Modulated IR for optical transmission of encoder data (TSOP 4840)
You need to look pretty closely at the gotchas of IR remote receivers. They can be a good easy solution but

They have internal AGC arrangements which are tuned for the kind of pulse widths that remotes produce. Different TSOPXXXX have different tunings.

When you deviate too far, they behave strangely, with varying sensitivity and pulse widths. 
They generally have a maximum and a minimum pulse width they are rated to work with.

When they are in sunlight, the behaviour changes again. You can also be too close/too much signal.

You need to ask the question (and test) what happens when the rotation rate is 0, or slow or $whatever?

However a situation of continuously sending your modulated signal to them, is favorable for the AGC.

They are not very frequency selective, so a crystal resonator is not really needed (i.e. if you get some other frequency variant, you can just use an RC and try them.)