Absolute strain demodulation with acoustooptic scanning low-coherence fiber interferometry

A traveling acoustic pulse in a two-mode fiber can act as a moving beam splitter coupling light from LP01 to LP11 mode. The intermodal delay varies as the acoustic pulse advances in the fiber, The device can be used in low-coherence interferometry to scan time delay for making a fast extended range absolute measurement without involving complex signal processing, As an illustration, the time delay as a function of axial strain applied to a section of highly birefringent fiber is scanned. Absolute measurement of strain over 900 mu epsilon with a resolution of the order of 25 mu epsilon is reported.