In-situ Tapering of Chalcogenide Fiber for Mid-infrared Supercontinuum Generation

Supercontinuum generation (SCG) in a tapered chalcogenide fiber is desirable for broadening mid-infrared (or mid-IR, roughly the 2-20 μm wavelength range) frequency combs^{1, 2} for applications such as molecular fingerprinting, ³ trace gas detection, ⁴ laser-driven particle acceleration, ⁵ and x-ray production via high harmonic generation. ⁶ Achieving efficient SCG in a tapered optical fiber requires precise control of the group velocity dispersion (GVD) and the temporal properties of the optical pulses at the beginning of the fiber, ⁷ which depend strongly on the geometry of the taper. ⁸ Due to variations in the tapering setup and procedure for successive SCG experiments-such as fiber length, tapering environment temperature, or power coupled into the fiber, in-situ spectral monitoring of the SCG is necessary to optimize the output spectrum for a single experiment.In-situ fiber tapering for SCG consists of coupling the pump source through the fiber to be tapered to a spectral measurement device. The fiber is then tapered while the spectral measurement signal is observed in real-time. When the signal reaches its peak, the tapering is stopped. The in-situ tapering procedure allows for generation of a stable, octave-spanning, mid-IR frequency comb from the sub harmonic of a commercially available near-IR frequency comb. ⁹ This method lowers cost due to the reduction in time and materials required to fabricate an optimal taper with a waist length of only 2 mm.The in-situ tapering technique can be extended to optimizing microstructured optical fiber (MOF) for SCG¹⁰ or tuning of the passband of MOFs, ¹¹ optimizing tapered fiber pairs for fused fiber couplers¹² and wavelength division multiplexers (WDMs), ¹³ or modifying dispersion compensation for compression or stretching of optical pulses.^14-16