The research addresses a long-standing gap in the modeling of these systems. Published in Scientific Reports, the work unifies multimodal excitation, incoherent fluorescence emission, and multimodal signal collection into a single, physically consistent model.
D-shaped optical fibers are widely used in sensing technologies, particularly for refractive index measurements and surface plasmon or lossy mode resonance (SPR/LMR) sensing.
These approaches track changes in the intensity of transmitted light or the resonance wavelength. Fluorescence-based sensing, by contrast, relies on detecting light emitted by fluorophores interacting with the fiber’s evanescent field and can provide high chemical specificity.
Despite its promise, fluorescence sensing in D-shaped fibers has lacked a comprehensive theoretical description. Previous studies often treated excitation, emission, or collection in isolation, making it difficult to predict overall sensor performance or guide design choices.
This issue is especially critical in biophotonics, where fluorescence signals are often weak.
A Unified Excitation-Emission-Collection Model
The new model addresses this gap by starting from the physics of a single fluorescent dipole and extending it to a homogeneously distributed ensemble of incoherent emitters.
Fluorophores are treated as independent dipoles with random orientations and phases, allowing the model to capture realistic incoherent emission while still accounting for multimode interference effects.
The sensor geometry is divided into three segments: an excitation segment, a D-shaped sensing segment, and a collection segment. A single guided mode in the excitation fiber is coupled into all guided modes of the D-shaped section, where fluorophores located at the core-external medium interface are excited.
The resulting fluorescence is then captured by all guided modes in the sensing region and coupled into the collection fiber for detection. The total fluorescence signal is obtained by summing contributions across all dipoles and all supported modes.
Polishing Depth Sets Optimal Fluorescence
Applying the model to multimode D-shaped fibers revealed a clear dependence of fluorescence output on polishing depth.
For fundamental-mode excitation, fluorescence increased as polishing progressed toward the middle of the fiber core. In this regime, the growing number of excited dipoles and the strengthening evanescent field outweighed reductions in mode count and excitation coupling efficiency.
Beyond the core midpoint, however, further polishing became counterproductive. The number of available dipoles and guided modes dropped sharply, and excitation coupling weakened, resulting in a decline in the collected fluorescence despite a still-strong evanescent field.
The model predicts optimal polishing depths of approximately 50 to 60 % of the core diameter, depending on the core size.
Smaller Cores are Key, But Have Limitations
Core diameter plays a central role: Smaller-core fibers produce higher maximum fluorescence signals because their modes are less confined, leading to stronger evanescent fields at the sensing surface.
This enhanced field compensates for the fact that smaller cores support fewer guided modes and fewer fluorophores.
Larger-core fibers, while easier to handle in practice, suffer from weaker fluorescence collection under fundamental-mode excitation, unless the excitation strategy is changed.
Higher-Order Modes Unlock Performance
One of the study’s most significant findings is the impact of higher-order mode (HoM) excitation. Exciting the D-shaped fiber with HoMs consistently increased fluorescence output compared to fundamental-mode excitation, due to the stronger evanescent fields associated with these modes.
For a 14-micrometre-core fiber, excitation with a specific higher-order mode produced a fluorescence signal roughly five times stronger than that obtained with the fundamental mode.
The advantage is most pronounced in larger-core fibers, where HoM excitation can offset intrinsic fluorescence collection limitations, shifting optimal performance to shallower polishing depths.
Design Guidance for Real-World Sensors
Although the study is theoretical, it provides practical guidance for sensor design.
It demonstrates how polishing depth, core size, and excitation mode interact, and how higher-order mode excitation can expand the usable design space for reliable, highly sensitive sensors.
By outlining a unified description of the full excitation-emission-collection process, the model lays a foundation for optimizing fluorescence-based fiber optic sensors used in medical diagnostics, chemical analysis, and environmental monitoring.
Journal Reference
Baghapour S., Zhang W.Q., et al. (2025). A rigorous theoretical model of fluorescence-based fiber optic sensors: application to D-shaped fibers. Scientific Reports. DOI: 10.1038/s41598-025-33244-8