To meet current and expected increases in bandwidth demand, cable operators have begun to roll out fiber deep and Node+0 architectures to support their DOCSIS 3.1 and Distributed Access Architecture strategies. This article is the second in a series that covers various aspects of multiwavelength transmission that enables operators to take advantage of existing infrastructure as part of their bandwidth expansion plans. The first article in the series provided background information on fiber deep and wavelength division multiplexing (WDM). Here, we discuss WDM component technology and specification parameters. Other articles in the series cover multiwavelength transmission choices and the impairment factors that should be considered, and network architecture and cabling strategies.
WDM Filter Types
Most telecom equipment commonly uses two different types of “filters” to multiplex (“mux”) and demultiplex (“demux”) wavelengths: arrayed waveguide gratings (AWG) and thin film filters (TFF), as shown in Figure 1.
AWGs use diffraction of light as the operating principle. Diffraction refers to a specific kind of interference (constructive or destructive) of lightwaves. This interference effect enables the muxing or demuxing.
In TFFs, wavelengths are either reflected or refracted through the filter. Reflection involves a change in direction of lightwaves when they bounce off a barrier. Lightwave refraction shifts the direction of waves as they pass from one medium to another.
The type of filter that is used for a DWDM fiber deep network will be based on the requirements of the network and the specifications needed on the device. Figure 2 shows an example of a DWDM filter with different specification parameters.
DWDM component specifications
An understanding of specification parameters is helpful when determining components for DWDM transmission. Specifications such as channel spacing, passband, isolation loss, gain ripple and insertion loss help determine performance of the DWDM component.
Channel Spacing: DWDM channel spacing is addressed in ITU-T 694.2. Spacing of 100 GHz enables but is not limited to 32, 40 and 44 channels. Spacing of 50 GHz doubles the potential channel count, enabling 64, 80 and 88 channels. Figure 3 offers an example of DWDM filter channel spacing.
Passband: Measured in GHz (gigahertz), passband bandwidth is the spectral width centered on the ITU grid at a given power level, usually 0.5 dB below the minimum insertion loss point. DWDM filters typically use channel spacing of 100 GHz with a working channel passband of +/-12.5 GHz from the wavelength’s center.
Isolation Loss: Isolation loss, measured in dB (decibels), is the ability to prevent light power at wavelengths outside the passband from passing through the channel passband port. Isolation loss is the difference of the maximum insertion loss within the filter passband and the minimum loss occurring within other filtering passbands.
Gain Ripple: Gain ripple is the maximum peak-to-peak variation in dB of insertion loss across a filter passband.
Insertion Loss: Insertion loss of a DWDM filter is given in dB as the maximum insertion loss occurring at the channel port with the highest loss. Insertion loss is the power difference between the 0-dB level and the minimum power level in a given passband window (see Figure 4).
See Figure 5 for other key specification parameters.
With this understanding of WDM technology and specification parameters, you might be interested in the final two articles in this series, which address multiwavelength transmission choices and the impairment factors that should be considered, and network architecture and cabling strategies. For additional background on fiber deep and WDM, refer to our first article.
David Kozischek is an applications marketing manager at Corning Optical Communications.