WO2015022027A1

WO2015022027A1 - Fluidically controlled optical router

Info

Publication number: WO2015022027A1
Application number: PCT/EP2013/067033
Authority: WO
Inventors: Daniel Kopp; Philipp Müller; Hans Zappe; Andreu LLOBERA; Jordi VILA-PLANAS; Xavi MUNOZ BERBEL
Original assignee: Albert-Ludwigs-Universität Freiburg; Centre Nacional De Microelectronica
Priority date: 2013-08-14
Filing date: 2013-08-14
Publication date: 2015-02-19

Abstract

The present invention relates to a system and method for input and output optical channel selection. The channels are selected by mirrors generated in specifically structured regions, resulting from selective filling of certain areas with a fluid. In particular, an optical router for switching at least one incident radiation beam towards at least one output, comprises at least one input radiation channel for receiving said radiation beam, at least one switching unit for changing the optical path of the radiation beam, at least one output radiation channel for outputting the deflected radiation. Said switching unit comprises a control unit and at least one fluid/fluid interface mirror that is deformed under the influence of the control unit to deflect the radiation towards a selected output radiation channel, said switching unit further comprising at least one phaseguide structure for mechanically aligning said fluid/fluid interface mirror.

Description

FLUIDICALLY CONTROLLED OPTICAL ROUTER

The present invention relates to a system and method for input and output optical channel selection. The channels are selected by mirrors generated in specifically structured regions, resulting from selective filling of certain areas with a fluid. Specific management of the fluidic filling and emptying may be accomplished by changes in pressure, flow, or volume. The system has a wide dynamic range of wavelengths, this allows that the system works in multiple zones of a spectrum simultaneously.

Optical routers have been investigated for many different applications, but mostly in the field of telecommunications. In telecommunications there exist two main groups of routers:

(i) routers requiring a signal transduction from optical to electrical and then to optical again (OEO) and

(ii) all-optical routers.

Both types of routers have to work at high switching frequencies so as to provide data transmission rates of Tb/s (see e. g. White, I., Penty, R., Webster, M., Chai, Y. J., Won- for, A., IEEE communications Magazine, 2002, 74-81 ). However, telecommunication routers are expensive, fragile, with a small range of wavelength, being 101 nm in the best case (White, I., Penty, R., Webster, M., Chai, Y. J., Wonfor, A., IEEE communications Magazine, 2002, 74-81 ), and are focused to work around a central 1550 nm wavelength, which is the basic working wavelength in telecommunications.

The configuration and architecture of the above-mentioned routers have been intended to be used only in telecommunications. However, there exist other applications where redirectioning of a given light beam is required, for instance the parallel screening of multiple cuvettes using white light. Here, the switching speed and the working wavelengths generally differ from those used in telecommunication routers.

An overview over known optical switches and their drawbacks will be given in the following.

Commercially available systems employ optoelectronic transponders based on semiconductor optical amplifiers (SOAs) (see for example White, I., Penty, R., Webster, M., Chai, Y. J., Wonfor, A.: Wavelength switching components for Future Photonic Net- works. IEEE communications Magazine, 40(9): 74-81 , 2002). Concepts for all-optical switching (for example based on wavelength converters and arrayed waveguide gratings (AWG)) are investigated within the scientific community. Typical requirements for telecommunication switches are: high speed, a limited wavelength range (typically a few tens of nm, but generally < 101 nm), designed to work around 1550 nm wavelength, low power consumption, and a cascadable structure.

Typical performance that has to be reached is a switching frequency of up to GHz and a data transmission rate up to Tb/s.

Since optical routers for telecommunications are limited in the range of wavelength that can be routed, other types of routers have been developed, in particular to be able to use a broad wavelength range in the visible region. Examples of such broad-band routers are Thorlabs OSWxx-yyyyE series (http://www.thorlabs.de/newgroup- page9.cfm?objectgroup_id=4336 (last checked: 2013-07-03)). These routers can be operated as a lossless, high-speed, optical isolation switch, a full-range variable optical attenuator (VOA), or an optical shutter. The OSWxx-yyyyE is not centered on a single wavelength but the wavelength must be larger than 480 nm. The wavelength working range is approx. 200 nm. The switching frequency is 1 -2 kHz, the cost is around 1 k€. This known router is difficult to integrate into other systems.

The MPM-2000 Ocean Optics Optical Multiplexer (http://www.oceanoptics.com/Prod- ucts/mpm2000.asp (last checked: 2013-07-03)) has a wavelength working range of 250 nm - 800 nm or 350 nm - 2000 nm. Due to its large size it is difficult to integrate and furthermore has a slow switching frequency (6.5 Hz).

These state-of-the-art broad-band routers are still bulky systems, which are difficult to integrate with other Micro-Opto-Electro-Mechanical Systems (MOEMS). In particular for photonic lab-on-a-chip (PhLoC) applications, it would be desirable to have a robust (no moving mechanical elements) router at hand, capable of selecting optical channels, and working in a wide range of wavelengths from visible to near IR.

As laid out in this invention, optofluidic technology is an alternative approach fo realizing an optical router. In recent years, optofluidic technology, a new paradigm for micro- optics, has developed from a niche known to a small number of academics to a technology with broad impact on different fields. Optofluidic devices are optical devices which exploit the interaction of light with a fluid (mostly liquids but also gases) to achieve a specific optical function. Currently, three major fields of applications of optofluidic devices can be identified: (i) photonic lab-on-a-chip (PhLOC), for example micro-scale sensors based on fluorescent, absorption, or spectral analysis of a miniature liquid sample,

(ii) tunable optofluidic devices, for example tunable liquid lenses, tunable irises, tunable liquid mirrors, optofluidic switches, electrowetting displays etc. Such devices have been applied to compact cameras, cell phones, medical endoscopes, dental video cameras, bar code scanners, or ophthalmic devices, and

(iii) displays.

Photonic lab-on-a-chip systems (PhLOC) are miniaturized biochemical laboratories shrinked to a cm-sized chip. The long term vision of PhLOC is to integrate all analytical steps from sample handling to detection into a single system. Using light for interrogating the analyte, which is especially interesting in the field of cellular biology, requires light-routing functionality on-chip. In particular, it is desirable to be able to address several optical sensing channels with one common input light source, or to collect the light from several channels into one common optical output (typically a fiber or a lightguide).

Generally, tuning mechanisms for optofluidic devices include but are not limited to flu- idic exchange of one liquid by another, adjusting liquid flow (very often in the laminar regime), electro-optical effect, optical control (e.g. optical tweezers), thermal actuation, electrowetting-on-dielectrics (EWOD), dielectrophoresis (DEP), pressure, external forces applied via membranes, and inertial forces.

The developments described above are mainly driven by the new functionality offered by optofluidic technology, which can often not be achieved on a similar size scale using classical optics (based on solid materials such as glass or polymers). There is also an ongoing trend for miniaturization in optics: digital cameras, medical endoscopes, biomedical sensors, mobile devices, etc., and new technologies such as electrowetting displays are being commercialized.

Other than the mirrors described in this invention, state-of-the art mirrors are based on highly reflective metallic surfaces or dielectric stacks. There also exist mirrors based on total internal reflection (TIR) at solid-gas interfaces. For example in Llobera, A., S. Demming, R. Wilke, and S. Buttgenbach: Multiple internal reflection poly(dimethylsiloxane) systems for optical sensing. Lab on chip, 7:1560-1566, 2007, fixed, curved mirrors produced by air pockets within a transparent polydimethylsiloxane (PDMS) bulk were presented. Furthermore, there exist rotating liquid mirrors (first conceived by Isaac Newton, first realized in 1872), which are used in so called liquid mirror telescopes for astronomy. Most commonly, mercury is used as the liquid (other liquids used are low melting alloys of gallium), which provides a highly reflecting metallic surface. By rotating the container in which the liquid is hold, the liquid is forced into a paraboloidal shape, which is the perfect shape for a focusing mirror.

In Wan, Zhiliang, Hongjun Zeng, and Alan Feinerman: Area-tunable micromirror based on electrowetting actuation of liquid-metal droplets. Applied Physics Letters, 89:201 107 (3pp), 2006, a flat circular micro-mirror tunable in its area by EWOD was demonstrated. The radius of the mirror was tuned from 13 μηι to 88 μηι by deforming a liquid metal droplet in contact with a flat cover glass.

The publication Bucaro, Michael A., Paul R. Kolodner, J. Ashley Taylor, Alex Si- dorenko, Joanna Aizenberg, and Tom N. Krupenkin: Tunable liquid optics: electrowet- ting-controlled liquid mirrors based on self-assembled janus tiles. Langmuir, 25:3876 - 3879, 2009, shows a curved concave micro-mirror that was built up from hundreds of hexagonal micro-mirror plates, self-assembled on a deformable surface of an oil droplet. The droplet shape was deformed by electrowetting-on-dielectrics (EWOD) using a transparent, conductive, but insulated cover glass, which allowed tuning of the mirror's focal length.

Optofluidic technology has also been applied for achieving optical switching functionality. A large number of optofluidic switching concepts have been demonstrated based on pneumatic actuation of membranes (Campbell, K., A. Groisman, U. Levy, L. Pang, S. Mookherjea, D. Psaltis, and Y.Fainman: A microfluidic 2x2 switch. Applied Physics Letters, 85f(25): 61 19-6121 , 2004., Song, W. and D. Psaltis: Pneumatically tunable optofluidic 2 x 2 switch for reconfigurable optical circuit. Lab on a chip 1 1 (14), 2397- 402, 201 1 ), liquid filling of cascaded micro-prisms (Seow, Y.C., S. P. Lim, H. P. Lee: Micro-light distribution system via optofluidic cascading prisms. Microfluid Nanofluid 1 1 :451 -458, 201 1 ) or blazed diffracting gratings (Groisman, A., S. Zamek, K. Campbell, L. Pang, U. Levy, and Y. Fainman: Optofluidic 1 x4 switch. Optics Express 16 (18), 13499-508, 2008), adaptable liquid-core-liquid cladding waveguides (Wolfe, D. B., R. S. Conroy, P. Garstecki, B. T. Mayers, M. a. Fischbach, K. E. Paul, M. Prentiss, and G. M. Whitesides. Dynamic control of liquid-core/liquid-cladding optical waveguides. Proceedings of the National Academy of Sciences of the United States, Lim, J.-M., J. P. Urbanski, T. Thorsen, and S.-M. Yang: Pneumatic control of a liquid-core/liquid- cladding waveguide as the basis for an optofluidic switch. Applied Physics Letters 98 (4), 044101 , 201 1 , Chung, A. J., and D. Erickson: Optofluidic waveguides for recon- figurable photonic systems. Optics Express, 19(9): 8602-8609, 201 1 ), hydrodynamic spreading (Nguyen, N.-T., T.-F. Kong, J.-H. Goh, and C. L.-N. Low: A micro optofluidic splitter and switch based on hydrodynamic spreading. Journal of Micromechanics and Microengineering 17 (1 1 ), 2169-2174, 2007), or photosensitive nematic liquid crystals on gratings (De Sio L, J. G. Cuennet, A. E. Vasdekis, and D. Psaltis: All-optical switching in an optofluidic polydimethylsiloxane: Liquid crystal grating defined by cast- molding. Applied Physics Letters, 62: (3pp), 2010).

As a severe limitation, none of the above concepts offers integrated actuation allowing direct electrical control of the optical switching; many of the above concepts require bulky external pumps for operation. For these reasons, state-of-the-art optofluidic switching concepts have so far not been commercialized.

In the year 2000, an optical switch based on TIR at air bubble to liquid interface was demonstrated (Fouquet, J.: Compact optical cross-connect switch based on total internal reflection in a fluid-containing planar lightwave circuit. Optical Fiber Communication Conference. Technical Digest Postconference Edition. Trends in Optics and Photonics 37(1 ):204-206, 2000). The bubbles were created by micro-heaters positioned at cross- points of a liquid filled network acting as a waveguide. Based on this concept, a 32x32 optical cross-connect switch was implemented with fast switching times in the range of a few milliseconds. This research was performed within Agilent Laboratories, California, USA, but did not result in a commercial product.

Furthermore, various reflectors based on fluid/fluid interfaces are disclosed in EP 2 187 375 A1 , US 6,447,081 B1 and WO 2004/102251 A1 .

Only a few of the above concepts can maintain a switched state for a long period of time (minutes to hours) and none of the concepts mentioned so far can maintain a switched state at low power consumption.

Therefore, there is a need to obtain robust optical routing devices without moving mechanical parts, which are able to select both input and output channel in a quick, inexpensive, continuous, and automatic way, and which are working in a wide range of wavelengths.

The aim of the present invention is to overcome the aforementioned drawbacks by developing an optofluidic, robust, low cost, and multichannel device, capable of selecting the input and output optical channels in a quick, simple, and automatic way, with the ability to be functional within a wide spectral range.

In accordance with this goal, a device has been designed and manufactured that integrates two basic elements, namely:

(1 ) tunable mirrors for routing light, and

(2) fluidic structures for selective and controlled filling of the areas of interest with a fluid.

Concerning light-routing functionality, the necessary beam steering was performed using optical elements based on fluid/fluid interface mirrors (either liquid-gas or liquid- liquid). Specifically, the operation of the router is based on the formation of mirrors based on total internal reflection (TIR). In order to fulfill TIR conditions, it is required to make use of two materials with different refractive index (Rl). Light initially injected in the material with the highest Rl (material A), undergoes TIR if it reaches the material B (with the lowest Rl) with an incidence angle greater than the critical angle. Materials used for this invention have been silicone oil (polydimethylsiloxane - PDMS) and air, although other materials could also be used. Thus, the material A/material B interface forms mirrors where light is reflected.

To generate the mirrors as well as to provide the necessary mobility, the device requires a system suitable to allow its partially filling and emptying. One option is to use phaseguides as shown in the article Vulto, P., Podszun, S., Meyer, P., Hermann, C, Manz, A., Urban, G. A., Lab on a chip, 201 1 , 1 1 , 1596-602, that increase localized fluidic resistance without increasing the manufacture complexity. Thus, the fluid is stopped in selected areas, thereby creating air mirrors. From a fluidic point of view, a pair of hydrophobic/hydrophilic (material A/material C or material C/material A) materials or a step in the topography is required. Due to these properties and the presence of phaseguides an increased fluidic resistance is achieved and therefore, stable mirrors can be obtained.

With the optical router system according to the present invention and the control over where the interfaces are formed, an equivalent of a tunable mirror without moving mechanical parts is formed. The tunable mirror allows the selection of both input and output optical channels but with a much higher level of robustness as compared to any other commercial device. The above working principle gives a great flexibility to the system:

(i) The design of the system can be modified such that the number of input or output optical channels can be increased or decreased.

(ii) The response time can be varied with a modification of the material volume and/or the type of materials, depending on the application.

(iii) The system can easily be integrated with other optical scanning systems.

(iv) It has two distinct working mechanisms, since multiple wavelengths can be coupled to a single output channel (integration) or an initial input signal can be sequentially injected into several output channels (differentiation).

From a fluidic point of view, structures may also be adapted to change the geometry of the phaseguide, resulting in a variation of the mirrors' curvature (for example, converging or diverging mirrors).

The described system has multiple advantages as compared to the already mentioned examples. It is extremely flexible in both design (different numbers of input or output optical channels, mirrors geometries and frequency response) and materials used (from photocurable resins as SU-8 to elastomers such as PDMS and even rigid plastics, PC or PMMA, or glass). To achieve a high efficiency router, it is only required to know two basic properties: (i) TIR conditions, taking into account the materials with which the mirrors are implemented and (ii) hydrophobicity/hydrophilicity conditions of constituent materials (material A / material C or material C / material A) or changes in topography that allow selective filling.

Besides, the proposed system can easily be integrated with other optical analysis or scanning systems. Even greater flexibility can be provided by the use of optical fibers.

As previously described, the system has the advantage of working within a wide range of wavelengths, depending on the materials chosen for manufacturing. These properties make the system unique in the market.

According to an advantageous embodiment of the present invention, an optical router for switching at least one incident radiation beam towards at least one output comprises at least one input radiation channel for receiving said radiation beam, at least one switching unit for changing the optical path of the radiation beam, and at least one output radiation channel for outputting the deflected radiation. The switching unit com- prises a control unit and at least one fluid/fluid interface mirror that is deformed under the influence of the control unit to deflect the radiation towards a selected output radiation channel, said switching unit further comprising at least one phaseguide structure for mechanically aligning said fluid/fluid interface mirror. According to a particular embodiment, said control unit comprises an electric control element having at least one electrode for deforming the fluid/fluid interface by means of electrowetting-on- dielectrics (EWOD).

Such a fully integrated electro-optical router system offers many advantages over conventional systems.

Other than for example optical switches based on semiconductor optical amplifiers (SOAs) White, I., Penty, R., Webster, M., Chai, Y. J., Wonfor, A., IEEE communications Magazine, 2002, 74-81 , the proposed fluid/fluid mirror concept offers a wide wavelength range that can be switched. The wavelength range is only limited by the transmission of the first, high Rl liquid and the condition for TIR at the fluid/fluidmirror (dispersion of refractive indices η,(λ) has to be taken into account).

The fluid/fluid mirrors also offer an exceptionally high optical surface quality, due to the inherently smoothness (sub-nm roughness) of the fluid/fluid interface. This smoothness is obtained from liquid-self-assembly without the need for any sophisticated surface treatment.

Due to the high mobility and deformability of liquids, the fluid/fluid mirror approach allows definition of smooth mirrors with straight or curved shapes. Consequently, light can not only be deflected but also focused.

The fluid/fluid mirrors can be used in conjunction with liquid lightguides or waveguides, which are produced by sandwiching a liquid of high refractive index (e.g. chloronaph- talene, n > 1 .6) between materials of lower refractive index. By using this approach, the fluid/fluid mirrors can be used with μm-sized optical fibers, lightguides, or even waveguides.

As the mirrors have no moving parts, light switching can be accomplished virtually free of wear, resulting in a potentially long life-time of such a device.

The optofluidic concept of using EWOD as the actuation mechanism has the advantages of enabling ultra-compact design of the tunable mirror and compatibility with fiber optics, invulnerability to mechanical shock and vibrations (if the liquid densities are closely matched), and low power consumption (< 5 mW) at moderate drive voltages (50 V to 70 V, at 1 kHz AC).

The accompanying drawings are incorporated into and form a part of the specification to illustrate several embodiments of the present invention. These drawings together with the description serve to explain the principles of the invention. The drawings are merely for the purpose of illustrating the preferred embodiment, and are not to be construed as limiting the invention to only the illustrated and described embodiments; alternative examples of how the invention can be made and used may be easily conceived based on the principles laid out herein. Furthermore, several aspects of the embodiments may form— individually or in different combinations— solutions according to the present invention. Further features and advantages will become apparent from the following more particular description of the various embodiments of the invention, as illustrated in the accompanying drawings, in which like references refer to like elements, and wherein:

Figure 1 shows an overview of the system used in the present invention;

Figure 2 schematically shows the process of filling and emptying of different areas separated by phaseguides;

Figure 3 shows the repeatability of the filling and emptying of the same area five times;

Figure 4 illustrates the working principle of light switching using tunable fluid/fluid mirrors;

Figure 5 shows a schematic cross-sectional view of a liquid of refractive index n_g, sandwiched between two substrates of lower refractive index n_c, to form a liquid lightguide;

Figure 6 illustrates actuation schemes for achieving liquid motion and hence mirror actuation by means of electrowetting-on-dielectrics (EWOD) (arrows indicate the direction of liquid movement);

Figure 7 shows further types of optical router systems that can be formed with tunable fluid/fluid mirrors.

The present invention will now be described in more detail with reference to the Figures. Figure 1 shows a first embodiment of an optical router 100 according to the pre- sent invention which incorporates four coupling sites for optical output fibers 108 and a group of phaseguides 1 14, which delimit the different fill/drain zones 103. The radiation is input via the site of an input fiber 106.

As discussed above, the system 100 is operable to select the input and/or output channel as the system is partially filled with a given material with high Rl (material A).

In a first example, the materials chosen are PDMS as material A, air as material B and water as material C. This choice is justified by the low price and their availability. The material of the system can be changed, the only limitation is that the material must be sufficiently transparent in the spectral region where the optical router 100 is intended to work.

Figure 2 schematically shows the filling and emptying process of the structure, creating a mobile interface which can change the angle and/or position of the input and output light beams.

Figure 3 shows an experimental validation of the presented system, with the subsequent filling and emptying of a given area. It is observed that when the water/PDMS interface is generated the light reaches a detector (curve 300). Instead, when the system is filled with water on both sides of the phaseguide (as shown by curve 102) the light is not reflected and the signal is zero. Furthermore, repeating the cycle of filling and emptying five times, this behavior remains virtually unchanged, showing a good repeatability of the system.

According to a further preferred embodiment, the present invention relates to an optical router using a tunable fluid/fluid interface. This router can, for example, be used to select optical channels for a incident radiation beam. The reflection of light is produced by total internal reflection (TIR) at a fluid/fluid interface. The thus obtained fluid/fluid mirror may be electrically tuned by integrated actuators, for example actuators based on elec- trowetting-on-dielectrics (EWOD). By moving the fluid/fluid interface into a beam path using such actuators, light can be deflected over a wide range of angles, which can be accurately chosen by designing the mirror shape and position. The proposed fluid/fluid mirrors can reflect light over a wide wavelength range, which allows working in multiple zones of a spectrum simultaneously.

Figure 4 shows an example of an optical router 100 using switching unit 107 with a fluid/fluid mirror as described in this invention. A first, unpolar liquid 1 1 6 of high refractive index ni_iquidi (e.g. an oil such as chloronaph- talene) is sandwiched between two substrates 1 18, 120, coated with a material 1 25of low refractive index n_c (e.g. glass coated with a fluoropolymer film) to act as a liquid lightguide (see Figure 5). The substrate 1 1 8, 1 20 itself or an additional surface layer 125 constitutes the cladding at which light is totally internally reflected.

Using either external light sources or fiber optics in conjunction with thin film metallic mirrors 1 05 and cylindrical collimation lenses 1 04, light may be coupled into the liquid lightguide and guided within the liquid 1 16, as shown in Figure 5. Such a configuration may be used as a liquid lightguide (lightguide because the thickness of the liquid layer is assumed to be large in comparison to the wavelength of the light guided, for example 350 μηι), into which light may be coupled from the side through transparent windows 109, obtained, for example, from polymers such as SU-8. If the unpolar ambient liquid 1 16 is filled into a planar microfluidic chamber, light propagation is free in two dimensions (x,y) and limited in the third dimension (z) (cf. Figure 4 and Figure 5).

A defined volume of a second conductive, polar liquid 122 of low refractive index ni_iquid2 (e.g. a water drop with dissolved anorganic salt), immiscible with the first liquid 1 1 6, is dispensed within the first liquid 1 16.

At the contact of the two liquids, a very smooth interface is formed with sub-nanometer roughness. If the structure height is small (for example a chamber height of < 2 mm), the interface 124 will show a constant radius of curvature R (as shown in the side view depicted in Figure 4), due to dominance of surface tension forces over gravitational forces at that size scale. The interface 1 24 constitutes an optical surface which reflects any beam of wavelength λ impinging on the surface at an angle larger than the critical angle of the fluid/fluid interface 0_C , which is given by d_c (A) = arcsin[/¾_u/d2 (A)/ n,_iquid1(A)].

A large difference in refractive index between both liquids is preferred, as it results in a high critical angle for TIR, in other words, a large angular range over which the fluid/fluid mirror is functional. For typical values, for example n_liquid1 = 1 .63 and n_liquid2 = 1 .33, the critical angle will be 0_C = 54.68 °.

In conclusion, a fluid/fluid mirror built up from typical materials can be practically used for light incident on the mirror at angles Θ, > 55 °.

Figure 4 also explains means of achieving electrical control of a fluid/fluid mirror 124. As shown by the cross-sectional view in Figure 4, for each output channel 108 a first non-insulated electrode 126 is in direct contact with the second conductive liquid 122, and supplies a GND potential to said liquid 122.

A second actuation electrode 128 is deposited on the lower substrate 120 and insulated from the second conductive liquid 122 by means of a thin dielectric film. This film may be composed of a first dielectric 123 of high breakdown strength (e.g. Parylene-C) and a second dielectric 125 (e.g. a fluoropolymer such as Cytop) which offers a high hydrophobicity (low surface energy).

Applying a voltage (either DC or AC) to the actuation electrode 128 results in electromechanical forces, which pull the second conductive liquid 122 onto the actuation electrode 128. This liquid movement is due to the effect of electrowetting-on-dielectrics (EWOD) and is indicated by arrow 130.

For each output channel 108, a small pinning structure (called„phaseguide") 1 14 is positioned at the edge of the actuation electrode 128, offering a sharp kink and preferably also a change in the wettability as compared to the dielectric surface above the electrode. This pinning structure 1 14 may be used to accurately align the advancing liquid front to a defined shape, which may be either straight or curved. The pinning structure 1 14 may be implemented, for example, by a slim stripe of patterned dry film resist of a few tens of μηι in height. Alternatively, a phaseguide 1 14 may also be formed directly from the substrate material, e.g. as a step.

As soon as the liquid has completely aligned all along the pinning structure, a highly stable mirror is formed whose shape is precisely defined in two dimensions (x,y).

The mirror curvature R (see side view in Figure 4) in the third dimension (z) is defined by the contact angles of the second liquid on the lower and upper substrate 1 18, 120 and the distance h between the substrates (see side view in Figure 4). As long as the curvature is sufficiently large, light guiding in the liquid lightguide (liquid 1 ) is not affected.

When the voltage is removed from the actuation electrode 128, the second liquid 122 dewets from the hydrophobic electrode and retracts into its initial position. This passive back-movement is driven by a competition between the liquids for wetting the solid substrate, as a result of differences in surface tensions of the liquids. Important for a robust actuation of the liquid mirrors 124 is an accurate positioning of the first liquid 1 16 with respect to the actuation electrode 128. One method known to those skilled in the art is to structure hydrophobic surface coatings 125 for defining a reservoir region into which the first liquid can retract. Another possible way is to use differences in chamber height to force an aqueous liquid into regions of large height, assuming a chamber with hydrophobic coatings.

For immediate actuation, an overlapbetween the first liquid 1 16 and the actuation electrode 128 is required in the Off-voltage state. Such a positioning may be achieved by designing an offset s between the edge of the actuation electrode 128 and the opening in the upper hydrophobic layer 125 used for defining the liquid reservoir, as illustrated by the dashed lines in the side view sketch of Figure 4(a).

The actuation scheme discussed above is only one of many possible schemes. Other possible EWOD actuation schemes are displayed in Figure 6. Depending on the complexity of the structure, two actuation electrodes on top and bottom substrates may be used for achieving higher actuation speed.

Other possible mirror configurations can of course also be provided according to the present invention. In particular, any other shape of the fluid/fluid mirror (convex, concave, spherical, aspherical, etc.) can be used, the only limitation being that the condition for TIR at the fluid/fluid interface must be fulfilled.

Using a second actuation electrode on the top substrate allows controlling both contact angles of the fluid/fluid interface on the upper and lower substrates. Such a concept thus enables control of both the curvature in the plane of light reflection (x,y) AND in the plane perpendicular to the plane of light reflection (e.g. xz). Hence light could also be reflected out of plane (in z-direction).

It could be demonstrated that the optofluidic router system according to the present invention is capable of routing light from one input channel (either external, collimated light or light emitted by a fiber on chip and collimated using 2D-cylindrical micro-lenses) into five optical output channels (either fibers or lightguides).

Photographs taken during the actuation of a fluid/fluid mirror, using EWOD actuators on a bottom substrate, prove that the actuation of the mirror is highly reversible. Actuation speed of such a mirror is in the range of a 1 ...2 seconds. Importantly, the mirror shape can be maintained in a highly stable position for minutes to hours, at a power consumption level of a few mW. Figure 7 shows other types of router systems, which could be created using the switching units 107 having fluid/fluid mirrors according to the present invention.

In particular, the optical routers shown in Figures 7 A1 to A4 use a combination of at least two fluid/fluid mirrors for each optical path. As shown in Figure 7A1 , in each selectable optical path a tunable EWOD fluid/fluid mirror 101 is combined with a fixed air mirror fabricated for instance in a transparent SU-8 material 102. At least one lens 104 focuses the radiation beam. For deflecting the light beam into the output fibres 108 fixed air prisms 1 10, which are formed in the SU-8 material, can be used.

Instead of the lenses 104 also an externally collimated light beam can be used, thus reducing the size of the setup, as shown in Figure 7A2. A further variant is shown as Figure 7A3, where each of the output optical fibres 108 is coupled via lenses 104. Here, the fixed air mirrors 102 are formed as planar mirrors. Of course, further fixed mirrors 1 12 can also be provided in the light path, as shown in Figure 7A4. A combination of the tunable fluid/fluid mirrors 101 with fixed air prisms 1 10 is shown as Figure 7B1 and B2. Here, either integrated SU-8 lenses are used together with an optical input fibre 106, or an externally collimated light source is used together with a fixed aperture, as shown in Figure 7B1 .

Finally, as shown in Figures 7C1 to C3, the tunable EWOD fluid/fluid mirror can also have a curved form or even different sizes for each optical path. As shown in Figures 7C1 and C2, of course the input channel and the output channel do not have to include an angle of 0° and 90°, respectively, but can also be arranged with any other suitable angle between them, as shown in Figures 7C1 and C2. Again, prisms can be used for deflecting the desired part of the radiation beam into the output channel 108, as this is shown in Figure 7C3.

Claims

1 . Optical router for switching at least one incident radiation beam (134) towards at least one output, said optical router (100) comprising: at least one input radiation channel (106) for receiving said radiation beam (134), at least one switching unit (107) for changing the optical path of the radiation beam (134), at least one output radiation channel(108) for outputting the deflected radiation, wherein said switching unit (107) comprises a control unit and at least one fluid/fluid interface mirror (101 , 124) that is deformed under the influence of the control unit to deflect the radiation beam (134) towards a selected output radiation channel (108), said switching unit (107) further comprising at least one phaseguide structure (1 14) for mechanically aligning said fluid/fluid interface mirror (101 , 124).

2. Optical router according to claim 1 , wherein said radiation beam (134) is guided by means of total internal reflection along its path through the optical router (100).

3. Optical router according to claim 1 or 2, wherein said control unit comprises an electric control element having at least one electrode for deforming the fluid/fluid interface by means of electrowetting-on-dielectrics (EWOD).

4. Optical router according to claim 3, wherein said electric control element comprises at least one reference electrode (126) which is electrically connected to one of the fluids (122) for connecting this fluid to a reference potential, and at least one electrically insulated control electrode (128) for controlling the wettability of a surface adjacent to the control electrode (128).

5. Optical router according to one of the preceding claims, wherein said fluid/fluid interface (124) is formed by a first and a second non-miscible liquids (1 16, 122), preferably a non-polar and a polar liquid.

6. Optical router according to claim 5, wherein the first liquid (1 16) comprises a high reflective index oil, preferably chloronaphtalene, and the second liquid (122) comprises water.

7. Optical router according to one of the preceding claims, wherein said switching unit (107) comprises a chamber formed by a first and a second substrate (1 18, 120) for retaining the fluids producing said fluid/fluid interface, wherein said optical path extends in a direction which is essentially in parallel to the first and second substrates (1 18, 120).

8. Optical router according to claim 7, wherein said first and/or second substrate (1 18, 120) comprises a conductive layer forming an electric control element.

9. Optical router according to claim 7 or 8, wherein said first and/or second substrate (1 18, 120) comprises a pinning structure, preferably a step-shaped stripe of patterned dry film resist for forming said phaseguide structure (1 14).

10. Optical router according to one of the preceding claims, wherein said fluid/fluid interface mirror (101 , 124) has a curved shape for focussing or dispersing said radiation beam (134).

1 1 . Optical router according to one of the preceding claims, further comprising at least one lens (104) arranged at the input radiation channel (106) and/or output radiation channel (108) for shaping the radiation beam (134) when being incident and/or being output.

12. Optical router according to claim 1 1 , wherein said at least one lens (104) comprises an air chamber formed in a transparent resist.

13. Optical router according to one of the preceding claims, wherein said at least one input radiation channel (106) for receiving said radiation beam (134), said at least one switching unit (107) for changing the optical path of the radiation beam (134), and said at least one output radiation channel (108) for outputting the deflected radiation are integrally fabricated as a Micro-Electro-Mechanical System (MEMS).

14. Optical router according to one of the preceding claims, further comprising at least one fluidic inlet and at least one fluidic outlet for filling the device with the fluids forming said at least one fluid/fluid interface mirror (101 , 124). Optical router according to one of the preceding claims, further comprising at least one micro-prism (1 10) for deflecting the radiation beam (134) when being incident and/or being output.

Optical router according to one of the preceding claims, wherein said input and/or output radiation channels (106, 108) comprise at least one light guide, optical fiber or waveguide structure.