JPH07253312A - Method and apparatus for measuring scanning accuracy in scanning optical system - Google Patents

Abstract

PURPOSE:To provide a method and an apparatus which enables accurate measurement even when there is a difference in the quantity of light reaching two sensors by arranging the two optical sensors within a scanning surface of a scanning optical system using a rotary polygonal mirror to measure the interval therebetween. CONSTITUTION:A beam B emitted from a light source 11 is reflected on a rotary polygonal mirror 13 reaching an imaging surface passing through an ftheta lens 15 and enters two optical sensors 20a and 20b placed on the imaging surface. Then, a time difference t1 is measured from outputs of the two optical sensors. Then, the direction of rotation of the polygonal mirror 13 is reversed to measure a time difference t2 likewise. Then, a correct time difference DELTAt) is determined by t=(t1+t2)/2. Measuring errors DELTAt are canceled by turning the polygonal mirror 13 both normally and oppositely.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to measurement and evaluation of a writing scanning optical system using a rotary polygon mirror, and more particularly to a method and apparatus for measuring scanning accuracy including measurement of jitter and magnification error of a scanning beam.

[0002]

2. Description of the Related Art As a device or method for measuring the scanning accuracy of a laser beam emitted from a scanning optical system using a rotating polygon mirror, the applicant has proposed the device shown in FIG. In the figure, 1 is a light source for emitting a laser beam L, 2 is a rotating polygon mirror, 3 is an fθ lens, 4 and 5 are photosensors including photodiodes, 6 and 7 are movable stages equipped with photosensors, and 8 is The pulse stage which carries these movable stages and moves is shown. The optical sensors 4 and 5 are
The pulse stage 8 and the movable stages 6 and 7 are movable in the image plane of the fθ lens 3 (that is, in the scanning plane).

In the above structure, first, the laser beam L from the light source 1 is applied to the rotary polygon mirror 2 which rotates in the direction of the arrow. The laser beam is reflected by the rotating polygon mirror and fθ
The light enters the lens 3 and goes toward the scanning surface on which the optical sensors 4 and 5 are placed. The laser beam L directed to the scanning surface is scanned by the rotation of the rotary polygon mirror 2 from the position at the upper end of the scanning surface to the lower position through the center position as shown by L1 → L2 → L3. Then, when one movable stage 6 is fixed to the upper end of the pulse stage 8 in the drawing and the other movable stage 7 is moved in the scanning plane to separate the two optical sensors 4 and 5, the laser beam is detected by the optical sensor. It is incident on 4,5 with a time lag and is detected.

The detection outputs of the optical sensors 4 and 5 have a waveform as shown in FIG. 8 through a waveform shaping circuit (not shown). The value of 50 to 90% of this output waveform is the threshold value S
And the time t (interval) from the time when the output of the optical sensor 4 reaches this threshold to the time when the output of the optical sensor 5 reaches this threshold is measured. Next, the fixed and movable movable stages 6 and 7 are reversed, and the interval is measured in the same manner as above. Further, the optical sensors 4 and 5 are moved at a constant interval so that the centers of the optical sensors 4 and 5 are f
The magnification error (linearity) can be calculated by comparing the time difference t at the other position with the time difference t in the state where it coincides with the optical axis X-X of the θ lens 3.

[0005]

However, in the above measuring method, if the amount of light of the scanned laser beam reaching each optical sensor is different, the problem shown in FIG. 9 arises. That is, in FIG. 9, if the amount of light reaching the optical sensor 5 is the same as the amount of light reaching the optical sensor 4, the output of the optical sensor 5 is as shown by the dotted line, but the amount of light reaching the optical sensor 5 is If the number is small, the output will be as shown by the solid line. Since the threshold values are the same, the original t cannot be measured as the time difference, and t'is measured.
An error of Δt will be included.

When the laser beams reaching the optical sensors 4 and 5 have uneven light amount due to internal reflection or interference,
As shown in FIG. 10, the output waveform of the optical sensor is disturbed,
When t ″ is measured with respect to the time difference t, the error Δt ′ is included.

An object of the present invention is to solve the above-mentioned problems, and an object thereof is to provide a measuring method of scanning accuracy capable of accurately measuring even if there is a difference in the amount of light reaching an optical sensor. I am trying. Another object of the present invention is to provide a scanning accuracy measuring method and an apparatus for measuring the scanning accuracy in which the light amounts reaching a plurality of photosensors are equalized and an error due to a light amount difference does not occur.

Further, the present invention provides a scanning accuracy measuring method and a measuring apparatus capable of accurately measuring the light reaching the optical sensor even if the light reaching the optical sensor includes disturbance due to internal reflection. It is an object.

[0009]

To achieve the above object, the present invention reflects light from a light source on a reflecting surface of a rotary polygon mirror and forms an image of the reflected light on an image forming surface of an optical system. In the scanning optical system for scanning, two optical sensors are provided along the scanning direction on the scanning surface, the rotating polygon mirror is rotated in one direction, and the time for scanning between the two optical sensors is obtained, Next, the rotary polygon mirror is rotated in the opposite direction at the same speed, the time for scanning between the two optical sensors is obtained, and both times are averaged to obtain the scanning time.

In addition, a configuration is used in which internal reflection components are removed from the beams incident on the two photosensors by using a polarization filter, or at least one of the two photosensors is moved so that both of the two positions at each position on the scanning surface. It is desirable to have a configuration in which the scanning time between the optical sensors is obtained.

In the apparatus of the present invention, a scanning optical system using a rotary polygon mirror is provided with an optical sensor, at least one of which is movable, on the image plane, and scanning is provided with an apparatus for measuring a scanning time between the pair of optical sensors. A scanning accuracy measuring device in an optical system is characterized in that at least one of the two optical sensors is provided with a filter so that the amount of light reaching each of the optical sensors becomes equal. Further, it is desirable that a polarization filter is provided in front of the two optical sensors.

[0012]

The beam emitted from the light source is reflected by the rotating polygon mirror, passes through the scanning optical system and reaches the image plane, and is rotated on the image plane to be placed on the image plane. It is incident on two other optical sensors. Then, the time difference t ₁ is measured from the outputs of the two optical sensors. Next, the rotation direction of the polygon mirror is reversed, and the time difference t ₂ is measured in the same manner. And
The correct time difference t is obtained by t = (t ₁ + t ₂ ) / 2. The measurement error Δt can be canceled by rotating the polygon mirror both forward and backward.

Alternatively, each light sensor is provided with a filter, and the transmittance of the beam is appropriately set to equalize the amount of light incident on each light sensor and measure the time difference t. In this case, it is not necessary to reverse the rotation direction of the polygon mirror.

Further, each optical sensor is provided with a polarization filter,
The internal reflection component in the incident beam is cut and made incident on the optical sensor. As a result, the output waveform is not disturbed, and a shaped waveform can be obtained. In the case of this method, it is used in combination with either the method of turning the polygonal mirror forward or backward, or the method of providing a filter to appropriately set the transmittance.

[0015]

Embodiments of the present invention will be described below with reference to the drawings. FIG. 1 (a) is a block diagram of an apparatus for carrying out the method of the present invention. A scanning optical system 10 as a unit to be measured shown in the figure includes a light source 11, a cylindrical lens 12, a rotary polygon mirror 13, a motor 14, an fθ lens 15 and a toric lens 16.

Reference numeral 17 designates the mechanical part of the device for measuring the scanning accuracy. This device 17 includes two slits 18a and 18b, two polarization filters 19a and 19b,
Optical sensors 20a, 20b, motorized stages 21a, 21
b and one pulse stage 22.
Slits 18a, 18b and polarization filters 19a, 19
b is integrally attached to each of the optical sensors 20a and 20b, of which the polarization filter is rotated and fixed at a position where internal reflection is minimized. Each optical sensor is fixed to the motorized stages 21a and 21b.
Each of a and 21b is independently movable on the pulse stage 22 according to an instruction from the pulse stage 22. Further, the pulse stage 22 includes the optical sensors 20a and 20b.
It is possible to detect the position where is.

FIG. 2 is a block diagram showing the electrical construction of the apparatus of FIG. 1 (a). The optical sensors 20a and 20b include
The waveform shaping circuits 23a and 23b are connected to each other, and their outputs are input to the time interval high speed counter 24 to measure the time for scanning between the two optical sensors.

The output of the time interval high-speed counter 24 is input to an arithmetic unit 25 composed of a microcomputer, and the result is displayed on a display unit 26 such as a CRT or a printer.
Output to. In addition, from the arithmetic unit 25 to the motor driver 2
7 is instructed to rotate the motor 14 forward or backward. The output of the arithmetic unit 25 is connected to the pulse stage 22 via the controller 28 and moves the positions of the optical sensors 20a and 20b.

Next, the operation of the apparatus of the present invention will be described with reference to FIGS. When the measurement is started, a beam B is emitted from the light source 11 on the unit under measurement 10, the beam B is reflected by the rotary polygon mirror 13, passes through the fθ lens 15 and the toric lens 16, and is emitted from the unit under measurement 10. The light is emitted and reaches the scanning surface of the optical sensors 20a and 20b.

Next, the arithmetic unit 25 includes a controller 28.
The optical sensor 2 by issuing an instruction to the pulse stage 22 via
0a and 20b are moved to desired positions. next,
The arithmetic unit 25 receives the polygon mirror 13 via the motor driver 27.
The beam B to B ₁ → B ₃
And scan.

The beam passes through the slits 18a, 18b and the polarization filters 19a, 19b and passes through the optical sensors 20a, 20b.
20b, and the output curves shown by the solid lines in FIG. 1B are obtained from the optical sensors 20a and 20b. Therefore, an appropriate threshold value S is determined and the interval t ₁ is measured. This time t ₁ includes an error Δt due to the difference in the amount of light incident on the optical sensor.

When the above measurement is performed, the calculating means 25 stores all necessary numerical values, keeps the positions of the optical sensors 20a and 20b as they are, and causes the motor driver 27 to drive the motor 14
Order the reversal at the same speed. Then the beam is B ₃ →
Since scanning is performed in the opposite direction to the direction B ₁ , the time (interval) t ₂ required for the optical sensors 20b to 20a can be measured as shown in FIG. 1 (b).

As can be seen from FIG. 1B, the normal rotation time t ₁ is the correct time t plus Δt, and the reverse rotation time t ₂ is the correct time t minus Δt. .
Therefore, an accurate interval t can be obtained by t = (t ₁ + t ₂ ) / 2. The movable stage 21b on which the optical sensor 20b is mounted is moved downward in FIG. 1 (a) and the above measurement is repeated until the positions 20b 'and 21b' are reached, and the interval at each position can be obtained. The measurement result and the calculation result are output as a table or a figure on the display device 26.

Further, the optical sensors 20a and 20b are moved from one end to the other end of the scanning range while keeping a constant interval, and the interval between the optical sensors at each position is measured. The centers of the optical sensors 20a and 20b are fθ. The magnification error of the scanning beam can be calculated by calculating the ratio of the interval at each position with the interval when the optical axis of the lens 15 overlaps as a reference.

In the embodiment shown in FIG. 1, the polarization filters 19a and 19b are used. However, this makes it possible to eliminate the light internally reflected from the photosensor, as described with reference to FIG. Accurate measurement is possible by preventing the disturbance of the output waveform.

FIG. 4 shows the configuration of another embodiment of the present invention. This corresponds to the optical sensors 20a, 2a in the embodiment of FIG.
The filters 30a and 30b having different transmittances are attached in front of 0b. Optical sensor 20a, 20b
Since the amount of light incident on the scanning line is determined by the position on the scanning line, the transmittance of the filter 30 corresponding to the position of each optical sensor is reduced.
When a and 30b are attached, each photosensor receives the same amount of light. Therefore, filters having various transmittances are prepared for the filters 30a and 30b, and the filters having the required transmittance are exchanged. According to this method, it is not necessary to rotate the motor both forward and backward.

As described above, since the amount of light incident on the photosensor is determined by the position on the scanning line, a well-known filter whose transmittance can be continuously changed is used as the filters 30a and 30b. The relationship between the scanning position and the amount of light reaching is input to the arithmetic unit 25 in advance, and the pulse stage 22 uses the optical sensors 20a,
When 20b is moved, the transmittance automatically adapted to the position can be obtained.

FIG. 5 is a flow chart showing the operation of the apparatus of FIG. Basically, it is the same as the flowchart of FIG. 3, except that the filter is switched, that is, the transmittance is set instead of determining the rotation direction of the motor. It should be noted that if the method shown in FIG. 4 is further used in combination with the method of rotating in the forward and reverse directions, the measurement accuracy can be improved as compared with the case of using it alone.

FIG. 6 shows a jitter measuring method, in which the optical sensors 20a and 20b shown in FIG. 4 are fixed to both ends of the scanning surface.
Same as above. In this case, only the jitter is measured.

[0030]

As described above, according to the present invention,
Two optical sensors are provided along the scanning direction on the scanning surface, the time for scanning between the two optical sensors is obtained by rotating the rotary polygon mirror in the forward or reverse direction, and both times are averaged to obtain an interval. In this configuration, or at least one of the two photosensors is provided with a filter so that the amount of light reaching any of the photosensors becomes equal, so that even if there is a difference in the amount of light reaching the photosensor due to scanning, Accurate scanning accuracy can be measured without being affected by the difference.

Further, if the structure using the polarization filter is used, the internal reflection can be cut, and the output waveform of the optical sensor can be shaped for accurate measurement. If at least one of the two optical sensors is moved and the scanning time between the optical sensors at each position on the scanning surface is obtained, the intervals at various positions can be measured and the jitter magnification error of the scanning beam is also obtained. be able to. If the filter is configured so that the transmittance can be continuously changed according to the scanning position, the filter replacement work becomes unnecessary and the measurement time can be shortened.

[Brief description of drawings]

1A is a diagram showing a configuration of an apparatus for carrying out the measuring method of the present invention, and FIG. 1B is a diagram explaining the measuring method of the present invention.

FIG. 2 is a block diagram showing an electrical configuration of an apparatus for carrying out the measuring method of the present invention.

FIG. 3 is a flow chart illustrating the operation of the apparatus of FIGS.

FIG. 4 is a diagram showing a configuration of another embodiment for carrying out the measuring method of the present invention.

5 is a flow chart illustrating the operation of the apparatus of FIG.

FIG. 6 is a diagram showing the configuration of another example for carrying out the measuring method of the present invention.

FIG. 7 is a diagram showing a configuration of an apparatus for performing a conventional measuring method.

FIG. 8 is a diagram illustrating a conventional measurement principle.

FIG. 9 is a diagram illustrating a problem in conventional measurement.

FIG. 10 is a diagram illustrating a problem in conventional measurement.

[Explanation of reference numerals] 10 unit to be measured 11 light source 13 rotating polygon mirror 15 fθ lens 18a, 18b slits 19a, 19b polarization filter 20a, 20b optical sensor 30a, 30b filter

Claims (6)

[Claims] 1. A scanning optical system in which light from a light source is reflected by a reflecting surface of a rotating polygonal mirror, and the reflected light is imaged and scanned on an image forming surface of an optical system. Two photosensors are provided along the optical axis, the rotary polygon mirror is rotated in one direction to obtain the time for scanning between the two photosensors, and then the rotary polygon mirror is rotated in the opposite direction at the same speed. A method for measuring scanning accuracy in a scanning optical system, characterized in that the scanning time between two optical sensors is obtained and both times are averaged to obtain the scanning time. 2. The method for measuring scanning accuracy in a scanning optical system according to claim 1, wherein an internal reflection component is cut from the beams incident on the two optical sensors by using a polarization filter. 3. The scanning optical system according to claim 1, wherein at least one of the two optical sensors is moved to obtain a scanning time between the optical sensors at each position of the scanning surface. How to measure accuracy. 4. A scanning optical system in which at least one optical sensor is provided on an imaging surface of a scanning optical system using a rotary polygon mirror, and a device for measuring a scanning time between the pair of optical sensors is provided. In the accuracy measuring device, at least one of the two photosensors is provided with a filter so that the amount of light reaching any of the photosensors is equalized. 5. The scanning accuracy measuring device in a scanning optical system according to claim 4, wherein a polarization filter is provided in addition to the two optical sensors. 6. The scanning accuracy measuring device in a scanning optical system according to claim 4, wherein the filter is capable of continuously changing the transmittance according to the scanning position.