Objective Fiber-optic strain sensors have attracted considerable attention owing to their inherent advantages of compact structure, immunity to electromagnetic interference, high sensitivity, and excellent repeatability, making them promising candidates for applications in aerospace, structural health monitoring, and precision measurement. Among various interferometric configurations, the Mach-Zehnder interferometer (MZI) offers flexible design through the manipulation of interference length and mode coupling processes, and features the merits of all-fiber integration, straightforward fabrication, and low cost. However, existing MZI-based strain sensors often rely on complex fabrication techniques such as fiber tapering, core-offset splicing, or the incorporation of specialty fibers including photonic crystal fibers, which inevitably increase fabrication difficulty, reduce reproducibility, or elevate material costs. To address these limitations, this study proposes and investigates a novel MZI fiber-optic strain sensor based on a no-core fiber-multi mode fiber-no-core fiber (NCF-MMF-NCF) structure. The NCF segments serve as mode splitters and combiners to efficiently excite and recombine high-order cladding modes, while the MMF segment functions as the sensing interference region. This configuration combines the mode excitation capability of NCF with the stable multimode propagation characteristics of MMF, achieving enhanced intermodal interference through a simple and cost-effective fabrication process that requires only standard fusion splicing.

Methods The proposed sensor was fabricated by sequentially fusion-splicing NCF and MMF segments between two standard single-mode fibers. All splicing operations were performed using a Fujikura 66S fusion splicer in automatic mode. To optimize the structural parameters, the beam propagation method (BPM) was employed to simulate the optical field distribution within the NCF-MMF-NCF structure. Simulations were conducted for NCF lengths with a fixed MMF length, to investigate the influence of NCF length on the energy distribution between the core and cladding modes. Experimentally, the output spectra of sensors with varying MMF lengths and NCF lengths were measured using a broadband light source and an optical spectrum analyzer. Fast fourier transform (FFT) analysis was applied to the measured spectra to extract the dominant spatial frequency components, followed by in inverse fast fourier transform (IFFT) to reconstruct the filtered interference spectra and determine the free spectral range (FSR). Strain sensing experiments were conducted at room temperature (20 °C) by mounting the sensor between a fixed stage and a precision translation stage separated by 20 cm. Axial strain was applied in increments of 50 με over the range of 0–450 με, and the corresponding spectral shifts were recorded. Stability was evaluated by monitoring the central wavelengths of multiple interference dips over a 60-minute period, and repeatability was assessed through two consecutive stretching cycles.

Results and Discussions BPM simulation results reveal that when the NCF length is 1.5 cm, a self-focusing point forms near the NCF-MMF interface, concentrating optical energy predominantly in the MMF core region and resulting in insufficient cladding mode excitation. When the NCF length is increased to 4 cm, the self-focusing effect at the interface is effectively suppressed, enabling more efficient coupling of optical energy into the MMF cladding and thereby enhancing intermodal interference. Further increasing the NCF length to 7 cm and 10 cm leads to excessive propagation loss without additional benefit in cladding energy distribution. Experimental results demonstrate that the MMF length primarily governs the FSR of the output spectrum, with measured FSR values for MMF lengths, which are in good agreement with theoretical predictions. In contrast, varying the NCF length from 0.5 cm to 4 cm produces negligible changes in FSR, confirming that NCF length predominantly affects the cladding energy distribution rather than the interference period. FFT analysis further corroborates these findings: different MMF lengths yield distinct dominant spatial frequencies, whereas different NCF lengths maintain nearly identical frequency positions. Based on the optimization results, the final sensor configuration employs an MMF length of 2 cm and an NCF length of 4 cm. Strain sensing measurements show that the interference spectrum undergoes a blue shift with increasing axial strain. Linear fitting of the dip wavelength shift as a function of applied strain yields a sensitivity of 4.05 pm/με with a correlation coefficient of R² 0.973 over the 0–450 με range. Compared with previously reported MZI strain sensors, the proposed sensor achieves superior sensitivity while maintaining significantly simpler fabrication requirements. Stability tests confirm that the central wavelengths of the monitored interference dips exhibit only minor fluctuations within the 60 min observation window, and two consecutive strain loading cycles produce consistent sensitivity values, demonstrating good stability and measurement repeatability.

Conclusions This cost-effective MZI fiber-optic strain sensor based on the NCF-MMF-NCF structure has been proposed, optimized and experimentally validated. The sensor achieves a strain sensitivity of 4.05 pm/με over the range of 0–450 με with good linearity and stability. The NCF segments enable efficient high-order mode excitation and recombination through standard fusion splicing alone, eliminating the need for fiber tapering, core-offset alignment, or specialty fiber components. The MMF length determines the FSR of the output spectrum, while the NCF length controls the cladding energy distribution and interference contrast. The demonstrated advantages of simple fabrication, low cost, and enhanced sensitivity make this sensor a promising candidate for practical strain sensing applications in structural health monitoring and precision measurement.