Cisco Optics Podcast Ep 51. If single-mode fiber seems like magic to you, listen to this (8 of 8)

[00:00:09] Hello everyone and welcome back to the Cisco Optics Podcast where we talk about pluggable optics for networks. Single-mode fiber transformed telecommunications in the 1970s and 1980s by raising the ceiling for communication bandwidth over transoceanic distances. It took years of R&D to get there though. Remember, we're sending light through tens of kilometers of glass without regeneration. I mean, imagine how pure the glass of an 80-kilometer thick window needs to be in order to see through it.

[00:00:36] This is episode 51 and we conclude our conversation with Rasheen McCool, expert in fiber networks for telecommunications and scientific instrumentation at Corning Optical Communications. We talk about fiber dispersion's effect on communication and coherent optical signals. Rasheen McCool is a senior market and technology develop manager for Corning Optical Fiber and Cable, specializing in single-mode fiber and access networks. She provides technical and market insight for Corning's global fiber and cable product line management teams.

[00:01:03] Rasheen has 30 years of experience in engineering and optical communications systems and subsystems, including many years designing networks for advanced radio telescopes. Rasheen has held project management and coordination lead roles in diverse teams throughout her career. Rasheen is the acting chair of the Women and Fiber Committee for the FTTH Council Europe and is the winner of the Corning Optical Communications You Make a Difference Award 2022 for her leadership of diversity initiatives in Corning.

[00:01:30] Rasheen holds a master's degree in electrical and electronic engineering from the University of Nottingham, United Kingdom, and is a chartered member of the Institute of Engineering and Technology. And now join me as I talk with Rasheen McCool. The first installation of that fiber was between New York and Washington, D.C. So I heard a story, I don't know if you've heard this or not,

[00:01:59] but I heard a story that those three people you mentioned who made the first low-loss fiber said, if we had known when we started that it would be so this hard and take this long, we never would have started. Have you heard that at all? That's what someone told me.

[00:02:26] I haven't heard that, but it sounds very similar to the code of the software engineer, which was, we did it because we thought it would be easy. Well, fortunately, they were perseverant and just didn't give up. Yes, exactly.

[00:02:46] And then since that time, then, you know, it's that story of improvement on attenuation, addition of characteristics that are useful to specific applications, the quality architecture so that every fiber is, you know, does what it says it will do and that people can rely on that.

[00:03:15] And then all the way down to 2017, where Corning delivered its 1 billion kilometers. Wow. We didn't talk about dispersion. Yeah, dispersion matters. And it's really interesting because it mattered for long-haul networks for a long time. So what is dispersion, right? What's chromatic dispersion?

[00:03:41] So chromatic dispersion happens because a pulse of light in the fiber will have very small differences in frequency. And those frequencies will travel at slightly different rates within the fiber, right? So what happens is you're going to have some frequencies that are traveling faster,

[00:04:10] some frequencies that are traveling slower. And that, at the receiver, what that does is it spreads the pulse. And that difference in the velocity is that's dispersion, right? That's what dispersion is. Yeah. Yeah. Yeah, exactly. Exactly. Now, what happens is if you get enough dispersion, subsequent pulses are going to crash in to the pulse before and the pulse after.

[00:04:40] And that's called inter-symbol interference. And that's a real problem in transmission systems because it makes it difficult for the receiver to identify whether it's got a zero or a one. Right. Now, the... And in our labs, we often look at eye diagrams. And so you can see the effect of that as the eye closure. It'll close the eye diagram. Yeah. Yeah, yeah, exact. Exactly. It'll close the eye diagram.

[00:05:11] Now, in the past, that has been a problem on long hauling because the further you go, the more of that dispersion effect you've got. But what happened recently in... Well, I say recently, but in this past decade, is that coherent transmission has become pervasive in long haul networks.

[00:05:34] And coherent transmission has digital signal processing, which will extract this chromatic dispersion impact. And so chromatic dispersion has become less of an issue in modern long haul networks.

[00:05:53] However, the short haul links, so the intensity modulation direct detect links, they're getting faster and faster and faster. And of course, as your bit pulse gets smaller and smaller, the more likely chromatic dispersion is to have an impact on it. Right, because they're closer together.

[00:06:18] So even just a little bit of bleeding, if they're closer together, is going to cause that intersymbol interference. Yeah, yeah. And because the shorter the pulse, then for the same delay, you're going to have more of a percentage of that pulse which is impacted. Mm-hmm.

[00:07:19] And so that's something that we have to take care of. And the other aspect of noise within a system that can impact the ability of the receiver to receive error-free transmission is polarisation mode dispersion.

[00:07:38] Now, polarisation mode dispersion happens because fibre is what is called an anisotropic material or an anisotropic crystal, which means that when light hits the fibre, it actually transmits, although it's a single mode, it transmits in two orthogonal polarisations. Mm-hmm. And those polarisations are always orthogonal.

[00:08:06] But they travel through the fibre and they sort of scramble their direction. And as they travel through the fibre, what can happen is that one of those polarisations may travel faster than another. And again, at the fibre, you end up with constructive and destructive interference because of the difference in rate of travel of these two orthogonal polarisations.

[00:08:36] And that creates what's called fading. So you get signal and then it fades away and then you get some more signal and it fades away. Okay. That's the impact of polarisation mode dispersion. Okay. I think we have at least covered dispersion there for single-mode fibre.

[00:08:57] With the polarisation mode dispersion, isn't there also the pulse spreading effect similar to chromatic dispersion where the two... I mean, they travel at such... If they go far enough, they'll travel at such different velocities that the two pulses will spread a little bit. Yeah. What you see in the... Because the receiver is a power...

[00:09:23] A squale or receiver, what you end up seeing is both of those orthogonal polarisations as they hit the receiver. And you'll see the positive and negative... Positive... So constructive and destructive interference. So what you see on the output of the receiver is what looks like a spread pulse with a dip in the middle. Hmm. And that's the fade in.

[00:09:52] If you get too much dip in the middle, you're effectively... That's where the fade in will come in. I see. Okay. Because the receiver circuitry will see that as a zero when it's actually a one. Well, this has been great. I think you can go have your tea now. Thank you. Thank you. And you can have a break as well. All right. Well, thank you so much. I really appreciate it. I've enjoyed it. Thank you.

[00:10:22] Yeah, me too. That was the eighth and final part of my conversation with Rashi Nkool. Hope you enjoyed it as much as I did. Hey, we have a new website. It's optics.podcastpage.io. That's optics.podcastpage.io.

[00:10:49] You can either listen there or use the same podcast platform you've been using all along. Please subscribe. Better yet, leave a review, especially if you use Apple Podcasts. Remember, we're part of the Cisco Podcast Network where you can find other great Cisco podcasts too. We also have educational videos on YouTube. Just go to youtube.com and search on Cisco Optics. Thank you for listening. This is Pat Chow, Product Manager at Cisco Optics. Until next time.