Thulium-doped fiber lasers (TDFL) emitting around 2 µm receive growing attention. The Tm3+ ions may be pumped around 0.8 µm into the 3H4 level, but the maximum achievable efficiency in theory reaches only 40 %. Much higher efficiencies are achievable in practice thanks to the cross-relaxation (CR) effect, also called „2-for-1“ process. To trigger the CR effect, very high contents of Tm3+ ions are required, which places significant requirements on the material design; high concentrations of Al2O3 are typically needed to prevent concentration quenching in highly-doped fibers. MCVD method combined with nanoparticle doping is one of the most perspective methods for the preparation of highly doped alumino-silicate fibers. In this contribution, a large set of thulium-doped fibers was prepared by various methods. The fibers were analyzed with emphasis on the fluorescence lifetime and laser performance, and the nanoparticle doping method was evaluated in comparison with conventional fabrication methods.

An analytical model of holmium-doped fiber laser is proposed and verified experimentally. The model is based on a two-level model, assumes only transitions between levels 5I8 and 5I7 of the Ho3+ ion in silica, and enables to express the threshold pump power and laser slope efficiency in a closed form. The effect of intrinsic losses is modeled by a physically motivated semi-empirical formula. The model is compared with a numerical one and its application limits are discussed.

Ultra-stable lasers are instrumental in fields such as optical atomic clocks, precision spectroscopy, and gravitational wave detection. Their application has been extended to diverse fields such as monitoring earth environment or radio astronomy. The state-of-the-art ultra-stable lasers are usually housed in research-grade environmentally-controlled laboratories. To extend their application reach beyond the laboratory walls, they need to be made transportable or their signal needs to be distributed to external users. Both portability and distribution can be addressed by using optical fibres. Optical fibre delay lines are emerging as a potential alternative to optical cavities for stabilizing lasers [1]. Additionally, optical fibres enable the distribution of optical ultra-stable signals to remote places. However, the thermal sensitivity of Single Mode Fibre (SMF) results in frequency drifts substantially higher than those of cavities. When distributing optical signals in SMF, non-linear effects are induced at relatively low injected power levels, limiting the amount of power that can be transferred. This limitation is particularly significant when transferring frequency combs [2]. In hollow-core optical fibres, light propagates through air rather than silica. This results in a 20 times lower thermal sensitivity, 30 dB lower backscattering, and 3-4 orders of magnitude lower non-linear coefficient, with respect to traditional glass-core SMF [3]. In the presentation, we will review the latest hollow core fibre technology and show how this enables ultra-stable laser stabilisation and several advantages in optical frequency distribution, including optical frequency combs, with respect to SMF. [1] F. Kéfélian, H. Jiang, P. Lemonde, and G. Santarelli, "Ultralow-frequency-noise stabilization of a laser by locking to an optical fiber-delay line," Optics letters, vol. 34, no. 7, 914-916, (2009). [2] Z. Feng et al., "Stable Optical Frequency Comb Distribution Enabled by Hollow‐Core Fibers," Laser & Photonics Reviews, vol. 16, no. 11, 2200167, (2022). [3] E. N. Fokoua, S. A. Mousavi, G. T. Jasion, D. J. Richardson, and F. Poletti, "Loss in hollow-core optical fibers: mechanisms, scaling rules, and limits," Advances in Optics and Photonics, vol. 15, no. 1, 1-85, (2023).

In this contribution, we study different types of mode interaction in high-average power, polarization maintaining (PM) ytterbium-doped fiber amplifiers. We analyze how they limit the performance of the fiber amplifier depending on the polarization of light used and introduce restrictions in the configuration of the fiber amplifier architecture. Avoided-crossings between core and cladding modes are studied in detail, with numerical simulations and systematic experiments, revealing that they are stronger when the input polarization is aligned parallel to the fast-axis of the fiber. We will show how the temporal dynamic characteristic of transverse mode instabilities depends on the polarization input angle of the seed laser. Moreover, a dramatic and unexpected deformation of the output beam was observed when operating one of our large-mode area PM fibers in the fast-axis, with a high mode content of the first high order mode in the slow-axis.

This study introduces constructions of the structural (multiring) doping profile for Large Mode Area fibers, incorporating Tm3+ and Tm3+/Ho3+ layer profiles. The presentation includes a numerical analysis of modal properties and beam shape. The chelate doping technology (CDT) of modified chemical vapour phase deposition (MCVD), known for its low attenuation active preforms fabrication method, was employed in optical fiber manufacturing. The multi-stage deposition in the MCVD-CDT system enables the fabrication of optical preforms with up to 20 layers. Concentrations of lanthanides were optimized to achieve broadband emission in the eye-safe spectral range of 1.55 to 2.10 µm. The fiber construction employed Tm3+ doping design used for a laser construction utilizing the Fabry-Perot resonator for single-mode laser beam generation at a wavelength of 1940 nm, with an M2 value of approximately 1.1. Acknowledgments: The project was funded by the National Science Centre (Poland) granted on the basis of decision no. UMO-2020/37/B/ST7/03094.

Anti-resonant hollow-core fibers have proven great replacement for the conventional gas cells in spectroscopic systems. Although this new technology has been so far demonstrated almost only in laboratory conditions, it shows great potential for practical applications. New methods for enhancing the performance of hollow-core-fibers-based gas sensors will be presented in several novel methane sensing setups operating in both near- and mid-infrared. The performance of the setups will be discussed in terms of their applications in real-world sensing scenarios.

The diameters of the microscopic anti-resonant (AR) elements inside AR hollow-core fibres can be non-destructively measured in real-time during fibre drawing using interference measurements of light scattered from the side of the fibre, but suffers from measurement deviations as the fibre rotates, as well as other disadvantages. We demonstrate an improved measurement principle based on Fabry-Pérot interference, which also allows direct measurement of the wall-thickness of the AR cladding elements. The method is therefore more robust, and provides more information during fibre drawing than previously possible.

We present the fabrication and light transmission through a hollow core fiber bundle with seven cores arranged in a hexagonal pattern. Each individual core facilitates independent light propagation and exhibits several transmission windows in the near infrared region. The lowest measured loss is 2.5 dB/m near 600 nm, with the broadest transmission window located between 1250 nm and 1450 nm with a loss of 4 dB/m.

Recent advancements in anti-resonant hollow-core fiber design have yielded fibers with ultra-low loss. However, preserving polarization alongside low-loss performance remains a challenge. Our study explores different fiber geometries, revealing an achievable birefringence of 2x10^-5 and minimal confinement loss (< 10 dB/km). These enhanced types of fiber designs hold promise for transformative applications in optical and quantum communication networks and sensing.

After fabrication, the gas pressure inside the core and cladding of hollow-core fibres is significantly below atmospheric pressure. If such an “as-drawn” fibre is then exposed to the atmosphere, pressure-driven flow will push gas from the surrounding environment into the core and cladding holes at different rates, affecting its performance. We use optical time-domain reflectometry to study the length-distributed gas flow dynamics in an as-drawn double-nested antiresonant nodeless fibre (DNANF). For the first time, we show measurement of the initial “as-drawn" pressure distribution along the fibre length and its subsequent evolution over time. Experimental measurements are then compared with gas flow simulations, to enable prediction of pressure equalisation times.

In the collective drive for pioneering innovation, the GSST (Galileo Solar Space Telescope Mission) initiative aims to enhance the construction process and integrate additional functionalities into the IFUs. In this context, lithium niobate appears as a versatile material due to its exceptional properties. The project tailored monocrystalline fibers LiNbO3 properties via micro-pulling down (μ-PD) techniques to produce high-quality single fibers.

A new concept for the sensing of media outside the cladding boundary of standard unmodified fibers will be presented. Light in the single optical mode is used to stimulate mechanical modes of the entire cladding cross-section. The boundary conditions for the oscillations of the mechanical modes are modified by surrounding substances: the outward dissipation of mechanical waves manifests in faster decay rates. The process is monitored through photoelastic scattering of additional optical probe waves. Point-measurements, spatially distributed analysis, and monitoring of coating layers will be presented.

This study discusses the design of three straightforward experiments replicating the Michelson stellar interferometer's operation. Stellar sources' emissions are simulated using light from circular end-faces of step-index polymer optical fibers, enabling the simulation of both single and double stars. As light passes through two identical pinholes on a lid covering a telescope's objective, interference fringes are generated. A digital camera, coupled to a telescope with a variable double aperture lid, captures interference fringes. By measuring fringe visibilities, simulated star diameters and separations are estimated with errors below 18%. The setup enables determining the size of extended sources located approximately 75 meters from the telescope. Additionally, the experiment explores spectral variations in fringe visibilities to extract information about the geometry of different stellar sources or components. These experiments are tailored for postgraduate students, offering an opportunity to delve into light coherence theory and gain practical experience in optical stellar interferometry, specifically utilizing plastic optical fibers or any other highly multimode fiber.

In the last few decades, interest in multimode optical fibers has been aroused for a wide range of applications in sensing, imaging and communication. In this work, the spatial domain of multimode fibers is examined, increasing both, the data rate and data security. In a first step, modes of the MMF are shaped by a spatial light modulator and coupled into the fiber. Camera-based intensity measurement is used to examine the modes supported by the fiber towards space division multiplexing (SDM). The obtained modes are restarted and the modal scrambling within the MMF is measured using digital holography. To investigate the optical setup’s coupling conditions, both the signal propagating through the MMF, including modal scrambling, and the internal portion reflected back from the output facet and propagating back to the transmitter side are measured through transmission matrix technology.

We demonstrate the highly efficient re-expansion formulation to determine the modes of a complex micro-structured optical fibers. The key to the success of this method is the inclusion of geometry-adapted minimal longitudinal basis set to effectively replicate the discontinuities in complex fiber permittivity profile, particularly solving the problem of the Gibbs phenomenon.

Our latest work in softphotonics features the seamless integration of liquid-core fibers into standard fiber systems, achieving unprecedented advancements in nonlinear optics and signal processing. Utilizing liquids like CS2 and halocarbonates, we have developed a reliable splicing technique with standard fibers, enabling low-loss all-fiber interconnectivity. This breakthrough provides insights into intra-fiber thermodynamics and enables robust, long-term applications in optical signal processing in the near-infrared. Our method opens new avenues for exploring negative pressures and facilitating multi-laser excitation for cross-phase modulation, showcasing the potential of liquid-filled fibers in future optical technologies.

Thulium fiber amplifiers (TDFA) operating in the spectral region extending the standard telecom C- and L- bands should increase the robustness of telecom infrastructure. We present the development and achieved experimental results for a TDFA applicable over a broad spectral range from 1650 nm to 2020 nm.
While the amplifier optimized for small-signal amplification in the spectral range 1820-2020 nm is based on a thulium-doped silica-based optical fiber with a standard refractive index profile, the amplifier optimized for amplification in the range 1650-1820 nm is based on a thulium-doped optical fiber with a specially tailored internal waveguide structure effectively suppressing guided modes at the longer wavelengths of the thulium emission band and simultaneously promoting mode propagation at the shorter wavelengths. We present the values of the output properties for both types of amplifiers, such as small-signal gain or noise figure.

We propose an all-optical control of the mode dynamics in GRaded INdex (GRIN) multimode waveguides, by a Non-Hermitian potential (simultaneous modulation of the refractive index and of the gain/loss coefficient). Such potential induces a unidirectional mode-coupling. Depending on the modulation parameters it yields to the enhancement/ reduction of the excitation of higher order transverse modes. In the latter case, this leads to an all-optical mode-cleaning. The proposal is supported by analytical predictions based on a coupled mode theory for 1D waveguides, which is numerically proven solving the wave propagation equation. The proposal is generalized to the more involved case of 2D waveguides for different geometries controlling the unidirectional mode-coupling and final beam shape.