The measurement of Brillouin gain spectrum with ultra-short optical fibers

The Brillouin scattering is one of the most important nonlinear effects in optical fibers. Measuring and characterizing the Brillouin gain spectrum is vital for any practical Brillouin applications. The Brillouin spectrum is commonly measured with very long optical fibers of at least a few hundred meters to several kilometers in length to ensure the generation of sufficient Brillouin gain. Not only the measuring system becomes cumbersome, but more importantly, what if the fiber under test cannot be made long under no circumstance. In this paper, we propose and demonstrate a specially designed Brillouin gain measuring system, based on an ultra-narrow linewidth pump laser with a heterodyne detection scheme, that is capable of characterizing the Brillouin spectra for optical fibers as short as a few meters in length. Single-mode optical fibers in 10-meter-long and 1-meter-long segments have been successfully measured and analyzed, while the minimal working length is expected to be shrinking down to a couple of meters even for large-mode-area fibers by theoretical prediction.