JPH0330843B2 - - Google Patents

Description

[Detailed description of the invention] The present invention relates to an image recording device.

In an image recording apparatus that uses a laser diode as a light source, conventionally, as shown in FIG. A technique for arranging them in correspondence with each other in the same direction as the sub-scanning direction y is known. Specifically, for example, as shown in FIG. 2, the laser beam from the laser diode 1 is transmitted through a coupling lens 3, a prism 7,
An image is formed on the scanning surface 2 as an elliptical spot whose major axis coincides with the sub-scanning direction y through an optical system including a slit 4, a rotating mirror 5, an f/θ lens 6, etc. (see FIG. 3d).

In addition, in FIG. 2, the optical axis L of the laser beam
Above, the general shape of the laser beam at each location indicated by , , on the cut plane in the direction perpendicular to the optical axis corresponds to a, b, c, and d in FIG. 3, respectively, and the laser diode in this example The direction S-S of the joint surface of No. 1 is arranged to correspond to the main scanning direction x.

In addition to the above example, the laser diode 1 may be arranged in the direction S-S of the junction surface, as described above.
In some cases, the prism 7 is arranged in alignment with the sub-scanning direction y, but even in this case, by replacing the prism 7 with another prism with different optical performance, etc., the laser image finally formed on the scanning surface 2 can be adjusted. The light beam is arranged into an elliptical spot whose long axis coincides with the sub-scanning direction y, as shown in FIG. 3d.

In this way, the reason why the long axis of the elliptical spot is aligned with the sub-scanning direction y is due to the fluctuation in the tendency of the mirror axis of mechanical scanning devices such as the galvanometer mirror and rotating mirror 5 to fall when rotated. This is a consideration to minimize the influence on pitch unevenness of scanning lines.

By the way, in view of the above requirements, when the laser diode 1 is arranged parallel to or perpendicular to the main scanning direction y in the direction S-S of the junction surface of the laser diode 1, the reflection of the rotating mirror 5 is The incidence angle dependence is recognized in the rate,
There is a drawback that exposure unevenness occurs in response to fluctuations in the angle of incidence as the rotating mirror 5 rotates.

For example, the complex refractive index of aluminum is n-ik
(n=1.9, k=7.0), the reflectance R _p of the component parallel to the incident plane and the reflectance R _s of the component perpendicular to the plane of incidence can be expressed by the following equations (1) and (2).

R _p = (n ² + k ² ) cos ² θ−2n cos θ + 1/(n ² + k ² ) cos
² θ + 2 n cos θ + 1……(1) R _s = (n ² + k ² ) − 2 n cos θ + cos ² / (n ² + k ² ) + 2 n cos
θ＋cos ² θ……(2) (However, θ is the angle of incidence) The result of calculating each formula (1) and (2) above is the graph in Figure 5, and the reflective surface of a mechanical scanning device is usually Since it is an aluminum vapor-deposited surface, the reflectance shown in FIG. 5 is followed. Since the laser beam from the laser diode is linearly polarized in the direction of its junction surface, by adjusting the arrangement of the laser diode, for example, by aligning the direction of the junction surface with the main scanning direction or the sub-scanning direction, R _s shown in Figure 5?
It follows the reflectance characteristics of only one of R _p . For example, due to the optical layout, as shown in Fig. 4, the average incident angle δ of the laser beam onto the rotating mirror 5 is often around 45°, so the incident angle of the laser beam is about 45°±15°. Since the reflectance varies within the range of , and the reflectance also varies, exposure unevenness occurs on the scanning surface 2.

This is true not only for the rotating mirror 5 having an aluminum vapor-deposited surface as a reflective surface, but also for other reflective surfaces,
For example, Al vapor deposited film + protective film, Al vapor deposited film + SiO ₂
A similar tendency is shown in mechanical scanning devices having a reflective surface made of +TiO ₂ or a dielectric multilayer film.

By the way, even with such a mechanical scanning device that is highly dependent on the incident angle, if the direction of the junction surface of the laser diode is arranged at 45 degrees to the main scanning direction, its reflectance characteristics will be as follows. As shown by the broken line in FIG. 5, the dependence on the incident angle is eliminated and the exposure unevenness is eliminated.

However, on the other hand, the scanning spot on the scanning surface 2 also becomes an ellipse inclined at 45 degrees with respect to the main scanning direction This makes it susceptible to the unevenness of the pitch of the scanning line due to fluctuations, resulting in a decrease in print quality.

The present invention has been made in view of the above circumstances, and uses a laser diode as a light source, makes the exposure spot shape on the scanning surface an elliptical spot having a long axis in the sub-scanning direction, and has less uneven exposure. The purpose is to provide an image recording device.

The present invention will be explained in detail below.

The image recording device according to the present invention is provided between a laser diode and a mechanical scanning device that scans by rotating or swinging a reflective surface, such that the polarization direction of the laser beam is relative to the scanning direction of the mechanical scanning device. omitted
An optical element is arranged that rotates the plane of polarization to 45° and causes it to enter the mechanical scanning device, or an optical element that converts the direct polarization of the laser beam into circular or elliptical polarization and makes it enter the mechanical scanning device. It is characterized by

FIG. 6 shows an image recording apparatus as an embodiment of the present invention.

In the figure, numeral 1 indicates a laser diode,
Reference numeral 2 indicates a scanning plane, and between the two, in order of proximity to the laser diode 1, a coupling lens 3, a beam shaping optical shape 8, an optical element 9, a rotating mirror 5,
An f/θ optical system 10 and the like are arranged.

In the example shown in the figure, the direction S-S of the junction surface of the laser diode 1 is arranged on the scanning surface 2, corresponding to the main scanning direction 8, the spot is imaged on the scanning plane 2 as an elliptical spot having its major axis in the sub-scanning direction y. Even if the junction surface direction S-S of the laser diode 1 is arranged on the scanning surface 2 and corresponds to the sub-scanning direction y, the laser beam has a long axis on the scanning surface 2 in the sub-scanning direction y. The image is formed as an elliptical spot.

In FIG. 6, before the laser beam is incident on the rotating mirror 5, it passes through an optical element 9, which is a crystal plate with a phase difference of λ/2, so that the main polarization direction of the laser beam is changed. Since it rotates at an angle of approximately 45° to the scanning direction,
The reflectance characteristics are as shown by the broken line in FIG. 5, and variations in reflectance due to variations in the angle of incidence are eliminated.

As the optical element 9, a crystal plate having a phase difference of λ/4 for converting the linearly polarized light of the laser beam into circular or elliptically polarized light may be used instead of the above-mentioned crystal having a phase difference of λ/2. You can also obtain the same effect.

[Brief explanation of the drawing]

Figure 1 is a diagram explaining the relationship between the scanning direction and the direction of the junction surface of the laser diode, Figure 2 is a diagram explaining the configuration of the optical system of the image recording device, and Figure 3 is the optical axis in Figure 2. Figure 4 is a diagram explaining the cross-sectional shape and size of the beam mentioned above, Figure 4 is a diagram explaining the variation of the incident angle due to the rotation of the rotating mirror, and Figure 5 is a diagram explaining the relationship between the incidence angle and the reflectance on the aluminum reflecting surface. FIG. 6, which is a diagram explaining the relationship, is a perspective view showing the main structure of an image recording apparatus as an embodiment of the present invention. 5... Rotating mirror (as an example of a mechanical scanning device), 9... Optical element.

Claims (1)

[Claims] 1. A laser diode is used as a light source,
The laser diodes are arranged so that the direction of the bonded surface is parallel or perpendicular to the main scanning direction on the scanning surface, and the direction of the bonded surface is parallel or perpendicular to the main scanning direction on the scanning surface. In either case, due to the properties of the optical system provided between the laser diode and the scanning surface, the laser beam that is finally imaged on the scanning surface is formed into an elliptical spot whose long axis coincides with the sub-scanning direction. In an image recording device, between a laser diode and a mechanical scanning device that scans by rotating or swinging a reflective surface, the polarization direction of the laser beam is such that the direction of polarization of the laser beam is different from the scanning direction of the mechanical scanning device. Arrangement of an optical element that rotates the plane of polarization to approximately 45° and causes the plane to enter the mechanical scanning device, or an optical element that converts linearly polarized light of the laser beam into circular or elliptically polarized light and causes the plane to enter the mechanical scanning device. An image recording device characterized by: