Therefore we need additional transitions at the beginning of each bit to set the proper start level if a series of equal bits is sent. We don't need this additional transition if the next bit has a different value. This is also called a 'biphase' code. A logic one is represented by a burst of oscillations. The reason is, that you can use a filter tuned to the carrier frequency to distinguish the signal from noise in the ambient light.
Fluorescent lamps are the main source of such noise.
Photodiodes behind an optical filter which transmits infrared light but blocks visible light are used as detectors. The signal from the photodiode is fed through a filter tuned to the carrier fequency and then amplified. The amplified signal is demodulated just like the carrier is demodulated in any AM radio receiver. It can be a lot of pain to design a sensitive receiver that does'nt start to oscillate. It is also necessary to have some automatic gain control to avoid overload of the amplifier at close distance to the emitter.