Patent Publication Number: US-2009225642-A1

Title: Switchable optical element

Description:
FIELD OF THE INVENTION 
     The present invention relates to a switchable optical element, particularly, but not exclusively, suitable for use in an optical scanning device for scanning an information layer of an optical record carrier; and to an optical scanning device including such an element. 
     BACKGROUND OF THE INVENTION 
     Switchable optical elements which have different states for introducing different wavefront modifications into radiation beams are known. Switching the optical element between different states changes the wavefront modification introduced into a radiation beam when passing through the optical element. 
     International patent application WO 04/027490 describes a switchable optical element which has two different states. The element has a chamber, which is filled with one fluid in one state and is filled with a different fluid in the other state. A wavefront modifier modifies a wavefront of a radiation beam passing through the chamber. The wavefront modification of the radiation beam depends upon both the state, which the optical element is switched into, and a wavelength of the radiation beam. 
     The switchable optical element disclosed in WO 04/027490 is included within an optical scanning device for scanning the information layer of different formats of optical record carriers, for example a Compact Disc (CD), a conventional Digital Versatile Disc (DVD) and a Blu-Ray™ Disc (BD). The wavelength of the radiation beam used to scan the record carriers is different for the different formats. Each format has a cover layer lying between the information layer and an exterior of the record carrier, which has a different thickness. By switching between different states of the switchable optical element these different thicknesses are accommodated so that the different formats of record carrier can be scanned correctly. 
     The variety of wavefront modifications which can be introduced into a radiation beam by the switchable optical element is relatively limited by the element&#39;s construction. Furthermore, the switchable optical element does not provide a functionality, which allows an optical scanning device to scan optical record carriers having multiple information layers. The element is relatively bulky as the element has a conduit, lying outside of an optically operative area, which is filled with one fluid whilst the other fluid fills the chamber. 
     International patent application WO 03/069380 discloses a variable focus lens having a meniscus, which separates two fluids. The fluid meniscus has a curvature, which introduces a focus into a radiation beam passing through the meniscus. Application of a voltage to electrodes of the lens varies a curvature of the meniscus in order to vary the focus of the radiation beam. An optical scanning device is disclosed which includes the variable focus lens and which can scan different information layers of a record carrier by varying the focus of the lens. 
     International patent application WO 04/051323 discloses apparatus for forming variable fluid meniscus configurations. The apparatus comprises a fluid meniscus, which separates two fluids. By applying different patterns of voltages to a configuration of electrodes surrounding the meniscus, different configurations of the meniscus can be obtained. In one embodiment, a radiation beam passes through a transparent electrode and is reflected by the fluid meniscus, which has a planar configuration, through a further transparent wall of the apparatus. In a different embodiment, the pattern of voltages applied to the electrodes causes the meniscus to adopt an asymmetric curvature, which introduces an applied focus into a radiation beam passing through the meniscus. Variation of the voltages causes the curvature of the meniscus to change. 
     SUMMARY OF THE INVENTION 
     It is an object of the present invention to provide a switchable optical element, which introduces different wavefront modifications into a radiation beam in a relatively efficient manner. 
     In accordance with one aspect of the present invention, there is provided a switchable optical element having an optical path for a radiation beam and having a first state and a second state, said element comprising: 
     a) a first fluid and a second, different, fluid which are immiscible and which are separated from each other by a fluid meniscus; 
     b) a first, transparent, wall part and a second, transparent, wall part spaced from each other along the optical path; and 
     c) a fluid switching system which is arranged to apply forces to said first and/or said second fluid in order to switch said element between said first and said second state, 
     wherein the first wall part includes a non-planar wavefront modifier, and wherein the fluid switching system is arranged to apply said forces so that: 
     when the element is in the first state, said first wall part is covered by the first fluid; and when the element is in the second state, said first wall part is covered by the second fluid, 
     characterised in that the fluid switching system is arranged to switch between said first state and said second state such that when the element is in the first state said second wall part is covered by the second fluid, and when the element is in the second state said second wall part is covered by the first fluid, wherein when the element is in both the first state and the second state at least part of said fluid meniscus is located on the optical path. 
     When a radiation beam passes along the optical path the switchable optical element, when in the first state, introduces a desired wavefront modification into the radiation beam. By switching the element to the second state a different, desired, wavefront modification can be introduced into a radiation beam. The form of each introduced wavefront modification depends upon factors including: a wavelength of the radiation beam, a material of the first fluid and of the second fluid, a structure and a material of the non-planar wavefront modifier, and whether the first and the second wall parts are covered by the first or the second fluid. Each of these factors provides a degree of freedom to a designer designing the switchable optical element to introduce particular wavefront modifications into different radiation beams. 
     With the fluid meniscus being located on the optical path when the element is in the first state and the second state, the first fluid and the second fluid are both located on the optical path. Switching between the first and the second states causes the two fluids to change places within the element so that they cover different of the first and second wall parts. It is therefore possible to switch between different forms of introduced wavefront modifications. The fluid meniscus may at least partially contribute to the form of an introduced wavefront modification in dependence on a configuration of the meniscus and/or on the materials of the first and the second fluids. In this way a further degree of design freedom is provided. 
     As a result of these different degrees of design freedom, the switchable optical element of the present invention has a relatively high level of customisation so that a relatively large variety of different and desired wavefront modifications can be introduced into at least one radiation beam and the wavefront modification can be changed simply by switching the state of the element or by changing a characteristic of the radiation beam, such as a wavelength. This is advantageous as the element can be designed and constructed to meet a relatively wide range of criteria required by optical applications needing to introduce a wavefront modification into a radiation beam. 
     In accordance with a further preferred embodiment of the present invention, when the element is in at least one of the first and the second state, said fluid meniscus is capable of being arranged to focus a radiation beam travelling along the optical path. 
     With the fluid meniscus being capable of focusing a radiation beam travelling along the optical path, a wavefront modification can be introduced such that a radiation beam is focused as a spot onto a desired position. This allows the switchable optical element to be used in optical applications where it is desirable to focus a radiation beam onto a desired position. 
     Preferably, in accordance with an embodiment of the present invention, when said element is in at least one of the first and the second state, said fluid meniscus has a curvature which determines an amount of said focus and said element comprises a variable focus system capable of varying the curvature of the fluid meniscus. 
     Variation of the curvature of the fluid meniscus allows the focus of the radiation beam to be varied in a relatively efficient and controlled manner. This, consequently, allows the switchable optical element to be used in optical applications where it is desirable to vary the position, which a radiation beam is focused onto. 
     In a preferred embodiment of the present invention, said fluid switching system includes a first and a second switching electrode, wherein 
     said fluid switching system is arranged to apply a voltage to said first switching electrode in order to switch said element to said first state, and wherein 
     said fluid switching system is arranged to apply a voltage to said second switching electrode in order to switch said element to said second state. 
     Application of a voltage to different electrodes in order to switch the state of the element allows the switching to be performed in a relatively rapid and efficient manner. This minimises any delay between the introduction of one wavefront modification and the introduction of a different wavefront modification into a radiation beam. 
     In a further preferred embodiment of the present invention said optical element includes a further electrode, wherein 
     when switching between the first state and the second state, said fluid switching system is arranged to apply a voltage to said further electrode, and wherein 
     when varying the curvature of the fluid meniscus, said variable focus system is arranged to apply a voltage to said further electrode. 
     Application of a voltage to the further electrode when switching between the first and the second state changes the positions of the first and the second fluids within the element in a relatively rapid manner such that the switching of the element is relatively efficient. 
     Preferably, in an embodiment of the present invention said non-planar wavefront modifier includes at least one of: an aspherical lens; a phase structure having a periodic profile; and a phase structure having a non-periodic profile. 
     Selection of a specific structure of the non-planar wavefront modifier allows a desired wavefront modification to be introduced into a radiation beam travelling along the optical path. In the case where the wavefront modifier includes more than one different type of wavefront modifier, different forms of wavefront modifications can be introduced into radiation beams having different predetermined wavelengths. 
     In a preferred embodiment of the present invention, said first fluid has a first refractive index and said second fluid has a second, different, refractive index, wherein said non-planar wavefront modifier is formed of a material having a refractive index which is approximately the same as said second refractive index. 
     With the material of the non-planar wavefront modifier having the same refractive index as that of the second fluid, and when the element is in the second state with the second wall part covered by the first fluid, the wavefront modifier introduces a zero wavefront modification in a radiation beam passing along the optical path. In this way a functionality of the wavefront modifier, and therefore the introduction of a wavefront modification into the radiation beam, can be switched on and off by switching between the states of the switchable optical element. 
     According to a further aspect of the present invention, there is provided an optical scanning device for scanning a record carrier having an information layer, wherein said optical scanning device comprises: 
     a) a switchable optical element in accordance with the invention; 
     b) a radiation beam source system arranged to provide a radiation beam having a predetermined wavelength; 
     c) an objective lens system arranged to focus said provided radiation beam to a scanning spot on said information layer; and 
     d) a detection system arranged to detect the radiation beam after scanning of the information layer and to convert radiation of the detected radiation beam into electrical signals. 
     The switchable optical element included in the optical scanning device is adapted to introduce a wavefront modification into a radiation beam for scanning a record carrier in a relatively accurate manner. Furthermore, with suitable adaptation of the switchable optical element, the optical scanning device can scan optical record carriers of different formats and/or comprising a plurality of information layers in a relatively accurate manner. 
     Further features and advantages of the invention will become apparent from the following description of preferred embodiments of the invention, given by way of example only, which is made with reference to the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  shows schematically a cross section of a switchable optical element in a first state in accordance with an embodiment of the present invention. 
         FIG. 2  shows schematically a cross section of the switchable optical element, in accordance with an embodiment of the present invention. 
         FIG. 3  shows schematically a cross section of the switchable optical element in an intermediate state in accordance with an embodiment of the present invention. 
         FIG. 4  shows schematically a cross section of the switchable optical element in a second state in accordance with an embodiment of the present invention. 
         FIG. 5  shows schematically a cross section of a switchable optical element in a first state in accordance with a different embodiment of the present invention. 
         FIG. 6  shows schematically a cross section of the switchable optical element, in accordance with the different embodiment of the present invention. 
         FIG. 7  shows schematically a cross section of the switchable optical element in an intermediate state in accordance with the different embodiment of the present invention. 
         FIG. 8  shows schematically an optical scanning device in accordance with an embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       FIG. 1  shows schematically a cross section of a switchable optical element  2  in accordance with a first embodiment of the present invention.  FIG. 2  shows schematically a cross section of the element  2  taken along the line A-A indicated in  FIG. 1 . The element  2  comprises a cylindrical electrode  4  having a longitudinal axis, which is coincident with an optical path OP of the element  2 . A first end of the cylindrical electrode  4  is covered by a first wall  6  and a second end of the cylindrical electrode  4  is covered by a second wall  8 . The first wall  6 , the second wall  8  and the cylindrical electrode  4  define a cylindrical chamber  10  which encloses a first fluid  12  and a second, different, fluid  14  which are immiscible with each other. A fluid meniscus  16  separates the first fluid  12  from the second fluid  14  and is located at least partly on the optical path OP. In this embodiment the meniscus  16  is centred about the optical path OP. The chamber  10  has a radius r taken perpendicular from the optical path OP. 
     In this embodiment the first fluid  12  is an electrically conductive liquid with a first refractive index, for instance a solution of potassium chloride (KCI) in water having a concentration of 0.1M and a refractive index of 1.34, and the second fluid  14  is an electrically insulative liquid with a second refractive index, for instance a copolymer of polyphenylmethylsiloxane and polydimethylsiloxane having a refractive index of 1.50. It is envisaged that the second fluid may alternatively be electrically conductive, but have a lower electrical conductivity than the first fluid  12 . 
     The first wall  6  comprises a first plate  18  and the second wall  8  comprises a second plate  20 . Each of the first and second plates  18 ,  20  are circular, planar, centred about the optical path OP, and formed of a transparent material which, in this example, is glass. A first switching electrode  22  lies in contact with both a surface of the first plate  18 , which faces towards the chamber  10 , and a peripheral surface of the first plate  18 . The first switching electrode  22  is circular, has a U-shaped cross section, is centred about the optical path OP and is formed of a transparent and electrically conductive material which, in this example, is indium tin oxide (ITO). A second switching electrode  24  lies in contact with both a surface of the second plate  20  which faces towards the chamber  10  and with a peripheral surface of the second plate  20 . The second switching electrode  24  is circular, has a U-shaped cross section, is centred about the optical path OP and is formed of a transparent and electrically conductive material which, in this example, is indium tin oxide (ITO). A hydrophobic layer  26  which is electrically insulative and transparent, coats a surface of the second switching electrode  24  which faces towards the chamber  10 . The hydrophobic layer  26  also coats an inner surface of the cylindrical electrode  4  and separates and insulates the first and the second switching electrodes  22 ,  24  from end surfaces of the cylindrical electrode  4 . In this example the hydrophobic layer  26  is formed of Teflon™ AF 1600 produced by Dupont™. 
     A part of the first wall  6  and a part of the second wall  8  are transparent and are spaced from each other along the optical path OP. The first wall part and the second wall part each have a radius, in a direction perpendicular the optical path OP, which is the same as the radius r of the chamber  10 . The first wall part and the second wall part each have a thickness, in a direction parallel the optical path OP, which is the same as a thickness, taken in the same direction, of the first wall  6  and the second wall  8 , respectively. 
     The first wall part includes a non-planar wavefront modifier  28 , which includes at least one of an aspherical lens; a phase structure having a periodic profile; and a phase structure having a non-periodic profile. In this embodiment the non-planar wavefront modifier  28  is an aspherical lens  30 , which includes a non-periodic phase structure (NPS)  32 . The NPS  32  is an annular phase structure comprising a plurality of annular protrusions, which have a non-periodic profile. The NPS  32  is formed within a surface of the aspherical lens  30 . A surface of the aspherical lens  30  and a surface of the NPS  32  constitute a wavefront modifier surface  34  of the first wall part which is coated with the hydrophobic layer  26 . The wavefront modifier  28  is mounted on a portion of the first switching electrode and is centred about the optical path OP such that the aspherical lens  30 , which is circular, and the NPS  32 , are centred about the optical path OP. The wavefront modifier  28  is constructed in accordance with specific dimensions such that a desired wavefront modification can be introduced by the wavefront modifier  28  into a radiation beam passing along the optical path OP. The dimensions of the wavefront modifier  28 , as illustrated in  FIG. 1 , are not shown according to scale and are not intended to indicate the specific dimensions of the wavefront modifier  28 . In this embodiment the wavefront modifier  28  is formed of a material, which has approximately the same refractive index as the second refractive index, for example hexanediol diacrylate (HDDA) which has a refractive index, for a radiation beam having a predetermined wavelength of 408 nm, of 1.51. 
     The element  2  comprises a ground electrode  36  which is inserted through an opening in the cylindrical electrode  4  and which lies in contact with the first fluid  12 . The ground electrode  36  is electrically connected to a ground terminal  40 . The hydrophobic layer  26  coats an inner surface of the opening in the cylindrical electrode  4  in order to insulate the ground electrode  36  from the cylindrical electrode  4 . 
     The fluid meniscus  16  is capable of being arranged to focus a radiation beam travelling along the optical path OP. The fluid meniscus  16  has a curvature, which determines an amount of this focus. The curvature is rotationally symmetric about the optical path OP and is shown in  FIG. 1  as having a convex curvature when viewed from the second wall  8 . The element  2  comprises a variable focus system (not indicated) which is capable of varying the curvature by applying electrowetting forces to the first and the second fluids  12 ,  14 . The variable focus system comprises a first voltage source V 1  which is electrically connected to the cylindrical electrode  4  and to the ground terminal  40 . Application of a voltage by the first voltage source V 1  to the cylindrical electrode  4  determines an extent of a contact angle α between the fluid meniscus  16  and a portion of the hydrophobic layer  26  which covers the inner surface of the cylindrical electrode  4 . The extent of the contact angle α determines the curvature of the meniscus  16 . Variation of the voltage applied by the first voltage source V 1  varies the extent of the contact angle α and consequently varies the curvature of the meniscus  16  such that convex, concave or planar curvatures may be obtained. 
     The element  2  comprises a fluid switching system (not indicated), which is arranged to apply electrowetting forces to the first and/or the second fluids  12 ,  14  in order to switch the element  2  between a first state and a second state. The first state and the second state will be described later in further detail. The fluid switching system comprises the first switching electrode  22 , the second switching electrode  24 , a second voltage source V 2  and a third voltage source V 3 . The fluid switching system comprises an electrical connection system  38  which electrically connects the second voltage source V 2  to the second switching electrode  24  and to the ground terminal  40 , and which electrically connects the third voltage source V 3  to the first switching electrode  22  and to the ground terminal  40 .  FIG. 1  shows the element  2  in the first state where the first wall part, including the wavefront modifier  28 , is covered by the first fluid  12  and the second wall part is covered by the second fluid  14 . The fluid switching system applies electrowetting forces to the first and second fluids  12 ,  14  by application of an appropriate voltage from the third voltage source V 3  to the first switching electrode  22 . In this first state, the second voltage source V 2  does not apply a voltage to the second switching electrode  24 . 
       FIG. 3  shows the element  2  in an intermediate state, which occurs during switching the element  2  between the first state and the second state. In this state, the second voltage source V 2  does not apply a voltage to the second switching electrode  24 , the third voltage source V 3  does not apply a voltage to the first switching electrode  22  and the first voltage source V 1  does not apply a voltage to the cylindrical electrode  4 . The first fluid  12  has a configuration, which minimises contact of the first fluid  12  with the hydrophobic layer  26 . At least one further fluid meniscus  42 , which is different to the fluid meniscus  16  described previously, maintains the separation of the first and the second fluids  12 ,  14  when the element  2  is in the intermediate state. In further embodiments of the present invention, when the element  2  is in the intermediate state, a voltage may also be applied by the second voltage source V 2  to the second switching electrode  24  in order to accelerate switching of the element from the first state to the second state. Similarly, when switching to the first state from the second state, the third voltage source V 3  may apply a voltage to the first switching electrode  22  when the element is in the intermediate state. 
       FIG. 4  shows the element  2  in the second state where the first wall part, including the wavefront modifier  28 , is covered by the second fluid  14  and the second wall part is covered by the first fluid  12 . The fluid switching system applies electrowetting forces to the first and second fluids  12 ,  14  by application of an appropriate voltage from the second voltage source V 2  to the second switching electrode  24 . In this second state, the third voltage source V 3  does not apply a voltage to the first switching electrode  22 . In  FIG. 4  the fluid meniscus  16  is shown having a curvature, which is convex when viewed from the first wall  6 . The first voltage source V 1  applies an appropriate voltage to the cylindrical electrode  4  in order to achieve this convex curvature. 
       FIGS. 5 ,  6  and  7  show an optical switchable element  44  of the present invention in accordance with a different embodiment. Features described in accordance with this embodiment are similar to those described in accordance with the previous embodiment. Such features are referenced using the same reference numerals incremented by 100 and similar descriptions should be taken to apply here also. 
       FIG. 5  shows the switchable optical element  44  in the first state and  FIG. 6  shows a cross-section of the element  44  taken along the line B-B indicated in  FIG. 5 . In this embodiment there is no cylindrical electrode, as described for the previous embodiment. The chamber  110  is defined by the first and second walls  106 ,  108 , a semi-cylindrical electrode  46  and a further semi-cylindrical electrode  48 . The semi-cylindrical electrodes  46 ,  48  are aligned relatively to each other to form a cylinder. The first wall  106  and the second wall  108  cover a first end and a second end of this cylinder, respectively. The hydrophobic layer  126  covers an inner surface of this cylinder and an insulating layer  50  is located along longitudinal edges of the semi-cylindrical electrodes  46 ,  48  in order to separate and insulate the two semi-cylindrical electrodes  46 ,  48  from each other. This insulating layer  50  is, for example, formed of parylene, polyethylene, or Teflon™ 1600 produced by DuPont™. 
     The semi-cylindrical electrode  46  is electrically connected to the first voltage source V 1 . A fourth voltage source V 4  is electrically connected to the further semi-cylindrical electrode  48  and to the ground terminal  40 . The ground electrode  136  is inserted through, and insulated from, the further semi-cylindrical electrode  48  in a similar manner to that described previously for the cylindrical electrode. 
     In this embodiment, with the element  44  in the first or the second state, the variable focus system determines the curvature of the fluid meniscus  116  by application of an appropriate voltage from the first voltage source V 1  to the semi-cylindrical electrode  46 . The variable focus system also controls the fourth voltage source V 4  to apply a voltage, which is the same as the appropriate voltage applied by the first voltage source V 1 , to the further semi-cylindrical electrode  48 . In this way the curvature of the fluid meniscus  116  is determined and the curvature is rotationally symmetric about the optical path OP. In order to vary the curvature of the fluid meniscus  116 , the voltage applied by the first voltage source V 1  to the semi-cylindrical electrode  46  and the voltage applied by the fourth voltage source V 4  to the further semi-cylindrical electrode  48 , are varied in unison such that these two voltages remain identical to each other. 
       FIG. 7  shows the switchable optical element  44  in an intermediate state during switching of the element  44  between the first state and the second state. In the intermediate state the first voltage source V 1 , the second voltage source V 2  and the third voltage source V 3  do not apply a voltage to the semi-cylindrical electrode  46 , the first switching electrode  122  and the second switching electrode  124 , respectively. The fluid switching system controls the fourth voltage source V 4  to apply an appropriate voltage to the further semi-cylindrical electrode  48 . This voltage applies electrowetting forces to the first and second fluids  112 ,  114  such that the first fluid  112  covers a portion of the hydrophobic layer  126  lying between the chamber  110  and the further semi-cylindrical electrode  48  and such that the first fluid  112  lies in contact with the ground electrode  136 . The second fluid  114  covers a portion of the hydrophobic layer  126  lying between the chamber  110  and the semi-cylindrical electrode  46 . Both the first wall part and the second wall parts are partially covered by the first fluid  112  and the second fluid  114 . 
       FIG. 8  shows schematically an optical scanning device  51  including a switchable optical element  52  in accordance with an embodiment of the present invention. The optical scanning device can scan a first format, a second, different, format and a third, different, format of optical record carrier with a first radiation beam having a first predetermined wavelength λ 1 , a second radiation beam having a second, different, predetermined wavelength λ 2 , and a third radiation beam  55  having a third, different, predetermined wavelength λ 3 , respectively. The third format of optical record carrier  53  is illustrated in  FIG. 8  and has a first information layer  54  and a second information layer  56  which are scanned by means of the third radiation beam  55 . The third format of optical record carrier  53  includes a cover layer  57 , formed for example of polycarbonate, on one side of which the first information layer  54  is arranged. The side of the second information layer  56  facing away from the cover layer  57  is protected from environmental influences by a further cover layer  58 . The first information layer  54  and the second information layer  56  are separated by a separating layer  60 . The cover layer  57  and/or the separating layer  60  act as a substrate for the third optical record carrier  53  by providing mechanical support for the first and second information layers  54 ,  56 . The cover layer  57  may, alternatively, have the function of protecting the first information layer  54 , while the mechanical support is provided by at least one of the separating layer  60 , the second information layer  56  and the further cover layer  58 . The first information layer  54  has a first information layer depth d 1  within the record carrier, which corresponds to the thickness of the cover layer  57 . The second information layer  56  has a second information layer depth d 2  within the record carrier, which corresponds to the combined thickness of the cover layer  57  and the separating layer  60 . The first and second formats of optical record carrier have at least one information layer lying at a different first information layer depth, which corresponds with the thickness of the cover layer of the first and second format of optical record carriers, respectively. The first information layer depth d 1  of the third format of optical record carrier  53  is less than the first information layer depth of the second optical record carrier, which is less than the first information layer depth of the first optical record carrier. 
     The first information layer  54  and the second information layer  56  are surfaces of the third optical record carrier  53 . Similarly the first information layer of the first and second formats of optical record carrier, are surfaces. Each of these surfaces contains at least one track, i.e. a path to be followed by the spot of a focused radiation on which path optically-readable marks are arranged to represent information. The marks may be, e.g., in the form of pits or areas with a reflection coefficient or a direction of magnetisation different from the surroundings. In the case where the third format of optical record carrier  53  has the shape of a disc, the following is defined with respect to a given track: the “radial direction” is the direction of a reference axis, the X-axis, between the track and the centre of the disc and the “tangential direction” is the direction of another axis, the Y-axis, that is tangential to the track and perpendicular to the X-axis. In this embodiment the first format of optical record carrier is a compact disc (CD) and the first information layer depth is approximately 1.2 mm, the second format of optical record carrier is a conventional digital versatile disc (DVD) and the first information layer depth is approximately 0.6 mm, and the third format of optical record carrier  3  is a Blu-ray™ disc (BD) and the first information layer depth d 1  is approximately 0.075 mm. The second information layer depth d 2  of the third format of optical record carrier  53  is approximately 0.1 mm. 
     As shown in  FIG. 8 , the optical scanning device  51  has an optical path OP and includes a radiation source system  62 , a collimator lens  64 , a beam splitter  66 , an objective lens system comprising the switchable optical element  52  and an objective lens  67 , and a detection system  68 . Furthermore, the optical scanning device  51  includes a servo circuit  70 , a focus actuator  71 , a radial actuator  72 , and an information processing unit  73  for error correction. 
     The radiation source system  62  is arranged for consecutively or simultaneously producing the first radiation beam, the second radiation beam and/or the third, different, radiation beam  55 . For example, the radiation source system  62  may comprise either a tunable semiconductor laser for consecutively supplying the radiation beams or three semiconductor lasers for simultaneously or consecutively supplying these radiation beams. In this embodiment the third wavelength λ 3  is shorter than the second wavelength λ 2 . The second wavelength λ 2  is shorter than the first wavelength λ 1 . In this embodiment the first, second and third wavelength λ 1 , λ 2 , λ 3 , respectively, is within the range of approximately 770 to 810 nm for λ 1 , 640 to 680 nm for λ 2 , 400 to 420 nm for λ 3  and preferably approximately 785 nm, 650 nm and 405 nm, respectively. The first, second and third radiation beams have a numerical aperture (NA) of approximately 0.5, 0.65 and 0.85 respectively. 
     The collimator lens  64  is arranged on the optical path OP for transforming the third radiation beam  55  into a third substantially collimated beam  74 . Similarly, it transforms the first and second radiation beams into a first substantially collimated beam and a second substantially collimated beam (not illustrated in  FIG. 8 ). 
     The beam splitter  66  is arranged for transmitting the first, second and third collimated radiation beams toward the objective lens system. Preferably, the beam splitter  66  is formed with a plane parallel plate that is tilted with an angle β with respect to the optical path OP and, preferably, β=45°. 
     The objective lens system is arranged to focus the first, second and third collimated radiation beams to a desired focal point on the first, second and third optical record carriers, respectively. The desired focal point for the first, second and third radiation beams is a first, second and third scanning spot  75 , respectively. Each scanning spot corresponds to a position on the first information layer of the appropriate optical record carrier or in the case of the third format  53 , alternatively a position on the second information layer  56 . Each scanning spot is preferably substantially diffraction limited and has a wave front aberration, which is less than 70 mλ. 
     The switchable optical element  52  in accordance with this embodiment may have the form of the embodiment described in relation to  FIGS. 1 to 4 , or the form of the embodiment described in  FIGS. 5 to 7 . In this embodiment the wavefront modifier  28 ;  128  comprises the aspherical lens  30 ;  130  and the NPS  32 ;  132 . The wavefront modifier  28 ;  128  has specific dimensions which introduce a different predetermined wavefront modification into the first radiation beam and into the second radiation beam. The wavefront modifier  28 ;  128  is formed of hexanediol diacrylate (HDDA) and the part of the hydrophobic layer  26 ;  126  which covers the wavefront modifier surface  34 ;  134  has a thickness which is smaller than the first, second and third wavelength λ 1 , λ 2 , λ 3  so that the hydrophobic layer  26 ;  126  does not modify the wavefront of the first, second and third radiation beams. The first fluid  12 ;  112  is salted water and the second fluid  14 ;  114  is oil. The oil has the same, or at least approximately the same, refractive index for a radiation beam having a predetermined wavelength of 408 nm as the refractive index of the material of the wavefront modifier  28 ;  128 . The radius r of the chamber  10 ;  110  is approximately 5 mm. 
     The objective lens  67  has a NA of approximately 0.85 and is optimised to focus a radiation beam having a predetermined wavelength of approximately 405 nm onto an information layer lying at a depth of approximately 0.1 mm within an optical record carrier. 
     During scanning, and with the switchable optical element  52  in the second state, the third optical record carrier  53  rotates on a spindle (not illustrated in  FIG. 8 ) and the first information layer  54  is then scanned through the cover layer  57 . The focused third radiation beam  74  reflects on the first information layer  54 , thereby forming a reflected third radiation beam which returns on the optical path OP of the forward converging focused third radiation beam provided by the objective lens system. The objective lens system transforms the reflected third radiation beam to a reflected collimated third radiation beam  76 . The beam splitter  66  separates the forward third radiation beam  74  from the reflected third radiation beam by transmitting at least a part of the reflected third radiation beam  76  towards the detection system  68 . 
     The detection system  68  includes a convergent lens  78  and a quadrant detector  80  which are arranged for capturing said part of the reflected third radiation beam  76  and converting it to one or more electrical signals. One of the signals is an information signal I data , the value of which represents the information scanned on the information layer  54 . The information signal I data  is processed by the information processing unit  73  for error correction. Other signals from the detection system  68  are a focus error signal I focus  and a radial tracking error signal I radial . The signal I focus  represents the axial difference in height along the optical path OP between the third scanning spot  75  and the position of the first information layer  54 . Preferably, this signal is formed by the “astigmatic method” which is known from, inter alia, the book by G. Bouwhuis, J. Braat, A. Huijser et al, entitled “Principles of Optical Disc Systems,” pp. 75-80 (Adam Hilger 1985) (ISBN 0-85274-785-3). A device for creating an astigmatism according to this focusing method is not illustrated. The radial tracking error signal I radial  represents the distance in the XY-plane of the first information layer  54  between the first scanning spot  75  and the center of a track in the information layer  54  to be followed by the first scanning spot  75 . Preferably, this signal is formed from the “radial push-pull method” which is known from, inter alia, the book by G. Bouwhuis, pp. 70-73. 
     The servo circuit  70  is arranged for, in response to the signals I focus  and I radial , providing servo control signals I control  for controlling the focus actuator  71  and the radial actuator  72 , respectively. The focus actuator  71  controls the position of a lens of the objective lens system along the optical path OP, thereby controlling the position of the scanning spot  75  such that it coincides substantially with the plane of the first information layer  54 . The radial actuator  72  controls the position of the objective lens  67  of the objective lens system along the X-axis, thereby controlling the radial position of the scanning spot  75  such that it coincides substantially with the centre line of the track to be followed in the first information layer  54 . 
     When scanning the first information layer  54  of the third format of optical record carrier  53 , the switchable optical element  52  is in the second state and the variable focus system controls the fluid meniscus  216  so that the curvature is convex when viewed from the second wall  208  and has a curvature of approximately 0.02 mm −1 . The variable focus system can vary the curvature of the fluid meniscus  16 ;  116  to a planar curvature of 0.00 mm −1  in order to move the scanning spot  75  of the third radiation beam  74  from the first information layer  54  onto the second information layer  56 . 
     When scanning the first format of record carrier and the second format of record carrier using the first and second radiation beam, respectively, the switchable optical element  52  is switched into the first state and the variable focus system controls the fluid meniscus  16 ;  116  so that the curvature is planar. 
     When scanning the first information layer of any of the formats of optical record carrier the cover layer introduces an amount of spherical aberration into the radiation beam scanning the optical record carrier. When scanning the second information layer  56  of the third format of record carrier  53 , the cover layer  57  and the separating layer  60  introduce an amount of spherical aberration into the third radiation beam  74 . 
     When scanning the first information layer of one of the formats of optical record carrier, the objective lens system introduces an amount of spherical aberration, into the radiation beam, which has an opposite sign and which is of approximately the same amount as the amount of spherical aberration introduced by the cover layer. When scanning the second information layer  56  of the third format of optical record carrier  53  the objective lens system introduces an amount of spherical aberration, into the third radiation beam  74 , which has an opposite sign and which is of approximately the same amount as the amount of spherical aberration introduced by the cover layer  57  and the separating layer  60 . Switching the switchable optical element  52  into the appropriate state and varying the curvature of the fluid meniscus  16 ;  116  determines the amount of spherical aberration introduced by the objective lens system and ensures that the radiation beam is focused accurately to the scanning spot. When scanning the first, second and third formats of optical record carrier, a root mean square (RMS) wavefront aberration of the scanning spot is approximately 13 mλ, 9 mλ and 15 mλ, respectively. 
     The above embodiments are to be understood as illustrative examples of the invention. Further embodiments of the invention are envisaged. For example, in further embodiments where it is desired for the switchable optical element to introduce a different wavefront modification into a radiation beam of a same or a different predetermined wavelength to those described previously, the refractive index of the first and second fluids, the refractive index of the material of the wavefront modifier, the structure of the non-planar wavefront modifier, materials of the switchable optical element, a volume of the first and the second fluids and dimensions of the switchable optical element such as the radius of the chamber or a thickness or configuration of the first and/or the second wall parts, may be different to those described above. 
     In the embodiments of the present invention described, the second fluid has a refractive index, which is the same as the refractive index of the material of the wavefront modifier. In further embodiments the material of the wavefront modifier may alternatively have a refractive index which is different to the refractive index of the first and the second fluid. It is further envisaged that at least one of the fluids may be a gas, instead of being a liquid. 
     In further embodiments, in addition to the first wall part including a wavefront modifier, the second wall part may also include a non-planar wavefront modifier, which includes, for example, at least one of an aspherical lens, a non-periodic phase structure and a periodic phase structure. 
     In further embodiments the electrodes for switching the state of the element and/or for varying the curvature of the fluid meniscus, may be arranged according to a different configuration, or may have different shapes. For example, the first switching electrode may alternatively be arranged across the surface of the wavefront modifier. In further embodiments the switchable optical element may have a configuration of electrodes without the ground electrode. In such embodiments a voltage is applied to at least two of the electrodes of the element in order to position the first and second fluids as desired. For example, for the element described previously with reference to  FIGS. 5 to 7 , but without the ground electrode, the element may be switched from the first state, where a voltage is applied to the first switching electrode and to each of the semi-cylindrical electrodes, to the second state, where a voltage is applied to the second switching electrode and to both of the semi-cylindrical electrodes, by applying a voltage to both the first switching electrode and the further semi-cylindrical electrode and then by applying a voltage to both the second switching electrode and the further semi-cylindrical electrode. In this way the first fluid and second fluid are moved within the chamber. The magnitude of the applied voltages is relatively higher than that of the applied voltages described for previous embodiments and the first fluid adopts a voltage with a magnitude less than that of the applied voltages, for example, approximately half the magnitude. 
     It is further envisaged that the fluid switching system and/or the variable focus system may apply different types of forces, which are not electrowetting forces, to the fluids in order to switch the state of the element and to vary the curvature of the fluid meniscus, respectively. In different embodiments where a variable focus of a radiation beam is not desired, the switchable optical element does not include the variable focus system described. In such embodiments, the element may be arranged to provide a fixed focus to a radiation beam. 
     In previously described embodiments it has been described that convex, concave and planar curvatures of the fluid meniscus are obtainable, all of which are rotationally symmetric about the optical path OP. In further embodiments, curvatures of the fluid meniscus may be achieved which are not rotationally symmetric about the optical path OP. 
     In further embodiments of the present invention, the element may have a conduit which is fluidly connected to the chamber, or the element may comprise two concentric cylinders, which facilitate movement of the first and the second fluids within the element during switching of the state of the element. 
     The switchable optical element is described as being included within an optical scanning device for scanning three formats of optical record carrier, one format having two information layers. In further embodiments the switchable optical element may be adapted such that, when included within an optical scanning device and by switching the states of the switchable optical element, the optical scanning device can scan different formats of optical record carrier, which may include CD, DVD or Blu-Ray™ and which may have one or a plurality of information layers. By varying the curvature of the fluid meniscus, it is envisaged that different information layers can be scanned of a record carrier having a plurality of information layers. 
     It is to be understood that any feature described in relation to any one embodiment may be used alone, or in combination with other features described, and may also be used in combination with one or more features of any other of the embodiments, or any combination of any other of the embodiments. Furthermore, equivalents and modifications not described above may also be employed without departing from the scope of the invention, which is defined in the accompanying claims.