The light source is chopped at some frequency which produces pulses at the optical receiver. The limiter is just a high gain amplifier that clips the output at the maximum level. The bandpass filter passes the frequency of the chopped light and rejects other frequencies. The demodulator is a rectifier that outputs DC. The integrator charges it's capacitor with the DC signal, and if the signal lasts long enuf, turns on the output transistor. If the signal does not last long enuf, the transistor is not turned on.