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Hi, I’m John George, Senior Director of Solutions Engineering and Fusion Splicers at OFS. What’s new in my world is our new MDU!Click® Solution for fiber deployment in buildings.
With the MDU!Click Solution we can defer the cost of a splitter module until we have a higher take rate on each floor. This is for a pay-as-you-grow kind of deployment. It’s used for a second, or third entrant into the building, fiber to the business deployments, or maybe high take rates aren’t expected initially and there’s a desire to defer the cost of the build as much as possible with subscriber growth.
The way the system works is we have a MDU!Click SlimBox® Flex Indoor Module in the MDF the entry point of the building with a one by four or one by eight splitter feeding through the EZ-Bend® 12 or 24 fiber riser cable. You can see this is a very compact cable that can fit in limited spaces. We’re breaking out a single fiber so we can support one subscriber per floor in the initial deployment. Then, we can connect that first subscriber by plugging in one of our EZ-Bend Jumpers – a drop cable assembly that can go many hundreds of feet if needed.
The EZ Bend cable has a 2.5 millimeter bend radius to handle the cornering in the buildings that’s often required. Then, to add more than one subscriber on each floor, we simply put in place our MDU!Click SlimBox Flex Indoor Splitter Module and expand from one to eight ports. We can reconnect our initial subscriber and connect seven more subscribers on that floor, in order to get a higher take rate as we get new subscribers in the building.
That’s what’s new in my world, the MDU!Click® Solution for fiber in the building.
As transmission speeds over optical fiber networks in the enterprise increase to 10 Gigabits per second (Gb/s) and beyond, a relatively new term – “laser-optimized fiber” – has crept into the industry’s vocabulary. What is laser-optimized fiber? What do you need to know about it? And what exactly does the term “laser- optimized” mean? Understanding the answers to these questions will help you prepare for the latest wave in optical communications for enterprise networks.
Why have optical fibers been “optimized” for use with lasers?
Older “legacy” optical fiber systems (Token Ring, Ethernet, FDDI, ATM) used in premises applications operated at relatively slow speeds in the range of 4 to 155 Megabits per second (Mb/s). These systems utilized inexpensive light sources called Light Emitting Diodes (LEDs), which were perfectly adequate for these slower speeds. Multimode fibers used in these systems were rated to certain minimum bandwidths, typically:
160 MHz/km over 62.5/125 μm fiber at 850 nm
500 MHz/km over 50/125 μm fiber at 850 nm
500 MHz/km over both products at 1300 nm
These fibers were tested for bandwidth using an Overfilled Launch (OFL) test method, which accurately replicated real-life performance with an LED.
As the demand for bandwidth and higher throughput increased, especially in building and campus backbones, LEDs could not keep pace. With a maximum modulation rate of 622 Mb/s, LEDs would not support the 1 Gb/s and greater transmission rates required. One could make use of traditional lasers (Fabry-Perot, Distributed Feedback) typically used over single-mode fiber. However these are considerably more expensive due to the higher performance characteristics required for long-distance transmission on single-mode fiber.
In response, the industry developed a new high-speed laser light source called a Vertical Cavity Surface Emitting Laser (VCSEL). These VCSELs are inexpensive and well suited for low-cost 850 nm multimode transmission systems, allowing for data rates of 1 Gb/s and 10 Gb/s in the enterprise. With the emergence of these VCSELs, multimode fiber had to be “optimized” for operation with lasers.
VCSELs provide higher power, narrower spectral width, smaller spot size and faster data rates than LEDs. All of these advantages add up to a significant performance boost. This assumes, of course, the fiber itself does not hinder performance. To understand why this could occur, we need to recognize the differences between VCSELs and LEDs and how they transmit signals along a multimode fiber.
All LEDs produce a smooth, uniform output that consistently fills the entire fiber core and excites the many hundreds of modes in the fiber. The bandwidth of the fiber is determined by the aggregate performance of all the modes in the fiber. If a few modes lag behind or get ahead due to modal dispersion, they have little impact on bandwidth because many other modes are carrying the bulk of the signal.
The energy output of a VCSEL is smaller and more concentrated than that of an LED. As a result, VCSELs do not excite all the modes in a multimode fiber, but rather only a restricted set of modes. The bandwidth of the fiber is dictated by this restricted set of modes, and any modes that lag or get ahead have a much greater influence on bandwidth.
Typically, a VCSEL’s power would be concentrated in the center of the fiber, where older fibers were prone to defects or variations in the refractive index profile (the critical light-guiding property in the core of the fiber), resulting in poor transmission of the signal. That is why some fibers may actually perform poorly with a VCSEL compared to an LED.
To complicate matters, the power profile of a VCSEL is nonuniform and fluctuates constantly. It changes sharply across its face, varies from VCSEL to VCSEL and changes with temperature and vibrational fluctuations. Consequently, individual VCSELs will excite different modes in a certain fiber at any given time. And because different modes carry varying amounts of power, the fiber’s bandwidth can vary in an unpredictable manner.
Why are laser-optimized fibers the best choice for use with VCSELs?
With the advent of VCSELs, it became apparent that the traditional multimode fiber deployed for LED systems did not take full advantage of the performance benefits of VCSELs.
To fully capitalize on the benefits that VCSELs offered, fiber manufacturers developed laser-optimized multimode fiber (LOMMF). LOMMF is specifically designed, fabricated and tested for efficient and reliable use with VCSELs.
LOMMFs should have a well-designed and carefully controlled refractive index profile to ensure optimum light transmission with a VCSEL. Precise control of the refractive index profile minimizes modal dispersion, also known as Differential Mode Delay (DMD). This ensures that all modes, or light paths, in the fiber arrive at the receiver at about the same time, minimizing pulse spreading and, therefore, maximizing bandwidth. A good refractive index profile is best achieved through DMD testing.
VCSELs and LOMMF provide tremendous flexibility and cost efficiency in “freeing up” bandwidth bottlenecks in the enterprise today and well into the future. LOMMF is completely compatible with LEDs and other fiber optic applications (there are no special connectors or termination required and no effect on attenuation). LOMMF can be installed now and utilized at slower data rates until the need arises to increase network speed to 1 or even 10 Gb/s. At that point, you only need to upgrade the optics modules to VCSEL-based transceivers. There is no need to pull new cable.
Can you use any laser-optimized fiber for 10 Gb/s?
No — it is important to note that not all laser-optimized fiber is 10 Gb/s capable. If 10 Gb/s capacity is in your future, you must make sure that the LOMMF you’re installing now is capable of handling 10 Gb/s. The first laser-optimized fibers, introduced to the market in the mid-1990s, were designed for 1 Gb/s applications. Available in both 62.5/125 µm and 50/125 µm designs, these fibers extended the reach capability of 1 Gb/s systems beyond what the industry standards stated. For instance, OFS 1 Gb/s Laser Optimized 62.5 Fiber can go 300 meters in cost-effective, 1 Gb/s 850 nm (1000BASE-SX) systems. 50/125 µm fibers offer even greater performance, with a reach of 600 meters or more. These 1 Gb/s LOMMFs, coupled with 850 nm VCSELs, allow for the lowest systems cost for building backbones and short-to medium-length campus backbones
How do you measure bandwidth for laser-optimized fiber?
Since LEDs have a uniform and consistent power profile that excites all the modes in a multimode fiber, the traditional OFL method of bandwidth measurement accurately predicts bandwidth of fiber for LED applications. But because VCSELs only excite some of the modes in a fiber, and in a varying manner, the OFL bandwidth measurement cannot predict what the fiber’s bandwidth would be if the fiber were to be used in a VCSEL application.
It should become clear now why fiber manufacturers developed laser-optimized fiber, and why DMD testing is so important. The refractive index has to be well designed and controlled to ensure that all modes exhibit minimal DMD and all arrive at the other end of the fiber at the same time. No matter which modes in the fiber are actually guiding the light, those modes will have minimal DMD and provide high bandwidth.
What should you look for in DMD testing?
DMD testing provides a clear picture of how individual mode groups carry light down the fiber, and which mode groups are causing DMD. In fact, that picture is so clear that the standards require fiber to be DMD-tested to ensure adequate bandwidth to the rated distances for 10 Gb/s applications.
Tony Irujo is sales engineer for optical fiber at OFS, a world-leading designer, manufacturer and provider of optical fiber, fiber optic cable, connectivity, fiber-to-the-subscriber (FTTx)and specialty photonics products. Tony provides technical sales and marketing support for multimode and single-mode optical fiber.
Tony has 25 years of experience in optical fiber manufacturing, testing and applications. He started with SpecTran in 1993 as a quality and process engineer and transitioned to more customer-focused roles with Lucent and OFS. He represents OFS in the Fiber Optic LAN Section (FOLS) of the TIA, has authored several papers on fiber technology and applications and is a frequent speaker at industry events. Tony has a Bachelor of Science degree in Mechanical Engineering from Western New England College in Springfield, MA.
Using optical fiber networks, people can access and share information at an amazing level. They can communicate, work and learn from virtually anywhere there’s an Internet connection. For people in rural communities that lack wireless or broadband services, their ability to obtain information is clearly unequal. Even getting a signal for a cellphone or laptop can mean driving miles to a more populated area. Life is much easier with an available high-speed optical fiber network.
Leveling the Playing Field
Implementing optical fiber helps to “level the playing field” by providing more equal access to information and opportunities for rural residents. In reality, optical fiber and wireless services can transform rural communities.
When optical fiber arrives, one obvious plus is being able to access a cell signal from home. That wireless service requires optical fiber, which acts as the nervous system of a network. Fiber to the Tower and Fiber to the Building lay the actual groundwork for wireless communications including LTE and 4G, and soon to come 5G. The benefits of this connectivity can be seen in three distinct areas as follows.
Rural Healthcare
Digital revolution through high-speed optical fiber Internet helps medical facilities provide better treatment for patients in rural areas in a number of ways, including:
Physicians can search files, consult with specialists and use remote diagnostics and alternative healthcare delivery methods;
Healthcare professionals may use connected devices to directly monitor and care for patients;
Patients practice “self-care” by accessing health-related information on the Internet.
Education
Teachers need optical fiber connectivity for video lectures and e-learning that can be widely shared. Students also need access to home Internet to complete homework and expand their learning. Colleges and universities require high-speed optical fiber Internet access to stay competitive and ensure their degree programs stay relevant.
Growth in Rural Communities
With 25% of rural residents lacking Internet access, fiber optic infrastructure build-outs are still needed. More people move into rural areas when they can maintain their standard of living. When optical fiber connectivity is optimal, existing or new businesses can reach and attract highly-qualified employees no matter where they live.
In rural areas where high-speed Internet is available, even small businesses and farms can benefit. The Internet of Things (IoT), another product of this digital revolution, makes Smart Farming possible. By applying sensing technologies through Smart Farming, farmers can practice more precise and scientific agriculture that results in increasingly bountiful, high-quality harvests.
Last month, internet speeds in Jackson, Mississippi, jumped from 1 Gb to 100 Gb. This leap forward is part of the city’s work to light up “dark fiber” in the robust fiber optic network that it owns.
The Origin of Dark Fiber
The term “dark fiber” refers to unused or underused fiber optic infrastructure (optical fibers, fiber cables and repeaters). Because it’s expensive to deploy cable (especially under oceans), companies typically install more fiber than they will need. This fact was especially true during the dot.com boom of the 1990s. However, after the bust of the early 2000s, many companies either went bankrupt or merged. The result is that today, in addition to “lit fibers” (fibers currently transmitting data by light), there are many “dark fibers” (unused fibers) within the same networks.
Cities Lighting UpBecause it’s possible to buy or lease these fibers, some cities and companies see using dark fiber as an appealing way to save money or create a new revenue stream. However, there are other factors to consider because using dark fiber isn’t straightforward. Buying and managing a fiber network takes skills that many organizations simply don’t have. Also, when a group starts selling network bandwidth, it takes on the role of becoming an ISP.
And on top of this, success isn’t always guaranteed. Take the example of California’s Santa Monica CityNet. In 2014, CityNet became the first 100 gigabit municipal network in the country. However, in its efforts to lease dark fiber, CityNet has signed up less than 2 percent of the business market since 2006. It has also collected only about $2.1 million in revenue over that time.
Other Uses
At least for now, dark fiber still has staying power. One factor driving its use is cloud computing which requires greater bandwidth. However, dark fiber could face greater competition as cities get “smarter” and 5G wireless communications roll out.
Ironically, dark fiber’s strength may come through uses besides connectivity. At the Lawrence Berkeley National Laboratory, Dr. Jonathan Ajo-Franklin is using dark fiber to measure seismic signals. Dr. Ajo-Franklin’s team gained permission to access a section of a dark fiber network between Sacramento and Calusa, California. During a seven-month experiment, the team collected about 300 terabytes of data. Ultimately, they found that the same dark fiber installed for communications was also useful in making distributed measurements. These measurements included seismic wave fields, temperature, strain and vibrations that can affect infrastructure (such as the number of cars on a road). In other words, an unused fiber installed for a telecom network might also be used in sensing.
What’s more, by using dark fiber, the team saved a substantial amount of money by replacing a critical, huge array of thousands of individual point sensors with an existing, installed fiber optic cable.
No — it is important to note that not all laser-optimized fiber is 10 Gb/s capable. If 10 Gb/s capacity is in your future, you must make sure that the LOMMF you’re installing now is capable of handling 10 Gb/s. The first laser-optimized fibers, introduced to the market in the mid-1990s, were designed for 1 Gb/s applications. Available in both 62.5/125 µm and 50/125 µm designs, these fibers extended the reach capability of 1 Gb/s systems beyond what the industry standards stated. For instance, OFS 1 Gb/s 
Using optical fiber networks, people can access and share information at an amazing level. They can communicate, work and learn from virtually anywhere there’s an Internet connection. For people in 