JPH02190814A - Laser output controller - Google Patents

Abstract

(57) [Summary] This bulletin contains application data before electronic filing, so abstract data is not recorded.

Description

DETAILED DESCRIPTION OF THE INVENTION Field of the Invention The present invention relates to a laser output control device for a laser printer, a laser plate making machine, etc. that uses a laser light source.

2. Description of the Related Art Conventionally, in this type of laser printer, etc., it has been required to be able to variably control the intensity of a laser beam on an image forming surface in order to deal with, for example, variations in photoreceptor sensitivity.

For this reason, various laser output control methods have been proposed.

As a first method, there is a method disclosed in Japanese Patent Application Laid-Open No. 101813/1983. This involves rotating a polarizing beam splitter placed on the beam optical path of a laser light source around the optical axis, and monitoring the reflected beam from the polarizing beam splitter with an optical sensor that rotates together with the polarizing beam splitter. By feeding back this monitor output to the rotation drive section of the polarization beam splitter, the rotation angle of the polarization beam splitter is controlled to obtain a laser output with a set amount of light.

As a second method, there is a method disclosed in Japanese Unexamined Patent Publication No. 63-74815. This uses a variable attenuator (for example, an ND filter that continuously changes density) and a beam splitter that electrically attenuates light, and the light from the beam splitter is received by a light receiving element, and the light is converted into an electric light. The output is compared with a reference voltage in a comparator, and the rotation of the ND filter is controlled to control the optical output.

The third method is the method disclosed in Japanese Unexamined Patent Publication No. 63-97060. This involves receiving the output of the laser diode with a light receiving element, photoelectrically converting it, comparing it with a reference voltage, and variably controlling the current flowing through the laser diode using an up/down counter and a D/A converter according to the comparison output.
It is designed to control the exit point of a laser beam from a laser diode.

Problems to be Solved by the Invention However, in the case of the first method, since the rotating part consists of a polarizing beam splitter and a light receiving element, the rotating part becomes large, and the drive motor for the rotation also becomes large. Furthermore, since the light receiving element is also rotated and displaced, a flexible cable is required for wiring to the light receiving element. Furthermore, if the polarizing beam splitter 1 is eccentric as shown in FIG. 7, its rotation will cause an optical axis deviation Δ, and great care must be taken when installing the polarizing beam splitter. Further, in the case of this method, the polarizing beam splitter must be rotated by 90' around the optical axis angle in order to change the output beam intensity between the maximum and minimum values.

Next, in the case of the second method, a large-area ND filter is required, resulting in an increase in the size of the apparatus. Further, as described in the same publication, in the case where a beam splitter is used to make the light incident on the light receiving element, the loss in the amount of light is large and the efficiency is poor.

Furthermore, in the case of the third method, the output of the laser diode is controlled by changing the current to the laser diode using the output of the D/A converter (digital control).
In order to control the output of the laser diode over a wide range with high precision, a D/A converter with a large number of bits is required. In addition, when controlling the laser diode to a low output, the S/
N deteriorates. For example, FIG. 8 is a characteristic diagram showing the relationship between the input current and optical output of a semiconductor laser, and the input t
The laser does not emit light until the first current reaches the threshold current Ith.L
ED emits light. Therefore, it is possible to use a semiconductor laser with low output power, for example.
1, the S/N, that is, LB/L, (LB
(LED emission component) becomes large. Here, L
Since the EDI light component is not coherent light and cannot be focused on a small spot, when using a laser beam narrowed down to a small spot like in a laser printer, the LEI)i light component is a flare (noise) component. This results in deterioration of image quality.

Means for solving the problem A laser beam from a laser light source is placed on the optical axis.
A polarizing beam splitter comprising a two-wavelength plate and a rotational driving means for rotationally displacing the half-wavelength plate around the optical axis angle, and disposed on the optical axis of the transmitted laser beam from the half-wavelength plate. a comparison means for comparing the photoelectric conversion output of the photoelectric conversion element with a reference value; and control means for driving and controlling the rotary drive means in accordance with the comparison output from the comparison means.

The laser beam from the working laser light source is linearly polarized light, and when this plane of polarization is incident on the 1/2 wave plate at an angle θ, the transmitted laser beam becomes linearly polarized light with the plane of polarization rotated by 20. Therefore, by rotationally displacing this 1/2 wavelength plate around the optical axis by the rotation driving means, it becomes possible to continuously change the polarization plane of the transmitted laser beam.

When such a transmitted laser beam is incident on a polarizing beam splitter, the P polarized light component and the S polarized light component are separated according to the rotation of the polarization plane.
Since the polarization component changes, the ratio between the reflected laser beam and the transmitted laser beam from the polarizing beam splitter changes accordingly. Therefore, the intensity of one beam emitted from the polarizing beam splitter is detected by the photoelectric conversion element, and compared with a reference value by the comparison means, and the rotation driving means is feedback-controlled by the control means so that the difference becomes 0. By controlling the rotation angle of the 1/2 wavelength plate, it becomes possible to adjust the laser beam intensity to correspond to the reference value.

Embodiment An embodiment of the present invention will be explained based on FIGS. 1 to 6. First, a laser light source 2 that emits a laser beam is provided. A 1/2 wavelength plate 4 is provided on the laser beam optical axis 3a from the laser light source 2. This 1/2 wavelength plate 4 is rotatably supported around the optical axis 3a, and is connected to rotation drive means 5. This rotational drive means 5 can be configured as appropriate, but in this embodiment, a drive motor 6 and a drive gear provided on the axis of the drive motor 6 and meshed with a gear 7 on the outer periphery of the 1/2 wavelength plate 4 are used. It consists of 8. Further, a polarizing beam splitter 9 is provided on the optical axis 3b of the transmitted laser beam from the 1/2 wavelength plate 4. Further, a photoelectric conversion element 10 that receives the reflected laser beam from the polarizing beam splitter 9 is provided. The photoelectric conversion output from the photoelectric conversion element 10 is input to a comparison circuit 11 as comparison means for comparing it with a base voltage as a predetermined reference value. The comparison output from this comparison circuit 11 is fed back to the drive motor 6 via a rotation angle control circuit 12 as a control means.

In such a configuration, the laser beam emitted from the laser light source 2 is linearly polarized in terms of waves. Such a linearly polarized laser beam has a plane of polarization at an angle of O with respect to the crystal axis (optical axis direction of the crystal) of the 1/2 wavelength plate 4.
When the laser beam is incident with a polarization angle of 20 degrees, the polarization plane of the transmitted laser beam is refracted within the main cross section that includes the crystal axis, so that the polarization plane of the transmitted laser beam becomes a straight line with the polarization plane rotated by an angle of 20, as shown in Figure 2. It becomes polarized light. That is, by using the 1/2 wavelength plate 4, the polarization plane of the laser beam can be rotated without rotating the laser light source 2 itself, and the polarization plane of the transmitted laser beam is formed by the incident beam and the main cross section. It can be rotated by an angle twice the angle θ. Therefore, such 1/
The polarization plane of the transmitted laser beam can be continuously changed by rotating the two-wavelength plate 4 around the optical axis and changing the crystal axis direction with respect to the incident laser beam, instead of making it fixed.

Furthermore, as is well known, the polarizing beam splitter 9 placed after the half-wave plate 4 is a cube-shaped polarizing element made by bonding the slopes of a pair of right-angled prisms, and the bonding surface (
The slope) is coated with a dielectric material. Therefore,
As shown in Figure 3, in the incident laser beam, a light component with a plane of polarization parallel to the slope (this is called a P-polarized light component) is transmitted, and a light component with a plane of polarization perpendicular to the slope is transmitted. (This is called the S-polarized component) is reflected to separate both components.

Therefore, the operation when such a polarizing beam splitter 9 and the half-wave plate 4 are combined will be explained with reference to FIG. 4. This is based on the assumption that the half-wave plate 4 is rotated around the optical axis at an angular velocity ω. Figure (a) shows the case where the rotation angle ω = O, and Figure (b) shows the case where the rotation Angle ω=45°
The case is shown below. First, Figure (a) shows a state in which the polarization plane of a linearly polarized (mostly S-polarized) incident laser beam (intensity I0) coincides with the crystal axis of the 172-wave plate 4, and the transmitted laser beam has a polarization plane. The beam enters the polarizing beam splitter 9 without rotating. Then, since most of the beam is an S-polarized component, it is reflected on the slope and is emitted as a reflected laser beam (2), and the remaining part is a P-polarized component and is transmitted as a transmitted laser beam (2). Closed and ejected. Polarization characteristics of the incident laser beam, 1/2 wavelength plate 4
Although it depends on the characteristics of the polarizing beam splitter 9, the reflected laser beam 8 has an intensity of Ig > 0.97 Io, and the transmitted laser beam ID is I. It has an intensity of <0.02Io. On the other hand, even in the case of incident laser beams having the same polarization characteristics, the rotation angle at
When the crystal axis of the half-wave plate 4 is rotated by 45 degrees due to =45 degrees, the polarization plane of the transmitted laser beam is rotated by 90 degrees, and becomes almost P-polarized component light, which enters the polarizing beam splitter 9. Therefore, the intensity of the laser beam emitted from this polarizing beam splitter 9 has an opposite relationship between the reflection side and the transmission side. That is, 1 reflected laser beam ■5 is Is≧0.02
A transmitted laser beam with an intensity of To. is Io <
It has an intensity of 0.97 Io.

Therefore, when the polarization plane of the laser beam incident on the polarization beam splitter 9 is rotated by rotating the 172-wavelength plate 4, the P of the linearly polarized light changes according to the rotation.
Since the ratio between the polarized light component and the S-polarized light component changes, the ratio between the reflected laser beam and the transmitted laser beam emitted from the polarizing beam splitter 9 can be continuously changed. Now, rotate the 1/2 wavelength plate 4 continuously around the optical axis (
If the rotation angle is 0° to 2π), the light intensity of the P and S polarized components emitted from the polarizing beam splitter 9 is
It is standardized as shown in the figure. According to this characteristic, if the light intensity of the -force component is known, the relative light intensity of the other component can also be found.

For this reason, for example, when using the transmitted laser beam from the polarizing beam splitter 9 for optical writing, etc., the reflected laser beam from the polarizing beam splitter 9 is monitored by the photoelectric conversion element 10, and the photoelectric conversion output is compared. A comparison is made in circuit 11 with a desired reference voltage. The comparison output (difference between charging conversion output voltage and reference voltage) from this comparison circuit 11 is fed back to the rotation angle control circuit 12,
The drive motor 6 is driven so that this voltage difference becomes O.
By controlling the rotation angle of the /2 wavelength plate 4, it is possible to adjust the optical output so that the transmitted laser beam intensity corresponds to a desired reference voltage. FIG. 6 shows the characteristics of the optical intensity of the transmitted laser beam from the polarizing beam splitter 9 and the output of the photoelectric conversion element 10 for the reflected laser beam with respect to the rotation angle of the 1/2 wavelength plate 4 in this embodiment. .

Note that if the reflected laser beam from the polarizing beam splitter 9 is used as the emitted beam, the intensity of the transmitted laser beam may be monitored and detected and fed back.

As described above, according to this embodiment, by controlling the rotation angle of the half-wave plate 4 around the optical axis according to the reference voltage applied to the comparator circuit 11, the optical intensity of the incident laser beam is
% to 98% (as mentioned above, depending on the polarization characteristics of the laser beam, etc.), it is possible to obtain an emitted laser beam over a wide range of light intensity. Therefore, even when recording materials (photosensitive materials) with very different sensitivities are used in, for example, a laser printer, a laser engraving machine, etc., it can be easily handled. In particular, according to this embodiment, since the 1/2 wavelength plate 4 is rotated, even when the intensity of the emitted laser beam is varied between the maximum and the minimum, this 1/2 wavelength plate 4 can be rotated.
It is sufficient to rotate the /2 wavelength plate 4 by 45 degrees, and there is no need to rotate it by 90 degrees as in the conventional polarization beam splitter rotation method. Further, the only rotating member is the 1/2 wavelength plate 4, and even if its drive system is included, the size will not be particularly large. Further, as shown in the characteristics shown in FIGS. 5 and 6, the intensity of the emitted laser beam can be controlled steplessly while being constantly monitored by the photoelectric conversion element 10, so that highly accurate output control is possible. Further, according to this embodiment, since the polarizing beam splitter 9 is used after the half-wave plate 4, the P-polarized component (the polarizing beam splitter 9 When the reflected light from the polarizing beam splitter 9 is used as the emitted laser beam, only the S-polarized light component is transmitted (reflected when the reflected light from the polarizing beam splitter 9 is used as the emitted laser beam). Even if there is, the noise component can be halved and the S/N ratio can be improved.

Effects of the Invention The present invention provides a rotation drive means for rotationally displacing the 1/2 wavelength plate disposed on the optical axis of the laser beam from the laser light source around the optical axis as described above. A polarizing beam splitter is provided along with the optical axis of the transmitted laser beam from the 1/2 wavelength plate, and a photoelectronic device receives either the reflected laser beam or the transmitted laser beam from the polarizing beam splitter. A conversion element is provided, a comparison means for comparing the charging conversion output of the optical t-conversion element with a value, and a control means for driving and controlling the rotation driving means in accordance with the comparison output from the comparison means. By controlling the rotation angle of the half-wave plate around the optical axis according to the reference voltage for the comparison means, it is possible to arbitrarily select an output laser beam having a wide range of light intensities relative to the light intensity of the incident laser beam. Therefore, even when recording materials with very different sensitivities are used in laser printers, laser engraving machines, etc., it can be easily handled. Due to displacement,
Even when varying the intensity of the emitted laser beam between maximum and minimum, it is only necessary to rotate this 1/2 wavelength plate by 45 degrees, and only this 1/2 wavelength plate is rotated.
Even including the drive system, it does not become particularly large, and furthermore, the intensity of the emitted laser beam can be continuously monitored and controlled steplessly with a photoelectric conversion element, making it possible to control the output with high precision. Since a polarizing beam splitter is used after the 1/2 wavelength plate, the noise component can be halved and the S/N can be improved even when the laser light source is used at low output. It is.

[Brief explanation of the drawing]

1 to 6 show an embodiment of the present invention,
Fig. 1 is a schematic perspective view, Fig. 2 is a perspective view showing the 016 optical plane rotation function of the 1/2 wavelength plate, Fig. 3 is a perspective view showing the polarization separation function of the polarizing beam splitter, and Fig. 4 is the 1/2 wavelength plate. A perspective view showing the function of a combination of a 2-wave plate and a polarizing beam splitter. Figure 5 is a normalized light intensity characteristic diagram when the 1/2-wave plate is continuously rotated. Figure 6 is a 1/2-wave plate and a polarizing beam splitter. FIG. 7 is a schematic side view showing a conventional method, and FIG. 8 is a current-output characteristic diagram of a semiconductor laser.

Claims (1)

[Claims] 1/ placed on the optical axis of the laser beam from the laser light source.
a two-wavelength plate, a rotational drive means for rotationally displacing the half-wavelength plate around an optical axis, and a polarizing beam splitter disposed on the optical axis of the transmitted laser beam from the half-wavelength plate;
A photoelectric conversion element that receives either the reflected laser beam or the transmitted laser beam from the polarizing beam splitter, a comparison means for comparing the photoelectric conversion output of this photoelectric conversion element with a reference value, and a comparison means for comparing the photoelectric conversion output of this photoelectric conversion element with a reference value. A laser output control device comprising: control means for driving and controlling the rotational drive means in accordance with a comparison output.