JPH01232322A - Turn-back mirror of laser beam scanning optical system - Google Patents

Abstract

PURPOSE:To easily and inexpensively correct the nonuniformity in the exposure on a photosensitive surface by forming an increase reflecting film which is gradually increased in film thickness from off-the-optical axis to on-the-optical axis to the reflecting face of the turn-back mirror. CONSTITUTION:The increase reflecting films 7a, 8a are provided in common used as protective films to the turn-back mirrors 7, 8. The increase reflecting film 8a of said films is so formed that the film thickness thereof increases gradually from the off-the-optical axis to the on-the-optical axis. A laser beam is, therefore, changed in the reflectivity by the thickness of the film 8a so that the reflectivity is small on the optical axis and is large off the optical axis. As a result, the reflectivity is small and the exposure decreases on the optical axis where the scanning speed is low. The reflectivity is large and the exposure increases off the optical axis where the scanning speed is high. The nonuniformity in the exposure on the photosensitive surface is thereby easily and inexpensively corrected even if a costly ftheta lens is not used.

Description

Detailed Description of the Invention (Industrial Application Field) The present invention relates to a laser beam scanning optical system, in particular optical correction of uniform velocity of scanning of a laser beam reflected by a rotating scanner such as a polygon mirror using an fθ lens or the like. This invention relates to a folding mirror used in a laser beam scanning optical system that does not perform

(Prior Art) A polygon mirror in an L/-q beam scanning optical system is rotated at a constant speed. Therefore, the laser beam reflected from the reflecting surface whose direction is changed by the rotation thereof is deflected about one point at the same speed as the rotation of the polygon mirror. By the way, when the photosensitive surface is scanned with this deflected laser beam, it is no longer scanned at a constant speed. For this reason, conventionally, uniform velocity scanning has been performed using a lens called an fθ lens, which actively uses distortion aberration to correct the inconsistency.

(Problems to be Solved by the Invention) However, since the fθ lens is extremely expensive, it greatly affects the cost of the laser beam scanning optical system.
It is not suitable for printers for personal computers, which require a low price. Therefore, electrical correction can be considered in which the output pitch of the laser beam is made smaller where the scanning speed is high and increased where the scanning speed is low, without using an fθ lens. This electrical correction enables image formation at an image forming pitch equal to constant speed scanning.

However, the actual scanning speed of the laser beam is not constant. Therefore, the exposure amount is small on the outer side of the axis where the scanning speed is high, and the exposure amount is large on the upper side of the axis where the scanning speed is low. This results in non-uniform image density.

To further correct this electrically, it is necessary to control the output of the laser beam with a special control circuit, which is expensive, and there are limitations to output control, so it is not perfect.

Therefore, the present invention proposes a laser beam scanning optical system that can easily and reliably correct the non-uniformity of the exposure amount due to the non-uniform speed of scanning by setting the reflection increasing film of the folding mirror, which is an existing optical component of the laser beam scanning optical system. The purpose is to provide a folding mirror for the system.

(Means for Solving the Problems) In order to achieve the above object, the present invention directs a laser beam reflected and deflected by a rotating scanner onto a photoreceptor through a folding mirror without using an fθ lens, and In a laser beam scanning optical system that scans from one side to the other,
The thickness of the reflection-enhancing film formed on the reflective surface is gradually increased from off the optical axis to on the optical axis, and the amount of reflected light within the scanning area is corrected so that it gradually increases from on the optical axis to off the optical axis. It is characterized by the following.

(Function) The laser beam is reflected by a rotating scanner that rotates at a constant speed and is deflected to scan the photosensitive surface from one side to the other. The scanning speed of the laser beam at this time is inconstant.

In other words, it is slow on the optical axis and becomes faster toward the outside of the optical axis.

However, when the laser beam that reaches the photoreceptor after deflection passes through a folding mirror on the way, it is affected by a reflection-enhancing film provided on the folding mirror.

The thickness of the reflection-enhancing film gradually increases from outside the optical axis of the folding mirror to on the optical axis, so the reflection angle of the laser beam that is incident on each part of the folding mirror and reflected is almost constant. Therefore, the reflectance of the laser beam reflected at each part of the folding mirror changes depending on the thickness of the reflection-enhancing film at that part, and the reflectance is small on the optical axis where the film is thick, and the film is thin. The reflectance increases on the outside of the optical axis.

As a result, the laser beam is reflected more on the outside of the optical axis than on the optical axis, so the amount of scanning light on the outside of the optical axis is increased compared to the amount of scanning light on the optical axis, and the scanning speed is slower on the optical axis. It is possible to correct the non-uniformity of the exposure amount due to the speed increasing toward the outside of the axis.

(Example) FIG. 1 is a schematic perspective view showing the basic configuration of a laser beam printer. In this figure, 1 is a semiconductor laser driven according to an image signal, and the laser beam 2 emitted from this semiconductor laser 1 is The light passes through a collimator lens 3 and a cylindrical lens 4, and is reflected by a polygon mirror 5, which has a large number of mirrors formed around a rotation axis. At this time, the laser beam 2 is deflected at a constant speed in one direction about one point each time the polygon mirror 5 rotates. The laser beam 2 deflected by the polygon mirror 5 then passes through the toro and ideal lens 6, becomes a parallel beam whose inclination angle has been corrected, is reflected by the folding mirror 7.8, reaches the photosensitive drum 9, and the photosensitive drum 9. The surface of the drum 9 is scanned from one side to the other in the drum axis direction (main scan).

However, even if the scanning on the photosensitive drum 9 by the laser beam 2 deflected at a constant velocity achieves uniform velocity in image formation through electrical correction, the actual scanning velocity remains unchanged. As a result, the amount of exposure on the photosensitive drum 9 becomes non-uniform. For this reason, it becomes necessary to correct the amount of light so that the exposure amount is lower on the optical axis where scanning is slow than on the optical axis where scanning is fast.

When calculating the amount of light intensity correction based on the fact that the angle of view of the laser beam scanning optical system that can realistically correct electrical uniformity is within 30°, the angle of view θ is 0°≦θ≦30. ', and the displacement of the beam on the surface of the photosensitive drum 9 is y
=ftan (wt) (where W: angular velocity of the polygon mirror), and the scanning speed is v =dy/dt =w
It can be expressed as f/cos2 (wt).

Here, the scanning speed when wt=0 is V-VO.

Therefore, the exposure amount around the scan area is 374 in the center.
become.

Therefore, in order to obtain an image with uniform density in the main scanning direction, it is necessary to correct the exposure amount by some method so that it becomes constant over the entire scanning area. This correction can be performed by inserting a filter in which the amount of transmitted light gradually changes from on the optical axis to off the optical axis, but this increases the number of special parts and increases the optical N loss.

By the way, the higher the reflectance on the folding mirror 7.8, the better, and for this purpose, the folding mirror 7.8 is provided with reflective coatings 7a, 8.
A is also provided as a protective film. The present invention focuses on this, and forms at least one of the reflection increasing films 8a so that it is thick on the optical axis and gradually becomes thinner toward the outside of the optical axis, as shown in FIG. As a result, the reflectance on the folding mirror 8 gradually decreases from the outside of the optical axis to the upper side of the optical axis, and most of the laser beam 3 is reflected on the outside of the optical axis to obtain a high scanning light amount, while the reflectance on the upper side of the optical axis is slightly reduced, a slightly lower amount of scanning light is obtained, and it is possible to correct non-uniformity of exposure amount due to uneven scanning speed.

Specifically, the maximum ratio of scanning speeds on the optical axis and off the optical axis is 3:4 in the above case, and the resulting difference in exposure amount is 25%. This is the oscillation wavelength λ of the semiconductor laser. -780nm
In order to correct this, as shown in FIG.
5nll to the aluminum layer! The reflection increasing film 8a is formed on top of the reflective film 8a, and the thickness of each layer of nL and n□ gradually increases from off the optical axis to on the optical axis.

Here, nL is a low refractive index substance, such as silicic acid (S, 0
□), magnesium fluoride (M, F2), cryolite (A
fF3.3NaF), etc., and nH is a high refractive index substance such as titanium oxide (T, 0□), cerium oxide (C, O□),
Zirconium oxide (Z, 0□), tantalum oxide (Ta
zOs), hafnium oxide (HfOZ), and the like.

Also aluminum layer A! It's gold (Au)! I (A
g) High brightness metals such as copper (Cu) and rhodium (Rh) may be used.

Note that a multilayer mirror in which the metal is made of a dielectric material can also be used, but an enhanced reflection method using aluminum is cheaper. Moreover, six layers of the reflection increasing film 8a were suitable for correcting the difference in exposure amount of 25%.

Note that even if the incident angle of the folding mirror 8 is the same on the optical axis and off the optical axis as seen from the side as shown in Fig. 2, for example, 45 degrees, the actual incident angle on the optical axis is Incident angle θ.

is equal to that, whereas the actual incident angle θ1 outside the optical axis (half angle of view 30°) is 49″, which is slightly larger than 45°, due to the expansion of the optical path.

In consideration of this, the reflection increasing film 8a is formed on the folding mirror 8.
Set the film thickness distribution as shown in the graph of Figure 5, and
When formed as shown in the figure, the oscillation wavelength λ of the semiconductor laser is as shown in FIG. The reflectance at =780 nm is 25% higher for the off-axis characteristic with a half angle of view of 30° than the characteristic a on the optical axis.

This makes it possible to eliminate the 25% difference in exposure amount due to non-uniform scanning speed.

The reflection enhancing film 8a as described above can be easily formed on the reflecting surface of the reflection mirror 8 by using a vacuum evaporation apparatus as shown in FIG. To explain this, the vacuum layer lI
A dome-shaped mirror holder 12 that rotates around a central axis at a constant rate is provided at the upper part of the interior, and a folding mirror 8 is held in this holder 12, and a reflection-enhancing film 8a having a desired film thickness ratio is formed by a vapor deposition material 14 placed below. I am trying to evaporate it.

For this reason, the folding mirrors 8 are held in the holder 12 with their reflective surfaces facing downward, and the mirrors 8 are arranged radially at equal intervals in the circumferential direction. Vapor deposition substance 1
4, the vapor deposition particles 14a fly out due to the impact of the electron beam 15 from below and travel straight toward the mirror holder 12, forming a vapor deposition film on the lower surfaces of the mirror holder 12 and the folding mirror 8. This vapor deposited film becomes one of each layer of the reflection increasing film 8a.

Here, the interior of the vacuum evaporation apparatus is adjusted so that the evaporated film is uniformly formed over the entire lower surface of the mirror holder 12. Then, the film thickness formation conditions for vapor deposition differ from those for the lower surface of the mirror holder 12 for the reflection surface of the folding mirror 8 by the extent that it protrudes from the surface of the mirror holder 12 in a planar state.

The reflection surface of the folding mirror 8 is flat and protrudes most from the bottom surface of the holder 12 on the optical axis, so that the incident angle of the vapor deposition particles 14a at each part and the vapor deposition substance 14 are different.
The thickness of the deposited film differs depending on the distance from the point. In other words, it is thicker on the upper side of the optical axis where the angle of incidence is small and the distance l is small as it moves forward from the holder 12.
The angle of incidence is large and the distance from the holder 12 is only small! A thin film is formed outside the optical axis, that is, on both end portions, where the value is larger.Such a vapor-deposited film is formed for each layer to obtain a predetermined reflection-enhancing film 8a.

In order to correctly form such a film thickness distribution according to the set values shown in the fifth article, it is preferable to correct it by changing the installation position and shape of the film thickness correction plate 16.

(Effects of the Invention) According to the present invention, since it has the above-described structure and operation, in a laser beam scanning optical system that does not correct optical uniformity, the difference in exposure amount due to the inconsistency of scanning is reflected by a mirror. This can be corrected by a reflection-enhancing film formed on the reflective surface of the mirror, and in a system in which a reflection-enhancing film is provided, it can be easily and inexpensively achieved by simply providing a predetermined distribution in the thickness.

[Brief explanation of the drawing]

The drawings show one embodiment of the present invention, and FIG. 1 is a schematic perspective view showing the basic configuration of a laser beam printer using a folding mirror according to the present invention, and FIG. Figure 3 is a side view of the folding mirror shown in Figure 2. Figure 4 is an explanatory diagram of the configuration of the reflective multilayer film of the folding mirror shown in Figure 2. Figure 5 is the film thickness distribution state of the reflective mirror. Figure 6 is a graph showing the reflectance characteristics of the folding mirror shown in Figure 2 on the optical axis and off the optical axis, and Figure 7 is a graph showing the vacuum evaporation equipment used to form the reflective mirror of the folding mirror shown in Figure 2. FIG. 1 −−−−−−−−−−−−−−−−−Semiconductor laser 2
-------1-----Laser beam 5--------
−−−− Polygon mirror 8−−−−−−−−1−−−
-----Folding mirror 8a--------------・-
・-Reflection-enhancing film 9-=------

Claims (2)

[Claims] (1) In a laser beam scanning optical system that directs a laser beam reflected and deflected by a rotating scanner onto a photoconductor via a folding mirror without using an fθ lens, and scans the surface from one side to the other, a laser beam is formed on a reflective surface. The thickness of the reflection-enhancing film gradually increases from off the optical axis to on the optical axis, and the amount of reflected light within the scanning area is corrected so that it gradually increases from on the optical axis to off the optical axis. Features a folding mirror in the laser beam scanning optical system. (2) A folding mirror for a laser beam scanning optical system according to claim 1, wherein the reflection-enhancing film comprises a high-brightness metal layer and six layers of reflection-enhancing coating films.