OM4 : the logical evolution of the optical fiber

he explosion in bandwidth demand for corporate networks in recent years is forcing the cable industry to excel in developing technologies that can track that reality. The very vocation of many of these companies has literally changed, while video games have become incredibly realistic, movies are available in 3D and almost 80% of the North American population has internet access. Behind these modern phenomena, a multitude of stakeholders are working hard for the sole purpose of enabling the effective transmission of data.

Have you ever wondered, for example, how many times a single video game sequence is passed from one developer to another when creating a new game? Knowing that this sequence is several tens of megabytes and that hundreds of sequences like this one must be designed, we can imagine the importance of transmitting at increasingly high and effective speeds. Or again, the manufacturer who develops a new part, will do it first on a specialized software, very often even in 3D and by the collaboration of several engineers who must exchange the plans through the network of the company. These are just a few examples of why, among other things, datacenters are building at an infernal pace and that business networks have become much more complex and demanding, or even crucial, for the survival of these companies.

It is not enough to say, « Let's go with fiber, we must carefully choose which one, according to current and future needs ».

The speed of networks
While in the early 2000s the majority of users were connected to the network at speeds of 10Mbp / s, electronic equipment such as switches communicated between them in the frame (commonly called backbone) at 100Mbps. Then came the network cards at 100 Mbps, while the frame has migrated at a speed of 1Gbps, to fi nally get to today while users communicate with each other at 1Gbps and that the framework is now returned to 10Gbp / s and soon 40Gbps.

What type of optical fiber to use?
All these needs have obviously brought new needs as to the type of optical fiber to be used in the framework. Too many infrastructure managers or integrators do not pay enough attention to the type of optical fiare vs. their needs or even the distance to go. It is not enough to say, “Let’s go with fiber”. We must carefully choose which one, according to current and future needs. But what are the possibilities? Most of you who have worked with optical fiber know that there are two distinct types of optical fiber, multimode and single mode. Grosso Modo, we know that single-mode fiber allows greater distances than the multimode. Why? Simply because the multimode fiber is designed to work with more economical electronic transmitters, but also less accurate and less powerful. The light source, typically a diode, uses a fiber having a much larger core and diffuses the light signal onto the walls of the heart in question. The defect of this method is that when the signal collides with these walls, it is attenuated more rapidly, thus reducing the possible distance. Among the types of optical cables found in this category are OM1 (62.5 / 125 microns) and OM2 (50/125 microns).

In contrast, the single mode fiber is designed for transmission with a laser. Although much more expensive, the laser sends a finer, more precise and more powerful signal. In addition, the laser uses the center of the heart, avoiding reflections and thus the attenuation of the signal too fast. So, why not use singlemode fiber at all times? Simply because equipment built with lasers is 3 to 4 times more expensive than
those with diodes or their derivatives (see below). With the increasing speed of the networks, the limits of multimode optical fiber OM1 and OM2 as well as transmitters to the diodes have been reached. Hence the appearance of the VCSEL type transmitters and the multimode optical fiber optimized for the laser. This approach is to use a “hybrid” transmitter, halfway between the diodes and the laser. VCSEL (Vertical Cavity Surface Emitting Laser, pronounced vixel) emits a much more accurate signal than traditional diodes, while being designed to transmit on multimode optical fi ow. The multimode optical fibers OM1 and OM2 have been originally designed to slow down the modes traveling in the center of the heart in order to swing them with those using the outside of it which they travel faster. Since the VCSEL makes greater use of the center of the heart, the OM1 and OM2 fibers are not ideal for this type of transmitter. Hence the need has come to produce a fiber optimized for the laser, that is to say that does not slow down the modes in the center of the heart. The optic fiber optimized for the laser, OM3 has therefore appeared.

Now that applications such as 40Gb / s and 100Gb / s are on the horizon, it must be ensured that the cabling in place is able to withstand these speeds at acceptable distances. What is an acceptable distance? Well, let’s start with the fact that according to studies done by the industry, the majority of the links of frames installed are within 300 meters, in a proportion of 75% approximately. It is therefore appropriate to keep this initial idea when it comes to choosing a type of optical fiber. The industry has been working for several years to produce a type of fiber optic that offers greater bandwidth than that offered by OM3. The latter, according to industry standards, must allow a bandwidth of 2000 MHz / km, while the OM4 is specified by the standard EIA / TIA 492AAAD at 4700 MHz-km. Mainly, the goal is to provide additional leeway to ensure the operation of the new applications mentioned above. Of course, all this in the distances specified for each of them, ie 550 meters for the 10Gbps and 150 meters for the 40Gbps and 100Gbps.

More than just OM4 fiber
When it comes to the operation of 40 Gbps and 100 Gbps applications, it’s not just a question of setting up an OM4 cable, far from it. Signal transmission and reception equipment, while using VCSEL technology as a signal transmission method, must now separate the signal on several sending channels on the fiber optic cable. The 40Gbps will require a 12-core cable and will transmit on 8 of them while the 100Gbps will use a 24-core cable with 20 in use. It’s actually adding up 10Gbps transmission channels to each other, 4 times for 40Gbps and 10 times for 100Gbps. Given the number of fiber optic strands used with this method, it is important to review the size of the cables planned during the planning stage, as this becomes very different from the old ways that used (or still use). ..) 1 pair of fibers per transmission channel.

Why not single-mode fiber?
The question that becomes inevitable, in the light of all these new technologies, method of transmission, etc., is the following; Would not it be wiser to install single-mode optical fiber, which does not have these limitations and complexities? And the trend among many customers with a relatively large infrastructure, such as universities, hospitals and others, is to implement hybrid solutions. The single-mode cable is not really more expensive than the multimode, it is rather the electronic equipment for signal transmission that are, as mentioned at the beginning of the article. An increasingly common practice is to install multimode cables and monopolies, the latter being in place for future needs. The multimode cable is used on the first day to support the current applications, while the single-mode cable is kept in reserve for future more demanding applications and for the eventuality that the market would reserve a lower cost for electronic equipment for the fiber. singlemode.

Optical fiber with a sharper radius of curvature
One of the physical constraints of optical fiber since it is used in infrastructures is its fragility to suffer from bending or curvature abuse. Without necessarily damaging the cable or glass, excessive flexing may result in attenuation of the signal and thus compromise system operation, especially in the case of new applications more sensitive to the loss of signal budget. As a result, the industry has developed Bend Optimized Multimode Fiber (BTE) optimized 50/125 micron multimode cables. These cables increase reliability in tighter curvature conditions. The allowed radius of curvature is now around 7.5mm, which is smaller than a 10-cent coin! This will not only facilitate the task of the installers, but will allow installation in environments traditionally less suitable for fiber optics while ensuring the integrity of the signal

Everything happens at the planning stage
The OM3 or OM4 fiber therefore seems to become an increasingly obvious choice when it comes to designing or planning the framework of a structured cabling system. Whether you are a user or an integrator, it is worth asking about this and possibly consulting an expert in the field if you are not sure of your choices. Manufacturers will also be able to advise you wisely on the right choice to make, who are actively involved in technology development and industry standards development. RCDD * specialists will also be able to advise you and even proceed with the development of the complete cabling solution as needed.

Patrick Desnoyers, RCDD / NTS is President of X Solutions, a company specializing in the design and integration of structured cabling and fiber optic cabling systems. Having worked in the telecommunications industry for more than 20 years, he holds BICSI RCDD and NTS certifications. During his career, he worked at a major manufacturer, supporting several major projects in Canada and abroad. He has also contributed to a number of standards development committees and continues to be involved in the evolution of structured cabling in Quebec and Canada.