Jun Zhu

Turbulence-degraded orbital angular momentum (OAM) mode recognition is critical for free-space optical (FSO) communication but is limited by atmospheric distortion. We propose a cascaded receiver integrating a vision transformer-based Uformer-OAM intensity-restoration network, an SE-ResNet-18 neural demodulator, and an entropy-modulated scaling (ETS) strategy for low-density parity-check (LDPC) soft decoding. In split-step beam propagation method (SS-BPM) simulations, Uformer-OAM achieves 31.34 dB peak signal-to-noise ratio (PSNR) and 96.9% recognition accuracy under strong turbulence ( C _n ² =10 ⁻¹³ m ^−2/3 , z =1000m), outperforming UNet by 7.19 dB and 18.3 points, and the full cascade attains bit error rate (BER) −6 at z =1300m where baselines exhibit error floors.

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.

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.

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.

A Bessel function method incorporating carrier modulation is introduced, which accurately determines the phase shift amplitude of vibration signals by analyzing the amplitude relationship between the carrier’s main frequency and its harmonics. Experiments with a programmable signal generator and interferometer probe confirmed its feasibility and superiority over traditional Bessel function methods in weak signal calculations. In the experiment, this method can precisely calculate phase shifts down to 3.9 mrad and achieve a minimum detectable phase shift of 22 μrad, making it suitable for signal calibration in weak signal scenarios and analysis of phase carrier modulation signals.

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.