Cisco Optics Podcast Ep 54. The smartest data center operators use multi-mode fiber - do you? (3/7)

[00:00:09] Hello everyone and welcome back to the Cisco Optics Podcast where we talk about

[00:00:13] pluggable optics for networks.

[00:00:15] Multi-mode fiber has been the workhorse for fiber optic communication within a

[00:00:18] data center for years

[00:00:20] as well as a choice for ethernet backbones in office buildings.

[00:00:23] It has a rich history that goes all the way back to the first

[00:00:26] optical fiber standards when 100 megabits per second

[00:00:30] was a huge data raid. Anybody remember those days?

[00:00:33] This is episode 54 and we continue our conversation with Hao Dong

[00:00:38] market technology development manager for Corning's optical communications

[00:00:41] business.

[00:00:42] We go into more detail about multi-mode fiber and its overall solution level

[00:00:46] cost benefits.

[00:00:47] Hao Dong is a market technology development manager for Corning's

[00:00:51] optical communications business. Within this role

[00:00:54] he focuses on technology and market trends to identify, evaluate

[00:00:58] and develop leading optical network solutions that formed the backbone of

[00:01:01] today's connected world.

[00:01:02] Hao has over 18 years of industry experience and has held various

[00:01:06] positions in engineering,

[00:01:08] development and marketing within the fields of optical components,

[00:01:11] fiber lasers and optical communications. Hao holds a bachelor of science in

[00:01:15] physics from Wuhan University

[00:01:17] and a PhD in physics from the University of Connecticut. And now join me

[00:01:21] as I talk with Hao Dong.

[00:01:34] Back to multi-mode. So

[00:01:37] what exactly is a mode?

[00:01:41] What exactly is a mode?

[00:01:45] Usually we, I mean from the physics standpoint you can

[00:01:50] look at the mode

[00:01:53] view the mode as a distribution of your

[00:01:57] power distribution of your

[00:02:01] optical wave. So essentially your

[00:02:05] optical signal is kind of like

[00:02:09] it's a

[00:02:12] electromagnetic wave, right? So

[00:02:16] for any kind of a magnetic wave

[00:02:20] so certainly you have to, you know it's governed by

[00:02:23] the Maxwell equations and your boundary conditions.

[00:02:27] By solving those equations and you eventually

[00:02:30] you will get the different solutions, right?

[00:02:33] Those solutions mathematically

[00:02:36] is called a mode.

[00:02:40] I mean from other perspective if you look at the

[00:02:44] those modes propagating inside the fiber

[00:02:48] I think they could be viewed as

[00:02:52] as a different optical

[00:02:56] groups traveling inside of the

[00:03:00] fiber by following different paths. Right, like different angles

[00:03:06] of incidence against the core cladding interface?

[00:03:10] That's right, yeah. Like those paths?

[00:03:14] So that would be like a ray

[00:03:18] the light ray model, right?

[00:03:22] Exactly, yeah. The ray optics model.

[00:03:26] I always like to think of like a very simple

[00:03:30] analogy like with a rope. If you tie a rope to a doorknob

[00:03:34] and then you start shaking the rope up and down

[00:03:38] if you do it at the right speed, you'll get a vibrational mode, right?

[00:03:42] Where the, if it's just kind of going up and down

[00:03:46] as a single hump going up and down, up and down, that's like a first order mode, right?

[00:03:50] And if you do it at twice the rate, you'll kind of get like a

[00:03:54] two hump thing that kind of

[00:03:58] alternates, right? That's right. So

[00:04:02] for those modes I think more specifically

[00:04:06] we call them a spatial mode, right? Spatial mode. So from that perspective

[00:04:10] you can imagine I think literally

[00:04:14] it's just this very specific

[00:04:18] power distribution in a spatial

[00:04:22] dimension. So like I said, in theory the propagation

[00:04:26] the propagating optical signal is kind of an

[00:04:30] electromagnetic wave, right? And therefore

[00:04:34] at the end of the day it's a power distribution within a fiber

[00:04:38] and also it's governed by Maxwell equation in certain boundary

[00:04:42] condition. And if you really

[00:04:46] come to the solution, the solution to these equations

[00:04:50] that's basically how we got the spatial mode.

[00:04:54] So I think

[00:04:58] the reason

[00:05:02] why we care about the spatial mode is because

[00:05:06] those different modes behave very differently inside the fiber, especially

[00:05:10] for the multimodal propagation. And they

[00:05:14] follow different paths at a different speed. When they see the different

[00:05:18] reflux index, their

[00:05:22] speed will be different. So that's why

[00:05:26] we care about those modes, especially for multimodal fiber.

[00:05:30] So what impact does that have? So why do we care about the fact

[00:05:34] that these modes travel at different speeds

[00:05:38] down the fiber?

[00:05:42] In the multimodal fiber

[00:05:46] there is a thing called the mode of dispersion.

[00:05:50] So the mode dispersion exists because different light

[00:05:54] with different modes have a different path length

[00:05:58] and they travel at a different speed. Therefore

[00:06:02] if you imagine when those rays

[00:06:06] most entering the fiber at the same time, but afterwards

[00:06:10] when they access the fiber at the other end, they access

[00:06:14] the fiber at a different time. So if the pulse

[00:06:18] goes in, so with this kind of time difference

[00:06:22] the pulse will be spread out. So that's what we call

[00:06:26] the mode of dispersion.

[00:06:30] So if you send in an optical pulse, like just a burst of energy

[00:06:34] a burst of optical energy, it may come in

[00:06:38] with a certain duration but by the time it comes out, it's actually much longer

[00:06:42] than the duration. Yes, when you put those energy

[00:06:46] when you put those pulses in a format of signal

[00:06:50] 1, 0, and those series signals together

[00:06:54] so the pulse dispersion

[00:06:58] and the mode of dispersion could be a serious issue because

[00:07:02] it blends those signals together. So certainly

[00:07:06] from the fiber design standpoint, you need to take that into

[00:07:10] consideration in trying to address that issue.

[00:07:14] That's a big concern when it comes to the multimodal

[00:07:18] fiber design as well.

[00:07:22] So modal dispersion is a big

[00:07:26] thing because I guess it's something you think about like every day.

[00:07:30] Exactly, yeah, the modal dispersion is a big thing. But I think

[00:07:34] from fiber design standpoint we have a

[00:07:38] well established way to address it.

[00:07:42] That's what you mentioned before, the grid and index of fiber.

[00:07:46] So the way how it works is

[00:07:50] for the grid index fiber, basically

[00:07:54] for those different

[00:07:58] modes as we said, the multimodal

[00:08:02] going into the fiber. The interesting thing

[00:08:06] is for the fundamental mode or lower order mode

[00:08:10] they tend to stay in the center of the core.

[00:08:14] For the higher order mode, they basically

[00:08:18] distribute, the power distribute

[00:08:22] closer to the edge, closer to the clad.

[00:08:26] So they're closer to the cladding.

[00:08:30] What kind of shapes can these higher order modes take?

[00:08:34] What would they look like?

[00:08:38] It could be symmetric

[00:08:42] rings or

[00:08:46] flowers, but typically

[00:08:50] they're in a symmetric shape.

[00:08:54] Symmetric, like circularly symmetric or

[00:08:58] X-Y symmetric? It could be either way.

[00:09:02] Symmetrically, circularly symmetric or like I said, like a donut

[00:09:06] shape. But versus this

[00:09:10] for the fundamental mode it's a

[00:09:14] power, right? It's a power that is kind of like more

[00:09:18] concentrating in the center. So that's the way we like

[00:09:22] we want the power to be more concentrated,

[00:09:26] to be more efficient in terms of carrying the information along the fiber.

[00:09:30] What are the main applications of multimode fiber?

[00:09:35] So for the

[00:09:39] when it comes to the applications, I think

[00:09:43] you have to basically look at the key metrics

[00:09:47] associated with the multimode fiber. So for example,

[00:09:51] the reach is a very important metric you need to consider

[00:09:55] when it comes to the network design. For example,

[00:09:59] if your network is two kilometers

[00:10:03] you need to kind of like connect in your data center

[00:10:07] like two kilometers away or five kilometers away, you know your multimode

[00:10:11] fiber is not be able to deliver

[00:10:15] such connection. So multimode fiber is

[00:10:19] not an option. Because the modal dispersion will

[00:10:23] limit that reach?

[00:10:27] Exactly, because of the modal dispersion. Yeah, modal

[00:10:31] dispersion in that kind of

[00:10:35] application, the modal dispersion is the number one

[00:10:39] concern. So naturally for, I mean

[00:10:43] when you consider typically for any

[00:10:47] distance longer than five kilometers, I would say now these days

[00:10:51] the single mode fiber will be the natural option.

[00:10:55] So the reach is a very important

[00:10:59] factor to consider. The other thing is the cost.

[00:11:03] Right? Rather than speaking the multimode solution

[00:11:07] is a lower cost solution comparing with the single mode.

[00:11:11] So if your application is

[00:11:15] kind of like super sensitive to cost,

[00:11:19] cost is a higher priority, then we recommend to go with

[00:11:23] the multimode to consider the multimode if the reach

[00:11:27] is short distance. Is that because of the cost of the fiber itself or the cost of

[00:11:31] connectivity? That's a very good question. So here

[00:11:35] when we talk about the cost, so we should

[00:11:39] take all things considered together including the fiber

[00:11:43] cabling connectivity and also the laser source.

[00:11:47] So we typically say multimode is relatively

[00:11:51] more cost effective solution

[00:11:55] versus single mode. The real

[00:11:59] fundamental reason is because the laser source

[00:12:03] because the multimode fiber

[00:12:07] transmission typically we use the VIXO which is very cost effective

[00:12:11] versus the DIV laser. From many factors standpoint,

[00:12:15] the VIXO, we always say the multimode

[00:12:19] fiber is designed to match the multimode,

[00:12:23] designed to match the VIXO. Eventually

[00:12:27] as you can tell, the core size of the multimode is way bigger than single mode.

[00:12:31] So when it comes to the alignment, the VIXO

[00:12:35] launching into the multimode versus the DIV laser

[00:12:39] launching into a small core of the single mode, obviously

[00:12:43] it's way easier for the multimode to finish the job.

[00:12:47] So that contributes a lot to the lower cost of the multimode

[00:12:51] solution. But that's a

[00:12:55] laser source part

[00:12:59] of the story. But when it comes to the fiber, the multimode actually

[00:13:03] is not necessarily cheaper than single mode because

[00:13:07] the core size is bigger, you need to put more

[00:13:11] higher quality material in the core.

[00:13:15] So end of the day, the fiber or cable

[00:13:19] might be more expensive than single mode but when it comes to the laser

[00:13:23] side to the light source and transceiver, the multimode

[00:13:27] is cheaper. So overall end to end, you still come out lower cost

[00:13:31] going with the multimode and the VIXO based

[00:13:35] transceivers. Yes, yes, exactly.

[00:13:41] Actually you brought up a very interesting

[00:13:45] part of the discussion.

[00:13:49] In early days we know the multimode is

[00:13:53] very much more cost-effective than

[00:13:57] single mode. If you look at the 4G situation,

[00:14:01] 100G situation, it's no brainer for people to go with the

[00:14:05] multimode for short and majority reach.

[00:14:09] But when the transceiver speed

[00:14:13] migrated to higher speed like 400G, especially

[00:14:17] when the silicon photonics getting more mature,

[00:14:21] people start to question the

[00:14:25] cost-effectiveness of the multimode. For us,

[00:14:29] one of the work we did is to quantify or

[00:14:33] quantitatively analyze the installation cost

[00:14:37] of the multimode versus single mode. Of course,

[00:14:41] in this analysis you need to take into consideration

[00:14:45] everything, your transceivers, optical contributions,

[00:14:49] cabling, connectivity.

[00:14:53] Long story short, our conclusion is even at

[00:14:57] 400G if your application distance is short, like

[00:15:01] less than 100 meter, multimode is still more cost-effective

[00:15:05] than single mode.

[00:15:09] Because of the lower cost, I think that's

[00:15:13] one of the two most important factors

[00:15:17] or features of the multimode so that the multimode is

[00:15:21] still being widely deployed in data center applications today.

[00:15:25] Yeah, and I think from at least from my perspective,

[00:15:29] I mean Cisco has been in the data center business

[00:15:33] for literally decades, right? And multimode was

[00:15:37] always the choice for the data center reaches which are short

[00:15:41] reach. There's such a huge

[00:15:45] install base and not just, you know, I would say not just physically

[00:15:49] install base but the mindset of people, right? Because

[00:15:53] I think a lot of people out there including probably people listening

[00:15:57] to this, they're

[00:16:01] not optics and lasers people. They haven't spent their life

[00:16:05] like you have working on

[00:16:09] splicing lasers, working on designing

[00:16:13] optical fibers. So they're thinking more about the equipment side

[00:16:17] and if they're comfortable with multimode fiber, it's just a very nice comfort zone.

[00:16:21] And so even at 400 gig,

[00:16:25] you know, if that's available, then

[00:16:29] yeah, that's what they're going to gravitate toward.

[00:16:33] Yeah, so especially like we mentioned

[00:16:37] before, right? For multimode solution,

[00:16:41] not only we consider it as a lower cost solution but

[00:16:45] also there is a great feature of the lower power consumption.

[00:16:49] That was the third part of my conversation with Hao Dong. Next time

[00:17:02] we'll get into power consumption considerations. We have a new

[00:17:06] website. It's optics.podcastpage.io

[00:17:10] You can either listen there or use the same podcast platform you've been using all along.

[00:17:14] Please subscribe. Better yet, leave a review

[00:17:18] especially if you've been using Apple Podcast. Remember, we're part

[00:17:22] of the Cisco Podcast Network where you can find other great Cisco podcasts too.

[00:17:26] We also have educational videos on YouTube. Just go to youtube.com

[00:17:30] and search on Cisco Optics. Thank you for

[00:17:34] listening. This is Pat Chow, Product Manager at Cisco Optics. The next episode

[00:17:38] is part four of my conversation with Hao Dong. Until next time.