Tapered suspended-core fibers for broadband supercontinuum generation and an f-2f interferometer.

Octave-spanning supercontinuum (SC) generation typically requires high input pulse energies and long fiber lengths. Achieving such high pulse energies is feasible only with low repetition rate lasers, operating in the kHz to MHz range. In contrast, high repetition rate lasers, such as microresonator-based frequency combs (microcombs), provide lower input pulse energy. This necessitates the use of fibers with high nonlinearity, high coupling efficiency with laser sources, and tailored dispersion properties for broadband supercontinua. In this work, we present tapered fibers fabricated from lead-bismuth-gallate (PBG) glass-a material with high nonlinearity-for octave-spanning supercontinuum generation spanning from 760 to 1860 nm at low input pulse energies (0.03-0.05 nJ). The untapered sections of these fibers feature a large core diameter (10 μm) to assure high coupling efficiency with standard single-mode fibers or laser sources. At the tapered waist, the core diameter is reduced to 1.5 μm, significantly enhancing the nonlinear coefficient and enabling octave-spanning supercontinuum generation. The supercontinuum spectra align well with the optical properties of periodically poled lithium niobate (PPLN-SHG6), making these tapered fibers ideal for f-2f interferometers to measure the carrier-envelope offset frequency of microcombs.