from the access channel 508 to the row microchannel 506 and delivered via electro-wetting forces to another access channel 508 in the same row. In an illustrative example, the figure shows a droplet 510 (solid line) at access channel 508Cb, corresponding to optical switch 404Cb. By selective application of specific voltages to the electrodes of the microchannel network 500, the droplet 510 may be moved to another access channel 508, such as access channel 508Cd, (where the droplet 510 is illustrated in dashed lines). FIG. 6 schematically illustrates the microchannel network 500 (solid lines) overlying the EWOD-activated optical switch network 402 (dashed lines), showing how the various access channels correlate with their respective optical switches.

One of the parameters that may be useful for controlling a microfluidically- controlled optical switch is the position of the liquid droplet. In many embodiments, this cannot be done optically because the liquid channel is enclosed. An approach to determining the liquid droplet position discussed herein is based on a capacitance measurement between the electrodes of the electro-wetting fluid driving mechanism system. These measurements are possible because of the difference in the relative permittivity of the different fluids present in the fluid microchannel. The liquid droplet that is used to drive the switching mechanism typically has a relatively high polarity and high relative permittivity, whereas the filler fluid (liquid or gas) typically has a lower polarity and lower relative permittivity. Thus, as the liquid droplet moves within the channel, the capacitance between electrodes, for example measured between one of the addressable electrodes and the common electrode, also changes.

FIG. 7 schematically illustrates an embodiment of a control system 700 for the microfluidic system 702 that may be used to control an array of optical switches as described above. The control system 700 is coupled to the microfluidic system 702, which contains a number of electrodes for controlling the motion of one or more liquid droplets in relation to a number of optical switches. In the illustrated embodiment, one conductor 704 couples between the control unit 700 and a single electrode in the microfluidic system 702. Accordingly, in such an embodiment there is one conductor 704 provided for each electrode in the microfluidic system. This is not a requirement, and in other embodiments, electrodes may share a conductor 704 via a multiplexing system.

The control system 700 includes an electrode activation unit 706, which selectively applies a voltage to one or more specific electrodes in sequence in the microfluidic system so as to move a liquid droplet from one position to another. The control system 702 also includes a droplet position detection unit 710 that is used to capacitively probe the electrodes in the microfluidic system 702 for determining the droplet location.

The droplet position detection unit 710 may have to address many different electrodes, for example in an optical chip having multiple optical switches, as exemplified in FIGs. 4-6. Consequently, the control system may also include a multiplexer unit 712 that connects between the droplet position detection unit 710 and selected electrodes in the microfluidic system 702. In such a case, the multiplexer unit 712 may include multiplexer integrated circuits that have low charge injection, for example less than 20 pC, and with low channel to channel leakage.

FIG. 8 schematically illustrates a cross-section through part of an optical switch 800 that includes the microfluidic system. The cross-section shows the fluid channel 806 between the upper structure 808 and the lower structure 810. The upper structure 808 is provided with a common electrode 826, which may be separated from the channel 806 by an anti-wetting layer 816 on top of the upper dielectric layer 812. The lower structure 810 is provided with addressable electrodes 828a, 828b, and may be separated from the channel 806 by an anti-wetting layer 818 and the lower dielectric layer 814. The figure also shows a liquid droplet 802 in the channel 806.

The capacitance of the structure shown in FIG. 8 may be estimated using the approximations shown in FIG. 9, where the droplet 902 is assumed to have flat sides in the vertical direction, rather than curved sides. The common electrode 826 and one of the addressable electrodes 828b are assumed to be electrically tied to ground, while the other addressable electrode 828a is held at a potential of V. In another embodiment the common electrode 826 can be left floating. In still another embodiment the common electrode 826 may not be present. The droplet 902 is assumed to overlap the addressable electrode 828a by a distance of x. The distance dl is the distance separating the droplet 902 from the addressable electrodes 828a, 828b and, in the illustrated embodiment is equal to the thickness of the anti-wetting layer 818 and the dielectric layer 814. The distance d2 is the distance separating the droplet 902 and the common electrode 826 and, in the illustrated embodiment, is equal to the thickness of the anti -wetting layer 816 and the dielectric layer 812.

The droplet 902 is considered to be conductive for ac currents, because it is formed of a polar material. Accordingly, the first capacitance, Cl, is the capacitance between that part of the droplet 902 overlapping the addressable electrode 828a, having a length x, and that part of the addressable electrode 828a overlapped by the droplet 902, and is indicated by the volume delineated by the bold rectangle marked Cl. The second capacitance, C2, is the capacitance between that part of the droplet 902 overlapping the addressable electrode 828b, and that part of the addressable electrode 828b overlapped by the droplet 902, and is indicated by the volume delineated by the bold rectangle marked C2. The third capacitance, C3, is the capacitance between the droplet 902 and that part of the common electrode 826 overlapped by the droplet 902, and is indicated by the volume delineated by the bold rectangle marked C3. Accordingly, the capacitance between the voltage source, at voltage V, and ground can be approximated by the circuit shown in FIG. 10 A, where the values of Cl, C2 and C3 represent those capacitances defined above in FIG. 9.

A schematic plan view of the droplet within the channel is presented in FIG. 10B, showing the droplet 902 within the channel 806, overlapping the two addressable electrodes 828a, 828b. The area Al is that area of the droplet 902 that overlaps the activated electrode 828a and the area A2 is that area of the droplet 902 that overlaps the grounded electrode 828b. A3 is the total area of the droplet 902 in the plan view, which is the area of the droplet that overlaps the grounded electrode 826.

Making the assumption that the capacitance, C, can be calculated from the parallel plate formula, C = kA/d, where k is the permittivity of the material between the plates, A is the overlapping plate area and d is the plate separation, the values of Cl, C2, C3 can be calculated as follows:

Cl = kl Al(x)/dl,

C2 = kl A2(x)/dl, and

C3 = kl A3/d2,

where kl is the permittivity of the dielectric layers 812 and 814, Al(x) is the area of the droplet that overlaps the activated electrode 828a, which is a function of the overlap distance x, and A2 is the area of the droplet that overlaps the grounded electrode 828b, which is also a function of the overlap distance x. Thus, both Cl and C2 are dependent on the overlap distance, x, where Cl increases with increasing x and C2 decreasing with increasing x. The value of C3 is independent of the transition distance, x, because the area of the droplet 902 overlapping the grounded electrode 826, A3, remains constant in this model, regardless of its position.

The capacitance of the volume outside the droplet 902 can be ignored. Those areas of the electrodes covered by the nonpolar ambient fluid, rather than the polar liquid droplet 902, form a capacitor across the channel 806. Therefore, the capacitance of this volume has an effective interplate separation of d3, which is significantly greater than the values of dl and d2. Accordingly, the value of this capacitor is significantly smaller than Cl, C2 and C3, and can be ignored.

A liquid droplet may be moved via the application of a static (DC) voltage, or more usually by the application of a chopped DC voltage of the order of a few lOs of Volts, e.g. around 50 V. As the liquid droplet moves along the microchannel, the capacitance across the electrodes also changes. By measuring the capacitance across the fluid channel, after removing the voltage applied to move the droplet, the relative magnitude of the capacitance can be determined to give the droplet’s position.

In the simple capacitance model shown in FIG. 10 A, the capacitance, Ct(x), between the applied voltage source, V, and ground is given by:

Ct(x) = Cl(x)(C2(x) + C3)/(Cl(x) + C2(x) +C3)

Thus, after calibration, a measurement of Ct(x) can be used to calculate the position of the edge of the droplet, x. The electrode has a width, w, measured in the direction of droplet travel. A typical liquid droplet, for example a droplet of

hydroxypropylene carbonate (HPC), around 900 pm in length in a 450 pm wide channel can change the capacitance by around 5 pF - 10 pF, although it should be understood that the change in capacitance is dependent, at least in part, on the size of the droplet and the type of liquid used.

A first example of a circuit 1100 that may be used in a droplet position detection unit, to make capacitance measurement across electrodes in an EWOD optical switch cell, is schematically illustrated in FIG. 11. A pulse generator 1102, having a known peak voltage amplitude, for example around 10 V, is coupled across the target capacitor 1104 for measurement. In the illustrated example, the pulse generator 1102 is connected across C3 of an electrode structure, but it may also be connected across Cl or C2. The other side of the target capacitor 1104 is connected to the input of an operational amplifier (op-amp) circuit 1106. The op-amp circuit 1106 is based on an LTC7652 Chopper OpAmp 1108, for example as is obtainable from Linear Technology Corp, Milpitas, California. The op- amp circuit 1106 may be protected by a protection circuit 1110 that uses, for example, an LT1010 operational amplifier 1112, to reduce the possibility of accidental over-driving of the Chopper OpAmp 1108 due to excessive input currents. All other components of the circuit listed in the diagram of the circuit 1100 are standard electronic components, known to one of ordinary skill.

The voltage pulses applied to the target capacitor 1104 inject an AC current whose value is proportional to the capacitance being measured. This current can be measured using the op-amp circuit 1106, which generates output voltages of lmV/pF and 10 mV/pf, respectively, using the outputs 1114 and 1116 for the circuit 1100.

Such an approach permits accurate capacitance measurements to be made with a resolution of about 100 fF. Thus, the working noise floor of the system is around 100 fF. This is an acceptable resolution when compared to the expected maximum signal of around 6 pF, i.e. a maximum signal to noise ratio of around 60: 1. Since the resolution of the capacitance measurement system is a fraction of the total capacitance of the droplet, the system is able to detect what fraction of the electrode is covered by the droplet, and thus determine the position of the droplet with a precision less than the electrode width, w.

By measuring the capacitance values over multiple adjacent electrodes both the position and the size of the droplet can be measured. This will be facilitated if the droplet covers multiple electrodes.. For example, the change in capacitance of the electrodes 828a and 828b in FIG. lOb is related to the areas of these electrodes 828a, 828b covered by the droplet 902, related to x and x’ respectively. By measuring the capacitance of adjacent electrodes it is, therefore, possible to calculate the amount by which each electrode is covered by the droplet and hence determine both the position and size of the droplet. For example, this may be useful to check that the size of a droplet, that has been budded out of a reservoir, lies within an acceptable range of sizes.

The capacitance measurement can also be used to monitor the droplet liquid quality over time. In case the polar liquid would mix over time with the non-polar ambient liquid, its effective permittivity will change and this can be measured with the capacitance measurements.

Another circuit 1200 that may be used in a droplet position detection unit, for measuring the capacitance of an EWOD cell, is schematically illustrated in FIG. 12. This circuit 1200 is advantageously well-suited to measuring the electrode capacitance in the presence of different parasitic capacitances that may be present in an optical switch array that contains multiple optical switches, each optical switch having an attendant number of electrodes.

The circuit 1200 may use a high precision, leakage compensated Switch Capacitor integrated circuit (IC) 1202, an LTC6943 chip available from Linear Technology Corp, Milpitas, California, which has multiple switched capacitor sections, enabling a small footprint solution. Low leakage, of the order of picocoulombs, is one feature of the circuit that permits measurement of the low capacitances characteristic of EWOD optical switch measurements, i.e. a few pF.

A precision reference signal, which may be a square wave, is generated in the IC

1202 and applied to node A by switching a precision temperature compensated voltage reference, Vref, (typically 2.5 V) between the reference voltage and a ground potential.

The reference voltage is generated from IC2 1204, an LT17902B micropower SOT-23 low dropout chip available from Linear Technology Corp., and buffered by IC13 1206, an LT1013 dual precision op-amp chip also available from Linear Technology Corp. The frequency of the chopping waveform is set, typically at 500 Hz, by a capacitor 1208, although the exact value of the chopping rate may differ from this value. An opposite polarity voltage signal is applied to node B. Node B is connected to a reference capacitance Cref and the unknown (EWOD) capacitance, Cx, is connected to node A. Node C is a summing point/middle point of the capacitors. The paths from Cref and Cx may be split physically into two paths cl, c2 on the printed circuit board and become common at the input to a multiplexer, as is discussed below. FIG. 13 schematically shows how the capacitances are connected to nodes A, B and C and the split paths cl, c2.

The center point return, node C is demodulated by a switch in the IC 1202 that is synchronous with the precision reference voltage signal. The demodulated output is directed to an op-amp 1210 that converts the capacitance current to an output voltage, Vout. In one embodiment, the op-amp 1210 may be a component of an LT1013 op-amp, obtainable from Linear Technology Corp., Milpitas, California. In one embodiment, the output current is integrated in the op-amp 1210 and fed back to the capacitor being measured, Cx, as a servo output by an integration capacitor 1212. The integration capacitor 1212 may be a 1.2 nF CGA series capacitor, available from TDK Corp., Tokyo, Japan. A second stage 1214 of the op-amp 1212 may be used to apply positive gain and scale directly to the full scale input 1216 of an analog-to-digital converter 1214 of a subsequent digital signal processor. Faster settling times may be achieved by increasing the chopping frequency of the reference signal, but this may also result in a decrease the common-mode rejection ratio (CMRR). It is advantageous to maintain the CMRR above around 100 dB.

The capacitance measurement circuit may be used to detect the position of several different liquid droplets. For example, the multiplexer unit 712 may sequentially couple the capacitance measurement circuit to different electrodes associated with a first optical switch to determine the location of a liquid droplet associated with the first optical switch, and then may sequentially couple the capacitance measurement circuit to different electrodes associated with a second optical switch to determine the location of a liquid droplet associated with the second optical switch. In this manner, the capacitance measurement unit of the droplet position detection unit 710 may be sequentially connected to all the optical switches of the microfluidic system 702 to determine the location of each liquid droplet associated with each optical switch. The circuit described above with regard to FIG. 12 is a self-balancing/servo design, having a primary function to keep point C at zero AC voltage. The overall circuit is a ratio-metric design and the relationship between Cx and Cref produces a voltage that is dependent only on ratio of these capacitances and the Vref voltage. Thus, the output voltage, Vout, is given by:

Vout = (Cx/C ref)/ Vref.

A more realistic approximation of the capacitances encountered in an actual implementation of the capacitance measurement circuit considers the parasitic capacitance contributions of the various circuit components, such as the printed circuit board, the connectors and their interfaces, and the like. An example of such a consideration is shown in FIG. 14, where Cref and Cx are the reference and unknown capacitances, as before, and C4-C8 represent various parasitic capacitive contributions. C5 and C6 represent parasitic capacitances arising from objects such as metal work and ground planes and the like, and can vary from zero pF to lOs of pF. C4 and C7 represent shunt capacitances, in other words capacitances arising from plastic connectors and PVC ribbon cables and the like, and can vary from around 2 pF to around 10 pF per connector, when using an IDC connector. C8 represents the overall shunt crosstalk across the bridge network.

When implementing the droplet position measurement unit it is advantageous to maintain the phantom and parasitic elements of the circuit at similar levels on either side of the input, i.e. to maintain the phantom and parasitic capacitances across the Cref side of the circuit to be approximately the same as those across the Cx side of the circuit. In other words, it is preferred that C4 + C5 is approximately the same as C6 + C7. This requires considerations of the layout of the printed circuit board, connector interfaces, cable designs and the like to maintain parasitic equality to the measurement signal paths A, B and C and cl and c2, described below. If the design is correctly symmetrical, then the servo action of the bridge will self-balance to an adaptive reference point, within limits. Too great an imbalance will shift the operating point into a potential saturation operating condition.

As was discussed above, the typical capacitance in an EWOD cell, Cx, is around 6 pF, and the noise floor is around 100 fF. In the circuit described above, the integration time is around lOOms. This integration time is not perceived to be a problem in the operation of an EWOD optical switch, as the time to move a liquid droplet from a starting location to a final location is typically a few seconds in duration. When remotely monitoring an EWOD optical switch array, the Cref capacitor is preferred to be as close as possible to the Cx capacitor. In some embodiments, the reference capacitor may be positioned next to the EWOD cell and may be incorporated as an interdigitated capacitor or a real surface mount capacitor or embedded on or in the interconnected PCB forward assembly. The reason for this is because the error in the ratio equation increases as the Cref capacitor is moved back towards the measurement circuit, further away from the Cx capacitor.

The 3 -wire interface may be split to a 4-wire interface to gain access to an independent Cref path. FIG. 13 shows an embodiment having separate wiring that is output from the circuit to dedicated Cref pins on the connector i.e. B - c2. The signal path B, to Cref, does not pass through the multiplexor unit. Instead, it bypasses the multiplexor unit so that the same Cref is used for all capacitance measurements. The separate paths cl and c2 may be commoned at the input to the multiplexor unit, which is typically mounted on a printed circuit board.

The circuit discussed above was evaluated in a test bed using a number of test capacitances over a range of 0 - 10 pF. The voltage output from the circuit, Vout, is shown in the graph in FIG. 15, plotted as line 1502. A linear fit is shown as line 1504 for comparison. As can be seen, the output from the circuit is approximately linear, which will result in the position of the liquid droplet in the fluid channel being discernable.

Example

The circuit discussed above with regard to FIG. 12 was used for measuring the capacitance in the presence of a polar liquid droplet, HPC, and for distinguishing the measured capacitance of a non-polar liquid, diphenyl sulfide (DPS). With no liquid in an EWOD cell, the background values for a given pair of electrodes varied between 0.5 pF and 5 pF. The background capacitance values were attributable to the structure of the interconnect and the non-ideal position of the calibration capacitor. When an HPC droplet was added across an electrode pair having a background capacitance of 5 pF, the resulting measured capacitance was 12 pF, an increase of 7 pF which was easily measurable. This increase in capacitance was also found to be repeatable. When the HPC droplet was placed over an electrode pair having a low baseline capacitance, 0.5 pF, the increase in capacitance was even larger, around 9 pF. When the HPC droplets were replaced with DPS droplets, the measured capacitance values fell back to the background level. Thus, the presence and location of HPC droplets can be verified and distinguished over a DPS ambient fluid using a capacitive approach. Moreover, it was confirmed that the droplet capacitance value is sufficiently high and the system measurement error sufficiently low that the system is able to detect what fraction of the electrode is covered by the droplet, and thus determine the position of the droplet to within a fraction of the electrode dimension.

As noted above, the present invention is applicable to fiber optical communication and data transmission systems. 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. 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.

Claims

Claims What we claim as the invention is:

1. A method, comprising:

providing an optical switch, the optical switch comprising at least a droplet of a first liquid within a fluid channel, a state of the optical switch being changeable by movement of the droplet of first liquid within the fluid channel, the optical switch further comprising a plurality of electrodes disposed proximate the fluid channel;

measuring capacitance of at least one electrode of the plurality of electrodes, the at least one electrode being proximate the droplet of first liquid; and determining a position of the droplet of first liquid based on the capacitance measurement.

2. A method according to claim 1, wherein the optical switch is a totally internally reflecting waveguide optical switch.

3. A method according to claim 1, wherein the optical switch is an electro- wetting on dielectric (EWOD) coupler switch.

4. A method as recited in claim 1, further comprising applying a first alternating reference voltage to the at least one electrode and detecting a current that depends on the capacitance value of the at least one electrode.

5. A method as recited in claim 4, further comprising applying a second alternating reference voltage, having a polarity opposite to the polarity of the first alternating reference voltage, to a reference capacitor.

6. A method as recited in claim 4, further comprising connecting a selected electrode of the optical switch to the first alternating reference voltage via a multiplexer unit and measuring capacitance of the at least one electrode comprises measuring capacitance of the selected electrode.

7. A method as recited in claim 1, wherein the at least one electrode has a width, w, measured in the direction of droplet travel, and wherein determining the position of the droplet comprises determining the position of the droplet with a precision of less than w.

8. A method as recited in claim 1, further comprising determining a size of the droplet of first liquid based on the extent to which capacitance of the at least one electrode is changed by the droplet.

9. A method as recited in claim 1, wherein the plurality of electrodes are configured to apply an electric field to the droplet of first liquid so as to move the droplet of first liquid via a resultant electro-wetting force.

10. An optical switching system, comprising:

an optical switch comprising at least one waveguide, a fluid channel proximate the at least one waveguide and a droplet of a first liquid within the fluid channel;

a set of electrodes located proximate fluid channel;

a droplet position detection unit comprising a circuit couplable to electrodes of the set of electrodes for capacitive detection of the position of the droplet of first liquid relative to the set of electrodes.

11. An optical switching system as recited in claim 10, further comprising a multiplexer unit, the circuit of the droplet position detection unit being couplable to a selected electrode of the set of electrodes through the multiplexer unit.

12. An optical switching system as recited in claim 10, wherein the circuit of the droplet position detection unit comprises a first AC reference voltage signal generator that generates a first reference AC voltage signal, couplable to a selected electrode of the set of electrodes.

13. An optical switching system as recited in claim 12, wherein the circuit of the droplet position detection unit comprises a second AC reference voltage signal generator that generates a second reference AC voltage signal having a polarity opposite the polarity of the first reference AC voltage signal, the second reference AC voltage signal being coupled to a reference capacitor.

14. An optical switching system as recited in claim 10, wherein the optical switch is an electro-wetting on dielectric (EWOD) coupler switch.

15. An optical switching system as recited in claim 10, wherein the optical switch is a totally internally reflecting waveguide optical switch.

16. An optical switching system as recited in claim 10, wherein the one or more electrodes has a width, w, measured in the direction of droplet travel and the droplet position detection system is capable of determining the position of the droplet of first liquid with a precision less than w.

17. An optical switching system as recited in claim 10, wherein the droplet position detection unit further measures droplet size based on capacitance change of one or more electrodes of the set of electrodes.

18. An optical switching system as recited in claim 10, further comprising an activating electrical circuit connected to the set of electrodes to selectively apply a voltage signal to one or more of the electrodes to change a position of the droplet of first liquid within the fluid channel.