JPH0513286B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention relates to a recording optical system using a laser as a light source. Conventionally, when this type of optical system is used in a scanning device, as shown in FIG. The rotating polygon mirror for deflection consisted of a deflecting means 3, a scanning lens 4, and a photosensitive medium 5. In this conventional device, the laser light source itself was expensive, and the modulation device was also expensive. In addition, not only is the laser light source itself large, but a space of 200 mm to 300 mm is required for arranging the modulator in the optical path, and the ratio of the space occupied by the laser light source and the modulator to the entire device cannot be ignored. On the other hand, it is possible to achieve a low-cost, compact recording optical system using a semiconductor laser that can be directly modulated as a light source with performance comparable to that of conventional devices. In this way, an optical system using a semiconductor laser as a light source was introduced in RCA Review vol.35 September 1974.
P335~P340 and RCA Review vol.36 December
1975 P744 to P758, but the laser mode covered in those documents changes drastically as the current value changes. Along with this, the size of the imaging spot also changes drastically. When such a multimode light source is used in a recording system, it is difficult to obtain a high quality image. An object of the present invention is to provide a recording optical system that uses a laser as a light source and can obtain a beam spot for good recording. In the recording optical system according to the present invention, the imaging optical system is configured such that the shape of the beam spot that scans the photosensitive medium is longer in the direction perpendicular to the scanning direction than the length in the scanning direction. The system forms a beam of light from a laser light source onto a photosensitive medium. The present invention will be explained in detail below. Figure 2 is a schematic diagram of the radiation situation of the light beam of the semiconductor laser, and 6a is the plane S perpendicular to the laser bonding surface.
6b is the beam waist position in the plane J parallel to the joining surface of the laser. Semiconductor lasers can be modulated at high frequencies, and their use in scanning systems has the advantage of achieving high-speed recording. FIG. 3 is a layout diagram of a scanning system using a semiconductor laser as a light source, in which the laser beam 7 is emitted from a beam waist position 6a in a plane perpendicular to the bonding surface and a beam waist position 6b in a plane parallel to the bonding surface. The objective lens 8 receives the light beam, and the cylindrical lens system 9 collimates the light beam emitted from the objective lens 8. In addition, the cylindrical lens system 9 has a refractive power in a direction parallel to the cemented surface of the laser 7, but has no refractive power in a direction perpendicular to the cemented surface, and has a function of expanding the beam diameter in the direction parallel to the cemented surface. has. The focal point of the objective lens 8 coincides with the beam waist position in the direction perpendicular to the cemented surface of the laser 7, and the focal point of the composite system of the objective lens 8 and the cylindrical lens system 9 in a plane parallel to the cemented surface of the laser 7 is the focal point of the combined system of the objective lens 8 and the cylindrical lens system 9. It exists at a position within the diffraction limit range with respect to the beam waist position in a plane parallel to the cemented surface. This point will be discussed in more detail. The cylindrical lens system 9 has a configuration as shown in FIG. It consists of a plano-concave cylindrical lens 9a whose surface is flat and a plano-convex cylindrical lens 9b whose light beam incident side is flat and whose exit side is convex. and plano-concave cylindrical lens 9
The focal point of the plano-convex cylindrical lens 9b coincides with that of the cylindrical plano-convex lens 9b, and when a parallel light beam is incident on the cylindrical plano-concave lens 9a, the light beam exiting from the cylindrical plano-convex lens 9b is an afocal system. In a plane perpendicular to the laser junction surface, this cylindrical lens system 9 has no refractive power as shown in FIG. 4b, and the incident light beam becomes the output light beam as it is. The light beam emitted from the cylindrical lens system 9 is deflected by a deflection mirror 10 shown in FIG. 3, and imaged on a photosensitive medium 12 by a scanning lens 11.
This photosensitive medium 12 is placed near the focal plane of the scanning lens 11. At this time, when the astigmatism difference in the beam waist position in the orthogonal direction is as shown in FIG. 2, the objective lens 8 and the cylindrical lens system 9
And, by the entire optical system of the scanning lens 11, the position where the light beam emitted from the beam waist position in the plane perpendicular to the cemented surface of the laser 7 is imaged, and the position in the plane parallel to the cemented surface of the laser 7 are focused. Positions 6a', 6 where the beam radiated from the beam waist position is imaged
b′ is different. Let the distance at this position be ΔS', and let the diameter of the aperture 13 that limits the beam diameter in FIG. 3 in the direction parallel to the laser junction surface be D _J ,
Let D _S be the diameter in the direction perpendicular to the laser bonding surface,
When the focal length of the scanning lens 11 is f ₁₁ and the emission wavelength of the laser is λ, ΔS′≦2.44λ {(1/D _S ) ² + (1/D _J ) ² }f ₁₁ ² (1) Experience has shown that if the following conditions are satisfied and the photosensitive medium is placed at an appropriate position between the two imaging positions having the above-mentioned difference in distance, good imaging performance almost at the diffraction limit can be obtained. Furthermore, if the focal length of the trindrical plano-concave lens in FIG. 4a is f ₉ a, the focal length of the cylindrical plano-convex lens is f ₉ b, and the focal length of the objective lens ₈ in FIG. The permissible difference ΔS in the beam waist position of the semiconductor laser in the orthogonal direction to obtain good performance must satisfy ΔS≦(f ₉ b/f ₉ a) ²・(f ₈ /f ₁₁ ) ²・ΔS′(2). Therefore, from equations (1) and (2), get. In other words, the focal length ratio of the two lenses constituting the afocal cylindrical lens system 9 is
When formula (3) is satisfied, it is possible to obtain good imaging performance at the diffraction limit level by arranging the photosensitive medium at an appropriate position in a scanning optical system as shown in Figure 3. . On the other hand, when the minimum amount of light required to reach the photosensitive medium is ε, and the total amount of light emitted by the semiconductor laser in the direction of the photosensitive medium is Eo, the amount of light transmitted onto the photosensitive medium by the optical system described above is E. ′ should satisfy the condition E′≧γo・Eo, where γo≡ε／Eo (4). Here, the luminous flux emitted from the semiconductor laser can be approximated as a Gaussian distribution with respect to its radiation angle, and the dispersion of the distribution in the planes perpendicular and parallel to the laser junction plane can be expressed as σ _S ² , σ _J ² .
It is assumed that the optical system receives the light beam emitted from this laser in the ranges of -θ _S to θ _S and -θ _J to θ _J in each plane. Then, the light receiving rate in each plane γ _S , γ _J
teeth It is. Now, γ≡γ _S・γ _J (7) Let us consider γ as the ratio of the amount of light received by the optical system to the amount of laser light emitted over the entire solid angle. At this time, if the above-mentioned relationship γ>γo (8) is satisfied for γo, the necessary amount of light to reach the photosensitive medium can be secured. Here, equation (5) is illustrated as shown in Fig. 5.
The horizontal axis represents θ _S /σ _S and the vertical axis represents γ _S. Similarly, when formula (6) is illustrated, it becomes as shown in FIG.
The horizontal axis represents θ _J /σ _J and the vertical axis represents γ _J. From these curves, by setting the dispersion σ _S ² , σ _J ² and acceptance angles θ _S , θ _J of the Gaussian distribution of the semiconductor laser, γ _S , γ _J can be found, and γ can be calculated from equation (7). . Now, as a procedure for setting up the optical system, if we determine the diameter D _S of the aperture 13 in a plane perpendicular to the cemented surface of the semiconductor laser and the focal length f ₈ of the objective lens 8, the acceptance angle θ _S in this plane is θ Since it is determined from _S = sin ^-1 (D _S /2f ₈ ), γ _S can be obtained from FIG. Furthermore, γ _J =
Find θ _J that satisfies γo/γ _S from Figure 6. If the value of θ _J is θ _J o, in order for the light quantity ratio γ actually received by the optical system to be larger than γo, the acceptance angle θ _J in the plane parallel to the junction surface of the semiconductor laser must be θ _J > θ _J o (9) is sufficient. At this time, the diameter of the aperture 13 in this plane
Since D _J is D _J =2・|f ₉ b/f ₉ a|・f ₈ sinθ _J (10), from equations (9) and (10), |f ₉ b/f ₉ a|<D _J / We get 2f ₈ sinθ _J o (11). That is, from the above equations (3) and (13), the focal length ratio of the constituent lenses of the cylindrical lens system 9 |f ₉ b/f ₉ a | If it falls within this range, it is possible to obtain good imaging performance on the order of the diffraction limit and to transmit the amount of light necessary for recording onto the photosensitive medium. In the cylindrical lens system 9, the reason why the plane of the cylindrical plano-concave lens is not provided on the incident side of the light flux is to prevent the reflected light from that plane from traveling backward into the laser. When the reflected light travels back into the laser, a so-called self-coupling phenomenon occurs, which changes the emission characteristics of the laser, which is undesirable. Next, in Fig. 3, between the aperture diameter D _J in the direction parallel to the laser bonding surface and the focal length f ₉ b of the cylindrical plano-convex lens in Fig. 4 a, f ₉ b/D _J > 5 (13). When the relationship is satisfied, the parallelism of the emitted light beam can be improved in the configuration of the cylindrical lens 9. In FIG. 4a, over-corrected spherical aberration occurs in the cylindrical plano-concave lens with respect to the incident parallel light beam, and under-corrected spherical aberration occurs in the cylindrical plano-convex lens. When the relationship of equation (4) above is satisfied, the amount of spherical aberration generated in each lens can be reduced, and it is possible to satisfactorily correct spherical aberration in a composite system of both lenses. If the above formula (13) is not satisfied, the amount of aberration generated in each lens increases, making it difficult to satisfactorily correct spherical aberration with a composite system of both lenses. In the above optical system, for example, the focal length f ₈ of the objective lens 8 is f ₈ = 10 mm, the focal length f ₁₁ of the scanning lens 11 is f ₁₁ = 500 mm, the aperture diameter D _S in the vertical direction of the laser cemented surface is D _S = 10 mm, the difference ΔS between the respective beam waist positions in the orthogonal direction is ΔS = 0.02 mm, and a semiconductor laser with a single mode wavelength λ of λ = 8000 Å is used. According to equation (3), the distance between the beam waist positions is parallel to the laser bonding surface. Aperture diameter D _J
The relationship between the absolute value |f ₉ b/f ₉ a| of the focal length ratio of each of the cylindrical plano-convex lens and the cylindrical plano-concave lens constituting the cylindrical lens system 9 is illustrated as a curve a in FIG. 7. Furthermore, the distribution of the semiconductor laser is determined by assuming that the dispersion perpendicular to the junction plane is σ _S ² = (15°) ² and the dispersion parallel to it is σ _J ² (40°) ² , and the minimum required for recording on the photosensitive medium. The required light amount ratio γo is set to γo=0.8. The acceptance angle θ _S in the plane perpendicular to the laser bonding surface is θ _S = sin ^-1 (D _S /2f ₈ ) =
30°, so θ _S /σ _S = 2, and from Fig. 6, γ _S =
0.955 is required. At this time, in order to ensure the necessary amount of light to reach the photosensitive medium, the above-mentioned (7),
From equation (8), it is sufficient if γ _J > γo/γ _S = 0.838. That is, from FIG. 6, θ _J o=1.4×σ _J =5.6° is obtained. Substituting the above obtained values into the right side of equation (11), the focal length ratio of the lenses constituting the cylindrical lens system 9 to the aperture diameter D _J in the direction parallel to the cemented surface of the semiconductor laser | f ₉ b / f ₉ a The maximum of | is shown by straight line b in FIG. That is, the value of the focal length ratio |f ₉ b/f ₉ a| should be within the shaded area in FIG. Here, when D _J =3.33 mm, |f ₉ b/f ₉ a|=1, the cylindrical lens system 9 no longer performs its function, and the result is good even without the cylindrical lens system 9. Generally, cylindrical lens system 9
The condition that can be omitted is that the value of D _J is However, when the value is κ≡(f ₈ /D _S ) ²・2.44λ/ΔS. That is, if the value of the aperture diameter D _J in the direction parallel to the laser bonding surface is approximately the value on the right side of equation (14), it is possible to obtain relatively good imaging performance even without the cylindrical lens system 9. be. For values of D _J <3.33, the value of |f ₉ b/f ₉ a| becomes less than 1. Since this is difficult to achieve with the configuration shown in FIG. 4a, it is excluded from consideration here. When the recording optical system as described above is used in a scanning device, the advantage of a semiconductor laser that can be directly modulated at a high frequency can be utilized. That is, as shown in FIG. 3, the light beam from the semiconductor laser is deflected using a rotating polygon mirror that rotates at a constant angular velocity as a deflecting means.
In cases such as scanning over photosensitive media, it becomes possible to achieve high speed recording. In this case, the scanning imaging lens 11 is an fθ lens (y' = f ₁₁ θ; y' is the distance from the optical axis to the scanning position, f ₁₁ is the focal length of the lens, and θ is the deflection incident on the lens. By using the angle that the light beam makes with the optical axis of the lens, the scanning speed on the photosensitive medium can be made constant, and the ratio of the various modulation signal frequencies given to the semiconductor laser and the fundamental clock frequency can be made into an integer ratio. This makes it possible to simplify the electrical circuit. Alternatively, instead of the constant angular velocity rotating mirror described above, the time t
On the other hand, when using a deflection means such as a galvano mirror that oscillates sinusoidally with an amplitude φo, an arcsine lens (y′=2φofsin ⁻¹
(θ/2φo); the meanings of y' and θ are as described above), the same effect as described above can be obtained. In these scans, the spot formed on the photosensitive medium can be formed into a desired shape by appropriately selecting the aperture diameters D _S and D _J described above. In the present invention, in the scanning system, for example, as shown in FIG.
When the image is scanned in the y' direction, if the spot diameter py' in that direction is made smaller than the spot diameter pz' in the z' direction, which is perpendicular to the y' direction, the spot will be scanned within the modulation time of one pixel as shown in Fig. 20. The spot is shifted from 13 to 13', and the recorded spot becomes larger, like 14. As a result, it becomes possible to make the size of the recording spot of one pixel equal in the y' and z' directions, and good recording is possible. Alternatively, it is possible to modulate the semiconductor laser at a higher frequency than in the above example and overlap multiple pixels as shown in Figures 21a and 21b to obtain image quality with high contrast in the scanning direction or in the direction perpendicular to it. be. As described above, by selecting the aperture diameter in the scanning system, it becomes possible to try to control the image quality in various ways. Examples of the present invention will be described in detail below. FIG. 8 shows the light distribution characteristics of the semiconductor laser used in the present invention, with the horizontal axis showing the spread angle and the vertical axis showing the intensity. The symbol S indicates the light distribution characteristic distribution in the direction perpendicular to the laser bonding surface, and the symbol J indicates the light distribution characteristic distribution in the direction parallel to the laser bonding surface. The distribution characteristics correspond to almost a Gaussian distribution in both directions perpendicular and parallel to the laser bonding surface, and when the dispersion in each direction is σ _S ² and σ _J ² , σ _S ≒1 ₅ °, σ _J ≒4°.
Further, the emission wavelength λ of this laser is λ≒8000 Å, it has a single mode, and the difference between the respective beam waist positions in the orthogonal direction is about 0.02 mm. 9 shows the objective lens 8, FIG. 10 shows the cylindrical lens system 9, and FIG. 11 shows the scanning lens 11.
A cross-section of the configuration is shown. Table 1 lists their constituent values. Ri, di, li, and Ni are the radius of curvature of the i-th surface, the axial thickness of the lens, the air gap, and the refractive index of the lens, respectively. The objective lens shown in Table 1 is a so-called retrofocus type lens in which a concave lens with negative refractive power is placed on the light output side, and this is used to lengthen the working distance between the laser light emitting surface and the objective lens. It was established in This is effective for increasing the working distance compared to the focal length when the focal length of the objective lens becomes short. However, if the focal length is long enough, it is possible to eliminate the concave lens on the exit side as shown in FIG. These values are shown in Table 2. Further, as shown in FIG. 13, it is also possible to use one aspherical lens as the objective lens. The constituent values are shown in Table 3. The shape of this aspherical surface is expressed by the following equation. However, in this case, _R24 = -7.833 A = 1.44747 x 10 ^-4 B = 3.52553 x 10 ^-6 . Further, in the cylindrical lens system, as shown in FIG. 14, even with a single lens, an equivalent system to the system shown in FIG. 10 can be obtained. This configuration data is shown in Table 4. At this time, the aforementioned f ₉ a and f ₉ b are equal to the respective focal lengths of the 25th and 26th surfaces. The various objective lenses and cylindrical lens systems described above may be combined in any manner. In particular, when used in combination with the single aspherical objective lens shown in FIG. 13 and the single cylindrical lens shown in FIG. 14, the configuration becomes simple, assembly and adjustment are easy, and costs can be reduced. Furthermore, when the number of constituent surfaces decreases, the light transmittance also increases, which is advantageous because the amount of light transmitted onto the photosensitive medium increases. The aperture used in this example has a diameter of 9.3 mm when the four-lens objective lens shown in FIG. 9 is used. Alternatively, good imaging performance can be obtained by setting the laser beam to 9.3 mm in the direction perpendicular to the joining surface and 16.8 mm in the parallel direction. The value of the focal length ratio |f ₉ b / f ₉ a | of a cylindrical lens system using an aperture of φ9.3 mm and the minimum light intensity ratio γo necessary for recording is γo = 0.686 is 2.1 from the above equation (12). It is sufficient if it falls between ≦｜f ₉ b/f ₉ a｜<6.0. In this example, the 10th
The cylindrical lens shown in the figure and Table 1 has |f ₉ b/f ₉ a|=4.13, and the cylindrical lens shown in FIG. 14 and Table 4 has |f ₉ b/f ₉ a|=3.83. Or, 9.3mm perpendicular to the laser joint surface.
If the parallel direction is 16.8 mm and the calculation is performed in the same manner according to equation (12), it is sufficient that 2.6≦|f ₉ b/f ₉ a|<10.8. In this case as well, the above embodiment is satisfied.
FIG. 15 shows the collimation performance of the objective lens shown in FIG. 9 and Table 1. FIG. 16 shows the performance of the emitted parallel light beam of the cylindrical lens shown in FIG. 10 and Table 1. 17a and 17b show the performance of the scanning lens shown in FIG. 11 and Table 1, where a shows spherical aberration, and b shows astigmatism and field curvature. FIG. 18 shows the performance of the objective lens shown in FIG. 12 and Table 2. In FIGS. 15, 16, and 18, the horizontal axis indicates wavelength aberration, 1λ=0.8×10 ^-3 mm, and the vertical axis indicates the height of the emitted light beam. The vertical axis in FIG. 17b is the angle formed with the optical axis of the deflected light beam incident on the scanning lens.
M and S indicate field curvature in the meridional section and sagittal section, respectively. The performance of the objective lens shown in FIG. 13 and the cylindrical lens shown in FIG. 14 is approximately the same as that of the objective lens shown in FIG. 9 and the performance of the cylindrical lens shown in FIG. 10, respectively, so their description will be omitted. As is clear from the performance of each lens shown, the performance is good at the diffraction limit.

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[Table] Next, a laser beam recording device using an electrophotographic optical system according to the present invention will be described.
This device may be substantially similar to the device of FIG. However, it is necessary to apply a modulation signal to the semiconductor laser 7 to control the light emission of the semiconductor laser 7, and to arrange an electrophotographic photosensitive material or a photosensitive drum on the scanning surface 12. CdS photosensitive materials are desirable as photosensitive materials for electrophotography. The reason for this is that the light from the semiconductor laser is near-infrared light, which matches the spectral sensitivity characteristics of the CdS photosensitive material. In other words, as a typical electrophotographic photosensitive material,
Examples include ZnO, amorphous Se, and CdS. However, in the case of Se photoreceptors, the high sensitivity region is biased towards the short wavelength side and does not match in the near infrared.
It is known that by mixing As or Te into Se, the sensitivity shifts toward longer wavelengths. Such a method can also be used. However, the addition of As or Te may exhibit undesirable characteristics as an electrophotographic photosensitive material, such as a decrease in dark resistance, so it is necessary to take measures such as forming a two-layer photosensitive layer. In addition, in ZnO, the inherent sensitivity of ZnO is 350 ~
400 mμ and can be dye-sensitized to be compatible with semiconductor laser light. Its absolute sensitivity is less than one-tenth that of the CdS photoreceptor described below. Therefore, it is desirable to use a DdS sensitive material. Furthermore, this CdS sensitive material preferably has a three-layer structure consisting of a conductive support, a CdS photoconductive layer, and an insulating layer.

[Brief explanation of the drawing]

Fig. 1 is a diagram showing a conventional scanning device, Fig. 2 is a schematic diagram of the radiation situation of a semiconductor laser beam, Fig. 3 is an optical layout diagram of a scanning system using a semiconductor laser as a light source, and Fig. 4 is a diagram of a scanning system using a semiconductor laser as a light source. Figures 5 and 6 are graphs showing the relationship between the acceptance angle and the acceptance rate when light from a semiconductor laser is received by the optical system, and Figure 7 is a graph explaining the details of the cylindrical system shown in the figure. Figure 3 is a graph showing the relationship between the minimum absolute value of the focal length ratio of the cylindrical lens plano-convex lens and the plano-concave lens, Figure 8 is a diagram showing the light distribution characteristics of the semiconductor laser, and Figure 9 is the graph showing the relationship between the minimum absolute value of the focal length ratio of the cylindrical lens plano-convex lens and the plano-concave lens. Figure 10 is a cross-sectional view of the cylindrical lens system in Figure 3, Figure 11 is a cross-sectional view of the scanning lens in Figure 3, and Figure 12 is a modified example of the lens in Figure 9. FIG. 13 is a cross-sectional view of the deformed objective lens.
Figure 4 is a cross-sectional view of the modified cylindrical lens, the first
Figure 5 is a diagram showing the collimation performance of the objective lens in Figure 9, Figure 16 is a diagram showing the performance of the emitted parallel light beam of the cylindrical lens in Figure 10, and Figure 17 is a diagram showing the performance of the scanning lens in Figure 11. 18 is a diagram showing the performance of the objective lens in FIG. 12, FIG. 19 is a diagram showing the state of the spot in the recording optical system according to the present invention, and FIG. 20 is a diagram showing the moving state of the spot in FIG. The figure shown in FIG. 21 is a diagram showing a method of recording each spot in an overlapping manner. In the figure, 6 is a light emission origin, 7 is a semiconductor laser, 8 is an objective lens, 9 is a cylindrical lens system, 10 is a deflector, 11 is a scanning lens, and 12 is a scanning surface.

Claims (1)

[Claims] 1. When a light beam from a semiconductor laser light source is collimated by an objective lens, passed through a deflector, and then focused on a photosensitive medium by a scanning lens, the light beam is collimated between the deflector and the objective lens. A diaphragm is provided at the spot, and this diaphragm changes the shape of the beam spot on the photosensitive medium.
A recording optical system characterized in that the length in the direction perpendicular to the scanning direction is longer than the length in the scanning direction.