Aspects of the present disclosure provide techniques for spatial filtering of multilevel Pulse Amplitude Modulated (PAM) optical signals. An example method is provided for operations which may be performed by components of an optical network, including but not limited to an optical transceiver. The example method generally includes receiving an optical signal that is pulse amplitude modulated (PAM) to encode information of a network packet as a series of optical signal pulses transmitted via an optical fiber, performing spatial filtering, at a transition from the optical fiber to a receiver, on the optical signal, and performing demodulation of the spatially filtered optical signal to decode the information of the network packet.

An optical transmitter includes: an amplitude control circuit, an E/O (Electrical-to-Optical) circuit, a detector and an optical power control circuit. The amplitude control circuit controls an amplitude of an input electric signal to generate a constant amplitude electric signal. The E/O circuit generates a modulated optical signal from the constant amplitude electric signal by a direct modulation. The detector detects an amplitude modulation component of the input electric signal. The optical power control circuit controls a power of the modulated optical signal based on the amplitude modulation component detected by the detector.

An optical transmitter includes: an amplitude control circuit, an E/O (Electrical-to-Optical) circuit, a detector and an optical power control circuit. The amplitude control circuit controls an amplitude of an input electric signal to generate a constant amplitude electric signal. The E/O circuit generates a modulated optical signal from the constant amplitude electric signal by a direct modulation. The detector detects an amplitude modulation component of the input electric signal. The optical power control circuit controls a power of the modulated optical signal based on the amplitude modulation component detected by the detector.

A transmission apparatus includes: a first obtaining section configured to obtain a first transmission characteristic in communication of a multi-carrier signal through a transmission path between the transmission apparatus and a distant transmission apparatus; a second obtaining section configured to obtain a second transmission characteristic under a predetermined usage condition within a specified performance range of the transmission apparatus; a correcting section configured to correct the first transmission characteristic according to the second transmission characteristic; and a determining section configured to determine an amount of allocation for each subcarrier included in the multi-carrier signal, according to the first transmission characteristic that has been corrected by the correcting section.

A transmission apparatus includes: a first obtaining section configured to obtain a first transmission characteristic in communication of a multi-carrier signal through a transmission path between the transmission apparatus and a distant transmission apparatus; a second obtaining section configured to obtain a second transmission characteristic under a predetermined usage condition within a specified performance range of the transmission apparatus; a correcting section configured to correct the first transmission characteristic according to the second transmission characteristic; and a determining section configured to determine an amount of allocation for each subcarrier included in the multi-carrier signal, according to the first transmission characteristic that has been corrected by the correcting section.

A method of transmitting a data signal using an optical transmitter of an optical communications system. The optical transmitter is configured to modulate an optical carrier in successive signalling intervals to generate an optical signal. A modulation scheme is provided which comprises a multi-dimensional symbol constellation. The modulation scheme is designed such that an average degree of polarization of a modulated optical signal output from the optical transmitter has a first value when averaged across a first signalling interval, and has a second value when averaged across more than one and fewer than 100 signalling intervals. The second value is less than 10 percent of the first value. During run-time, an encoder of the optical transmitter encoding a data signal to be transmitted as symbols of the constellation, and a modulator of the optical transmitter modulating available dimensions of the optical carrier in accordance with the symbols.

An optical system includes an optical transmitter and a controller that determines a particular wavelength for a channel of the optical transmitter. The controller causes the optical transmitter to transmit a first optical signal with a first data sequence that is determined based on the particular wavelength, and determines a first optical power that is generated based on the first optical signal and the first data sequence. The controller causes the optical transmitter to transmit a second optical signal with a second data sequence that is determined based on the particular wavelength, and determines a second optical power that is generated based on the second optical signal and the second data sequence. The controller calculates a power difference between the first optical power and the second optical power, and causes the particular wavelength for the channel to be modified based on the calculated power difference.

A method for preventing nonlinear interference in an optical communication system. The method may include selecting an optical signal of a first optical channel. The method may include determining an estimate of inter-channel nonlinear interference to the optical signal of the first optical channel. The inter-channel nonlinear interference may be generated by one or more optical signals transmitted over a second optical channel in the optical communication system. The method may include determining one or more linear filters based on the estimate of the inter-channel nonlinear interference. The method may include pre-distorting an optical signal for transmission over the second optical channel using the one or more linear filters. The pre-distorted optical signal may be configured for reducing the inter-channel nonlinear interference to the first optical signal of the first optical channel. The method may include transmitting the pre-distorted optical signal over the second optical channel through an optical transmission medium.

A method for preventing nonlinear interference in an optical communication system. The method may include selecting an optical signal of a first optical channel. The method may include determining an estimate of inter-channel nonlinear interference to the optical signal of the first optical channel. The inter-channel nonlinear interference may be generated by one or more optical signals transmitted over a second optical channel in the optical communication system. The method may include determining one or more linear filters based on the estimate of the inter-channel nonlinear interference. The method may include pre-distorting an optical signal for transmission over the second optical channel using the one or more linear filters. The pre-distorted optical signal may be configured for reducing the inter-channel nonlinear interference to the first optical signal of the first optical channel. The method may include transmitting the pre-distorted optical signal over the second optical channel through an optical transmission medium.

Methods and systems for superchannel power pre-emphasis may adjust power levels of selected subcarriers of the superchannel. The power pre-emphasis may be performed at a transmission stage using a laser source, a variable optical attenuator, or a wavelength selective switch. The power pre-emphasis may be performed in-line at a reconfigurable optical add-drop multiplexer node. The power pre-emphasis may be performed using feedback control based on a receiver output. The power pre-emphasis may be performed using feedforward control based on optical path computations.