JPH0339287B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a focus adjustment device for a light beam, and more particularly to a focus adjustment device for a light beam that automatically adjusts the focal position of a light beam such as a laser beam.

Conventionally, in the field of drawing devices using laser light, devices having a mechanism for moving a lens parallel to an optical axis have been known as devices for automatically adjusting the focal position of a light beam. However, since these conventional devices require mechanical operation to move the lens, they have the disadvantage that high-speed operation is difficult due to the inertia of the mass of the lens.

For example, when scanning a plane from one end of a plane to the other by reflecting a laser beam from a laser light source onto a rotating polygon mirror through a lens, the laser beam reflected by the rotating polygon mirror is reflected onto the scanning plane. In a general configuration in which the optical path length between the lens and the scanning point increases as it moves in a straight line from one end to the other, the laser beam is moved closer to the rotating polygon mirror as the optical path length increases. However, it is not practical to change the lens position rapidly in synchronization with the rotation of the rotating polygon mirror.

SUMMARY OF THE INVENTION An object of the present invention is to provide a focusing device for a light beam that eliminates such conventional drawbacks. In the above example, the object is to provide a focusing device for a light beam that can change the spread angle of light incident on a lens instead of moving the position of the lens.
When scanning light using a rotating polygon mirror as in the above example, the position of the lens is fixed, and as the light reflected by the rotating polygon mirror moves from one end of the scanning surface to the other, By widening the divergence angle of the incident light on the lens, the effect is equivalent to bringing the lens itself closer to a rotating polygon mirror.
That is, the light beam focus adjustment device of the present invention deflects a parallel light beam using an acousto-optic deflector, and deflects the parallel light beam using the deflector to a light correction plate that corrects the focal position of the light beam by a different amount depending on the incident position of the light beam. The incident position of the light beam is determined, and the light beam that has passed through the light correction plate is reflected by a plane mirror placed close to the back of the light correction plate, the incident optical path is reversed, and the light beam is output from the deflector in the opposite direction, thereby adjusting the deflection angle. It is characterized by producing emitted light with a corresponding spread angle.

Hereinafter, the present invention will be explained with reference to drawings of embodiments. FIG. 1 is a perspective view of an embodiment of the invention, and FIGS. 2a and 2b are a plan view and a side view of the embodiment of the invention.

A polarizing prism filter 2 and an acousto-optic deflector 3 (hereinafter referred to as deflector 3) for deflecting the light beam are disposed on the optical axis 1 at the entrance of the light beam of the light beam focusing device of the present invention.

(The point at which the deflector 3 deflects the light beam 4 traveling along the optical axis 1 is called a tendency point 5. Starting from the deflection point 5, the center line in the direction of the central angle of the angle range at which the deflector 3 deflects the light beam is referred to as the central axis 6, and the surface including the optical axis 1 and the central axis 6 is referred to as the deflection surface.) On the central axis 6, a λ/4 wavelength correction plate 7, a lens 8, cylindrical lenses 9, 10, A lens 11, a light correction plate 12 and a plane mirror 13 close to the lens 11 are arranged in this order. The distance u between the deflection point 5 and the lens 8 is
Release it.

The distance between lens 8 and lens 11 is equal to the sum of their focal lengths (referred to as f ₁ and f ₂ ). Cylindrical lenses 9 and 10 are separated by a distance S. cylindrical lens 9,
10 orients the axis of the cylinder in a direction perpendicular to the deflection plane (hereinafter referred to as the Y direction).

(The direction of the central axis 6 is called the Z-axis, and the direction perpendicular to it and perpendicular to the _Y direction is called the Let the distance to the cylindrical lens 10 be a.

Each interval u, S, and a is set so as to satisfy the following conditions.

(b) The cylindrical lenses 9 and 10 act on the X-direction component of the light beam 4. If the focal length of the cylindrical lenses 9 and 10 is f ₃ , then the distance S between them is S=√2・f ₃ ... (1).

By doing so, the combination of the cylindrical lenses 9 and 10 becomes an optical system whose focal length F is given by the following equation.

F=(2+√2)/2·f ₃ (1-1) Also, both the front focal point and the rear focal point of this optical system are the midpoint between the two lenses.

(b) A combined optical system of cylindrical lenses 9 and 10 collects the collimated light beam 4 at the focal point of the lens 8, and converts it into a lens 11, a light correction plate 12, and a plane mirror 13.
The interval a is determined so that the parts are assembled again at the position.

That is, a is given by the following equation.

a=f ₃ /√2+(f ₃ ) ² /f ₂・(1+Δa)/(6−4・√2) …(2) Δa=4・(F/f ₂ ) ⁴ /(1−2・(F/ _f2 ) ² +√1-4( ₂ ) ² )...(2-1) Here, _f3 is made sufficiently smaller than _f2 . As a result, F also becomes sufficiently smaller than _f2 , so Δa can be ignored.

(c) The interval u is the distance between the X-direction component of the light beam 4 that is deflected at the deflection point 5 of the deflector 3, passes through the lens 8, cylindrical lenses 9 and 10, and the lens 11, and is reflected by the mirror 13, and then returns to the deflection point. Set to return to 5.

That is, u is expressed by the following formula.

u=f ₁ + (f ₁ ) ² /f ₂ /(1-Δu) ...(3) Δu=2・F/f ₂ ...(3-1) Here, similarly to condition (2), Δu can be ignored.

Next, the light correction plate 12 will be explained with reference to FIG.

In a coordinate system with X, Y, and Z axes set in the X, Y, and Z directions, the thickness (t) of the optical glass plate is given by the following equation (4) using f(x) as a function of X. A light correction plate 12 having a thickness is used.

t=(constant)−f(x)・Y ² ...(4) In addition, the lens 11, the light correction plate 12, and the plane mirror 13
It is also possible to use one that is integrally formed by bonding the surfaces together. In addition, the lens 11 and the light correction plate 12
and the plane mirror 13 may be replaced with a correction concave mirror having a concave surface given by the following equation (5).

Z=(constant)−(X ² +Y ² )÷(4·f ₂ )−f(x)·Y ² (5) As a specific example, the focal length of each lens is selected as follows.

f ₁ = 100mm f ₂ = 100mm f ₃ = 10mm Then, each distance interval u, S and a are calculated by formulas (1), (2),
According to (3), the following values are required.

u = 200 mm, S = 14 mm, a = 10 mm Next, the operation of the light flux focusing device of the present invention will be explained as follows.
This will be explained with reference to Figures a and b.

A light beam 4 is incident along the optical axis 1 into the light beam focusing device of the present invention. The light beam 4', which first passes through the polarizing prism filter 2, travels back and forth on the optical path to the plane mirror 13, and returns to the polarizing prism filter 2 again, travels back and forth through the λ/4 wavelength correction plate 7, so that the plane of polarization is rotated by 90 degrees. Therefore, it is reflected by the polarizing prism filter 2 and output in the output direction 15. Lens 8, cylindrical lenses 9 and 10, and lens 11 act on the component parallel to the deflection plane of light beam 4.

At the deflection point 5, the angle that the light beam 4 makes with the central axis 6 after being deflected by the deflector 3 is θ, and the height of the light beam 4 from the center axis 6 is h.

At the position of the plane mirror 13, the angle formed by the component parallel to the deflection plane of the light beam 4 with the central axis 6 is Ω, and the height from the central axis 6 is x.

According to condition (1), the angle θ that the light beam 4 forms with the central axis 6 at the deflection point 5 defines the height x of the light beam 4 from the center axis 6 at the position of the plane mirror 13. This height x is given by the following equations (6) and (7).

x=b・sinθ ……(6) b=f ₁・f ₂ /f ₃・(2−√2)・(1−Δb) ……(7) Δb=(Δa・(1/f ₂ +1/ F)-(F/ _f2 ) ² )/(1+F/ _f2 +Δa/F)...(7-1) As a premise for this calculation, it is assumed that the sine condition holds in the optical system.

Here, Δb can be ignored as in condition (1).

On the other hand, since condition (2) holds, the angle Ω that the light beam 4 forms with the central axis 6 at the position of the mirror 13 defines the height h of the light beam from the central axis 6 at the position of the deflection point 5. , which is given by the following equation (8).

h=b・sin(Ω) ...(8) According to equation (7), even if the angle Ω that the component parallel to the deflection plane of the light beam 4 forms with the central axis 6 changes at the position of the plane mirror 13, the deflection point remains unchanged. does not affect the advancing angle θ of the light beam 4 at 5.

That is, the component parallel to the deflection circle of the light beam 4 reflected by the plane mirror 13 and reciprocating to the deflection point 5 of the deflector 3 is the forward direction and the reverse direction at the deflection point 5, regardless of the orientation of the surface of the plane mirror 13. The direction of travel becomes parallel. Therefore, the focusing device for a light beam according to the present invention has a feature that automatically satisfies the traveling direction conditions of the light beam 4 necessary for causing the light beam 4 to reciprocate in the deflector 3.

The lenses 8 and 11 and the light correction plate 12 act on the component of the light beam 4 that is perpendicular to the deflection plane and parallel to a plane containing the central axis 6 (hereinafter referred to as a vertical plane).
If the height of the component parallel to the deflection plane of the light beam 4 incident on the light correction plate 12 from the central axis 6 is x, then
It functions as a cylindrical lens whose focal length (denoted as F) changes depending on the height x as shown in the following equation (9).

F=1÷[2·f(x)·(n−1)] (9) Here, n is the refractive index of the optical glass plate that is the material of the light correction plate 12.

Even if the deflector 3, lenses 8, 11, light correction plate 12, and plane mirror 13 are replaced with equivalent cylindrical lenses and plane mirrors, the optical effect on the component parallel to the vertical plane of the light beam 4 is equivalent. An equivalent cylindrical lens and plane mirror are present at the deflection point 5 with the deflector 3 removed.

The focal length (referred to as f) of the equivalent cylindrical lens changes depending on the angle at which the light beam 4 is actually deflected by the deflector 3. f is given by the following equation (10).

f=(f ₁ ÷ f ₂ ) ÷ [2・f(x)・(n-1)] ...(10) In other words, the light flux focusing device of the present invention calculates the focal length f of the equivalent cylindrical lens. It has characteristics that can be changed without mechanical action.

Equivalently, a cylindrical lens and a plane mirror exist at a polarization point 5 on the optical axis 1 with the deflector 3 removed. Therefore, the light beam 4 that has gone back and forth between the deflection point 5 and the plane mirror and returns will always pass through the same position where it first passed through the deflection point 5. That is, it has a feature that the light beam 4 automatically satisfies the positional condition in which the deflection point 5 reciprocates.

To state the above qualitatively, even if the incident light beam 4 is parallel and has a circular cross section, when it enters the light correction plate 12, the cross section of the light beam is directed in the Y-axis direction due to the action of the cylindrical lenses 9 and 10. It has an elongated flat shape. The light beam having a flat cross section will be incident on different positions along the X-axis direction on the light correction plate 12 depending on the amount of deflection in the X-axis direction by the deflector 3. In the flat light beam incident on the light correction plate 12, only the Y-time direction component is refracted in the Y-axis direction depending on the incident position, and the beam is incident on the plane mirror 13. Of the light beam reflected by the plane mirror 13, only the Y-axis direction component is refracted and returned to the deflector 3, where it passes through the polarizing prism filter 2.
The light is reflected by the light beam and is emitted as emitted light 15. When this emitted light is optically scanned by a rotating polygon mirror, it goes without saying that the deflection of the deflector 3 and the rotation of the rotating polygon mirror are synchronized. As the light beam moves from one end to the other end on the scanning plane, the deflection angle of the deflector 3 is increased. The light beam that is largely deflected as shown by the solid line in FIG.
Since it passes through the end in the axial direction (the concave lens portion in FIG. 3), only the Y-axis direction component is greatly affected by the refraction effect of the concave lens.

On the other hand, when the deflection angle is small, the light beam passes through the convex lens section shown in FIG. 3, and only the component in the Y-axis direction is subjected to the refraction effect of the convex lens. The cross-section of the light beam irradiated onto the light correction plate 12 has the same flat cross-sectional shape regardless of the amount of deflection; As for refraction, the more the deflection angle is covered, the more the concave lens effect is applied, and the smaller the deflection angle, the more the convex lens effect is applied. As a result, the polarizing prism filter 2
The divergence angle of the emitted light 15 on the exit end face becomes larger as the light is deflected more greatly as it is affected by the concave lens action, and the spread angle of the light 15 becomes smaller as the light is deflected to a smaller extent as it is more affected by the convex lens action. Therefore, when this emitted light 15 is reflected by a rotating polyhedron through a lens to scan a plane, the same effect as when the lens is kept fixed and moved according to the optical scanning distance is produced. That will happen.

() Since the focus of the light beam can be adjusted without using mechanical operation, high-speed focus adjustment can be easily performed.

() The conditions necessary for reciprocating the luminous flux from the acousto-optic deflector to the plane mirror and returning the luminous flux to the original position are satisfied by the arrangement of the lens system including the cylindrical lens. The optical mechanism can be adjusted very easily. It has the following advantages.

[Brief explanation of the drawing]

FIG. 1 is a perspective view of an embodiment of the invention, FIGS. 2a and 2b are a plan view and a side view of an embodiment of the invention,
FIG. 3 is a perspective view of the light correction plate of the present invention. [Symbols in the figure], 1... optical axis, 2... polarizing prism filter, 3... (acousto-optic) deflector,
4, 4'... Luminous flux, 5... Deflection point, 6... Central axis,
7...λ/4 wavelength correction plate, 8... Lens, 9,1
0... Cylindrical lens, 11... Lens, 12... Light correction plate, 13... Plane mirror, 14... Focal position (after lens 8), 15... Outgoing direction.

Claims (1)

[Claims] 1. an acousto-optic deflector, and a deflection device in which the deflector deflects a light beam along an axis (hereinafter referred to as the central axis) in the direction of the central angle of the angular range in which the deflector deflects the light beam. a first lens arranged at a first interval from the point; and a cylindrical axis extending from the first lens in a direction perpendicular to the deflection plane on which the deflector deflects the light beam (hereinafter referred to as the Y direction). a second cylindrical lens disposed at a second interval; a third cylindrical lens having a cylinder axis in the Y direction and disposed at a third interval from the second cylindrical lens; and a third cylindrical lens disposed at a third interval from the second cylindrical lens. A focusing device for a light beam, comprising: a light correction reflection optical mechanism arranged at a fourth interval from the center at a distance satisfying the following condition. (1-1) The light beam deflected at the deflection point passes through the first lens, second and third cylindrical lenses, is reflected by the light correction reflection optical mechanism, and returns to the deflection point again. (1-2) A component parallel to the deflection plane of the parallel light beam in the deflector is focused on one point by a light correction reflection optical mechanism. 2. The second cylindrical lens and the third cylindrical lens have the same focal length, and the third interval is a distance obtained by multiplying the focal length by the square root of 2. Focusing device for light flux. 3 When the light correction reflection optical mechanism is set to a lens, the X axis is set in a direction perpendicular to the central axis of the light beam and parallel to the deflection plane, and the Y axis is set in the Y direction, the thickness t of the optical glass plate is f(x) as a function of X and t=
(Constant) -f(x)・^Y2 A light correction plate having a thickness given by the formula: (constant) - f(x)・Y Focusing device for light flux. 4. The light correction reflection optical mechanism is configured such that the Z-axis is set parallel to the central axis, and the X-axis, Y-axis
In the coordinate system set along with the axis, the surface of the mirror is expressed as Z = (constant) - (X ² + Y ² ) ÷ (4・f ₂ ) − f(x)・2. A focusing device for a light beam according to claim 1, further comprising a correction mirror having a surface expressed by the formula ^Y2 .