Carrier-Suppressed Return-to-Zero (CSRZ) is an optical line code.
In CSRZ the field intensity drops to zero between consecutive bits (RZ), and the field phase alternates by àradians between neighbouring bits, so that if the phase of the signal is e.g. 0 in even bits (bit number 2n), the phase in odd bit slots (bit number 2n+1) will be ÃÂ, the phase alternation amplitude. In its standard form CSRZ is generated by a single MachâÂÂZehnder modulator (MZM), driven by two sinusoidal waves at half the bit rate B<sub>R</sub>, and in phase opposition. This gives rise to characteristically broad pulses (duty cycle 67%).
The signal format Alternate-Phase Return-to-Zero (APRZ) can be viewed as a generalisation of CSRZ in which the phase alternation can take up any value ÃÂæ (and not necessarily only ÃÂ) 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 (light on=1, light off=0), 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 (still at a spacing of B<sub>R</sub>) are shifted by B<sub>R</sub>/2 with respect to RZ, so that no peak is present at the carrier and power is ideally zero at the carrier frequency (hence the name).
Compared to standard RZ-OOK, the CSRZ-OOK is considered to be more tolerant to filtering and chromatic dispersion, thanks to its narrower spectrum.