Inline measurement of modal phase differences for the characterization of intermodal cross-phase modulation

Schnack M, Seck F, Lüpken NM, Fallnich C

Abstract

We present an inline measurement technique for the phase difference between two transverse fiber modes. This technique proves to be well suited for direct measurement of an all-optically changed phase difference between two transverse modes of picosecond probe pulses, induced via intermodal cross-phase modulation by co-propagating picosecond control pulses. We achieved unprecedented agreement with the corresponding analytical description for time-independent changes of the phase difference as well as for time-dependent changes on an intra-pulse timescale. The simple and accurate predictability combined with the ability to measure modal phase differences inline poses a key step towards full all-optical control over transverse modes in fibers. This may lead the way to a variety of novel functionalities for all-fiber applications, like a spatial degree of freedom in fiber-sensing technology by using transverse fiber modes or all-optical spatial and temporal pulse shaping inside a fiber.