JPH0587806B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a light beam scanning device, and more particularly to a light beam scanning device using a diffraction grating, typically a hologram.

Optical deflectors used in optical beam scanning devices include mechanical optical deflectors such as rotating polygon mirrors and galvanometers, acousto-optic optical deflectors that utilize the interaction between ultrasonic waves and light, and moving holograms. Holographic optical deflectors, etc., have been known in the past. Among these, hologram optical deflectors are mainly composed of holograms that can be easily mass-produced by photographic processes or thermocompression bonding processes, and the hologram optical deflectors themselves have tilt errors and wobbling. Even if the hologram occurs, its effect is unlikely to appear on the deflected beam, the hologram itself can have a lens effect, so a focusing lens can be omitted, and the direction of light deflection can be set in the direction of the hologram's motion. It has the advantage of being able to be freely selected independently from the other devices, and is attracting expectations for simpler and lower-cost equipment.

The principle of the hologram optical deflector is to change the direction of the first-order diffracted beam to be reproduced by changing the relative positional relationship between the hologram plate and the illumination beam for reproduction. Examples of holographic optical deflectors that embody this principle include those in which reflective holograms are arranged in the direction of rotation of a rotating spherical surface (convex or concave), and those in which holograms are arranged on the side of a rotating cylinder or rectangular tube. , one in which holograms are arranged in the circumferential direction of a rotating disk, etc. are known.

Among these, those in which reflective holograms are arranged on a spherical surface and those in which holograms are arranged on the side of a cylinder or rectangular tube use a hologram reproducing system without aberration, so they can resolve more than 10,000 points per scan. It is also possible to obtain a large number of image points, which is useful for constructing a high-resolution laser scanning device. However, in order to realize a hologram optical deflector in the form of a sphere or cylinder, it is necessary to coat a photosensitive material such as a photographic emulsion, a photoresist, or a photopolymer on the sphere or cylinder, which is technically difficult. Furthermore, in the case of a hologram optical deflector in the form of a rectangular cylinder, although the individual holograms are formed in the shape of a flat plate, high precision is required to assemble them, and there is a limit to how high the speed can be increased due to large windage losses.

On the other hand, a configuration in which holograms are arranged in the circumferential direction of a disk has a very simple structure, does not have the above-mentioned drawbacks, and is considered to be the most suitable form for practical use. However, so far, there has been no example of practical use other than the case where scanning lines in various directions can be obtained from a plurality of holograms constituting a hologram optical deflector and the holographic optical deflector is incorporated into a barcode reader. The biggest reason for this is that when trying to obtain scanning lines on one plane, the scanning lines become arcuate, which is inconvenient as a device for recording or reading document information or image information.

Attempts to solve this problem in disk-shaped hologram optical deflectors are disclosed in U.S. Pat. No. 4,289,371, U.S. Pat.
It is described in the specification of No. 57-85018.

The method described in US Pat. No. 4,289,371 is
This is a method in which the ratio λ/d between the wavelength λ of the light beam used and the period d (constant) of the diffraction grating (hologram) is set to 1 to 1.618. But with this method,
For example, if the wavelength λ of the light beam used is 0.488 μm, the period d of the diffraction grating must be 0.30 μm to 0.49 μm. It is not easy to manufacture a high-quality diffraction grating (hologram) with such a fine period. The first problem is that the vibration of the holographic exposure device and the disturbance of the surrounding air must be strictly eliminated, and the holographic grating pattern with the above-mentioned period must be faithfully exposed onto the recording material. The difficulties are well known to those in the art. The second problem is that it is difficult to obtain a practical recording material for recording a diffraction grating having the period described above.

The method described in US Pat. No. 3,721,486 is
This is a method in which a light beam is diffracted twice by two diffraction gratings that rotate in opposite directions at equal speeds. However, in order to rotate the two diffraction gratings at equal speeds and in opposite directions, a complicated rotation transmission mechanism is required, and a technically difficult problem remains.

The method described in Japanese Patent Application Laid-Open No. 57-85018 is
This is a method in which a light beam is diffracted twice using two types of diffraction gratings (holograms) that rotate in synchronization with each other. However, this method is complicated because it requires two types of diffraction gratings connected to each other in a predetermined relationship.

By the way, in a disk-shaped hologram optical deflector,
Generally, the rotation angle of the disk and the deflection angle of the light beam are not in a proportional relationship. Therefore, when a deflected light beam is focused by a focusing lens to scan a light spot on a scanning surface, the light spot generally does not move at a constant speed. The method of keeping the moving speed of the light spot almost constant on the scanning plane is JC.
Wyant in Applied Optics, Volume 14, Issue 5, pp. 1057-1058. This method uses a distortion-free lens as a focusing lens to approximately keep the light spot moving speed on the scanning plane constant, and is not exact.

A first object of the present invention is to form a linear scanning line that is not arch-shaped on a plane when constructing an optical beam scanning device that uses a rotating plate-shaped optical deflector made of a diffraction grating, typically a hologram. It's about getting.

A second object of the present invention is to enable the use of a diffraction grating with a relatively large grating period in order to achieve the first object described above, thereby facilitating the creation of the diffraction grating. be.

A third object of the present invention is to eliminate the need for a complicated rotation transmission mechanism in order to achieve the above first object.

A fourth object of the present invention is to achieve the above-mentioned first object without requiring two types of diffraction gratings,
This means that only one type of diffraction grating is required.

A fifth object of the present invention is to achieve the above-mentioned second object, thereby making it possible to use a diffraction grating that does not necessarily require the holograph technique and can also be created using the ruling technique. be.

A sixth object of the present invention is to make the scanning speed of the light spot on the scanning plane constant with high precision.

A seventh object of the present invention is to improve the performance of the device by achieving the above-mentioned first and sixth objects, as well as to achieve the second, third, fourth, and fifth objects. The aim is to improve the reliability of the device and reduce the cost.

The present invention uses a rotating plate provided with at least one linear planar diffraction grating with equal pitch, and a light beam parallel to the rotating axis (the light beam is a wavelength of which is approximately half the grating period of the planar diffraction grating, and the light beam diffracted by the planar diffraction grating is reflected by a planar reflective surface perpendicular to the incident surface. Guide the light beam again to the plane diffraction grating so that the projection onto the incident surface forms an angle of 180° with the diffracted light beam, and scan the light beam diffracted by the plane diffraction grating again by the θ lens. A light beam scanning device is characterized in that the light beam is focused on a surface, and the focused light spot scans the scanning surface as the rotary plate rotates.

The present invention will be explained in detail below.

FIG. 1 shows an example of a rotating plate used in the present invention. As is clear from the drawing, a plurality of linear planar diffraction gratings 3-1, 3-2, . ing. The number of diffraction gratings is not necessarily limited to six as in this example, but may be any number greater than or equal to one. Further, the direction of the grating lines of the diffraction grating is preferably a direction tangent to the rotation direction of the rotary plate 1.

Diffraction gratings can be created by various methods, but the case where they are directly created using holographic techniques will be explained with reference to FIG. For a rotating plate 1 with a recording medium provided on its surface,
Plane waves 4 and 5 having mutually coherent wavelengths λ are made incident. Interference fringes formed by the interference of the plane waves 4 and 5 are exposed a predetermined number of times through a certain mask. This number of times is equal to the number of diffraction gratings to be provided on the rotating plate; for example, when six diffraction gratings are provided as shown in FIG. 1, exposure is performed six times.
The rotary plate is rotated by a predetermined angle (60° in the case of six diffraction gratings) between each exposure. After exposure in this manner, a process suitable for the recording medium, such as development, is performed to obtain a rotary plate provided with the required diffraction grating. If the incident angles of the two plane waves 4 and 5 during exposure are equal and symmetrical angles α with respect to the normal 6, then the period d of the grating (interference fringes) is d=λ/(2sinα)...( 1) becomes. As an example, the wavelength of a plane wave is λ=
If the grating period is 0.488 μm and the incident angle is α=14.5°, then the grating period is d=0.976 μm. Further, the direction of the grating (interference fringes) is perpendicular to the plane (the paper plane in FIG. 2) formed by the chief rays of the two plane waves 4 and 5.

FIG. 3 is a side view showing one embodiment of the light beam scanning device of the present invention. The surface of the rotary plate 1 that includes the rotation axis 2 is used as an incident surface, and a collimated light beam 7 with a wavelength λ' parallel to the rotation axis 2 is made incident. That is, in the present invention, the entrance plane is defined as a plane that includes the rotation axis 2 and the incident light beam 7. In FIG. 3, the plane of incidence is taken within the plane of the paper without loss of generality. The incident light beam 7 is parallel to the rotation axis 2 of the rotating plate 1, that is, the incident light beam 7 is perpendicular to the plane of the diffraction grating 3, so that the direction of the grating is aligned with the incident plane (in FIG. The diffraction angle α' of the light beam 8a that is generated when the light beam is perpendicular to ) is an angle determined by d=λ'/sin α' (2), and this light beam 8a is also within the plane of incidence. Here, d is the grating period of the diffraction grating 3. However, when the direction of the grating changes from the direction perpendicular to the plane of incidence as the rotating plate 1 rotates, the diffracted light beam 8b disappears within the plane of incidence, and the component in the direction perpendicular to the plane of incidence disappears. have. This means that the light beam 8b is deflected in a direction perpendicular to the plane of incidence. However, light beam 8b
is deflected in a direction perpendicular to the plane of incidence, and at the same time the projection angle β on the plane of incidence changes with the direction of the grating and becomes no longer equal to α'. Therefore, the light beam 8a
and 8b are directly focused by a focusing lens to form a light spot on the scanning surface and a scanning line is drawn by rotating the rotary plate 1, the scanning line deviates from a straight line and takes on an arched shape. . Therefore, in the present invention, in order to eliminate the arching of the scanning line on the scanning plane,
A light beam that has been diffracted once by the diffraction grating is guided back to the diffraction grating and diffracted again. A specific example of this method is as shown in FIG. 3, in which the light beams 8a and 8b are arranged between two flat reflecting surfaces 9 and 10 perpendicular to the plane of incidence so that the reflecting surfaces are not at right angles to each other. It is reflected and guided to the diffraction grating 3 again. By doing this, when the light beam 8a is guided to the diffraction grating 3 again, it remains within the incident plane (the plane of the paper in FIG. 3), and its re-incidence angle is α', so
The light beam 11a generated by being diffracted again by the diffraction grating 3 is within the plane of incidence and perpendicular to the plane of the diffraction grating 3. On the other hand, when the light beam 8b is guided to the diffraction grating 3 again, the projection angle on the incidence plane is β, and the projection angle of the light beam 11b generated by being diffracted by the diffraction grating 3 again on the incidence plane is Interestingly, the angle γ with the plane is exactly 90°. This means that when the light beams 11a and 11b are focused on the scanning surface 14 by the focusing lens 13, a light spot is formed at exactly the same position 15 in the Y direction, and therefore the scanning line is shaped into an arch. Indicates complete removal.

By the way, when the direction of the grating changes by an angle φ from the direction perpendicular to the plane of incidence as the rotary plate 1 rotates, the angle θ that the light beam 11b that has been diffracted twice by the diffraction grating 3 makes with the plane of incidence is In the case of the embodiment shown in FIG. 3, the analysis revealed that θ=sin ^-1 (2λ'/dsinφ) (3). A very characteristic aspect of the present invention is derived from equation (3). In other words, in the case of d=2λ'...(4), by making the grating period d of the diffraction grating 3 equal to twice the wavelength λ' of the light beam, θ=φ...(5), and the deflection The deflection angle of the light beam 11b can be made proportional to the rotation angle of the diffraction grating 3, that is, the rotation angle of the rotating plate 1. Therefore, by rotating the rotating plate 1 at a constant angular velocity and using a so-called θ lens, which is now readily available in this field, as the focusing lens 13, the light spot focused on the scanning surface 14 is focused in the X direction. The scanning line moves at a constant speed, and as described above, the scanning line does not arch at all. The wavelength of the light beam is λ′=
In the case of 0.488μm, the grating period of the diffraction grating is d=
This becomes 0.976 μm, and when converted to the spatial frequency of the grating pattern, 1/d=1024 lp/mm. Such a diffraction grating can of course be created easily by using a holographic technique, but also by a ruling technique other than a holographic technique.

In addition, in the present invention, from equations (2) and (4), α′=30°……(6) It is becoming.

In the embodiment shown in FIG. 3 described above, the light beam is guided again to the diffraction grating by the two reflecting surfaces 9 and 10 whose reflecting surfaces are perpendicular to each other, but the number of reflecting surfaces is limited to two. Never. In short, when the light beams 8a and 8b generated by the first diffraction are guided to the diffraction grating again, the reflecting surfaces are configured so that they each form an angle of 180° when projected onto the incident surface. do it. In addition, there are various specific examples of the reflective surface, such as one in which a reflective film is provided on the front or back surface of an optical glass substrate, and one in which total reflection of a prism is utilized.

Furthermore, methods for creating diffraction gratings include not only direct application of holographic techniques, but also methods of photocopying or creating replicas using a single original hologram, and methods that are completely independent of holographic techniques. Created using the technique of
A method of creating a replica using the original plate can be adopted.

Furthermore, the rotary plate on which the diffraction grating is provided does not necessarily have to be circular, but may be polygonal.

In this way, according to the present invention, in a light beam scanning device using a rotating plate-shaped optical deflector using a diffraction grating, it is possible to completely eliminate arching of the scanning line.

In addition, when correcting the arcuate shape of the scanning line, a relatively large grating period (in the example, the grating period d=
A diffraction grating of 0.976 μm (described above) can be used, making it extremely easy to create a diffraction grating and making it possible to use various techniques.

Furthermore, in the present invention, since it is sufficient to rotate one rotary plate, a complicated rotation transmission mechanism is not required.

Furthermore, in the present invention, arching of the scanning line can be corrected simply by using only one type of diffraction grating.

Furthermore, according to the present invention, the scanning speed of the light spot on the scanning surface can be kept constant with high precision.

As described above, according to the present invention, a high performance and highly reliable optical beam scanning device can be easily realized at a low cost.

[Brief explanation of the drawing]

FIG. 1 is a plan view showing an example of a rotary plate used in the present invention, FIG. 2 is a side view showing one method for creating a plane diffraction grating used in the present invention, and FIG. 3 is a light beam scanning method of the present invention. FIG. 2 is a side view showing an embodiment of the device. 1... Rotating plate, 2... Rotating shaft, 3... Planar diffraction grating, 9, 10... Planar reflecting surface, 13... θ lens.

Claims (1)

[Claims] 1 A rotating plate provided with at least one equally spaced linear simple plane diffraction grating, with the surface including the rotational axis of this rotating plate as the incident plane and parallel to this rotational axis,
a light beam source that makes incident a light beam having a wavelength approximately half the period of the planar diffraction grating; a light beam diffracted by the planar diffraction grating is reflected by a planar reflective surface perpendicular to the incident surface; an optical system that guides the light beam again to the plane diffraction grating so that the projection onto the incident surface makes an angle of 180° with the diffracted light beam, and scans the light beam diffracted by the plane diffraction grating again. 1. A light beam scanning device comprising a θ lens that focuses on a surface, and a focused light spot scans the scanning surface as the rotary plate rotates.