This article investigates using optical fibers with enhanced backscattering profiles to improve distributed fiber sensor performance and reduce instrumentation costs. Using a femtosecond (fs) laser direct writing technique, the Rayleigh backscattering profile of a standard telecom fiber was enhanced by more than 40 dB to improve the signal-to-noise ratio (SNR) for optical frequency domain reflectometry (OFDR). The enhanced backscattering signals enable effectively distributed strain measurements using a low-cost tunable laser (TL). Median filtering is applied to denoise cross correlation results to further improve measurement outcomes. Results presented in this article show that a TL with a 1-nm tuning range, which is far less than the tuning range used in commercial OFDR interrogators, can perform effectively distributed strain measurements using sensing fibers with enhanced backscattering profiles. The sensing fiber with over 40-dB backscattering enhancement achieved 4.8-cm spatial resolution in strain measurements with a root mean square accuracy of less than 2.70 $\mu \varepsilon $ when 10–50 $\mu \varepsilon $ were exerted to the sensing fiber. Results presented in this article reveal both the potential and limitations of sensing fibers with enhanced backscattering for OFDR-based distributed fiber sensors.

This paper presents a method of reel-to-reel femtosecond laser direct writing that enables the continuous inscription of low-loss and high-temperature stable Rayleigh scattering centers inside the core of single-mode optical fibers for distributed temperature sensing up to 1000°C. By examining the correlation between the Rayleigh backscattering profile and the cross-section morphology of femtosecond laser-induced nanograting in fiber cores, this paper reveals the mechanisms that underlie the fabrication of high-temperature stable distributed fiber sensors with low loss. By fine-tuning laser exposure conditions, the femtosecond laser-fabricated Rayleigh scattering enhanced section could achieve an optimized propagation loss of 0.01 dB/cm with an increased signal-to-noise ratio of over 35 dB for meters of lengths. Long-term high-temperature stability of the Rayleigh scattering enhanced section was successfully demonstrated, with improved thermal stability and signal-to-noise ratio. The fabrication method presented here provides a promising technique to improve the performance of Optical Frequency-Domain Reflectometry based distributed sensing for harsh environment applications.

We present a highly reproducible method of fabricating a tapered intrinsic Fabry-Perot interferometer (IFPI) device with 5~6-μm diameter at the taper waist. A femtosecond laser was applied to inscribe an IFPI with 3-cm cavity length in a single-mode fiber. A CO ₂ laser heated tapering process enabled by digitally controlled mirrors and a precision motorized fiber feed system was used to create a stable heating zone with the desired temperature profile for tapering the fiber IFPI cavity. The well-engineered tapering process produced tapered IFPI devices with insertion loss less than 0.3-dB at 1550-nm. A strong evanescent field exposed by the taper section was explored for refractive index sensing. Using swept optical frequency domination reflectometry, the tapered IFPI fiber sensor achieved a minimal refractive index sensing resolution 2×10 ^-5 . This paper demonstrates an integrated laser fabrication technique to produce tapered fiber optic devices for sensing applications.

This paper presents a high-resolution, signal-jump-free demodulation method for Fiber Bragg Grating (FBG) systems employing a low-cost spectrometer. Utilizing a modified Logarithm Gaussian Levenberg-Marquardt (LGLM) method, the proposed approach enables precise and stable Bragg wavelength tracking. The principles of the LGLM method were thoroughly analyzed and derived, affirming the feasibility and robustness of the FBG system. Static experiments with 9 sampling points demonstrated that the LGLM method achieves a standard deviation (STD) of 1.7 pm, representing a 6.9-fold improvement over conventional Gaussian fitting. Strain experiments confirmed a strong linear relationship between strain and Bragg wavelength, with dynamic strain measurements at a 6 kHz sampling rate showing that the LGLM method effectively eliminates signal jumps observed in Gaussian fitting. At 500 Hz, the minimum detectable strain (MDS) of the LGLM method was determined to be 0.0173 με/Hz^−1/2, more than 10 times better than Gaussian fitting. Temperature experiments conducted over a range of 100–700 °C further validated the method's performance, with the LGLM achieving an STD of 1.56 pm and a variation of 3.4 pm, representing improvements of 10.25 times and 52.9 times, respectively, compared to Gaussian fitting. These findings demonstrate the exceptional sensitivity, stability, and robustness of the LGLM method for demanding applications such as high-temperature monitoring and structural health diagnostics.

This paper presents a low-cost integrated intrinsic Fabry-Perot interferometer (IFPI) sensor array demodulation algorithm based on local spectral analysis, which exploits multiple local spectra instead of the entire spectrum to achieve ultrasonic vibration measurement well beyond the sweeping period of the tunable laser source. By balancing the demodulation range and the demodulation resolution of IFPIs, this paper divides the entire spectrum produced by a distributed feedback (DFB) laser into 152 local spectra by a sliding window for IFPIs demodulation respectively, thereby implementing up to 160 kHz vibration measurement. To reduce the load on the data acquisition system and simplifies the interrogation system, this paper adopts a nonlinear tuning error compensation method based on an in-line IFPI sensor. In addition, chirp Z-transform is used in this paper to replace the traditional zero-padding-based fast Fourier transform to achieve the local fineness of demodulated optical path difference (OPD) spectrum, so as to reduce the waste of computation caused by zero-padding.

This article examines the impacts of mode hopping on the sensing performance of optical frequency domain reflectometry (OFDR) and explores the potential for developing economical OFDR interrogators employing low-cost distributed feedback (DFB) lasers. By conducting numerical simulations, this study reveals that mode hopping has minimal effects on distance sensing measurements in free space due to the limited duration of beat interference signal at the incorrect frequency within the coherence length. Additionally, the simulations indicate that mode hopping only slightly affects the distributed strain sensing of OFDR, resulting in an error range of less than ±$1\mu$$\mathsf{\epsilon }$ when $100\mu$$\mathsf{\epsilon }$ is applied to the sensing fiber. These findings highlight the potential of using low-cost DFB lasers with a 1-nm wavelength sweep range and a 1-MHz linewidth as tunable laser sources in OFDR while maintaining reliable and accurate sensing performance.