The invention relates to a method for creating a control channel in an optical transmission signal, wherein the optical transmission signal (S.sub.DS,i, S.sub.US,i) includes an optical carrier frequency component, a higher frequency modulation component carrying user information to be transported from a first end to a second end of an optical transmission link and a lower frequency modulation component carrying control information, the higher frequency modulation component realizing a user channel and the lower frequency modulation component realizing the control channel, and wherein the lower frequency modulation component is created by amplitude modulation. According to the invention, the lower frequency modulation component includes a binary digital pilot tone signal component which corresponds to a pilot tone signal having a predetermined pilot tone frequency (f.sub.i).

To generate, in an optical transmission device, a response signal corresponding to executed control even when said optical transmission device does not comprise one or both of a main signal photoelectric conversion function and a main signal optical amplification function, an optical transmission device comprises: an extraction unit that outputs, from a first optical signal including a main signal and a control signal, a signal including control information included in the control signal; a control unit that executes control on the basis of the control information; and a response signal output unit that outputs, according to the control, a response signal in a wavelength band different from the main signal.

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 signal processing apparatus and method for monitoring channel spacing which may be configured in a receiver and includes: a first determining unit to determine a frequency range of a pilot of a center channel and a frequency range of a pilot of a neighboring channel using a receive signal; a second determining unit to determine a center channel frequency offset of the center channel pilot according to the center channel frequency range, and determine a frequency offset of the neighboring channel pilot according to the neighboring channel frequency range; and a third determining unit to determine channel spacing between the center channel and the neighboring channel according to the center channel frequency offset, the neighboring channel frequency offset and a frequency of a pilot signal at a transmitter side.

In one embodiment, a system for analyzing optical networks includes a network analyzer optically coupled to a fiber optic network. The network analyzer may transmit a test signal to the fiber optic network, receive a reflected signal in response to transmitting the test signal, wherein the reflected signal is generated in response to the test signal interacting with a defect of the fiber optic network. The analyzer may then analyze a power of the reflected signal, wherein the power of the reflected signal corresponds to the defect in the fiber optic network, calculate a transmit time of the reflected signal, wherein the transmit time corresponds a location of the defect in the fiber optic network, identify a component of the fiber optic network corresponding to the defect and the location of the defect in the fiber optic network, and generate a maintenance report based on the analyzed reflected signal.

Methods and apparatuses for restoring lost signal in a network transmission line are disclosed. A first optical signal transmitted from a first optical module is received at an optical switch, the first optical signal having a first optical spectrum with data encoded into the first optical signal. A second optical signal having a second optical spectrum corresponding to the first optical spectrum without data encoded into the second optical signal, is received at the optical switch, the second optical signal the second optical signal transmitted from an amplified spontaneous emission source. Detecting, at a first photo detector, a loss of optical spectrum in the first optical signal, and, in response to detecting the loss of optical spectrum in the first optical signal, switching the optical switch from passing the first optical signal to passing the second optical signal thereby supplying at least one idler carrier without data imposed.

A dual wavelength Optical Time Domain Reflectometer (OTDR) system, embedded in a network element, includes a first OTDR source for wavelength λ.sub.1; a second OTDR source for wavelength λ.sub.2; an OTDR measurement subsystem adapted to measure backscatter signals λ.sub.1.sub._.sub.BACK, λ.sub.2.sub._.sub.BACK associated with the wavelength λ.sub.1 and the wavelength λ.sub.2; and one or more ports connecting the first OTDR source, the second OTDR source, and the OTDR measurement subsystem to one or more fiber pairs; wherein wavelength λ.sub.1 and wavelength λ.sub.2 are each outside of one or more signal bands with traffic-bearing channels, thereby enabling operation in-service with the traffic-bearing channels.

A system for testing reflections within a data transmission signal includes a data transmission line configured to transmit the signal in a downstream direction, and a test probe configured to electrically contact a contact point on the transmission line and measure a magnitude of a frequency response of the signal therein. The system further includes a spectrum capturing device in operable contact with the test probe, and configured to collect and arrange data of frequency response magnitudes measured by the test probe. The data transmission line includes at least a first impedance mismatch corresponding to a first reflection point along the transmission line, and the spectrum capturing device is configured to determine a severity of the first reflection based on a comparison of a first voltage V.sub.1 with a second voltage V.sub.2, where V.sub.1 represents a DC term, and where V.sub.2 represents a reflected energy of a subsequent impulse.

A method and apparatus for monitoring in-line signal quality and a system. The method for monitoring in-line signal quality includes: according to signal to noise ratios (SNRs) of subcarriers obtained in a transmission initialization period, setting a subcarrier with a highest SNR to be a pilot subcarrier and other subcarriers to be data subcarriers; determining bit allocation and power allocation of the pilot subcarrier and bit allocation and power allocation of the data subcarriers; setting data-decision-based SNR measurement thresholds for the data subcarriers according to the bit allocation of the data subcarriers; and comparing the SNRs of the data subcarriers obtained through data-decision-based SNR measurement in a transmission period with the SNR measurement thresholds of the data subcarriers, and when an SNR of a data subcarrier is less than its SNR measurement threshold, trigger pilot-based SNR measurement of the data subcarrier. Hence, not only temporally continuous in-line signal quality monitoring may be provided, but also accuracy of the monitoring result may be guaranteed.

A line monitoring system may include a laser source to launch a probe signal over a first bandwidth, a polarization maintaining tap to receive and split the probe signal, into a first portion and a second portion, a polarization rotator to receive the first portion and send the first portion to a transmission system, a return tap to receive the second portion and to receive a return signal from the transmission system, wherein the return signal being derived from the first portion, a photodetector coupled to receive an interference signal from the return tap, wherein the interference signal is generated by a mixing the return signal and the second portion, where the photodetector is arranged to output a power signal based upon the interference signal, and a power measurement system to measure the power signal at a given measurement frequency over a second bandwidth, comparable to the first bandwidth.