JPS6318169B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a device for detecting images of characters, figures, photographs, etc. using laser light, and particularly to an optical system used in a facsimile transmitter.

The method of detecting the shape and image of an object by scanning a laser beam over an object or paper using a polarizer and a condensing lens is as follows:
Because it has the characteristics of being able to obtain high speed and high resolution, it is used in optical scanning devices such as POS (point of sight) and facsimile transmitters. Generally, in the above laser beam scanning device, the size of one pixel from which information is obtained at one time is determined by the diameter of the light beam focused on the scanning surface. Further, generally required resolution differs depending on the optical scanning device, and the diameter of the focused light beam must be arbitrarily set in accordance with the different resolutions. A commonly used method to meet this requirement is to use an optical system such as a beam expander to convert the diameter of the light beam incident on the optical scanning optical system, thereby changing the focused beam. Generally, the spatial energy distribution of the light beam emitted from a laser light source is a Gaussian distribution, so the light energy distribution in one pixel irradiated by one light beam is always a Gaussian distribution regardless of the beam diameter. Become. This means that information obtained from reflected light reflects information from the center of one pixel more strongly than information from the periphery. This effect is noticeable in facsimile machines, where information is missing between scanning lines of the light beam. For this reason, there was a problem in that increasing the diameter of the light beam lowered the resolution in the scanning line direction.

An object of the present invention is to provide a scanning optical system that focuses a light beam in the main scanning direction and expands the light beam in the sub-scanning direction (a direction orthogonal to the main scanning direction) in order to maintain high resolution.

The scanning optical system of the present invention is provided with a light scanning means for scanning a light beam from a monochromatic light source on the emission side of the monochromatic light source, and a signal light reflected from the scanning surface is transmitted above the scanning surface scanned by the light beam from the scanning means. The structure is such that a photodetector for detecting is provided, and a light beam converter for converting the light beam on the scanning surface into an elliptical beam is provided between the monochromatic light source and the light scanning means.

Next, the present invention will be explained in detail with reference to the drawings.

FIG. 1 is an example of a conventional laser scanning optical system shown for explaining the present invention.

Laser light source 1, lens 3, lens 4, scanning mirror 6, fθ lens 7, and scanning surface 8 are arranged in this order,
Lens 3, lens 4, and fθ lens 7 are placed so that their optical axes coincide with laser light 2 emitted from laser light source 1. The photodetector 10 is in the scanning plane 8
The photodetector 1 is installed parallel to the main scanning line direction above the scanning surface so that the reflected light from the photodetector 1 can be effectively detected.
The photocathode 0 forms an angle of about 45° with respect to the scanning plane 8. A laser beam 2 emitted from a laser light source 1 becomes a collimated light beam 5 having an enlarged beam diameter by lenses 3 and 4 provided on the emission side of the laser light source 1. After being reflected by the provided scanning mirror 6, it is focused by the f/theta lens 7 onto the scanning plane 8 located on the xy plane. The focused beam 9 moves the scanning mirror 6 xz
scanned in the x direction by rotating in the plane;
Each time scanning in the x direction is completed, the scanning plane 8 sequentially moves in the y direction. The reflected light from the scanning surface 8 is detected by the photodetector 10.
The information is read out and two-dimensional scanning is performed. In this case, the spatial energy distribution of the laser beam is usually a Gaussian distribution, so the focused beam 8
The energy distribution of is also a Gaussian distribution. Generally, the diameter of a laser beam with a Gaussian distribution has an energy of 1/
It is expressed by the diameter of the point e ² (e is the base of the natural logarithm). When the diameter of the light beam 5 is 2ω ₀ and the focal length of the f·θ lens 7 is f, the diameter 2ω of the focused beam 9 is expressed by the following formula, where λ is the wavelength of the laser beam.

2ω=2fλ/πω ₀ Therefore, if the diameter 2ω ₀ of the light beam 5 is decreased, the focused beam diameter 2ω will be increased, and if the diameter 2ω ₀ of the light beam 5 is increased, the focused beam diameter 2ω will be decreased. The diameter of the light beam 5 is usually changed by changing the ratio of the focal lengths of the lenses 3 and 4.

Now consider scanning in a facsimile transmitter. In a facsimile transmitter, the resolution in the main scanning direction (x direction) is limited by the band of the transmission system, and the resolution in the sub scanning direction (y direction) is determined by the distance the scanning plane 8 is sent for each scan. It will be done.

Usually, the resolution in the main scanning direction is better than the resolution in the sub-scanning direction.

FIG. 2 is a diagram for explaining the relationship between a scanning plane and a focused beam on the scanning plane. FIG. 2a shows the relationship between the diameters of the focused beams on the scanning plane, where the focused beam 12 scans on the scanning line 11.
A focused beam 12 is drawn for each resolution point in the main scanning direction. If the focused beams 12 have diameters that are adjacent to each other in the main scanning direction, there will be areas that cannot be scanned because they do not overlap in the sub-scanning direction. If the focused beams 13 have diameters that are adjacent to each other in the sub-scanning direction, they will overlap in the main-scanning direction, resulting in lower resolution. Therefore, the optimal focused beam in both directions is a focused beam 14 with an elliptical shape. Furthermore, since the light energy distribution of the focused beam is a Gaussian intensity distribution as shown in FIG. 2b, it must be noted that the light is spread out to the periphery compared to a rectangular intensity distribution. The focused beam diameter 2ω is defined as the point where the optical energy becomes 1/e ² , but since information outside this diameter 2ω will also be obtained, it is better to make the focused beam intensity distribution as close to a rectangular intensity distribution as possible.

FIG. 3 is a diagram showing an embodiment of the scanning optical system according to the present invention. The difference from the conventional one is that a light beam converter 23 is provided between the light source 21 and the lens 25. A laser beam 22 emitted from a laser light source 21 is converted into an elliptical light beam 24 by a beam converter 23 provided on the emission side of the laser light source 21, and is converted into an elliptical light beam 24 by a lens 25 provided on the emission side of the beam converter 23. A light beam 27 is further expanded by a lens 26. Further, the light beam 27 is reflected by a scanning mirror 28 and then becomes a focused beam 31 which is focused by an f/θ lens 29 onto a scanning plane 30 on the xy plane. The focused beam 31 is scanned in the x direction by rotating the scanning mirror 28 in the xz plane.
moves sequentially in the y direction. Reflected light from the scanning surface 30 is received by a photodetector 32, information is read out, and two-dimensional scanning is performed. The scanning mirror 28 rotates a reflecting mirror to deflect the light beam 27, and can be a galvanometer mirror or a rotating polygon mirror.

It is also possible to use an ultrasonic deflector.

FIG. 4 is a diagram for explaining the light beam conversion action by the light beam converter 23. Figure 4a shows the light beam conversion effect when a conventional cylindrical lens or prism is used as the light beam converter.The beam diameter of the laser beam 22 in Figure 3 is the same in both the y and z directions. It is assumed that the light beam has a shape 41. By the light beam converter 23
The light beam shape 41 is unchanged in the direction y
The light beam 24 is converted into a light beam 24 having a light beam shape 42 with a smaller beam diameter in the direction. Since the light beam 24 then passes through three lenses, the shape of the focused beam 31 on the scanning surface 30 becomes a Franhofel diffraction image (Fourier transform image) of the light beam 24. Therefore, the beam diameter becomes larger on the scanning surface 30 in the direction in which the beam diameter is smaller. Since the y and z directions of the light beam 24 correspond to the y and x directions, respectively, on the scanning plane 30, a focused beam 31 that spreads in the y direction on the scanning plane 30 is obtained.

FIG. 4b shows the light beam conversion effect when the diffraction grating according to the present invention is used as a light beam converter.
shows the angular dependence of light energy. It is assumed that the angular dependence of the laser beam 22 incident on the light beam converter 23 is a profile 43 in both the y and z directions.

The light beam 24 after the light beam conversion has the same profile 43 in the z direction, but has a plurality of peaks like the profile 44 in the y direction, and has a wide angle dependence. As already mentioned,
Since the shape of the focused beam 31 on the scanning surface 30 is a Fourier transformed image of the light beam 24, the width increases in a direction with a widened angular dependence. Therefore, a focused beam 31 is obtained which spreads in the y direction on the scanning plane 30.

FIG. 5 specifically shows an example of a light beam converter using a diffraction grating. The laser beam 62 enters a sine wave diffraction grating 63 and diffracted beams 6 are formed on both sides of the 0th order beam 64.
4',64'' is obtained.

At this time, the angle θ formed by the optical axes of the diffracted lights 64', 64'' and the zero-order light 64 is expressed by the following equation, where p is the pitch of the diffraction grating and λ is the wavelength of the laser beam.

sinθ=λ/p Although the beam diameters of the incident light, zero-order light, and diffracted light do not change, the amount of light depends on the degree of modulation of the diffraction grating. The reason why the intensity of the 0th order light and the ±1st order light are equal is because of the phase difference.
When it was 1.44rad.

FIG. 6 is a diagram showing the overlap of focused beams on the scanning plane. Now, if the beam diameter of the focused beam 65 by the zero-order light 64 is 2ω, then the diffracted light 64',
In order to obtain the shape of the focused beam 66 by overlapping the focused beams 65' and 65'' by 64'', the following condition should be satisfied.

ff ₁ λ/f ₂ p~ω Here, f is the focal length of the f·θ lens, and f ₁ and f ₂ are the focal lengths of the lenses 25 and 26 in FIG. 3, respectively.

FIG. 7 is a diagram showing a manufacturing method for obtaining a holographic diffraction grating used in the present invention. The monochromatic light 72 emitted from the monochromatic light source 71 is sent to the shutter 73
through the half mirror 74 into two beams 72',
72'', and the beam 72' is reflected by a reflecting mirror 75 and a half mirror 76.The beam 72'' is reflected by a reflecting mirror 75', passes through the half mirror 76,
Interference fringes with the light beam 72' are recorded on a substrate 77 coated with photoresist. The angle θ between the pitch p of the recorded sinusoidal interference fringe and the two beams 72' and 72''
The relationship with is determined by the following formula.

1/p=2sin(θ/2)/λ Here, λ is the wavelength of monochromatic light, and to expose the photoresist, Ar laser, He-
It is necessary to use a short wavelength laser such as a Cd laser.

FIG. 8 shows the optical phase difference due to the unevenness of the interference fringes recorded on the photoresist. When the exposure time is constant, there is a linear relationship between the phase difference of the diffraction grating and the development time, and the required phase difference of the diffraction grating can be controlled by changing the development time. As already mentioned, the phase difference at which the amount of diffracted light of the ±1st-order light and the amount of 0th-order light become equal is preferably around 1.44 rad.

I to E in FIG. 9 show the manufacturing process of replicating a diffraction grating onto a thermoplastic material.

In (a), a metal film 81 is formed by electroless plating on the surface of the diffraction grating 80 made on the photoresist base. For the metal film 81, it is easy to use nickel.

Next, in (b), the thickness of the metal film 81 is increased by electrolytic plating to form a metal film 82 having a thickness of at least 1 μm or more.

Next, in C, a thermoplastic resin 83, such as an acrylic resin or a vinyl chloride resin, is pressed against the metal film 82 by applying heat and pressure to deform it.

Finally, in step d, the thermoplastic resin 83 is cooled and peeled off to form a diffraction grating 84 engraved in the thermoplastic resin.
is obtained. Since the refractive index of the photoresist and the thermoplastic resin are slightly different, the phase difference of the diffraction grating 80 formed on the photoresist base must be created taking this difference into consideration. By using a diffraction grating in this way, an inexpensive and simple light beam converter can be obtained.

As described above, according to the present invention, it is possible to obtain a laser scanning optical system that scans with an elliptical light beam and can perform dense scanning in the sub-scanning direction without lowering the resolution in the main-scanning direction.

[Brief explanation of the drawing]

Figure 1 is a diagram for explaining an example of a conventional laser scanning optical system, Figures 2a and b are diagrams for explaining the relationship between a scanning plane and a focused beam on the scanning plane, and Figure 3 is a diagram of the present invention. A diagram showing an example of a scanning optical system according to
Figure 4a is a diagram for explaining the light beam conversion effect of a conventional light beam converter, Figures 4b and 5.
6 are diagrams for explaining an example of a light beam converter used in the present invention, and FIGS. 7, 8, and 9 A to D are diagrams showing the manufacturing process of a diffraction grating used in the present invention. It is. In the figure, 1, 21, 71 are laser light sources,
2, 5, 9, 22, 24, 27, 31. 64, 6
4′, 64″, 72, 72′, 72″ are laser beams,
3, 4, 7, 25, 26, 29 are lenses, 6, 2
8 is a scanning mirror, 8 and 30 are scanning surfaces, 10 and 32 are photodetectors, 12, 13 and 14 are focused beams, 11 is a scanning line, 41, 42, 43, 44, 65, 65',
65″, 66 are light beam profiles, 63, 8
0,84 is a diffraction grating, 73 is a shutter, 74,
76 is a half mirror, 75 and 75' are reflective mirrors, 7
7 is a photoresist base, 81 and 82 are metal films,
83 is a thermoplastic resin.

Claims (1)

[Claims] 1. A light scanning means consisting of a light deflector and a condensing lens for scanning a light beam from a monochromatic light source is provided on the emission side of the monochromatic light source, and a light beam from the scanning surface is provided above the scanning surface scanned by the light beam from the scanning means. In a scanning optical system provided with a photodetector for detecting reflected light and reading information, a light beam converter having a periodic grating shape is provided between the monochromatic light source and the light scanning means, and the light beam converter generates diffracted light in a one-dimensional direction, and causes the diffracted light and undiffracted light to overlap on the scanning plane by the condensing lens, thereby converting the light beam on the scanning plane into an elliptical beam. scanning optical system.