JP2003315712A - Optical scanner and image forming apparatus - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To convert a luminous flux emitted from each light emitting point to a divergent luminous flux by a coupling optical system, to reduce the magnification in the sub scanning direction of the optical system between the light source and a surface to be scanned, and also, to effectively reduce the deterioration of an image such as uneven density and the fluctuation of vertical lines, as to optical scanning introducing a multibeam scanning system. <P>SOLUTION: The luminous flux emitted from each light emitting point of the light source 1 is converted to the divergent luminous flux by the coupling optical system 2. The 1st optical system 4 arranged between the coupling optical system 2 and a deflecting means 7, and constituted of at least one lens has a positive refractive power in the main scanning direction. <P>COPYRIGHT: (C)2004,JPO

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an optical scanning device and an image forming apparatus.

[0002]

2. Description of the Related Art As an optical scanning method capable of improving a writing speed, a multi-beam scanning method has been proposed in which a plurality of scanning lines are simultaneously optically scanned by a plurality of light spots. In an optical scanning device of a multi-beam scanning system, a plurality of light beams emitted from a plurality of light emitting points (point-like light emitting sources) such as a semiconductor laser array are usually converted into parallel light beams by a coupling optical system, It is imaged as a "long line image in the main scanning direction" near the deflecting / reflecting surface of the polygon mirror by the cylindrical lens.

The polygon mirror rotates at a constant speed to deflect each reflected light beam at a constant angular velocity. The respective deflected light fluxes are guided onto the surface to be scanned by the scanning and imaging optical system and are condensed as a plurality of light spots separated in the sub-scanning direction, and the plurality of scanning lines are simultaneously optically scanned by these light spots.

One of the important matters in the multi-beam scanning type optical scanning device is that the "scanning line pitch", which is the interval between scanning lines (moving loci of each optical spot) optically scanned by a plurality of optical spots, is stable. Is what you are doing.

If the assembling state of the light spot changes due to environmental changes such as temperature change or changes over time, and the interval P in the sub-scanning direction between adjacent light emitting points changes by ΔP,
The interval ΔS between the scanning lines is ΔS according to “the lateral magnification in the sub-scanning direction of the optical system between the light source and the surface to be scanned: m”.
= MΔP changes.

In order for the scanning line pitch to be "more stable" with respect to changes with time and environmental changes, it is preferable that the lateral magnification: m is small. Therefore, in the optical scanning device of the multi-beam scanning system, in order to suppress the lateral magnification: m to be low,
It is preferable that the light beams emitted from the plurality of light emitting points are converted into "divergent light beams" by the coupling optical system.

However, when the light flux from the light emitting point is converted to divergent by the coupling lens, the following problems occur.

That is, assuming a semiconductor laser array as the light source, a plurality of light emitting points are arranged on a straight line, but if the light emitting points are arranged "inclined with respect to the sub-scanning direction", the cup The principal rays of each light beam coupled by the ring lens have "an angle that opens toward the deflecting means in the main scanning direction," and as a long line image in the main scanning direction near the deflection reflection surface of the polygon mirror by the cylindrical lens. When formed, each line image is separated in the sub-scanning direction, but also separated in the main scanning direction.

Then, when the principal rays of each light beam head toward the "same image height position in the main scanning direction" on the surface to be scanned, they head in the same direction as viewed from the sub-scanning direction (that is, in the main scanning direction). , The respective chief rays are in a state of being “parallelly offset” from each other. Since each light beam has an angle so as to open in the main scanning direction with respect to the polygon mirror, the main rays of each light beam do not simultaneously go to the same image height in the main scanning direction on the surface to be scanned, Although the principal rays of the above-mentioned heading toward the same image height are deviated in time, the direction of each principal ray when heading toward the same image height is a problem, ignoring this time difference.

At this time, when the scanning image forming optical system (usually, the fθ lens) has a function of "forming an image of a parallel light beam", the focus position of each deflected light beam (image forming point position of the divergent light beam) Is a crossing position of the principal rays (when the principal rays parallel to each other toward the same image height in the main scanning direction are considered to be virtually parallel rays in the same light flux, the scanning and imaging optical system is used to (The position where a typical light beam is imaged))
When the position of the surface to be scanned is matched with the focus position, the light spots that should have the same image height are displaced from each other in the main scanning direction,
When writing is performed by the multi-beam scanning method with such a plurality of light spots, “dot position shift” occurs for each light spot.

When the scanning image forming optical system has a function of forming an image of a divergent light beam, each divergent light beam forms an image on the surface to be scanned, but the chief ray of the light beam traveling to the same image height is as described above. Similarly, since they are parallel to each other when viewed in the sub-scanning direction, the principal rays intersect each other on the deflecting means side with respect to the surface to be scanned due to the action of the scanning optical system. The spots are deviated from each other in the main scanning direction, and when writing is performed by the multi-beam scanning method with such a plurality of light spots, “dot position deviation” occurs for each light spot.

Such a dot position shift occurs on the writing start side and the writing end side as well, but since the angles of the deflecting and reflecting surfaces on the writing start side and the writing end side are different, it is possible to move to the writing start side. The above "parallel displacement of parallel chief rays" of the traveling light flux
Is different from the shift amount of the principal ray of the light flux heading toward the writing end side. Therefore, the writing width (scanning width) of each light beam differs from each other, and a deviation occurs between the light beams.

If the arrangement direction of the light emitting points of the semiconductor laser array is matched with the sub-scanning direction, the dot position deviation and the writing width deviation do not occur, but considering the existence of an assembly error and the like, the arrangement of the light emitting points is arranged. It is difficult to adjust the direction so as to completely match the sub-scanning direction.

As described above, when the light flux after passing through the coupling optical system is converted into a divergent light flux, due to the dot position shift in the main scanning direction and the writing width of each light flux, density unevenness, vertical line fluctuation, etc. Causes image deterioration.

[0015]

SUMMARY OF THE INVENTION In optical scanning of a multi-beam scanning system, the present invention diverges the luminous flux from each light emitting point of a light source by a coupling optical system so as to be advantageous for stabilizing the scanning line pitch. By converting into a light flux, the lateral magnification m of the optical system between the light source and the surface to be scanned in the sub-scanning direction is reduced, and the dot position deviation in the main scanning direction and the writing width between the light fluxes are reduced. The problem is to effectively reduce image deterioration such as density unevenness and vertical line fluctuation due to deviation.

[0016]

An optical scanning device according to the present invention is a "multi-beam scanning optical scanning device", which comprises a light source, a coupling optical system, a deflecting means, a scanning imaging optical system, It has a first optical system and an aperture stop.

A "light source" is a plurality of light emitting points (point-like light emitting sources)
For example, a semiconductor laser array can be preferably used, and a method of combining light fluxes from a plurality of semiconductor lasers using a combining prism can be used.

The "coupling optical system" is an optical system for coupling the light flux from each light emitting point of the light source, and can be configured as a lens system or a mirror system, or a combined system in which a lens and a mirror are combined.

The "deflecting means" is a means for simultaneously deflecting the respective light beams emitted from the coupling optical system, and swings the polygon mirror, the rotating single-sided mirror, the rotating two-sided mirror, and the deflective reflecting surface. A galvanometer mirror or the like can be used.

The "scanning and imaging optical system" is an optical system that guides each light beam deflected by the deflecting means onto the surface to be scanned and condenses each as a light spot on the surface to be scanned.

The "aperture stop" is provided between the coupling optical system and the deflecting means and performs "beam shaping" for setting the spot diameter of each light spot on the surface to be scanned.

The "first optical system" is arranged between the coupling optical system and the deflecting means, and is composed of at least one lens.

The feature of the present invention is that "the coupling optical system converts the light flux from each light emitting point of the light source into a divergent light flux, and the first optical system is positive in the main scanning direction. It has a refractive power ”(Claim 1).

When the first optical system "has a positive refracting power in the main scanning direction", the principal ray of each light beam converted into a divergent light beam by the coupling lens "mainly scans toward the light deflecting means. Even if the light beams have an angle with respect to each other, the spread of the light beams due to the above angle is reduced by the action of the "positive refractive power in the main scanning direction" of the first optical system.

Therefore, the "deviation in the main scanning direction" of each light beam at the position of the deflecting / reflecting surface of the deflecting means also becomes small, and the dot position deviation and the writing width deviation due to this deviation can be reduced.

A plurality of light emitting points in the light source of the optical scanning device according to claim 1 can be “separatedly arranged in the main scanning direction and the sub scanning direction” (claim 2). For example, the case where the arrangement direction of the light emitting points of the semiconductor laser array is tilted in the sub-scanning direction corresponds to this case.

In the optical scanning device according to the first or second aspect, the positive refracting power of the first optical system in the main scanning direction is "the coupled divergent light beams are substantially parallel light beams in the main scanning direction. It is preferably set to “do” (Claim 3). With this arrangement, by using a scanning image forming optical system having the function of "forming an image of a parallel light beam in the main scanning direction", it is possible to effectively reduce or prevent the occurrence of dot position deviation and writing width deviation. .

The aperture stop in the optical scanning device according to claim 1 or 2 or 3 comprises a coupling optical system and a first optical system.
Between the optical system and the optical element closest to the deflecting means of the coupling optical system to the aperture stop: L1, and the distance from the aperture stop to the optical element closest to the light source of the first optical system: L2. , Condition: It is preferable to arrange at a position satisfying L1 <L2 (claim 4). The significance of this condition will be described later.

The optical scanning device according to any one of claims 1 to 4 can have "a plurality of pairs of a light source and a coupling optical system" (claim 5). In this case, each light source has a plurality of light emitting points.

In the optical scanning device according to the fifth aspect, it is preferable that the light flux from each pair of the light source and the coupling optical system "crosses in the main scanning direction" in the vicinity of the deflecting / reflecting surface of the deflecting means. 6).

The image forming apparatus of the present invention is an "image forming apparatus for forming an image by performing optical scanning on a photosensitive image carrier", and an optical scanning device for performing optical scanning of a multi-beam scanning system on the image carrier. The optical scanning device according to any one of claims 1 to 6 is provided (claim 7).

Various types of "photosensitive image carrier" can be used. For example, a "silver salt film" can be used as the image carrier. In this case, a latent image is formed by writing by optical scanning, and this latent image can be visualized by processing by a normal silver salt photographic process. Such an image forming apparatus can be implemented as an "optical plate making apparatus" or an "optical drawing apparatus" for drawing a CT scan image or the like.

As the photosensitive image carrier, it is also possible to use a "coloring medium (positive photographic paper) which develops color due to the heat energy of the light spot during optical scanning". A visible image can be formed on.

As the photosensitive image bearing member, a "photoconductive photosensitive member" can be used. As the photoconductive photoconductor, a sheet-like one such as zinc oxide paper can be used, or a selenium photoconductor, an organic photo-semiconductor or the like "a drum-shaped or belt-shaped repeatedly used one" can be used. You can also

When a photoconductive photoconductor is used as an image carrier, an electrostatic latent image is formed by uniform charging of the photoconductor and optical scanning of a multi-beam scanning system by an optical scanning device. The electrostatic latent image is visualized as a toner image by development. The toner image is directly fixed on the photoconductor when the photoconductor is in the form of a sheet such as zinc oxide paper, and the transfer paper or the OHP sheet when the photoconductor is reusable. It is transferred and fixed on a sheet-shaped recording medium such as (plastic sheet for overhead projector).

The transfer of the toner image from the photoconductive photoconductor to the sheet-shaped recording medium may be carried out directly from the photoconductor to the sheet-shaped recording medium (direct transfer system), or once the intermediate image is transferred from the photoconductor. After transferring to an intermediate transfer medium such as a transfer belt,
You may make it transfer from this intermediate transfer medium to a sheet-shaped recording medium (intermediate transfer system). Such an image forming apparatus can be implemented as an optical printer, an optical plotter, a digital copying machine, a facsimile machine, or the like.

In the image forming apparatus of the present invention, a plurality of the photoconductors are arranged along the conveying path of the sheet-shaped recording medium, and an electrostatic latent image is formed for each photoconductor by using a plurality of optical scanning devices. Then, the toner image obtained by visualizing these can be transferred and fixed on the same sheet-shaped recording medium, and can be embodied as a “tandem image forming apparatus” that synthetically obtains a color image or a multicolor image.

[0038]

BEST MODE FOR CARRYING OUT THE INVENTION Embodiments will be described below.

In FIG. 1A, reference numeral 1 is a semiconductor laser array as a "light source", reference numeral 2 is a coupling lens as a "coupling optical system", reference numeral 3 is an "aperture stop", and reference numeral 4 is a "first". 1 optical system ", reference numeral 5 is a mirror, reference numeral 6 is soundproof glass, reference numeral 7 is a polygon mirror as" deflection means ", reference numerals 8 and 9 are scanning lenses, reference numeral 10 is a mirror, reference numeral 10 is a synchronization detection means, reference numeral 12 Indicates the surface to be scanned.

The scanning lenses 8 and 9 constitute a "scanning / imaging optical system". A plurality of light fluxes emitted from the semiconductor laser array 1 having a plurality of light emission points (point-shaped light emission sources) are converted into divergent light fluxes by the coupling lens 2. Each luminous flux (divergent luminous flux) that has passed through the coupling lens 2 is
Aperture stop 3 for obtaining a desired spot diameter on the surface to be scanned
“Beam shaped” after passing through, and is incident on the first optical system 4. The first optical system 4 is composed of one lens and has a positive refractive power in both the main scanning direction and the sub scanning direction.

Each light beam incident on the first optical system 4 is converted into a substantially parallel light beam in the main scanning direction by the positive refracting power of the first optical system 4 in the main scanning direction, and has a positive refracting power in the sub scanning direction. Each of them is focused into a light beam in the sub-scanning direction, reflected by the mirror 5, and then formed as a "long line image in the main scanning direction" in the vicinity of the deflecting reflection surface of the polygon mirror 7.

These line images are separated in the sub-scanning direction and are offset from each other in the main scanning direction.

When the polygon mirror 7 rotates at a constant speed, the light beams reflected by the deflecting / reflecting surface are deflected at a constant angular velocity while passing through the scanning lenses 8 and 9 forming a scanning image forming optical system.
The light is guided onto the surface to be scanned 12 and is condensed as a plurality of light spots divided into sub-scanning directions, and a plurality of scanning lines on the surface to be scanned 12 are simultaneously optically scanned (multi-beam scanning).

The scanning and image forming optical system is an fθ lens, which has the function of "forming an image of a parallel light beam" in the main scanning direction, and the deflection reflection surface position of the polygon mirror 7 and the surface to be scanned in the sub scanning direction. It has the function of establishing a "geometrically-optically substantially conjugate relationship".

Prior to the optical scanning of the effective writing width, at least one of the deflected light beams is guided by the mirror 10 to the synchronization detecting means (photodetector) 11, and the synchronization detecting means 11 is detected.
"Write" is started after a certain time has elapsed after detection by. At this time, since it is necessary to match the writing start positions of a plurality of light fluxes, it is preferable that the detected deflected light fluxes are focused on the synchronization detection means 11 in the main scanning direction. The light receiving surface of the detecting means 11 is arranged at the “equivalent position” with the surface to be scanned 12.

The surface 12 to be scanned is actually the photosensitive surface of a "photosensitive image bearing member" such as a photoconductive photosensitive member.

As described above, the optical scanning device shown in FIG. 1A is a multi-beam scanning type optical scanning device, and includes a light source 1 having a plurality of light emitting points, and a light flux from each light emitting point of the light source. A coupling optical system 2 for coupling the optical system, a deflecting means 7 for simultaneously deflecting the light beams emitted from the coupling optical system, and a light beam deflected by the deflecting means for guiding the light beams onto the surface 12 to be scanned. , A first optical system which is provided between the scanning and imaging optical systems 8 and 9 for converging as a light spot on the surface to be scanned and the coupling optical system 2 and the deflecting means 7 and which is composed of at least one lens. It has a system 4 and an aperture stop 3 arranged between the coupling optical system 2 and the deflecting means 7, and the coupling optical system 2 transforms the luminous flux from each light emitting point of the light source 1 into a divergent luminous flux. It is to convert, the first optical system 4 Having positive refractive power in the scanning direction (claim 1).

The light flux from each light emitting point is made into a divergent light flux by the coupling lens 2 and the first optical system 4 has a positive refracting power in the main scanning direction (claim 1). Each light beam after passing can be converted from a divergent light beam into a substantially parallel light beam in the main scanning direction. Further, since the scanning image forming optical system has an "action of forming an image of a parallel light beam" in the main scanning direction, the focus position of the deflected light beam (image forming point of the substantially parallel light beam)
And the positions where the principal rays of the respective light beams traveling toward the same image height in the main scanning direction intersect each other and the surface to be scanned is aligned with the focus position, the occurrence of dot position deviation in the main scanning direction is suppressed. You can

In addition, "the angle of the deflecting / reflecting surface of the deflecting means is different" between the image height on the writing start side and the image height on the writing end side, and the "principal ray" between the chief rays of each light beam heading toward these image heights. Even if the "deviation amount in the scanning direction" is different between the writing start side and the writing end side, the image heights of the respective light fluxes are substantially the same in the main scanning direction, so that the deviation of the writing width can be eliminated and the density unevenness can be eliminated. The occurrence of vertical line fluctuations can be reduced.

The scanning and imaging optical system also has a function of making the position of the deflecting and reflecting surface of the polygon mirror 7 and the surface to be scanned "geometrically optically substantially conjugate" in the sub-scanning direction.
The surface tilt in the polygon mirror 7 is effectively corrected.

In the embodiment of FIG. 1 (A), since each light flux transmitted through the coupling lens 2 is divergent, as compared with the case where the coupling lens is used as a collimator lens to convert it into a parallel light flux. As a result, the following effects can be obtained.

That is, firstly, in order to obtain the same spot diameter as in the above-mentioned embodiment by making the light flux into a parallel light flux by the coupling optical system, the light flux width: ω shown in FIG. 2 is made the same. There is a need. Compared with the case where the light flux emitted from the light emitting point P1 is converted into a parallel light flux by the coupling optical system CL1 (upper diagram in FIG. 2), the light is converted into a divergent light beam (lower diagram in FIG. 2), the coupling optics is used. The effective diameter of the system CL can be made smaller, and as a result, the lens outer diameter of the coupling optical system CL can be made smaller, so that the size and cost of the light source device can be reduced, and the "effective wavefront aberration" can be made by reducing the effective diameter. Therefore, it becomes possible to improve the optical performance. In FIG. 2, the first optical system 4A is a cylindrical lens.

Secondly, when the light flux emitted from the semiconductor laser array is converted into a parallel light flux by the coupling optical system,
As shown in FIG. 3A, the “ghost light reflected by the aperture stop 3” is condensed by the coupling lens 2 and returns to the emission point, so that the intensity of the emitted light may become unstable. If it becomes unstable, uneven density may occur.

As in the present invention, when the action of the coupling optical system is a divergent action, "the ghost light reflected by the aperture stop 3 is condensed by the coupling lens 2 as shown in FIG. 3B. However, since it does not return to the light emitting point ”, it is possible to obtain a stable radiant light intensity and suppress the occurrence of the uneven density.

Third, the "magnification in the sub-scanning direction" of the optical system between the light source and the surface to be scanned can be lowered, and the light emitting point position ( It is possible to reduce the deterioration of the scanning line pitch on the surface to be scanned due to the change of the light emitting point interval).

In the embodiment shown in FIG. 1A, the light source 1
A "semiconductor laser array" is used as. In general,
The distance between the light emitting points of the semiconductor laser array can be shortened to only about several tens of μm in order to eliminate the influence of thermal crosstalk. On the other hand, it is necessary to set the scanning line pitch on the surface to be scanned 12 to a "value corresponding to the pixel density".

The light emitting point interval in the sub-scanning direction of the light source is "P1s", the scanning line pitch on the surface to be scanned is "Ps",
When the “lateral magnification in the sub-scanning direction” of the optical system between the light source and the surface to be scanned is “m”, the scanning line pitch Ps is “Ps = m · P”.
1s ”. Therefore, an interval that can correspond to a high scanning line pitch (21.2 for 1200 dpi)
In order to obtain (μm), there is no choice but to reduce m or P1s.

In the present invention, the function of the coupling optical system is "the function of converting the light beam from the light emitting point into the divergent light beam" to reduce "m", but "m" can be reduced without limitation. Do not mean. As the "m" becomes smaller, the "aperture diameter in the sub-scanning direction" of the aperture stop becomes relatively smaller than the divergence of the light flux, and the light utilization efficiency becomes low, and a sufficient amount of light on the surface to be scanned is obtained. Will not be obtained.

There is the above-mentioned limit in reducing P1s by reducing the light emitting point interval of the semiconductor laser array.

Therefore, in order to reduce the distance between the light emitting points of the semiconductor laser array in the sub-scanning direction, as shown in FIG. 1B, the plurality of light emitting points Ch1 to Ch4 are sub-scanned in a plane perpendicular to the optical axis. The tilt angle is θ with respect to the direction (claim 2), and the light emitting point interval is 1 in the sub-scanning direction as “l · cos θ”.
It is better to compress like.

As in the embodiment described above,
The positive refracting power of the first optical system 4 in the main scanning direction is set so as to "make each coupled divergent light beam into a substantially parallel light beam in the main scanning direction" (claim 3).
By combining with a scanning image forming optical system that has the function of forming an image of a parallel light beam, as described above, it is possible to suppress the occurrence of dot position deviation in the main scanning direction, reduce the deviation of the writing width, and reduce the density unevenness. It is possible to reduce vertical line fluctuations.

In the embodiment described with reference to FIG. 1A, the aperture stop 3 is also arranged between the coupling optical system 2 and the first optical system 4, but the aperture stop 3 is a cup. Distance from the optical element closest to the deflecting means of the coupling optical system 2 to the aperture stop 3, which is provided close to the ring optical system 2, is L1, and the optical element closest to the light source of the first optical system 4 from the aperture stop 3 The distance to: L2 satisfies the condition: L1 <L2.

FIG. 4A shows a model of the power arrangement and the position of the aperture stop 3 in the sub-scan section. FIG. 4B shows the distances L1 and L2. As the position of the aperture stop 3, consider the position: A (solid line) and the position: B (broken line) in FIG. Position: In A,
The aperture stop 3 is arranged at a position satisfying L1 <L2,
Position: In B, the position of the aperture stop 3 is L1> L2.

Since the aperture stop 3 functions as "the entrance pupil of the optical system thereafter", a plurality of light beams are projected from the conjugate point (exit pupil) of the aperture stop 3 with "an angle in the sub-scanning direction". Head to the scan plane 12.

At this time, if the conjugate point of the aperture stop 3 is close to the surface 12 to be scanned (“the conjugate point of B” in the figure), the angles of the plurality of light beams toward the surface 12 to be scanned become relatively large, and Due to the influence of the field curvature in the scanning direction, the scanning line pitch largely changes depending on the image height. Also, the depth margin of the spot diameter in the sub-scanning direction is reduced due to the influence of diffraction.

The arrangement position of the aperture stop 3 satisfies the condition L1 <L2.
By satisfying the above condition, the distance between the conjugate point of the aperture stop 3 in the sub-scanning direction and the surface to be scanned increases (“conjugate point of A” in the figure), and the angle formed by the plurality of light beams in the sub-scanning direction is relatively large. And the fluctuation of the scanning line pitch due to the image height is effectively reduced. Moreover, the influence of diffraction can be reduced, and the depth margin of the spot diameter in the sub-scanning direction can be increased.

FIG. 6 is a diagram for explaining another embodiment.

The light emitting point of the current semiconductor laser array is 2
A point or 4 points is normal. For example, in order to optically scan four scanning lines simultaneously by the multi-beam scanning method using a semiconductor laser array having two light emitting points, two semiconductor laser arrays are required.

The optical scanning device of FIG. 6 uses two semiconductor laser arrays each having two light-emitting points and two coupling optical systems corresponding to these semiconductor laser arrays, that is, "a light source and a coupling optical system. As a pair, multiple pairs (two pairs) "
It is an embodiment of the optical scanning device (claim 5) having.

A plurality of (two) light beams emitted from each of the semiconductor laser arrays 11A and 11B are coupled by a coupling optical system (coupling lenses 12A and 12B) to become a divergent light beam, and the aperture stop is used. Each beam is shaped by 13 and has a substantially parallel light beam in the main scanning direction and a focused light beam in the sub scanning direction by the first optical system 14 having a positive refractive power in both the main and sub scanning directions.
The image is formed as a long line image in the main scanning direction at the position of the deflecting / reflecting surface of the polygon mirror 17 which is the deflecting means.

The light beam reflected by the polygon mirror 17 is deflected at a constant angular velocity as the polygon mirror 17 rotates at a constant speed, and the scanning lenses 18-1 and 18-that form a scanning image forming optical system.
After passing through Nos. 2 and 19, the optical path is bent by the mirror 20, and the light is guided to the photosensitive surface of the photoconductive photoconductor 21 that is the actual surface to be scanned, forming four light spots separated in the sub-scanning direction. Then, the four scanning lines are optically scanned at the same time.

Also in this embodiment, the "scan imaging optical system" has the "action of forming an image of a parallel light beam" in the main scanning direction.
And has a function of making the position of the deflective reflection surface of the polygon mirror 17 and the position of the scanned surface in the sub-scanning direction "geometrically-optically substantially conjugate relationship".

Therefore, also in this embodiment, the deviation of the dot position and the deviation of the writing width can be effectively reduced, and the vertical line fluctuation and the density unevenness can be effectively suppressed.

As shown in FIG. 6, in this embodiment, the light beams emitted from the semiconductor laser arrays 11A and 11B "cross" in the main scanning direction in the vicinity of the deflective reflection surface of the polygon mirror 17. That is, the light sources 11A and 11
The light beams from each pair of B and the coupling optical systems 12A and 12B intersect in the main scanning direction in the vicinity of the deflecting reflection surface of the deflecting means 17 (claim 5). The technical significance of doing so will be described with reference to FIG.

In FIGS. 5A and 5B, reference numeral D
₁ represents a semiconductor laser one of the light beam irradiated from the array 11A but when reaching the image height Q on the scan surface 21, the state position of the deflection reflection surfaces of the polygon mirror 17, the symbol D
Reference numeral ₂ represents the position of the deflecting / reflecting surface when one of the light beams emitted from the semiconductor laser 11B reaches the image height Q.

When each of the luminous fluxes (the principal rays of the luminous flux) is incident on the deflecting / reflecting surface of the polygon mirror 17, a certain angle: Δα
This angle: Δα corresponds to a time delay (D ₁ and D _{1) in the} position of the deflective reflection surface for reaching the image height Q.
₍ Time to rotate the angle formed by ₂ ) occurs.

In FIG. 5B, the respective light beams are incident on the deflective reflecting surface while being distant from each other in the main scanning direction, so that the two light beams reach the image height Q through "a considerably different optical path". . On the other hand, in FIG. 5B, since the respective light fluxes intersect in the main scanning direction in the vicinity of the deflection reflection surface, they reach the image height Q through substantially the same optical path.

As shown in FIG. 5B, each light beam reaching the same image height Q is converted into each optical element 18-1, 1 of the scanning image forming optical system.
When passing through different positions 8-2 and 19, "different optical action" is received by the scanning and imaging optical system, and therefore, "aberration of aberrations and the like of two light fluxes reaching the same image height Q in the main scanning direction on the surface to be scanned". The optical characteristics are different, and in particular, the influence of the scanning line pitch on the variation between image heights becomes very large.

In the case of the present invention (FIG. 5 (a)),
If the light beams from the semiconductor laser arrays 11A and 11B (main rays thereof) are made to intersect in the main scanning direction in the vicinity of the deflecting reflection surface of the polygon mirror 17, the respective light beams are "same in the main scanning direction on the surface to be scanned." When the image is formed at the image height Q, each light beam is reflected by each optical element 18-1, 18-2, 19 of the scanning image forming optical system.
It passes through almost the same position, and "optical characteristics such as aberration"
Is also the same, and the fluctuation of the scanning line pitch between image heights can be effectively reduced.

Further, the "variation of the writing position in the main scanning direction" between the light fluxes due to the "variation of each component" on the image plane 21 side of the polygon mirror 17 is approximately the same for all light fluxes,
The deviation of the writing position between the light beams in the main scanning direction can be suppressed.

Further, all the light fluxes that form an image at the same image height pass through the “substantially the same position in the main scanning direction” of the scanning and imaging optical system, so that the aberrations of the respective lenses constituting the scanning and imaging optical system are The influence can be suppressed to a small level, and the image-forming position in the main scanning direction can be accurately matched with each light beam. After synchronization detection, "a delay time (time from detection to writing start is common to each detected light beam. Even if ") is set", the positional deviation in the main scanning direction at the image height at the beginning of writing can be suppressed. Further, by making it as shown in FIG. 5A, the radius of the inscribed circle of the polygon mirror 17 can be minimized.

[0082]

EXAMPLES Specific examples of the embodiment shown in FIG. 1 will be given below. Pixel density: 1200 dpi Light source: Semiconductor laser array Number of emission points: 4 Adjacent emission point spacing: 14 μm Emission wavelength: 780 nm Inclination angle with respect to sub-scanning direction: θ = 56.5 degrees (see FIG. 1 (b)) Optical system "Rm: paraxial radius of curvature in the main scanning direction, Rs: paraxial radius of curvature in the sub scanning direction, N: refractive index at the wavelength used (780 nm), X: distance in the optical axis direction surface number Rm (mm) Rs (mm) X (mm) N Remark Light source--1.05-Semiconductor laser array 1 ∞ ∞ 0.3 1.511 Cover glass 2 ∞ ∞ 22.3--3 ∞ ∞ 4.5 1.685 Coupling lens 4 * -18.5 -18.5 1.0--5 ∞ ∞ 70.1-Aperture stop 6 ∞ 36.1 3.0 1.511 First optical system (lens) 7 -500 ∞ 70.1--8----Deflection surface The surface marked with * is a "coaxial aspherical surface". Although not shown, the wavefront aberration of each light beam emitted from the coupling lens is well corrected.

The first optical system has the "action of converting the transmitted light flux into a parallel light flux" in the main scanning direction. The polygon mirror, which is the deflecting means, has an inscribed circle radius of 18 mm and a deflective reflecting surface of 6
The angle between the optical axis of the scanning imaging optical system and the optical axis of the coupling lens is 60 degrees. "Optical system after deflecting means" Surface number Rm (mm) Rs (mm) X (mm) N Remark 0 ∞ ∞ 52.6 − Deflector 1 * -312.6 -312.6 31.4 1.511 Scan lens 2 * -83.0 -83.0 109.4 ― − 3 ** -500 -47.7 -47.7 1.685 Scanning lens 4 -1000 -23.38 143.6 − − 5 − − − − Scanned surface The surface marked * is the distance in the optical axis direction: X, the distance in the optical axis orthogonal direction. , Y, conic constant: K, and high-order aspherical surface coefficients: A, B, C, D ... Are expressed by the following equation (1).

X = (Y ² ) / R / [1 + √ {1− (1 + K) (Y / R) ² }} + A · Y ⁴ + B · Y ⁶ + C · Y ⁸ + D · Y ¹⁰ + ·・ (1) For surface number 1, K = 2.667, A = 1.79E-07, B = -1.08E-12, C = -3.18E-14, D =
3.74E-18 Surface number 2 is K = 0.02, A = 2.50E-07, B = 9.61E-12, C = 4.54E-15, D = -3.0
The shape is specified by 3E-18.

The surface marked with ** has a non-arcuate shape in the main scanning direction, and the radius of curvature in the sub scanning direction continuously changes depending on the lens height Y in the main scanning direction.

The shape of surface number 3 in the main scanning direction is as described in (1) above.
In the formula, Y is expressed as the coordinate in the main scanning direction, K = -71.73, A = 4.33E-08, B = -5.97E-13, C = -1.28E-16
, D = 5.73E-21.

The state of "change in curvature in the sub-scanning direction" of the surface of surface number 3 is the distance Y from the optical axis in the main scanning direction: Y
"The radius of curvature in the virtual plane cross section orthogonal to the main scanning direction: Rs (Y)", which is a variable, is expressed by the polynomial: Rs (Y) = Rs (0) + Σbj · Y ^j (j = 1, 2, 3, ...) (2), the change of the curvature in the sub-scanning direction in the main scanning direction is symmetric with respect to the optical axis, and each constant of the above polynomial is as follows.

Rs (0) =-47.7, b2 = 1.60E-03, b4 = -2.32E-0
7, b6 = 1.60E-11, b8 = -5.61E-16, b10 = 2.18E-20, b12 =
-1.25E-24 In this embodiment, a soundproof glass (refractive index: 1.511) having a thickness of 1.9 mm is arranged so as to be inclined by 8 degrees in the deflecting surface. The divergence angle of the semiconductor laser array is half value, θn = 31 degrees in the direction orthogonal to the active layer, and θp in the parallel direction.
= 9 degrees.

The scanning image forming optical system composed of two scanning lenses has an "action for forming an image of a parallel light beam in the main scanning direction". The aperture stop is on the divergent beam and L1
Since it is arranged at a position that satisfies (= 1.0 mm) <L2 (= 70.1 mm), a “sufficient image plane depth width” of the spot diameter can be obtained.

When the aperture diameter of the aperture stop was 3.6 mm (main scanning direction) × 1.26 mm (sub scanning direction), a stable spot diameter was obtained at each image height as follows.

[0091] When L1 = 1 mm, the conjugate point of the aperture stop in the sub-scanning direction is -80 mm from the surface to be scanned, and L1 = 6
It is farther from the surface to be scanned by 49 mm than when it is 0 mm, and it is possible to make the scanning line intervals of a plurality of beams uniform and to expand the sub-scanning beam spot diameter depth range.

[0092] described above in Example, since the coupling lens is the coupling optical system is a "plano-convex", the focal length: f is the refractive index: n = 1.685, the object-side curvature radius: r ₁ = ∞, radius of curvature on image side: r ₂ = -1
From _8.5, f = -R 2 /(n-1)=27.00mm
Is. On the other hand, the optical distance from the light source to the coupling lens is 23.80, which is shorter than the focal length: f. Therefore, the light flux transmitted through the coupling lens has a weak divergence while suppressing the divergence angle emitted from the light emitting point. It becomes sex.

In the above-described embodiment, the function of the coupling lens is to "make the transmitted light flux weakly divergent",
The lateral magnification m of the optical system between the light source and the surface to be scanned in the sub-scanning direction is 2.8, and the action of the coupling lens serves as a collimating lens, and the refracting power of the first optical system in the main scanning direction is 0. Then, the lateral magnification can be reduced by 0.1 as compared with the same lateral magnification of 2.9, and the scanning line pitch can be made “more stable” with time and environmental changes.

The first optical system converts each light beam from the light source side into a parallel light beam in the main scanning direction, and the scanning imaging optical system has an action of forming a parallel light beam in the main scanning direction. There was no dot position deviation or writing width deviation, and it was possible to prevent density unevenness and vertical line fluctuation.

[0095]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS FIG. 7 shows a first embodiment of an image forming apparatus.
The morphology is shown. This image forming apparatus is a laser printer. The laser printer 100 has a “photoconductive photosensitive member formed in a cylindrical shape” as a photosensitive image carrier 111. Around the image carrier 111, a charging roller 112 as a charging unit, a developing device 113, a transfer roller 114,
A cleaning device 115 is provided. A "corona charger" can also be used as the charging means.

Further, an optical scanning device 117 for performing optical scanning of the multi-beam scanning system by the laser beam LB is provided, and "exposure by optical writing" is performed between the charging roller 112 and the developing device 113. There is.

In FIG. 7, reference numeral 116 is a fixing device, reference numeral 118 is a cassette, reference numeral 119 is a registration roller pair,
Reference numeral 120 is a paper feed roller, reference numeral 121 is a transport path, and reference numeral 12
Reference numeral 2 denotes a pair of discharge rollers, reference numeral 123 denotes a tray, and reference numeral P denotes a transfer sheet as a “sheet-shaped recording medium”.

When an image is formed, the image carrier 111, which is a photoconductive photoconductor, is rotated at a constant speed in the clockwise direction, its surface is uniformly charged by the charging roller 112, and the laser beam LB of the optical scanning device 117 is used. An electrostatic latent image is formed by being exposed by the "multi-beam scanning optical writing".
The formed electrostatic latent image is a so-called "negative latent image", and the image portion is exposed.

This electrostatic latent image is reversely developed by the developing device 113, and a toner image is formed on the image carrier 111.

The cassette 118 containing the transfer paper P can be attached to and detached from the main body of the image forming apparatus 100. In the mounted state as shown in the figure, the uppermost one of the stored transfer papers P is the paper feed roller 120. The transfer paper P that has been fed by the above is caught by the registration roller pair 119 at the leading end thereof.

The registration roller pair 119 is the image carrier 11
The transfer paper P is sent to the transfer portion at the same timing as the toner image on the transfer position 1 moves to the transfer position. The transferred transfer paper P is superposed on the toner image at the transfer portion, and the toner image is electrostatically transferred by the action of the transfer roller 114.

The transfer paper P on which the toner image is transferred is sent to the fixing device 116, the toner image is fixed on the fixing device 116, passes through the conveying path 121, and the pair of paper discharge rollers 122.
Are discharged onto the tray 123. The surface of the image carrier 111 after the toner image is transferred is cleaned by a cleaning device 115 to remove residual toner, paper dust, and the like.

As the optical scanning device 117, the device described in the embodiments with reference to FIGS. 1 and 6 can be used as appropriate.

That is, the image forming apparatus of FIG. 7 is an apparatus for forming an image by optically scanning the photosensitive image carrier 111, and is an optical scanning device for optically scanning the image carrier 111. The optical scanning device according to any one of 1 to 6 can be used (Claim 7).

[0105]

As described above, according to the present invention, a novel optical scanning device and a novel image forming apparatus using the optical scanning device can be realized. The optical scanning device of the present invention is of a multi-beam scanning type, but an optical system between the light source and the surface to be scanned is formed by converting the light flux from each light emitting point of the light source into a divergent light flux by the coupling optical system. In addition, it is possible to reduce the magnification in the sub-scanning direction, stabilize the scanning line pitch against changes over time and environmental changes, and effectively suppress or prevent dot position deviation and writing width deviation, and to eliminate density unevenness. It is possible to effectively suppress or prevent the occurrence of vertical line fluctuations.

Further, the effective diameter of the coupling optical system can be reduced, the light source device can be downsized and the cost can be reduced, and the small effective diameter can improve the deterioration of the wavefront aberration and improve the optical quality. Performance can be realized. Further, it is possible to prevent the ghost light reflected by the aperture stop from being condensed and return to the light emitting point, obtain a stable light emission intensity, and suppress the occurrence of density unevenness.

Therefore, the image forming apparatus of the present invention, by using such an optical scanning device, can execute high-speed and good image writing with good multi-beam scanning, and can realize good image formation. As a light source, a semiconductor laser array is illustrated above as a light source having a plurality of light emitting points, but the form of the light source is not limited to this. For example, the light source having a plurality of light emitting points may be combined by a prism or the like.

[Brief description of drawings]

FIG. 1 is a diagram for explaining an embodiment of an optical scanning device.

FIG. 2 is a diagram for explaining an effect of making a light flux from a light source divergent by a coupling optical system.

FIG. 3 is a diagram for explaining another effect of making the light flux from the light source divergent by the coupling optical system.

FIG. 4 is a diagram for explaining the invention described in claim 4;

FIG. 5 is a diagram for explaining the invention described in claim 6;

FIG. 6 is a diagram for explaining another embodiment of the optical scanning device.

FIG. 7 is a diagram for explaining the first embodiment of the image forming apparatus.

[Explanation of symbols]

1 light source 2 Coupling optical system 3 aperture stop 4 First optical system 7 Deflection means 8, 9 Scanning optical system 12 Scanned surface

─────────────────────────────────────────────────── ─── Continued Front Page (51) Int.Cl. ⁷ Identification Code FI Theme Coat (Reference) H04N 1/113 H04N 1/04 104A F Term (Reference) 2C362 AA07 AA14 BA84 BB22 2H045 AA01 BA32 CA03 CB63 DA02 5C051 AA02 CA07 DB22 DB24 DB30 DC04 5C072 AA03 DA02 DA04 DA21 HA02 HA06 HA09 HA13 HB08 XA05

Claims (7)

[Claims] 1. An optical scanning device of a multi-beam scanning system, comprising a light source having a plurality of light emitting points, a coupling optical system for coupling light fluxes from the respective light emitting points of the light source, and the coupling optical system. Deflecting means for simultaneously deflecting the respective light beams emitted from the scanning optical imaging optical system for guiding the respective light beams deflected by the deflecting means onto the surface to be scanned and converging them as light spots on the surface to be scanned. A system, a first optical system provided between the coupling optical system and the deflecting means, the first optical system including at least one lens, and an aperture stop provided between the coupling optical system and the deflecting means. The coupling optical system converts a light beam from each light emitting point of the light source into a divergent light beam, and the first optical system has a positive refractive power in the main scanning direction. Characteristic light running Apparatus. 2. The optical scanning device according to claim 1, wherein the plurality of light emitting points in the light source are arranged separately in the main scanning direction and the sub scanning direction. 3. The optical scanning device according to claim 1, wherein the positive refracting power of the first optical system in the main scanning direction couples the divergent light fluxes coupled to each other in the main scanning direction. An optical scanning device, characterized in that 4. The optical scanning device according to claim 1, wherein an aperture stop is arranged between the coupling optical system and the first optical system, and the optical element closest to the deflecting means of the coupling optical system. From the aperture stop to the aperture stop: L1, and the distance from the aperture stop to the optical element closest to the light source of the first optical system: L2, the condition: L1 <L2 is satisfied. 5. The optical scanning device according to any one of claims 1 to 4, wherein a plurality of pairs of a light source and a coupling optical system are provided. 6. The optical scanning device according to claim 5, wherein the light flux from each pair of the light source and the coupling optical system intersects in the main scanning direction in the vicinity of the deflection reflection surface of the deflection means. Scanning device. 7. An image forming apparatus for forming an image by performing optical scanning on a photosensitive image carrier, wherein the optical scanning device performs optical scanning of a multi-beam scanning method on the image carrier. 2. An image forming apparatus having the optical scanning device according to any one of 1.