Faster transponders that operate at 200 Gbit/s and 400 Gbit/s are becoming more and more commercially available. However, many optical spectrum analyzer (OSA) users need to decide what type of OSA is required to analyze these signals in the field: traditional grating-based OSAs or a new generation of OSAs? There have been some comments that operators should select the high-resolution OSAs, which provide higher resolution bandwidth (RBW), for optimal analysis. Is this hype really founded?
Let’s get back to basics. An OSA is a wavelength-dependent power meter. A filter, or slit, lets through a certain amount of light onto a detector, which takes the measurement. The OSA then displays the measured power associated with the central wavelength of the filter and, moving the filter across the entire wavelength band of interest, a graph is then generated showing power versus wavelength. Narrower slits will have higher resolution, as broad slits average the information from a broader range, which inhibits the view of closely spaced features.
RBW can therefore be defined as the width of the slit inside the OSA. A smaller RBW will enable one to see finer details of a DWDM spectrum, such as clearly separating close channels.
But is a smaller RBW always better?
Not necessarily. The reason? The RBW must be wide enough to let through sufficient optical noise in a single scan, while at the same time avoiding crosstalk from neighboring signals. Studies prove that the best OSA for a given application is one that offers the perfect balance between a smaller RBW to separate channels and a larger RBW to provide good noise measurements. Here is a rule of thumb on how to select the right OSA from EXFO, depending on the application:
EXFO’s latest white paper, Choosing the Best Field-based Optical Spectrum Analyzer for Analysis of 200G/400G and Flexgrid Signals, provides an in-depth look into the inherent differences between grating-based and high-resolution OSAs and, supported by three case studies, explains why high-resolution OSAs are not always the best choice for network equipment manufacturers (NEMs) and operators. Before acquiring any new OSAs for your 200G/400G deployments, this white paper is a must-read to weed out the hype from technical facts.