1. A method for monitoring optical signal-to-noise ratio (OSNR) in optical signals, the method comprising: receiving a first optical signal for OSNR monitoring, the first optical signal including a first optical channel; optically filtering the first optical signal to generate an in-band signal over a passband corresponding to the first optical channel; measuring an optical power of the in-band signal over the passband, wherein an electrical power signal indicative of the optical power over the passband is generated; electrically amplifying the electrical power signal to generate an amplified power signal; digitizing the amplified power signal to generate power waveform data; and processing the power waveform data using a digital signal processor to calculate OSNR of the first optical channel, wherein the first optical signal is modulated using orthogonal dual-polarization modulation including an X-polarization component and a Y-polarization component, and further comprising: after the optically filtering, optically selecting, in a sequential alternating manner, one of the X-polarization component and the Y-polarization component for the in-band signal; generating, in the sequential alternating manner, first power waveform data for the X-polarization component and second power waveform data for the Y-polarization component; and processing the first power waveform data and the second power waveform data using the digital signal processor to calculate first OSNR for the X-polarization component and second OSNR for the Y-polarization component.