Abstract

Frequency combs are of constant significant interest for their use in diverse areas of physics ranging from metrology to biomedical and environmental spectroscopy. One of interesting techniques to obtain multi-octave comb-like optical spectra is based on generation of higher-order stimulated Raman scattering (SRS) in a hollow-core photonic crystal fibers (HC-PCFs) filled with hydrogen gas [1]. The ability of these types of fibers to strongly confine together gases and laser pump, while keeping their interaction length over several meters, has allowed to reduce, by six-order of magnitude, the laser power in comparison to previous equivalent techniques using a gas cell, which have required a GW level of peak powers. However, the power threshold to generate intense Stokes and anti-Stokes Raman lines is still in the order of tens of kW [2,3]. Moreover, a main inconvenience of using a hydrogen gas in HC-PCF is a high hydrogen permeability of silica, which requires of using special actions to protect the fiber from gas leakage. To overcome this undesired and destructive issue, in our work we use instead CO2 gas and we study its Fermi-dyad n1/2n2 band through SRS. This rovibrational Q-branch, shifted by 41.64 THz from the pump laser, is of fundamental interest because of its unique extremely narrow spectral width of the order of 300MHz (at FWHM) at around 1 bar of CO2. Indeed, at the pressure of about few tens of mbar to 1 bar, this band undergoes a strong spectral compression due to combined effects of Dick narrowing and collisional linemixing. Moreover, although it is well-known that SRS of CO2 is less efficient that the one of hydrogen, interestingly the Q-branch of CO2 may exhibit a strong collapse, which makes this band very intense [4].

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