System for recording and reproducing hologram

What is claimed is:

1. A method for recording information as Fourrier transform holograms which, in reproducing, are read out by relative, continuous scan with a reproducing light beam, comprising the steps of;

preparing information in a form adapted to be recorded as holograms by dividing the original information to be recorded into a number of information groups each consisting of a predetermined number, N, of unit-informations each consisting of a given amount of information, said predetermined number N being at least two,

supplying the prepared information to an information input device having a single or a plurality of unit-information input positions beginning from a first unit-information of a first group sequentially, said plurality of unit-information input positions being arranged on an information input surface without substantially overlapping one another,

exposing each unit-information supplied to said unit-information input position to an object light beam to effect modulation thereof, and

recording each unit-information as a hologram on a recording medium with a plurality of reference light beams one by one or a single reference light beam such that one-to-one different combinations of said single unit-information input position and said plurality of reference light beams or one-to-one different combinations of said plurality of unit-information input positions and said single reference light beam are used in sequence to record a series of holograms aligned on a track of the recording medium.

2. A method according to claim 1 wherein said recording step includes using a single reference light beam and N successive unit-informations are supplied sequentially to spatially separate N unit-information input positions.

3. A method according to claim 1 wherein said recording step is carried out by using a plurality (M) of reference light beams having different respective incident angles, M being less than or equal to the number N, and N/M, (an integer) spatially separated unit-information input positions, N/M being less than the number N, such that all possible one-to-one combinations of said reference light beams and unit-information input positions is equal to the number N, and wherein a first hologram is recorded by using said first unit-information input position and a second reference light beam or by using a second unit-information input position the first reference light beam, and other holograms are recorded in sequence in a similar procedure by using in sequence all said possible combinations of the unit-information input positions and the reference light beams.

4. A method according to claim 1 wherein said recording step is carried out by using a plurality (M) of reference light beams having different incident angles, where M is less than or equal to the number 2N, and a single or a plurality 2N/M, (an integer) of unit-information input positions, where 2N/M is less than the number 2N, all possible one-to-one combinations of said reference light beams and unit information input positions being equal to the number 2N and divided into two channels each including N of said combinations, recording holograms on odd-numbered tracks by using one channel of N of said combinations and holograms on even-numbered tracks by using the other channels of N of said combinations, respectively.

5. A method according to claim 1 wherein the number N is 2, 3 or 4.

6. A method according to claim 1 wherein said recording step includes using a single reference light beam and two spatially separated input channels each of N spatially separated unit-information input positions, successive unit-informations being supplied sequentially to the N unit-information input positions of the first channel to record holograms of said units of information as a first track on the recording medium, and subsequent successive unit-informations being supplied sequentially to the N unit-information input positions of the second channel to record holograms of said units of information as a second track adjacent said first track, and any subsequent tracks being recorded such that all even numbered tracks are made by exposure to the second channel and all odd numbered tracks by exposure to said first channel.

7. A method according to claim 6 wherein adjacent tracks are recorded in a partially overlapped manner.

8. A method for reproducing information through an inverse Fourier transform by continuously scanning holograms recorded on a recording medium as a series of holograms comprising the steps of;

sequentially scanning adjacent said holograms in a track by a reproducing light beam having a diameter r.sub.1 in the direction of scan which meets the relation

where L(.epsilon.-l, 0) represents value of either .epsilon.-l or 0 whichever is larger, l is the length of a hologram in the direction of scan, .epsilon. is the hologram-to-hologram separation, and N is the number of units of information in each group of information, and said reproducing light beam having a diameter r.sub.2 in the transverse direction to the scan direction which is confined not to illuminate a second neighbouring track, and

among a plurality of images reproduced on a plurality (N or 2N) of substantially spaced image positions on the image plane, by means of photoelectric transducer devices installed thereon, sequentially reading out images free from optical crosstalk with images reproduced on the same image positions from holograms in which recorded units of information are designated with the same information unit numbers regardless of information group numbers.

9. A method according to claim 8 wherein the N image positions are arranged in one channel on the image plane and the diameter r.sub.2 of the reproducing light beam is set not to overlap a first neighbouring track.

10. A method according to claim 8 wherein two channels each of N image positions are arranged in the image plane, reading out holograms on odd-numbered tracks and even-numbered tracks with one and the other channels, respectively, of N photoelectric tranceducer devices installed on said image positions.

11. A hologram reproducing method according to claim 8 wherein each unit-information imaged from holograms is arranged in a two-dimensional bit array.

12. A system for recording information as Fourier transform holograms comprising

a coherent light source,

a light splitting means for splitting a light beam from said coherent light source into a reference light beam and an object light beam,

information input means disposed in a plane perpendicular to the optical axis of said object light beam for sequentially exposing discrete units of information to said object light beam, said information input means having a single or a plurality of unit-information input positions substantially spaced with one another,

information supply means to supply said units of information to said information input means, said information supply means processing original information into a form in which the original information is divided into a number of groups each consisting of a predetermined number N, (larger than or equal to two) of said units of information, each unit consisting of a given amount of information, each of said units of information being supplied sequentially to said information input means as a spatial pattern,

lens means for converging said object light beam on a Fourier transform plane,

a recording medium located in the Fourier transform plane formed by said lens means,

means for directing said reference light beam to said Fourier transform plane with a fixed single angle or with a plurality of predetermined incident angles sequentially so as to cause interference with said object light beam on said recording medium,

driving means for moving said recording medium in said Fourier transform plane, and

a control circuit connected to said information supply means, said reference light directing means and said driving means for achieving cooperation therebetween to form a series of Fourier transform holograms on said recording medium, such that one-to-one different combinations of said single unit-information input position and said plurality of incident angles of said reference light beam or one-to-one different combinations of said plurality of unit-information input positions and said fixed single incident angle of said reference light beam are used in sequence to record said series of holograms, each of said holograms storing a respective unit-information supplied in a predetermined sequence.

13. A system according to claim 12, wherein said information input means has the same number of substantially spaced unit-information input positions as the number (N) of information units in each group of the original information, and said reference light directing means produces a single reference light beam having a predetermined incident angle with respect to the Fourier transform plane, and wherein said control circuit controls such that the kth unit-information input position of said information input means whereby the original information is recorded as successive holograms forming a single track or a plurality of tracks non-overlapping on the recording medium.

14. A system according to claim 12, wherein said information input means has unit-information input positions (2N) two times the number of units of information (N) constituting each group of the original information and said unit-information positions are aligned in two channels each consisting of N unit-information positions, and wherein said reference light directing means produces a single reference light having a fixed incident angle with respect to the Fourier transform plane, and wherein said control circuit controls its associated means such that the kth unit-information of each of a predetermined number of groups of information are supplied to the kth input position of a first channel of said information input device sequentially for recording on odd-numbered tracks of said recording medium and then the kth unit-information of each of another predetermined number of groups of information are supplied to the kth input position of a second channel of said information input device sequentially for recording on even-numbered tracks of said recording medium, and wherein adjacent tracks are separated or partially overlapped with each other.

15. A system according to claim 12, wherein said information input means has a plurality of unit-information input positions N/M less than the number of units of information constituting one group of information, where M is an integer larger than one and N/M is an integer larger than or equal to 1 and less than N, and wherein said reference light directing means produces a reference light which takes different M incident angles with respect to the Fourier transform plane, and wherein said control circuit controls associated means such that N different combinations are formed by all of said M incident angles and all of said N/M unit-information input positions and the kth unit-information of each group is recorded by use of the kth combination of said N different combinations so that adjacent tracks on said record medium do not overlap.

16. A system according to claim 12, wherein said information input means has a plurality of unit-information input positions 2N/M less than the two times the number of units of information constituting one group of information, where M is an integer larger than one and 2N/M is larger than or equal to 1 and less than 2N, and wherein said reference light directing means produces a reference light which takes M different incident angles with respect to the Fourier transform plane, and wherein said control circuit controls associated means such that 2N different combinations divided into two channels each including N combinations by all of said M incident angles and all of said 2N/M unit-information input positions and the kth unit-information of each of a predetermined number of groups is recorded on odd-numbered tracks by use of the kth combination of a first channel of N combinations and the kth unit-information of each of another predetermined number of groups is recorded on even-numbered tracks by use of the kth combination of a second channel of N combinations so that adjacent tracks are separated or partially overlapped.

17. A system for reproducing information from holograms recorded on a track or tracks of a record medium comprising

a light source for producing a reproducing light beam having a beam diameter r.sub.1 in the direction of scan, which meets the following relation:

where L(.epsilon.-l, 0) means value of either .epsilon.-l or 0 which is larger, l is the same of the hologram in the direction of scan, .epsilon. is the hologram-to-hologram separation, and N is the number of units of information of each group of information,

and said reproducing light beam having a beam diameter r.sub.2 in the transverse direction to the scan direction which is confined not to illuminate second neighbouring tracks,

means to scan the holograms relative to said reproducing light beam,

means to effect the inverse Fourier transform of a refracted light of said reproducing light beam through the holograms,

photoelectric transducer means having a plurality of unit-information detecting positions substantially, spaced and disposed on the inverse Fourier transform plane,

readout means connected to said photoelectric transducer means and including a timing mark detection circuit and switching circuit for reading out each of units of information reproduced by said photoelectric transducer means sequentially, and

control circuit connected to said scan means and said readout means to control cooperation therebetween.

18. A system for reproducing according to claim 17, wherein the holograms recorded on a record medium are constituted by a single or a plurality of tracks without overlapping between adjacent tracks, wherein

the size of said r.sub.2 is selected not to overlap first neighbouring track, and

said photoelectric transducer means has N unit-information detecting devices.

19. A system for reproducing according to claim 17, wherein holograms recorded on a record medium are constituted by a plurality of tracks separated from or overlapping partially with each other, wherein

the size of said r.sub.2 is selected as large as at least not to overlap second neighbouring track, and said photoelectric transducer means has 2N unit-information detecting devices.

20. A Fourier transform holograms recording and reproducing system comprising;

a coherent light source,

a light splitting means for splitting a light beam from said coherent light source into a reference light beam and an object light beam,

information input means disposed in a plane perpendicular to the optical axis of said object light beam for sequentially exposing information to said object light beam, said information input means having one or a plurality of unit-information input positions,

information supply means to supply original information to said information input means, said information supply means processing and storing the original information in a form in which the original information is divided into a number of groups each consisting of a predetermined number (N, larger than or equal to two) of units of information each consisting of a given amount of information, each of said unit-information being supplied sequentially to said information input means as a spatial pattern,

lens means for converging said object light beam on a Fourier transform plane,

a recording medium located in the Fourier transform plane formed by said lens means,

means for directing said reference light beam to said Fourier transform plane with a single or one of a plurality of predetermined incident angles so as to cause interference with said object light beam on said recording medium,

driving means for moving said recording medium in said Fourier transform plane,

a control circuit connected to said information supply means, and reference light directing means and said driving means for achieving cooperation therebetween to form a series of Fourier transform holograms on said recording medium, each of said holograms containing corresponding unit-information supplied in a predetermined sequence, said formation of a series of holograms being performed by interferences of all possible N or 2N one-to-one combinations among said unit-information input means and said reference light beams,

a light source for producing a reproducing light beam having a beam diameter r.sub.1 in the direction of scan, which meets the following relation:

where L(.epsilon.-l, 0) means value of either .epsilon.-l or 0 which is larger, l is the size of the hologram in the direction of scan, .epsilon. is the hologram-to-hologram separation, and N is the number of units of information of each group of information,

and said reproducing light beam having a beam diameter r.sub.2 in the transverse direction to the scan direction which is confined not to illuminate second neighbouring tracks,

means to scan the holograms relative to said reproducing light beam,

means to effect the inverse Fourier transform of a refracted light of said reproducing light beam through the holograms,

photoelectric transducer means having a plurality of unit-information detecting devices substantially spaced and disposed on the inverse Fourier transform plane,

readout means connected to said photoelectric transducer means and including a timing mark detection circuit and switching circuit for reading out each of units of information reproduced by said photoelectric transducer means sequentially, and

control circuit connected to said scan means and said readout means to control cooperation therebetween.

Description Submit all comments and votes

The present invention relates to a system for recording and reproducing information using Fourier transform holography, and more particularly to such a system and a method which eliminate crosstalk between adjacent holograms and enables continuous reproduction.

One of the characteristics of the Fourier transform holography is the image immobility due to the shift function of the Fourier transform. By this function, even if the hologram progresses on a hologram plane, an image from the hologram is reproduced in the same position on an image plane. Accordingly, when a hologram is to be read from one end to the other end, the above characteristic of the Fourier transform hologram can be directly utilized. One example of such a system is disclosed in U.S. Pat. No. 3,657,473 issued to John W. Corcoran on Apr. 18, 1972. When the amount of information is small such as in an identification card or credit card, all information can be recorded in one hologram and the required information can be retrieved by reproducing that hologram. However, when the amount of information is large as is the case of a computer tape, it is not generally possible to record the whole of the information in one hologram. In this case, the original information must be divided and recorded in a plurality of holograms. When those divided original information is recorded as a number of Fourier transform holograms a track or tracks of a film and those holograms are sequentially read out by continuously illuminating reproducing light while moving the film, optical crosstalk between adjacent holograms occurs on the imaging plane so that the information cannot be read out correctly. Namely, although the function of the image immobility in the Fourier transform holography presents an advantage of the reproduction of an image at a fixed position independently of the position of the hologram, it also brings about a disadvantage of reproducing the images from adjacent holograms at the same image position so that the individual information cannot be retrieved correctly. The present invention provides means for resolving the problems encountered in the use of Fourier transform holography. It is possible, by a known method, to prevent the simultaneous reproduction of two adjacent holograms, for example, by setting a sufficiently wide gap between adjacent holograms and setting the diameter of reproducing light beam to less than the size of the hologram. However this approach cannot resolve the problem of the decrease in effective record density of the record medium. It is another object of the present invention to resolve the above problem.

When information is read out of a computer magnetic tape, it may be effected asynchronously. However, when a PCM (pulse code modulation) coded audio signal is read out, it is desirable that bit information is read out of the record medium in synchronized form. It is yet another object of the present invention to allow continuous reproduction, in time sequence, of PCM coded audio signal bit information recorded on a number of Fourier transform holograms.

Regarding the arrangement of bit information recorded on the hologram, a one-dimensional bit array is shown in many references. However, this type of arrangement does not increase the record density of the record medium. It is a further object of the present invention to provide a system which enables the reproduction of information from a number of holograms having two-dimensional bit information recorded, in synchronous or asynchronous manner while eliminating the crosstalk between adjacent holograms.

The above objects relate to the system for recording a reproducing a hologram without causing crosstalk between adjacent holograms in the same track. When a number of holograms are recorded along the track, they may be recorded on one track but usually they are recorded in a plurality of tracks extending longitudinally of the film or tranversely. In this case, in order to prevent crosstalk between adjacent tracks, the illumination area by the reproducing light should not overlap an adjacent track. To this end, a gap between the tracks should be set wide or a diameter of the reproducing light beam which is transverse to the direction of scan should be confined to the same order as the track width. It is another object of the present invention to provide a reproducing method which allows the elimination of optical crosstalk between adjacent tracks on an imaging plane of a reproducing optical system even when a plurality of hologram tracks are closely arranged.

It is yet another object of the present invention to permit a large allowable tolerance in the diameter of the reproducing light beam in a reproducing system which diameter is transverse to the direction of scan, without sacrificing a mean record density on a hologram record medium.

It is yet another object of the present invention to allow proper reproduction of the hologram even when the adjacent tracks are recorded in partially overlapped manner and hence to increase the mean record density on the record medium.

It is yet another object of the present invention to provide a recording method which enables the above reproducing methods.

In the present invention, at least two substantially non-overlapping image positions are provided on an image plane of a Fourier transform hologram reproducing optical system, and photoelectric transducer means are arranged at respective positions. Assuming two such positions for simplification, an image is reproduced from a first hologram among a number of holograms to be reproduced, to the first image position, another image is reproduced from a second hologram to the second image position, and another image is reproduced from a third hologram to the first position, and so on. In this manner the optical crosstalk between the reproduced images from adjacent holograms can be eliminated. Furthermore, by setting the diameter of a reproducing light beam to span over two adjacent holograms, while information can be read out from the crosstalk-free image reproduced from the first hologram by the first photoelectric transducer means, a crosstalk-free image can be reproduced from the second hologram on the second photoelectric transducer means. Consequently, in addition to asynchronous reproduction of the information, the information can be continuously reproduced in synchronous manner. While the holograms arranged in one track have been considered, by separating the image reproducing positions for adjacent tracks, a gap between the tracks can be narrowed and the mean record density can be further increased.

To record the hologram which assures the separation of the image reproducing positions of adjacent holograms, a plurality of substantially non-overlapping information input positions may be provided on an information input plane of a Fourier transform hologram recording optical system. Alternatively, it may be attained by a single information input position and a reference light having a plurality of incident angles.

The foregoing and other objects, features and advantages of the present invention will become more apparent from the following detailed description of the preferred embodiments of the invention when taken in conjunction with the accompanying drawings, in which;

FIG. 1 illustrates a principle of recording Fourier transform hologram in accordance with the present invention.

FIG. 2 illustrates a principle of reproducing Fourier transfer holograms in accordance with the present invention.

FIGS. 3 and 4 show examples of a hologram record format and reproducing light projection area in the hologram reproducing method of the present invention.

FIG. 5 shows an example of the hologram reproducing method of the present invention, in which the number of image positions N = 2.

FIG. 6 shows a hologram scan chart corresponding to that shown in FIG. 5.

FIG. 7 shows a time sequence chart of a reproduced image intensity corresponding to FIG. 6.

FIG. 8 is a time sequence chart of the information reproduction corresponding to FIG. 7.

FIG. 9 shows a hologram scan chart in one example of the hologram reproducing method of the present invention, in which N = 3, and a time sequence chart of the reproduced image intensity.

FIG. 10 shows one embodiment of the method of recording Fourier transform hologram of the present invention which is different from FIG. 1.

FIG. 11 shows reproducing light and the image positions in reproducing the hologram recorded by the method of FIG. 10.

FIG. 12 shows an embodiment of a Fourier transform hologram recording system according to the present invention.

FIG. 13 shows an embodiment of a reproducing system of the present invention for the hologram recorded by the system of FIG. 12, in which the number of the image positions N = 3, and one channel is used.

FIG. 14 shows another embodiment of the recording system different from FIG. 12.

FIG. 15 shows an embodiment of the reproducing system of the present invention for reproducing the hologram recorded by the system of FIG. 14, in which the number of the image positions per channel N = 2, and two channels are used.

A method of dividing original information to be recorded in the hologram is first explained. The original information is divided into a number of information groups each including a given number N (N .gtoreq. 2) of information units each including a given amount of information. For example, where the original information is animation recorded on a movie film, one frame of the movie film constitutes one information unit, and two frames or two information units forms one information group (N = 2). In this manner the entire animation of the movie film is divided into a number of information groups alternatively, three frames or three information units (N = 3) may constitute one information group to divide the entire animation of the movie film into a number of information groups. While many dividing methods may exist, an appropriate dividing method is employed considering the configuration of the hologram recording optical system and the reproducing optical system. As another example, where the original information is bit information recorded on a computer magnetic tape, one digit (e.g. 9 bits) of the bit information constitutes one information unit, and four digits (36 bits) or four information units (N = 4) constitute one information group. In this manner the whole of the bit information of the magnetic tape is divided into a plurality of information groups. In any case, the original information is divided into a number of groups each consisting of N information units and one information unit is recorded in one hologram as a Fourier transform hologram, and a number of holograms are arranged in a track or tracks. In the Fourier transform hologram reproducing optical system, N image reproducing positions per channel are provided in an image plane. The respective hologram is assigned with a unit number and a group number corresponding to a unit number and a group number of the divided original information. The image reproducing positions from the holograms are thus specified only by the unit number independently of the group number of the hologram. One example of recording the holograms in the above manner is explained with reference to FIG. 1.

FIG. 1 shows an information input section of the Fourier transform hologram recording optical system and a record medium, in which a laser device, a recording lens system and a shutter, etc. are omitted. It comprises an information input apparatus 10, a hologram 14 and a record medium 15 in the form of tape. The information input device 10 comprises two channels of N unit-information input device 10 (1, 1), 10 (1, 2), . . . 10 (1, N) and 10 (2, 1), 10 (2, 2), . . . 10 (2, N). Each of the unit-information input devices has a capacity capable of receiving one unit-information. For example, when one information unit consists of 16 bits and N = 3, the information input device 10 may be a page composer consisting of a 6-row, 16-column bit array. The page composer may use a liquid crystal or an electro-optical crystal, or it may use a 35 mm film. A laser beam 11 is modulated by the unit-information input device 10 (s, k) (where s = 1, 2; k = 1, 2, . . . N), and Fourier transformed by a lens system not shown to produce a signal light beam 12. A reference light beam 13 is generated from the same laser device as the laser beam 11 is generated. The signal beam 12 and the reference beam 13 interfere on the record medium 15 located in a Fourier transform plane of the system to form the Fourier transform hologram 14. On the record medium, a number of holograms 14 are arranged and recorded on a number of tracks 16. The unit-information devided from the original information to be hologram-recorded is represented by D (j, k), where j (= 1, 2, 3 . . . ) represents a group number and k (= 1, 2, . . . N .gtoreq. 2) represents a unit number. First, a unit-information D (1, 1) is applied to an information input device 10 (1, 1) to record a hologram 14 (1, 1), then a unit-information D (1, 2) is applied to an information input device 10 (1, 2) to record a hologram 14 (1, 2), and so on and a unit-information D (1, N) is applied to an information input device 10 (1, N) to record a hologram 14 (1, N). Subsequently, a unit-information D (2, 1) is applied to an information input device 10 (1, 1) to record a hologram 14 (2, 1). In the same manner, using the unit-information input devices 10 (1, k) in the first channel, the unit-information D (j, k) are recorded on a first track 16-1 on the hologram record medium 15. Then, to record holograms on a second track 16-2, the unit-information D (j, k) are applied to the unit-information input devices 10 (2, k) of a second channel. In a similar way, using the unit-information input devices of the first channel, holograms are recorded on odd-numbered tracks, and using the unit-information input devices of the second channel holograms are recorded on even-numbered tracks.

A method for reproducing the holograms thus recorded is explained with reference to FIG. 2. FIG. 2 particularly illustrates a hologram tape in a Fourier transform hologram reproducing optical system, and a photoelectric transducing apparatus. A laser device, reproducing lens system or the like are omitted from the drawing. The photoelectric transducing apparatus 20 is arranged on an image plane of the optical system at a real image reproducing position. Numeral 21 represents a reproducing light beam, and 22 represents the hologram tape which is derived by developing the record medium 15 shown in FIG. 1. Numerals 14 and 16 represent the hologram and the track, respectively, as in FIG. 1. In FIG. 2, the hologram tape 22 is continuously moving in the direction of the arrow, and the holograms are relatively scanned by the reproducing light beam 21 along the track. To scan a different track, the reproducing light beam 21 may be moved in parallel to illuminate a desired track. The photoelectric transducing apparatus 20 comprises two channels of N unit-information photoelectric transducing devices (hereinafter referred to as unit-information detectors) 20 (1, 1), 20 (1, 2), . . . 20 (1, N) and 20 (2, 1) 20 (2, 2), . . . 20 (2, N), which correspond to the information input devices. Each of the unit-information detectors is adapted to convert optical images reproduced from the holograms into electric signals. For example, when the information are bit information, a phototransistor array may be used. In FIG. 2, images from the holograms 14 (j, k) are reproduced on the unit-information detectors of either the first channel or the second channel, depending on whether the track on which the holograms have been recorded is an odd-numbered or even-numbered, and the images from the holograms in the same track are reproduced to a position specified only by the unit number k independently of the group number j of the hologram 14 (j, k), in other words at a position 20 (s, k) where s = 1 or 2), as will be apparent to those skilled in holography technology from the above description relating to the recording method.

Assuming that r.sub.2 represents a diameter of the reproducing light beam which is transverse to the direction of scan, it has been necessary in prior art method to select r.sub.2 such that the beam does not overlap a next adjacent track. According to the present invention, r.sub.2 may be selected such that the beam does not overlap a next but one track. Accordingly, an allowable tolerance to r.sub.2 can be widened. This is explained in conjunction with FIG. 3.

In FIG. 3, numeral 30 represents the area illuminated with a reproducing light beam, the dimension of which is r.sub.1 in diameter along the direction of scan and r.sub.2 indiameter transversely thereto. The reproducing light beam is shown as scanning over i-th track 16-i. Since r.sub.2 is larger than the track width, the reproducing light beam overlaps the tracks 16-(i-1) and 16-(i+1). However, as seen from FIGS. 1 and 2, the holograms on the (i-1)th track and the holograms on the (i+1)th track are reproduced on the unit-information detectors in the same one channel and the holograms on the i-th track are reproduced on the unit-information detectors in the other channel. Namely, in FIG. 3, if i is an even number, the holograms 14 (m, k) and 14 (m, k+1) are reproduced separately on the unit-information detectors 20 (2, k) and 20 (2, k+1), respectively, shown in FIG. 2, while the holograms 14 (j, k) and 14 (n, k) are both reproduced on the unit-information detector 20 (1, k) in overlapped manner and the holograms 14 (j, k+1) and 14 (n, k+1) are both reproduced on the unit-information detector 20 (l, k+1), in overlapped manner. Accordingly, in reproducing the holograms in the i-th track no crosstalk with the (i.+-.1)th tracks (the next adjacent tracks) occurs.

While gaps are provided between tracks, in FIG. 3, adjacent tracks may be partially overlap with each other. This will be explained in conjunction with FIG. 4. In FIG. 4, the same reference numerals as in FIG. 3 are used. In FIG. 4, like in FIG. 3, the images from the holograms on the i-th track to be read out are reproduced to the unit-information detectors in a channel which is different from the channel in which the images from the holograms on the (i.+-.1) tracks are reproduced. Accordingly, even if the adjacent tracks are recorded in partially overlapped manner as shown, the crosstalk between the tracks does not occur provided that the diameter r.sub.2 does not overlap the next but one track.

It has been described that the crosstalk between the tracks can be eliminated by the present invention. A method for reproducing information from the holograms in the same track is now explained. FIG. 5 shows a Fourier transform hologram reproducing optical system similar to FIG. 2, and it shows only major portions thereof for the above purpose. It comprises a photoelectric transducing apparatus 50, a reproducing light beam 51 a beam spot 52, a hologram tape 53, holograms 14 and tracks 55. The hologram tape 53 is continuously moving in the direction of the arrow 56 and the holograms on the i-th track 55-i are being relatively scanned in continuous manner by the reproducing light beam 51. The original information has been divided into a plurality of groups each consisting of two information units. In the photoelectric transducing apparatus 50, only one channel of unit-information detectors 50-1 and 50-2 are shown. It shows the channel of unit-information detectors which corresponds to the i-th track of holograms. Thus, when recording is effected by the two channel unit-information input apparatus as shown in FIG. 1 and reproduction is effected by the reproducing light beam having relatively large diameter r.sub.2 as shown in FIG. 3 or 4, one may consider that only that channel of the two channel unit-information detectors of FIG. 2 which corresponds to the i-th track is shown in FIG. 5. Alternatively, one may consider that FIG. 5 shows the reproduction of the holograms along the i-th track when a unit-information input apparatus of one channel N-unit is used in the hologram recording optical system and a unit-information detecting apparatus of one channel is used in the reproducing optical system and the diameter r.sub.2 of the reproducing light beam is selected not to overlap the next adjacent track. In FIG. 5, X-axis is assumed to extend over the i-th track and dimension of a hologram 14 (j, k) on the track along the X-axis is represented by l. Assuming that the holograms have been recorded in the range between X.sub.1 (j, k) and X.sub.2 (j, k) on the X-axis, then X.sub.2 (j, k) - X.sub.1 (j, k) = l. Also assuming thet the hologram 14 (j, k) is next to the hologram 14 (j', k'), hologram-to-hologram separation .epsilon. may be defined by; .epsilon. = X.sub.1 (j', k')- X.sub.1 (j, k). The dimension of the reproducing light beam in the X-direction is r.sub.1. FIG. 6 is a hologram scan chart showing a manner when the reproducing light beam 51 in FIG. 5 scans over the track 55-i. The time sequence changes in reproduced image intensity in the unit-information detectors 50-1 and 50-2 are shown by 7-1 and 7-2 in FIG. 7.

In FIG. 6, an abscissa represents a time and an ordinate represents the X-axis. When X.sub.1 (1, 1) = 0, and N = 2, then

X.sub.1 (j, k) = (2j + k-3).epsilon. (1) X.sub.2 (j, k) = (2j + k-3).epsilon. + l (2)

The trace of the reproducing light beam illumination area can be represented by an area encircled by;

R.sub.1 (t) = Ut (3) R.sub.2 (t) = Ut + r.sub.1 (4)

Where U is the moving velocity of the film. Therefore, the time at which the hologram 14 (j, k) is reproduced can be derived from the equations (1) through (4). Assuming that the hologram 14 (j, k) enters the reproducing light beam illumination area at time t = t.sub.1 (j, k) and moves out of that area at time t = t.sub.2 (j, k), then;

t.sub.1 (j, k) = (1/U)[(2j + k-3).epsilon. - r.sub.1 ] (5) t.sub.2 (j, k) = (1/U)[(2j + k-3).epsilon. + l] (6)

Irrespective of the value of j, all of the holograms 14 (j, 1) which belong to the unit number 1 are sequentially reproduced on the unit-information detector 50-1 and all of the holograms 14 (j, 2) which belong to the unit number 2 are sequentially reproduced on the unit-information detector 50-2, by the scan of the reproducing light beam. The changes, in time, of the reproduced image intensity on each of the unit-information detectors are shown in FIG. 7, in which an abscissa represents a time and an ordinate represents the reproduced image intensity, and 7-1 and 7-2 are time sequence charts of the reproduced image intensity on the unit-information detectors 50-1 and 50-2, respectively. In FIG. 7, the change in the intensity of the reproduced images from one hologram is shown schematically. 14A(j, k) shows a reproduced image from the hologram 14(j, k). As is apparent from FIG. 7, when

t.sub.1 (j, k) < t.sub.2 (j-1, k)

crosstalk occurs between the reproduced images 14A(j-1, k) and 14A(j, k) during the period of t.sub.1 (j, k) to t.sub.2 (j-1, k), but if

t.sub.2 (j-1, k) < t.sub.1 (j+1, k) (7)

is met, there always exists a period while the hologram 14(j, k) is reproduced, that is, between the times t.sub.1 (j, k) and t.sub.2 (j, k), during which period no crosstalk occurs between the holograms 14(j, k) and 14(j-1, k) and between the holograms 14(j, k) and 14(j+1, k), respectively, that is, there always exists a period .tau. = t.sub.1 (j+1, k) - t.sub.2 (j-1, k) during which only the hologram 14(j, k) is reproduced. Under the condition of the formula (7), if

t.sub.2 (j-1, 1) < t.sub.1 (j, 2) (8)

is met, a crosstalk free reproduced image of the hologram 14(j, 1) and a crosstalk free reproduced image of the hologram 14(j-1, 2) are reproduced on different unit information detectors simultaneously for a period (.DELTA..tau.) from t.sub.2 (j-1, 1) to t.sub.1 (j, 2). The condition which meets the formulas (7) and (8) is;

2.epsilon. - l < r.sub.1 < 3.epsilon. - l (9)

If

t.sub.2 (j-1, k) < t.sub.1 (j, k)

is met, no crosstalk occurs between the reproduced images 14A(j-1, k) and 14A(j, k). Accordingly, if

t.sub.1 (j, 1) < t.sub.2 (j-1, 2)

is met, the holograms 14(j-1, 2) and 14(j, l) are reproduced simultaneously on the unit-information detectors 50-2 and 50-1, respectively, for the period .DELTA..tau. = t.sub.2 (j-1, 2) - t.sub.1 (j, 1) without any crosstalk therebetween.

The condition therefor is;

.epsilon. - l < r.sub.1 < 2.epsilon. - l (10)

From the formulas (9) and (10), it is derived that;

.epsilon. - l < r.sub.1 < 3.epsilon. - l (11)

Thus, in FIG. 5, if the scan is effected by the reproducing light beam having the beam diameter r.sub.1 as defined by the formula (11), the reproduced images from adjacent holograms in the same track can be reproduced without any optical crosstalk therebetween while assuring the period (.DELTA..tau. > 0) during which the reproduced images from the adjacent holograms simultaneously appear on different unit-information detectors. Thus, by switching the output circuits of the two unit-information detectors on which the images are coincidently reproduced, during said period (.DELTA..tau.), the information from the adjacent holograms can be continuously reproduced in time sequence. FIG. 8 shows a timing chart therefor with correspondence to FIG. 7. In FIG. 8, 8-1 and 8-2 emphasize those portions 14B(j, k) of the reproduced image intensity changes 14A(j, k) in 7-1 and 7-2 of FIG. 7 in which no crosstalk occurs. In 8-1 and 8-2, an abscissa shows a time and an ordinate shows the reproduced image intensity. 8-3 shows the flow of information alternately derived from two unit-information detectors. 14C(j, k ) represents information photoelectrically converted from the crosstalk free reproduced image 14B(j, k). In observing FIG. 8, it should be noted that in 8-1 and 8-2 the reproduced images 14B(j, k) appear on an image plane only for the period .tau. and they are stationary but in 8-3 the reproduced information 14C(j, k) are signals derived by electronically scanning the reproduced images 14B(j, k). For example, when the reproduced images 14B(j, k) are in the form of a 9-row by 10-column bit pattern and the detectors 50-1 and 50-2 each consist of a 9-row by 10-column photo-diode array and the nine channels are electronically scanned in parallel in photo-electrically converting the reproduced images, then 14C(j, k) represents nine channels of time sequential outputs with each channel comprising ten bits.

In the examples shown in FIGS. 5 through 8, one information group consisted of two information units or N = 2, and there existed gaps between adjacent holograms or .epsilon. > l. It should be understood that N may be equal to or larger than 2, and .epsilon. may be 0 < .epsilon. .ltoreq. l. That is, when N, l and .epsilon. are selected in implementing the reproducing method of the present invention, the diameter r.sub.1 of the reproducing light beam in the direction of the scan may be selected in the range of:

L(.epsilon.-l, 0) < r.sub.1 < (2N-1).epsilon. - l (12)

where L(.epsilon.-l, 0) represents .epsilon.-l or 0, whichever larger. As an example, an application where N = 3 is explained below. FIG. 9-1 shows a hologram scan chart similar to FIG. 6. In FIG. 9, the abscissa represents time and the ordinate represents an X-axis drawn along the track on the hologram tape. Numeral 91 represents a sectional area taken along the X-axis of the hologram tape. 92(j, k) represents a hologram on which the k-th unit-information of the j-th group are recorded. In FIG. 9-1, hologram-to-hologram separation .epsilon. is equal to the size l of the hologram in the direction of the track. Thus, there is no gap between adjacent holograms. In this case, from the formula (12) the diameter r.sub.1 of the reproducing light beam is defined by 0<r.sub.1 <4l. The area encircled by R.sub.1 (t) and R.sub.2 (t) represents a trace of the reproducing light beam when r.sub.1 =l and the area encircled by R.sub.1 (t) and R'.sub.2 (t) represents a trace of the reproducing light beam when r'.sub.1 = 2l.

FIGS. 9-2 and 9-3 show the reproduced image intensity time sequence charts corresponding to FIG. 7 when r.sub.1 =l and r'.