Abstract:
The invention relates to a variable-focus lens ( 60 ) for focussing light rays in light paths passing through the lens along an optical axis (Δ). The lens comprises an arrangement of first and second immiscible liquids ( 67, 68 ) that have different refractive indices and are in contact over a moveable refractive optical interface ( 69 ), a volume of gas ( 72 ) in contact with one of said liquids, and a retention measure ( 70, 74 ) for keeping the volume of gas away from the light paths of the light rays passing through the lens for focussing.

Description:
CROSS-REFERENCE TO RELATED PATENT APPLICATIONS  
       [0001]     The right of foreign priority is claimed under 35 U.S.C. §119(a) based on France Application No. 0551737, filed Jun. 23, 2005, the entire contents of which, including the specification, drawings, claims and abstract, are incorporated herein by reference.  
       BACKGROUND OF THE INVENTION  
       [0002]     The present invention relates to a variable-focus lens and more particularly to a lens involving the deformation of a body of liquid (“drop”) by electrowetting effects.  
         [0003]     A variable-focus lens usually comprises an enclosure, bounded by two transparent windows, which contains at least two immiscible liquids of different refractive indices. The two liquids are in contact over a moveable refractive interface through which the light rays received by the lens pass. The liquid lens includes a system for deforming the moveable refractive interface by electrowetting effects, thus making it possible to modify the optical power of the lens.  
         [0004]     The housing for such a lens generally constitutes a rigid structure. The pressure of the liquids in the housing may increase substantially, for example, during the operations of assembling the components of the housing, or, once the housing has been assembled, upon an increase in temperature of the liquids of the lens, which have higher expansion coefficients than the expansion coefficients of the constituent materials of the housing.  
         [0005]     Excessive pressure of the liquids contained in the housing increases the risk of causing the transparent plates to deform, resulting in an undesirable optical distortion. In the worst case, if the increase in pressure of the liquids is too high, this may result in fracture of the transparent plates. Special precautions therefore have to be taken when assembling the mount for the lens and/or to limit the temperature range permitted for storing and using such a lens.  
         [0006]     Patent application U.S. Ser. No. 11/284125, which is commonly owned and not yet published (not prior art), describes a housing for a variable-focus lens that includes a compensating device for the expansion of the liquids contained therein. The disclosure of this prior application is incorporated by reference into the present application.  
         [0007]      FIG. 1  is substantially similar to  FIG. 3  of patent application U.S. Ser. No. 11/284125 and shows a variable-focus lens mount  10 , having an optical axis Δ, which comprises an upper part  12  and a lower part  14  which, when they are assembled, define an internal volume  15 . The lower part  14  comprises a body  16  having a base  17  through which a central opening  18  passes, the base being extended by a cylindrical lateral portion  20 . The base  17  comprises a corrugated portion  23 , the cross section of which in a plane containing the axis Δ has the exact or approximate form of an “S”. A transparent cylindrical plate  24  is fastened to the body  16  by adhesive  22 . The upper part  12  of the mount  10  comprises a cover  30  through the central part of which a cylindrical opening  32  passes. The upper part is extended by a cylindrical lateral wall  34 . The cover  30  includes an elastic portion  36  provided between the opening  32  and the cylindrical lateral wall  34 . The elastic portion  36  comprises a corrugated portion, the cross section of which in a plane containing the axis Δ has the exact or approximate form of an “S”. A transparent cylindrical plate  38  is fastened to the cover  30  by adhesive  40 . An intermediate piece  42  is placed in the internal volume  15  in electrical contact with the body  16 . Passing through the intermediate piece  42  is an opening that defines a truncated conical surface  48  adjacent to the glass plate  24 . The intermediate piece  42  is made of a conducting material and is covered with an insulating layer  49  on the surfaces in contact with the liquids. A seal  50  is placed between the body  16  and the cover  30 .  
         [0008]     A volume (“drop”) of an insulating liquid  52  is placed on the conical surface  48 , and the rest of the internal volume  15  is filled with an electrically conducting liquid  54 , which is immiscible with the insulating liquid, has a different refractive index from and has substantially the same density as the insulating liquid. By electrowetting effects, it is possible to modify the curvature of the contact surface between the two liquids, as a function of a voltage V applied between the intermediate piece  42  and the cover  30 , which form two electrodes. During this change in the curvature of the liquid-liquid interface, the edge of the interface between the conducting liquid  54  and the insulating liquid  52  moves along the conical surface  48 . For example, the contact surface passes from the initial, e.g., concave shape, denoted by the reference A, to the convex shape illustrated by the dashed curve and denoted by the reference B. Thus, a light beam passing through the cell orthogonally to the plates  38  and  24  will be focused to a greater or lesser extent according to the applied voltage. In general, the conducting liquid comprises an aqueous liquid, and the insulating liquid comprises an oily liquid.  
         [0009]     The “S”-shaped corrugated portions  23 ,  36  are able to deform when the liquids contained in the internal volume  15  expand, so as to limit the increase in internal pressure of the lens.  
         [0010]     One possible limitation of such a lens is that a certain degree of deformation of the corrugated portions  23 ,  36  may result in a change in the shape of the lens housing, especially the distance separating the two transparent plates  24 ,  38 . This may lead to the appearance of additional optical defects. Furthermore, the fact that the housing is deformable may make it difficult for the components of the housing to be precisely positioned, one with respect to another. Thus, it may prove difficult to keep the optical part of the lens centered with respect to a reference external to the lens. In addition, the production of the “S”-shaped portions  23 ,  36  requires specific stamping steps, which complicates the manufacture of such a lens. Thus, the present invention can be employed together with other measures for controlling the pressure inside the lens, such as the device described in the commonly owned earlier application, or it may be employed as the sole pressure controlling measure in a lens system.  
       SUMMARY OF THE INVENTION  
       [0011]     One object of the present invention is to provide a variable-focus lens that is easy to manufacture and that makes it possible to limit the variation in the internal pressure of the lens when there is a temperature change, while keeping the structure of the lens rigid.  
         [0012]     For this purpose, the invention provides an electrowetting variable-focus lens comprising at least first and second liquids and a volume of gas in contact with one of the liquids, the volume of gas comprising for example one or more bubbles of gas, and a retention measure for keeping the volume of gas away from the paths of the light rays passing through the lens for focusing.  
         [0013]     According to one preferred embodiment of the present invention, the lens comprises a first chamber containing first and second immiscible liquids, of different refractive indices, in contact over a refractive optical interface that can be deformed by electrowetting effects; a second chamber containing the first liquid and a volume of gas; and a passage for the first liquid to pass between the first and second chambers.  
         [0014]     According to another preferred embodiment of the present invention, the second chamber comprises at least one wall in contact with the volume of gas, and this wall comprises or is covered by a material that has low wettability by the first liquid.  
         [0015]     According to another preferred embodiment of the present invention, the passage comprises walls that comprise or are covered by a material that has high wettability by the first liquid.  
         [0016]     According to another preferred embodiment of the present invention, the second chamber is bounded by first, second and third walls, the first wall being inclined at a first angle to the second wall, the second wall being inclined at a second angle to the third wall, and the first wall being inclined at a third angle to the third wall, the first angle being smaller than the second angle and smaller than the third angle.  
         [0017]     According to another preferred embodiment of the present invention, the passage and/or the second chamber are formed by pores of a porous material.  
         [0018]     According to another preferred embodiment of the present invention, the second chamber has the form of a tube, wherein one end of the tube opens into the first chamber and the opposite end of the tube is closed.  
         [0019]     According to a further preferred aspect of the present invention, there is provided a method of manufacturing an electrowetting variable-focus lens, comprising the steps of forming a partial enclosure formed of a enclosure member; filling the partial enclosure with at least one liquid; forming a volume of gas in the partial enclosure, wherein the volume of gas is in contact with said liquid in a region through which light rays passing through the lens do not pass; and closing off the enclosure with a second enclosure member to form a sealed enclosure.  
         [0020]     According to one preferred method of implementing the present invention, the three last-recited steps of the above method are carried out under reduced pressure, i.e. below atmospheric pressure.  
         [0021]     These and further objects, features and advantages of the present invention will be explained in detail in the following description of particular preferred, non-limiting exemplary embodiments, when considered in relation to the accompanying figures of drawing. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0022]     In the drawings:  
         [0023]      FIG. 1 , described above, is a section through a variable-focus lens that includes a device for compensating for the expansion of the liquids contained in the lens according to an earlier commonly owned proposal;  
         [0024]      FIG. 2  illustrates the general principle of a liquid lens according to one embodiment of the invention; and  
         [0025]     FIGS.  3  to  13  show more detailed exemplary embodiments of a liquid lens according to alternative embodiments of the invention. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0026]     For the sake of clarity, identical elements have been denoted by the same reference numerals in the various figures.  
         [0027]     The present invention relates to intentionally introducing a volume of gas into contact with one of the liquids contained in the lens, taking care to prevent the volume of gas from being present in the region through which the light rays pass. Retention measures are used to prevent the volume of gas from being displaced into the light path. When the temperature changes, the liquids contained in the lens expand, and this expansion is compensated by the volume of gas, which by nature is very compressible, thus limiting the change in internal pressure of the lens. The gas may be, for example, air, an inert gas or a mixture of inert gases, or, alternatively or in combination, the vapour of one of the liquids contained in the lens.  
         [0028]     According to the invention, the volume of gas could comprise, for example, one or more bubbles of gas contained in the lens.  
         [0029]      FIG. 2  shows generally one example of a variable-focus lens  60  employing the compensation principle according to the present invention. The lens  60  comprises a liquid chamber  61  bounded by two transparent plates  62 ,  64  fastened on their periphery to an intermediate piece  66 . The liquid chamber  61  is entirely filled with two liquids  67 ,  68 , the contact surface of which defines a moveable refractive interface  69 . The variable-focus lens  60  further contains a system to deform the moveable refractive interface  69  by electrowetting. For example, the liquid  67  is a conductive liquid, the liquid  68  is an insulating liquid, and the intermediate piece  66  is made of a conductive material coated with an insulating layer (not illustrated specifically), thus forming a first electrode, while a second electrode is formed, for example, by deposition of a conductive transparent layer  63  on the internal surface of the plate  62 . Application of a voltage V between the electrodes results in the deformation of the refractive interface  69 .  
         [0030]     According to the embodiment of  FIG. 2 , the intermediate piece  66  comprises an expansion chamber  70  partly filled with the liquid  67 , with the remainder corresponding to a gas bubble  72 . The expansion chamber  70  plays no part in the optical properties of the lens  60 . The expansion chamber  70  is connected to the liquid chamber  61  via a passage  74  which, in  FIG. 2 , is represented by a duct. The shape or the nature of the walls of the expansion chamber  70  and/or of the passage  74  ensures that the gas bubble  72  remains in the expansion chamber  70  and does not penetrate into the liquid chamber  61 . When the liquids expand, a greater or lesser amount of liquid  67  penetrates into the expansion chamber  70  or leaves the expansion chamber  70 , and causes a change in the volume of the gas bubble  72 . Several separate expansion chambers may be provided, each being connected to the liquid chamber  61  so as to provide several small gas bubbles. This makes it possible to minimize the risks of the gas bubbles  72  moving when subjected to mechanical shocks.  
         [0031]      FIGS. 3 and 4  show an overall cross section and a detailed cross section of a first more preferred exemplary embodiment of the variable-focus lens  60  according to another embodiment of the invention. The intermediate piece  66  corresponds to an annular ring having an optical axis Δ, which includes a central opening that defines the liquid chamber  61  containing the two immiscible liquids  67 ,  68 , the interface of which forms the moveable refractive interface  69 . The annular ring  66  comprises an internal wall  78  along which the refractive interface  69  can move by electrowetting effects induced by application of a voltage, for example, the same way as described in reference of  FIG. 2 . For example, the internal wall  78  is preferably conical. In the present example, the liquid  67  is, for example, an aqueous liquid and the liquid  68  is an oily liquid.  
         [0032]     In the present embodiment, the expansion chamber  70  has symmetry of rotation about the axis Δ. It is defined by an upper wall  84  corresponding to a portion of the lower wall of the upper plate  62 , a lower wall  86  inclined to the upper wall  84  at an angle α, and an end wall  88  inclined to the lower wall  86  at an angle β and to the upper wall  84  at an angle γ. The lower wall  86  and the end wall  88  correspond to portions of the upper wall of the annular ring  66 . The passage  74  corresponds, in the first embodiment, to an annular interstice of thickness d (narrow gap), via which the expansion chamber  70  communicates with the liquid chamber  61  so that some of the aqueous liquid  67  can move between the expansion chamber  70  and the liquid chamber  61 . Preferably, the thickness d is less than a few tens of microns, i.e., preferably less than 50 microns and preferably within a range of 10 to 50 microns. The interstice  74  need not have a constant thickness, and can be obtained by the upper plate  62  simply pressing on the annular ring  66 , the surface irregularities of the plate  62  and of the annular ring  66  being sufficient to ensure the presence of communicating channels between the liquid chamber  61  and the expansion chamber  70 .  
         [0033]     The walls defining the annular interstice  74  are advantageously covered with a hydrophilic material, so that the capillary forces prevent the gas bubble  72  from passing into the annular interstice  74 . The angle α is advantageously 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  72  is pushed back against the end wall  88 . So as to make it even easier to position the gas bubble  72  on the end wall  88 , the upper and lower walls  84 ,  86  may be covered with a hydrophilic material, and the end wall  88  may be covered with a hydrophobic material.  
         [0034]      FIG. 5  shows a second more preferred embodiment, similar to the first more preferred embodiment, differing by the fact that the passage comprises a ring  82  of a porous material placed between the expansion chamber  70  and the liquid chamber  61 . The porous material may be a hydrophilic material, or the pores of the porous material may be covered with a hydrophilic material. This second more preferred embodiment has the advantage of allowing the gas bubble  72  to be properly stabilized in the expansion chamber  70 , since the gas bubble  72  cannot easily penetrate the pores of the porous ring  82 .  
         [0035]      FIG. 6  shows a third more preferred embodiment, in which the expansion chamber  70  comprises a region having a symmetry of revolution about the axis Δ, bounded by a lower wall  90  and an upper wall  92  that are inclined to each other. The cross section of these inclined walls, when viewed in a plane containing the axis Δ, corresponds to a “V” of angle γ. The passage  74  corresponds to an annular region that is an extension of the expansion chamber  70 . The walls  90 ,  92  are covered with a hydrophobic material, so that the gas bubble  72  is naturally localized in the corner of angle γ.  
         [0036]      FIG. 7  shows a fourth more preferred embodiment, in which the expansion chamber  70  has an annular shape. The cross section of annular chamber  70  in a plane containing the axis Δ corresponds to a “V” of angle γ, wherein the converging point is directed toward the liquid chamber  61 . The passage  74  corresponds to an annular interstice that opens into the expansion chamber  70  on the opposite side from the corner of angle γ. Compared with the third more preferred embodiment, the fourth more preferred embodiment makes it possible to further reduce the risk of the gas bubble  72  penetrating into the liquid chamber  61 .  
         [0037]      FIG. 8  shows, very schematically, a fifth more preferred embodiment, in which the expansion chamber  70  is formed by the pores of the central region of a block  94  of a porous material. The block is placed in contact with the liquid  67  of the liquid chamber  61 , in such a manner that it does not impede the path of the light beams. The dotted line  96  shows the boundary between the gas bubble  72  and the liquid  67 . The central region of the block  94  comprises or is covered by a highly hydrophobic material, so that the liquid has no tendency, through a capillary effect, to expel the gas bubble  72  out of the block  94  of porous material. The passage  74  then corresponds to the peripheral region of the block  94 .  
         [0038]      FIG. 9  shows a variant of the fifth more preferred embodiment, in which provision is made for the peripheral region of the block  94  of porous material (which is bounded in  FIG. 9  on the side facing liquid chamber  61  by dotted lines  97 ) in contact with the liquid  67  of the liquid chamber  61  to comprise or be covered by a hydrophilic material, so as to prevent the passage of the gas bubble  72  trapped in the block  94  of porous material.  
         [0039]      FIGS. 10 and 11  show a cross section and a top view, respectively, of a sixth more preferred embodiment, in which the expansion chamber  70  consists of a groove spiralled around the axis Δ and produced on the upper face of the annular ring  66 . One end  98  of the spiral emerges in the liquid chamber  61 , while the opposite end  99  is closed. The gas bubble is localized at the closed end  99  of the groove. The walls of the groove are covered with a hydrophobic material, at least at the closed end  99 , in order to encourage retention of the gas bubble in this part of the groove. Such an embodiment effectively prevents the gas bubble from escaping out of the groove in the event of shocks.  
         [0040]     The first, second, third, fourth and sixth more preferred embodiments have the advantage that the expansion chamber  70  is accessible throughout the process of manufacturing the lens, up to the final steps before the upper plate  62  is fitted. In this way, surface treatment processes, if necessary, may be easily carried out.  
         [0041]     In general, it is advantageous to place one or more baffles or labyrinth-forming members in the expansion chamber  70  and/or in the passage  74 , because this makes it possible to further reduce the risks of the gas bubble  72  penetrating into the liquid chamber  61 , especially in the case of sudden movements of the lens  60 . A baffle may be provided in a similar manner to that shown in  FIG. 7 , in the form of a sharply angled region or several sharply angled regions in the expansion chamber  70 . The baffle may also be provided in the passage  74 , or between the two. A baffle may also be produced in the form of one or more protuberances placed in the expansion chamber  70 , in contact with the liquid  67  or in the passage  74 .  
         [0042]      FIG. 12  shows another preferred embodiment, according to which baffles or labyrinth-forming members are placed in the passage  74  between the liquid chamber  61  and the expansion chamber  70 . In this example, the passage  74  is formed of a curved duct, having, for example, an “S” shape, therefore further reducing the risk of the gas bubble penetrating into the liquid chamber.  
         [0043]     According to another preferred embodiment of the invention, the gas can be in contact with either or both of the two liquids whose interface forms the moveable refractive interface.  FIG. 13  represents an embodiment, similar to one depicted in FIGS.  3  to  5 , except that the expansion chamber  70  is arranged on the side of the lens close to the plate  64 , whereby the gas bubble  72  is in contact with the liquid  68 .  
         [0044]     The method of manufacturing a lens according to embodiments of the invention may include a step of immersing the lens in the aqueous liquid at ambient pressure, and placing the oily liquid into the liquid chamber  61  before or after immersion of the lens  60  in the aqueous liquid. In this case, when the lens  60  is closed (that is to say when the plates  62 ,  64  have been fastened to the annular ring  66  in the embodiments described above), the internal pressure of the lens increases due to an excess amount of aqueous liquid that is trapped. As a result, even when closed at ambient pressure, an overpressure is obtained in the lens after closure.  
         [0045]     The embodiments described above are particularly suitable for the case in which the lens is filled with aqueous and oily liquids under partial vacuum, so that the liquids that the lens contains are naturally degassed. The internal pressure of the lens  60 , after closure of the lens  60 , is then equal to the saturation vapour pressure of the aqueous liquid. The saturation vapour pressure of the aqueous liquid is, in general, quite low. To give an example, in the case of water, it is of the order of 2.3 kPa at 20° C., 12.3 kPa at 50 C., 47.4 kPa at 80° C. and 101 kPa at 100° C. The pressure in the lens therefore remains below atmospheric pressure over the entire normal operating temperature range of the lens  60 . The upper and lower plates  62 ,  64  may therefore be placed on either side of the annular ring  66 , as shown in  FIG. 3 , so that the adhesive or the weld for fastening the plates  62 ,  64  to the annular ring  66  always works in compression. Furthermore, the change in internal pressure remains relatively small, even over a large temperature range. This is because, for temperatures varying from −40° C. to 80° C., the internal pressure of the lens  60 , according to embodiment of the invention, that is produced at a sub-atmospheric pressure, varies by less than one atmosphere.  
         [0046]     Of course, the teaching or concept of the present invention is capable of various alternative embodiments and modifications that will be apparent to those skilled in the art.  
         [0047]     In particular, in the case of the first, second, third and fourth more preferred embodiments, the expansion chamber need not be annular but may correspond to ring sectors distributed on the periphery of the central opening of the annular ring  66 .  
         [0048]     Moreover, the embodiments described above relate to a lens  60  comprised of three parts  62 ,  64  and  66 . However, it is clear that alternative embodiments of the present invention can be implemented for lenses of different structure, including a larger or smaller number of parts.  
         [0049]     A number of preferred embodiments of the invention have been described having a refractive interface moveable by the electrowetting effect. The present invention can also apply to embodiments of variable focus lenses in which the refractive interface between two liquids is moved by other phenomena, for example, by application of pressure.  
         [0050]     The volume of gas included in the liquid lens according to different embodiments of the invention can take different forms, including, but not limited to, one or a more gas bubbles, gas dispersed in the pores of a porous material, etc. The applicant has established that the percentage of volume of the gas compared to the overall internal volume of the lens (i.e., the volume containing the first and second liquids) is advantageously comprised between 5% and 50%, more advantageously between 10% and 20%, more advantageously around 15% (under atmospheric pressure), in order to compensate for the expansion.  
         [0051]     Although not limited to the application of the variable focus lens in a camera module to be integrated in a mobile phone, the invention is especially well suited for this application. The compensation for the change in temperature can most preferably be obtained with a rigid structure that does not have any elastic parts inducing deformation of the lens, and that can be manufactured using only a small number of pieces.  
         [0052]     Having thus described at least one illustrative embodiment of the invention, various alterations, modifications and improvements will readily occur to those skilled in the art. Such alterations, modifications and improvements are intended to be within the spirit and scope of the invention. Accordingly, the foregoing description is by way of example only and is not intended to be limiting. The invention is limited only as defined in the following claims and the equivalent thereto.