Abstract:
An optical information recording/reproducing apparatus using holography comprises a signal generation unit that modulates input data, adds at least one control bit to each group of N bits, performs an NRZI-modulation on the modulated data, determines the at least one control bit such that a digital sum value of the NRZI-modulated data is 0, performs NRZI modulation on the data whose at least one control bit was determined, and rearranges the data to generate 2-dimensional data; a pickup that records the 2-dimensional data in a hologram disc and reproduces the 2-dimensional data from the hologram disc; and a signal processing unit that corrects the 2-dimensional data reproduced by the pickup, performs NRZI-modulation on the 2-dimensional data that has undergone a binarization operation, removes the at least one control bit added during the recording, and demodulates the data according to a modulation rule used during the recording.

Description:
INCORPORATION BY REFERENCE 
     The present application claims priority from Japanese application JP2008-131454 filed on May 20, 2008, the content of which is hereby incorporated by reference into this application. 
     BACKGROUND OF THE INVENTION 
     The present invention relates to an apparatus and a method for recording information on and/or reproducing information from optical information recording media by using holography. 
     Optical disc products with a recording density of about 50 GB are being commercialized that are based on Blu-ray Disc (BD) standard and High Definition Digital Versatile Disc (HD DVD) standard using a blue semiconductor laser. 
     Optical discs are expected to have an increased capacity of as large as 100 GB to 1 TB, comparable to that of HDD (Hard Disc Drive), in the future. 
     However, to realize such an ultrahigh density with the current optical discs, a novel storage technology is required, different from the conventional trend of high density technologies that attempts to increase the storage capacity by shortening a wavelength and increasing NA of an object lens. 
     With a wide-ranging studies on next generation storage technologies under way, a hologram recording technology is available that records digital information using holography. 
     Among the hologram recording technologies is one disclosed in JP-A-2004-272268. This patent document describes a so-called angle-multiplexing recording method which focuses a signal beam flux on an optical information recording medium through a lens and at the same time throws a reference beam of collimated rays to the medium to cause interferences to record a hologram and displays different pages of data on a spatial light modulator by changing an incidence angle of the reference beam to the optical recording medium to realize multiplex recording. The patent document also discloses a technology that puts an aperture (spatial filter) at a beam waist of a lens-focused signal beam to shorten the intervals of adjoining holograms, thereby increasing the recording density and capacity, compared with those of the conventional angle-multiplexing recording method. 
     Another hologram recording technology is disclosed in, for example, WO2004-102542. This document describes an example of shift multiplexing hologram recording method which, in one spatial light modulator, focuses a light from inner pixels as a signal beam and a light from outer ring-like pixels as a reference beam onto an optical recording medium through one and the same lens to cause interferences between the signal beam and the reference beam at near the focus plane of the lens to record a hologram. 
     There is an encoding method used for the above hologram recording, such as one disclosed in JP-A-9-197947. This patent document describes a 2-dimensional encoding method for hologram recording which throws at least one light wave through a 2-dimensional spatial light modulator to determine information to be recorded, characterized in that four adjoining pixels or 4-multiples of pixels in the 2-dimensional spatial light modulator are taken as one set and that one fourth of the number of pixels making up each set is made to pass the light and the remaining three fourths are made to interrupt it. 
     Another example of the conventional technology is JP-A-2005-190636, which provides “a holographic recording method, a holographic memory reproducing method, a holographic recording apparatus and a holographic memory reproducing apparatus, designed to improve an encoding rate by preventing variations in reproduced imaged intensity even if pixel blocks of different numbers of ON pixels are mixedly used.” 
     SUMMARY OF THE INVENTION 
     In the method described in JP-A-2004272268 which applies the encoding technique of JP-A-9-197947 or in the method described in WO-2004-102542 which applies the encoding technique of JP-A-9-197947, 2-dimensional data of  FIG. 12C  is obtained by performing the encoding of  FIG. 12B  on data strings of  FIG. 12A . These methods however, have a drawback of consuming a four-bit area to produce 2 bits of information and therefore being unable to improve the recording density. The method that simply transmits light with “1” and blocks light with “0” results in light transmissivity varying from one page to another. This difference in transmissivity causes different pages, when reproduced, to have different levels of reproduced image brightness, giving rise to a possibility that an erroneous decision may be made when the reference values for binarization decision are equal. There is another problem that consumption of dynamic range in a hologram recording medium is not constant. 
     Further, JP-A-2005-190636 does not take into account a possibility of the light transmissivity varying among different pages when pixel blocks with different ON-pixel numbers are mixedly used. 
     An object of this invention is to provide an encoding method capable of improving the recording density while keeping the transmissivity constant among different pages. 
     The object of this invention can be realized by, for example, controlling a 2-dimensional data arrangement. 
     In the recording of digital information using holography, this invention allows for improvement of the digital density while keeping the transmissivity constant among different pages. 
     Other objects, features and advantages of the invention will become apparent from the following description of the embodiments of the invention taken in conjunction with the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is an overall configuration of an optical information recording/reproducing apparatus according to an embodiment. 
         FIG. 2  is a schematic view showing an example of a pickup in the optical information recording/reproducing apparatus. 
         FIGS. 3A-3C  are flow charts showing an example of operation flow performed by the optical information recording/reproducing apparatus. 
         FIG. 4  is a flow chart showing an example of operation performed by the optical information recording/reproducing apparatus during data recording. 
         FIG. 5  is a flow chart showing an example of detailed operation flow performed by the optical information recording/reproducing apparatus during data reproduction. 
         FIGS. 6A-6G  show examples of encoding method performed by the optical information recording/reproducing apparatus. 
         FIG. 7  is a flow chart showing an example of detailed operation flow performed by the optical information recording/reproducing apparatus during data recording. 
         FIG. 8  is a flow chart showing an example of detailed operation flow performed by the optical information recording/reproducing apparatus during data reproduction. 
         FIGS. 9A-9F  show examples of encoding methods performed by the optical information recording/reproducing apparatus. 
         FIG. 10  is a flow chart showing an example of detailed operation flow performed by the optical information recording/reproducing apparatus during data recording. 
         FIGS. 11A and 11B  show examples of modulation performed by the optical information recording/reproducing apparatus. 
         FIGS. 12A-12C  show examples of conventional encoding methods performed by the optical information recording/reproducing apparatus. 
     
    
    
     DETAILED DESCRIPTION OF THE EMBODIMENTS 
     Now, embodiments of this invention will be described below. 
     Embodiment 1 
       FIG. 1  shows an overall configuration of the optical information recording/reproducing apparatus to record and/or reproduce digital information by using holography. 
     The optical information recording/reproducing apparatus  10  has a pickup  11 , a phase conjugate optical system  12 , a disc-cure optical system  13 , a disc rotation angle detecting optical system  14  and a rotating motor  50 . The optical information recording medium  1  can be turned by the rotating motor  50 . 
     The pickup  11  emits a reference beam and a signal beam to the optical information recording medium  1  to record digital information by using holography. 
     At this time, the information signal to be recorded is sent by a controller  89  through a signal generation circuit  86  to a spatial light modulator described later in the pickup  11  where the signal beam is modulated by the spatial light modulator. 
     When the recorded information in the optical information recording medium  1  is reproduced, a phase conjugate beam of the reference beam emitted from the pickup  11  is generated by the phase conjugate optical system  12 . The phase conjugate beam is a light wave that propagates in a direction opposite the incident light while keeping the same wave plane. A light reproduced by the phase conjugate beam is detected by an optical detector described later in the pickup  11  and is processed by a signal processing circuit  85  to reproduce the signal. 
     The time during which the reference beam and the signal beam are irradiated to the optical information recording medium  1  can be adjusted by controlling a shutter open-close time described later by the controller  89  through a shutter control circuit  87 . 
     The disc-cure optical system  13  has a function of generating an optical beam used for pre-curing and post-curing the optical information recording medium  1 . The pre-cure means a step of preliminarily applying a predetermined optical beam before irradiating the reference beam and the signal beam to a desired position when recording information at the position of interest in the optical information recording medium  1 . The post-cure means is a step of applying a predetermined optical beam after having recorded information at a desired position in the optical information recording medium  1  in order to make the desired position unrecordable. 
     The disc rotation angle detecting optical system  14  is used to detect a rotation angle of the optical information recording medium  1 . Adjusting the optical information recording medium  1  to a predetermined rotation angle can be done by detecting a signal corresponding to the rotation angle by the disc rotation angle detecting optical system  14  and controlling the rotation angle of the optical information recording medium  1  by the controller  89  using the detected signal through a disc rotating motor control circuit  88 . 
     A light source drive circuit  82  supplies a predetermined amount of light source drive current to light sources inside the pickup  11 , the disc-cure optical system  13  and the disc rotation angle detecting optical system  14 , so that these light sources can emit light beams of a predetermined light quantity. 
     The pickup  11 , the phase conjugate optical system  12  and the disc-cure optical system  13  are each provided with a mechanism that allows them to slide in a radial direction of the optical information recording medium  1 . So their positions are controlled by these mechanisms through an access control circuit  81 . 
     The recording technology using holography can record information of ultrahigh density and therefore tends to have very small allowable errors with respect to variations in inclination and position of the optical information recording medium  1 . So a servo mechanism may be provided in the optical information recording/reproducing apparatus  10  to correct variations of, for example, inclination and position of the optical information recording medium  1 , for which allowable errors are very small, through a servo control circuit  84  by installing a device in the pickup  11  to detect these variations and by generating a servo control signal in a servo signal generation circuit  83 . 
     The pickup  11 , the phase conjugate optical system  12 , the disc-cure optical system  13  and the disc rotation angle detecting optical system  14  may be arranged commonly for some optical systems or all of the optical systems for simplification. 
       FIG. 2  shows an example configuration of the optical system for the pickup  11  in the optical information recording/reproducing apparatus  10 . 
     A light beam emitted from the light source  301  passes through a collimate lens  302  and enters into a shutter  303 . When the shutter  303  is open, the light beam passes through it and is controlled in its polarization direction by an optical element  304  constructed of ½ waveplate so that a ratio between P-polarization and S-polarization is a desired one. Then the beam enters a PBS (Polarization Beam Splitter) prism  305 . 
     The light beam that has passed through the PBS prism  305  is expanded in its diameter by a beam expander  309 , before passing through a phase mask  311 , a relay lens  310  and a PBS prism  307  and entering into a spatial light modulator  308 . 
     The signal beam that was given information by the spatial light modulator  308  passes through the PBS prism  307  and then travels through a relay lens  312  and a spatial filter  313 . Then, the signal beam is focused by an object lens  325  onto the optical information recording medium  1 . 
     A light beam reflected by the PBS prism  305  works as a reference beam. After being set in a predetermined polarization direction by a polarization direction conversion element  324  depending on whether the operation is being performed for recording or for reproduction, the beam passes through a mirror  314  and a mirror  315  and enters into a galvanometer mirror  316 . The galvanometer mirror  316 , since it adjusts its angle by an actuator  317 , can set to a desired angle the incident angle of the reference beam entering into the optical information recording medium  1  after passing through a lens  319  and a  320 . 
     By having the signal beam and the reference beam overlap each other in the optical information recording medium  1  as described above, interference patterns are formed in the recording medium and then written into the recording medium to record the information. Further, since the incidence angle of the reference beam entering the optical information recording medium  1  can be changed by the galvanometer mirror  316 , the angle multiplexing-based recording can be performed. 
     In reproducing the recorded information, the reference beam is applied to the optical information recording medium  1 . The light beam that has passed through the optical information recording medium  1  is reflected by a galvanometer mirror  321  to generate a phase conjugate beam. 
     The reproduced light beam generated by this phase conjugate beam propagates through the object lens  325 , the relay lens  312  and the spatial filter  313 . Then, the reproduced light beam is reflected by the PBS prism  307  and enters into an optical detector  318  to reproduce the recorded signal. 
     The optical system configuration of the pickup  11  is not limited to  FIG. 2 . 
       FIGS. 3A-3C  show the operation flow for the recording and reproduction in the optical information recording/reproducing apparatus  10 . Here, a recording and reproduction flow using holography will be explained. 
       FIG. 3A  shows an operation flow from the optical information recording medium  1  being inserted into the optical information recording/reproducing apparatus  10  until the recording or reproduction is ready.  FIG. 3B  shows an operation flow from the standby state until the information is recorded in the optical information recording medium  1 .  FIG. 3C  shows an operation flow from the standby state until the information recorded in the optical information recording medium  1  is reproduced. 
     When a medium is inserted (S 301 ) as shown in  FIG. 3A , the optical information recording/reproducing apparatus  10  makes a disc check to see if the inserted medium is intended for recording or reproducing digital information using holography (S 302 ). 
     If the disc check result finds that the disc is intended to record or reproduce digital information using holography, the optical information recording/reproducing apparatus  10  reads control data for the optical information recording medium and retrieves, for example, information about the optical information recording medium and information about various setting conditions for recording or reproduction (S 303 ). 
     After the control data has been read out, the optical information recording/reproducing apparatus  10  makes various adjustments according to the control data and executes learning processing concerning the pickup  11  (S 304 ). Now the optical information recording/reproducing apparatus  10  is ready for recording or reproduction (S 305 ). 
     The operation flow from the standby state to the recording of information, as shown in  FIG. 3B , involves receiving data to be recorded and sending information corresponding to the data to the spatial light modulator in the pickup  11  (S 306 ). 
     Then, to record high quality information in the optical information recording medium, various learning processing is executed in advance as required (S 307 ) and, at the same time, seek operation (S 308 ) and address regeneration (S 309 ) are repeated to put the pickup  11  and the disc-cure optical system  13  at a predetermined position on the optical information recording medium. 
     Then, a light beam emitted from the disc-cure optical system  13  is applied to the medium to pre-cure a predetermined area (S 310 ). The reference beam and the signal beam emitted from the pickup  11  are used to record data (S 311 ). 
     After the data is recorded, the data is verified as necessary (S 312 ) and the light beam emitted from the disc-cure optical system  13  is used for post-curing (S 313 ). 
     In the operation flow from the standby state to the reproduction of the recorded information, as shown in  FIG. 3C , various learning processing is executed as necessary in advance (S 314 ). Then, the seek operation (S 315 ) and the address regeneration (S 316 ) are repeated to put the pickup  11  and the phase conjugate optical system  12  at a predetermined position on the optical information recording medium. 
     After this, the reference beam is emitted from the pickup  11  to read information recorded in the optical information recording medium (S 317 ). 
     The encoding method in this example will be described in detail by referring to  FIG. 4 ,  FIG. 5  and  FIGS. 6A-6G . 
       FIG. 4  shows a detailed operation flow of S 306  in  FIG. 3B .  FIG. 5  shows a detailed operation flow of S 317  in  FIG. 3C .  FIGS. 6A-6G  show examples of processing. 
     First, detailed operations during recording will be explained. When the signal generation circuit  86  receives one page of recording data (S 401 ) ( FIG. 6A ), it modulates data strings by using a modulation table (S 402 ). This modulation is done to facilitate detection of data during reproduction by preventing the same data “0” or “1” from repeating continuously and also to control spatial frequency characteristics of patterns to be finally recorded. This modulation, however, may not be performed. Next, the modulated data is divided into units of N bits, to each of which one control bit is added (S 403 ) ( FIG. 6B ). The control bit is determined to be “0” or “1” according to a method described below. First, let us assume that a control bit at a certain position is “0” (S 404 ). An NRZI (None Return to Zero Inverted) modulation is performed on a data string up to the next control bit. This modulation leaves the value unchanged if the bit is “0” and inverts the value if it is “1” (S 405 ). Unlike the NRZI modulated data, a DSV (Digital Sum Value), which is an accumulated value of 1 in the data string taken as +1 and 0 as −1, is calculated up to the next control bit (S 406 ). To make the value of page data constant among different pages, it is desired that the DSV be a sum value not only for the data string between the control bits but also for the data following the entire NRZI modulation up to the control bit. Next, when the control bit is assumed to be “1”, operations similar to S 404 , S 405  and S 406  are also executed. Here, the DSVs calculated in S 406  to S 409  are compared to determine the control bit added in S 403  to make DSV close to 0 (S 410 ) ( FIG. 6C ). These operations from S 404  to S 410  are repeated (S 411 ) to determine all the added control bits. On the data strings for which control bits were determined, the NRZI modulation is performed to generate data strings to be recorded (S 412 ) ( FIG. 6D ). 
     Then, two-dimensional data is constructed as shown in  FIG. 6E , with “0” taken as “non-transmissive” and “1” as “transmissive” (they may be reverse). For each unit to which a control bit is inserted, an area of n (vertical)×m (horizontal) pixels is set ( 601 ,  602 ,  603 ,  604 ) and bits are arranged there. This bit arrangement for each unit is repeated the same number of times as a page of data to create one page of 2-dimensional data (S 413 ). In the example of  FIG. 6F , the bit arrangement in the unit and the unit arrangement in the page are done by placing data beginning with the upper left and moving toward right and, when the right end is reached, moving one line down and then toward right. The data arrangement is not limited to this method.  FIG. 6G  shows an example configuration where n=1. 
     A marker that works as a reference during reproduction is added to the 2-dimensional data constructed as described above (S 414 ). The data marked in this way is transferred to the spatial light modulator  308  (S 415 ). 
     Next, a detailed operation during reproduction will be explained. First, image data retrieved from the optical detector  318  is transferred to the signal processing circuit  85  (S 501 ). The image position is detected with an image marker taken as a reference (S 502 ). The image data undergoes a distortion correction, including image inclination, magnification and distortion (S 503 ). The corrected image is then subjected to a binarization operation (S 504 ) and removed of markers (S 505 ) to obtain 2-dimensional data (S 506 ). Although the binarization generally employs a method of comparing adjoining bits, other methods may be employed. By reversing the recording procedure, the 2-dimensional data is rearranged into 1-dimensional data, which then undergoes the NRZI modulation (S 507 ). The data is removed of the added control bits (S 508 ) and demodulated into the original data strings by using the modulation table used for recording, thus reproducing the original data (S 509 ) (S 510 ). 
     The explained drive construction and operation are just one example and this invention can employ other constructions and can be applied not only to the angle-multiplexing method but also to the shift multiplexing method. The same is true of the following embodiments. 
     With the above operation, 2-dimensional data can be created whose ratios of transmissive and non-transmissive bits are always even in the entire page data although they may differ among different units. This in turn allows the data to be recorded with the transmissivity kept constant among pages. During recording, a signal beam modulated by the spatial light modulator  308  is focused by the object lens  325  onto the recording medium, so a Fourier-transformed image is recorded. This means that if the transmissivity of created 2-dimensional data differs among different units, the recording medium is not affected. 
     Embodiment 2 
     The second embodiment differs from embodiment 1 in the 2-mensional data generation method of S 306  and the data reproducing method of S 317 .  FIG. 7  shows a detailed operation flow of S 306  in  FIG. 3B .  FIG. 8  shows a detailed operation flow of S 317  in  FIG. 3C .  FIGS. 9A-9F  show examples of processing. 
     First, a detailed operation during recording will be explained. When the signal generation circuit  86  receives one page of recording data (S 701 ) ( FIG. 9A ), it modulates data strings by using a modulation table (S 702 ). Next, the data is divided into units of N bits, to each of which a control bit is added (S 703 ) ( FIG. 9B ). The control bit is one bit in this embodiment but may be multiple bits. First, assuming that a control bit at a certain position is “0” (S 704 ), a DSV up to the next control bit is calculated (S 705 ). It is desired that the DSV be a sum value not only for the data between the control bits but also for all data up to that control bit. Next, when the control bit is assumed to be “1” (S 706 ), an inversion operation of inverting “0” to “1” and “1” to “0” is executed on data strings up to the next control bit (S 707 ). The inverted data strings are used to calculate a DSV up to the next control bit (S 708 ). This data inversion may be performed commonly for several bits by using a table. The DSVs calculated by S 705  and S 708  are compared to determine a control bit added to make the DSV close to 0 (S 709 ). The operations from S 704  to S 709  are repeated (S 710 ) to determine all the control bits added in S 703 . The data strings to which this control bit was added are subjected to the inversion operation for each unit according to the control bit to generate the data strings to be recorded (S 711 ) ( FIG. 9C ). 
     After this, 2-dimensional data is constructed as shown in  FIG. 9D , with “0” taken as non-transmissive and “1” as transmissive (they may be opposite). For each unit to which a control bit is inserted, an area of n (vertical)×m (horizontal) pixels is set ( 901 ,  902 ,  903 ,  904 ) and bits are arranged there. This bit arrangement for each unit is repeated the same number of times as a page of data to create one page of 2-dimensional data (S 712 ). In the example of  FIG. 9E , the bit arrangement in the unit and the unit arrangement in the page are done by placing data beginning with the upper left and moving toward right and, when the right end is reached, moving one line down and then toward right. The data arrangement is not limited to this method.  FIG. 9F  shows an example configuration where n=1. 
     The 2-dimensional data constructed as described above is attached with a marker that works as a reference during reproduction (S 713 ). The data marked in this way is transferred to the spatial light modulator  308  (S 714 ). 
     Next, a detailed operation during reproduction will be explained. First, image data retrieved from the optical detector  318  is transferred to the signal processing circuit  85  (S 801 ). The image position is detected with an image marker taken as a reference (S 802 ). The image data undergoes a distortion correction, including image inclination, magnification and distortion (S 803 ). The corrected image is then subjected to a binarization operation (S 804 ) and removed of markers (S 805 ) to obtain 2-dimensional data (S 806 ). Although the binarization generally employs a method of comparing adjoining bits, other methods may be employed. By reversing the recording procedure, the 2-dimensional data is rearranged into 1-dimensional data, which is then inverted for each unit (S 807 ). The inverted data is removed of the added control bits (S 808 ) and demodulated into the original data strings by using the modulation table used for recording, thus reproducing the original data ( 8509 ) (S 810 ). 
     With the above operation, 2-dimensional data can be created whose ratios of transmissive and non-transmissive bits are always even in the entire page data although they may differ among different units. This in turn allows the data to be recorded with the transmissivity kept constant among pages. During recording, a signal beam modulated by the spatial light modulator  308  is focused on the recording medium by the object lens  325 , so a Fourier-transformed image is recorded. This means that if the transmissivity of created 2-dimensional data differs among different units, the recording medium is not affected. 
     Unlike embodiment 1, this embodiment does not perform such operations as NRZI modulation and thus can record data at high speed. It is noted, however, that since the data inversion depends on the control bits, the reading of the control bits becomes important as shown in  FIG. 9E  ( 905 ,  906 ,  907 ,  908 ). It is therefore effective to represent the control bit with multiple bits, as described above, add error correction codes or placing data at a central area of the page where reading errors are not likely to occur. 
     Embodiment 3 
     This embodiment differs from embodiment 1 in the control bit determination rule in S 410 . In embodiment 1, the DSVs calculated from S 406  to S 409  are compared and the control bit added in S 403  is determined so as to make the DSV close to 0 (S 410 ). In this embodiment, the control bit is determined so as to make the DSV close to a preset target value (S 1010 ). 
     Further, to make it easy to shift the DSV in a certain direction, it is useful to modulate the data in advance so as to make the ratios of “0” and “1” of the NRZI-modulated data uneven. For example, in the modulation operation of S 1002  in  FIG. 10 , the modulation is executed using a modulation table such as one shown in  FIG. 11A . This operation is characterized in that the modulated data has “1” appear the even number of times, which, when the data is NRZI-modulated, makes the frequencies of appearance of “0” and “1” differ. Although the use of this table renders the ratio of “0” and “1” uneven, the method of modulation is not limited to this table and other methods may be used as long as they can change the ratio of “0” and “1”. While this table performs a modulation from 2-bits to 3 bits, other bit numbers may be used. 
     The above method can similarly applied also to embodiment 2. It is noted, however, that since embodiment 2 does not perform the NRZI modulation, it is useful to make a greater number of “0s” (or “1s”) appear in the data modulated by S 1002 . For example, in the modulation operation S 1002  of  FIG. 10 , a modulation table, such as shown in  FIG. 11B , is used. Although the use of this table can render the ratio of “0” and 1” uneven, the modulation method is not limited to this table and other methods may be used as long as they can change the ratio of “0” and “1”. While this table performs a modulation from 2 bits to 3 bits, other bit numbers may be used. 
     With the above operation, 2-dimensional data can be created whose ratio of transmissive and non-transmissive bits are always even in the entire page data although they may differ among different units. This allows the data to be recorded so that the transmissivity is kept constant among different pages. During recording, a signal beam modulated by the spatial light modulator  308  is focused on the recording medium by the object lens  325 , so a Fourier-transformed image is recorded. This means that if the transmissivity of created 2-dimensional data differs among different units, the recording medium is not easily affected. 
     Further, in embodiment 1 since the ratio of “0” and “1” are equal, the transmissivity in one page can be set only at 50% by the spatial light modulator  308 . In contrast to embodiment 1, this embodiment is characterized by the ability to set an arbitrary transmissivity. For example, the transmissivity can be made small by first setting the DSV target value at a negative value to make the ratio or frequency of “0” high, determining the control bit and then creating the 2-dimensional data with “0” taken as non-transmissive bit. This suppresses the consumption of a dynamic range of the medium, the level of multiplexing can be raised. Further, depending on the content of data, the advantage of this embodiment can be realized by setting the ratio of “0” and “1” within a predetermined range (e.g., 45%-55%). This in turn reduces loads during recording. What is described here also applies to other embodiments. 
     It should be further understood by those skilled in the art that although the foregoing description has been made on embodiments of the invention, the invention is not limited thereto and various changes and modifications may be made without departing from the spirit of the invention and the scope of the appended claims.