Oct 19

A Primer on WDM-PON

Fiber to the home (FTTH) has been available for some years now and is widely (although not universally) deployed. FTTH technology is still evolving to meet both present and future demands. Technologies such as wavelength division multiplexing (WDM) and time division multiplexing (TDM) have existed for decades. Today, a combination of those technologies, TWDM, presents one solution to feeder fiber exhaust and growing bandwidth demands in the last mile.

Legacy passive optical network (PON) systems employed an optical splitter to link an optical line terminal (OLT) to up to 64 optical network units (ONUs) using a single feeder fiber. The optical distribution network (ODN) was completed by the installation of a single fiber from each splitter output port to a subscriber. NG-PON(2), as described by the ITU G.989 recommendation, is designed to coexist with legacy PON and XG-PON and allow a graceful upgrade path to NG-PON and NG-PON2.

The ITU-T G.989 NG-PON2 recommends 100 GHz channel spacing for downstream transmission and either 50, 100 or 200 GHz for upstream wavelength channel spacing. The G.989 recommendation proposes addresses both TWDM and an optional P2P WDM architecture for optical transmission.

  • Option 1 – TWDM: TWDM systems use legacy splitters (power splitting) for coexistence. The ITU recommends the optical spectrum of 1524 to 1544 nanometers for upstream TWDM transmission and 1596 to 1603 nanometers for downstream TWDM transmission. In this case the ONUs are NG-PON2 ONUs that are equipped with both a tunable transmitter and tunable receiver. In this configuration the OLT can support up to 64 ONUs.

Courtesy BT Research
Figure 1. Examples of flexible deployment options for coexistence and greenfields
  • Option 2 – Dedicated P2P WDM:  The G.989.2 standard allows for an optional P2P WDM links. These links would pass through a branching node providing wavelength selection. In this case the ONU downstream wavelength is determined by its physical connectivity. This system can span distances greater than 20 km by bypassing the power splitter and its 15~18 dB of optical attenuation.

Designers, planners, and managers must know how to provision these updated FTTH architectures, and especially how to gracefully and elegantly migrate from legacy architectures to next-generation architectures. Note that the scope of the ITU G.989 standard does not address all future proposed commercial PON architectures.

But what does all of this mean for those who install, test, and troubleshoot WDM‑PON systems?

One clear impact is the increased sensitivity to macrobends when operating at longer wavelengths. This is critical to consider when working with splice trays and other fiber/cable management products. Installing bend-insensitive single-mode fiber can mitigate this effect but this is not always practical, nor is it safe to assume that previously-installed fibers are bend insensitive. Testing installations with an OTDR at 1650 nanometers is recommended in order to ensure macrobending losses do not impact the system performance. Additionally, new optical test equipment such as wavelength selective optical power meters may be required to identify the specific wavelengths provisioned.