Fourier plane analysis and refinement of SLM calibration
The present disclosure relates to phase-sensitive calibration of an SLM system. In particular, it relates to selecting among local calibrations when there is more than one calibration that satisfies a calibration intensity criteria. It utilizes information available before Fourier filtering, from non-zeroth order diffraction components to counter drift among alternative local calibrations.
1. A method of calibrating an SLM (spatial light modulator) having a multitude of SLM elements for which linear combinations of multiple alternative calibrations produce practically indistinguishable intensity patterns on a detector in an image plane, by using analysis of multiple diffraction orders in image data projected from the SLM, the method including:
projecting from the SLM at least one image of a calibration pattern onto an intensity detector in an image plane and iteratively adjusting groups of the SLM elements to produce a uniform intensity;
at least occasionally during the iteratively adjusting the elements, collecting and comparing images of at least two diffraction orders on opposite sides of the zeroth diffraction order and utilizing differences between the images of the diffraction orders to guide the iteratively adjusting the elements;
adjusting orientation of an intermediate reflective device in the projection optics; to direct the diffraction orders through the aperture in the Fourier plane where the zeroth diffraction order of the image would otherwise pass.
2. The method of claim 1 , further including adjusting positioning of an intermediate reflective device in the projection optics to direct the diffraction orders through the aperture in the Fourier plane where the zeroth diffraction order of the image would otherwise pass.
3. A calibration subsystem usable with an SLM (spatial light modulator), the SLM including a multitude of SLM elements, an SLM controller that drives the SLM elements, projection optics that relay electromagnetic radiation images from the SLM through an aperture in a Fourier plane to a workpiece in an image pane, the calibration subsystem including:
detection means for imaging a zeroth diffraction order of a calibration image in the image plane;
alternate order detection means for imaging at least two diffraction orders on opposite sides of the zeroth diffraction order
wherein the alternate order detection means comprises an adjustable mount for the SLM that adjusts orientation of the SLM to direct the two diffraction orders through the aperture in the Fourier plane where the zeroth diffraction order of the image would otherwise pass;
computing resources coupled to the detection means and the alternate order detection means and to the SLM controller, the computing resources including memory, logic and resources adapted to
process data from the detection means and iteratively adjust the SLM elements to calibrate the SLM;
compare data from the two diffraction orders on the opposite sides of the zeroth order to guide the iterative adjustment of the SLM element; and
signal the SLM controller to drive groups of the SLM elements and iteratively adjust the SLM elements.
4. The subsystem of claim 3 , wherein the adjustable mount includes piezoelectric actuators that adjust orientation of the SLM.
5. A calibration subsystem usable with an SLM (spatial light modulator), the SLM including a multitude of SLM elements, an SLM controller that drives the SLM elements, projection optics that relay electromagnetic radiation images from the SLM through an aperture in a Fourier plane to a workpiece in an image pane, the calibration subsystem including:
detection means for imaging a zeroth diffraction order of a calibration image in the image plane;
alternate order detection means for imaging at least two diffraction orders on opposite sides of the zeroth diffraction order
wherein the alternate order detection means comprises a steering mirror in an optical path of the projection optics that successively directs the two diffraction orders through the aperture in the Fourier plane where the zeroth diffraction order of the image would otherwise pass;
computing resources coupled to the detection means and the alternate order detection means and to the SLM controller, the computing resources including memory, logic and resources adapted to
process data from the detection means and iteratively adjust the SLM elements to calibrate the SLM;
compare data from the two diffraction orders on the opposite sides of the zeroth order to guide the iterative adjustment of the SLM element; and
signal the SLM controller to drive groups of the SLM elements and iteratively adjust the SLM elements.