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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.

A resonant fiber ring cavity based on a photonicbandgap hollow-core fiber is described and characterized. We explore the fine spectral dependence of the transmission losses of this cavity and their impact on the performances of the resulting resonator fiber optic gyroscope (RFOG).

International audience; Ce travail rapporte la conception, la fabrication et la caractérisation d'une fibre optique mircrostructurée air/silice effilée permettant une conversion de fréquence large bande par le processus de mélange à quatre ondes vers le moyen infra-rouge. Nous démontrons expérimentalement une bande de gain paramétrique de près de 100 nm. Une utilisation envisagée de ce taper réside dans le développement de sources laser femtosecondes.

International audience; In this communication we present results dealing with the precise management of dispersion coefficients into two highly nonlinear fiber structures (a tapered PCF and an all-solid fiber) for far-detuned frequency conversion towards 2µm.

While optical fibers display excellent performances in the infrared, visible and ultraviolet ranges remain poorly addressed by them. Obtaining better fibers for the short-wavelength range has been restricted, in all fiber optics, by scattering processes. In hollow-core fibers, the scattering loss arises from the core roughness and represents the limiting factor in reducing their loss regardless of the fiber cladding confinement power. To attain fibers performing at short wavelengths, it is paramount developing means to minimize the height variations on the fiber microstructure boundaries. Here, we report on the reduction of the core surface roughness of hollow-core fibers by modifying their fabrication technique. In the novel process proposed herein, counter directional gas fluxes are applied within the fiber holes during fabrication to attain an increased shear rate on its microstructure. The effect of the process on the surface roughness has been quantified by optical profilometry and the results showed that the root-mean-square surface roughness has been reduced from 0.40 nm to 0.15 nm. The improvement in the fiber core surface quality entailed fibers with ultralow loss in the short-wavelength range. We report on fibers with record loss values as low as 50 dB/km at 290 nm, 9.7 dB/km at 369 nm, 5.0 dB/km at 480 nm, and 1.8 dB/km at 719 nm. The results reveal this new approach as a promising path for the development of hollow-core fibers guiding at short wavelengths with loss that can potentially be orders of magnitude lower than the ones achievable with their silica-core counterparts.

In this communication, we will first review the recent advances of hollow-core photonic crystal fibers. Then, the possibility offered to tailor their optical properties by making tapers will be discussed.

International audience; We numerically demonstrate a broadband parametric gain around 2 µm using a tapered Photonic crystal fiber.

We demonstrate strong and stable phase-locking between the pump and the quantum generated Stokes/anti-Stokes spectral-pair in a comb generated in the regime of a single spatio-temporal coherent mode transient stimulated Raman scattering in H2-filled HCPCF.

International audience; We report on a nonlinear compression down to 20 fs in air-filled HCPCF. Novel spectral-temporal dynamic is observed and analysed. The results represent a promising pathway for strong and stable compression of current commercial ultra-short-pulse lasers

International audience; We report on the development of new state-of-the-art ultralow-loss hollow-core fibers guiding in the short-wavelength range. The new record low attenuation values are as low as 50.0 dB/km at 290 nm, 9.7 dB/km at 369 nm, 5.0 dB/km at 480 nm, 0.9 dB/km at 558 nm, and 1.8 dB/km at 719 nm. Notably, the core roughness levels of the new state-of-the-art fibers are under the thermodynamic equilibrium limit set by the surface capillary waves scenario in amorphous silica.

International audience; This communication will review our recent developments in inhibited-coupling hollow-core fibers, ranging from understanding the optical guiding mechanism to new state-of-the-art performances. These results are expected to have strong impacts in a new landscape of research and technology.

National audience; Nous présentons les premiers résultats d'amplification par fibres optiques coniques dopées par des ions Er 3+ réalisées par la voie poudre. Cette architecture de fibre spéciale permet la conservation du comportement monomode spatial et l'amplification à forte énergie d'impulsions brèves sub-MW.

International audience; We report on a nonlinear compression down to 20 fs in air-filled HCPCF. Novel spectraltemporal dynamic is demonstrated. Spectral bouncing of the compressed pulse is triggered by the interplay between Raman redshift and plasma blueshift.

International audience; We propose a hybrid Kagome-tubular lattice hollow-core fiber for ultra-low loss and single-mode operation. The fiber displays a minimum loss of 1.6dB/km at 1050nm and a higherorder modes extinction of 47dB for a 10m-long fiber.

We report on hollow-core fibers displaying rms core roughness under the surface capillary waves thermodynamic limit and record loss in the short-wavelength range (50.0 dB/km at 290 nm, 9.7 dB/km at 369 nm, 5.0 dB/km at 480 nm, 0.9 dB/km at 558 nm, 1.8 dB/km at 719 nm)., 5 pages, 3 figures

Remarkable recent demonstrations of ultra-low-loss inhibited-coupling (IC) hollow-core photonic-crystal fibres (HCPCFs) established them as serious candidates for next-generation long-haul fibre optics systems. A hindrance to this prospect and also to short-haul applications such as micromachining, where stable and high-quality beam delivery is needed, is the difficulty in designing and fabricating an IC-guiding fibre that combines ultra-low loss, truly robust single-modeness, and polarisation-maintaining operation. The design solutions proposed to date require a trade-off between low loss and truly single-modeness. Here, we propose a novel IC-HCPCF for achieving low-loss and effective single-mode operation. The fibre is endowed with a hybrid cladding composed of a Kagome-tubular lattice (HKT). This new concept of a microstructured cladding allows us to significantly reduce the confinement loss and, at the same time, preserve truly robust single-mode operation. Experimental results show an HKT-IC-HCPCF with a minimum loss of 1.6 dB/km at 1050 nm and a higher-order mode extinction ratio as high as 47.0 dB for a 10 m long fibre. The robustness of the fibre single-modeness is tested by moving the fibre and varying the coupling conditions. The design proposed herein opens a new route for the development of HCPCFs that combine robust ultra-low-loss transmission and single-mode beam delivery and provides new insight into IC guidance., Hollow-core photonic crystal fibres: Low loss with hybrid-lattice cladding A hybrid microstructured cladding significantly reduces confinement loss and preserves single-mode operation in hollow-core photonic crystal fibres. The hybrid cladding was conceptualised and designed by Fetah Benabid of XLIM Institute, CNRS, and colleagues in France and Italy. It is made of an inner cladding of six tubes connected to an outer basket weave-like ‘Kagome’ lattice via a number of small, thin tubes. Hollow-core photonic crystal fibres surrounded by this hybrid cladding demonstrated a minimum loss figure of 1.6 decibels per kilometre at wavelengths of 1050 nanometres with robust single-mode operation in a ten-metre-long fibre. The design demonstrates the concept’s potential for single-mode waveguides and could contribute towards realising the next generation of long-haul optical fibres.

International audience; We report on the generation of pulse trains from a hydrogen-filled hollow-core fiber pumped by 10 ps pulses for wave synthesis. Experimental results show ultrashort pulse trains separated by 57 fs with 20 fs duration.

International audience; We propose an ultra-compact double wavelength source 266-289 nm developed for DIAL LIDAR of Ozone in the troposphere thanks to a hollow-core photonic crystal fiber filled in with Deuterium.

International audience; We report on simultaneous CARS and two-photon excited fluorescence imaging using tailored double-clad hypocycloid core-contour Kagome fiber optimized for broad bandwidth lowloss, high bioimaging collection efficiency and reduced bend loss sensitivity.

International audience; Generation of sub-Doppler spectral lines in compact devices is very useful in bringing functionalities such as laser-frequency stabilization, slow light application or quantum sensing to a wider and friendlier use. An all-fiber Photonic MicroCell (PMC), which consists of a gas-filled hollow core photonic crystal fiber (HCPCF) hermetically spliced to a solid optical fiber [1], is one of the most promising of such devices because of its ultra-long interaction length. Thus far, the PMC assembly processes are either limited to a short range of HCPCF core diameter [2], or entails the use of glue [3], or exhibit high injection losses (~10dB) [4]. Here, we report on a 7-meter contaminant free and single-mode C2H2-PMC. The process consists of end-capping a 80µbar acetylene-filled HCPCF via heat-collapsing a fused borosilicate sleeve on the two HCPCF tips. The process requires no helium gas [2] nor glue [3], which are potential pollution sources, and keeps the physical integrity of the HCPCF intact. The used HCPCF exhibits a novel cladding lattice and operates in a robust single-mode fashion [5]. The fiber has a 30 µm core diameter, a 10dB/km transmission loss at 1540 nm. The resulted PMC has polished and mounted ends to FC/APC connectors for integration (Fig. 1(a)), exhibits a total insertion loss of ~2dB and a motion-resistant single-mode guidance (see reconstructed near field in Fig. 1 (b)). The PMC was then used to generate saturable absorption spectroscopy (SAS) measurement (Fig. 1 (b-c))). The observed saturable absorption peak reaches 25% contrast and 20MHz linewidth (FWHM) for ~190mW of pump power that is the highest observed contrast in a fully sealed PMC [6]. The evolution of the SA peak characteristics over pump power are represented in Fig. 1 (e), and shows for pump power of less than 100 mW a transit-time limited linewidth (19MHz), thus indicating no additional collisional dephasing that could be induced by contaminant during the assembly process. The linewidth was monitored over 3 months for 23 mW pump power with no significant variation (Fig.1(f)), demonstrating thus its hermeticity, and its suitability for long time and mobile applications. Fig. 1. (a) FC/APC patch-cord like final PMC. (b) Near field intensity profile at the output of polished PMC. (c) Normalized SAS measurement on P9 absorption line of acetylene (1530.371nm) in purple line, simulation of SAS signal for 31 µbar pressure in dashed line. (d) SA contrast zoomed in SA peak with numerical calculation in dashed line. (e) Experimental (dots) and calculated (line) SA transparency peak contrast and linewidth evolution with pump power. The dashed line correspond to the transit time limit. (f) Evolution of SA FWHM during 95 days for a pump power of 23mW.

International audience; We report on inhibited-coupling hollow-core optical fibers exhibiting specific modal operation and low loss in the short-wavelength range. We show that judicious choice of the fiber cladding structure allows obtaining fibers that favor the propagation of higher-order modes and fibers which combine effective single-mode operation and low loss. Moreover, we report on recent improvements in hollow-core fibers with ultralow loss in the visible and ultraviolet range.

International audience; We demonstrate a technique for compensation of the idler angular chirp from an optical parametric amplifier in non-collinear geometry. This method enables generating broadband CEP- stable pulses for seeding high-energy optical parametric amplifiers.

We measure the light backscattered by resonant optical cavities based on two types of hollow-core photonic bandgap fibers, namely, 7-cell and 19-cell fibers. The measurement of the intensity backscattered by the cavity at resonance permits to deduce the value of the fiber backscattering coefficient. We find backscattering coefficients of the order of 2.0 × 10-6 and 1.0 × 10-6 m-1 for the 7-cell and 19-cell fibers, respectively, two orders of magnitude larger than the one for standard solid-core single-mode fiber.

International audience

International audience; We report on a compact multi-line Raman source with a spectrum spanning from 250nm to 750nm with spectral densities ranging from 0.2 to 4µW/nm between 250-300nm and 20 to 700 µW/nm for 300-350 nm.

International audience; We demonstrate a novel energy-efficient method for seeding CEP-stable OPCPAs. We couple the CEP-stable idler of a broadband OPCPA into a Kagome fiber thus compensating for its angular chirp. We show the pulse compressibility. OCIS codes: 190.4970 Parametric oscillators and amplifiers; 060.4005 Microstructured fibers The quest for isolated attosecond pulses (IAPs) in the water window drives the research of carrier-envelope phase (CEP) stable high-energy, short-pulse sources in the infrared. The spectral region around 2-m is a good compromise between high harmonic generation (HHG) efficiency and cutoff extension for reaching the water window spectral region. To produce few-cycle pulses at 2-m, necessary for driving IAPs, the preferred optical scheme consists in pumping optical parametric chirped pulse amplifiers (OPCPAs) with 1-m lasers. One difficulty of this approach is the generation of a few-cycle CEP-stable seed starting from a non-CEP-stable laser. Degenerate optical parametric amplifiers (OPAs) or OPCPAs with type-I -barium borate (BBO) crystals are able to produce few-cycle signal and idler pulses, in which the idler has a stable CEP [1]. Such broadband OP(CP)As can be generated either collinearly or non-collinearly. In a collinear setup, the idler is angular-chirp free, it is spatially overlapped with the signal, has the same spectral content and the same polarization, but a different CEP. Therefore it is not possible to spatially separate the CEP-stable idler from the non-CEP-stable signal beam. In a non-collinear setup, the idler has a very strong angular chirp due to different phase matching for each signal-idler wavelength pair. In both cases, the idler cannot be used as a seed for successive amplification stages. An optical design [2,3] was introduced to address this problem, consisting in using a narrowband collinear or non-collinear OP(CP)A with signal and idler at different spectral regions with a negligible angular chirp compared with the natural beam divergence of the idler. The CEP-stable idler or its second harmonic then pumps a CEP-stable broadband supercontinuum which can be used as a seed for successive amplification stages. This method was used in several works and it is of great advantage in particular for optical parametric synthesizers due to the possibility to produce a CEP-stable seed over multiple octaves [4,5]. In this work, we introduce a novel scheme which allows saving pump energy for the generation of a few-cycle CEP-stable seed for a single channel OPCPA chain. The method consists in using a broadband non-collinear OPCPA with a strongly angularly-chirped idler and focusing it into a Kagome fiber [6]. The Kagome fiber is used in a linear regime and it homogenizes the wave-vector directions of all idler wavelengths, similarly to the technique used for homogenizing diode lasers [7]. At the output of the fiber, the CEP-stable broadband idler has a well-defined propagation direction, i.e. no angular chirp, and can be directly used to seed the following amplification stages. We believe that our technique is simpler than other techniques developed to compensate for the idler angular chirp of an OPA [8,9]. In our setup (Fig. 1a) we utilized 308 J of total pump energy from the same home-built Yb:KYW regenerative amplifier [10] used in [5] and built a 2-stage degenerate OPCPA with BBO crystals. The idler output energy of the second OPCPA stage is 10.6 J and it has a strong angular chirp which we measured to be ~3.6°. We focused the idler into a Kagome fiber with ~22-cm length (7-cell core design with 63 core diameter). The fiber output energy is 3.8J, which can be further improved by optimizing the coupling efficiency (currently 36%) into the Kagome fiber. The spectra of the idler before and after the Kagome fiber are shown in Fig. 1 (b). The fiber output has no residual detectable angular chirp.

The development of silicon-core fibers have drawn strong interest over the last decade. Indeed, silicon is a material offering interesting properties such as mid-wave infrared transparency (1–7 μm), high refractive index (3.48 @ 1.55 μm) and large third order non linearity (≈ 1.5×10−13 cm2/W). Until now, three fabrication processes have been developed to achieve glass-clad silicon-core fibers: the high-pressure microfluidic chemical deposition method [1], the molten core method from silicon rods [2] and the powder-in-tube method [3]. This last one shows a very high potential to achieve hybrid optical fibers as it allows not only to use a wide range of materials but it is also compatible with the stack-and-draw process; in this way, many different designs of fibers can be especially drawn. Unfortunately, fibers fabricated by this method are very short, only a few centimetres long because of the mechanical stresses accumulation at the interface between the two materials which have different thermal expansion coefficients (Si: 3×10−6 K−1; SiO 2 : 5×10−7 K−1), and because of the presence of air between the grains.

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.