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Fiber testing facts

With 10G and 40G SONET and Ethernet rates being installed, other parameters of the fiber must be measured and managed in order to guarantee 5 9's performance that the world now expects.


September 1, 2005  


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Testing fiber during network upgrades is nothing new. All you need are traditional tools such as an Optical Time Domain Reflectometer (OTDR) and Loss Test set during installation and network provisioning.

But with 10G and 40G SONET and Ethernet rates being installed, other parameters of the fiber must be measured and managed in order to guarantee 5 9’s performance that the world now expects

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In the beginning there was light… then fiber, and it was all good: Compared to other technologies we use everyday, fiber optics is starting to sound mainstream.

The first tools for testing the integrity of these links connected an optical source at one end and a calibrated power meter set to the same target wavelength at the other.

Loss was measured and monitored to ensure enough transmitted signal got down the fiber to the receiver.

In the late 1980s the OTDR was introduced and remains a primary tool for measuring fiber length, determining fiber attenuation (dB/km), locating breaks, verifying splice loss and checking for bending (higher loss at higher wavelengths).

In the 1990s, transmission rates were well below Gigabit rates.

Loss and integrity were the main parametrics of link quality. Thousands of OTDR’s and tens of thousands of power meter and sources were manufactured for installation and maintenance of early links.

The need for speed: The introduction of the Internet and ever-cheaper computers caused an explosion in the amount of data network providers needed to carry. Five years ago, the scales tipped globally in the volume of data vs. telephony traffic transported.

Data continues to have a much higher growth rate. To transport the aggregate volume, higher speeds were needed across the network. The core network (Metro and long haul) saw higher rates first.

Network providers each have different business cases for installing higher rates, but when network transmission equipment prices fall and they can provide four times the existing rate with only a two times increase in cost, it makes sense to provision the higher rate.

Other times, lack of available fibers on a certain route necessitates the need to put more traffic on an existing fiber.

All of the above cause deployment of 10 and 40 Gigabit per second transmission systems. If SONET is being used, these are labeled OC-192 and OC-768 rates respectively.

Blink and you will miss it. About 200 million times: In today’s Megapixel and Gigabyte world, it is easy to forget how fast data is switching at 10G and 40G. If you are impressed with nano-technology, then hold onto your stopwatch, because transport networks are way beyond that.

Let’s look at little closer at the speed of transmissions. Bit width is length of time needed to register a single digital one or zero. For a SONET system running at OC-48 rates (2.5Gigabits per second) the bit width is about 0.4 nanoseconds (ns). That is fast.

Now increase the transmission speed a factor of four to 10G (OC-192). That bit width is reduced to only 100 pico-seconds (ps). A picosecond is 1/1000 of a nanosecond or one trillionth (0.000000000001) of a second.

With bit widths in the low picosecond range, measuring impairments that effect transmission need to have resolutions in the sub-pico second range. 0.01 of a picosecond is 10 femtoseconds.

Return loss and Reflectance: As speeds increase, a system’s susceptibility to returned energy and echo signals caused by high reflectances throughout the link increase as well. Return Loss (ORL) is a measure in dB of total amount of energy reflected back by a link. Reflectance is a measure also in dB energy reflected at individual events such as connectors or mechanical splices.

As we move to 10 and 40G systems, both values should be better than 30dB — a good goal is 40dB across the network.

Strong reflections can cause changes in modulation as well as increased noise at the receiver end increasing bit error rates (BER).

An OTDR that can measure high and low reflectance is the best tool to certify your link for reflectance and locate problems.

Fiber’s speed limit: Error-free digital transmission depends on receivers identifying a one or zero correctly. Dispersion is a small effect in fiber that becomes a larger percentage of the bit width as speeds rise. It affects the receiver’s ability to clearly distinguish bits.

If we imagine a data stream as a series of pulses, then dispersion results in the pulses spreading out making the receiver’s decision on what is a one or a zero more difficult and resulting in a higher bit error rate.

Effects such as chromatic dispersion and polarization mode dispersion are a result of the refractive index in the fiber not being constant in one way or the other.

Chromatic Dispersion (CD) is the variation in the speed of propagation of a light wave signal with wavelength. It leads to spreading of the light pulses and eventually to inter-symbol-interference (ISI) with increased bit error rate (BER).

Simply put, any signal consists of different wavelengths, and CD causes some wavelengths to travel slower then others resulting in part of the pulse arriving at different times and it spreading out.

CD is a result of the combination of material dispersion and wave-guide dispersion. The total delay caused by CD is measured in picoseconds for a fictional signal that has wavelengths spread over a nanometer.

Polarization Mode Dispersion is caused by the small difference in refractive index for a particular pair of orthogonal polarization states, a property called birefringence.

Fiber imperfections: This means that the speed of light depends on the path it takes along the fiber.

In contrast to an ideal fiber, a real fiber and especially fibers manufactured before 1994 can exhibit several kinds of imperfections such as impurities and fiber asymmetry.

These imperfections are partly inherent to the manufacturing process of the fiber, cabling, and partly caused by the quality of fiber deployment.

When light is coupled into a fiber, it takes different paths traveling through the fiber. Differential group delay is the difference in time that the components of the light pulse needs to travel along the fiber path it takes. The Average of Differential Group Delay (DGD) over wavelength is Polarization Module Dispersion. (PMD).

So I’ve introduced you to CD and PMD. But why do we need to measure this? Because the delay caused by dispersion becomes a significant part of the bid width.

Complex effects, easy measurements: The good news is that these new measurements are not difficult to make.

A useful CD measurement can be as easy as connecting to one end of the fiber and running an automatic measurement.

Total CD at the wavelength you will be using as well as the fiber type can be displayed. If the limits given by the equipment manufacturer are exceeded because of fiber length, then information is provided to specify CD compensation modules.

A DGD/PMD measurement has also become a one-button, single person measurement. A technician can hook up a source to one or all the desired fibers at nearest end of the link, then travel to the far end, hook up fiber sequentially to the DGD/PMD analyzer measure.

DGD across wavelength and its average (PMD) is displayed automatically. Even pass/ fail limits can be entered certifying the fiber with a check mark or X indicating pass/fail.

Certify and match your fiber to a service: With known limits, you and your customers will want to know if their link will support a service. Pass Fail limits on measurements can help anyone do this. By testing fiber for loss, reflectance, Return loss, and dispersion, you can certify individual fibers to support certain speeds of service.

Consider certifying all your fibers and allocating each existing service to the worst fiber that can still support it.

For example if you have a OC-12 link, use the fiber with the worst PMD value.

It will not effect service and it frees up fibers with lower values of PMD for higher speeds or future rates.

Fiber Certification means testing dispersion, reflectance and loss to limits. Decide the accuracy you want on measurements and measure all the fibers you can and save the best fibers for tomorrow. You will be glad you did.

Peter Schweiger is the business development manager for Agilent Technologies Inc.’s optical network test, fiber optics and systems division. He can be reached at peter_schweiger@agilent.com.