JP2939766B2 - Laser scanning spot diameter measuring device - Google Patents

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 an image forming surface, and particularly measures a spot diameter of the laser beam on the image forming surface. To a device that

<Prior Art> As this kind of conventional technique, for example, as shown in FIG. 9, a drum 62 having a slit 61 is rotated at a constant speed, and a light amount of a laser beam incident through the slit 61 is reduced. There is one that measures the spot diameter by measuring with an optical sensor 63 provided behind 61. That is, the larger the spot diameter of the laser beam, the smaller the amount of light per unit area. Therefore, the amount of the laser beam entering from the spot 61 is also small, so that the spot diameter can be measured.

In addition, there are those applied to laser printers such as those disclosed in JP-A-2-51119 and JP-A-60-100113.

In the former method, a laser beam is applied to a filter having a lattice pattern in which a transparent portion and a light-shielding portion are intermittent in the scanning direction. As the spot diameter of the laser beam increases, the amount of light passing through the transparent portion decreases and the light passes through the light-shielding portion. Because the amount of leakage light is large, the output change detected by the light-receiving sensor behind the filter is small, and when the spot diameter is small, the opposite relationship is used. Is what you do.

In the latter, the laser beam reflected by the mirror provided on the image plane is applied to the intersecting central part of the four divided light receiving elements via a cylindrical lens, and the spot shape that changes according to the defocus position is received by each light receiving element. The spot diameter is measured by detecting the light amount ratio of the element.

<Problems to be Solved by the Invention> However, in the first conventional example, since the measuring device is large and expensive and can measure only a stopped laser beam, a laser printer or a laser scanner that scans the laser beam is used. However, it is not suitable for adopting a method for adjusting the spot diameter while measuring the spot diameter.

Further, the second conventional example has a problem that a filter is required in addition to the detector, and the measured value varies depending on the accuracy of a grid pattern formed on the filter.

Further, the third conventional example has a problem that a cylindrical lens is required for spot diameter measurement, and the position of the lens needs to be adjusted.

The present invention has been made in view of such a conventional problem, and has a simple configuration in which a detection unit includes only a sensor, and can measure a spot diameter with high responsiveness and high accuracy to a scanned laser beam. It is an object of the present invention to provide a laser beam scanning system spot diameter measuring apparatus as described above.

<Means for Solving the Problems> For this reason, the present invention relates to a laser scanning system that scans a laser beam on an image forming surface, in which a plurality of divided light receiving elements are brought close to a scanning path of the laser beam on the image forming surface. In addition, the time required for the laser beam to move from one light receiving element to an adjacent light receiving element is measured from the output state of each light receiving element, and the spot diameter of the laser beam on the image plane is determined according to the moving time. Is provided with a measuring means for measuring.

For example, the light receiving element may be divided into two by a dividing line orthogonal to the laser beam scanning direction, and the measuring unit may measure a diameter of the spot in the laser beam scanning direction (hereinafter, referred to as a lateral diameter).

Further, the light receiving element is divided into two by a dividing line oblique to the laser beam scanning direction, and the measuring means measures the diameter of the spot in a direction perpendicular to the laser beam scanning direction (hereinafter, referred to as a vertical diameter). It can be.

Further, the light receiving element may be divided into three parts by a dividing line orthogonal to the laser beam scanning direction and a dividing line obliquely inclined, and the measuring means may measure the horizontal diameter and the vertical diameter of the spot. .

<Operation> When the spot diameter of the laser beam is small, from one light receiving element to the adjacent light receiving element. It moves in a short time, and at this time, the output change of a pair of adjacent light receiving elements is sharp, and when the spot diameter is large, it takes time to move,
Output change is gradual.

Therefore, the spot diameter can be measured by measuring the time required for movement between the light receiving elements from the output state of each light receiving element.

In the case where the light receiving element is divided into two by a dividing line orthogonal to the laser beam scanning direction, the movement is completed after one end of the lateral diameter of the spot contacts the dividing line and the other end contacts the dividing line. Therefore, the lateral diameter of the spot can be measured by the measuring means.

In the case where the light-receiving element is divided into two by a dividing line that is oblique to the laser beam scanning direction, a portion near one end of the vertical diameter of the spot comes into contact with the dividing line, and then the other end of the vertical diameter Since the movement is completed until the close part touches the dividing line, the vertical diameter of the spot can be measured by the measuring means.

Further, when the light receiving element is divided into three parts by a dividing line orthogonal to the laser beam scanning direction and a dividing line obliquely inclined, the measuring means measures the lateral diameter and the longitudinal diameter of the spot for the above-mentioned reason. be able to.

<Example> Hereinafter, an example of the present invention will be described with reference to the drawings.

In FIG. 1 showing the overall configuration of a laser printer provided with a spot diameter measuring device according to the present invention, a laser beam 2 emitted from a semiconductor laser 1 is converted into a parallel beam by a collimator lens 3 and then reflected by a mirror 4. After that, the light enters a light deflector 5 such as a rotary polygon mirror. The laser beam 2 is reflected and deflected by the optical deflector 5,
Photosensitive material (hereinafter referred to as film) passed through a converging lens 6
7 is scanned (main scanning) in a direction substantially perpendicular to the arrow direction.

At the same time, the film 7 is conveyed in the direction of the arrow Y by the conveying device 8 to perform sub-scanning, and therefore, the laser beam 2 is irradiated onto the film 7 two-dimensionally.

The semiconductor laser 1 irradiates the laser beam 2 immediately before or immediately after the transport device 8 to form an image.

The laser beam 2 is modulated based on an image signal output from an image signal output device, and an image carried by the image signal is provided on a film 7 exposed to the modulated laser beam. Is recorded as

The image signal is a digital signal, is passed through a signal converter, is converted into a signal based on a predetermined conversion table, is modulated into an analog signal by a D / A converter, and is input to a modulation circuit. The modulation circuit controls the driver of the semiconductor laser 1 based on an analog image signal, so that the laser beam 2 is modulated as described above.

The exposed film 7 is then sent to a known developing machine, where it undergoes development, fixing, washing, and drying.
By this processing, the photographic latent image is developed, and the image carried by the image signal is stored on the film 7 as a visible image.

The conveying device 8 for the film 7 includes a driving roller 9 that is rotationally driven by a motor (not shown) and the like.
A sub-scanning roller (main roller) composed of a plurality of driven rollers (nip rollers) 10 in the scanning direction rotating in contact with the roller 9 is provided, and the film 7 sandwiched between the rollers 9 and 10 is transported.

The spot diameter measuring device according to the present invention is provided in a laser printer having such a basic configuration.

That is, the optical sensor 11 is disposed on the same surface as the film 7, that is, on the image forming surface or a position optically equivalent to the image forming surface and at a position where the laser beam to be scanned hits.
The optical sensor 11 has light receiving elements 11a and 11b divided by a dividing line orthogonal to the scanning direction so as to be arranged close to the scanning direction as shown in FIG. 2, and the laser beam hits first. The output of the light receiving element 11a on the side is input to the + input terminal of the differential amplifier 12, and the output of the light receiving element 11b on the side to be applied later is input to the-input terminal of the differential amplifier 12.

The output of the differential amplifier 12 has two comparators 13, 1
Entered in 4. Here, one comparator 13 outputs a high level when the input voltage V _S from the differential amplifier 12 is higher than the comparison voltage V _A , and the other comparator 14 outputs the comparison voltage V _B (< When the input voltage V _S from the differential amplifier 12 is smaller than V _A ), the output is at a high level.

The outputs of the two comparators 13 and 14 are input to a timer 15, which outputs a time Δt ₂ from the falling edge of the pulse signal input from the comparator 13 to the rising edge of the pulse signal input from the comparator 14. Is measured.

The output of the timer 15 is input to a spot diameter calculation circuit 16, and the spot diameter calculation circuit 16 calculates the spot diameter from a table in which the relationship between this time and the spot diameter is stored in advance in ROM based on the measured time Δt. Ask.

In addition, the differential amplifier 12, the comparators 13, 14, the timer 15,
The spot diameter calculation circuit 16 constitutes a measuring means.

Here, when the spot diameter is small and when the spot diameter is large,
When the output change state of the differential amplifier 12 is compared, as shown in FIG. 3, in the former, the time Δt ₁ from the maximum output to the minimum output (Δt ₁ > Δt ₂ is set for a reason to be described later), that is, the light receiving element 11a It can be seen that the time required for the spot to move to the light receiving element 11b is short, and that the latter is long.

Incidentally, when the spot shape of the laser beam is a perfect circle, the spot diameter is uniformly determined with respect to the moving time. Therefore, when the perfect circle is the target shape of the spot and the diameter is adjusted by only one operation, the spot diameter may be adjusted by performing an operation based on the measurement result.

However, the spot shape of the laser beam is intentionally made to be an elliptical shape (for example, when the diameter of the spot diameter in the direction perpendicular to the scanning direction is corrected as a correction for the inclination of each reflecting surface of the rotary polygon mirror) or the target shape. Is a perfect circle, but the spot diameter in the scanning direction (horizontal diameter) and the diameter in the direction perpendicular to this (vertical diameter) are independent in order to correct an elliptical distortion due to accuracy or environmental changes. In a device having a two-dimensional operation mechanism for adjusting the horizontal and vertical axes, it is necessary to measure the horizontal and vertical diameters independently. Alternatively, it is conceivable to adjust only one of the horizontal diameter and the vertical diameter.

In that case, the light receiving sensor 11 having the light receiving elements 11a and 11b divided in a direction perpendicular to the scanning direction described above,
It becomes a sensor for measuring the lateral diameter. In other words, in the output change of the differential amplifier 12 shown in FIG. 4, the output from the maximum output from the time when one end of the lateral diameter of the spot comes into contact with the light receiving element 11b by scanning until the other end of the lateral diameter comes into contact with the light receiving element 11b. since changes to a minimum, by the movement time Delta] t _1, the lateral diameter is determined.

However, since the slope of the waveform between the initial part and the last part of the movement time Δt ₁ is gentle, it is preferable that a trigger point (comparison voltage of the comparators 13 and 14) causes a large error to occur around this point. Since there are no such points, for example, the points A and B where the amplitude of the waveform becomes 1/2 are used as trigger points to measure the movement time Δt ₂ of the portion where the output changes abruptly, thereby improving the accuracy. In this case, if the elliptical shape is maintained even if the vertical diameter is different, the output waveform does not change, so that the horizontal diameter can be determined. FIG. 5 shows the transit time Δt ₂ and the differential amplifier
This is a graph showing equal horizontal diameter lines for 12 output values (values when the maximum value is 1). For example, the moving time Δt ₂ is 108n
The lateral diameter in the case of s is 116 μm.

Further, the gap between the light receiving elements 11a and 11b of this type of split type sensor may be about 10 μm,
When measuring a spot diameter of μm or more, there is not much problem. When measuring a spot diameter smaller than that, it is desirable to reduce the gap width. If the spot diameter is about 1/5 or less, the waveform distortion does not pose a problem. When the lateral diameter measured in this way is distorted with respect to the reference value for some reason, the lateral diameter is always feedback-controlled by the lateral diameter adjusting mechanism provided in the optical system of the main unit, so that the lateral diameter is always controlled. The lateral diameter can be kept at the optimum value.

It should be noted that although it is possible to measure for a time only by passing through one light receiving element without using such a split type sensor, the split type sensor can use the steep waveform portion as described above, Measurement accuracy is improved.

According to such a spot diameter measuring apparatus (the same applies to the following embodiments), the apparatus can be manufactured in a small size and at low cost, can be easily incorporated into a main body device such as a laser printer, and can scan laser beams. Can be measured with good response.

In the case of a scanning system in which the spot diameter is minimized, it is preferable to dispose the light receiving sensor at a position separated from the position equivalent to the imaging plane by the depth of focus or more. Thereby, it is possible to determine whether the focal position of the laser beam is shifted to the near side or far side with respect to the imaging plane.

Furthermore, in this measurement, a high-speed response is required, and therefore, it is desirable that the sensor is small. However, in order to keep the mounting accuracy from becoming strict, the sensor may be vertically elongated.

Next, an example of measuring the vertical diameter of the spot diameter will be described.

In the case of vertical diameter measurement, as shown in FIG. 6, a light receiving sensor 21 having light receiving elements 21a and 21b divided into two so that a dividing line is slightly inclined with respect to a laser beam scanning direction is used. The other circuit configuration and measurement principle are the same as those in the case of the lateral diameter measurement shown in FIG. 2, and as shown in the figure, when the output V _S of the differential amplifier 12 becomes equal to or less than the comparison value _{VA, the} comparison value V _B The vertical diameter is obtained from a table stored in the ROM in advance, based on the time Δt ₂ between AB until the following.

The inclination angle of the dividing line with respect to the scanning direction is, theoretically, the smaller the inclination angle, the smaller the influence of the horizontal diameter and the more independent the vertical diameter because the smaller the inclination angle, the closer to the dividing line at the lower and upper ends of the spot vertical diameter. Although the measurement can be performed, the mounting accuracy of the light receiving sensor 21 becomes strict because the laser beam crosses the dividing line. On the other hand, if the inclination angle is too small, the mounting angle error affects the measured value, which is not preferable. Although the mounting angle of the sensor differs depending on the size of the light receiving sensor and the vertical diameter to be measured, it is preferable to incline the sensor by about 15 ° or more for a sensor of about 1 mm or about 5 ° or more for a sensor of about 3 mm or more. In that case, if there is enough space, it is advantageous to use a sensor that is long in the scanning direction, because the inclination angle can be reduced while maintaining the mounting accuracy. vice versa,
If the inclination angle is too large, not only does the detection sensitivity to a change in the vertical diameter become dull, but also the influence of the horizontal diameter on the measured value of the vertical diameter becomes large.
However, this is the case where only the vertical diameter is measured, and if used together with the horizontal diameter measurement as in the embodiment described later, the influence of the horizontal diameter can be removed by calculation, so that the inclination angle can be further increased.

When it is necessary to measure the horizontal diameter and the vertical diameter, or
As an embodiment in which the measurement of the vertical diameter is used in combination with the measurement of the horizontal diameter in order to accurately measure the influence of the horizontal diameter except for the influence of the horizontal diameter, the two embodiments are used as they are, and the two types of light receiving sensors 11 and 21 are used.
May be arranged in the scanning direction, and the horizontal and vertical diameters may be measured from these detection signals. In that case, as described above, when measuring the longitudinal diameter, when removing the influence of the lateral diameter by the measured value of the lateral diameter, the inclination angle of the dividing line that is obliquely inclined is compared with the case where the longitudinal diameter is measured alone. , To some extent.

In a more rational embodiment, as shown in FIG.
Light receiving sensor 3 having light receiving elements 31a, 31b, 31c divided into three by a dividing line orthogonal to the scanning direction and an inclined dividing line
Using 1, the horizontal diameter may be measured by the time required to move from the light receiving element 31a to the light receiving element 31b, and then the vertical diameter may be measured by the time required to move from the light receiving element 31b to the light receiving element 31c. As described above, when measuring the vertical diameter, the measurement accuracy of the vertical diameter can be improved by removing the influence of the horizontal diameter from the measured value of the horizontal diameter.

FIG. 8 shows the relationship between the moving time and the vertical diameter when the inclination angle of the dividing line is 20 ° and the horizontal diameter is 70 μm. For example, when the movement time Δt ₂ is 370 ns, the vertical diameter is 145 ns.
μm.

<Effects of the Invention> As described above, according to the present invention, a laser beam spot diameter can be measured with high accuracy and responsiveness, while being a small and inexpensive measuring device. It is of high utility value as a measurement for adjusting the spot diameter by being incorporated into the device.

Also, by using only the light receiving sensor that makes the dividing line of the light receiving element orthogonal to the scanning direction, the horizontal diameter of the spot diameter can be measured, and by using the light receiving sensor that is inclined with respect to the scanning direction, the vertical diameter can be measured. it can.

Furthermore, by using two of these two types of sensors side by side,
Alternatively, by using a sensor divided into three by the orthogonal dividing line and the inclined dividing line, it is also possible to measure the lateral diameter and the longitudinal diameter at the same time. It is also possible to perform highly accurate measurement by removing the influence of.

[Brief description of the drawings]

FIG. 1 is a perspective view showing the overall configuration of an embodiment for measuring the spot lateral diameter of the present invention, FIG. 2 is a circuit diagram of a portion relating to the spot diameter measuring apparatus of the above embodiment, and FIGS. FIG. 4B is a diagram showing an output change state of the light receiving sensor when the spot diameter is small and when the spot diameter is large in the embodiment. FIG. 4 is a diagram for explaining a measurement time portion of the actual light receiving sensor in the embodiment. FIG. 6 is a diagram showing the relationship between the spot moving time and the lateral diameter with respect to the output of the light receiving sensor of the above embodiment. FIG. 6 is a diagram showing the light receiving sensor of the embodiment for measuring the spot longitudinal diameter of the present invention and its output state. FIG. 7 is a diagram showing a light receiving sensor according to an embodiment of the present invention for simultaneously measuring the horizontal and vertical diameters of the spot and the output state thereof. FIG. FIG. 9 shows a conventional spot diameter measurement. Is a perspective view showing an example of a device. 2: laser beam, 7: film, 11, 21, 31: light receiving sensor, 11a, 11b, 21a, 21b, 31a, 31b, 31c: light receiving element, 12 ...
Differential amplifier, 13,14… Comparator, 15… Timer, 16…
Spot diameter calculation circuit

Claims (4)

(57) [Claims] In a laser scanning system for scanning a laser beam on an image forming surface, a plurality of divided light receiving elements are arranged close to a scanning path of the laser beam on the image forming surface, and the laser beam is emitted. Measuring means for measuring a time required to move from one light receiving element to an adjacent light receiving element from an output state of each light receiving element and measuring a spot diameter on an imaging plane of a laser beam according to the moving time; Characteristic spot diameter measuring device for laser scanning system. 2. The laser scanning system according to claim 1, wherein the light receiving element is divided into two by a dividing line orthogonal to the laser beam scanning direction, and the measuring means measures a diameter of the spot in the laser beam scanning direction. Spot diameter measuring device. 3. The light receiving element is divided into two by a dividing line oblique to the laser beam scanning direction, and the measuring means measures the diameter of the spot in a direction perpendicular to the laser beam scanning direction. The spot diameter measuring device for a laser scanning system according to item 1. 4. The light-receiving element is divided into three parts by a dividing line orthogonal to the laser beam scanning direction and a dividing line inclined obliquely.
2. The spot diameter measuring apparatus for a laser scanning system according to claim 1, wherein the measuring means measures the diameter of the spot in the scanning direction of the laser beam and the diameter of the spot in a direction perpendicular thereto.