I think I bought a similar thing from a surplus outfit and they were dubbed an intensity controller. Mine are marked 043H. Somewhere I could have a datasheet and the full part number. I have three.
They belong in a "Photo IC" category: **broken link removed**
OP's chip is bigger, though. Seems to be too much stuff on there for just level detection... unless they are doing A/D with a serial output or give you programmable trip points or something. It has 5 actual bond connections to the lead frame, and one of those is hooked straight to the detector - so with Vcc and Gnd that leaves 2 pins to get the signal out. Synchronous serial communications generally uses 3. Still looks like a demodulator to me.