The invention relates to raster output scanners, and more particularly, to a technique for producing both synchronization timing signals and laser beam intensity signals using a single light sensor. 
     The following patent assigned to the assignee hereof is incorporated by reference: U.S. Pat. No. 4,952,022. 
     BACKGROUND OF THE INVENTION 
     Electrophotographic marking is a well known method of copying or printing documents by exposing a substantially uniformly charged photoreceptor to an optical light image of an original document, discharging the photoreceptor to create an electrostatic latent image of the original document on the photoreceptor&#39;s surface, selectively adhering toner to the latent image, and transferring the resulting toner pattern from the photoreceptor, either directly to a marking substrate such as a sheet of paper, or indirectly after an intermediate transfer step. The transferred toner powder image is fused to the marking substrate using heat and/or pressure to make the image permanent. Finally, the surface of the photoreceptor is cleaned of residual developing material and recharged in preparation for the creation of the next image. 
     While many types of light exposure systems have been developed, a commonly used system is the raster output scanner (ROS) comprised of a laser beam source, a means for modulating the laser beam (which, as in the case of a laser diode, may be the action of turning the source itself on and off) so that the laser beam contains image information, a rotating polygon mirror having one or more reflective surfaces, pre-polygon optics for collimating the laser beam, post-polygon optics to focus the laser beam into a well-defined spot on the photoreceptor surface and to compensate for the mechanical error known as polygon wobble, and one or more path folding mirrors to reduce the overall physical size of the scanner housing. The laser source, modulator, and pre-polygon optics produce a collimated laser beam which is directed to strike the reflective polygon facets. As the polygon rotates, the reflected beam passes through the post-polygon optics and is redirected by any folding mirrors to produce a focused spot that sweeps along the surface of the charged photoreceptor in a straight scan line. Since the photoreceptor moves in a direction substantially perpendicular to the scan line, the swept spot covers the entire photoreceptor surface in a raster pattern. By suitably modulating the laser beam in accordance with the position of the exposing spot at any instant, a desired latent image can be produced on the photoreceptor. 
     To assist the understanding of the present invention, several things should be noted and described in further detail. First, the phenomenon known as scan line jitter is caused by the failure of pixels in successive scan lines to be precisely aligned with each other. It is common practice to position a photodetector element in the scan line path just ahead of the latent image area in order to establish an accurate measure of beam timing on successive scans. When the laser beam crosses the photodetector, a start-of-scan signal is produced which initializes the pixel clock controlling the data stream that modulates the laser beam. Second, in high quality imaging systems it is important that the laser beam have a stabilized intensity so that optimum exposure can be maintained. This enables optimization of the charging and development systems which are critical to producing high quality images. Having known beam intensities becomes even more important when multiple laser beams are used, such as in a color printer. Since the intensity of the laser beam from a laser source driven by a fixed current is strongly effected by operating temperature and changes with time due to aging, and since the output power of different laser sources driven by the same current can be quite different, the ability to dynamically regulate the intensity of the laser beams is important. Such regulation is typically implemented using a dedicated photodetector. 
     Normally, the production of the start-of-scan signal and the regulation of the laser beam intensity are carried out independently with separate photodetectors and separate preamplifiers, plus sufficient electrical support which includes connectors, wiring, and physical space for the two light sensing systems. The use of separate systems unnecessarily increases cost and both manufacturing and assembly overhead while potentially reducing system reliability. Therefore, a technique of achieving start-of-scan detection and dynamic beam intensity regulation using a single photodetector system for both functions would be beneficial. 
     SUMMARY OF THE INVENTION 
     The principles of the present invention provide for producing both a start-of-scan signal and a laser beam intensity control signal using a single photodetector. A raster output scanner according to the present invention is comprised of a laser source for generating a beam of laser light; a rotating polygon having at least one reflecting mirror facet for sweeping the laser beam along a scan line plane, an optical fiber with a light receiving end positioned at a predetermined location in the scan line plane to collect a portion of the light flux in the sweeping laser beam, and a photodetector for receiving the flux emitted from the exit end of the optical fiber and for converting the emitted flux into a beam-dependent electrical current. The raster output scanner further comprises a scan detection circuit for producing a start-of-scan signal from the beam dependent current, and a beam intensity circuit for producing an electrical output signal which depends upon the magnitude of the beam dependent current and thus upon the laser beam intensity. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 schematically illustrates an electrophotographic printing machine which incorporates the principles of the present invention; 
     FIG. 2 is a top view of the raster output scanner used in the electrophotographic printing machine illustrated in FIG. 1; 
     FIG. 3 schematically illustrates a network which produces a start-of-scan and which also establishes the beam intensity of the laser in the raster output scanner of FIG. 2; 
     FIG. 4 illustrates the operation of the start-of-scan detector used in the network shown in FIG. 3; 
     FIG. 5 shows the front of the optical fiber used in the raster output scanner of FIG. 2; 
     FIG. 6 illustrates the optical fiber of FIG. 5 during fabrication; and 
     FIG. 7 schematically illustrates a network which detects the start-of-scan and which establishes the beam intensity of four laser beams. 
     In the drawings, like numbers designate like elements, Additionally, the text includes directional signals which are taken relative to the drawings (such as right, left, top, and bottom). Those directional signals are meant to aid the understanding of the present invention, not to limit it. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     FIG. 1 illustrates an electrophotographic printing machine  8  designed to produce original documents. Although the principles of the present invention are well suited for use in such machines, they are also well suited for use in other devices. Therefore it should be understood that the present invention is not limited to the particular embodiment illustrated in FIG. 1 or to the particular application shown therein. 
     The printing machine  8  includes a charge retentive component in the form of an Active Matrix (AMAT) photoreceptor  10  which has a photoconductive surface and which travels in the direction indicated by arrow  12 . Photoreceptor  10  is mounted on drive roller  14  and tension rollers  16  and  18 , with drive roller  14  turned by drive motor  20 . 
     As the photoreceptor advances, each part passes through the subsequently described processing stations in sequence. For convenience, a single section of the photoreceptor, referred to as the image area, is identified. The image area is the part of the photoreceptor processed by the various stations to produce toner layers. While the photoreceptor may have numerous contiguous image areas, each is processed in the same way. Therefore, a description of the processing of one image area suffices to explain the operation of the printing machine. 
     As the photoreceptor  10  advances, the image area passes through a charging station A. At charging station A a corona generating scorotron  22  charges the image area surface to a relatively high and substantially uniform potential, for example −500 volts. While the image area is described as being negatively charged, it could be positively charged if the voltage levels and polarities of the other relevant sections of the printing machine are appropriately reconfigured. It is to be understood that the scorotron  22  is supplied electrical power as required for proper operation. 
     After passing through the charging station A, the photoreceptor is advanced to an exposure station at B where the charged image area is exposed by laser-based raster output scanning assembly  24  which illuminates the image area with a raster representation of a first color image, say black. The optical laser beam flux in the raster representation discharges the image area in a pattern corresponding to input control data thereby creating a first electrostatic latent image. While various aspects of the raster output scanning assembly  24  are described in more detail subsequently, it should be understood that the raster output scanning assembly includes an optical fiber  102  strategically placed in the path of the output laser beam  104  so that flux collected by optical fiber  102  is guided to sensor network  106 . Sensor network  106  is configured to generate both an output start-of-scan signal  108  and an output beam intensity signal  110  from the detected flux in a manner which is subsequently described. 
     After passing through the exposure station B, the exposed image area passes through a first “discharged area development” station C where a negatively charged development material  26  comprised of black toner particles is advanced to the image area. The development material is attracted to the less negative discharged sections of the image area and repelled by the more negative unexposed sections. The result is a first toner pattern on the image area that corresponds to the first electrostatic latent image. It will be recognized by those practiced in the art that the present invention can be applied in the case of charged area development, and that the development structures illustrated in FIG.  1  and labeled C, F, G, and H, are of a design suitable for advancing toner particles suspended in a liquid solution to the surface of photoreceptor  10 . However, it should be understood that the present invention is not limited to the particular embodiment shown therein. 
     After passing through the first development station C the image area advances to a transfusing module D that includes a positively charged transfusing member  28 , which may be a belt as illustrated in FIG. 1, or a drum, forming a first transfer nip  29  with the photoreceptor surface. The first transfer nip is characterized by a first region of compression or pressure between the photoreceptor  10  and the surface of transfusing member  28  where negatively charged toner layer on the photoreceptor is attracted by the positive potential of the transfusing member. 
     After the first toner image is transferred to the transfusing member  28 , the image area passes to a cleaning station E which removes untransferred development material and other residue from the surface of photoreceptor  10  using one or more cleaning brushes contained in housing  32 . 
     The image area is advanced through the charge-expose-develop-transfer-clean sequence for a second color of developer material (say yellow). Charging station A recharges the image area and exposure station B illuminates the recharged image area with an optical raster representation of a second color image (yellow) to create a second electrostatic latent image. The image area is advanced to a second development station F where second negatively charged development material  34  comprised of yellow toner particles is deposited on the image area in a pattern corresponding to the second electrostatic latent image. The image area and adhered toner pattern advances to the transfusing module D where the second color toner is transferred to the transfusing member  28 . 
     The image area is cleaned by the cleaning station E, and the charge-expose-develop-transfer-clean sequence is repeated for a third color of development material  36  (say magenta) using development station G, and finally for a fourth color  38  (cyan) of development material using development station H. 
     The transfusing member  28  is entrained between a transfuse roller  40  and a transfer roller  44 . The transfuse roller is driven at constant velocity by a motor, which is not shown, such that the transfusing member advances in the direction  46  at the same velocity as photoreceptor  10 . The spacing between successive image areas is regulated to match the circumference of transfusing member  28  to maintain mechanical synchronism and allow the various toner images to be transferred to the transfusing member  28  in proper registration. 
     Still referring to FIG. 1, transfusing module D includes a backup roller  56  which rotates in direction  58 . The backup roller  56  located opposite the transfuse roller  40  forms a second nip with the transfusing member  28  which is under pressure and acts as a transfusing zone. When a substrate  60  such as paper passes through the transfusing zone, the composite toner layer on the surface of transfusing member  28  is heated by thermal energy accumulated from a radiant preheater  61  or from a conductive preheater  62 , and heat conducted directly from the transfuse roller  40 . The combination of heat and pressure in the nip fuses the composite toner layer onto the substrate surface making a permanent color image. 
     The present invention is functionally associated most closely with the raster output scanning assembly  24 . Referring now to FIG. 2, the raster output scanning assembly  24  includes a modulated laser diode  150  which is excited to form laser beam  104  according to input image data from a data source and laser driver  152  (which may be physically remote from the raster output scanning assembly  24 ). The output flux from laser  150  is collimated by optical element  154  and reflected by fold mirror  156 . The collimated beam is then focused on reflective facets  157  of rotating polygon  158  by cylindrical lens  160 . Each facet of rotating polygon  158  deflects the beam which is focused into a well defined spot on the surface of photoreceptor  10  (also see FIG. 1) by scan lens elements  162  and  164 . 
     As polygon  158  rotates, the sharply focused spot formed by laser beam  104  traces a narrow path on the surface of photoreceptor  10  that defines the scan line. The input end  166  of the optical fiber  102  is positioned along the scan line path just ahead of the active image area of photoreceptor  10  and oriented to collect light flux from beam  104  that is incident directly on the fiber end. The optical fiber transmits the intercepted light flux to the sensor network  106 . 
     FIG. 3 illustrates the sensor network  106  in more detail. Light flux emerging from the output end  168  of optical fiber  102  is directed onto a fast photodetector  170 . The photodetector is reverse biased with its cathode connected via a resistor  172  to a power supply  174 , and its anode connected to a common voltage node comprising one lead of a capacitor  176 , the drain of a field effect transistor  178  in a common source configuration, and the input of a voltage comparator as is subsequently described. Light flux collected when the scanned spot formed by laser beam  104  strikes the input end of the optical fiber illuminates the photodetector. The resulting photoinduced current pulse develops a voltage pulse on the cathode of the photodetector and delivers a charge pulse to the capacitor  176 . As is described below, the voltage pulse on the cathode of the photodetector is used to generate a start-of-scan logical transition essentially calibrating the data bit stream clock with respect to the starting time of each scan. The rate at which the capacitor voltage increases as current pulses accumulate charge on the capacitor depends on the collected light flux which is used to control the intensity of the laser beam  104  through the operating level of modulated laser diode  150 . 
     Still referring to FIG. 3, the voltage pulse on the photodetector cathode passes through DC blocking capacitor  180  to buffer amplifier  182  and drives delay line  184  and attenuator  186  in parallel. The time delayed pulse delivered by delay line  184  is applied to the non-inverting input of a fast comparator  188 , while the attenuator  186  output is applied to the inverting input of the comparator. The comparator generates a start-of-scan positive going logical transition output  108  that is invariant with respect to the amplitude of the pulse delivered by buffer amplifier  182 . 
     The operation of the comparator  188  is described with reference to FIG.  4 . The temporal profile of the attenuator output is illustrated by trace  200 , while the output of the delay line on the same scale for the same pulse input is illustrated by trace  202 . The comparator input is biased so that the quiescent voltage of input  b  always exceeds the quiescent voltage of  a  by a small margin. A voltage pulse delivered to the parallel inputs of the attenuator and delay line causes the voltage of trace  202  to exceed the voltage of trace  200 . At the crossover point, the comparator output is a positive going pulse transition or edge indicated by trace  206  which defines the start-of-scan signal  108 . It will be understood by those practiced in the electronic art that when the amplitudes of traces  200  and  202  are proportional, the crossover point can be chosen to coincide with the steepest rising slope of trace  202  in order to provide the least uncertain timing of the start-of-scan signal. It will also be understood that the flat portion at the top of trace  200  can be a natural consequence of the focused spot formed by laser beam  104  being smaller than the aperture of the input end  166  of optical fiber  102 . As shown in FIG. 6, a fiber aperture of about 0.005 inches is substantially larger than the focused spot of about 40 microns associated with a printing machine  8  capable of imaging 600 spots per inch. Electronic pulse stretching means can also be provided as part of the network defining amplifier  182  to ensure that traces  200  and  202  exhibit relatively flat top portions. 
     Refer now once again to FIG. 3 for a description of the components used to control the laser beam intensity. The gate of the field effect transistor  178  is controlled by state machine  210  which is clocked by the start-of-scan signal  108 . The voltage on the capacitor  176  is buffered by amplifier  212  and is applied to the non-inverting input of comparator  214 . The inverting input is established at a reference voltage from a voltage divider  216 . As shown in FIG. 3 the output of the comparator  214  controls the incrementing direction of an Up/Down counter  218 , which is clocked by the state machine  210  on a line  211 . The Up/Down counter contains a binary integer which determines the analog output level of digital-to-analog (D to A) laser diode current control circuit  220 . The D to A output level  110  controls the drive current applied to the laser diode, and thus the intensity of the laser beam  104  in the “on” state. 
     The operation of the components used to control the laser beam intensity will now be explained. First, after receipt of a start-of-scan signal indicating that the laser beam is positioned at the critical point in the input aperture of optical fiber  102 , the state machine  210  drives the gate of the field effect transistor  178  positive and fully discharges capacitor  176 . After a fixed discharge time, the field effect transistor is turned off and a predetermined number of start-of-scan events is counted. Each time the laser beam sweeps across the optical fiber, the photoinduced current from the photodetector  170  accumulates in capacitor  176 . The capacitor voltage is buffered by amplifier  212  and compared with the fixed voltage from the voltage divider  216  by the comparator  214 . When the predetermined number of start-of-scan events is reached, the output of the comparator  214  will be in one of two states depending on the capacitor voltage which is a measure of the accumulated charge. At the next start-of-scan event the state machine  210  applies a clocking transition to the Up/Down counter  218  via line  211 . If the state of the comparator output indicates that the capacitor voltage is less than the fixed reference voltage, the binary integer stored by the Up/Down counter is incremented such that the beam intensity control signal causes the laser drive current, and thus the laser beam intensity, to increase. Conversely, if the state of the comparator indicates a capacitor voltage higher than the fixed reference voltage, the contents of the Up/Down counter is decremented, reducing the laser drive current, and thus decreasing the laser beam intensity. 
     Beneficially the optical fiber  102  has a shaped receiving end  166  of relatively constant width, which delivers fast rise and fall time light pulses to photodetector  170 . Referring now to FIG. 5, the receiving end is elongated and is orientated with the laser beam  104  sweeping along a line substantially perpendicular to the axis of elongation. Beneficially, part of the optical fiber is embedded in epoxy  230  such that the receiving end may be conveniently adjusted and spatially fixed in the path of the laser beam  104 . 
     Referring now to FIG. 6, a major step in fabricating the receiving end  166  is to heat and reshape an optical fiber to have an elongated cross-section about 5 mils thick. The elongated portion is severed and can produce pairs of optical fibers with elongated ends. The elongated fiber ends are orientated and potted with epoxy in a ferrule or other suitable housing (with the body of the optical fiber remaining exposed) for mounting in the electrophotographic printing machine  8 . The elongated end is machined flat and polished with a polishing paste at a predetermined angle. The output end  168  of optical fiber  102  may be similarly potted in a suitable housing for ease of mounting in the electrophotographic printing machine  8 . Because of the specially fabricated receiving end  166 , the optical fiber is beneficially comprised of a plastic optical fiber. 
     For examples of methods for forming the ends of optical fibers, reference is made to U.S. Pat. No. 4,952,022 which is hereby incorporated by reference. 
     While the foregoing has described a raster scanner assembly which uses a single laser diode, the principles of the present invention are equally applicable to multiple laser diode raster scanner assembly systems. For example, FIG. 7 illustrates in schematic form a network which can produce start-of-scan signals and beam intensity control signals in a four laser diode raster scanner assembly system. It is to be understood that the optical fiber  102  is placed in the path of the four sweeping laser beams, and that those beams have a predetermined spatial and temporal relationship. In addition, it is to be understood that the state machine  302  sequentially controls which one of the four laser diodes is operational and provides illumination during the short period that the start-of-scan pulse is generated (the others being turned off). 
     Assume in the following that a laser diode which produces a beam  1  is illuminated at the initiation of the scan and generates a start-of-scan transition as described above. In response to the start-of-scan input, state machine  302  first drives the gate of the field effect transistor  178  positive for a fixed discharge time to fully discharge capacitor  176 , and then turns the field effect transistor off to allow photoinduced currents in photodetector  170  to accumulate on capacitor  176  for a predetermined number of scans. The voltage on the capacitor is buffered by amplifier  212  and compared with the reference voltage from voltage divider  216  by the comparator  214 . The output of the comparator, which is in one of two states, is applied to the direction control of all four Up/Down counters  304 . One of those, designated A, is related to the beam  1 . The four counters are also connected via a common clock line to the state machine  302 . In addition, each counter is connected to the state machine by an individual enable line  310 . One of those enable lines, designated B, is related to the Up/Down counter designated  A  (and thus to the beam  1 ). When the predetermined number of start-of-scan events has been reached, the state machine  302  applies a true state on the enable line designated  B  followed by a clock transition to the clock input of all of the Up/Down counters in parallel. Only the Up/Down counter  304  designated  A  reacts to the state of the comparator  214  and the clock transition from the state machine  302 . If the state of the comparator is such that the capacitor voltage is less than the fixed reference voltage, the integer stored in the Up/Down counter designated  A  is incremented causing laser diode current control  320  to increase the laser beam intensity of beam  1 . Conversely, if the state of the comparator indicates a capacitor voltage higher than the reference voltage after the designated accumulation cycles, the contents of the Up/Down counter designated  A  is decremented, reducing the laser drive current, and thus the laser beam intensity of beam  1 . 
     To control the beam intensity of all four laser diodes, each diode is selected sequentially in turn by the state machine  302  through select signals  300 , and each is regulated by incrementing or decrementing its associated Up/Down counter to control the individual currents and hence the individual intensities. Since the spatial and temporal relationship between the individual diodes are know, the synchronization of the modulation of the individual laser diodes can be calibrated to prevent scan line misalignment. It will be understood by those in the electronic art that the basic control system outlined in FIG. 7 can be extended so that the direction of each of the Up/Down counters  304  is controlled by separate reference voltage sources and separate comparators connected to the output of a common buffer amplifier  212  thereby allowing individual intensity calibration of the four laser diode sources. It will be further understood that the regulating cycle of state machine  302  can be enhanced to include a “rapid startup mode” where the contents of the Up/Down counters are initially zeroed and incremented or decremented by more than one clock pulse on each cycle when the difference between the voltage on capacitor  176  and the reference voltage exceeds a predetermined threshold. 
     It is to be understood that while the figures and the foregoing description illustrate the present invention, they are exemplary only. Skilled workers in the applicable arts will recognize numerous modifications and adaptations which will remain within the principles of the present invention. Therefore, the present invention is to be limited only by the following claims.
A raster output scanner in which both the start-of-scan and the beam intensity of a laser beam are determined using a single photodetector. The raster output scanner has a laser source for generating a laser beam; a rotating polygon for sweeping the laser beam along a scan line plane, an optical fiber with a light receiving end positioned at a known position in the scan line plane and which intercepts at least a portion of the sweeping laser beam, and a photodetector for converting the intercepted laser beam into a beam current. The raster output scanner further includes both a scan detection circuit for producing a star-of-scan signal from the beam current and a beam intensity circuit for producing an electrical output which depends upon the magnitude of the beam current.