JP2727444B2 - Optical scanning device - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION [Industrial Application Field] The present invention relates to an optical scanning apparatus used in a laser beam printer or a laser beam copying apparatus which has an electrophotographic process and requires high-definition image output, that is, The present invention relates to an optical scanning device that converges and scans a light receiving surface with a light beam from a semiconductor laser or an imaging optical system.

[Prior Art] Conventionally, in a laser beam printer or a laser beam copying machine as described above, the optical path length from a semiconductor laser to a light receiving surface or an object surface is made as invariant as possible with respect to environmental changes, aging or time variations. As a way to try to keep
The following are known.

One is to form a support that supports the semiconductor laser and the collimator lens that collimates the light from this laser with a metal having a small linear expansion coefficient, and to change the distance between the emission point of the semiconductor laser and the collimator lens to environmental fluctuations. On the other hand, they try to keep them unchanged. The other one is to periodically adjust the semiconductor laser device and perform image correction.

[Problems to be Solved by the Invention] However, in these methods, it is unavoidable that the members are expensive and maintenance is troublesome. Another drawback is that it is not possible to completely guarantee that the diameter or the beam diameter always satisfies the specifications.

On the other hand, the focused beam diameter is detected at a position equivalent to the light receiving surface, and based on this, the optical scanning device actuates the lens system to set the light receiving surface within the focal point depth range of the focused beam. An optical scanning device using an autofocus mechanism that is performed as needed has been proposed.

However, even in such a conventional example, there is a disadvantage that the device is inevitably complicated and expensive.

Therefore, an object of the present invention is to solve the above-mentioned drawbacks,
Select the wavelength of the semiconductor laser so that the optical path difference between the light receiving surface and the imaging surface of the optical device, which fluctuates due to environmental fluctuations and time fluctuations, is included within the depth of focus corresponding to the required high-definition image. An object of the present invention is to provide an optical scanning device configured to satisfy the expression.

[Means for Solving the Problems] In order to achieve the above object, in an optical scanning device of the present invention in which a light beam from a semiconductor laser is imaged on a light receiving surface by an imaging optical system, the following relational expression is used. The components are configured so as to satisfy the requirements, and the need for maintenance and the complexity or high price of the device are avoided by performing only one adjustment at the time of assembly.

W: diameter of light beam condensed on the light-receiving surface required for high-definition images (diameter at which point image intensity is 1 / e ² with respect to peak intensity), λ: wavelength of semiconductor laser, k: comparison constant 2.05, ▲ F ^ef _NO ▼: Effective F number of the light beam condensed on the light receiving surface, β: Lateral magnification of the conjugate point on the light receiving surface side with respect to the conjugate point on the semiconductor laser side by the image forming optical system, l: Imaging optical system Amount of movement in the optical axis direction due to environmental and temporal fluctuations of the conjugate point on the semiconductor laser side due to the fluctuation, α: Amount of movement in the optical axis direction due to environmental and temporal fluctuations of the conjugate point on the light receiving surface side by the imaging optical system, γ: The amount of field curvature of the laser beam waist that is focused and scanned on the light receiving surface by the image optical system.

[Operation] In the above relational expression, 2 (β ² l + α) represents relative movement in the optical axis direction of the optical scanning surface due to environmental and temporal fluctuations (not only fluctuations of the light receiving surface itself, but also fluctuations of the light emitting point of the semiconductor laser and collimator). 2 ▲ F ^ef _NO ＷW-γ means the depth of focus of the optical scanning surface in consideration of the curvature of field of the image of the light beam.

Therefore, the basic idea of the present invention is As can be understood from the relational expression, 1 / e with respect to the peak intensity of the point image intensity distribution determined by the laser wavelength and the effective {F ^ef _NO ▼ of the optical scanning optical system, where λ · k · ▲ F ^ef _NO ▼ = W _O. ^second diameter, i.e., focused beam minimum on an optical scanning surface of diameter, relative to 1 / e ² diameter W to the peak intensity of a point image intensity distribution required as a high-definition image, change the environment and time Is set so that the focal depth 2 ▲ F ^ef _NO Ｗ W-γ on the optical scanning surface side is larger than the optical axis direction moving amount 2 (β ² l + α) of the optical scanning image surface due to Is to select the laser wavelength λ to perform this.
(If you try to select the effective F _NO of the optical scanning optical system, 2F
^ef _NO ▼ W-γ also fluctuates at the same time, and there is a case where no solution exists.) Embodiment FIG. 1 schematically shows an embodiment of the present invention. In FIG. 1, a light beam oscillated from a semiconductor laser 1 is converted into a parallel light beam by a collimator lens 2. The parallel light beam is condensed on the reflecting mirror surface 5a of the polygon mirror 5 only in the sub-scanning direction by the cylindrical lens 3 having power only in the sub-scanning direction perpendicular to the scanning surface formed by the scanning light beam. (In the direction perpendicular to the sub-scanning direction), a parallel light beam remains as a linear light beam and enters the reflecting mirror surface 5a. Here, the linear light beam is deflected and scanned as the mirror 5 rotates. In FIG. 1, 3 of the reflecting mirror surface 5a is shown.
The light beam is deflected at two positions. After the deflection, it has f · θ characteristics in the main scanning direction (a characteristic in which the ideal image height is given by the product of the focal length f and the light beam incident angle θ) and has a conjugate point between the light receiving surface 10 and the vicinity of the reflecting mirror surface 5a in the sub scanning direction. The luminous flux is narrowed down on the light receiving surface 10 by the anamorphic condenser lenses 6, 7, and 8 having the above, and the light is scanned on the light receiving surface 10. First
In the figure, between the polygon mirror 5 and the condenser lenses 6, 7, 8,
Flat glass panes 4 and 9 are arranged between the condenser lenses 6, 7, and 8 and the light receiving surface 10, respectively.

First, in such a configuration, the case where the wavelength of the semiconductor laser 1 does not satisfy the above relational expression will be described in comparison with the present invention.

When the wavelength of the semiconductor laser 1 is 780 nm, to provide a high-definition image, for example, an output equivalent to 600 dpi (dots per inch), the intensity distribution of the laser beam on the light receiving surface 10 is 1 / e ² width by empirical rule
It is necessary to narrow down to about 45 μm. In order to secure this beam diameter over the entire scanning area, it is necessary to set the beam diameter at a minimum value of about 40 μm (1 / e ² width). From this minimum beam diameter, the effective F number ▲ F ^ef _NOの of the laser beam entering the light receiving surface 10 is about 30 as a plane wave, and when a Gaussian beam from the semiconductor laser 1 is used, the end of the passing beam due to the beam regulation by the diaphragm About 28.5
(The laser wavelength is 780n as described above.
m).

From this effective F number, the length of the beam waist due to diffraction is determined. In other words, the laser wavelength is 780nm
Then, when the effective F-number is 28.5, the focal depth with a 1 / e ² width luminous flux diameter of 45 μm or less is one luminous flux (one angle of view).
2.2mm and anamorphic condenser lens (f
When a lens system having a curvature of field of about ± 0.3 mm at a half angle of view of 30 ° is used, the focal depth in the entire scanning area is 1.4 mm.

This is shown in FIG. In FIG. 2 (a), W ₁ is the beam waist, and θ ₁ is the effective F of the luminous flux.
This is the angle related to the number. Further, in FIG. 2 (b), A ₁ is a scanning surface, A ₁ 'and A ₁ "represents a range of relative variations due to environmental and temporal fluctuations in the scanning plane, B ₁ is the best of the optical scanning optical system The image plane (with field curvature), B ₁ ′ and B ₁ ″ are 1 /
e indicates a depth of focus of a light beam diameter of 45 μm having a width of ² widths, and G indicates the depth of focus of the entire scanning area. As shown here, the depth of focus C ₁ (1.4 m
m) is smaller than the range of the environment / time variation of the scanning plane, so that the output of a high-definition image cannot be guaranteed by the environment / time variation.

On the other hand, in this embodiment, the following settings are made.

The wavelength of the semiconductor laser 1 is 680 nm or 670 nm. In this case, the effective F-number of the light beam is 33 in order to make the light beam diameter of 1 / e ² width 45 μm or less over the entire scanning area. Becomes shorter in inverse proportion to F
(The number may be large.) At that time, the focal depth for one light beam is about 2.3 mm, and the focal depth over the entire scanning area is 1.7 mm for the same reason as described above.

On the other hand, the range of the relative variation of the scanning surface 10 depending on the environment and time is as follows.

Due to the thermal expansion of the optical scanning device and the frame on the scanning surface and the movable displacement of the light receiving surface, the light receiving surface 10 fluctuates in environment and time of about ± 0.1 mm, and the focal length due to the environmental temperature change (0 to 60 ° C.) The focus (beam waist) moves about ± 0 due to fluctuations.
If the focus movement of the laser emitting unit on the collimator lens 2 side is ± 0.002 mm (l) due to the environmental temperature change, the focal length of the collimator lens 2 is about 20 mm, and the imaging optical system 6
Vertical magnification (horizontal magnification β
Is 225 times (= (300/20) × ² ), and the focus shift on the light receiving surface 10 is about ± 0.45 mm, and the sum of the variation factors is about ± 0.75 mm. This is 1.5mm in range
In the comparative example of FIG. 2, the focal depth exceeds the depth of focus C ₁ (1.4 mm), but in this embodiment, as shown in FIG.
Fits within C ₂ (1.7mm).

In FIGS. 3A and 3B, the suffix 1 of each code in FIG. 2 is 2, but each code indicates the same one.

In this way, in this embodiment, the laser wavelength is shortened so that high-definition image output can always be maintained even if there are environmental and temporal fluctuations.

[Effects of the Invention] With the configuration described above, in the present invention, high-definition image output is maintained only by adjustment at the time of assembling without causing the maintenance and the apparatus to be complicated.

[Brief description of the drawings]

FIG. 1 is a schematic configuration diagram of one embodiment of the present invention, and FIG.
FIG. 2B is a diagram illustrating a beam waist when a predetermined light beam diameter is generated with respect to a comparative example, and FIG. 2B is a diagram illustrating a field curvature, a depth of focus of a predetermined light beam diameter, and a relative change of a scanning surface with respect to the comparative example. FIGS. 3 (a) and 3 (b) are diagrams similar to FIGS. 2 (a) and 2 (b) relating to the embodiment of the present invention. 1 ... Semiconductor laser, 2 ... Collimator lens, 3 ...
Cylindrical lens, 5 ... polygon mirror, 6,
7, 8 anamorphic condenser lens, 10 scanning plane

Claims (1)

(57) [Claims] An optical scanning device for deflecting a light beam from a semiconductor laser diode by an optical deflector, condensing the light beam on a light receiving surface by an imaging optical system, and optically scanning the light beam. An optical scanning device at a light emitting point of a diode, wherein a wavelength of a semiconductor laser diode satisfies the following relational expression. W: diameter of light beam condensed on the light-receiving surface required for high-definition images (diameter at which point image intensity is 1 / e ² with respect to peak intensity), λ: wavelength of semiconductor laser, k: proportional constant 2.05, ▲ F ^ef _NO ▼: Effective F number of the light beam condensed on the light receiving surface, β: Lateral magnification of the conjugate point on the light receiving surface side with respect to the conjugate point on the semiconductor laser side by the image forming optical system, l: Imaging optical system , The amount of movement of the conjugate point on the semiconductor laser side in the optical axis direction due to environmental and temporal fluctuations, α: the amount of movement of the conjugate point on the light receiving surface side by the imaging optical system in the optical axis direction, γ: the imaging optical system Field curvature of the laser beam waist focused and scanned on the light receiving surface by