Understanding Multimode Launching Conditions and TIA TSB-178
What is EF? It is a new method to define the launch conditions of a light source EF measurement is based on the power distribution measurement on the connector end face of the light source, reference jumper or conditioner
Why is EF important? The new proposal (TIA TSB-178) requires light sources used for multimode IL measurement to fit in the Encircled Flux (EF) template This new specification is more stringent than previous standards or proposals Consequently, it has an impact ($$$) on test equipment and optical component manufacturers
The Real Problem MM IL measurement is highly sensitive to the source launching conditions Example: IL of a connector varies greatly depending on whether a VCSEL or LED is used to perform the measurement Source Launching Conditions Overfilled (ex: surface emitted LED) Slightly under filled (targeted conditions) Under filled (ex: VCSEL or edge emitting LED) Measured Connector IL Over evaluated (IL = 1dB to1.5db) On target (IL = 0.5dB) Under evaluated (IL = 0.02dB)
Before Why should anyone care? Variations of IL measurements were common and tolerated Previous launching conditions (requirements or definitions) didn t provide repeatable launching conditions Tolerated because: MM networks were transmitting at relatively low speeds and optical loss budgets were large (ex: Fast Ethernet had > 12dB for loss budget )
Why should anyone change? Today Today 1 or 10 Gigabit/second systems transmit up to 300 meters the loss budgets are much tighter ~ 2.6 db for 10 Gigabit Ethernet ~ 3.5 db @ 1Gigabit Ethernet Therefore IL must now be measured precisely to ensure system performance The industry now needs repeatable and consistent IL measurements The response to this new need is the Encircled Flux template for attenuation and loss measurements (TIA TSB-178)
How is EF Measured? 1. Near Field Radius Step 1: A picture of the connector end face is taken. Light intensity of each pixel is measured. 2. EF = Normalized EF Radius (µm) Step 2: A function is applied to the raw data to calculate the EF
3. How ER is Measured? (cont d) EF template Normalized EF Radius (µm) EF 0 0,1 0,2 0,3 0,4 0,5 0,6 0,7 0,8 0,9 1,0 Example: At this point 50% of the measured power, within a 17 µm radius Radial Offset (μm) Target 10 0.3350 15 0.6550 20 0.9200 22 0.9751 Template Target 0 2 4 6 8 10 12 14 16 18 20 22 24 26 28 Radius (μm) The measured EF has to fit within the EF template Step 3: The result has to fit within the defined template Note: Different templates exist for 50 µm and 62.5 µm fibers and for 850nm and 1300nm
Other Changes EF compliance requires: Correct launching as determined by EF template Central launch obtained by EF reference grade launch cord or EF conditioner EF compliance = EF compliant source + EF reference grade launch cord or EF conditioner Source 850 nm Source 850 nm EF OK EF Conditioner or EF Reference grade launch cord EF compliant set-up
EF Reference Grade Launch Cord An EF reference grade launch cord is a launch cord made of: Reference grade connectors Reference grade multimode fiber
Reference Grade Connector Launch cord Device Under Test (DUT) Source Power Meter EF ok EF ok The tolerances of some standard MM connectors are not tight enough to guarantee a central launch and required concentricity of reference grade connection
Reference Grade Connector (cont d) IL < 0.1 db EF ok EF ok Reference Grade connectors have tighter tolerances than the standard ones The proposal (TIA TSB-178) defines reference grade connectors as: the expected insertion loss between a pair of such connectors should be less than 0.1 db for multimode fiber
Reference Grade Fiber (cont d) To guarantee EF conditions are unchanged the core diameter and core/cladding concentricity have tight tolerances EF ok EF ok Constant Core size Cladding/core concentricity
EF Mode Conditioners Using an external mode conditioner can also guarantee EF compliance and in some cases regardless of the initial launching conditions In other cases, EF compliance is product specific. Compliance is only guaranteed when used with a particular model of light source or configuration.
EF Measurement EF Metrology Challenges There is currently no EF measurement traceability to any national or international standards body such as to NIST, NPL, PTB or METAS
Conclusion Time line for new proposals (soon to be standard) TIA TSB-178 (published September 2008) IEC-61280-4-1 (publication in 2009) The EF standard will likely replace the CPR standard in TIA 568 The EF standard will radically change the way multimode components are tested
Any Questions? Contact Info: Varis C. Hicks varis.hicks@exfo.com 418-683-0913 ext 3106