Carrier-Suppressed Return-to-Zero (CSRZ) is an optical

line code In telecommunication, a line code is a pattern of voltage, current, or photons used to represent digital data transmitted down a communication channel or written to a storage medium. This repertoire of signals is usually called a constrained c ...

. In CSRZ the field intensity drops to zero between consecutive bits ( RZ), 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 (bit number 2''n''), the phase in odd bit slots (bit number 2''n''+1) will be π, the phase alternation amplitude. In its standard form CSRZ is generated by a single Mach–Zehnder modulator (MZM), driven by two sinusoidal waves at half the bit rate B_R, and in phase opposition. This gives rise to characteristically broad pulses (duty cycle 67%). The signal format

Alternate-Phase Return-to-Zero Alternate-Phase Return-to-Zero (APRZ) is an optical line code. In APRZ the field intensity drops to zero between consecutive bits, and the Field (physics), field Phase (waves), phase alternates between neighbouring bits, so that if the phase of th ...

(APRZ) can be viewed as a generalisation of CSRZ in which the phase alternation can take up any value ''ΔΦ'' (and not necessarily only π) and the duty cycle is also a free parameter. CSRZ can be used to generate specific optical modulation formats, e.g. CSRZ-

OOK Ook, OoK or OOK may refer to: * Ook Chung (born 1963), Korean-Canadian writer from Quebec * On-off keying, in radio technology * Toksook Bay Airport (IATA code OOK), in Alaska * Ook!, an esoteric programming language based on Brainfuck * Ook, th ...

, 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_R'') are shifted by ''B_R''/2 with respect to RZ, so that no peak is present at the carrier and power is ideally zero at the

carrier frequency In telecommunications, a carrier wave, carrier signal, or just carrier, is a waveform (usually sinusoidal) that is modulated (modified) with an information-bearing signal for the purpose of conveying information. This carrier wave usually has a ...

(hence the name). Compared to standard RZ-OOK, the CSRZ-OOK is considered to be more tolerant to filtering and

chromatic dispersion In optics, and by analogy other branches of physics dealing with wave propagation, dispersion is the phenomenon in which the phase velocity of a wave depends on its frequency; sometimes the term chromatic dispersion is used for specificity to o ...

, thanks to its narrower

spectrum A spectrum (plural ''spectra'' or ''spectrums'') is a condition that is not limited to a specific set of values but can vary, without gaps, across a continuum. The word was first used scientifically in optics to describe the rainbow of colors ...

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