JP2757308B2 - Light beam scanning optical device - Google Patents

Description

Description: BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a light beam or a laser scanning optical device used for, for example, a laser beam printer or a digital copying machine, and more particularly to a high-resolution laser scanning optical device.

FIG. 2 shows a configuration of a typical example of a laser scanning optical device.
In FIG. 1, a divergent light beam emitted from a semiconductor laser 1 as a light source unit is collimated by a collimator lens 2 as a coupling lens.
Is converted into substantially parallel light by the laser beam, and is focused by the cylindrical lens 3 only in the sub-scanning direction (the direction perpendicular to the main scanning plane formed by the deflected scanning beam), and is located near the focal position of the cylindrical lens 3 in the sub-scanning direction. It is directed to the deflecting / reflecting surface 4 'of the polygon 4 which is the deflecting means arranged.

The cylindrical lens 3 is used for correcting the inclination of the polygon 4 (that is, the light beam is formed substantially unchanged on the medium to be scanned even if the deflecting / reflecting surface 4 'of the polygon 4 is inclined in the sub-scanning direction). Thus, a line image formed by the cylindrical lens 3 and extending in the main scanning plane is formed substantially coincident with the polygon reflection surface 4 '.

Since the polygon 4 is rotated by the motor 5 about the axis 0 perpendicular to the main scanning plane in the direction of the arrow 6, the light beam hitting on the polygon reflecting surface 4 'is reflected and deflected there. The deflected light beam has an f · θ characteristic (the ideal image height of the optical system is a focal point) formed by the spherical lens 7 and the toric lens 8 which is an anamorphic lens (the refractive power is different between the main scanning direction and the sub-scanning direction). The anamorphic imaging optical system 9 having the characteristic of the product of the distance f and the light beam incident angle θ) forms a spot image 11 on the photosensitive member 10 as a photosensitive medium and extends in the main scanning direction by deflection scanning. Form.

Since the photoconductor 10 is rotating in the direction of the arrow 12, the line images 11 are sequentially formed in the sub-scanning direction, and an image is thereby created.

In such a laser scanning optical device, a good image can be obtained by forming an imaging spot on the surface 13 of the photoconductor 10 by the imaging optical system 9. Therefore, it is necessary that the imaging spot is always stopped at a predetermined position and formed.

[Problems to be Solved by the Invention] However, in the above-described conventional example, the distance between the semiconductor laser 1 and the collimator lens 2 fluctuates due to an environment, particularly a temperature change. There is a disadvantage that the surface moves and the spot is blurred on the photoconductor surface 13.

This will be described with reference to FIG. 3 which shows only the main scanning section in FIG. The polygon reflecting surface 4 'is denoted by P, and the same components as those in FIG. 2 are denoted by the same reference numerals. The optical path of the initial setting (the spot is formed on the photosensitive member surface 13) is shown by a solid line, and the optical path at the time of temperature change is shown by a broken line. That is,
In the initial setting, the distance between the semiconductor laser 1 and the collimator lens 2 was l, but this distance changed by Δl due to thermal expansion due to a change in temperature, and 2 ′ indicated by a broken line at the position l ′. when the collimator lens came, the light beam will be image surface becomes as indicated by a broken line is changed by △ S _K to from 13 '13.

This amount is expressed as follows. Usually, a collimator lens 2 having an F number of about F _NO1 = 2 is used in order to effectively use the amount of divergent light beam from the semiconductor laser 1. Also, since the spot diameter required on the photoconductor 10 is about 80 μm, the F-number F
_NO2 = about 60 is used. Therefore, assuming that the beam diameter of the parallel light is D, all the optical systems 2 and 9 Considering, the relationship between △ S _K and △ l is When the above numerical values are substituted into this, △ S _K = 900 × △ l, and if △ l is, for example, 5μ (a change of this degree is unavoidable), △ S _K becomes as large as 4.5 mm. However, in this case, since the spot diameter required is about 80μ depth of focus is about 5 mm, △ S _K is once within an acceptable range.

However, a spot diameter for a high definition image demanded in recent years is required to be about 30 μm. In such a case, the F-number F _NO2 of the imaging optical system 9 becomes about 22, mm, which is almost equal to the depth of focus proportional to the square of _FNO2 . That is, by F _NO2 decreases △ S _K even smaller is reduced no less depth of focus. Therefore, when the temperature changes, a predetermined spot diameter on the surface of the photoconductor cannot be maintained.

Therefore, in a conventional scanning optical device for obtaining a high-definition spot diameter, in order to solve such a problem, it has been proposed to detect a light beam emitted from a collimator lens and perform an AF (automatic focusing) operation. This has the drawback that the apparatus becomes complicated and bulky, and the cost increases.

Therefore, an object of the present invention is to solve the above problem by imposing an appropriate condition on the F-number _FNO1 of the coupling lens so that a high-definition spot diameter is stable on the surface to be scanned even if the environment fluctuates. It is another object of the present invention to obtain a light beam scanning optical device obtained by the above method.

[Summary of the Invention] Generally, the following relationship is established with respect to the depth of focus Z at which a desired spot diameter is guaranteed on the surface to be scanned.

Z = κF _NO2 ² · λ where κ is a truncation factor,
λ is the working wavelength.

This depth is ideal, and the depth excluding the amount taken by the error of the optical component and the mechanical component is the depth that can be given to the environmental fluctuation. In the case of a high-definition light beam scanning optical device, about 1 / of the above equation Z can be used for this.

Using the above relational expression, There will be a desired spot size is obtained larger than the variation △ S _K of the image plane due to an environmental change.

Therefore, Is obtained. That is, in the light beam scanning optical device according to the present invention, the F number F of the coupling lens is used.
_{When NO1} satisfies the above conditional expression, a high-definition spot is always obtained on the surface to be scanned.

As for Δl, a proposal has already been made to reduce Δl by utilizing the difference in the thermal expansion of the lens barrel of the collimator lens. However, taking into account that even if this is used, a fluctuation of about 5μ cannot be avoided, △ l = 5μ, κ = 1.8, λ = 780
By adding nm, F _NO1 ≧ 3.3.

That is, in a more specific embodiment of the present invention, by setting the F-number F _NO1 of the collimator lens 2 to 3.3 or more,
It is possible to stably obtain a high-definition spot diameter.

Embodiment FIG. 1 shows an embodiment of the present invention. In FIG. 1, the same components as those in FIGS. 2 and 3 indicate the same parts.

In the embodiment shown in FIG. 1, the F-number F _NO1 of the collimator lens 22 is four. Because of the F-number of such a size, the coupling efficiency of the semiconductor laser 21 is degraded. As the semiconductor laser 21, instead of the conventional 5 mW, use a semiconductor laser having an output about four times higher than this. I have.

The F-number F _NO2 of the imaging optical system 9 is about 22, so that a spot diameter of about 30 μ is obtained on the surface 10 to be scanned. In such a configuration, the depth is about 0.67 mm.
Further, the image plane when due to a temperature change △ l is 5 [mu] (FIG. 1 indicated by a broken line) of the variation △ S _K is Which is sufficiently smaller than the depth value. Therefore, by using the collimator lens 22 with _FNO1 of 4, a stable and high-definition spot diameter is always maintained even if the environment changes.

In this embodiment, the output of the semiconductor laser is increased to compensate for the deterioration of the coupling efficiency. However, the same object can be achieved by increasing the sensitivity of the medium to be scanned.

[Effect] As described above, by imposing an appropriate condition on the F-number of the coupling lens, more specifically, this F-number can be obtained.
By setting the number to 3.3 or more, the image plane of the imaging optical system can be stabilized without complicating the apparatus, and a stable and high-definition spot diameter can be maintained on the medium to be scanned.

[Brief description of the drawings]

FIG. 1 is a sectional view of a main scanning plane according to an embodiment of the present invention;
FIG. 2 is a perspective view of a conventional light beam scanning optical device, and FIG. 3 is a main scanning sectional view for explaining a conventional example. 3 ... cylindrical lens, 9 ... imaging optical system, 20 ...
... scanned surface, 21 ... semiconductor laser, 22 ... collimator lens

Claims (3)

(57) [Claims] 1. A deflecting means for deflecting a light beam emitted from a coupling lens that converts a light beam from a light source unit into substantially parallel light, and an imaging optical system for forming an image of the light beam deflected by the deflecting means on a medium to be scanned. In the light beam scanning optical device having, the F number of the coupling lens is F _NO1, and λ is the working wavelength,
Let Δl be the change due to environmental changes in the distance between the light source and the coupling lens, and κ be the truncation factor and F _NO1 A light beam scanning optical device characterized by satisfying the following relationship: 2. The semiconductor device according to claim 1, wherein said light source is a semiconductor laser.
A light beam scanning optical device according to claim 1. 3. The F number F _{NO1 of the} coupling lens.
3. The light beam scanning optical device according to claim 1, wherein F _NO1 ≧ 3.3.