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We report on the development of all-fiber stand-alone iodine-filled photonic microcells demonstrating record absorption contrast at room temperature. The microcell’s fiber is made of inhibited coupling guiding hollow-core photonic crystal fibers. The fiber-core loading with iodine was undertaken at 10−1-10−2mbar vapor pressure using what, to the best of our knowledge, is a novel gas-manifold based on metallic vacuum parts with ceramic coated inner surfaces for corrosion resistance. The fiber is then sealed on the tips and mounted on FC/APC connectors for better integration with standard fiber components. The stand-alone microcells display Doppler lines with contrasts up to 73% in the 633 nm wavelength range, and an off-resonance insertion loss between 3 to 4 dB. Sub-Doppler spectroscopy based on saturable absorption has been carried out to resolve the hyperfine structure of the P(33)6-3 lines at room temperature with a full-width at half maximum of 24 MHz on the b4 component with the help of lock-in amplification. Also, we demonstrate distinguishable hyperfine components on the R(39)6-3 line at room temperature without any recourse to signal-to-noise ratio amplification techniques.

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.

International audience; We report on the use and qualification of the recently developed single-mode hybrid hollow-core photonic crystal fiber for a robust and long-life acetylene photonic micro-cell based optical frequency reference.

Gas-filled Hollow Core Photonic Cristal Fibres (HCPCF) or Photonic MicroCell (PMC) [1] has proven to offer an outstanding gas-cell for laser metrology and spectroscopy applications. Sub-Doppler spectral features, generated via electromagnetically induced transparency (EIT) or saturated absorption (SA), have been generated in molecular gases despite the weak strength of their optical transitions [1–4], and an optical frequency reference with excellent accuracy has been demonstrated [1]. So far, the reported stand-alone PMCs are either made by hermetically splicing the HCPCF tips to an all-solid optical fibre [1,2] or by encapsulating them to a glass cell [3]. All these techniques show limitations. For example, the splicing based all-fibre PMC technique is not adapted to all fibres as it requires matching the outer-diameter and/or mode-field-diameters of the two fibres. Also the use of fibre tapering to reduce this mismatch [2] remains too cumbersome for the obtained splice-loss figures, which are >2 dB and exhibit asymmetric loss. Furthermore, the use of Helium (He) gas during the "air-ambient" splicing process to avoid air pollution in the fibre core renders this technique too sensitive to residual gas contamination. Conversely, the technique of encapsulating the fibre tip in an enlarged borosilicate capillary [3] presents potential contamination during and after fabrication because of the use of Epoxy glue, and the ensued outgassing, which is detrimental for optimized observation of sub-Doppler features and for the life time of the PMC.

In this paper, we report on the ultrafast laser-induced birefringence, refractive index changes, and enhanced photoluminescence properties in the volume of neodymium (Nd), yttrium (Y) co-doped strontium fluoride (SrF

International audience; High refractive index contrast, birefringence and luminescence properties of written channel waveguides inside fluoride crystals written by femtosecond (fs) laser have been studied. Herein, fs laser-induced stress affected zone is an efficient tool to create birefringence and high refractive index contrast in Nd 3+ , Y 3+ codoped SrF 2 crystal for wide range of pulse energies and repetition rates. In fabricating the waveguides at high repetition rate (typ. 500 kHz), we avoided too much heat accumulation, and thus they exhibit lower propagation loss in codoped SrF 2 crystal (1.63 ± 0.21 dB/cm for TM-polarization). The measured retardance can be interpreted as related to stress-induced birefringence in response to free of stress volume expansion photo-induced in the non-spherical irradiated zone. The photoluminescence and lifetime measurements are also carried out in order to understand local changes of the network in and around femtosecond laser induced waveguides written in codoped SrF 2 crystal.

In this paper, we report on the ultrafast laser-induced birefringence, refractive index changes, and enhanced photoluminescence properties in the volume of neodymium (Nd), yttrium (Y) co-doped strontium fluoride (SrF2) and Nd, Y co-doped calcium fluoride (CaF2) crystals. The optical waveguides written with 500 kHz repetition rate provided lowest propagation loss of 1.63±0.21 dB cm−1 for transverse magnetic (TM) polarization at 632.8 nm in Nd,Y:SrF2 crystal. The measured retardance can be interpreted by stress-induced birefringence related to the permanent volume expansion, photo induced by a non-spherical irradiated zone. The absorption, steady-state, and time-resolved photoluminescence properties are also carried out in and out of the laser irradiated zone, enabling the local changes of the Nd and Y network in Nd,Y:SrF2, as well as well-preserved Nd fluorescence in the written optical waveguides.

We report on the development of an acetylene-filled photonic microcell based on an assembly process that is contaminant free and requires no helium buffer gas nor gluing procedure. The microcell consists of a 7-m-long and 30 µm core-diameter inhibited-coupling guiding hollow-core photonic crystal fiber filled with acetylene gas at a pressure in the range of 80 µbar, sealed by capping its ends with fusion-collapsing a glass-tube sleeve, and mounted on FC connectors for integration. The microcell shows a robust single-mode behavior and a total insertion loss of ∼ 1.5 d B . The spectroscopic merit of the formed microcell is tested by generating electromagnetic induced transparency and saturated absorption on R13 and P9 absorption lines, respectively. The sub-Doppler transparencies show a close to transit time limited linewidth of 17 ± 3 M H z . The latter was monitored for over 3 months. As a demonstration, the microcell was used to frequency stabilize a laser with fractional frequency instability improvement by a factor 50 at 100 s integration time compared to free running laser operation.

We reported on the femtosecond (fs) laser induced permanent refractive index changes, stress-induced birefringence, and micro-photoluminescence properties of Nd3+, Y3+ codoped CaF2 crystals. Permanent changes in the linear optical properties were studied for various pulse energies (0.01 - 5 µJ) at two different repetition rates (10 kHz and 500 kHz). Optimum quantitative phase changes are close to - 1.7π radians within an irradiated area, surrounded by positive phase changes due to the stress field that can reach + 0.7π. Provided that, we avoid heat-accumulation at 500 kHz, written laser lines offer higher linear birefringence on the order of λ/4 at 200 nm. From the spectroscopic point of view, we observed the Raman changes and micro-photoluminescence spectra of irradiated fs-laser Nd3+, Y3+ doped CaF2.

We have investigated that femtosecond laser induced optical waveguides in AF32 and in Borofloat-33 glasses. Type-I positive and type-II negative refractive index change mechanisms were established in AF32 and in Borofloat-33 glass waveguides, respectively. Lowest propagation loss of 1.1 ± 0.31 dB/cm could be attained in AF32 glass, which has a typically higher index change than in Borofloat-33 glass. Resultant losses are directly correlated with densification and non-bridging oxygen hole centers, which can be authenticated by Raman and photoluminescence studies.

International audience; A standalone FC/PC connectorized and sealed Iodine filled hollow core fiber with coupling efficiency as high as 75% and an absorption contrast reaching 65% on the P(33) 6-3 transition at room temperature is demonstrated.

Harsh environmental sensing using fiber Bragg grating (FBG) technologies is a rapidly growing field of research. A new generation of devices that operate in extreme environments (particularly those operating above 800°C) including environments with additional challenges such as radiation and intense optical fields (e.g. high power lasers), must be capable of withstanding gradual annealing and degradation, or aging, over time while preserving their intrinsic advantages (multiplexing capabilities, electromagnetic immunity, low intrusivity, mechanical reliability) in comparison to thermocouples. Of particular local relevance these include live diagnostics with optical sensor embedded into high-temperature composites for aircraft, helicopters and space infrastructure, temperature and pressure sensing in increasingly deeper and hotter oil bores, and the monitoring and mapping of temperature distribution at power plants, furnaces and chemical reactor. Much of the answer is related to inherent glass structure and properties, particularly viscous flow, chemical migration and stress relaxation. Thermal stability may therefore be achieved by judicious application of glass preparation, glass annealing and glass laser irradiations. FBG regeneration process [1] is the only current approach that can enable photonic technologies to operate in such harsh environments at elevated temperatures. In this paper we investigated the mechanisms of FBG thermal regeneration and in particular glass relaxation processes using micro-Raman, surface topography measurements and SEM.