JPS63239419A - Laser optical system - Google Patents

Abstract

PURPOSE:To eliminate the side lobe of the periphery of a converged light point by providing a filter which passes only the center part of luminous flux in one dimensional direction and decreases in transmissivity toward its peripheral part. CONSTITUTION:A semiconductor laser 1 emits laser light and natural light. The laser light has a constant intensity distribution within a specific radiation angle range and elliptically sectioned light which has a difference between a longitudinal and a lateral spread. The natural light has various angle components and is emitted even in directions where the laser light is not emitted. A density filter 3 is arranged so as to reduce and approximate the diameter of the converged light point of light in a low-output area where the natural light is dominant to the convergent light point diameter of the laser light, and the passing rate of the luminous flux decreases toward the periphery. The light passed through the density distributed filter 3 is converged on a specific position P through a convergent lens 4 and then the side lobe of the periphery of the converged light point is eliminated.

Description

DETAILED DESCRIPTION OF THE INVENTION (Field of the Invention) The present invention relates to a laser optical system equipped with a laser light source, and in particular, it relates to a laser optical system equipped with a laser light source, and more particularly, when light emitted from a laser light source is converged, side lobes are generated around the convergence spot. This relates to a laser optical system that does not cause this phenomenon.

(Prior Art) Laser optical systems have been widely used as scanning light generating means in various scanning recording devices and scanning reading devices. Among these, semiconductor lasers are used as the laser light source for laser optical systems. Semiconductor lasers are smaller, cheaper, and consume less power than gas lasers, etc., and they also have so-called analog direct modulation that changes the output by controlling the drive current. This is possible, and when used in a scanning recording device, it is very convenient because the above-mentioned direct modulation can be performed by a signal emitted in accordance with image information.

By the way, in the above-mentioned laser optical system, a beam diameter adjustment plate having an aperture that allows only the light in the center of the beam to pass through is sometimes provided on the optical path of the light emitted from the laser light source. There are various purposes for arranging such a beam diameter adjustment plate. For example, it is known that such an adjustment plate as described above is provided on the optical path in order to increase the depth of focus when converging light. (Japanese Patent Publication No. 58-20015) In some cases, the above adjustment plate is provided for beam shaping. Furthermore, when the laser light source is a semiconductor laser, the beam diameter adjusting plate described above is used to use the light in the low power range of the light emitted from the semiconductor laser without increasing the convergence spot diameter at the convergence position. may be provided. The relationship between the output of the semiconductor laser and the convergence spot diameter and the function of the beam diameter adjusting plate will be explained below.

It is known that there are two types of light emitted from a semiconductor laser: laser oscillation light and light in the spontaneous luminescence region.The relationship between the drive current of the semiconductor laser, the laser oscillation light and the light in the spontaneous luminescence region is It is shown in the figure. In the graph shown, the line a° indicates the relationship between the drive current and the output of light in the spontaneous emission area (hereinafter referred to as spontaneous emission light), and the line a indicates the relationship between the drive current and the output of the laser oscillation light. It is.

As shown in the graph, when a current is applied to the semiconductor laser, no laser oscillation light is output until the current exceeds the threshold current 1o, and only spontaneous luminescence light is output. The output of naturally emitted light increases little by little as the drive current increases, but at a current threshold of 1. When the output of the laser oscillation light exceeds 1, and the output of the laser oscillation light increases, its proportion to the total emitted light becomes small, and substantially only the laser oscillation light is output. The relationship between the total amount of light emitted from the semiconductor laser, which is a combination of naturally emitted light and laser oscillation light, and the amount of current is represented by a curve C.

However, since the spontaneously emitted light has various angular components mixed together compared to the laser oscillation light, there is a problem in that the laser oscillation light cannot be converged to a small spot diameter when converged by a converging lens. For this reason, when a semiconductor laser optical system is used in a scanning recording device that requires recording light to be modulated over a wide dynamic range and used even in the low-power region where naturally emitted light is dominant, it is difficult to A problem arises in that the beam diameter becomes large and the spatial resolution of scanning is impaired. In order to solve this problem, if a beam diameter adjustment plate is provided on the optical path of the light emitted from the semiconductor laser, which has an aperture that allows only the central part of the light beam to pass through, it is possible to reduce the convergence spot diameter even for naturally emitted light. Can be made smaller.

(Problem to be Solved by the Invention) However, when a beam diameter adjusting member having an aperture as described above is provided on the optical path of light emitted from a laser light source, the light is obstructed by the outline of the aperture, so the beam diameter When the light after passing through the adjustment member is converged, a so-called side lobe occurs in which weak light is generated as primary light, secondary light, etc. around the zero-order light that is converged at a predetermined position. When such side lobes appear, various problems occur, such as the appearance of image ghosts when a laser optical system is used in a scanning recording apparatus and image information is recorded using converged light.

SUMMARY OF THE INVENTION Therefore, an object of the present invention is to provide a laser optical system that can extract only the light in the central part of the luminous flux and that does not generate side lobes when the extracted light is converged. . .

(Means for Solving the Problems) The laser optical system of the present invention is provided with a laser light source and on the optical path of the light emitted from the laser light source, and allows only the light in the central portion of the light beam to pass in at least one-dimensional direction. This device is characterized by being equipped with a concentration distribution filter that gradually reduces transmittance toward the periphery.

Note that the concentration distribution filter in the present invention may be either an absorption type that absorbs light in a high concentration area or a reflection type that reflects light in a high concentration area.

(Function) Since the concentration distribution filter adjusts the passage of light according to its optical density, there is no risk of cutting off part of the incident light as with a beam diameter adjustment member having an aperture, and the filter does not pass through the filter. Side lobes no longer occur when the subsequent light is converged.

Further, since the concentration distribution filter can freely select the concentration distribution, it is easy to freely adjust the beam shape of the light passing through the filter by changing the distribution.

(Example) Hereinafter, an example of the present invention will be described with reference to the drawings.

ff11 is a side view showing an outline of a semiconductor laser optical system which is an embodiment of the laser optical system of the present invention.

When a current is applied to the semiconductor laser 1, the semiconductor laser 1 emits light IA with an amount of light emitted according to the amount of current, and the light IA oscillated from the semiconductor laser 1 enters a collimator lens 2 provided on the optical path and is collimated. After being turned into light, a density distribution filter 3 is formed in which the central part has a low optical density and the peripheral part has a high optical density, and only the low density part substantially transmits light.
incident on .

The semiconductor laser 1 emits two types of light, the above-mentioned laser oscillation light and spontaneous luminescence light, depending on the applied current, and among these, the laser oscillation light exhibits a constant intensity distribution within a predetermined radiation angle range. The light has an elliptical cross-sectional shape with different spread angles in the vertical and horizontal directions. On the other hand, naturally emitted light, unlike laser oscillation light, has various angular components and is emitted even in directions where laser oscillation light is not emitted.

The concentration distribution filter 3 is disposed to reduce the convergence spot diameter of light in the low output region where naturally emitted light is dominant to bring it closer to the convergence spot diameter of laser oscillation light, and reduces the size of the low concentration portion. The smaller the diameter of the passing light, the smaller the convergence spot diameter in the low output region. This concentration distribution filter 3 is shown in FIG.
In this case, a rectangular low-density portion 3a is formed inside a high-density portion 3b that does not substantially allow light to pass through. The light transmittance of this concentration distribution filter 3 in the horizontal direction (X direction) and the vertical direction (y direction) is as shown in the figure, and the size of the low concentration portion 3a is larger than the beam diameter of the laser oscillation light in both the x and y force directions. is also smaller. Note that the shape of the low concentration portion 3a may be any shape as long as the vertical and horizontal widths are suitable for extracting light of a desired diameter, such as an ellipse as shown in FIG. 3(b). Good too. When the low concentration portion 3a has an elliptical shape, the light transmittance in the x and y force directions is as shown in the figure. Further, the concentration distribution of the concentration distribution filter 3 is not limited to the one in which the low concentration portion and the high concentration portion gradually change continuously as described above, but may be formed so that the concentration gradually decreases toward the center of the filter. You can leave it there. In either case, as shown in FIG. 3, it is desirable that the maximum value T wax and the minimum value T in of the transmittance of the density distribution filter are separated by at least one digit. It should be noted that the density distribution of the filter does not necessarily have to be provided in a two-dimensional direction, and if expansion of the convergence spot diameter of naturally emitted light is not a problem in one direction, it is possible to provide a density distribution only in the one-dimensional direction. Good too.

The light that has passed through the concentration distribution filter 3 as described above enters the converging lens 4 as shown in FIG.
It is converged at . If the concentration distribution filter 3 is not provided on the optical path of the light emitted from the semiconductor laser 1, the relationship between the output of the semiconductor laser 1 and the convergence spot diameter will be as shown by the broken line in FIG. 4. . In addition,
The vertical axis in FIG. 4 is the relative convergence spot diameter, with the convergence spot diameter when the output is 3 mW, in which substantially only laser oscillation light is output, being 1. If a concentration distribution filter is not provided in this way, the convergence spot diameter of only the naturally emitted light or the light in the low output region where the naturally emitted light is dominant will be extremely large, but the optical system of this example In this case, the spot diameter of the light in the low output region can be reduced by the action of the concentration distribution filter 3. That is,
- As an example, as shown in FIG. 3, the width W of the portion having a transmittance from the maximum value Tg+ax of the light transmission weight distribution of the concentration distribution filter 3 to the value T■ax/2, which is 1/2 of the maximum value, is The relationship between the output of the semiconductor laser and the above-mentioned relative convergence spot diameter is shown by the solid line in FIG. 4 when the diameter of the incident light on the filter 3 is 0.6 times the diameter of the incident light on the filter 3, which is parallelized by the collimator lens 2 in both the vertical and horizontal directions. Therefore, even in the low power range, the convergence spot diameter can be made much smaller than before. Moreover, no side lobes appeared at the convergence position, and convergence light consisting only of zero-order light was obtained.

Note that in the above embodiment, the concentration distribution filter 3 is arranged between the collimator lens 2 and the converging lens 4;
The filter 3 may be placed at any position on the optical path between the semiconductor laser 1 and the convergence position P. It is also possible to put the concentration distribution filter 3 inside the collimator lens 2 or the converging lens 4. The size of the low concentration portion of the concentration distribution filter may be appropriately adjusted to a size suitable for the beam diameter of the incident light depending on the position where it is placed.

Note that, as shown in Figure 1, when the concentration distribution filter is arranged perpendicular to the optical axis of the incident light, the light reflected from the surface of this concentration distribution filter passes through the collimator lens. It is conceivable that the semiconductor laser will be reverted to a semiconductor laser. If the reflected light is returned to the semiconductor laser in this way, problems such as mode hopping may occur and the output of the semiconductor laser may fluctuate due to light interference. The effect of the return of reflected light also occurs in lasers other than semiconductor lasers. For example, in the case of gas lasers such as He-Ne lasers, the return light causes interference noise inside the resonator, so it is difficult to emit light from the laser. There is a disadvantage that the output of the light emitted varies. In particular, if this density distribution filter is a so-called reflective density distribution filter that suppresses the transmission of light by reflecting the incident light at the high density area, the amount of light reflected at the high density area is large and the effect is large. . Therefore, if the concentration distribution filter 3' is arranged at an angle with respect to the light IA emitted from the semiconductor laser 1, as shown in FIG. 5, the optical path of the light IB reflected by the concentration distribution filter will move diagonally upward. It is no longer returned to the semiconductor laser 1, and the above-mentioned inconvenience can be avoided. It goes without saying that the concentration distribution (reflectance distribution) of the concentration distribution filter may be formed to have an appropriate slope depending on the angle of the filter.

Further, even in an optical system as shown in FIG. 6, in which the optical output of a semiconductor laser is automatically controlled using a photodetector 5, it may be preferable to arrange the concentration distribution filter 3' at an angle. That is, in the illustrated optical system, the ° support base 6
The semiconductor laser chip 1' held by the semiconductor laser chip 1' is configured such that the optical IC emitted from the rear thereof is detected by the photodetector 5, and the photodetector 5 detects the semiconductor laser chip according to the output of the optical IC. The drive current is controlled to keep the amount of light IA emitted from the semiconductor laser chip 1' constant. In such an optical system, if the reflected light from the reflective concentration distribution filter 3' is returned in the direction of the laser chip 1', the returned light will interfere with the optical IC, making it impossible to perform accurate control. Therefore, if the concentration distribution filter 3' is tilted as shown, the reflected light IB will not interfere with the optical IC, so that automatic control of the laser power can be performed effectively.

Next, a usage example in which the semiconductor laser optical system according to the present invention is incorporated into an optical scanning device will be described with reference to FIG.

In the illustrated device, light 11A emitted from a semiconductor laser 11 passes through a collimator lens 12 to become parallel light, and then enters a concentration distribution filter 13.

Note that since this filter 13 is of a light absorption type, the light lIA
is arranged perpendicular to the optical axis of the The beam shape of the light ILA is a horizontally elongated ellipse as shown by the broken line in the figure, and the low concentration portion 13a provided inside the high concentration portion 13b of the concentration distribution filter 13 is located along the short axis of the light 11A. Passage is restricted only in the vertical direction, and the beam shape of the light 11A' that has passed through the filter 13 is further contracted in the vertical direction. Concentration distribution filter 1
The light IIA' that has passed through the polygon mirror 16 is converged only in the vertical direction by a cylindrical lens 15 provided on the optical path, and is incident on a rotating polygon mirror 16 rotating in the direction of arrow A as a line image perpendicular to its drive axis. The rotating polygon mirror 16 emits light 11
A' is reflected and deflected in the main scanning direction, and the deflected light 11A'
passes through an fθ lens 14 made up of two lenses, then passes through a cylindrical mirror 17 provided on the optical path extending in the main scanning direction, and is transported in the direction of arrow B (sub-scanned). Repeat arrow A on scanned surface I8
Main scan in the ' direction. The cylindrical mirror 17 is configured to converge the light 118' only in the sub-scanning direction on the scanned surface 18 by the incident concentration distribution filter 13, and the distance from the fθ lens 14 to the scanned surface 18 is is equal to the focal length of the entire fθ lens 14. In this way, in this device, the cylindrical lens and the mirrors 15 and 17 are arranged, and by converging the light 11A' only in the sub-scanning direction on the rotating polygon mirror 16, the rotation polygon mirror 16 is free from surface tilt and axial vibration. Even if this occurs, the scanning position of the light 11A' on the scanned surface 18 does not shift in the sub-scanning direction, and it is possible to form scanning lines with equal pitches and without gaps in the sub-scanning direction. In addition, the accuracy of the convergent spot diameter on the scanned surface 18 in such a scanning device is particularly required in the sub-scanning direction, but in this device, as described above, in the optical path before entering the rotating polygon mirror, the Since the convergence spot diameter is adjusted in the direction by the concentration distribution filter 13, there is no problem such as blurring of the beam in the sub-scanning direction on the scanned surface 18. Further, the adjustment of the convergence spot diameter is performed using a concentration distribution filter, and since the light is not completely eclipsed during adjustment, side lobes will not occur on the scanned surface 18, which is the convergence position of the light 11A'. . Therefore, according to the present apparatus, highly accurate scanning can be performed without blurring the beam diameter in the sub-scanning direction and without the appearance of side lobes.

Above, the laser optical system of the present invention has been explained mainly using a semiconductor laser optical system as an example, but the present invention also applies to other laser optical systems where it is necessary to limit the beam diameter in order to increase the depth of focus, etc. The laser light source is not limited to a semiconductor laser. It goes without saying that the desired concentration distribution of the concentration distribution filter also changes depending on the purpose of adjusting the beam diameter.

Furthermore, for example, if the diameter of the collimator lens is set smaller than the diameter of the incident light, the entire lens will function as an aperture, and a portion of the light will be eclipsed by these lenses, but such an optical system By providing a concentration distribution filter on the optical path, in addition to the above-mentioned original purpose of the filter, it is also possible to achieve the effect of cutting off the eclipsed light and preventing the generation of side lobes.

(Effects of the Invention) As described in detail above, according to the laser optical system of the present invention, the beam diameter can be effectively adjusted using the concentration distribution filter without generating side lobes. Therefore, by converging the light that has passed through the concentration distribution filter, it is possible to increase the depth of focus and eliminate blurring of the beam diameter, and it is possible to obtain well-converged light at the convergence position.

[Brief explanation of the drawing]

FIG. 1 is a side view showing a semiconductor laser optical system which is an embodiment of the laser optical system of the present invention; FIGS. 2(a) and (b) are schematic diagrams showing an outline of a concentration distribution filter and its light transmittance; Fig. 3 is a graph showing the light transmittance of a density distribution filter; Fig. 4 is a graph showing the relationship between light output and convergence spot diameter when a density distribution filter is provided, compared to when no filter is provided; 5 and 6 are side views showing a semiconductor laser optical system according to another embodiment of the present invention, FIG. 7 is a perspective view of an optical scanning device using the semiconductor laser optical system according to the present invention, and FIG. 8 is a side view showing a semiconductor laser optical system according to another embodiment of the present invention. 3 is a graph showing the relationship between the driving current of a semiconductor laser and the output of spontaneously emitted light and laser oscillation light. l...Semiconductor laser IA, IA'...Light 3
．． 3'...Concentration distribution filter 4...Convergent lens Fig. 1 Fig. 2 Fig. 3 Fig. 4 IN History 77 (mW) Fig. 7 Purity of 7N! F (No change in content) Figure 5 Figure 6 Old electric nine-

Claims (1)

[Scope of Claims] 1) A laser light source and a laser light source provided on the optical path of the light emitted from the laser light source, which allows only the light in the central portion of the light beam to pass in at least one dimension, and whose transmittance decreases as it moves toward the peripheral portions. Laser optics with gradually decreasing concentration distribution filter. 2) The laser optical system according to claim 1, wherein the laser light source is a semiconductor laser. 3) The laser optical system according to claim 1 or 2, wherein the concentration distribution filter is arranged at a predetermined angle with respect to the optical path of the light.