How can I determine the length of a Multimode fiber?

[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.

 

[Baluncore]

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