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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

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.

The advent of photonic bandgap (PBG) guiding hollow- core photonic crystal fiber (HC-PCF) sparked the hope of guiding light with attenuation below the fundamental silica Rayleigh scattering limit (SRSL) of conventional step-index fibers. Unfortunately, the combination of the strong core-cladding optical-overlap, the surface roughness at the silica cladding struts and the presence of interface-modes limited the lowest reported transmission-loss to 1.2 dB/km at 1550 nm. This hope is recently revived by the introduction of hypocycloid core- contour (i.e. negative curvature) in inhibited-coupling (IC) guiding HC-PCF, and the reduction of their confinement loss to a level that makes them serious contenders for light transmission below the SRSL in UV- VIS-NIR spectral range. Here, we report on several IC guiding HC-PCFs with a hypocycloid core-contour and a cladding structure made of a single ring from a tubular lattice. The fibers guide in the UV-VIS and NIR, and among which we list one with a record transmission loss of 7.7 dB/km at ~750 nm (only a factor ~2 above the SRSL), and a second with an ultra-broad fundamental-band with loss in the range of 10-20 dB/km spanning from 600 to 1200 nm. Both fibers present near-single mode guidance and very low bend loss sensitivity. The results show that the limit in the transmission is set by confinement loss for wavelengths longer than ~1 {\mu}m and by surface- roughness for shorter wavelengths, thus indicating that transmission loss well below the SRSL in the visible is possible with a surface roughness reduction and would open the possibility of the first UV low-loss light- guidance.

Optica 3(8), 816 - 822(2016). doi:10.1364/OPTICA.3.000816, Over the last decade, the development of ultrafast laser pulses in the mid-infrared (MIR) region has led to important breakthroughs in attosecond science and strong-field physics. However, as most such broadband MIR laser sources are near-IR pumped, the generation of high-intensity, long-wavelength MIR pulses is still a challenge, especially starting from picosecond pulses. Here we report, both experimentally and numerically, nonlinear pulse compression of sub-millijoule picosecond pulses down to sub-300 fs at 2050 nm wavelength in gas-filled Kagome-type hollow-core photonic crystal fibers for driving MIR optical parametric amplifiers. The pump laser is comprised of a compact fiber laser-seeded 2 μm chirped pulse amplification system based on a Ho:YLF crystal at 1 kHz repetition rate. Spectral broadening is studied for different experimental conditions with variations of gas pressure and incident pulse energies. The spectrally broadened 1.8 ps pulses with a Fourier-limited duration of 250 fs are compressed using an external prism-based compressor down to 285 fs and output energy of 125 μJ., Published by OSA, Washington, DC

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Since the advent of atom laser-cooling, trapping or cooling natural molecules has been a long standing and challenging goal. Here, we demonstrate a method for laser-trapping molecules that is radically novel in its configuration, in its underlined physical dynamics and in its outcomes. It is based on self-optically spatially-nanostructured high pressure molecular hydrogen confined in hollow-core photonic-crystal-fibre. An accelerating molecular-lattice is formed by a periodic potential associated with Raman saturation except for a 1-dimentional array of nanometer wide and strongly-localizing sections. In these sections, molecules with a speed of as large as 1800 m/s are trapped, and stimulated Raman scattering in the Lamb-Dicke regime occurs to generate high power forward and backward-Stokes continuous-wave laser with sideband-resolved sub-Doppler emission spectrum. The spectrum exhibits a central line with a sub-recoil linewidth of as low as 14 kHz, more than 5 orders-of-magnitude narrower than in conventional Raman scattering, and sidebands comprising Mollow triplet, molecular motional-sidebands and four-wave-mixing., Comment: 28 pages 1-12 for main manuscript 13-28 for Methodes and appendices 4 figures for Main manuscript 12 figures for the Methods part

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Last years have seen the emergence of a new class of hollow-core photonic crystal fibers (HC-PCF) no longer relying on photonic bandgap guidance but on inhibited-coupling (IC) mechanism. The first specie of this fiber family took the form of Kagome-lattice [1]. The guiding rule for enhancement of such a guidance (minimizing the coupling between the core mode and cladding mode, i.e (φ clad |Δn 2 | φ core ) → 0) led to the introduction in 2010 of hypocycloidal (or negative curvature) core contour [2] in Kagome HC-PCFs where record loss values (8.5 dB/km at 1 μm [3]) have been demonstrated. Thanks to this design, three reasons explain the reduction of such a criterion: i) the HE 11 mode field spatial overlap with silica is reduced to the tangent sections of the inner cups of the hypocycloid contour, ii) the larger perimeter of the hypocycloid contour results in a higher azimuthal-like number in the silica core-surround modes and hence, stronger transverse phase-mismatch with the core mode. iii) the spatial overlap between the core-mode with connecting nodes supporting low azimuthal number modes is strongly reduced. Recently, another hollow-core fiber design has been proposed that fully fulfils the mentioned features for IC guidance, relying on a tubular lattice with a one ring of isolated (with no connecting nodes) thin glass tubes [4]. Here, we report on the fabrication and characterization of ultra-low loss single-ring tubular lattice HC-PCFs (SR HC-PCF) guiding m the UV-VIS-NIR with absolute record loss value for HC-PCF at 750 nm with 7.7 dB/km, supplanting bandgap fiber performances in this spectral range.

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