arXiv subjects

Markus Walther

Publications and source records attributed to Markus Walther.

11 recordsLinked to original sources

arXiv2026

Fabrication-Aware Design of a Hybrid Metasurface-Bragg Mirror for Low-Noise Precision Optics

High-reflectivity coatings for precision interferometry must simultaneously minimize optical losses and thermally driven displacement noise. Dielectric Bragg mirrors provide robust high reflectance but rely on thick multilayer coatings, whereas metasurface mirrors provide high reflectance and low noise but are sensitive to fabrication-induced deviations. A fabrication-aware hybrid mirror concept is introduced that combines a resonant single-layer metasurface, an etch-stop layer, an antiresonant spacer, and a reduced Bragg reflector. Fabrication effects, including geometric tolerances and line-edge roughness, are explicitly considered in the optical design. Full-wave electromagnetic simulations are combined with a truncated-Gaussian Monte Carlo analysis to determine the performance distribution under the assumed fabrication conditions. The ideal metasurface design exceeds $99.999\%$ reflectance. After roughness-aware reoptimization, the modeled non-reflected power remains below $2.88\cdot10^{-4}$ at $95\%$ fabrication yield. For the cryogenic ETpathfinder gravitational-wave testbed, three Bragg layer pairs reduce the non-reflected power of the complete stack to approximately $6.4\,\mathrm{ppm}$. The estimated thermal displacement-noise amplitude spectral density is $8.4\cdot10^{-21}\,\mathrm{m\,Hz^{-1/2}}$ at $100\,\mathrm{Hz}$. The architecture connects fabrication robustness, optical performance, and thermal-noise reduction within a single design framework.

physics.optics↗

arXiv2026

Beyond Bragg-Mirrors for Gravitational Wave Telescopes: A Fabrication Tolerant Hybrid Metasurface-Bragg Mirror Design

Coating thermal noise in high-reflectivity test-mass mirrors is a major limitation for future gravitational-wave detectors, especially in the 10--300 Hz band. ET-Pathfinder therefore requires mirror coatings that combine very high reflectance at 1.55 micrometer with low thermal noise under cryogenic conditions. Conventional dielectric Bragg mirrors provide high reflectance but require thick coatings, whereas metasurface mirrors can reduce coating-related noise but are limited by fabrication tolerances and line-edge roughness. We present a hybrid metasurface--Bragg mirror concept tailored to ET-Pathfinder. The design combines a fabrication-tolerant one-layer metasurface, an anti-resonant Fabry--Perot spacer, and a reduced dielectric Bragg stack. Optical performance is evaluated using full-wave electromagnetic simulations, while fabrication robustness is assessed with a truncated-Gaussian Monte Carlo analysis. Line-edge roughness is included as a systematic edge-smoothing effect. The resulting reflectance distributions are used to determine the minimum Bragg-stack support required to meet system-level specifications. The ideal metasurface exceeds 99.999% reflectance. When fabrication uncertainties and line-edge roughness are included, the metasurface reflectance is limited to about 99.9% at the 95% yield level. The remaining transmission can be compensated by a supporting Bragg stack with as few as seven layer pairs. For this configuration, the hybrid mirror achieves a total thermal displacement noise about one order of magnitude below the projected ET-Pathfinder coating-noise budget. These results show that fabrication-limited metasurface reflectance can be compensated within a hybrid architecture, enabling reduced coating thickness and thermal noise for next-generation gravitational-wave detectors.

astro-ph.IM↗

arXiv2015

Wavevector Selective Metasurfaces and Tunnel Vision Filters

Metasurfaces offer unprecedented flexibility in the design and control of light propagation, replacing bulk optical components and exhibiting exotic optical effects. One of the basic properties of the metasurfaces, which renders them as frequency selective surfaces, is the ability to transmit or reflect radiation within a narrow spectral band that can be engineered on demand. Here we introduce and demonstrate experimentally in the THz domain the concept of wavevector selective surfaces -- metasurfaces transparent only within a narrow range of light propagation directions operating effectively as tunnel vision filters. Practical implementations of the new concept include applications in wavefront manipulation, observational instruments, vision and free-space communication in light-scattering environments, as well as passive camouflage.

physics.optics↗

arXiv2011

Suspended core subwavelength fibers: practical designs for the low-loss terahertz guidance

In this work we report two designs of subwavelength fibers packaged for practical terahertz wave guiding. We describe fabrication, modeling and characterization of microstructured polymer fibers featuring a subwavelength-size core suspended in the middle of a large porous outer cladding. This design allows convenient handling of the subwavelength fibers without distorting their modal profile. Additionally, the air-tight porous cladding serves as a natural enclosure for the fiber core, thus avoiding the need for a bulky external enclosure for humidity-purged atmosphere. Fibers of 5 mm and 3 mm in outer diameters with a 150 μm suspended solid core and a 900 μm suspended porous core respectively, were obtained by utilizing a combination of drilling and stacking techniques. Characterization of the fiber optical properties and the near-field imaging of the guided modes were performed using a terahertz near-field microscopy setup. Near-field imaging of the modal profiles at the fiber output confirmed the effectively single-mode behavior of such waveguides. The suspended core fibers exhibit broadband transmission from 0.10 THz to 0.27 THz (larger core), and from 0.25 THz to 0.51 THz (smaller core). Due to the large fraction of power that is guided in the holey cladding, fiber propagation losses as low as 0.02 cm-1 are demonstrated. Low-loss guidance combined with the core isolated from environmental perturbations make these all-dielectric fibers suitable for practical terahertz imaging and sensing applications.

physics.optics↗

arXiv2000

Temiar Reduplication in One-Level Prosodic Morphology

Temiar reduplication is a difficult piece of prosodic morphology. This paper presents the first computational analysis of Temiar reduplication, using the novel finite-state approach of One-Level Prosodic Morphology originally developed by Walther (1999b, 2000). After reviewing both the data and the basic tenets of One-level Prosodic Morphology, the analysis is laid out in some detail, using the notation of the FSA Utilities finite-state toolkit (van Noord 1997). One important discovery is that in this approach one can easily define a regular expression operator which ambiguously scans a string in the left- or rightward direction for a certain prosodic property. This yields an elegant account of base-length-dependent triggering of reduplication as found in Temiar.

cs.CL↗

arXiv2000

Finite-State Reduplication in One-Level Prosodic Morphology

Reduplication, a central instance of prosodic morphology, is particularly challenging for state-of-the-art computational morphology, since it involves copying of some part of a phonological string. In this paper I advocate a finite-state method that combines enriched lexical representations via intersection to implement the copying. The proposal includes a resource-conscious variant of automata and can benefit from the existence of lazy algorithms. Finally, the implementation of a complex case from Koasati is presented.

cs.CL↗

arXiv1999

One-Level Prosodic Morphology

Recent developments in theoretical linguistics have lead to a widespread acceptance of constraint-based analyses of prosodic morphology phenomena such as truncation, infixation, floating morphemes and reduplication. Of these, reduplication is particularly challenging for state-of-the-art computational morphology, since it involves copying of some part of a phonological string. In this paper I argue for certain extensions to the one-level model of phonology and morphology (Bird & Ellison 1994) to cover the computational aspects of prosodic morphology using finite-state methods. In a nutshell, enriched lexical representations provide additional automaton arcs to repeat or skip sounds and also to allow insertion of additional material. A kind of resource consciousness is introduced to control this additional freedom, distinguishing between producer and consumer arcs. The non-finite-state copying aspect of reduplication is mapped to automata intersection, itself a non-finite-state operation. Bounded local optimization prunes certain automaton arcs that fail to contribute to linguistic optimisation criteria. The paper then presents implemented case studies of Ulwa construct state infixation, German hypocoristic truncation and Tagalog over-applying reduplication that illustrate the expressive power of this approach, before its merits and limitations are discussed and possible extensions are sketched. I conclude that the one-level approach to prosodic morphology presents an attractive way of extending finite-state techniques to difficult phenomena that hitherto resisted elegant computational analyses.

cs.CL↗

arXiv1998

Processing Unknown Words in HPSG

The lexical acquisition system presented in this paper incrementally updates linguistic properties of unknown words inferred from their surrounding context by parsing sentences with an HPSG grammar for German. We employ a gradual, information-based concept of ``unknownness'' providing a uniform treatment for the range of completely known to maximally unknown lexical entries. ``Unknown'' information is viewed as revisable information, which is either generalizable or specializable. Updating takes place after parsing, which only requires a modified lexical lookup. Revisable pieces of information are identified by grammar-specified declarations which provide access paths into the parse feature structure. The updating mechanism revises the corresponding places in the lexical feature structures iff the context actually provides new information. For revising generalizable information, type union is required. A worked-out example demonstrates the inferential capacity of our implemented system.

cs.CL↗

arXiv1998

Computing Declarative Prosodic Morphology

This paper describes a computational, declarative approach to prosodic morphology that uses inviolable constraints to denote small finite candidate sets which are filtered by a restrictive incremental optimization mechanism. The new approach is illustrated with an implemented fragment of Modern Hebrew verbs couched in MicroCUF, an expressive constraint logic formalism. For generation and parsing of word forms, I propose a novel off-line technique to eliminate run-time optimization. It produces a finite-state oracle that efficiently restricts the constraint interpreter's search space. As a byproduct, unknown words can be analyzed without special mechanisms. Unlike pure finite-state transducer approaches, this hybrid setup allows for more expressivity in constraints to specify e.g. token identity for reduplication or arithmetic constraints for phonetics.

cs.CL↗

arXiv1996

OT SIMPLE - a construction-kit approach to Optimality Theory implementation

This paper details a simple approach to the implementation of Optimality Theory (OT, Prince and Smolensky 1993) on a computer, in part reusing standard system software. In a nutshell, OT's GENerating source is implemented as a BinProlog program interpreting a context-free specification of a GEN structural grammar according to a user-supplied input form. The resulting set of textually flattened candidate tree representations is passed to the CONstraint stage. Constraints are implemented by finite-state transducers specified as `sed' stream editor scripts that typically map ill-formed portions of the candidate to violation marks. EVALuation of candidates reduces to simple sorting: the violation-mark-annotated output leaving CON is fed into `sort', which orders candidates on the basis of the violation vector column of each line, thereby bringing the optimal candidate to the top. This approach gave rise to OT SIMPLE, the first freely available software tool for the OT framework to provide generic facilities for both GEN and CONstraint definition. Its practical applicability is demonstrated by modelling the OT analysis of apparent subtractive pluralization in Upper Hessian presented in Golston and Wiese (1996).

cmp-lg↗

arXiv1994

Coupling Phonology and Phonetics in a Constraint-Based Gestural Model

An implemented approach which couples a constraint-based phonology component with an articulatory speech synthesizer is proposed. Articulatory gestures ensure a tight connection between both components, as they comprise both physical-phonetic and phonological aspects. The phonological modelling of e.g. syllabification and phonological processes such as German final devoicing is expressed in the constraint logic programming language CUF. Extending CUF by arithmetic constraints allows the simultaneous description of both phonology and phonetics. Thus declarative lexicalist theories of grammar such as HPSG may be enriched up to the level of detailed phonetic realisation. Initial acoustic demonstrations show that our approach is in principle capable of synthesizing full utterances in a linguistically motivated fashion.

cmp-lg↗