Many factors need consideration when planning an increase in network capacity: availability of fiber, construction costs, network architecture and type of nodes deployed. Operators have worked hard to standardize their networks — typically using a 1310 nm transport for shorter hauls when there was sufficient fiber in place, as well as 1550 nm technologies to cover longer distances and/or to overcome fiber scarcity. Today, fiber availability is still a major challenge to network growth due to the high cost of installing new fibers.

A variety of wavelength division multiplexing (xWDM) techniques have evolved to address the problems both of fiber scarcity and continuing demands for increased network capacity. Several of these techniques have been standardized on wavelength "grids" across the optical spectrum, and Figure 1 is a simplified graphical representation of some of these wavelength plans.

Figure 1. Representation of the ITU Wavelength Grid

Click on diagram to enlarge

DWDM

Since the 1990s, equipment manufacturers have been pioneering Dense Wavelength Division Multiplexing (DWDM) technology for the distribution of narrowcast signals. This technique enables more and more wavelengths, and hence narrowcast services, to be carried on the same fiber. These DWDM wavelengths are positioned between 1530 nm and 1565 nm (C-band) as shown in Figure 1. This technology has evolved from early 8-wavelength systems on a 200 GHz spaced grid to the current industry-standard (ITU-T G.694.1) 40-wavelength system with 100 GHz optical spacing. Initially, each DWDM wavelength carried just 8 RF QAM narrowcast channels, containing a mix of video, high-speed data and voice. However, the technology has matured to a 32 QAM channels per wavelength standard, with more advanced manufacturers supporting 75 QAM channels (450MHz—1GHz BW) per wavelength. While DWDM is now widely deployed, it has primarily been a bandwidth expansion tool for MSOs who had standardized on 1550 nm hybrid fiber/coax (HFC) networks.

CWDM

Like DWDM, Coarse Wavelength Division Multiplexing (CWDM) is centered in the C-band. However, the wavelengths here are spaced 20 nm apart. Initially the standard extended from 1470 nm to 1610 nm, with up to eight wavelengths supported. However, ITU-T Recommendation G694-2 was approved in June 2002, extending this down to 1270 nm and supporting up to 18 wavelengths (see Figure 1), assuming no "water-peak". (At the end of 2003, it was further decided to jog the wavelength grid by 1 nm to align it with current industry practice while maintaining symmetrical nominal central wavelength deviations.) For systems installed today, Aurora has taken advantage of 15 of the CWDM wavelengths, avoiding the "water-peak," to provide a very cost-effective solution for its digital return technology.

LcWDM^®

O-band (1260 to 1360 nm) dense WDM solutions for CATV optical transport have been investigated for some time but until recently there were no obvious cost-effective solutions to the many non-linear effects associated with optical transmission in close proximity to the zero-dispersion wavelength of the fiber. It should be noted that basically every fiber optimized for the 1310 nm window of operation, including SMF-28^® and SMF-28e^® fibers, and installed since the early 1980s, shares these similar zero-dispersion characteristics.

A breakthrough has now made it possible for MSOs to deploy more densely spaced multi-wavelengths in the O-band. This is represented in Figure 1. This O-band multi-wavelength solution, which Aurora has trademarked under the name LcWDM (Low Cost Wavelength Division Multiplexing), enables up to 8 separate wavelengths to be carried in this band in the downstream on one fiber — and for a reach up to 30 km. Previously, the limitation was just one wavelength in this band — using traditional 1310 nm DFBs which have been extensively deployed in our industry for the last 15–20 years.

Multi-wavelength solutions for the O-band were first introduced in 2006. After careful study of O-band non-linearity effects, it quickly became apparent that it was choice of wavelength and their relative position to one another which would be critical to developing a field-deployable system. The correct wavelength positioning "neutralizes" or avoids the impact of fiber phenomena such as fiber dispersion, four-wave mixing, SRS and cross phase modulation on RF signals modulated onto the laser light. These phenomena, interacting with laser transmitter characteristics in the multi-wavelength system, all cause distortions, mostly composite second order (CSO), in the RF spectrum. Further research has led to the tightening of specifications for optical filters while still incorporating the same robust and reliable filter technology that has been proven over many years in DWDM C-band applications.

The first introduction of this dense WDM O-band technology was limited to just two wavelengths; subsequent technology advances and production refinements have resulted in 8 wavelengths now being supported. Today, these systems can support reaches up to 30 km, depending upon the number of wavelengths and the desired receive optical power at the forward path receiver. However, further refinement in the choice of wavelengths, improved laser technology specifically focused in areas where laser parameters interact with fiber characteristics and non-linearities, and improvement in electronics to further counter non-linear fiber effects will continue to further evolve this technology towards more wavelengths and longer distances over a single fiber.

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