Mode Field Diameter (MFD) is a measure of light intensity in the core of a single mode fiber. It is traditionally defined as the width where intensity falls to 1/e of its peak value, but standards now define it via the Petermann II integral of the far-field intensity distribution. MFD represents the effective area of light propagation in both the core and cladding. It provides important information about a cable's performance and impacts from bending or improper source-fiber coupling that could lead to excessive loss. MFD is tested using an optical time domain reflectometer to obtain the far-field profile and calculate the Petermann II integral to determine the MFD value.

1. Mode Field Diameter (MFD) Overview

2. • Mode Field Diameter is a measure of
light intensity in a single mode fiber
cross section
• Traditionally, MFD is the width of the
curve where the intensity falls below
1/𝑒"
of its peak. While still a good way
to visualize the MFD, the definition has
improved
What is MFD?
• International standards define MFD via
the Petermann II definition which
involves integration of angles obtained
from the far-field intensity distribution
• MFD, as the effective area of the fiber,
is larger than the core with light also
propagating through a portion of the
cladding

3. • Provides information about the cable’s performance and the impact of sources of
loss such as macro and micro bending
• Appropriately matched fibers and sources avoids excessive loss or underfilling
(source-to-fiber and fiber-to-fiber coupling loss)
Why is MFD important?
Deceptively good loss going right to left
Really bad loss going left to right

4. • OP1021 Launch Condition Analyzer
• Farfield profile obtained via the direct
farfield method, which is the preferred
reference method for MFD
• Calculations per Petermann II integral
done in OPL-LCA
• MFD included in results and test report
Testing Solution for MFD

5. • TIA/EIA-455-191A and IEC 60793-1-45
• Standards highlight four methods for calculating MFD, with the direct farfield method
being the most direct and thus used to resolve disputes
• MFD is calculated by measuring the farfield
pattern and evaluating the Petermann II
integral using the measured data
• Evaluation of this integral is done in
OPL-LCA following the process
detailed in Annex A of both
standards documents
Standards

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