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OFS AcoustiSens® Optical Fibers used in random OPO system demonstration
By Regina Pynn, OFS Industrial Sensing & Networking Market Manager
Once again, OFS optical fibers are paving the way for researchers to bring cutting-edge technology out of the lab and into practical applications. This time, we’re delving into the realm of optical fiber sensing – a technology that relies on a carefully tuned light source with specific traits like wavelength, power, and pulse width.
Generally optical fiber sensing starts with a laser, but they come with a catch: lasers have their materials carefully selected to emit stable light pulses at a specific desired wavelength, limiting their flexibility. A system with wavelength modulation promises exciting innovations for fields as diverse as quantum computing and LiDAR sensing.
OPOs can use the deliberate scattering in AcoustiSens optical fiber to change the wavelength of light pulses
Enter the optical parametric oscillator (OPO). It transforms regular laser light into controlled wavelength pulses by guiding the laser light into an optical cavity, bouncing it around nonlinear crystals and resonators. As the light moves through the cavity and is sent back over itself multiple times the system changes wavelengths and creates parametric amplification.
However, there’s a hiccup in this dazzling performance: OPOs are quite sensitive to temperature and environmental changes. Even small changes impact the wavelength and power of the light as it exits the cavity, confining OPOs mostly to high-maintenance lab settings.
Researchers theorized that a random laser, which encourages scatter in the light source, would make the system more robust because the scattering would come from the controlled design of the laser and not be at the mercy of environmental changes in the optical cavity.
A groundbreaking paper from the University of Ottawa validates this concept. A team demonstrated, for the first time, that an augmented sensing optical fiber like OFS’ AcoustiSens can make this idea a reality. AcoustiSens is manufactured with enhanced Rayleigh scattering and this scattering allowed the OPO system to have stable, tuned wavelengths in a simple and robust optical cavity.
Congratulations to the University of Ottawa team and to all the technologists working to unshackle OPOs from the lab.
Dual-Brillouin-Peak Optical Fiber was designed and fabricated by researchers from OFS
In an era of advanced sensing technologies, the dual-Brillouin-peak optical fiber emerges as a new practical solution forresolving the strain-temperature cross-sensitivity that exists in almost all optical fiber sensors. Its potential spans across a multitude of fields, demanding precision over long distances and high resolutions. This groundbreaking technology is set to redefine the boundaries of Brillouin scattering based distributed fiber sensing.
Dual-Brillouin-peak single-mode optical fiber can measure both strain and temperature at the same time. This is a very useful feature for applications such as structural health monitoring, oil and gas exploration, and power transmission.
Dual-Brillouin-peak single-mode optical fiber has two distinct peaks in its Brillouin gain spectrum with similar amplitude levels. By measuring the frequency shifts of these two peaks, we can determine both the strain and the temperature along the fiber.
This is different from conventional single-mode optical fibers, which have only one dominant Brillouin peak and can only measure either strain or temperature, but not both at the same time. To measure both parameters, we would need to use two different fibers or a special fiber with a coating that has a different thermal expansion coefficient that usually results in an ill-conditioned discrimination.
The dual-Brillouin-peak optical fiber has several advantages over these methods. First, it simplifies the measurement system by reducing the number of components and connections. Second, it eliminates the need for calibration or compensation of the thermal expansion coefficient. Third, it increases the accuracy and resolution of the measurements by enhancing the Brillouin gain of the higher-order acoustic mode.
The researchers demonstrated the performance of their optical fiber in a 25-kilometer sensing length with 5-meter spatial resolution. They achieved a temperature resolution of 2°C and a strain resolution of 40 microstrain.
The fiber and standard single-mode telecom fibers are interchangeable with low splicing loss. The fiber is fully compatible with existing BOTDR/BOTDA (Brillouin Optical Time Domain Reflectometer/Analyzer) interrogators in the market. The dual-Brillouin-peak optical fiber is a promising technology for simultaneous distributed strain and temperature measurement. It has potential applications in various fields that require long-distance and high-resolution sensing.
With the $42.5 billion Broadband Equity, Access, and Deployment (BEAD) state allocations announced on June 26, states are now proactively creating plans to build high speed broadband networks to the unserved and underserved – all the way down to the last home or business on the farthest dirt road of every community.
In this On Topic, you’ll get a deep look at the critical impact fiber is making in rural communities. You’ll see how providers are able to make such a profound difference in these economies, as well as the technical “how-to” that makes these accomplishments possible.
Hi, I’m Mark Boxer, Technical Manager with OFS, and here’s what’s new in my world.
The first waves of fiber deployments were focused on single family homes and for good reason. There wasn’t as much demand for bandwidth and MDU or multi dwelling units and 15 to 20 years ago we just really didn’t have the same products that are now routinely used in MDUs. You know MDUs 15-20 years ago were some expensive science projects. So much has changed and it really can now make sense to do MDUs first.
So, first, broadband demand rises in single family homes and MDUs, and the commercial and regulatory environments are also changing. And very importantly the industry has responded with new products and installation methods that dramatically simplify the installation process and then to use. So as an industry we’ve learned that fiber installations and MDUs need to be fast, easy, and not visible – to preserve the building decor. The fiber also needs to be able to withstand the many bends of an apartment. InvisiLight Solutions enable these types of installations.
We’ve also discovered that with these tools, fiber installations for MDUs really have now flipped the script so these solutions are fast. They’re easy and inexpensive to install, meaning these installations can also be very profitable for network operators to deploy.
At a time when outside plant cable lead times may be long, indoor fiber products may have shorter lead times, which means that service could be turned up more quickly. We’ve also discovered that with these tools, the products can be installed with the minimal amount of training. A crew can be up and running within minutes to hours. Or, as we’ve heard people say before, on Thursday he’s a taxi driver, on Friday he’s an InvisiLight installer.
It can make sense for a service provider to consider MDUs first. So, let’s take a closer look. The Ultra bend insensitive fiber standard that’s used in these builds is G.657B3 and the OFS brand name for this fiber type is EZ-Bend fiber. You can bunch it up and tie it into knots. You can run into very tight angles and staple it with little added loss.
Many MDUs have lots of bends, and if you use a typical outside plant cable to do what I just did with the knot, then that would turn out the lights. However, we routinely deploy EZ-Bend and InvisiLight in environments with dozens of 90-degree angles, with no issues.
So, I was on a MDU build a couple of weeks ago where we went through forty-five 90-degree angles, two splices, and two connectors. The result was a loss of less than half a DB from the splitter to the ONT, and frankly that’s impressive.
The term InvisiLight is a nod to the light that goes into the fiber and the fact that it’s also almost invisible to the resident. Out of sight, out of mind. These products are deployed in minutes.
Deploying to MDUs can be fast and inexpensive to deploy, inconspicuous with low optical loss, and reasonably available supply chain. When you add these benefits together, it’s worth considering MDUs first. That’s what’s new in my world.
Visit our step-by-step Invisilight MDU/ILU product configurator for starting a Bill of Materials for your building.
This configurator is designed for buildings with riser spaces and indoor hallways. Additional options are available.
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.
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.
Executive Summary
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.
Transcript:
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.
What is Fiber Optic Cable Made up of?
Fiber optic cables are made up of several components: a core, cladding, jacket, and strength members.
The core is the optical fiber itself which is a continuous strand of ultra-thin glass.
Within the core, there are two highly specialized glass coatings called cladding and jacketing.
The cladding helps bounce back imperceptible light signals as they travel along the cable by reflecting off of its walls.
The jacketing protects the delicate optical fibers from mechanical damage and environmental effects.
Lastly, strength members such as aramid yarns or steel wires are used to reinforce and protect the cable further against bending or stretching forces. Together these components form a fiber optic cable that carries light signals over long distances without signal loss or interference.
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.
Cabling
OFS makes two main types of cables – stranded cables and central tube cables.
Stranded cables are made by stranding tubes of fibers, flat ribbons, or rollable ribbons around a central member. Stranded cables are often used in applications requiring frequent access to fibers.
For direct-buried cables, one or more steel armor layers may be added to provide rodent resistance and toning capability.
Outer Jacket
Different versions of polyethylene are used for the majority of outside plant cables. For inside plant or indoor/outdoor cables, materials are chosen to include appropriate flame and smoke resistance.
Information Printing
Type of cable, date of manufacture, length and a unique serial number that can enable traceability through the manufacturing process is printed.
Final Testing
Once manufacturing is complete, the finished cable undergoes final testing for length and optical properties.
Shipping
Cables are then packaged for shipment and loaded on trucks to their final destination.
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.
