Distributed Measurement of Brillouin Gain Spectrum in Photonic Crystal Fibre

Distributed measurements of the Brillouin gain spectrum in a photonic crystal fibre have been carried out for the first time to our knowledge. These measurements confirm the strong increase of the Brillouin threshold as resulting from the broadband and multimode nature of the Brillouin gain spectrum all along the fibre. INTRODUCTION The periodic wavelength-scale air-hole microstructure of solid-core photonic crystal fibres (PCFs) drastically alter their acoustic properties, compared with what is commonly observed in all-silica fibres [1,3,4]. It thus significantly impacts on stimulated Brillouin scattering (SBS), recent results have shown that the SBS spectrum in ultra-small core PCF is broadened and even turns multimode, leading to a higher SBS threshold than in standard fibres [2,5]. The aim of the present work is to clearly identify and understand the broadband and multimode nature of SBS spectra in PCFs as resulting from either the structural irregularities of the PCF or the large numerical aperture that allows for the simultaneous generation of several acoustic modes. To this end, we present distributed measurements of the SBS frequency shift in a 160m-long PCF using a recently-developed technique [6]. The results of our measurements indicate that the structural irregularities of the PCF have actually little impact on the SBS frequency shift. We also show that this PCF exhibits a strong 3-fold increase in the Brillouin threshold compared with a uniform all-silica fibre, in good agreement with the Brillouin linewidth broadening by 80MHz.