Testing basics

As fibre installations increase, it is necessary for contractors to equip their technicians with better ways to test fibre sections. Harley Lang III, marketing manager at Fluke Networks writes on the benefits of both OLTS and OTDR testing and how they complement each other.

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By  Harley Lang III Published  November 14, 2007

Fibre is playing an increasing role in the majority of network installation contracts. According to a report entitled "Structured Cabling Systems" by FTM Consulting, fibre cabling revenues will exceed unshielded twisted pair (UTP) revenues for the first time in 2008.

The result is that more attention is being focused on the primary tools for certifying fibre optical cable, optical loss test sets (OLTS) and optical time domain reflectometers (OTDR). While the measurements taken by these instruments are similar, they typically perform different roles in the certification process. Rather than being competitive, OLTS and OTDR are actually complementary tools that both play a role in the majority of fibre installation projects. This article will explain how each method works, describe its advantages, and provide some suggestions for contractors on how to develop a testing strategy.

OLTS operation and benefits

The increasing role played by fibre means that it is becoming more important than ever to understand and take advantage of the primary methods used often to test and certify fibre cabling systems.

Rather than being competitive, OLTS and OTDR are actually complementary tools that both play a role in the majority of fibre installation projects.

OLTS has long been the primary method of testing premises fibre optic cabling. The test is designed to determine the total amount of light loss over the fibre link. Other terms used to refer to this technology are loss/length and power meter/light source (PMLS). The test is performed with a stable light source that produces a continuous wave at specific wavelengths. The light source is connected to one end of the fibre. A power meter with a photo detector is installed at the opposite end of the fibre link. The detector measures optical power at the same wavelengths as the light source. These two devices determine the total amount of light loss.

This loss/length certification is described in certification standards such as Telecommunications Industry Association's (TIA's) TSB140 bulletin entitled "Additional Guidelines for Field-Testing Length, Loss and Polarity of Optical Fibre Cabling Systems" as basic or Tier 1certification that is required for all fibre optic cabling links. The Tier 1 tests are attenuation (insertion loss), length and polarity.

A key innovation in recent years is the availability of fibre loss/length modules that can be attached to copper test sets to make them function as an OLTS. Some of these instruments can test two fibres at a time in order to verify polarity, certify the fibre lenght and reduce the time required for certification.

The copper tester mainframe with fibre loss/length module is used at one of the fibres and the remote at the other end. A reference power level is set using test reference cords before separating the two instruments and plugging each end of the fibre to be tested into them. Then with the press of a single button, both fibres are tested at two wavelengths to measure their length and loss and determine a pass or fail status in less than 12 seconds. The polarity can be quickly reversed to provide bi-directional results. The approach provides an efficient and accurate method to certify that the fibre link meets the loss budget for a specific application such as 10Gbit/s Ethernet.

OTDR operation and benefits

With tighter loss budgets and less room for error in high bandwidth fibre backbones, network owners and designers are now setting specifications not only for overall loss budgets but also for individual splices and connectors. Because light sources and power meters are not able to perform this type of test, many standards organisations such as TIA and the International Organization for Standardization (ISO) are recommending extended or Tier 2 certification.

Tier 2 certification involves the acquisition of a trace from an OTDR. An OTDR can pinpoint the location of faults on a fibre link and certify the workmanship involved in an installation. OTDRs find and characterise both reflective and non-reflective events in optical fibre runs. The result is that the OTDR is able to certify every fibre optic connector and splice and ensure that there are no unplanned loss events due to poor cable management or installation.

OTDRs use specialised pulsed laser diodes to transmit a series of very short high-power light pulses into a fibre. As the pulse of the OTDR travels down the fibre, most of the light travels in the direction of the fibre. High-gain light detectors measure the light that is reflected or backscattered as each pulse travels down the fibre. The OTDR uses these measurements to detect events in the fibre that reduce or reflect the power in the source pulse.

For example, a small fraction of the light is scattered in a different direction due to the normal structure of and small defects in the glass that makes up the fibre. The phenomenon of light being scattered by impurities is called Rayleigh backscattering. A certain amount of backscatter is expected for a specific fibre length based on its attenuation coefficient specification.

When a pulse of light meets connections, breaks, cracks, splices, sharp bends or the end of the fibre, it reflects due to the sudden change in the refractive index. These reflections are called Fresnel reflections. The amount of light reflected, not including the backscatter from the fibre itself, 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 reflectances that indicate poorer connections.

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