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. The results show that the distributed thermal strain difference of two fiber rings will cause additional phase fluctuation, which leads to higher low-frequency noise. Therefore, a dual-fiber-ring MZI with matched distributed thermal strains is proposed to suppress the phase noise caused by the thermal strain, and the best suppression is as high as 45.6 dB. This is very important for the research and design of low noise fiber seismometer.

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. By optimizing the noise factor, the performance of vectorial holography over an MMF is improved compared with the conjugate transpose and inverse TM methods. Our results might shed new light on the optical communication and detection mediated by MMFs.

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, µÉ› level measurement sensitivity and 104 µÉ› 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. Meanwhile, the spatial position correction rectifys the position error in the spatial domain caused by segmentation, which reduces the error from the 10 m level to the mm level, enabling precise splicing of spectra and expanding the spectral range, thus extending the strain range. In our experiments, we achieved a strain sensitivity of ±3.2 µÉ› (3σ) over a length of 1 km with a spatial resolution of 1 cm and extended the strain measurement range to 10,000 µÉ›. This method provides, what we believe to be, a new solution for achieving high accuracy and wide range OFDR sensing at the km level.

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. In this Letter, we propose an online polarization error suppression method using Bayesian optimization. Our measurement results are verified by a commercial OVA instrument that uses the offline alignment method. The OVA featuring online error suppression will be widely used in the production of optical devices, not just in the laboratory.

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. The features of long distance (9.8 km), ultrawide dynamic range (107.8 dB), short measurement time (2 sec), and signal-to-noise ratio improvement against ambient noise are experimentally demonstrated. The method makes it possible to evaluate a long polarization-maintaining fiber in an environment whose state changes rapidly.

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. The advantages and disadvantages are discussed, and the current technical requirements are concluded, respectively. This review can provide a reference for studying submarine cable-based seismic monitoring.

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. Bandwidth of the FBG's reflection spectrum is therefore broadened that increases its reflected light power. The chirp rate of the FBG reduces with airflow velocity since the temperature gradient is weakened under the cooling effect of the airflow, resulting in a certain relationship between the reflected power of the FBG and airflow velocity. In the experiment, by detecting the reflected power of the FBG, airflow velocity measurement is achieved successfully with a high sensitivity up to -28.60 µW/(m·s-1) at airflow velocity of 0.1 m/s and a dynamic response time of under one second. The measurement range is up to 0 to 11 m/s. The intensity interrogation scheme of the FBG hot-wire anemometer reduces its cost greatly and makes it a promising solution for airflow velocity measurement in practical applications.

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. Accessibly, the upper envelope is only sensitive to the refractive index (RI) owing to the characteristics of the open-cavity FPI, while the lower one is immune to the RI and employed to detect the temperature by taking advantage of the FSI. Most importantly, the sensitivities of RI and temperature can be significantly improved simultaneously without crosstalk. The experimental results show that the RI sensitivity is -19844.67 nm/RIU and the temperature sensitivity is -46.14 nm/°C, which can be used for high-precision temperature and RI simultaneous measurement.

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. The simulation results show that the refractive index sensitivity S = 7.343 THz/RIU has been achieved. More importantly, this is the first report of tunable PIT effects that can produce up to quintuple PIT windows by using the BP and graphene hybrid structure. The high refractive index sensitivity of the quintuple PIT system for each peak is 3.467 THz/RIU, 3.467 THz/RIU, 3.600 THz/RIU, 4.267 THz/RIU, 4.733 THz/RIU and 6.133 THz/RIU, respectively, which can be used for multiple refractive index sensing function.

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. The proposed sensor also demonstrates longer sensing distance and continuous absolute measurement compared to other sensors. The proposed model can be applied to the end tracking of soft robotics and structural health monitoring of civil infrastructures.

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. RH sensitivity of 0.192 nm/%RH is achieved within a measurement range of 20-80%RH. Dynamic measurement shows a response and recovery time of 240 and 350 ms, respectively. Sensor performance testing shows good repeatability and stability at room temperature but also reveals slight dependence of the RH sensitivity on environmental temperature. Therefore, a fiber Bragg grating is cascaded to the FPI sensing probe to monitor temperature simultaneously with temperature sensitivity of 10 pm/°C.

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. In the experiment, an RI sensitivity of 39.08 nm/RIU was achieved by bending a single-mode fiber to a radius of 4 mm. Additionally, the proposed sensor maintains its buffer coating intact, which boosts its practicability and application adaptability.

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 2.02 cm range could be monitored correctly. The corresponding Brillouin frequency shift variations were in the range of 91.7 to 9 MHz, which agrees well with theoretical calculations. The sensitivity of the Er-FMF device increased inversely with the bending radius, with the optimal sensitivity of 292.7 MHz/cm obtained at a radius of 0.3 cm. To the best of our knowledge, this is the smallest radius of curvature detected and highest sensitivity reported to date, indicating potential applications in distributed sharp-bend sensing, such as intelligent robotics and structural health monitoring.

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). Owing to the inhomogeneity of gold coating induced by the manufacturing process, it is necessary to select the GCF with the gold coating of better homogeneity via DPP-BOTDA. Due to the residual stress that solidified in the GCF during the cooling process of coating, the GCF would first undergo plastic deformation and then elastic deformation in the strain measurement. After one-time strain measurement to remove the residual stress of GCF, the standard deviation of the strain coefficients at room temperature and high temperature are $ \pm {0.5}\% $±0.5% and $ \pm {1.3}\% $±1.3%, respectively, which is mainly due to the nonuniform thickness of the gold coating and the nonuniformity of silica fiber at high temperature.

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. Owing to the residual stress accumulated during the cooling process of coating and the low yield strength of gold, a pre-pulling test is essential to measure the strain of a gold-coated fiber. An equal axial force model is used to recalculate the strain distribution induced by the large temperature difference within the furnace. The high-temperature strain coefficient of an annealed gold-coated fiber decreases with temperature, i.e. from ~0.046 MHz/µÎµ at 100 °C to ~0.022 MHz/µÎµ at 1000 °C, mainly due to the increase in Young's modulus of silica with temperature. To the best of our knowledge, this is the first time that an annealed gold-coated fiber has been applied for distributed high-temperature strain sensing, which demonstrates the potential applications for strain monitoring in complex, high-temperature devices such as jet engines or turbines.

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 1.08 km standard single-mode fiber (SSMF) with 20.48 m spatial resolution and 0.59 °C temperature accuracy. Neither frequency scanning, nor polarization scrambling, nor averaging is required in our scheme. All the data are obtained through only one-shot measurement, indicating that the sensing speed is only limited by the length of fiber.

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). The BFS at the far end of 24-km sensing fiber has been measured with 0.54 MHz standard deviation which corresponds to a 0.5°C temperature accuracy. This technique is simple and cost effective because it is implemented using the similar experimental setup of the standard BOTDA, however, it should be noted that the consecutive small-scale events have to be separated by a minimum length corresponding to the spatial resolution defined by the pulse width difference.

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. For a single-slope assisted BOTDA, dynamic strain measurement with the maximum strain of 2467.4µÎµ and the vibration frequency components of 10.44Hz and 20.94Hz is obtained. For a multi-slope-assisted BOTDA, dynamic measurement with the strain variation up to 5372.9µÎµ and the vibration frequency components of 5.58Hz and 11.14Hz is achieved by using FAT to extend the strain range.

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. We demonstrate a proof-of-concept experiment of the transient PS-BDG and show the notch frequency changing by tuning the phase shift. The proposed PS-BDGs have important potential applications in microwave photonics, all-optical signal processing and RoF (radio-over-fiber) networks.

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 GeO2-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 ±2.4°C for the SMF and ±3.6°C for the PCF. The BFS hopping is a highly temperature-dependent behavior, which means that a high temperature (>800°C) would accelerate this process to reach a stable state. After BFS hopping, both the SMF and PCF show good repeatability for temperatures higher than 1000°C without annealing. The process of coating burning of a silica fiber not only introduces a loss induced by micro-bending, but also imposes a compressive stress on the bare fiber, which contributes to an additional BFS variation at the temperature period of the coating burning (∼300°C-500°C).