JPH05294005A - Image forming device - Google Patents

Abstract

PURPOSE:To provide a high speed optical scanner or an image forming device which is small in size and low in cost. CONSTITUTION:A plurality of laser beams injected from a semiconductor laser array 21 having a light emitting section arranged in a two-dimensional way are deflected by a rotary polygon mirror 3. On a surface to be scanned, an image-formation spot is arranged for scanning line in accordance with given regulation so as to scan a plurality of scanning lines simultaneously. Thus, an image forming device having simple structure of low cost, small in size, and having high reliability can be obtained.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an image forming apparatus for forming a latent image on an image bearing member by using a laser beam scanning device for scanning a plurality of laser beams, and particularly to a scanning line on a surface to be scanned. Regarding placement of image spots.

[0002]

2. Description of the Related Art Optical scanning devices, particularly laser scanning devices using semiconductor lasers, are widely used in image forming devices such as image display devices and image recording devices because of their simple structure, high speed, and high resolution. Has been. Among them, it is most suitable as an exposure device for electrophotographic printers, and many products are marketed as laser beam printers.

However, in recent years, there has been an increasing demand for higher speeds and higher resolutions of image forming apparatuses, and it is desired to improve the scanning speed. A high-speed deflecting device is required to realize high-speed scanning, but for example, when a rotating polygon mirror is used as the deflecting device, there is an upper limit to the increase in the number of rotations.
As one of the solutions, for example, JP-A-51-1007
As shown in 42, there is proposed an image forming apparatus using a so-called multi-beam scanning device which scans a plurality of laser beams which can be independently modulated and simultaneously scans a plurality of scanning lines in one scanning.

FIG. 11 shows an example of a conventional optical scanning device using such a plurality of laser beams. As a light source that emits multiple laser beams, a gas laser was time-divided with a modulator, a beam splitter was used, and a method using multiple light sources was previously devised. Therefore, the semiconductor laser array 1 is often used. This is one in which multiple diodes are formed on a single (that is, monolithic) element substrate, and the lighting of the semiconductor laser is independently controlled by individually controlling the current flowing to each diode. Is.

Here, the structure of the semiconductor laser array will be described in detail. FIG. 12 shows a structural diagram of a semiconductor laser array that has been conventionally put to practical use. In FIG. 12, the light emitted from the bonding layer 51 is reflected by the reflecting surfaces 52 at both ends of the element substrate and reciprocates in the bonding layer 51 to cause laser oscillation. On the other hand, the refractive index or the gain is different in the direction orthogonal to the reciprocating direction of the light, and the light is not reflected and does not oscillate. The reflecting surfaces 52 at both ends generally need to have a high reflectance and are formed by cleaving the semiconductor substrate. One of the reflection surfaces 52 on both sides has a slightly lower reflectance than the other, and the laser beam 53 is emitted. Since the laser beam is emitted from the end surface orthogonal to the semiconductor substrate surface in this way, it is also called an "end surface emitting laser".

In the array-shaped semiconductor laser, a plurality of band-shaped bonding layers are arranged in parallel, and the light emitting portions thereof are also arranged in a line on the end face, and parallel laser beams are emitted. The laser beams emitted from the semiconductor laser array 1 are shaped into parallel laser beams by the collimator lens 2. The directions of these laser beams are periodically and repeatedly deflected by the rotating polygon mirror 3. The deflected laser beam forms an imaging spot 6 on the surface to be scanned by the scanning lens 4.

The image bearing member 5 is placed on the surface to be scanned, the charge on the surface of the portion irradiated with the laser disappears, and the developer is selectively electrostatically attached to the portion for development. .. Further, this developer is also electrostatically transferred to a transfer material (paper in many cases), and finally melted and fixed while applying pressure with a roller or the like.

In a laser beam printer, since an output image forms an image as an output of a computer, it is a digital binary image in most cases, and the image is represented by pixels (dots) arranged at regular vertical and horizontal intervals. It

[0009]

The problems to be solved by the present invention will be presented below by exemplifying simple numerical values in a typical conventional image forming apparatus. In the following description of the image forming method, it is assumed that the surface to be scanned moves substantially at right angles to the scanning line, and the direction of the scanning line is the "main scanning direction",
A direction substantially orthogonal to the main scanning direction within the surface to be scanned, that is, the feeding direction of the surface to be scanned is referred to as a "sub-scanning direction".

Scanning lines by which the respective imaging spots scan the surface to be scanned are substantially parallel to each other. Further, since the light emitting portions of the semiconductor laser array of the light source are arranged in a straight line, the imaging spots on the surface to be scanned are also arranged in a substantially straight line.

The distance between adjacent image-forming spots arranged in a line on the surface to be scanned is such that each laser beam passes through the distance δ between the light-emitting portions corresponding to each image-forming spot on the semiconductor laser array serving as a light source. It is a value obtained by multiplying the imaging magnification M of the optical system. The image forming magnification M is substantially equal to a value obtained by dividing the focal length Fs of the scanning lens 4 by the focal length Fc of the collimator lens 2. In addition, in FIG.
Although it includes a surface tilt correction optical system to be described later, it may be considered by including this in a scanning lens and a collimator lens to consider a focal length.

The interval p between scanning lines is usually equal to the pixel (dot) pitch in the sub-scanning direction. This pixel pitch is expressed by the reciprocal of the resolution of the printer, for example, 300 dpi.
(Dot per inch: 1 inch [25.4m
The number of dots per m] is 84.7 μm.
On the other hand, the focal length fc = 1 of the collimator lens described above
When the focal length fs of the scanning lens is fs = 160 mm, the imaging magnification of the optical system becomes 16 times. In addition, the distance δ between the light emitting portions at the end surface of the element substrate in the semiconductor laser array
Is about 100 μm at the smallest. Therefore, the interval Δ between the image forming spots on the surface to be scanned is 1.6 mm. That is, since the interval between the image forming spots is much larger than the scanning line interval, the following two methods can be considered in order to fill the entire surface to be scanned with scanning lines having a constant pitch.

First, the first method is disclosed in JP-A-54-15825.
As shown in FIG. 1, a straight line formed by connecting the image forming spots is tilted at a constant angle, and scanning is performed while shifting the timing of each scanning line by a constant time. FIG. 13 shows the positional relationship between the imaging spot and the scanning line on the surface to be scanned when scanning is performed with four laser beams. Each imaging spot S1 to S
4 scans adjacent scanning lines with a fixed delay. This scanning line and the imaging spots S1 to S4
As can be seen from the figure, the inclination γ of the straight line formed by is defined by sin γ = p / Δ from the interval Δ of the imaging spot and the scanning line pitch p.
Becomes In the numerical example shown above, γ is about 3.0 °.

However, in an image forming apparatus such as a laser beam printer, unless the pitch error between adjacent scanning lines is suppressed to a certain value or less, it appears in the form of unevenness and deteriorates the image quality. In the case of the example described here, the scan line 4
The light and shade of the image will occur for each book, which will be very unsightly.

Therefore, for example, when it is required that the error of all the scanning line intervals be within 10%, four adjacent scanning lines are drawn in this example, so three times the maximum pitch (that is, 3p) is p. Error of 1/10 or less must be suppressed to 3.3% or less. From the above equation, the error of the angle γ must be suppressed to 3.3% or less, that is, 0.10 ° or less. However, it is not easy to adjust the mounting angle of the semiconductor laser array or the feeding direction of the surface to be scanned to such a small angle error.

Therefore, in practice, the above adjustment is performed to adjust the scanning line pitch to a certain value, and then the feed speed is finely adjusted so that the surface to be scanned is moved by the number of scanning lines to be scanned at one time. It becomes essential to do. However, if the feeding speed can be finely adjusted, the timing of the entire image forming process is affected, and the device design becomes difficult.

Further, there is a difference in the time taken for each of the image forming spots S1 to S4 to pass through a certain point in the main scanning direction (for example, the line A shown in FIG. 9), so that the laser is modulated by the image data. In this case, it is necessary to accurately generate a delay according to the interval of the imaged spot and transfer the data. Therefore, in order to accurately align the positions of the pixels in the main scanning direction, it is necessary to accurately set the delay amount for each scanning line. For example, in FIG. 9, the interval in the main scanning direction of S1 to S4 is 3 p / tan γ. This is represented, and in the above example, it is about 1.598 mm, which corresponds to about 18.8 pixels.
For the same reason as explained in the previous example, the deviation of each dot in the main scanning direction also needs to be suppressed to a certain error or less. Generally, the allowable value of the pixel shift in the main scanning direction is less than the allowable value of the pitch error of the scanning line, but it is desirable to be at least half the pixel pitch or less. In this case, the value is about 43.
Since it is μm, the allowable error of the delay amount is 0.043 / 1.5
98 should be less than about 2.7%. With the current technology, it is possible to electrically set such an accurate delay amount, but inevitably the configuration of the laser drive circuit becomes complicated. Further, it is difficult to set this value in consideration of the variation of the imaging magnification due to the variation of the parameters of the optical system and the dimension of the mechanical portion.

The second method is disclosed in Japanese Patent Laid-Open No. 56-110.
As disclosed in 960, there is a method of arranging image forming spots at right angles to the scanning lines and scanning every fixed number of scanning lines. FIG. 14 shows the positional relationship between the scanning line and the imaging spot by this method. According to this method, since the image forming spots are arranged in a line in the sub-scanning direction, no delay is required between the pixels during scanning, and the laser may be modulated at the same timing. According to the above publication, scanning is performed at intervals of the imaging spots by the number of scanning lines obtained by adding 1 to the value obtained by multiplying the number m of scanning lines drawn simultaneously by an integer k. The scanning order in this method will be described in more detail below.

Since k = 2 in FIG. 14, of the four laser beam imaging spots in the first scan,
The scanning line L1 is scanned by the spot S1. In the next second scanning, the surface to be scanned has moved by four scanning lines, so the scanning line L5 is drawn by the spot S1.
Further, in the third scanning, the scanning line L is formed by the spot S1.
Scan 9. In the next fourth scanning, the spot S1
Scans the scanning line L13, but for the first time, the lasers of the spot S2 are simultaneously modulated and scan the scanning line L4.
The table below shows the correspondence between spots and scanning lines in each scan. In this way, adjacent scan lines on the surface to be scanned are drawn by different scans, and therefore a special device is required for sending out the image data.

[0020]

[Table 1]

Further, applying the above numerical examples, k must be 5 or more in order to secure the interval δ of the light emitting portions of 100 μm or more. Returning again to the example of FIG. 14, although the imaging spots S1 and S4 are separated by 2.286 mm on the surface to be scanned, the drawn scanning lines are adjacent to each other. At this time, if the straight line formed by the image forming spots S1 to S4 is slightly inclined in the direction perpendicular to the scanning line, the pixel positions on the adjacent scanning lines are displaced in the main scanning direction. For example, in the example shown in FIG. 14, when tilted as shown in FIG. 15, the deviations of the spots S1 and S4 occur on the scanning lines L1 and L2 by h. That is, the mounting angle of the semiconductor laser array or the feeding direction of the surface to be scanned must be precisely adjusted in the same manner as in the first method.

Further, since the value of k can take only an integer, the interval Δ of the imaging spot on the surface to be scanned is m × k + 1 times the scanning line pitch p (in the above example, scanning with four beams is performed). Therefore, it is necessary to design the scanning device so that the value of m = 4). However, the imaging magnification M of the optical system is almost unconditionally determined if the divergence angle of the beam emitted from the semiconductor laser array and the size of the imaging spot on the surface to be scanned are determined. The interval δ of the light emitting portions of can take only the value of p × (m × k + 1) / M,
There is no degree of freedom in design, and it is not possible to set the optimal light emitting portion spacing in terms of the device structure. For example, based on the above example, δ
To make the value around 100 μm, when k = 4, δ = 9
When 0.0 μm and k = 5, δ = 111.1 μm, k = 6
In this case, δ = 132.3 μm, and δ = 100 μm or δ = 120 μm cannot be set.

Further, as in the case of the above first method,
If the precision of the interval Δ of the imaged spots is low, a periodical error occurs in the scanning line interval, and the image density unevenness appears. However, when the image forming spots are arranged in the sub-scanning direction in this manner, there is no method for finely adjusting the interval Δ,
As described in the first method, the scanning line pitch can be finely adjusted only by adjusting the feed rate of the surface to be scanned, which makes the device design more difficult.

Both the first method and the second method have been described for the case of writing with four laser beams at the same time, but the above problem becomes more difficult as the number of laser beams increases. It can be easily inferred. Simultaneous scanning of, for example, as many as 10 laser beams is virtually impossible with the current technology as long as the method described here is used.

As described above, when either of the first and second methods described above is used, It is difficult or impossible to finely adjust the spacing between the scanning lines, and it is necessary to finely adjust the feed speed of the surface to be scanned.

2. When the image forming spot interval (Δ) has a distance in the main scanning direction, it is necessary to accurately provide a time delay to the image data for modulating each laser, which complicates the configuration of the laser drive circuit.

3. When the distance between the image forming spots is determined and the image forming magnification of the optical system is determined, there is no freedom in designing the distance between the light emitting portions of the semiconductor laser array.

There are problems such as It is also clear that the above problem becomes more serious as the number of simultaneously scanned beams (ie m) increases.

Therefore, an object of the present invention is to perform scanning with a plurality of laser beams using a semiconductor laser array, without using delicate and precise adjustment and using a laser driving circuit having a simple structure. The purpose is to clarify the positional relationship between the scanning line and the image forming spot, and obtain an image forming apparatus capable of high-speed scanning.

[0030]

An image forming apparatus according to the present invention includes a light source that emits a plurality of laser beams that can be independently modulated from a light emitting section that is two-dimensionally arranged, and each of the plurality of laser beams. By using an optical scanning device having a laser beam deflector that periodically deflects the plurality of laser beams and a scanning optical system that forms an image of the plurality of laser beams on a surface to be scanned. In an image forming apparatus that forms a latent image on an image carrier whose surface moves relative to a direction substantially perpendicular to the scanning direction, the number of the plurality of laser beams is n, and the number of laser beams connected to the image carrier is n. The image spot numbers are S1 to Sn in order from the beginning in the direction orthogonal to the scanning direction, and from the image formation spot S1,
Letting L2 to Ln be distances measured at right angles to the scanning directions to the respective imaging spots S2 to Sn, values D2 to Dn obtained by dividing each of L2 to Ln by the scanning line pitch p are substantially integers. The above problem is achieved by using an optical scanning device in which the remainders M2 to Mn obtained by dividing D2 to Dn by n are different natural numbers less than n.

Further, the light source emits a laser beam having an optical axis substantially vertical to the surface of the element substrate, and each of the light emitting portions is a semiconductor laser array capable of individually controlling its lighting and light amount. That at least two of the imaging spots S1 to Sn are on the same straight line parallel to the feed direction of the image carrier, or scanning of the imaging spots S1 to Sn. The number of positions in the scanning direction is smaller than the number n of the plurality of laser beams, the scanning spots are equally spaced, and the respective imaging spots occupying the same positions in the scanning direction have the same spacing in the sub-scanning direction. The above-mentioned problems can also be achieved by the following.

[0032]

According to the image forming apparatus of the present invention, a plurality of (n) laser beams emitted from the semiconductor laser array are deflected by the deflector, and a spot is formed on the surface to be scanned through the scanning lens. Performs high-speed scanning. The feed amount of the surface to be scanned in the sub-scanning direction per scanning cycle is always n times the scanning line pitch. When arranging the imaging spot with respect to the scanning line, a value obtained by dividing the interval in the sub-scanning direction by the scanning line pitch is set to be an almost integer, and further dividing the integer by the number n of laser beams. As long as the residuals are different from each other, even if the surface to be scanned is sent in the sub-scanning direction, the scanning lines do not overlap but are written at a predetermined pitch. Since the respective image forming spots do not have to be arranged on one straight line, the arrangement of the light sources can be determined in a form closer to the optimum optical coupling magnification required from the image forming characteristics.
Further, it is possible to freely select the arrangement of the light sources so that the maximum value is reduced while the minimum value of the distance between the imaging spots is kept large.

[0033]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS An embodiment of the image forming apparatus according to the present invention will be described below.

FIG. 1 is a schematic view of a portion of an optical scanning device which best shows the features of the image forming apparatus according to the embodiment of the present invention. Here, the plurality of laser beams emitted from the plurality of light emitting portions of the semiconductor laser array 21 are collimated (collimated) by a collimator lens 2 into a laser beam having a predetermined beam diameter. This laser beam is incident on the rotary polygon mirror 3 and is deflected as it rotates. The laser beam that has passed through the scanning lens 4 forms an image on a spot 6 on an image carrier 5 placed on the surface to be scanned. Focusing the laser beam to form an image means that it is at the beam waist position of the Gaussian beam,
The spot has a circular or elliptical shape and a Gaussian distribution-like light intensity distribution with the center at the apex. The structure of the semiconductor laser array 21 will be described later in detail.

A reflection mirror 11 is provided at a position within the deflection range of the laser beam and which is not involved in scanning the surface to be scanned, and the laser beam reflected by the reflection mirror 11 is incident on the photodetector 12. In order to detect the laser beam with the photodetector 12, the photodetector 1 must be detected during the scanning period.
Control is performed so that the laser is turned on immediately before the laser beam is deflected to 2 and turned off before the scanning range necessary for the original scanning. The horizontal synchronization signal output from the photodetector controls the start of laser modulation in accordance with the image data.

The function of the scanning lens 4 is roughly divided into two. The first function is an image forming function of forming an image of the laser beam on the scanning plane at any scanning position. In particular, when the resolution is high and the size of the spot of the laser to be imaged is small, the depth of focus becomes shallow, so the optical field curvature (in this case, including astigmatism) correction function becomes important. The second function is the so-called "fθ function",
It has a function of converting a laser beam deflected at a constant angular velocity by a rotating mirror into a constant linear velocity scanning on the image carrier.

Generally, when a rotary polygon mirror is used for the scanner,
Due to the parallelism error of each mirror surface with respect to the rotation axis (so-called “face tilt”), the imaging point of the reflected laser beam is largely displaced in the sub-scanning direction. In order to avoid this, it is possible to form an image at the same position in the sub-scanning direction by an optical system conventionally called an "tilt correction optical system". This is based on the principle that light rays emitted from one of the optical conjugate points are always focused on the other conjugate point, and the image plane (scanned surface)
It is realized by providing an optical system for making the deflection surface as an optical conjugate point. However, since this conjugate relationship only needs to be established in a plane perpendicular to the scanning plane, it becomes an anamorphic scanning optical system. Specifically, in this embodiment, the toroidal lens 8 in FIG. 1 has this function.

In the case of having such a conjugate type tilt correction optical system, the laser beam is formed as a long line image in the main scanning direction on the deflection surface. For this reason, a cylindrical lens 7 having optical power only in a plane perpendicular to the scanning plane is provided behind the collimator lens to form a line image on the deflection surface.

As described above, the scanning optical system is an anamorphic optical system, but when viewed from the light source to the image plane, the image forming magnifications are almost equal in the direction orthogonal to the scan plane. Therefore, in the following description of the embodiments, it is not particularly distinguished whether or not the imaging magnification is within the scanning plane.

Next, the image forming process will be described.
FIG. 2 is a diagram showing a printing process of the image forming apparatus in the embodiment of the present invention. The process of obtaining a print result on the transfer material 101 is based on a so-called electrophotographic process. As the image carrier 5, in an electrophotographic printer using a semiconductor laser as a light source, an organic photoconductor (O
PC) is often used. The image carrier 5 is first charged by the charger 102 to a constant surface potential, and then the laser beam scanning device 103 performs optical writing, that is, exposure. The laser beam scanning device 103 axially scans the image carrier 5 with a plurality of laser beams 104 whose light intensities are independently modulated according to image information, and generates charges for canceling the surface potential only in the exposed portion. The absolute value of the surface potential of the part becomes small. As a result, a surface potential distribution corresponding to the image, that is, an electrostatic latent image is formed on the image carrier. The electrostatic latent image is developed by the developing device 105 by selectively attaching a developer according to the surface potential. This developer is transferred to the transfer material 1 by the transfer device 106.
01 (usually paper). The transfer material 101 is fixed by heat and pressure by the fixing device 107 and is discharged.

Next, the positional relationship between the imaging spot and the scanning line will be described.

FIG. 3 shows the relationship between the scanning line and the image spot on the surface to be scanned in the optical scanning device used in the embodiment of the image forming apparatus of the present invention. In this embodiment, four laser beams are simultaneously scanned. The scanning lines are scanned at equal intervals with a pitch p. In the figure, the imaging spots S1 to S4 of the laser beam periodically move from left to right at a substantially constant speed. The surface to be scanned moves in the direction shown in the figure at a constant speed. Two-dimensional scanning is realized by the combination of these two movements. Regardless of the positional relationship of the spots to be scanned, the moving speed of the surface to be scanned is 4 scanning line segments per scanning cycle.

Now, the scanning lines are referred to as L1, L2, L3, ... From the front end in the moving direction of the surface to be scanned. In the first scan, the spot S1 scans the scanning line L4 and the spot S2 scans the scanning line L2. During scanning, the laser is turned on / off according to the image data or intensity modulation is performed according to the image density. The start of this modulation or lighting control is performed with reference to the horizontal synchronizing signal obtained from the photodetector 12 already described. At this time, since the spots S3 and S4 do not contribute to scanning, the laser does not operate.

In the next scanning, the scanning line L8 is scanned by the spot S1 and the scanning line L6 is scanned by the spot S2.
However, the spots S3 and S4 still do not participate in scanning. In the next scanning, the scanning line L is formed by the spot S1.
12, the scanning line L10 is scanned by the spot S2. Only at this time, the scanning line L3 is scanned by the spot S3 and the scanning line L1 is scanned by the spot S4.
Two laser beams work simultaneously. After that, by repeating this operation, the entire surface to be scanned is filled up with scanning lines at equal intervals. The following table shows the correspondence between the imaging spot and the scanning line for each scan.

[0045]

[Table 2]

Here, the spacing between the spot S1 and the other three spots S2, S3, and S4 when viewed in the sub-scanning direction is 2p, 9p, and 11p, respectively, when expressed by the scanning line pitch p. The remainders obtained by dividing this by the number of spots (that is, the number of scanning lines simultaneously scanned) 4 are 2, 1 and 3, respectively. As described above, if the distances measured from one spot to another spot in the sub-scanning direction are divided by the scanning line pitch, and the remainders divided by the number of scanning lines simultaneously scanned are natural numbers different from each other, they will always overlap. It is possible to fill the surface to be scanned with evenly spaced scanning lines.

Here, spots S1 and S3 in the sub-scanning direction,
The interval between S2 and S4 is 9p. Further, the interval E in the main scanning direction between the spots S1 and S2 and S3 and S4 can be set arbitrarily. Since the spots S1 and S3 and S2 and S4 are present at the same position in the main scanning direction, the two respectively transfer image data at the same timing, and transfer the image data to S2 and S4 compared to the spots S1 and S3. You can delay the time.

FIG. 4 shows a circuit block diagram relating to the transfer of image data. Image data to be written by scanning is stored in the image memory 61 as binary data for each pixel (dot). In FIG. 4, when the laser beam crosses the photodetector 12, a pulse-shaped horizontal synchronizing signal Hsync is generated.
Is generated, and the pulse is input to the two delay circuits 51 and 52. The delay circuit 51 outputs the delayed signal delayed for a fixed time to the selection circuit 53. If the read start signal is input to the selection circuit, the laser drive circuit 5 outputs the image data of a predetermined scanning line based on the delay signal.
Transfer to 4, 56. Laser drive circuits 54, 55, 5
Reference numerals 6 and 57 denote light emitting portions 41 of the semiconductor laser array 21,
They are connected to 42, 43, and 44, and each independently controls the drive current of the laser so that a predetermined amount of light is emitted at the time of lighting.

The laser beams emitted from the light emitting portions 41, 42, 43 and 44 are spots S1, S2 and S, respectively.
It corresponds to 3 and S4. The delay time generated in the delay circuit 51 is set shorter than the delay time generated in the delay circuit 52 by the time obtained by dividing the distance E in FIG. 3 by the scanning speed, and the spots S1 to S4 are on the surface to be scanned. Then, pixels can be formed at the same position in the main scanning direction.

As already described in the description of the prior art, the distance δ between the light emitting portions of the semiconductor laser array and the distance Δ between the spots are uniquely determined from one to the other depending on the imaging magnification M of the optical system. Therefore, in order to be able to freely select the interval of the light emitting portions, it is desirable that the interval of the spots be as large as possible.
According to the above requirements, considering this embodiment, in the case shown in FIG. 5, the intervals between the spots S1 and S2, S3, S4 are 1p, 10p, 11p when expressed by the scanning line pitch p, and are shown in FIG. In this case, they are 2p, 7p, and 9p, respectively. As in the example given as the problem of the prior art, when it is attempted to set the light emitting portion distance δ to about 100 μm, the distance Δ between the spots S1 and S3 (same for S2 and S4) is 18 p,
It can be set to 19p and 21p, and it is understood that the intervals of the light emitting portions at this time can be selected to be 95.3 μm, 100.5 μm, and 111.1 μm, which are considerably fine.

As described above, the distance between the imaging spots in the sub-scanning direction and the distance between the light emitting portions on the semiconductor laser array are not limited at all, but the values can be selected within a practically sufficient range. On the other hand, the interval between the image forming spots in the main scanning direction is
It can be arbitrarily designed by setting the difference between the delay times of the two delay circuits.

When the image forming spots are arranged in a parallelogram shape as in this embodiment, the accuracy of the scanning line spacing is determined by the imaging magnification of the optical system, the light emitting portion spacing on the semiconductor laser array, and the spot for the scanning line. There are three factors that contribute to the angle of the parallelogram. Of these, the former two are equivalent to the prior art and will not be described here.

The angle of the parallelogram formed by the imaging spots, which is the third factor, will be described below. In order to keep the error of the scanning line pitch p within 10%, as described above in the first method of the problem of the prior art, in this embodiment of the present invention, E.tan in FIG.
γ must be within 10% of p. Since Δ is 1.6 mm when the distance between the light emitting portions is 100 μm, it is desirable to set E to about 1.6 mm. Therefore, the above error of γ may be set within 0.30 °. This is about 1/3 of the value shown in the above-mentioned prior art problem. In other words, the allowable value of the angular error is relaxed by the difference between the fact that three intervals of the imaging spots have to be taken into consideration or only one, as in this embodiment.

Further, in the surface emitting type semiconductor laser array as will be described later, since the divergence angle of the laser beam from the light emitting portion can be made small, the image forming magnification of the entire optical system can be made small, resulting in a more angular error. Is acceptable.

The same applies to the positional accuracy of the image forming spot in the main scanning direction, and the perpendicularity of the straight line formed by S1 and S3 to the scanning line is greatly relaxed. But,
As described above, the positional accuracy of the imaging spot in the main scanning direction is lower than that required in the sub scanning direction, and therefore it is sufficient to manage only the scanning line interval (sub scanning direction).

Since the four spots are arranged in a parallelogram as described above, the four laser beams are close to each other when the cross section perpendicular to the optical axis is taken. It is possible to reduce the effective diameter of the optical components such as the collimator lens, the tilt correction lens, the scanning lens, and the reflecting surface of the deflector. Similarly, the chip size of the semiconductor laser array can be reduced.

Further, when the optical paths of a plurality of laser beams are largely separated from each other in each lens, each aberration of the lens acts on each beam and, for example, the degree of curvature of each scanning line on the surface to be scanned becomes large. Incidentally, adjacent scanning lines may not be parallel, but this does not occur in the present invention because the laser beams are adjacent to each other as described above.

In the above embodiment, four image forming spots are arranged in a parallelogram shape, but it is also possible to arrange three nine image forming spots, for example, as shown in FIG.
In this case, the surface to be scanned is sent in the sub-scanning direction by nine scanning lines. As described above, the accuracy of the array is twice as severe as in the case of four, but as described in the problem of the prior art, it is necessary to arrange nine imaging spots in one row for scanning. Can be said to be sufficiently practical as compared with the fact that it is impossible to be precise.

As another embodiment, FIG. 8 shows an example in which the image forming spots are arranged in a square shape, which is disadvantageous in that all four delay amounts when transferring image data to four lasers are different. However, since the arrangement of the light emitting portions on the semiconductor laser array can be made square, there is an advantage that the directionality of the element substrate of the laser array is eliminated and the manufacturing becomes easy.

Furthermore, in the case of scanning with three laser beams as another embodiment, if the laser beams are arranged as shown in FIG. 9, the spots S2 and S3 may be written with image data at the same timing, and the three spots may be written. Since they can be arranged in an equilateral triangle shape, the intervals of the light emitting parts on the semiconductor laser array can be placed at the same positions, so that even if there is thermal or optical interference between the light emitting parts on the device substrate, Can be even.

In order to realize the embodiment described above, a semiconductor laser array in which light emitting portions are two-dimensionally arranged is essential. Of course, arranging two edge emitting laser arrays as mentioned in the description of the prior art does not change the effect of the present invention, but a monolithic structure in which the light emitting portions are two-dimensionally arranged on one semiconductor chip. Structure is desirable. Such a semiconductor laser array has
It is more preferable to use a so-called surface emitting semiconductor laser. In this surface emitting semiconductor laser, the cross-sectional area of the emitting portion of the laser beam can be made larger than that of the conventional edge emitting semiconductor laser, so that the divergence angle of the laser beam becomes small.

In such a surface emitting semiconductor laser, current and light can be efficiently confined in the laser resonator, so that heat generation per one light emitting portion can be reduced and at the same time, a plurality of light emitting portions are adjacent to each other. Mutual optical,
Electrical and thermal interference can be reduced. Therefore, the distance between the light emitting portions can be made smaller than that of the conventional semiconductor laser.

FIG. 10 is a cross-sectional view of one of the light emitting portions two-dimensionally arranged on the element substrate of this surface emitting type semiconductor laser array. Is formed by stacking several tens of AlGaAs layers of the semiconductor multi-layered reflective film 23, and AlG is formed on each of them.
A clad layer 24 made of aAs, an active layer 25, a clad layer 26, and a contact layer 27 are laminated, and finally a SiO ₂ dielectric multi-layered reflective layer 28 is formed. Also GaAs
Window-shaped electrodes 29 and 30 are formed on the entire back surface of the substrate 22 and around the dielectric multilayer reflection layer on the front surface, and the whole constitutes an optical resonator. The light generated in the active layer reciprocates between the upper and lower reflection layers 27 and 23 in the direction perpendicular to the substrate surface and oscillates, so that the optical axis of the laser beam 31 is substantially perpendicular to the substrate surface.

A buried layer 32 is formed around the optical resonator.
Group II-VI compound semiconductors are embedded. Examples of II-VI group compound semiconductors include Zn and C as group II elements.
d, Hg, O as a group VI element, S, Se, and Te are 2 to
It is desirable that the combination of the four elements and the lattice constant of the compound be matched with the lattice constant of the semiconductor layer including the clad layer 24, the active layer 25, and the clad layer 26. This II
-VI group compound semiconductors have very high electrical resistance,
While efficiently confining the current in the optical resonator,
Since there is a difference in the refractive index from the AlGaAs semiconductor layer forming the optical resonator, light traveling inside the optical resonator at an angle perpendicular to or close to the substrate surface of the device is buried in the buried layer 3
It is totally reflected at the interface with 2 and can be efficiently trapped. Therefore, when such a semiconductor laser is used, laser oscillation starts with a much smaller current as compared with the conventional semiconductor laser. That is, the threshold current is low and the amount of heat loss in the element substrate is small. The diode is formed on the GaAs substrate 22, and the light generated in the active layer 25 is reflected by the reflective layer 2.
The laser beam 31 oscillates back and forth between 3 and 28, and the laser beam 31 is emitted from the reflecting layer 28 having a slightly lower reflectance among the two reflecting layers.
Is emitted perpendicularly to the substrate surface of the device.

The embodiment described above is only one embodiment of the present invention, and even if the constituent elements in the embodiment are replaced with other ones, the same effect is obtained. For example, as the deflecting device, in addition to a rotary polygon mirror, a device such as a galvano mirror or a hologram disk that can periodically deflect the direction of light,
It has the same effect as the above embodiment. Further, the collimator lens, the scanning lens, and the tilt correction optical system are not essential in the configuration of the optical system, and the presence or absence thereof does not affect the effects of the present invention.

The arrangement of the image forming spots with respect to the scanning line in the above-mentioned embodiment only shows a few examples in which the technical effect is the maximum, and satisfies the conditions described in the claims of the present invention. If so, it has a sufficient effect as compared with the conventional technology.

Alternatively, the structure of the surface-emitting type laser device shown in the above embodiment is one exemplification that can be realized, and other structures exhibit the same effect.

Further, it goes without saying that the image forming apparatus of the present invention can be applied to not only printing apparatuses such as printers and copying machines, but also facsimiles and displays with the same effect.

[0069]

As described above, in the image forming apparatus of the present invention: When scanning a plurality of laser beams, some image spots are placed at the same position in the main scanning direction, so that image data can be written with the same delay amount and the circuit can be simplified.

2. By arranging the image-forming spots in the main scanning direction and the sub-scanning direction two-dimensionally, the angular accuracy of attachment of the semiconductor laser array or the surface to be scanned in the feed direction can be relaxed, and the device can be easily manufactured. In addition, a large allowable error means that there is a large margin for deformation, deterioration, and environmental changes after manufacturing, which improves the reliability of the device.

3. The maximum distance between the laser beams in the optical path from the light source (semiconductor laser array) to the surface to be scanned is smaller than that of the conventional one in which the imaging spots are arranged on a straight line, that is, a plurality of beams occupy The cross-sectional area can be reduced, the size of the deflecting surface of the deflector and the size of each lens in the optical path can be reduced, and this greatly contributes to downsizing, weight reduction, and cost reduction of the device. Similarly, the size of the semiconductor laser array can be minimized.

4. There is a degree of freedom in the spacing between the light emitting parts and the placement of the light emitting parts on the semiconductor laser array, and it is possible to set the optimal spacing and placement when constructing the elements on a monolithic substrate, and a semiconductor laser with higher performance and reliability. Arrays can be used.

In the present invention, particularly when a surface-emitting type semiconductor laser array is used as a light source, a two-dimensional image forming spot can be easily arranged and the interval between the light emitting portions can be made smaller than in the conventional case. The above effects can be maximized.

[Brief description of drawings]

FIG. 1 is a schematic view of an optical scanning device used in an image forming apparatus according to an embodiment of the present invention.

FIG. 2 is a schematic diagram of a printing process of the image forming apparatus according to the embodiment of the present invention.

FIG. 3 is a layout diagram of image forming spots and scanning lines according to an embodiment of the present invention.

FIG. 4 is a circuit block diagram relating to transfer of image data according to an embodiment of the present invention.

FIG. 5 is an arrangement diagram showing another arrangement example of the image forming spots and the scanning lines in the embodiment of the present invention.

FIG. 6 is a layout view showing still another layout example of the imaging spots and scanning lines in the embodiment of the present invention.

FIG. 7 is an arrangement diagram showing another arrangement example of the image forming spots and the scanning lines in the embodiment of the present invention.

FIG. 8 is a layout view showing another layout example of the image forming spots and the scanning lines in the embodiment of the present invention.

FIG. 9 is a layout diagram showing another layout example of the imaging spots and scanning lines in the embodiment of the present invention.

FIG. 10 is a cross-sectional view of one light emitting portion of a surface emitting semiconductor laser array used in one embodiment of the present invention.

FIG. 11 is a schematic view of an optical scanning device used in a conventional image forming apparatus.

FIG. 12 is a conceptual diagram of an edge emitting semiconductor laser in a conventional example.

FIG. 13 is an arrangement diagram of image forming spots and scanning lines in a conventional example.

FIG. 14 is an arrangement diagram of image forming spots and scanning lines in another conventional example.

FIG. 15 is an explanatory diagram of an image forming spot and a tilt of a scanning line in another conventional example.

[Explanation of symbols]

 1, 21 Semiconductor laser array 2 Collimator lens 3 Rotating polygonal mirror 4 Scanning lens 5 Image carrier 6 Imaging spot 7 Cylindrical lens 8 Toroidal lens 9 Scanning line 11 Reflecting mirror 12 Photodetector 41, 42, 43, 44 Light emitting part 51 , 52 Delay circuit 53 Selection circuit 54, 55, 56, 57 Laser drive circuit 61 Image memory

Front page continuation (72) Inventor Kata Takada 3-3-5 Yamato, Suwa City, Nagano Seiko Epson Corporation

Claims (4)

[Claims] 1. A light source that emits a plurality of laser beams that can be independently modulated from a light emitting section that is two-dimensionally arranged, and a laser beam deflector that periodically deflects each of the plurality of laser beams. And a scanning optical system for forming an image of the plurality of laser beams on the surface to be scanned, the surface of the optical scanning device is substantially perpendicular to the scanning direction of the deflected laser beams. In an image forming apparatus that forms a latent image on a moving image carrier,
The number of the plurality of laser beams is n, and the numbers of the image forming spots formed on the image carrier are S1 to Sn in the direction orthogonal to the scanning direction from the beginning.
1, the distances L2 to Ln measured at right angles to the scanning directions to the respective imaging spots S2 to Sn are L2 to Ln.
An optical scanning device in which the values D2 to Dn obtained by dividing each n by the scanning line pitch p are substantially integers, and the remainders M2 to Mn obtained by dividing D2 to Dn by n are natural numbers less than n different from each other. An image forming apparatus characterized by being used. 2. The semiconductor laser array, wherein the light source emits a laser beam having an optical axis substantially vertical to the surface of the element substrate, and each of the light emitting portions can individually control its lighting and light amount. The image forming apparatus according to claim 1, wherein: 3. The image forming apparatus according to claim 1, wherein at least two of the image forming spots S1 to Sn are on the same straight line parallel to the feed direction of the image carrier. .. 4. A position occupied by the imaging spots S1 to Sn in the scanning direction is the number n of the plurality of laser beams.
4. The image forming method according to claim 3, wherein the image forming spots are located at a smaller number of locations, are equally spaced in the scanning direction, and the respective imaging spots occupying the same position in the scanning direction are also equally spaced in the sub-scanning direction. apparatus.