Scanning Beam Detector

The invention broadly relates to a radiant energy beam scanning system. More particularly, the invention concerns a method and means for generating timing and power level reference signals responsive to the detection of the center of the power distribution within the beam at a predetermined point along a scanned path.

In conventional light beam recording devices, an intensity modulated light beam (e.g. a laser) is repetitively scanned across the surface of a photosensitive medium to produce an imagewise rendition of the desired information. The cyclical scanning pattern is typically established with the use of a moving beam deflector, such as a rotating polygonal mirror or an oscillating galvanometer mirror.

For quality image resolution, the recorded characters are usually formed as composite images of individual picture elements generated on consecutive scan lines. To avoid blurred or deformed characters, these individual picture elements must be aligned, from scan to scan, in the vertical, or cross-scan, direction (i.e., orthogonal to the direction of scan). Various forms of detectors have been proposed heretofore to minimize the effects of this form of image deformation, commonly referred to as jitter. Normally such detectors are positioned at the start of each scan line at a predetermined distance from the edge of the recording surface. A signal generated by the detector is utilized as a timing, or synchronizing, signal to assure that the modulation of the beam begins in precisely the same vertical plane of the beam relative to the edge of the recording surface.

In most commercially feasible systems, however, such detectors do not totally compensate for jitter introduced by irregularities in the beam deflecting mechanism, such as the lack of reflective or surface flatness uniformity between adjacent facets of the polygonal mirror, rotational asymmetry associated with the polygon drive mechanism, and the like. While it is possible to correct for such unavoidable jitter by manufacturing the polygon and drivers to strict tolerances, such measures are generally cost prohibitive for commercial printers.

The foregoing effects are compounded in the systems which employ a solid state diode laser as the recording beam. As is quite well known, the power output of such lasers varies both spatially and in amplitude over time. Conventional beam detectors do not compensate for these variations and, hence, unavoidably produce jitter when utilized in a diode laser environment. Exemplary of these known detectors are the so called slit detectors, which compare the amplitude of a photodiode output signal against a predetermined, fixed reference voltage. When the amplitude of the diode signal passes through this reference threshold, an indicator signal is generated. With a diode laser scanning system, the intensity distribution pattern of the formed beam is generally gaussian. The signal generated by the detector diode will track, in amplitude, this gaussian shape as the beam sweeps across the face of the detector. The outputs produced by beams having different power levels will, necessarily, pass through the fixed reference level at different relative times, resulting in the generation of indicator signals at different points in time in relation to the time reference base of the sweep of the beam. Since the synchronization of the scanner system is keyed to the time difference between the generation of the indicator signal and the transit time of the beam from the detector location to the targeted edge of the recording medium, this differential triggering effects a translation of information horizontally, or in the scanned direction, from line to line so that the picture elements do not align properly in the cross-scan direction. These effects are present for all fixed reference single edge detectors, whether triggered by the leading or trailing edge of the diode output signal.

A known means for overcoming the problem associated with comparing against a fixed reference, is the so called split detector. Such detectors utilize a two photodiode-dual comparator configuration to compensate for variation in beam output power. In operation, the sweep of the beam over the first detector sets an associated first comparator. The output of this first comparator is supplied as a reference for the second comparator, which is thereafter triggered by the sweep of the beam across the second diode detector to provide the indicator signal. While this form of detector performs quite satisfactorily, there is an attendant cost tradeoff.

In accordance with the present invention there is provided a means and technique for detecting a scanning beam of radiant energy which enables synchronization of a radiant energy beam scanning system relative to the center of the power distribution within the beam so that there is proper alignment of information recorded on consecutive scan lines. Compensation is provided for variations in both the power level and focus of the beam. The invention is particularly well suited for use as a start of scan detection in a diode laser based recording system.

In a preferred embodiment, a single diode detector, partially masked in a cross-scan direction, is located at a predetermined position in a scan path of the beam. The output signal produced when the diode detector is irradiated by the scanning beam is coupled to an input of an op amp voltage follower. The output of the op amp is filtered and stored as a reference signal on a capacitor. Since the output of the voltage follower is a unity gain, in phase replica of the applied diode output signal, the stored reference signal represents the average total power within the incident beam. This reference signal is divided by a fixed ratio (preferably a division in half) and supplied to the non-inverting input of a comparator. The diode output signal, which is coupled to the non-inverting input, initially sets the comparator low. As the beam crosses the edge of the mask, the detector output signal decreases rapidly towards the zero level, pulling the non-inverting terminal of the comparator with it. When this falling output signal reaches the level of the fixed ratio of the reference signal stored on the capacitor, the comparator switches high, creating the indicator signal. When the division ratio is selected to divide in half the average total power level (as stored on the capacitor associated with the output of the op amp) the generation of the indicator signal coincides with the passage of half the beam power across the edge of the mask. Since half of the detected beam power still remains within the unmasked active diode area, it will be seen that the mask edge provides a reference point along the scan path which marks the center of the beam power distribution coincident with the generation of the indicator signal. For a gaussian-like distribution of beam power, the synchronization of the system relative to this detection of the beam power center enables positioning the maximum "writing" energy where intended on the recording medium. Since the centers of the beam power distribution will likewise be detected in consecutive scans, the respective spatial positions of the beam power centers will also be positioned in the desired locations on the recording medium. Thus, marks intended to be made will, in fact, be made in proper relation to previously recorded marks.

In addition to avoiding the problem of jitter by compensating for variations in beam power level and focus, there is also provided, by virtue of the potential developed and stored on the capacitor, a DC reference signal which is representative of the average total power of the beam. This reference signal is available for feedback control of the beam source. When utilized as a start of scan detector, this signal enables correction of the output power level of the beam source at the beginning of a scan line, before the initiation of recording.

FIG. 1 is an electrical schematic diagram of the preferred detector according to the invention.

FIG. 2 is a front view of a masked detector diode.

FIG. 3 shows various signals provided by the apparatus of FIG. 1.

Referring initially to FIG. 2, there is shown a front view of a diode detector 10, across which there is shown a beam spot 20 travelling in the direction of arrow 22. Since the present invention has applicability to any radiant energy beam scanning system, it will be appreciated by those having skill in the art that various means could be employed to effect the transit of beam spot 20 across detector 10. Since the invention functions equally well with an out of focus beam, it is likewise not essential that the beam be formed into a circular, or otherwise symmetrical, spot as shown in FIG. 2. To the contrary, the invention is expressly intended to compensate for such variations in beam spot size, shape, or spatial power distribution. It is only necessary that the active diode area 12 be electrically responsive to the presence of the particular radiant energy beam being utilized. In the diode laser context herein described, it has been found that a pin photodiode functions quite satisfactorily. Although not drawn to scale, FIG. 2 suggests the relative dimensions of an approximately 4.2 mils beam spot traversing a detector active area of approximately 100 mils. It will be seen that an out of focus, or "sloppy", spot will also fully irradiate the active diode area. As will be discussed more fully hereinafter, the power level of the beam is sensed during its traverse of the active area 12, a proportional measure thereof being stored as a reference. After the beam passes the edge of mask 24, the active diode area 12 is no longer irradiated so that the output of detector 10 decreases. While it is preferred that the mask present a sharp edge in the cross-scan direction so that the detector responds sharply, it will be apparent that various other forms of a mask, or, in fact under certain conditions, no mask at all, may be employed consistently with the disclosure herein.

In the preferred configuration of FIG. 1, diode detector 10 is reversed biased to operate in a photoconductive mode to provide fast response. Accordingly, it functions to deliver an output current through a load resistor 14 when the beam 20 is incident upon the active diode area 12. The detector output signal, or voltage developed across load resistor 14, will, of course, rise and fall in amplitude in proportion to the power of the beam, since the preferred pin photodiode has a substantially linear response. The output signal so developed is coupled to the non-inverting input terminal 32 of operational amplifier 30. It will be readily apparent that, by virtue of the feedback to the inverting terminal 34, op amp 30 is configurated as a voltage follower so that the output thereof is a unity gain, in phase replica of the input voltage, i.e. the voltage developed by diode detector 10 and its load resistor 14 in response to the beam.

The output of op amp 30 is connected to ground through diode 31, resistor 36, and capacitor 38. Resistor 36 and capacitor 38 form a filter which functions to smooth or average, the maximum output of op amp 30 so that there is stored on capacitor 38 a reference signal which represents the average total power of the detected beam. Thus, any variations, or ripples, which are caused by fluctuations in the detected beam power level, will be smoothed out by the RC time constant of the filter, providing an average maximum value which is stored on capacitor 38. Diode 31 is included in the output circuitry of op amp 30 to enable retention of the charge on capacitor 38. Its presence allows capacitor 38 to remain charged while the detector output signal and, hence, the output of op amp 30, go toward zero. Thereafter, diode 31 permits slow decay of the charge on capacitor 38.

The reference voltage stored on capacitor 38 is proportionately divided by the network comprising resistors 33 and 35. The values of resistors 33 and 35 are selected so that a fixed percentage of the reference signal stored on capacitor 38 is supplied as a reference potential to the non-inverting input terminal 42 of comparator 40. Preferably the ratio is selected to deliver 50 percent of the potential stored on capacitor 38. With this selection, a signal representing 50 percent of the average total power of the detected beam is established as the reference for comparator 40. The output of the diode detector is directly coupled to the inverting input terminal 44 for comparison with the representative half power level.

Operation of this circuit is more easily understood with reference to the wave forms shown in FIG. 3. For illustrational purposes, the three wave forms are shown superposed in approximate time wise relationship to each other. It will be understood that each of the three curves is drawn relative to a unique amplitude axis, these axes not being shown in relative scale. Wave form A represents the detector output signal developed as beam spot 20 traverses the diode detector 10. As noted above, this signal is supplied to op amp 30 and, as well, to the inverting input 44 of comparator 40. Wave form B illustrates the reference signal supplied to the non-inverting input terminal 42 of comparator 40. This signal, it will be recalled, is derived through division of the potential stored on the capacitor 38. Curve C represents the output wave form of comparator 40 in response to the input signals shown in curves A and B.

When beam spot 20 enters the active diode area 12, detector 10 produces a current which develops an output voltage V.sub.o which rises in accordance with the beam power level as illustrated in curve A of FIG. 3. This transition is sensed by comparator 40, which sets, or switches, low as shown in wave form C. The total beam power is sensed as the beam transits the active diode area 12 producing the initial slight overshoot of wave form A, followed by a generally flat maximum output level. When the beam passes into the masked region, the output falls sharply as indicated in curve A. As shown by wave form B, the potential applied as reference to the non-inverting input terminal 42 of comparator 40 rises to a level representative of the 50 percent power level. Because of the RC time constant of resistor 36 and capacitor 38, once developed, this potential is maintained at a generally even level, against which the diode output is compared by comparator 40. When this detector output falls to a half power level (V.sub.o/2), comparator 40 triggers, or goes high, as shown by wave form C this transition provides the indicator signal.

Referring to wave form B, it will be noted that this curve also represents the shape of the voltage developed across capacitor 38. It will be appreciated that the filtered output of the voltage follower op amp 30 will rise and stabilize at an amplitude equal to the average maximum amplitude of the detector output signal. The level thus reached, and consequently stored on the capacitor 38, represents the average total power of the sweeping beam. Since this potential decays slowly relative to the transitions of the detector output signal and the comparator output signal, it is available concurrently with the generation of the indicator signal, i.e. when comparator 40 goes high. As suggested by the arrow adjacent capacitor 38 this measure of the total power level of the beam is available for feedback control of the beam generator.

With the selection of the preferred divide by two ratio for the resistive network, the generated indicator signal enables system synchronization so as to place the center of the power distribution within the beam at the point desired on the recording medium. Since the beam velocity as well as the distance between detector and target area on the record medium are known, this desired placement may be synchronized and provided in a known manner. The divide by two choice is most advantageous for achieving vertical alignment of picture elements in consecutive scans. By triggering always relative to the detection of the center position of the beam power distribution, the maximum available energy available for marking will always be positioned where intended on the recording medium.

Referring again to FIG. 3, curve B is shown as having an initial level of about 0.2 volts. This minimum reference potential for the positive input 42 of comparator 40 is provided to prevent noise from falsely triggering comparator 40. To prevent input 42 from going to ground, positive hysteresis is provided by resistor 46. To eliminate the error which would normally result when using hysteresis in signal detection, diode 48 is provided in the hysteresis loop. When the signal is present, this diode reverse biases, effectively removing resistor 46 from the circuit.