Alternate-Phase Return-to-Zero

Alternate-Phase Return-to-Zero is an optical line code.
In APRZ the field intensity drops to zero between consecutive bits, and the field phase alternates between neighbouring bits, so that if the phase of the signal is, for example, 0 in even bits, the phase in odd bit slots will be ΔΦ, the phase alternation amplitude.

Special cases

Return-to-zero can be seen as a special case of APRZ in which ΔΦ=0, while Carrier-Suppressed Return-to-Zero can be viewed as a special case of APRZ in which ΔΦ=π.
APRZ can be used to generate specific optical modulation formats, for example, APRZ-OOK, in which data is coded on the intensity of the signal using a binary scheme. APRZ is often used to designate APRZ-OOK.

The characteristic properties of an APRZ signal are those to have a spectrum similar to that of an RZ signal, except that frequency peaks at a spacing of B_R/2 as opposed to B_R are observed.