In-service skew monitoring in a nested Mach-Zehnder modulator structure using pilot signals and balanced phase detection

An apparatus, comprising:

• a drive circuit that supplies, during a first time interval, first in-phase drive signals that are modulated in accordance with a first electrical tone and first quadrature drive signals that are modulated in accordance with a second electrical tone, the first electrical tone having a first frequency and the second electrical tone having a second frequency,
• wherein during a second time interval, the drive circuit supplies second in-phase drive signals that are modulated in accordance with the second electrical tone having the second frequency and second quadrature drive signals that are modulated in accordance with the first electrical tone having the first frequency, the first frequency being different than the second frequency,
• such that, based on the first in-phase and first quadrature drive signals during the first time interval, a modulator outputs a first optical signal including:
• a first in-phase component modulated to have a first optical tone at the first frequency;
• and a first quadrature component modulated to have a second optical tone at the second frequency, and
• based on the second in-phase and second quadrature drive signals during the second time interval, the modulator outputs a second optical signal including:
• a second in-phase component modulated to have the second optical tone at the second frequency;
• and a second quadrature component modulated to have the first optical tone at the second frequency.

An apparatus, comprising:

• a drive circuit that selectively supplies a first plurality of electrical signals and a second plurality of electrical signals;
• a modulation circuit coupled to the drive circuit, such that:
• based on the first plurality of electrical signals, the modulation circuit outputs a first optical signal having a first in-phase component and a first quadrature component, the first in-phase component being modulated in accordance with a first tone having a first frequency and the first quadrature component being modulated in accordance with a second tone having a second frequency different than the first frequency, and
• based on the second plurality of electrical signals, the modulation circuit outputs a second optical signal having a second in-phase component and a second quadrature component, the second in-phase component being modulated in accordance with the second tone having the second frequency and the second quadrature component being modulated in accordance with the first tone having the first frequency;
• and
• a detector circuit that supplies an error signal based on a portion of at least one of the first and second optical signals, the detector circuit supplying third electrical signals to the modulation circuit based on the error signal.