Structured light, optical fields engineered in their spatial, polarization, or phase degrees of freedom, has become a key resource across advanced communication, sensing, imaging, and quantum technologies. Optical fibers nowadays play an essential role in this landscape, providing stable and scalable platforms for guiding and amplifying complex modes such as vector and orbital angular momentum (OAM) beams. In this work, we demonstrate an actively spun ring-shaped tapered fiber as a gain medium for efficient amplification of OAM modes, preserving their modal purity and polarization topology. OAM beams with topological charges [ = 1 and [ =2 carrying 60 ps pulses at 15 MHz repetition rate at 1030 nm wavelength are amplified over 1.2 W average power with modal purity over 95%. The spatially resolved measurement of the OAM beam polarization topology revealed a small distortion due to the coupling into neighbouring modes. These results demonstrate the high potential of active spun ring-shaped tapered fibers for power scaling of complex beams, preserving their phase and polarization structure simultaneously.

We demonstrate a pathway for producing large-core fiber preforms with optimized characteristics for high-power fiber laser applications. Reactive powder sintering (REPUSIL) was used for producing large volumes of doped silica fiber preforms with predictable, homogeneous, and precise compositional profiles, focusing on formulations near the equimolar dopant ratio of P:Al = 1 to suppress Yb-related photodarkening (PD). Spectroscopic and structural properties are related to PD performance for both preform and optical fiber materials. All preform samples exhibit a radial dip-free, step-like refractive index profile with relative average index fluctuations of less than 2%. Reduced excess PD loss of 10 dB /m after 21 h exposure time and lower PD rates are obtained when the P content is adapted at a slight excess over Al, even when the overall Al content is high.

Zinc oxide (ZnO)-doped silica exhibits radioluminescent properties, attributed to the presence of the crystalline zinc silicate willemite ( Zn2SiO4), making it a promising material for fiber-based radiation sensing. To maintain optimal waveguiding properties, a uniform ZnO distribution in the silica host material is essential. This study investigates two distinct fabrication methodologies for ZnO-doped silica preforms and their characterization of chemical and optical parameters, evaluating radial ZnO concentration and refractive index (RI) profiles. The first approach utilizes a nanoparticle- suspension doping technique via modified chemical vapor deposition (MCVD), where a porous silica layer inside a fused silica tube is infiltrated with a ZnO nanoparticle suspension. This approach results in a 1.2 mm preform core diameter with a ZnO peak concentration of 1.35 mol%, and an overall asymmetrical radial profile with an MCVD-typical central dip. The second approach uses a powder-based synthesis technique, resulting in a 14 mm preform core diameter with a nearly rectangular concentration profile at an average doping level of similar to 0.77 mol% ZnO. A larger, more uniform core offers the potential for enhanced radioluminescent signal output.

We will present an overview on recent advances in high-volume synthesis starting from the powder-based concept of REPUSIL to create high-purity silica preform materials on demand. Emphasis is placed on precise index-matching of preform couples at defined level between each other, reaching an accuracy better than 5x10(-5). The synthesized twin material will be used to fabricate fully aperiodic large pitch fibers (FA-LPF) to be used in lasers for material processing.

Crystals and fibers doped with Ce3+ are highly valued for their wide-ranging fluorescent emission, which covers both visible and near-infrared wavelengths [1], [2]. Among these, their broad fluorescence spectrum makes them suitable for potential use in lighting technologies [3]. Additionally, Ce3+-doped silica fibers hold particular importance in non-invasive biomedical techniques like Optical Coherence Tomography (OCT) [4]. This work focuses on examining the fluorescence behaviour of a Ce3+-doped silica fiber and glass when pumped with 405 nm.

This paper presents the development and experimental demonstration of all-fiber Master Oscillator Power Amplifier (MOPA) systems capable of delivering high-average-power and high-peak-power picosecond pulses. The systems incorporate two distinct all-glass gain modules utilizing active ytterbium-doped spun tapered double-clad fibers (sT-DCFs) with core diameters of 60 mu m and 92 mu m. Utilizing the 60 mu m core diameter sT-DCF amplifier, 7.3 ps pulses at 26 MHz repetition rate were amplified to achieve 200 W average power and a peak power of 1 MW. The 92 mu m core diameter sT-DCF amplifier demonstrated 7 ps pulses with 600 W average power at a high repetition rate of 182 MHz. Both systems delivered near-diffraction-limited beam quality (M-2 < 1.4), indicating excellent spatial coherence and the capability to mitigate transverse mode instability (TMI) at high power levels. The alignment-free, all-glass design incorporates Ampliconyx APMX-TGM-xx/1040 1 kW gain modules highlight the potential of these MOPA systems for compact, efficient, and robust ultrafast fiber laser solutions. This design is particularly suited for a wide range of industrial applications requiring high average/peak power, superior beam quality, and reliable operation.

During recent years, the optical-fiber-based simultaneous sensing of strain and temperature has attracted increased interest for different applications, e.g., in medicine, architecture, and aerospace. Specialized fiber layouts further enlarge the field of applications at much lower costs and with easier handling. Today, the performance of many sensors fabricated from conventional fibers suffers from cross-sensitivity (temperature and strain) and relatively high interrogation costs. In contrast, customized fiber architectures would make it possible to circumvent such sensor drawbacks. Here, we report on the development of a high-quality coupled-core fiber and its performance for sensors—from the initial fiber layout via elaboration of the preform and fiber up to the sensor evaluation. A compact, high-speed, and cost-effective interrogation unit using such a specialized coupled-core fiber has been designed to monitor reflectivity changes while even being able to distinguish the direction of the force or impact. Several fiber core material techniques and approaches were investigated, which made it possible to obtain a sufficient volume of material for the required fiber core number and a specialized fiber core geometry in terms of core distances and radial refractive index profile, whilst handling the non-symmetrical fiber architectures of such modeled, complex structures and balancing resources and efforts.

We will report on recent advances in fabrication of large volume silica based, doped fiber preform materials synthesized via powder-based processes. Recently, there has been increased interest for power scaling in fiber based laser applications that requires large core volumes with excellent homogeneity in refractive indices, but also chemical variety (in terms of high dopant concentrations, different dopants). A structural fiber variety requires dedicated large volume core material of reproducible and tailorable chemical composition. Established technologies such as modified chemical vapor deposition (MCVD) or crucible melting rely on complex thermal processing, and are limited in accessible chemistries, dopant concentration, achievable functionalities, and in case of MCVD in achievable core sizes. The current process development thus targets to overcome such draw-backs by including novel approaches to enable extreme material combinations, enhanced reactivity, or novel functions.

Amplifying short pulses directly within a single fiber laser system has proven to be a challenging task, primarily due to thermally induced transverse mode instabilities and detrimental nonlinear effects. Another demanding aspect is preserving the linear polarization state at high power levels, which is even more pronounced for ultra-large-mode area fibers. This study demonstrates significant advancement in the direct amplification of narrow linewidth short pulses from tens of mW to several hundreds of Watts in a single-stage amplification, maintaining a high degree of linear polarization at the maximum output power. Through a comprehensive experimental investigation, two distinct types of Ytterbium-doped tapered double-clad fibers (T-DCFs), namely, PANDA (PT-DCF), with high built-in birefringence, and spun (sT-DCF), with ultra-low built-in birefringence, are examined. The unique geometrical architecture of the amplifiers is exploited for the realization of a compact and highly efficient picosecond fiber-based laser system, achieving more than 75% slope efficiency. In a single amplification stage, 50 ps pulses at a repetition rate of 20 MHz and an average power of 65 mW are amplified up to 457 W and 573 W of average power using PT-DCF and sT-DCF amplifiers, respectively. Both amplifiers exhibit near diffraction limited beam quality, M-2 < 1.4 at the highest power level. At the maximum power levels, the system maintains a high degree of linear polarisation, achieving similar to 90% and similar to 94% for the sT-DCF and PT-DCF, respectively. These ultra-large mode area fiber amplifiers are verified as versatile solutions for direct amplification of short pulses up to half-kW level with excellent spectral, spatial, and polarization characteristics.

Fiber optic bending sensing has potential use in industrial and medical applications. Thus, so far, several configurations have been reported with that end, but the state-of-the-art sensors are either complex, temperature dependent, or cannot be multiplexed easily. To circumvent these important limitations, we have developed a sensing platform based on an asymmetric coupled-core optical fiber that is combined with conventional Bragg gratings. The asymmetric fiber was designed with three cores arranged in an equilateral triangle. The said fiber supports supermodes that suffer drastic changes when it is bent. Consequently, the reflection of a Bragg grating inscribed close to the asymmetric fiber changes drastically, but its wavelength position is not altered. We demonstrate experimentally that our sensing platform allows the development of highly sensitive bending sensors that have important practical assets. The latter include compactness and simple fabrication, capability of distinguishing the direction of bending and simultaneous detection of temperature and bending. Moreover, the sensors can be multiplexed easily and can be interrogated with commercially available fiber optic sensor read out units.

We demonstrate the dramatic progress in Yb-doped spun tapered double-clad fiber amplifiers delivering up to 550 W of average power with single mode spatial profile and 50 ps pulses at 20 MHz repetition rate. The special geometrical architecture of the fiber enables the direct amplification of short pulses from tens of mW to hundreds of watt levels in a single amplification stage, leading towards the realization of a compact and highly efficient picosecond fiber-based laser system with excellent output spatial and temporal characteristics.

Applying the highly versatile and flexible MCVD technology at Leibniz-IPHT two new designs for optical sensing fibers were realized by co-doping of fused silica. For FBG sensing a Ge/B co-doped fiber with a mode field diameter adapted to standard single mode telecom fibers was prepared. The influence of boron on the attenuation at the inscription wavelength 1550 nm is visible. For distributed Brillouin sensing applications a preform with lateral separated germanium and aluminum doped regions and nearly step-index characteristic was fabricated by the MCVD in combination with the solution doping technique. Theoretical analysis of the acoustic properties and Brillouin spectrum have been shown, that this design is a potential candidate for strain and temperature discrimination. Because of the high temperatures during the preparation processes the radial refractive index and dopant concentration profiles of both fiber designs are influenced by diffusion.

Applying the highly versatile and flexible MCVD technology at Leibniz-IPHT two new designs for optical sensing fibers were realized by co-doping of fused silica. For FBG sensing a Ge/B co-doped fiber with a mode field diameter adapted to standard single mode telecom fibers was prepared. The influence of boron on the attenuation at the inscription wavelength 1550 nm is visible. For distributed Brillouin sensing applications a preform with lateral separated germanium and aluminum doped regions and nearly step-index characteristic was fabricated by the MCVD in combination with the solution doping technique. Theoretical analysis of the acoustic properties and Brillouin spectrum have been shown, that this design is a potential candidate for strain and temperature discrimination. Because of the high temperatures during the preparation processes the radial refractive index and dopant concentration profiles of both fiber designs are influenced by diffusion.

The average shift of high-frequency IR and Raman spectra, calculated as their centre of gravity (COG), correlates linearly with optical basicity and allows for estimating the average degree of covalent/ionic bonding by vibrational spectroscopy.

Active large mode area (LMA) tapered double-clad fibers (T-DCF) have been widely used in high-power pulsed fiber Master Oscillator Power Amplifier (MOPA) systems as a result of the capability of such systems to deliver a few MW-level peak power and several hundred watts of average power with several tens of microjoules pulse energy [1]. Cylindrical-vector beams perform various advantages for industrial and scientific applications, such as particle acceleration and trapping, high-resolution microscopy, optical data storage and material processing. Up to the present time, CW laser, nanosecond MOPA and high-power MOPA systems delivering radially and azimuthally polarized output beams have been demonstrated [2]. Typically, they are represented by a MOPA system in which an isotropic LMA fiber is used for the last amplification stage. However, there are some limitations that come with choosing isotropic LMA fiber - in particular, uncontrolled distortion of the spatial distribution and low contrast between the bright and dark zones of the doughnut-shaped beam. Recently, our group has demonstrated a new type of T-DCF - so-call spun T-DCF (sT-DCF) with small birefringence supporting the propagation and amplification of radially polarized light, which we propose for the solution to existing problems [3].

Indium tin oxide (ITO) coatings have been proposed to reduce thermal emission losses for solar thermal applications. Unfortunately, ITO also has a large amount of free charge carriers (-1 x 1020 per cm3), which absorb sunlight. To address this issue, we propose a nano-patterned ITO-coated quartz exhibiting both anti-reflectivity (to maximize solar transmission) and low emissivity (to minimize long wavelengths radiative losses). A record small-size nanosphere (-60 nm) etch mask was prepared via double self-assembly, followed by dry etching and characterisation. In parallel, alternative nanopattern geometries were modelled using the Lumerical FDTD software to optimise short wavelength transmission without diminishing the inherently low emissivity of unetched ITO. It was found that an inverted moth's eye pattern (height = 250 nm and spacing = 80 nm) gave the best results at various solar concentrations (1 sun @ 100 degrees C, 10 suns @ 400 degrees C, and 100 suns @ 600 degrees C), resulting in -7% improvement in the solar weighted transmission as well as a similar boost in the overall efficiency factor for selectivity. It was concluded that if the proposed deposition/etching processes can be cost-effectively scaled in a continuous process, it would provide a net performance boost for most solar thermal technologies.