Hector Martin

As far as I know, without the optical feedback from the pickup itself onto the pre-groove, focus are tracking effectively become open-loop.

Angular tracking is a problem too (normal burners just don't care except roughly on average). Friend built some FPGA-based PLL thing.

... which worked fine for visual purposes but for nm-scale tracking you really need some accurate angular feedback too.

PLL with a small number of feedback markers and high quality OCXO might suffice?

The tricky bit I guess is that you need a high angular feedback frequency to compensate for mechanical instability.

I.e. no matter how good your PLL is, one pulse per revolution isn't enough for nm-scale accuracy.

To avoid need for precise even spacing of marks, could you have one primary pulse per rev and many minor ones...

...and measure timing between each one (average over many revs) before start to get phase offsets for each to feed into PLL?