Ultrafast measurement of vector spatial modes by using two-dimensional linear optical sampling

To the best of the authors' knowledge, we present the first-ever demonstration of the two-dimensional linear optical sampling of fiber spatial modes. The images of the fiber cross sections excited by the LP₀₁ or LP₁₁ modes are directly projected onto a two-dimensional photodetector array and coherently sampled by local pulses with a uniform spatial distribution. Consequently, the spatiotemporal complex amplitude of the fiber mode is observed with a time resolution of a few picoseconds by using electronics with a bandwidth of only a few MHz. Such ultrafast and direct observation of vector spatial modes enables the characterization of the space-division multiplexing fiber itself with high time accuracy and wide bandwidth.