Single-Mode Sapphire Fiber Micro-Displacement Sensing via Femtosecond Laser Direct Writing

doi:10.16553/j.cnki.issn1000-985x.2026.0079

Abstract

Abstract: Non-contact micro-displacement measurement is important for monitoring critical components in high-temperature harsh environments. Optical fiber sensors are attractive for such applications because of their compact size， electromagnetic immunity， and remote interrogation capability. Reflective Fabry-Pérot sensors with an external air cavity are particularly suitable for non-contact displacement measurement because the variation in the distance between the fiber end face and an external reflector can be converted into a change in the air-cavity length， which can be demodulated from the reflection spectrum. However， conventional cladding-free sapphire fibers support multimode transmission， which may induce mode coupling， spectral fluctuations， and unstable Fabry-Pérot interference signals. In this work， a reflective external Fabry-Pérot air-cavity micro-displacement sensor based on a femtosecond-laser-written single-mode guiding structure in sapphire fiber was proposed and fabricated. The external Fabry-Pérot air cavity was formed between the sapphire fiber end face and a gold-coated mirror. To improve the transmission characteristics of the sapphire fiber， refractive-index modification was introduced inside the fiber by femtosecond laser direct writing. A depressed-cladding structure was constructed in which the laser-modified peripheral region acted as an effective low-index cladding， while the unmodified central region served as the guiding core. With this design， single-mode transmission over a length of 4 cm was achieved， and multimode-transmission-induced interference in the sapphire fiber was effectively suppressed. The resulting clearer interference fringes provided a stable spectral basis for displacement demodulation. The displacement response of the sensor was experimentally investigated under different displacement step sizes. For the 5 μm step test， the cavity length was demodulated by calculating the average free spectral range of the reflection spectrum. The demodulated cavity length exhibited excellent linearity with the applied displacement， with a fitted slope of 1.000 0 μm/μm and a coefficient of determination R² of 0.999 97. These results indicate that the applied displacement can be accurately converted into the variation of the Fabry-Pérot air-cavity length. To further evaluate the response under smaller displacement variations， displacement tests with step sizes of 500， 100 and 50 nm were carried out. In these tests， a selected interference dip was tracked， and the displacement-wavelength response relationship was established. The displacement-wavelength sensitivities are obtained as 3.558 3， 3.368 5 and 3.037 6 nm/μm for the 500， 100 and 50 nm step tests， respectively. These results demonstrate that the proposed sensor provides reliable response and high-resolution tracking capability for displacement steps down to 50 nm. The main innovation of this work is the integration of a femtosecond-laser-written single-mode guiding structure in sapphire fiber with a reflective external Fabry-Pérot air cavity for non-contact micro-displacement measurement. The depressed-cladding structure improves the transmission state of the sapphire fiber and reduces the influence of multimode interference on the reflection spectrum， thereby enhancing the reliability of displacement demodulation. Benefiting from the excellent high-temperature stability of sapphire fiber， the proposed sensing structure shows potential for non-contact micro-displacement measurement in high-temperature environments. This study provides a useful reference for the development of single-mode sapphire-fiber-based displacement sensors for harsh-environment applications.

Key words: sapphire fiber; displacement measurement; single-mode operation; Fabry-Pérot air external cavity; femtosecond laser direct writing; depressed-cladding structure

CLC Number:

TN253

XU Jing, GAO Shan, ZHANG Bowen, LIU Yan, ZHU Yao, ZHANG Zhibo, YANG Jing, GUAN Chunying. Single-Mode Sapphire Fiber Micro-Displacement Sensing via Femtosecond Laser Direct Writing[J]. Journal of Synthetic Crystals, 2026, 55(7): 1135-1142.

Figures/Tables 9

Fig.1 Schematic of the reflection-type Fabry-Pérot sapphire fiber sensor. （a） Overall structure and optical path；（b） cross-section structure of the modified region

Fig.2 Schematic of the femtosecond laser processing system

Fig.3 Microscopic images of sensor structure. （a） Side view；（b） end-face view

Fig.4 Schematic of the displacement measurement system

Fig.5 Comparison of interference reflection spectra before and after single-mode modification of sapphire fiber

Fig.6 Displacement-cavity length response curve at a 5 μm step size

Fig.7 Displacement response results at a 500 nm step size. （a） Response spectra；（b） displacement-wavelength relationship

Fig.8 Displacement response results at a 100 nm step size. （a） Response spectra；（b） displacement-wavelength relationship

Fig.9 Displacement response results at a 50 nm step size. （a） Response spectra；（b） displacement-wavelength relationship

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