In-band optical signal-to-noise ratio measurement

A method for in-band measurement of an optical signal-to-noise ratio (OSNR) by a demodulation device, the method comprising:

• receiving an optical payload containing a low-frequency signal;
• filtering a first passband of the optical payload in an optical channel;
• converting the first passband-filtered optical signal to a first target electrical signal;
• measuring DC and AC components of the first target electrical signal;
• filtering a second passband of the optical payload within a bandwidth of the optical channel;
• converting the second passband-filtered optical signal to a second target electrical signal;
• measuring DC and AC components of the second target electrical signal;
• and
• determining the OSNR of the optical channel, wherein the OSNR of the optical channel is based on a first ratio between the DC components of the first and second target electrical signals and a second ration between the AC components of the first and second target electrical signals.