That requires the next phase I'm waiting on: nm-scale lithographic printing on silicon.
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As far as I know, without the optical feedback from the pickup itself onto the pre-groove, focus are tracking effectively become open-loop.
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Angular tracking is a problem too (normal burners just don't care except roughly on average). Friend built some FPGA-based PLL thing.
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... which worked fine for visual purposes but for nm-scale tracking you really need some accurate angular feedback too.
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PLL with a small number of feedback markers and high quality OCXO might suffice?
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The tricky bit I guess is that you need a high angular feedback frequency to compensate for mechanical instability.
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I.e. no matter how good your PLL is, one pulse per revolution isn't enough for nm-scale accuracy.
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To avoid need for precise even spacing of marks, could you have one primary pulse per rev and many minor ones...
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...and measure timing between each one (average over many revs) before start to get phase offsets for each to feed into PLL?
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