A distributed optical fiber refractive index sensor based on etched Ge-doped SMF in optical frequency domain reflection (OFDR) was proposed and demonstrated. The etched Ge-doped SMF was obtained by only using wet-etching, i.e., hydrofluoric acid solution. The distributed refractive index sensing is achieved by measuring the spectral shift of the local RBS spectra using OFDR. The sensing length of 10 cm and the spatial resolution of 5.25 mm are achieved in the experiment. The refractive index sensing range is as wide as 1.33–1.44 refractive index units (RIU), where the average sensitivity was about 757 GHz/RIU. Moreover, the maximum sensitivity of 2396.9 GHZ/RIU is obtained between 1.43 and 1.44 RIU.

A high-spatial-resolution OFDR distributed temperature sensor based on Au-SMF was experimentally demonstrated by using step-by-step and image wavelet denoising methods (IWDM). The measured temperature between 50 and 600 °C could be successfully demodulated by using SM-IWDM at a spatial resolution of 3.2 mm. The temperature sensitivity coefficient of the Au-SMF was 3.18 GHz/°C. The accuracy of the demodulated temperature was approximately 0.24 °C. Such a method has great potential to expand the temperature measurement range, which is very useful for high-temperature applications.

A multi-channel OFDR strain sensor was demonstrated based on femtosecond-laser-inscribed wavelength/space division multiplexing weak fiber Bragg grating array (WFBGA) in standard SMF. At a sensing spatial resolution of 3 mm, the number of channels and the measurement strain range were eight and 9000 µϵ, respectively. The reflection spectral of each WFBG for each channel were successfully captured using spectral splitting method based on OFDR, thereby achieving strain demodulation for each channel. Compared with WDM WFBGAs, the maximum strain was only 1000 μϵ based on two-channel SMF.

A real-time and high spatial resolution optical frequency domain reflectometry (OFDR) distributed strain sensing system that entirely employs a field-programmable gate array (FPGA) for data acquisition and cross-correlation shift calculation was demonstrated. The key factors in achieving real-time OFDR are the use of 2-D fast Fourier transform (2D-FFT) and a parallel computing framework designed using DDR4. The effect of the total data size and the number of padded zeros on the FPGA processing time is studied. Compared to the serial computing of 1436 ms with a central processing unit (CPU), the processing time is reduced to 67 ms using FPGA, enhancing nearly 20 times. The two-point spatial resolution is 0.07 mm when the total data size is 4 MS. The strain standard deviation is $3.49~\mu \varepsilon $ with a processing time of 268 ms using the proposed FPGA-based OFDR when the total data size is 1 MS and the sliding window is padded with zero to 8192.

A high-spatial resolution and wide-range distributed strain sensor based on UV laser exposed single mode fiber (E-SMF) has been experimentally demonstrated, of which the maximum measurable strain was up to 9000 με under a spatial resolution of 1.5 mm. To address the tradeoff between the strain resolution and sensing spatial resolution, an efficient adaptive zero padding (AZP) was proposed to calculate the number of padding zeros to optimize the number of zeroing padding (NZP). Compared with traditional zero padding, the cross-correlation calculation time was improved from 138.842s to 0.476s by using proposed AZP method under the spatial resolution of 1.5 mm, indicating that the cross-correlation calculation time was reduced by nearly 300 times. Moreover, the distance compensation method (DCM) was employed to improve the deteriorated similarity induced by position deviation between the reference and measurement signal. could be greatly improved by using DCM. The strain property of the E-SMF and SMF was compared and investigated by DCM based on AZP method. The strain profile of the E-SMF could be successfully and well demodulated in the zero- and stretched-strain section at the spatial resolution of 1.5 mm by using DCM, when the applied strain was increased from 1000 to 9000 με.

Fiber-optic sensing systems based on a forward transmission interferometric structure can achieve high sensitivity and a wide frequency response over long distances. However, there are still shortcomings in its ability to position multi-point vibrations and detect low-frequency vibrations, which limits its usefulness. To address these challenges, we study the viability of merging long-range forward-transmission distributed vibration sensing (FTDVS) with high spatial resolution optical frequency-domain reflectometry (OFDR), forming the first reported hybrid distributed sensing method between these two methods. The probe light source is shared between the two sub-systems, which utilizes stable linear optical frequency sweeping facilitated by high-order sideband injection locking. As a result, this is a new approach for the FTDVS method, which conventionally uses fixed-frequency continuous light. The method of nearest neighbor signal replacement (NSR) is proposed to address the issue of discontinuity in phase demodulation under periodic external modulation. The experimental results demonstrate that the hybrid system can determine the position of vibration signals between 0 and 900 Hz within a sensing distance of 21 km. When the sensing distance is extended to 71 km, the FTDVS module can still function adequately for high-frequency vibration signals. This hybrid architecture offers a fresh approach to simultaneously achieving long-distance sensing and wide frequency response, making it suitable for the combined measurement of dynamic (e.g., gas leakage, pipeline excavation warning) and quasi-static (e.g., pipeline displacement) events in long-distance applications.