Since when are CD reader optics interferometers? The linked video shows a CW homodyne LIDAR used for measuring vibration frequency and counting vibration amplitude in fringes.

Last I looked CD readers used a 4-detector sensor's differential low-pass signals for closed-loop track-following so the rotation need not be optically centered. I also see no reason why optical disc readouts would need homodyne let alone heterodyne readout.

The pits and valleys are spaced λ/4 apart. The reason there is such a stark difference in intensity is because this λ/4 spacing causes interference at pit->valley and valley->pit transition points. Of course this not a standard interferometer but rather a purpose build one. Wikipedia says:

> Interferometry is a technique which uses the interference of superimposed waves to extract information.

And a blu ray player directly uses interference of superimposed waves to extract information. It squarely fits in the homodyne category.