CWDM4-OCP Optical Module Specifications
Dec 18, 2025| What are the specifications of the CWDM4-0CP optical module? Facebook's OCP project defined the module specifications.
The 100G CWDM4 module used in the OCP (Open Compute Project) has the following specifications, which are consistent with the MSA-defined CWDM4 module specifications.
| Parameter | Standard CWDM4 | CWDM4-OCP | Notes |
|---|---|---|---|
| Rate | 4×25 Gb/s | 4×25 Gb/s | Unchanged |
| Package | QSFP28 | QSFP28 | Unchanged |
| Transmission Distance | 2 km | 500 m | Reduced |
| Tx OMA Transmit Optical Power | -4 dBm | -5 dBm | Transmit power reduced by 1 dB |
| Rx Sensitivity | -10 dBm | -9.5 dBm | Receive sensitivity reduced by 0.5 dB |
| Channel Loss | 5 dB | 3.5 dB | Total loss reduced by 1.5 dB |
| Operating Temperature (Module External Temperature) | 0~70 °C | 15~55 °C | Relaxed |
In general, the following points have been relaxed:
(1) The operating temperature range has been broadened, from the original commercial grade temperature to 15~55℃.
(2) The transmission distance has been reduced from 2km to 500m, which saves 1.5 dB in the fiber optic channel power budget (originally a 5dB budget). This 1.5 dB allows for more relaxed requirements for the optical module's transmission and reception specifications.
Facebook allocates it as follows: the output power of the transmitter is relaxed by 1 dB; the receiver sensitivity is relaxed by 0.5 dB. Other optical module specifications remain consistent with CWDM4, still using the QSFP28 package and still featuring four channels, each with a data rate of 25 Gb/s.
Facebook chose this type of optical module for several reasons:
(1) The vast majority of fiber optic cabling distances in data centers are less than 500 meters.
(2) In the past, at lower data rates, both multimode and single-mode fibers were used, requiring separate deployment. However, as optical module speeds increase, the transmission distance of multimode fiber decreases. Therefore, promoting overall single-mode fiber cabling is more convenient for future distance and speed expansion, and avoids the hassle of managing both single-mode and multimode fibers separately.
(3) Previous 500m transmissions used parallel fibers, but now with CWDM4 modules, only 1/4 of the fiber count is needed. This is a significant improvement! Optical module technology is now advanced enough to support internal wavelength division multiplexing, and the price has also decreased to an affordable level.
100G short-wavelength (850~940nm) wavelength division multiplexing multi-source protocol
Transmitting and receiving optical path specifications:
| Main Parameter | Requirement | Unit | Notes |
|---|---|---|---|
| Average Transmit Power | -7.5 ~ 2 dBm | dBm | |
| Average Transmit Power | -5.5 ~ 3 dBm | dBm | |
| Extinction Ratio | 2 dB | dB | |
| Receive Power (Average) | -9.5 dBm | dBm | |
| Receive Sensitivity (OMA, Average Method) | TBD | To Be Determined |
Link metrics:
| Main Parameter | OM3 | OM4 | OM5 | Unit | Notes |
|---|---|---|---|---|---|
| Transmission Distance | 75 | 100 | 150 | m | |
| Insertion Loss | 1.8 | 1.9 | 2.9 | dB | |
| Effective Modal Bandwidth @ 850 nm | See notes below | See notes below | See notes below | MHz · km | See below |
| Channel Cost | L0: 1.8 L1: 1.8 L2: 1.8 L3: 1.7 | L0: 1.9 L1: 1.9 L2: 1.8 L3: 1.8 | L0: 2.0 L1: 1.9 L2: 1.9 L3: 1.9 | dB | Likely different channel insertion loss budgets (L0 to L3 possibly representing different configurations or wavelengths) |
What is effective modal bandwidth?
In early multimode fiber designs, LEDs were considered as light sources. LEDs have a larger divergence angle than the fiber core, resulting in a very large spot size, hence the term "full injection bandwidth."
Later, it was found that using VCSELs as light sources was more convenient and less expensive. VCSELs have a smaller divergence angle than LEDs, smaller than the fiber core, so the concept of full injection bandwidth was no longer applicable. Therefore, the concept of effective modal bandwidth was introduced, which is a product of bandwidth and distance.
Low signal bandwidth allows for longer transmission distances.
High signal bandwidth results in shorter transmission distances.
For example, with a 25 Gb/s 940 nm signal, if the device has a bandwidth of 16 GHz (the theoretical limit for 25 Gb/s NRZ is 12.5 GHz), then the effective modal bandwidth of OM5 is 2,500 MHz·km, allowing for a transmission distance of (2,500/16,000) = 156 m.
OM4 has an effective modal bandwidth of 1859 MHz·km, which allows for transmission over (1859/16000) = 116 meters. This MSA specifies a transmission distance of 100m for OM4 and 150m for OM5. The effective modal bandwidth is a key performance indicator in the MSA.
| Wavelength (nm) | Effective Modal Bandwidth (MHz · km) - OM3 | Effective Modal Bandwidth (MHz · km) - OM4 | Effective Modal Bandwidth (MHz · km) - OM5 | Unit |
|---|---|---|---|---|
| 850 | 2000 | 4520 | 4190 | MHz · km |
| 880 | 1667 | 3076 | 3700 | MHz · km |
| 910 | 1426 | 2329 | 2880 | MHz · km |
| 940 | 1243 | 1859 | 2600 | MHz · km |