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IEC standard 61280-4-1 Ed. 2.0, which was published in 2009 specifies conditions that a light source has to comply with in order to be used for making meaningful measurements of link attenuation. The standard details the specific modal launch conditions that must be used, at 850nm, 1300nm, and for both standard fibre sizes; 50um and 62.5um. This is done in terms of the Encircled Flux parameter.

Most lights sources, LEDs and OTDRs, do not naturally comply with this standard unless they are specifically designed to do so. The 'ModCon' Mode Controller was designed to deliver a stable, IEC compliant, modal distribution that is completely independent of the input distribution.

The photo shown right is the packaged mode controller.

As an example of the ModCon in action, the insertion loss of a link of 11 concatenated patchcords was measured with diffent light sources. These were a singlemod source, a commercial LED tester, a mode scrambler launch, and an offet singlemode source. The graph, right, shows that the loss ragned from 0.5dB to 5.1dB. A ModCon was thien inserted between each light sorce and the link under test. The results, shown in red, were stable to +/-0.1dB.

Click here for a paper on the ModCon submitted to IET Optoelectronics (.pdf 740k).

Click here for a PhD on mode control. (.pdf 4.2M)

In addition to complying with the IEC Encircled Flux specification, the mode controller can be made with bespoke modal distributions. For example, the plots below show a 85/85 launch condition. This is where the width of the near-field intensity distribution in the fibre is equal to 85% of the core diameter of the fibre, and the far-field intensity distribution is equal to 85% of the maximum numerical aperture of the fibre. According to fibre measurement standards IEC60793-1-20 and EIA/TIA-455-47B, the near-field and far-field widths are defined at the 2.5% and 5% intensity threshold level, respectively.