Abstract:
The present invention relates to a method of manufacturing an electrowetting-based variable-focus lens, comprising: (a) providing an enclosure having a cavity ( 17 ) and at least one channel ( 27, 28; 59 ) communicating at one end with the cavity and at the other end emerging at the exterior surface of the lens; (b) filling the cavity with first and second liquids ( 44, 47 ) that are immiscible and of different refractive indices via the channel; and (c) hermetically scaling the channel.

Description:
FIELD OF THE INVENTION 
       [0001]    The present invention relates to a method of manufacturing a variable-focus lens and more particularly to a method of manufacturing a lens involving the deformation of a drop of liquid by electrowetting effects. The present invention also relates to an electrowetting-based variable-focus lens obtained by such a method. 
       SUMMARY OF THE PRIOR ART 
       [0002]    A variable-focus lens comprises an enclosure, generally bounded by two transparent parallel plates, which contains at least two immiscible liquids of different refractive indices. In general, one of the liquids is an aqueous liquid and the other liquid is an oily liquid. The interface between the two liquids defines a movable refractive interface through which the light rays received by the lens pass. The lens comprises means for deforming the movable dioptric interface by electrowetting effects, thus making it possible to modify the optical power of the lens. 
         [0003]    Such electrowetting-based variable-focus lenses are described in general in European patent 1 019 758. 
         [0004]    The filling of the lens with the two liquids is generally carried out when one of the transparent plates has not yet been fastened to the enclosure. The enclosure is then immersed in the aqueous liquid, which fills the entire enclosure. A drop of oily liquid can then be put into place, for example by means of a syringe. The transparent plate is then fitted in order to close off the enclosure. 
         [0005]    With such a method of assembly, it may be difficult for the position of the drop of the oily liquid in the enclosure of the aqueous-liquid-filled lens to be accurately controlled. Furthermore, when closing off the enclosure of the lens, it may be difficult for the internal pressure of the lens to be accurately controlled. This is because the step of closing off the enclosure of the lens generally comprises a step of compressing a seal. At the end of the method of assembling the lens, there is therefore an overpressure in the lens that cannot be easily controlled accurately. For some applications, it may be desirable to accurately fix the internal pressure of the lens at the end of assembly. In particular, it may be desirable for the internal pressure of the lens to be fixed at a value lower than atmospheric pressure. 
       SUMMARY OF THE INVENTION 
       [0006]    The aim of the present invention is to provide a method of manufacturing an electrowetting-based variable-focus lens that allows simple and precise positioning of the liquids contained in the lens, and also an electrowetting-based variable-focus lens obtained by such a method. 
         [0007]    Another object of the present invention is to allow the internal pressure of the lens when assembling the lens to be fixed in a simple and precise manner. 
         [0008]    For this purpose, according to a first aspect of the present invention there is provided a method of manufacturing an electrowetting-based variable-focus lens, which comprises the following steps of:
       (a) providing an enclosure having a cavity and at least one channel communicating at one end with the cavity and at the other end emerging at the exterior surface of the lens;   (b) filling the cavity with first and second liquids that are immiscible and of different refractive indices via the channel; and   (c) hermetically sealing the channel.       
 
         [0012]    According to one example of a method according to the present invention, the lens comprises first and second channels, each of the first and second channels communicating at one end with the cavity and at the other end emerging at the external surface of the lens. The method comprises the following successive steps of: introducing the first liquid via the first channel, in order to at least partly fill the cavity; introducing the second liquid via the first or the second channel until the cavity is completely filled with the first and second liquid; and hermetically sealing the first and second channels. 
         [0013]    According to a further example of a method according to the present invention, step (b) comprises the steps of creating a vacuum in the cavity; and introducing in succession by suction, the first and second liquids via the channel in order to fill the cavity. 
         [0014]    According to a further example of a method according to the present invention, step (c) comprises the steps of hermetically sealing the channel with a malleable material; and holding the malleable material in place by an adhesive. 
         [0015]    According to a further example of a method according to the present invention, step (c) comprises the steps of applying an overpressure to the enclosure; partially filling the channel with a curable liquid material, in other words a liquid material capable of hardening; in releasing the overpressure applied to the enclosure, which results in the curable liquid material penetrating further into the channel; and hardening the curable liquid material. 
         [0016]    According to a further example of a method according to the present invention, step (c) comprises the steps of heating the enclosure; partially filling the channel with a curable liquid material; cooling the enclosure, which results in the curable liquid material penetrating further into the channel; and hardening the curable liquid material. 
         [0017]    According to one example of a method according to the present invention, step (c) is preceded by a step of forming, via the channel, a gas bubble in contact with the first liquid in a region that is not liable to be traversed by light rays passing through the lens. 
         [0018]    According to a further aspect of the present invention, there is provided an electrowetting-based variable-focus lens comprising an enclosure that comprises a cavity containing at least two immiscible liquids of different refractive indices. The enclosure comprises at least one channel communicating at one end with the cavity and at the other end emerging at the exterior surface of the lens, said channel being hermetically sealed by a plug. 
         [0019]    According to one embodiment of the present invention, the lens comprises at least two channels, each channel communicating at one end with the cavity and at the other end emerging at the external surface of the enclosure, each channel being hermetically sealed by a plug, said cavity being bounded by first and second transparent plates opposite each other and by an intermediate component, one channel communicating with the cavity at the join between the first plate and the intermediate component and the other channel communicating with the cavity at the join between the second plate and the intermediate component. 
         [0020]    According to one embodiment of the present invention, the channel comprises a portion of reduced cross-section, the plug comprising at least one part made of a malleable material compressed in said portion of reduced cross-section. 
         [0021]    According to one embodiment of the present invention, the plug comprises at least one part made of an adhesive. 
         [0022]    According to one embodiment of the present invention, the lens further comprises a chamber containing one liquid from among the two liquids and a gas bubble, a means for said one liquid to pass between the chamber and the cavity, and a means for retaining the gas bubble in the chamber. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0023]    These objects, features and advantages, together with others of the present invention, will be explained in detail in the following description of non-limiting specific exemplary embodiments in relation to the appended figures in which: 
           [0024]      FIG. 1  is an exploded sectional view of a first exemplary embodiment of a variable-focus lens according to the invention; 
           [0025]      FIGS. 2 to 4  illustrate successive steps of one example of a method of assembling the lens of  FIG. 1 ; and 
           [0026]      FIG. 5  is a partial cross-section of a second exemplary embodiment of the variable-focus lens according to the invention. 
       
    
    
     DETAILED DESCRIPTION 
       [0027]    For the sake of clarity, identical elements have been denoted by identical references in the various figures. 
         [0028]      FIG. 1  is a sectional exploded view of a first exemplary embodiment of a variable-focus lens  10  according to the invention, comprising upper and lower circular transparent plates  12  and  14  of axis Δ and an annular ring  16  of axis A. The ring  16  has a central opening  17  which is bounded by a lower truncated-cone-shaped wall  18  (in other words a conical frustum) and an upper truncated-cone-shaped wall  20  that are separated by a shoulder  22 . The ring  16  further comprises, on its upper surface, a planar face  24  on which the upper plate  12  is intended to bear, in order to form the topside of an enclosure. Similarly, the ring  16  comprises, on its lower face, a planar bearing surface  26  on which the lower plate  14  is intended to bear, in order to form the underside of the enclosure. The plates  12  and  14  and the ring  16  may be made of rigid insulating materials, for example glass in the case of plates  12 ,  14 , which must be transparent to the intended operating wavelengths of the lens, and a ceramic material, for example alumina, in the case of the ring  16 . 
         [0029]    The ring  16  comprises channels  27 ,  28  which each connect the central opening  17  with the side wall of the ring  16 . In the present exemplary embodiment, the channel  27  comprises a cylindrical opening  29 , the axis of which is perpendicular to the axis A and one end of which opens into the side wall of the ring  16 . The opening  29  is extended by a cylindrical opening  30  with a closed end of the same axis and of smaller diameter, defining a shoulder  31  with the opening  29 . The ring comprises a groove  32  on the upper face  24  of the ring  16 , which has one end opening into the upper truncated conical wall  20  of the central opening  17  and a closed end. The ring  16  further comprises an opening  33  with a closed end of axis parallel to the axis A, which opens into the groove  32  and communicates with the opening  30 . In the present exemplary embodiment, the channel  28  has a structure substantially similar to the structure of the channel  27 . The channel  28  comprises a cylindrical opening  34 , the axis of which is perpendicular to the axis A and one end of which opens into the side wall of the ring  16 . The opening  34  is extended by a cylindrical opening  35  with a closed end of the same axis and of smaller diameter and which, with the opening  34 , defines a shoulder  36 . The ring  16  comprises a groove  37  on the lower face  26  of the ring  16 , which has one end opening into the lower truncated conical wall  18  of the central opening  17  and a closed end. The ring  16  further comprises an opening  38  with a closed end of axis parallel to the axis A, which opens into the groove  37  and communicates with the opening  35 . 
         [0030]    More generally, the channels  27 ,  28  may be of any shape. However, it is desirable for the region where each channel  27 ,  28  opens into the central opening  27  to be placed so as not to disturb the operation of the lens  10 . 
         [0031]    The external periphery of the lower face of the upper plate  12  is coated with a conducting film  39 . Conducting film  40  is also deposited on the planar face  24 , that film extending towards the external periphery of the ring  16  and being extended towards the internal periphery as far as level with the upper truncated conical portion  20 . In  FIG. 1 , the film  40  is shown covering the walls of the groove  32 . The conducting film  40  is intended to come into contact with the conducting film  39 . The conducting film  40  extends sufficiently to come into contact via its internal part with the conducting liquid that will be contained in the lens. The conducting films  39  and  40  are made of materials that are electrically conducting sufficiently to act as electrodes and adhere, on one side, to the plate  12  and, on the other side, to the ring  16 . These materials may for example be a gold-tin alloy, an indium-based alloy, a bismuth-based alloy, etc. 
         [0032]    The upper face of the lower plate  14  is coated with a conducting film  41  on its external periphery facing the bearing surface  26  of the lower face of the ring  16 . The lower face of the ring  16  is coated with a conducting film  42  which is extended over the lower truncated conical portion  18 . In  FIG. 1 , the film  42  is shown covering the walls of the groove  37 . On the lower truncated conical wall  18 , the conducting film  42  is coated with an insulating film  43  that is extended slightly onto the shoulder  22  and onto the lower face of the ring  16 , penetrating into the groove  37 . The materials making up the conducting films  41  and  42  are of the same nature as the materials of the conducting films  39  and  40 . The electrodes of the lens  10  correspond to the conducting films  40  and  42 . The films  49 ,  40 ,  41 ,  42  and  43  are shown only in  FIG. 1  and have not been drawn to scale. 
         [0033]      FIG. 2  shows the structure obtained after the ring  16  has been fastened to the upper and lower plates  12  and  14 . The bonding between each plate  12 ,  14  and the ring  16  may be provided by localized heating of the periphery of the structure. In one particular exemplary embodiment of the present invention, this may be produced by heating the periphery of the ring  16  by laser irradiation. The conducting films  39 ,  40 ,  41  and  42  then melt and form an impermeable bond. In an alternative embodiment, the conducting films  39  and  41  may be omitted, it being possible for the upper and lower plates  12 ,  14  to be fastened to the ring  16  by means of an adhesive. 
         [0034]      FIG. 3  illustrates the step of the method of filling the lens  10  with the conducting and insulating liquids. Such a step firstly comprises the filling of the lens  10  with the conducting liquid  44 . To do this, the conducting liquid is introduced via the channel  28  (along the path  45 ), the channel  27  then acting as a vent. The conducting liquid is introduced until it completely fills the central opening  17  and escapes via the channel  27  (along the path  46 ). The insulating liquid is then introduced via the channel  28  (along the path  45 ) until a drop  47  is obtained on the truncated conical wall  18 . When no voltage is applied between the electrodes  40  and  42 , the refractive optical interface separating the insulating liquid  47  from the conducting liquid  44  is level with the upper part of the truncated conical wall  18 . 
         [0035]      FIG. 4  shows the structure obtained after the channels  27 ,  28  have been closed off by means of balls  48  of a malleable material, for example gold. The balls  48  are squashed into the shoulders  31 ,  36  so as to close off the openings  30 ,  35 . The balls  48  are held in place by depositing an adhesive  49 , for example an epoxy adhesive, in the openings  29 ,  34 . The squashed balls  48  ensure that the lens  10  is sealed and the adhesive  49  provides mechanical retention of the squashed balls  48 . The hermetic sealing of the channels  27 ,  28  is achieved after the internal pressure in the lens  10  has been set to a desired value. 
         [0036]    According to an alternative example of a method according to the present invention, once the lens  10  has been filled with the conducting and insulating liquids the channels  27 ,  28  are closed off in the following manner:
       the lens  10  is pressurized, for example by applying pressure to the upper and lower plates  12 ,  14 ;   the openings  29 ,  34  are filled with a liquid adhesive, for example an epoxy adhesive that can be cured by being exposed to ultraviolet radiation, which is compatible with the insulating and conducting liquids, i.e. immiscible with the insulating and conducting liquids;   the pressure applied to the lens  10  is released so that the liquid adhesive penetrates into the openings  30 ,  35  by suction; and   the adhesive is cured, for example by exposing the adhesive to a source of ultraviolet radiation.       
 
         [0041]    According to another alternative example of a method according to the present invention, once the lens  10  has been filled with the conducting and insulating liquids the channels  27 ,  28  are closed off in the following manner:
       the temperature of the lens  10  is raised, causing the liquids contained in the lens to expand;   the openings  29 ,  34  are filled with a liquid adhesive, for example an epoxy adhesive that can be cured by being exposed to ultraviolet radiation, which is compatible with the insulating and conducting liquids, i.e. immiscible with the insulating and conducting liquid;   the lens  10  is cooled so that the liquid adhesive penetrates into the openings  30 ,  35  by suction resulting from the reduction in the volume of the liquids contained in the lens  10 ; and   the adhesive is cured, for example by exposing the adhesive to a source of ultraviolet radiation.       
 
         [0046]    In general, the channels  27 ,  28  may be closed off with any material that is compatible with the insulating and conducting liquids and allows a sealed closure of the channels to be obtained. 
         [0047]    In the present exemplary embodiment, both channels  27 ,  28  are made in the ring  16 . According to an alternative embodiment, provision may be made for at least one of the channels  27 ,  28  to be made partly in the ring  16  and partly in one of the upper and lower plates  12 ,  14 . For example, it is possible to provide, for one channel, an opening in the periphery of a plate  12 ,  14 , which opens into a groove provided in the ring  16  and connected to the central opening  17 . Such an opening can then be closed off as described above, especially by squashing a gold ball into the opening. According to another alternative embodiment, the channels  27 ,  28  may both be in the plates  12 ,  14  and open into regions of the central opening  17  so as to cause little or no disturbance to the path of the light rays. 
         [0048]    According to an alternative embodiment, the lens comprises a single channel, for example the channel  28  of the exemplary embodiment described above. The lens can then be filled in the following manner:
       a vacuum is created in the internal volume of the lens  10 ;   the internal volume of the lens is partially filled with the conducting liquid, by introducing conducting liquid into the channel, the lens filling by sucking up the conducting liquid;   the drop of insulating liquid is formed by completing the filling of the internal volume of the lens, by introducing the insulating liquid into the channel, the lens filling by sucking up the insulating liquid; and   the channel is closed off.       
 
         [0053]    In an alternative example of a method according to the present invention, a gas bubble is intentionally introduced so as to come into contact with one of the liquids contained in the lens, taking care to prevent the gas bubble from being present in the region through which the light rays pass. When the temperature changes, the liquids contained in the lens expand at the expense of the gas bubble, which by nature is highly compressible, thus limiting the change in internal pressure of the lens. The gas may be air, an inert gas or a mixture of inert gases or the vapour of one of the liquids contained in the lens. 
         [0054]      FIG. 5  is a detailed sectional view of a second exemplary embodiment allowing the formation of a gas bubble in the lens. The ring  16  comprises, on the upper face  24 , a circular groove  50  about the axis a of the lens  10 , which groove is connected to the central opening of the ring  16  via an annular interstice  52  of thickness d along the axis A. The groove  50  is bounded by an upper wall  54  corresponding to a portion of the lower wall of the upper plate  12 , a lower wall  56  inclined to the upper wall  54  at an angle Δ, and an end wall  58  inclined to the lower wall  56  at an angle β and to the upper wall  54  at an angle γ, the lower wall  56  and the end wall  58  corresponding to portions of the upper wall of the ring  16 . Preferably, the thickness d of the interstice is less than a few tens of microns. The interstice  52  need not have a constant thickness, and can be obtained by the upper plate  12  simply pressing on the annular ring  16 , the surface irregularities of the plate  12  and of the annular ring  16  being sufficient to ensure the presence of communicating channels between the groove  50  and the central opening of the ring  16 . 
         [0055]    The upper plate  12  comprises a channel  59  that connects the groove  50  to the upper face of the upper plate  12 . The channel  59  comprises an opening  60 , the axis of which is parallel to the axis A and one end of which opens into the groove  50 . The opening  60  is extended by a larger-diameter cylindrical opening  62  with which it defines a shoulder  63 . The opening  62  opens into the upper face of the upper plate  12 . According to the second exemplary embodiment, the channel  59  acts as the channel  27 . The lens  10  further comprises the channel  28  described previously in relation to the first exemplary embodiment. 
         [0056]    After the lens  10  has been filled, a gas may be introduced into the groove  50  so that a gas bubble  64  forms in the groove  50 . This may be achieved by sucking in liquid via the channel  28 . After the air bubble has been formed, the channel  59  is hermetically sealed, for example as described previously by means of a squashed gold ball  65  and a plug of adhesive  66 . According to a variant of the second exemplary embodiment, the channel  27  is present, the groove  50  being locally interrupted on either side of the groove  32  of the channel  27 . The channels  27  and  28  can then be used for filling the lens  10 , while the channel  59  and one or other of the channels  27  and  28  are used to form the gas bubble  64 . 
         [0057]    The walls that define the interstice  52  are covered with a hydrophilic material so that the capillary forces prevent the gas bubble  64  from passing into the annular interstice  52 . The angle α is smaller than the angles β and γ so that the aqueous liquid is spontaneously attracted into the corner of angle α and the gas bubble  64  is pushed back against the end wall  58 . So as to make it even easier to position the gas bubble  64  on the end wall  58 , the upper and lower walls  54 ,  56  may be covered with a hydrophilic material and the end wall  58  may be covered with a hydrophobic material. 
         [0058]    Of course, the present invention is capable of various alternative embodiments and modifications that will be apparent to those skilled in the art. In particular, in the exemplary embodiments described above, the lower truncated conical wall  18  may be extended, in the lower part, by a cylindrical wall. Furthermore, in the second exemplary embodiment, the circular groove  50  may be replaced with a spiralled groove, one end of which opens into the central opening  17  and the opposite end of which is closed.