JPH0497114A - Spot diameter adjustment device for laser scanning system - Google Patents

Abstract

PURPOSE:To facilitate spot diameter adjustment and to reduce the size, weight, and cost of the device by sticking a mirror plate on a piezoelectric element and forming a reflecting surface and providing a reflecting surface curvature control means which feeds and drives the piezoelectric element electrically to control the curvature of the reflecting surface and adjust a spot diameter on a scanned surface. CONSTITUTION:The reflecting surface of a reflecting mirror 13 is formed by sticking the mirror plate 13b on the piezoelectric element 13a. The signal from a spot diameter sensor 17 which is provided at a specific distance l from the extended surface of the scanned surface 16 in the travel direction of a laser beam is inputted to a piezoelectric element driving circuit 18. The piezoelectric element driving circuit 18 while setting the driving signal of the piezoelectric element 13a by arithmetic according to the spot diameter on the scanned surface 16 outputs the driving signal to the piezoelectric element 13a, which is curved. Then the surface of the mirror plate 13b is curved to curvature corresponding to the quantity of fed electricity to vary the convergence rate of the laser beam reflected by the surface of the mirror plate 13b, thereby controlling the spot diameter on the scanned surface 16 to proper size. Consequently, the spot diameter is adjusted by the small-sized, lightweight structure with high accuracy.

Description

[Detailed Description of the Invention] <Industrial Application Field> The present invention relates to a laser scanning system such as a laser printer that scans a laser beam on a scanning surface, and particularly to a device for adjusting the spot diameter of a laser beam on a scanning surface. Regarding.

<Prior art> As a conventional technology of this kind, for example, Japanese Patent Application Laid-Open No. 2-511
There is something like the one shown in Publication No. 19.

An outline of this will be explained based on FIGS. 6 to 9.

The image processing section 61 outputs an image output signal P to the control section 60, and also outputs an image signal S to the laser driver 51, which causes the fixed laser element 52 to blink at a predetermined timing.

The laser beam emitted from the solid-state laser element 52 is
After being made into substantially parallel light by the collimator lens system 53,
It is reflected and deflected by the rotating polygon mirror 54, and the fθ lens group 5
6a, 56b, and 56c.

The converged laser beam is imaged into a spot on the scanning surface of the photosensitive drum 59. On the surface of the photosensitive drum 59,
An exposure distribution for an image-scan is formed, and the photosensitive drum 59 is further rotated by a predetermined amount for each scan to form a latent image on the drum 59 having an exposure distribution according to the image signal S, using a well-known photographic process. This is recorded as a visible image on recording paper.

Next, the operation of the focus position adjustment mechanism will be explained.

The laser beam converged by the fθ lens groups 56a, 56b, and 56c is also guided onto the photodetector element 58 via the barcode filter 57. In FIG. 7, when the barcode filter 57 is viewed from the photodetector element 58 side, the scanning spot S of the laser beam moves in the direction of the arrow C, and when it passes through the transparent portion 57a of the barcode filter 57 and when it passes through the light-blocking portion 57b. When passing, the photodetecting element 5
8, the amount of light detected changes periodically.

At this time, if the imaging condition is good and the scanning spot S is small in the scanning direction, the ratio of the transmitted light to the entire spot increases as shown in FIG. The amount of light leaking from the adjacent transmitting part 7a when passing through the light shielding part 7b becomes smaller.

On the other hand, as shown in FIG. 8(B), if the imaging condition is poor and the scanning spot S is large in the scanning direction, the ratio of the transmitted light to the entire spot decreases, and the amount of transmitted light when passing through the transmitting section 7a decreases. The amount of light leaking from the adjacent transmitting section 7a when passing through the light shielding section 7b increases.

FIGS. 9A and 9B show changes in the amount of light when the imaging condition is good and when the imaging condition is poor, respectively.

Then, the contrast V is determined by the following formula based on the maximum light amount θaaax and the minimum light amount θwin, and the position of the collimator lens system 53 in the optical axis direction is adjusted by the focal position adjustment means 54 so that the contrast V is maximized. Adjust the spot diameter to the optimum size.

<Problem to be solved by the invention> However, in such a conventional spot diameter adjustment device,
It has various problems listed below.

First, a highly accurate mechanism is required. That is, since the spot diameter is adjusted by moving the collimator lens, strict adjustment accuracy is required, and it is also susceptible to optical axis misalignment due to the movement of the collimator lens.

In the specific example, the focal length f of the collimator lens is f=5m
It is short, about m, and has a narrow depth of focus. There are various definitions of the depth of focus, but if it is defined as the position where the intensity ratio at the center of the spot is 80%, it becomes πωo'/8λ,
Assuming that the collimator lens has three effective diameters and a wavelength of 0.8 μm, the depth of focus is only about 1.4 μm. The allowable depth of focus varies depending on each individual system, and although it is not possible to apply the above numerical values to each other, precision on the order of μm is required.

Furthermore, even if it were possible to adjust the depth of focus to within the depth of focus, it would be extremely difficult to fix components having movable parts with high precision, considering the effects of vibrations and the like. In particular, in a system that constantly detects the focus state and applies feedback, the convergence state of the heel constantly changes even during recording operation.

Regarding the influence of optical axis deviation, accuracy on the order of μm is still required, and it is difficult to provide such a movable mechanism without backlash. These maladjustments may cause blurring or unevenness in the output image, kicking of the light beam, or abnormal H-3YNC signal detection.

Furthermore, since the adjustment mechanism is large-scale, the cost is high. In other words, is there a way to move a lens that does not require adjustment precision (for example, an f-theta lens or a lens placed between a collimator lens and an f-theta lens), or is there a way to move a lens that does not require adjustment precision? requires a mechanism,
This will result in high costs.

Furthermore, luminous flux responsiveness cannot be obtained. In other words, the conventional methods are not suitable for correcting gradual changes such as defocus due to fluctuations in environmental temperature, or are suitable for correcting fast changes such as fluctuations in spot diameter that occur with each scan. Unable to follow.

In a specific example, there is a problem in correcting out of focus due to the difference in curvature of each reflecting surface of a rotating polygon mirror. That is, although a reflective surface is usually created to have highly accurate flatness, it is difficult to stably create an area with an area degree of λ/4 or more. Figure 10 shows an enlarged image of the output image when a vertical line is drawn for each pixel using a laser printer. This applies to each reflective surface (6 in this example).
This phenomenon occurs because only one surface among the surfaces (surfaces) has not achieved surface accuracy and has a slight curvature. Periodically (when scanned by a reflective surface with poor area coverage)
The spot diameter becomes large, and as a result, it connects with the adjacent vertical line, making it appear as if there is a horizontal black line.

Although such fluctuations in the spot diameter are only extreme values, it becomes a time problem when outputting a special pattern such as the one described above.

Attempting to correct this would require the focus adjustment mechanism to respond several times the scanning frequency, which was previously impossible.

Another example is when an imaging lens is placed in front of a rotating polygon mirror, the optical path differs depending on the scanning position (see the dotted line in Figure 3), as described later, so that the focal position is on the scanning plane. However, in order to correct this, the lens moving mechanism is required to have a frequency response that is at least higher than the scanning frequency.

The present invention was developed in view of these conventional problems, and has a small and lightweight structure that can withstand not only changes in the environment but also fluctuations in spot diameter that occur from scan to scan.
Moreover, it is an object of the present invention to provide a laser scanning spot diameter adjusting device that can easily and accurately adjust the spot diameter.

<Means for Solving the Problems> For this reason, the present invention provides a laser scanning system in which a laser beam is scanned directly on a scanning surface through a path of reflection on a reflecting surface, in which a mirror plate is attached to a piezoelectric element on the reflecting surface. At the same time, the configuration includes a reflecting surface curvature control means for adjusting the spot diameter of the laser beam on the scanning surface by controlling the curvature of the reflecting surface by energizing and driving the piezoelectric element.

The reflective surface curvature control means may include a spot diameter measuring means for measuring the spot diameter of the laser beam on the scanning surface, and the piezoelectric element may be energized and driven based on an output signal from the spot diameter measuring means. good.

In that case, the spot diameter measuring means may be arranged at a position separated by a predetermined distance from the scanning surface in the traveling direction of the laser beam.

Further, the reflective surface curvature control means may be configured to energize and drive the piezoelectric element based on a predetermined correction signal.

Further, the correction signal may be determined for each reflecting surface of a rotating polygon mirror that deflects the laser beam, depending on the state of the reflecting surface.

Furthermore, the correction signal may be determined to vary depending on the scanning position of the laser beam.

<Operation> When the piezoelectric element is energized and the curvature of the reflective surface is controlled by the reflective surface curvature control means, the degree of convergence of the laser beam changes, thereby adjusting the spot diameter of the laser beam on the scanning surface to the optimum size. It can be adjusted to

In addition, when the reflective surface curvature control means includes a spot diameter measuring means, the spot diameter is measured and the energization of the piezoelectric element is feedback-controlled so as to have an optimum size.

If the spot diameter measuring means is installed at a position a predetermined distance away from the scanning surface in the direction in which the laser beam travels, changes in the spot diameter can be accurately measured in areas outside the depth of focus, thereby improving the accuracy of spot diameter adjustment. or improve.

On the other hand, as the reflecting surface curvature control means, if a correction signal is determined by experiment etc. to optimize the spot diameter,
Based on the correction signal, the piezoelectric element can be controlled to be energized by an oven loop.

In that case, if the correction signal is determined for each reflecting surface of the rotating polygon mirror, the spot diameter can be optimally adjusted by scanning each reflecting surface in accordance with the correction amount that differs for each reflecting surface.

Furthermore, if the amount of correction differs depending on the scanning position of the laser beam, the correction signal can be determined according to the scanning position and the spot diameter can be adjusted to the optimum spot diameter according to the scanning position.

<Example> Embodiments of the present invention will be described below based on the drawings.

In FIG. 1 showing the first embodiment, a laser element 11
The laser beam emitted from the collimator lens system 12 passes through the collimator lens system 12 to become substantially parallel light, is reflected by the reflecting mirror I3, is further reflected and deflected by the rotating polygon mirror 14, and is converged through the fθ lens group 15. The image is formed on a scanning surface (imaging surface) 16 consisting of the surface of a photosensitive ram, etc., and is scanned. The other laser driver, image processing section, and control section are the same as those in the prior art example described above, so their explanation will be omitted.

To explain the constituent parts according to the present invention, the reflecting surface of the reflecting mirror 13 is connected to the piezoelectric element 1 as shown in FIG. 2(A).
It is formed by attaching a mirror plate 13b to 3a.

On the other hand, a spot diameter sensor 17 as a spot diameter measuring means is provided at a position a predetermined distance E away from the extended surface of the scanning surface 16 in the direction of travel of the laser beam, and a signal from the sensor 17 is used as a reflecting surface curvature control means. Piezoelectric element drive circuit 1
Let 8 do the manual work. The piezoelectric element drive circuit (2) 8 sets a drive signal for the piezoelectric element 13a by calculation based on the spot diameter on the scanning surface 16 measured by the spot diameter sensor 17, and outputs the drive signal to the piezoelectric element 13a.
The piezoelectric element 13a is curved by the electrostrictive effect. As a result, the surface of the mirror plate 13b that is integrated with the piezoelectric element 13a is curved to have a curvature according to the amount of current applied, and the convergence rate of the laser beam reflected from the surface of the mirror plate 13b is changed to optimize the spot diameter on the scanning surface 16. control the size.

Specifically, when the beam waist state occurs behind the imaging plane and the spot diameter is determined to be too large on the imaging plane, the central part of the reflecting surface is concave as shown in Figure 2 (A). This increases the degree of convergence of the laser beam and reduces the spot diameter.

FIG. 2(B) shows the structure and circuit function of such a piezoelectric element 13a. This type of piezoelectric element 13a is called a bimorph actuator, and has two pieces of metal elastic plate 131 as a center electrode. Thin plate-shaped piezoelectric element 132.133
It has a structure in which these two elements 13 are pasted together.
2.133 is bent by the voltage V applied between the metal elastic plate 131 which is the center electrode. The amount of curvature U when no load is specifically shown in the figure.

is expressed by the following equation.

Uo =6-d, (I!/l) 2 (1+t,/l
)V [m] However, dz+: equivalent piezoelectric constant (230xl
O-12 [m/V] ■, applied voltage t: thickness t8 of piezoelectric element 13a, thickness 1 of metal elastic plate 131, total length of metal elastic plate 131! :The free length of the metal elastic plate 131 (approximately 85% of β) allows the spot diameter to be adjusted while driving the lightweight and small piezoelectric element 13a, making the entire device smaller and lighter and lower in cost. It is easy to control and has excellent responsiveness.

Incidentally, is it possible to use a piezoelectric element to control the position of the laser element or lens, or to control it by changing the curvature of the reflecting surface, or to increase the amount of change in the spot diameter with respect to the displacement of the piezoelectric element? It is also excellent in terms of high-speed response.

The spot diameter is measured by the spot diameter sensor I7 as follows:
When making corrections for relatively gradual changes in spot diameter due to environmental changes (such as thermal deformation of lens system components), it is sufficient to measure only the first time during recording or at predetermined intervals. Are there any reflective surfaces 14 of the rotating polygon mirror 14?
When correcting for fluctuations in the spot diameter that occur for each scan due to differences in surface accuracy for each a (the size is smaller than that caused by environmental changes), the spot diameter should be adjusted for each scan. Although it is necessary to measure and make adjustments, it is good to use them depending on the situation.

Here, the thickness of the end plate 13b used is preferably on the order of 1 to 3. If it becomes too thick, a large force will be required to give distortion to the end plate 13b, making it difficult to implement.

Furthermore, in order to attach the mirror plate 13b to the piezoelectric element 13a,
A good and easy method is to use adhesive, or a certain degree of rigidity is required. This is because the strain caused by the piezoelectric element 13a, which is extremely small compared to the material of the end plate 13b, is absorbed by the adhesive portion.

Next, the reason why the spot diameter sensor 17 is arranged at a predetermined distance l from the scanning surface 16 in the laser beam traveling direction in this embodiment will be explained. Here, the predetermined distance l is set to about 2 to 3 times (20 to 30 times) the depth of focus of the laser beam.

In a typical optical scanning system, the beam waist of the laser beam is adjusted to be on the scanning surface 16.

Therefore, if the spot sensor 17 is placed near the scanning surface 16, even if the spot diameter changes, the spot diameter will increase regardless of whether the focus is on the front side or the rear side of the scanning surface 16. This makes it difficult to judge in which direction the change occurred. Therefore, by measuring the spot diameter at a position away from the beam waist, we can determine which side is in focus by checking the size relative to the reference size measured when the scanning plane 16 is in focus. It is possible to accurately determine whether the spot is misaligned, and the spot diameter can be easily adjusted.

In addition, in order to increase the cost and improve accuracy, spot diameter sensors are placed before and after the scanning surface, and the drive of the piezoelectric element is controlled so that the difference in output between the two becomes O. You can.

FIG. 3 shows a second embodiment. Components common to those in the first embodiment will be described with the same reference numerals (the same applies to other embodiments).

This device is applied to a device in which an imaging lens 21 is placed in front of a rotating polygon mirror 14. This type (Po
The objective type (st-Objective type) has a simple lens system, and has the advantage of being low cost.As shown by the dotted line in the figure, the focus position is curved with respect to the scanning plane, which is called curvature of field. This will cause a misalignment.

Therefore, this field curvature is corrected. Regarding correction of field curvature, since the amount of defocus varies depending on the scanning position, it is meaningless to measure the spot diameter at the scanning end as in the above embodiment.

Therefore, a pattern of the correction amount for the field curvature with respect to the scanning position is written in the ROM 22 in advance, and the main scanning synchronization (H-3YNC) sensor 23 provided at the scanning end is used.
The control circuit 18 drives the piezoelectric element 13a in accordance with the pattern according to the synchronization signal from the synchronous signal from the control circuit 18 to correct the curvature of field. Specifically, a drive signal may be applied in a pattern that does not change the convergence state of the laser beam near the center of the scanning position, but shifts the convergence position of the laser beam backward as it approaches the end.

Thereby, the convergence position of the laser beam can be aligned substantially on the scanning plane. It may be difficult to completely match the in-focus position with the scanning plane, but there is no practical problem if it falls within 4 to 5 mm.

Further, instead of correcting field curvature using the method of this embodiment, it is also possible to perform feedback correction of drift-like out-of-focus by using the method of the first embodiment.

FIG. 4 shows a third embodiment applied when the state of each reflecting surface of the rotating polygon mirror is known in advance.
A correction signal (piezoelectric element drive signal) corresponding to each reflective surface 14a of the rotary polygon mirror 14 is written in the ROM 22, while a surface sensor 31 (for example, connected to the rotation axis of the rotary polygon mirror 14) is written to determine each reflective surface 14a of the rotary polygon mirror 14. The piezoelectric element 13a is driven using a correction signal corresponding to the reflective surface 14a determined based on the rotation angle, etc.). In this case, the piezoelectric element 13a can be driven depending on the state of the reflective surface 14a for the next scan, which has already been detected, at the same time as the image area is completed, so it is possible to provide some margin for responsiveness. can.

Furthermore, as a fourth embodiment that is a modification of the third embodiment, the state (flatness) of each reflecting surface 14a of the rotating polygon mirror 14 differs depending on the scanning position (for example, Po5t shown in FIG. 3). - For objects that cause curvature of field, such as an objective type, a pattern signal that differs depending on the scanning position is written in the ROM 22 for each reflective surface 14a, and the piezoelectric element 13a is driven and controlled according to the pattern signal. Make it.

Further, as shown in FIG. 5, a mirror plate 42 is fixed to the end of a laminated piezoelectric element 4I, and a reflecting surface with a variable curvature is formed by pushing and pulling the central part of the mirror plate 42. However, since the cost is high or the power obtained is large, it is best to use them depending on the purpose.

<Effects of the Invention> As explained above, according to the present invention, the spot diameter can be adjusted simply by driving the light and small piezoelectric element integrated with the end plate, making it possible to reduce the size, weight, and cost of the entire device. Not only can it be adjusted depending on the environment, but the spot diameter can be greatly changed due to the displacement of the piezoelectric element, so it is extremely responsive to fluctuations in the spot diameter for each scan due to differences in flatness of each reflecting surface of the rotating polygon mirror. It is also possible to make good adjustments to changes in the spot diameter with respect to the scanning position due to curvature of field or the like.

[Brief explanation of the drawing]

FIG. 1 is a system configuration diagram showing a first embodiment of the present invention;
FIG. 2(A) is an enlarged sectional view showing the structure of the reflector plate used in the above embodiment, and FIG. 2(B) is the structure 1 of the piezoelectric element.
3 is a system configuration diagram showing the second embodiment of the present invention, FIG. 4 is a system configuration diagram showing the third embodiment of the present invention, and FIG. 5 is a diagram showing the structure of the reflector. FIG. 6 is a system configuration diagram showing an example of a conventional spot diameter adjusting device; FIG. 7 is an enlarged view showing a bar code filter portion of the conventional example; FIG. 8 (A). (B) is a cross-sectional view showing the amount of transmitted light depending on the spot position, FIGS. 9(A) and (B) are diagrams showing the relationship between the spot diameter and the amount of transmitted light, and FIG. FIG. 3 is a diagram showing the state of the spot diameter when one of the flatness is poor. 11... Laser element 12... Collimator lens 13... Reflection plate 13a, 41-... Piezoelectric element 13b, 42... Mirror plate 14... Rotating polygon mirror
15... fθ lens 16... scanning surface 17.
・・Spot diameter sensor 18・・Control circuit 21・
...Imaging lens 22...ROM 23...Main scanning synchronization sensor 31...Surface sensor Figure 1 Patent applicant Konica Corporation representative Patent attorney Fujio Sasashima Figure 2 (A) (B) (A ) n (A) Figure 9 Iminari 1 Figure 10 (B)

Claims (6)

[Claims] (1) In a laser scanning system in which a laser beam is scanned on a scanning surface via a path of reflection on a reflecting surface, the reflecting surface is formed by attaching a mirror plate to a piezoelectric element, and the piezoelectric element is driven with electricity. 1. A spot diameter adjusting device for a laser scanning system, comprising a reflective surface curvature control means for adjusting the spot diameter of a laser beam on a scanning surface by controlling the curvature of the reflective surface. (2) The reflective surface curvature control means includes a spot diameter measuring means for measuring the spot diameter of the laser beam on the scanning surface, and is configured by energizing and driving a piezoelectric element based on an output signal from the spot diameter measuring means. The spot diameter adjustment device for a laser scanning system according to claim 1. (3) The spot diameter adjusting device for a laser scanning system according to claim 2, wherein the spot diameter measuring means is disposed at a position a predetermined distance away from the scanning surface in the traveling direction of the laser beam. (4) The spot diameter adjusting device for a laser scanning system according to claim 1, wherein the reflective surface curvature control means is configured by driving a piezoelectric element by energizing it based on a predetermined correction signal. (5) The spot diameter adjustment device for a laser scanning system according to claim 4, wherein the correction signal is determined for each reflecting surface of a rotating polygon mirror that deflects the laser beam, depending on the state of the reflecting surface. (6) The spot diameter adjusting device for a laser scanning system according to claim 4 or 5, wherein the correction signal is determined to change depending on the scanning position of the laser beam.