Holographic deflection device

A holographic deflection device having a tunable laser, and at least one hologram for deflecting light incident thereupon from the tunable laser, so that the deflection of the light can be controlled by a change of the wavelength of the beam from the tunable laser.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to an optical deflection device. More 
precisely, it relates to a holographic deflection device by which the 
deflection angle can be controlled only by controlling the wavelength of 
the light beam source and in which it is not necessary to move the lens. 
2. Description of the Related Art 
In conventional optical deflection devices, the deflection angle is usually 
controlled by a mechanical displacement of a rotating polygon mirror or an 
oscillating mirror by a motor. It is also known to use a hologram scanner 
in which a hologram is mechanically moved. 
In these types of known optical deflection devices having mechanically 
movable portions, the devices are relatively large, lubrication of the 
movable portions is necessary, and unavoidable wear of the movable 
portions occurs. Also, the precision of the control of the deflection 
angle depends on the limited mechanical precision of the mechanical 
movable portions. The scanning speed is limited by the mechanical 
movement. 
Also known is an optical deflector which incorporates a solid element, such 
as an acoustooptic element, but this optical deflector can realize only a 
small deflection angle and thus has only a limited use. 
In view of the above, and contrary to the usual practice of the prior art 
in which it is considered that the deflection angle can be controlled only 
by a mechanical displacement of a lens or a mirror, the inventors of the 
present invention decided to investigate a non-mechanical control of the 
deflection angle. 
Namely, the primary object of the present invention is to provide an 
optical deflection device by which the deflection angle can be optically 
controlled without moving the lens or mirror. 
SUMMARY OF THE INVENTION 
To achieve the object mentioned above, according to the present invention, 
there is provided an optical deflection device which comprises a tunable 
semiconductor laser, in which the wavelength of the beam emitted therefrom 
can be varied, and at least one hologram lens which is inclined from the 
optical axis at a predetermined angle, whereby the deflection angle can be 
controlled by changing the wavelength of the laser beam incident upon the 
hologram. 
With this arrangement, the diffraction grating can be changed by only a 
change in the wavelength of the laser beam, resulting in an easy control 
of the deflection angle of the laser beam by the hologram.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
In FIG. 1, which shows a basic principle of the deflection device according 
to the present invention, the device essentially comprises a hologram lens 
11 having a focal length depending on the wavelength of the beam 
transmitted therethrough, and a tunable laser 13. The hologram lens 11, as 
is well known, converges light by a diffraction effect and this function 
is largely dependent upon the wavelength of the beam. The hologram lens 11 
is inclined from the optical axis X of the tunable laser 13 at a 
predetermined angle .theta., so that when the wavelength of the laser beam 
incident upon the hologram lens 11 varies, the deflection angle of the 
hologram lens 11 also varies, as shown by an imaginary line. Consequently, 
the focal point on an image plane M is scanned in the directions shown by 
arrows in FIG. 1. The present invention utilizes this characteristic of 
the hologram lens to realize a laser scanner which can scan the laser beam 
only by controlling the wavelength of the laser beam and without a 
mechanical displacement of the hologram lens 11. Note that when the 
present invention is applied to a deflection device unable to carry out 
beam scanning, the hologram lens 11 can be replaced by a common plane 
hologram having plane relief type gratings and not able to converge light. 
It should be noted that the focal length of the hologram changes slightly, 
in addition to the change of the deflection angle, when the wavelength of 
the laser beam is changed, as is well known, and accordingly, this should 
be taken into consideration when the position of the image plane M is 
determined. 
Supposing that the spatial frequency (1/pitch of the gratings) of a 
diffraction hologram is f and the wavelength is .lambda., then the 
following equation stands, as is well known: 
EQU sin .theta..sub.2 +sin .theta..sub.1 =f..lambda. 
wherein, .theta..sub.1 =incident angle, .theta..sub.2 =exit (deflection) 
angle. 
Accordingly, for example, if f=1813/mm, .theta..sub.1 =.theta..sub.2 
=45.degree., .lambda.=780 nm, the change .DELTA..lambda. of .lambda. by 
.DELTA..lambda.=2 nm, and .DELTA..lambda.=20 nm causes 0.29.degree. and 
3.00.degree. changes in the deflection angle, respectively. In this case, 
when the distance between the hologram and the image plane M is 1000 mm, 
the scanning width is 5.1-52.0 mm. 
The deflection angle can be determined by the angle .theta. by which the 
hologram lens 11 is inclined from the optical axis of the laser beam 
source 13, and in practice, it may not be possible to obtain a desired 
deflection angle by a single hologram lens. Therefore, it is possible to 
provide a plane hologram (not hologram lens) 15, in an angled (angular 
spiral) arrangement, as shown in FIG. 2, so that the deflection angle can 
be integrated when light is transmitted through the plane hologram 15. 
In the illustrated embodiment, the plane hologram 15 is inclined at 
approximately 45.degree. with respect to the beam incident thereupon, so 
that when a laser beam having a predetermined wavelength is passed through 
the plane hologram 15, the laser beam is deflected by approximately 
90.degree.. It should be noted that the angular displacement of the laser 
beam can be optionally determined in accordance with the position of the 
image plane M and the number of plane holograms 15. 
The arrangement shown in FIG. 2 is directed to a realization of a hologram 
scanner similar to FIG. 1. In the arrangement shown in FIG. 2, the first 
and final holograms are formed as hologram lenses 11 to converge the light 
at a point (focal point) on the image plane M. However, if a convergence 
of the light is not necessary, the final hologram lens 11 can be replaced 
with a plane hologram 15. 
It will be also easily understood that the final hologram lens 11 can be 
replaced with a common optical lens, such as a glass lens, if the lens is 
needed only to converge the light, i.e., only to focus the beam without 
controlling the deflection angle in accordance with a change of the 
wavelength of the beam. 
Generally speaking, the scanning range (width) for a hologram scanner 
increases in proportion to the increase of the number of plane holograms 
15 and hologram lenses 11. Theoretically, the deflection angle and the 
scanning width are increased N times when the number of hologram stages is 
N. 
Although there are various known tunable lasers 13 which can be used in the 
present invention, a tunable semiconductor laser is preferably used in the 
illustrated embodiment. 
FIG. 3 shows another embodiment of the present invention, in which the 
deflection device is applied to a device for reading optical information, 
such as a bar-code reader (pen reader). In FIG. 3, the light emitted from 
the tunable semiconductor laser 13 is collimated by a collimating lens 19 
into parallel light and then reflected at 90.degree. to the right in FIG. 
3 by a half mirror 21 which is inclined at an angle of 45.degree. from the 
optical axis of the laser 13. The reflected light is then deflected by the 
plane hologram 15 which is inclined at an angle of 45.degree. with respect 
to the reflected light, and then converged onto an information medium 
(e.g. bar-code) S by the hologram lens 11. The light reflected by the 
information medium S is deflected again by the hologram lens 11 and the 
plane hologram 15, is transmitted through the half mirror 21, and is 
focussed on an optical detector 25 by a collecting lens 23. In this 
arrangement, when the wavelength of the laser beam is changed by the 
tunable laser 13, the light emitted from the hologram lens 11 is scanned 
to read the bar-code on the medium S, as mentioned above. In this 
embodiment, the change of the wavelength enables the beam to be 
non-mechanically scanned, and the combination of this deflection device of 
the present invention with an optical detector enables a bar-code reader 
to be easily realized. 
FIG. 4 shows an embodiment in which the deflection device of the present 
invention is applied to a tracking servo-system for an optical head of an 
optical disc apparatus. The basic construction of the arrangement shown in 
FIG. 4 is similar to that shown in FIG. 3 and, accordingly, the elements 
shown in FIG. 4 corresponding to those in FIG. 3 are denoted by the same 
numerals. Note that the collecting lens 24 is adapted to scan the light on 
the optical disc 30, and the information on the optical disc 30 is 
detected by the optical detector 25. In FIG. 4, two plane holograms 15 are 
utilized. 
It should be noted that an achromatic lens can be located behind the plane 
hologram or hologram lens(es) to correct any chromatic abberation 
occurring due to the associated hologram or hologram lens. 
FIGS. 5 to 7 show an embodiment of a compact hologram module or hologram 
unit forming a deflection device according to the present invention, in 
which a desired number of holograms are integrally incorporated in a 
transparent polygonal body in the following special arrangement. 
As shown in FIG. 5, four plane diffraction holograms 15A-15D are located in 
a parallelepiped glass body 32 in a diagonal arrangement. When the laser 
beam emitted from the tunable laser 13 is incident at a slant from above 
upon the glass body 32 at a predetermined angle, the light is successively 
diffracted downward by the holograms 15A to 15D substantially along an 
angle spiral path (having one and a half turns in the illustrated 
embodiment) and finally emitted outward from the final hologram (hologram 
15B in the illustrated embodiment). In this arrangement, when the 
wavelength of the laser beam is varied by the tunable laser 13, the light 
emitted from the hologram module can be scanned from point 101 to point 
103 in the direction A shown in FIG. 5. 
This will be described below in more detail with reference to FIGS. 6 and 
7. 
In FIG. 6, which shows only the plane holograms 15A-15D perpendicular to 
each other, one of the holograms, i.e., the first hologram upon which the 
light is incident from the tunable laser 13, i.e., the hologram 15A, has a 
non-hologram portion 31 in which a hologram is not formed. This will be 
referred to as a beam incident portion 31. Similarly, the hologram from 
which the deflected light is emitted, e.g., the hologram 15C, has a 
non-hologram portion 33 in which a hologram is not formed. This will also 
be referred to as a beam emitting portion 33. Preferably, the beam 
incident portion 31 is formed in an upper edge portion of the hologram 15A 
and the beam emitting portion 33 is formed in a lower edge portion of the 
hologram 15C. 
The light incident upon the beam incident portion 31 (non-hologram portion) 
of the hologram 15A at a slant from above and toward the second hologram 
15B through the beam incident portion 31 is first diffracted by second 
hologram 15B toward the third hologram 15C at a diffraction angle 
.theta..sub.1. The light is then diffracted by the third hologram 15C 
toward the fourth hologram 15D at a diffraction angle .theta..sub.2. This 
diffraction is repeated. The light is sent downward substantially along 
the angled spiral path every time the diffraction occurs. In the 
illustrated embodiment, the light is emitted outward from the beam 
emitting portion 33 (non-hologram portion) of the third hologram 15C after 
the light has passed through about one and a half turns of the angled 
spiral. The change of the wavelength of the laser beam incident upon the 
incident portion 31 of the first hologram 15A causes the change in the 
deflection angle of the plane holograms, to enable a scan of the emitted 
rays between points 101 and 103. 
Alternatively, instead of the beam being incident upon the incident portion 
at a slant from above, it is also possible to incline the diffraction 
grating of at least one hologram, e.g., the hologram 15A, at a 
predetermined inclination angle .DELTA..theta., from the axis of symmetry 
(center axis of the four holograms), as shown in FIG. 6. The inclined 
gratings cause the light perpendicularly incident upon the side face of 
the parallelepiped glass body 32 to be diffracted downward. The above 
alternative, in which the hologram has an inclined diffraction grating, 
can be applied to any hologram, but preferably is applied to the hologram 
which first diffracts the incident beam, i.e., the hologram 15B, in the 
illustrated embodiment. Note that, in FIG. 6, although the hologram 15A is 
shown with inclined diffraction gratings, this is only for clarification, 
since the inclination of the diffraction gratings can be seen clearest in 
the hologram 15A rather than the hologram 15B. 
FIGS. 8 and 9 show a concrete example of the construction of the hologram 
module shown in FIGS. 5 to 7. 
In FIG. 8, the hologram module has four rectangular prisms 50 having a 
height of 20 mm and a side length of 10 mm, and having a right-angle 
triangular cross section. Two of the prisms 50 have hologram material 
applied to the adjacent side faces defining a right angle. The hologram 
material can be, for example, PVCz (polyvinyl carbazole), but is not 
limited thereto. The hologram material is subject to a holographic 
exposure to form a diffraction grating 53 f=2738/mm. The holographic 
exposure is carried out, for example, by the interference (interference 
having an equal angle of 26.4.degree.) of two beams from an He-Cd laser 
(.lambda.=325 nm) or by the interference (interference having an equal 
angle of 37.2.degree.) of an He-Cd laser having .lambda.=441.6 nm. One of 
the two rectangular prisms having the diffraction grating (holograms) 53 
has non-hologram portions having a width of 5 mm in the height direction 
on an upper edge portion of one side face and on a lower edge portion of 
the other side face, to provide the incident portion 31 and the emitting 
portion 33 (FIG. 9). 
The two rectangular prisms having holograms formed thereon and opposed to 
each other, and the remaining two rectangular prisms 50 having no hologram 
formed thereon and opposed to each other, are bonded together by an 
adhesive to form a parallelepiped hologram module (assembly) in which the 
latter two prisms 50 having no hologram are located between the first two 
prisms 50 having holograms, so that the assembly has a hologram (grating) 
53 located in a diagonal arrangement in the parallelepiped transparent 
body. 
FIG. 10 shows a modified embodiment of FIG. 8. 
In FIG. 10, each of the rectangular prisms 50 has one hologram (grating) 53 
on one of the side faces defining a right angle. Namely, in the 
modification shown in FIG. 10, four identical rectangular prisms 50 having 
the holograms 53 are bonded together, similar to FIG. 8, to realize a 
hologram module equivalent to the hologram shown in FIG. 8. 
Laser beams 4 mm in diameter are incident upon the hologram module shown in 
FIG. 8. The laser beam source is, for example, a tunable semiconductor 
laser by which the wavelength of 780 nm can be varied within the range of 
.+-.5 nm. When the collimated light is incident upon the incident portion 
(non-hologram portion) 31 of the rectangular prism 50 at an incident angle 
of, for example, 22.8.degree. with respect to an axis X normal to the side 
face of the associated prism 50, the incident light is refracted by the 
glass surface (refractive index =1.51) of the prism 50, so that the 
refracted light is orientated in the direction of 14.9.degree. in the 
glass. 
The light in the glass is diffracted by the first hologram 53. Generally 
speaking, the diffraction angle .theta..sub.m of the light by the m'th 
(m-order) hologram is given by the following equation: 
EQU sin .theta..sub.m =f(.lambda./n)-cos .theta..sub.m-1 .circle.1 
wherein n is a refractive index. 
When the diffraction angle is varied by .DELTA..theta..sub.m ' due to the 
change .DELTA..lambda. of the wavelength, and if it is supposed that 
.theta..sub.m =.pi./4+.DELTA..theta..sub.m ', the equation .circle.1 can 
be approximately represented by the following equation: 
##EQU1## 
wherein 
##EQU2## 
From this, the change of the diffraction angle by a diffraction of N times 
can be represented by: 
EQU .DELTA..theta..sub.m '=2N.DELTA..lambda./.lambda. .circle.3 
Finally, the exit angle .theta. of the light emitted from the emitting 
portion 33 of the hologram module is given the following equation, in 
accordance with Snell's law: 
EQU .theta.=sin.sup.-1 (nsin .DELTA..theta..sub.m ') .circle.4 
For example, when the light is diffracted eight times, the following 
equation is obtained from equations .circle.3 and .circle.4 : 
EQU 66 .theta..sub.8 '=5.9.degree. 
EQU 74 =8.9.degree. 
wherein .DELTA..lambda.=5 nm, .lambda.=780 nm. 
FIG. 12 shows the application of the device of the invention to a known 
laser printer. In this application, the beam emitted from the tunable 
semiconductor laser 13 is scanned along a straight line (scanning line) on 
a photosensitive recording drum 57 of the laser printer by the hologram 
module 40 of the present invention, as mentioned above, through a known 
f.theta. lens 55 for focussing the scanning beam, which otherwise would be 
scanned in an arc as shown at 110 in FIG. 12, onto a plane. The subject of 
the present invention is not directed to the laser printer itself, and 
accordingly, a detailed explanation of the construction of the laser 
printer will not be given herein. In the arrangement shown in FIG. 12, to 
obtain the diffraction angle of .theta.=60.degree., the following equation 
is obtained from equations .circle.3 and .circle.4 : 
EQU .DELTA..theta..sub.m '.apprxeq.19.3.degree.=0.34 rad 
Therefore, 
EQU 2N.DELTA..lambda./.lambda.=0.34. 
If it is supposed that .lambda.=780 nm, then 
.DELTA..lambda.=16 nm when N=8, 
.DELTA..lambda.=11 nm when N=12, and 
.DELTA..lambda.=8.2 nm when N=16. 
It is confirmed that, although an increase of N leads to a decreased 
efficiency (.eta.) in the use of the light, .eta. is more than 66% when 
N=8, since each of the holograms has a high efficiency of light usage of 
more than 95%. 
FIG. 13 shows another embodiment, in which the laser beam light emitted 
from the tunable semiconductor laser is collimated by a collimating lens 
59, so that parallel beams of light are incident upon the hologram module 
40. The f.theta. lens is not provided in this embodiment. In this 
embodiment, the beam is actually scanned in an arc, as mentioned before. 
It is also possible to provide an additional hologram in order to correct 
the focussing points along the arc, to scan the beam in a straight line. 
In the arrangement shown in FIG. 13, the last hologram of the hologram 
module corrects the beam. 
In FIG. 12, it is also possible to provide a difference in the variation of 
the wavelength per unit time, as shown in FIG. 14, i.e., to carry out a 
timed control of the sweep of the wavelength and thus realize scanning at 
the same speed. 
The invention can be also applied to a bar-code reader or an optical head, 
etc. 
FIGS. 15 to 17 show another embodiment of the present invention, in which a 
combination of a transmission hologram and reflecting mirrors is used to 
realize a hologram module of the invention. 
In FIG. 15, the module essentially has two reflecting surfaces 71, 72 and a 
transmission hologram 150 located between the reflecting surfaces 71, 72. 
In the illustrated embodiment, the reflecting surfaces 71 and 72 are 
embodied by reflecting mirrors formed on adjacent side faces of a 
rectangular optical glass 74, so that the light entering the hologram 
module is reflected by the reflecting surfaces. Also, in the illustrated 
embodiment, the reflecting surface 71 has partial non-reflecting portions 
71a and 71b, through which the light is transmitted, so that the incident 
light can pass through the non-reflecting portion 71a into the hologram 
module and the outgoing light can be emitted therefrom through the 
non-reflecting portion 71b, which is located lower than the non-reflecting 
portion 71a, as shown in FIG. 15. 
The reflecting surfaces 71 and 72 intersect each other at a predetermined 
angle P (e.g. 90.degree.), at an intersecting axis (z-axis). Preferably, 
the plate-like hologram 150 is extended so that it intersects the z-axis. 
Also preferably, the hologram 150 has plane gratings with grating grooves 
parallel to the z-axis. The parallel arrangement of the grating grooves to 
the z-axis is not always necessary. 
The optical glass 74, which is an optical media, is formed, for example, by 
BK7. In theory, the optical medium can be air. In this case, the 
reflecting surfaces 71 and 72 can be formed on respective plate-like 
reflecting mirrors which intersect each other at a predetermined angle P. 
The laser beam (wavelength .lambda.) is incident upon the hologram module 
through the non-reflecting portion 71a of the reflecting surface 71 from 
the tunable laser 13, preferably at an incident angle of .alpha. with 
respect to the surface plane thereof, as shown in FIG. 17. The incident 
light is preferably parallel but can be convergent. 
The incident light enters the optical medium (optical glass 74) through the 
non-reflecting portion 71a without being reflected. The light reaches the 
hologram 150 and is diffracted thereby toward the reflecting surface 72. 
The light is then reflected again by the reflecting surface 72 toward the 
hologram 150, so that the light is diffracted by the hologram 150 toward 
the reflecting surface 71. The reflection by the reflecting surfaces 71 
and 72 and the diffraction by the hologram 150 are repeated, and finally, 
the light is emitted from the hologram module through the nonreflecting 
portion 71b. 
In FIGS. 15 to 17, the solid line shows one example of the beam track and 
the points represented by 1, 2, 3 and 4 are points on the hologram 150 at 
which the light is diffracted. The points at which the light is diffracted 
by the hologram 150 are successively moved downward in the order of 1, 2, 
3 and 4, as shown in FIG. 17. 
The imaginary line in FIGS. 15 to 17 shows one example of a beam track when 
the wavelength of the laser beam is changed from .lambda. to 
.lambda.'=.lambda.+.DELTA..lambda.. When the wavelength is increased by 
.DELTA..lambda., the diffraction angle is increased accordingly, and thus 
the diffraction angle is successively increased every time the light is 
diffracted by the hologram. The points represented by 1', 2', 3' and 4' 
are points on the hologram 150 at which the light having a wavelength of 
.lambda.' is diffracted. The light is finally emitted through the 
non-reflecting portion 71b from the hologram module. It should be recalled 
that the angle and the position of the light emitted from the hologram 
module through the non-reflecting portion 71b vary in accordance with the 
change of the wavelength of the beam, as mentioned above, so that the 
light can be scanned by a control of the wavelength of the incident beam. 
The following is an example of the design of the hologram module shown in 
FIGS. 15 to 17. 
The optical glass 74, which is made of BK7 (refractive index n=1.51), has a 
square section 20 mm .times.20 mm and a plane grating 150 which is 
inclined at 45.degree. with respect to the reflecting surfaces 71 and 72 
formed on the two adjacent side faces of the optical glass 74. The 
hologram 150 is embedded in the glass body (optical glass). The plane 
grating of the hologram is formed by the holographic interference of two 
fluxes of the beam from an Ar laser (.lambda.=488 nm) at the same incident 
angle of .+-.41.9.degree.. The hologram is preferably a phase hologram and 
the material of the hologram can be polyvinyl carbazol or gelatin 
dichromate, etc. The reflecting surface 71 has non-reflecting portions 
corresponding to the incident portion 71a and outgoing portion 71b in the 
direction perpendicular to the z-axis. The reflectivity of the reflecting 
surfaces is more than 99%, which is realized by forming multilayers of 
film on the reflecting surfaces. 
The collimated light (beam diameter: about 0.8 mm) from the tunable laser 
13 is incident upon the side face of the glass body through the 
non-reflecting portion (incident portion) 71a at an incident angle of 
about 2.degree. (.alpha.=2.degree.) substantially in parallel with the 
reflecting surface 72. The angle of the light entering the glass body is 
changed from .alpha. to .alpha.', which can be obtained from the following 
Snell's law: 
EQU .alpha.'=sin.sup.-1 (nsin .alpha.).apprxeq.1.32.degree. 
wherein n=refractive index of the glass body =1.51 
(i) First, the wavelength of the laser beams is set at .lambda. (=780 nm). 
The incident light is incident upon point 1 of the hologram 150, at an 
incident angle of .theta..sub.i.sup.(1) (=45.degree.), when projected in a 
plan view (FIG. 15). The outgoing angle .theta..sub.o.sup.(1) of the light 
diffracted by the hologram 150 at the point 1 is given by the following 
equation: 
EQU sin .theta..sub.o.sup.(1) =f.lambda./n-sin .theta..sub.i.sup.(1) 
=45.degree. 
Then, the light is reflected by the reflecting surface 72 and reaches point 
2 of the hologram 150, where the light is similarly diffracted by the 
hologram 150. 
The foregoing operations are repeated, so that the light is successively 
moved downward. If the diffraction by the hologram takes place twice, the 
light is moved downward by 40 nm.times.tan .alpha.'=0.92 mm, and if the 
diffraction takes place ten times, the downward displacement of the light 
is about 4.61 mm. Accordingly, in the latter case, if the non-reflecting 
portion 71b is provided in an area which extends downward from a point 
below the incident portion 71a by about 4.61 mm, the light can be emitted 
from the hologram module through the non-reflecting portion 71b. 
(ii) Subsequently, the wavelength of the laser beam is changed to .lambda.' 
(=785 nm). Also, in this case, the light is incident upon point 1 of the 
hologram 150 at an incident angle of 45.degree.. The diffraction angle 
(sin .theta.'.sub.o.sup.(1)) at point 1 of the hologram 150 is as follows: 
EQU sin .theta.'.sub.o.sup.(1) =f.lambda.'/n-sin .theta..sub.i.sup.(1) 
From .lambda.'=.lambda.+.DELTA..lambda. is obtained: 
EQU .theta.'.sub.o.sup.(1) .apprxeq.45.75.degree. 
Accordingly, the light is incident upon and reflected by the reflecting 
surface 72 at an angle of about 0.74.degree.. The reflected light is 
incident upon point 2' at an incident angle of .theta..sub.i.sup.(2) 
=45.degree.-0.74.degree.=44.26.degree.. 
Generally speaking, in the hologram module of the present invention, the 
light incident upon the hologram at an incident angle of .theta..sub.o is 
incident again upon the hologram at an incident angle of sin.sup.-1 (cos 
.GAMMA..sub.o) after reflection by the associated reflecting surface. In 
the repetition of the diffraction by the hologram, the change in the angle 
of the hologram causes a reduction of the incident angle of the subsequent 
incidence of the light upon the hologram, and accordingly, an increase of 
the diffraction angle. This results in a synergetic effect of a change of 
the angle by the hologram and the reduction of the incident angle, 
resulting in a large change in angle. 
FIG. 18 shows one example of a beam track, which can be mathematically 
obtained, in the present invention. The light is subject to repeated 
diffraction and reflection to cause the change in the outgoing angle and 
the position thereof. 
Note that the numbers of the diffraction points (1',2', . . . 11') and the 
reflection points (1,2, . . . 10) in FIG. 18 do not correspond to those in 
FIGS. 16 and 17. 
FIG. 19 shows a relationship between the number of diffractions by the 
hologram and the change in angle when the wavelength varies by +5 nm and 
-5 nm, respectively. The change in angle referred to means a difference in 
angle occurring when the wavelength changes and when the wavelength does 
not change. Two change curves are shown in FIG. 19, one for in the air and 
the other for in the glass. 
For, example, when the diffraction takes place ten times, the change of +5 
nm in the wavelength causes a change of 22.7.degree. in angle in the air, 
and the change of -5 nm in wavelength causes a change of -7.98.degree. in 
angle. If eleven diffractions occur, the light is then emitted from the 
hologram module through a non-reflecting portion which is provided on the 
reflecting surface 72, similar to the non-reflecting portion 71b of the 
reflecting surface 71, and the change in angle (.DELTA..theta.) is 
34.0.degree.. 
FIG. 20 shows a relationship between the change in angle (.DELTA..theta.) 
and the change of wavelength (.DELTA..theta.) in the case of eleven 
diffractions (number N of the diffraction is 11). Note that the change of 
-5 nm in wavelength causes a -8.3.degree. change in angle. Therefore, a 
.+-.5 nm change in wavelength can produce an about 42.degree. change in 
angle. This change in angle is sufficient for the application of the 
hologram module of the present invention, for example, to a laser printer. 
The efficiency .eta. of light usage is given by the following equation, 
provided that the efficiency of the hologram is 95% and the reflectivity 
of the reflecting surfaces is 99%. 
EQU .eta.=0.99.sup.10 .times.0.95.sup.11 =0.51 
When the present invention is applied to a laser printer, a beam focussing 
function and a constant speed scanning function are required. The beam 
focussing can be achieved by a known additional optical system, such as an 
optical head in an optical disc apparatus which has a beam converging 
function and which can be provided at a portion of the hologram module 
from which the light is emitted, e.g., at the non-reflecting portion 71b. 
The constant speed beam scanning can be achieved by control of the 
electric current of the tunable laser which controls the change of th 
wavelength in relation to time, as mentioned above with reference to FIG. 
14. 
FIGS. 21 and 22 show another application of the present invention, in which 
more than two tunable lasers (three tunable lasers 13a, 13b, and 13c are 
shown in the illustrated embodiment) are provided to realize a multi-beam 
scanner. Namely, the beams are emitted from three separate tunable lasers 
and are reflected by respective additional mirrors M toward the 
non-reflecting portion (incident portion) 71a of the reflecting surface 
71. The incident angles of the three beams can be either identical to or 
different from each other. In the arrangement shown in FIG. 21, the 
outgoing portion, i.e. the exit non-reflecting portion 72b, is formed on 
the reflecting surface 72 rather than on the reflecting surface 71. 
In FIG. 22, the three beams are incident upon the incident portion 71a at 
an incident angle .alpha.' with respect to a line normal to the incident 
surface, along an oblique line, unlike FIG. 21 in which the three beams 
are incident upon a vertical line. 
The beam deflection is non-mechanically effected. In theory, it is possible 
to scan the beam at a high speed up to a limit of the response to the 
wavelength of the laser, which is at least one thousand times the scanning 
speed (about 300 m/sec at the most) of the mechanical scanning of the 
existing polygon mirror. 
FIGS. 23 and 24 show modified embodiments of FIGS. 21 and 22. In FIG. 23, 
the reflecting surface 72 is inclined by .DELTA..phi. with respect to a 
vertical plane, which is perpendicular to a vertical plane including the 
reflecting surface 71. With this arrangement, it is possible to make the 
light incident upon the incident portion (non-reflecting portion) 71a 
normal thereto (i.e. .alpha. or .alpha.'=0.degree.), since the light 
incident upon the incident portion 71a is diffracted by the hologram 150 
toward the reflecting surface 72 which reflects the diffracted light 
downward at an angle determined by the inclination angle .DELTA..phi. of 
the reflecting surface 72, so that the light is finally emitted from the 
hologram module through the non-reflecting portion 72 formed on the side 
face thereof, similar to the arrangement shown in FIGS. 21 and 22. 
In FIG. 24, the diffraction grating of the hologram 150' is inclined at an 
angle .DELTA..phi. with respect to the vertical, instead of the 
inclination of the reflecting surface 72 as shown in FIG. 23. The 
arrangement shown in FIG. 24 is functionally equivalent to the arrangement 
shown in FIG. 23. Namely, even if the light is incident upon the incident 
portion 71a perpendicularly thereto, the light can be emitted from the 
hologram module through the lower non-reflecting portion 71b formed on the 
side face including the reflecting surface 71 or 72. 
With the arrangements illustrated in FIGS. 23 and 24, since the light can 
be incident upon the incident portion 71a perpendicularly thereto, an 
easier optical arrangement of the incident light can be expected. 
In FIGS. 21 to 24, numerals 101 and 103 designate incident light and 
outgoing light, respectively. 
FIGS. 25A, 25B, 26, and 27 show three modified embodiments in which the 
most significant feature resides in the use of a reflection hologram 
instead of the transmission hologram in the above-mentioned embodiments. 
In FIGS. 25A and 25B, a hologram module has a glass body 63 with a 
triangular section which is provided, on adjacent side faces defining an 
angle P which is less than 90.degree. in the illustrated embodiment, with 
a reflecting surface 71 and a reflection hologram 65. Alternatively, it is 
also possible to provide separate plates having the reflecting surface 71 
and the reflection hologram 65, respectively, so that the plates intersect 
each other at an angle P, without the glass body 63. In this alternative, 
the optical medium is air, not glass. 
The side face of the hologram module including the reflecting surface 71 
has a partial non-reflecting portion 71a (FIGS. 21, 23, 24, etc.) upon 
which the light 101 is incident. 
Similarly, the side face including the reflection hologram 65 has a partial 
non-hologram portion 65b through which the light 103 or 103' is finally 
emitted, as shown in FIG. 25B. 
The light 101 incident upon the non-reflecting portion (incident portion) 
from the tunable laser 13 is diffracted by the reflection hologram 65 
toward the reflecting surface 71, by which the diffracted light is 
reflected. The light, after repeated diffraction by the reflection 
hologram 65 and repeated reflection by the reflecting surface 71, similar 
to the foregoing embodiments, is emitted from the hologram module through 
the non-hologram portion 65b. 
In the arrangement shown in FIGS. 25A and 25B, the light is incident upon 
the incident portion 71a at a predetermined angle, similar to, for 
example, FIG. 21. 
The hologram module shown in FIGS. 25A and 25B is functionally almost 
equivalent to the aforementioned embodiments. The reflection and 
diffraction angle increases as the angle P defined by the reflecting 
surface 71 and the reflection hologram 65 increases. 
Note that the spatial frequency f of the hologram is given by the following 
equation. 
EQU 2 sinp=f.lambda./n 
wherein n is the diffraction index of the optical medium 63 and .lambda. is 
the wavelength of the beams. 
When P=45.degree., the tracks of the light are almost identical to those of 
FIG. 18. If the incident portion upon which the light from the tunable 
laser 13 is incident and the outgoing portion from which the light is 
finally emitted from the hologram module are provided on the side face 
including the reflecting surface 71, similar to FIGS. 15 to 17, and when 
N=10, an about 22.7.degree. of variation .DELTA..theta. of the angle is 
obtained by the variation .DELTA..lambda. of the wavelength of the beam by 
+5 nm. 
Numeral 103' designates one example of the light which is finally emitted 
from the hologram module when the wavelength of the incident beam is 
changed. 
In FIG. 26, two reflection holograms 65A and 65B are provided on the 
adjacent side faces of the glass body 63. Namely, the reflecting surface 
71 in FIG. 25A is replaced with the additional reflection hologram 65B in 
FIG. 26. In the hologram module shown in FIG. 26, the light 101 from the 
tunable laser 13 is incident upon the incident portion (non-hologram 
portion) which corresponds to the incident portion 71a, for example, in 
FIG. 21, and which is provided on one of the side faces including the 
reflection hologram 65B or reflection hologram 65A, at a predetermined 
incident angle. In this embodiment, the incident light is repeatedly 
diffracted and reflected between the two reflection holograms 65A and 65B. 
In FIG. 26, the light 103 and 103' is emitted from a non-hologram portion 
which corresponds to the non-hologram portion 65b shown in FIG. 25B and 
which is provided on the side face including the reflection hologram 65A, 
similar to the aforementioned embodiments. 
In FIG. 26, when the angle P defined by the two reflection holograms 65A 
and 65B is 90.degree. (P=90.degree.), a variation .DELTA..theta. similar 
to that in FIG. 18 can be expected. 
In a modification illustrated in FIG. 27, a transmission hologram 150 and 
reflection holograms 65A and 65B are provided. The transmission hologram 
150 is located between the opposite side faces of two identical glass 
bodies having a right angle triangle cross section. The reflection 
holograms 65A and 65B are aligned perpendicularly to the transmission 
hologram 150. In this embodiment, the incident portion (non-hologram 
portion) upon which the light 101 from the tunable laser 13 is incident 
and the outgoing portion (non-hologram portion) from which the light 103 
is finally emitted are both provided on the transmission hologram 150. In 
this embodiment, the light can be scanned in accordance with the change in 
wavelength of the incident beam, similar to the aforementioned 
embodiments. 
In FIGS. 25A, 26 and 27, numerals having a prime, such as 1', 2', etc., 
designate the order of the diffraction and reflection of the incident 
light in the hologram module. 
When plane gratings of, for example, f=2740/mm.sup.2, at an equal pitch of 
the hologram are formed to realize the hologram module of the invention, 
it should be borne in mind that they may have different incident angles at 
the incident points. For example, referring to FIG. 25A, 26 or 27, it can 
be easily understood that the light is incident upon the points 1, 2', 3', 
4', etc., of the hologram 65, 65A, 65B, or 150. From this view point, it 
is preferably necessary to form a hologram having constant pitch gratings 
in which the light can be always incident upon any grating (points) at a 
Bragg angle. Such a hologram having constant pitch gratings and different 
Bragg angles can be produced in the process as disclosed, for example, in 
Japanese Unexamined Patent Publication (Kokai) No. 58-158678 corresponding 
to U.S. application Ser. No. 467,773 now abandoned. 
FIG. 28 shows an example of how to produce a hologram used in the present 
invention, in which a master hologram with 2740/mm.sup.2 of equi-pitch 
gratings is first formed by interference exposure process of two groups of 
coherent light (plane waves at 41.9.degree. and -41.9.degree.). Then, a 
copy hologram plate 201 with a photosensitive layer on which a copy 
hologram is to be formed is located below tee master hologram 200 through 
an index matching liquid (e.g. xylene or ethyl alcohol, etc.) 203. When 
the master hologram is copied on the photosensitive layer of the cop 
hologram plate to form a copy hologram, the copying light 207 is incident 
upon the master hologram 200 at different incident angles .theta..sub.c 
(x), which are properly selected to be optimum, depending on the position 
x of the master hologram 200. Such light having different incident angles 
can be easily created, for example, by a cylindrical lens or the like. 
The angle P in FIGS. 15, 25A, and 26 is less than 180.degree., and 
preferably less than 90.degree..