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

line code In telecommunications, a line code is a pattern of voltage, current, or photons used to represent digital data transmission (telecommunications), transmitted down a communication channel or written to a storage medium. This repertoire of signal ...

. 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 phase alternates between neighbouring bits, so that if the phase of the signal is, for example, 0 in ...

(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 A duty cycle or power cycle is the fraction of one period in which a signal or system is active. Duty cycle is commonly expressed as a percentage or a ratio. A period is the time it takes for a signal to complete an on-and-off cycle. As a for ...

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, the ...

, 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 periodic waveform (usually sinusoidal) that conveys information through a process called ''modulation''. One or more of the wave's properties, such as amplitude or fre ...

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

filtering Filtration is a physical process that separates solid matter and fluid from a mixture. Filter, filtering, filters or filtration may also refer to: Science and technology Computing * Filter (higher-order function), in functional programming * Fil ...

and

chromatic dispersion Dispersion is the phenomenon in which the phase velocity of a wave depends on its frequency. Sometimes the term chromatic dispersion is used to refer to optics specifically, as opposed to wave propagation in general. A medium having this commo ...

, thanks to its narrower

spectrum A spectrum (: spectra or spectrums) is a set of related ideas, objects, or properties whose features overlap such that they blend to form a continuum. The word ''spectrum'' was first used scientifically in optics to describe the rainbow of co ...

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