A Summary of SWDM

swdm

The continued growth in high bandwidth applications and the population that uses them drives the need for higher data rates in all segments of optical networks–including data centers, local area networks, optical backplanes, and other short reach applications. Many data center operators are already upgrading their data center connection technologies from 10Gbps and 40Gbps to 25Gbps and 100Gbps. SWDM is one of the latest technologies adapted to boost data rates in short reach applications. In this article, a summary of some of the key aspects of SWDM technology, including its advantages and the type of optical fiber required to enable it are presented.

What is SWDM?

Shortwave wavelength division multiplexing (SWDM) is a technology that uses multiple wavelengths at the shorter wavelength end of the fiber optic spectrum, around 850nm, in a single multimode fiber (MMF). In other words, the same well know CWDM technology is applied to short reach MMF systems.

SWDM systems use four channels centered at 850, 880, 910 and 940nm (30nm spacing) to achieve higher data rates. 40Gbps is implemented as 10Gbps x 4λ and 100Gbps is implemented as 25Gbps x 4λ.

Why SWDM?

The cost effectiveness of MMF based systems in short reach applications is enabled by low cost vertical-cavity surface emitting lasers (VCSELs) which operate at 850nm. Unfortunately, VCSELs are prone to modulation challenges making it difficult to modulate them fast enough to transmit high data rate signals. Although work is in progress to develop a 50Gbps standard, the highest standardized single channel data rate for MMF systems to date is 25Gbps (IEEE P802.3by).

In 2010, IEEE ratified 802.3BA 40G/100G recommendation to meet increasing bandwidth demand. This standard uses parallel optics – transceiver and optical fiber arrays – to achieve high data rates of 40Gbps and 100Gbps. 40Gbps is implemented as 10Gbps x 4f (40GBASE-SR4) and 100Gbps is implemented as 10Gbps x 10f (100GBASE-SR10) or 25Gbps x 4f (100GBASE-SR4). 12 or 24f ribbon cables are used to achieve 40 or 100Gbps bidirectional transmission.

There are a few challenges to parallel optics implementation of 40G and 100G MMF systems and they include:

  • Users with laser optimized OM3 and OM4 duplex cable already installed must replace their cable with more expensive ribbon cable with multiple optical fibers.
  • Optical skew – due to slight variations in optical fiber length and group index of refraction in a ribbon cable, component signals of the high data rate signals arrive at slightly different times thereby distorting the signal
  • Slight transceiver/fiber array mismatches may result in signal degradation.

SWDM addresses these issues by using multiple wavelengths in a single duplex fiber optic cable.

WideBand MMF (OM5) for SWDM

Laser optimized cables OM3 and OM4 are optimized for transmission at one wavelength of 850nm. A new fiber standard, TIA-492AAAE WideBand MMF, requiring the specification of effective modal bandwidth (EMB) and attenuation at 953nm in addition to 850nm was published in 2016. Guaranteeing bandwidth performance at 850nm and 953nm ensures adequate bandwidth at all wavelengths of interest for SWDM.

ISO/IEC 11801 chose OM5 as the official designation for cabling containing the WideBand MMF.

In a nutshell

SWDM technology boosts transmission capacity by sending multiple signals in four wavelengths of a single fiber. The development of Wideband MMF enables adequate bandwidth for the specified SWDM wavelengths.

To learn more about optical networking, enroll in a Certified Optical Network Associate (CONA) course.

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