JPS6061715A - Optical scanning method - Google Patents

Abstract

PURPOSE:To realize a linearization of a high level by setting an incident angle to a hologram disk, to an optimum incident angle, setting a lattice surface of a hologram lattice and a scanning surface to a conjugate relation with regard to a specified direction, and forming an image on the scanning surface by a scanning beam. CONSTITUTION:A diffracted light LB10 which has been made incident on a hologram disk 1 at an optimum incident angle and generated is made incident on a spherical lens 40 through a plane mirror 3, passes through a cylindrical lens 8 and reaches a scanning surface 5. The spherical lens 40 and the lens 8 link a lattice surface of a hologram lattice 10-i and a scanning surface to a conjugate relation in the direction orthogonal to the scanning direction. According to such a constitution, a position of an image on the scanning surface of an optical beam is determined clearly with respect to a position of the optical beam on the lattice suface, and since an incident position of the optical beam to the lattice surface is constant and invariable, a position of an image on the scanning surface is also constant and invariable with regard to the direction orthogonal to the scanning direction, the scanning line becomes a straight line theoretically, and a shift from the straight line can be reduced to the extent that it is scarcely measured.

Description

[Detailed description of the invention] (Technical field) The present invention relates to an optical scanning method.

(Prior art) Coherent surface light such as laser light is made incident on a hologram disk having a hologram grating.

An optical scanning method is known in which a diffracted light is obtained by a hologram grating, and the diffracted light is deflected by rotating a hologram disk to scan a scanning surface, and is intended to be applied to printers, reading scanners, and the like.

Figure 1 shows an example of such an optical scanning type scanning device.
It is illustrated diagrammatically.

In Fig. 1, numeral 1 is a hologram disk, numeral 2 is a motor, numeral 3 is a plane mirror, numeral 4 is a spherical lens,
Reference numeral 5 indicates a scanning surface, and reference numeral LB indicates a light beam of laser light as coherent light.

As shown in FIG. 2, the hologram disk 1 includes hologram gratings 10-1, 10-2, .

. . are arranged in an annular shape, and as shown in FIG. 1, the motor 2 is equipped with a mounting portion 11 and can be rotated by the motor 2. Note that the hatching attached to the hologram grating in FIG. 2 does not represent the grating itself. Of course, the hologram grating is an equally spaced linear grating.

When the light beam LB enters the hologram disk 1 (
(see FIG. 1), the light beam LB is a hologram grating 10-
The light enters one of the first order lights, a part of it is transmitted straight as the 0th-order light LBO, and a part becomes the diffracted light LBI.

After being reflected by the plane mirror 3, the diffracted light LBI enters the spherical lens 4, and is reflected by the spherical lens 4.

It is focused into a spot on the scanning surface 5.

When the hologram disk 1 is rotated in a fixed direction by the motor 2, a relative movement occurs between the light beam BL and the hologram grating on which this light beam is incident, and the direction of the grating and the 6 relative movement to the light beam BL are generated. The positional relationship changes,
This deflects the diffraction field LBI. As the hologram disk 1 rotates, the hologram grating on which the light beam LB is incident is sequentially switched. As a result, the deflection of the diffracted light LBI is periodically repeated.

When the diffracted light LBI is deflected, the spot of the light beam on the scanning surface 5 moves on the scanning surface 5 and scans the scanning surface 5 accordingly.

By the way, one of the problems with this type of optical scanning method is the problem of straightening the scanning line. With the polarization of the diffracted light,
The light beam spora on the scanning plane moves. The locus of this spot on the scanning plane is called a scanning line, and it is originally desirable that this scanning line be a straight line, so various attempts have been made to make the scanning line straight. This problem is a long and difficult one.

For example, if we take the apparatus shown in Fig. 1 as an example, if we consider a straight line that is perpendicular to the drawing on the scanning plane 5 as an ideal scanning line in Fig. No scan line completely overlaps this ideal scan line. For example, if the focal length of the spherical lens 4 is 500 degrees and the scanning angle, that is, the deflection angle of the scanning beam is ±21 degrees, then even if the actual scanning line is made as straight as possible,
Set the deviation between the ideal scanning line and the actual scanning line to the maximum value.

100 μm, reaching above the IJ. Due to such errors in actual scanning lines, it is difficult to realize high-definition scanning.

(Purpose) Therefore, an object of the present invention is to provide a novel optical scanning method that makes it possible to linearize an actual scanning line to an extremely high degree.

(composition) The present invention will be explained below.

The features of the present invention are as follows.

That is, first, the angle of incidence of the coherent light incident on the hologram disk is
The optimum angle of incidence is determined.

This optimal angle of incidence is determined by setting the wavelength of coherent light to λ,
When the grating pitch of the hologram grating is d, it is uniquely determined using a dimensionless quantity of λ/4 as a parameter.

Second, the optical system that guides the diffracted light to the scanning surface allows the grating surface of the hologram grating and the scanning surface to be aligned in a specific direction.
In a conjugate relationship, the scanning beam, ie, the beam scanning the scanning plane, is substantially imaged onto the scanning plane. The above-mentioned specific direction is a direction perpendicular to the scanning direction, that is, a direction perpendicular to an ideal straight scanning line on the scanning plane.

Before going into the description of specific examples, first, the above-mentioned optimum angle of incidence will be explained.

Deflection of coherent light by a hologram disk having a hologram grating has a characteristic called a deflection characteristic. This will be explained with reference to FIGS. 3 to 5.

As shown in FIG. 3, a light beam LB is made incident on one hologram grating of the hologram disk 1, and the diffracted light is applied in the form of a screen SvC spot.

When the hologram disk J is rotated, the trajectory of the diffracted light deflected by one hologram grating on the screen S is examined. This trajectory differs depending on the incident angle θ.

In FIG. 4, the Y-axis is a direction perpendicular to the drawing in FIG. 3, and the Y-axis is a direction perpendicular to the Y-axis on the screen S.

Generally, when the incident angle θ is small, the locus is upwardly convex and parabola-like, as shown by curve 4-1 in FIG. As the angle of incidence θ gradually increases, the trajectory gradually approaches the Y-axis, but it does not coincide with the Y-axis, and at a certain angle of incidence, curve 4 is the closest form to the Y-axis.
-2, and then as the incident angle becomes larger, it becomes a downwardly convex, parabola-like locus as shown by curve 4-3. Comparing curve 4-1.4-3 and curve 4-2, curve 4. It can be seen that curve 4-2 has two inflection points, whereas curve 4-1.4-3 has no inflection points.

The angle of incidence that provides a trajectory closest to the Y-axis, as shown in curve 4-2, is called the optimal angle of incidence.

The magnitude of the optimum incident angle and the specific shape of the locus at the optimum incident angle are determined according to the dimensionless quantity '/a obtained from the grating pitch d of the hologram grating and the wavelength λ of the light beam.

As an example, the case where λ/d=1.4 is shown in FIG.

The horizontal axis is the deflection angle, and the vertical axis is the distance from the deflection point to the screen divided by the Y-axis coordinates, that is, the deviation of each trajectory from the Y-axis in radians.

In curve 5-1 of FIG. 5, the incident angle is 4200, curve 5
-2, 5-3, and 5-4 (the incident angles are 4425°, 460°, and 470°, respectively. Curve 5-2
From this, it can be seen that the optimal angle of incidence in this case, that is, when λ/d = 1.4, is 44.25°.

Earlier, it was stated that in the case of a light scanning optical system as shown in Fig. 1, it is not possible to perfectly match the actual scanning line with the ideal scanning line, but this can be explained with reference to Fig. 6. I will explain.

As mentioned above, the scanning angle of the scanning beam in this device is ±
21 degrees, the focal length of the spherical lens 4 is 50C nm,
If we look at the relationship between the maximum amount of deviation (fin) between the scanning line of the kernels and the ideal one within the scanning angle range of ±21 degrees and the angle of incidence, we get something like the one shown in FIG.

The horizontal axis in FIG. 6 indicates the incident angle, and the vertical axis on the left side indicates the maximum deviation amount. The right vertical axis indicates the maximum deviation amount as a relative value. The parameters λ are curves 6-1, 6-2,
6-3, 1゜2.1.4, 1.6, respectively.
It is.

As is immediately clear by referring to Fig. 6, the first
In the device shown in the figure, even if the angle of incidence is set to the optimum angle of incidence, a deviation of more than the maximum ZooRn will occur with respect to the ideal scanning line. It can be seen that when the deviation occurs, the above deviation increases rapidly.

As is clear from this, it is impossible in principle to completely straighten the scanning line with the apparatus shown in FIG.

The invention will now be described with reference to specific embodiments with reference to Figures 7 and 7 below.

In order to avoid complications, items that are considered to have a high risk of confusion are shown in Figure 1 in Figures 7 and 8.
The same reference numerals as in FIG. 2 are used.

In the embodiment shown in FIG. 7, reference numeral 6 indicates a cylindrical lens, reference numeral 7 indicates a plane mirror, and reference numeral 8 indicates a cylindrical lens. FIG. 8 shows the embodiment shown in FIG. 7 viewed from above, and shows only the parts necessary for explanation.

A light beam LB of laser light as a coherent light is first focused in one direction, finally in a direction perpendicular to the scanning direction, by a cylindrical lens 6, and enters the hologram disk 1 via a plane mirror 7. At this time,
Of course, the angle of incidence is determined to be the optimum angle of incidence.

Diffracted light LB generated by the hologram grating. enters the spherical lens 40 via the plane mirror 3, further passes through the cylindrical lens 8, and reaches the scanning surface 5. The cylindrical lens 8 is in the vertical direction in FIG.
It has power in the direction perpendicular to the scanning direction.

As shown in FIG. 8, when the hologram disk 1 is rotated in the direction of the arrow, the scanning beam moves along the scanning surface 5 in the direction of the arrow.
That is, the drawing is scanned from the top to the bottom.

Now, as an optical system that guides the diffracted light LB+o to the scanning surface, a plane mirror 3, a spherical lens 40, . There is a cylindrical lens 8, of which the spherical mirror lens 4o and the cylindrical lens 8 are important.

Note that it is preferable to use an ifθ lens as the spherical lens 4o in which the incident angle and the image height are proportional, from the viewpoint of uniform scanning speed.

Now, the spherical lens 4o and the cylindrical lens 8 are the lattice plane of the porogram grating in the direction orthogonal to the scanning direction, that is, the hologram grating that actually generates the diffracted light, in other words, the hologram grating on which the light beam is incident. The lattice plane of the hologram grating 10-i (in accordance with FIG. 8, the lattice plane of the hologram grating 10-i and the scanning plane are connected in a conjugate relationship).

Therefore, the image of the light em on the grating plane of the hologram grating is
An image is formed on the scanning plane 5 in a direction perpendicular to the scanning direction.

Since the light beam is focused in the above-mentioned direction on the grating plane of the hologram grating, the image of the light beam is also focused on the scanning plane 5 in the above-mentioned direction. In the scanning direction, the diffracted light LB1o+'li is focused onto the scanning surface 5 by the spherical lens 40 (see FIG. 8).

In this way, a scanning beam substantially imaged onto the scanning plane can be obtained.

Now, as mentioned above, since the lattice plane of the hologram grating of the hologram disk 1 and the scanning plane 5 are in a conjugate relationship, the position of the image of the light beam on the lattice plane on the scanning plane in the direction orthogonal to the scanning direction is , is uniquely determined with respect to the position of the light beam on the grating plane, but since the incident position of the light beam on the grating plane remains constant, the position of the image on the scanning plane also changes in the direction perpendicular to the scanning direction. In other words, in the optical scanning method according to the present invention, the scanning line is in principle a straight line.

Of course, in reality, it is unavoidable that the scanning line will deviate slightly from a perfect straight line due to installation errors of each device, manufacturing errors of lenses, etc., but in actual implementation conditions,
The deviation of the scanning line from a straight line can be made so small that it is almost unmeasurable.

91st g! , the curve denoted by 9-2 is the first
In the apparatus shown in the figure, the scanning line shape when the optimum incident angle is set, curve 9-2, shows the scanning line shape when the present invention is implemented under the same conditions. The vertical axis indicates the relative value of the amount of deviation of the scanning line from the straight line. The scanning line 9-1 is substantially a straight line, and the maximum deviation amount is 1/1 of that of the curve 9-2.
It is 4o or less.

Note that λ/d is 611.4 and the scanning angle is ±210.

In the case of the apparatus shown in FIG. 1, when the angle of incidence on the hologram disk 1 deviates from the optimum angle of incidence, the deviation of the scanning line from the straight line increases rapidly (see FIG. 6). However, in the case of the present invention, even if the angle of incidence deviates from the optimum angle of incidence, the deviation increases much more slowly.

For example, λ/d = 1. , 4, the scanning angle is ±2
Taking the case of 10 as an example, the maximum deviation amount between the scanning line and the straight line is, in relative value, in the case of the apparatus shown in FIG.
In contrast to the curve 100 in the figure, in the case of the present invention, the curve is like the curve 101 in the figure.

In addition to the combination of the above-mentioned spherical lens and cylindrical lens, the optical system for establishing a conjugate relationship between the grating plane and the scanning plane in the direction perpendicular to the scanning direction includes only a cylindrical lens, only a cylindrical mirror, Alternatively, a combination of a cylindrical mirror and a spherical lens can be considered.

(Effects) As described above, according to the present invention, a novel optical scanning method can be provided. In this method, since the grating plane and the scanning plane have a conjugate relationship in the direction perpendicular to the scanning direction, highly linearized scanning lines can be obtained. In addition, since the angle of incidence is set to the optimum angle of incidence, fluctuations in the direction perpendicular to the scanning direction of the deflected diffracted light are small, and the accuracy of the optical system near the optical axis to realize the above conjugate relationship is small. The above conjugate relationship can be realized with high precision.

Since the scanning line obtained by the present invention is highly linearized (see FIG. 9), it is fully compatible with high resolution scanning.

On the other hand, if the linearity of the scanning line is to be achieved to the extent achieved by the apparatus shown in FIG. I think it is possible.

For example, considering the parallelism of both sides of the disk and the grating pitch precision of the hologram disk 1, if we try to make the scanning line as straight as possible using the apparatus shown in Figure 1, the permissible error in parallelism is 1~ If it takes 2 seconds, the tolerance range of grating pitch accuracy is on the order of 110-3n. However, in the method of the present invention, the permissible error for the parallelism is 1 root degree, and the tolerance for the grating pitch accuracy is % 10".
It is on the order of 1.nm, which means that these precisions can be relaxed to a wide range.It is also clear from FIG. 10 that the precision of the incident angle can be relaxed.

[Brief explanation of drawings]

1 to 6 are diagrams for explaining conventionally known optical scanning systems and their problems, and FIG. 7 is a front view schematically showing one embodiment of the present invention. FIG. 8 is a plan view of the embodiment shown in FIG. 7, illustrating only the parts necessary for explanation, and FIGS. 9 and 10 are diagrams for explaining the present invention in detail. DESCRIPTION OF SYMBOLS 1... Hologram disk, 2... Motor, 3゜7... Flat i [, 6, 8... Cylindrical lens,
40... Spherical lens, 5... Scanning surface, LB.・・・
・Diffraction light.

Claims (1)

[Claims] A hologram disc having a hologram grating has a coherence 1
A method for optically scanning a scanning surface by making a non-coherent light incident and obtaining a diffracted light by the hologram grating, and deflecting the diffracted light by rotating the hologram disk, the method comprising: a hologram disk using coherent light; The angle of incidence on the hologram grating is set to the optimum angle of incidence determined according to λ/4 (λ: the wavelength of coherent light, d: the grating pitch of the hologram grating), and the optical system guides the diffracted light to the scanning surface. The scanning beam is characterized in that a scanning beam that substantially forms an image on the scanning plane p1 is obtained by establishing a conjugate relationship between the locus plane and the scanning plane in a direction perpendicular to the scanning direction. Optical scanning method.