What is coherent optical technology and why is it important?

Coherent optical technology uses advanced modulation and digital signal processing to dramatically increase capacity and reach of fiber optic systems.

Traditional (Direct Detection) vs Coherent

How Coherent Works

1. Modulate light with amplitude, phase, and polarization
2. Local oscillator laser at receiver
3. Coherent mixing captures full signal state
4. ADC digitizes for DSP
5. DSP compensates dispersion, PMD, nonlinearities

Key Enabling Technologies

• High-speed DACs and ADCs (60+ GSps)
• Sophisticated DSP algorithms
• Polarization multiplexing
• High-order QAM modulation
• Integrated silicon photonics

Modulation Formats

• QPSK: 4 states, most robust, 100G typical
• 8QAM: 8 states, 150G capacity
• 16QAM: 16 states, 200G-400G, shorter reach
• 64QAM: 64 states, highest capacity, shortest reach

Coherent Pluggable Optics

• 400G-ZR: 100G reach in QSFP-DD/OSFP
• 400G-ZR+: 1000+ km with amplification
• 800G coherent modules emerging
• Enables "IPoDWDM" — routers direct on DWDM

Impact on Network Design

• Simplified architecture (no transponders)
• Higher capacity per fiber
• Longer reach without regeneration
• Automatic dispersion/PMD compensation
• Reduced power per bit

Deployment Trends

• Metro DCI: 400G-ZR widely deployed
• Long-haul: 400G ZR+ replacing older systems
• Backbone: 800G coming
• Access: Not yet coherent (cost)