· An ideal photodetector which has unity quantum efficiency and which produces no dark current, that is no electron holde pairs are generated in the absence of an optical pulse.
· This condition, it is possible to find the minimum received optical power required for a specific nit error rate performance in a digital syatem. This minimum received power level is known as the quantum limit.
· Assume an optical pulse of energy E falls on the photodeetector in a time interval t. This can only be interpreted by the receiver as a 0 pulse if no electron hole pairs are generated with the pulse with the pulse present.
· The probability that n=0 electrons are emitted in a time interval t is
· Thus for a given probability Pr(o), we can find the minimum energy E required at a specific wavelength l.