Published June 21, 2026 • 7 min read • Telecom Insights
Wavelength Division Multiplexing (WDM) — and its dense variant DWDM — is the technology that multiplies fibre capacity by transmitting multiple optical carriers at different wavelengths simultaneously on a single fibre. Modern DWDM systems support 80–96 wavelengths in the C-band (1528–1566 nm, 4.8 THz spectrum), each carrying 400 Gbps or 800 Gbps, for aggregate fibre capacities exceeding 40 Tbps. With the extension into the L-band (1565–1625 nm), capacities approach 80 Tbps per fibre pair.
Key Takeaway: A single DWDM fibre pair carrying 80 wavelengths at 800 Gbps each delivers 64 Tbps of aggregate capacity — equivalent to streaming 16 million simultaneous 4K video streams over a strand of glass thinner than a human hair.
ITU-T Grid: Fixed Grid vs Flex Grid
DWDM wavelengths are organised on an ITU-T G.694.1 frequency grid anchored at 193.1 THz. The traditional fixed grid uses 50 GHz or 100 GHz channel spacing — sufficient for 10G/100G wavelengths but wasteful for mixed-rate systems. The flex grid (G.694.1, 2012) defines channels in 12.5 GHz slices, with a single wavelength occupying 4–12 slices (50–150 GHz). This enables efficient packing of mixed-rate wavelengths — 400G signals might use 75 GHz while 100G signals use 50 GHz on the same fibre. Super-channels combine multiple optical carriers to exceed the baud rate of a single laser.
ROADM: Reconfigurable Optical Add-Drop Multiplexer
ROADMs replace fixed optical filters with wavelength-selective switches that can dynamically add, drop, or pass individual wavelengths without manual patch panel changes. The core technology is the Wavelength Selective Switch (WSS) — using LCoS (Liquid Crystal on Silicon) or MEMS mirror arrays to steer individual wavelengths between ports. A Colourless-Directionless-Contentionless (CDC) ROADM allows any wavelength to be added/dropped at any port without wavelength or direction restrictions — the holy grail of optical flexibility. CDC-F (with Flex-grid) adds flexible channel width, completing the full ROADM capability set.
Coherent Optical Detection
Modern DWDM systems universally employ coherent detection using DP-QPSK, DP-16QAM, or DP-64QAM modulation with digital signal processing (DSP). The coherent receiver recovers both amplitude and phase of the optical field, enabling: polarisation multiplexing (2× spectral efficiency), electronic dispersion compensation (eliminating optical DCMs), and constellation shaping (probabilistic or geometric) for near-Shannon-limit performance. A modern 800G coherent DSP (e.g., Ciena WaveLogic 6, Nokia PSE-VI) processes ~100 billion samples/second with 7 nm ASIC technology.
400G and 800G Wavelengths
The industry is transitioning from 100G/200G to 400G ZR/ZR+ (pluggable QSFP-DD/OSFP coherent modules with ~120 km reach) and 800G line-side wavelengths for metro/long-haul. 800G uses ~128 GBaud symbol rate with DP-64QAM (6 bits/symbol/polarisation) or DP-16QAM probabilistic shaping to optimise reach vs capacity. Pluggable coherent optics (400ZR, OpenZR+) in QSFP-DD form factors bring coherent technology to router/switch ports, collapsing the traditional transponder layer — a paradigm shift in optical networking economics.
Amplification: EDFA and Raman
Erbium-Doped Fibre Amplifiers (EDFAs) provide gain across the C-band (1528–1562 nm) with noise figures around 4–5 dB — standard in every DWDM system. Raman amplification uses the transmission fibre itself as the gain medium, providing distributed amplification with lower effective noise figure (~0 dB), extending reach by 5–8 dB. Hybrid EDFA+Raman is standard for ultra-long-haul (>2000 km) submarine and terrestrial systems.
DWDM technology continues to push toward higher spectral efficiency, with research into S-band extension, space-division multiplexing (multi-core/multi-mode fibres), and hollow-core fibre with ultra-low latency for the next decade of capacity growth.
