A proactive and non-obtrusive channel probing scheme is provided to accurately predict channel power, gain, and optical signal to noise ratio (OSNR) without disrupting the existing connections. In one example, using a probe signal with 5 s pulse duration in a single-hop network, rapid wavelength switching is achieved with power excursions less than or equal to 0.2 dB for different loading configurations.

A network element includes a transmitting amplifier configured to transmit to a first optical fiber, wherein the transmitting amplifier has a pump laser; and an optical monitor connected to a second optical fiber and configured to detect a portion of optical power thereon; wherein the pump laser is modulated to convey a signal to a second optical monitor in a second network element connected to the first optical fiber, when the transmitting amplifier is one of in a safety mode and has no input.

Provided is a signal loopback circuit which, in order to loop back a monitoring signal in a relay device for relaying optical signals of a plurality of wavelength bands, connects between a channel of first direction and a channel of second direction through which an optical signal of first wavelength band and an optical signal of second wavelength band are transmitted, wherein the signal loopback circuit is provided with a first coupler for branching the optical signal on the channel of first direction, a first filter for extracting at least one of a monitoring signal of first wavelength band and a monitoring signal of second wavelength band that are used in the channel of first direction from the optical signal branched by the first coupler, and a second coupler for causing the monitoring signal extracted by the first filter to be joined to the channel of the second direction.

A wavelength control system, method, and apparatus are described in the present disclosure. An example method include: adjusting powers of subcarriers on a super channel to a same power, where the subcarriers of the super channel includes consecutive subcarriers, a subcarrier i1, a subcarrier i, and a subcarrier i+1; obtaining Q values of the subcarrier i1 and the subcarrier i+1, where the Q values indicate performance of the subcarriers; calculating a Q value difference between the Q value of the subcarrier i+1 and the Q value of the subcarrier i1, and calculating a difference between the Q value difference and a pre-obtained reference value of the subcarrier i; determining whether the absolute value of the difference is not less than the pre-obtained allowable frequency offset value, and adjusting a center wavelength of the subcarrier i according to the difference.

A proactive and non-obtrusive channel probing scheme is provided to accurately predict channel power, gain, and optical signal to noise ratio (OSNR) without disrupting the existing connections. In one example, using a probe signal with 5 s pulse duration in a single-hop network, rapid wavelength switching is achieved with power excursions less than or equal to 0.2 dB for different loading configurations.

A polarimeter includes a Polarization Maintaining (PM) coupler with an input configured to receive input light and split the input light to a first output and a second output; a first PM fiber coupled to the first output; a second PM fiber coupled to the second output; a first polarization device coupled to the first PM fiber; a second polarization device coupled to the second PM fiber; and a plurality of detectors coupled to the first polarization device and the second polarization device, wherein outputs i.sub.1, i.sub.2, i.sub.3, i.sub.4 are determined based on outputs of the plurality of detectors, the outputs i.sub.1, i.sub.2, i.sub.3, i.sub.4 are linear projections of corresponding Stokes Parameters of the input light.

Provided is a signal loopback circuit which, in order to loop back a monitoring signal in a relay device for relaying optical signals of a plurality of wavelength bands, connects between a channel of first direction and a channel of second direction through which an optical signal of first wavelength band and an optical signal of second wavelength band are transmitted, wherein the signal loopback circuit is provided with a first coupler for branching the optical signal on the channel of first direction, a first filter for extracting at least one of a monitoring signal of first wavelength band and a monitoring signal of second wavelength band that are used in the channel of first direction from the optical signal branched by the first coupler, and a second coupler for causing the monitoring signal extracted by the first filter to be joined to the channel of the second direction.

Methods and systems for optical signal transmission, particularly with carrier-less amplitude and phase (CAP) modulation and direct detection, are disclosed. In one exemplary aspect, a method of optical signal transmission is disclosed. The method includes receiving information bits at an input interface; mapping the information bits to a plurality of modulation symbols; separating in-phase (I) and quadrature (Q) components of the plurality of modulation symbols such that the I and Q components form a Hilbert pair in a resulting signal; pre-dispersing the resulting signal with an inverse of a phase delay of an expected chromatic dispersion to obtain a pre-dispersed signal; converting the pre-dispersed signal from digital domain to analog domain using a digital to analog conversion circuit; performing modulation of an output of the digital to analog conversion circuit to generate an output signal; and transmitting, over an optical transmission medium, the output signal from the modulation.

Using pump-probe measurements on multi-span optical links may result in the determination of one or more of the following: 1) wavelength-dependent power profile and gain evolution along the optical link; 2) wavelength-dependent dispersion map; and 3) location of regions of high polarization-dependent loss (PDL) and polarization-mode dispersion (PMD). Such measurements may be a useful diagnostic for maintenance and upgrade activities on deployed cables as well as for commissioning new cables.

A polarimeter system integrated into an optical line system includes a transmitter coupled to a transmit filter communicatively coupled to an output port in an optical line device, wherein the transmitter is configured to generate a polarization probe signal, and wherein a wavelength of the polarization probe signal is configured to operate in-service with traffic-bearing channels on the output port; and a polarimeter receiver coupled to a receive filter communicatively coupled to an input port in the optical line device, wherein the polarimeter receiver is configured to vary arrangement of input light from the filter and to measure various outputs of the varied arrangement to derive measurement of State of Polarization (SOP) of the input light.