SWDM in New Installations
Along with the excitement that SWDM is bringing to the future of Data Center communications, there is complementary work in the Telecommunications Industry Association subcommittee, TIA TR-42 to standardize a new type of wideband multimode fiber (WBMMF) to optimize the benefits of VCSEL based shortwave wavelength division multiplexing (SWDM).
Some of the goals and benefits of standardizing a new fiber with SWDM include:
- Retain legacy application support of OM4 (backward compatibility)
- Increase capacity to >100G per fiber
- Reduce fiber count by 4x
- Enable 100G-SR, 400G-SR4, 1600G-SR16
- Increase MMF utility and value to customers
A data center operator could deploy WBMMF today using conventional 850 nm optics with performance comparable to today’s OM4, yet having deployed the infrastructure to maximize the full reach possible when SWDM optics are deployed. Figure 1 illustrates an experiment done at 28Gb/s with a 940nm VCSEL, which is anticipated to be the reach-limiting wavelength used in near-term SWDM deployments. Two 100m fiber spools were characterized: one compliant to OM4 specifications and the other WBMMF. The BER sensitivity plots with each of these fibers were measured along with a reference back-to-back measurement shown in Figure 2. Test setup consists of a bit error rate tester (BERT, with a Pseudo random bit sequence Pattern Generator, PPG, and Error Detector, ED), transmitter (TX) evaluation board, fiber under test (FUT), variable optical attenuator (VOA), optical receiver (RX), and an external limiting amplifier.
Figure 2 below shows the results of this experiment, which is limited by the optical receiver used. Nevertheless, it illustrates the potential benefit of WBMMF over standard OM4.
While the OM4 fiber is compliant at 850nm, operation near the upper range of the SWDM wavelength range has almost a 2dB penalty over WBMMF specified to fit this application. That is 2dB that may be employed to maximize the reach, support more complex channel topologies (i.e. more patch panels), or provide comfortable operating margin in a SWDM deployment.
Demonstrations at tradeshows and in laboratories have shown 500m operation at 40G and 300m at 100G with this WideBand fiber and SWDM transceivers. However, the specified reach for each application is yet to be determined. As channel rates increase to 50Gbaud, having fiber that optimizes the performance of all SWDM wavelengths will be key to optimize channel design flexibility.
Today, 40G SWDM deployments are possible, and soon, 100G (25G x 4λ) SWDM will be possible over duplex fibers. 400G will be possible using parallel optical connectors (either via eight fibers with 25G x 4λ per fiber or via four fibers with 50G x 4λ per fiber).
|LC duplex fiber||12-fiber MPO for a parallel transceiver|
An investment in WBMMF made today will have years of benefit by enabling several future steps in data throughput as more complex modulation schemes are introduced. 1.6T on an MPO connection is expected to become a possibility. SWDM technology with WBMMF has a bright future.