Encircled Flux" for higher accuracy in fibre optic measurements

What is "Encircled Flux"?

How does "Encircled Flux" make itself felt when measuring on multimode fibre optic cabling links?

Why is Encircled Flux multimode fibre measurement technology important for the installer?

Measurement inaccuracy

Multimode fibre optic cabling links in so-called "application-neutral communication cable systems" are usually evaluated either by means of an attenuation measurement (LSPM = Light Source + Power Meter, corresponds to measurement type "Tier 1") or an OTDR (OTDR = Optical Time Domain Reflectometer, corresponds to measurement type "Tier 2").

Particularly with Tier 1 attenuation measurement, there are always major deviations in the results, depending on the measurement equipment used. Since the attenuation budgets between transmitter and receiver are becoming smaller and smaller in modern application protocols, the measurement results must become more and more precise. In order to obtain reliable and comparable measurement results, new definitions for the attenuation measurement of multimode fibre optic cabling have been agreed upon in the standardisation process. Part of the improvement in accuracy is achieved by optimising the referencing methods and the permissible tolerances of the measuring equipment used, the second part is achieved by specifying the test signal to be used. In this article we will focus on this point, the new requirements of the "Encircled-Flux" definition for the test signal used.


As already described very often in many different technical articles, cleanliness and freedom from damage are the top priorities in fibre optic cabling and of course also in fibre optic measurement technology. In order to get a picture of the cleanliness and condition of the fibre optic connectors used on the reference cables (measuring cables) and on the fibre optic sections to be measured, it is essential to look at all the fibre optic connector end faces involved using a fibre optic microscope before each measurement. Then, if they are dirty, the connector end faces can be cleaned, then inspected again and if everything is then clean and free of damage, only then can the plug connection of the inspected and cleaned fibre optic connectors be made.

Any other procedure would be "flying blind" and careless with the connector faces of the fibre optic connectors, which can lead to further soiling, damage and even total failure of fibre optic cabling sections (see Fig. 1).

Optical attenuation

In order to be able to explain the influence of "Encircled Flux" on measurement technology, one must first deal with the basics of fibre optic attenuation measurement. How do you measure the attenuation of fibre optic cabling? It actually sounds relatively simple, you use a light source (Light Source =LS) and a power meter (Power Meter = PM) and use them to make a measurement of the light power lost on the fibre link. So the attenuation:

L [dB] = 10 * log (Pin/Pout)

Or if the level meter used can display the measured light power levels directly in dBm, then you can simply calculate the attenuation as the subtraction of the transmit power (PLS [dBm]) minus the receive power (PPM [dBm]):

L [dB] = PLS [dBm] – PPM [dBm]

This looks very simple, but the pitfall here is in the detail, as one must first perform a "zeroing" ("normalisation" or, officially according to the standard, "reference procedure") to eliminate the measurement equipment's contribution to the overall result. Especially the quality of the measuring cables and plugs contributes significantly to the accuracy of the result. In order to obtain the correct relative measured value, the power emitted by the light source (PLS) must be determined and stored in the power meter as a reference value. But this is where the drama begins.


The standards have provided for between one and three reference measurement cables for this zeroing and for the excitation conditions - i.e. how and how much light (more precisely light modes) are coupled into the optical fibres - there have also been different methods based on the light sources used. In recent years, the topic of light coupling has been taken up again and the definition of "encircled flux" ("limited luminous flux") has been integrated into the standards. What was it like before? Initially, the use of LED light sources was standardised. These produce a so-called "overfill" excitation (see Fig. 2 - Part 2a).

However, this "overfill" was not very clever, because by flooding the LED light source with light, light modes were coupled into both the glass fibre core and the glass fibre cladding. These were then travelling in the glass fibre as stable modes of low order (close to the glass fibre core) and as unstable modes of higher order (further away from the glass fibre core, into the glass fibre cladding). With short reference measurement cables, this has led to too much light being measured with a power meter during zero adjustment. This in turn led to faulty measurements because the measured reference power was too high due to the sheath modes. In addition, with longer cabling distances, the unstable higher order modes in the fibre optic cladding and sometimes also in the fibre optic core may have disappeared after several metres and are then no longer involved in the measurement. In order to achieve more stable conditions, the so-called cladding modes and the unstable higher-order modes were filtered out before zeroing by means of a mode filter (mandrel) (see Fig. 3).

"EF" to the supply point

If the light source itself has the "Encircled Flux" excitation condition, this can be achieved by using special reference measuring cables, the so-called transparent mode. This means that it is possible for the light to be coupled into the wiring sections to be measured to be exactly "Flux encircled" at the output of the reference measuring cable/plug. The advantage of this method over other methods of measuring the encircled flux (e.g. a mode conditioner connected between the light source and the reference plug) is that the wear and tear of a reference plug at the end of the reference measuring cables (jumpers) does not become an insanely expensive consequence, but remains relatively inexpensive due to the replacement with a new transparent mode reference cable.

If one now compares measurements of different measuring instruments, one comes to results whose deviations, due to the clear definition of the mode distribution at the reference plug by "Encircled Flux", are clearly below 10 %.


More than ever, the use of measurement equipment that works with "Encircled Flux" compatible light sources will be imperative in the future in order to achieve reliable and accurate results in the measurements of multimode fibre optic cabling links with decreasing attenuation budgets.

This is the only way to ensure in advance that modern high-speed fibre optic applications can be transmitted without problems.

Thomas Hüsch
Technical Support & Training
Softing IT Networks GmbH