Subcarrier based adaptive equalization of electrical filtering effects on sub-carrier multiplexed signals

An apparatus, comprising:

• a laser that supplies light;
• a modulator circuit, the modulator circuit receiving the light from the laser;
• a digital-to-analog circuit that supplies a drive signal to the modulator circuit, the modulator circuit modulating the light to provide an optical signal that includes in-phase and quadrature components, the digital-to-analog circuit including first and second paths that carry electrical signals associated with the in-phase and quadrature components of the optical signal;
• and
• a digital signal processor that provides a digital signal to the digital-to-analog circuit, the digital signal processor including circuitry that corrects for impairments that would occur in the optical signal in the absence of the circuitry, the impairments being associated with at least a difference in a characteristic between the first and second paths,
• wherein the optical signal includes a plurality of subcarriers, and
• wherein each of the plurality of subcarriers has a corresponding one of a plurality of first frequencies, one of the plurality of subcarriers having one of the plurality of first frequencies, said one of the plurality of first frequencies being is greater than a second frequency, and another one of the plurality of subcarriers having another one of the plurality of first frequencies, said one of the plurality of second frequencies being is less than the second frequency, the impairments being associated with an image of said one of the plurality of subcarriers at said another one of the plurality of first frequencies, the second frequency corresponding to a frequency of the light from the laser and received by the modulator.

An apparatus, comprising:

• a laser that supplies light;
• a modulator circuit, the modulator circuit receiving the light from the laser;
• a digital-to-analog circuit that supplies a drive signal to the modulator circuit, the modulator circuit modulating the light to provide an optical signal that includes in-phase and quadrature components, the digital-to-analog circuit including first and second paths that carry electrical signals associated with the in-phase and quadrature components of the optical signal;
• a digital signal processor that provides a digital signal to the digital-to-analog circuit, the digital signal processor including circuitry that corrects for impairments that would occur in the optical signal in the absence of the circuitry, the impairments being associated with at least a difference in a characteristic between the first and second paths,
• wherein the optical signal includes a plurality of subcarriers,
• wherein the circuitry in the digital signal processor includes a multiple-input, multiple-output (MIMO) circuit
• wherein the MIMO circuit includes a finite impulse response (FIR) filter, which includes a plurality of stages,
• wherein each of the plurality stages includes a respective one of a plurality of delay circuits and a corresponding one of a plurality of multiplier circuits, whereby each of the plurality of multiplier circuits multiplies an input to each of the plurality of stages by a respective one of a plurality of coefficients, and
• wherein each of a plurality of first values is associated with a corresponding one of the plurality of coefficients, each of the plurality of first values being changed to a corresponding one of second values, based on a detected version of the optical signal at a receiver.

An apparatus, comprising:

• a coherent receiver that receives an optical signal having in-phase and quadrature components;
• an analog-to-digital circuit that receives an analog signal from the coherent receiver, the digital-to-analog circuit including first and second paths that carry electrical signals associated with the in-phase and quadrature components of the optical signal;
• a digital signal processor that receives a digital signal from the analog-to-digital conversion circuit, the digital signal processor including circuitry that corrects for impairments that would occur in the digital signal in the absence of the circuitry, the impairments being associated with at least a difference in a characteristic between the first and second paths,
• wherein the optical signal includes a plurality of subcarriers,
• wherein each of the plurality of subcarriers has a corresponding one of a plurality of first frequencies, one of the plurality of subcarriers having one of the plurality of first frequencies, said one of the plurality of first frequencies being is greater than a second frequency, and another one of the plurality of subcarriers having another one of the plurality of first frequencies, said one of the plurality of second frequencies being is less than the second frequency, the impairments being an image of said one of the plurality of subcarriers at said another one of the plurality of first frequencies, the second frequency corresponding to a frequency of the light from the laser and received by the modulator.