Single Mode to Multi-mode back to Single Mode

[Warr]

Alright. I'm having a little trouble getting started on this design of an interferometer. Essentially what I want is to couple a laser into the single mode fiber (SMF), which is fusion spliced to a multi-mode fiber (MMF). Assuming the fiber is unperturbed, the modes will not mix and you will no interference pattern at the end of the MMF (or perhaps you will have an interference pattern characteristic of the 'unperturbed' state, I'm not really sure) . However, if the fiber is say, stretched, the index of refraction will change, causing an interference pattern. I'm wondering how I could measure this. If this were to be used to measure an oscillatory deformation of the MMF (lets pretend its change in length delta L is a sine function of some angular frequency omega), what would I see in terms of the output of the SMF. I'm pretty confused here...

Some more information. From wikipedia http://en.wikipedia.org/wiki/Speckle_pattern

"In the output of a multimode optical fiber, a speckle pattern results from a superposition of mode field patterns. If the relative modal group velocities change with time, the speckle pattern will also change with time. If differential mode attenuation occurs, modal noise results."

 

[berkeman]

Your 2nd MMF fiber will not support the single mode that is injected, IMO.

 

[anorlunda]

PF has several threads that may have the information you seek:
https://www.physicsforums.com/threads/modes-of-optical-fiber-propagation.946536/post-5991156
https://www.physicsforums.com/threa...rnal-reflection-in-a-mm-sm-fiber-work.912464/
https://www.physicsforums.com/threads/single-mode-vs-multi-mode-fiber-coupling.966496/post-6135391
https://www.physicsforums.com/threads/optical-fibers-field-shape.968920/
https://www.physicsforums.com/threads/single-mode-vs-multi-mode-fiber-coupling.966496/
https://www.physicsforums.com/threads/whats-the-mode-of-light-in-single-mode-fiber.963179/
https://www.physicsforums.com/threads/modes-of-optical-fiber-propagation.946536/
https://www.physicsforums.com/threa...es-in-birefringent-single-mode-fibers.922854/

 

[tech99]

I think if you splice the SMF fibre to the MMF fibre, when you apply the laser the light entering the MMF will develop many modes. This will result in a speckle pattern on a screen near the output. If you now deform the MMF I think the speckle pattern will change.
So it is a matter of how to measure the changes in speckle pattern, which in these times I suppose will be done by software, subtracting one image from the other say. Or using some sort of spatial correlator.