Optical Scanning Apparatus Having an Array of Light Sources

1 Field of the Invention

This invention relates to an image recording and displaying apparatus which utilizes an array of aligned light sources such as a semiconductor laser array.

2 Description of the Prior Art

There are known image recording and displaying apparatuses having a single source of light. FIG. 1 shows an example of such apparatuses wherein a divergent beam emitted from a semiconductor laser 1 is collimated by means of a collimator lens 2 and falls on a rotary multifaceted mirror 3. The beam which has been reflected and deflected by the rotary multifaceted mirror is then brought to a focus on a surface 5 to be scanned through an imaging lens 4 such as an f.sub..theta. lens or the like. By modulating the semiconductor laser, the desired image can be formed on the surface to be scanned.

If an array of aligned light sources are used in such an apparatus, various advantages will be obtained. The term "an array of aligned light sources" is defined herein as such a construction that a plurality of semiconductor lasers 1a, 1b and 1c which can be independently driven and modulated are brought into a line as shown in FIG. 2. If these light sources are used instead of a single light source, one can obtain the following advantages:

1. The apparatus can be run at higher speed because a plurality of scanning lines are utilized for recording and displaying.

2. For this reason, a rotary multifaceted mirror, a galvano mirror and the like can be operated at lower speed.

3. Semiconductor lasers can be used with lower power resulting in prolonged life.

However, if such light sources are used in the prior art optical systems without modifying, there can be provided such disadvantages as described hereinbelow.

FIG. 3 illustrates an example of an optical system wherein three semiconductor lasers 1a, 1b and 1c are used as a light source portion 1. Beams emitted from the semiconductor lasers 1a, 1b and 1c are oscillated to distribute with maximum intensity in a direction perpendicular to the end face of the light source portion 1. When the beams are collimated by a collimator lens 2, the beam from the semiconductor laser 1b on the optical axis only becomes parallel to the axis while the remaining beams from the semiconductor lasers 1a and 1c located off of the axis are collimated to intersect the optical axis with a finite angle .theta.. This angle .theta. is obtained by the use of the following formula:

where f is a focal length in the collimator lens, 1 is a pitch in the light source array and n is the number of light sources. It is considered that the pitch 1 in the semiconductor laser array must be about 0.1 mm minimum because of various problems accompanied by heat emanation, manufacturing techniques and the like. Now, if the focal length f of the collimating lens is 10 mm, and the number of lasers n is 8, the angle .theta. is

Further, if the distance d between the focal plane 6 of the collimator lens and the multifaceted mirror is 100 mm, a light spread .DELTA. on the reflecting surface on the multifaceted mirror is

This value cannot be neglected. As a result, if light sources 1a, 1b, 1c and 1d are disposed along such a line that is parallel or substantially parallel to the primary scanning direction as shown in FIG. 4A, light spots would be spread on the reflecting surface 3a of the multifaceted mirror 3 in the rotational direction thereof as shown in FIG. 5. Consequently, each of the beams 7a, 7b and 7c collimated by the lens 2 will have a quantity of reflected light per rotational angle .theta. of the multifaceted mirror 3 which is distributed as shown in FIG. 6. As seen from FIG. 6, the range of rotational angle 2.theta..sub.o in which the scanning can be effectively made is extremely narrow in comparison with the case of a single source of light.

In order to overcome such a problem, the multifaceted mirror must be increased in diameter. This means that the multifaceted mirror cannot be easily driven or otherwise a motor for driving this mirror should have higher power resulting in increased cost and size.

If the light sources 1a, 1b, 1c and 1d are located along such a line that is perpendicular to the primary scanning direction as shown in FIG. 4B, the multifaceted mirror may have the same diameter as in a single light source. However, it must be increased in thickness since the beam spread on the reflecting surface is widened to distribute along the rotational axis of the multifaceted mirror. Similarly, such an arrangement will have the same problems as in the multifaceted mirror having its increased diameter.

It is an object of this invention to provide an improved optical scanning apparatus having an array of aligned light sources which may include a small-sized deflector and which have a compact structure manufactured inexpensively.

Another object of this invention is to provide an improved optical scanning apparatus of the above type which is adapted to correct any pitch variation of beam on a surface to be scanned due to inclination in the deflecting surface of a deflector or in the rotating shaft thereof.

According to this invention, the above objects can be accomplished by converging a plurality of light beams on an area as small as possible adjacent the deflecting surface of the deflector.

In one aspect of this invention, the optical scanning apparatus comprises an array of aligned semiconductor lasers as an light-source array which is adapted to emit nonparallel rays, a deflector such as a multifaceted mirror, a galvanomirror or the like which includes a polygonal deflecting surface, and a collimator lens for collimating the rays from the light sources, which has an exit pupil at the side of said deflector, the position of said exit pupil or a position conjugate therewith being extremely close to the deflecting surface of said deflector. In other words, this invention utilizes a concept of a deflective scan plane which can be formed with time by beams deflected at the deflector. Considering ray components of each beam emitted from said light-source portion in the deflective scan plane, the chief ray in these ray components is incident parallel to the optical axis in the collimator lens. The collimator lens has a focal position at the side of the deflector or a conjugate position therewith which is extremely close to the deflecting and reflecting surface of the deflector. Thus, the deflector can be maintained with the same size as in a single light source, even if the number of light sources is increased.

In another aspect of this invention, the collimator lenses can be divided into two groups of lens with one group of negative power being disposed at a position close to the light sources while the other group of positive power being located at another position close to the deflector, if the collimator lens has its small focal length so that the reflecting surface of the deflector cannot be disposed close to the focal position of the collimator lens. Thus, the primary plane of the collimator lens groups at the side of the deflector can be positioned as close as possible to the deflector so that the distance therebetween will be increased.

In still another aspect of this invention, the optical scanning apparatus is able to have a function for correcting any inclination by comprising a light-source portion consisting of an array of aligned light sources, a deflecting portion for simultaneously deflecting the beams from said light-source portion, a scanned surface adapted to be scanned simultaneously by the deflected beams from the deflector, a first anamorphic optics member disposed between said light-source and deflecting portions, and a second anamorphic optics member located between said deflecting portion and said scanned surface. In a plane parallel to said deflective scan plane, the position of the pupil in said first anamorphic optics member at the side of the deflecting portion or a position optically conjugate therewith is close to the polygonal deflecting surface of said deflecting portion. Thus, the deflecting portion may have its deflecting surface of small size. Furthermore, the first anamorphic optics member is adapted to cause the beams from the light-source portion to be incident on the deflecting surface of said deflecting portion substantially under parallel-ray state in the plane parallel to said deflective scan plane and also to cause them to be incident on the deflecting surface under such a state that the beams are imaged thereon in a plane perpendicular to said deflective scan plane. Consequently, each of the beams from the light-source portion is imaged on the deflecting surface along a line in a direction parallel to said deflective scan plane. Each beam imaged in a line is further imaged on the scanned surface through said second anamorphic optics member to form a better spot. The deflecting surface of the deflecting portion is now held at an optically conjugate position with the scanned surface through the second anamorphic optics member in the plane parallel to the deflective scan plane so that the influence due to any inclination in the deflecting portion will be corrected.

If the optical scanning apparatus according to this invention is provided with semiconductor lasers as light sources, it would be preferred that said first anamorphic optics member comprises a collimator lens for collimating the beams from the light-source portion and a cylindrical lens for converging the light components of the parallel beams from the collimator lens which are positioned in a plane perpendicular to the deflective scan plane.

FIG. 1 shows an example of the prior art optical scanning apparatus;

FIG. 2 shows an example of semiconductor laser array;

FIGS. 3, 5, and 6 illustrate various disadvantages in the prior art wherein light sources are aligned in an array;

FIGS. 4A and 4B illustrate known light source arrays; parameters considered in devising the present invention where light sources are aligned;

FIG. 7 is a plan view of an optical system which is an embodiment of this invention;

FIG. 8 is a front view of the light-source portion shown in FIG. 7;

FIGS. 9A and 9B are illustrative views of a collimator lens's structure which can be used in the optical system according to this invention;

FIGS. 10A and 10B show another embodiment of the optical scanning system according to this invention;

FIG. 11 illustrates an afocal system used in the optical scanning system shown in FIG. 10;

FIG. 12 shows still another embodiment of this invention;

FIGS. 13A and 13B show further embodiment of this invention;

FIGS. 14A, 14B and 15A, 15B show a positional relationship between a collimator lens and a cylindrical lens in the optical scanning system shown in FIG. 13;

FIGS. 16A and 16B show a positional relationship in optics between a light-source portion and a deflecting portion in the optical scanning system shown in FIG. 13;

FIGS. 17A and 17B show the optical scanning system of FIG. 10 wherein the afocal optical system thereof has a function for correcting any inclination; and

FIG. 18 is a view showing an optical scanning apparatus of this invention which is integrally formed.

FIG. 7 is a plan view showing an embodiment of the optical scanning apparatus according to this invention. The apparatus comprises a light-source portion 11 consisting of a plurality of semiconductor lasers which are aligned in an array. The emitting surface 12 of the light-source portion 11 is disposed in one focal plane of a collimator lens 13. FIG. 8 is a view in which the emitting surface 12 is viewed from the side of the collimator lens 13. It is understood that light-emitting portions 11a, 11b and 11c are in progressively closer in position relative to the rotating shaft 14a of a multifaceted (polygonal) mirror 14. A chief ray in the beam emitted from each light-emitting portion is incident on the collimator lens 13 under such a state that it is parallel to the optical axis in the collimator lens 13. The focal position of the collimator lens 13 at the side of the polygonal mirror 14, which is a position corresponding to an exit pupil of the collimator lens 13 in the illustrated embodiment, is close to the deflecting surface 14b of the polygonal mirror for deflecting the beams. The beams deflected by the polygonal mirror 14 are imaged upon photosensitive surface 16 in a cylindrical photoreceptor through a imaging lens 15.

If the reflecting surface 14b of the polygonal mirror 14 cannot be disposed at a position close to the focal position 17 of the collimator lens 13 because the focal length of the collimator lens 13 is too short, such a problem can be overcome by arrangement as shown in FIGS. 9A and 9B. In an arrangement of FIG. 9A, the collimator lens 13 has its primary plane 18 located at a position as close as possible to the deflector so that the focal length f of the collimator lens 13 will be smaller than a distance a between the focal plane 17 of the collimator lens 13 and the end face thereof at the side of the deflector. FIG. 9B shows a structure of the collimator lens 13 for providing the arrangement shown in FIG. 9A. In this structure, the collimator lenses 13 is divided into two groups of lens 13a and 13b with one lens group 13a of negative power being located close to the light-source portion while the other lens group 13b of positive power being disposed close to the deflector. As a result, said primary plane 18 will be shifted toward the deflector so that the distance between the deflector and the collimator lens can be increased. Thus, even if the focal length of the collimator lens 13 is too short, the deflecting surface of the deflector can be disposed nearby the focal plane 17 of the collimator lens 13.

FIG. 10A is a plan view showing another embodiment of the optical scanning apparatus according to this invention, and FIG. 10B is a front view of this embodiment in which the light-emitting face 23 of a light-source portion 21 including semiconductor lasers is viewed from a collimator lens 22. As shown in FIG. 10B, the light-emitting portions 23a, 23b and 23c, which are all semiconductor lasers, are slightly shifted from one another in a vertical direction as viewed along the optical axis of the collimator lens 22. Beams 21a, 21b and 21c emanating from the light-emitting portions are collimated by the collimating lens 22 and then passed through the exit pupil 24 to be incident upon an afocal system 25 which consists of two positive lens elements 25a and 25b. The afocal system 25 is adapted to emanate parallelray beams. With respect to this afocal system 25, the exit pupil 24 of the collimator lens 22 is optically conjugate with the deflecting and reflecting surface 26a of a polygonal mirror 26. As a result, the image of the exit pupil 24 will be formed close to the deflecting and reflecting surface 26a. Thereafter, the beams are deflectively scanned by the polygonal mirror 26 and then imaged through an imaging lens system 27 upon a photoreceptive drum 28 which provides a surface to be scanned. Therefore, the drum surface will be scanned by a plurality of beam spots as the polygonal mirror is rotated. Since the light-emitting portions are positioned at different levels in the vertical direction as shown in FIG. 10B, the beam spots imaged on the drum also are slightly shifted from one another in a direction that is perpendicular to the drawing.

FIG. 11 illustrates a principle in the above afocal system 25 which consists of a positive lens group 25a having a focal length fa and a positive lens group 25b having a focal length fb. The exit pupil 24 of the collimator lens 22 is positioned at the forward focal plane of the positive lens group 25a while the deflecting and reflecting surface 26a of the deflector 26 is located at the rearward focal plane of the positive lens group 25b. The lens groups 25a and 25b are spaced away from each other by the sum (fa +fb) of the focal lengths fa, fb. In such an arrangement, the conjugate plane of the exit pupil 24 of the collimator lens 22 can be formed in the reflecting surface of the deflector 26. This is more advantageous than that in which the deflecting surface 26a of the scanner is directly located in the exit pupil plane 24 of the collimator lens in the following points.

(i) By providing the focal length fb of the rearward lens group 25b which is longer than the focal length fc of the collimator lens 22, a space between the afocal lens system and the deflector can be increased resulting in facility in designing.

(ii) By changing the focal lengths fa, fb of the forward and rearward lens groups 25a, 25b in the afocal lens system 25, an angular magnification .gamma. can be optionally changed as shown by the following formula:

where .theta.a is an incident angle into the forward lens group and .theta.b is an emanant angle from the rearward lens group. For example, if the angular magnification .gamma. is decreased, a distance S between the adjacent imaged spots on the imaging surface is also decreased as represented by the following formula:

Therefore, flexibility in design will be increased.

FIG. 12 is a plan view showing still another embodiment of the optical scanning apparatus according to this invention wherein an afocal system 29 comprises two paraboloidal mirrors 29a and 29b. This afocal system has the same function as in the last-mentioned afocal lens system and will not be further described herein.

FIGS. 13A and 13B show respectively plan and side views of a further embodiment according to this invention in which the optical scanning apparatus has an inclination-correcting function. In this illustrated embodiment, the optical scanning apparatus includes a light-source portion 31 constructed by an array of aligned semiconductor lasers as a plurality of light sources. The light-emitting face 32 of this light-source portion 31 is disposed in one focal plane of a collimator lens 33a which defines an anamorphic optical system 33. The beams emitted from the semiconductor lasers are collimated by the collimator lens 33a and then passed through a cylindrical lens 33b to be incident on a rotary multifaceted mirror 34. The beams reflected by the rotary multifaceted mirror 34 are imaged on a photoreceptive drum 36 through an imaging lens 35.

In accordance with this invention, the position of a pupil of the collimator lens 33a at the side of the rotary multifaceted mirror or a position optically conjugate with such a pupil is extremely close to the reflecting surface 34b of the rotary multifaceted mirror 34 in a deflective scan plane in which the beams are deflected by the rotary multifaceted mirror, that is, in a plane of FIG. 13A. In other words, the light-source portion is located relative to the collimator lens such that a chief ray of each beam from the respective light-emitting point in the light-source portion is passed through the collimator lens 33a parallel to the optical axis thereof. In addition, the reflecting surface 34b of the rotary multifaceted mirror 34 is extremely close to the focal position of the collimator lens. In such an arrangement, the beam spread on the reflecting surface of the deflector will be the same as in a single light source even if the number of light sources is increased.

As shown in FIG. 13B, the cylindrical lens 33b has its power in a plane perpendicular to the deflective scan plane, which power causes the beams from the lasers to form a linear image on the reflecting surface 34b of the multifaceted mirror 34 in a direction perpendicular to the rotating shaft 34a thereof. This means that irregular pitches in images due to any inclination in the reflecting surface of the rotary multifaceted mirror can be optically corrected as disclosed in U.S. Pat. No. 3,750,189, for example. In other words, the reflecting surface 34b of the rotary multifaceted mirror is positioned close to the focal plane of the collimator lens in the deflective scan plane and at the focal plane of the cylindrical lens 33b in a plane perpendicular to the deflective scan plane.

The imaging lens 35 is similarly an anamorphic system and adapted to cause the parallel-ray beams reflected by the reflecting surface 34b to form images on the photoreceptive drum 36 in the deflective scan plane and also to maintain the reflecting surface 34b in an optically conjugate relationship with the surface of the photoreceptive drum 36 in the plane perpendicular to the deflective scan plane.

As will be apparent from the foregoing, this invention provides an optical scanning apparatus which is provided with a rotary multifaceted mirror of smaller size rotated at higher speed and in which irregular pitches due to any inclination in the reflecting surface of the rotary multifacted mirror can be positively corrected. In the actual application, however, there is a problem in an arrangement when it is desired that the focal length fa' of the cylindrical lens 33b is longer than the focal length fc of the collimator lens. This invention also settles such a problem.

FIGS. 14A, 14B, 15A and 15B illustrate some solutions to the above problem. FIGS. 14A and 14B show a solution when the focal length fa' of the cylindrical lens is shorter than the focal length fc of the collimator lens. In this solution, the cylindrical lens 33b is disposed between the collimator lens 33a and the deflecting surface 34b of the rotary multifaceted mirror 34 so that the focal plane of the collimator lens will substantially coincide with that of the cylindrical lens 33b. FIG. 14B illustrates optical paths in the deflective scan plane which are represented only by chief rays 37a, 37b and 37c from the respective light-emitting portions.

On the other hand, when it is desired that the focal length fa' of the cylindrical lens be longer than that of the collimator lens, the cylindrical lens 33b is divided into a convex lens 33c and a concave lens 33d as shown in FIG. 15A. In such an arrangement, the primary plane of the cylindrical lens can be located at the side of the light-source portion so that the cylindrical lens 33b will be disposed between the collimator lens 33a and the rotary multifaceted mirror 34 as shown in FIG. 15A. FIG. 15B illustrates optical paths in the deflective scan plane which are represented merely by chief rays 38a, 38b and 38c.

Developing these solutions, the following matters can be said with respect to FIGS. 16A and 16B.

(1) Considering a condensing lens 39 formed integrally by the collimator and cylindrical lenses, the light-emitting point 32 of the semiconductor laser and the reflecting surface 34b of the rotary multifaceted mirror are conjugate with respect to such a direction as is perpendicular to the deflective scan plane as shown in FIG. 16A.

(2) An infinite object point and the reflecting surface 34b of the rotary multifaceted mirror are conjugate in a plane perpendicular to the last-mentioned direction as shown in FIG. 16B.

If a condensing lens is designed to satisfy the above two functions, it can be used at the incident side as an optical system for correcting irregularities due to any inclination in the deflecting surface when a plurality of lasers are used. It will be thus unnecessary to collimate the beams from the lens 33a. As a result, the lenses can be freely designed. Furthermore, a focus can be adjusted by defocusing the overall condensing lens.

FIG. 17 shows an example of said afocal system 25 where it is given a function for correcting irregularities due to any inclination in the system shown in FIG. 10. FIG. 17A illustrates optical paths in the deflective scan plane while FIG. 17B represents optical paths in a plane perpendicular to the deflective scan plane. As shown in FIGS. 17A and 17B, components of scanning beams in a direction of optical axis, that is, in a direction perpendicular to the deflective scanning direction are imaged on the deflecting and reflecting surface 26a of the deflector 26 in a linear configuration. In order to accomplish this, the positive lens 25a shown in FIG. 10A is replaced by a cylindrical lens 25a' the generating line of which is perpendicular to the rotational axis of the deflector, that is, the deflective scan plane. In the deflective scan plane shown in FIG. 17A, the beams are imaged as in the case of FIG. 10A. In a plane perpendicular to the deflecting surface, on the other hand, the beams from the collimator lens are not imaged by the cylindrical positive lens 25a' but condensed by the positive lens 25b to form a linear image on the deflecting surface 26a. In such an afocal system, an imaging lens system 27 is anamorphic in that the linear image on the deflecting and reflecting surface 26a is maintained optically conjugate with the photoreceptive drum 28 in a plane perpendicular to the deflective scan plane. Accordingly, the beams will not be adversely affected by any inclination in the deflecting and reflecting surface in the plane perpendicular to the deflective scan plane. In the deflective scan plane of the imaging lens system 27, on the other hand, the beams which are deflected by the deflecting and reflecting surface 26a are imaged on the photoreceptive drum 28 as shown in FIG. 10A. In this connection, the paraboloidal mirror 29a can be replaced by a paraboloidal mirror of cylindrical configuration so that the above correction with respect to any inclination will be made.

FIG. 18 is a perspective view showing an integral unit into which the optical system shown in FIG. 7 is assembled. In such an integral unit, the deflector is mounted in a casing 19 on which housings 13a and 15a for the collimator lens 13 and imaging lens system 15 are mounted. The semiconductor laser array 11 is mounted on the housing 13a of the collimator lens. This integral unit can be more accurately assembled in such a manner that the relative positional relationship between the optical elements will not be disturbed by any external cause such as vibration or the like.