Explain coherent optical communication and its advantages over direct detection.

Coherent optical communication mixes received signal with local oscillator laser before detection, preserving phase and enabling complex modulation formats. Advantages: Improved receiver sensitivity (approaching quantum limit), Access to full optical field (amplitude and phase), enabling PM-QPSK, 16-QAM etc., Linear channel response enables digital signal processing for impairment compensation, and Flexible wavelength selection without optical filters. DSP algorithms compensate chromatic dispersion, PMD, and laser phase noise digitally. Enables 100G/400G per wavelength transmission over long distances. Challenges include laser linewidth requirements and DSP complexity.