Design of an optical communication system starts by deciding how to convert electrical signals to optical signals. The output light from the source is modulated by applying the electrical signals directly either to the optical source or to an external modulator.
Two format options are used when modulating an optical signal such as Return to Zero or Non-return to zero. These are abbreviated as RZ and NRZ respectively. In a Return to Zero format, optical pulses represented by bit 1 is shorter than the bit slot. The amplitude of such optical pulses returns to zero before the bit duration is over. In contrast, in a Non-Return to Zero format, the optical pulse remains ON throughout the bit slot and its amplitude does not return or drop to zero between two or more successive 1 bits
Pulse width remains same in Return to Zero (RZ) format. Pulse width varies depending on the bit pattern in a Non-Return to Zero (NRZ) format. Return to zero needs more bandwidth while Non-Return to Zero can save on bandwidth by almost half of that of RZ format. In RZ format, the ON-OFF process happens without stopping. In a NRZ format, the ON-OFF proess is reduced to a fewer times. NRZ format calls for tighter control of pulse width. Also it may lead to pattern-dependent effects if the optical pulse spreads during transmission.
Due to the savings it offers in bandwidth, NRZ format is preferred in optical transmission.
Post a Comment