Cheng Zuo

A fiber optic multi-parameter sensor is proposed based on a Mach-Zehnder interferometer (MZI). The in-line MZI is constructed by inscribing a linear waveguide and a helical waveguide within a segment of no-core fiber (NCF) via a femtosecond laser, with the entire structure positioned between two segments of multi-mode fiber (MMF). Owing to its unique helical structure, the sensor demonstrates high sensitivity to torsion and curvature, and possesses the capability to distinguish torsion and curvature directions. Experimental results demonstrate that it achieves a maximum torsional sensitivity of -1.382 nm/rad/m within a counterclockwise (CCW) torsional range of 0-6.108 rad/m, and a maximum sensitivity of 1.274 nm/rad/m within a clockwise (CW) torsional range of 0-6.981 rad/m. In the curvature range of 0-2 m^-1, the sensor exhibits a maximum curvature sensitivity of 13.468 nm/m^-1 in the 180° bending direction. Over the temperature range of 40-80 °C, it presents a temperature sensitivity of -35.33 pm/°C. Moreover, by building a sensitivity matrix, it is possible to measure the curvature, torsion, and temperature parameters simultaneously. Featuring high sensitivity, compact design, and robust performance, the proposed sensor is promising for applications in flexible tactile sensing for intelligent robots, adaptive intelligent sensing systems, and structural health monitoring fields.

We propose a in-line Mach-Zehnder interferometer (MZI) structure based on femtosecond laser-inscribed dual waveguides in a no-core fiber (NCF). The in-line MZI comprises two waveguides with refractive index differences in NCF. Through systematic simulation studies, critical parameters including coupler length, waveguide length, and inter-waveguide spacing were optimized to achieve optimal sensor performance. Experimental results demonstrate that the proposed sensor exhibits superior sensitivity to both temperature and curvature, with a distinct directional selectivity observed in curvature detection. Within the curvature range of 1-1.5 m^-1, the sensor achieves a maximum curvature sensitivity of 34.727 nm/m^-1. Over a temperature range of 45 to 75 °C, the sensor exhibits a temperature sensitivity of 109.57 pm/°C. Furthermore, by constructing a sensitivity matrix, parameters of curvature and temperature can be measured simultaneously. With high sensitivity, a compact design, and robustness, the proposed sensor is appropriate for construction, bridge engineering, and aerospace applications.

A high-performance interferometric fiber optic hydrophone (FOH) for the deep sea is designed and developed in the paper. The two sensing arms of the Michelson interferometer are wound on the elastic and rigid layers of the mandrel, which enhance the sensitivity and hydrostatic pressure resistance of the FOH. By modulating a narrow linewidth laser, the linearity of modulation depth and optical path difference (OPD) with hydrostatic pressure is investigated, and the mismatch of OPD between the inner and outer wound fibers is corrected to ensure high phase resolution of the FOH system. The experimental results show that the FOH can maintain a sensitivity of about - 140 dB re rad/μPa within a hydrostatic pressure of 0 – 40 MPa. The measured total OPD change, which varies linearly with hydrostatic pressure, is about 0.604 m. The phase resolution of the corrected FOH is optimized by at least 11 dB compared to the FOH of the uncorrected OPD. At a hydrostatic pressure of 40 MPa, the minimum detectable pressure (MDP) of the FOH is about 7.08 μPa/√Hz at 1 kHz and 31.62 μPa/√Hz at 100 Hz, indicating that the FOH has good detection capability of weak acoustic signals in the deep-sea.

In a fiber sensing array, the time delay between channels significantly impacts the suppression of laser-introduced phase noise (LPN) through the reference interferometer scheme. A method for suppressing the impact of time delay, which includes the measurement and compensation, is proposed. Time delay in the entire 5-element sensor array is determined by calculating the phase difference between the calibrated signals, while compensating for the time delays between the sensing signals and the reference signal is achieved through temporal shifting of digital signal sequences. The experimental results show that the time-delay effect is suppressed in four channels of delay-sensing signals, with the suppression effect of 23.1-26.8 dB. Additionally, the compensated sensing signal exhibits a background noise ratio that differs by 5-8 dB compared to the non-delayed sensing signal. The correlation between time-delay residual and sampling rate is verified by theoretical analysis and experiments. If time-delay residual can be effectively reduced, the compensation effect can be optimized whether the sampling rate is increased or decreased. The proposed method effectively compensates for inter-channel time delay and introduces an optimization approach to enhance the compensation effect, thereby facilitating effective suppression on the influence of LPN in the sensing array.

This paper proposes a high-performance fiber optic interferometric hydrophone based on composite structure. Based on the air-backed push-pull fiber optic hydrophone structure, the inner elastic tube is replaced by a thick hollow column, introducing axial acoustic pressure sensing to improve the detection sensitivity of the fiber optic hydrophone. The influence of axial stress on the deformation of the composite structure is investigated by theoretical analysis and numerical simulation, and the structural design of fiber optic hydrophone with high acoustic pressure sensitivity and wide frequency response range is realized. The experimental results indicate that the average sensitivity of the fiber optic hydrophone based on the composite structure reaches –113.25 dB re rad/µPa with a frequency range of 10 Hz-3 kHz, approximately twice as much as compared with the push-pull structure, which is basically in accordance with the theory and simulation. Besides, the minimum detectable pressure (MDP) of the fiber optic hydrophone is about 2 µPa/√Hz at 1 kHz and 0.56 mPa/√Hz at 10 Hz. Compared to other state-of-the-art mandrel fiber optic hydrophones, the composite structure has improved MDP by at least 6 dB at 1kHz, which provides excellent potential for application in underwater weak signal detection.

Frequency-domain measurement method of time-delay based on laser intensity or phase noise as multi-frequency signal source is proposed. The frequency-spectrum distribution of noise signal caused by the time-delay is analyzed to extract the characteristic frequencies, for derive the time-delay. The experimental results with intensity noise source show that for the time-delay fiber ranging from 0.5 km to 15 km in length, the measurement deviation compared to OTDR results ranges from -0.0514% to 0.0949%, demonstrating good measurement accuracy. For multiple measurements of 5 km-long time-delay fiber, the measurement deviation range with intensity noise source is from 0.0417% to 0.0689%, with a characteristic frequency fluctuation of 11 Hz, corresponding to time-delay measurement resolution of 6.69 × 10 ^-9 s. The deviation range of the measurement with phase noise source is from -0.2437% to 0.1257%, with a characteristic frequency fluctuation of 150 Hz, corresponding to time-delay measurement resolution of 9.11 × 10 ^-8 s. Therefore, the time-delay measurement resolution for the two noise sources are lower than the minimum interval of 2 × 10 ^-6 s, corresponding to sample rate of 500 kHz. It also meets the precision requirements of shifting discrete-signal timing needed for common-mode suppression on phase or intensity noise in optical fiber sensor arrays.

For hydrophones with very-low frequency (VLF) and high detection resolution, elliptic-fitting phase-difference demodulation technology (EF-PDDT) will be investigated for suppressive effect on laser phase noise (LPN) and advantage of measuring VLF signal. The suppressive effect was analyzed by examining two types of phase noise resulting from center-frequency drift and linewidth of laser. The experimental results demonstrate that EF-PDDT achieves a suppressive effect of 44 dB on the phase signal, and the effect of 48 dB can be achieved on a wider linewidth laser. In the detection of VLF signals, EF-PDDT has higher information-utilization efficiency compared to quadrature phase demodulation technology, resulting in a reduction of data volume by 66 dB and circumventing the limitations imposed by long-term sampling due to equipment performance, thereby enhancing spectral resolution. Hence, EF-PDDT is an effective solution for improving the detection resolution of VLF signals, and finding applications in fields such as underwater communication and seismic-wave monitoring.

We propose and experimentally demonstrate an ultrasensitive temperature and strain sensor based on a hybrid fiber interferometer (HFI) with the Vernier effect. The HFI consists of a fiber coupler, a section of hollow core photonic crystal fiber (HCPCF), and a certain length of high birefringence polarization maintaining fiber (Hi-Bi PMF) to form a composite structure with a FPI built into the FSI. The HCPCF-based FPI is insensitive to temperature and strain and is an ideal reference interferometer. By adjusting the length of the HCPCF and Hi-Bi PMF, the Vernier effect can be created, which significantly increases the sensitivity of the sensor. Experimental results show that the proposed sensor has high temperature and strain sensitivities up to 50.671 nm/°C and -81.69 pm/με, respectively, with a high resolution of ± 0.0018 °C and ± 0.86 με. The proposed sensor is low-cost, compact, and easily fabricated, making it ideal for applications that require high-precision temperature and strain measurement.