Wound Fiber-Optic Vibration Sensors

• Wound fiber-optic vibration sensors are systems where fibers are helically wrapped to convert mechanical vibrations into optical changes, offering distributed sensing and enhanced low-frequency sensitivity.
• They leverage modalities such as phase modulation, speckle analysis, and polarimetric demodulation with techniques like ϕ-OTDR and Floquet-engineered clocks to achieve high resolution and dynamic range.
• Critical design factors, including winding geometry, fiber type, and signal processing, determine performance in applications from geophysical sensing to structural health diagnostics.

Wound fiber-optic vibration sensors are engineered systems in which optical fibers are helically or spirally wrapped around an inertial form or structure to transduce mechanical vibrations into optical phase, intensity, polarization, or speckle changes. These platforms enable highly sensitive and distributed detection of vibrational and acoustic phenomena over extended bandwidths and spatial ranges, and can be tailored by leveraging advanced interrogation schemes such as Floquet-engineered optical lattice clocks, phase-sensitive optical time-domain reflectometry (ϕ-OTDR), and multimodal speckle-polarization analysis. Their performance is governed by fiber geometry, winding topology, optical interrogation protocols, environmental coupling, and post-processing methodologies. Recent research demonstrates that wound configurations are essential for achieving enhanced sensitivity at low frequencies, high spatial resolution, and robust discrimination of vibrational signatures, surpassing conventional interferometric and single-modality approaches (Yin et al., 21 Jan 2026, Fedorov et al., 2015, Monteiro et al., 26 Sep 2025).

1. Fundamental Principles of Wound Fiber-Optic Vibration Sensing

Wound fiber-optic vibration sensors utilize the mechanical-to-optical transduction inherent in guided-wave photonics. Wrapping an optical fiber around a mandrel, pipeline, or inertial block translates external vibration or acceleration into local or distributed strain along the fiber, which modulates its optical properties. The principal mechanisms employed in documented sensor systems include:

• Phase modulation: Mechanical strain induces an optical path length change, imparting a time-dependent phase shift to the propagating light field, measurable via interferometry or clock-based spectroscopy (Yin et al., 21 Jan 2026, Fedorov et al., 2015).
• Speckle variation: Multimode interference pattern changes encode localized deformations or vibration events, extractable via high-speed imaging (Monteiro et al., 26 Sep 2025).
• Polarization change: Strain-induced birefringence alters the polarization state of transmitted light in single-mode sections, amenable to polarimetric demodulation (Monteiro et al., 26 Sep 2025).

Structurally, sensitivity to vibration is enhanced by the number of fiber turns $N$ and the winding geometry, as each turn contributes to cumulative strain effects. Spatial resolution is influenced by fiber type, pulse duration (in time-domain reflectometry), and speckle pattern decorrelation length.

2. Floquet-Engineered Demodulation with Optical Lattice Clocks

The Floquet-engineered optical lattice clock demodulation architecture (Yin et al., 21 Jan 2026) implements ultra-sensitive, low-frequency vibration measurement by integrating a wound fiber with an FBG-based retro-reflection in the lattice path of a cold-atom clock. Vibration-induced displacements $z_0(t) = A_0 \sin(\omega_v t)$ modulate the position of the lattice potential and the phase of the interrogating clock laser. In the comoving frame, the system Hamiltonian is presented as:

$$H_\text{com}(t) = \frac{p^2}{2m_a} + V_0 \cos^2(k_L z) - F(t) z,$$

with $F(t) = -m_a A_0 \omega_v^2 \sin(\omega_v t)$.

Probing the clock transition $|g\rangle \rightarrow |e\rangle$ with a phase-modulated Rabi drive leads to a Fourier-expanded Rabi frequency with modulation index $\beta = k_c A_0$. The vibrational dynamics are mapped to resolvable Floquet sidebands in the Rabi transition spectrum via the Jacobi–Anger expansion:

$$\Omega(t) = \Omega_0 e^{i\beta \sin(\omega_v t)} = \Omega_0 \sum_{m=-\infty}^{\infty} J_m(\beta) e^{i m \omega_v t}.$$

The resulting sideband amplitudes $J_m(\beta)$ provide a direct readout of vibration-induced phase excursions. Notably, this approach achieves a phase-change sensitivity exceeding $6\times10^3~\text{rad/g}$ over frequencies from 0.5 Hz to 200 Hz using a 4 km wound fiber (2 dB/km loss), a regime where conventional Fabry–Pérot or homodyne sensors rapidly lose fidelity due to noise and phase ambiguity (Yin et al., 21 Jan 2026).

Key advantages include sub-Hz operational bandwidth, immunity to $2\pi$ ambiguity, and a transportable package integrating vacuum, optical, and control subsystems on a compact rack.

3. Distributed Sensing via Phase-Sensitive OTDR and KZ Filtering

ϕ-OTDR-based wound fiber sensors employ time-resolved Rayleigh backscattering to map distributed strain. A pulsed, narrow-linewidth laser traverses the fiber, and the spatial origin of returning backscatter is determined through time-of-flight:

$z = \frac{c}{2n} t.$

The dynamic-strain-induced phase change along a gauge length $L_g$ is

$$\Delta\phi(z, t) = \frac{4\pi n}{\lambda} \epsilon(z, t) L_g.$$

Winding the fiber multiplies sensitivity by $N$: $$\Delta\phi_\text{total} \approx N \frac{4\pi n}{\lambda} \epsilon_\text{local} L_g$$. KZ filtering is deployed to suppress seismic and instrument noise, using recursive moving averages:

$$KZ_{m,k}(x_t) = S_m[ S_m[ \ldots S_m(x_t) \ldots ] ],$$

with $k$ nested applications of an $m$-point average.

This approach yields highly robust detection, localization error $\sigma_\text{loc} \sim 1~\mathrm{m}$ (pulse width 200 ns), and recognition probabilities $P_D > 0.95$ (human), with the false alarm rate $<2\times10^{-2}$ per km·h. Winding details, such as radius ($R\geq10$ cm to suppress bend loss) and turns ($N=10$–50), are critical for balancing sensitivity and loss (Fedorov et al., 2015).

Event types (localized, delocalized in time/space) can be distinguished by interpreting KZ-filtered deviation signatures, facilitating applications in seismic, intrusion, and monitoring domains.

4. Multimodal Speckle-Polarization Fiber Sensing

Recent multimodal wound fiber sensors synergize speckle-based localization and high-bandwidth polarimetric waveform reconstruction (Monteiro et al., 26 Sep 2025). The system comprises:

• A speckle-localization arm: 532 nm CW laser delivered through MMF (50 μm core) helically wound on the structure, with output speckle patterns recorded at 4,000 FPS on a camera. The spatial resolution, set by modal group-delay spread, achieves $\Delta z_\text{res} \sim 3$ cm over 4 m MMF.
• A polarimetric monitoring arm: 1550 nm CW laser through co-routed SMF (9 μm core), with a high-speed polarimeter (200 kHz) to decode waveform content from induced birefringence.

Localization and waveform data are fused via pseudo-inverse processing, calibrated zone-wise for the speckle arm and globally for the polarization arm. This architecture achieves bandwidth of 100 Hz–40 kHz (polarimetric SNR >20 dB mid-band), dynamic range ≥60 dB, and zone discrimination accuracy exceeding 99% (see Table).

Modality	Resolution/Bandwidth	Key Performance
Speckle (MMF)	3 cm / 2 kHz	Zone localization, >99%
Polarimetric	— / 40 kHz	Waveform SNR >20 dB

Environmental robustness is ensured by periodic recalibration and suitable winding radii (≥3 cm for MMF, ≥5 cm for SMF). The fused approach addresses the bandwidth-resolution trade-off inherent in single-modality fiber sensors.

5. Implementation Considerations and Sensor Engineering

Sensor performance and practicality hinge on multiple engineering parameters:

• Winding geometry and materials: The number of turns ($N$), helix angle ($\theta \sim 45^\circ$), and winding radius impact sensitivity, mechanical robustness, and environmental noise coupling.
• Fiber handling: Buffer layers or polymer coatings reduce mechanical cross-talk (“loop–loop” interactions), while maintaining sufficiently gentle bends suppresses macro-bend and modal loss.
• Optoelectronic integration: For Floquet-engineered schemes, clock laser residual linewidth (sub-Hz) and ultra-low phase noise ($<-100$ dBc/Hz) are critical for resolving narrow spectral features (Yin et al., 21 Jan 2026). In OTDR architectures, pulse width determines spatial resolution but must be balanced against signal-to-noise.
• Calibration and signal processing: Multimodal systems employ zone-specific and global calibration for speckle and polarization arms, respectively, with real-time processing on high-throughput digitizers or GPUs (Monteiro et al., 26 Sep 2025).
• Environmental controls: Maintaining temperature stability ($\pm0.1$ K), acoustic isolation, and minimizing thermal drift are essential across all platforms for long-term stability and low false alarm rates.

A typical field-deployable Floquet-engineered sensor includes a coil of 50–100 turns, an FBG element, and a low-creep inertial block. The assembly, including optoelectronic subsystems, can be realized in compact, rack-mounted form factors (Yin et al., 21 Jan 2026).

6. Comparative Performance and Practical Limits

New-generation wound fiber sensors using Floquet demodulation attain phase-change sensitivity $>6\times10^3~$rad/g at frequencies down to 0.5 Hz, exceeding conventional interferometric and OTDR schemes (typically $10^2$–$10^3$ rad/g above 10 Hz, with substantial rolloff at low frequencies) (Yin et al., 21 Jan 2026). Multimodal approaches achieve centimeter-scale spatial localization and kHz–tens of kHz bandwidth, bypassing the resolution–bandwidth trade-off inherent to conventional OTDR.

Limitations are set by:

• Photon budget and atomic shot noise (Floquet/clock platforms).
• Optical loss and modal dispersion (long-wavelength and multimodal interrogations).
• Environmental drift and thermal coupling, which affect phase referencing and calibration.
• Trade-off between spatial resolution and SNR (ϕ-OTDR).

Increasing sensor length, turn number, and fiber quality and employing advanced coding or signal processing can push spatial and frequency limits further, subject to engineering tolerances in winding and optoelectronic readout.

7. Future Prospects and Integration Pathways

Wound fiber-optic vibration sensor systems demonstrate substantial scalability for structural health monitoring, sub-Hz geophysical sensing, non-intrusive surveillance, and industrial machinery diagnostics. Future directions include integrating machine-learning classifiers with KZ-filtered features to suppress false alarms in non-stationary backgrounds (Fedorov et al., 2015), implementing adaptive multimodal fusion for robust discrimination of co-located vibrational sources, and further miniaturization of atomic clock-based readouts (Yin et al., 21 Jan 2026). Advances in fiber design, low-loss FBG integration, and multi-scale calibration protocols are expected to broaden the operational envelope and fidelity of wound sensor platforms for distributed, low-maintenance deployments.

References:

• Floquet-engineered wound fiber-optic vibration sensors (Yin et al., 21 Jan 2026)
• Distributed ϕ-OTDR wound sensors and KZ filtering (Fedorov et al., 2015)
• Multimodal speckle–polarization fiber vibration sensors (Monteiro et al., 26 Sep 2025)