JPH0371691B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION (a) Technical Field of the Invention The present invention relates to a light beam scanning device, and more particularly to a light beam scanning device that uses the diffraction effect of a hologram.

(b) Background of the Technology In recent years, a light beam scanning method that utilizes the diffraction effect of a hologram has been used in barcode reading devices in POS (Point-of-sale) systems, laser printers, and the like.

This method allows for faster scanning than the conventional light beam scanning method using a rotating mirror, and the optical system configuration to obtain a wide scanning width is relatively small and simple. benefits can be obtained.

In devices that use a light beam scanning method using a hologram as described above, as seen in laser printers, etc., the light beam in the scanning plane generally requires high resolution, and it is necessary to minimize the scanning light beam diameter as much as possible. However, increasing the distance between the scanning plane and the hologram disk to obtain a wide scanning width tends to reduce resolution.

(c) Prior Art and Problems The present inventors have discovered that the above-mentioned decrease in resolution can be improved by shifting the plane wave incident on the photosensitive surface from the vertical direction when creating a hologram (Japanese Patent Laid-Open No. 53 −75950). Furthermore, the present invention has disclosed that a diffraction wave obtained by irradiating a scanning hologram in the opposite direction with a diverging wave using a light beam imaging point on a scanning surface as a point light source is formed by interference with another wave surface. proposed a method for reducing astigmatism by using a hologram as a wavefront conversion element and making a light beam incident on the scanning hologram through the wavefront conversion element (Japanese Patent Application No. 54448/1983).

On the other hand, when using a semiconductor laser as a light source,
The wavelength of the output light fluctuates due to temperature changes in the semiconductor laser element, and as a result, the position of the light beam on the scanning plane fluctuates, reducing scanning accuracy. A method has been proposed in which a hologram plate having a function of diffracting a light beam in a direction opposite to the direction of light beam diffraction by the disk is placed between the laser light source and the hologram disk (Japanese Patent Publication No. 56-70517). .

In the above conventional method, it is necessary to prepare a special optical system for creating the wavefront conversion element and scanning using the same, or a scanning device is required to make the light beam perpendicularly incident on the hologram plate. Because of the necessity of constructing a system, the cost of the device is high, and it also has drawbacks such as restrictions on usage conditions, which are obstacles to practical use.

(d) Object of the Invention The present invention solves the problems in the conventional methods described above, and provides a light beam scanning device in which both the occurrence of aberration on the scanning plane and the variation in scanning position due to wavelength variation of a semiconductor laser light source are prevented. The purpose is to make it possible to provide.

(e) Structure of the Invention The present invention provides a light beam scanning device using a hologram disk composed of hologram facets, in which a wavefront conversion element on which a hologram identical to the hologram facet is formed is connected between a light source and the hologram disk. The main feature is that the scanning light beam is incident on the hologram disk through the wavefront conversion element, and in this case, the hologram disk and the wavefront conversion element are parallel and close to each other, and By making the incident beam to the wavefront conversion element incident at a slight angle shift from the direction of the output beam from the hologram disk, or by focusing the incident beam to the wavefront conversion element near the irradiation point of the output beam from the hologram disk. The transmitted light (0th-order diffracted light) can be separated by creating a beam of It is characterized by:

(f) Embodiments of the invention Examples of the invention will be described below with reference to the drawings.

FIG. 1 is a diagram showing a general configuration of a light beam scanning device according to the present invention, in which a light beam 1 is emitted from a light source (not shown) such as a semiconductor laser and is focused by a lens (not shown). is incident on the hologram disk 2 which rotates in the direction of the arrow around the axis X-X, and is diffracted here toward the scanning surface 3, for example, an electrostatic recording drum in a laser printer, and as shown by reference numeral 31, it is diffracted on the scanning line. Moving. Generally, in light beam scanning devices, it is necessary to repeatedly scan the light beam on the scanning surface at high speed.
The hologram disk 2 is composed of a portion called a facet as shown by the reference numeral 21, and as the hologram disk 2 rotates, one scan is performed each time the facet 21 passes through the irradiation position of the light beam 1. will be held. Incidentally, when the light beam is repeatedly scanned in a straight line in the same direction on the scanning surface 3 as in a laser printer, the same facet 21 will be arranged on the hologram disk 2.

In the optical beam scanning device of the present invention, a wavefront conversion element 4 is provided parallel to and adjacent to the hologram disk 2. The wavefront conversion element 4 has the same hologram as the facet 21 formed thereon, and is fixed on the optical path of the light beam 1. Second
The figure shows the optical path of the light beam 1 in a cross section centered near the wavefront conversion element 4. The incident light beam 1 is diffracted by the wavefront conversion element 4, and the diffracted light 1
It becomes 1. The diffracted light 11 is transmitted to the hologram disk 2
It enters the upper facet 21 and is further diffracted,
This becomes diffracted light 12. The presence of the wavefront conversion element 4 prevents astigmatism of the light beam on the scanning surface and fluctuations in the scanning position due to wavelength fluctuations of the light beam. The principle will be explained in detail below.

As shown in FIG. 3, when a spherical wave S is generally incident on the facet 21 of the hologram disk 2, the projection beam R on the scanning surface 3 has aberrations.
Causes DS. In order to avoid this, a wavefront Q such that the diffracted light is focused on one point P on the scanning plane may be made incident on the facet 21 as shown in FIG. In order to obtain such a wavefront Q, it is sufficient to utilize the fact that when a wavefront Rc that is conjugate to the output light wavefront R is incident on the facet 21, a wavefront Qc that is conjugate to the wavefront Q is obtained. That is, when a hologram identical to the facet 21 is placed as the wavefront conversion element 4 in front of the facet 21 and a beam of light with the wavefront Rc is incident thereon, the wavefront Qc is incident on the facet 21. ,
If the wavefront Qc is considered to be equal to the wavefront Q, a wavefront R without aberration is emitted. In addition, the wave surface
As Rc, a focused light obtained by an ordinary lens or the like may be used.

By the way, the 0th-order diffracted light Po that has passed through the wavefront conversion element 4 appears, but the distance d between the facet 21 and the wavefront conversion element 4, the parallelism between the facet 21 and the wavefront conversion element 4, or the incident angle of the wavefront Rc is The projection beam P (normally 1st-order diffracted light) on the scanning surface and the 0th-order diffracted light Po can be separated by changing the minute amount.
By providing a slit 5 as shown in the figure, Po can be easily removed.

Next, the principle of the effect of preventing scanning position shift based on the wavelength fluctuation of emitted light when a semiconductor laser is used as a light source by providing the wavefront conversion element 4 will be explained using FIG.

In the same figure, for simplicity, we will use facet 2.
1 and the wavefront conversion element 4 are parallel, the wavelength of the incident light to the wavefront conversion element 4 is λ1, the incident angle is θi, the output angle of the diffracted light by the wavefront conversion element 4 is θq1, and the incident angle to the facet 21 is θj1. , and the emission angle of the diffracted light R is .theta.r1, and the spatial frequencies of the holograms of the wavefront conversion element 4 and the facet 21 are fi and fj, respectively, and the following relationship exists between them.

sinθq1=fiλ1−sinθi...(1) sinθr1=fjλ1−sinθj1...(2) Since the wavefront conversion element 4 and facet 21 are parallel, sinθq1=sinθj1...(3) Therefore, sinθr1=(fi −fj)λ1+sinθi ……(4) When the wavelength of the incident light is λ2, and if each subscript is represented by 2, sinθr2=(fi−fj)λ2+sinθi ……(5) From equations (4) and (5), sinθr2=sinθr1=(fi−fj)(λ2−λ1) ≡Δf・Δλ……(6) Since the wavefront conversion element 4 and facet 21 are the same hologram, Δf is extremely small. Therefore, we obtain θr2≒θr1...(7). In other words, even if a wavelength variation Δλ occurs in the emitted light when a semiconductor laser is used as a light source,
Fluctuations (θr1 to θr2) in the emission direction of the light beam R from the facet 21 are extremely small, and scanning position deviations on the scanning plane can be reduced.

In the explanation of FIG. 4, it is assumed that the wavefront Q and the wavefront Qc are approximately equal, and this means that the smaller the distance d between the wavefront conversion element 4 and the facet 21 in FIG. 6, the greater the effect of the present invention. do.

FIG. 7 shows another embodiment of the present invention, in which a filter 6 having a transmittance-angle of incidence characteristic as shown in FIG. 8 is provided between the wavefront conversion element 4 and the facet 21. This is a method of attenuating the transmitted light (0th-order diffracted light) of the wavefront conversion element 4, and the filter 6 may be bonded to the output side of the wavefront conversion element 4 as shown in the figure. Further, as the filter 6, an appropriate interference filter can be used.

The main advantages of the present invention over conventional methods are that it is possible to reduce both astigmatism in a light beam scanning device using a hologram as well as a decrease in scanning accuracy due to fluctuations in the light source wavelength; As a hologram that is exactly the same as the facet 21 is used, it is easy to manufacture, the light beam does not need to be incident perpendicularly to the wavefront conversion element 4, and the wavefront conversion element 4
Since the facet 21 and the facet 21 are provided substantially parallel to each other, the light beam scanning system can be made smaller and the structure can be simplified, and furthermore, the manufacturing cost of the light beam scanning device can be reduced.

(g) Effects of the Invention According to the present invention, it is possible to provide a low-cost light beam scanning device that is capable of high scanning accuracy and high quality light beam scanning.

[Brief explanation of drawings]

FIGS. 1 and 2 are diagrams showing an outline of the configuration of a light beam scanning device according to the present invention and diagrams showing an optical path in a partial cross section, and FIGS. FIGS. 7 and 8 are diagrams for explaining the principle of preventing the occurrence of scanning position deviation based on the above-described method, and are diagrams showing other embodiments of the present invention. In the figure, 1 is a light beam, 2 is a hologram disk, 3 is a scanning surface, 4 is a wavefront conversion element, 5 is a slit, 6 is a filter, 11 and 12 are diffracted lights, 21 is a facet, and 31 is a scanning line.

Claims (1)

[Claims] 1. In a light beam scanning device using a hologram disk composed of hologram facets, a wavefront conversion element on which a hologram identical to the hologram facet is formed is fixed between a light source and the hologram disk. 1. A light beam scanning device, characterized in that the light beam scanning device is provided as a hologram disk and makes a scanning light beam incident on the hologram disk via the wavefront conversion element. 2. Claims characterized in that the hologram disk and the wavefront conversion element are arranged in parallel and close to each other, and the incident beam to the wavefront conversion element is made to be incident on the wavefront conversion element with a slight angle shift from the direction of the output beam from the hologram disk. 2. The light beam scanning device according to item 1. 3. The optical beam scanning device according to claim 1, wherein the incident beam to the wavefront conversion element is a beam that is focused near the irradiation point of the output beam from the hologram disk. 4. The light beam scanning device according to claims 1 to 3, characterized in that a filter is provided between the hologram disk and the wavefront conversion element, the transmittance of which is dependent on the angle of incidence. .