Fiber Optic Training Sessions

Launch Conditions forMultimode Testing
March 2019

+1 (805) 987-1700
sales@optotest.com
engineering@optotest.com
www.optotest.com

What are Launch Conditions?
• Launch Condition: the way a light source fills
the fiber it is propagating through.
• Almost all insertion loss (IL) measurements on
multimode (MM) fiber are extremely affected by
the launch into the reference cable.
• Depending on the launch of the reference cable,
the results could be overly optimistic or overly
pessimistic in relation to the actual IL of thedevice under test (DUT) when used in the field.

LEDs & Lasers
For testing, light sources should be repeatable and test the performanceof most of the core.
LEDs• Typically have a very broad emission cone.
• Often fill both the core and cladding.
• This is bad for testing since it causes otherwise good cables to failsimply because the light at the core/cladding boundary gets
attenuated off.
Lasers• Don’t fill the core of MM fiber and typically propagate unevenly; thelight is not always centered in the core.
• This is bad for IL testing since unstable measurements and loss
readings only account for part of the fiber rather than the entire core;
this can lead to overly optimistic results and cables that pass IL tests
in production but fail in the field.

Standards• Some end-users require that strict and specific launch
conditions be met, often requiring documentation as proof.
• To help standardize the requirements of specific industries,
their relevant standards bodies often set the launch
conditions necessary for testing based on the system
requirements that the DUT will be deployed into.

• For example, the IEC has set a standard for the launch condition for MM cables under IEC 61280-4-1.
This condition is known as Encircled Flux (EF). The standard focuses on controlling the distribution oflight relative to the distance from the center of the core.
• Certain military, aerospace, and harsh environment standards specify their own launch conditions, such as
M80 and 70/70, with requirements based on the NA of the fiber, spot size of the source, and other factors.

Encircled Flux
• Specific launch conditions allow for testing that mimics
certain conditions that DUTs may face when deployedin the field.
• EF in particular was developed to mimic the worst casescenario a component would see in the field; not to be
confused with the EF launch that applies to transceivers.
• A typical test for EF compliance checks the amount of light at five points across the diameter of the core
to make sure that it fits within certain bounds.
• Functionally, these bounds create a slight underfill of the multimode fiber, testing a large area of the core
without including lossy higher order modes to the outermost edge of the core.
• It also creates reliable results from the time of production which are easily replicated during quality tests
by the end user.

Keep in Mind
• The Encircled Flux launch condition standard for testing fiber optic cables is different than the standard of
the same name that applies to transceiver launch requirements.
• Launch condition standards for patch cords and other fiber optic assemblies overwhelmingly apply only tothe test system used for qualifying insertion loss. Whether the cables themselves maintain a launch
condition would not be covered under the same standard. It would typically be a separate requirement thatwould be explicitly called out by the customer.
• Typically, launch conditions have specific requirements for each wavelength and fiber core size combinationand it is important to make sure that the devices you use to verify this can cover the necessary
wavelengths, numerical apertures, etc., of the standards that apply. Many launch condition analysis
modules are valid only around the 850nm wavelength and will not support the launch condition verification
for the full spectrum of light that multimode fibers can carry.

Verifying Compliance to Launch Condition Standards• These graphs and results are generated by
the OPL-LCA software that controls theOP1021 Launch Condition Analyzer.
• The OP1021 verifies that light sources
adhere to the specific launch conditions
they are designed for.

OP1021 Launch Condition Analyzer
The OP1021 comes with 2 internal sources. These
are usually one 850nm LED and one 1300nm LED.
Both sources are coupled into a 105/125µm stepindex fiber to provide an overfilled launch condition
into both 50µm and 62.5µm fibers.

Farfield Measurement• +/- 0.5 radians scan angle• 0.001rad step size
• measure numerical aperture NANearfield Measurement• +/- 250 µm scan range• 0.1 µm step size
• Software computes Encircled Flux
• Auto-align feature to center and focus Nearfield scanner
Overall Features• Wavelength Independent: measures 850nm to 1625nm• Farfield and Nearfield analysis in one instrument• Integrated OPL LCA software• Directly links to Excel spreadsheet – quick print

For more details, see our Application Note:
AN-135 Launch Conditions for Multimode Testing
https://www.optotest.com/an-135-launch-conditions-for-multimode-testing/Contact us:+1 (805) 987-1700
sales@optotest.com
engineering@optotest.com
www.optotest.com