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We report on transversally structured beam effect on saturated absorption spectroscopy in acetylene-filled hollow-core fiber. Experimental results reveal frequency shift and contrast enhancement of absorption lines using LP11 guided mode rather than gaussian-like mode. © 2022 The Author(s) Sub-Doppler spectroscopy such as electromagnetically induced transparency (EIT) or saturated absorption spectroscopy (SAS) is usually performed with a Gaussian mode profile both in free-space and guided optics configurations. Recently, in free space, works have been reported where Laguerre-Gaussian (LG) modes have been set as pump or probe beams in EIT [1-5] and SAS [6] schemes in Rb vapors. Radial variation of LG mode induces narrowing of Doppler-free transparencies profiles. Chanu et al. [1] demonstrated decrease of resonances linewidths of Rb of almost factor 6 switching from Gaussian to LG mode. For [5], LG modes contribute to increase the Rb Zeeman level lifetime and so decrease the FWHM. Furthermore, influence of LG modes order has also been studied in [2] demonstrating that larger order induces narrower signal due to its azimuthal component and authors of [4] present the influence of transverse mode overlap (pump and probe) on the signal. Gas-filled HCPCF technology already demonstrated its potential and impact on the enhancement of the light-gas interactions phenomena and especially in spectroscopy. The various works deriving from the seminal demonstration [7] have been limited to pump (or pump-probe) Gaussian beam configurations due to the fact that most of the developments on HCPCFs have been focused on the design and fabrication of fibers supporting the lowest loss LP01 like mode with the main objective to increase transmission performances. Moreover, the intrinsic cladding modal properties of microstructured hollow-core fibers render difficult the control on the loss mode hierarchy and to guide light in a high order single mode fashion. This restriction has been lifted by the emergence of the tubular cladding HCPCF providing a large flexibility on the tubes cladding position. Thanks to alteration of the azimuthal position of the cladding tubes, Osorio et al. [8] demonstrated, theoretically and experimentally, for the first time in a HCPCF, an inversion of the classical loss hierarchy in fibers developing fibers whose lowest loss modes are the LP11 and LP21, instead of the LP01. In this study, we specifically design and fabricate a fiber based on this geometry allowing to guide preferentially high order mode (HOM) around 1550nm without any specific light coupling precaution, we report for the first time to the best of our knowledge on SA spectroscopy generated in HCPCF with a LP11 high-order mode control beam. Comparison with Gaussian control beam configuration demonstrates an increase of the contrast and peak position shift. |
