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Tata Communications, Huawei, and Huawei Marine have announced the successful completion of a 400G field trial on a subsea network over 6,000 km. The test results demonstrated an optical transmission of 400G signals, an industry-first for a submarine cable system of this length. 

Huawei and Huawei Marine say their technical solution ‘adopted the modulation format of dual carrier polarisation division multiplexing quadrature phase shift keying (DC-PDM-QPSK), an innovative ‘Faster-Than-Nyquist’ (FTN) compensation and recovery algorithm, proprietary clock recovery technology and soft decision forward error correction (SD-FEC) technology to address the problems of high-speed signal distortion and unstable clocks’. The use of such advanced technology underpins Huawei and Huawei Marine’s significant commitment to investing in research and development to meet the needs of their customers, the companies say.

Hon Kit Lam, vice president for international transmission and IP business at Tata Communications, said: ‘The 400G technology of Huawei & Huawei Marine demonstrates that our existing subsea network asset is capable of supporting future, next-generation transmission technology as shown in the 400G trial. Tata Communications constantly assesses new technology to expand our capabilities and to enhance our ability to support traffic growth demand from our customers.’

Jack Wang, president of Huawei Transmission Network Product Line, added: ‘Huawei has always taken a customer-centric approach to our research and development efforts. Our 400G technology will not only help operators manage larger levels of traffic faster, but also optimise operating and network maintenance costs. We are pleased to be working with leading international carriers like Tata Communications on this 400G technology trial.’

What is Fiber Optic Splicing

Knowledge of fiber optic splicing methods is vital to any company or fiber optic technician involved in Telecommunications or LAN and networking projects.

Simply put, fiber optic splicing involves joining two fiber optic cables together. The other, more common, method of joining fibers is called termination or connectorization. Fiber splicing typically results in lower light loss and back reflection than termination making it the preferred method when the cable runs are too long for a single length of fiber or when joining two different types of cable together, such as a 48-fiber cable to four 12-fiber cables. Splicing is also used to restore fiber optic cables when a buried cable is accidentally severed.

There are two methods of fiber optic splicing, fusion splicing & mechanical splicing. If you are just beginning to splice fiber, you might want to look at your long-term goals in this field in order to chose which technique best fits your economic and performance objectives.

Mechanical Splicing vs. Fusion Splicing

• Mechanical Splicing:
Mechanical splices are simply alignment devices, designed to
hold the two fiber ends in a precisely aligned position thus enabling light to pass from one fiber into the other. (Typical loss: 0.3 dB)

• Fusion Splicing:
In fusion splicing a machine is used to precisely align the two fiber ends then the glass ends are “fused” or “welded” together using some type of heat or electric arc. This produces a continuous connection between the fibers enabling very low loss light transmission. (Typical loss: 0.1 dB)

• Which method is better?
The typical reason for choosing one method over the other is economics. Mechanical splicing has a low initial investment ($1,000 – $2,000) but costs more per splice ($12-$40 each). While the cost per splice for fusion splicing is lower ($0.50 – $1.50 each), the initial investment is much higher ($15,000 – $50,000 depending on the accuracy and features of the fusion splicing machine being purchased). The more precise you need the alignment (better alignment results in lower loss) the more you pay for the machine.

As for the performance of each splicing method, the decision is often based on what industry you are working in. Fusion splicing produces lower loss and less back reflection than mechanical splicing because the resulting fusion splice points are almost seamless. Fusion splices are used primarily with single mode fiber where as Mechanical splices work with both single and multi mode fiber.

Many Telecommunications and CATV companies invest in fusion splicing for their long haul singlemode networks, but will still use mechanical splicing for shorter, local cable runs. Since analog video signals require minimal reflection for optimal performance, fusion splicing is preferred for this application as well. The LAN industry has the choice of either method, as signal loss and reflection are minor concerns for most LAN applications.

Fusion Splicing Method
As mentioned previously, fusion splicing is a junction of two or more optical fibers that have been permanently affixed by welding them together by an electronic arc.

Four basic steps to completing a proper fusion splice:

Step 1: Preparing the fiber – Strip the protective coatings, jackets, tubes, strength members, etc. leaving only the bare fiber showing. The main concern here is cleanliness.

Step 2: Cleave the fiber – Using a good fiber cleaver here is essential to a successful fusion splice. The cleaved end must be mirror-smooth and perpendicular to the fiber axis to obtain a proper splice. NOTE: The cleaver does not cut the fiber! It merely nicks the fiber and then pulls or flexes it to cause a clean break. The goal is to produce a cleaved end that is as perfectly perpendicular as possible. That is why a good cleaver for fusion splicing can often cost $1,000 to $3,000. These cleavers can consistently produce a cleave angle of 0.5 degree or less.

Step 3: Fuse the fiber – There are two steps within this step, alignment and heating. Alignment can be manual or automatic depending on what equipment you have. The higher priced equipment you use, the more accurate the alignment becomes. Once properly aligned the fusion splicer unit then uses an electrical arc to melt the fibers, permanently welding the two fiber ends together.

Step 4: Protect the fiber – Protecting the fiber from bending and tensile forces will ensure the splice not break during normal handling. A typical fusion splice has a tensile strength between 0.5 and 1.5 lbs and will not break during normal handling but it still requires protection from excessive bending and pulling forces. Using heat shrink tubing, silicone gel and/or mechanical crimp protectors will keep the splice protected from outside elements and breakage.

Mechanical Splicing Method
Mechanical splicing is an optical junction where the fibers are precisely aligned and held in place by a self-contained assembly, not a permanent bond. This method aligns the two fiber ends to a common centerline, aligning their cores so the light can pass from one fiber to another.

Four steps to performing a mechanical splice:

Step 1: Preparing the fiber – Strip the protective coatings, jackets, tubes, strength members, etc. leaving only the bare fiber showing. The main concern here is cleanliness.

Step 2: Cleave the fiber – The process is identical to the cleaving for fusion splicing but the cleave precision is not as critical.

Step 3: Mechanically join the fibers – There is no heat used in this method. Simply position the fiber ends together inside the mechanical splice unit. The index matching gel inside the mechanical splice apparatus will help couple the light from one fiber end to the other. Older apparatus will have an epoxy rather than the index matching gel holding the cores together.

Step 4: Protect the fiber – the completed mechanical splice provides its own protection for the splice.

Tips for Better Splices:

1. Thoroughly and frequently clean your splicing tools. When working with fiber, keep in mind that particles not visible to the naked eye could cause tremendous problems when working with fiber optics. “Excessive” cleaning of your fiber and tools will save you time and money down the road.

2. Properly maintain and operate your cleaver. The cleaver is your most valuable tool in fiber splicing. Within mechanical splicing you need the proper angle to insure proper end faces or too much light escaping into the air gaps between the two fibers will occur. The index matching gel will eliminate most of the light escape but cannot overcome a low quality cleave. You should expect to spend around $200 to $1,000 for a good quality cleaver suitable for mechanical splicing.

For Fusion splicing, you need an even more precise cleaver to achieve the exceptional low loss (0.05 dB and less). If you have a poor cleave the fiber ends might not melt together properly causing light loss and high reflection problems. Expect to pay $1,000 to $4,000 for a good cleaver to handle the precision required for fusion splicing. Maintaining your cleaver by following manufacturer instructions for cleaning as well as using the tool properly will provide you with a long lasting piece of equipment and ensuring the job is done right the first time.

3. Fusion parameters must be adjusted minimally and methodically (fusion splicing only). If you start changing the fusion parameters on the splicer as soon as there is a hint of a problem you might lose your desired setting. Dirty equipment should be your first check and them continue with the parameters. Fusion time and fusion current are the two key factors for splicing. Different variables of these two factors can produce the same splice results. High time and low current result in the same outcome as high current and low time. Make sure to change one variable at a time and keep checking until you have found the right fusion parameters for your fiber type.

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Blue Valley Tele-Communications (BVTC) is reportedly upgrading its fiber-optic network in 17 communities in northeastern Kansas via equipment from Adtran. The fiber-to-the-home (FTTH) deployment will secure the economic future of nearly 7500 residential and business customers in the area, the equipment provider asserts. “NEOCLEAN As an early adopter of fiber-optic networks, BVTC has always understood how technology can enable for rural communities the same innovation, growth, and prosperity found in larger metropolitan areas,” said a representative for Adtran.

“Not only are businesses in northeastern Kansas driving commerce growth via BVTC’s voice, video, and data services, but local schools are participating in distance learning,” added the company spokesperson. “The community is also benefiting from stronger medical care, as doctors are now able to consult and share digitally transmitted medical data with colleagues around the world from the local hospitals.”

In this most recent fiber-optic network upgrade, BVTC replaced its existing equipment (apparently from Tellabs) with Adtran’s integrated Total Access 5000 platform with Optical Networking Edge (ONE) capabilities to support the delivery of gigabit FTTH and Carrier Ethernet services through its access infrastructure, as well as leverage packet-optical transport to carry those premium services across its footprint. BVTC says it can now support stringent cloud and mobile backhaul service-level agreements as well as deliver IPTV, Ideal 45-163 voice over IP (VoIP), and high-speed Internet.
“Blue Valley Tele-Communications has always believed in the impact technology can have on the economic progress and quality of life for our customers. It truly has the ability to keep our communities on an even playing field with national and even global competitors,” says Jon Novak, project manager, Blue Valley Tele-Communications, Inc. Novak concludes, “The versatility of Adtran’s broadband solutions enable us to have a single vendor to partner with us in deploying transport and access services. That combined with Adtran’s overall knowledge of the market helps us deliver best-in-class services while providing growing economic opportunities for our residents, businesses and community as a whole.”

A higher power version of the fiber tracer called a visual fault locator (VFL, visual fault finder) uses a visible laser that can also find faults. The red laser light is powerful enough for continuity checking or to trace fibers for several kilometers, identify splices in splice trays and show breaks in fibers or high loss connectors. You can actually see the loss of light at a fiber break by the bright red light from the VFL through the jacket of many yellow or orange simplex cables (excepting black or gray jackets, of course.)  It’s most important use is finding faults in short cables or near the connector where OTDRs cannot find them.

You can also use this gadget to visually verify and optimize mechanical splices or prepolished-splice type fiber optic connectors. By visually minimizing the light lost you can get the lowest loss splice. In fact- don’t even think of doing one of those prepolished-splice type connectors without one. No other method will assure you of high yield with those connectors.

A note on VFL eye safety. VFLs use visible light. You will find it uncomfortable to look at the output of a fiber illuminated by a VFL. That’s good, because the power level is high and you should not be looking at it. When tracing fibers, look from the side of the fiber to see if light is present.

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General Fiber Optic Inspection and Fiber Optic Cleaning Procedures

This section describes the connector cleaning process. Additional sections provide more detail on specific fiber optic inspection and fiber optic cleaning techniques.

General Cleaning Process

Complete these steps:

1. Inspect the fiber connector, component, or bulkhead with a fiber microscope.

2. If the connector is dirty, clean it with a dry cleaning technique.

3. Inspect the connector.

4. If the connector is still dirty, repeat the dry cleaning technique.

5. Inspect the connector.

6. If the connector is still dirty, clean it with a wet cleaning technique followed immediately with a dry clean in order to ensure no residue is left on the endface.

   Note: Wet cleaning is not recommended for bulkheads and receptacles. Damage to equipment can occur.

7. Inspect the connector again.

8. If the contaminate still cannot be removed, repeat the cleaning procedure until the endface is clean.

Figure 1 shows the connector cleaning process flow.

Figure 1

Note: Never use alcohol or wet cleaning without a way to ensure that it does not leave residue on the endface. It can cause equipment damage.

Connector Inspection Technique

This inspection technique is done with the use of fiber microscope in order to view the endface.

A fiber microscope is a customized microscope used in order to inspect optical fiber components. The fiber microscope should provide at least 200x total magnification. Specific adapters are needed to properly inspect the endface of most connector types, for example: 1.25 mm, 2.5 mm, or APC connectors.

Tools

• Clean, resealable container for the endcaps

• Fiber microscope 

• Bulkhead fiber optic inspection probe

Figure 2 shows different kinds of fiber microscope

Figure 2

The bulkhead fiber optic inspection probe is a handheld fiber microscope used in order to inspect connectors in a bulkhead, backplane, or receptacle port. It should provide at least 200x total magnification displayed on a video monitor. Handheld portable monitors are also available. Specific adapters are needed in order to properly inspect the endface of most connector types.

Figure 3 shows a handheld fiber microscope with probe and adapter tip for 1.25 mm connector.

Figure 3

Figure 4 shows two types of handheld fiber microscope.

Figure 4

Caution: Read the reminders and warnings before you begin this process.

Complete these steps in order to inspect the connector:

1. Make sure that the lasers are turned off before you begin the inspection.

   Warning: Invisible laser radiation might be emitted from disconnected fibers or connectors. Do not stare into beams or view directly with optical instruments.

2. Remove the protective cap and store it in a clean resealable container.

3. Verify the style of connector you inspect and put the appropriate inspection adapter or probe on your equipment.

4. Insert the fiber connector into the fiber optic microscope adapter, and adjust the focus ring so that you see a clear endface image. Figure 5 shows a clean single mode connector endface.

   Figure 5

   

5. Or, place the tip of the handheld probe into the bulkhead connector and adjust the focus.

   Figure 6 shows the handheld probe inserted into a bulkhead connection.

   Figure 6

   

6. On the video monitor, verify that there is no contamination present on the connector endface.

7. Clean the endface and reinspect, as necessary. Refer to the appropriate section:

   • Cleaning Techniques for Pigtails and Patch Cords

   • Cleaning Techniques for Bulkheads and Receptacles

8. Immediately plug the clean connector into the mating clean connector in order to reduce the risk of recontamination.

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At the turn of the millennium, fiber inspection was performed with a microscope. This required technicians to stick their eye on a potentially live fiber, which meant risking personal injury every time they had to assess fiber endface quality.

In mid-2005, the eye care community breathed a sigh of relief when the first fiber inspection probes made their way to the market. These probes were able to display an image of the endface on an LCD rather than directly on a tech’s retina. However, this image had to be interpreted. What constituted a defect or contaminant had to be based on user knowledge and gut instinct; depending on the quality of the focus, image centering, and several other parameters, there was always a chance of misinterpretation.

By 2010, the first intelligent analysis software for fiber inspection, which was based on the IEC standard, had arrived. The software automatically detected and analyzed any defect, highlighted it on the display screen, and gave an overall pass-fail status, thereby removing the burden of interpretation and human error from the equation. Or did it?

Regardless of the power of the on-board intelligence, poor focus and poor image capture will lead to errors. More often than not, an out- of-focus speck, scratch, or trace will simply not appear on the screen. The intelligent software will give the connector a thumbs up, when in reality it should not have. This is what is referred to as a “false positive.” As the saying goes, garbage in, garbage out.

Figure 1 below compares manual centering and focusing to automatic centering and focusing. The methodology involved inspecting the connector, cleaning it, and inspecting it again (with the manual focus probe) until a pass was obtained. The same port was then inspected with an automated unit.

Figure 1. Manual versus automatic focusing.

Many papers and studies have shown the impact that connector cleanliness has on network issues and failures. Unfortunately, very few technicians, operators, and managers acknowledge this. As mentioned before, regardless of the on-board intelligence and analysis software, when the endface is slightly out-of-focus or slightly overexposed or the image is slightly off-center, false positives will occur. To truly rid the world of the connector cleanliness plague, the last remaining unknowns and variables must be removed from the equation.

The following are examples of the impact that not-so-squeaky-clean connectors can have.

Impact on higher data rates
A Tier 1 data center test covers link budget only. If it fails, the cause of the failure is not analyzed. The connectors are changed, and if that does not work, the link is broken up and shortened. This is expensive, but often cheaper than locating and troubleshooting the issue.

Since the standardization of Gigabit Ethernet (i.e.,1000GBASE-SX) in 2002, the 3.56-dB total channel insertion loss (IL) for 50/125-micron multimode fiber was reduced to 2.6 dB for 10GBASE- SR and to 1.9 dB for 40GBASE-SR4 (and 100GBASE-SR10; see the table below). Consequently, for 40GBASE-SR4, a maximum connector loss of 1.0 dB is required for a 150-meter channel containing multiple connector interfaces and high-bandwidth OM4 fiber. Therefore, data center upgrades to higher data rates such as 40G and 100G may fail because the tolerance to IL becomes much tighter.

Since 2010, the ISO/IEC-11801 specification on general-purpose telecommunication cabling systems has also tightened the loss budget for connectors:

At higher speeds on OM4 fibers, 50% of the fibers must have a maximum IL of 0.35 dB or less. Therefore, the need to properly inspect connectors has never been greater.

Impact on other test results
Since a dirty connector will typically exhibit more reflectance and loss, the optical return loss (ORL) and IL readings taken by an OTDR will be higher. Figure 2 below illustrates this common problem. The experiment was conducted on a very short, 101.4-meter singlemode fiber link. Fiber loss in itself accounts for approximately 0.003 dB at 1310 nm, which is deemed inconsequential. The ORL and IL reading at 1310 nm for Connector 2 is 0.638 dB, with a reflection of -31.5 dB. The link ORL is 27.86 dB.

Figure 2. ORL and IL readings on an uncleaned connector.

After cleaning the connectors, the loss reading dropped to 0.053 dB at 1310 nm, with a reflectance of -55.9 dB. The link ORL also dropped to 50.4 dB. Everything is back to normal (Figure 3).

Figure 3. ORL and IL readings on a cleaned connector.

If we apply these results to the data center example above, only three of these bad connections would have failed at 40G data rates and higher.

Figure 4 shows Connector 2 before and after cleaning. It is interesting to note that the contaminant here is not grease or oil from the technician’s fingers, but simply dust collected from the environment (e.g., drywall, concrete, skin particles, and sand). Therefore, even when a technician does not touch the connector endface, it can still be contaminated.

Figure 4. Connector 2 before and after cleaning.

Impact on OTN bit error rate tests
Another example involves erratic readings during 40G or 100G Optical Transport Network (OTN) bit error rate tests (BERTs). Dirty connectors reduce the signal-to-noise ratio (SNR) at the receiver, and most PIN receivers react the same way to noise: with a proportional increase in BER. Problems such as forward error correction (FEC), alarm indication signal (AIS), or backward defect indicator (BDI) may also occur and lead to the unnecessary troubleshooting of Tx and Rx equipment. This means sending a technician to the site to retest the link to obtain clear results. This can be a very time-consuming, especially when you consider that a BERT needs to be error-free for 24 hours.

Case in point, a major operator in America used pre-installed fibers to deploy a 40-Gbps system across three states in 2013. They were using the “clean and connect” method without any inspection. They had to perform three BERTs because errors were showing up after 14 hours of testing.

The lesson here is that paying attention to fiber inspection will save time and eliminate the need to perform additional BERTs.

Impact on ORL
Every system has a maximum ORL, and clean connectors are vital to it. One area where ORL can be extremely detrimental is in high-speed coherent transmission (40G and 100G transport). In most of these deployments, whether they are greenfield or brownfield, low loss amplification is required to optimize distance. This means deploying a mix of Erbium-doped fiber amplifiers (EDFA) and, more recently, Raman amplifiers.

Raman is a low-noise amplification technique that uses the fiber itself as the amplifying media. It can easily be added to any existing infrastructure with little engineering. However, since the fiber is the amplifier, all light traveling within it will be amplified (i.e., both the signal and unwanted reflections). Reflections must therefore be kept to a minimum in every Raman-amplified system.

Getting the right result
To recap, false positives and the four examples above can be avoided by implementing the best troubleshooting and maintenance practices, which includes proper connector inspection and cleaning.

In today’s telecommunication environment, where opex is the name of the game, long, tedious, and ultimately misdirected troubleshooting efforts are unwelcome. Field technicians and engineers will waste precious time looking for issues at the fiber level (macrobends, splice points) or the transmission level (transmission and receiving cards) before checking the connectors. A fiber inspection probe that not only analyzes the connector endface image but auto-centers, auto-focuses, and freezes it will ensure the integrity and repeatability of inspection results.

By Francis Audet, Vincent Racine, and Gwennaël Amice

Francis Audet is advisor, CTO Office, Vincent Racine is product line manager, and Gwennaël Amice is senior application engineer at EXFO Inc.

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Google Fiber is nationally seeking candidates for the position of Outside Plant (OSP) Field Construction Manager. These positions will be based in one of the following cities: Portland, OR; Salt Lake City, UT; Phoenix, AZ; San Antonio, TX; Nashville, TN; Raleigh/Durham, NC; Charlotte, NC; and Atlanta, GA. Job applicants are asked to indicate a location preference in their cover letters.

Stated responsibilities for the Outside Plant (OSP) Field Construction Manager include the following: “Support the Metro Project Manager to manage the construction of Google Fiber’s OSP Fiber to the Home (FTTH) network; Interface with the Google Fiber OSP network team to coordinate project construction activities, progress and financial reporting, invoice review, and change management; Work with contractors and staff to develop construction schedules, monitor production, and ensure adherence to specifications; Manage production within budget and schedule constraints; Coordinate with cross-functional teams to seamlessly turn over completed network to Operations and Customer Service.”

Preferred qualifications for the position are as follows: “BS degree in Construction Management; 8 years of experience in managing large, highly-complex, outside plant projects, FTTH or outside plant; Familiar with GIS (Geographic Information Systems), ESRI and shapefile functionality; Knowledge of network drawings, route maps and scopes of work and interpreting fiber test results and auditing projects for compliance with scopes of work; Robust knowledge of inside and outside plant fiber optic network infrastructure, engineering design and construction, and the ability to work cross-functionally to design and build scalable construction, installation, and support processes.”

In the listing for the position, the company states, “As a member of [Google’s Network Engineering] team, you have a direct impact on design and feature enhancements to keep our systems running smoothly. You also ensure that network operations are safe and efficient by monitoring network performance, coordinating planned maintenance, adjusting hardware components and responding to network connectivity issues. Google’s complex network generates a constant stream of challenges which require you to continually be innovative with an evolving set of technologies. Keeping the network reliable ensures that our users stay connected with our suite of applications, products and services.”

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Market research firm Vertical Systems Group says that 39.3% of businesses in the United States with at least 20 employees can access services via a fiber-optic network connection.

This figure represents an improvement of just over 3% from the 36.1% total of 2012.

The remaining 60.7% of buildings reside in what Vertical Systems Group refers to as “The Fiber Gap.”

“During the past year, network operators narrowed the business fiber gap through construction and acquisitions,” explains Rosemary Cochran, principal at Vertical Systems Group. “The majority of new fiber deployments were focused on connecting medium and smaller buildings in the metro areas surrounding major cities across the U.S.”

Fiber cable is the optimal wireline access technology for delivery of higher-speed network services, the market research firm asserts. Carrier Ethernet and IP/MPLS VPNs, cloud and Internet connectivity, and mobile backhaul applications would all benefit from fiber connections, the company says.

“Broader accessibility to on-net fiber has started to shake up the services markets,” adds Vertical’s Cochran. “Fiber-based providers and cable MSOs are capitalizing on the reach and cost advantages of their footprints juxtaposed to legacy infrastructures. Customers are reaping the benefits of more service options, more competitive pricing, and faster service installations.”

The business connection statistics come from the @Fiber research track of Vertical Systems Group’s ENS (Emerging Networks Service). The research covers 2004 through 2013, and includes quantification by customer segment (large enterprise and SMB) and four building sizes (20-50 employees, 51-100 employees, 101-250 employees, >250 employees).

Introduction

It is important that every fiber connector be inspected and cleaned prior to mating. This document describes inspection and cleaning processes for fiber optic connections.

The procedures in this document describe basic inspection techniques and processes of cleaning for fiber optic cables, bulkheads, and adapters used in fiber optic connections.

Note: This document is intended for use by service personnel, field service technicians, and hardware installers.

Inspection and Cleaning are Critical

Clean fiber optic components are a requirement for quality connections between fiber optic equipment. One of the most basic and important procedures for the maintenance of fiber optic systems is to clean the fiber optic equipment.

Any contamination in the fiber connection can cause failure of the component or failure of the whole system. Even microscopic dust particles can cause a variety of problems for optical connections. A particle that partially or completely blocks the core generates strong back reflections, which can cause instability in the laser system. Dust particles trapped between two fiber faces can scratch the glass surfaces. Even if a particle is only situated on the cladding or the edge of the endface, it can cause an air gap or misalignment between the fiber cores which significantly degrades the optical signal.

• A 1-micrometer dust particle on a single-mode core can block up to 1% of the light (a 0.05dB loss).

• A 9-micrometer speck is still too small to see without a microscope, but it can completely block the fiber core. These contaminants can be more difficult to remove than dust particles.

By comparison, a typical human hair is 50 to 75 micrometers in diameter, as much as eight times larger. So, even though dust might not be visible, it is still present in the air and can deposit onto the connector. In addition to dust, other types of contamination must also be cleaned off the endface. Such materials include:

• Oils, frequently from human hands

• Film residues, condensed from vapors in the air

• Powdery coatings, left after water or other solvents evaporate away

These contaminants can be more difficult to remove than dust particles and can also cause damage to equipment if not removed.

Caution: With the high powered lasers now in use for communications systems, any contaminant can be burned into the fiber endface if it blocks the core while the laser is turned on. This burn might damage the optical surface enough that it cannot be cleaned.

When you clean fiber components, always complete the steps in the procedures carefully. The goal is to eliminate any dust or contamination and to provide a clean environment for the fiber-optic connection. Remember that inspection, cleaning and re-inspection are critical steps which must be done before you make any fiber-optic connection.

General Reminders and Warnings

Review these reminders and warnings before you inspect and clean your fiber-optic connections.

Reminders

• Always turn off any laser sources before you inspect fiber connectors, optical components, or bulkheads.

• Always make sure that the cable is disconnected at both ends or that the card or pluggable receiver is removed from the chassis.

• Always wear the appropriate safety glasses when required in your area. Be sure that any laser safety glasses meet federal and state regulations and are matched to the lasers used within your environment.

• Always inspect the connectors or adapters before you clean.

• Always inspect and clean the connectors before you make a connection.

• Always use the connector housing to plug or unplug a fiber.

• Always keep a protective cap on unplugged fiber connectors.

• Always store unused protective caps in a resealable container in order to prevent the possibility of the transfer of dust to the fiber. Locate the containers near the connectors for easy access.

• Always discard used tissues and swabs properly.

Warnings

• Never use alcohol or wet cleaning without a way to ensure that it does not leave residue on the endface. It can cause damage to the equipment.

• Never look into a fiber while the system lasers are on.

• Never clean bulkheads or receptacle devices without a way to inspect them.

• Never touch products without being properly grounded.

• Never use unfiltered handheld magnifiers or focusing optics to inspect fiber connectors.

• Never connect a fiber to a fiberscope while the system lasers are on.

• Never touch the end face of the fiber connectors.

• Never twist or pull forcefully on the fiber cable.

• Never reuse any tissue, swab or cleaning cassette reel.

• Never touch the clean area of a tissue, swab, or cleaning fabric.

• Never touch any portion of a tissue or swab where alcohol was applied.

• Never touch the dispensing tip of an alcohol bottle.

• Never use alcohol around an open flame or spark; alcohol is very flammable.

Best Practices

• Resealable containers should be used to store all cleaning tool, and store endcaps in a separate container. The inside of these containers must be kept very clean and the lid should be kept tightly closed to avoid contamination of the contents during fiber connection.

• Never allow cleaning alcohol to evaporate slowly off the ferrule as it can leave residual material on the cladding and fiber core. This is extremely difficult to clean off without another wet cleaning and usually more difficult to remove than the original contaminant. Liquid alcohol can also remain in small crevices or cavities where it might re-emerge.

Measuring attenuation in a fiber-optic cable is a vital ingredient to obtaining the maximum performance from a system designs. But, for designers, just starting to work in the fiber-optic design space, measuring attenuation can seem like a monumental task.

In this tutorial, we’ll take a look at the basics behind attenuation as well as at the impact Maxwell’s equations and a power detector have on attenuation measurements. We’ll then examine the two simple measurement techniques that designers can employ during to ease the attenuation measurement process.

Attenuation Basics
Light propagates in a glass tube because of reflection at the cylinder’s surface by the angle at which the optimal path is totally internal. But light moves in alternating electric and magnetic components at 90 degrees to each other that can be construed as a block waveguide over time.

Attenuation is tested more than predicted. It has been categorized according to some common experiences, and this should be a warning that you are quite capable of coming up with different predictions based on different experience levels.

In the manufacturing process, crystallization of the glass out of a melt results in minor “kinks” in the final product due to impurities. OH water molecules called “high water” are another deformity incorporated into the material during the manufacture that results in absorption. Differences also occur between the surface and the bulk material out of the melt.

Composition, temperature, and pressure are interrelated variables in the manufacturing process, and they all affect the transition from melt to solid. The transition itself is a stage process in which crystallization changes these variables of composition, temperature, and pressure throughout the transition. Inconsistency is the result and inconsistency will cause imperfections resulting in scattering and absorption.

To avoid interpretation differences, designers can use loss measurements to evaluate attenuation on a cable. The calculation of loss is established as a ratio between power launched and power output measured in watts. The decibel conversion is:

Because attenuation features are incorporated in every fiber, they also increase their effect with the length of that fiber. Macrobending, for example, happens in a gradual curve of the tube with the accumulation of kinks; a kind-of systems-effect in distance because the wave propagation takes place over time.

The attenuation number on most data sheets is the quantity of dB/km, where:

Other complicating variables occur from environmental conditions in humidity, temperature and vibration. Loose or tight-buffered construction distinguishes indoor from outdoor use, but these environmental factors are also important in a laboratory when trying to measure at the most sensitive levels.

Maxwell’s Equations
Wave propagation is understood by Maxwell’s equations describing the field effects. Field effects explain how the fiber core is penetrated, and inform the use of the cladding, but they are generally important for the entire discipline of fiber-optic technology. The wave equations have an equivalent relationship with each other, and translate by integral as well as second-order partial differential equations. These equations explain the nature of electromagnetic (EM) waves propagating in a lossy medium, and the propagation in waveguides as well as resonance.

Maxwell’s four equations describe wave propagation. The first equation  represents Gauss’s law that bounds the charge by the enclosed surface and relates to the total refractive index.

Maxwell’s second equation  represents Faraday’s law of electromagnetic induction where the electromotive force recommends fiber optic bit rate and bandwidth compared with purely electrical conduction.

The third equation  describes the relationship of electric and magnetic field coordinates that extends the wave propagation outside the core, and the fourth equation  relates to the universal constant of vacuum permeability μ₀.

In general, these equations let us know how the fields of an EM wave relate to one another, as well as propagate in a medium. Light travels as an ideal, unbounded dielectric where both vectors, electric and magnetic, are perpendicular to the direction of propagation (the z axis) according to the second and third laws.

Fiber limitations in conductivity, permitivity, and permeability of the medium relate to features of the first and fourth law with the result that an electromagnetic field propagating in a medium takes the form of damping waves. This has an attenuation constant also known as the propagation constant because of the dependent frequency effects in the conducting fiber. EM radiation in the high frequency of light is perfectly conducted, and yet glass is only a perfect diaelectric for a low-frequency alternating current. The result is attenuation.

In addition to attenuation, the solutions for Maxwell’s equations yield another feature important in fiber-optic cable technology: discrete, eigenvalue waveguides. Waveguides describe stable field patterns that are one of the advantages of digital transmission in stability with modal features compared to analog signals. These are important for the design and fabrication of multi-mode fibers, but it relates here as background to indicate why the unique EM wave properties are an advantage in fiber optic technology. With single-mode fibers, the phase properties that are displayed as modes are less important than the polarization features of light. Thus, a polarization mode dispersion (PMD) value heads most single mode fiber data sheets.

There are several tricks to compensate for attenuation, and we want to conclude here that attenuation is to-be-expected. Our question was how much attenuation is taking place in the cable, but the complicating system includes the laser source and the power meter.

Determining Bit Rate
Bit rate is one area that attenuation has a big impact in. The bit rate that can be transmitted, measured in bit/s, relates to the bandwidth, measured in Hertz, that is the frequency range within which a signal can be transmitted without significant deterioration. Bit rate and bandwidth (BW) are often related as BW = BR/2because one period of sine wave requires two bits of characterization information.

A fiber-optic provides between 100 and 1000 THz in light frequency compared to radio frequencies of 500 kHz to 100 MHz, coaxial cable up to 100 MHz, and copper wire, over a short distance, up to 1 MHz.

Multiplexing
Maximizing the bit rate transmission are modulation schemes that take advantage of the wave nature in combination with the refractive control. Control distinguishes one carrier from another, and the wave features allow several configurations each of which can be beneficially overlapped, one with the other.

Sonet features carrier switched data in frames transmitting in intervals of 51.84 Mbit/s. As many as 48 of these OC-n levels are possible for a total of 48 * 51.84 = 2.488 Gbits. Groups of OC-ns are called “superframes.”

Communication companies use multiplexing to increase the bandwidth by improving these devices at the ends without having to install new fiber. With the right type of fiber, however, you can, in principle, have a device that does both multiplexing and demultiplexing.

The nature of the signal informs the varying ways that multiplexing can be accomplished. Consider signals coded in close sequence with time division multiplexing (TDM), or carrying signals at different frequencies in wave division multiplexing (WDM). Add these together and you can achieve multiple waves at different times with dense wave division multiplexing (DWDM).

These complex multiplexing combinations use statistical methods to keep track of the overlapping patterns. The nature of the signal interacts between theory, device, and mathematics resulting in a rich communication channel.

The Detector
Power detection is a vital element for determining the exact attenuation on a fiber-optic cable. A typical, medium-grade detector is a power meter that registers 0.001-dB typical polarization dependent response with a single input port for both connectors and bare fiber measurements (Figure 1).

Fiber connector adapters are usually included with the power detector along with a changeover configuration from bare fiber measurements. These connectors can be threaded, or simply slipped into the adapter hopefully comparable with a bare fiber where the adapter locates the fiber ferrule at the same place in the integrating cavity as the bare fiber. The angle of the cleave, possibly magnified by the connection, reflections, and absorptions will introduce measurement noise just as they do between the source and the fiber.

The power meter measures light by turning it into electrical current where an efficiency rating called the responsivity is measured in units of amperes per watt (A/W). A photodiode typically converts optical power to current, but thermal detectors are also used because sometimes a measure independent of the wavelength is important. The source problems are duplicated in the detector with efficiency calibration of the photodiode because the responsivity is a function of wavelength.

Thermal detectors, on the other hand, directly convert optical to electrical power like a large sink into which all the splashing water can be measured as it is caught in the basin. It won’t tell you about the flow of power, however, and a photodiode is more sensitive in that regard. Photodiodes are capable of measuring power levels down to less than 1 pW (-90 dBm). The thermal plates, in contrast, produce a proportional measure where linearity is irrelevant because the only aim is to achieve equal temperature between two layers.

The highest performance solution is to use a tunable laser as the source and an optical spectrum analyzer (OSA) to cover a broad wavelength range. An OSA provides additional filtering to reject some of the broadband noise emission from tunable lasers, and the combination provides a large measurement range and fine wavelength resolution in most measurements.

In this article, lets consider a bootstrap method that can be done as an initial test of two attenuation measures for insertion loss apart from the electronics of bandwidth broadcast and power metering. One of the reasons for this simpler approach is that OSA’s can be expensive, but another is that automated detectors only solve part of the measurement problem.

The craft of working with fiber optic cable dominates the management of source, connectors, and meter with temperature stabilization, power range non-linearity, polarization, reflection, interference, angled fiber cleaves, and other, systematic certainties and uncertainties. Variations on spectrum analysis, for example, can be devised using interferometry techniques just as variations in measurement can be devised simply with configurations of circulators and isolators.

Simple tests are possible because fiber-optic expertise is a craft that automation has not overcome. As long as the source and the detector consistently operate in the same way over some assuring time period, we can perform two simple insertion loss experiments for measuring attenuation.

Simple Attenuation Measurements
As mentioned in the paragraph above, two simple insertion loss experiments can be used to measure for attenuation (Figure 2). If we assume consistency at the ends, we can use an attenuation effect to measure two quantities just within the cable. Light is attenuated in the ratio dB/km, and we can measure the difference between two configurations; one where the length or bending is minimal, compared (measured) against one where the length or bending is extensive.

The beauty of this measurement is that it does not depend on the lack of control inherent in any system. Reasonable stability is the only requirement, and we can reasonably measure that over some reassuring time period.

The most important step is simply confirming that you have a signal. Designers might be delayed at this point while you develop skill in the cleaving process.

With a signal in place, time measures will help designers become comfortable with the system’s stability. Wiggle the fiber, jump up and down on the floor, and open the windows to vary the environmental factors.

Designers can change features of the system like the length of the fiber as well as changing and recording environmental conditions. In our laboratory, our experimentalist bought several lengths of fiber in graduations of feet to test a more sophisticated measure of coherence length for reflection concerns. But the principle is the same that even sophisticated answers can be tested in simple configuration tests.

Calculating the principle in mathematics provides a numerical reference of this analysis. Will you get a graduated measure of attenuation due to length because of the dispersion features in the wave propagation and the manufacturing defects in the cable gradually accumulating over distance? Defining this one feature of attenuation with the attendant control problems will not be unlike defining the most sophisticated fiber optic attenuation questions. Testing the limits of attenuation in length is a simple approach that nevertheless involves many complicating features of working with fiber-optic cable. This playful part of experimentation further entangles designers with the system as they get more comfortable swapping modules in and out and watching the results.

Attenuation from bending can be done in the same way. Wrapping the fiber around cylinders of different diameters will logically result in degradations of the signal due to attenuation. Snell’s index of internal reflection will cease at some coiled limit, the attenuation might gradually increase as the coil tightens, or the cable will simply break.

Designers looking to use the methods described above should see if they can obtain the three following results: no effect, attenuation matched to diameter, and broken coil. Number one and three shouldn’t be too hard. But, designers must grapple with understanding at what point does attenuation turn into an interrupted signal?

Designers should also try to achieve result number two to reassure themselves that a null result isn’t poor experimentation, and to justify the money spent on broken coil.

Wrap Up
These two simple measures are a beginner’s way to get familiar with the measurement of signals in a fiber optic cable by independently making two measures of attenuation. Attenuation results from an interesting combination of theory, devices, and configuration where unravelling the signal features will depend on craftsmanship. The system can become increasingly complex with just a few components, and this beginning will lay a foundation for understanding, building, and trouble-shooting more extensive networks. In any system, we commonly find a source, a transmission line, and the receiver. Connecting and testing these in a fiber optic cable system is one approach to understanding the nature of measurement which is the nature of error.