Multi parameter discrimination using multiple spectral troughs in a cascaded fiber sensor

TL;DR

This work addresses cross-sensitivity in optical fiber sensing by engineering a cascaded SMF–MMF–LPFG platform that hosts multiple spectral troughs from distinct physical mechanisms. By treating each trough as a vector response to temperature, strain, and refractive index and applying a calibrated sensitivity-matrix inversion, the authors demonstrate robust multi-parameter discrimination within a single spectrum using wavelength-based demodulation. The configuration optimization shows clear separation of spectral features, with troughs exhibiting high linearity and modest cross-coupling, enabling accurate temperature, strain, and RI retrieval even under coupled disturbances; dual-parameter decoupling is supported by a 2×2 matrix formulation. Experimental validation, including dynamic concentration tracking and thermo-mechanical decoupling, confirms the approach’s practical potential for compact, low-comocial, and field-deployable multi-parameter sensing with improved resolution and stability.

Abstract

Accurate monitoring of temperature, axial strain, and refractive index is critical for structural health monitoring, industrial process control, and environmental sensing. However, conventional optical fiber sensors are often limited by strong parameter cross sensitivity, poor discrimination capability, and increased system complexity when multiple sensing units are required. In this work, a compact multi-parameter optical fiber sensing platform is proposed based on a cascaded single-mode fiber, multimode fiber, and long-period fiber grating structure, combined with a wavelength-based spectral demodulation strategy. Within the cascaded configuration, multiple characteristic spectral troughs arising from distinct physical mechanisms coexist in a single transmission spectrum. Interference-induced troughs are generated by the multimode fiber section, while a resonance-induced trough is introduced by the long-period fiber grating. Although none of these troughs responds exclusively to a single parameter, each exhibits simultaneous and linearly independent responses to temperature, axial strain, and refractive index with distinct sensitivity magnitudes and trends. Consequently, each trough can be described by a unique sensitivity vector, enabling robust multi-parameter discrimination through multi-wavelength spectral demodulation.

Figures (5)

• Figure 1: Optical path configuration and spectral responses of the cascaded LPFG--PMF sensing system. (a) Schematic diagram of a Sagnac interferometer incorporating both a long period fiber grating (LPFG) and a polarization-maintaining fiber (PMF) within the same loop. (b) Measured output spectrum of the LPFG--PMF composite Sagnac configuration, where the LPFG resonance trough spectrally overlap with the Sagnac interference fringes. (c) Enlarged view of the coupling region between the LPFG and PMF, illustrating spectral distortion and resonance splitting induced by local over-coupling effects. (d) Optimized sensing configuration, in which the PMF solely forms the Sagnac interferometric loop, while the LPFG operates in-line in transmission along the main optical path. (e) Representative spectral responses of the optimized system under external perturbations, together with the corresponding shifts of selected characteristic wavelengths. (f) Linear dependence of the characteristic wavelength shifts on the applied external physical parameters.
• Figure 2: Comparison of spectral responses for different fiber connection configurations. (a) Schematic illustration of configuration I, employing an SMF--MMF--SMF structure with a controlled lateral offset at the splicing interfaces to induce multimode interference. (b) Schematic illustration of configuration II (MMF--SMF--MMF), where the inserted SMF section acts as a spatial mode filter and largely suppresses effective multimode excitation in the adjacent MMF segment. As a result, the transmitted spectrum is dominated by smooth insertion-loss variations rather than well-defined interference-induced troughs, which limits its suitability for wavelength-tracking-based sensing. (c) Transmission spectra obtained from configuration I under different offset conditions, showing that an offset of approximately 50% yields the most distinct and reproducible resonance features. (d) Transmission spectra corresponding to configuration II, exhibiting weak spectral modulation and the absence of well-defined characteristic trough.
• Figure 3: Comparison of sensing responses obtained using different spectral demodulation schemes. (a) Transmission spectra of the multimode fiber measured at different temperatures (40, 60, 80, and 100 $^\circ$C), where the spectral response is dominated by transmission intensity variation with no well-defined wavelength shift. (b) Temperature response extracted from intensity-based demodulation of the multimode fiber spectra in (a), exhibiting pronounced data dispersion and limited linearity. (c) Wavelength shift of a characteristic spectral trough measured at different temperatures (30, 60, and 90 $^\circ$C), demonstrating a clear and monotonic wavelength response. (d) Linear fitting of the characteristic wavelength as a function of temperature, from which the temperature sensitivity and linearity are extracted. (e) Wavelength shift of the characteristic spectral trough under different applied axial strain levels (0, 3, and 6 $m\varepsilon$). (f) Linear fitting of the characteristic wavelength as a function of axial strain, showing improved linearity and repeatability compared with intensity-based demodulation.
• Figure 4: Spectral responses of the SMF--MMF--LPFG cascaded sensor under multiple external perturbations. (a) Schematic illustration of the cascaded SMF--MMF--LPFG sensing configuration. (b) Transmission spectra measured at discrete temperatures of 30, 60, and 90 $^\circ$C, showing multiple characteristic spectral troughs. (c) Wavelength evolution of three representative spectral troughs as the temperature increases from 30 to 90 $^\circ$C with a step of 10 $^\circ$C. (d) Wavelength shifts of the spectral troughs under discrete applied axial strain levels of 0, 3, and 6 $m\varepsilon$. (e) Continuous wavelength evolution of the spectral troughs as the axial strain varies from 0 to 6 $m\varepsilon$ with a step of 1 $m\varepsilon$. (f) Spectral responses of the characteristic troughs measured at discrete relative refractive index levels of 0%, 10%, and 20%. (g) Continuous wavelength evolution of the spectral troughs as the relative refractive index increases from 0% to 20% with a step of 2%.
• Figure 5: Experimental validation of the proposed multi-parameter decoupling strategy under coupled temperature disturbances. (a) Dynamic NaCl concentration tracking under periodic temperature modulation. The concentration ground truth is programmed as stepwise levels of 0%, 3.5%, and 5%, while the temperature is continuously varied between approximately 30 $^\circ$C and 40 $^\circ$C. The conventional single-parameter method exhibits pronounced temperature-induced concentration fluctuations, whereas the proposed method accurately tracks the true concentration with significantly reduced error. (b) Temperature--strain decoupled mechanical sensing under concurrent strain loading and temperature steps. The conventional method shows substantial apparent strain drift caused by temperature variations, while the proposed method effectively suppresses thermal cross-sensitivity and faithfully reproduces the strain ground truth.