Abstract:
The invention relates to a system for shifting at least one light spot ( 903 ), said system comprising: —a phase-modulator ( 906 ) for generating, from an input light beam, a light beam which is phase-modulated according to a controllable phase profile, —an array of apertures ( 902 ) for generating said at least one light spot ( 903 ) from said phase-modulated light beam, a change of the phase profile implying a shift of said at least one light spot ( 903 ). Use: Optical storage

Description:
FIELD OF THE INVENTION  
       [0001]     The invention relates to a system for shifting at least one light spot, for example over an information carrier in view of data recovering.  
         [0002]     The invention may be used in the field of optical data storage.  
       BACKGROUND OF THE INVENTION  
       [0003]     Use optical storage is nowadays widespread for content distribution, for example in storage systems based on the DVD (Digital Versatile Disk) standards. Optical storage has a big advantage over hard-disk and solid-state storage in that information carriers are easy and cheap to duplicate.  
         [0004]     However, due to the large amount of moving parts in the drives, known applications using this type of storage are not robust to shocks when performing read operations, considering the required stability of said moving parts during such operations. As a consequence, optical storage cannot easily be used in applications which are subject to shocks, such as in portable devices.  
       OBJECT AND SUMMARY OF THE INVENTION  
       [0005]     It is an object of the invention to propose a new system for shifting at least one light spot, for example over an information carrier in view of data recovering. To this end, the system for shifting said at least one light spot comprises: 
        a phase-modulator for generating, from an input light beam, a light beam which is phase-modulated according to a controllable phase profile,     an array of apertures for generating said at least one light spot from said phase-modulated light beam, 
 
 a change of the phase profile implying a shift of said at least one light spot. 
       
 
         [0008]     The shift of the light spot is realized by exploiting the Talbot effect created by the array of apertures, in combination with the action of the phase-modulator. The shift of the light spot can be a lateral shift intended to scan the information carrier surface during a reading operation, or can be an axial shift intended to accurately focus the light spot on the surface of the information carrier for improving the quality of the reading operation.  
         [0009]     The shift of the light spot is realized without any mechanics, so that the system defines a robust mechanical solution. This system can thus advantageously be implemented in a portable device which is subject to shocks.  
         [0010]     This solution is applicable to the shift of a single light spot on an information carrier, but it is also applicable to an array of light spots which are simultaneously applied to the surface of an information carrier.  
         [0011]     The invention also relates to a reading apparatus for reading data stored on an information carrier, said reading apparatus comprising a system according to the invention.  
         [0012]     Detailed explanations and other aspects of the invention will be given below. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0013]     The particular aspects of the invention will now be explained with reference to the embodiments described hereinafter and considered in connection with the accompanying drawings, in which identical parts or sub-steps are designated in the same manner:  
         [0014]      FIG. 1  depicts a first system according to the invention,  
         [0015]      FIG. 2  depicts a second system according to the invention,  
         [0016]      FIG. 3  depicts a detailed view of components dedicated to macro-cell scanning used in systems according to the invention,  
         [0017]      FIG. 4  illustrate the principle of macro-cell scanning according to the invention,  
         [0018]      FIG. 5  depicts a three-dimensional view of the first system according to the invention,  
         [0019]      FIG. 6  depicts a first arrangement for moving the systems according to the invention over an information carrier,  
         [0020]      FIG. 7  depicts a second arrangement for moving the systems according to the invention over an information carrier,  
         [0021]      FIG. 8  depicts detailed elements of the second arrangement for moving the systems according to the invention over an information carrier,  
         [0022]      FIG. 9  depicts a third system according to the invention,  
         [0023]      FIG. 10  depicts a three-dimensional view of the third system according to the invention,  
         [0024]      FIG. 11  depicts an electrically controllable liquid crystal cell and a voltage control curve of such a liquid crystal cell,  
         [0025]      FIG. 12  illustrates various apparatus and devices comprising a system according to the invention. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0026]     The system according to the invention aims at reading data stored on an information carrier. The information carrier is intended to store binary data organized according to an array, as in a data matrix. If the information carrier is intended to be read in transmission, the states of binary data stored on the information carrier are represented by transparent areas and non-transparent areas (i.e. light-absorbing). Alternatively, if the information carrier is intended to be read in reflection, the states of binary data stored on the information carrier are represented by non-reflective areas (i.e. light-absorbing) and reflective areas. The areas are marked in a material such as glass, plastic or a material having magnetic properties.  
         [0000]     The system according to the invention comprises:  
         [0000]    
       
         
           
              an optical element for generating an array of light spots from an input light beam, said array of light spots being intended to scan said information carrier,  
              a detector for detecting said data from an array of output light beams generated by said information carrier.  
           
         
       
     
         [0029]     In a first embodiment depicted in  FIG. 1 , the system according to the invention for reading data stored on an information carrier  101  comprises an optical element  102  for generating an array of light spots  103  from an input light beam  104 , said array of light spots  103  being intended to scan the information carrier  101 .  
         [0030]     The optical element  102  corresponds to a two-dimensional array of micro-lenses to the input of which the coherent input light beam  104  is applied. The array of micro-lenses  102  is placed parallel and distant from the information carrier  101  so that light spots are focused on the information carrier. The numerical aperture and quality of the micro-lenses determines the size of the light spots. For example, a two-dimensional array of micro-lenses  102  having a numerical aperture which equals 0.3 can be used. The input light beam  104  can be realized by a waveguide (not represented) for expanding an input laser beam, or by a two-dimensional array of coupled micro lasers.  
         [0031]     The light spots are applied on transparent or non-transparent areas of the information carrier  101 . If a light spot is applied on a non-transparent area, no output light beam is generated in response by the information carrier. If a light spot is applied on a transparent area, an output light beam is generated in response by the information carrier, said output light beam being detected by the detector  105 . The detector  105  is thus used for detecting the binary value of the data of the area to which the optical spot is applied.  
         [0032]     The detector  105  is advantageously made of an array of CMOS or CCD pixels. For example, one pixel of the detector is placed opposite an elementary data area containing one data (i.e. one bit) of the information carrier. In that case, one pixel of the detector is intended to detect one data of the information carrier.  
         [0033]     Advantageously, an array of micro-lenses (not represented) is placed between the information carrier  101  and the detector  105  for focusing the output light beams generated by the information carrier on the detector, for improving the detection of the data.  
         [0034]     In a second embodiment depicted in  FIG. 2 , the system according to the invention for reading data stored on an information carrier  201  comprises an optical element  202  for generating an array of light spots  203  from an input light beam  204 , said array of light spots  203  being intended to scan the information carrier  201 .  
         [0035]     The optical element  202  corresponds to a two-dimensional array of apertures to the input of which the coherent input light beam  204  is applied. The apertures correspond for example to circular holes having a diameter of 1 μm or much smaller. The input light beam  204  can be realized by a waveguide (not represented) for expanding an input laser beam, or by a two-dimensional array of coupled micro lasers.  
         [0036]     The light spots are applied to transparent or non-transparent areas of the information carrier  201 . If a light spot is applied to a non-transparent area, no output light beam is generated in response by the information carrier. If a light spot is applied to a transparent area, an output light beam is generated in response by the information carrier, said output light beam being detected by the detector  205 . Similarly as the first embodiment depicted in  FIG. 1 , the detector  205  is thus used for detecting the binary value of the data of the area on which the optical spot is applied.  
         [0037]     The detector  205  is advantageously made of an array of CMOS or CCD pixels. For example, one pixel of the detector is placed opposite an elementary data area containing a data of the information carrier. In that case, one pixel of the detector is intended to detect one data of the information carrier.  
         [0038]     Advantageously, an array of micro-lenses (not represented) is placed between the information carrier  201  and the detector  205  for focusing the output light beams generated by the information carrier on the detector, improving the detection of the data.  
         [0039]     The array of light spots  203  is generated by the array of apertures  202  in exploiting the Talbot effect which is a diffraction phenomenon working as follows. When a coherent light beams, such as the input light beam  204 , is applied to an object having a periodic diffractive structure (thus forming light emitters), such as the array of apertures  202 , the diffracted lights recombine into identical images of the emitters at a plane located at a predictable distance z 0  from the diffracting structure. This distance z 0  is known as the Talbot distance. The Talbot distance z 0  is given by the relation z 0 =2.n.d 2 /λ, where d is the periodic spacing of the light emitters, λ is the wavelength of the input light beam, and n is the refractive index of the propagation space. More generally, re-imaging takes place at other distances z(m) spaced further from the emitters and which are a multiple of the Talbot distance z such that z(m)=2.n.m.d 2 /λ, where m is an integer. Such a re-imaging also takes place for m=½+an integer, but here the image is shifted over half a period. The re-imaging also takes place for m=¼+an integer, and for m=¾+an integer, but the image has a doubled frequency which means that the period of the light spots is halved with respect to that of the array of apertures.  
         [0040]     Exploiting the Talbot effect allows to generate an array of light spots of high quality at a relatively large distance from the array of apertures  202  (a few hundreds of μm, expressed by z(m)), without the need for optical lenses. This allows to insert for example a cover layer between the array of aperture  202  and the information carrier  201  to prevent the latter from contamination (e.g. dust, finger prints . . . ). Moreover, this facilitates the implementation and allows to increase in a cost-effective manner, compared to the use of an array of micro-lenses, the density of light spots which are applied to the information carrier.  
         [0041]      FIG. 3  depicts a detailed view of the system according to the invention. It depicts a detector  305  which is intended to detect data from output light beams generated by the information carrier  301 . The detector comprises pixels referred to as  302 - 303 - 304 , the number of pixels shown being limited to facilitate the understanding. In particular, pixel  302  is intended to detect data stored on the data area  306  of the information carrier, pixel  303  is intended to detect data stored on the data area  307 , and pixel  304  is intended to detect data stored on the data area  308 . Each data area (also called macro-cell) comprises a set of elementary data. For example, data area  306  comprises binary data referred to as  306   a - 306   b - 306   c - 306   d.    
         [0042]     In this embodiment, one pixel of the detector is intended to detect a set of data, each elementary data among this set of data being successively read by a single light spot generated either by the array of micro-lenses  102  depicted in  FIG. 1 , or by the array of apertures depicted in  FIG. 2 . This way of reading data on the information carrier is called macro-cell scanning in the following.  
         [0043]      FIG. 4 , which is based on  FIG. 3 , illustrates by a non-limitative example the macro-cell scanning of an information carrier  401 .  
         [0044]     Data stored on the information carrier  401  have two states indicated either by a black area (i.e. non-transparent) or white area (i.e. transparent). For example, a black area corresponds to a “0” binary state while a white area corresponds to a “1” binary state.  
         [0045]     When a pixel of the detector  405  is illuminated by an output light beam generated by the information carrier  401 , the pixel is represented by a white area. In that case, the pixel delivers an electric output signal (not represented) having a first state. On the contrary, when a pixel of the detector  405  does not receive any output light beam from the information carrier, the pixel is represented by a cross-hatched area. In that case, the pixel delivers an electric output signal (not represented) having a second state.  
         [0046]     In this example, each set of data comprises four elementary data, and a single light spot is applied simultaneously to each set of data. The scanning of the information carrier  401  by the light spots  403  is performed for example from left to right, with an incremental lateral displacement which equals the distance between two elementary data.  
         [0047]     In position A, all the light spots are applied to non-transparent areas so that all pixels of the detector are in the second state.  
         [0048]     In position B, after displacement of the light spots to the right, the light spot to the left is applied to a transparent area so that the corresponding pixel is in the first state, while the two other light spots are applied to non-transparent areas so that the two corresponding pixels of the detector are in the second state.  
         [0049]     In position C, after displacement of the light spots to the right, the light spot to the left is applied to a non-transparent area so that the corresponding pixel is in the second state, while the two other light spots are applied to transparent areas so that the two corresponding pixels of the detector are in the first state.  
         [0050]     In position D, after displacement of the light spots to the right, the central light spot is applied to a non-transparent area so that the corresponding pixel is in the second state, while the two other light spots are applied to transparent areas so that the two corresponding pixels of the detector are in the first state.  
         [0051]     The scanning of the information carrier  401  is complete when the light spots have been applied to all data of a set of data facing a pixel of the detector. It implies a two-dimensional scanning of the information carrier. Elementary data which compose a set of data opposite a pixel of the detector are read successively by a single light spot.  
         [0052]      FIG. 5  depicts a three-dimensional view of the system as depicted in  FIG. 2 . It comprises an array of apertures  502  for generating an array of light spots applied to the information carrier  501 . Each light spot is applied and scanned over a two-dimensional set of data of the information carrier  501  (represented by bold squares). In response to this light spot, the information carrier generates (or not, if the light spot is applied to a non-transparent area) an output light beam in response, which is detected by the pixel of the detector  503  opposite the set of data which is scanned. The scanning of the information carrier  501  is performed in displacing the array of apertures  502  along the x and y axes.  
         [0053]     The array of apertures  502 , the information carrier  501  and the detector  503  are stacked in parallel planes. The only moving part is the array of apertures  502 .  
         [0054]     It is noted that the three-dimensional view of the system as depicted in  FIG. 1  would be the same as the one depicted in  FIG. 5  in replacing the array of apertures  502  by the array of micro-lenses  102 .  
         [0055]     The scanning of the information carrier by the array of light spots is done in a plane parallel to the information carrier. A scanning device provides translational movement of the light spots in the two directions x and y for scanning all the surface of the information carrier.  
         [0056]     In a first solution depicted in  FIG. 6 , the scanning device corresponds to an H-bridge. The optical element generating the array of light spots (i.e. the array of micro-lenses or the array of apertures) is implemented in a first sledge  601  which is movable along the y axis compared to a second sledge  602 . To this end, the first sledge  601  comprises joints  603 - 604 - 605 - 606  in contact with guides  607 - 608 . The second sledge  602  is movable along the x axis by means of joints  611 - 612 - 613 - 614  in contact with guides  609 - 610 . The sledges  601  and  602  are translated by means of actuators (not represented), such as by step-by-step motors, magnetic or piezoelectric actuators acting as jacks.  
         [0057]     In a second solution depicted in  FIG. 7 , the scanning device is maintained in a frame  701 . The elements used for suspending the frame  701  are depicted in a detailed three-dimensional view in  FIG. 8 . These elements comprise: 
        a first leaf spring  802 ,     a second leaf spring  803 ,     a first piezoelectric element  804  providing the actuation of the scanning device  801  along the x axis,     a second piezoelectric element  805  providing the actuation of the scanning device  801  along the y axis.        
 
         [0062]     The second solution depicted in  FIG. 7  has less mechanical transmissions than the H-bridge solution depicted in  FIG. 6 . The piezoelectric elements, in contact with the frame  701 , are electrically controlled (not represented) so that a voltage variation results in a dimension change of the piezoelectric elements, leading to a displacement of the frame  701  along the x and/or the y axis.  
         [0063]     The position Pos 1  depicts the scanning device  701  in a first position, while the position Pos 2  depicts the scanning device  701  in a second position after translation along the x axis. The flexibility of the leaf springs  702  and  703  is put in evidence.  
         [0064]     A similar configuration can be built with four piezoelectric elements, the two extra piezoelectric elements replacing the leaf springs  702  and  703 . In that case, opposite pair of piezoelectric elements act together in one direction in the same way as an antagonistic pair of muscles.  
         [0065]      FIG. 9  depicts an improved system according to the invention in which the scanning of the information carrier  901  is realized without moving components.  FIG. 9  is based on  FIG. 2  but additionally comprises a phase-modulator  906  placed in the path light of the input light beam  904 .  
         [0066]     The non-mechanical scanning is realized by applying a phase profile defined by the phase-modulator  906  to the input light beam  904 , and in varying this phase profile. The phase-modulator  906  varies the phase of the input light beam  904  with respect to the lateral distance x (and/or y).  
         [0067]     It is noted that the phase-modulator  906  can also be placed between the array of apertures  902  and the information carrier  901  (not represented).  
         [0068]     When the phase-modulator  906  acts so as the phase φ(x) varies in a linear way with respect to the position x, this leads to a lateral shift Δx of the array of light spots  903  along the lateral axis x. It can be defined that the phase φ(x) and the lateral position x are linked by the following relation:  
               ϕ   ⁡     (   x   )       =         2   ⁢   π     a     ·     x   λ               Eq   .           ⁢   1             
 
         [0069]     where 
        x is the lateral position (taken for example from the extreme left side of the phase-modulator  906 ),     λ is the wavelength of the input light beam  904 , a is a variable parameter.        
 
         [0072]     It can be shown that if a phase profile as defined by Eq.1 is performed by the phase-modulator  906 , the lateral shift Δx of the array of light spots  903  is given by the following relation: 
 
Δx=a.Z  Eq.2 
        where Z is fixed value corresponding advantageously to the Talbot distance z 0 , or to an integer multiple or a sub-multiple of the Talbot distance z 0 ,        
 
         [0074]     The parameter a allows modifying the linearity factor of the phase profile in view of changing the lateral shift Δx. For each value of the parameter a, a different phase profile is defined. A variation of the parameter a results as a consequence in a shift spots in x.  
         [0075]     For scanning all the surface of the information carrier  901 , each macro-cell data of the information carrier must be scanned by a light spot of the array of spots. The scanning of a macro-cell data thus corresponds to a two-dimensional scanning along the x and y axis. This two-dimensional scanning is performed in defining simultaneously a linear phase modulation according to the x and y axis, the defined phase profile resulting from a linear combination of a linear phase profile according to the x (as defined by Eq.1) axis and a linear phase profile according to the y axis (similarly as defined by Eq.1 in substituting x by y).  
         [0076]     The phase-modulator  906  comprises advantageously controllable liquid crystal (LC) cells. For example, pixelated linear nematic LC cells can be used such that each aperture of the array of apertures  902  has its own LC cell, and can be given its own phase φ(x). Thus, the phase-modulator  906  corresponds to a two-dimensional array of LC cells. Nematic substances can be aligned by electric and magnetic fields, resulting in a phase change. Nematic cells are optically equivalent to a linear wave plate having a fixed optical axis, but whose birefringence is a function of the applied voltage. As the applied voltage varies, the birefringence changes, resulting in a change of the optical path length, thus in a phase change.  
         [0077]     Since each of the apertures of the array of apertures  902  has its own LC cell, the phase profile defines a ramp having incremental steps, the ramp globally fitting a linear equation defined by Eq.1.  
         [0078]     Alternatively, the phase-modulator  906  may define a continuous preset phase profile which is electrically controllable for varying the linearity factor of the phase profile.  
         [0079]     Alternatively, electro-wetting cells can be used for defining the phase profile. In this case, the phase profile according to the lateral position x is changed by applying different high voltages to the electro-wetting cell.  
         [0080]     Alternatively, the array of light spots can be shifted on the information carrier in varying the incident angle of the input light beam  904  from the axial direction z. This solution is advantageous since it no more requires a phase-modulator placed on the path light. It requires however actuation means (not represented) for varying the angle of the light beam  904 . When the angle of the light beam is a with respect to the surface normal of the aperture array, the phase profile corresponds to the one defined by Eq.1.  
         [0081]      FIG. 10  depicts a three-dimensional view of the system as depicted in  FIG. 9 . It comprises an array of apertures  1002  for generating an array of light spots applied to the information carrier  1001 . Each light spot is applied and scanned over a two-dimensional set of data of the information carrier  1001  (represented by squares in bold). In response to this light spot, the information carrier generates (or not, if the light spot is applied to a non-transparent area) an output light beam detected by the pixel of the detector  1003  opposite the set of data which is scanned. The scanning of the information carrier  1002  along the x and y axis is performed by means of a phase-modulator  1006  placed below the array of apertures  1002 , without moving any elements.  
         [0082]     The phase-modulator  1006 , the array of apertures  1002 , the information carrier  1001  and the detector  1003  are stacked in parallel planes.  
         [0083]      FIG. 11  depicts an example of a LC cell. It comprises an LC layer  1101 , a glass substrate  1102 , transparent electrodes  1103 , alignment layers  1104 . In this figure, the parameter d corresponds to the cell thickness, while θ corresponds to the angle of the LC molecules. As the liquid crystal molecules rotate, due to the applied electric field by the voltage generator  1105 , linearly polarised light propagating through the cell will experience a different effective refractive index, resulting in a phase change.  
         [0084]     As an example, the resulting graph of a phase change Δφ versus voltage is also illustrated in  FIG. 11 . The characteristics of the curve depend on the LC material which is used, the wavelength of the light and the cell thickness d.  
         [0085]     When the phase-modulator  906  acts so as the phase φ(x) of the input light beam  904  varies in a quadratic way with respect to the position x, this leads to an axial shift Δz of the array of light spots  903  along the axial axis z. It can be defined that the phase φ(x) and the lateral position x are linked by the following relation:  
               ϕ   ⁡     (   x   )       =         2   ⁢   π       2   ⁢   R       ·       x   2     λ               Eq   ⁢           .           ⁢   3             
 
         [0086]     where x is the lateral position, 
        λ is the wavelength of the input light beam  904 ,     R is a variable parameter corresponding to the radius of curvature of the phase profile,     Δz is the axial shift with respect to the position for z=0.        
 
         [0090]     It can be shown that if a phase profile as defined by Eq.3 is performed by the phase-modulator  906 , the axial shift Δz of the array of light spots  903  can be accurately approximated by the following relation:  
               Δ   ⁢           ⁢   z     ≈       Z   2     R             Eq   .           ⁢   4             
        where Z is fixed value corresponding advantageously to the Talbot distance z 0 , or to an integer multiple or a sub-multiple of the Talbot distance z 0 .        
 
         [0092]     The variable parameter R allows modifying the quadratic factor of the phase profile in view of changing the axial shift Δz. For each value of the parameter R, a different phase profile is defined. A variation of the parameter R results as a consequence in a shift Δz. The light spots  903  will thus focus closer to or further away from the surface of the information carrier  901 . The quadratic phase profile plays the same role as a focus actuator in more traditional recording, but without using any mechanical elements.  
         [0093]     Advantageously, the phase profile defined by the phase-modulator  906  may result from a linear combination of a linear phase profile as defined by Eq.1 (according to the x and/or y axis) and a quadratic phase profile as defined by Eq.4. This allows performing at the same time a two-dimensional scanning of the light spots, while setting accurately the focus of the light spots on the surface of the information carrier  901 .  
         [0094]     As illustrated in  FIG. 12 , the system according to the invention may advantageously be implemented in a reading apparatus RA (e.g. home player apparatus . . . ), a portable device PD (e.g. portable digital assistant, portable computer, a game player unit . . . ), or a mobile telephone MT. These apparatus and devices comprise an opening (OP) intended to receive an information carrier IC as previously described, and a system according to the invention for shifting light spots over said information carrier IC in view of data recovering.  
         [0095]     Use of the verb “comprise” and its conjugations does not exclude the presence of elements or steps other than those stated in the claims. Use of the article “a” or “an” preceding an element or step does not exclude the presence of a plurality of such elements or steps.