Fiber Optic Cable Testing - Article 2

In my previous article we have seen two different ways of testing optical fiber cables. We will discuss further more methods of optical fiber testing.

3. Optical Loss Test Set(OTLS) or Light source power meter(LSPM).

The fiber optic link attenuation is tested using Optical Loss Test Set(OTLS) or a Light source power meter(LSPM). This type of testing is the most accurate testing available and is the most accurate characterization of the fiber optic cables capability. The OLTS tests for the total amount of light loss on the fiber link(namely attenuation). Testing with OLTS/LSPM can be conducted at one or more wavelengths, but a minimum is recommended that testing be performed at the wavelengths that the network will operate(for example 850nm for a laser-optimized fiber network where VCSEL will be used for data transmission). Unidirectional testing is the minimum level but the system owner may require that a bidirectional test be performed (testing performance from both directions). With the use of OTLS/LSPM testing, you can perform the link length testing, polarity verification while performing attenuation testing. But this capability completely depends on the features of OTLS devices that you are using.

As the name indicates ‘light source power meter(LSPM)’ we use a light source on one end of the link and a power meter on the other. The light source is acting as the transmitter and power meter as the receiver. We can calibrate these two together and they determine the total amount of light lost on a link. The measured loss is compared to a specified loss budget for the link to determine if it passes this “Basic” or “Tier 1” certification. Tier 1 certification is described in standards such as the Telecommunications Industry Association’s (TIA’s) TSB140 bulletin entitled “Additional Guidelines for Field-Testing Length, Loss and Polarity of Optical Fiber Cabling Systems.” Tier 1 certification is required for virtually all fiber optic links today.

Listed below are TIA/EIA- 568 insertion loss limits for the various components. Specific installations or protocols call for stricter limits.

Loss budget (TIA/EIA specification limits)

Element	Insertion Loss
Splice	< 0.3 dB at all wavelengths
Connector Pair	< 0.75 dB at all wavelengths

Test results should be compared with the link attenuation allowance calculated as follows:

Link Attenuation Allowance (dB) = Cable Attenuation Allowance (dB) + Connector Insertion Loss Allowance (dB) + Splice Insertion Loss Allowance (dB)

A recent development in OLTS solutions is the availability of OLTS modules for copper cable analyzers. These modules can test two fibers at a time to verify polarity as well as a test for loss in each direction. 
Looking at the video from FS.com is as simple as possible for you to understand the procedure. How to Use Optical Light Source and Power Meter.

Tier 2 Fiber Testing

While Tier 1 fiber optic tests can identify problems in terms of pass or fail, they cannot determine the root cause or location of the problem. Tier 2 fiber optic testing is used to pinpoint root-cause locations and the amount of loss and optical return loss (ORL) from each problem contributor and is performed selectively in addition to Tier 1 testing under specific conditions and situations. Tier 2 fiber testing provides a deeper level of link visibility, unlike any other fiber infrastructure tests. The optical time-domain reflectometer (OTDR) is used to perform Tier 2 fiber optic testing.

1. Optical time-domain reflectometer (OTDR)

With the rapid advancements in fiber optic technology, OTDR testing has become an indispensable method to build, certify, maintain and troubleshoot fiber optic systems. OTDR is an instrument used to create a virtual “picture” of a fiber optic cable route. The analyzed data can provide insight into the integrity of the fibers, as well as any passive optical component such as the connections, splices, splitters, and multiplexers along the cable path.

Tier 2 certification involves acquiring a trace from an OTDR. An OTDR trace finds and characterizes reflective and non-reflective events in a fiber run. This pinpoints the location of any fault and certifies the workmanship of the installation. Tier 2 certification ensures that there are no unplanned loss events due to poor cable management or errors in installation. Once this information has been captured, analyzed and stored, it can be recalled as needed to evaluate the degradation of the same cable over time.

The OTDR is also the only tool capable of troubleshooting any fiber optic cable failures by locating the distance to the fault and identifying the type of fault-like breaks, bends and any excessive loss. An OTDR instrument can be portable or it can be rack-mounted and placed for permanent monitoring in the network such that an alarm can be triggered if the fiber is compromised.

OTDRs use special pulsed laser diodes to transmit high-power light pulses into a fiber. As the pulses travel down the fiber, most of the light travels in that direction. High-gain light detectors measure any light that is reflected from each pulse. The OTDR uses these measurements to detect events in the fiber that reduce or reflect the power in the source pulse.

For example, a small fraction of the pulse light is scattered in a different direction due to the normal structure of fiber and small defects in the glass. This phenomenon of light scattered by impurities in the fiber is called Rayleigh backscattering. A certain amount of backscatter is expected based on a fiber’s attenuation coefficient specification.

When a pulse of light meets connections, breaks, cracks, splices, sharp bends or the end of the fiber, it reflects due to the change in the refractive index. These reflections are called Fresnel (pronounced frA-NEL) reflections. The amount of light reflected, not including the backscatter, relative to the source pulse is called reflectance. It is expressed in units of dB and is usually expressed as a negative value for passive optics, with values closer to 0 representing larger reflectance, poorer connections, and greater losses.

OTDRs display trace results by plotting reflected and backscattered light versus distance along the fiber as shown in figure 4. The Y-axis represents power level and the X-axis shows distance. When you read the plot from left to right, the backscatter values decrease because the loss increases as the distance increases.

OTDR traces have several common characteristics. Most traces begin with an initial input pulse that is a result of a Fresnel reflection occurring at the connection to the OTDR. Following this pulse, the OTDR trace is a curve sloping downward and interrupted by gradual shifts. The gradual decline results from backscattering as light travels along the fiber. This decline may be interrupted by sharp shifts that represent a deviation of the trace in the upward or downward direction. Loss events appear as a step down on the plot. These shifts or point defects are usually caused by connectors, splices or breaks. The end of the fiber can be identified by a large spike after which the trace drops dramatically down the Y-axis. Finally, the output pulse at the end of the OTDR trace results from reflection occurring at the output of the fiber end face.

An OTDR trace is valuable because it makes it possible to certify that the workmanship and quality of the installation meet the design and warranty specifications, for current and future applications. For example, common requirements are that the loss associated with a splice should be no larger than 0.3 dB and that associated with a connector should be no more than 0.75 dB. These event losses are completely invisible to an OLTS. With an OTDR, the performance of each splice and connector can be measured. If they do not meet specifications, they can be corrected during the installation process, not afterward when the network is live. Many contractors perform Tier 2 certification as preventative maintenance and to document their workmanship on a completed installation.

Another recent development in fiber optic testing is the availability of OTDR modules for copper cable analyzers. OTDR modules greatly simplify the task of performing Tier 2 testing of fiber links. Anyone familiar with copper certification can now easily perform Tier 2 fiber certification because they see a familiar user interface, commands, and diagnostics. This shortens the learning curve and extends the value of the existing copper tester.

Summary

One may ask, if an OTDR is used is an OLTS still necessary? The answer is yes because an OLTS directly measures total fiber losses and length while these values can only be inferred from an OTDR. Products that perform Tier 1 and Tier 2 tests make it easier to offer total fiber testing and certification. Fiber optic inspection, cleaning and testing are essential for accurate data transmission. 

Certain precautions should always be taken when performing any type of optic testing. The use of appropriate mating adaptors for launch cords is necessary to ensure reliable test results. All launch cords(which connects the OTDR to the link-under-test, discloses the insertion loss and reflectance of the near-end connection) and adapters need to be clean and free of defects prior to and during testing. It is highly recommended that reference-grade launch cords to be used for end-to-end attenuation testing. Launch cables should be used for OTDR testing to enable a stable launch and to ensure that the first event in the fiber link is visible in the trace. Using high-quality fiber testers and tools would be helpful to achieve the purpose. 

Knowledge Credits: www.flukenetworks.com

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