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Carrier-Suppressed Return-to-Zero is an optical line code. In CSRZ the field intensity drops to zero between consecutive bits , and the field phase alternates by π radians between neighbouring bits, so that if the phase of the signal is e.g. 0 in even bits , the phase in odd bit slots will be π, the phase alternation amplitude. In its standard form CSRZ is generated by a single Mach–Zehnder modulator , driven by two sinusoidal waves at half the bit rate BR, and in phase opposition. This gives rise to characteristically broad pulses.
The signal format Alternate-Phase Return-to-Zero can be viewed as a generalisation of CSRZ in which the phase alternation can take up any value ΔΦ and the duty cycle is also a free parameter.
CSRZ can be used to generate specific optical modulation formats, e.g. CSRZ-OOK, in which data is coded on the intensity of the signal using a binary scheme , or CSRZ-DPSK, in which data is coded on the differential phase of the signal, etc. CSRZ is often used to designate APRZ-OOK.
The characteristic properties of an CSRZ signal are those to have a spectrum similar to that of an RZ signal, except that frequency peaks are shifted by BR/2 with respect to RZ, so that no peak is present at the carrier and power is ideally zero at the carrier frequency.