Abstract:
In a laser printer wherein a photoconductive drum is scanned by utilizing a plurality of laser beams, there are detected spacings between the plural laser beams, such detection being confined to a blanking time. On the basis of the detected results, the spacings between the plural laser beams are controlled; and this state is maintained for a printing time. With this arrangement, it is possible to accurately detect the spacings between the plural laser beams thereby to obtain the practicable laser printer employing the plural laser beams.

Description:
BACKGROUND OF THE INVENTION 
     The present invention relates to a laser printer which is capable of printing high grade characters and images by scanning laser beams. 
     A conventional laser printer whose printing velocity is medium or low usually utilizes a single diode laser beam. On the other hand, a printer in which the printing is effected at an extra-high speed is at present arranged such that the single gas laser beam is intactly employed, or this single gas laser beam is divided into plural gas laser beams by means of an acoustic optical deflector; and both the frequency band region of a modulator and the rate at which a rotating polygonal mirror rotates are reduced by scanning the plural gas laser beams simultaneously. 
     In either case, however, so far as the gas laser is employed as mentioned above, it is impossible to steer clear of the large consumption of electricity and miniaturize the apparatus. Under such circumstances, there is recently focussed an introduction of a semiconductor laser which is capable of executing direct modulation for itself. However, this kind of semiconductor laser involves defects wherein the wavelength thereof is 1.5˜2-fold as long as that of the gas laser, which requires a large-sized configuration of the rotating polygonal mirror, and it has been quite difficult to constitute a laser printer by using a single laser beam on account of deteriorated photoconductivity which responds to a long wavelength of the foregoing semiconductor laser. 
     Such being the case, there is a growing expectation in a method where parallel scanning is effected by making use of a plurality of semiconductor laser beams. Some means for actualizing this end have been disclosed in the specifications of U.S. patent application No. 680497, West German patent application No. P3445751.8, U.S. patent application No. 804940 and West German patent pplication No. P3543472.4 with which some of the inventors of the present invention are associated. However, provided that the printing is performed during the detection of the spacings between plural semiconductor laser beams, it is inconveniently unfeasible to accurately detect it. 
     SUMMARY OF THE INVENTION 
     Accordingly, it is a primary object of the present invention which obviates the above-described problems to provide a laser printer which is capable of precisely detecting spacings between a plurality of laser beams thereby to control the desired spacings. To accomplish this end, the present invention is characterized such that there is prescribed a timing at which to detect the spacings between the aforementioned plural laser beams. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a view showing a constitution of a laser printer of one embodiment according to the present invention, the laser printer utilizing a plurality of laser beams to control the spacings therebetween; 
     FIG. 2 is a view showing the timing at which locations of the laser beams relative to this invention are detected; and 
     FIG. 3 is a view showing a constitution of a photodetector for detecting the spacings between the plural laser beams relative to the present invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Referring now to FIG. 1, there is shown a laser printer which employs two semiconductor lasers of one embodiment according to the present invention. 
     Laser beams respectively emitted from two lasers 11, 12 pass through a synthesizing prism 5 where the laser beams are synthesized in the same direction at such a spacing that the two laser beams slightly deviate from each other. In the wake of this, the beams are directed toward a rotating polygonal mirror 6 and then turns to an Fθ lens 7 serving as a scanning lens. On a photoconductive drum 8, the two laser beams are adjusted to a predetermined spacing, and at the same time, it is required to render this spacing immutable. As shown in FIG. 1, for this purpose is proposed an apparatus in which the spacing between the two laser beams is read with the aid of a divided detector 13 by making use of the light which is partially leaked out of the prism 5 designed for synthesizing the beam; and the above-described spacing is controlled by virtue of the resultant signal. 
     The laser printer employing the semiconductor, however, creates the following drawback. It is feasible for the laser itself to modulate at a printing time, so that the modulated light enters the divided detector 13. As a result, intensity of the laser beams varies and hence it is difficult to detect the position thereof. Consequently, as for the present invention, the arrangement is such that the detection is carried out within a blanking time for which the printing is not effected; the spacings between the laser beams are controlled on the basis of the thus detected results; and this state is maintained in a printing time. If the rotating polygonal mirror is commonly used, an angle at which the light is theoretically able to scan in the case of an n-faced mirror can be given by: 
     
         360/n 
    
     Inasmuch as the light which is incident upon the rotating polygonal mirror usually has a specified magnitude, it is common that the actual angle at which the photo-scanning is performed on the polygonal mirror comes to 60-70% of the above-described theoretical value. 
     Accordingly, a period of 60-70% is, as described above, employed for printing, whereas a period of 30-40% is defined as non-scanning time. Generally speaking, this non-scanning time is considered to be rather disadvantageous, since effective utilization of the laser beams is unfeasible during this time. The present invention, however, is characterized in detecting the position by setting the two laser beams in a state of &#34;ON&#34; for this blanking time. FIG. 2(a) shows the timing relative to modulation of semiconductor lasers (marked with 11, 12 in FIG. 1) of an embodiment according to the present invention. Namely, in case that the positional control is conducted with respect to the two laser beams which are respectively emitted from the semiconductor lasers 11, 12 by the use of a signal (hereinafter referred to as a spacing error signal) which is detected for the blanking time, it is, as mentioned before, not easy to obtain the spacing error signal for the printing time. Hence the so-called sample holding mode is adopted, wherein the aforecited positional control is performed by using the spacing error signal obtained for the blanking time only, and such a state is arranged to be kept for the printing time. A blanking time 9 can be detected as follows. It can be observed through FIG. 1 that the photodetector 3 is disposed outside a print starting position on the scanning line for the laser beam, this photodetector 3 being designed for setting the start timing at which the laser beam is scanned. To be specific, the time just when the scanning laser beam passes through the photodetector 3 is defined as the scanning start timing. The timing positions are indicated by the reference numeral 120 in FIGS. 2(a), 2(b). It is to be noted that FIG. 2(b) shows the timing of an output signal emitted from the photodetector 3. A width T of a scanning period 10 for which the scanning is effected with respect to a predetermined width measured from the point 120 is expressed such as: 
     
         T=L/(4πnf) 
    
     where L is the printing width of a rotating polygonal mirror, n is the number of revolution thereof, and f is the focal length of the Fθ lens 7 employed therein. After traversing the width T, the blanking time 9 is present. For this period, the laser beam continue to light up on direct current. Namely, the blanking time 9 is a time till the laser beam in the next scanning plan comes to the photodetector 3 again. The divided type photodetector 13 detects the spacing error for the blanking time. A configuration of such a photodetector is, as described at full length in the specifications of U.S. patent application Ser. No. 680,497 or West German patent application No. P3445751.8, typified by that of the photodetector 13 shown in FIG. 3. The photodetector 13 is constituted by two pairs of divided detectors 141, 142 and 143, 144 and is arranged to read the positions of the respective laser beams by a differential motion. 
     In FIG. 3, suppose that the laser beams on the detectors 151, 152 are so located as to be impartially applied to the divided detectors 141, 142 and 143, 144, the foregoing differential output is zero. However, if the beams are located so that they are asymmetrically applied thereto, there appears a differential signal which displays the differential output corresponding to the positions of the laser beams. Consequently, there are provided controllers 61, 62 consisting of, for example, a gain adjusting unit or the like which has a function to effect a negative feedback with a view to controlling actuators 21, 22. With this arrangement, the differential signal remains to be zero, this signal being obtained by means of differential devices 41, 42 (for instance, a differential amplifier or the like) shown in FIG. 1. 
     Coupling lenses 1, 2 are moved by dint of the motion of the actuators 21, 22 in such a way that the surfaces of the lenses are so directed as to be perpendicular to the optical axes, whereby two spots irradiated with the laser beams are adjusted in order that no positional deviation is produced. 
     With the aid of the signal (See FIG. 2(b)) created when the scanning laser beam shown in FIG. 1 passes through the detector 3, the controller 60 shown in the same Figure performs controlling operations with respect to &#34;ON&#34; and &#34;OFF&#34; of the laser beam, the sample holding processing, detection of the spacing error signal and the timing of various kinds of operations as in the case of the actuators 21, 22 by which the above-described differential signal comes to zero. 
     As can be clarified from the description thus far made, the present invention yields the following effects. 
     An output-load of the laser for use is reduced by executing the positional control of the aforementioned laser beams. Moreover, it is feasible to actualize an extra-speed laser printer which is capable of effecitively decreasing a light modulation frequency of the laser.