How can I determine the length of a Multimode fiber?

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SUMMARY

The discussion focuses on determining the length of a multimode fiber using the measured period of oscillations observed in the spectral response collected by an Optical Spectrum Analyzer. The user has a step index SMS (single-multi-single mode fiber) and seeks guidance on calculating fiber length, neglecting dispersion. The recommended method is to utilize an Optical Time Domain Reflectometer (OTDR), which measures the time of light reflection to calculate the fiber length based on group velocity adjustments for refractive index. The discussion also highlights the significance of standing wave patterns due to refractive index mismatches.

PREREQUISITES
  • Understanding of Optical Time Domain Reflectometry (OTDR)
  • Familiarity with refractive index concepts
  • Knowledge of spectral response analysis using Optical Spectrum Analyzers
  • Basic proficiency in MATLAB for analyzing oscillation periods
NEXT STEPS
  • Research the principles of Optical Time Domain Reflectometry (OTDR) for fiber length measurement
  • Study the effects of refractive index mismatches on optical signals
  • Learn about the analysis of spectral responses in multimode fibers
  • Explore MATLAB techniques for signal processing and oscillation analysis
USEFUL FOR

Optical engineers, fiber optic technicians, and researchers involved in fiber optic sensing and measurement techniques will benefit from this discussion.

mefisto
I have a step index SMS (single-multi-single mode fiber) with an equal lateral splicing offset at both junctions.
Refractive indexes, offset and radius of cores are given.
This device is fed by a broad-band source and the spectral response collected by an Optical Spectrum Analyzer is given.

The spectral response has an evident oscillating behavior and I have measured the period of oscillations thanks to Matlab

Can you suggest me how to determine the length of the multimode fiber using the measured period ? I can neglect dispersion. I have looked at articles that investigate the use of this structure as a sensing device , but I did not find a relation, since the length was always given .
thanks in advance.
 
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To measure fibre length, use an Optical Time Domain Reflectometer, OTDR. It sends short pulses of light up the line to reflect from the mismatch, or the open far end. The time to reflection, adjusted for group velocity due to RI, gives twice the line length.

If there is a refractive index or impedance mismatch in the system then you will see standing wave patterns. I expect the SW pattern will be due to some small junction interface, so will not give fibre length. How long do you think your fibre is, 1mm, 1m or 1km?

Can you please post the graph that shows the oscillation, with a clearly readable horizontal axis.

Edit:
Look at pages 20 to 25 in this file.
http://122.physics.ucdavis.edu/sites/default/files/files/Electronics/TransmissionLinesPart_II.pdf
 
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