The use of ronchi rulings for high precision, two-dimensional galvo deflection systems

Ronchi rulings can serve as low cost, high precision position references in galvo-based, actively-stabilized laser deflection systems. An overview of beam position sensing principles is presented, including a comparison of the design trade offs between monotonic and periodic position sensors. Next, methods are described to use a Ronchi ruling as a mask to define a series of identical locations on a media. Each location is the gap between two bars of the Ronchi ruling. By focusing the beam onto the grating, and introducing a high frequency, low amplitude dither in the beam position, a position signal can be generated which is zero when the beam is centered between two grating bars. This signal is suitable for use in a position stabilization loop which keeps the beam centered at the particular location. A method of using a pair of quadrature signals (including the dither position signal) is described in which the beam position can be tracked while scanning across many locations. A dual mode demonstration system is described which accesses any of 63 positions with a settling time less than two milliseconds. The spot size is 50 microns, resolution 1 micron and drift is not measurable. Techniques for extending the one-dimensional deflection to two-dimensional (x-y) operation are presented. © 1984 SPIE.