JP2574400B2 - Scanning light distortion correction device - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION [Summary] Regarding scanning for correcting distortion of scanning light in an optical scanning device using a laser beam, in a scanning optical system using a parabolic mirror, a scanning beam on a projection surface is A beam scanning optical system that scans a beam deflected by a rotating deflecting mirror in a predetermined direction with a parabolic mirror with the aim of correcting the beam so as to draw a substantially linear trajectory. , The beam is incident from an oblique direction, and the resulting trajectory of the deflected beam is configured to draw a trajectory that substantially cancels the curved trajectory of the scanning beam by the parabolic mirror.

[Industrial applications]

The present invention relates to an optical scanning device used for laser recording, laser image reading, laser processing, laser surface inspection, and the like using a laser beam, and more particularly to an apparatus for correcting a bending distortion of the scanning light.

[Conventional technology]

Generally, in an optical system that scans a focused laser beam, a laser beam from a laser light source is scanned by a rotating polygon mirror (polygon mirror) and converged on a recording medium by an F • Θ lens system. On the other hand, an optical system using a parabolic mirror (parabolic mirror) has been developed and put into practical use instead of the expensive F / Θ lens system (for example, Japanese Patent Application Laid-Open No. 52-4109,
JP-A-52-56547 and JP-A-56-155916, or Abe and Matsuda co-authored, "Light Beam Scanning Deviation Correction Method Using Parabolic Mirror", Optics p67-74, published April 1977).

As is well known, an optical system using a parabolic mirror has the following advantages as compared with an optical system using an F • Θ lens system.

(1) Since it is a mirror, it is not affected by the wavelength and therefore has no chromatic aberration.

(2) A relatively large scanning length can be easily obtained.

(3) A high-performance system can be realized at relatively low cost.

(4) A large amount of reflected light can be detected.

[Problems to be solved by the invention]

However, on the other hand, in an optical system using a parabolic mirror, it is necessary to shift the optical axis of the incident beam and the optical axis of the reflected beam, that is, to provide a so-called off-axis angle between both beams. (Otherwise the reflected beam coincides with the incident beam and cannot be detected), which has the disadvantage that the scanning beam is curved on the projection surface.

FIGS. 13 and 14 are diagrams for explaining the curvature of the scanning beam. In FIG. 1, a laser beam 10 is irradiated from a predetermined direction in a horizontal plane (a plane parallel to the Z axis), and is reflected and scanned in a predetermined direction by a rotating mirror (polygon mirror, galvano mirror, etc.) 13. The beam 10 applied to the parabolic mirror 11 is reflected there and forms an image on a scanning plane (projection plane) 17.

The incident position of the scanning beam on the parabolic mirror 11 changes sequentially with the rotation of the rotating mirror 11, but the parabolic mirror 11 has a three-dimensional curved shape like a parabolic antenna.
As indicated by 0 ', the projection surface 17 is changed according to the position of reflection on the parabolic mirror 11.
Continuously changes the optical distance from
The above image forming position shifts and curves as shown by 10C. This degree of curvature is represented by the height difference d of the scanning beam. In addition, Θ
(FIG. 14) is the off-axis angle.

Various adverse effects such as a scanning beam curvature may cause a reading error when used as a beam scanner in a POS system, or may cause printing defects when used as a beam scanner in a reeser printer. Effect.

SUMMARY OF THE INVENTION It is an object of the present invention to realize an optical system capable of realizing linear scanning by correcting such a curvature of a scanning beam on a projection surface.

[Means for solving the problem]

The inventor of the present application has focused on the fact that the above-mentioned problem arises because the beam is incident perpendicular to the rotation axis of the deflecting mirror (rotating mirror). That is, the light reflected by the parabolic mirror is projected onto the projection surface.
This is for drawing a curved trajectory corresponding to the curved beam 10C on 17. Therefore, this problem can be solved by creating reflected light by a parabolic mirror that draws a locus that cancels out the curvature of the beam on the projection surface. Therefore, the most characteristic feature of the present invention is that the light is basically incident obliquely with respect to the optical axis of the deflecting mirror.

FIG. 1 shows the basic principle of curvature correction in the present invention.

In general, with respect to the rotation axis O of the deflection mirror M enters the light beam vertically, as shown in FIG. 1 (1), the trajectory B _o of the deflected beam is a straight line when viewed in a projection plane S. On the other hand, when the beam is incident obliquely with respect to the mirror rotation axis O, the trajectory B _{0 of the} deflected beam will be curved in an arc shape as shown in FIGS. 1 (2) and (3) because the optical path lengths will not be the same. The phenomenon is seen. FIG. 1 (2) shows a case where light is incident from obliquely above, and FIG. 1 (3) shows a case where light is incident from obliquely below. The degree of curvature depends on the angle of incidence of the beam. That is, it is possible to substantially desired curvature ray trajectory B _o of the deflected beam by designing the beam incident angle to an appropriate value.

 The present invention utilizes this principle.

That is, the present invention is based on the idea that the curvature of the deflected beam (reflected beam) cancels out the curvature caused by the parabolic mirror generated later.

In order to achieve the above object, according to the distortion correction apparatus of the present invention, a beam is incident obliquely at a predetermined incident angle with respect to the rotation axis of the deflecting mirror, and the trajectory of the resulting deflected beam is emitted. The configuration is characterized by drawing a trajectory that substantially cancels out a curved trajectory of the scanning beam by the object mirror.

Preferably, a predetermined off-axis angle is given to the parabolic mirror, and the beam incident angle on the deflecting mirror is set to approximately 0.65 times the off-axis angle of the parabolic mirror.

Preferably, the beam incident on the deflecting mirror is made incident from the front of the deflecting mirror when the deflecting mirror faces the parabolic mirror.

Preferably, the rotation axis of the deflecting mirror is inclined with respect to the perpendicular of the optical axis of the parabolic mirror by the same angle as the angle of incidence of the beam on the deflecting mirror.

[Operation] When the beam is incident on the deflecting mirror from an oblique direction, the deflected beam follows a curved trajectory as described above.

On the other hand, the scanning beam by the parabolic mirror necessarily draws a curved trajectory as described above.

Since the curved trajectory of the deflected beam by the deflecting mirror is a curve in a direction that cancels out the curved trajectory of the scanning beam by the parabolic mirror, the scanning line on the scanning plane is eventually substantially straight.

It has been confirmed that the best linearity of the scanning beam is obtained when the beam incident angle on the deflecting mirror is set to approximately 0.65 times the off-axis angle of the parabolic mirror.

If the beam incident on the deflecting mirror is made to enter from the front of the deflecting mirror when the deflecting mirror is facing the front with respect to the parabolic mirror, the deflecting mirror sets the required range of deflection angle in a small area only at the center. The cover can be covered, and the size of the deflection mirror can be reduced.

By tilting the rotation axis of the deflecting mirror with respect to a line perpendicular to the optical axis of the parabolic mirror by the same angle as the angle of incidence of the beam on the deflecting mirror, the reflected light of the deflecting mirror is converted to the optical axis of the parabolic mirror. Parallel to

Examples Examples of the present invention will be described below.

First, a method of determining a specific arrangement of each element of the optical system according to the present invention will be described (second, third, fourth).
Figure).

1. Derivation of normal vector of deflection mirror M Incident angle β of beam 10 to deflection mirror (rotating mirror) M
(Hereinafter referred to as “curvature correction angle”) is a geometric calculation in the illustrated orthogonal three-dimensional XYZ coordinate system from the focal length, off-axis angle, scan length, and deflection position of the parabolic mirror P. Required by

The rotation angle of the deflecting mirror M is α in a counterclockwise direction (counterclockwise) when viewed from the X-axis direction (the optical axis direction of the parabolic mirror). The inclination of the rotation axis OO of the deflecting mirror M is defined as β in a clockwise direction (clockwise) when viewed from the Y-axis direction (a vertical plane direction orthogonal to the optical axis of the parabolic mirror).

First, the normal vector of the mirror when α = β = 0 is
If N ₀ , then N _o = (0,0, −1) (1).

The rotation matrix at this time is as follows.

The normal unit vector N of the deflecting mirror is expressed by the following equation.

_{N = N o R x (α} ) R y (β) = (0, sinα, -cos α) R y (β) = (cos αsin β, sin α, -cosαcos β) ......
(4) 2. Derivation of the formula of the reflected light from the deflecting mirror If the vector of the beam incident on the deflecting mirror is I (the direction is opposite to the beam) and the vector of the reflected beam is R (the direction is the same as the beam), I = (Tan2β, 0, -1) (5) A = IN = tan2βcos αsin β + cos αcos β
(6) N ₁ = (IN) N = AN (7) R = 2N ₁ −I = 2AN−I = 2 (tan2βcosαsinβ + cosαcosβ) x (cosαsinβ, sinα, −cosαcosβ) ) - (tan2β, 0, -1 ) = (r x, r y, r z) ...... (8) Therefore, the coordinates of the reflection point of the deflecting mirror (m _x, m _y, When m _z), reflecting The beam is represented by the following equation.

(X, y, z) = uR + (m x, m y, m z) = (ur x + m x, ur y + m y, ur z + m z) ...... (9) Here, u is a parameter.

3. Derivation of the coordinates of the intersection of the reflected light and the parabolic mirror The paraboloid is expressed by the following equation.

z = (x ² + y ² ) / (4f) (13) where f is the focal length of the paraboloid.

By substituting equation (13) into equation (12), the following equation is obtained: (x ² + y ² ) / (4f) = ur _z + m _z .

 Also, the equations (10) and (11) are substituted into the equation (14).

｛(Ur _x + m _x ) ² + ur _y + m _y ) ² ｝ / (4f) = ur _z + m _z ...
(15) Let u be u _{k at} this time.

u _k when _{α = 0 = (4fm z +} m x 2 -m y 2) / (2 (r x m x + r y m y -2fr z)) ...... (17) the intersection K of the reflected light and the parabolic Are represented by the following equation (k _x , k _y , k _z ).

4. Equation of reflected light of parabolic mirror The normal unit vector N _{k at} the intersection K of the parabolic mirror is obtained by the following equation.

When the vector of the reflected light of the paraboloid and S, S = 2 (R · N k) N k -R S o = (- k x r x / f-k y r y / f + 2r x) / ((k ^{_{x / 2f) 2 + (k}} y / 2f) 2 +1) Therefore, the reflected light of the parabolic mirror is given by the following equation.

Where t is a parameter.

5. Beam / Spot Irradiation Position The coordinates of the beam irradiation plane (projection plane) are expressed by the following formula.

z = tan θx + f (28) The equation (28) is substituted into the equation (27).

t = (tan θx + f−k _z / s _z ) (29) By substituting this t into the equations (25) and (26), the scanning position can be obtained. Therefore, it is necessary to divide the X position (X _s ) by cos θ, so that X _s = (s _x t + k _x ) / cos θ (30) The trajectory (curvature) of the scanning light is obtained by the above calculation. .

5 and 6 show the arrangement structure of the scanning optical system according to one embodiment of the present invention.

In terms of hardware, it is basically the same as that shown in FIG. That is, the deflection mirror faces the parabolic mirror 11.
13 is placed rotatably about a rotation shaft 13a. Rotary shaft 13a
Is connected to a rotation drive source such as a motor (not shown), and is rotated about a rotation axis OO. The deflecting mirror 13 may be a rotating mirror (polygon), and a galvano mirror or the like may be used. Preferably, the rotation axis OO of the deflection mirror 13 is inclined with respect to the X axis by an angle equal to the curvature correction angle β. This is to make the reflected beam parallel to the optical axis (Z axis) of the parabolic mirror 11 when the incident beam enters the deflection mirror 13 at an incident angle of the curvature correction angle β as described later. It is. That is,
Since the parabolic mirror 11 has a function of converging the parallel light to its focal point (ie, the projection plane 17 located at a focal distance f from the parabolic mirror 11), the beam incident on the parabolic mirror 11, that is, Preferably, the beam reflected by the deflecting mirror 13 is made parallel to the Z axis.

Conversely, the deflecting mirror (especially when using a polygon mirror) is arranged so that its rotation axis is vertical,
On the other hand, it goes without saying that the parabolic mirror 11 and the projection surface 17 may be inclined relative to the deflection mirror by β.

As a result, the beam that has entered the deflection mirror 13 at the curvature correction angle β is reflected at the same reflection angle β and enters the parabolic mirror 11 as parallel light.

In the illustrated embodiment, the focal length f of the parabolic mirror is 300 mm (f = 3
300 mm), the axis of the parabolic mirror off angle theta of 10 ° (theta = 10 °), the beam scanning length (necessary length of the scanning beam B _o) was 250 mm. The curvature correction angle β was selected to be 6.5 ° (β = 6.5 °). In this case, in the related art (beam incidence perpendicular to the deflecting mirror, that is, the curvature correction angle β = 0 °), as shown in FIG. 7, a curvature of about 3 mm was generated. According to the above, it was experimentally confirmed that the scanning curvature was ± 50 μm or less and was substantially a straight line.

The incident direction of the incident beam is preferably
Is zero, that is, the deflecting mirror is a parabolic mirror 11
From the Z-axis or a direction parallel to the Z-axis when the mirror is directed straight ahead. This situation is the eighth
It is shown in FIG. FIG. 8 (1) shows a conventional beam incident method on the deflecting mirror, which is incident on the deflecting mirror 13 from a direction at 90 ° to the optical axis of the parabolic mirror 11. Therefore, in order to scan the beam over a predetermined angle range, the deflection mirror 13 needs a certain length l. That is, if the deflecting mirror 13 is short, for example, if the deflecting mirror 13 gradually rises (substantially becomes vertical) substantially parallel to the parabolic mirror 11 with its rotation, the incident beam Deviates from both ends of the deflecting mirror. However, according to the embodiment of the present invention, since the incident beam is made to enter from the front of the deflecting mirror 13 as shown in FIG. 8 (2), the incident beam irradiates only a small area near the center of the deflecting mirror 13. Therefore, the size of the deflecting mirror can be reduced. According to a simple calculation, the length 1 of the deflecting mirror of FIG. 8B is about 70% of that of the deflecting mirror of FIG. 8A.

FIG. 9 shows another embodiment of the present invention, in which the relationship between the reflected beam and the incident beam is opposite to that of the above embodiment. In FIG. 9, the deflecting mirror 13 is placed at the focal point of the parabolic mirror 11, and its rotation axis is inclined at an angle equal to the bending correction angle β with respect to the vertical direction. Therefore, when the beam is incident on the deflection mirror 13 at the curvature correction angle β, the reflected beam reflected by the parabolic mirror 11 becomes parallel to the Z axis.
In other words, the embodiment shown in FIG. 9 is the optical axis (Z axis) of the parabolic mirror.
This is an embodiment for generating a reflected beam parallel to the above.

10 to 12 are diagrams showing experimental results for confirming the effect of the present invention.

In this experiment, f and Θ were set to appropriate values generally used, and the trajectory of the scanning beam when β was changed was calculated. The trajectory is shown only in the right half.

FIG. 10 shows that when f = 300 mm, Θ = 5 ゜, β = 3.2 ゜, 3.3
The results are shown in the case of changing by {゜}, 3.4 ゜ and 0.1 ゜. As understood from the figure, a locus closest to a straight line is drawn in the case of β {3.3}. FIG. 10 merely shows that the best result was obtained when β {3.3}.
Generally, the degree of curvature is practically no problem up to about ± 50 μm. Therefore, when the scanning length is used in a range of, for example, about 130 mm (one side), β {3.2} or β {3.4} may be used. FIG. 11 shows that when f = 300 mm, Θ = 10 ゜, β = 6.4 ゜, 6.5
The results when {}, 6.6% and 0.1% are changed are shown. No.
From the results shown in FIG. 11, the best results were obtained when β = 6.5 °. In this case, as in the case of FIG. 10, depending on the scanning length or other conditions, β = 6.
4 ° or β = 6.6 ° may be used.

In each case, the inventor of the present application has confirmed that the scanning line becomes substantially straight when β = 0.65 °.

FIG. 12 shows the result (β-Θ diagram) of obtaining β at which the scanning line becomes substantially straight by setting f and に to various values.
Thereby, it was confirmed that β {0.65} was satisfied.

In general, the off-axis angle の of the parabolic mirror is often used at about {10}.

Generally, the deflecting mirror is placed at a position 2/3 (2f / 3) of the focal length f of the parabolic mirror.

〔The invention's effect〕

As described above, according to the present invention, the beam is incident on the deflection mirror at a predetermined curvature correction angle,
The curvature of the scanning line by the parabolic mirror is corrected, and a scanning line that can be regarded as a straight line in practical use can be created.

Further, by setting the incident angle of the beam on the deflection mirror to approximately 0.65 times the off-axis angle of the parabolic mirror, the linearity of the scanning line is optimized.

By making the incident beam of the deflecting mirror enter from the front of the deflecting mirror when the deflecting mirror faces the front with respect to the parabolic mirror, the size of the deflecting mirror can be reduced.

In addition, by tilting the rotation axis of the deflecting mirror with respect to the perpendicular to the optical axis of the parabolic mirror by the same angle as the angle of incidence of the beam on the deflecting mirror, the reflected light of the deflecting mirror is converted to the optical axis of the parabolic mirror And the scanning lines can be effectively imaged at the focal point of the parabolic mirror.

BRIEF DESCRIPTION OF THE DRAWINGS FIGS. 1 (1), (2) and (3) show the basic principle of the present invention, and are diagrams for explaining the relationship between the beam incident angle with respect to a deflection mirror and the locus of a reflected beam; FIG. 2 is an illustrative view showing a basic optical coordinate arrangement of the present invention, FIG. 3 is an illustrative view explaining reflection of a deflecting mirror, FIG. 4 is an explanatory view showing coordinate positions of an irradiation spot on a projection surface, FIG. 5 is an illustrative view showing one example of a specific arrangement structure of the optical system according to the present invention, FIG. 6 is a front view of FIG. 5, FIG. 7 is a diagram showing a curvature correction effect of the present invention,
FIGS. 8 (1) and 8 (2) are diagrams for explaining an arrangement relationship between a parabolic mirror and a deflecting mirror according to the related art and the present invention, respectively.
Figure is a diagram showing the arrangement of the optical system according to another embodiment of the present invention,
FIGS. 10 and 11 are diagrams showing experimental results on the trajectory of the scanning line in the present invention, and FIG. 12 is a diagram showing experimental results on the relationship between the curvature correction angle and the off-axis angle according to the present invention. FIG. 13 is an illustrative view showing a conventional scanning optical system, and FIG. 14 is a front view of FIG. 11, P: deflecting mirror, 13, M: deflecting mirror, Θ: Off-axis angle, β: Curve correction angle.

 ──────────────────────────────────────────────────続 き Continuing on the front page (72) Inventor Noriyuki Hiraoka 1015 Uedanaka, Nakahara-ku, Kawasaki City, Kanagawa Prefecture Inside Fujitsu Limited (72) Inventor Hiroyuki Tsukahara 1015 Kamiodanaka, Nakahara-ku, Kawasaki City, Kanagawa Prefecture Fujitsu Limited ( 72) Inventor Yoshinori Sudo 1015 Uedanaka, Nakahara-ku, Kawasaki City, Kanagawa Prefecture Inside Fujitsu Limited (56) References JP-A-52-49850 (JP, A) JP-A-56-167118 (JP, A) 1974-4109 (JP, A)

Claims (4)

(57) [Claims] In a scanning optical system for scanning a beam deflected by a rotatable deflecting mirror (13, M) in a predetermined direction by a parabolic mirror (11, P), a rotation axis (O-axis) of the deflecting mirror is used.
A beam is obliquely incident on O) at a predetermined incident angle (β), and the resulting trajectory of the deflected beam describes a trajectory substantially canceling the curved trajectory of the scanning beam by the parabolic mirror. Scan light distortion correction device. 2. The method according to claim 1, wherein a predetermined off-axis angle (Θ) is given to the parabolic mirror, and a beam incident angle on the deflection mirror is set to approximately 0.65 times the off-axis angle of the parabolic mirror. Claim 1
The distortion correction device according to the above. 3. The distortion correcting device according to claim 1, wherein the beam incident on the deflecting mirror is incident from the front of the deflecting mirror when the deflecting mirror faces the parabolic mirror. . 4. The deflecting mirror according to claim 1, 2 or 3, wherein the rotation axis of the deflecting mirror is inclined with respect to the perpendicular of the optical axis of the parabolic mirror by the same angle as the beam incident angle on the deflecting mirror. Distortion correction device.