1. Field of Invention
The invention relates to a multibeam scanner capable of scanning with constant light amounts of laser beams at a high scan cycle.
2. Description of Related Art
As is well-known in the art, a laser beam scanner is put to a number of uses. For example, a laser printer uses a laser beam scanner comprising a laser diode for emitting a laser beam. The laser beam scanner modulates the laser beam based on image signal and deflects the modulated laser beam by using a deflector, such as a polygon mirror, thereby sequentially scanning a photosensitive medium with the laser beam in a main-scanning direction to form a scanning line on the photosensitive medium. At the same time, the laser beam scanner moves the photosensitive medium in a moving direction opposite to a sub-scanning direction to form successive scanning lines. In such a scanning operation, a latent image is formed on the photosensitive medium, developed with toner as a visible image and, then, transferred to a recording sheet by a fixing unit.
However, the laser diode has a fundamental difficulty in stabilizing the light amount emitted. Even if the laser diode is driven with a constant driving current, the temperature of the laser diode increases due to its light emission. The luminous efficiency of the laser diode decreases with the temperature increase.
Therefore, in order to overcome such a drawback, there is performed, as disclosed in Japanese Patent Application No. 63-42432, automatic power control (APC) for feeding a driving current to the laser diode, so that the laser diode always emits the laser beam of appropriate light amount. More specifically, in APC, a control circuit controls the laser diode to emit the laser beam, whereat a photoreceptor detects the emitted laser diode and determines the light amount of the laser beam to give a feedback on the light amount to the control circuit. Upon reception of the feedback, the control circuit properly controls a constant-current circuit to feed an appropriate driving current based on the received feedback, so that the laser diode emits a laser beam of appropriate light amount.
Further, there is provided a beam detector (BD) positioned a predetermined distance before the scan start position in the laser scanner. The BD enables the start of scanning with the laser beam, deflected by the deflector at high speed, on the photosensitive medium. After a predetermined time duration has elapsed since the timing at which the BD detected the laser beam, the laser beam scanner starts scanning the photosensitive medium with the laser beam modulated based on image signals. As the BD only detects a laser beam of more than a threshold light amount, the variations in light amounts of the laser beam may cause a detection fault to destroy the above-mentioned timing. It is therefore preferable that such scan start timing detection is performed by the BD after completing APC.
There is enough time for a single-beam scanner comprising a single laser diode, which has been mentioned above, to perform APC and to detect the scan start timing after finishing forming a scanning line and before starting to form a next scanning line.
Against the above-described backdrop, there has been recently proposed a multibeam scanner that comprises a plurality of laser diodes for emitting a plurality of laser beams simultaneously. This multibeam scanner has an advantage of scanning the photosensitive medium with the laser beams, along a plurality of scanning lines, in the main-scanning direction simultaneously, and thereby is capable of exposing the photosensitive medium in a very short time.
The multibeam scanner may have therein a plurality of photoreceptors to detect the respective light amounts of the plurality of laser beams. It is, however, still difficult for the multibeam scanner to detect the light amounts of the laser beams while avoiding interference between the laser beams, because the laser beams are emitted in very close proximity to each other. The multibeam scanner will have to have an extremely complicated structure to resolve the above-mentioned problem.
The multibeam scanner can have a simple structure with a single BD, but cannot perform APC for all the laser diodes simultaneously. Thus, it is necessary to perform APC separately for every laser diode as disclosed in, for example, Japanese Patent No. 63-42432. FIG. 8 is a timing chart illustrating control timing for such a conventional multibeam scanner. The horizontal axis in FIG. 8 represents elapsed time. xe2x80x9cSOS (start of scanning)xe2x80x9d and xe2x80x9cEOS (end of scanning)xe2x80x9d stand for the start timing and the end timing of scanning, respectively. Further, timings for switching on and off laser diodes LD1, LD2 to emit laser beams are represented under headings of xe2x80x9cLD1xe2x80x9d and xe2x80x9cLD2xe2x80x9d, respectively. Timings of performing APC for the LD1 and LD2 are represented under headings xe2x80x9cAPC/LD1xe2x80x9d and xe2x80x9cAPC/LD2xe2x80x9d, respectively, and the timing at which the BD detects the laser beam is represented under a heading of xe2x80x9cBDxe2x80x9d.
As shown in FIG. 8, in the case where there are secured time durations separately for performing APC for the LD1 and LD2 and a time duration of BD beam detection, it is inevitable that the time durations for performing APC are shortened. Accordingly, the multibeam scanner does not have enough time to perform APC to precisely determine driving currents for the LD1 and LD2, and is incapable of stabilizing the laser beams emitted therefrom. There have been attempts to work around such a problem, in terms of circuitry of the multibeam scanner, by providing an A/D converter. However, this complicates the structure of the multibeam scanner and, at the same time, makes its cost-performance worse.
Also, in the case where the multibeam scanner scans at short scan cycle by, for example, speeding up the rotation of polygon mirror, the multibeam scanner does not get sufficient time for performing APC for the laser diodes. Thus, the laser beams will not be sufficiently stabilized.
The invention has been developed to resolve the above-mentioned and other problems, and especially to provide a multibeam scanner capable of scanning with laser beams with steady light amounts at a high scan cycle.
According to the invention, there is provided a multibeam scanner for scanning an image forming area with a plurality of laser beams and forming a plurality of scanning lines corresponding to the plurality of laser beams, comprising:
a plurality of beam emitting points that emit a plurality of laser beams, respectively;
a scan position changing unit that changes scan positions which the laser beams scan in the image forming area and in a non-image forming area;
a timing controller that controls, in synchronism with the scan position changing unit, each of the plurality of beam emitting points to emit the laser beam during a respective beam emitting period one by one in the non-image forming area, the timing controller controlling the beam emitting point that last emits the laser beam among the plurality of the beam emitting points to emit the laser beam at a predetermined beam detecting position in the non-image forming area during its beam emitting period;
a photoreceptor that detects each of the laser beams emitted one by one under the control of the timing controller, and outputs a beam amount signal indicating a beam amount of the laser beam;
a beam amount controller that controls, based on the corresponding beam amount signals, the beam amounts of the laser beams emitted by the beam emitting points, respectively;
a scan start timing detector that detects the last emitted laser beam at the predetermined beam detecting position in the non-image forming area, and determines a scan start timing based upon the detection of the laser beam at the beam detecting position; and
a scan controller that controls the beam emitting points, based on the scan start timing, to emit the laser beams, each of which beam amounts being controlled by the beam amount controller, thereby forming the plurality of scanning lines in the image forming area.
According to the invention, the multibeam scanner performs APC on each of the plurality of beam emitting points one by one during corresponding time durations, respectively, in the non-image forming area. The laser beam emitted last among the plurality of laser beams is controlled to reach the beam detecting position within the corresponding time duration by the timing controller. The beam detector detects the laser beam emitted at the beam detecting position, which has been stabilized during APC, and determines the scan start timing with very high precision.
In other words, the time duration provided to the beam emitting point that last emits the laser beam under the control of the timing controller, is used not only for performing APC for that last emitted laser beam, but also for identifying the scan start timing by using the beam detector. This allows the laser diodes to avoid wasting time again and again, as rise time for stabilizing the laser beam, and to secure the time durations as long as possible. It is therefore possible to perform APC against the laser diodes properly during the sufficient time durations and, at the same time, possible to determine the scan start timing precisely by detecting the stabilized laser beam.