Parallel beam to beam power correction

An electro-optical system for adjusting beam to beam power non-uniformity in a multi-beam scanning system is disclosed. The electro-optical system consists of N×M array of light beams and N×M array of photodetectors wherein optical means deflect the light beams onto a photodetector array. A circuit consisting of N×M array of programmable laser drivers programmed with uniformity values for each light beam and having an N×M array of feedback loops is used to adjust the power intensity of each beam through the programmable laser drivers wherein each programmable laser driver uses the photodetector array summed with non-linearity inputs to adjust for beam to beam power uniformity correction.

CROSS-REFERENCE TO RELATED APPLICATION

Attention is directed to copending application Ser. No. 10/762,179, entitled, “Parallel Beam to Beam Uniformity Correction” filed concurrently herewith. The disclosure of this copending application is hereby incorporated by reference in its entirety.

BACKGROUND

The present invention relates generally to the field of optical imaging. More specifically, the invention relates to xerographic printers and, more particularly, to xerographic printers that incorporate a Vertical Cavity Surface Emitting Laser (VCSEL) array whose output is corrected by beam intensity variations through the use of process control electronics.

Polygon Raster Output Scanner (ROS) printers typically consist of a modulating laser light source, a polygon scanning beam deflector, an optical system of lenses and mirrors, a xerographic marking engine and the electronics to control printer operation. The ROS is positioned in an optical scan system to write an image on a moving photoreceptor surface. In the ROS system, a modulated beam is directed onto the facets of a rotating polygon mirror, which then sweeps the reflected beam across the photoreceptor surface. Each sweep exposes a raster line to a linear segment of a video signal image.

However, the use of a rotating polygon mirror presents several inherent problems. Bow and wobble of the beam scanning across the photoreceptor surface result from imperfections in the mirror or even slight misangling of the mirror or from the instability of the rotation of the polygon mirror. These problems typically require complex, precise and expensive optical elements between the light source and the rotating polygon mirror and between the rotating polygon mirror and the photoreceptor surface. Additionally, optically complex elements are also needed to compensate for refractive index dispersion that causes changes in the focal length of the imaging optics of the ROS.

The modulating laser light source may consist of a Vertical Cavity Surface Emitting Laser (VCSEL) array. The VCSEL array may be either a one or two-dimensional array of individual laser sources. Each individual laser source in the VCSEL array has a corresponding drive circuit which may be used to generate a beam to expose a corresponding area on a moving photoreceptor in response to video data information applied to the drive circuits of the VCSEL array. The photoreceptor is advanced in the process direction to provide a desired image by the formation of sequential scan lines generated by the beam to beam exposure delivered from the VCSEL array.

Current beam to beam uniformity correction in multi-beam ROS systems multiplexes one photo detector and one loop-back system among all the beams. This is accomplished by sequentially selecting each beam and comparing the output of the photo detector for that beam with a “reference” to decide whether to increase or decrease the power intensity in the selected beam. This sequential detection and power adjustment process takes a few micro-seconds per beam. In a high performance multi-beam ROS system the beam to beam uniformity correction has to be fine-tuned for each beam per scan line. In a VCSEL ROS system incorporating a two-dimensional array for producing a plurality of beams, the sequential beam to beam uniformity correction may take up to a few hundred micro-seconds. This uniformity correction scheme presents an amount of delay time for each line to be printed such that it renders the sequential multiplexing of all the beams unusable for high speed/high performance platforms.

SUMMARY

An electro-optical system for adjusting beam to beam power non-uniformity in a multi-beam scanning system is disclosed. The electro-optical system consists of N×M array of light beams and N×M array of photodetectors wherein optical means deflect the light beams onto a photodetector array. A circuit consisting of N×M array of programmable laser drivers programmed with uniformity values for each light beam and having an N×M array of feedback loops is used to adjust the power intensity of each beam through the programmable laser drivers wherein each programmable laser driver uses the photodetector array summed with non-linearity inputs to adjust for beam to beam power uniformity correction.

DETAILED DESCRIPTION

A multi-beam scanning system comprising an array of light sources each having a programmable reference mechanism or driver programmed with calibrated uniformity values for producing a corresponding light beam for producing a corresponding power intensity of light beams is described. The multi-beam scanning system includes a beam splitter to deflect the light beams onto a photodetector array wherein an array of feedback loops simultaneously adjust the power intensity in the fast/slow scan direction for each light beam by using the programmable reference mechansims or drivers. Each programmable driver uses a photodetector on the photodetector array to adjust for parallel beam to beam power correction produced from the array of light sources.

Reference is now made toFIG. 1wherein there is shown a schematic view of a ROS printing system incorporating an optical source10. The optical source may be either a one or two-dimensional array of light sources each light source producing light beams of unequal power intensity. The optical source10shown is a two dimensional N×M laser array10of Vertical Cavity Surface Emitting Lasers (VCSELs), all emitting nominally the same wavelength and same polarization state. The individual VCSELs12in the VCSEL N×M array10are arranged vertically and horizontally in each scan plane direction with equal center to center spacing between the individual VCSELs12. The VCSEL N×M array10may be monolithic in one embodiment wherein “N” and “M” may be any combination of integer values greater than zero to describe the two-dimensional array.

Returning to the line projection architecture of the ROS printing system shown inFIG. 1, the VCSELs12each emit beams14through a collimator lens60wherein the beams14pass through a beam splitter or mirror16and then through a pre-polygon lens system23. The lens system23focuses the beams14into a controlled energy distribution beam25that is reflected from the mirrored facets27of a rotating polygon scanner70. With the rotation of polygon70, the light beam25is reflected from each illuminated facet27and passes through a series of post-polygon lenses15and19, respectively, which scans the beam25across a surface of a photoreceptor20.

Referring now toFIG. 2, there is shown a prior art timing diagram illustrating the amount of delay time using sequential correcting for beam to beam uniformity. Beam to beam uniformity correction in multi-beam ROS systems multiplexes one photo detector and one loop-back system among all the beams. This may be accomplished by sequentially selecting each beam32and comparing the output of a photo detector for that beam with a “reference” to decide whether to increase or decrease the power intensity in the selected beam32. As shown inFIG. 2, a sequential detection and power adjustment process may take greater than 100 micro-seconds for beam to beam correction during line synchronization30. What would be desirable is a parallel detection and power adjustment scheme. As shown inFIG. 3using parallel beam to beam uniformity correction reduces the time40to that of adjusting the power correction for an individual VCSEL beam14when calibrating and adjusting for all the beams38during the line synchronization operation30.

Turning back toFIG. 1, each individual beam14from each individual VCSEL12is split by the beam splitter16for generating a beam18for passing through an imaging lens62for receipt on an photo detector plane64having photo detectors42. The photo detector plane64has the photo detectors42arranged in a N×M array to match the corresponding VCSELs12on the VCSEL N×M array10. All the VCSELs in the N×M array10may now be addressed at the same time in parallel on the photo detector plane64. The output of each photo detector42may now be used for each beam18corresponding to a beam14from the VCSEL array10as a “reference” to decide whether to increase or decrease the power intensity for a given selected VCSEL12.

FIG. 5is a simplified circuit diagram for calibrating and providing parallel beam to beam uniformity and power correction for a VCSEL ROS of N×M array beams using the architecture shown inFIG. 1. The following is one method in embodiments that may be used to calibrate the multi-beam VCSEL array10for parallel beam to beam uniformity correction. One method used to obtain calibration values is to first turn all the beams32“ON” and close the switch S144shown inFIG. 5. Next, the intensity of each beam18at the photo detector plane64is either manually or automatically calibrated for a nominal value on the surface of the photoreceptor20through use of a programmable uniformity control reference mechanism or driver. The programmable uniformity control reference mechanism or driver may be in one embodiment an eight “8” bit digital to analog (D to A) converter28that varies the amount of current into each light source for varying the intensity of each corresponding light beam produced. A photo detector42is used to measure the output power intensity of each beam18for the corresponding beam14. Therefore, each VCSEL12has a corresponding calibrated D to A converter47used to uniformly adjust the intensity of each beam14such that they are all of equal strength.

The calibrated uniformity values corresponding to each beam may be stored in a non-volatile memory location.FIG. 4illustrates a graph showing one example of a correction coefficient curve for long term power compensation wherein the uniformity values may be modified to compensate for long term aging of the VCSELs12at predetermined intervals. This may be determined through cycle testing during machine set-up time, or by multiplying the uniformity values by a time varying empirically/mathematically derived coefficient to compensate for long term power compensation. The correction coefficient may also be implemented within the D to A converters as part of the calibration process and for real time operation as described below.

Referring once again toFIG. 5, for real time operation, the calibrated uniformity reference values for each beam are set by loading these values into the D to A converters28through47for each beam during machine set-up time. Next, switch S144is closed simultaneously for each VCSEL12to establish a closed loop for beam to beam uniformity correction. Summing amplifier46uses the value from the photo detectors42and sums it with the different reference values stored in each of the D to A converters. By dedicating one cycle time, ≈2 Usec or less, per scan line, parallel beam to beam uniformity correction is achieved. More specifically, during this cycle, all beams are turned to “ON” and the closed-loop automatically modifies the light intensity of each beam to match that of the reference value. This reduces the existing sequential/serial approach to the power adjustment of the light beams from an unacceptable level of a few hundred Usec per line to approximately 2 Usec per line or lower which provides a manageable overhead in very high performance imaging applications.

For real time parallel beam to beam power correction in the fast/slow scan direction the following method in embodiments may be implemented. As described above, the reference value for each beam is set by loading uniformity values into the D to A converters28during machine set-up time. Next, a closed-loop is once again established by closing switch S144and dedicating one cycle time, ≈2 Usec or less, per scan line for parallel beam to beam static power correction, i.e., loading a new value of a Smile52correction value into the corresponding input.

Real time/dynamic power correction50in the fast/slow scan direction may be implemented by driving other inputs into a summing amplifier48as depicted inFIG. 5. These inputs could represent Droop54or other system non-linearity(s)56like vibration that may be corrected by modulating the beam intensity statically or dynamically.