Oh, you edited your original response
Ok, so you want 14400bps simplex to the PIC & were concerned about noise & also about driving RS232.
To remove the noise you can use an RC LPF followed by a schmitt trigger inverter. You need an inverter due to the RS232 having opposite polarity to a PIC UART (note: some pics can set the polarity/inversion the uart); the schmitt trigger part helps remove some of the noise. Also, make sure the RC time delay is bigger than the glitches and smaller than the bit times.
As far as using IR - that's also very feasible, if you wanted to try. Just don't use a phototransistor as the receiving element as they're very slow. Try a photodiode and AC amplifier (a few transistors). Mounting the photodiode shouldn't be that critical; while the IR transmitter will produce a nonuniform wide angle pattern, due to the high speed of transmission (i.e. the bittimes) vs the low speed of rotation of the detector, the intensity shouldn't change much during a bit time.
EDIT: you mentioned RS232 using TTL - no, it doesn't. A PC serial port will accept inverted TTL serial data; it outputs positive and negative voltages though. Normally you use a MAX232, or a resistor & diode clamp + inverter to make your uC able to stomach it.