CWDM vs DWDM: Expanding Capacity Without New Fiber

The core idea

Wavelength division multiplexing sends multiple independent signals down one fiber, each on its own wavelength. The fiber count stays the same; the capacity multiplies. There are two mainstream variants, and they are chosen for different reasons.

CWDM — coarse wavelength division multiplexing

• Channel spacing: 20 nm
• Typical channel count: up to 18, spanning roughly 1270 nm to 1610 nm
• Light source: uncooled DFB lasers
• Cost: lower per channel, because uncooled lasers are cheaper and consume less power
• Reach: typically up to 80–120 km, but practical distances are often limited by the fact that CWDM channels are not compatible with standard erbium-doped fiber amplifiers (EDFAs)

DWDM — dense wavelength division multiplexing

• Channel spacing: 100 GHz (~0.8 nm) or 50 GHz (~0.4 nm)
• Typical channel count: 40 to 80+ in the C-band
• Light source: cooled, wavelength-stabilised lasers
• Cost: higher per channel, because the lasers must be temperature-stabilised and precisely tuned
• Reach: designed to work with EDFA amplification, which is what makes multi-hundred-kilometre spans feasible

The decision table

| | CWDM | DWDM | |---|---|---| | Spacing | 20 nm | 100 / 50 GHz | | Channels | Up to 18 | 40–80+ | | Laser | Uncooled | Cooled | | Per-channel cost | Lower | Higher | | Amplifier compatible | No (standard EDFA band) | Yes | | Typical reach | 40–80 km | 80 km to several hundred km | | Best fit | Metro access, capacity on a budget | Long-haul, high channel count, amplified spans |

How to choose

Two questions usually settle it:

1. Do you need amplification? If the span needs an EDFA, DWDM is the answer. CWDM channels fall outside the standard amplification band.
2. How many channels do you actually need? If the answer is under a dozen and the distance is inside 80 km, CWDM delivers the same result at materially lower cost per channel.

A common pattern is CWDM in the metro access layer and DWDM in the core, with an OADM or ROADM handoff between them.

What to plan before ordering

• The full channel plan, not just today's channels — the filter and mux/demux units are sized once.
• Insertion loss for every passive component, including patch panels and OADMs. Long-reach WDM links fail on optical budget far more often than on module quality.
• Optical testing on the link before commissioning. This is standard practice and worth insisting on.

Canjin Technology supplies CWDM and DWDM optics from 1.25G SFP to 10G SFP+, in 40 km, 80 km and 120 km classes with full channel listings by part number.