Cisco Optics Podcast Ep 53. The smartest data center operators use multi-mode fiber - do you? (2/7)

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

[00:00:12] pluggable optics for networks.

[00:00:14] Multi-mode fiber has been the workhorse for fiber optic communication within a data

[00:00:18] center for years

[00:00:19] 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

[00:00:29] was a huge data raid. Anybody remember those days?

[00:00:33] This is episode 53 and we continue our conversation with Hao Dong,

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

[00:00:42] We get into the difference between single-mode and multi-mode fiber.

[00:00:46] Hao Dong is a market technology development manager for Corning's

[00:00:50] optical communications business. Within this role

[00:00:53] he focuses on technology and market trends to identify, evaluate,

[00:00:57] and develop leading optical network solutions that formed the backbone of

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

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

[00:01:05] positions in engineering,

[00:01:07] development, and marketing within the fields of optical components,

[00:01:10] fiber lasers, and optical communications. Hao holds a Bachelor of Science in

[00:01:14] Physics from Wuhan University

[00:01:16] and a PhD in physics from the University of Connecticut. And now join me

[00:01:20] as I talk with Hao Dong.

[00:01:34] Details when it comes to the thermal management.

[00:01:37] You know it's funny we're talking about this now because this is a big

[00:01:40] topic in data centers, right?

[00:01:43] Not for the fiber itself but just all the equipment

[00:01:47] keeping it all cool. People are looking into

[00:01:50] liquid cooling of network equipment.

[00:01:53] Exactly, so that's why

[00:01:56] we always think

[00:02:01] the multi-mode is valuable when it comes to the power.

[00:02:04] Because it's a feature of the low power

[00:02:08] consumption and the low cost especially

[00:02:11] the low power. People talk a lot about the lower cost and value of the

[00:02:16] multi-mode fiber but I think sometimes we

[00:02:18] totally underestimate its value of the low

[00:02:23] power consumption especially for the data center applications.

[00:02:26] Okay, before we keep going, can you explain for people who

[00:02:31] are not familiar with it? Single mode fiber, multi-mode fiber, what is a mode

[00:02:35] and what is difference between the two? Yeah, of course.

[00:02:39] So we talk about the single mode

[00:02:42] and the multi-mode. The big difference is the fiber

[00:02:46] as a waveguide. So if you look at the

[00:02:50] fiber structure

[00:02:53] there was a core in the middle and also there was

[00:02:57] cladding around it.

[00:03:00] That actually comes to the question of

[00:03:06] the fiber profile. Basically for a fiber

[00:03:10] person we can't talk about the fiber without mentioning

[00:03:14] the fiber profile. So

[00:03:18] for fiber as a

[00:03:22] waveguide, the big issue is the core size.

[00:03:28] When a core is small enough the waveguide can only carry on

[00:03:32] one single mode or fundamental mode. For this case basically we call it the

[00:03:36] single mode fiber. But if you keep increasing your

[00:03:40] core size to a certain degree, your waveguide can carry

[00:03:45] more than one mode. So in that case we call it the multi-mode

[00:03:48] fiber. Okay, so if we could just visualize for a moment

[00:03:53] the core and the cladding. If you could picture the cross

[00:03:58] section of a strand of fiber, the cross section has basically a core

[00:04:02] which is a concentric, a circle concentric with the

[00:04:06] cladding which is a bigger circle. The difference between the core

[00:04:11] and the cladding is aside from the chemical makeup is

[00:04:15] basically index of refraction primarily.

[00:04:20] Exactly. So when it comes to the

[00:04:23] fiber, you want to have a

[00:04:27] design of the core ahead of the time.

[00:04:31] And then when it comes to the making process,

[00:04:34] so fortunately we have a tool, we have a way

[00:04:39] of controlling your profile. For example if you

[00:04:43] are doping some dopant like germanium, you can

[00:04:46] certainly raise the index and if you dope the fluorine

[00:04:50] your refract index will be dropped. So by following those

[00:04:54] simple rows basically you can control your profile.

[00:04:59] So the profile consists of what is the actual value of the

[00:05:04] index of refraction for both the core and the cladding?

[00:05:07] And then also the dimensions? Yes, so I think there are

[00:05:13] two things. One is again if you look at the fiber it's a wave

[00:05:17] guide. It's guided the wave by following

[00:05:22] a physical phenomenon called internal total internal reflection.

[00:05:27] So how well you design your fiber, how well you design your profile

[00:05:32] eventually will determine the transmission performance.

[00:05:37] Especially here we talk about the single mode and multi-mode.

[00:05:40] So you have to put all the factors into consideration together like

[00:05:46] refraction index, the fiber NA, the core size

[00:05:51] to make sure your sort of like a V number

[00:05:56] to be proper. For example if you want to keep a single mode

[00:05:59] operation your V number will be 2.4 around that value.

[00:06:06] And how is the V number defined? The V number I think is defined

[00:06:10] as a function of your index and your fiber NA

[00:06:15] and the core radius. NA numerical aperture? Numerical aperture, yeah.

[00:06:19] Numerical aperture as you know is, I mean when it comes to the fiber

[00:06:25] is directly related to the index of the core and index of the cladding.

[00:06:33] Right and I think if from what I remember if you were to visualize the

[00:06:37] NA it's kind of like the angle the the angle of the cone that would

[00:06:42] be emitted from the fiber. Exactly.

[00:06:47] The light that comes out of the fiber. Depending on

[00:06:52] the parameters you mentioned that cone can be narrow or wide.

[00:06:56] That's right. That's the NA. Yes, yes that's NA.

[00:07:00] The reason why we have a different NA design for the fiber because

[00:07:05] you have to be you know design you have to design the fiber to be

[00:07:08] compatible with your light source right. For example

[00:07:11] if your light source is say Vixo you know it's a multi-mode and

[00:07:20] and you know you have a different requirement to accept to capture the

[00:07:26] light versus a single mode of design. Right and so you want to match the

[00:07:33] light source NA to the fiber NA right if you want to get all the light

[00:07:39] into the fiber efficiently right. Right, right and also depending on different

[00:07:45] you know different different light source and different applications

[00:07:49] your profile could be either step index of fiber or graded index of fiber.

[00:07:53] Right so there are a lot of you know considerations here when it comes to

[00:07:57] the profile in the in the specific fiber design.

[00:08:03] And graded index means? Graded index fiber means you know if you

[00:08:10] visualize refractive index as a function of the radius so at the

[00:08:15] center of the you know at the center of the fiber the refract index

[00:08:20] will reach to the peak and then it gradually drop to the edge.

[00:08:25] So basically it will look like a parabolic shape.

[00:08:32] Okay instead of a step index which would look like

[00:08:36] what kind of shape? Which is like a step right so in the core the whole

[00:08:41] core area across you know all the radius it's pretty much at the same level when it comes to

[00:08:47] the refract index. It's a flat it's like a plateau and then drop to the bottom drop to the

[00:08:54] bottom when it comes to the cladding. Okay okay back to multimode so what exactly is a mode?

[00:09:17] That was the second part of my conversation with Hao Dong. Next time we'll get into more

[00:09:22] detail about multimode fiber. We have a new website it's optics.podcastpage.io. You can

[00:09:29] either listen there or use the same podcast platform you've been using all along. Please

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[00:09:40] we're part of the Cisco Podcast Network where you can find other great Cisco podcasts too.

[00:09:45] We also have educational videos on YouTube just go to youtube.com and search on Cisco Optics.

[00:09:52] Thank you for listening this is Pat Chow, product manager at Cisco Optics. The next episode

[00:09:56] is part three of my conversation with Hao Dong. Until next time.