1.  Field of the Invention
This invention relates to a light beam scanning apparatus.  This invention particularly relates to a light beam scanning apparatus utilizing a diffraction grating, typically a hologram. 
2.  Description of the Prior Art
As light deflectors used in light beam scanning apparatuses, there have heretofore been known a mechanical light deflector such as a multi-face rotating mirror or a galvanometer mirror, an acousto-optic light deflector utilizing the interaction between ultrasonic waves and light, a holographic light deflector utilizing a moving hologram, and the like.  Among these light deflectors, the holographic light deflector is advantageous in that (i) the hologram used as the main component of the light deflector can be easily reproduced in large number by a photographic process, a thermocompression bonding process, or the like, (ii) the beam deflected by the holographic light deflector is scarcely affected by any error in the angle of inclination or wobbling of the holographic light deflector, (iii) no focusing lens need be used since the hologram itself can be made to act in the same way as a lens, and (iv) the direction of light deflection can be selected independently of the moving direction of the hologram.  Thus the holographic light deflector is expected to be suitable for simplifying the construction of light beam scanning apparatuses and reducing the cost of these apparatuses. 
The holographic light deflector is based on the principle that, by changing the position of the hologram plate with respect to a reconstruction beam, the direction of the first-order diffracted component of the reconstructed beam is changed.  As forms of the holographic light deflector embodying this principle, there have heretofore been known a form wherein reflection type holograms are positioned on a rotatable (convex or concave) spherical surface in the rotating direction thereof, a form wherein holograms are positioned on the side face of a rotatable cylindrical body or on the side faces of a rotatable prism-like body, a form wherein holograms are positioned on a rotatable disk in the circumferential direction thereof, and the like. 
Among the aforesaid forms of the holographic light deflectors, the holographic light deflector wherein reflection type holograms are positioned on a spherical surface and the holographic light deflector wherein holograms are positioned on the side face of a cylindrical body or on the side faces of a prism-like body are advantageous for the construction of a laser scanning apparatus exhibiting a high resolution.  This is because, since a holographic reconstruction system which has no aberration is utilized, it is possible to obtain more than 10,000 resolution points per scan.  However, in order to realize a holographic light deflector comprising the holograms positioned on the spherical surface or on the cylindrical body, it is necessary to apply a light-sensitive material such as a photographic emulsion, a photoresist or a photopolymer on the spherical surface or on the surface of the cylindrical body.  From the technical viewpoint, it is not always possible to accurately apply the light-sensitive material on the spherical surface or on the surface of the cylindrical body.  Further, in the holographic light deflector comprising the holograms positioned on the side faces of a prism-like body, though each hologram is fabricated in the form of a flat plate, a high assembling accuracy is required, and there is a limit to increasing of the operation speed due to a large air turbulence loss. 
On the other hand, the holographic light deflector comprising the holograms positioned on a disk in the circumferential direction thereof does not exhibit the aforesaid drawbacks since the construction is very simple, and this form of deflector is expected to be most suitable for practical use.  To date, however, the only case in which holographic light deflector of this type that has been put into practice is that wherein it is incorporated into a bar code reader so that scan lines can be obtained in various directions from a plurality of holograms constituting the holographic light deflector.  This is mainly because, when a scan line is formed on a flat surface, the scan line becomes curved or bowed.  This scan line "bow" is inconvenient for recording and read-out of document information and image information. 
As disclosed, for example, in U.S. Pat.  Nos.  4,289,371 and 3,721,486, and Japanese Unexamined Patent publication No.  57(1982)-85018, various attempts have been made to solve the scan line bow problem arising in the holographic light deflector comprising the holograms positioned on a disk in the circumferential direction thereof. 
In the method disclosed in U.S. Pat.  No.  4,289,371, the diffraction grating is constructed so as to have a .lambda./d ratio [wherein .lambda.  is the wavelength of the reconstruction beam, and d is the period (constant) of the diffraction grating (hologram)] of between 1 and 1.618.  In this method, however, when the wavelength .lambda.  of the light beam used for reconstruction is e.g. 0.488 .mu.m, the period d of the diffraction grating must be between 0.30 .mu.m and 0.49 .mu.m.  It is not easy to accurately make a diffraction grating (hologram) having such a very short period for the following reasons.  First, it is necessary to completely eliminate vibration of the photographic exposure apparatus and away of ambient air.  As is well known by the experts in the art, it is not always possible to accurately expose a recording material to a holographic grating pattern having a very short period within the aforesaid range.  Second, it is not always possible to obtain a recording material practically suitable for recording a diffraction grating having a period within the aforesaid range. 
In the method disclosed in U.S. Pat.  No.  3,721,486, a light beam is diffracted twice by use of two diffraction gratings rotating reversely at an equal speed.  However, in order to rotate two diffraction gratings in reverse directions at a speed equal to each other, a complicated rotation transmitting mechanism becomes necessary, and a problem which is technically difficult to solve arises. 
In the method disclosed in Japanese Unexamined Patent Publication No.  57(1982)-85018, a light beam is diffracted twice by use of two kinds of diffraction gratings (holograms) rotating synchronously with each other.  However, this method requires two kinds of diffraction gratings coupled in predetermined relation to each other, and is complicated.