JPH0259442B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention relates to an optical scanning device that uses a semiconductor laser as a light source and scans a laser beam using a mechanical deflector such as a rotating polygon mirror. In an optical scanning device that uses a rotating polygon mirror as an optical deflector, it is impossible to prevent each reflective surface of the rotating polygon mirror from being tilted with respect to its rotation axis due to manufacturing errors, which causes uneven pitch of the scanning line. If this error is to be eliminated by increasing the precision of the rotating polygon mirror, it will result in an extreme increase in cost. Therefore, in the so-called sub-scanning direction parallel to the rotation axis of the rotary mirror, an optical system with a so-called tilt correction function that puts the reflective surface and the scanning surface in a geometrically conjugate relationship with respect to the imaging lens system is used. introduced.
These optical systems introduce optical systems with different refractive powers in the scanning plane and in the plane perpendicular to this, that is, cylindrical lenses and toroidal lenses, and in the direction perpendicular to the scanning direction, the so-called sub-scanning direction,
The deflection surface and the surface to be scanned are arranged in a conjugate relationship with respect to the composite system of these lens systems and the imaging lens. Also, at this time, condenser lenses are arranged so that the light beam from the light source is focused on the deflection plane. On the other hand, within the scanning plane, the light from the light source enters the deflection plane as a parallel beam,
Scanning is performed by rotating the deflection surface. For this reason, a cylindrical lens is also introduced into the condenser lens system, and generally a linear spot in the scanning direction is formed on the deflection surface. In conventional optical systems of this type, a linear spot was always formed on the deflection surface as described above, which required a certain distance between the light source and the deflector, which made it difficult to miniaturize the device. There was a limit. On the other hand, instead of creating a linear spot on the reflective surface, a cylindrical lens having refractive power in the sub-scanning direction is placed in front of the polarizer, and a light beam that diverges in the sub-scanning direction is projected onto the deflection surface. It has been proposed to use an imaging lens to form a substantially parallel light beam, and to form an image on a scanning plane using a convex cylindrical lens with a short focal length. (Japanese Patent Application Laid-Open No. 54-49152) This is because the tilt correction effect only utilizes the short focal length of the convex cylindrical lens, so the correction effect is not complete. Moreover, the light source emits a parallel light beam. It is limited to lasers and cannot be used for point-emitting laser light sources such as semiconductor lasers. In the present invention, in a so-called Gaussian beam such as a laser beam, the positional relationship of the beam waist that is conjugate to the lens is different from that in geometric optics. By taking advantage of the fact that the beam waist does not necessarily have to be present on the deflection surface, the present invention attempts to obtain an optical scanning device that has an inclination correction function but does not have the above-mentioned drawbacks. A detailed explanation will be given below with reference to the drawings. FIG. 1 is a perspective view of the main parts of the scanning optical system of the present invention, in which a beam from a light source that emits a Gaussian beam, here a semiconductor laser 1, is condensed by a coupling lens 2 and polarized by a rotating polygon mirror, etc. A convex cylindrical lens 3 is disposed between the coupling lens 2 and the deflector 4, and the light enters the deflector 4 as diverging light in the main scanning direction and converging light in the sub-scanning direction. The beam scanned by the rotation of the deflector 4 forms an image on the surface to be scanned 7 in the main scanning direction by the imaging lens 5 having an fΘ characteristic, but after once forming a beam waist in the sub-scanning direction, An image is formed on the scanned surface 7 by the convex cylindrical lens 6 . Here, the deflection surface and the scanned surface are in a conjugate relationship with respect to the system of the imaging lens 5 and the convex cylindrical lens 6, and the spot position on the scanned surface changes in the sub-scanning direction due to the inclination of the deflection surface. is prevented. The optical path of this optical system is shown in FIGS. 2 and 3 in the main scanning direction and the sub-scanning direction. FIG. 2 is an optical path diagram in the main scanning direction, in which the diverging light beam from the semiconductor laser is subjected to the focusing action of the coupling lens 2, and becomes a diverging light beam having an imaginary beam waist ω ₂ at the front focal position of the imaging lens 5. In the Gaussian beam optical system, referring to FIG. 4, the focal length of the lens is f, the beam waist radius on the object side is ω ₀ , the beam waist radius on the image side is ω ₁ ,
If the beam waist positions are d ₁ and d ₂ , then 1/ω ² / _I = 1/ω ² / _O (1−d ₁ /f) ² + 1/f ² (π
ω _O /λ) ² …(1) d ₂ = f + (d _{1 -} f) f ² / (d _{1 -} f) ² + (πω _O ² / λ) ²
…(2) holds true. Therefore, the front focus of the imaging lens 5, i.e. d ₁
A beam of light with a beam waist at position = f is d ₂ =
f, and a beam waist is generated on the scanned surface 7 at the rear focal point. The cylindrical lenses 3 and 6 placed in the optical path have no refractive power in this direction. FIG. 3 is an optical path diagram in the sub-scanning direction, and the laser beam forms an imaginary beam waist ω' ₂ by the coupling lens 2. The light beam forms a beam waist ω' ₃ at a position beyond the scanned surface 7 by a convex cylindrical lens 3 having refractive power in the sub-scanning direction. This focused light beam is further subjected to the focusing action of the imaging lens 5, and
A beam waist ω' ₄ is formed, and this beam waist is connected to the scanned surface 7 by a convex cylindrical lens 6.
An image of the desired beam waist size is formed on the top. At this time, the first cylindrical lens 3 and the second cylindrical lens
The refractive power and arrangement of the cylindrical lens 6 are determined so that a desired beam waist can be formed on the surface to be scanned based on the emission size of the laser 1 in the sub-scanning direction.
Further, as shown in FIG. 3b, the deflection surface and the scanned surface are geometrically optically conjugate to each other by the imaging lens and the second cylindrical lens. If the above relationship is established and the refractive power of each lens is determined so that the desired beam waist size ω ₅ and ω′ ₅ can be obtained on the scanned surface, the tilt correction function will be achieved.
An optical system for light scanning having desired spot diameters in the scanning direction and the sub-scanning direction can be obtained. Specific examples will be shown below with numerical values. The meanings of the symbols are as follows.

[Table] The numerical values are f ₂ = 271.3, S ₁ = 93.58 as an imaging lens.
An fΘ lens was used, and the beam spot size on the scanned surface was set to ω ₅ =0.0525 ω′ ₅ =0.06. The oscillation wavelength of the semiconductor laser is λ = 780mm, and the emission size is 1 x 2μ.
m is used. In Example 1, the distance between the coupling lens 2 and the deflector 4 is S ₂
Example 2, an example of making the device smaller and making the device more compact
assumes that since the beam divergence angle of the semiconductor laser is large, vignetting occurs in the coupling lens 2, and the beam waists ω ₂ and ω′ ₂ due to the coupling lens become twice the theoretical value shown in equation (1). Example, Example 3 is an example in which the distance between the coupling lens 2 and the deflector 4 is large, and the semiconductor laser is rotated 90 degrees under the same conditions as Example 4 and Example 1, and its active layer is rotated in the main scanning plane. This is an example of using it in a position parallel to .

【table】

[Table] Fig. 5 shows an example in which the hologram scanner 8 is used as a beam deflector, and the symbols in the figure indicate the first
Corresponds to the figure. Note that 9 is a reflecting mirror.

[Brief explanation of drawings]

FIG. 1 is a perspective view of the main parts of the optical scanning device of the present invention;
Figure 2 is a diagram of the optical path in the main scanning direction, Figure 3 is a diagram of the optical path in the sub-scanning direction, Figure 4 is an explanatory diagram of the imaging characteristics of the Gaussian beam, and Figure 5 shows the use of a hologram scanner as an optical deflector. FIG. 1: Semiconductor laser, 2: Coupling lens, 3, 6:
Cylindrical lens, 4: Deflector, 5: Imaging
fΘ lens, 7: scanned surface, 8: hologram scanner.

Claims (1)

[Claims] 1 Consists of a semiconductor laser, a coupling lens that condenses light from the laser, a deflection device that scans the beam from the coupling lens, and an imaging lens that forms an image of the scanning light on the surface to be scanned.In the main scanning direction, The coupling lens produces a beam waist at the front focal position of the imaging lens, and the imaging lens produces a beam waist on the scanned surface that coincides with the rear focal position.On the other hand, in the sub-scanning direction, the coupling lens and A beam waist is created at a position beyond the scanned surface by the coupling lens and the first convex cylindrical lens arranged in the deflector, and a beam waist is created in front of the scanned surface by the imaging lens. A beam waist is created on the scanned surface by a second convex cylindrical lens disposed between the imaging lens and the second convex cylindrical lens. A scanning optical system having a tilt correction function using a semiconductor laser, characterized in that a deflecting surface and a surface to be scanned are placed in a conjugate relationship.