Conference Agenda

“Girl with a Pearl Earring”, nicknamed the Mona Lisa of the North, is considered one of Johannes Vermeer's masterpieces. The painting represents the imaginary portrait (it is a tronie) of a young woman, sublimated by the Dutch painter's masterful use of light and technical mastery. What also attracts attention in this painting, and sets it apart from other portraits, is the presence of this pearl, a presence that seems to hesitate between the brilliance of reflected light and discretion as it blends into its surroundings.

This painting and pearl dilemma can be seen as an allegory for nanoparticles in optical fibers. While the leitmotif of glass for optical fibers is to be as homogeneous as possible, optical fibers containing nanoparticles are distinguished by the deliberate introduction of heterogeneities into the glass. Induced light scattering introduces a dilemma: it must be minimized for applications such as lasers, or enhanced for sensors.

In this presentation, we will discuss the different processes for preparing optical fibers containing nanoparticles. In particular, we'll show how, thanks to the use of light (a femtosecond laser), it's possible to control the characteristics of nanoparticles in the core of optical fibers.

Optical trapping in air of dielectric micro-spheres is reported using a dual fiber optical tweezers. The spheres are trapped on the interference fringes created by the two counter-propagating optical trapping beams. The use of polarization maintaining fibers allowed us to obtain very high and reproducible trapping efficiencies.

Colour as an information carrier widely exists in nature for camouflage and signalling, inspiring recent interest in bio-inspired colour-changing materials. This study introduces an optical fibre dissipation sensor pair integrated into a soft continuum robot, allowing for dynamic colour expression in response to shape changes. The leaked light was used for real-time colour changes to interact with environmental changes. The system successfully demonstrated colour mixing and transitions during bending, offering combined sensing and visual signalling for applications for visual strain expression.

In this work, we present a novel method to accurately retrieve

temperature information from two LPGs arranged in sequence within a Mach-

Zehnder interferometric configuration. By carefully analysing specific spectral

features and formulating the inversion problem in an unconventional way, we

successfully separate the contributions of each grating, enabling reliable sensing

performance.

Multi-band systems have demonstrated to be a viable solution to sustain capacity growth required by optical communication systems, thanks to the availability of wide bandwidth amplification technologies, like the Raman amplifier (RA). However, extreme levels of optimization are needed to extract all the potential, requiring super-fast and accurate evaluation of the impact of nonlinear effects. This is a tricky task when the transmission bandwidth is very large, as all fiber parameters becomes frequency dependent and the number of data channels and RA pumps is large. Also, the inter-channel stimulated Raman scattering (ISRS) become impactful.

Optimization approaches based on Gaussian Noise (GN) models turn to be very complex, with a consequent slow down of the whole design process. Even using the fast GN-based closed-form-models (CFMs), it requires a full spectral and spatial knowledge of the signal power profile along the fiber span. This is particularly computational heavy when backward RA is considered. We propose an approach based on machine learning (ML) and neural networks (NN) to accelerate the process. The method, tested for a super-(C+L) system (12 THz bandwidth) and backward Raman amplification, guarantees a high level of accuracy and a significant speed increase.

Advances in fibre design, particularly through spatial multiplexing strategies such as multi-core and multi-mode fibres, have shown significant promise. This talk reviews key fibre design innovations aimed at capacity maximisation, exploring core design optimisation, modal dispersion management, and reduction of inter-channel interference. Practical challenges, recent breakthroughs, and future directions in designing optical fibres for maximised capacity and improved performance in next-generation high-capacity systems will be discussed.

We use X-ray computed nanotomography to characterize an optical fiber taper used for nanofiber-based sensors. The resolution achieved goes far beyond the capability of standard optical computed tomography devices.

In this work, we numerically investigate the gain properties of an Yb3+:Er3+:Tm3+:Ho3+ co-doped optical fiber amplifier. The numerical simulations show that a gain higher than 15 dB in a wavelength range of 300 nm can be achieved. The erbium amplifies in the well-known C-band, while thulium and holmium amplify from 1760 nm to 2030 nm.

We propose an efficient method of mitigating Stimulated Brillouin Scattering in a single-frequency multimode fiber amplifier. By applying wavefront shaping to the continuous wave seed, we excite many modes in our Yb-doped multimode fiber amplifier reducing the backward propagating Stokes power. Simultaneously, we can control the output profile ensuring good beam quality. In the experiment, our multimode fiber amplifier achieves 503~W amplified signal power, its slope efficiency is 82~\%, the amplified signal has 18~kHz spectral linewidth and the propagation factor of the output beam is less than 1.35.

We present measurements of the evolution of the core and cladding diameters in tapered silica microfibers by LC-OCT. The results will help to refine the models of propagation of modes in tapers.

We report the observation of classical light thermalization to the negative tem-

perature Rayleigh-Jeans (RJ) equilibrium states. We extend theoretically these

equilibrium states to the Bose-Einstein quantum regime (BE) through the anal-

ysis of the thermodynamic properties.

We demonstrate a building block validation strategy for structural health monitoring (SHM) of aerospace composite structures using optical fibre Bragg grating (FBG) sensors. Standard FBGs are surface-mounted for global strain monitoring, while microstructured FBGs (MOFBGs) are embedded to resolve directional and through-thickness strain. We validate this approach from coupon to fuselage scale. The sensors detect impact-induced damage, and their outputs are fused into a Global Damage Index (GDI) that quantifies structural integrity. We show that the sensor technologies and damage evaluation methods scale effectively across component size and complexity, offering a certified path toward embedded SHM in aerospace composites.

In this work, a switchable multiwavelength L-band fiber ring laser based on a polarization-dependent booster optical amplifier (BOA) and a motorized polarization controller is experimentally demonstrated. Lasing wavelengths selection is achieved by automatically adjusting the polarization state, enabling single, dual, or triple line emission.

We report a theoretical and experimental investigation of far-detuned intramodal (FWM) in the fundamental mode of optical microfibers (OMF) depending on their diameter. We demonstrate that the signal wavelength can be tuned over a wide spectral range simply by varying the OMF diameter. Using a pump at 1064 nm, signal wavelengths ranging from around 750 to 950 nm are generating by adjusting the OMF diameter from approximately 8.5 to 6.5 µm.

This work aims to optimize the Kerr effect in optical nanofibers made from various glass materials with refractive indices close to that of silica and immersed in acetone. Key factors considered include the nanofiber diameter, the optical properties of the core material, and the effective area of the fundamental HE11 mode, both within the core and in the surrounding medium. The study highlights the influence of these parameters on enhancing the nonlinear optical response through the evanescent field.

We present numerical calculations and experimental measurements of Brillouin scattering efficiency in a nanofiber gas cell. The results demonstrate highly efficient nonlinear conversion within the nanofiber gas cell

Optical nanofibers (ONFs) made from fluorophosphate glass (OHARA - FPM) enable strong confinement, low losses, and enhanced evanescent fields for nonlinear optics. We show that the HE11 mode achieves high Raman gain (11.62 m−1·W−1) in a compact 10 cm ONF with a 300 nm radius. These results optimize ONF fabrication, lower the Raman threshold, and expand the Raman effect’s operational range.

Interferometry has long been used to measure the phase of light signals. Combined with a heterodyne detection scheme, it allows to simultaneously and unambiguously record amplitude and phase variations. In this work, we exploit these well-known techniques to evaluate the nonlinear phase induced by the optical Kerr effect during the propagation of a laser pulse in a nonlinear medium. We show that the nonlinear index can easily be retrieved when the accumulated phase remains small, but counter-intuitive results can be observed at higher powers.

We report a significant reduction in taper transmission loss by processing ZBLAN fibre under a controlled argon environment. The contrast between tapers processed under argon and ambient air is highlighted through quantitative loss measurement as well as by investigating the surface morphology of the tapers using optical and electron microscopy.

Composite optical fibers with Yb³⁺ doped crystals embedded in glass matrices combine the benefits of crystalline and glassy phases for photonic applications. This work demonstrates the feasibility of preparing composite fibers within glass systems. YbPO₄ crystals were incorporated into silica via solution doping of the soot layer during MCVD, followed by collapse and fiber drawing. Electron and Raman microscopy confirmed the survival of ~100 nm crystals after processing up to 2100 °C. Another fiber was prepared by embedding LiNbO₃:Yb³⁺ crystals in phosphate glass using a direct doping method. Both fibers demonstrate the potential of post-treatment-free, crystal-in-glass fibers for laser applications.

To the best of our knowledge the inscription of tilted fibre Bragg

gratings via the point by point method, without beam shaping, has been demon-

strated for the first time. Using two parallel inscribed FBGs we have been able

to create cladding and core mode coupling effects indicative of a tilted FBG.

A negative curvature hollow-core fiber design with double pole-anchored cladding elements is numerically proposed for spectral filtering. The fiber structure is investigated for improvement in filtering ability through manipulation of the pole length. The findings reveal reduced confinement losses as low as 0.0003 dB/km for filtered and 0.0054 dB/km for unfiltered wavelengths yielding enhanced loss modulation depth.

We demonstrate the first all-fiber mid-infrared ring cavity laser. The laser comprises a single-mode ZBLAN optical fiber coupler, a tapered pump combiner, a polarization controller, and an Er:ZBLAN fiber. The laser is characterized with a pumping wavelength of 0.976 μm, exhibiting continuous wave emission in the wavelength band of 2.8 μm, with maximum output power of 36 mW.

Adaptive Optics (AO) is a key technology to enhance the imaging performance of ground based astronomical telescopes, compensating the atmospheric aberrations through an adaptive mirror. To enhance the efficiency of these AO systems, many observatories aim to integrate the adaptive mirror within the secondary mirror of the telescope, resulting in an adaptive secondary mirror (ASM).

TNO is developing ASM’s based on a unique and highly efficient electromagnetic actuator technology, which high force output enable the use of relatively thick mirror shells (>3,5mm), and no need for active cooling through their high energy efficiency. These aspects lead to an overall highly robust and reliable ASM system, which is considered a key requirements for such active components that are built into the heart of the telescope.

TNO has realized two prototypes ASM systems for the NASA IRTF telescope (Ø24cm,and 36 actuators, readily installed and tested) and the UH-88 telescope (Ø62cm and 204 actuators, currently going through factory acceptance testing). In this talk the recent results of these ASM’s will be presented. Furthermore, an outlook will be provided on the future developments including the ASM for the KECK telescope (Ø1.4m and ~3400 actuators).

This work discovers two hidden cases of blockwise recurrence in Zernike computations. Based on these findings, a new computation scheme for Zernike polynomials is proposed. It uses one Pascal triangle for all internal factors, thus avoiding calculations of factorials, cos/sin, inverse matrix, etc., and meets the requirements of computatonal accuracy, high speed, low memory footprint, and flexibility.

Mid-Spatial Frequencies (MSFs) are structured surface errors on

optical elements that often require a full wave-optics treatment beyond the limits

of geometrical approximations. Within the context of paraxial scalar wave-optics,

we introduce the Grating Mode Series Expansion (GMSE) method—an

efficient approach for modeling MSF propagation through astigmatic, first-order

optical systems. By decomposing wavefront perturbations into discrete grating

modes on top of an envelope beam, GMSE enables the analytical propagation of

each mode, accurately capturing interference effects and spatial energy redistribution.

The framework is further developed into an algorithm that sequentially

models both form errors as well as MSFs, at arbitrary distances from pupil

planes. From this method, we derive intuitive shift relations that reveal three

key phenomena: (a) MSFs can induce measurable beam shifts at apertures, (b)

even weak MSFs can produce significant non-uniformities in pupil-plane irradiance

due to interference, and (c) the transition between imaging degradation

and stray-light contribution is formalized as a function of system parameters.

The method and its implications are demonstrated through representative case

studies and form the back-bone for the description of MSFs propagation in nonparaxial

systems.

Global optimization in optical design is particularly challenging for aspheric and freeform surfaces due to their complex, non-symmetric nature and the presence of numerous local minima in high-dimensional design spaces. Traditional optimization methods often struggle to efficiently escape these local minima, leading to suboptimal solutions. To address this, we propose the Simple Saddle Point Detection (SSPD) Algorithm, which enhances optimization by systematically identifying transition points that connect different design regions. By leveraging these pathways, the algorithm enables a more structured exploration of the design space, improving the convergence toward high-performance solutions. This study applies the SSPD approach to optimize complex optical systems, including catadioptric and multiple (folded) imaging mirror systems, where conventional methods face significant limitations. The results demonstrate that this approach is highly effective in refining aspheric and freeform optical designs, facilitating more efficient and reliable global optimization. Finally, we present the global search results as a closed network, highlighting the capability of SSPD to navigate complex design landscapes and achieve superior optical performance.

Increasing accuracy requirements for optical systems require taking thermal disturbances into account at an early stage of the design process. Therefore, a simulation method is presented with a mesh-based absorption algorithm, to account for the temperature distribution in a lens. Having this information, e.g., temperature dependent material properties can be used or thermal deformations can be considered.

In this presentation, I will review the latest developments on the use of AI in optical engineering. We'll see how AI can be used for teaching optical engineering, in particular using AI generative tools. Over the last few years, several uses of AI in optics have been published but few developments have really changed the life of an optical engineer. We'll also take a closer look at how generative AI could be used by optical designers. Finally, we'll look to the future and how we want to build the future of optical engineering with these new tools.

The design of off-axis augmented reality display systems based on freeform holographic optical elements (HOE) is presented. The complex freeform HOE is fabricated using freeform optics. Joint optimization method of imaging and recording systems is proposed considering the imaging performance, diffraction efficiency and design constraints. Prototypes including near-eye display and head-up display systems are developed. Larger FOV and more compact structure are realized, compared with the systems using traditional HOEs.

We propose a method to design freeform imaging systems using inverse methods from nonimaging optics. A linear optical map in phase space implies that the ratio of source and target energy distributions is a constant. The performance of the inverse freeform design is compared to a classical design by raytracing parallel beams of light and comparing the corresponding spot sizes. The inverse design significantly outperforms the classical design.

In the present paper we investigate how optimization algorithm can be tailored to improve the lens design process. We replaced gradient-based optimisation methods by the Covariance Matrix Adaptation Evolution Strategy (CMA-ES). This stochastic algorithm is considered more robust and is well suited to avoid local optima often found in optical design. In addition, the algorithm is paired with an augmented Lagrangian method to incorporate constraints handling inside the computation framework. Performances are illustrated on a photographic zoom lens.

We present a fully optical system for the convolution of spatial features in photochromic media. The method relies on imprinting the Fourier transform of a mask into a photochromic sample using 405 nm light, and subsequently performing a convolution using a second, non-activating wavelength (470 nm). Our approach offers reconfigurability and dynamic training capabilities, overcoming the rigidity of traditional optical correlators. Experimental and simulated results demonstrate the system’s ability to distinguish spatial features with high selectivity, enabling compact and efficient optical pattern recognition. This work paves the way for new applications in optical signal processing, machine vision, and embedded neuromorphic photonic hardware.

We consider the inverse problem from nonimaging optics: given a source and its light distribution, find the optical surfaces that transform

the light into a desired target distribution. We present models for optical systems with a parallel or point source. The surfaces (lens or reflector) are freeform. Our models are based on Hamilton's characteristic functions and energy conservation.

Phase elements can enhance the performance of imaging systems while reducing their size, by adding continuous phase gradients on top of the geometrical surface. Compared to their typical implementation as diffractive elements, Fresnel optics can achieve similar performance, while maintaining broadband functionality. This work introduces a methodology to optimize such Fresnel surfaces, resulting in high-performance, compact imaging designs.

We present an inverse method to compute freeform reflector and lens surfaces for generalized zero-étendue sources. The initial position and direction of a light ray is parameterized by two source planes and the final position and direction of a light ray is parameterized by two target planes. We use energy conservation to determine optical mappings, and we use the optical path length to derive equations for the optical surfaces. In two numerical examples, we illustrate the algorithm's capabilities to tackle complex light distributions.

Phase space ray tracing is an alternative to (Quasi-)Monte Carlo ray tracing in 2D. We introduce a 3D phase space algorithm and apply it to the compound parabolic concentrator. Our results show that phase space ray tracing outperforms Quasi-Monte Carlo ray tracing in 3D.

We present a hybrid method for reflector design with finite light sources, combining a neural-network-based solver with a deconvolution-inspired iterative correction scheme. Our approach addresses the limitations of classical techniques, which often assume idealized point or parallel sources, by solving a simplified problem using a neural network and refining the solution via feedback from ray-traced simulations of the full finite-source system. We demonstrate the effectiveness of our method on a representative example, showing improved convergence toward a prescribed far-field intensity distribution compared to the approximate problem's solution.