Fiber Optic Blog | M2 Optics

Free Webinar: "The Need for Speed - Migrating Your Test Lab to 100G"

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Free Webinar by ONPATH Technologies, QualiSystems, and M2 Optics
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Thursday, February 9th at 1pm EST/10am PST
 

Webinar Description:

Whether you're an early adopter and have 100GbE ports installed, or are just learning about the need to support 100G, ONPATH, QualiSystems, and M2 Optics will discuss the common challenges businesses are encountering and offer practical advice with lasting solutions.

Learn How To:

• Save Capital:  Maximize your resources as you migrate to 100G

• Save Time:  Avoid compatibility issues due to the various methods used to achieve 100G speeds

• Save Resources:  Design, store, and recall multiple 100G network topologies in order to reduce time-to-market

• Save Space:  Enable real-world, physical layer test environments for 100G
   

Network Communication Speed Comparison Chart

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As the demand for faster speeds has grown rapidly over recent decades, new communications networking equipment and systems are constantly being developed to accommodate these requirements .   A result of these efforts is a series of established networking protocols and terminology that are used to identify various speed thresholds and the related equipment.

The chart below is intended as a quick reference guide to help readers associate the various networking speeds with their respective protocol/service names.

 
Network Communication Speed Comparison Chart

Chart courtesy of ONPATH Technologies 
Whitepaper “Optical Switching: Implementing a Future-Proof Infrastructure” 

Calculating Optical Fiber Latency

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Latency is a term that is used to describe a time delay in a transmission medium such as a vacuum, air, or a fiber optic waveguide.  In free space, light travels at 299,792,458 meters per second.  This equates to 299.792 meters per microsecond (µs) or 3.34µs per kilometer.  In fiber optics, the latency of the fiber is the time it takes for light to travel a specified distance through the glass core of the fiber.  Light moving through the fiber optic core will travel slower than light through a vacuum because of the differences of the refractive index of light in free space and in the glass.

In a fiber optic waveguide, it is necessary to know the refractive index of the glass in the core before the speed of light (and thus latency) in the core can be calculated.  Since the refractive index of light in the fiber core is dependent on the slight variations of the refractive index of the glass at different wavelengths, it is necessary to know the wavelength being used in the system as well as the refractive index at that wavelength.

To demonstrate this, we used two types of single mode optical fibers (G.652 and G.655) from two leading manufacturers that are commonly deployed in networks around the world.   The wavelength dependent refractive index values shown in the table below are based on their published specifications:

* Distance = Speed of Light (299.792m/µs) / Refractive Index
 

For example, using a 100km length of Brand B (G.652) fiber with a 1310nm laser would result in a latency of 489.34µs (100,000/204.357 = 489.34).  At 1550nm, the same fiber length would have a latency of 489.67µs (100,000/204.220 = 489.67).  A small difference of only 0.33 microseconds occurs as the result of using the two different wavelengths in the same fiber.

Note that only in the longest, most sensitive applications are the minor variations in refractive index important.  A refractive index of 1.47 can typically be used to calculate the latency or time delay of a fiber length to a generally acceptable accuracy.

Using all this information, a rule of thumb for quickly calculating latency in single mode fiber is using 4.9 microseconds per kilometer with 1.47 as the refractive index.  With a little less accuracy, you can use 5 microseconds per kilometer as a rounded guide for ease of calculation in determining overall latency of a known length fiber link.

OTDR Launch Fibers - Importance and Usage

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An Optical Time Domain Reflectometer (OTDR) is an optical measurement instrument designed to detect faults, splices and bends in optical fiber cables.  It functions by launching pulses of light into the optical fiber and measuring the back reflections created by the faults, splices, and bends.  It can identify the exact location of the fault by measuring the round trip time from the launch to the detection of the reflected returning pulse.  The time is determined by the speed of light in the glass core of the optical fiber.

With so many factors affecting the launch and detection process, problems such as unreliable traces of measurements are likely to be seen especially when only a little amount of light comes back to the OTDR for analysis.  This occurs when trying to look at a very long length optical fiber.  If you are trying to look at a very long optical fiber, it is necessary to launch a lot of power to see the end.  When a lot of optical power is launched, the pulse width of the launched optical signal is increased.  The large pulse width decreases the resolution of the measurement and can be as much as several hundred meters.  Faults near to the launch end are then masked because of the hundreds of meters between the launch pulse and the receiver being able to see the reflected pulse.  If there is a fault near the launch point, it can also create large reflections that saturate and overload the receiver.  This length of fiber is sometimes called the Dead Zone because the faults are masked in the length close to the OTDR.  The receiver requires an amount of time to recover from saturation.  Depending on the OTDR design, wavelength, and magnitude, the OTDR may take up to 500 meters or more to fully recover.

Most OTDR manuals suggest the use of launch fibers to resolve these issues.  Launch fibers place the necessary length of fiber between the OTDR and the actual fiber being measured providing time for the receiver to settle and also for the pulse width dependent resolution to be overcome.  When launch fibers are used, faults close to the end of the fiber being measured can be seen by the OTDR.  They do not interfere with the actual fiber being measured and thus are very secure and proven as a technique for identifying faults in the total length of fiber from the first interface to the last.

An OTDR launch fiber, often available on a small spool or within a “launch box”, is used to create the proper conditions for testing another similar optical fiber for faults.  This method avoids undesirable variations in loss and distance measurements.  A launch fiber will help to overcome the blind spot or Dead Zone of an OTDR brought about by high launch power or faults near the launch end of the fiber.  In summary, an OTDR launch fiber provides both the time and distance required for the OTDR to effectively look at and measure the characteristics of the entire length of fiber being tested, especially the length closest to the OTDR.
 

Fiber Optic Splitters - Common Types

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Fiber optic splitters enable a signal on an optical fiber to be distributed among two or more fibers.  Since splitters contain no electronics nor require power, they are an integral component and widely used in most fiber optic networks.  As a basic example, the diagram below shows how light in a single input fiber can split between four individual fibers (1x4):

Splitters can be built using a variety of single mode and multimode optical fibers and with most connector types for various applications.

From a technology standpoint, there are two commonly used types of optical splitters: 

• Fused Biconic Tapered (FBT)
• Planar Lightwave Circuit (PLC)

As with most technology, each type has both advantages and disadvantages when deploying them in a passive optical network.

Fused Biconic Tapered Splitters

FBT is the traditional technology in which two fibers are placed closely together and fused together by applying heat while the assembly is being elongated and tapered.  A signal source is used to determine the point at which the desired coupling ratio has been met, which then stops the process.

As this technology has been developed over time, the quality of FBT splitters is very good and they can be deployed in a cost-effective manner.  FBT splitters are widely accepted and used in passive networks, especially for instances where the split configuration is smaller (1x2, 1x4, etc). 

A drawback of this technology occurs when larger split configurations (1x16, 1x32, 1x64, etc) are required.  FBT technology is limited in the number of quality splits that can be achieved in a single instance, so several must be spliced together when a larger split configuration is required.   As a result, the physical size increases due to multiple splitters, along with the excess loss from the splices.  Thus, for these instances, PLC splitter are more ideal as we will discuss in the next section.

Example:  1x2 FBT Splitter in LGX Module

Planar Lightwave Circuit Splitters

A more recent technology, PLC splitters offer a better solution for applications where larger split configurations are required.  To achieve this, waveguides are fabricated using lithography onto a silica glass substrate, which allows for routing specific percentages of light.  As a result, PLC splitters offer very accurate and even splits with minimal loss in an efficient package.

With the rapid growth of FTTx worldwide, the requirement for larger split configurations (1x32, 1x64, etc) in these networks has also grown in order to serve mass subscribers.  Due to the performance benefits and overall low cost to deploy, PLC splitters are now the ideal solutions for these types of applications.

Example:  1x32 PLC Splitter Module

Chromatic Dispersion in Optical Fibers

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Chromatic dispersion is a phenomenon that is an important factor in fiber optic communications.  It is the result of the different colors, or wavelengths, in a light beam arriving at their destination at slightly different times.  The result is a spreading, or dispersion, of the on-off light pulses that convey digital information.  Special care must be taken to compensate for this dispersion so that the optical fiber delivers its maximum capacity.

Chromatic dispersion is commonplace, as it is actually what causes rainbows - sunlight is dispersed by droplets of water in the air.  Sir Isaac Newton observed this phenomenon when he passed sunlight through a prism and saw it diverge into a spectrum of different colors.  This dispersion occurs because different colors, or light frequencies, act slightly differently as they pass through a medium such as glass.  In fiber-based systems, an optical fiber, comprised of a core and cladding with differing refractive index materials, inevitably causes some wavelengths of light to travel slower or faster than others.

Chromatic dispersion is a serious consideration in long-haul optical fibers.  Its effect is essentially to stretch or flatten the initially sharply-defined binary pulses of information.  This degradation makes the signals (1s and 0s) more difficult to distinguish from each other at the far end of the fiber.  The result is that at any given length, the effective information capacity, or bandwidth, of the fiber optic cable can be significantly reduced.  Dispersion is added as the modulated beam of light, consisting of a number of closely spaced wavelengths, travels down this nearly transparent waveguide.

The bottom line is that chromatic dispersion becomes a major consideration and must be accounted for when developing or deploying fiber optic equipment for use in telecommunications, cable TV, or other high-speed optical networks.

Fortunately, techniques have been developed that help compensate for the negative effects of chromatic dispersion.  One method involves pre-compensating the signal for the anticipated dispersion before it's sent down the optical fiber.  Another method calls for using dispersion compensating fiber at the end of a length to correct or reverse the dispersion that was realized as the signal traversed the optical fiber.  As a result, these techniques are widely used to help solve the problem of chromatic dispersion.

Latency in Fiber Optic Networks

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The term latency refers to the time delay in a particular system. For communications systems, latency is an important factor, because transmission delays can affect the quality and reliability of the system. In the case of fiber optic networks, latency is the time delay that affects light as it travels through the fiber optic network.

The speed of light in a vacuum is the ideal maximum speed for a fiber optic system, but given the delays caused by refraction within the fiber optic cable, transmission speed is considerably less in fiber optic systems. Also, the latency increases over the distance traveled, so this must also be factored in to compute the latency for any fiber optic route.

The quality of fiber optic cable is an important factor in reducing latency in a network. If the cable is of low quality, then the light tends to be delayed as it travels through it. So it is important to start with the highest quality cable, and keep the network well maintained. The best way to maintain the quality of the network is to make cable runs as straight and normal as possible.

Another way to minimize latency in a fiber optic network is through careful design and construction. Each time a fiber optic cable is turned or routed it decreases the speed of the signal. Although it is impossible to make all networks straight and flat, it is important to maximize the potential speed of the light flowing through the fiber optic cable at all times.

There are other methods of reducing latency in networks that have become industry standard. Amplifiers or regenerators can also be employed to boost signal speeds at points along the network to counter the effects of latency. Amplification can also add its own latency, however, so its use must be carefully considered and added using precise engineering standards.

Minimizing latency is critical for high speed networks that serve mission critical functions like telecommunications or financial services. If latency in a system is allowed to reach high levels, then the time delays in the data being processed will fail to make use of the available cycles in the data processing systems handling the data. This can cause decreased reliability or quality of service, or ultimately system failure.

All fiber optic networks must address latency in their design and construction. Using the highest quality components, and applying engineering principles for proper design, latency can be minimized and effectively handled in today’s fiber optic networks.

Fiber Network Simulation – 4 Tips for Improving Test Results

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Engineers performing fiber network simulation testing with the goal of certifying that their equipment will work as intended once deployed in the field, often require the use of optical fiber spools to complete these procedures.  Since it is crucial these tests produce both correct and reliable results, below are some tips for ensuring the most positive results.

Tip #1 – Invest in a quality testing platform

Using a quality testing platform with integrated fiber spools provides a number of benefits for engineers.  First, the valuable fiber and connectors are protected from common, accidental damage which can lead to poor results and shorten the usage life of the fiber.  Second, having a sturdy enclosure means that results will be consistent for every test, which is also a key factor in these procedures.  Lastly, test platforms make life easier for the user, as packaged fiber allows for worry-free handling while enhancing the look and organization of your test lab or demonstrations.  While it may cost less in the short term to just use bare fiber spools, a small investment in addition to the fiber itself for a test platform/enclosure will be well worth it in the long run for achieving ideal results.

Tip #2 – Use only new or existing high quality fiber

With engineering departments often under tight budget constraints, engineers are sometimes forced to seek alternative routes when acquiring test fiber, such as purchasing used fiber from a 3^rd party source (auction website, used equipment providers, etc).  While used fiber is often easily found and offered at well below market price for new fiber, it should be avoided as there is never any guarantee as to what is being received.  Aside from often not being the exact length required, used fiber can have issues such as bends/crimps, damaged connectors, splices that should not be there, and more that a user is not aware of until working with it.  Purchasing or using new and high quality fiber from a reputable, proven source guarantees receipt of exactly what is needed for testing procedures.

Tip #3 – Match the test fiber type with the type deployed in the field

In recent years, industry organizations (ITU, etc) have helped to establish standards that ensure optical fibers meet certain specifications in order to allow various types of equipment and networks to operate seamlessly around the world.  While G.652 single mode fiber, for example, is very similar across different manufacturers due to these standards, engineers should still match their test fiber with the type of fiber that is deployed in the field whenever possible.  Although the fiber is principally the same, the various manufacturers do produce it using different, proprietary methods and equipment.  The result is that fibers may often have slightly different performance specs in certain areas.  This being the case, if engineers want to ensure their equipment works exactly as intended when deployed in the network, they will want to test using the same fiber, so as to have an “apples-to-apples” comparison in simulating the network exactly.

Tip #4 - Clean connectors frequently

It is very important to keep connectors and adapters clean, as dirt on a connector can have a negative impact on equipment performance.  As most are already aware, one of the first steps in troubleshooting equipment and systems that are not performing optimally is to check to make sure all connections are secure and clean – the same thinking applies when testing equipment in the lab.  Remember to clean connectors prior to testing, especially during times of increased handling or after longer periods of minimal use.

Benchmarking CATV System Performance with OMI

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Benchmarking is a term that is used widely to evaluate one's performance against a standard, which is typically based on the best organizations or processes in your industry.  In most cases, the goals of benchmarking are often centered around key factors such as quality, time, and cost.  In the performance of fiber optic communications systems, benchmarking generally applies to the performance quality of the information provided, but on a second level has an extremely significant impact on both time and cost.

In CATV systems, the quality of the delivered information is readily apparent and highly dependent on the quality of the systems delivering that information.  One of the most important performance components in a CATV system is the laser transmitter, which is used to translate electrical signal information into equivalent optical signals for transmission through fiber cables.  There are many factors, such as aging, noise sources, and distortions that can affect the quality of the signal and thus impact the viewing experience.  Questions that should routinely be addressed include:

• Is the laser dying or wearing down?
• Are the laser transmitter electronics deteriorating?
• Are the RF drive levels too high or too low?
• Is there a bad splice or RF connector?

All of these potential issues can have an adverse affect on how the viewing public sees the operator that delivers their information.  Therefore, it is in the best interest of the CATV system operators to address these potential issues and develop standards in order to ensure their systems are performing at an optimal level at all times.

The one parameter that can be used to accomplish this is the Optical Modulation Index (OMI) of the laser transmitter.  Proper setting of the OMI not only ensures that the laser is operating at maximum power and with minimal distortions, but in turn verifies that all components are secure and working as expected.  Once the optimal OMI is set and known for the laser transmitters, an operator should then routinely check the OMI on a periodic basis against their standard in order to always deliver superior quality to their customers.  The result is an increase in quality of service to customers, while saving time and money as potential failures can be spotted in advance.

For many CATV operators, checking and setting proper OMI in order to establish a standard has been difficult, as it is very time consuming and costly using traditional equipment.  However, new instruments like the FOS 1000A OMI meter have made this into a relatively simple task, as it removes all of the time consuming detail by internally incorporating all of the required, calibrated functions and automating the outcome with a precise OMI value (per channel and total OMI %).  As a result the setting, optimizing, and benchmarking of OMI for superior performance becomes an easy, reliable, and repeatable task.

The CATV operators in today's market that receive the highest recognition from customers and peers for their system performance are benchmarking with OMI.  In these cases, OMI is used as a standard at several points:

• Laser transmitter setup before and during initial deployment
• Periodic maintenance to ensure ongoing performance levels
• Troubleshooting to quickly identify and resolve network issues when they do occur

In order to achieve the best results, retain customers, and run more efficient operations, benchmarking system performance with OMI is a necessary but easy way to reach the highest standard.

Learn More About OMI - Free Whitepaper

Single Mode Optical Fibers by ITU Standards

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The International Telecommunication Union (ITU) is a United Nations agency involved with the development of worldwide standards for communications technology.  With the explosive growth and use of fiber optic technology around the world, a number of single mode optical fibers have been designed over the years for various applications.  Since integration of a variety of optical fibers and systems is required to achieve seamless worldwide communication, ITU developed standards for fibers that help to ensure this can happen.

     G.652 - Standard single mode optical fiber (SMF)

     G.653 - Dispersion-shifted optical fiber

     G.654 - Cut-off shifted optical fiber

     G.655 - Non-zero dispersion-shifted optical fiber (NZDSF)

     G.656 - Non-Zero dispersion for Wideband Optical Transport fiber

     G.657 - Bending loss insensitive optical fiber for access networks

Although most users of optical fiber will often refer to single mode fiber types by their popular brand names from leading manufacturers, understanding the industry specifications above will help to avoid confusion in instances where the G-specification is used instead.

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