Chirp-pulse pair φOTDR.

We proposed a novel, to the best of our knowledge, chirp-pulse pair phase-sensitive optical time-domain reflectometry (CPP-φOTDR) technique, enhanced by an adaptive filtering algorithm. This technique utilizes a pair of chirp pulses: one with a low chirp rate and another with a high chirp rate, with their Rayleigh backscattering (RBS) processed through a low-pass (LP) electrical filter. The adaptive filtering algorithm effectively preserves the extensive measurement range afforded by the high chirp rate pulse while enhancing the sensitivity provided by the low chirp rate pulse.

Deep empirical neural network for optical phase retrieval over a scattering medium.

Supervised learning, a popular tool in modern science and technology, thrives on huge amounts of labeled data. Physics-enhanced deep neural networks offer an effective solution to alleviate the data burden by incorporating an analytical model that interprets the underlying physical processes. However, it completely fails in tackling systems without analytical solution, where wave scattering systems with multiple input multiple output are typical examples.

High-sensitivity hot-wire anemometer using cobalt-doped fiber-based long-period gratings.

A high-sensitivity hot-wire anemometer is proposed for use with a cobalt-doped fiber (CDF) based long-period grating (LPG) heated optically by a 1480 nm laser. The CDF-LPG absorbs laser power and generates heat inherently, thereby eliminating the need for both metal coating and mode coupling devices that are usually required in optical fiber grating anemometers. The dip wavelength of the CDF-LPG shifts with airflow velocity due to the cooling effect of the airflow.

Physical layer security--enhanced optical communication based on chaos masking and chaotic hardware encryption.

The security and confidentiality of information are crucial in contemporary communication systems. In this work, we propose a physical layer security-enhanced optical communication scheme based on dual-level protection with chaos masking (CMS) and chaotic hardware encryption. The integration of CMS and chaotic hardware encryption contributes to enhancing the security of the system.

Phase noise suppression of optic flexural disk accelerometer by studying the thermal stability of optical fiber ring.

As the core sensing elements of ultra-long fiber interferometer, the distributed thermal strain difference of the fiber rings can cause extra noise of the flexural disk, resulting in a penalty of the deterioration accuracy. In this paper, the thermal strain distribution characteristics of the fiber ring are firstly analyzed by the finite element method (FEM), and the distribution result is consistent with that demonstrated by the Rayleigh optical frequency-domain reflectometry (R-OFDR) strain measurement. The interferometer phase noise caused by the distributed strain difference is further studied by constructing a fully symmetric polarization-maintaining fiber-ring Mach-Zehnder interferometer (MZI) with an arm length of over 100 meters.

Vectorial holography over a multimode fiber.

Vectorial holography through a strongly scattering medium can facilitate various applications in optics and photonics. However, the realization of vectorial holography with arbitrary distribution of optical intensity is still limited because of experimental noise during the calibration of vectorial transmission matrix (TM) and reconstruction noise during the retrieval of input wavefront for a given holographic target. Herein, we propose and experimentally demonstrate the vectorial holography with arbitrary distribution of optical intensity over a multimode fiber (MMF) using the Tikhonov regularization.

Non-orthogonal optical multiplexing empowered by deep learning.

Orthogonality among channels is a canonical basis for optical multiplexing featured with division multiplexing, which substantially reduce the complexity of signal post-processing in demultiplexing. However, it inevitably imposes an upper limit of capacity for multiplexing. Herein, we report on non-orthogonal optical multiplexing over a multimode fiber (MMF) leveraged by a deep neural network, termed speckle light field retrieval network (SLRnet), where it can learn the complicated mapping relation between multiple non-orthogonal input light field encoded with information and their corresponding single intensity output.

Principles and Applications of Seismic Monitoring Based on Submarine Optical Cable.

Submarine optical cables, utilized as fiber-optic sensors for seismic monitoring, are gaining increasing interest because of their advantages of extending the detection coverage, improving the detection quality, and enhancing long-term stability. The fiber-optic seismic monitoring sensors are mainly composed of the optical interferometer, fiber Bragg grating, optical polarimeter, and distributed acoustic sensing, respectively. This paper reviews the principles of the four optical seismic sensors, as well as their applications of submarine seismology over submarine optical cables.

Improve accuracy and measurement range of sensing in km-level OFDR using spectral splicing method.

In this paper, we propose and demonstrate a spectral splicing method (SSM) for distributed strain sensing based on optical frequency domain reflectometry (OFDR), which can achieve km level measurement length, µɛ level measurement sensitivity and 10 µɛ level measurement range. Based on the traditional method of cross-correlation demodulation, the SSM replaces the original centralized data processing method with a segmented processing method and achieves precise splicing of the spectrum corresponding to each signal segment by spatial position correction, thus realizing strain demodulation. Segmentation effectively suppresses the phase noise accumulated in the large sweep range over long distances, expands the sweep range that can be processed from the nm level to the 10 nm level, and improves strain sensitivity.

Optical frequency domain polarimetry based on a self-referenced unbalanced Mach-Zehnder interferometer.

Optical frequency domain polarimetry (OFDP) is an emerging distributed polarization crosstalk rapid measurement method with an ultrawide dynamic range. However, interferometric phase noise induced by the laser source and ambient noise results in a trade-off between measurement length and dynamic range. In this Letter, we solve this problem with a self-referenced unbalanced Mach-Zehnder interferometer.

Online polarization error suppressed optical vector analyzer based on Bayesian optimization.

An optical vector analyzer (OVA) based on orthogonal polarization interrogation and polarization diversity detection is widely used to measure an optical device's loss, delay, or polarization-dependent features. Polarization misalignment is the OVA's primary error source. Conventional offline polarization alignment using a calibrator greatly reduces the measurement reliability and efficiency.

Multiple plasmon-induced transparency based on black phosphorus and graphene for high-sensitivity refractive index sensing.

A hybrid bilayer black phosphorus (BP) and graphene structure with high sensitivity is proposed for obtaining plasmon-induced transparency (PIT). By means of surface plasmon resonance in the rectangular-ring BP structure and ribbon graphene structure, a PIT effect with high refractive index sensitivity is achieved, and the surface plasmon hybridization between graphene and anisotropic BP is analyzed theoretically. Meanwhile, the PIT effect is quantitatively described using the coupled oscillator model and the strong coherent coupling phenomena are analyzed by adjusting the coupling distance between BP and graphene, the Fermi level of graphene, and the crystal orientation of BP, respectively.

Ring-core few-mode fiber and DPP-BOTDA-based distributed large-curvature sensing eligible for shape reconstruction.

This study proposes a distributed large-curvature sensor based on ring-core few-mode fiber (RC-FMF) and differential pulse-pair Brillouin optical time-domain analysis (DPP-BOTDA). The RC-FMF is adhered to a thin steel substrate and an asymmetric hump shape is reconstructed using the Frenet-Serret algorithm. The proposed curvature sensor demonstrates a larger curvature-sensing range, excellent tolerance to bending-induced optical loss, and increased Brillouin gain coefficient.

Intensity-interrogated hot-wire anemometer based on chirp effect of a fiber Bragg grating.

An intensity-interrogated optical fiber hot-wire anemometer based on the chirp effect of fiber Bragg grating (FBG) is presented. The FBG is coated with a silver film and heated optically by a 1480 nm laser beam, which is coupled into the fiber cladding by a long-period grating (LPG) and absorbed by the silver film to convert to thermal heat. Due to the gradual decrease of laser power along the length of the FBG, a temperature gradient is formed that induces a chirp effect to the FBG.

Optical Fiber Interferometric Humidity Sensor by Using Hollow Core Fiber Interacting with Gelatin Film.

An optical fiber Fabry-Perot interferometer (FPI) is constructed for relative humidity measurement by fusion splicing a short hollow core fiber (HCF) to the end of a single-mode fiber and coating the tip of the HCF with a layer of gelatin. The thickness of the gelatin film changes with ambient humidity level and modulates cavity length of the FPI. Humidity measurement is therefore realized by measuring the wavelength shift of the interreference fringe.

Combined-Vernier effect based on hybrid fiber interferometers for ultrasensitive temperature and refractive index sensing.

A kind of hybrid fiber interferometer consisting of a fiber Sagnac interferometer (FSI), a closed-cavity Fabry-Perot interferometer (FPI), and an open-cavity FPI is proposed for generating combined-Vernier-effect. Through adjusting the polarization-maintaining fiber (PMF) length of the FSI, the free spectral range (FSR) is tailored to be similar to that of the parallel-connected reference FPI for producing the first Vernier effect, of which the spectrum is used to match the sensing FPI spectrum for obtaining the second Vernier effect. Noticeable lower and upper spectral envelopes are achieved in the first and second Vernier effects, respectively, so called the combined-Vernier spectrum.

Distributed refractive index sensing based on bending-induced multimodal interference and Rayleigh backscattering spectrum.

A distributed refractive index (RI) sensor based on high-performance optical frequency domain reflectometry was developed by bending a piece of standard single-mode fiber to excite sets of higher-order modes that penetrate the surrounding medium. External variations in RI modifies the profiles of the sets of excited higher-order modes, which are then partially coupled back into the fiber core and interfere with the fundamental mode. Accordingly, the fundamental mode carries the outer varied RI information, and RI sensing can be achieved by monitoring the wavelength shift of the local Rayleigh backscattered spectra.

Bending-loss-resistant distributed Brillouin curvature sensor based on an erbium-doped few-mode fiber.

We developed a bending-loss-resistant distributed Brillouin curvature sensor based on an erbium-doped few-mode fiber (Er-FMF) and differential pulse-width pair Brillouin optical time-domain analysis. With Er ion amplification compensating for bending-induced optical loss, radii in the ∼0.3 to 2.

Investigation of the effect of gold coating of gold-coated fiber on distributed strain measurement by differential pulse pair Brillouin optical-time analysis.

Gold-coated fiber (GCF) shows the potential to sense strain at high temperature owing to the hermeticity of gold coating that prevents hydrogen penetration. Nevertheless, there are trivial details of the gold coating of GCF that need to be addressed before using GCF to measure strain at high temperature. In this study, we thoroughly investigate the effect of the gold coating of GCF on strain measurement both at room temperature and high temperature up to 700°C with differential pulse pair Brillouin optical-time analysis (DPP-BOTDA).

Distributed Brillouin optical fiber temperature and strain sensing at a high temperature up to 1000 °C by using an annealed gold-coated fiber.

In this study, the distributed temperature and strain sensing with an annealed single mode gold-coated optical fiber over a wide temperature range up to 1000 °C is demonstrated by using the differential pulse pair (DPP) Brillouin optical time domain analysis (BOTDA). Owing to the protection provided by the gold coating, the fiber can withstand high temperature environments and maintain a high strength, which enables the gold-coated fiber acting as a repeatable high-temperature sensor. After annealing twice to remove the internal stress, the temperature coefficient of the gold-coated fiber is stable and consistent with a nonlinear function.

Slope-assisted BOTDA based on vector SBS and frequency-agile technique for wide-strain-range dynamic measurements.

We present a slope-assisted BOTDA system based on the vector stimulated Brillouin scattering (SBS) and frequency-agile technique (FAT) for the wide-strain-range dynamic measurement. A dimensionless coefficient K defined as the ratio of Brillouin phase-shift to gain is employed to demodulate the strain of the fiber, and it is immune to the power fluctuation of pump pulse and has a linear relation of the frequency detuning for the continuous pump and Stokes waves. For a 30ns-square pump pulse, the available frequency span of the K spectrum can reach up to 200MHz, which is larger than fourfold of 48MHz-linewidth of Brillouin gain spectrum.

Detecting cm-scale hot spot over 24-km-long single-mode fiber by using differential pulse pair BOTDA based on double-peak spectrum.

In distributed Brillouin optical fiber sensor when the length of the perturbation to be detected is much smaller than the spatial resolution that is defined by the pulse width, the measured Brillouin gain spectrum (BGS) experiences two or multiple peaks. In this work, we propose and demonstrate a technique using differential pulse pair Brillouin optical time-domain analysis (DPP-BOTDA) based on double-peak BGS to enhance small-scale events detection capability, where two types of single mode fiber (main fiber and secondary fiber) with 116 MHz Brillouin frequency shift (BFS) difference have been used. We have realized detection of a 5-cm hot spot at the far end of 24-km single mode fiber by employing a 50-cm spatial resolution DPP-BOTDA with only 1GS/s sampling rate (corresponding to 10 cm/point).

Single-shot distributed Brillouin optical time domain analyzer.

We demonstrate a novel single-shot distributed Brillouin optical time domain analyzer (SS-BOTDA). In our method, dual-polarization probe with orthogonal frequency-division multiplexing (OFDM) modulation is used to acquire the distributed Brillouin gain spectra, and coherent detection is used to enhance the signal-to-noise ratio (SNR) drastically. Distributed temperature sensing is demonstrated over a 1.

1200°C high-temperature distributed optical fiber sensing using Brillouin optical time domain analysis.

In this paper, up to 1100°C and 1200°C high-temperature distributed Brillouin sensing based on a GeO-doped single-mode fiber (SMF) and a pure silica photonic crystal fiber (PCF) are demonstrated, respectively. The Brillouin frequency shift's (BFS) dependence on temperatures of the SMF and PCF agrees with a nonlinear function instead of a linear function, which is mainly due to the change of the acoustic velocity in a silica fiber. BFS hopping is observed in both kinds of fibers between 800°C-900°C in the first annealing process, and after that, the BFS exhibits stability and repeatability with a measurement accuracy as high as ±2.

Phase-shifted Brillouin dynamic gratings using single pump phase-modulation: proof of concept.

Two novel phase-shifted Brillouin dynamic gratings (PS-BDGs) are proposed using single pump phase-modulation (SPPM) in a polarization maintaining fiber (PMF) for the first time to our knowledge. Firstly, based on the stimulated Brillouin scattering (SBS), a transient PS-BDG with a 3-dB bandwidth of 354MHz is written by a 2-ns pump1 pulse and a 100-ps pump2 pulse, where the phase of pump1 pulse is shifted with π from its middle point through phase modulation. Then, with a high repetition rate of 250MHz for both pump pulses, an enhanced PS-BDG with a deep notch depth is obtained and its notch frequency can be easily tuned by changing the phase shift.