Unified Optical Scanning Technology

Chapter 5 - Control of Scanner Beam Displacement

Two forms of beam misplacements are discussed here. The first, known as "cross-scan" error, results in small shifts of the beam in that direction that can cause a perceptible defect called 'banding.' This typically develops from small angular errors of the rotating shaft (wobble) and/or from angular nonuniformities of polygon facets in the cross-scan direction that displace the beams during successive scans. Extensive research has been conducted to relieve this problem without the burden of stringent mechanical tolerances, resulting in effective control methods. Attention is devoted here to these successful options.

The second form is a less frequently encountered beam misplacement that forms a 'ghost image.' In contrast to the above-described source of error, this effect does not result from a mechanical deviation of polygon facets or from a flawed lens design. Rather, it is a consequence of the propagation paths of the input and scanned beams and of the redirection and refocusing of the scatter from the original focused beam that is incident properly on the storage medium. Two effective correction methods are discussed.

An unexpected interrelationship between these two entirely different types of beam misplacements is introduced, which appears between one of the procedures for ghost abatement and one for the above-noted cross-scan error. An excellent alternative eliminates the ghost image while avoiding conflict with the control of cross-scan error.

5.1 CROSS-SCAN ERROR AND ITS CORRECTION

The discussion of beam deflection in Chapter 4 identified some imperfections in the positional accuracy of several scanning devices, along with an indication of some control methods (Sections 4.2, 4.3, 4.3.1, 4.4.1.1, 4.5.3, 4.5.4, and 4.6.3 and Table 4.1). This reiteration serves not only to introduce the topic of this chapter, but to highlight its operational significance. The requirements for optical scanning transcend the basic need for efficient beam positioning by seeking its accomplishment with high positional integrity. Although almost all scanning devices execute beam deflection along one axis (the "along-scan" direction), and may be accompanied by positional errors that require correction in that direction, it is the quadrature axis ('cross-scan' direction) that is most difficult to access, hence most insidious and awkward to control. After exercising reasonable care in minimizing scan perturbations, the along-scan direction benefits from the basic property of being time related, allowing complementation of the residual scan nonuniformities with timely indexing of the data stream. A classic method of accurate tracking is to derive a synchronous auxiliary or pilot beam from the same deflector, scan it across a precise grid, detect the equally timed impulses, and then trigger the signal data stream accordingly to provide a corresponding spatial integrity [Toy]. Various related techniques are instituted to take advantage of this time-space relationship. A prevalent example is the precise initiation of data scans that are triggered by "start-of-scan" (SOS) pulses that are detected optically from the actual or auxiliary deflected beam.

Because the cross-scan errors appear in quadrature to the along-scan errors, entirely different correction methods must be considered and instituted. Depending on their sources and periodicity (which may be pseudorandom), the defects may be short or long fractions of the scan period. When complete scan lines are misplaced, this formation appears often as the very perceptible and therefore insidious problem called 'banding.' The intensive effort that is directed to alleviate cross-scan errors has achieved substantive success. This section concentrates on these error-reduction disciplines and techniques.

 

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