Measuring fibre cabling and the problem of encircled flux loss

Last week I went on a very interesting training day courtesy of Nexans - data cable & parts supplier. I went looking forward to learning all about the new standards surrounding catagory-8 cabling for 40 and 56 Gigabit ethernet (a massive 1600Mhz of bandwidth down a twisted pair cable!) and the new GG45 connector; but those things will have to wait for another blog post! The thing that really tickled my fancy is the new standard for measuring the response of multi-mode fibre.

Multi-mode fibre works in a fundamentally different fashion to single mode (they are as different as twisted-pair and coaxial copper cable; but they look very similair). If you want a bit of a primer on fibre then Hugh & I did an episode of The Engineer's Bench a couple of years ago on the subject.

As we've gone from one-gig to greater than 10Gigbits/sec in OM3 and OM4 cable and engineers have often noted the lack of consistency between different manufacturers light-source testers. You might get as much as 0.5dB of difference between say an Owl and a JDSU calibrated light source and detector. We typically use a 20dB(m) laser at 850nM to test OM3 and we always just deliver the loss figres to the client, but it would be good to know if your absolute reading is of any use at all?

Well, the answer is that LED or VCSEL (Vertical-cavity surface-emitting laser) will tend to "overfill" the fibre and high-order modes of light travel (to a degree) down the cladding of the cable.

Launch conditions correspond to how optical power is launched into the fiber core when measuring fiber attenuation. Ideal launch conditions should occur if the light is distributed through the whole fiber core.

Transmission of Light in Multimode Fiber in Underfilled Conditions 

Transmission of Light in Multimode Fiber in Overfilled Conditions

An overfilled launch condition occurs when the launch spot size and angular distribution are larger than the fiber core (for example, when the source is a light-emitting diode [LED]). Incident light that falls outside the fiber core is lost as well as light that is at angles greater than the angle of acceptance for the fiber core. Light sources affect attenuation measurements such that one that underfills the fiber exhibits a lower attenuation value than the actual, whereas one that overfills the fiber exhibits a higher attenuation value than the actual. The new parameter covered in the IEC 61280-4-1 Ed2 standard from June 2009 is known as Encircled Flux (EF), which is related to distribution of power in the fiber core and also the launch spot size (radius) and angular distribution.

All the manufacturers are producing EF-compliant testers so you don't need to worry about inaccurate reading due to these high-order modes, but for now there are some suggestions.

Multimode launch cables allow for the signal to achieve modal equilibrium, but it does not ensure test equipment will be EF-compliant based on the IEC 61280-4-1 standard.
Multimode launch cables are used to reveal the insertion loss and reflectance of the near-end connection to the link under OTDR test. They also reduce the impact of possible fiber anomalies near the light source on the test.

If the fiber is overfilled, high-order mode power loss can significantly affect measurement results. Fiber mandrels that act as “low-pass mode filters” can eliminate power in high-order modes. It effectively eliminates all loosely coupled modes that are generated by an overfilled light source while it passes tightly coupled modes on with little or no attenuation. This solution does not make test equipment EF-compliant.

Mode conditioning patch cords reduce the impact of differential mode delay on transmission reliability in Gigabit Ethernet applications, such as 1000Base-LX. They also properly propagate the laser VCSEL light along a multimode fiber. This solution does not make test equipment EF-compliant