Optical & Photonic RF

CWDM

Q: What is the difference between CWDM and DWDM?

CWDM uses a wide 20 nm channel spacing (about 2500 GHz at 1550 nm) defined by ITU-T G.694.2, supporting up to 18 channels from 1271 to 1611 nm. DWDM uses tight 0.8, 0.4, or 0.2 nm spacing (100, 50, or 25 GHz) per ITU-T G.694.1, packing 40 to 96 or more channels into the C-band. The wide CWDM grid means transmitter wavelength can drift across temperature without leaving its passband, so CWDM uses uncooled DFB lasers with no thermoelectric cooler. This cuts cost and power by roughly 3 to 5 times versus DWDM, at the expense of channel count and reach. CWDM is typically passive and not amplified because most channels fall outside the EDFA gain window.

Q: Why can CWDM use uncooled lasers while DWDM cannot?

A directly modulated DFB laser drifts about 0.08 to 0.1 nm per degree C of junction temperature. Over a 0 to 70 degree C industrial range that is roughly 5.6 to 7 nm of total wavelength shift. CWDM allocates a wide passband (the 20 nm grid gives each filter roughly a plus or minus 6.5 nm window after accounting for laser tolerance and aging), so an uncooled laser stays within its channel without a thermoelectric cooler. DWDM channels are only 0.4 to 0.8 nm apart, so even 1 nm of drift would cross into the neighboring channel; DWDM lasers must be locked to within plus or minus 0.02 to 0.03 nm using a TEC and wavelength locker. Removing the TEC is what makes CWDM transceivers cheaper and lower power.

Q: What is the typical reach and channel count of a passive CWDM link?

A passive (unamplified) CWDM mux/demux pair adds about 1.5 to 3.0 dB insertion loss per device. With a typical 23 to 28 dB transceiver link budget and single-mode fiber loss of about 0.25 dB per km at the longer wavelengths rising to 0.35 dB per km or more on the short-wavelength channels, single-mode reach is commonly 40 to 80 km point to point. The 1310 to 1470 nm short-wavelength channels suffer higher fiber attenuation (and the 1391 nm water-peak region is often skipped on legacy G.652 fiber), so reach varies by channel. Full-spectrum systems use up to 18 channels on G.652.C/D low-water-peak fiber; legacy fiber that retains the 1383 nm OH absorption peak is usually limited to 8 channels in the longer-wavelength bands.

/koars dub-uhl-yoo dee em/ (Coarse Wavelength Division Multiplexing)

Short for coarse wavelength division multiplexing, an optical transport scheme standardized in ITU-T G.694.2 that combines up to 18 separate wavelengths on a 20 nm grid spanning 1271 to 1611 nm onto one single-mode fiber. The wide channel spacing (about 2,500 GHz at 1550 nm) lets transmitters drift with temperature without crossing into neighboring channels, so CWDM uses low-cost uncooled DFB lasers with no thermoelectric cooler. This makes it roughly 3 to 5 times cheaper and lower power than DWDM, at the cost of channel count and amplified reach. CWDM is the workhorse of access, enterprise, and short-haul transport, and increasingly carries digitized antenna data in radio-over-fiber mobile fronthaul.

Standard: ITU-T G.694.2

Channel Grid: 20 nm (1271 to 1611 nm)

Max Channels: 18

The 20 nm Grid and Why It Matters

CWDM trades spectral efficiency for cost. Where dense wavelength division multiplexing crams channels onto a 0.4 to 0.8 nm grid inside the narrow C-band, CWDM spreads 18 channels across 340 nm of spectrum on a fixed 20 nm grid. ITU-T G.694.2 anchors the grid at center wavelengths of 1271, 1291, 1311, and so on through 1611 nm, each labeled by its nominal value. The enormous spacing relative to DWDM is the entire point: a directly modulated distributed-feedback (DFB) laser drifts roughly 0.08 to 0.1 nm for every degree Celsius of temperature change, so across an industrial 0 to 70 degree C range it wanders 5.6 to 7 nm. A 20 nm channel passband swallows that drift comfortably, eliminating the need for a thermoelectric cooler, wavelength locker, and the power and cost they carry.

Each CWDM channel is separated by a thin-film filter (TFF) mux/demux that passes a roughly plus or minus 6.5 nm window and rejects adjacent channels by 30 dB or more. Because the filters are passive and broadband, a CWDM multiplexer is small, requires no power, and adds only 1.5 to 3.0 dB of insertion loss per pass. The penalty is reach. Most CWDM channels fall outside the 1530 to 1565 nm erbium-doped fiber amplifier gain window, so CWDM links are typically unamplified and limited by the transceiver link budget to about 40 to 80 km. Short-wavelength channels near 1310 nm also see higher fiber attenuation, and the 1391 nm channel sits near the OH water-absorption peak that legacy G.652 fiber retains.

Channel Loss and Link Budget

For an RF or microwave engineer feeding an analog or digitized signal over fiber, the relevant question is whether the optical link budget closes. The available budget is the difference between transmitter launch power and receiver sensitivity, and it must cover fiber attenuation, both mux and demux insertion losses, connectors, and a margin for aging. On low-water-peak G.652.C/D fiber the full 18-channel plan is usable; on older fiber retaining the 1383 nm peak the practical count drops to about 8 longer-wavelength channels.

Channel center wavelengths (ITU-T G.694.2, 20 nm grid):
λ_n = 1271 nm + (n × 20 nm), n = 0, 1, 2, … 17 → 1271 to 1611 nm

Optical link budget:
B = P_tx − S_rx ≈ (α × L) + IL_mux + IL_demux + L_conn + M_aging

Laser thermal wavelength drift:
Δλ ≈ 0.09 nm/°C × ΔT (≈ 6.3 nm over a 70 °C span)

Where α ≈ 0.25 to 0.35 dB/km, L = fiber length, IL ≈ 1.5 to 3.0 dB per mux/demux, M_aging ≈ 3 dB. Example: B = 28 dB, α = 0.30 dB/km, two filters at 2 dB, 3 dB margin → reach ≈ 70 km.

CWDM Versus DWDM and Other Optical Transport

Scheme	Standard	Channel Spacing	Max Channels	Laser / Cooling	Typical Reach
CWDM	ITU-T G.694.2	20 nm (≈2,500 GHz)	18	Uncooled DFB, no TEC	40 to 80 km (passive)
DWDM	ITU-T G.694.1	0.8 / 0.4 / 0.2 nm	40 to 96+	Cooled DFB + TEC + locker	80 to 2,000+ km (amplified)
LWDM / LAN-WDM	IEEE 802.3 (400GbE)	4.5 nm (800 GHz)	4 to 8	Cooled, O-band	2 to 10 km
Single wavelength (grey)	n/a	1 channel	1	Uncooled FP / DFB	0.5 to 40 km

Common Questions

Frequently Asked Questions

What is the difference between CWDM and DWDM?

CWDM uses a wide 20 nm grid (about 2,500 GHz at 1550 nm) per ITU-T G.694.2, supporting up to 18 channels from 1271 to 1611 nm with uncooled lasers. DWDM uses 0.8, 0.4, or 0.2 nm spacing per G.694.1, packing 40 to 96+ channels into the C-band with cooled, wavelength-locked lasers. The wide CWDM grid removes the thermoelectric cooler, cutting cost and power roughly 3 to 5 times, at the expense of channel count and amplified reach. CWDM is usually passive and unamplified because most channels sit outside the EDFA gain window.

Why can CWDM use uncooled lasers while DWDM cannot?

A DFB laser drifts about 0.08 to 0.1 nm/°C, or roughly 6.3 nm over a 70 °C industrial range. The 20 nm CWDM grid gives each filter about a plus or minus 6.5 nm window, so an uncooled laser stays inside its channel without a TEC. DWDM channels are only 0.4 to 0.8 nm apart, so even 1 nm of drift crosses into the neighbor; DWDM lasers must hold plus or minus 0.02 to 0.03 nm using a TEC and wavelength locker. Removing the TEC is what makes CWDM transceivers cheaper and lower power.

What is the typical reach and channel count of a passive CWDM link?

A passive mux/demux pair adds about 1.5 to 3.0 dB insertion loss each. With a 23 to 28 dB transceiver budget and fiber loss of 0.25 to 0.35 dB/km, single-mode reach is commonly 40 to 80 km point to point. Short-wavelength channels near 1310 nm see higher attenuation, and the 1391 nm water-peak region is skipped on legacy G.652 fiber. Full 18-channel operation needs low-water-peak G.652.C/D fiber; legacy fiber is usually limited to about 8 longer-wavelength channels.

Related Terms

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