JPH03132710A - Scan exposing method using plural laser beams - Google Patents

Abstract

PURPOSE:To prevent an image from going to irregular in density owing to heterodyne interference and to form a correct image corresponding to an image signal by determining the intensity distribution shape of each laser beam and its scanning speed on a photosensitive surface so that the distribution of the quantity of exposure when a scan is made with plural laser beams is the sum of distributions of the quantities of exposure when scans are made with the respective laser beams. CONSTITUTION:A scanning optical system is so constituted that laser beams which are separated in a subscanning direction to a large diameter in a main scanning direction form an image on the mirror surface 22 of a polygon mirror 20. Heterodyne interference is caused at overlap parts of the laser beams and the intensity distribution varies with time. For the purpose, at lest the intensity distribution shape of each laser beam and its scanning speed on the photosensitive surface are so determined that the distribution of the quantity of exposure when the scan is made with the plural laser beams is the sum of the distributions of the quantities of exposure when the scans are made with the respective laser beams. Consequently, even when the heterodyne interference is caused at the overlap parts of the laser beams and the intensity distribution varies with time, an image with density corresponding to the image signal can be formed.

Description

[Detailed Description of the Invention] [Industrial Application Field] The present invention relates to a multiple laser beam and a scanning exposure method, and particularly to a multi-frequency beam that divides an incident laser beam into a plurality of beams according to the frequency of an incident ultrasonic wave. The present invention relates to a plurality of laser beams and a scanning exposure method in which a plurality of laser beams are generated using several acousto-optic elements and the plurality of laser beams are simultaneously scanned for exposure.

Regarding.

[Prior Art and Problems to be Solved by the Invention] Light modulation devices using multi-frequency acousto-optic elements have been known (Japanese Patent Publication No. 63-5741, Japanese Patent Application Laid-Open No. 54-5455, JP-A-57-41618, JP-B-Sho 53-9856, etc.). In an optical modulation device using such a multi-frequency acousto-optic element, a plurality of laser beams arranged so as to partially overlap on a photosensitive surface are simultaneously scanned in a direction orthogonal to the direction in which the laser beams are arranged, with a main scanning direction. Scanning exposure will be performed.

On the other hand, Doppler shift occurs in the laser beam emitted from the multi-frequency acousto-optic element, in which the frequency of the laser beam shifts depending on the direction of incidence of the ultrasonic wave. Heterodyne interference occurs in which the intensity distribution of the signal changes over time.

Regarding this heterodyne interference, two laser beams whose intensity distributions are normally distributed and whose amplitudes are modulated at angular frequencies ω1ω2 will be described. As shown in FIG. 1, the intensity distribution of each laser beam is a normal distribution (Gaussian distribution) as shown by curves A and B. Assuming that the Rede beam is not scanned, the difference between each frequency Δω (=ω1−ω
When the product Δωt of 2) and time t is 0, the intensity distribution of the composite wave is as shown by curve C, and Δωt = (2n+1)π/
4, the intensity distribution becomes like a curve, Δωt=n
The intensity distribution at π/2 is as shown by curve E. In addition,
In Figure 1, n is a natural number, r is the radius from the center of the normal distribution of the part where the intensity is 1/e2 (e is the base of the natural logarithm) with respect to the center intensity, and d/2 is the center of the composite wave. is the distance from to the center of each laser beam. In this way, the intensity distribution of the composite wave changes over time due to heterodyne interference.

When exposing a photosensitive surface using a laser beam to form an image, the density of the image is determined by the product of the intensity of the laser beam and the exposure time, that is, the amount of exposure. FIG. 2 shows the relationship between this exposure amount and the intensity distribution of the laser beam undergoing heterodyne interference. 21stffl
As shown in (1), when two laser beams are heterodyne-interfering, the intensity distribution on the photosensitive surface when L2 is scanned in the direction of the arrow changes as the intensity distribution of the composite wave changes as shown in Figure 1. Therefore, it changes as shown in FIG. 2 (2). At this time, the intensity distribution at the center of the composite wave has a frequency undulation that corresponds to the frequency difference between the two laser beams, so if the scanning speed is fast, the exposure amount due to the composite wave will be the same as that of the second laser beam.
It changes as shown by curve F in Figure (3). Note that if the scanning speed is slowed down, the exposure amount is averaged, so that the exposure amount becomes constant as shown by the dotted line G.

Therefore, when scanning exposure is performed using a laser beam with heterodyne interference, density unevenness will occur in the image due to heterodyne interference unless the scanning speed and the like are appropriately determined.

In the case of scanning exposure using a plurality of laser beams, the present invention prevents the exposure dose distribution from changing over time even if the intensity distribution of the overlapping portion of the laser beams changes over time due to heterogain interference, so that the image signal It is an object of the present invention to provide a multiple laser beam and scanning exposure method that can form correct images corresponding to the following.

[Means to solve the problem]

In order to achieve the above object, the present invention arranges a plurality of Rede beams that have heterogain interference in their overlapping portions so that they partially overlap on a photosensitive surface, and simultaneously scans and exposes a plurality of laser beams in a direction intersecting the arrangement direction of the laser beams. For this purpose, the exposure amount distribution when scanning with multiple laser beams is the sum of the exposure amount distribution when scanning with each laser beam.
The method is characterized in that at least the shape of the intensity distribution of each laser beam and the scanning speed on the photosensitive surface are determined.

In this invention, the intensity distribution of each laser beam is a normal distribution, and the shape of the intensity distribution is determined by the radius r from the center of the part where the intensity is - with respect to the central degree.1. It is possible to determine the scanning speed V on the photosensitive surface, the radius r from the center of the part where the distance between adjacent laser beams is 1/e2 or more, and the scanning speed V. This predetermined value is preferably 1.5.

The plurality of laser beams that have hetero gain interference in the overlapping portion can be generated by an acousto-optic element that deflects the incident laser beam in a direction according to the frequency of the incident ultrasonic wave.

[Effect]

In the present invention, a plurality of laser beams having heterogain interference at the overlapping portions are arranged on the photosensitive surface so that some of the laser beams overlap,
A plurality of laser beams are simultaneously scanned and exposed in a direction crossing the array direction of the laser beams. Therefore, heterogain interference occurs in the overlapping portion of the laser beams, and the intensity distribution changes over time.

Therefore, at least the intensity distribution shape of each laser beam and the scanning speed on the photosensitive surface should be adjusted so that the exposure distribution when scanning with multiple laser beams is the sum of the exposure distribution when scanning with each laser beam. decide. The exposure amount is expressed as the product of the laser beam intensity and the exposure time.The intensity is related to the intensity distribution shape, and the exposure time is related to the scanning speed on the photosensitive surface, so the laser beam intensity distribution shape and the photosensitive surface By determining the above scanning speed, the exposure amount distribution can be determined as described above. In this way, since the exposure dose distribution when scanning with multiple laser beams is the sum of the exposure doses when scanning with each laser beam, heterogain interference occurs in the overlapped part of the Rede beams, causing the intensity distribution to change over time. It is possible to form an image with a density that corresponds to the image signal even if the density changes with the passage of time.

According to experiments conducted by the present inventor, the intensity distribution of each laser beam is normal distribution, and the shape of the intensity distribution is defined by the radius r from the center of the part where the intensity is 1/e2 with respect to the center intensity. When the above scanning speed is v1 and the frequency difference between adjacent laser beams is Δf, if the radius r and the scanning speed v1 are determined as the frequency difference Δf so that v/(r・Δf) is less than a predetermined value, multiple It was confirmed that the exposure distribution when scanning with one laser beam is the sum of the exposure distribution when scanning with each laser beam.

Figure 3 (1) is v= l 50m/s e cSr=
1°95μm, Δf=33.3Mf (z, i.e. V/
It shows the distribution of exposure NE on the photosensitive surface (xy plane) when (r·Δf) is 2.46. Also, Figure 3 (2)
is v=80m/sec, r=1.95μm, Δf=33
．． 3M) It shows the exposure amount distribution when Iz, that is, v/(r・Δf) is 1623, and FIG.
”/=54m/sec, r=1.95μm, 8f=25
M) fz, that is, the exposure amount distribution when V/(r·Δf)=1.10. When the ratio v/(r・△f) is 1.23 or less, the exposure amount distribution due to the composite wave is approximately the algebraic sum of the exposure amount distribution of each laser beam, and when the ratio is 2.46 or more, the exposure amount The distribution is affected by heterodyne interference. When we investigated the density of dots when scanning with the multi-frequency acousto-optic device turned on and off, we confirmed that if the ratio was set to 1.5 or less, no density unevenness was observed and correct dots were formed corresponding to the on/off signals. It was done.

〔Effect of the invention〕

As explained above, according to the present invention, since the exposure distribution when scanning with multiple laser beams is the sum of the exposure distribution when scanning with each laser beam, the time lapse due to heterodyne interference is In addition, even when using a Radhe beam whose intensity distribution changes, it is possible to form a correct image corresponding to the image signal.

〔Example〕

The present invention will be explained in detail below. First, a scanning optical system of a laser printer to which the present invention is applicable will be described. This scanning optical system is configured to image a laser beam, which has a large diameter in the main scanning direction and is separated into a plurality of beams in the sub-scanning direction, on a mirror surface 22 of a polygon mirror 20. As shown in Figure 4, a multi-frequency acousto-optic device (AO
On the laser beam exit side of M) 10, a converging lens 12 with a focal length f is arranged so that the polarization plane 24 of the AOMIO is located in the first focal plane. On the laser beam exit side of the converging lens 12, there is a sub-scanning direction beam shaping cylindrical lens having lens power only in the sub-scanning direction and having a focal length fbf so that the first focal plane overlaps with the second focal plane of the converging lens 12. 14 are arranged. On the laser beam exit side of the sub-scanning direction beam shaping cylindrical lens 14, there is a focal length fbsO main scanning direction beam shaping cylindrical lens having a lens power only in the main scanning direction so that the first focal point overlaps the second focal point of the converging lens 12. A lens 16 is arranged. Further, on the laser beam exit side of the main scanning direction beam shaping cylindrical lens 16, a focal point having lens power only in the sub scanning direction is provided such that the first focal plane overlaps with the second focal plane of the sub scanning direction beam shaping cylindrical lens 14. The distance is f. , a tilt correction cylindrical lens 18 is arranged. The polygon mirror 20 is arranged so that the second focal plane of the tilt correction cylindrical lens 18 and the mirror surface 22 overlap. Therefore, the mirror surface 22, A○M polarization surface 24, etc. become beam waist surfaces.

An optical system consisting of an fθ lens 32 and a cylindrical lens 34 having lens power only in the sub-scanning direction is arranged on the laser beam reflection side of the polygon mirror.

In the main scanning direction of this optical system, the first focal plane is the mirror surface 22.
The second focal plane overlaps with the photosensitive surface of the photosensitive material 36, and the mirror surface 22 and the photosensitive surface are in a conjugate relationship with respect to the sub-scanning direction.

The operation of this embodiment will be explained below. In the main scanning direction, only the converging lens 12 and the main scanning direction beam shaping cylindrical lens 16 have lens power. For this reason, a beam expander is configured by the converging lens 12 and the main scanning direction beam shaping cylindrical lens 16, and the laser beam irradiated from the cylindrical lens 16 is converted into a parallel laser beam spread in the main scanning direction, and the mirror surface 22 is irradiated.

In the sub-scanning direction, the converging lens 12, the sub-scanning beam shaping cylindrical lens 14, and the U (M misalignment correction cylindrical lens 18) have lens power. 14 and the tilt correction cylindrical lens 18 overlap, the focal planes of the converging lens 12 and the tilt correction cylindrical lens 18 are relayed in order from the converging lens 12 to the cylindrical lens 14 and from the cylindrical lens 14 to the cylindrical lens 16 on the Fourier transform plane, and the Fraunhofer beam at the AOM deflection position is placed on the mirror surface 22. - A diffraction image is formed.As a result, a plurality of laser beams separated in the sub-scanning direction are formed on the mirror surface 22.The image on the focal plane 22 is a Fraunhofer diffraction image. Become.

As a result of the above, a laser beam whose diameter in the main scanning direction is larger than the diameter in the sub-scanning direction and which is separated into a plurality of pieces in the sub-scanning direction is imaged on the mirror surface 22.

The image of the surface of the polygon mirror 20 is formed on the photosensitive surface by the fθ lens 32 and the optical lens 34, but the image of the surface of the polygon mirror 20 is
If we consider the θ lens 32 and the linear lens 34 as one combined lens, the image on the surface of the polygon mirror 20 becomes the object point for this combined lens, and the image on the photosensitive surface becomes the image point in the sub-scanning direction. has a conjugate relationship. Therefore, all the laser beams emitted from the object point are always converged on the image point, so even if the polygon mirror 20 has a tilted surface, this tilt can be corrected.

The image on the photosensitive surface becomes a plurality of circular spots arranged in the sub-scanning direction.

Using the above-mentioned scanning optical system, the intensity distribution becomes a normal distribution, the frequency difference △f is 25M) (z), and the beam diameter r is divided into two beams of 1.95 μm, and the beams are scanned on the photosensitive surface. When scanning exposure was carried out at a scanning speed of 54 m/sec, the exposure dose distribution on the recording material became as shown in Figure 3 (3), which was the algebraic sum of the exposure doses of the two laser beams. v/(r·Δf) was 1.10.

Further, when the AOM was modulated by the image signal, the density of the dots on the photosensitive material was correctly recorded in accordance with the image signal.

[Brief explanation of the drawing]

Figure 1 is a diagram to explain heterodyne interference, Figure 2 is a diagram to explain heterodyne interference.
Figures (1), (2), and (3) are diagrams showing changes in exposure amount when heterodyne interference occurs;
2) and (3) are diagrams showing the exposure dose distribution when the magnitude of V/(r・Δf) is changed using a laser beam in which heterogain interference occurs, and Fig. 4 is a diagram to which the present invention is applied. 1 is a perspective view of a possible scanning exposure optics; FIG. 12... Converging lens, 14.16.18.34... Cylindrical lens 20... Polygon mirror

Claims (4)

[Claims] (1) A plurality of laser beams that cause heterodyne interference at the overlapping portions are arranged so that they partially overlap on the photosensitive surface, and when performing exposure by simultaneously scanning multiple laser beams in a direction that intersects with the direction in which the laser beams are arranged, scanning with multiple laser beams is performed. At least the intensity distribution shape of each laser beam and the scanning speed on the photosensitive surface are determined so that the exposure distribution when scanning with each laser beam is the sum of the exposure distribution when scanning with each laser beam. A scanning exposure method using a laser beam. (2) The intensity distribution of each laser beam is made into a normal distribution, and the shape of the intensity distribution is determined by the radius r from the center of the part where the intensity is 1/e^2 with respect to the center intensity, and the scanning speed on the photosensitive surface is determined. The plurality of laser beams according to claim (1), wherein at least the radius r and the scanning speed V are determined so that V/r·Δf is equal to or less than a predetermined value, where V is the frequency difference between adjacent laser beams and Δf. scanning exposure method. (3) The scanning exposure method using a plurality of laser beams according to claim (2), wherein the predetermined value is 1.5. (4) The plurality of laser beams according to claim (1), (2), or (3), wherein the plurality of laser beams are generated by an acousto-optic element that deflects the incident laser beam in a direction according to the frequency of the incident ultrasonic wave. A scanning exposure method using a laser beam.