EYE ON FIBER

How to Comply With TIA/EIA-568 B.1 and B.3 When

-Testing Insertion Loss

Introduction

During the last four years, a number of standards have changed. TIA/EIA-568 B.1 and B.3. have replaced TIA/EIA- 568A. B.1 and B.3 reference the latest versions of additional testing standards. In this column, I will highlight the manner in which the combined effects of these multiple changes impact the insertion loss test procedure you need to follow.

Multimode Testing

Clause 7.1 of the TIA/EIA-568 B.3 requires that multimode insertion loss testing be performed in accordance with Method B of OFSTP-14A, otherwise known as TIA/EIA-526-14A. Method B is also known as the )single reference lead method) (Figures 1 and 2). This same clause requires that the light launch conditions meet the requirements of TIA/EIA-455-50 B (FOTP-50 B). Many, but not all, light sources are LEDs which overfill the fiber core and therefore, do not meet the requirements of FOTP-50 B.

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Figure 1: Method B Reference

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Figure 2: Method B Test

KEY POINT 1: You need to know whether your light source is a Category 1 source, as designated in Annex A of TIA/EIA-526-14A.

Wait. Do not go out to buy a new light source. Your light source may create the required condition internally. If not, you can achieve the required launch condition with a mode filter to strip out higher order modes. These modes can result in field measured power losses which are higher than those stated by the fiber and cable manufacturers.

A mode filter, also called an equilibrium mode simulator (EMS) in TIA/EIA-171, is a round tube of a specific diameter. The diameter is determined by the fiber being tested (Table 1). To use the mode filter to create the proper launch condition, you will wrap the light source reference lead (Figure 3) five times around the mandrel of the proper diameter. This requirement appears in Clause 11.3.3 of TIA/EIA-568 B.1.

Fiber Type

Cable Type

900 µm

2 mm cable

2.4 mm cable

3.0 mm cable

50 µm

24.1 mm

23 mm

22.6 mm

22 mm

62.5 µm

19.1 mm

18 mm

17.6 mm

17 mm

Table 1: Mandrel Diameter Vs. Cable Diameter and Fiber Type

Figure 3: Reference Cable Wrapped Around a Mode Filtering Mandrel (Courtesy; Noyes Fiber Optics)

KEY POINT 2: You will need to wrap the reference lead around a mandrel prior to setting the launch power.

This change results in both good news and bad news. The good news is that this change in launch conditions results in standardized testing conditions regardless of the type of light source in the testing equipment. If your test equipment is out for calibration, you can use different equipment and test under the same conditions.

Now for the bad news: the test results with a mandrel may not simulate the power loss which the transmitter experiences. If the transmitter is a LED which overfills the fiber core, use of the mandrel can result in a power loss which is less than that experienced by the transmitter. If the transmitter is a VCSEL, the use of mandrel wrap can result in a power loss which is greater than that experienced by the transmitter.

KEY POINT 3: troubleshooting of power problems becomes more complicated. The power loss can be acceptably low and the receiver may receive insufficient power. Conversely, power loss can measure unacceptably high and the receiver may receive sufficient power. This complication arises because the generic test method may not simulate operation.

The solution to troubleshooting such power problems is the use of a light source which simulates the operation of the source in the transmitter. That is, if the transmitter has an overfilling LED, the test source should have an overfilling LED. If the transmitter has a VCSEL, in which launched most of the light is launched into the fiber in a donut shaped area with a diameter of approximately 35 µm, the test source should have a VCSEL with the same characteristics.

Singlemode Testing

Clause 7.2 of the TIA/EIA-568 B.3 requires that singlemode insertion loss testing be performed in accordance with Method A.1 of OFSTP-7, otherwise known as TIA/EIA-526-7. This method requires that the reference operates as singlemode. To accomplish this requirement, the source reference cable needs a 2 inch diameter loop to filter out the higher order modes which may be present in a short launch cable. This loop is often overlooked by field installers.

For additional information on fiber optic testing, see TSB 140, Additional Guidelines for Field-Testing Length, Loss and Polarity of Optical Fiber Cabling Systems. This document will be issued in the first half of 2003.

Respectfully submitted for your consideration,

Eric R. Pearson, CPC, CFOS

President

Pearson Technologies Inc.

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