When you’re trying to decide on which fiber optic cable to choose for an application. You have many choices of various constructions and fiber types of as has made this step easier for you. With the help of our new cable selection guide, which consists of cables for industrial applications, what the selection guide is designed to do is to make your job easier to determine the most appropriate and cost-effective cable for your project or installation. The selection guide is going to focus on the office industrial multimode fiber optic cables that are designed for, and common in, many factories and substations, as well as other harsh environment applications.
For those of you who work in industrial environments, whether it be a steel mill, paper mill, a substation, or any type of harsh environment, the HCS product line is ideal for you for many reasons. Unlike traditional fibers, the primary fiber coating makes bonds to the fiber and enhances the strength during the draw or production process. Additionally, because the coating remains on the glass during the termination process, the fiber maintains its inherent strength. This makes it unique in that there is never bare glass exposed to the environment such as humidity, dirt, and dust. Those environmental factors are all known to detract from the strength of the fiber optics. When you’re considering the product line, here’s what you’re getting a rugged and robust construction resistance to abrasion in industrial chemicals. Reliable and repeatable termination process. A short learning curve for terminations. So selection considerations include extreme temperatures of Florida, Arizona, to the frigid temperatures of the Canadian tundra or Alaska. Humidity also plays a factor in states like Texas and Louisiana. You will also consider installation pull strength, the compressive strength of the cable. What kind of weight can the cable bear during the installation?
The first page here we have 62.5 micron cables, which are indoor zipcord cables for data center type applications. Indoor outdoor cables which can be used for either these are breakout cables and all the outdoor cables will have a water block in the cable as well as some of them will have a glass armor for road and deterring because you’re going to deter the road they’re trying to chew through the cable.
And looking at our 200 micron step index cable from simplex to zip cords to breakouts through indoor applications, we have riser rated we have plenum rated, we have indoor outdoor cables. And we also have cables that are designed strictly for outdoor applications. Again, with the water block, as you see at the bottom here, we have a schematic which shows the termination process of a typical O of this connector.
First, we’re going to remove the cable jacket, then we’re going to strip the fiber. Then we’re going to crimp the connector on, and then we’re going to cleave the glass. And for a pristine finish on the end of the connector. What I’ll do in my next talk is share with you information on the connector systems that go along with the cables we just talked about. And that’s what’s new in my world.
Mark Boxer, OFS Technical Manager, reveals what makes Rollable Ribbon so special. To form these ribbons, fibers are partially bonded to each other at intermittent points. This design not only enables mass fusion ribbon splicing but allows for easier individual fiber breakout than flat ribbons. The preferential bending plane of the rollable ribbons facilitates rolling and routing in smaller closures and splice trays, similar to individual fibers.
Hi. I’m Mark Boxer with OFS. Today I’d like to talk about Rollable Ribbon and if you haven’t seen it before, it is this stuff, so it’s pretty neat. It collapses upon itself when it’s rolled into a tight cylinder so you can kind of collapse it.
And then you can also easily separate it out to pull out an individual fiber. So, I’ll do that and get a little bit closer. So you can see that and then it just snaps back into place so you can slice it again. So, it splices like a ribbon. It can be rolled very, very tightly, providing the ability of tight fiber density in a very, very small package.
So, you compare this to a flat ribbon. This is a flat ribbon and so there is no there is no moving of this or no rolling this without breaking the ribbon. So why this is important really comes back to geometry. So, this is an 864 fiber central to flat ribbon cable. So, if you look at it, you can see that there is a lot of space in between the flat ribbons and the tube.
And that’s because we’ve got ribbons that are fundamentally rectangles. And the cable is fundamentally a circle. When you have a circle and a rectangle, then it those don’t fit that efficiently. If you look at a rollable version of this cable with 864 fibers, you can see here there’s not very much space between the fibers and the tube.
And let me go ahead and put both of these side by side. And you can see that this is the flat ribbon 864. This is the rollable 864. Now there’s a significant difference in size between the two.
Rollable Ribbon was dreamed up in Japan back in the 2000’s and went through a development path that was actually similar to the SC Connector that happened in the early 1990s.
And what I mean by that is that NTT in Japan farmed out the concept to various companies who ultimately brought it to market. For the case of Rollable Ribbon OFS parent company, Furukawa was one of those companies that introduced Rollable Ribbon and so we brought it back to brought it to market during the middle part of the 2010’s.
So there are so many benefits to Rollable Ribbon. You know, we see it in a lot of different environments, and I call this the buffet of benefits using Rollable Ribbon. It’s small. The ribbons themselves are very small so they can be rolled. That means that cables can be much smaller and lighter for a given size.
That can gives the ability to fit more fiber into smaller ducts. It can mean smaller hand holds more cable on a real for longer lengths or fewer slice points. And then for aerial installations also since everything’s smaller, also less weight on the pole, less amount of ice to build up. Smaller in general typically makes things easier. Some customers like it because it can be placed in thinner slice trays enabling more trays and a closure or a smaller closure for a given fiber count.
All of these cables are gel free, so they’re easier to prepare for slicing. And of course, these are ribbon. So there can be mass fusion spliced 12 at a time. The initial installations were primarily used to connect data centers together because data centers use very high fiber count cables, you know, think of 1728s, 3456s.
But what I’m really excited about is the concept of using rollable ribbons in lower fiber count networks. Including fiber to the home, backbone, and distribution networks. For those applications, we have typically used, lose tube cables in the past because it’s easier to pull out an individual fiber to connect a subscriber. But now with Rollable Ribbon, it’s easier to pick out an individual fiber versus a loose tube. The ribbons or clearly marked, the fiber color is always going to be in the same place. You just basically pick out whatever you need.
So now the cables can be smaller, they can be easier to work with in the field. If you compare these so these 288 fiber count cables. The first is a loose tube. It’s actually pretty big. And this is a flat ribbon cable 288.
So now look at the rollable – and so we’ve got a couple of different versions of this but actually the larger rollable you can see that even the larger rollable is much smaller than either the loose tube or the flat ribbon.
All of these cables are GR20 rated. So you have the same rugged, crush, impact, and tinsel performance that we’ve come to rely on for decades. So give Rollable Ribbon some thoughts for your network. Now you can get the benefits of ribbon slicing when you can use it. Or also single fiber access. You can do either one, whenever or where ever you need to.
It can act as a ribbon. It can also act as a single fiber giving you a lot of freedom to use either platform in a much smaller package.
So overall, we think we’re going to see a lot more customers move towards Rollable Ribbon in their fiber to the home distribution networks. And now in the future.
Sandy Oregon located about 30 miles from downtown Portland is blessed with beautiful mountain scenery but unfortunately its great location also leads to challenges when it comes to being connected to the outside world. In 2002 city officials had the vision to start their own internet service provider and offer affordable internet service to its residents. The result was SandyNet. From the beginning it was a huge success. People wanted it. They were hungry for it.
We had been in municipal ISP for quite some time starting off with DSL and wireless and outgrown that technology and so really the only next step for technology was to go to fiber. So, in 2014 SandyNet partnered with OFS to bring high-speed fiber to Sandy’s businesses and residents. They set some specific goals at the time that included deploying a future-proof fiber optic network. Providing all neighborhoods with the same service enabling residents the ability to access videos, e-learning, gaming, and government services, increasing Sandy’s competitiveness to attract new business and offering the city-owned network as a utility. In the years since upgrading to a fiber optic network, how has Sandy done with meeting their goals?
Sandy’s seeing some unprecedented growth right now both on the residential side and a lot of commercial opportunities. Going forward, SandyNet will continue to help lure people here and increase our tax base, increase our residence and increase the virtual learning environment that a lot of our students are partaking in. Right now, over the course of the last six seven years, during that time period I’ve talked to a number of people in Sandy who have moved here to the community and while it wasn’t the sole decision that they made the fact that gigabit fiber was available at their residence in a wired solution was the deciding factor for several people to buy a house in this community.
Our community has just thrived with having that big fat fiber pipe. That’s one of the things for us where we were always struggling with the amount of capacity that our previous internet service providers were giving to us. Now we don’t have that problem anymore. We have over capacity in a sense. We have enough to where people can stream and listen to music while they’re working and we can still get all our work done. We can download the data that we need to get it done in minutes, seconds. Instead of an hour or two, we can upload stuff into our servers, into our clients servers, quickly just all of that greatness that comes with having a really fast internet connection.
It’s a great asset to have our SandyNet charges on the same utility bill that our water and sewer charges are on. It’s an easy one payment a month to have this service that you know is much more affordable than a lot of the other providers that are out both in our area and nationally.
But back in 2014 when SandyNet was deploying fiber to try and make its network future proof no one could have foreseen what fiber would mean to not just the town but the entire world in 2020. There was virtually no one whose life was not affected by the COVID-19 pandemic. Having a high speed fiber optic network became the difference between those who were able to keep pace with the world and those who were left wanting something better. It was amazing that we got our fiber put in place when we did. It was built out to so many homes and had gone in front of hundreds of the homes in Sandy. Those that didn’t initially take it, could get it during that time. There was no rationing. We were in school one day and then everyone was at home the next day. Not just students, but also people working from home.
When you have a strand of fiber going to your house providing gigabit capacity it makes working from home very easy and I think a lot of people have been very happy with the service that they’ve gotten here in Sandy. From council meetings being held virtually, to team meetings, and other department meetings at the administrative level, we were able to continue our operations seamlessly with the help of SandyNet During the pandemic we did see an increase in speed upwards of 25 across our overall traffic patterns. Mostly traffic did increase during the workday. Our network is designed to be able to carry that load.
Once the city of Sandy decided to go all in with fiber who better to turn to than OFS. The industry leading experts at OFS were able to design, engineer, and deploy a network that was easily able to meet all of Sandy’s goals for a robust future-proof network.
OFS is able to combine several products that are designed to work with the gigabit, and 10 gigabit, 25 gigabit fiber. OFS can integrate solutions together to provide a complete turnkey solution for fiber to the home to our customers. We have to thank OFS a lot for SandyNet. We can’t go out and buy a book on how to set up fiber to the home network. Working with people that have been in the industry for 30 some years and having a great team that has worked on fiber the home projects in the past has been great. OFS has done all the heavy lifting. They’ve figured out all the optical budget information and then just provided this seamless solution and high-quality product.
Sandy’s been a fantastic success. We really appreciate all the opportunities from the city of Sandy for OFS. A complete OFS product solution along with the design the engineering, the build has provided the city of Sandy residents with great economic enhancement and quality of life improvement for decades to come. It would be a lot harder to differentiate Sandy from any other city of 12,000 people in Oregon. We still have many things that make us unique, but SandyNet really is the biggest one and I think it will have the most impact on our community over time.
To read the audio transcript for The Last 100 Meters of Fiber to and Into the Home click here.
AN OFS WHITE PAPER: NEW TECHNOLOGIES ENABLE FASTER, EASIER FIBER INSTALLATION INTO HOMES AND MDUs
For immediate access to our exclusive FTTH whitepaper, click here.
EZ-Bend Cabling and InvisiLight Solutions enable fast, easy and accepted fiber deployments to and into MDUs, homes and offices.
Driven by unprecedented demand for Gigabit and fast emerging 10 Gigabit Broadband, Fiber to the Home (FTTH) deployment is expected to reach record numbers in the coming years. According to iDate, FTTH connections will more than double in Europe over the next six years, while RVA forecasts that FTTH investment in North America will double in the next five years compared to the previous five. In addition to homes, service providers are bringing fiber to the living unit in multiple dwelling units (MDUs) and into commercial and institutional buildings. In all these scenarios, a cost-effective and compact optical network terminal (ONT) is typically placed inside each living unit or office to enable Gigabit or even 10 Gigabit connectivity. However, there are aesthetic and cost challenges to placing fiber inside residences or buildings. While service providers typically prefer to deploy ONTs deep within subscribers’ units with co-located Wi-Fi:
there may be no existing fiber ducts or pathways to the ONT location.
installing new ducts or cutting and patching walls can be very expensive and disruptive.
surface mounting conventional fiber cables can be unsightly and result in optical signal loss when the cables are bent around the many sharp corners on the pathway to the ONT.
wire molding or tape systems to house fiber are typically cost prohibitive, visible. and very slow to install.
Good day, my name is John George. I’m Senior Director of Solutions and Professional Services with OFS. Today I want to share what’s new in my world that could help you build better fiber to the home (FTTH).
Originally, FTTH deployments were fiber to outdoor ONTs on the side of a house. From there, copper typically was used to reach an indoor residential gateway inside the home. Over time we discovered indoor ONTs are smaller, and less expensive. They could be deployed inside the home, co-located with the wi-fi in the center of the building and achieve much better wi-fi coverage. This would reduce the cost, but there had to be a way to get the fiber into the home to that indoor ONT.
For MDU’s and apartment buildings, it evolved from fiber to the building, and trying to use the existing in-building copper to bring the internet service to each living unit. That proved to be insufficient from a bandwidth perspective for MDUs. To support 10 gigabit data rates that are being deployed now, it’s desired by service providers to get fiber into each living unit of an apartment building.
I’ll walk you through some innovative technologies we offer, that help solve those significant problems. The first one is EZ-Bend Cabling Solution. This is a three-millimeter diameter cable. It can have connectors on both ends. It can simply be stapled around any corner, and bends sharply in half with no concern about loss whatsoever. It has a 2.5 millimeter minimum bend radius – that’s twice as tight as anything else you can get on the market. The best from others is only five millimeters.
The EZ-Bend 2.5 millimeter bend radius technology is even more necessary as we move to from gigabit, to 10 gigabit. 10 gigabit use longer wavelengths in the transmission system. Longer wavelengths have much higher bend loss than what’s used for the gigabit. So, cable bends in a 10 gigabit transmission is a bigger challenge, but we solve it with the EZ-Bend. You can literally bend it any way without concern for signal loss.
The next technology we offer is the OFS InvisiLight Solution. It‘s used to solve issues inside the home or apartment. Originally, stapling cables inside the home was acceptable. Now, many customers don’t want to see the fiber at all inside the living unit. The OFS InvisiLight Solution is fiber installed inside an apartment, or home that blends into its environment; nearly invisible to anyone looking directly at it. The OFS InvisiLight Solution gets the fiber deep inside to the ONT. We have connectors on both ends of the InvisiLight spool making it easier to deploy a single part number, that can cover up to 132 feet, getting fiber into a home or an MDU living unit.
We also have a 12 fiber version, and a 16 fiber version for MDU hallways. These editions to go down the hallway and drop off the single fiber InvisiLight into the living unit.
Finally, we have the EZ-Bend Single Family Drop Solution that includes drop cabling, from the pole or pedestal to the home, and into the home. This is a double end connectorized assembly that’s toneable orange and can be laid out as a temporary drop, then buried later. What’s beautiful about this is it has the InvisiLight fiber inside the cable, so you can run this to the house below grade, direct buried, or induct, then wrap the house with it, and then to extend into the home to reach the ONT. All that is needed is to strip the jacketing off and then what you have is the fiber outside going through the wall to the inside where we’ve got the InvisiLight deployed to the ONT. Essentially this is a single piece of glass all the way from the curb, into the home, and into connecting the ONT. Quite simple, one part number, you don’t need a Network Interface Device (NID). You don’t need the time to install the NID. You save dollars if you avoid conduit and the NID by going with the EZ-Bend Single Family Drop Solution to the home into the home straight to the ONT.
That’s what’s new in my world. Thank you for listening.
We invite you on a tour of our fiber optic cable manufacturing facilities in Carrollton, Georgia, USA. View the highly automated OFS manufacturing process that produces a wide variety of fiber optic cables and products for telecommunications applications. Loose tube, microcables, flat ribbon, ADSS, ultra high density rollable ribbon cables and premise cables are all made here.
The manufacturing facility is registered in compliance with the ISO 9001, ISO 14000, and TL 9000 standards. Traceability is maintained through every step of the process and ultimately back to the incoming fiber. The facility also has a fully functional product qualification lab and cable installation test track.
OFS Uses Both 200 and 250 Micron Fibers
OFS makes several different fiber structures in the Carrollton facility, including loose tube, flat ribbon and rollable ribbon structures. These structures are used in different cable types and applications.
Statistical Process Control Techniques
Each stage in the manufacturing process is highly controlled with appropriate dimensional targets and tolerances.
Colored ink is applied to the Fiber
The industry standard color code is used to provide clear identification of the fibers over their lifetimes. Colored ink is applied to specified thicknesses, cured, and respooled for the next step in the process.
Buffer Tube Manufacturing Process
To make loose tubes, fibers or ribbons are paid off of their spools and a buffer tube is extruded around them. The Carrollton plant makes gel-free and gel-filled buffer tubes of different materials, including polypropylene and PBT. Different sized buffer tubes are used for different product types. Buffer tubes used in outside plant applications include either water blocking materials impregnated with super absorbent polymer or gel.
Ribbon Manufacturing Process
A matrix material is applied to the fibers to bind them together so they can be spliced as a group. 12 and 24 fiber flat ribbons are most common. Fiber color code alignment and geometric specifications are very important so ribbons can be spliced and connected in the field.
Rollable ribbons are only partially bonded together, enabling them to be rolled into a cylindrical package. Rollable ribbons are only partially bonded together, enabling them to be rolled into a cylindrical package. Since circles are more space efficient than rectangles, rollable ribbons cables can hold twice the fibers as comparable sized flat ribbon cables. Since these fibers are partially bonded, they can be easily spliced either as single fibers or as a ribbon, giving more deployment flexibility to the network operator.
OFS makes two main types of cables – stranded cables and central tube cables.
The first Article in this series focused on growth in bandwidth demand and attenuation in optical fibers. Article 2 concentrated on the several types of dispersion that exist in fiber today, closely followed by Article 3 – Fiber strength and reliability. Article 4 featured single-mode fiber geometries and now the latest release from OFS – Article 5, deals with “Cut-off Wavelength” (COW). This latest article will help the user to understand what “cut-off wavelength” is, why does it matter and how is it measured.
CUT-OFF WAVELENGTH MAKES A COMEBACK IN IMPORTANCE
Take an intro to fiber course, and you’ll learn about attenuation, dispersion, fiber geometry, maybe fiber strength. However, buried in the fine print of fiber specs is a parameter called cutoff wavelength. Although fiber manufacturers and some fiber users are aware of cutoff wavelength, it’s not as famous a parameter. Users still focus primarily on attenuation, maybe dispersion, as critical propagation properties.
Due to relatively new operating wavelength requirements below 1310 nm, some astute end users are taking a renewed look at cutoff wavelength specifications. Relatively new Passive Optical Network (PON) protocols are looking at wavelengths as low as 1270 nm, which, optical spectrum-speaking, is a hop and a jump away from the historical cable cutoff wavelength specification λcc, 1260 nm.
Different fibers do different jobs in today’s networks. Some fibers are bend insensitive. Some fibers enable more power to increase signal-to-noise ratios at ultra-high speeds. Cutoff wavelength is important for the performance of these fibers, but these fibers may require differences in measurement methods versus “standard” G.652-type fibers. We’ll talk about measurement methods later in this paper.
WHAT IS CUT-OFF WAVELENGTH?
In an optical fiber a number of different light-“modes” may exist. Modes are different types of light waves which may each carry different portions of light from the input to the output of the fiber.
A multimode fiber, by definition, may carry several modes of light (several hundred), but in a single-mode fiber, only one mode is carried.
The wavelength at which the fiber is at the cusp of changing from single-mode to multi-mode is called the Cut-off Wavelength. Typically, a fiber is considered to be single mode for wavelengths longer than the Cut-off Wavelength (COW) – and multimode at shorter wavelengths. In real life, the transition from single mode to multi-mode transmission does not occur abruptly at an isolated wavelength – but rather relatively smoothly over a range of wavelengths. The single-wavelength number on a specification is a simplification.
The most common way to make a fiber single-mode is to reduce its core-size (diameter), but the contrast between the refractive index of the fiber core and the fiber cladding is also important. These two properties determine the “effective core size” which determines if the fiber is single mode or not at a given wavelength.
WHY IS IT IMPORTANT?
The whole idea of a single-mode fiber is to keep the other modes out of the optical transmission. The reason is why Multi-Mode fibers are used for short distance transmission only: different modes traveling through a fiber take different paths.
An optical pulse injected into a multi-mode fiber will be transmitted via different modes reaching the end of the fiber after slightly different travel times. Once the modes are recombined at the output of the fiber, the shape of the input pulse will have been distorted (blurred). This distorting effect on the pulse is known as modal dispersion and affects the bandwidth (MHz-km) of a multimode fiber. Single-mode fiber does not have modal dispersion, and consequently has much higher bandwidth over dramatically longer distances.
The concept of cutoff wavelength is receiving renewed attention as next generation PON systems begin operating at wavelengths shorter than 1310 nm. This will be discussed in more depth later in the paper.
“ONLY ONE LIGHT MODE IN MY SINGLE MODE FIBER, PLEASE!”
The light mode intended for transmission in a single mode fiber is called the Fundamental Mode (also called LP01). All other modes are called Higher Order Modes, the most important of those is the Secondary Mode (LP11).
And one might ask: “Where do those higher order modes come from?”
These modes may be generated at splices and connections between fibers. The higher the splice/connection loss, the more powerful the Higher Order Modes (HOM) tend to be generated.
HOMs are also called “leaky modes” because they are bound only loosely to the fiber core and tend to leak out of the fiber after traveling a relatively short distance – for wavelengths longer than the Cut-off Wavelength. The longer the distance, the more of those modes will have leaked out.
The Cut-off Wavelength is defined as the wavelength at which the power level of the Higher Order Modes has been reduced by 19.3 dB relative to the level of the Fundamental Mode (strictly speaking this is true only for the Second Order Mode).
MODAL NOISE PROBLEMS
Do we need to worry about modal noise in today’s systems? Without wanting to raise unwarranted concerns, we want to highlight possible situations where modal noise may arise and be problematic.
As briefly mentioned, splices or connectors will cause some of the light in the Fundamental Mode to be coupled into higher order modes. And likewise, light from Higher Order light modes may be coupled down into the fundamental mode by similar splices or connectors.
Situations may arise where two splices or connectors are situated closely to each other. If they are too close, or if the Cut-off Wavelength is too high, parts of the HOMs generated at the first splice/connector may be coupled back down into the Fundamental Mode at the second splice/connector and mixed with light from the initial Fundamental Mode.
This may cause a problem because the travelling time for the light signal may typically be different in the Higher Order Modes than in the Fundamental Mode – and so the two signals may be somewhat out of phase when mixed together. Because of polarization and other effects, such phase difference may change as a result of temperature variation and stress, and this can result in a type of noise called Modal Noise.
To create significant levels of modal noise two joints with large connection losses must exist (one is not enough). Furthermore, the two joints must be so closely spaced that Higher Order Modes have not leaked out of the fiber before reaching the second joint. And finally, the laser used must exhibit some level of mode partitioning.
WHY ARE THERE DIFFERENT TYPES OF CUT-OFF WAVELENGTH?
The Cut-off Wavelength of a fiber depends on the length of the fiber, the longer the fiber the lower the Cut-off Wavelength tends to be. For that reason, there are 3 types of Cut-off Wavelengths defined to match different applications:
CABLE CUT-OFF WAVELENGTH (λcc): It is very easy to mistake this term for “Cabled COW” – but in principle it does not really matter whether the fiber is cabled or not. The initial intention with this parameter was to simulate a situation with two closely spaced cable splices, for example in a repair situation. 20 meters splicing distance was considered a relevant minimum – and to simulate fiber being deployed in splice cassettes, a 1-meter fiber length including one 80 mm loop was added in each end for the measurement set up. Effectively the full length of the fiber measured is 22 meters.
JUMPER CUT-OFF WAVELENGTH (λcj): As the name suggests it simulates a jumper cable. It is measured on a 2-meter length of fiber with one winding which diameter may be freely defined – which in the US is typically 152 mm.
FIBER CUT-OFF WAVELENGTH (λcf): As the name suggests it simulates a fiber being bent in only large diameters. In principle it is measured on a 2-meter length of fiber with one 280 mm diameter, but as explained later special care must be taken during measuring – especially for bend-insensitive fibers.
Because Cable COW is measured on a 22 m fiber sample whereas Fiber COW is measured on 2 m fiber, Fiber COW is typically higher than Cable COW.
Normally it is possible to find a good statistical correlation between the Cable COW and the Fiber COW. Since only 2-meter fiber is needed when measuring Fiber COW, it is easier to measure Fiber COW than Cable COW where 22 meters are needed. And because of the correlation Fiber COW measurements are often sufficient to ensure that the Cable COW is within limits.
Also, cabling fibers often has an effect of stripping out higher order modes, due to either macro or micro-bending of the fibers inside the cable sheath. In situations where this effect is significant, Cable COW measured on a cabled fiber may be lower than Cable COW measured on a non-cabled fiber (and of course lower than Fiber COW).
It depends on the fiber type and design, but a realistic example for a step-index fiber is a fiber cutoff of 1350 nm could have a corresponding cable COW of 1260 nm.
WHY IS CABLE CUT-OFF WAVELENGTH SPECIFIED AS 1260 nm IN IEC AND ITU-T RECOMMENDATIONS?
Initially Single Mode fibers were intended for 1310 nm operation, and manufacturing variability of lasers were rather large, so lasers sold as “1310 nm lasers” could in fact emit light at rather different wavelengths than 1310 nm. So, to create a certain “guard band”, the maximum Cable COW was defined to be 1260 nm.
Today, laser wavelengths may be more tightly controlled and extremely accurate, but those lasers may also be more costly than less accurate lasers. Furthermore, some FTTH/PON transmission formats (especially newer ones such as XGS-PON) use wavelengths close to 1260 nm. So, the 1260 nm COW today has renewed relevance.
BEND-INSENSITIVE FIBERS – AND PROBLEMS MEASURING THEIR ATTENUATION
This may seem like a subject entirely different from Cut-off Wavelength – but measurement problems are quite similar and may not be obvious at all.
To determine the Cut-off Wavelength, two measurements of the power on the output of the fiber are compared:
A. Power in the Fundamental Mode (LP01) only
B. Power in the Fundamental Mode (LP01) and the Higher Order Modes (which in practical terms means the Second Order Mode: LP11)
“B” is just a simple measurement of output of the fiber – including all modes. But in order to measure “A” we need a filter to get rid of the Higher Order Modes. They are loosely bound modes, and in a standard G.652 fiber they will tend to leak out of the fiber relatively quickly – especially when the fiber is bent in small diameter loops. Two 80 mm loops – perhaps with an added 25 mm loop – in a 2-meter G.652 fiber will do the trick, so this is often used as a HOM filter.
MEASURING ATTENUATION USING THE CUT-BACK METHOD
When measuring the attenuation of a Bend Insensitive fiber – including some advanced Large Area G.654 fibers – different methods may be used. The Cut Back measurement method is often considered the reference method. Light is injected into the Fiber Under Test (FUT) and the aim is to measure the power injected into the fiber (at the beginning of the fiber) and compare that to the measured power at the end of the fiber. Subtract the two and the fiber attenuation is found – divide this by the fiber length, and the attenuation in dB/km is found.
To determine the exact input power level, the fiber is often cut a short distance from the actual input end of the FUT, and the input power level is measured.
It is always important to avoid Higher Order Modes at the input of the measured fiber, since they will leak out of the FUT over a relatively short distance and be missing at the output of the FUT. HOMs are like ghosts – appearing at the input of the FUT and then disappearing along the fiber length. With these around, one would tend to measure a “too high” power level at the input of the fiber – and the resulting calculated fiber attenuation will then be too high. Such increased attenuation will tend to be more pronounced at shorter wavelengths.
Previously, such attenuation measurements were predominantly made on standard G.652 fiber and that was easy. You could grab the first 2 meters of the G.652 FUT, make the desired 2 or 3 loops directly on the fiber itself within those first 2 meters of fiber and that would give you your HOM filter and get rid of the Higher Order Modes. Your cut-back point for attenuation measurement could then be situated right after the first 2 meters of the FUT – so you would only waste 2 – 3 meters of fiber for each measurement.
But on bend insensitive fibers it is not quite as easy to get rid of the higher order modes. These fibers tend to better restrict the light from leaking out during fiber bends. Unfortunately, that is true for the higher order light modes as well – those modes which we would want to get rid of when measuring the fiber.
However, the 2-meter G.652 standard fiber with two 80 mm and one 25 mm loop may still be used. It may be spliced to the Fiber Under Test and if a good splice is obtained only an insignificant level of Higher Order Modes may be generated at that splice point – ensuring a good measurement if the splice point is also used as the Cut Back Point.
Another possibility is to use the first 22 meters of the FUT including the two loops of 80 mm recommended for measuring Cut-off Wavelength. Since that is the recommended test set up for COW measurement, you know that after 22 meters the HOM level is very low at the FUT’s COW or longer wavelengths. The Cut Back Point may be chosen to be just after the first 22 meters of the FUT but if very high precision measurement is required, or if FUT is relatively short, more than 22 meters may be needed.
On OTDR measurements problems caused by HOMs are often hidden. The reason for this is that an OTDR will typically have a “Dead-Zone” during which the light detector of the OTDR is recovering and as such unable to detect the incoming light signal. This may cover some 500 meters during which length the HOMs have since long leaked out of the fiber.
Hi, I’m Mark Boxer Technical Manager for Solutions and Applications Engineering for OFS and this is what’s new in my world.
We are seeing a tremendous amount of recognition around the country that fiber is the answer for connectivity now and into the future. We’ve seen just unprecedented interest in fiber networks around the country and the benefits that bandwidth brings to an area and specifically with a focus in rural areas. I live in a rural area this is an old, restored farmhouse and we are seeing a lot of interest in broadband even in very remote and rural areas. The Fiber Broadband Association is in the process of publishing some projections showing that demand for a household will exceed a gigabit per second.
This demand is coming from a lot of different places. It’s coming from the use of high-resolution video, all of the different applications that we’re running, as well as augmented reality and virtual reality. We’re starting to see a lot of these trickle into daily life today. We expect that these types of technologies will continue to get more accepted over the decade leading to a lot of increase in demand for bandwidth. Fiber is really the only way to build a future proof network. We are also seeing a high level of interest in next generation PON platforms including XGS-PON, NG-PON2. These are platforms that use different wavelengths than the networks that we’ve been deploying primarily for the last 15 to 20 years. These wavelengths are a little bit different because they are a little bit longer and they’re also a little bit more bend sensitive. Which means that we’re also a little bit more focused on bend sensitivity and making sure that these networks are viable for decades to come as more of these applications and more of these protocols come online and onto the networks.
If you’ve been using OFS fiber for years you know this is something you’ve probably heard about for a long period of time. It’s something your network is well prepared for. There has been lots of innovation especially over the last few years with fiber. There have been some groundbreaking technologies introduced and one of those is the concept of rollable ribbon. We see rollable ribbon making its way really into all parts of the network and we think that’s going to bring a tremendous amount of benefits to network operators including higher density. Once you have higher fiber density you can put more fibers into a smaller space. That frees up a lot of benefits with underground networks because you can pack more fibers into a smaller duct. For aerial networks now you can go longer distances with less sag or less weight on your poles so a lot of different benefits there. Also, since it’s a ribbon then you get the benefit of ribbon splicing.
We’re seeing a lot of interest in rollable ribbon being applied throughout the network and on top of that we’re seeing a lot of interest in builds to the home, builds to the MDU. These networks have a lot of bends and so they require that their fibers have a lot of bend insensitivity and so we’ve seen a lot of builds of those kinds over the last six months to year.
At OFS we bring a lot of different solutions. If you’re a network operator don’t get overwhelmed that you have to use this technology or that technology. A lot of these are just different tools in the toolbox and you can mix and match to make your network the most sustainable and viable for the for the future and OFS can really help with that.
So, one of the things that we are happy to do is spend some time with you looking at the choices that go into the network. So if you’re interested in learning more about some of the choices and the trade-offs in their network and how we could potentially help with that please feel free to contact me or your OFS representative and we’d be happy to help.
The first Article in this series focused on growth in bandwidth demand and attenuation in optical fibers. Article 2 focused on several types of dispersion that exist in fiber, followed by Article 3 – Fiber strength and reliability.
This article, the fourth in the series, will focus on single-mode fiber geometries.
When speaking about fiber geometries, we typically consider diameters of core, MFD (Mode Field Diameter), cladding and coating. Also, the concentricities of those and the ovalities – and then the actual curl of the fiber. More on that later however.
The primary impact of fiber geometry occurs in the splicing and connectorization processes. Fibers with good and consistent geometry tend to splice and connect better with lower connectorization losses than do others. However, as highlighted previously in Article 2, fiber concentricity could also play an important role for polarization mode dispersion (PMD) performance and is an important parameter. For high quality fibers geometry has been good for a long time, and we may have been so accustomed to it that we sometimes take it for granted. However, this may not always be the case.
We’ll work our way through a typical fiber specification, highlighting the importance of various single-mode fiber geometry specifications. In the first figure we present the basic properties of the geometry of a fiber.
CLADDING DIAMETER (OUTER GLASS DIAMETER)
Cladding diameter is the outer diameter of the glass portion of the fiber. For telecommunications fibers, this diameter has been 125 microns (μm) for a very long time. On the other hand, the diameter tolerance has not always been 0.7 μm.
During the 1980s, optical fibers had outer diameter tolerances as high as +/- 3.0 μm. As illustrated in Figure 2, trying to match up 8 micron fiber cores when cladding diameters varied between 122 and 128 μm in diameter could result in very high losses, since the two cores may be significantly misaligned even though the claddings of the two fibers are perfectly aligned. This situation is why fusion splicing machines required additional technology to help align the actual fiber cores. This extra technology, however, increased the price of the splicing units.
As the industry matured, single-mode fiber diameters remained the same at 125 μm. However, over the same time period, the specification tolerance declined to 0.7 μm with typical variation along the length of the fiber becoming even tighter.
From a manufacturing perspective, such tolerances were not easy to achieve. When fiber was first invented, the developers had to create manufacturing methods along with ways to measure fiber diameter. When manufacturing to tolerances of tenths of a micron, inputs such as stray air currents, vibrations or particulate in the glass can cause significant diameter variability. These factors require top- tier fiber manufacturers to have very tight control over their processes and procedures.
As diameter variability has decreased, splicing machines have reduced the alignment technologies needed. While there has been a significant decrease in the price of these machines, there has been no corresponding substantial increase in splice loss. Core alignment splicing machines still provide the best performance, however smaller “fixed V-groove” machines with lower prices and limited alignment capability have significantly reduced the performance gap. The typical splice loss for OFS AllWave®+ Zero Water Peak (ZWP) Optical Fiber, spliced using a core alignment splicing machine, is roughly 0.03 dB, whereas the same fibers spliced with a fixed V-groove machine have an average loss of approximately 0.05 dB. In a comparison of the absolute values, that is a significant difference. However, in the context of use in most fiber optic network applications, the difference is actually rather insignificant.
Enabled by tighter fiber geometry, the reduced cost of splicing machines is one of the factors that have contributed to the overall decrease in the cost of building fiber networks. In fact, this change has ultimately enabled fiber to the home to become a reality.
MODE FIELD DIAMETER (MFD)
Mode field diameter (MFD) is another specification related to fiber geometry. In a typical G.652.D- compliant single-mode fiber, not all of the light travels in the core; in fact, a small amount of light travels in the fiber cladding. The term MFD is a measure of the diameter of the optical power density distribution, which is the diameter in which 95% of the power resides.
MFD is important for two main reasons.
The first reason is that fiber bending loss is typically correlated with MFD. Unless special fiber designs are used, as the MFD increases, bend loss also increases, and vice versa. Historically, fibers with smaller mode field diameters are less bend sensitive. That being said, modern fiber designs has enabled fiber manufacturers to make bend insensitive, single-mode fibers with a nominal mode field diameter of 9.2 μm – which is the same as the vast majority of classical standard G.652.D fibers. However, there are also fibers with MFD as high as 8.6 micron offering superior bending performance, with very low bending losses even for bending diameters as low as 5 mm – which is half of the smallest bending diameter specified by ITU-T for G.657.B3 fibers.
Second, because of the ease of use, OTDR measuring instruments are often used to measure attenuation. However, OTDRs only give correct results if the measurement conditions are perfect. A sudden jump in MFD is definitely not a perfect measurement condition. So in the case where two fibers of different mode field diameters are spliced together, the OTDR will errantly show either a power gain, known as a “gainer”, or elevated loss, depending on in which direction the measurement is taken. When measured from the larger MFD into the smaller, a gainer is produced. When measured from the smaller MFD into the larger, an elevated loss is seen, as shown below. This is an artifact of the OTDR measurement method and does not affect transmission properties. Breaking and re-splicing the fibers will typically not change the result, unless there’s a bad cleave or some other anomaly at the splice interface. The correct way to measure splices overall is bi-directional OTDR, which is even more important for fibers with MFD mismatches.
This fact shows why it may be advantageous to use bend-insensitive fiber with MFDs of 9.2 micron. Since experience with installation and measurements of fibers is almost always gained working with standard G.652.D fibers with MFDs of 9.2 micron, such bend insensitive G.657 fibers will behave in a very familiar way in terms of splicing and control measurements. Especially when splicing to the installed base of 9.2 μm MFD single-mode fiber.
CORE-CLAD CONCENTRICITY ERROR
Core/clad concentricity error (CCCE or CCE) is also called Core-Clad Eccentricity and measures how well the core is centered in the fiber. CCCE is measured in microns and, of course, the closer the core is placed to perfect center, the lower the CCCE value and the better it is. Again, core aligning splicers tend to give the lowest splice losses.
Clad non-circularity is also called Cladding Ovality and measures a fiber’s deviation from being perfectly circular – becoming an oval rather than a circle. It is measured as a percentage difference between the “long” and the “short” diameter of an oval. If the Cladding Non-Circularity is zero the cladding forms a perfectly round circle. Similar to other fiber properties, better cladding non-circularity can result in improved splicing and connectorization performance.
While coating specifications are not as stringent as glass specifications, they are also important – especially when fibers are used in ribbons. The two main parameters are Coating Diameter (Uncolored) 237 – 247 μm and Coating-Clad Concentricity Error of max. 0.5 micron.
For roughly the first 30 years of single-mode fiber manufacturing, a coating nominal diameter of approximately 245-250 μm was standard in the industry. However, in 2014, OFS launched a 200 μm fiber in response to the need for higher fiber density in fiber optic cable designs.
Although the difference between 200 and 250 μm is not tremendously large, smaller diameter fibers can enable twice the fiber count in the same size buffer tube in a cable, while also still preserving long-term reliability. This fact has led to many new compact cable designs, including extremely small micro-cables, loose tube duct cables and all-dielectric, self-supporting (ADSS) aerial cables. As the demand for higher fiber density continues to increase, we can expect to see even more cable designs taking advantage of smaller diameter coatings. Important though, is that the coating will still be able to sufficiently shield the fibers from micro-bending, which may otherwise cause increased losses in the fiber when the fiber is inadvertently being “squeezed” in the cable – especially with low temperature.
Another possibility is to make the glass fiber itself less sensitive to such potential problems – so it’s not just a simple task of reducing the thickness of the fiber coating, but obtaining a sufficiently good fiber performance too.
Besides inherent size, coating diameter control is extremely important. Coating diameter can affect the size of the overall bundle in fibers. If the coating is too thick, the overall bundle may incur strain sooner than expected. If, on the other hand, coating concentricity is not good, there can be additional concerns particularly when splicing ribbons.
The final parameter we will discuss is fiber curl.
Fiber curl assesses the non-linearity of bare glass. In other words, fiber curl measures how straight the glass fiber is when no external stressors are present. If imbalanced stresses are frozen into a fiber during the draw process, curl can result. This curl can show up during the splicing of fiber optic ribbons or when fixed V-groove splicing machines are used.
If curl occurs, the two ends of the fiber will not be straight or match up during the splicing process. This situation leads to both high losses and difficulty splicing. Curl is measured in meters of curl, with a typical specification being > 4m. When optical fiber comes out of the fiber draw, it is annealed during the manufacturing process to reduce the effects of curl. As a result of this process, for users of top-quality fiber, fiber curl poses no concern for typical telecom applications.
Fiber geometry is often taken for granted by end users, primarily because it has been very good for so long. However, it has taken hard work and the contributions of innumerable people over many years for fiber geometry quality to reach its current level.
Article 1 in this series focused on growth in bandwidth demand and attenuation in optical fibers caused by bending, and the built-in mechanisms scattering and absorption. Article 2 focused on several types of dispersion that exist in fiber.
This article, the third in the series, will focus on fiber strength and reliability.
Except for products specifically intended for short time use only, lifetime and durability are normally of rather high importance for the user. As for optical fibers, the installation of the cabled fibers is typically both time consuming and very costly, and so reliability and lifetime may be even more important.
Historically this has resulted in a lot of focus on fiber lifetime, and although materials for optical fiber manufacture have been under constant improvement over the last 30 – 40 years, the longevity of even some of the first fiber optic cables is impressive.
Unlike most other fiber properties, we are here dealing with statistical properties which are difficult and time consuming to measure, and the exact lifetime of an individual fiber cannot be predicted.
As such this is not different from lifetime considerations for other products, but failure consequences may typically be more severe.
Fiber has a reputation for excellent reliability, and that hasn’t been acquired by luck. For most end users, fiber just works, and they don’t give the topic much thought. However, this performance is based on a deep understanding of the mechanical performance of the glass to the molecular level. Researchers have spent entire careers acquiring this understanding and translating this knowledge to product design, manufacturing, and installation recommendations.
WHY DO FIBERS BREAK?
For starters, pristine glass optical fiber is stronger than steel, when measured diameter to diameter. Although the actual intrinsic strength of glass can vary according with sample preparation, it is often greater than 700,000 pounds/square inch (PSI), compared to 70,000- 85,000 PSI for cold rolled steel.
Although glass is very strong, it is brittle so it cannot be stretched for large amounts without breaking. Imperfections and mechanical flaws may weaken the glass significantly as may impurities even as small as fractions of a micron.
In order to limit the number of impurities to an absolute minimum, OFS use synthetic silica to make our glass. Synthetic silica (SiO2) is hence made from ultra-pure chemicals which are pure to the parts per billion level, and the actual core of the fiber is pure to the 10 parts per trillion level. It would resemble allowing only one bad rice-grain in a 20 ft standard shipping container filled with good rice. By keeping such an extremely high glass purity, the risk of fiber weakening because of impurities is minimized. Other manufacturing methods may not provide similar high purity of the finished glass fiber.
But impurities are not the only reasons that may cause a fiber to break. A glass surface typically contains a number of cracks of different sizes. This is true for fiber glass surfaces as well, and when stress is applied to the fiber, the cracks will grow larger. As might be expected, a large stress on the fiber will cause fiber cracks and other flaws to grow more rapidly than if only a small stress is applied to that fiber.
There may be a huge difference in the speed by which the cracks grow, and this may not always seem obvious. More on that later.
Another issue which may not seem obvious is that stress is also applied to a fiber simply by bending that fiber. As shown in Figure 3 the outermost surface of a fiber bend will be stretched and result in tension – whereas the innermost surface will be compressed. Compression does not cause cracks in the fiber surface to grow larger – but tension does.
It probably seems more obvious that tension applied along the length of the fiber (typically by pulling the fiber) will also cause cracks and flaws to grow.
When cracks or flaws grow larger the strength of the fiber is reduced because a smaller cross section of the fiber is now holding the fiber together. So, a fiber with large cracks or flaws will break at a lower strain than a pristine fiber containing only small (or no) cracks.
To ensure that no large cracks or flaws are present in the fiber, a manufacturing step called “proof testing” is added after the actual making of the fiber. During proof testing, every 1 – 2 meter section of the fiber is subjected to a relatively large strain. So large that it makes the fiber 1% longer. Under such large strain cracks and flaws in the fiber will grow quickly, and if the cracks are large enough the fiber will simply break. This ensures that after proof testing no large cracks or flaws exist in the fiber anymore. However, small cracks and flaws will still be present and that is why the glass production method is so important. The purer the glass, the less likely problems will occur long term.
The proof test itself is simple. Fiber is typically guided around a pulley with a 1 kg weight attached to it for 100 kpsi proof test (Figure 6).
Fibers for use in submarine cables are normally proof tested even more stringently (2% elongation) to ensure even better lifetime performance.
An undamaged fiber which is proof tested 1% (100 kpsi) and used in standard applications and cable constructions will have a very long lifetime – typically 40 years or more. But fibers may get damaged. If that happens before proof testing, the fiber will break and the problem is eliminated.
However, damage to the fiber after proof testing may happen on rare occasions. Fibers are wound on spools for easy and safe handling and transportation and if the wound fiber “package” is somehow affected by a mechanical impact – as for example if the fiber spool is accidently dropped and hits the corner of a table – this may create large cracks in the fiber surface, and the fiber may suddenly not possess the expected strength anymore.
Likewise, the actual glass surface of the fiber may be “scratched” – potentially leaving large cracks in the fiber surface. Such scratches may have different causes but probably the most obvious of those causes is that the tool used to strip the coating of the glass fiber may inadvertently inflict scratches on the fiber surface. Also, sharp small objects may penetrate the fiber coating if the fiber is being forced against them – again possibly scratching the fiber glass surface.
In “loose tube” cable constructions fibers are placed in plastic tubes, which are helically wound around the cable-core. Such a helical construction ensures that cables may be pulled – and thereby elongated – quite a bit during installation before strain is transmitted to the actual fibers. The effect is the same as seen in a classical “spiral” telephone handset cord which can be extended quite a long way without putting additional strain on the copper wires within the cord.
Most cable constructions are similarly able to “absorb” significant strain on the cable before transmitting any additional strain to the fibers themselves, and in general such a “maximum pulling force” is always specified for cables – although different names may be used for it.
Nevertheless, a small risk does exist if for example heavy machinery stretched sections of a cable so much that the actual fibers were unintentionally subjected to excess tension, leading to fast growth of cracks in the fiber surface. As a result, the fiber lifetime may have been reduced – possibly without leaving traces on the cable or the fiber themselves. With modern equipment, training and knowledge, fiber damage problems are, however, highly uncommon.
In some situations, thermal variances may result in the movement of cables relative to the closures, and that may put the fibers under additional tension from time to time.
Because of helical structures present in many cable constructions the fiber is effectively situated in large diameter bends along the full cable length. Furthermore, it is common practice to coil a small amount of excess fiber in 50 mm or 60 mm coils in splice cassettes. So, in reality, the fiber is almost always under constant – but small – stress during its operational lifetime. Since the bending diameters are typically large the resulting lifetime for an un-damaged fiber is still very long. The breaking risk of a high-quality fiber under such conditions is therefore negligible. A very important feature for cables carrying signals to thousands of users.
For a damaged fiber, however, a rather puzzling situation may arise. The fiber may be fully installed, control measured, and all parameters may appear to be fine. However, because of damage the fiber may contain rather large cracks, which may propagate and hence weaken the fiber. Even with commonly used cable constructions and splice cassettes, damaged fibers may break rather quickly. It’s possible that fiber breaks can start appearing months after installation when everything appeared to be in order at the time of installation and even when nobody has actually touched the fibers since.
Some fiber cables are specifically intended for applications where small bending diameters are required. This could be drop cables to cell towers or 5G small cells – and in particular cables intended for installation in homes and apartments where it is important to be able to route the cable closely around door frames, windows and ceilings in order to make the cable as invisible as possible. Cables as thin as 0.6 mm are available, which may be bent with a radius as tight as 2.5 mm, while maintaining a failure risk which is still much smaller than that of typical electronic consumer equipment.
In figure 8) the results of a typical lifetime test are shown. The test registers the lifetime of a high-quality fiber when that fiber is bent at different diameters. What is important to notice is that lifetime is hugely dependent on the bending radius. For the fiber in the test, the lifetime changes from 1 minute to 40 years by changing the bending radius from 1.0 mm to 1.8 mm. Although very long lifetime predictions tend to be somewhat questionable, it is obvious that a bending radius of 2.5 mm – like the one recommended for some ultra-bend insensitive fibers – will still give a very long lifetime.
However, rather than focusing on lifetime it may be much more interesting for the user to have a prediction of the number of expected failures over a reasonable period of time. For example, computer hard disk drives are often characterized by the Annualized Failure Rate (AFR) representing the expected average number of failures over one year. Some investigations have shown approximately 1% of such failures expected over a year.
To compare fibers with that, and using internationally recognized lifetime models, a fiber cable used inside an apartment with for example 12 quarter-circle bends of 2.5 mm radius will have an expected failure rate of 45 ppm over 30 years. That is 0.0045% failure risk over 30 years (not in just 1 year).
So, fiber optic cables in typical indoor home installations with bend radii as low as 2.5 mm still have exceptionally less failure risks than the typical electronic household equipment which we know and use in our daily life. And since a – very rare – fiber break will affect only a few users, the possibility of having almost invisible cable installations using thin cables and 2.5 mm radius bending outweighs the small risk of failure for many users.
As a quick refresher, article 1 in this series focused on growth in bandwidth demand. We also looked at attenuation in optical fibers caused by factors external to the fiber e.g. bending, and built in attenuation mechanisms i.e. scattering and absorption.
In this second article, we will focus on the several types of dispersion that exist in fiber.
DISPERSION – WHAT IS IT?
Much, but not all, of the traffic traveling through fiber networks takes the form of pulses of laser light. Such a pulse is created by turning a laser on and off, creating light pulses where “no light” represents a digital “0” – and “full light” represents a digital “1”. Digital information is consequently a series of “no light” and “full light” transmitted in a code which a receiver at the other end of the fiber understands and can convert to a digital electrical signal.
Illustrating such a signal would be a series of square pulses as shown in Figure 1.
Whenever such a signal is affected by dispersion, the edges of the square pulses will be rounded, and the pulse will be spread out over time. So dispersion broadens the pulses.
If the dispersion is small, the detector at the other end of the fiber will still be able to detect the signal correctly. Once the dispersion grows too large, the broadened pulses will overlap each other and the detector will start misreading the signal, creating errors that will effectively hamper the transmission quality. A measure of that quality is the BER (Bit Error Rate) which states the number of transmission errors relative to the total number of transmitted bits.
Since a faster transmission rate requires pulses to be of a shorter duration, this also means that a given level of dispersion will be more harmful to faster transmission rate signals. Furthermore, dispersion is almost always dependent on the fiber length – the longer the fiber, the greater the dispersion.
Hence transmission is limited by: A) The dispersion of the fiber B) The transmission rate, and C) The length of the fiber. Dispersion can be described as a “speed limiter”- and the 3 main types are:
Modal Dispersion, Chromatic Dispersion and Polarization Mode Dispersion.
Modal Dispersion is the most serious of the dispersion types, and hence the most severe “speed limiter”.
Light “modes” are different types of waves carrying the light through the fiber. In a “Multi Mode” fiber, the core is rather large and may typically allow up to 17 different modes to propagate. In a “Single Mode” fiber, the core is so small that it will allow only one mode to propagate.
The problem is that the different modes follow different paths through the fiber – and these paths are of different lengths. Some modes travel close to the center of the core – others bounce against the outer edges of the core, and these modes travel a longer way than the ones close to the center. So the different modes travel different distances – and hence some tend to travel faster than others. Parts of the light being injected into the fiber will travel via one mode – other parts via another mode – and so on. If nothing is done to mitigate this, parts of the input signal will arrive at the output later than other parts – and this will cause the output signal to be “dispersed” relative to the input signal as illustrated in Figure 1.
To try to minimize the dispersion of the signal to the output of the fiber, the fiber core of a multimode fiber is designed to delay the Light Modes travelling close to the core (which is the shortest distance) and to speed up the modes travelling the longest distance. In a perfect world this would result in all modes bringing light simultaneously to the output of the fiber. Alas the world is less than perfect, and as such a bit of Modal Dispersion cannot be avoided in real life.
This means that, even though Multimode fibers are able to use very price efficient light sources (like LEDs or VCSELs) they are still limited to transmission distances of typically less than 2 km, actually often less than a few hundred meters.
The way to avoid Modal Dispersion is to shrink the size of the fiber core. In a small fiber core there is only room for one light mode to exist, called the Fundamental Mode. In such single-mode fibers, higher order modes may indeed be generated at splices or connectors, but they will leak out of the fiber after traveling a short distance through the fiber.
Having now found a way to avoid the most important speed limiter we can turn our attention to the next in line.
Chromatic Dispersion means that light of different wavelengths travel with different speeds along the fiber. Again, such a difference results in the “blurring” of the signal on the output side of the fiber and effectively acts as a speed limiter.
One might wonder why this should be such a problem, since lasers used to inject light into the fiber have very precisely defined and stable wavelengths. However, quickly turning a laser light on and off actually by itself generates a number of new wavelengths close to the original laser wavelength. Most of these new wavelengths are luckily quite weak and will not cause problems – but unfortunately as the laser light is turned on and off ever more quickly, the range of generated wavelengths broadens (Figure 5).
In such transmission systems the problems caused by Chromatic Dispersion worsen with increasing transmission speed and with longer fiber lengths (scaling linearly with fiber length).
Trying to minimize problems with Chromatic Dispersion the “Dispersion Shifted” (ITU-T G.653) fiber type was initially developed. In classical standard single-mode (ITU-T G.652) fibers the Chromatic Dispersion is zero around 1310 nm. The Dispersion Shifted fibers were targeted for the Chromatic Dispersion to be zero around 1550 nm, because the attenuation of the fiber is lower at 1550 nm and so this combination seemed ideal.
Basically, this worked fine right up until DWDM arrived. In DWDM systems a number of individual channels are transmitted over the same fiber. Each channel is assigned a unique wavelength, but unfortunately the fiber non-linearity called Four Wave Mixing (FWM) tends to cause unwanted noise problems in DWDM systems if the Chromatic Dispersion in the fiber is very low.
So realizing that some level of Chromatic Dispersion is preferable in order to limit fiber non-linearity problems in DWDM systems, Non-Zero Dispersion Shifted fiber (ITU-T G.655) was developed. This fiber type has a small amount of Chromatic Dispersion around 1550 nm (significantly smaller than standard G.652 fibers) so the “speed limitation” is smaller – but still the Chromatic Dispersion is high enough to reduce non-linearity problems very significantly. Later the G.656 Non- Zero Dispersion Shifted fibers were developed as a response to the demand for an increasing number of channels in DWDM systems. When the number of channels go up, the individual channels need to be packed more closely together – and that in turn requires more Chromatic Dispersion in the fiber to reduce the effect of the Four Wave Mixing.
In parallel with the development of new fiber types with different Chromatic Dispersion characteristics, special devices with negative Chromatic Dispersion were developed. Since transmission fibers normally have positive Chromatic Dispersion, a combination of those two can be used to reduce total Chromatic Dispersion for a full fiber link to almost zero.
With the ability to reduce the total chromatic dispersion of a transmission link, the higher Chromatic Dispersion of the G.656 fibers was consequently an acceptable technical compromise – leaving only cost issues still to be considered.
In many of the recent high-capacity transmission systems, the Chromatic Dispersion of the transmission fiber is compensated electronically with high efficiency, and for such systems fibers with high Chromatic Dispersion may actually be advantageous because it helps to limit fiber non-linearities.
Just to make confusion complete, a single-mode fiber will actually be able to carry TWO versions of the fundamental light mode. The reason for this is that light may exist in two different polarizations, the modes of which are perpendicular to one another. The phenomenon is known from some sunglasses which cut away one of those polarization modes. Reflected sunlight from the sea surface or a wet road will predominantly consist of light in one of these polarization modes – whereas light reflected by other objects will consist of a mixture of the two polarization modes. Cutting away the polarization mode of the reflected light will “kill” the reflections, but let the other polarization mode pass through the glasses, leaving other objects visible.
In an optical fiber, the two polarization modes will both exist, but may travel at different speeds through the fiber. Such speed-differences will arise if the fiber core is not perfectly circular and if stress is present in the fiber. Stress can be “frozen” into the fiber during manufacturing if the fiber geometry is not absolutely perfect, for example, if the cladding or coating is not circular, or if the center of the core is different from the center of the cladding or coating.
Even using state of the art, high-quality manufacturing process, the fiber will not be geometrically 100% perfect, hence there will be a speed-difference between the two polarization modes, dispersion will result, and it may limit high speed transmission through the fiber. Even if the fiber were 100% perfect, the slight bending of the fiber in a cable would introduce stress in the fiber – creating PMD. So this is our third speed limiter.
Looking at a fiber from a “PMD-perspective” it may be thought of as having a “fast” and a “slow” lane. An effective way of reducing PMD is by twisting the fiber back and forth during manufacturing so that a high number of shifts between the “fast” and “slow” lanes are effectively seen by the light travelling through the fiber.
Because stress is an important cause of PMD, externally applied stress will also affect fiber PMD. In reality just holding a fiber between two fingers may change PMD. As a result, the PMD of a fiber may be affected both by the cabling of the fiber and by external stresses, for example vibrations from a nearby railroad.
As with other dispersion types the effect of PMD increases with increased transmission distance (PMD scales with the square root of the distance) and increased transmission speed. For transmission rates of 2.5 Gbps and smaller, PMD is normally not a problem. For very high transmission rate systems, the compensation of PMD is today made electronically and built into the transmission system.