Hologram scanner and method of recording and reproducing hologram in the hologram scanner

Abstract: A hologram scanner includes a reproducing light source (17) for generating a collimated reproducing light beam (15), a hologram disk (12) being adapted to rotate, and a hologram (11) disposed on the hologram disk for deflecting the reproducing light beam by diffraction so as to scan an image surface (13) by the deflected reproducing light beam (16) during a rotation of the hologram disk. The hologram has interference fringes recorded by a mutual interference of an object light of divergent spherical wave and a reference light of divergent spherical wave. The object light is generated by an object light source positioned at a first position apart from a hologram recording plane of the hologram disk, on which the hologram is to be recorded, by a first distance. The reference light is generated by a reference light source positioned at a second position apart from the hologram recording plane by a second distance. The first and second distances are different from each other so that the deflected reproducing light beam is focused on the image surface.

Description: BACKGROUND OF THE INVENTION 1.Field of the Invention The present invention relates to a hologram scanner and a method of recording and reproducing a hologram in the hologram scanner, which is used in an image forming apparatus such as a laser beam printer, and an image reading apparatus. 2.Description of the Related Art In various kinds of scanning devices used in an image forming apparatus and an image reading apparatus, there is a hologram scanner, which utilizes a hologram as means for deflecting a light beam for scanning. The hologram scanner has a hologram disk on which a plurality of holograms are arranged along its circumferential direction. When the hologram disk rotates, the reproducing light beam is deflected by the diffraction effect of the hologram. The hologram scanner scans an image surface by the deflected diffraction light beam in a main scanning direction. Such a hologram is recorded and manufactured by a mutual interference of two light fluxes, i.e. an object light of laser beam and a reference light of laser beam. If a non-aberrational spherical wave or a non-aberrational plane wave are utilized as these two light fluxes, interference fringes are obtained which are represented by quadratic curves, such as circular curves, elliptic curves, parabolic curves, and hyperbolic curves. Accordingly, the hologram is recorded so as to have a desired diffraction power, a desired diffraction direction, a desired focusing power etc., by adjusting the pitch and the shape of the interference fringes. The hologram thus recorded has a function of focusing the reproducing light beam so as to form a light spot on the image surface as well as the function of deflecting the reproducing light beam. Methods of recording and reproducing such a hologram can be divided into some cases according to the kinds of the object light, the reference light and the reproducing light, as explained below. Namely, in a first case, a hologram recorded by the mutual interference of a spherical divergent wave as the object light and a plane wave as the reference light, is reproduced by a collimated reproducing light beam. In this first case, the distance from the hologram to the center of the reproducing light source is infinity, and thus the reproducing light beam diffracted by the hologram is converged at the focal point of the hologram. In a second case, a hologram recorded by the mutual interference of a spherical divergent wave as the object light and a plane wave as the reference light, is reproduced by a divergent reproducing light beam from a point farther than the focal point of the hologram. In this second case, the reproducing light beam is converged at such a far point as to enable a magnified scanning operation. In a third case, a hologram recorded by the mutual interference of a spherical divergent wave as the object light and a spherical convergent wave as the reference light, is reproduced by a divergent reproducing light beam, so that the aberration with respect to a relatively wide range of scanning angle is made small. Such a case is disclosed in Japanese Patent Laying Open No.54-104849. In a fourth case, a hologram recorded by the mutual interference of a spherical divergent wave as the object light and a spherical divergent wave as the reference light, is reproduced by a convergent reproducing light beam, so that a hologram can be obtained whose focusing power is small and whose focal length is long. In the above explained cases, the focusing ability of the hologram is realized by a gradient in the spatial frequency of the interference fringes of the hologram, while the deflecting ability of the hologram is realized by a change in the direction of the interference fringes of the hologram at its reproducing point in accordance with the rotation of the hologram. The focal length f of the hologram as a focusing lens is approximately given by the following equation (1). 1/f = nλ 2 / λ 1 (1/Z O -1/Z R ) wherein, λ 1 : wave length of the recording (object and reference) light λ 2 : wave length of the reproducing light beam n : order of diffraction Z O : distance between the hologram plane and the center of the object light source Z R : distance between the hologram plane and the center of the reference light source The sign of each symbol Z O and Z R is positive in case of the divergent light and negative in case of the convergent light. The following equation (2) is approximately effected as for the relationship between a distance Z c and a distance Z j , which corresponds to the relationship between the object point and the image point, where Z c represents the distance from the hologram plane to the reproducing light source, and Z j represents the distance from the hologram plane to the image surface. 1/Z j = 1/Z c + 1/f wherein, the sign of each symbol Z j and Z c is positive in the transmitting direction of the reproducing light beam with respect to the hologram plane. Accordingly, if the hologram having a focal length f is reproduced by a collimated light beam, the diffraction light beam is focused at the focal point of the hologram. If the hologram is reproduced by a divergent or convergent light beam, the diffraction light beam is focused at the image point as assuming the center of the reproducing light source as an object point. Thus, in the aforementioned first case of the related art in which the diffraction light beam is converged at the focal point, an obtainable scanning length of the diffraction light beam can be as short as about the moving amount of the hologram in company with the rotation of the hologram. In the aforementioned second case of the related art, the size of the light spot on the image surface can not be made small during the scanning operation since an abrupt change in the image forming length of the hologram happens as the scanning angle increases. In the aforementioned third case of the related art, since the hologram functions as a focusing lens having a short focal length, a change of the diffraction light beam due to the shift of the optical axis of the hologram is significantly increased. In addition, a large size lens or a concave mirror is necessary in order to converge the reference light onto the hologram recording plane of the hologram disk in the recording process, which is not preferable from a view point of the recording and manufacturing process of the hologram. In the aformentioned fourth case of the related art, if the hologram is recorded with such a specific condition as Z O = Z R , the hologram having no focusing ability is obtained. Then, if such a spherical light beam as being focused at the image surface is used as the reproducing light beam, the diffraction light beam is also focused at the image surface. Such a hologram is disclosed in Japanese Patent Laying Open No.60-194419. Thus, it is expected that a small light spot can be obtained at the image surface since the hologram itself hardly has an aberration. However, in this case, since such a reproducing light beam as converging in a great distance is required, the establishment of the reproducing optical system is very difficult. Accordingly, there is a first problem of those related arts mentioned above that it is difficult to obtain a hologram having both of a sufficient deflecting ability and a sufficient focusing ability. By the way, in order to enhance the ability of deflecting the reproducing light beam by use of the hologram, recorded in the above mentioned manners, as the hologram in the aforementioned hologram scanner, such a reproducing condition is required that the diffraction ability of the hologram itself is high and the incident angle of field of the reproducing light beam with respect to the hologram plane is also large, so as to increase the deflection angle of the diffraction light beam. However, the aberration of the diffraction light beam as the scanning light beam, at the image surface is inevitably generated to a great extent under such a reproducing condition. Main aberrations generated in the reproducing operation of the hologram are coma-aberration, astigmatism, and a curvature of field. The coma-aberration is generated in the sub-scanning direction (Yh direction), which is perpendicular to the main scanning direction (Xh direction). The astigmatism and the curvature of field are generated in the main scanning direction. Such aberrations thus generated make it difficult to reduce the size of the light spot formed out of the reproducing light beam on the image surface. As a countermeasure to the above explained aberrations, the hologram may be recorded to have such interference fringes that the aberration at the image surface in the reproducing operation of the hologram is reduced, by use of the object light and the reference light including aberration to cancel the aberration of the reproducing light beam. In fact, it is possible to cancel to some extent the aberration of the reproducing light beam by recording the hologram by use of the aberrational wave, which is generated by obliquely inputting a plane wave or a spherical wave to a spherical lens, as the object light or the reference light. Generally speaking, it is desired in a hologram scanner to make the light spot smaller and thus enable a scanning operation with a higher resolution. However, in the above mentioned method using the aberrational wave in the recording operation so as to cancel the aberration generated in the reproducing operation, the aberration in the reproducing operation can be only cancelled in either the main scanning direction or the sub-scanning direction. Namely, if the coma-aberration is effectively cancelled, the astigmatism and the curvature of field remain. On the contrary, if the astigmatism and the curvature of field are effectively cancelled, the coma-aberration remains. Consequently, the above mentioned method has a certain fundamental limit to reduce the size of the light spot on the image surface in the reproducing operation of the hologram. EP-A-0 122 783 and EP-A-0 334 631 each disclose a method of recording a hologram by mutual interference of an object light and a reference light on a hologram recording plane of a hologram disk. In EP-A-0 122 783, each of the object light and the reference light comprises a divergent spherical wave. The reference light source is located on the center axis of the hologram disk at a first distance from the recording plane. The object light source is located offset from said center axis at a second distance from the recording plane. In EP-A-0 334 631, the reference light comprises a divergent spherical wave and the object light comprises a coma wave. SUMMARY OF THE INVENTION The present invention aims to provide a method of recording a hologram, in which a hologram having both a sufficient deflecting ability and a sufficient focusing ability can be recorded by a relatively simple optical recording system, and can be reproduced by a collimated reproducing light beam. According to the present invention, the aim can be achieved by a method of recording a hologram, as defined by claim 1. A hologram scanner employing a hologram recorded by the method of claim 1 is defined by claim 4. The sub-claims 2, 3, 5 and 6 are directed to embodiments of the invention. According to the method of the present invention, since the first and second distances are different from each other, a focusing power as well as a deflecting power can be given to the hologram recorded by the mutal interference of the divergent spherical waves even though the reproducing light beam is a collimated type. Here, the difference is especially set so that the deflected reproducing light beam is focused on the image surface, i.e. the first and second distances and the distance from the image surface to the hologram satisfy the relationship prescribed by the aforementioned equations (1) and (2). Thus, in the reproducing operation, the collimated reproducing light beam is deflected by the hologram toward the image surface, and is focused on the image surface. Accordingly, in the method of the invention, since both of the object light and the reference light are divergent spherical wave, the recording optical system for recording a hologram having both a deflecting ability and a focusing ability can be made simple. At the same time, since the reproducing light beam is a collimated type, the establishment of the reproducing optical system in the hologram scanner is easy. According to the hologram scanner of the present invention, since it includes the hologram which is recorded and reproduced according to the abovementioned method, the hologram scanner having a relatively simplified optical system can perform a scanning operation with a good quality by use of the hologram having both a sufficient deflecting ability and a sufficient focusing ability which can be recorded by a relatively simple optical recording system and can be reproduced by a collimated light beam. BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 is a schematic perspective view showing a main portion of an embodiment of the present invention; Fig. 2 is a schematic side view of the embodiment for explaining the optical paths of the recording lights and the reproducing lights; Fig. 3 is a graph showing an example of the optimization of the positions of the recording light sources in the embodiment. Fig. 4 is a graph showing the scan-line bow, with respect to each incident angle, versus the scan position in the embodiment; Fig. 5 is a graph showing the wavefront aberration with respect to each incident angle, versus the scan position in the embodiment; Fig. 6 is a graph showing a relationship between the incident angle and the optical path length in the embodiment; and Fig. 7 is a graph showing the Bragg angle versus the incident angle in the embodiment. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Fig. 1 shows a hologram scanner in an embodiment of the present invention. Fig. 2 shows a recording and reproducing condition of the hologram in the embodiment by the optical paths. In Fig. 1, the reference numeral 12 designates a hologram disk. A plurality of holograms 11 are arranged on the hologram disk 12 along its circumferential direction. The hologram disk 12 is adapted to rotate around its rotational axis 14. The reference numeral 13 designates an image surface which is scanned by the hologram scanner. A collimated reproducing light beam 15 in a direction parallel to or crossing the rotational axis 14 is inputted to the hologram 11 from a light source 17 such as a semiconductor light source through a collimator lens 18. The reproducing light beam 15 is deflected by the diffraction effect of the hologram 11 and transmitted as a diffraction light beam 16 toward the image surface 13. The diffraction light beam 16 is focused on the image surface 13, and is moved for scanning as the hologram disk 12 rotates. A method of recording and reproducing the hologram 11 will be explained below. In Fig.1 and Fig.2, the reference sign H designates a rotational center of the hologram 11. The reference sign J designates a focusing point of the diffraction light beam 16 on the image surface 13. The reference signs O and R designate a center position of an object light source and a center position of a reference light source, respectively. In the recording process of the hologram 11, assuming that the center H is situated on the original point of the X,Y,Z coordinates system, the rotational axis 14 is situated on the Z axis, and the hologram recording plane of the hologram disk 12 on which the hologram 11 is to be recorded is situated on the XY plane, the focusing ability of the recorded hologram 11 depends on each Z coordinate of the positions O and R, while the deflecting ability of the recorded hologram 11 depends on each X coordinate and each Y coordinate of each positions O and R, respectively. The Z coordinates of those positions O and R are set to be different from each other so that the diffraction light beam 16 is to be focused on the image surface 13, that is, the focal length of the hologram 11 is to be made substantially the same as the distance between the hologram 11 and the image surface 13 in the relationship prescribed by the aforementioned equations (1) and (2). Such a difference of the Z coordinates of the positions O and R can be determined by mathematical programming, an example of which will be described later. Further, the positions O and R as for the X,Y coordinates as well as the Z coordinate are determined by mathematical programming so that the diffraction light beam 16 is to be sufficiently deflected by the hologram 11 in accordance with the rotation of the hologram disk 12, and so that the scan line of the diffraction light beam 16 on the image surface 13 is to be substantially a straight line. For example, in the mathematical programming process, the required deflecting ability and the focusing ability of the hologram are expressed as functions having design parameters of the positions of the recording optical system and the reproducing optical system. Then, by setting the deflecting ability as the constraint and setting the focusing ability as the performance function, the design parameters which gives the minimum value of the performance function under the constraint is turned to be the optimum. By use of the object light source at the position O thus determined, and the reference light source at the position R of thus determined, the recording process of the hologram 11 is performed. Namely, interference fringes are formed by the mutual interference of these two lights on the hologram recording plane which consists of a photosensitive recording material, so as to record the hologram 11. Assuming the rotation angle of the hologram 11 is zero, the positions O and R and a reproducing position P should be on the YZ plane including the Z axis in order to make the deflecting angle zero. Thus, only the Y,Z coordinates are to be considered as coordinates of the recording optical system i.e. the object light source and the reference light source. As those object and reference light sources, for example, an Ar (Argon) laser source with the wave length of 363.8 nm can be preferably used for the hologram scanner in which a semiconductor laser source with the wave length of 780 nm is used as the reproducing light source 17. Fig.3 shows an example of the optimization of the positions O (Y O ,Z O ) and R (Y R ,Z R ) with respect to the incident angle  in by the above mentioned mathematical programming, under the condition that the distance between the hologram disk 12 and the image surface 13 is assumed to be 300 mm. In this example, such a condition is assumed that the distance r between the point P and the center H is 35 mm, twelve holograms 11 are provided on the hologram disk 12, and the diffraction light beam 16 is moved for scanning on the image surface 13 by a scan length of 260 mm every time the reproducing light beam 15 goes across each hologram 11 in company with the rotation of the hologram disk 12. Fig.4 shows the scan-line bow of the diffraction light beam 16 on the image surface 13, with respect to each incident angle  in , versus the scan position on the image surface 13. In Fig.4, a position of the scan position 0 corresponds to the scan center. According to Fig.4, the scan-line bow is within as little as 0.1 mm. Fig.5 shows the change of each RMS (effective value) wavefront aberration at the image surface 13 with respect to each incident angle  in , versus the scan position on the image surface 13. According to Fig.5, the RMS wavefront aberration is within λ/14 i.e. within the tolerance of the RMS wavefront value defined as the SD (Strehl's Definition) value is not less than 80 %. That is to say, the diffraction light beam 16 having a small wavefront aberration can be obtained with respect to the whole portion of the scan line. Fig.6 shows a relationship between the incident angle  in of the reproducing light beam 15 and the optical path length of the diffraction light beam 16 at the scan center. According to Figs.5 and 6, the wavefront aberration increases and the optical path length reduces as the incident angle  in increases. The incident angle  in is related to the diffraction efficiency, and it is desirable to set the incident angle  in at an angle close to the Bragg angle  B in order to obtain a high diffraction efficiency. The Bragg angle  B is expressed by a following equation (3), where d represents the pitch of the interference fringes of the hologram and λ2 represents the wavelength of the reproducing light beam 15.  B = sin -1 (λ 2 /(2d)) By use of the equation (3), the Bragg angle  B is calculated from the pitch d as for the hologram optimized by the aforementioned mathematical programming in its recording process with respect to each incident angle  in . Fig.7 shows the thus calculated Bragg angle  B versus the incident angle  in . According to Fig.7, the incident angle  in coincides with the Bragg angle  B when  in is about 45 degrees. Consequently, the incident angle  in is preferably determined by taking into consideration the RMS wavefront aberration, the optical path length and the Bragg angle conditions. As described above, in the embodiment of the present invention, because of the prescribed difference between the Z coordinates of the positions O and R in the recording process, the focusing ability as well as the deflecting ability can be given to the hologram 11, with respect to the reproducing light beam 15 of a collimated type in the reproducing operation. Further because of the prescribed positioning of the positions O and R in the recording process, the reproducing light beam 15 inputted to the hologram 11 by an incident angle  in can be focused on the image surface 13 as the diffraction light beam 16, and the diffraction light beam 16 can be straightly moved on the image surface 13 in the reproducing operation.

Claims: A method of recording a hologram (11) by mutual interference of an object light and a reference light, the method comprising: generating said object light as a divergent spherical wave using an object light source disposed at a first position (O), and projecting the generated object light toward a hologram recording plane of a hologram disk (12) on which the hologram (11) is to be recorded, said first position (O) being apart from said hologram recording plane by a first distance; and generating said reference light as a divergent spherical wave using a reference light source disposed at a second position (R), and projecting the generated reference light toward said hologram recording plane, said second position (R) being apart from said hologram recording plane by a second distance different from said first distance; wherein the object light wave and the reference light wave are coherent and the light sources are the respective origins of the divergent spherical waves; characterized in that each of said first and second positions (O, R) lies offset from the center axis (14) of said hologram disk (12). A method according to claim 1, wherein said first and second positions (O, R) are determined by mathematical programming so as to record said hologram (11) having a desirable focusing power and a desirable deflecting power. A method according to claim 1 or claim 2, wherein an argon laser source is employed as said object and reference light sources. A hologram scanner comprising: a reproducing light source (17,18; 30,31) for generating a reproducing light beam (15; 35; 135); a hologram disk (12; 23; 123) being adapted to rotate; and a hologram (11; 22; 122) disposed on said hologram disk for deflecting said reproducing light beam by diffraction so as to scan an image surface (13; 24; 124) by said deflected reproducing light beam (16; 36; 136) during a rotation of said hologram disk; said hologram (11; 22; 122) being recorded on said hologram disk by the method of any preceding claim. A hologram scanner according to claim 4, wherein said reproducing light beam is a collimated light beam (15; 35; 135). A hologram scanner according to claim 4 or claim 5, wherein said reproducing light source comprises a semiconductor laser source (17; 30).

Inventors: TOYOSHIMA TETSURO, UEDA TETSUYUKI, TOYOSHIMA, TETSURO, UEDA, TETSUYUKI
Applicants: SHARP KK, SHARP KABUSHIKI KAISHA
Application year: 1990
Main classification: G02B26
IPC codes: G02B26