Library · Signals, sensing & imaging
What Optical Waveguides & Fiber Sensing got wrong, from 29 theses
Summary written by a model from the records below: The records describe various failure modes in optical waveguides and fiber optic sensing systems, including fabrication defects, packaging failures, and optical coupling losses. They also detail instances where fiber sensor configurations were rejected or beaten due to environmental disturbances, spatial resolution limits, and poor sensitivity compared to alternative methods. These records come from graduate theses, 2021 to 2025, each labeled with its degree where the repository states it, and each links to its thesis. They were extracted by language models reading the full text, so treat each as a lead to read, not a verdict.
Plus 7 problems the authors left open, listed at the end.
Failure modes, by theses reporting them
Select a mode to jump to its records.
Waveguide fabrication defects, phase matching errors, and modal dispersion issues degrade optical performance
Thermal drift, photosensitivity issues, and thickness variations disrupt grating inscription and phase matching in fabricated waveguides. In addition, low refractive index contrast, planarization problems, and bend geometry issues cause excessive radiative leakage, modal overlap, and inadequate rejection depths.
Tried and failed
photonic crystal waveguides on low-index-contrast substrates applied to optical waveguiding. Reason: insufficient refractive index contrast caused excessive out-of-plane radiative leakage
Near- and mid-infrared integrated photonic bio-chemical sensing devices with on-chip spectrometers · UT Austin · PhD
Tried and failed
photochemical self-trapping of optical beams applied to stimuli-responsive polymer hydrogels. Reason: laser beams underwent substantial self-defocusing instead of forming stable self-trapped optical waveguides
Spiropyran-Functionalized Hydrogels as a Designable Platform for Responsive Nonlinear Optics · Harvard · PhD
Tried and failed
sidewall poling of etched thin-film waveguides applied to nonlinear ultraviolet optical frequency conversion. Outcome: no signal. Reason: film thickness variations disrupted phase-matching, causing extremely low conversion efficiency
Nonlinear Frequency Generation in Periodically Poled Thin Film Lithium Niobate · Harvard · PhD
Lost to a baseline
Experimental integrated black phosphorus waveguide modulator achieved 8 dB/mm, falling short of the 40 dB/mm predicted by numerical simulations due to non-ideal planarization and substrate gate inefficiency.
Electromechanically control the optoelectronic properties of 2D semiconductors · University of Washington · PhD
Lost to a baseline
S-bend with two 10 µm radius standard circular bends completely failed to resolve input modes and overlapped signals randomly compared to the straight waveguide (de)MUX baseline.
Increased Capacity in WDM & MDM Networks Via Subwavelength Grating Engineering · Carleton University · Master's
Considered and rejected
Considered and rejected: Rejected adiabatic waveguide couplers for integrated OPA/AFC in thin-film lithium niobate because they lack sufficient broadband operation across the required wavelengths.
Broadband spectral control of quantum light with frequency conversion · Cornell · PhD
Inadequate optical interaction length, refractive index contrast, or material thickness limits detection sensitivity
Fiber Bragg gratings and waveguide absorption sensors fail to register detectable signals when mode overlap, acoustic strain sensitivity, or material thickness is insufficient. High substrate refractive index, index matching with silica, and excessive plasmonic losses also prevent adequate reflectance, optical modulation, and birefringence detection.
Tried and failed
fiber Bragg grating strain sensing applied to guided acoustic wave detection. Outcome: no signal. Reason: insufficient sensitivity to low-amplitude dynamic strain waves even under high actuation voltage
Condition monitoring of composite structure with Fibre Optic Sensors · Imperial · PhD
Tried and failed
transmission photoelasticity under crossed polarizers applied to local strain around embedded fibers. Outcome: no signal. Reason: optical birefringence induced by local electric-field deformation was below the detection threshold
Studies of Electric-Field Induced Surface Instabilities using Confocal Microscopy · Harvard · PhD
Tried and failed
direct absorption sensing using standard optical waveguides applied to trace gas detection. Outcome: no signal. Reason: optical interaction length and mode overlap with analyte were insufficient to produce detectable absorption at low concentrations
Integrated photonic devices for trace gas sensing and spectroscopy · UT Austin · PhD
Tried and failed
direct sample deposition on unpatterned slab waveguide applied to on-chip evanescent field spectroscopy. Outcome: no signal. Reason: high substrate refractive index and scattering/opacity caused poor coupling sensitivity and low signal-to-noise ratio
Developments in THz Polaritonics: Towards Integrated Nonlinear THz Spectroscopy · MIT · PhD
Considered and rejected
Considered and rejected: Rejected plasmonic waveguides for optical mode reduction due to excessive optical losses.
All-optical associative learning element · Oxford · PhD
Considered and rejected
Considered and rejected: Depositing ITO directly onto bare silica fiber without TiO2, rejected because the refractive index of ITO matches SiO2 at 1550 nm (~1.5), failing to provide sufficient reflectance (20-30%).
Opto-Electric Parametric Amplification of Mechanical Resonators for Thermal Noise Squeezing · Texas State University · Master's
Considered and rejected
Considered and rejected: Rejected thin-film surface coating of Terfenol-D on optical fiber due to insufficient thickness yielding low magnetic sensitivity.
Multimaterial Fiber Sensors for Physical Measurements · Virginia Tech · Doctorate
Sensor embedding and packaging lead to microbending, spectral distortion, and structural degradation
Embedding optical fibers inside composites or friction interfaces causes peak splitting, transmission loss, and reduced structural compressive strength. Thermal contraction during cryogenic cooldown and high temperature exposure cause adhesive seal failures, fiber slipping, and microbending signal attenuation.
Tried and failed
embedded fiber Bragg grating strain sensing applied to dynamic sliding friction interface. Outcome: no signal. Reason: optical peak splitting caused by transverse loading and debris ingress under dynamic conditions
Investigation of disc brake interface strain distributions using Fibre Bragg Grating sensors · Cranfield · PhD
Tried and failed
embedding optical fiber sensors applied to composite prepregs. Outcome: worse than baseline. Reason: caused optical transmission loss, reduced compressive strength, and distorted spectral signals during curing
Condition monitoring of composite structure with Fibre Optic Sensors · Imperial · PhD
Tried and failed
localized bonding of fiber optic sensors applied to cryogenic temperature sensing. Outcome: no signal. Reason: differential thermal contraction caused fiber microbending and slipping, resulting in severe signal attenuation during cooldown
Quench dynamics and fiber optic quench detection of VIPER high temperature superconductor cable · MIT · PhD
Considered and rejected
Considered and rejected: Rejected direct submerged fiber probe design for hot oil degradation because silica epoxy seal failed at high temperatures.
Hadamard-Transform Fluorescence Excitation-Emission-Matrix Spectroscopy: Rapid 3-Dimensional Fluorescence Data for Analytical and Industrial Applications · Queen's University · PhD
Considered and rejected
Considered and rejected: Rejected embedded FOS in favor of surface mounting due to structural compression knockdown (20%), inability to repair/replace broken fibers, and fabrication cure-induced spectrum distortion.
Condition monitoring of composite structure with Fibre Optic Sensors · Imperial · PhD
Considered and rejected
Considered and rejected: Rejected unbonded/partially bonded FBG fiber routing with Kapton tape covers because loose non-sensing regions caused micro-bending and optical signal attenuation during cooldown.
Quench dynamics and fiber optic quench detection of VIPER high temperature superconductor cable · MIT · PhD
Optical coupling losses, connector attenuation, and beam divergence exceed detector or system tolerances
Light transmitted across detachable connectors, standoff distances, and proximity gaps experiences severe intensity attenuation and beam divergence beyond detector areas. Waveguide tapers and fiber interrogators also fail when mode mismatches, lens collection limitations, or asymmetric index profiles prevent efficient optical coupling.
Lost to a baseline
Measured beam size of 0.65 mm at the mid-IR chalcogenide waveguide output on the beam profiler compared to the theoretical expected beam size of 2.4 mm due to lens collection numerical aperture limitations.
Investigation of femtosecond laser-enabled 3D micro-infiltration processes · EPFL · PhD
Considered and rejected
Considered and rejected: Rejected inverse tapers for air- and Nb2O5-clad waveguides because asymmetric refractive index profile forced optical modes into substrate rather than expanding them.
Germanium on Silicon Integrated Photonics for the Mid-Wave Infrared · MIT · PhD
Tried and failed
fiber-optic quadrature encoding through detachable connectors applied to rotary actuator position sensing. Outcome: no signal. Reason: optical signal suffered excessive light intensity attenuation across custom transmission-line disconnect interfaces
Design, Modeling, and Control of a Concentric Tube Robot for MR-Guided Intracerebral Hemorrhage Evacuation · Georgia Tech · PhD
Tried and failed
direct fiber-to-detector proximity coupling without relay optics applied to optical fiber signal collection to photodetector. Outcome: worse than baseline. Reason: beam divergence over the gap caused the beam size to exceed the detector active area
Development of a multibeam multiphoton multiwellplate microscope (M3M) for 2D/3D FLIM FRET assays · Imperial · PhD
Considered and rejected
Considered and rejected: Rejected standard fiber-based FBG interrogators for HOM characterization due to NA and core diameter mismatch blocking outer core energy
Investigation of Bragg Gratings in Few-Mode Fibers with a Femtosecond Laser Point-by-Point Technique · Virginia Tech · Doctorate
Fiber sensor configurations achieve lower sensitivity and accuracy than alternative sensing baselines
Several fiber designs exhibit reduced sensitivity or lower signal to noise ratios when evaluated against resonant cavities, Mach-Zehnder interferometers, and scintillating screens. Modifications such as cladding treatments or hollow fiber structures also fail to match the performance of unmodified fibers or fiber tip sensors.
Lost to a baseline
Filled-waveguide method was not sensitive or accurate enough to distinguish loss factor among polystyrene-metal mixtures, whereas resonant cavity clearly resolved them
Microwave nondestructive evaluation of metal powders used in additive manufacturing · Iowa State · PhD
Lost to a baseline
TEOS-modified cladded optical fibers exhibited lower sensitivity than unmodified cladded fibers due to reduced light scattering.
TUNING AND UNDERSTANDING SOFT SURFACE PHENOMENA FOR WEARABLE AND BIOMIMETIC APPLICATIONS · Cornell · PhD
Lost to a baseline
Hollow fiber electrochemical sensor showed sensitivity per surface area of 0.69 x 10^-6 A/mM/mm^2 compared to 8 x 10^-6 A/mM/mm^2 for the fiber tip sensor due to lower exposed carbon black
Ultimate Feature Sizes in Thermally Drawn Fibers: from Fundamental Analysis to Novel Functional Fibers · EPFL · PhD
Lost to a baseline
Single-fiber CCD readout suffered lower SNR at low DPP (<5 Gy/pulse), underestimating Gaussian beam tails compared to YAG scintillating screens.
VHEE dosimetry in a FLASH: ultrahigh dose rate dosimetry and real-time beam monitoring for very high energy electron FLASH radiotherapy · Oxford · PhD
Mechanical vibration, motion, and bending cause severe signal noise and polarization fading
Rapid descent, continuous motion, and environmental vibrations disturb optical cells and fiber paths during in-situ measurements. Endoscopic deployment and coherent fiber bundles also suffer from polarization-induced fading and degraded phase sensitivity compared to free-space optics.
Tried and failed
coherent fiber bundle imaging probe applied to differential phase contrast microscopy. Outcome: worse than baseline. Reason: resolution limited by fiber core pitch and optical phase sensitivity degraded tenfold compared to free-space optics
Quantitative Oblique Back-Illumination in the Study of Thick Biomedical Samples · Georgia Tech · PhD
Tried and failed
in-situ spectrophotometric calibration during dynamic sensor motion applied to submersible optical chemical sensors. Outcome: unstable. Reason: motion-induced mechanical vibration disturbed optical cells and fiber paths, degrading spectral quality
Steep spatial strain gradients and multiplexing limits impair distributed fiber sensing
Steep strain gradients near crack regions cause data loss, measurement inaccuracies, and errors in uniform strain averaging along fiber segments. Interferometric and coated Fabry-Perot configurations are also rejected due to structural perturbation, bend radius constraints, or an inability to multiplex sensors along a single fiber.
Tried and failed
distributed fiber optic sensing applied to high strain gradient crack regions. Outcome: no signal. Reason: steep strain gradients caused sensor data loss and measurement inaccuracies
Fatigue Strengthening of Damaged Steel Plates using Ultra-High Modulus Carbon Fibre-reinforced Polymer Plates · Queen's University · Master's
Tried and failed
piecewise uniform strain averaging along fiber segments applied to distributed shape sensing under load. Outcome: worse than baseline. Reason: linear averaging across high-gradient distributed loads failed without continuous spline interpolation to preserve curvature profile
Condition monitoring of composite structure with Fibre Optic Sensors · Imperial · PhD
Considered and rejected
Considered and rejected: Rejected Fabry-Pérot fiber optic humidity sensors with distal SnO2 coating due to lack of multiplexing capability along a single fiber line.
Radiation-tolerant Multichannel Dew Point Temperature Monitoring System for High Energy Physics Applications · EPFL · PhD
Left open by the authors
Problems the authors named and did not get to.
Left open
Design and experimentally demonstrate cross-axis two-mode fiber Bragg gratings for high-sensitivity multi-parameter bending sensing. Blocker: Requires physical femtosecond laser inscription setup, optical fiber laboratory equipment, and physical sensor fabrication
Investigation of Bragg Gratings in Few-Mode Fibers with a Femtosecond Laser Point-by-Point Technique · Virginia Tech · Doctorate
Left open
Perform optoelectrical characterization measuring open-circuit voltage, short-circuit current, bandwidth, and photovoltaic efficiency on the fabricated integrated in-fiber photodiode heterojunction. Blocker: Requires specialized physical lab equipment and the physical multimaterial fibers fabricated in the thesis.
Semiconductor-based device architectures in multimaterial fibers · EPFL · PhD
Left open
Disentangle and analyze piezoelectric resonances and acoustic mode coupling in SrTiO3 and BaTiO3 electro-optic transducers. Blocker: Requires experimental laboratory measurements and characterization of fabricated physical photonic/piezoelectric devices
Towards microwave-to-optical quantum transduction with barium titanate/strontium titanate integrated photonics · EPFL · PhD
Left open
Incorporate a fiber balanced bridge circuit with active and dummy FBGs to cancel mechanical strain in quench detection. Blocker: Requires physical fiber optic hardware (FBGs and bridge circuit) or specialized experimental setup to implement and test.
Quench dynamics and fiber optic quench detection of VIPER high temperature superconductor cable · MIT · PhD
Left open
Instrument cracked steel plates with polyimide-coated fiber optic cables at the interface to measure induced bending and crack opening stresses during fatigue testing. Blocker: Requires physical laboratory access, experimental fatigue testing equipment, and fiber optic sensing apparatus
Fatigue Strengthening of Damaged Steel Plates using Ultra-High Modulus Carbon Fibre-reinforced Polymer Plates · Queen's University · Master's
Left open
Analyze the validity of the Drucker-Prager criterion for fiber-reinforced adhesive specimens across various fiber orientations. Blocker: Requires physical testing apparatus and physical specimens with different fiber orientations.
Non-Proportional Fatigue by Example of Fiber-Reinforced Rotor Blade Adhesive · Leibniz Universität Hannover · PhD
Left open
Investigate why incorporating physics-based dimensionless numbers improves generalization on out-of-distribution test data but not in-distribution data. Blocker: Requires the private experimental microfiber fabrication dataset and models used in the thesis.
Deep neural network-driven enhancement of the microfiber design and fabrication process · Iowa State · PhD
Who has worked on this
Authors of the public theses cited on this page, most-cited first.
- Cheng Xu · Imperial · PhD · 2022
Condition monitoring of composite structure with Fibre Optic Sensors - Erica Salazar · MIT · PhD · 2021
Quench dynamics and fiber optic quench detection of VIPER high temperature superconductor cable - Tong Qiu · Virginia Tech · Doctorate · 2022
Investigation of Bragg Gratings in Few-Mode Fibers with a Femtosecond Laser Point-by-Point Technique - Liam Knoll · Queen's University · Master's · 2022
Fatigue Strengthening of Damaged Steel Plates using Ultra-High Modulus Carbon Fibre-reinforced Polymer Plates - William Flanagan · Imperial · PhD · 2025
Development of a multibeam multiphoton multiwellplate microscope (M3M) for 2D/3D FLIM FRET assays - Joseph Talbot Filbert · Iowa State · PhD · 2025
Microwave nondestructive evaluation of metal powders used in additive manufacturing - Annina Irina Riedhauser · EPFL · PhD · 2025
Towards microwave-to-optical quantum transduction with barium titanate/strontium titanate integrated photonics - Anthony Lawrence Gunderman · Georgia Tech · PhD · 2024
Design, Modeling, and Control of a Concentric Tube Robot for MR-Guided Intracerebral Hemorrhage Evacuation - Federico Presutti · Cornell · PhD · 2024
Broadband spectral control of quantum light with frequency conversion - Soumya Shankar Ghosh · Harvard · PhD · 2024
Nonlinear Frequency Generation in Periodically Poled Thin Film Lithium Niobate - Eric Rueyhao Sung · MIT · PhD · 2024
Developments in THz Polaritonics: Towards Integrated Nonlinear THz Spectroscopy - Ruixuan Wang · Virginia Tech · Doctorate · 2024
Multimaterial Fiber Sensors for Physical Measurements
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