The Lightwave Blog

Editor's Note: One of our visitors asked to start a new thread. Here it is.

Upon my first reading the heading, "A first step toward sanity," I'd have bet money that you were referring to Oki's breakthrough in the ultra long haul space, by achieving, reportedly, up to 160 Gbps at the present time over thousands of km's, thus appearing to find a pathway to breaking the back of PMD's stultifying effects on the upper bounds of throughput over ULH distances: http://tinyurl.com/4beawp.

Until now, the industry seems to have almost surrendered to the idea that parallel optics (i.e., using various methods of effecting "inverse multiplexing" over multiple wavelengths) is the only approach to achieving speeds greater than 10G over extended distances, and in many cases even 10G, never mind 100G, 160G, or beyond.

As recently as last month I read an account that covered this topic in Lightwave by an author who stated that PMD has little effect on singlemode fiber these days. (See the article here.) Of course, many manufacturers of "newer" single mode are affected less than most variants of "older" single mode due to recent improvements in manufacture, but PMD at some point (as symbol rates are raised sufficiently high) is an anomalous characteristic that affects all types of fiber, through both static and dynamic manifestations.

It occurs to me that many authors who make these assertions either have products to sell that use parallel optics, or they have come to a foregone conclusion for other reasons that symbol rates will remain at 10 Gigabaud or below indefinitely, or so it seems, while employing various forms of analog (duo-binary, quadrature-based, etc.) modulation techniques, hence making it easy to suggest that higher speeds can indeed be achieved in the presence of PMD.

While this may be true, at what cost is this occurring in terms of the number of wavelengths that must be used in support of ever-higher throughput rates? And at what cost is it occurring in terms of forfeiting optical-level compatibility between a growing number of dissimilar analog modulation schemes as they begin to converge in large numbers and require handing off to one another at optical network nodes? Thoughts?

Frank A. Coluccio, DTI Consulting Inc.
[email protected]