JPH0151802B2 - - 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 separation power differs depending on the optical scanning device, and the diameter of the focused light beam must be arbitrarily set in accordance with this different resolution. 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 in the scanning optical system for scanning the light beam from the scanning means, the monochromatic light source and the light scanning means are provided. The structure includes a one-dimensional striped phase grating that converts the light beam on the scanning plane into an elliptical beam between them. Next, the present invention will be explained in detail with reference to the drawings. As a description of the scanning optical system, a scanning optical system used in a transmitter will be described below, but the same holds true for a scanning recording optical system. FIG. 1 shows an example of a conventional laser scanning optical system. 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. Figure 2a shows the relationship between the focused beam diameters on the scanning plane, and shows the focused beam 12 scanning 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, reducing the 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. It is defined as the point where the optical energy of a focused beam diameter 2ω is 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. As a light beam converter, published patent 1983-
Prisms, cylindrical lenses, birefringent materials, and diffraction gratings are used as described in 61483 "Scanning Optics". In particular, diffraction gratings are small and do not require positional accuracy, making them ideal as light beam converters. However, the diffraction grating described in the above literature is a phase grating with a sinusoidal phase profile, and if the sinusoidal profile cannot be manufactured faithfully, higher-order diffraction light will be generated and the desired elliptical beam will not be obtained. do not have. In order to produce a phase profile, a method is used in which a pattern is exposed on a photoresist, but it is extremely difficult to produce a faithful sine wave profile due to the nonlinear characteristics of the photoresist. The present invention has been made in this respect, and relates to a scanning optical system using a striped phase grating, which can be easily manufactured, as a light beam converter. FIG. 4 shows the principle of a light beam converter used in the present invention. When the laser beam 40 enters the striped phase grating 41, a plurality of diffracted beams 4 are formed.
The diffracted beams are divided into 2, 43, and 44, and each diffracted beam is combined on a Fourier transform plane (X-Y plane) located at a position f behind the lens 45 to obtain a converted beam having an intensity distribution 46. By appropriately selecting the relationship between the laser beam diameter 2ω and the grating pitch P and the phase amount m of the grating, the intensity distribution of the converted beam can be made into a rectangular shape. The amplitude transmittance t(x,y) of a striped grating with a grating pitch P and a phase amount m radians is expressed by equation (1). t (x, y) = EXP {jmrect (2x/P) comb (x/P)} (1) where

[Formula] comb (x/p) = _∞ 〓 ^n=-∞ δ (x/p-n) When a laser beam (waist width w) is incident on this striped grating, on the lens focal point (on the ζ axis) The light intensity distribution B(ζ) of the converted beam is expressed by equation (2). B(ζ)=A∫ ^∞ _-∞ exp(−x ² /w ² )・t(x)・exp(i2πxζ)dx (2) Here, ζ=X/λ・f (2) can be analytically Since it cannot be solved, B is better than numerical calculation.
(ζ) can be found. The resulting light intensity distribution is the sum of 0th order light, 1st order light, 2nd order light, etc., so in order to obtain a rectangular converted beam profile, the light intensity of 0th order light and 1st order light must be equal. It is necessary to set it so that In FIG. 5, the amount of light from the striped grating to each order is plotted with the amount of phase on the horizontal axis. At a point P where the phase amount m is close to 2 radians, the light quantities of the 0th-order light and the 1st-order light become equal.
FIG. 6 is a plot of the converted beam profile when the phase amount m is 2 radians, using the ratio of the laser beam width 2w and the grating pitch P as variables.
When the value of 2w/p is 2.0, the 0th order and ±1st order light remain separated, but as the value of 2w/p decreases, they approach a rectangular profile, and at 2w/p = 0.8, the apex becomes quite linear. A profile with a shape close to a rectangle is obtained. Depending on the elliptical shape required for the scanning beam, the value of 2w/p is determined to be around 0.8. FIG. 7 shows a method for manufacturing a striped phase grating used in the present invention using steps a to e. First, a transparent substrate 50, such as a glass plate or an acrylic plate, is coated with a photoresist 51, such as AZ-1350, using a spinner (FIG. 7a). The thickness d of the photoresist is set to be equal to the phase amount m. Since the refractive index n of photoresist is around 1.5,
The phase amount m is obtained by equation (3). m=2π(n-1)d/λ (3) πd/λ To obtain m=2 radians, it is sufficient to manufacture a film with a thickness of d~0.4 μm. The above film thickness can be obtained by applying high concentration AZ-1350 with a spinner at a rotation speed of about 1000 rpm. Next, a mask 52 is closely placed on the photoresist film 51, and ultraviolet light 53 is irradiated (FIG. 7b). The mask 52 has striped shading recorded such that the pitch P is 2w/p to 0.8 with respect to the laser beam width 2w.
It is sufficient for the mask 52 to be manufactured by photographing a reduced size of a pattern cut on a strip coat on a silver salt dry plate. By developing the exposed photoresist, a relief pattern 53 having the same pattern as the mask 52 is obtained (FIG. 7c). The optical element having the relief pattern of the photoresist 53 on the transparent substrate 50 has a phase amount m=
Since it is a striped phase grating of 2w/p~0.8 with 2 radians, it can be used in the optical system of the present invention. Photoresist films have drawbacks such as being soluble in ketone solvents and being easily scratched, so it may be necessary to manufacture elements with even stronger films. A dielectric film, for example zirconium oxide, is then deposited on the relief pattern of the photoresist 53 (FIG. 7d). Since the refractive index of zircon oxide is approximately 2, the thickness of the deposited film is set to approximately 2000 Å. The sample after deposition is treated with acetone and a photoresist stripping solution to obtain a striped pattern of zirconium oxide (FIG. 7e). The zircon oxide phase grating produced in this way has a phase amount of 2 radians, is resistant to chemicals and scratches, and is fully usable for practical use. Additionally, Ni plating is applied to the phase grating formed by the relief pattern of the photoresist film, a press mold is made from the Ni plating film, and a phase grating made of acrylic resin is manufactured by press processing similar to that used for records. You can also do it. According to this method, phase gratings can be produced in large quantities at low cost, and characteristics similar to those obtained by the above method can be obtained. As described above in detail, according to the present invention, a scanning optical system using a highly reliable light beam converter can be obtained. Further, although the optical system of a scanning transmitter has been described as the scanning optical system, a similar device can be obtained for a scanning recording device.

[Brief explanation of drawings]

FIG. 1 is a conventional laser scanning optical system, FIG. 2 is a diagram for explaining the relationship between a scanning surface and a focused beam on the scanning surface, and FIG. 3 is a diagram showing an embodiment of a scanning optical system according to the present invention. Figure 4 is a diagram showing the principle of the optical beam converter used in the present invention, Figure 5 is a diagram showing the light intensity of each order of the converted beam, Figure 6 is a diagram showing the converted beam profile, and Figure 7 is a diagram showing the optical beam. It is a figure which shows the manufacturing method of a converter. In the figure, 1, 21 are laser light sources, 2, 5,
22, 24, 27, 40 are laser beams, 3, 4,
7, 25, 26, 29, 45 are lenses, 8, 30
is a scanning plane, 9, 12, 13, 14, 31 are scanning beams, 11 is a scanning line, 10, 32 are photodetectors, 6,
28 is a scanning mirror, 23, 41 are light beam converters, 42, 43, 44 are diffracted lights, 46 is a converted beam profile, 50 is a transparent substrate, 5
1, 53 is a photoresist, 52 is a mask, 53
is ultraviolet light, and 54 and 55 are dielectric films.

Claims (1)

[Claims] 1. In a scanning optical system in which a light scanning means 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 means is scanned, a light beam on a scanning surface is provided between the monochromatic light source and the light scanning means. It is a one-dimensional striped phase grating that converts a light beam placed into an elliptical beam into an elliptical beam.
1. A scanning optical system characterized by being provided with a phase grating whose ratio K=2w/p to 2w is around 0.8 and whose phase amount is around 2 radians.