Patent Publication Number: US-2006018182-A1

Title: Solid state microoptoelectromechanical system (moens) for reading photonics diffractive memory

Description:
FIELD OF THE INVENTION  
      The present invention generally relates to a photonics diffractive memory. In particular, the present invention relates to an apparatus for reading information from the photonics diffractive memory.  
     BACKGROUND OF THE INVENTION  
      The large storage capacities and relative low costs of CD-ROMS and DVDs have created an even greater demand for still larger and cheaper optical storage media. Holographic memories have been proposed to supersede the optical disc as a high-capacity digital storage medium. The high density and speed of the holographic memory comes from three-dimensional recording and from the simultaneous readout of an entire packet of data at one time. The principal advantages of holographic memory are a higher information density (10 11  bits or more per square centimeter), a short random access time (˜100 microseconds and less), and a high information transmission rate (10 9  bit/sec).  
      In holographic recording, a light beam from a coherent monochromatic source (e.g., a laser) is split into a reference beam and an object beam. The object beam is passed through a spatial light modulator (SLM) and then into a storage medium. The SLM forms a matrix of shutters that represents a packet of binary data. The object beam passes through the SLM which acts to modulate the object beam with the binary information being displayed on the SLM. The modulated object beam is then directed to one point on the storage medium by an addressing mechanism where it intersects with the reference beam to create a hologram representing the packet of data.  
      An optical system consisting of lenses and mirrors is used to precisely direct the optical beam encoded with the packet of data to the particular addressed area of the storage medium. Optimum use of the capacity of a thick storage medium is realized by spatial and angular multiplexing. In spatial multiplexing, a set of packets is stored in the storage medium shaped into a plane as an array of spatially separated and regularly arranged subholograms by varying the beam direction in the x-axis and y-axis of the plane. Each subhologram is formed at a point in the storage medium with the rectangular coordinates representing the respective packet address as recorded in the storage medium. In angular multiplexing, recording is carried out by keeping the x- and y-coordinates the same while changing the irradiation angle of the reference beam in the storage medium. By repeatedly incrementing the irradiation angle, a plurality of packets of information is recorded as a set of subholograms at the same x- and y-spatial location.  
      Previous holographic devices for recording information in a highly multiplexed volume holographic memory, and for reading the information out, require components and dimensions having a large size which places a limit on the ability to miniaturize these systems. Because previous holographic devices use motors and large-scale components such as mirrors and lenses, the addressing systems of these previous devices are slow. Furthermore, the mechanical components of these previous devices need frequent maintenance to correct errors and dysfunction coming, for instance, from wear and friction (i.e., tribology effect). Furthermore, previous addressing systems are expensive because they use complex systems for control. Thus, their prices cannot be lowered by mass production. Moreover, previous devices are not economical in their energy consumption. Even when previous addressing devices are accurate when new, the wear and friction of the interacting surfaces that are in relative motion lowers their accuracy with time.  
      In view of the foregoing, it would be desirable to provide one or more techniques which overcomes the above-described inadequacies and shortcomings of the above-described proposed solutions.  
     OBJECTS OF THE INVENTION  
      In view of the foregoing, it is an object of the present invention to provide an improvement in higher speed and smaller size of photonics diffractive memory reading systems.  
      It is a further object of the present invention to provide a miniaturization of a photonics diffractive memory reading system.  
      It is another object of the present invention to reduce the addressing system of a photonics diffractive memory reading system to a matchbox size.  
      It is a still a further object of the present invention to design a solid state reading system that can be rapidly manufactured in large quantities and low cost out of existing resources.  
     SUMMARY OF THE INVENTION  
      In order to achieve the above-mentioned objectives, the present invention comprises a solid-state system for reading information from a photonics diffractive memory. A coherent light source generates a convergent light beam which is then deflected by an acousto-optic deflector. A plurality of micro-mirrors receives the deflected light beam from the acousto-optic deflector at one of the micro-mirrors. A photonics diffractive memory having a plurality of points receives at one of the points the reflected light beam which is reflected from the micro-mirror. A detector has a plurality of light-detecting cells. At least one of the cells receives a portion of the reflected light beam transmitted through the point.  
      In a further aspect of the present invention, the micro-mirrors are configured as a matrix.  
      In another aspect of the present invention, there is a lens which forms the convergent light beam from the light source.  
      In still another aspect of the present invention, the convergent light source is selected from the group consisting of a low power laser and a light-emitting diode.  
      In yet another aspect of the present invention, the detector is a CCD detector array.  
      In a further aspect of the present invention, each of the plurality of points stores one or more diffraction patterns.  
      In yet another aspect of the present invention, the photonics diffractive memory comprises stored therein information located at the plurality of points of the memory and at a plurality of angles at each one of the points so as to form a plurality of packets of information at each one of the points.  
      In another aspect of the present invention, each of the micro-mirrors is a oscillatory scanning micro-mirror.  
      In a further aspect of the present invention, a computer is configured to coordinate the synchronization of the acousto-optic deflector and the oscillatory micro-mirrors so that the reflected light beam is directed to one of the points with a specific angle for a sufficient time to retrieve information from the point.  
      In yet another aspect of the present invention, each of the micro-mirrors is a oscillatory micro-mirror and the oscillation cycle of the micro-mirror is coordinated with the scanning of the acousto-optical deflector so as to direct said reflected light beam onto one of the points of the storage medium.  
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
      In order to facilitate a fuller understanding of the present invention, reference is now made to the appended drawings. These drawings should not be construed as limiting the present invention, but are intended to be exemplary only.  
       FIG. 1  shows a micro-mirror assembly according to the present invention.  
       FIG. 2  shows a perspective view of a micro-mirror assembly according to the present invention.  
       FIG. 2   a  shows a close up view of the actuator of the micro-mirror assembly according to the present invention.  
       FIG. 3   a  shows adding an epitaxial layer to a wafer as part of the MEMS fabrication process according to the present invention.  
       FIG. 3   b  shows the formation of the starting electrodes and deposition of a metal layer as part of the MEMS fabrication process according to the present invention.  
       FIG. 3   c  shows an anisotropical etch to remove the substrate underneath the designed mirror plate as part of the MEMS fabrication process according to the present invention.  
       FIG. 3   d  shows a cross section of the micro-mirror chip according to the present invention.  
       FIG. 4   a  shows a starting electrode of a micro-mirror assembly according to the present invention.  
       FIG. 4   b  shows operation of a micro-mirror being driven by a saw tooth signal according to the present invention.  
       FIG. 5  shows a solid state reading system according to the present invention.  
       FIG. 6  shows an acousto-optic deflector according to the present invention.  
       FIG. 7  shows a schematic representation of a diffractive optics recording process  
       FIG. 8  shows a matrix of points forming a storage medium according to the present invention.  
       FIG. 9  shows synchronization of the mirror of the solid state reading system according to the present invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION  
      The compact architecture for diffractive optics systems in accordance with the present invention integrates a number of components into a compact package, including an acousto-optic deflector and a microoptoelectromechanical system (MOEMS) device which reduces the addressing component of a reading system for a photonics diffractive memory to a matchbox size. The reading system is made of solid-state components. The mirrors are built in CMOS technology resulting in the advantage that the reading system can be mass-produced at low cost.  
      Various diffractive recording/reading processes have been developed in the art and further details can be found in the book  Holographic Data Storage, Springer  (2000) edited by H. J. Coufal, D. Psaltis, and G. T. Sincerbox. In this specification, the term “diffractive” is used throughout to differentiate prior art holographic technology used for 3-D image generation from diffractive technology necessary for the generation of a storage medium. For example, diffraction efficiency is critical to the viability of any material to be used as a diffractive storage medium. The quality of interference constituting a 3D-hologram is simple to achieve compared to the quality required to realize a storage medium. Moreover, a storage diffractive pattern can also be implemented by using other techniques than the interference of a reference and object beam, such as using as an e-beam etched on a material to generated diffraction patterns. For all these reasons, the specification herein introduces the concept of a broader diffractive optics technology.  
       FIG. 1  shows a top view of a scanning micro-mirror element  100  comprising a mirror plate  102  suspended by two or four torsional springs  122   a ,  122   b  which connect the mirror plate  102  to anchors  120   a ,  120   b , respectively. The anchors  120   a ,  120   b  are attached to the substrate  110 . The two comb like driving electrodes  105   a ,  105   b  create torque to move the mirror plate  102 . The mirror plate  102  of  FIG. 1  is an example of a microoptoelectromechanical system (MOEMS). A MEOMS is a system which combines electrical and mechanical components, including optical components, into a physically small size.  
       FIG. 2  shows a perspective view of the micro-mirror element  100  comprising the mirror plate  102  cut in a silicon substrate on which a reflected film is deposited, typically a film of aluminum with a typical thickness of about fifty nanometers. The plate  102  is suspended from the two or four twisting points  120   a ,  120   b  and is actuated by the two or four drive electrodes  105   a ,  105   b , depending on whether it is desired to have the mirror  102  rotates in one or two directions. The angle of deflection is in theory unlimited, but in practice it is about 60°.  
      The variation of the capacitance C  125  (C varies with angle φ) between the mirror plate  102  and the comb like driving electrodes  105   a ,  105   b  is used to generate the plate tortional movement. If a voltage U is applied by an energy source (not shown) to the driving electrodes  105   a ,  105   b , the generated electrostatic torque M is: 
 
 M =½ dC/dφU   2  
 
 where φ is the deflection angle of the plate. 
 
      The mirror plate  102  can have a size from 0.5×0.5 mm up to 3×3 mm. The actuators (the movement between mirror plate  102  and electrodes  105  as driven by the energy source) are resonantly excited, i.e., they are continuously oscillating. The scan frequency depends on the size of the mirror plate (0.14 KHz up to 20 KHz) and a mechanical scan angle of ±15° can be achieved at a driving voltage of only 20V.  
      When the actuator works in synchronous mode, it is possible to control the angular position of the mirror plate  102  by controlling the maximum deflection amplitude and oscillating period. Advantages of these mirrors is that the amplitude of the deflection can be monitored with the driving voltage U. For a large scan angle, the deflection angle varies linearly with the excitation voltage.  
      As shown in  FIG. 2A , the space lying between the mirror plate  102  and the drive electrodes  105   a ,  105   b  forms a variable capacitor. Thus, applying a voltage generates electrostatic torque acting on the plate and causing it to rotate and/or oscillate. Given the particularly small size of these micro-mirrors on the one hand, but also their mode of operation on the other, it becomes possible to reduce the size of the read device  400  (see  FIG. 5 ) significantly and hence achieve a very high level of integration.  
       FIGS. 3   a - 3   d  show the process for manufacturing a micro-mirror element  200  on a substrate  230  with starting electrodes  210   a ,  210   b . The fabrication is achieved using a CMOS-compatible technology. Referring to  FIG. 3   a , a wafer  230  serves as the base material. A buried oxide (BOX) layer  221  is produced in a SIMOX (Separation by Implantation of Oxygen) process. A 200-nm-thick silicon layer  205  on top of the BOX  221  is strengthened by a 20 um thick epitaxial layer. Referring to  FIG. 3   b , an oxide and a metal layer are deposited and patterned to form the starting electrodes  210   a ,  210   b . The metal layer is protected by an additional oxide. In the next step a 50-nm-thick layer  206  of Al is deposited forming the reflective coating in the mirror area. Referring to  FIG. 3   c ., the substrate underneath the designed torsional springs and the mirror plate  205  is removed by an anisotropical etch in a tetramethylammonium hydroxide (TMAH) solution leaving the remaining portions  230 . TMAH is a chemical solution used for antisotropical etching of the wafer substrate in which the micro mirrors are etched. After that the BOX layer is removed and the epitaxial layer is patterned using the Advanced Silicon Etch™-process, trenches  207  are formed. A cross section of the micro-mirror chip  200  at the end of the process is shown in  FIG. 3   d.    
       FIG. 4   a  illustrates the operation of the micro-mirror element  100 .  FIG. 4   a  shows the starting electrodes  210   a  used to start a motion of the mirror plate  205 . A voltage of a fixed frequency is applied on the starting electrode  210   a  which yield asymmetries. Assuming perfect symmetry of the actuator it is impossible to start the oscillation without external induced forces. Therefore, there is an additional starting electrode  210   a ,  210   b  which is located on top of each of the driving electrodes  221  and isolated from it by an oxide  209 . These electrodes  210   a ,  210   b  can be contacted separately and break the symmetry of the configuration. Once oscillation is initiated, the mirror actuation works in a synchronized mode where the mirror plate  205  oscillates in phase with the driving excitation of the voltage U generated by an energy source.  
       FIG. 4   b  shows synchronization of the mirror plate  102  as driven by a saw tooth signal  300 . The saw tooth signal  300  comprises the voltage U applied with a predetermined frequency per second. The operation of the mirror plate  102  is shown at five different positions  301 - 305  as the mirror plate  102  is driven by saw tooth wave  300  applied across the drive electrodes  105   a ,  105   b  (see  FIG. 1 ). In a full cycle comprising a movement from a positive angle, to zero degrees, to a negative angle, the mirror element  102  moves from positions  301  to  304  (a full cycle) and then begins the cycle again at position  305 .  
      Table 1 shows the eigenfrequency (resonance frequency) of the micro-mirror element  100  as a function of mirror size. The eigenfrequency depends on the mechanical and electrical characteristics of the micro-mirror element  100 . In the synchronized mode, the mirror oscillates at two times the eigenfrequency.  
                                           TABLE 1                                      1D Mirror   0.5   1   1.5   2   3           size (mm)           Resonance   2.32   0.4-7.5   .25-2.5   .14-1.5   .2           frequency           (Khz)                      
 
       FIG. 5  shows a reading system  400  comprising a separate unit on a platform  470  supporting an acousto-optic deflector  430 , a microoptoelectromechanical systems (MOEMS) matrix  440 , a matrix memory  450 , and an image sensor  460 , such as a CCD (charge-coupled device) detection system or other such image detection system. Additional devices located on or off the platform  470  comprise a light source  410  (e.g., a laser, laser diode) and a converging lens  420 .  
      The operation of the reading system proceeds with the light source  410  emitting a light beam  480   a  which is focused by the converging lens  420  from a plane wave to spherical wave  480   b . The spherical wave  480   b  is a convergent beam. The convergent beam  480   b  is deflected by the acousto-optic deflector  430  to form beam  480   c  which impinges on one of the micro-mirror elements of the MEOMS matrix  440 . The MEOMS mirror matrix  440  has a size that fits the constraints of the memory matrix addressing system. The matrix of micro-mirrors  440  is used to address the matrix of points of the memory  450  in which data are recorded by spatial and angular multiplexing. The beam  480   c  coming from the acousto-optic deflector  430  forms an area with a diameter that can fit within the diameter of each one of the mirror elements of the MEOMS matrix  440 . Additionally, the memory matrix  450  is spatially adjusted in such a way that the size of the laser beam  480   d  fits exactly the size of every point of the memory matrix  450 .  
       FIG. 6  shows the acousto-optic deflector  430  in greater detail. The acousto-optic (AO) deflector  430  directs the laser beam  480   b  at an angle to the micro-mirror array  440 . When acousto-optical crystals are subjected to stress, especially by means of a transducer usually consisting of a piezoelectric crystal, they modify the angle of diffraction of the light and, in general, of the electromagnetic wave which passes through them in order to modify the value of the diffraction angle of the emerging beam  480  c. Thus, modifying the actuating frequency of the piezoelectric transducer deflects the light beam  480   b  to form the light beam  480   c  at one of a plurality of angles.  
      Thus, as shown in  FIG. 6 , the variations in orientation along OX and OY (referring to the rectilinear co-ordinates of  FIG. 2 ) of the incident read beam  480   b  emanating from the low-power laser  410  are obtained by subjecting this beam to two acousto-optic components  121 ,  122 . Consequently it may be understood that, by varying the vibration frequency of the piezoelectric crystal associated with the acousto-optic component(s), it becomes possible to modify, very rapidly, the desired orientation of the grating within the rows and columns of the data-carrying matrix  450 . The limiting factor then becomes the response time of the mirror elements of the MEOMS matrix  440  which act on the angle of incidence of the read beam.  
       FIG. 7  and  FIG. 8  describe the contents of the diffractive storage medium. Referring to  FIG. 7 , in forming a diffractive pattern, or alternately a hologram, a reference beam  1  intersects with an object beam  4  to form a sub-hologram  8   a  (referred to alternately as a point) extending through the volume of storage medium  8 . There is a separate sub-hologram or point  8  a extending through the volume for each angle and spatial location of the reference beam  1 . The object beam  4  is modulated with a packet of information  6 . The packet  6  contains information in the form of a plurality of bits. The source of the information for the packet  6  can be a computer, the Internet, or any other information-producing source. The hologram impinges on the surface  8  a of the storage medium  8  and extends through the volume of the storage medium  8 . The information for the packet  6  is modulated onto the storage medium  8  by spatial multiplexing and angle multiplexing. Angle multiplexing is achieved by varying the angle a of the reference beam  1  with respect to the surface plane of the storage medium  8 . A separate packet  6  of information is recorded in the storage medium  8  as a sub-hologram for each chosen angle a and spatial location. Spatial multiplexing is achieved by shifting the reference beam  1  with respect to the surface of the storage medium  8  so that the point  8   a  shifts to another spatial location, for example point  8   a ′, on the surface of the storage medium  8 .  
      The storage medium  8  is typically a three-dimensional body made up of a material sensitive to a spatial distribution of light energy produced by interference of the object light beam  4  and the reference light beam  1 . A hologram may be recorded in a medium as a variation of absorption or phase or both. The storage material must respond to incident light patterns causing a change in its optical properties. In a volume hologram, a large number of packets of data can be superimposed, so that every packet of data can be reconstructed without distortion. A volume (thick) hologram may be regarded as a superposition of three dimensional gratings recorded in the depth of the layer of the recording material each satisfying the Bragg law (i.e., a volume phase grating). The grating planes in a volume hologram produce change in refraction and/or absorption.  
      Several materials have been considered as storage material for optical storage systems because of inherent advantages. These advantages include a self-developing capability, dry processing, good stability, thick emulsion, high sensitivity, and nonvolatile storage. Some materials that have been considered for volume holograms are photorefractive crystals, photopolymer materials, and polypeptide material.  
      Referring now to  FIG. 8 , there is shown in greater detail the storage medium  8  arranged in the form of a flat sheet, herein referred to as a matrix. In this example, the matrix is 1 cm 2 . Each of a plurality of points on the matrix is defined by its rectilinear coordinates (x, y). An image-forming system (not shown) reduces the object beam  4  to the sub-hologram  8   a  having a minimum adopted size at one of the x, y point of the matrix. A point in physical space defined by its rectilinear coordinates contains a plurality of packets  8   b.    
      In this case, a 1 mm 2  image  8   a  is obtained by focusing the object beam  4  onto the storage medium  8  centered at its coordinate. Due to this interference between the two beams  1 , 4 , a diffractive image  8   a  1 mm 2  in size is recorded in the storage material  8  centered at the coordinates of the matrix. Spatial multiplexing is carried out by sequentially changing the rectilinear coordinates. The object beam  4  focuses on the storage material  8  so that a separate image  8   a  is recorded at a unique position in the plane defined by its coordinates (x, y). This spatial multiplexing results in a 10 by 10 matrix of diffractive images  8   a . Angle multiplexing is carried out by sequentially changing the angle of the reference beam  1  by means of the mirror elements of the MEOMS matrix  440 . Angle multiplexing is used to create 15-20 packets of information  8   b  corresponding to 15 discrete variations of the angle of incidence of the reference beam. Additionally, it is possible to reach 20-25 packets by simple multiplexing and 40-50 packets by using double symmetrical angular multiplexing. A data packet is reconstructed by shinning the reference beam  1  at the same angle and spatial location in which the data packed was recorded. The portion of the reference beam  1  diffracted by the storage material  8  forms the reconstruction, which is typically detected by a detector array. The storage material  8  may be mechanically shifted in order to store data packets at different points by its coordinates (x, y).  
       FIG. 9  shows synchronization of the micro-mirrors  440 . Because the micro-mirrors  440  are continuously oscillating, it is necessary to synchronize the acousto-optic deflector (AOD)  430  and the micro-mirrors  440  in order to realize the addressing of a data packet of the memory  450 . By knowing mirrors parameters like amplitude of deflection and oscillating period, it is possible to control the switching time of the AOD  430 . This way, one of the micro-mirrors can be accessed which addresses a desired position on the memory  450 . The AOD  430  redirects the laser beam on a chosen mirror at a given time.  
      Two representative micro-mirrors  440   a ,  440   b  of the micro-mirror array  440  of  FIG. 5  are shown with each of the micro-mirrors at a different position. The rest position  441   a  is shown for the micro-mirror  440   a . The rest position  441   b  is shown for the micro-mirror  440   b . The coherent laser beam is directed by the AOD  430  at different times to one of the micro-mirrors  440   a ,  440   b  which reflect the light beam at a predetermined location and angle to the memory  450 . The lens  455  focuses the light energy onto the CCD array  460 . A CPU (not shown), such as a computer, microcontroller, or other such control device, controls the AOD  430 , the micro-mirrors  440 , and the CCD detector  460 . The CPU (not shown) receives inputs from sensors indicating the positions of the micro-mirrors  440   a ,  440   b  and receives inputs on the state of the AOD  430 . The CPU (not shown) then controls the mirror positions of the micro-mirrors  440  and the deflection angle of the AOD  430 . Synchronization of the micro-mirrors  440  with the AOD  430  is necessary to reach a maximum deflection angle. The maximum deflection angle is the maximum angle that can be reached by the processed beam. This means that the output beam of the acousto-optic device can reach a maximum value. Between the positive and negative value of this maximum will lie the angular range of the acousto-optic device. An other advantage of synchronization is that the maximum deflection can be monitored by the driving voltage control . That is, the deflection varies linearly with the driving excitation voltage U.  
       FIG. 9  illustrates synchronization between the micro-mirrors  440 , the AOD  430  and the CCD camera  460 . The synchronization is shown for two of the micro-mirrors  440   a ,  440   b  of the micro-mirror array  440 . Because the micro-mirrors  440   a ,  440   b  are continuously oscillating at low frequencies (i.e., 200 Hz), the micro-mirrors  440   a ,  440   b  can be considered as fixed mirrors compared to the switching time of the AOD (10 to 100 μs). At a switching time T, the micro-mirrors positions can be monitored so that the it is determined how to access a specific packet-of information from the memory  450 . In the present invention, the CPU (not shown) controls the mirror synchronization and calculates the switching time of the AOD  430  and the CCD  460  to read a given packet of the memory  450 . The positions of the micro-mirrors  440  are calculated to address every packet of the memory  450 . At a time T 1 , the AOD  430  is switched to address the micro-mirror  440   a  to read a packet of the memory  450 . At another time T 2 , the AOD  430  is switched to address the micro-mirror  440   b  to read a packet of the memory  450 . The micro-mirror  440   a  is shown at an angle α1 from the normal position  441   a . The micro-mirror  440   b  is shown at an angle α2 from the normal position  441   b . The lens  455  focuses the output waveform carrying the data packets onto the array of the CCD camera  460 .  
      The present invention is not to be limited in scope by the specific embodiments described herein. Indeed, various modifications of the present invention, in addition to those described herein, will be apparent to those of skill in the art from the foregoing description and accompanying drawings. Thus, such modifications are intended to fall within the scope of the appended claims.