Abstract:
The present invention provides an improved optical pickup device based on the developing electronically reconfigurable diffraction grating MEMS technology. The improved optical pickup device has applications that include but are not limited to CD and DVD for audio, video and computer technology. The present invention can provide improvements to this current and future technology with higher data storage density and faster retrieval. In a preferred embodiment, the optical pickup apparatus comprises an electronically reconfigurable diffraction grating modulating relative light intensities as among at least two different diffraction orders of light diffracted by the electronically reconfigurable diffraction grating; focusing optics for focusing the light diffracted by the electronically reconfigurable diffraction grating into diffractive spots corresponding with each of the diffraction orders and onto an optical storage medium, which light is then reflected by the optical storage medium; and a detector for detecting the light reflected by the optical storage medium and striking said detector.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
   This application claims the benefit of U.S. Provisional Application No. 60,149,856, filed Aug. 19, 1999. 

   FIELD OF THE INVENTION 
   This invention relates to the field of optical pickup devices, and particularly to electronically-controlled optical pickup devices. 
   BACKGROUND OF THE INVENTION 
   Various configurations and implementations of electronically reconfigurable diffraction gratings fabricated using MEMS technology are disclosed, for example, in U.S. Pat. No. 5,841,579 by Bloom, et al.; U.S. Pat. No. 5,757,536 by Ricco, et al.; and U.S. Pat. No. 5,999,319 by Castracane. These MEMS-based electronically reconfigurable diffraction gratings offer new and unique degrees of freedom in controlling diffraction of light from a grating, bringing qualitatively new potential to this centuries-old optical device. 
   The primary advantage provided by these reconfigurable diffraction gratings is the elimination of mechanical tuning and the advent of dynamic control and programmable tuning of the diffraction pattern. The practical applications of this device, however, have been limited so far. This device has found applications mainly in spectroscopy and digital display applications. As will be disclosed herein, a novel and nonobvious application of the electronically reconfigurable diffraction grating is to an improved optical pickup device that can be implemented in CD players, DVD players, computer storage devices, and laser based surface profilometers. 
   A typical layout of an existing optical pickup device, for example, in a CD player, is shown in  FIG. 1. A  solid state laser diode  102 , typically emitting in the near IR, emits optical power in a wedge shaped beam with a typical divergence of 10×30 degrees in the X and Y directions, respectively. A diffraction grating  104  splits the output laser beam into a main (zero order) beam  106  and two (1 st  order) side beams designated as first 1 st  order side beam  108  and second 1 st  order side beam  110 . In these existing prior art devices, only the zero and first order beams ( 106 ,  108  and  110  respectively) are used. The higher order beams (second order and above) are not used. The zero order beams are used to read content information, e.g., music, video, computer data, etc., from the disk. The 1 st  order side beams  108  and  110  are used for tracking the track on the disk which is being read (tracking information). The tracking servo mechanism in a typical CD player or other device that would use an optical pickup, maintains the 1 st  order side beams  108  and  110  by keeping the amplitude of the reflection of these two 1 st  order side beams  108  and  110  equalized, as measured by the system&#39;s photodetector in a feedback loop arrangement. 
   Next, the laser beam passes through a polarizer  111 , polarizing beam splitter  112 , a turning mirror  118 , a collimating lens  114 , a quarter wave plate  116 , and the objective lens  120  before reaching the optical storage media disk  122  (compact, digital video, etc.). The collimated laser beams (the main zero order beam  106  and the two 1 st  order side beams  108  and  110 ) pass through the objective lens  120  and are focused to diffraction-limited spots on the information layer of the disk, known as the pits. The reflected beam retraces the original path up until it passes through the polarizing beam splitter  112  at which point it is diverted toward the photodetector array  124 . Additional focusing optics  126  are used to focus the reflected main zero order beam  106  on the quadrant photodetector  128  and the 1 st  order side beams on individual photodetectors  130 , located on the side of the quadrant detector  128  in the photodetector array  124 , as shown in FIG.  3 . 
   For reference,  FIG. 2  shows a typical recorded fragment on a CD or alternative optical storage media. Shown in  FIG. 2  are the pits  232  and the coast  234 . The pits  232  comprise the information content storage layer of the disk and are where the main zero order beam  106  is focused to. Assuming the optical storage media  450  is round, a pit line  233  would contain all pits  232  located at the same radius and is represented in  FIG. 2  as the dotted line through the center of laterally adjacent pits. Similarly, the coast  234  is defined as the area between adjacent pits  232  both laterally and longitudinally. The coast line  235  is defined as the median line equidistant between successive pit lines  233 . The typical width of the pits  232  is 0.5 micron (shown as the vertical distance  231  across the pit) and the pitch is 1.5 micron (defined as the distance between pits lines  233  and shown as the vertical distance  237 ) which makes the width of the coast  234  1 micron (shown as vertical distance  239 ). This is where the 1 st  order side beams  108  and  110  are focused to. 
   As mentioned above, the photodetector array  124  typically comprises a quadrant photodetector  128  (labeled A, B, C, D) and two individual photodetectors  130  (labeled E and F) that are located on the wide extremes of the quadrant detector  128 . For reference, this photodetector array configuration is shown in  FIG. 3  along with the typical reflected beams. The individual photodetectors  130  located on the wide extremes are typically used to detect and measure the reflected 1 st  order side beams  108  and  110 , while the quadrant detector  128  is used to measure the reflected zero order beam  106 . 
   The two reflected 1 st  order beams  108  and  110  are used for horizontal tracking. When the focal spot shifts sideways from the center of the pits  232 , one of the side spots starts leaving the coast  234  and covering some of the pit  232  area creating an obvious change in reflected intensity. The resulting difference in the signals from the two individual photodetectors  130  is used as an error signal in the feedback loop for horizontal tracking. The width of the coast  234  is typically twice the width of the pits  232 , as shown in  FIG. 2 , to provide for the differential feedback signal. 
   The reflected zero order beam  106  is used in a feedback configuration to establish focus on the pit  232  of the optical storage media  450  for information content readout. When in focus, the reflected zero order beam  106  is circular on the quadrant photodetector  128  as shown in FIG.  3 . When out of focus in one direction, the reflected zero order  106  is diagonally elliptical across quadrants B and C as shown in  FIG. 3   a , while being out of focus in the opposite direction produces an ellipse across quadrants A and D, as shown in  FIG. 3   b . The focus is maintained by sampling the intensities of the diagonal quadrants and comparing. In other words, the sum of the intensities of quadrants B and C is compared to the sum of the intensities of quadrants A and D in a feedback loop in order to maintain focus. 
   This method of signal detection and processing limits the technology to non-overlapping diffraction limited reflections on the photodetector  128 . Any overlap in the zero and +/−first order reflected beams ( 106 ,  108 ,  110 ), would skew the signals to the photodetector  128 , and thereby falsify the horizontal tracking and information focusing feedback. Therefore, the prior art technology is limited in optical storage density to a configuration that provides diffraction limited nonoverlapping signals to the photodetector  128 . 
   The prior art technology, as described above, is further limited to using only the diffracted energy in the zero and first orders, in addition to being limited to nonoverlapping reflections on the readout device. This technology is described, for example, in U.S. Pat. Nos. 5,717,674; 5,475,670; 5,412,631; 5,231,620, 5,128,914, and 5,094,732. These patents teach multiple ways of implementing optical pickup devices utilizing three signals, one from the zero diffractive order and one from each the +/−first orders. In order to increase both the optical storage density and optical readout speed, improvements in the method of signal processing must be realized to overcome these existing limitations. 
   OBJECTS OF THE INVENTION 
   Therefore, it is desirable to provide an improved optical pickup device and method utilizing the advantages of an electronically reconfigurable diffraction grating. 
   It is also desirable to provide an improved optical pickup device and method with increased storage density and increased readout speed over the prior art. 
   It is also desirable to provide an improved optical pickup device and method that utilizes the zero order, +/− first orders and higher diffractive orders in the readout of optical storage devices. 
   It is also desirable to provide an improved optical pickup device and method which can utilize overlapping diffraction orders, thereby allowing information on an optical disk to be stored more compactly and read more rapidly. 
   It is also desirable to provide an improved optical pickup device and method that improves the readout speed at which optical disks are read. 
   SUMMARY OF THE INVENTION 
   The present invention applies an electronically reconfigurable grating in an optical pickup, offering higher-data storage density and faster retrieval in the future generations of CD and DVD technology, for audio, video, and computer applications. Currently, optical pickup devices are common to CD and DVD reader technology, however, the present invention is not limited to those devices. All applications of optical pickup devices, current and future, would benefit from the present invention. 
   In a preferred embodiment, the optical pickup apparatus comprises an electronically reconfigurable diffraction grating modulating relative light intensities as among at least two different diffraction orders of light diffracted by the electronically reconfigurable diffraction grating; focusing optics for focusing the light diffracted by the electronically reconfigurable diffraction grating into diffractive spots corresponding with each of the diffraction orders and onto an optical storage medium, which light is then reflected by the optical storage medium; and a detector for detecting the light reflected by the optical storage medium and striking said detector. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The features of the invention believed to be novel are set forth in the associated claims. The invention, however, together with further objects and advantages thereof, may best be understood by reference to the following description taken in conjunction with the accompanying drawings in which: 
       FIG. 1  is a detailed schematic of prior art optical pickup technology. 
       FIG. 2  is a zoomed view schematic of a typical fragment of a recording on an optical disk. 
       FIG. 3  is a top view of light reflected from an optical disk striking the photodetector array in focus according to the prior art. 
       FIG. 3   a  is a top view of light reflected from an optical disk striking the photodetector array out of focus according to the prior art. 
       FIG. 3   b  is a top view of light reflected from an optical disk striking the photodetector array also out of focus according to the prior art. 
       FIG. 4  is a schematic of the light path of the improved optical pickup device utilizing a reconfigurable diffraction grating in reflection mode. 
       FIG. 4   a  is a reflection type reconfigurable diffraction grating. 
       FIG. 4   b  is a schematic of the light path of the improved optical pickup device utilizing a reconfigurable diffraction grating in transmission mode. 
       FIG. 5  is a top view light reflected from an optical disk striking the photodetector array according to several alternative preferred embodiments of the invention. 
       FIG. 6  is a schematic view of a preferred embodiment of the diffracted laser illumination of the optical storage media. 
       FIG. 6   a  shows the light distribution on the photodetector corresponding to the preferred embodiment shown in FIG.  6 . 
       FIG. 7  is a schematic of an alternative implementation of the diffracted laser illumination of the optical storage media. 
       FIG. 7   a  is a secondary schematic of the alternative implementation of the diffracted laser illumination of the optical storage media shown in FIG.  7 . 
       FIG. 7   b  shows the light distribution on the photodetector corresponding to the alternative embodiment shown in  FIGS. 7 and 7   a.    
       FIG. 8  is a schematic of another alternative implementation of the diffracted laser illumination of the optical storage media. 
       FIG. 8   a  shows the light distribution on the photodetector corresponding to the preferred embodiment shown in FIG.  8 . 
       FIG. 9  shows a schematic detailing the phase shift measurement. 
   

   DETAILED DESCRIPTION 
   The present invention is an improved optical pickup device that incorporates an electronically reconfigurable diffraction grating utilizing the zero order and multiple higher diffractive orders. The present invention has new degrees of freedom which can offer many advantages compared to existing optical pickup technology. The main expected useful results are increased data storage density and readout speed. These implementations are not possible using a conventional diffraction grating and can only be realized by the optical pickup device that includes an electronically reconfigurable diffraction grating  442  as described.  FIG. 4  shows a schematic of the improved optical pickup device implemented with an optical delivery system similar to the prior art to allow for easy comparison. It is a schematic of the light path of the preferred embodiment of the improved optical pickup device, though variations can be achieved by someone of ordinary skill within the scope of this disclosure and its associated claims. 
   A laser diode light source  440  illuminates an electronically reconfigurable diffraction grating  442  that receives an input signal from the control system  444 . The multiple orders of diffracted light  446  are collected by the delivery and focusing optics  44 B, and focused onto the optical storage media  450  for data retrieval (pickup). The reflected light is returned through the same optical train and diverted by the polarizing beamsplitter  452  through additional focusing optics  454  onto a photodetector array  456 . Each of these main components will now be described in more detail. 
   The laser diode  440 , as described in the prior art, is typically a solid state laser diode emitting in the near IR, that emits optical power in a wedge shaped beam with a typical divergence of 10×30 degrees in the X and Y directions, respectively. These are the typical light sources found in such optical pickup devices due to their reliability, low power consumption and long lifetime attributes. 
   The electronically reconfigurable diffraction grating  442  is a programmable device, typically fabricated using microelectromechanical systems (MEMS) technology, which allows the user fine control over the spatial distribution of light intensity in the diffraction pattern. A typical reconfigurable diffraction grating  442  is shown in  FIG. 4   a . As shown in the figure, the array of rulings  437  are separated by equal ruling spacings  435  and are individually addressable by the common electrode  433  that runs underneath either every ruling  437  or under a periodic distribution of the rulings  437  (every other, every third, every fifth, etc.). The spatial distribution of light intensity is controlled by a voltage applied to each ruling  437  of the grating. The control system  444  applies a series of preprogrammed voltages to the grating to achieve the desired grating rulings  437  configuration and switching sequence. Programming a series of voltages allows automated processing at various spatial distributions. The pathway  439  between the reconfigurable diffraction grating  442  and the control system  444  is a two way pathway to provide feedback to the control system. Switching becomes automated with the electronically reconfigurable diffraction grating  442 , as opposed to the mechanical tuning required to adjust a conventional grating element. 
   The inclusion of the electronically reconfigurable diffraction grating  442  significantly increases the number of degrees of freedom in controlling the diffracted of light over the conventional diffraction grating technology. Incorporated into the present invention of the improved optical pickup device, the electronically reconfigurable diffraction grating provides additional processing capabilities, which result in higher speed of information retrieval and recording, as well as increased storage density. 
   The reconfigurable grating  442  is illustrated in reflection mode, but technological advances in MEMS-based grating designs could provide a transmission grating that can be used in future implementations of the present invention. It is understood that the illustrated reflection-mode reconfigurable diffraction grating  442  can be replaced with a transmission-mode reconfigurable diffraction grating as this technology is improved, in a configuration such as is illustrated by  FIG. 4   b , which shows an alternative embodiment of the present invention in which the grating element is an electronically reconfigurable transmission diffraction grating  443 . All other elements of the invention remain the same as shown in the preferred embodiment in FIG.  4 . The purpose, functionality and implementation of the transmission type grating  443  would be the same as presented with the reflection type grating  442  in the preferred embodiment of the present invention. 
   The optical delivery and focusing system  448  is designed specifically for the application in which the optical pickup device is used. The function of the optical delivery system  448  is to provide a focusing mechanism by which the diffracted light is focused on the optical storage media  450 , typically including an automated focusing lens. It also typically provides a return path for the light reflected from the optical storage media  450 . The optical delivery components in  FIG. 1  (polarizer  111 , beamsplitter  112 , turning mirror  118 , automated focusing lens  120 , collimating lens  114  and quarter wave plate  116 ) show a typical arrangement of the optical delivery and focusing system implemented by prior art optical pickup devices in CD players/readers. The present invention may utilize functionally similar optical configurations but are not limited to identical configurations of the optical delivery and focusing system since the primary purpose is to provide a means of focusing the diffracted light and a return path for the reflected light. For means of easy comparison, however,  FIG. 4  illustrates for a preferred embodiment, an optical delivery and focusing system  448  similar to that shown in FIG.  1 . It comprises a polarizer  458 , a polarizing beamsplitter  452 , a turning mirror  460 , a collimating lens  462 , a quarter wave plate  464  and an automatic focusing lens  466 . 
   The optical storage media  450  that typically utilize the optical pickup devices include CDs and DVDs for audio, video, and computer data storage. For reference,  FIG. 2  shows a typical recorded fragment on optical storage media  450 . Shown in  FIG. 2  are the pits  232  and the coast  234 . The pits  232  comprise the information content storage layer of the disk and are addressed by the pickup in order to read the content information stored on the optical disk. Typically the pits  232  are aligned along a track, and those aligned at the same radius from the center of the disk reside along the same pit line  233 . The coast  234  is defined as the spacing between the adjacent pits  232 , and is used for tracking. The coast line  235  which is equidistant between two radially successive pit lines  233 , is addressed by the optical pickup to tracking information in order to locate the pit lines  233 . 
   The reflected light collected from the optical storage media  450  is returned through the optical delivery system  448  and focused onto a photodetector array  456  for readout. Typically, the types of photodetector arrays  456  used are quadrant photodetectors that provide simple geometric arrangements of detection quadrants, such as shown in FIG.  5 .  FIG. 5  shows the top view of the photodetector array  456  incorporated in the improved optical pickup device. As shown in the figure, the photodetector array  456  is subdivided into a main array  460  with four quadrants (A, B, C, D) and two individual subarrays,  462  and  464  respectively, which lie on opposite sides of the main array  460 . 
   The distribution of intensity between the diffracted orders of light is modulated at a frequency which is high compared to the data readout frequency. This dynamic control of the grating configuration is only possible by utilizing an electronically reconfigurable grating  442 . As a result, scanning of the optical storage media  450  surface will take place in a stepwise fashion such that when the intensity of the diffracted light energy is concentrated in the higher orders, points farther from the center are mostly illuminated, and when the intensity of the diffracted light energy is concentrated in the zero order, the center is predominantly illuminated. The selection of the diffraction order that receives the majority of the light energy and therefore is mostly illuminated on the optical storage media  450  is defined by the voltage applied to the electronically reconfigurable grating  442 , with programmable sequential voltage steps implemented by the control system  444 . The selected diffraction order is therefore known and the individual diffraction orders do not need to be resolved in the image on the photodiode array  456 . This allows the diffraction orders to partially overlap with one another without compromising the information readout, and leads to the capability for higher optical disk storage densities and faster information retrieval. Knowledge of the modulation of the intensity in individual diffraction orders, as implemented in the electronically reconfigurable diffraction grating  442  by the control system  444 , is used to differentiate the signals at the photodetector array  456 . 
   The light distribution on the optical storage media  450  is shown in FIG.  6 . For simplicity of the figure, only zero and first order are shown, however second order and higher may also be included. The electronically controlled redistribution of light energy between the spots originating from different diffraction orders, namely zero order diffractive spot  666 , +/−first order diffractive spots  668  and  670 , is a variation of digital scanning of the optical storage media  450  surface across the pit line  233 . If the storage density is so high that the pit  232  size and the coast  234  width is small compared to the diffractive focal spot sizes ( 666 ,  668 ,  670 ), the stored information can still be detected due to the modulation of the intensity of that specific diffracted order by the grating and the lateral and vertical tracking is still possible if the transverse scanning is fast compared to the readout frequency and a simple deconvolution technique is applied. 
     FIG. 6   a  shows the reflected diffractive focal spots ( 666 ,  668 ,  670 ) on the photodetector array  456 . The selective modulation of the intensity of the diffractive focal spots ( 666 ,  668 ,  670 ) and subsequent processing by the photodetector  456  signals, features such as coast  234  of the optical storage media  450  can be resolved even if the diffraction-limited spots ( 666 ,  668 ,  670 ) overlap as illustrated in FIG.  6 . The intensity modulation is achieved dynamically by repositioning the associated rulings  437  of the electronically reconfigurable grating  442 . This improved processing technique allows for a higher storage density on the optical storage media  450  since that density is no longer limited by the size of the diffraction limited spots ( 666 ,  668 ,  670 ). Further, the need to optically differentiate these spots on, for example, the photodetector  456  is eliminated. 
     FIGS. 7 and 7   a  shows an alternative implementation of the preferred embodiment of the present invention. These embodiments essentially entail implementations of the present invention whereby a faster readout time is realized by utilizing multiple higher orders simultaneously. This alternative embodiment comprises the multiple diffractive orders to be used as follows; zero order, reading content information from a pit  232 ; +/−first order  668 / 670  reading content information from a pit  232 ; and and the +/−second orders  672 / 674  reading tracking information from a coast  234 . This alternative embodiment of the present invention can be implemented by focusing the +/−second order diffractive spots  672 / 674  (unused in prior art optical pickup devices) on the center of the coast  234  between the pit lines, while the first orders scan over the boundary between the pits  232  and the coast  234  as shown in FIG.  7 . Alternatively this can be implemented as shown in  FIG. 7   a  where the zero order  666  is focused on a pit  232 , the +/first orders  668 / 670  are focused on the next successive pit  232 , and the +/−second orders  672 / 674  are focused on the next coast  234 . This increases the sensitivity of error detection for lateral position and allows reduction of the coast  234  width, thereby allowing a higher storage density. In this embodiment, the overlapping diffractive spots can be differentiated by the intensity modulation means described above in the preferred embodiment. The five resultant diffractive spots would appear on the photodetector as shown in  FIG. 7   a . Shown in the figure are the zero order diffractive spot  666 , the +/−first order diffractive spots  668 / 670 , and the +/−second order diffractive spots  672 / 674 . 
   Alternatively, the preferred embodiment can be implemented, as shown in FIG.  8 . This alternative embodiment comprises the multiple diffractive orders to be used as follows; zero order, reading content information from a pit  232 ; +/−first order  668 / 670  reading tracking information from a coast  234 ; and the +/−second orders  672 / 674  reading tracking information from a coast  234 . This can be implemented by focusing the zero order  666  on a pit  232 , first order diffractive spots  668 / 670  on the coast lines  235  adjacent to the pit line  233 , as is done in the prior art, and, focusing the second order diffractive spots  672 / 674  on the coast lines  235  behind the next pit lines  233 . This can obviously be extended to 3-d and higher orders by focusing higher orders on the next successive pit lines  233 . The diffracted focal spots would appear on the photodetector array  456  as shown in  FIG. 8   a . In this embodiment, the overlapping diffractive spots can be differentiated by the intensity modulation means described above in the preferred embodiment. The five resultant diffractive spots would appear on the photodetector as shown in FIG. Ba. Shown in the figure are the zero order diffractive spot  666 , the +/−first order diffractive spots  668 / 670 , and the +/−second order diffractive spots  672 / 674 . 
   Techniques such as these described in these two alternative embodiments can be used to increase the readout speed by addressing multiple pits and/or coasts simultaneously. Shown in these two alternative embodiments are just two implementations of using the higher orders to address and readout multiple sites simultaneously. Obviously, the techniques described above can be used to address any combination of pits and coasts simultaneously, thereby increasing the readout speed. 
   A more precise way of measuring the output is to measure the phase shift at the modulation frequency between the AC components of the intensity in different diffraction orders. This can allow the application of modulation (carrier frequency) to the readout signal and use phase demodulation of the error signals for tracking and stabilization. The signal of each quadrant of the photodetector  454  is proportional to the brightness of corresponding area on the optical storage media  450  which, in turn, depends on reflectivity of the surface and illumination. When the data storage density is high, the feature sizes on the optical storage media  450  are smaller than the size of a diffraction-limited spot. The illuminating laser spots overlap, and the brightness of the area imaged onto the photodetector  454  results from a sum of intensities of adjacent spots illuminating the surface of the optical storage media  450 . Intensities in the different spots will oscillate with the same frequency but with different phase shifts. The phase shifts depend on the voltage applied to the electronically reconfigurable grating  442  and the angle of incidence of light onto the grating  442 . The phase of the signal from the photodetector  454  will be a function of intensities and phase shifts of the individual spots, as well as the location of features on the surface of the optical storage media  450  such as coast  234  of the track. For known intensities and phase shifts in the illuminating spots, the position of the coast  234  can be retrieved from the phase of the signal readout at the photodetector  454 . 
   The sensitivity of the phase measurement can be demonstrated with reference to the vector diagram in FIG.  9 . For example, if the measured intensity of the zero order and the first order are relatively similar, they can be differentiated by measuring their phase shift by the technique described above. The zero order intensity vector  990 , with zero order phase angle  991  is added to the first order intensity vector  992  with first order phase angle  993 , the resultant intensity vector  994  would have the measurable resultant phase shift  995 . 
   While only certain preferred features of the invention have been illustrated and described, many modifications, changes and substitutions will occur to those skilled in the art. It is, therefore, to be understood that this disclosure and its associated claims are intended to cover all such modifications and changes as fall within the true spirit of the invention.