JPH05215985A - Scanning optical system - Google Patents

Abstract

PURPOSE:To provide the scanning optical system which has a focal line position nearly matched with a Gaussian image plane even to oblique incident light and is free from defocusing on a scanned surface (image plane) even if the oscillation wavelength of a light source is fluctuated. CONSTITUTION:A 2nd image formation system is provided with a hologram cylindrical lens 62. The focal length fH of LZP constituting the hologram cylindrical lens 62 is a positive value and the focal length fG of a concave cylindrical lens is a negative value. Further, this system is provided with an image point adjusting mechanism which moves the hologram cylindrical lens 62 or a cylindrical lens 41 according to the quantity of fluctuation in the oscillation wavelength of the light source. The characteristics of the LZP and concave cylindrical surface to the oblique incident light are cancelled with each other to nearly match the focal line position to the oblique incident light with the Gaussian image plane.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a scanning optical system having a mechanism for correcting chromatic aberration associated with wavelength fluctuation of a light source.

[0002]

2. Description of the Related Art As is well known in the art, a scanning optical system deflects a light beam emitted from a light source such as a semiconductor laser by a polygon mirror and scans a surface to be scanned with the light beam through a scanning lens. Therefore, in order to perform good scanning, it is necessary to correct the surface tilt of the polygon mirror. Therefore, in the conventional scanning optical system, for example, the cylindrical lens 1 shown in FIG. 10 is arranged between the polygon mirror and the surface to be scanned, and the reflection surface and the surface to be scanned (image surface) of the polygon mirror are the scanning surface. A plane (xz plane in FIG. 10) orthogonal to is conjugated.

However, when the cylindrical lens 1 shown in FIG. 10 is used, the following problems occur. That is, as shown in FIG. 10, when a parallel light beam L ′ that is on the yz plane and forms an angle θ with respect to the z-axis enters the cylindrical lens 1, the radius of curvature of the cylindrical surface of the cylindrical lens 1 is apparently small, Focus line FL '
Is formed on the −Z side of the Gaussian image plane. Therefore, there is a problem that the image surface is curved. In the following description, the angle θ (θ ≠
Light incident at 0 ° is referred to as “oblique incident light”.

As a solution to this problem, a cylindrical lens 10 shown in FIG. 11 has been proposed by the present applicant. In this cylindrical lens 10, a linear zone plate (hereinafter referred to as “LZP”) 11 functioning as a diffractive element is provided on the −Z side, while a concave cylindrical surface 12 is on the image side (see FIG. Z side)
It is provided in.

Here, before explaining the function and effect of the cylindrical lens 10, first, the characteristics of the LZP 11 will be briefly described. As shown in FIG. 12, a plane wave is incident from the −Z side of FIG.
Consider the case where the first-order diffracted wave is converted into a cylindrical wave by 11. 2 if the optical path difference between the ray exiting the k-th band LB from the optical axis OA (z axis) and the ray passing through the optical axis is an integral multiple of the wavelength
The two lights strengthen each other. That is, x of the k-th linear band
The coordinate Xk is

[0006]

[Equation 1]

Is given by Where fH is this LZP
The focal length of 11, and λ is the wavelength of light. Therefore, by forming the unequal-spaced linear bands LB parallel to the y-axis so that the band density increases as the absolute value of the x-coordinate increases from the origin O, it is possible to provide a function equivalent to that of the convex cylindrical lens. it can.

More generally, the position of the straight line band can be expressed as an equiphase line in which the phase difference defined by the following equation is an integral multiple of 2π.

[0009]

[Equation 2]

However, A and B are constants.

If k = 1 in equation 1,

[0012]

[Equation 3]

Then, the coordinate X1 of the first band is obtained. Since fH >> λ

[0014]

[Equation 4]

It can be organized as follows.

By the way, the position of the first band is when the right side of Equation 2 is 2π, and since the value of x is small, the fourth-order term is ignored.

[0017]

[Equation 5]

Is obtained. And by substituting equation 4 into equation 5,

[0019]

[Equation 6]

[Mathematical formula-see original document]

[0021]

[Equation 7]

Is obtained.

Therefore, in the cylindrical lens 10, the LZP is set by setting the constant A appropriately.
The focal length fH of 11 has a positive value, and the characteristics of the LZP 11 with respect to obliquely incident light are completely opposite to those of the concave cylindrical surface 12. As a result, the respective characteristics are canceled out, and the focal line position for obliquely incident light substantially matches the Gaussian image plane.

The cylindrical lens 10 (FIG. 11) according to this proposal is replaced with the conventional cylindrical lens 1
In order to distinguish it from FIG.
The cylindrical lens 10 is referred to as a "hologram cylindrical lens".

[0025]

By the way, since the hologram cylindrical lens 10 uses the diffraction effect, the chromatic aberration becomes large. Therefore, when the oscillation wavelength of the semiconductor laser (light source) changes according to the ambient temperature, the image point moves in the optical axis direction z. In a typical example, when the wavelength of the semiconductor laser increases by 1 nm, the focal length decreases by 0.5 mm. On the other hand, since the depth of focus of a scanning optical system having a spot diameter of 30 μm is about ± 0.5 mm, if the wavelength fluctuates by ± 1 nm or more, defocus occurs on the surface to be scanned (image surface).

The present invention has been made to solve the above-mentioned problems, and the focal line position substantially coincides with the Gaussian image plane even with respect to obliquely incident light, and even if the oscillation wavelength of the light source fluctuates,
It is an object of the present invention to provide a scanning optical system in which no focus shift occurs on the surface to be scanned (image surface).

[0027]

In the scanning optical system according to the present invention, the light beam from the light source is condensed only in the rotational axis direction of the rotary polygon mirror by the first imaging optical system including the anamorphic surface. A light beam scanning device for scanning the surface to be scanned in a main scanning direction substantially orthogonal to the rotation axis direction by a light beam reflected and deflected by the rotating polygon mirror. In order to achieve the above object, the second imaging optical system has a plurality of linear bands extending in the main scanning direction arranged at unequal intervals in the rotation axis direction, and has a focal length fH. And a concave cylindrical surface having a focal length fG having an axis of symmetry parallel to the first direction, and
A cylindrical lens satisfying the following inequalities fH> 0 fG <0 is provided, and further a mechanism for moving the cylindrical lens or the anamorphic surface in the optical axis direction according to the wavelength variation of the light source is provided. ..

[0028]

In the scanning optical system according to the present invention, the second image forming optical system is provided with the cylindrical lens. The focal length fH of the linear zone plate that constitutes this cylindrical lens has a positive value, while the focal length fG of the concave cylindrical surface has a negative value, so the characteristics of the linear zone plate for obliquely incident light are those of the concave cylindrical surface. It has the opposite characteristics. As a result, the respective characteristics are canceled out, and the focal line position for obliquely incident light substantially matches the Gaussian image plane. Moreover, the cylindrical lens or the anamorphic surface is moved in the optical axis direction according to the wavelength variation amount of the light source, the image point displacement due to the wavelength variation is adjusted, and the image point coincides with the scanned surface.

[0029]

【Example】

A. First Embodiment FIG. 1 is a diagram showing a first embodiment of the scanning optical system according to the present invention. Further, FIG. 2 is a partial sectional view thereof. In addition,
In these figures, the rotation axis 50 of the polygon mirror 50
an x-axis extending parallel to a, a z-axis coinciding with the optical axis OA,
Description will be made using a three-dimensional space composed of the x-axis and the y-axis orthogonal to the z-axis.

In this scanning optical system, a semiconductor laser 30 as a light source is fixed on a base 20 made of aluminum. The semiconductor laser 30 is of the GaAlAs type, for example, and its oscillation center wavelength is 780 nm.
However, the oscillation wavelength has a dependency on the environmental temperature, and for example, as shown in FIG. 3, it increases by 1 nm for every 5 ° C. increase in the environmental temperature.

The light beam from this semiconductor laser 30 is transmitted through a cylindrical lens 41 and a collimating lens 4.
A linear image is formed on the reflecting surface of the polygon mirror 50 via the first image forming optical system 40 composed of two.

A second imaging optical system 60 is arranged between the polygon mirror 50 and the surface (image surface) 70 to be scanned, and the deflected light beam deflected by the reflecting surface of the polygon mirror 50 is scanned. An image is formed on the surface 70. The second image forming optical system 60 is composed of four lenses 61a to 61d, fθ.
It is composed of a lens 61 and a hologram cylindrical lens 62 having the same structure as in FIG.

Although not shown in FIG. 1,
Each lens is fixed to the base 20 by a suitable holder. Table 1 shows lens data of the scanning optical system.

[0034]

[Table 1] In the table, the refractive index is for a wavelength of 780 nm. The first surface shows the semiconductor laser 30. First
The NA on the light source side of the imaging optical system is 0.1. The second surface and the sixteenth surface are cylindrical surfaces, and the radii of curvature in the y direction are both infinite. The third surface is a cylindrical aspherical surface,

[0035]

[Equation 8]

However, it is expressed by K = -0.868 Cx = 1 / rx. The seventh surface is a polygon mirror surface, and its effective rotation angle is 25 °. The fifteenth surface is LZP, and the position of the straight line band is Φ (x) = 2πAx ² , provided that the right side of A = 19.68 is an integral multiple of 2π.

From Equation 7, the focal length fH of LZP is 32.
57 mm, and from Table 1, the focal length fG of the concave cylindrical surface is -38.8 mm. Therefore fH> 0, fG
<0.

Further, in this scanning optical system, the surface to be scanned 70
NA on the side is 0.023, and it is 3 at a spot of 30 μm.
It is configured to scan 50 mm.

By configuring the scanning optical system in this manner, the reflecting surface of the polygon mirror 50 becomes optically conjugate with the surface to be scanned 70 in the y direction, and the polygon mirror 5
The surface misalignment of 0 is corrected, and the image recorded on the scanned surface 70 is maintained in high quality. Moreover, since the hologram cylindrical lens 62 is provided in the second imaging optical system 60,
As described above, the focal line position for obliquely incident light can be matched with the Gaussian image plane.

By the way, as described above, the chromatic aberration of the hologram cylindrical lens 62 is large, and the rate of change of the chromatic aberration with respect to wavelength fluctuation is -0.42 × 10 ⁶ .
That is, as shown in FIG. 4, the ambient temperature rises by about 5 ° C. and the oscillation wavelength λ of the semiconductor laser 30 becomes the central wavelength λ.
When 0 nm (= 780 nm) increases by 1 nm, the focal length decreases by 0.42 mm. In the figure, the solid line shows the locus of the light beam L0 having the central wavelength λ0 (= 780 nm), and the dotted line shows the locus of the light beam L having the wavelength λ (= 781 nm).

Therefore, in this embodiment, an image point adjusting mechanism is provided, and the hologram cylindrical lens 62 is attached to the surface 7 to be scanned at a ratio of 0.42 mm with respect to an increase of the ambient temperature by 5 ° C.
It has moved to the 0 side. That is, as shown in FIGS. 1 and 2, a long high-expansion material 21 extending in the z direction is fitted into the recess of the base 20. The end of the high expansion material 21 on the semiconductor laser 30 side (-Z side in FIG. 1) is fixed to the base 20 by an adhesive, while the other end is free, so that it can be expanded and contracted in the z direction. Is becoming As the high expansion material 21, for example, polyacetal resin manufactured by Polyplastics Co., Ltd. can be used. In this case, the difference in linear expansion coefficient from the base (made of aluminum) 20 is 11 × 1.
It becomes 0 ^-5 (/ K). Therefore, if the length of the high-expansion material 21 made of polyacetal resin is set to 760 mm, the high-expansion material 21 is extended by 0.42 mm each time the environmental temperature rises by 5 ° C and is held by the holder 22 on the high-expansion material 21. The hologram cylindrical lens 62 is moved to the scanned surface 70 side. As a result, the expansion and contraction of the high expansion material 21 cancels the movement of the image point due to the change of the oscillation wavelength, so that the image point is not affected by the change of the environmental temperature.
Matches It is optimal to make the image point completely coincide with the surface to be scanned 70, but if the image point is within the range of the depth of focus, there is no problem even if the image point deviates from the surface to be scanned 70. Therefore, it suffices that the image point substantially coincides with the scanned surface 70 within this range.

Incidentally, the structure of the image point adjusting mechanism is not limited to the above, and for example, the base 20 is used.
It may be made of a high expansion material. In that case, since the base 20 expands and contracts not only in the optical axis direction z but also in the main scanning direction y, expansion and contraction of the base 20 in the direction y may apply unnecessary stress to the hologram cylindrical lens 62. Therefore, in order to avoid this stress, it is preferable to provide a gap between the hologram cylindrical lens 62 and the holder 22 so that the hologram cylindrical lens 62 and the holder 22 are relatively movable in the main scanning direction y. .. The distance between the semiconductor laser 30 and the first imaging optical system 40 and the fθ lens 6
It is desirable that the intervals between the lenses 61a to 61d of No. 1 are constant, and the semiconductor laser 30, the first imaging optical system 40, and the fθ lens 61 are fixed to a plate of a low expansion material and then attached to the base 20. Is good.

B. Second Embodiment Further, the image point adjusting mechanism may be configured as described below. That is, the hologram cylindrical lens 62
In the first embodiment, the difference in the coefficient of linear expansion between the base 20 and the high expansion material 21 is used to drive the laser beam in the optical axis direction z, but in the second embodiment, as shown in FIG. A holder guide 23 having a dovetail groove is fixed to the base 20, and the dovetail groove connects the holder 22 to the holder guide 23 so that the holder guide 23 can move in the direction z. Further, one end of the bimetal 24 is connected to the holder 22, and the other end is connected to the bimetal holder 25 protruding from the base 20. Various kinds of known bimetals can be used, and it is sufficient that the hologram cylindrical lens 62 is pushed toward the surface to be scanned (image surface) 70 at a rate of 0.42 mm per 5 ° C.

C. Third Embodiment Further, in the above-described embodiment, the case where the hologram cylindrical lens 62 is moved in the optical axis direction z to adjust the image point according to the temperature around the hologram cylindrical lens 62 has been described. Therefore, the oscillation wavelength of the semiconductor laser 30 may change. Therefore,
In the third embodiment shown in FIG. 6, the oscillation wavelength of the semiconductor laser 30 is directly measured, and the hologram cylindrical lens 62 is moved in accordance therewith, so that such a situation can be appropriately dealt with. Hereinafter, a detailed description will be given with reference to FIG.

In the third embodiment, a bimorph type piezo element 26 is used instead of the bimetal 24 as a drive source for moving the hologram cylindrical lens 62 in the optical axis direction z. Also, this piezo element 2
A control unit (described next) for controlling 6 is provided. Since the other structure is the same as that of the second embodiment (FIG. 5), its explanation is omitted here.

In the control unit, a reflective diffraction element 81 is provided between the cylindrical lens 41 and the collimating lens 42.
Most of the light beam that has passed through the cylindrical lens 41 is transmitted to the polygon mirror 50 side, and only the remaining part is reflected. Then, the reflected light beam is dispersed and further condensed by the lens 82 on the position sensor 83 such as PSD. Therefore, when the oscillation wavelength of the semiconductor laser 30 changes, the focus position on the position sensor 83 changes according to the change in wavelength, so that the change in wavelength of the semiconductor laser 30 can be known. The arrangement position of the reflection diffraction element 81 is not limited to the above, and it may be arranged at any position of the first image forming optical system 40, or the first image forming optical system 40 and the polygon mirror. You may arrange | position between 50.

Then, the position sensor 83 outputs a signal S1 relating to the oscillation wavelength of the semiconductor laser 30 to the data processing section 84. The data processing unit 84 calculates the optimum position of the hologram cylindrical lens 62 according to the oscillation wavelength of the semiconductor laser 30 based on the signal S1 and gives a control signal to the piezo driver 85.
Then, the piezo element 26 is driven by the piezo driver 85, and the hologram cylindrical lens 62 is positioned in the optical axis direction z so that the image point substantially coincides with the scanned surface 70. In this way, the hologram cylindrical lens 62 can be always arranged at an appropriate position according to the oscillation wavelength of the semiconductor laser 30, and the focus shift can be prevented.

D. Fourth Example Further, as a modified example (fourth example) of the third example, the temperature may be detected instead of the direct measurement of the oscillation wavelength of the semiconductor laser 30. FIG. 7 is a diagram showing a fourth embodiment of the scanning optical system according to the present invention. As shown in the figure, a thermocouple 86 is in contact with the semiconductor laser 30. Then, the signal S2 from the thermocouple 86 is given to the temperature measuring section 87, and the temperature of the semiconductor laser 30 is calculated. Further, the oscillation wavelength of the semiconductor laser 30 is indirectly obtained by the data processing unit 84 based on the signal from the temperature measuring unit 87, and the optimum position of the hologram cylindrical lens 62 is calculated in accordance therewith. After that, a control signal is output from the data processing unit 84 to the piezo driver 85,
The piezo element 26 is driven based on the signal, and as a result, the hologram cylindrical lens 62 is arranged at a suitable position.

As described above, since the oscillation wavelength of the semiconductor laser 30 is not directly measured in the fourth embodiment, the reliability is lower than that in the third embodiment (FIG. 6), but the cost is low. It has the advantage that it can be implemented in.

In the first to fourth embodiments, the case where the hologram cylindrical lens 62 is composed of the LZP 11 and the cylindrical surface 12 (FIG. 11) has been described, but as shown in FIG. 8, the LZP 11 is formed on one side. Is provided and the other side is provided with a first optical element 62a finished to a plane 13 parallel to the xy plane, and the concave cylindrical surface 12 is provided on one side and the plane 14 parallel to the xy plane is provided on the other side. Hologram cylindrical lens 62 'in combination with second optical element 62b
Also in, the same effect as described above can be obtained.

For example, the optical element 62a is the same as the LZP11 of the fifteenth surface shown in Table 1, and the optical element 62b is
Is the same as the concave cylindrical surface of the 16th surface shown in Table 1, the focal length of the optical element 62a is 32.57.
mm, and the focal length of the optical element 62b is −38.8 mm. The optical distance between the optical elements 62a and 62b is set to 6.62 m.
m, the combined focal length is 98.34 mm, and when only the optical element 62a is moved in the optical axis direction Z by 55 μm,
The combined focal length changes 0.42 mm. If a hologram cylindrical lens is formed by the two separated optical elements 62a and 62b and only one of them is moved, the moving amount can be reduced to about 1/8.

As described above, one of the optical elements 62a and 62b may be moved, or both may be independently movable in the optical axis direction z.

In the first to fourth embodiments, the LZP 11 is provided on the side of the semiconductor laser 30 and the surface to be scanned 70.
Although the case where the concave cylindrical surface 12 is disposed on the side has been described, the concave cylindrical surface 12 may be disposed on the semiconductor laser 30 side and the LZP11 may be disposed on the scanned surface 70 side, and the LZP11 in the embodiment of FIG. The arrangement order of the concave cylindrical surface 12 and the parallel planes 13 and 14 is arbitrary.

E. Fifth Example Further, in the first to fourth examples, the case where the hologram cylindrical lens 62 is moved by the image point adjusting mechanism has been described, but the element including the anamorphic surface in the first imaging optical system 40. That is, the cylindrical lens 41 may be moved. The fifth embodiment will be described below in comparison with the first embodiment (FIG. 1).

In the first embodiment, the hologram cylindrical lens 62 is provided with an image point adjusting mechanism (high expansion material 21), whereas in the fifth embodiment, the cylindrical lens 41 is provided with an image point adjusting mechanism (FIG. 9). Are largely different in that they are provided, and other configurations are the same.

Next, a specific structure will be described. As shown in FIG. 9, a support member 91 is fixed to the base 20. Then, the low expansion material 92 is provided so as to extend from the support member 91 in the optical axis direction z.
The semiconductor laser 30 is fixed by. Also,
Similar to the low expansion material 92, the high expansion material 93 is the low expansion material 9
2 extends in the optical axis direction z, and a cylindrical lens 41 is attached to its tip. Here, a 40 mm long polyacetal resin (linear expansion coefficient = 13 × 10 ⁻⁵ ) is used as the high expansion material 93, while a 33 mm long aluminum (expansion coefficient = 2.3 ×) is used as the low expansion material 92.
When 10 ⁻⁵ ) is used, the cylindrical lens 41 moves away from the semiconductor laser 30 at a rate of 22 μm every 5 ° C. increase in temperature. As a result, the scanned surface 7
A focus shift on 0 can be compensated. Here, it should be noted that, in the first to fourth embodiments, in order to compensate for the focus shift, the temperature of 0.
Hologram cylindrical lens 6 at a ratio of 42 mm
It was necessary to move 2 to the side of the surface to be scanned 70, but in the fifth embodiment, it is sufficient to move the cylindrical lens 41 by a slight distance. This is because the NA on the light source (semiconductor laser 30) side of the optical system is in the x direction and y.
Since the direction is 0.1 and the NA on the surface to be scanned 70 is 0.023 in both the x direction and the y direction, the angular magnification of the entire system is 0.23. Therefore, the vertical magnification of the whole system is 19
Therefore, the movement amount of the cylindrical lens 41 for adjusting the image point is only 1/19 of the movement amount of the hologram cylindrical lens 62 in the first embodiment.

Incidentally, in the fifth embodiment, the conjugate property between the reflecting surface of the polygon mirror 50 and the surface to be scanned 70 collapses as the focal length of the hologram cylindrical lens 62 changes, so that the effect of correcting the surface tilt of the polygon mirror 50 is reduced. Cannot be compensated. However, there is an effect that it can be realized at the lowest cost as compared with the other embodiments.

[0058]

As described above, according to the scanning optical system of the present invention, the focal length fH of the linear zone plate has a positive value, while the focal length fG of the concave cylindrical surface has a negative value. Since the characteristic of the linear zone plate for obliquely incident light is the opposite of that of the concave cylindrical surface, the focal line position for obliquely incident light can be made to substantially coincide with the Gaussian image plane. .. In addition, the cylindrical lens or anamorphic surface is moved in the optical axis direction according to the wavelength variation of the light beam from the light source to adjust the image point displacement due to the wavelength variation. Even if fluctuates, it is possible to prevent focus deviation on the surface to be scanned (image surface).

[Brief description of drawings]

FIG. 1 is a perspective view showing a first embodiment of a scanning optical system according to the present invention.

FIG. 2 is a partial cross-sectional view of FIG.

FIG. 3 is a graph showing an oscillation wavelength of a semiconductor laser with respect to ambient temperature.

FIG. 4 is a diagram showing a relationship between a wavelength of a light beam and an image point.

FIG. 5 is a perspective view showing a second embodiment of the scanning optical system according to the present invention.

FIG. 6 is a perspective view showing a third embodiment of the scanning optical system according to the present invention.

FIG. 7 is a perspective view showing a fourth embodiment of the scanning optical system according to the present invention.

FIG. 8 is a perspective view showing a modified example of the hologram cylindrical lens.

FIG. 9 is a partial sectional view showing a fifth embodiment of the scanning optical system according to the present invention.

FIG. 10 is a schematic diagram showing a relationship between a cylindrical lens and an image point in a conventional scanning optical system.

FIG. 11 is a schematic diagram showing a relationship between a hologram cylindrical lens and an image point in a scanning optical system.

FIG. 12 is a perspective view for explaining the characteristics of LZP.

[Explanation of symbols]

 11 Linear Zone Plate (LZP) 12 Concave Cylindrical Surface 30 Semiconductor Laser 40 First Imaging Optical System 50 Polygon Mirror 50a Rotation Axis 60 Second Imaging Optical System 62, 62 'Hologram Cylindrical Lens 70 Scanned Surface OA Optical Axis

Claims (1)

[Claims] 1. A light beam from a light source is condensed by a first imaging optical system including an anamorphic surface only in a rotation axis direction of a rotary polygon mirror, and is reflected and deflected by the rotary polygon mirror. A light beam scanning device that scans a surface to be scanned in a main scanning direction substantially orthogonal to the rotation axis direction via a second imaging optical system, wherein the second imaging optical system is arranged in the main scanning direction. A linear zone plate having a plurality of extended linear bands arranged at unequal intervals in the rotation axis direction and functioning as a diffractive element having a focal length fH;
And a concave cylindrical surface having a focal length fG having a symmetry axis parallel to the direction of, and a cylindrical lens satisfying the following inequality fH> 0 fG <0, and further according to the wavelength fluctuation amount of the light source. A scanning optical system comprising a mechanism for moving the cylindrical lens or the anamorphic surface in the optical axis direction.