We present a machine-learning approach to generate and dynamically reconfigure orbital angular momentum (OAM) beams in a multimode inhibited-coupling hollow-core photonic crystal fiber (IC-HCPCF). To achieve this, we trained a neural network-based digital twin for a 139 µm core IC-HCPCF guiding over 60 LP-like modes per polarization in the green spectral range. The accuracy of the neural network was confirmed by the high fidelity between experimental and predicted output intensity patterns in Fourier-conjugate planes, with median Pearson correlation coefficients exceeding 98%. These results highlight multimode IC-HCPCFs as a versatile platform for structured light transport and on-demand beam shaping.

Hollow-core photonic crystal fibers (HCPCF) have recently matured into a versatile platform for light guidance, offering properties such as ultralow loss and specific modal operation. By modifying the HCPCF cladding symmetry, it is possible to control the modal loss hierarchy and facilitate the propagation of specific higher-order modes. In this context, we here demonstrate the realization of an angle sensor utilizing a tubular HCPCF with a two-fold symmetric cladding. This specific fiber design enables the generation of an output intensity profile resulting from the superposition of LP01 and LP11-like modes, whose excitation and resulting output intensity distribution are dependent on the polarization angle of the incident light. Thus, by rotating the input beam's polarization and analyzing the evolution of the resulting output profile, we characterized a polarization rotation angle sensor exhibiting a sensitivity of (25 +/- 1) counts/degree and an estimated resolution of 0.4 degrees. We understand that this work broadens the framework of HCPCF applications, demonstrating that symmetry-modified hollow-core fibers can act as a promising platform for advanced sensing scenarios and polarimetric characterizations.

Efficient light transmission in the blue-visible regime remains a major limitation for fiber-fed astronomical spectrographs, where low photon flux and the intrinsic attenuation of conventional silica fibers reduce survey sensitivity and depth. Inhibited-coupling hollow-core fibers (IC-HCFs) with reduced surface roughness offer a promising alternative, providing guidance predominantly in air and enabling significantly lower loss across the visible spectrum. In this study, we present a comparative evaluation of IC-HCFs against standard multi-mode fibers used in current astronomical instrumentation. We assess the throughput loss that occurs due to bending, twisting, or pinching of the optical fibers when moved using one of the robotic fiber-positioner prototypes designed for next-generation telescopes. These measurements quantify the performance gains offered by IC-HCFs for blue-sensitive spectroscopy and assess their suitability for integration into future survey facilities.

Hollow-core photonic crystal fibers (HCPCF) have experienced tremendous advancements recently, leading to remarkable demonstrations of transmission loss reduction and deeper understanding of their guidance fundamentals. Indeed, this progress has entailed investigations into various HCPCF designs, allowing for attaining diverse properties of interest such as ultralow loss, polarization filtering, and specific modal operation. Among HCPCFs with tailored modal characteristics are fibers displaying microstructures with modified symmetry, which allow for changing the loss hierarchy between the guided modes, hence favoring the propagation of higher-order modes. In this context, we here demonstrate the realization of an angle sensor utilizing a tubular HCPCF with a two-fold symmetric cladding. This specific fiber design enables the generation of an output intensity profile resulting from the superposition of LP01 and LP11-like modes, whose excitation and resulting output intensity spatial distribution are dependent on the polarization angle of the incident light. Thus, by rotating the input beam's polarization and analyzing the evolution of the resulting output profile, we characterized a rotation angle sensor exhibiting a sensitivity of 25 counts/degree and an estimated resolution of 0.3°. We understand that this work broadens the framework of HCPCF applications, demonstrating that symmetry-modified hollow-core fibers can act as a promising platform for advanced sensing scenarios and polarimetric characterizations.

We report on the design and characterization of a picometer-resolution optical profilometer with a large dynamic range for assessing the surface roughness of the elements forming the microstructure of hollow-core photonic crystal fibers (HCPCFs). The characterization of the device allowed us to demonstrate a sensitivity of 2pm/Hz and, remarkably, a dynamic range of 1 cm. In addition, we were able to demonstrate the applicability of the profilometer in measuring the surface roughness levels inside different HCPCF designs. Our results validate the profilometer operation and reveal this device as a powerful tool to be integrated into HCPCF development chains.

We report on the development and characterization of an all-fiber photonic microcell (PMC) filled with iodine vapor, a compact and integrable frequency reference for optical metrology and precision timekeeping. The PMC's hollow-core photonic crystal fiber provides unprecedently high absorption contrast of iodine molecules at room temperature and 100 bar pressure-level thanks to combining the advantages of long optical interaction length and small modal area, resulting in an exceptionally high signal-to-noise ratio (SNR). Despite the relatively strong wall-collision broadening, an external cavity diode laser locked to a sub-Doppler transition of the PMC iodine exhibits an error signal-based Allan deviation of 7 x 10(-14) at 1 second integration time. The PMC manufacturing set-up is scalable for mass production. Given the increasing relevance of iodine-based frequency references for space, marine exploration, and on-the-field atomic clocks, this work offers a mass-manufacturable, fiber-integrated alternative to conventional bulk iodine vapor cells, our approach paves the way for more robust and practical implementations of iodine-based optical frequency standards.

This paper reviews the field of extreme nonlinear optics in optical fibers, highlighting key phenomena and advancements. It discusses multiple ionization effects caused by femtosecond laser pulses that generate plasma and induce permanent material modifications, as well as plasma luminescence and its dependence on material imperfections. The formation and dynamics of plasma filaments, including helical structures, are explored, along with the rainbow spiral emission pattern useful in communications and particle manipulation. The review covers the generation of spatial-temporal waves, supercontinuum broadening, and advanced modeling techniques, such as multimode unidirectional pulse propagation equations for describing optical pulse evolution. Experimental demonstrations involving discretized conical waves and supercontinuum generation optimization are detailed. The paper emphasizes the unique capabilities of photonic crystal fibers, especially hollow-core variants, in achieving broad supercontinua and Raman frequency combs, ultrashort pulse compression, high-harmonic generation, plasma formation, and nonclassical light production. Our outlook highlights ongoing research into spatiotemporal helicon waves, ultrashort pulse propagation, vacuum ultraviolet and mid-infrared supercontinuum generation, and innovative fiber technologies. Future directions focus on enhancing fiber performance, understanding multimodal wave dynamics, and expanding applications in telecommunications, sensing, and quantum science.

We report a new generation of HCPCFs demonstrating ultralow loss in the short-wavelength domain by the use of new fabrication process. The process consists of applying a shear stress on the fiber silica core-surround surface. This approach allows a significant silica surface roughness reduction demonstrated by using a home-made picometer-resolution and high dynamic range profilometer. The surface quality improvement is further corroborated by record optical performances measured in the UV spectral range with losses between 50-10 dB/km in the wavelength range of 280-400 nm. As a demonstration of the interest of such fibers, UV laser beam delivery at specific industrial wavelengths including 257/266 nm and 343/355 nm are presented.

We report on the development of inhibited-coupling hollow-core photonic crystal fiber with record-low transmission loss of ⪅50 dB/km at 266 nm, and solarization-free. 2 meter-long patchcords with SMA terminations were made and their UV handling and lifespan were tested. A laser beam from a 266 nm wavelength laser-source emitting 1 ns wide and 30 μJ energy pulses was injected into the hollow-core fiber patchcord, achieving a total transmission rate of 93%. The laser beam was kept continuously coupled to the patchcord for over 100 hrs. The results show excellent stability in transmitted power (fluctuations of less than 2.6%) and in mode quality. To our knowledge this the first fiber guidance of DUV laser that combines high energy handling and long lifespan. The results mark a major milestone in the adoption of fiber UV laser beam delivery by the industrial community.

To optimize the use of hollow-core photonic crystal fibers (HC-PCF), their cores are filled with an atomic gas for an ultra-enhanced interaction with an incident laser beam in applications such as atomic vapor microcells. One challenge in these gas-filled HC-PCFs is to control the physiochemical interactions between the gas medium and the silica inner surface of the fiber core surround. In this work, thus, the processing of ceramic coatings on glass substrates by chemical solution deposition is focused on. Also, the successful implementation of an original coating procedure for a deposition inside hollow-core fibers with complex microstructures is described. It is indeed possible to form a thin, dense, inorganic, and amorphous layer with a low thickness, low roughness, and high transparency. To obtain such a result, several parameters must be controlled, including the concentration of the solution, the technique and the deposition time, as well as the heat treatment undergone by the fiber. In particular, the selected aluminosilicate coatings, which are nonporous and present a 20-30 nm thickness, demonstrate a considerable improvement of the lifetime properties of the fibers filled with rubidium vapor, without modifying its original guiding properties. In this work, the deposition of ceramic coatings at the inner core of hollow-core photonic crystal fibers, the goal being to avoid gas/surface interactions and increase their lifetime, is described. The thin, dense, inorganic, and amorphous films are deposited from sol-gel-based solutions. In addition, their transmission is not different from the original fiber, demonstrating that they can be used as protective coatings in atomic vapor microcells.image (c) 2024 WILEY-VCH GmbH

We report on a Raman laser emitting in the yellow spectral range using a CO2-filled hollow-core photonic crystal fiber. Taking advantage of a state-of-art inhibited-coupling hollow-core photonic crystal fibre, exhibiting minimum transmission loss of similar to 1 dB/km in the 500-600 nm region, we were able to develop an extremely compact and simple yellow-Raman laser scheme, allowing to emit as much as 60 mW of average power at the 574.5 nm wavelength while using a compact, microchip laser as a pump source. This solution provides an innovative and scalable alternative for the other yellow laser schemes, which are of high demand in the field of biophotonics due to their effective interaction with hemoglobin and melanin.

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.

Photon-pairs generated by spontaneous four-wave mixing (SFWM) in inhibited-coupling hollow-core photonic crystal fiber (IC-HCPCF) filled with an inert gas (Xenon) combine Raman free generation, high signal-to-noise ratio, entanglement control and integrability in the telecommunications network [1]. Furthermore, this technology proved to be a promising platform for both single photon and entangled photon-pairs sources. However, increasing the brightness of such platform is required. Recently, we demonstrated an enhancement of this photon -pairs source brightness by an order of magnitude to reach $B_{0}=30\text{kHz}$ using GHz-repetition rate femtosecond laser [2].

While the cladding design of inhibited-coupling hollow-core photonic crystal fibers (IC-HCPCF) sets their transmission performance in the infrared wavelength range $(\lambda > 1\mu \mathrm{m})$ , the limiting factor in their transmission loss at shorter wavelengths is set by surface scattering loss (SSL) [1]. As the SSL depends on the HCPCFs' core surface roughness (SR) arising from thermally-induced surface capillary waves (SCW) occurring during fiber drawing, the characterization of its core surfaces, achieved by either atomic force microscopy (AFM) [2] or optical profilometry [3], becomes of paramount importance.

Recently, quantum technology has gained a lot of interest knowing that this field is expected to provide new opportunities, especially regarding high sensitivity and precision in sensing applications. However, despite the efforts, most quantum sensors are still bulky and hardly operate outside a dedicated environment. In this paper, we present a new approach and the first step in paving the way for an all-fibered cell for alkali atom cooling aiming toward miniaturization of the quantum sensors. The fibered cell consists of a hollow-core photonic crystal fiber (HCPCF) based on a Kagome-Tubular Hybrid cladding [1] placed between two solid Polarizing Maintaining (PM) fibers with high reflection Fiber Bragg Gratings (FBGs) forming a Fabry-Perot (FP) cavity.