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."