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CWDM4: 100G Coarse Wavelength Division Multiplexing Technology, Working Principles, and Key Specifications

LINK-PP Official 　·

Dec 03,2025

What is CWDM4?

CWDM4 (Coarse Wavelength Division Multiplexing 4) is a four-wavelength multiplexing specification that transmits four optical channels over a single fiber using coarse wavelength spacing. Each channel operates at a nominal wavelength around the 1310 nm band. CWDM4 is commonly used in 100G optical Ethernet systems, enabling compact, low-power, and cost-efficient parallel optical transmission with simplified wavelength control.

Technical Background and Evolution

CWDM originated as a method to reduce wavelength stabilization requirements compared to dense WDM systems. With the growth of 100G Ethernet and data center interconnect demand, a four-lane version—CWDM4—was introduced to aggregate four 25G optical carriers into a single 100G link.

CWDM4 was developed to solve key industry challenges:

Reducing cost and power consumption in high-volume data center optics
Enabling parallel optics without dense wavelength spacing
Maintaining compatibility with single-mode fibers
Avoiding stringent wavelength-locked lasers
Supporting longer reach than multimode parallel optics

CWDM4 evolved alongside 25G SerDes advances, external modulation improvements, and optical component integration (MUX/DEMUX filters, alignment, packaging).

How It Works

CWDM4 uses four optical wavelengths, each carrying a 25 Gbps lane (NRZ) or 26G line-coded equivalent, combined into a single fiber.

Core mechanisms include:

1. Laser Sources

Four lasers operate at coarse wavelengths (typically ~1271, 1291, 1311, 1331 nm).
Coarse spacing reduces thermal stabilization complexity.

2. Multiplexing (MUX)

A passive optical filter combines the four wavelengths with low insertion loss.

3. Fiber Transmission

A single-mode fiber transports all wavelengths simultaneously without interference due to the channel spacing.

4. Demultiplexing (DEMUX)

On the receiver side, the four wavelengths are optically separated.

5. Detection and Electrical Conversion

Each lane is received by a photodiode (often PIN) and processed by a TIA + SerDes.

6. Wavelength Stability and Temperature Management

CWDM4 relaxes wavelength tolerance compared to DWDM, enabling low-cost uncooled lasers.

Key Specifications

Typical CWDM4 parameters include:

Number of channels: 4
Nominal wavelengths: ~1271, 1291, 1311, 1331 nm
Channel spacing: ~20 nm
Lane data rate: 25–28 Gbps
Aggregate data rate: 100 Gbps
Transmission distance: typically up to 2 km on single-mode fiber
Encoding: NRZ; some variants support 25G FEC formats
Optical budget: defined by insertion loss, receiver sensitivity, and filter tolerances
Laser type: uncooled CWDM DFB or EML
Receiver type: usually PIN photodiodes (APDs optional but uncommon)

Typical Use Cases

100G Ethernet optical links (100GBASE-CWDM4)
Data center interconnects (leaf–spine fabrics)
Cloud infrastructure and hyperscale networks
5G/6G fronthaul mid-haul links
AI/ML cluster fabrics requiring 100G per-port connectivity
Enterprise and regional backbone systems

CWDM4 is widely used where moderate reach, low power, and cost efficiency are critical.

Related Technologies / Terms

100GBASE-LR4: Dense 4-wavelength LAN reach standard using tighter λ spacing.
CWDM: Coarse WDM with larger channel spacing (20 nm).
DWDM: Dense WDM with tightly spaced channels used for long-haul transport.
LAN-WDM: Intermediate wavelength spacing used in certain 100G optics.
PAM4: Used in 200G/400G parallel systems, not typical for CWDM4.
EML / DFB Lasers: Typical laser types supporting CWDM4 wavelengths.
MUX/DEMUX Filters: Passive components used for wavelength combining and separation.

Frequently Asked Questions (FAQs)

1. How is CWDM4 different from LR4?

CWDM4 uses wider (coarse) wavelength spacing and uncooled lasers, making it lower cost. LR4 uses tightly controlled wavelengths and supports longer distances.

2. What distance does CWDM4 typically support?

Up to 2 km over single-mode fiber, depending on insertion loss and link budget.

3. Does CWDM4 require cooled lasers?

No. CWDM4 is designed for uncooled DFB/EML lasers due to its large wavelength tolerance.

4. Is CWDM4 compatible with PAM4?

Standard CWDM4 uses NRZ at 25G lanes. PAM4 is used in 200G/400G systems; CWDM4 generally remains NRZ-based.

5. Why does CWDM4 use wavelengths around 1310 nm?

The O-band minimizes chromatic dispersion and supports low-cost lambda generation.

6. What determines the optical budget of CWDM4?

Loss from MUX/DEMUX, connector loss, fiber attenuation, and receiver sensitivity.

Industry Standards Involved

CWDM4 is referenced or defined in:

CWDM4 MSA (original specification for 100G CWDM4 optics)
IEEE 802.3bm / 802.3cu for 100GBASE-CWDM4 optical interfaces
IEC component performance and safety standards
MSA transceiver form-factor standards (SFP, SFP28, QSFP28, etc.)

These standards define wavelengths, optical budgets, transmitter requirements, and receiver sensitivity.

Summary

CWDM4 is a four-channel coarse wavelength multiplexing technology designed to support 100G optical transmission over single-mode fiber with relaxed wavelength control, low power, and reduced cost. It is a widely adopted solution in data centers and cloud networks, providing a balance between performance, reach, and manufacturability.