Methods, apparatuses, and systems are provided. A method includes obtaining a wavelength division multiplexing optical signal. The wavelength division multiplexing optical signal includes N pairs of optical signals. Each pair of optical signals includes a first optical signal and a second optical signal that are of different wavelengths and whose polarizations are orthogonal to each other. N is an integer greater than 1, and a frequency of at least one of N first optical signals and a frequency of at least one of N second optical signals are within a zero-dispersion frequency ZDF region of an optical fiber. The method further includes sending the wavelength division multiplexing optical signal through the optical fiber.

Systems and methods for calibrating a Raman amplifier (34, 50, 60) in a photonic line system (10) of an optical network are provided. A method (180), according to one implementation, includes the step of setting (182) the gain of a plurality of pump lasers of a Raman amplifier (34, 50, 60) to a safe level. For example, the pump lasers are configured to operate at different wavelengths. Also, the Raman amplifier (34, 50, 60) is connected to a fiber span (16) having a specific fiber-type. The safe can be defined as a level that keeps adverse intermodulation effects below a predetermined threshold regardless of the specific fiber-type. In addition, the method (180) includes the step of increasing (184) the gain of the pump lasers without prior knowledge of the specific fiber-type of the fiber span (16) while keeping the adverse intermodulation effects below the predetermined threshold.

Input light includes a multicarrier signal and first CW light of a first optical frequency. A transmitter generates a modulated optical signal based on an inverted signal of a dropped signal. A light source generates second CW light of a second optical frequency. A delay element adjusts a phase difference between the modulated optical signal and the second CW light. The multicarrier signal, the first CW light, the modulated optical signal and the second CW light are input to nonlinear optical medium. A detector detects beat frequency component between the modulated optical signal and the second CW light. A controller controls the delay element so as to increase the beat frequency component. A difference between the first optical frequency and an optical frequency of the dropped optical signal is substantially the same as a difference between the second optical frequency and an optical frequency of the modulated optical signal.

Input light includes a multicarrier signal and first CW light of a first optical frequency. A transmitter generates a modulated optical signal based on an inverted signal of a dropped signal. A light source generates second CW light of a second optical frequency. A delay element adjusts a phase difference between the modulated optical signal and the second CW light. The multicarrier signal, the first CW light, the modulated optical signal and the second CW light are input to nonlinear optical medium. A detector detects beat frequency component between the modulated optical signal and the second CW light. A controller controls the delay element so as to increase the beat frequency component. A difference between the first optical frequency and an optical frequency of the dropped optical signal is substantially the same as a difference between the second optical frequency and an optical frequency of the modulated optical signal.

An apparatus (104) mitigates cross-channel nonlinear distortion of an optical signal (138) carried on one of a plurality of wavelength channels (118) in a wavelength division multiplexed (WDM) transmission system (100). The apparatus includes a first optical receiver (126) which is arranged to detect a measure (134) of aggregate optical power of the plurality of wavelength channels. A nonlinear dispersion compensator includes means (144) for applying a phase modulation to the optical signal in proportion to the measure of aggregate optical power.

An apparatus (104) mitigates cross-channel nonlinear distortion of an optical signal (138) carried on one of a plurality of wavelength channels (118) in a wavelength division multiplexed (WDM) transmission system (100). The apparatus includes a first optical receiver (126) which is arranged to detect a measure (134) of aggregate optical power of the plurality of wavelength channels. A nonlinear dispersion compensator includes means (144) for applying a phase modulation to the optical signal in proportion to the measure of aggregate optical power.

A method includes calculating a phase of a light pulse that is propagating along an optical fiber of a fiber broadband communications network, calculating a change in the phase of the light pulse due to a nonlinearity, determining a cause of a portion of the change in the phase, wherein the cause is at least one of: a self-phase modulation, a cross-phase modulation, or a four-wave mixing, and initiating at least one action to mitigate the nonlinearity, wherein the at least one action is selected based on the cause that is determined.

A method includes calculating a phase of a light pulse that is propagating along an optical fiber of a fiber broadband communications network, calculating a change in the phase of the light pulse due to a nonlinearity, determining a cause of a portion of the change in the phase, wherein the cause is at least one of: a self-phase modulation, a cross-phase modulation, or a four-wave mixing, and initiating at least one action to mitigate the nonlinearity, wherein the at least one action is selected based on the cause that is determined.