Current communication data rates in local networks range from 10/100Mbps for Ethernet to 1Gbps in fibre distributed data interfaces (FDDI) or Gigabit Ethernet networks, writes Craig Buckingham.

The increasing demand for Internet Protocol (IP)-based services such as voice, video, and data commands higher speed and larger bandwidth and pushes 10 Gigabit Ethernet into enterprise networks.

In that context, the optical loss budget of multimode cable requires accurate measurement and repeatability, especially when it tends to reach a threshold of the required bandwidth.

Measurements of loss and bandwidth in multimode fibres highly depend upon the launch conditions of the light source used for the measurement. New standards give some guidance to test equipment manufacturers in order to make the optical loss budget measurement repeatable, regardless of the test equipment used.

This article will explain launch conditions in multimode fibres and their impact on the optical loss measurements.

Multimode Fibre
Multimode fibres have a much larger core than singlemode (50, 62.5mm or even higher), allowing light transmission through different paths (called modes).

Launch Conditions
Launch conditions correspond to how optical power is launched into the fibre core when measuring fibre attenuation.

Ideal launch conditions should occur if the light is distributed through the whole fibre core. Actually, multimode optical fibre launch conditions may typically be characterised as being underfilled or overfilled.

They are characterised as underfilled when most of the optical power is concentrated in the centre of the fibre, which occurs when the launch spot size and angular distribution are smaller than the fibre core (for example, when the source is a laser or virtual cavity surface-emitting laser [VCSEL]).

See Figure 1.

An overfilled launch condition occurs when the launch spot size and angular distribution are larger than the fibre core (for example, when the source is a light-emitting diode [LED]). Incidental light that falls outside the fibre core is lost as well as light that is at angles greater than the angle of acceptance for the fibre core.

See Figure 2.

Light sources affect attenuation measurements such that one that underfills the fibre exhibits a lower attenuation value than the actual, whereas one that overfills the fibre exhibits a higher attenuation value than the actual.

Underfilled/Overfilled—Which is best?
Neither underfilled or overfilled is optimal, because both result in measurement variations.

Measurement variations are not critical when the allowed loss budget is over-dimensioned versus the expected bandwidth. But it is important to know the variation range for loss budget when it is close to its limit. In that case, a 50% variation may be too important for certifying the network, thus requiring fine measurements.

The International Electrotechnical Commission (IEC) 61280-4-1 provides guidance to guarantee that attenuation variations remain within ±10%.

Using IEC 61280-4-1-compliant test equipment in the field ensures that attenuation measurements will vary less than ±10% for >1dB loss and ±0.07dB for <1dB loss among various test equipment.

Encircled Flux
The parameter covered in the IEC 61280-4-1 Ed2 standard is known as Encircled Flux (EF), which is related to distribution of power in the fibre core and also the launch spot size (radius) and angular distribution.

EF corresponds to the ratio between the transmitted power at a given radius of the fibre core and the total injected power. The EF value equals the ratio between the amount of light transmitted in that middle part and the total amount of light emitted into the whole core.

IEC 61280-4-1 Standard
The IEC 61280-4-1 standard recommendations are based on the defined lower and upper boundaries of EF values at four predefined radii of the fibre core (10, 15, 20 and 22mm), and for each wavelength (850 and 1300nm).

Field Solution
IEC 61280-4-1 requires that the light coming from the end of the launching cord complies with the recommended EF boundaries.

Checking EF Compliance in the Field
Performing tests with IEC 61280-4-1 qualified equipment does not ensure that launch conditions comply with the standard at the time of testing, as variations can exist in the launching cord used or other variables.

Technicians conducting field measurements cannot access the EF measurement needed to check the launch conditions; therefore, the IEC 61280-4-1 standard proposes using physical artifacts.

Technical Solution—EF Modal Controller
EF-compliance is provided through the use of a modal controller device that is either integrated into the test equipment (light source or OTDR), or an external device. This device can be inserted either between the source and the fibre under test (LSPM1 method), or between the OTDR and the launch cable (OTDR measurement). Some modal controllers may also include a launch cable for direct connection to the fibre under test during OTDR measurements.

The EF modal controller is a passive device that ensures that launch conditions meet the IEC 61280-4-1 requirements regardless of the light source used (LED or laser). Modal controllers exist for both 50 and 62.5mm core fibres, some with integrated launch cables.

Non-EF-Compliant Devices
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 fibre anomalies near the light source on the test.

If the fibre is overfilled, high-order mode power loss can significantly affect measurement results. Fibre 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 fibre. This solution does not make test equipment EF-compliant.

Impact of EF Modal Controller on Different Sources (LED and Laser)
Without control of the launch conditions (Figure 3), the measurement of loss budget may vary significantly depending on the source used (LED or laser).

Using a modal controller to control the launch conditions according to the IEC standard (example below) can guarantee that the difference between LB1 and LB2 is less than 10%.

Craig Buckingham RCDD has been involved with the ICT industry since beginning his career as Telecommunications Mechanic in the British Army 15 years ago. He has worked on all facets of the cabling industry including network configuration, cabling installation, design and consultation. Craig has been with AFC for 4 months now as their Technical Development Manager delivering technical Learning Sessions to Consultants and End Users alike.

Hero image supplied by JDSU.

Visit www.afcgroup.com.au.