High power laser delivery, telecommunication and fiber sensing technologies have been boosted by the development of the low loss hollow-core photonic crystal fibers (HC PCFs). The HC PCFs guide light in a hollow region with minor optic power propagated in the glass. Filling the hollow core with gases or liquids has enabled intense light-matter interaction over distances that are thousands of times longer than the Rayleigh length. This has caused advancements in quantum optics, atomic physics and gas spectroscopy. Usually, these experimental setups require gas cells in which laser light is launched in the system through windows and using standard optical components. The dimensions of the system are therefore limited to the size of the gas cells and hence scaling is challenging.
A remedy for this limitation was the direct fusion splicing of a solid-core single-mode fiber (SMF) to the HC PCF. However, efficient optical coupling was obtained only for HC PCF with mode field diameter (MFD) comparable to the one of standard SMF (about 10 μm). Furthermore the gas-air interface can be a problem at high power because of back reflections.
Researchers led by Professor Philip Russell at Max Planck Institute for the Science of Light in Germany reported a new technique for launching light from a SMF to a HC PCF. A nanospike, or in simple words a silica tip, obtained by thermally tapering a SMF, is inserted to the core of the HC PCF. Proper taper design ensures efficient adiabatic transmission from the nanospike into the hollow core without Fresnel reflections and a correct choice of the nanospike diameter permits coupling to HC PCFs of any core size. Nanospikes with smaller tip diameters are necessary for larger HC PCF cores. In addition an optomechanical backaction between the nanospike and the HC PCF allows optical trapping of the nanospike at the core center. Pennetta et al. exploit this idea to realize an integrated all fiber device that can operate as a compact gas cell, only a few centimeters long. To demonstrate its perfomances, the author use the proposed device to observe Raman scattering and molecular modulation of hydrogen.
Pennetta R., Xie S., Lenahan F., Mridha M., Novoa D., Russell P. St. J. Fresnel-Reflection-Free Self-Aligning Nanospike Interface between a Step-Index Fiber and a Hollow-Core Photonic-Crystal-Fiber Gas Cell. Physical Review Applied. 2017 Jul 14; 8(1):014014.
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