Laser optical system

Abstract: A laser optical system comprises a laser beam source (1), a beam diameter adjusting member (3) disposed in the optical path of a laser beam emitted by the laser beam source and having an aperture through which only the center portion of the laser beam is to be passed, a converging lens (4) for converging the laser beam (1A′) passing through the aperture of the beam diameter adjusting member (3), and a spatial filter (5) disposed in the vicinity of a convergence position of the laser beam for restricting the passage of the laser beam in at least one-­ dimensional direction and allowing passage of only a zero-­ order light component converged to a predetermined position. Alternatively, the laser optical system comprises a laser beam source, and a density distribution filter disposed in the optical path of the laser beam emitted by the laser beam source for allowing passage of only the center portion of the laser beam in at least one-dimensional direction and gradually decreasing the transmittance toward the peripheral portion of the laser beam.

Description: BACKGROUND OF THE INVENTION Field of the Invention This invention relates to a laser optical system provided with a laser beam source. This invention particularly relates to a laser optical system wherein no side lobe is generated to around a convergence spot at the time a laser beam emitted by the laser beam source is converged. Description of the Prior Art Laser optical systems have heretofore been widely used as, for example, a means for generating a scanning light beam in scanning recording apparatuses and scanning read-out apparatuses. Among the laser optical systems, the laser optical system using a semiconductor laser as the laser beam source is advantageous in that the semiconductor laser is smaller, cheaper and consumes less power than a gas laser or the like, and the direct analog modulation wherein the optical output is changed by control of the drive current can be effected, and therefore direct modulation may be carried out by use of signals generated in accordance with image information in the case where the laser optical system is used in a scanning recording apparatus. In the aforesaid laser optical system, a beam diameter adjusting plate having an aperture through which only the laser beam portion at the center of the laser beam emitted by the laser beam source is to be passed is often disposed in the optical path of the laser beam. The beam diameter adjusting plate is used for various purposes, for example for increasing the focal depth at the time the laser beam is converged, as disclosed in Japanese Patent Publication No. 58(1983)-20015. Also, in the case where a semiconductor laser is used as the laser beam source, the beam diameter adjusting plate is often disposed so that the light in a low output region in the laser beam emitted by the semiconductor laser can also be utilized without increasing the diameter of the convergence spot at the convergence position. The relationship between the output of the semiconductor laser and the diameter of the convergence spot and the effects of the beam diameter adjusting plate will be described hereinbelow. It has heretofore been known that the laser beam emitted by the semiconductor laser is classified into laser oscillation light and light in the natural light emission region. Figure 8 shows the relationships between the drive current for the semiconductor laser on one hand and the laser oscillation light and the light in the natural light emission range on the other hand. In Figure 8, line "a" indicates the relationship between the drive current and the output of the light in the natural light emission region (hereinafter referred to as the natural emission light), and line "b" indicates the relationship between the drive current and the output of the laser oscillation light. As shown, in the case where the drive current is applied to the semiconductor laser, no laser oscillation light is emitted and only the natural emission light is emitted until the level of the drive current exceeds a threshold current lo. As the level of the drive current increases, the output of the natural emission light increases gradually. However, the laser oscillation light starts to be generated when the level of the drive current exceeds the threshold current lo, and the output of the laser oscillation light becomes large thereafter. At this time, the ratio of the output of the natural emission light to the output of the laser beam as a whole diminishes, and substantially the laser oscillation light alone is emitted. The relationship between the level of the drive current and the total amount of the natural emission light and the laser oscillation light generated by the semiconductor laser is indicated by curve "c". The natural emission light contains various angle components as compared with the laser oscillation light, and therefore the natural emission light cannot be converged to a spot diameter so small as the spot diameter of the laser oscillation light. Therefore, in the case where the semiconductor laser optical system is used in a scanning recording apparatus wherein the recording light must be modulated over a wide dynamic range and even a light beam of the low output region in which the natural emission light is predominant must be utilized, the beam diameter in the low output region increases and the spatial resolution of the scanning deteriorates. Accordingly, in Japanese Patent Application No. 61(1986)-196352, the applicant proposed a semiconductor laser optical system provided with a beam diameter adjusting plate having an aperture through which only the center portion of the laser beam is to be passed on the basis of the findings that the diameter of the convergence spot can be reduced for the natural emission light as well when the beam diameter adjusting plate is disposed in the optical path of the laser beam emitted by the semiconductor laser. As mentioned above, the beam diameter adjusting plate is used for increasing the focal depth, or, in the case of the semiconductor laser, for preventing the diameter of the convergence spot of the natural emission light from increasing. In the case where a lens such as a collimator lens disposed in the optical path has a diameter smaller than the diameter of the incident light beam, the lens itself may act in the same manner as an aperture and may substantially constitute the beam diameter adjusting member. However, in the case where the beam diameter adjusting member mentioned above is disposed in the optical path of the laser beam emitted by the laser beam source, a part of the laser beam is eclipsed by the contour portions of the aperture of the beam diameter adjusting member. As a result, when the laser beam after passage through the beam diameter adjusting member is converged, a weak first-order light component, a weak second-order light component and the like are generated as side lobes around a zero-order light component converged to a predetermined position. When such side lobes are generated, various problems consequently arise, for example image ghosts appear in the case where the laser optical system is used in the scanning recording apparatus and recording of the image information is carried out by use of the converged laser beam. SUMMARY OF THE INVENTION The object of the present invention is to provide a laser optical system wherein only the center portion of the laser beam is taken up and no side lobes are generated when the laser beam portion taken up is converged. The present invention achieves its object by providing a laser optical system comprising the features set out in claim 1. In the laser optical system in accordance with the present invention, the density distribution filter may be of an absorption type for absorbing light at a high-density portion, or it may be of a reflection type for reflecting light at the high-density portion. With the laser optical system in accordance with the present invention wherein the density distribution filter adjusts the passage of the laser beam in accordance with the optical density thereof, there is no risk of a part of the incident laser beam being eclipsed as in the case of the beam diameter adjusting member having an aperture, and no side lobe is generated at the time the laser beam after passage through the density distribution filter is converged. Also, the density distribution of the density distribution filter can be selected as desired, and therefore the shape of the laser beam passing through the density distribution filter can easily be adjusted as desired by changing the density distribution of the density distribution filter. Therefore, with the laser optical system in accordance with the present invention, the beam diameter can be adjusted efficiently by use of the density distribution filter without side lobes being generated. Accordingly, by converging the laser beam passing through the density distribution filter, the focal depth can be increased and blurring of the beam diameter can be eliminated. Thus a good converged laser beam can be obtained at the convergence position. BRIEF DESCRIPTION OF THE DRAWINGS Figure 1 is a side view showing a semiconductor laser optical system in accordance with the present invention, Figure 2 is a graph showing the intensity distribution in accordance with the radiation angle of the laser oscillation light, Figures 3a and 3b are schematic views showing examples of the density distribution filters and optical transmittances thereof, Figure 4 is a graph showing the optical transmittance of the density distribution filter, Figure 5 is a graph showing the relationship between the optical output and the diameter of a convergence spot in the case where the density distribution filter is provided in comparison with the relationship in the case where no density distribution filter is provided, Figures 6 and 7 are side views showing semiconductor laser optical systems as further embodiments of the present invention, and Figure 8 is a graph showing the relationships between the drive current for a semiconductor laser on one hand and the output of the natural emission light and the output of the laser oscillation light on the other hand, Figure 9 is a perspective view showing the optical scanning apparatus wherein the semiconductor laser optical system in accorance with the present invention is utilized. DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention will hereinbelow be described in further detail with reference to the accompanying drawings. Figure 1 shows a semiconductor laser optical system in accorance with the present invention. The semiconductor laser 1 emits natural emission light and laser oscillation light as mentioned above in accordance with the level of the applied current. As shown in Figure 2, the laser oscillation light has predetermined intensity distribution in a predetermined radiation angle range, and the intensity in the middle of the radiation angle range becomes the maximum intensity L. The laser oscillation light has an elliptical cross-sectional shape wherein the spread angle is different between the longitudinal direction and the transverse direction, and the gradient of a curve 1 differs between the longitudinal direction and the transverse direction. On the other hand, the natural emission light contains various angle components and is radiated also in directions different from the directions of radiation of the laser oscillation light. Referring to Figure 1, a current is applied to a semiconductor laser 1, and the semiconductor laser 1 emits a laser beam 1A in a light emission amount in accordance with the level of the current. The laser beam 1A generated by the semiconductor laser 1 is collimated by a collimator lens 2 disposed in the optical path, and then impinges upon a density distribution filter 23 which is constituted so that the optical density at the center portion is lower and the optical density at the peripheral portion is higher, and only the low density portion substantially transmits light. The density distribution filter 23 is used for decreasing the diameter of the convergence spot of the laser beam in the low output region, wherein the natural emission light is predominant, to a value close to the diameter of the convergence spot of the laser oscillation light. The diameter of the convergence spot of the laser beam in the low output region can be made smaller as the size of the low density portion is made smaller to decrease the diameter of the laser beam passing through the low density portion. By way of example, the density distribution filter 23 is constituted as shown in Figure 3A so that a rectangular low density portion 23a is formed inward from a high density portion 23b, which substantially intercepts light. The optical transmittance of the density distribution filter 23 in the transverse direction (i.e. in the x direction) and the optical transmittance thereof in the longitudinal direction (i.e. in the y direction) are as shown in Figure 3A. The sizes of the low density portion 23a in the x direction and in the y direction are smaller than the beam diameter of the laser oscillation light. The low density portion 23a may have any shape insofar as the longitudinal and transverse widths are suitable for taking up the laser beam of a desirable diameter. For example, the shape of the low density portion 23a may be elliptical as shown in Figure 3B. In the case where the shape of the low density portion 23a is elliptical, the optical transmittance of the density distribution filter 23 in the x direction and the optical transmittance thereof in the y direction become as shown in Figure 3B. Also, the density distribution of the density distribution filter 23 need not be such that the low density portion and the high density portion are continuously changed gradually as mentioned above, and may be such that the density decreases stepwise toward the center of the filter. In any case, as shown in Figure 4, the maximum value Tmax and the minimum value Tmin of the transmittance of the density distribution filter 23 should preferably be different from each other by at least one order of ten. The density distribution of the filter need not be formed in two-dimensional directions, and may be formed only in a one-dimensional direction in the case where problems with regard to increase in the diameter of the convergence spot of the natural emission light and the like are allowable in one direction. As shown in Figure 1, the laser beam 1A' passing through the density distribution filter 23 is converged by the converging lens 4 to a predetermined convergence position Q. In the case where the density distribution filter 23 is not disposed in the optical path of the laser beam 1A emitted by the semiconductor laser 1, the relationship between the output of the semiconductor laser 1 and the diameter of the convergence spot becomes as indicated by the broken line in Figure 5. On the vertical axis of the graph shown in Figure 5, the diameter of the convergence spot relative to the diameter of the convergence spot at the time of the output of 3mW wherein substantially the laser oscillation light alone is generated, is taken as 1. Thus in the case where the density distribution filter 23 is not used, the diameter of the convergence spot of the laser beam in the low output region wherein only the natural emission light is generated or the natural emission light is predominant becomes very large. However, with this embodiment of the laser optical system in accordance with the present invention, the diameter of the convergence spot of the laser beam in the low output region can be reduced by the effects of the density distribution filter 23. Specifically, in the case where each of the longitudinal width W and the transverse width W of the portion of the density distribution filter 23 corresponding to the transmittance range from the maximum value Tmax to one-half of the maximum value Tmax, i.e. Tmax/2. in the optical transmittance distribution as shown in Figure 11 is. for example, 0.6 times the diameter (half width) of the laser beam 1A incident upon the density distribution filter 23 after being collimated by the collimator lens 2, the relationship between the output of the semiconductor laser 1 and the diameter of the convergence spot becomes as indicated by the solid line in Figure 5. Thus the diameter of the convergence spot can be made markedly smaller than in the conventional laser optical system in the low output region as well. Also. no side lobe is generated at the convergence position, and the converged beam constituted by the zero-order light component alone can be obtained. Though the density distribution filter 23 is disposed between the collimator lens 2 and the converging lens 4, the density distribution filter 23 may be disposed at any position in the optical path between the semiconductor laser 1 and the convergence position Q. Also, the density distribution filter 23 may be incorporated inside of the collimator lens or inside of the converging lens 4. The size of the low density portion of the density distribution filter 23 may be adjusted to a value suitable for the beam diameter of the incident laser beam in accordance with the location of the density distribution filter 23. In the case where the density distribution filter 23 is positioned normal to the optical axis of the incident laser beam 1A as shown in Figure 1, it may occur that a part of tne laser beam 1A reflected by the surface of the density distribution filter 23 passes through the collimator lens 2 and returns to the semiconductor laser 1. When the reflected laser beam is returned to the semiconductor laser 1 in this manner, mode hopping occurs or the output of the semiconductor laser 1 is caused to fluctuate by optical interference. The adverse effects of returning of the reflected laser beam returning also arise in the cases of lasers other than the semiconductor laser. For example, in the case of a gas laser such as a He-Ne laser too, interference noise is generated inside of the resonator by the returning laser beam, and therefore the output of the laser beam emitted by the laser fluctuates. Particularly in the case where the density distribution filter 23 is of the reflection type wherein the high density portion reflects the incident laser beam to restrict the passage of the laser beam, a large amount of the laser beam is reflected by the high density portion, and therefore the aforesaid adverse effects become large. To eliminate such problems, as shown in Figure 6, a density distribution filter 23' may be positioned obliquely with respect to the optical axis of the laser beam 1A emitted by the semiconductor laser 1. With this configuration, the optical path of a laser beam 1B reflected by the density distribution filter 23' is shifted upward obliquely, so that no laser beam is returned to the semiconductor laser 1 and the aforesaid problems can be avoided. The density distribution (reflectivity distribution) of the density distribution filter 23' may be adjusted to an appropriate condition in accordance with the angle of inclination of the density distribution filter 23'. Also in the optical system as shown in Figure 7 wherein the optical output of a semiconductor laser is automatically controlled by use of a photodetector 25, it is often desired to position the density distribution filter 23' obliquely. Specifically, in the optical system shown in Figure 7, a semiconductor laser chip 1' supported by a supporting base 26 is disposed so that a laser beam 1C emitted from the back of the semiconductor laser chip 1' is detected by the photodetector 25. The photodetector 25 controls the drive current for the semiconductor laser chip 1' in accordance with the output of the laser beam 1C, so that the optical amount of the laser beam 1A emitted forward by the semiconductor laser chip 1' is always kept constant. In such an optical system, when the laser beam reflected by the reflection type density distribution filter 23' is returned toward the semiconductor laser chip 1', the returning laser beam interferes with the laser beam 1C, and the control cannot be carried out accurately. Accordingly, the density distribution filter 23' is inclined as shown to eliminate interference of the reflected laser beam 1B with the laser beam 1C and to carry out the automatic control of the laser power accurately. An example of the optical scanning apparatus wherein the semiconductor laser optical system in accordance with the present invention is employed will be described hereinbelow with reference to Figure 9. In Figure 9, a laser beam 11A emitted by a semiconductor laser 11 is collimated by a collimator lens 12, and is then made to impinge upon a density distribution filter 33. The density distribution filter 33 is of the light absorbing type, and is therefore disposed normal to the optical axis of the laser beam 11A. The laser beam 11A has a horizontally long elliptical beam shape as indicated by the broken line in Figure 9, and a low density portion 33a formed inward from a high density portion 33b of the density distribution filter 33 is provided to restrict the passage of the laser beam 11A therethrough only in the shorter axis direction (i.e. in the vertical direction in Figure 9. Therefore, laser beam 11A' passing through the density distribution filter 33 has its beam shape contracted vertically as compared with the beam shape of the laser beam 11A. The laser beam 11A' passing through the density distribution filter 33 is converged by a cylindrical lens 17, which is disposed in the optical path, only in the vertical direction. The laser beam 11A' is thus made to impinge upon a rotating polygon mirror 18, which is rotating in the direction as indicated by the arrow A, as a linear image normal to the drive axis of the rotating polygon mirror 18. The rotating polygon mirror 18 reflects and deflects the laser beam 11A' in a main scanning direction, and the deflected laser beam 11A' is passed through an f lens 19 constituted by a combination of two lenses. The deflected laser beam 11A' is then passed through a cylindrical mirror 20 disposed in the optical path to extend in the main scanning direction, and repeatedly scans a surface 21, which is to be scanned and is being conveyed in a sub-scanning direction as indicated by the arrow B. in the main scanning direction indicated by the arrow A'. The cylindrical mirror 20 converges the incident laser beam 11A' only in the sub-scanning direction on the surface 21 which is to be scanned, and the distance between the f lens 19 and the surface 21 which is to be scanned is adjusted to be equal to the focal length of the f lens 19 as a whole. Thus in this optical scanning apparatus, the cylindrical lens 17 and the cylindrical mirror 20 are disposed, and the laser beam 11A' is converged onto the rotating polygon mirror 18 only in the sub-scanning direction. Therefore, even though surface inclination or axis deviation arises with the rotating polygon mirror 18, the scanning position of the laser beam 11A' on the surface 21 which is to be scanned does not deviate in the sub-scanning direction, and scanning lines free of splits in the sub-scanning direction can be formed at equal intervals. Also, in such an optical scanning apparatus, high accuracy of the diameter of the convergence spot on the surface 21 which is to be scanned is required, particularly in the sub-scanning direction. In the optical scanning apparatus shown in Figure 9, the diameter of the convergence spot is adjusted in the vertical direction, which corresponds to the sub-scanning direction, by the density distribution filter 33 in the optical path prior to the impingement of the laser beam 11A' upon the rotating polygon mirror 18 as mentioned above, the therefore the problem with regard to beam blur in the sub-scanning direction on the surface 21 which is to be scanned does not arise. Also, the adjustment of the diameter of the convergence spot is carried out by the density distribution filter 33, and the laser beam 11A is not completely eclipsed in the course of the adjustment. Therefore, no side lobe is generated on the surface 21 which is to be scanned and on which the laser beam 11A' is converged. Accordingly, scanning can be carried out accurately on the surface 21 which is to be scanned so that no beam diameter blur appears in the sub-scanning direction and no side lobe is generated. The laser optical system in accordance with the present invention is described above as an example. The desirable density distribution of the density distribution filter varies in accordance with the purpose of the adjustment of the beam diameter. Also, in the case where the diameter of the collimator lens is adjusted to be smaller than, for example, the diameter of the incident laser beam, the collimator lens as a whole acts in the same manner as an aperture, and a part of the laser beam is eclipsed by the collimator lens. However, when the density distribution filter is disposed in the optical path in such an optical system, the density distribution filter can achieve the effects of cutting off the eclipsed laser beam and preventing the generation of side lobes as well as the original purposes of the density distribution filter.

Claims: A laser optical system comprising: a semiconductor laser (1) of direct analog modulation type; a driving means for driving the semiconductor laser (1) both in the ranges of natural light emission and laser light emission; a converging lens (4) for converging a laser beam emitted by said laser to a predetermined convergence position (Q); and a density distribution filter (23) disposed in an optical path of said laser beam in front of said converging lens constituted such that the optical density at the center portion (23a) is lower, and the optical density at the peripheral portion (23b) is higher, whereby only the low density portion substantially transmits the light beam; and the high density portion substantially blocks the light beam such that the convergence spot of the light beam is prevented from increasing when the laser is driven in the region of natural light emission; and the transmittance gradually decreases from its center portion to its peripheral portion. A laser optical system as defined in claim 1 wherein said density distribution filter is inclined by a predetermined angle with respect to the optical path of said laser beam. A laser optical system as defined in claim 1 wherein said density distribution filter is formed so that the maximum value Tmax and the minimum value Tmin of the optical transmittance are different from each other by at least one order of ten.

Inventors: MIYAGAWA ICHIROU, MIYAGAWA, ICHIROU, MIYAGAWA, ICHIROU C/O FUJI PHOTO FILM CO.,LTD.
Applicants: FUJI PHOTO FILM CO LTD, FUJI PHOTO FILM CO., LTD.
Application year: 1987
Main classification: G02B27
IPC codes: G02B26, G02B27