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Why encircled flux plays a key role in optical fibre testing

Encircled flux is a metric for defining launch conditions on multimode optical fibre that reduces measurement uncertainty in link loss measurements shown by different test equipment. Adrian Young, senior technical support engineer at Fluke Networks, explains how to take account of encircled flux in Tier 1 optical loss measurement

With the introduction of low-loss fibre optic components such as LC/MPO cassettes, loss budgets for testing are becoming increasingly small. As a result, installers are finding that previous methods and assumptions about testing are no longer valid.

In Tier 1 optical fibre testing, it is common for two different testers to obtain a variability of up to 40 per cent. This is due to encircled flux (EF) – the ratio between the transmitted power at a given radius of an optical fibre core and the total injected power. EF correlates test results to conservative launch condition performances in gigabit Ethernet optical fibre transceivers, and is becoming particularly important when operating to custom loss budgets based on specifications provided by the vendor, which will result in increasingly tight margins.

To reduce the uncertainty in link loss measurements shown by different test equipment when testing multimode optical fibre, installers need to take account of encircled flux. Even if they are not yet required to be EF compliant, they should begin preparing now.

There are four aspects to successful Tier 1 optical loss measurement: the LED source, the jumper reference, the connectors and encircled flux. Each must be set correctly to achieve optimum results.

Choosing the right source
In theory users can test with either a vertical cavity surface emitting laser (VCSEL) or an LED. However, current ANSI/TIA regulations specify that the source must have a spectral width of 30-60nm, whereas a VCSEL source has a width of just 0.65nm.

If the optical fibre system is tested with a VCSEL, the cabling vendor may not accept the application warranty due to the uncertainty of the measurement. It is the responsibility of the individual testing and providing the warranty for the system to ask what type of source is to be used. If in doubt, we recommend that the tester reviews the test equipment vendor’s data sheet and verifies requirements with the vendor providing the warranty for the cabling system.

Setting the reference correctly
If the reference is set incorrectly it can result in optimistic and negative loss results. The latter are the largest cause of failed system acceptance and warranty denial, and suggest an amplification of the optical system, which is impossible in a passive system. It is essential to follow the industry standard and set a reference using a single test reference cord.

Common causes of error are:

  • Setting a reference through a bulkhead adapter, which adds uncertainty of up to 1.5 dB due to loss in the adapter. The reference has to be continuous from the source to the meter using one test reference cord (1 jumper);
  • Not coiling the fibre round a mandrel, which could make results pessimistic by up to 0.4dB; and
  • Using bent insensitive multimode fibre (BIMMF) test reference cords with dual wavelength testers.

To achieve reliable results the tester should use equipment with interchangeable adapters on the input ports. This allows them to set a 1 Jumper reference in accordance with TIA and cabling vendor requirements. It is also important to purchase the correct adapters and cabling reference cords.

Using reference grade connectors
Bad cords lead to poor and inconsistent test results. ISO/IEC 14763-3 defines reference grade connectors as having a loss of <0.1 dB for multimode and <0.2 dB for single mode. When a low loss cassette has a 0.15 dB LC connector, testing it with anything worse than a 0.15 dB LC connector is going to result in a pessimistic result or potential failure.

Allowing for encircled flux
To reduce the measurement uncertainty caused by different light sources, TIA standards defined the launch condition from a multimodal optical source in terms of coupled power ratio (CPR). However, this still allowed too much variation between sources, which was why the concept of encircled flux was introduced. This specifies the modal power across the entire end face of the test reference cord, which has to be maintained to the end of the cord.

TIA-TSB-4979 describes two methods for meeting EF requirements. The first is to use an external launch conditioner which replaces the mandrel. This can turn any LED source into an EF-compliant solution, avoiding the need to buy new test equipment, but external launch conditioners are expensive, bulky and need to be replaced when the connector at the end breaks. Many data centre managers operating to tight optical loss budgets tell installers to include the cost of launch conditioners in their bid.

The second option is to use an EF compliant source with a tuned reference cord that strips out the unwanted modes (see Figure 1). Although this is a proprietary solution and requires the purchase of new test equipment, the cords are less expensive and bulky than launch conditioners.  

Figure 1: encircled flux test reference cords

To address poor referencing, test equipment vendors are creating automated wizards to walk the technician through the reference procedure process

If existing test equipment has fixed input ports that do not allow a 1 Jumper LC reference, installers could consider jumping two generations of testers and make testing of optical fibre both EF and 1 Jumper compliant at the same time in accordance with the ANSI/TIA-526-14-B.

Best practice tips for installers
Tighter loss budgets and higher data rate systems are making EF increasingly important, and at some point EF compliance will become mandatory for optical fibre testing.

We recommend that organisations review their field test procedures and ensure their installers are following best practice. They should:

  • Use LEDs as the light source, not VCSELS;
  • Avoid setting references through bulkhead adapters;
  • Use mandrels to remove higher modes, while remembering that they are not a substitute for EF adapters;
  • Invest in optical fibre test equipment with interchangeable adapters on the input ports;
  • Verify test reference cords and not use BIMMF as a test cord;
  • Save measurements and make them part of test documentation; and
  • Create a strategy for EF compliance, bearing in mind that vendors may insist on a 1 Jumper EF-compliant measurement before sending an engineer to troubleshoot a failing system.

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