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Focus on Maintenance & Testing: OTDR Testing: The Second Tier

The majority of designers and end users want optical fiber cabling to be tested in two tiers -- with an OLTS and an OTDR.


July 1, 2002  


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Why should I use an optical time domain reflectometer (OTDR) for testing my fiber? I test it with a power loss meter already.”

That is a question asked by many installers of fiber systems. In contrast to this, nearly all designers and end users need optical fiber cabling to be tested in two tiers — with an optical loss test set (OLTS) and an OTDR.

The OLTS (a light source and power meter) is used for testing because the source that is used will most closely emulate the system that the cable will be connected to. For example, testing a channel with an 1300 nm light-emitting diode (LED) source will closely emulate a 100BASE-FX network since it uses an LED launch for its transmission. Additionally, Vertical Cavity Surface Emitting Lasers (VCSEL) sources are also being used to test for Gigabit Ethernet.

An OTDR, on the other hand, has been widely known for troubleshooting and characterizing a channel. For example, an OTDR can be used to determine the length of the fiber channel, the quality of the connections, and whether the cable installation was done without making sharp bends.

The need for testing these links with these two instruments has also been raised by the Telecommunications Industry Association. The TIA TR-42.8 Subcommittee has initiated a project to write a Telecommunications Systems Bulletin (TSB) that will clarify testing methods and help designers, installers and end users understand these two tiers of testing fiber cabling.

THE IMPORTANCE OF TESTING

Testing provides the end user with the confidence that a cabling system is going to work for the applications that will be placed on it. Prior to the acceptance of Gigabit Ethernet, cabling loss budgets were of little concern. However, the high data rate Gigabit systems require that the cabling loss budgets be much lower and that the maximum distances be reduced.

The charts on page 25 and 26 are used to illustrate just how technologies are being affected by maximum distance and cabling loss budget requirements as data rates increase:

When viewing these graphical illustrations, it is clear that testing has to be accurate for new fiber installations. In addition, all events along the cabling can be critical to the installation of an application over the fiber cabling.

Field-testing of multimode fiber loss measurements is carried out at specific wavelengths with an OLTS since performance is wavelength-dependent. Typically, cabling systems designers specify the testing of installed fibers with both an OLTS (at specific wavelengths) and an OTDR within their performance requirements. Unlike the OLTS that directly measures the cabling loss by closely reproducing the transmitter and receiver in a transmission channel, the OTDR works indirectly in characterizing an optical fiber channel.

THE OTDR OPERATION

An OTDR is an opto-electrical test instrument that is used to characterize optical fibers with a graphical signature on a display screen. The OTDR has the capability to measure the length of the fiber and determine the attenuation (signal loss) between any two points in an optical fiber channel or link, including the fiber, connectors, splices and other events (such as a sharp bend incurred during cable installation). These areas or events are measured by manoeuvring cursors along the fiber trace path on the display screen between the points of interest.

An OTDR sends high-powered pulses of light into a fiber-cabling segment and measures the strength of the reflections back to the instrument. The pulse is attenuated outbound and the reflection is also attenuated on the return. These reflections occur from points along the fiber segment in which about one-millionth of the light pulse is reflected back to the OTDR. The OTDR takes thousands of these measurements and displays the data points on the screen as a “trace” line sloping down from left to right with events such as connections, splices and fiber bends shown along the fiber length. The horizontal scale of the display screen provides distance, while the vertical scale provides signal level measured in decibels (dB). (Please see Figure 1).

The OTDR uses the effects of “Rayleigh Scattering” and “Fresnel Reflection” in measuring the characteristics of an optical fiber channel or link. Rayleigh Scattering, also known as backscatter, is the major loss factor in the optical fiber itself, where some of the transmitted light is reflected back to the transmitter.

Fresnel Reflection occurs when different material densities are encountered, such as air gap occurrences at connectors or the end of a fiber. Although the OTDR does not measure the transmitted light level, it closely correlates the effect of Rayleigh Scattering and Fresnel Reflection to the transmitted pulse. For instance, if the level of transmitted light decreases between two points, due to a tight bend or splice, the backscatter will drop the same amount.

The operation of the OTDR does not require reference measurements or access to both ends of a fiber channel or link to make its measurements, thereby only requiring one technician to secure the measurements. However, the OTDR’s distance resolution is limited by the transmitted pulse width set on the test instrument. It must use a launch cable of sufficient length (usually 100 metres or greater) to overcome the saturation of the receiver by reflections from the first connector and crosstalk in the coupler inside the test-instrument. The display of the OTDR may be interpreted into a table, or the OTDR operator can interpret the trace.

THE OLTS OPERATION

The most basic optical fiber measurement is that of received optical power. An OLTS includes an optical power meter combined with a light source. In operating the OLTS, the power meter is calibrated to the light source (better known as referencing). Once the power meter is referenced and connected to one end of a fiber, and the other end of the fiber is connected to the light source, the power meter measures the drop in optical power from the level emitted by the source to the level received at the detector. Power meters can also be used to measure the optical power level from other sources such as a fiber switch.

To make optical fiber loss measurements, the source must be compatible with the type of fiber and the wavelength for performing the test. For multimode systems, most sources are LEDs or VCSELs that are commonly used as transmitters in actual systems, making them representative of real applications. Typical wavelengths found in the market today are 850 and 1300 nm for LED based systems, and 850 nm for VCSEL. Since fiber is sensitive to wavelength, it is critical to use the proper source when measuring optical fiber loss. For example, a 1300 nm LED source would be used to test a 100BASE-FX network, while an 850 nm VCSEL source would be used to test 1000BASE-SX (Gigabit Ethernet).

The OLTS requires referencing and access to both ends of the cable under test. Although this requires two technicians, it can also provide verification of polarity and verification that one fiber pair is not transposed with another. For example, without verifying both ends of a fiber link in a fiber to the desk project, a link labelled number 7 in a telecommunications room patch panel may show up as link 3 on the floor area. In addition, the OLTS will most closely emulate the system to be tested.

WORKING TOGETHER

An OLTS source, with either an LED or VCSEL, should be used to emulate the actual systems being deployed. Testing the cabling using ANSI/TIA/EIA-526-14A, Method B is imperative to ensure all connectors are included in the measurement.

An OTDR will identify the quality of the installation by showing tight bends that may have been placed during installation, connector quality (including dirt), and whether additional splices or connections were placed that were not specified in the original design. The OTDR provides a visual indication that the installation was done according to specification and in a quality fashion.

The OLTS and the OTDR are complementary in testing optical fiber premises networks and should be used
for certifying fiber cable installations. Certification of the cabling will produce reports directly from the test instrument, or the data can be downloaded to a computer. The OLTS will most closely emulate the system to be tested, while the OTDR works indirectly to characterize the installed cabling channel. These attributes make the OLTS and the OTDR ideal complementary test instruments for acceptance and commissioning criteria of installed cabling.

Bob Jensen, RCDD, is the Fiber Optics Programs Manager for Fluke Networks. He has more than 25 years of technical and managerial experience in the telecommunications industry, and chairs both the TR-42 Engineering Committee and the TR-42.2 Subcommittee for Residential Infrastructure (TIA/EIA-570-A).