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• MCF with RC-units is smaller in size compared to WC-MCF with the same number of cores. • MCF with RC-units having smaller SMD can reduce MIMO complexity compared to FM-MCF. • The characteristics of the MCFs with three types of RC-units were compared. • 12-core MCF with a 3-core RC-unit arrangement can be treated as a better scheme. Randomly-coupled unit grouping multi-core fiber (RC-UG-MCF) is a new fiber structure that combines weakly and strongly coupled multi-core fiber technologies. Compared to the weakly-coupled MCF with a large cladding size, the RC-UG-MCF can shrink the cladding size by grouping the cores. On the other hand, the RC-UG-MCF only maintains a few cores in the strongly-coupled condition, which can reduce the multiple-input multiple- output digital signal processing (MIMO-DSP) complexity compared to the coupled MCFs with full MIMO-DSP for all the cores. In this paper, we intro- duce three types of randomly-coupled unit grouping 12-core fiber, which are randomly-coupled 2-core unit grouping MCF (RC-2CUG-MCF), RC-3CUG- MCF and RC-4CUG-MCF. Subsequently, we compare these three RC-UG- MCFs in terms of group delay spread, inter-unit crosstalk (XT), and cladding size. To achieve an inter-unit XT of −40 dB/ 100 km , RC-3CUG-MCF can provide a solution to maintain MIMO-DSP scale at the level of 6 × 6 with 180-µm cladding radius. However, the RC-2CUG-MCF has worse inter-unit XT , and the RC-4CUG-MCF has larger MIMO complexity. Based on the analysis, there is a trade-off relationship between the MIMO-DSP scale and the fiber cladding size, assuming that the target inter-unit XT is fixed. [ABSTRACT FROM AUTHOR]

We demonstrate a fiber optic accelerometer for low frequency vibration detection in this paper. A slider moving along a guide rail is suspended over an optical fiber into which a fiber Bragg grating (FBG) is inscribed. When acceleration signal acts on our sensing structure, the slider will vibrate vertically under the constraint of the optical fiber, inducing the change of FBG axial strain and we can therefore establish the relationship between vibrational acceleration and wavelength shift. Laboratory measurements validate the effectiveness of the derived theoretical results that the sensor owns a low natural frequency of 9.5 Hz and a high sensitivity up to 926.29 pm/g from 0.5 to 9 Hz. The sensor structure also shows a good ability to resist lateral interference that the cross-sensitivity is calculated as 0.3%. • Compact low-frequency FBG accelerometer using fiber itself as elastic element. • Low natural frequency of 9.5 Hz and high sensitivity up to 926.29 pm/g. • Cross sensitivity of 0.3% indicates its strong ability to resist lateral interference. [ABSTRACT FROM AUTHOR]

• Based on temperature field simulation and validation, as well as taper contour simulation and validation, this study proposed a method that effectively simulated the light transmission trajectories of a single fiber in tapered optical fibers. • The findings in this article provided insight into how the behavior of individual tapered fibers in a tapered optical fiber array affected imaging quality, and consequently provided theoretical design for eliminating halo defects and improving the uniformity of light transmission. • The transmittance and uniformity of TOFA were tested using self-developed transmittance detection equipment, providing a method for verifying simulation results. Tens of millions or billions of single tapered fibers are independent optical transmission elements of tapered optical fiber array (TOFA) for imaging. The optical transmission trajectories of these single tapered fibers, which directly affect imaging qualities of TOFA, are rarely analyzed. This paper provides a method to numerically simulate it for fibers located at different positions along the radial direction of TOFA. The tapered curves of TOFA were firstly simulated by Finite Element Analysis (FEA) software and experimentally verified by the same preparation parameters. It is shown that the slope of tapered curve increases with stretching length. Meanwhile, the taper angle gets greater, and the profile of tapered deformation zone expands outward, while the radius of straight zone remains stable. Based on these, the optical transmission trajectory of single tapered fibers located from center to edge in TOFA was simulated, and then further verified by a self-developed instrument. Both simulation and experiment illustrate that the relative transmittance of single tapered optical fiber is decreased along the radial direction within the same TOFA. In comparison with the transmittance of the central axial fiber, the simulated and experimental unevenness on relative transmittance of peripheral fibers is within 7.11% and 9.74%, respectively. With the relative transmittance at the central position of the optical fiber being identical, the difference of relative transmittance (Δ T) between the simulation and experiment is gradually increased from center to peripheral regions along the radial direction of TOFA, reaching the maximum Δ T of 4.23%. The simulated results are well in agreement with the experimental ones. [ABSTRACT FROM AUTHOR]

• We modeled honeycomb-like multicore tellurite glass fiber network beam propagation. • We show how to obtain the best localized beam possible by changing structural parameters. • Randomization of the position of each core does not improve the localization of the beam. • Randomization of the size of each core improves the localization of the beam. • The best simulation obtained show a localized and stable beam pattern at z = 10 cm distance. In this paper, we have demonstrated by simulation that the transport of NIR optical images in a 10-cm-long all-solid tellurite optical fiber (wavelength of 1.55 µm) has a highly localized beam pattern by using a honeycomb structure with transversely pseudo-disordered fiber with random core sizes and fixed nearest-neighbor core-core center distance (core size between 1.0 and 1.4 µm and core-core center distance fixed at 3 µm). This paper shows the steps and different simulations used to obtain this structure with such a high beam localization. [ABSTRACT FROM AUTHOR]

In this paper, we demonstrate a simple structure, ultra-narrow linewidth, extremely high stability and high optical signal to noise ratio single-longitudinal-mode (SLM) erbium-doped fiber laser (EDFL). Nested Fiber Ring (NFR), nested in the ring cavity of EDFL, is employed to serve as a high quality mode filter to eliminate the dense longitudinal mode of the EDFL. The mode-selection capability of the RIR is investigated and discussed theoretically and experimentally. Saturable absorber, which composes of a section of unpumped polarization maintaining erbium-doped fiber and a Fabry–Perot fiber Bragg grating (FP-FBG), is employed to produce a dynamic narrowband grating to ensure the SLM laser output. Saturable absorber, consisted of a section of unpumped polarization maintaining erbium-doped fiber, produces a dynamic narrowband grating to ensure the SLM laser output. We achieve a ultra-narrow linewidth SLM laser output of 550 Hz. The laser has an optical signal-to-noise ratio (OSNR) of more than 59 dB. In addition, the stability of the wavelength and power fluctuations below 0.03 nm and 0.02 dB, respectively. The single-longitudinal-mode fiber laser has a compact structure, ultra-narrow linewidth, and excellent performance, and is expected to be used in the fields of coherent communications, precision metrology, and so on. • We propose a new structure of ultra-narrow linewidth single-longitudinal-mode erbium-doped fiber laser based on Nested Fiber Ring structure and Saturable Absorber. • The new structure has the advantages of simple structure, easy preparation and high stability. • The output performance of the proposed fiber laser is excellent, the 3-dB linewidth is 550 Hz and the optical signal-to-noise ratio is 59 dB. • The filtering function of the Nested Fiber Ring was theoretically derived and its filtering effect was analyzed. [ABSTRACT FROM AUTHOR]

Multimode fiber (MMF) imaging based on compressive sensing (CS) is a promising technique for ultrathin endoscopic applications due to its advantages such as small size, low cost, and high speed. However, how to choose the most suitable MMF in the application has not been analyzed in detail. In this paper, the influences of fiber core diameter on the quality of CS MMF imaging are investigated theoretically and experimentally. Firstly, the relationship between the core diameter of MMF and the speckle size of its output speckle patterns is established. Then the speckle patterns with different speckle sizes are synthesized to explore their impact on imaging quality. Results show that a MMF with a larger core diameter yields better image quality in ideal conditions, while the most suitable core diameter decreases with the increasing noise or/and declining sampling rate. Finally, an experimental platform is built to verify the simulation, the same trends between experimental results and simulation results validate the feasibility of the simulation method. This paper provides a reference on how to choose a more suitable MMF to obtain a high-quality image in practical applications. • Impact of core diameter on the quality of multimode fiber (MMF) imaging is studied. • In ideal conditions, MMF with a larger core diameter yields better image quality. • With the increasing noise, the optimal core diameter of MMF decreases. • With the declining sampling rate, the optimal core diameter of MMF decreases. • The study provides a reference on selecting suitable MMF for ultrathin endoscopes. [ABSTRACT FROM AUTHOR]

In this paper we studied thermal-lens effect on transverse modes in the confined-gain Yb-doped fiber for the first time, to the best of our knowledge. It is found that the thermal-lens effect is enhanced with the reduction of doped area due to the higher thermal density in the doped area. Then, the variations of three low-order transverse modes (i.e., fundamental, LP 11 and LP 02 modes) with the heat load and doped diameter are studied, and the mode areas and overlapping factors with the doped area of these transverse modes are also discussed. The gain competition of these transverse modes is also investigated by calculating the differential-gain ratio with the purpose of achieving the single-mode operating. It is found that the stronger thermal-lens effect will result in the reduction of optimum doped diameter for suppressing high-order modes. This study can provide significant guidance for designing the confined-gain Yb-doped fibers for the applications in high-power fiber lasers and amplifiers. [ABSTRACT FROM AUTHOR]

• This paper proposes a strain and temperature sensing system based on an erbium-doped fiber laser. • An intermodal interference between a single-mode fiber and a 12.4-mm suspended core fiber is used as a frequency-selective device of the fiber laser. • From zero to 1200 με, the strain sensitivity of the system is 1.80 pm/με, and the linearity is 98.30%. • From 100 ℃ to 1000 ℃, the temperature sensitivity of the system is 1.88 pm/℃, and the linearity is 98.58%. In this paper, a strain and temperature sensing system based on an erbium-doped fiber laser (EDFL) is proposed and experimentally verified. The cavity mirror of the fiber laser is composed of an optical coupler (OC) and a circulator. An intermodal interference between a single-mode fiber and a 12.4-mm suspended core fiber (SCF) is used as a frequency-selective device. The period and wave height of the SCF transmission spectrum is 6.30 nm and 4.188 dB. The fiber laser has a peak wavelength and power of 1564.46 nm and −14.729 dBm, respectively. The peak wavelength of the fiber laser drifts toward a short wavelength as the SCF is stretched from zero to 1200 με. The laser undergoes the transition from a single wavelength to a dual wavelength. According to the measurement results, the system's strain sensitivity is 1.80 pm/με, and its linearity is 98.30%. A temperature rise experiment from 100 °C to 1000 °C is carried out on the SCF, and the peak wavelength of the fiber laser drifts toward a long wavelength. The temperature sensitivity of the system is 1.88 pm/°C, and the linearity is 98.58%. [ABSTRACT FROM AUTHOR]

• The successful experiments of distributed acoustic sensing system based on Φ-OTDR in the near-field target localization in shallow water is of great significance in its widespread application in the near-field localization of multiple targets, the recognition of surface and underwater target, tracking target in water, and so on. • At present, no scholars have studied the application of the distributed acoustic sensing optical fiber (DASF) to near-field target localization in shallow water. Based on the spherical wave theory, a near-field receiving signal model and the steps of localization based on DASF are presented in this paper. The feasibility and accuracy of using DASF to locate near-field target in shallow water are verified through simulations and experiments. • In this paper, taking the application of land-based DAS system to near-field target localization in shallow water as a breakthrough, theoretical modeling and experiments are mutually verified, which is consistent with the submission direction of " Optical Fiber Technology" and is suitable for publication. The distributed acoustic sensing optical fibre (DASF) is widely used for target detection and localisation on land as well as underwater optical fibre safety monitoring. To extend it to near-field target localisation in shallow water, DASF based on phase-sensitive optical time-domain reflectometry (Φ-OTDR) is used to receive near-field target signals. Given that the acoustic wavefront in the near field is spherical, the localisation method is based on the near-field localisation principle. The feasibility and accuracy of Φ-OTDR based on DASF for effectively locating near-field targets in shallow water are validated through simulations and practical experiments. Furthermore, the first experiment for applying DASF based on Φ-OTDR to near-field target localisation in shallow water is conducted, laying the theoretical and experimental groundwork for its improved application to underwater target detection. [ABSTRACT FROM AUTHOR]

• The position deviations of polarization-maintaining fiber (PMF) may lead to the deterioration of the polarization extinction ratio (PER) by affecting the energy coupling process of the fast and the slow axes in an 80 dB adjustable PER generator. • The axial distance has no influence on PER. The influences of lateral misalignment and angle deflection are a square exponential relationship, which is negligible below 50 dB and increases drastically above it. • This paper provides a reference for the traceability of ultra-high PER and the research on optical fiber technologies in silicon-based photonic chips, optical waveguides, and semiconductor lasers. This paper constructed an 80 dB adjustable polarization extinction ratio (PER) generator based on coaxial rotating polarization-maintaining fiber (PMF). The position deviations of PMF may lead to the deterioration of PER by affecting the energy coupling of the two axes. The quantitative relationship between PER and different position deviations is determined by simulations and experiments, and the PER uncertainty is analyzed. The axial distance has no influence on PER. The influences of lateral misalignment and angle deflection are a square exponential relationship, which is negligible below 50 dB and increases drastically above it. The expanded uncertainties (k = 2) in the ranges of 50 to 60 dB, 60 to 70 dB and 70 to 80 dB are about 0.2 dB, 0.6 dB and 2.7 dB, respectively. This paper provides a reference for the research of ultra-high PER traceability and other optical fiber technologies. [ABSTRACT FROM AUTHOR]

• An improved RLS method was proposed to enhance the SNR of dual wavelength distributed fiber acoustic sensing system. • The tap factor is self-iterated according to the parameters at the previous time under corresponding evaluation metrics. • The system allows detecting and locating acoustic signal on a single fiber without using additional sensitizing devices. • The SNR of the acoustic restoration and localization are improved by 14.98 ∼ 20.33 dB and 19.27 ∼ 29.71 dB, respectively. • The proposed system exhibits considerable potential for demodulating and reconstructing acoustic signal over long distances. In this paper, an improved recursive least squares (RLS) method has been proposed to enhance the signal-to-noise ratio (SNR) of dual wavelength distributed fiber acoustic sensing system. The raw detected signals of 1550 nm differential signal and 1310 nm interference signal are processed by wavelet decomposition and reconstruction techniques to obtain the estimation of the reference signal. The tap factor is self-iterated according to the parameters at the previous time under the corresponding evaluation metrics. The system allows detecting and locating acoustic signal at an arbitrary position on a single optical fiber without using additional sensitizing devices. Experimental results demonstrate that the SNR of the acoustic restoration and localization are substantially improved by 14.98 ∼ 20.33 dB and 19.27 ∼ 29.71 dB, respectively. The proposed system exhibits considerable potential for demodulating and reconstructing acoustic signals over long distances and in wide ranges. [ABSTRACT FROM AUTHOR]

In this paper, we investigate the optimal design of phase modulation waveforms using the theoretical model of Tm3+-doped fiber amplifiers. The proposed design suppresses the SBS effect in the 2 μm-band Tm3+-doped fiber amplifiers with the help of a reference point-based multi-device non-dominated sequential genetic algorithm (NSGA-III). At the same time, synthesis efficiency is ensured. Different from the previous use of conventional multi-objective genetic algorithms to suppress the SBS effect in narrow linewidth amplifiers in the 1 μm band, the maximum SBS threshold is 121 W at a signal linewidth of 496 MHz with a more uniform distribution of Pareto solutions under this scheme. The signal linewidth of 680.61 MHz increases the SBS threshold by almost 20 W and the operating speed by several times compared to a conventional multi-objective genetic algorithm at different modulation depths, modulation frequencies and modulation periods. The threshold enhancement factor of the SBS signal with optimal modulation is 2.12 times that of the sinusoidal signal with an extended bandwidth of 700 MHz. The 2 μm laser obtained by this scheme enables more accurate and longer distance ranges in coherent detection. [ABSTRACT FROM AUTHOR]

• The length of the designed PBS based on DNDHC-NCF is 1.38 cm, which solves the technical problems of long length of traditional PBS. • The operation bandwidth is 320 nm, covering S + C + L + U bands. • Is of great significance in promoting the application and development of DNDHC-NCF. In this paper, a polarization beam splitter（PBS） based on double nested dual-core negative-curvature fiber (DNHC-NCF) is designed. Two hollow cores are constructed by eight double nested cladding tubes arranged at certain angles. The influences of structural parameters on the performances of the DNHC-NCF PBS have been studied by using the finite element method (FEM). According to the simulation results, the structure of the proposed DNHC-NCF PBS is optimized. The final results indicate that the length of the proposed DNHC-NCF PBS is 1.38 cm and the extinction ratio (ER) is less than −20 dB in the wavelength range of 1.48–1.80 µm (320 nm, covers S + C + L + U bands). Besides, the higher order mode extinction ratio (HOMER) in the wavelength range of 1.3–1.71 µm is greater than 100, showing good single-mode characteristic as well. It is believed that the proposed double nested dual-core negative curvature fiber polarization beam splitter will have important applications in the fields of optical communication system. [ABSTRACT FROM AUTHOR]

This paper describes a newly developed butt joint type hollow-core fiber connector with protected fiber ends. It can typically realize nearly 0.5-dB insertion and 45-dB return loss without physical contact. It has been confirmed that the HCF connector is suitable for actual use because it maintains stable connection characteristics in mating cycles, and it can be cleaned with an ordinary connector cleaner. [ABSTRACT FROM AUTHOR]

• A numerical model of a dual-pass bismuth-doped fiber amplifier is established. • By analyzing the loss of the reflection module of the bismuth-doped fiber amplifier in a dual-pass configuration versus the gain of the amplifier, a reflector-based signal and pump dual-pass bismuth-doped fiber amplifier is designed. • The amplifier experimentally obtained a maximum net gain of 41.8 dB at 1325 nm with a noise figure of 5.8 dB. Due to the tendency of data traffic approaching the current capacity limit, the development of a high-performance o-band amplifier has become an important topic. In this paper, we construct a model of the dual-pass BDFA and compare it with the single-pass BDFA. In addition, the effect of the loss of the reflected part of the dual-pass structure on the gain is analyzed by numerical simulation. Utilizing the theory to guide the experiment, a reflector-based signal and pump dual-pass O-band bismuth-doped phosphosilicate fiber amplifier is proposed. The amplifier experimentally obtained a maximum net gain of 41.8 dB at 1325 nm with a noise figure (NF) of 5.8 dB under the total pump power of 1050 mW and the input signal power of −34 dBm. The 3 dB saturation output signal power of the BDFA is 16.8 dBm at a total pump power of 1050 mW, at which point the signal input power is –22 dBm and the NF is 6.6 dB. The gain coefficient is 0.064 dB/mW at the input signal power of −30 dBm. The measured 3 dB and 10 dB net gain bandwidths of this amplifier are 30 nm (1312–1342 nm) and 58 nm (1300–1358 nm), respectively. [ABSTRACT FROM AUTHOR]

• Novel acetone gas optical sensor using ZnO-TiO 2 nanorod on tapered multimode fiber. • Present the influence of titanium dioxide (TiO 2) as a catalyst to the zinc oxide (ZnO) nanorods. • Low-temperature detection of acetone gas at 87 °C for the acetone concentrations of 1000, 5000 and 10,000 ppm. • Notable output intensity responses were observed for ZnO-TiO 2 nanorods decorated with Au-Pd thin films (ZTNAP). This paper presents the influence of titanium dioxide (TiO 2) as a catalyst to the zinc oxide (ZnO) nanorods coated on the tapered MMF fiber utilized as a sensing layer for the detection of acetone gas at low temperatures. To enhance the sensitivity of the sensor ZnO-TiO 2 nanorods were decorated with Au-Pd film. Hydrothermal method was used to synthesize the ZnO nanorods while TiO 2 powder was incorporated by using a simple doping procedure during fabrication. The decoration of the Au-Pd thin films is integrated with the ZnO-TiO 2 by using DC sputtering method. The ZnO doped with 15 mM TiO 2 shows a strong and significant peak based on XRD analysis which can be indexed to the hexagonal phase of ZnO. A few globular structures of TiO 2 were randomly distributed as observed by the SEM image. Four different fiber sensing layers were prepared with respect to 10 mM ZnO precursor; ZnO nanorods (ZN), ZnO nanorods decorated with Au-Pd thin films (ZNAP), ZnO-15 mM TiO 2 nanorods (ZTN) and ZnO-15 mM TiO 2 nanorods decorated with Au-Pd thin films (ZTNAP). The spectrometry setup was used to measure the response of the sensor toward different concentrations of acetone gas in terms of intensity with respect to its wavelength. Notable output intensity responses were observed for ZTNAP in terms of sensitivity and standard deviation towards various acetone concentrations while the repeatability curve showed a high response of acetone gas at 10,000 ppm. All experiments were performed at the low temperature of 87 °C. [ABSTRACT FROM AUTHOR]

• A method to solve the lack of multiplexed ability for fiber sensor based on specklegrams. • A specklegram can be decomposed spatially according to different incident angle. • The annular specklegram can be formed by the incident of side-coupling light. • Transfer learning technology is employed to improve the accuracy of multiplexed sensor based on specklegram recognition. With the advancement of image processing and artificial intelligence technology, Fiber Specklegram Sensors (FSS) based on specklegram recognition have emerged as a novel sensing technology. The output speckles represent the interference between different transmission modes in the fiber, which vary with changes in fiber fluctuations such as the pressure on the fiber or ambient temperature. This sensor boasts high precision and low cost. However, when the optical fiber is required to perform multi-parameter or multi-position sensing simultaneously, the variety of specklegrams counterpart to external stimuli has become very large, significantly limiting the application scenarios of FSS. In this paper, a new structure based on side-coupling theory is proposed, where the output specklegram comprises a circular specklegram and an annular specklegram formed by end-coupling light and side-coupling light respectively. In this manner, the circular specklegram and annular specklegram can represent different sensing parameters, and the circular specklegram is completely unaffected by the annular specklegram, enabling multiplexed sensing. Multiplexed experiments for pressure and temperature sensing are conducted utilizing this structure, and deep learning technology is employed to process the different sets of specklegrams for regression prediction. The accuracy of pressure and temperature sensing tests reached 98.62% and 95.31% respectively, verifying the feasibility of this method for multiplexed sensing. [ABSTRACT FROM AUTHOR]

• This paper explores the design of a 1 × 4 linear array multicore fiber with 125-μm cladding for O-band transmission. • Fiber design for a balanced performance between inter-core crosstalk and the tunneling loss of outer cores near the cladding edge was identified. • We demonstrated error-free 400G LR4 and 100G LWDM transmission over 6.3 km of an MCF with a total data rate of 1 Tb/s. This paper explores in detail the design of a 1 × 4 linear array multicore fiber with 125-μm cladding for O-band transmission. Through modeling we identified fiber design parameters for a balanced performance between inter-core crosstalk and the tunneling loss of the outer cores near the cladding edge. Guided by the modeling results, we have fabricated a 1 × 4 multicore fiber with improved performance. Using such fiber, we were able to demonstrate error-free 400G LR4 and 100G LWDM transmission over 6.3 km with a total data rate of 1 Tb/s. [ABSTRACT FROM AUTHOR]

• This paper analyzes the state of mode coupling in a SI air-clad silica optical fiber. • The coupling coefficient of SI air-clad silica optical fiber is obtained. • At a length of 2100 m, the equilibrium mode distribution in the investigated fiber is established. • For the first time this paper reports on the strength of mode coupling in this fiber. • The results obtained can be exploited to employ SI air-clad fibers in broadband astro-photonics systems. This paper analyzes the state of mode coupling in a high numerical aperture (NA) step-index (SI) air-clad silica optical fiber using the power flow equation. The coupling coefficient of this equation was fine-tuned at first to allow it to correctly reconstruct previously reported experimental intensity distributions of output far-field (FF) patterns. At a length of 2100 m, the equilibrium mode distribution in the investigated fiber was obtained to be established. In our best knowledge for the first time, this paper reports on the strength of mode coupling and output beam quality of SI air-clad silica optical fiber. The results obtained can be exploited to employ SI air-clad fibers in broadband high-NA astro-photonics systems. [ABSTRACT FROM AUTHOR]

• In this paper, a new core modulation method is proposed for the first time. A novel long period fiber grating with arched fiber core is fabricated and the sensing characteristics are investigated. • The fiber etched by the CO 2 laser is put into the fiber fusion splicer for heating and melting. Under the action of liquid surface tension, the fiber core is deflected to the etching side, forming an arched core. • The experimental results show that the arched core long period fiber grating has excellent bending and strain characteristics. In this paper, the fabrication of arched fiber core based on single-mode fiber is first proposed and experimentally studied about the relevant sensing characteristics. A novel kind of long-period fiber grating (LPFG) with good sensing performance is formed by increasing the number of arched cores with a fixed cycle. The arched core long-period fiber grating (AC-LPFG) has excellent bending sensitivities with −25.06 nm/m−1, −4.26 nm/m−1, 16.84 nm/m−1, −4.92 nm/m−1 in four different directions, and it shows that the AC-LPFG can distinguish the bending direction very well. Meanwhile, the strain sensitivity and temperature sensitivity of the AC-LPFG are also experimentally tested to be −5.9 pm/μm and 84.5 pm/℃ respectively. The excellent bending and strain characteristics of the AC-LPFG indicate that the arched fiber core has similar effects as some special fibers, which will surely get closer attention and more research applications in the future. [ABSTRACT FROM AUTHOR]

• Our sensor has a significant intensity response of torsion in the range from −200° to 200°. • The sensitivity of ∼1.1 dB/(rad/m) is achieved with the maximum of 1.17 dB/(rad/m) and the measured error of less than 0.03 rad/m. • Our sensor has the ultra-low cross-sensitivities of 4.7 × 10−2 (rad/m)/m−1 and 1.2 × 10−2 (rad/m)/℃. • Our sensor has the merits of ease of fabrication and cost-efficiency. In this paper, a few-mode-fiber based in-line Mach-Zehnder interferometer is proposed and completed for directional torsion sensing by core-offset splicing technique. The optimal offset and fiber length for fabrication are gained and the comprehensive tests are conducted in terms of torsion, bending and temperature. The experimental results show that our sensor has obvious intensity variation with the applied shear strain. The torsion sensitivity reaches ∼1.1 dB/(rad/m) in the range from −13.08 rad/m to 13.08 rad/m, but with the wavelength shift of 0.102 nm/(rad/m). In addition, thanks for the tiny bending and temperature responses of −0.055 dB/m−1 and 0.014 dB/℃, the ultra-low cross-sensitivities of 4.7 × 10−2 (rad/m)/m−1 and 1.2 × 10−2 (rad/m)/℃ are gained simultaneously, which dramatically improves the practicality of fiber-optic torsion sensor. [ABSTRACT FROM AUTHOR]

• High accuracy and good generalization ability. • Easy saturation and unable to perceive continuous variation of parameters are solved. • The relationship between the specklegram and the accuracy is analyzed. • Analytical process is conductive to the selection of dataset. • Transfer learning is used to tackle the poor portability and reusability. A novel fiber specklegram torsion sensor based on multimode fiber (MMF) is proposed and experimentally demonstrated in this paper. The Residual Network (ResNet) is used to establish the relationship between the changing process of the specklegram and the torsion angle of the MMF. The experimental results indicate that the sensor exhibits the advantages of high accuracy and large measurement range. Within a torsion angle range of 0–360°, the prediction error is within ±2° for 99.1% of the specklegrams in test set. By analyzing the decorrelation process of the specklegrams from different kinds of MMFs, we find that the difference in torsion angle prediction accuracy of different MMFs stems from the decorrelation angle of the specklegram, and the larger decorrelation angle of specklegram holds higher torsion angle sensing accuracy of the trained ResNet. Meanwhile, the difference of the decorrelation angle of specklegram is attributed to the difference of the fiber normalized frequency. This analytical process may be applicable to the analysis of other specklegram based sensing schemes and the selection of dataset for fiber specklegram sensor. The possibility of transfer learning is also investigated to show an improved portability and reusability. In addition, the fiber specklegram torsion sensor owns other merits such as easy fabrication and good robustness, which shows its wide range of potential applications. [ABSTRACT FROM AUTHOR]

• We propose a 5G NR- multiband for Radio over Fiber link outfitted with an optimized magnitude selective affine (OMSA) method to cover enhanced mobile broadband (eMBB) situations for 2.14 GHz and 10 GHz, correspondingly. • We show experimentally that OMSA further reduces the complexity with better or at least similar performance to that of MSA by using a weighted function as a nonlinear operator to accurately capture the nonlinear behaviour of the system being modelled, which can lead to a reduction in the number of segments and improvement in the accuracy of the model. This means that it will also lead to a reduction in the number of coefficients needed. This helps to make the model more efficient and simpler to use. • The experimental results demonstrate that the OMSA model has lower complexity than the MSA model while achieving a slightly better performance that is assessed in terms of computations, error vector magnitude (EVM) or adjacent channel power ratio (ACPR). The application of analog radio over fiber (A-RoF) systems for 5G new radio (NR) multiband waveforms presents challenges, including nonlinearity introduced by the Mach Zehnder Modulator and the dispersion and attenuation of the optical signal over long fiber lengths. To address these challenges, this paper proposes a new version of the magnitude-selective affine (MSA) model, named the optimized magnitude-selective affine (OMSA) model, which incorporates a power-reliant weighting function to improve performance while reducing complexity via Digital Predistortion (DPD). The OMSA model is tested using 5G NR signals at 10 GHz with 50 MHz and flexible-waveform signals at 2.14 GHz with a 20 MHz bandwidth, transmitted over a 10 km fiber length using a Mach Zehnder Modulator and a 1550 nm optical carrier. The performance of the OMSA model is compared to the MSA and generalized memory polynomial (GMP) models in terms of adjacent channel power ratio, error vector magnitude, and complexity. The results show that the OMSA model outperforms the MSA and GMP models in terms of performance reducing the error vector magnitude to 1.9% as compared to 4% and 3% in case of GMP and MSA respectively. Additionally, in terms of complexity measured via coefficients, OMSA is 168 times more efficient as compared to GMP and offers 1.93 times higher efficiency as compared to MSA while maintaining lower complexity. The experimental demonstration of 5G NR multiband waveforms over A-RoF links using the OMSA model represents a significant step towards the practical implementation of high-performance, low-complexity 5G NR systems over fiber-optic links and provides a promising solution to the challenges associated with DPD linearization for A-RoF systems. [ABSTRACT FROM AUTHOR]

• Due to the production process, the refractive index of the fiber is different from the ideal refractive index. We studied the effect of this difference on the performance of the separating vector mode of the ring fiber. • Established a model to simulate how impact the ring thickness, the maximum refractive index of the ring and curve steepness have on the mode separation. • Fabricated two ring-core fiber and compared their performances. In this paper, the influences of the ring thickness, the maximum refractive index of the ring and curve steepness on the mode separation are analyzed. The model is established with considering the actual fiber fabrication, especially the transition range between two adjacent layers are studied. Then we manufacture the ring-core fiber with central air hole. This study has reference significance for the structure design of vortex optical fibers. [ABSTRACT FROM AUTHOR]

The all-fiber voltage sensor is difficult for application because of its complex structure, difficult signal demodulation, and sensitivity to the environment. In this paper, a wavelength modulated all-fiber voltage sensor with a simple structure is designed and proposed using straight tapered single-mode fiber and stacked piezoelectric crystal as a voltage sensing unit. The simulation analysis of its performance shows that it can realize the voltage measurement of 0 kV ∼ 10.41 kV, and the accuracy can reach 0.2%. Additionally, the tapered fiber has good strain sensitivity and temperature insensitivity, and its sensitivity is up to 0.03206 nm / μ ε with low bending loss. [ABSTRACT FROM AUTHOR]

• Two different structures of reflected fiber ethanol probes were compared. • Relationship of sensitivity and morphology parameters have been discussed. • Microfiber taper ethanol probe has been experimentally demonstrated. • Ethanol sensitivity of 7.18 × 10-4 dBm/ppm was obtained during 0–200 ppm. Reflected fiber probes can efficiently detect different environmental parameters in the narrow space. In this paper, the coreless fiber and the microfiber taper cascade-spliced structures were coated with the sensitive materials. Their refractive index sensing performance was studied and compared. The microfiber taper shows a sensitivity of −36.581 dBm/RIU, which is significantly higher than that of the coreless fiber structure with the same length (-9.482 dBm/RIU). The reflected microfiber taper ethanol gas sensor was made by surface-modified with the mixtures containing zinc oxide nanosheets and UV glue with low refractive index. In experiment, as the ethanol concentration increased, the intensity of the reflection spectrum increased, resulting in the sensitivity of 7.18 × 10-4 dBm/ppm. This miniature fiber structure can be integrated into the conventional optical fiber system to develop the miniature detectors with high performance. [ABSTRACT FROM AUTHOR]

This paper investigates the relationship between cross-sectional geometry and splicing loss in a four-core multicore fiber (MCF) with a standard 125-μm cladding diameter. We show that a binarized cross-sectional image is useful for determining the cladding/core center accurately. The validity of the measured geometry is demonstrated by comparing the measured and estimated splice loss histograms. The requirements for cladding/core concentricity error in a four-core MCF with a 125-μm cladding diameter are also discussed numerically. Our results show that a four-core MCF can potentially achieve a feasible splicing loss property by considering the most stringent geometrical tolerance in the existing standard. Further improvement to the MCF geometry and/or connector technology, however, will be required to ensure the lowest connector loss standard when an additional tolerance for connector ferrules is needed. [ABSTRACT FROM AUTHOR]

• The temperature sensitivity can be improved by controlling the axial and radial TECs. • The axial TEC of NCF is increased and radial TEC is suppressed to improve sensitivity. • The temperature sensitivity is improved to −355 pm/°C. • The sensor has the potential of refractive index sensing and biosensing. In this paper, a liquid sealed temperature sensor is assembled based on single-mode-no-core-single-mode (SNCS) optical fiber structure. Theoretical calculations and experiments verify the sensing characteristics of this structure. The structure improves the temperature sensitivity to −355 pm/°C by encapsulating an organic solvent with large thermal optical coefficient and using packaging material with large thermal expansion coefficient. The theoretical results are in good agreement with the experimental results. Theoretical simulations are completed on MATLAB using far from the cut-off condition. The SNCS structure is encapsulated in a microfluidic chamber by applying microfluidic processing technology. The axial thermal expansion coefficient of the no-core fiber (NCF) is increased and radial thermal expansion coefficient is suppressed by the polydimethylsiloxane (PDMS) mold stretching NCF to improve the temperature sensitivity. The enhancement effect of PDMS stretching NCF on temperature sensitivity is verified by contrast experiment. Due to the wrapping of PDMS, the sensor has strong resistance to mechanical damage and external interference. [ABSTRACT FROM AUTHOR]

• An all-fiber half waveplate is placed at the center of the whole spun highly birefringent fiber to compensate for the depolarization in the fiber. • The all-fiber half waveplate is made of the helically wound spun highly birefringent fiber without breaking the whole fiber. • The advantages of the proposed method are simple, effective and without breaking the fiber. • Simulations and experiments have demonstrated the excellent performance of the proposed method. Nowadays, the spun highly birefringent single-mode optical fiber is increasingly being considered for current-sensing applications. However, when a broadband polarized light is launched into the fiber, apparent depolarization will occur with its propagation. This leads to a decrease in the polarization sensibility. In this paper, we propose a kind of method to compensate for the depolarization in spun highly birefringent fibers. An all-fiber half waveplate made of the helically wound spun highly birefringent fiber is placed at the center of the whole fiber. To obtain this half waveplate, the torsion of the helix should be equal to the inherent polarization rotation of the spun highly birefringent fiber. The advantages of the proposed method are simple, effective and without breaking the fiber. Simulations and experiments have demonstrated the excellent performance of the proposed method. [ABSTRACT FROM AUTHOR]

• NCF length which consist of even self-image index has higher sensitivity compared to odd self-image index. • However, there were no significant sensitivity changes between them. • The best sensor performance was achieved by the NCF length of 2.90 cm with two self-image indexes. No-core fiber has been widely demonstrated as an alternative refractive index sensor over conventional optical fiber sensors. However, previous works demonstrated that the self-image length optimization of this sensor only focused on simulation and formed a conclusion without data analysis. Hence, the sensitivity provided does not follow the standard unit, providing a limited evaluation of the sensitivity for refractive index measurement. This paper investigates the no-core fiber at a fixed diameter of 125 μm and varies the length based on the self-image index ranging from 1 to 4 indexes to evaluate the sensor sensitivity at the analyte refractive index ranging from 1.3000 RIU to 1.4000 RIU via experimental study. The results showed that the dip wavelength shift non-linearly increased for all the samples as the analyte refractive index increased. The overall analysis showed that there were significant changes in the sensitivity between the odd and even self-image indexes, and there were no significant sensitivity changes between them. The best sensor performance was achieved by the NCF length of 2.90 cm with two self-image indexes. The sensitivity achieved by the sensor was 104.64 nm/RIU and 241.30 nm/RIU for analyte refractive index ranging from 1.3000 RIU to 1.3600 RIU and 1.36000 RIU to 1.4000 RIU, respectively, via a linear fitting plot. Meanwhile, the figure merit value obtained by the sensor was 1.45 RIU−1 and 3.75 RIU−1 at analyte RI of 1.3600 RIU and 1.4000 RIU, respectively. In the future, this can be used as a reference for no core fiber length optimization. [ABSTRACT FROM AUTHOR]

In this paper, we demonstrated a novel seed source for the continuous wave Yb-Raman fiber amplifier, which adopted an 1140 nm single mode laser diode (LD) as Raman signal seed and a conventional 1085 nm fiber oscillator as the Raman pump seed. By splicing a section of long passive fiber in the Raman pump seed cavity, optimizing the Raman pump seed power and the cooling condition, the higher order Raman threshold was doubled and over 500 W output Yb-Raman fiber amplifier was achieved. In addition, our experiment also proved that, for the first time, splicing long passive fiber in the seed oscillator cavity improved the temporal performance of the first order Raman in the conventional fiber master oscillator power amplifier, which could increase the higher order Raman threshold as a result. The seed structure based on a single mode LD and a long cavity length fiber oscillator, could provide a highly compact and cost effective seed source for the high power Yb-Raman fiber amplifier with enhanced higher order Raman suppression. [ABSTRACT FROM AUTHOR]

• Ring based Hybrid FSO- Fiber Optic Architecture. • Same wavelength transmission for downstream and upstream Transmission. • A reliable hybrid 4 × 10 Gbps fiber optic-FSO based ring architecture. • Backup protection at CO side as well as at user side. This paper proposes a reliable hybrid 4 × 10 Gbps fiber optic-FSO based ring architecture. The proposed architecture aims to provide reliable and bandwidth-efficient transmission. The proposed architecture has a fault protection capability in presence of the link failure occurring at the feeder network (FN) and distribution network (DN) towards the central office (CO) and optical networking unit (ONU) respectively. To avail the benefits of free space optics (FSO), it is integrated with the optical fiber at the user end. The architecture transmits the data on the ring in only a clockwise direction for both downstream and upstream transmission and therefore enables the utilization of the same wavelengths without any interference. i.e. the proposed architecture improves the bandwidth utilization and needs only half of the spectrum. In the proposed architecture, four 10 Gbps signals are propagated through a 20 km single-mode fiber (SMF) and a 200 m length of free-space/optical fiber link for the downstream and upstream transmissions. We also compared the signal transmission performance of architecture in terms of bit error rate (BER) for different weather conditions of the FSO link at the user side. The architecture works well for the different weather conditions and can transmit data through the FSO link for upto 700 m and 200 m for clear and heavy rain weather conditions respectively. Protection against faults in the fiber links; improves the survivability and reliability of the network. The proposed architecture is useful in rural and urban hilly areas where signal transmission interrupts frequently due to various geographical challenges. [ABSTRACT FROM AUTHOR]

• An MZI fiber optic temperature sensor was designed with a new structure of TCF-NCF-TCF (thin-core optical fiber, abbreviated to TCF; and no-core optical fiber, abbreviated to TNT) • The fiber optic sensor has been sensitized with PDMS, a material with high thermo-optical coefficient and thermal expansion coefficient, which increases the sensitivity from 57.14 pm/°C to 130 pm/°C. • The sensor exhibits high sensitivity and accuracy in temperature sensing, which has good stability in the range of 50 °C to 80 °C. • The MZI in this paper is simple to manufacture, low cost and easy to produce. It has great advantages for forming large-scale industrial production. A novel fiber optic sensor based on the Mach-Zender interference (MZI) principle and sensitized with polydimethylsiloxane (PDMS) is proposed. This sensor is primarily a sandwich structure with TCF-NCF-TCF (thin-core optical fiber, abbreviated to TCF; and no-core optical fiber, abbreviated to TNT) fused together sequentially, and its overall length is 3 cm. The TNT structure can effectively excite the cladding mode due to the fiber-core mismatch, which produces a significant interference spectrum. Because no-core optical fibers lack a core and cladding structure, they are equivalent to direct contact between the optical waveguide and the external environment, which is extremely sensitive to the variations in the external environment. The experimental results show the highest temperature sensitivity of 57.14 pm/°C ranges from 45 °C to 80 °C. PDMS, a material with high thermo-optical and thermal expansion coefficients, is coated on the surface of the fiber optic sensing zone. And the temperature sensitivity is effectively improved up to 130 pm/°C. Due to the small shape, compact structure, easy production, and steady mechanical strength, the sensor offers significant promise for temperature sensing applications. [ABSTRACT FROM AUTHOR]

• The crosstalk influence between the different cores in MCF on IFOG is analyzed and relevant experiment is designed. • By connecting four cores of a 300-m long MCF as fiber coil of IFOG, the ARW is 0.006°∕√h and the drift is 0.01°∕h, which is close to the performance of SCF with the same length. • Closed-loop scheme and DSP are used. This scheme is expected to put MCF into the products of IFOG in the foreseeable future. In this paper, multi-core fiber (MCF) is integrated in interferometric fiber optic gyroscope (IFOG) as the sensing coil to improve its sensitivity without increasing the fiber length or Shupe error. We have used a 300-m long 7-core fiber and connected 4 cores of MCF to each other by fan-in and fan-out (FIFO) devices of fused taper in closed-loop IFOG to replace the traditional single-core fiber (SCF). Before experiment, the main benefits and certain possible problems of using MCF are explained and analyzed. The angular random walk (ARW) of 0.006°/√h and drift of 0.01°/h have been achieved, which is close to the performance of IFOG using the 1200-m long SCF. Besides having a lower Shupe sensitivity, the MCF-IFOG also has small crosstalk effect when using broadband light source. The results show that the scheme of MCF for IFOG is expected to be used in practical application in the foreseeable future. [ABSTRACT FROM AUTHOR]

• A highly sensitive temperature and strain sensor based on Lyot filter with Vernier effect has been demonstrated. • Because the interference spectra of the Lyot filter and fiber Sagnac interferometer are both relatively uniform, it is easy to achieve the Vernier effect. • The FSRs of the Lyot filter and fiber Sagnac interferometer can be regulated conveniently, it is easy to achieve higher magnification based on Vernier effect. • Compared with the sensors based on Lyot structure with Vernier effect, the sensor in this work realized highest magnification and sensitivity. A highly sensitive temperature and strain sensor cascaded with a fiber Lyot filter (FLF) and a fiber Sagnac interferometer (FSI) is proposed in this paper. The temperature and strain characteristics of Lyot filter without and with Vernier effect are experimentally studied. Because the free spectral ranges (FSRs) of the two interferometers can be controlled simply by adjusting the length of polarization-maintaining fiber (PMF), the Vernier effect can be realized conveniently. High temperature and strain sensitivity measurement is obtained by this structure for the first time. The experimental results indicate that the temperature and strain sensitivity of the cascade structure can reach −61.52 nm/°C and 58.63 pm/με respectively, which is about 50 times higher than that of a single FLF whose temperature and strain sensitivity is 1.23 nm/°C and 1.21 pm/με. By carefully optimizing the structural parameter, the sensitivity can be further improved, which provides a new choice for temperature and strain detection. [ABSTRACT FROM AUTHOR]

Single-mode-no-core-single-mode (SNS) optical fiber structures have valuable potential to encapsulate as high-precision temperature sensors, due to their great sensitivity. This paper is to improve the temperature sensitivity of SNS fibers by studying the parameters of fiber length, diameter, and refractive index of the seal liquid. Transmission spectra of the fibers are given in both aspects of experiment and numerical simulation. We find that using a slim no-core fiber section sealed with liquid of high refractive index can effectively improve the sensitivity, while the choice of fiber length mainly affects the width of transmission peak. The results may contribute to the development of high-sensitivity optic temperature sensor. [ABSTRACT FROM AUTHOR]

• A hundred-Watt 980-nm MOPA fiber laser is demonstrated with the 20/125-μm DCYF. • In-band ASE limits the power up-scaling of the MOPA fiber laser. • Increasing seed power is beneficial to suppress the in-band ASE. • Split-step seed loading method is presented, with which 109-W output power is achieved. • The fiber laser provides a cost-effective solution for high-power core-pumping source. In this paper, a hundred-watt monolithic fiber laser operating near 980 nm is demonstrated by using a double-cladding Yb-doped fiber (DCYF) with 20-μm core and 125-μm inner-cladding diameter. The fiber laser is fabricated with the master oscillator power amplifier (MOPA) configuration to scale up the output power. In our experiment, it is found that the power scalability of the fiber laser is limited by the in-band amplified spontaneous emission (ASE) which can be effectively suppressed by increasing the seed power. Then, with the split-step seed loading method, 109-W output power around 980 nm has been achieved with in-band and 1030-nm ASE suppressed to 17.1 and 40.6 dB, respectively. The beam quality has also been measured, and the M2 factor is about 1.9 at the maximum output power. Our work provides a cost-effective solution to achieve hundred-Watt fiber lasers operating near 980 nm. [ABSTRACT FROM AUTHOR]

• We demonstrate a passively mode-locked laser operating at 2 um region based on a saturable absorber (SA) of FONPs deposited on the D-shaped fiber (DF). This is the first use of FONPs to achieve mode-locking at 2um. • The mode-locking operates in 1931 nm, the 3 dB bandwidth is 5.9 nm, the repetition frequency is 24 MHz, and the pulse width is 1.29 ps. In addition, the signal-to-noise ratio is ∼67 dB, indicating the stability of the output spectrum. As a unique type of transition metal oxides, ferroferric-oxide nanoparticles (FONPs) have attracted much attention due to their outstanding physical and chemical properties. In this paper, we demonstrate a passively mode-locked laser operating at 2 um region based on a saturable absorber (SA) of FONPs deposited on the D-shaped fiber (DF). FONPs were deposited on DF to form FONP-DF SA devices. As inserted into the thulium-doped fiber laser (TDFL), a mode-locked pulse was realized at 1931 nm. For all we know, this is the first demonstration of FONP-DF SA in fiber lasers at 2 um region. This work fills the gap of FNOP in the realization of 2 um mode-locking and lays the foundation for us to use FNOP-DF for magnetic field sensing in the future. [ABSTRACT FROM AUTHOR]

• An unclosed fiber optic current sensor based on the helically wound spun highly birefringent fiber half waveplate on the tunnel boring machine is proposed and experimentally demonstrated. • The unclosed fiber optic current sensor is designed to coil the spun highly birefringent fiber around the measured current clockwise and counter clockwise alternately in order to be mounted on the measured object with large diameter easily. • The helically wound spun highly birefringent fiber half waveplate is proposed to accumulate the Faraday effect. • The good performance of the unclosed fiber optic current sensor is confirmed by the laboratory experiment. In the application of the fiber optic current sensor that is mounted on tunnel boring machine (TBM) for measuring the probe current in tunnel geological prediction, the field installation and the influence of the linear birefringence caused by external perturbations are problems needing to be solved. In this paper, an unclosed fiber optic current sensor (UFOCS) based on the helically wound spun highly birefringent (SHB) fiber half waveplate is proposed and demonstrated. The UFOCS is designed to coil the fiber around the measured current clockwise and counter clockwise alternately in order to be mounted on the measured object with large diameter easily. The all-fiber half waveplate made by the helically wound SHB fiber is used to accumulate the Faraday effect. The simulation is carried out based on the polarization model and a linear response is obtained. The laboratory experiment is conducted and the results confirm the good performance of the UFOCS. [ABSTRACT FROM AUTHOR]

• Temperature effects of fiber-based luminescent solar concentrators. • Effect of thermal cycling on the luminescence of POFs doped with Lumogen dyes. • Almost full recovery in the output intensity of Lumogen red fiber in thermal cycles. • Temperature sensitivities of 0.07 nm/°C in this doped fibers was revealed. In this paper, we report the temperature effects on the luminescence of polymer optical fibers doped with Lumogen dyes (red and orange). Specifically, we have analyzed the intensity and the average wavelength of the spectra emitted by both fibers when they are heated from 30 °C to 90 °C. An almost full self–recovery of the signal in the fiber doped with Lumogen red has been observed when the fiber is subjected to thermal cycles with oscillating temperature variations between 30 °C and 90 °C. The emitted spectra are red/blue shifted when the fiber sample is heated/cooled with a final temperature sensitivity around 0.07 nm/°C when a stationary situation is reached. [ABSTRACT FROM AUTHOR]

• The cross-correlation between short and long pulses in BOTDR results in a narrow Brillouin spectrum width. • The subtraction between two optical pulses with slightly different duration offers sub-meter spatial resolution measurement. • The DCS-BOTDR technique offers a potential application for long distance fiber measurement with high measurement accuracy. In this paper, we report the effect of pulse duration on the Brillouin frequency shift (BFS) accuracy measurement in our previously proposed differential cross-spectrum Brillouin optical time domain reflectometry (DCS-BOTDR) fiber sensing technique. The DCS-BOTDR uses a pair of optical pulsed probes generated via intensity modulation scheme, in which the first probe consists of a long- and short duration of pulses separated by a short interval, while the second probe only has the long pulse with the same duration as the first one. From the experimental analysis, by varying the duration of the long pulse between 6 and 60 ns, we found that an optimum range of the long pulse duration between 14 and 22 ns provided good measurement of BFS accuracy around 5.3 MHz. Furthermore, by heating up a 2 m fiber section at the end of a 4 km test fiber, by using long- and short pulse durations of 18 ns and 4 ns respectively, we have successfully demonstrated 40 cm spatial resolution and 4 °C of temperature resolution measurement. [ABSTRACT FROM AUTHOR]

• A hollow-core PCF composed of magnesium fluoride was proposed to identify toxic and hazardous gases. • The structure is based on the quasi-crystal arrangement of air holes. • Numerical simulation results showed that the fiber proposed in this study has a sensitivity coefficient above 50%, relatively low loss, and low dispersion. • The device is suitable for identifying toxic and dangerous gases. • The detection of which using this tool prevents combustion and explosion in the drilling process. In this paper, a hollow-core photonic crystal fiber (PCF) composed of magnesium fluoride is designed to identify toxic and hazardous gases. Its band structure is calculated using the well-known plane wave expansion method. The numerical results show that the proposed structure has a relatively wide photonic bandgap (PBG) in the wavelength range of 1 to 3.3 μm covering the near- and mid-infrared wavelength ranges. Then, by applying optical pulses with a wavelength in the range of the PBG to the proposed PCF, its propagation behavior is calculated by solving the nonlinear Schrödinger equation using the split-step Fourier method. The type of gas can be identified by observing and comparing the frequency responses of the gas-filled PCF with the absence of gas injection. Simulation results show that the fiber proposed in this study has a sensitivity coefficient of more than 50%. The low loss, low dispersion, and high accuracy of the proposed fiber make it a suitable tool for identifying toxic and dangerous gases such as methane and hydrogen sulfide in the drilling and oil exploration industry. Identifying them using this tool prevents combustion and explosion in the drilling process. [ABSTRACT FROM AUTHOR]

• A novel remote node design has been presented with fiber fault protection for ring topology NG-PON2. • The presented NG-PON2 architecture is based on single fiber ring topology. • The proposed remote nodes are able to communicate in both clockwise and anticlockwise directions. • In case of fiber fault the remote nodes can change the direction of communication and service disruption can be avoided. The service disruptions in passive optical networks (PONs) due to fiber or node failure is an important issue. In the conventional star topology time and wavelength division multiplexing (TWDM) based next generation passive optical network stage 2 (NG-PON2), there is no provision to restore the service if the fiber between remote node (RN) and optical network unit (ONU) fails. In this paper, we proposed a novel entirely passive node design for ring based NG-PON2 which provides fiber fault protection against any fiber break between RNs. The working of the proposed node is verified by simulating normal conditions and fault conditions. It is observed that in the case of fiber failure, the affected ONUs can still communicate with the optical line terminal (OLT). The performance of the system has been analyzed in terms of bit error rate (BER), and the BER <10−9 confirms the correct working of the system. [ABSTRACT FROM AUTHOR]

• Dual U-shaped passive-active fibers for refractive index sensing. • Evanescent wave coupling effect inducing multiple modulations. • Sensitivity enhancement factors of 1.55, 1.48 and 2.90 for bend ratios of 6, 12 and 17. • Simple ray tracing analysis with experimental data corroboration. Numerous methods have been reported to further enhance the sensitivity of U-shaped fiber optic sensors towards external refractive index variation. This paper introduces a variant U-shaped fiber structure, which is inspired by the U-shaped and twisted fiber configurations. The U-shaped passive-active fiber arrangement consists of two fibers placed in a concentric semicircle manner and is expected to yield higher sensitivity than single U-shaped structure due to the multiple modulations at the boundaries of the fibers. This configuration is also beneficial in terms of its simple fabrication process (i.e. no twisting) and compact-size. Ray tracing analysis was performed, and the simulated data corroborates with the experimental data. Moreover, the sensor sensitivity is enhanced by a factor of around 1.55, 1.48 and 2.90 for bend ratios of 6, 12 and 17, respectively. In addition, the sensor demonstrated good sensing stability, which eliminates the intense signal fluctuation problem caused by the inconsistent gap between the fibers. Ultimately, the U-shaped passive-active fiber provides a promising route towards developing a low-cost, portable, and sensitive sensor, and allows the incorporation of a mixture of transducer coatings for further improvement in the sensing performance. [ABSTRACT FROM AUTHOR]

• BFS extraction technology based on K Nearest Neighbor Algorithm provides a new research idea for BOTDA to extract BFS in low SNRs. • BFS extraction accuracy decreases with the decrease of SNR and the increase of frequency step. • The uncertainty of BFS extraction by KNN can be 0.44 with frequency scanning step of 1MHz and SNR of 11dB. The extraction of Brillouin frequency shifts from a Brillouin gain spectrum is a common problem in the engineering applications of Brillouin distributed fiber sensors. In this paper, extraction of Brillouin frequency shifts from Brillouin gain spectrum in Brillouin distributed fiber sensors using K nearest neighbor algorithm is proposed and experimentally demonstrated. The Brillouin frequency shift extraction is treated as a model-free supervised classification problem. The influence of Signal-to-noise ratio and frequency scanning step is studied. The performance of Brillouin frequency shift extraction degrades with the increase of frequency scanning step, and with the decrease of signal-to-noise ratio. The result shows that the algorithm of K nearest neighbor algorithm is a better way to extract the Brillouin frequency shift from Brillouin gain spectrum, comparing with curve-fitting methods, the uncertainty can be 0.44 with frequency scanning step of 1 MHz and SNR of 11 dB. [ABSTRACT FROM AUTHOR]

• We fabricated the V-grooves in the step refractive index multimode fiber, sensing channel I is formed by coating gold film on the core surface, due to the V-grooves, the cladding mode of the fiber is excited, sensing channel II is formed by coating gold film on the 2 cm cladding area behind the sawtooth area, the dual-channel sensor is fabricated. • The dual-channel sensor adjusted the resonance wavelength range by utilizing the angle of the V-grooves to change the SPR resonance angle. • Due to the existence of V-shaped grooves, the total reflection angle of transmitted light in the sawtooth area changes, which significantly improves the SPR sensitivity, the sensitivity is 3114.86 nm/RIU, which is higher than that of traditional transmission type multimode fiber SPR sensor. • The sawtooth structure fiber SPR sensor proposed in this paper can effectively improve the sensitivity and realize the dual-channel measurement based on wavelength division multiplexing technology which will promote the pace of simultaneous detection of multiple analytes, and regardless of the working mode, the SPR resonance wavelength and sensitivity of the sensor have a good consistency. The fiber SPR sensor with a single sensing channel can not detect multiple analytes at the same time and introduce the reference channel to eliminate nonspecific interference. Hereby we proposed a sawtooth dual-channel fiber SPR sensor based on WDM technology. By CO 2 laser, multiple V-shaped grooves are processed on the step multimode fiber to form a sawtooth structure. The first stage of the sensor was fabricated by coating gold film on the sawtooth area, and the second stage was fabricated by coating gold film on the cladding area 2 cm behind the sawtooth area. The sensitivity and dynamic range of the two channels are 3114.86 nm/RIU and 1584.23 nm/RIU, 719.93 nm-854.2 nm, and 584.24 nm-667.62 nm, respectively. The proposed fiber SPR sensor employs the sawtooth structure to adjust the SPR resonance wavelength, so as to realize dual-channel sensing, which is of significance in the fields of simultaneous detection of multiple analytes and temperature compensation of reference channels. [ABSTRACT FROM AUTHOR]

A hybrid-word-length quantization scheme for the perturbation-based fiber nonlinearity compensation is proposed in this paper. The key feature of the proposed scheme is quantizing small-absolute-value coefficients with aggressive single-bit word length and large-absolute-value coefficients with multiple-bit word length. A significant complexity reduction can be obtained because of the simplified architecture of the single-bit multiplier. The proposed scheme was investigated in both simulation and experiment. Results show that the hardware complexity was reduced by approximately an order of magnitude compared with the uniform quantization scheme when the performance degradation was 0.2 dB, which validated the efficiency of the proposed scheme. [ABSTRACT FROM AUTHOR]

• A method based on CLMT is introduced to analyze adiabatically tapered MMFs. • The CLMT can greatly improve the calculation efficiency. • Tapered MMFs with different tapering shapes are simulated. Analyzing the mode evolution characteristics of light in a tapered multimode fiber is an important research topic for multimode fibers. In this paper, an analytical method based on coupled local mode theory is introduced, which can analyze adiabatically tapered multimode fiber with different taper parameters. Compared with traditional beam propagation methods, the coupled local mode theory can greatly improve the calculation efficiency and directly reflect the mode evolution process. Therefore, it has important application prospects in the optimal design of tapered fibers. Based on the coupled local mode theory, the multimode fibers with different taper shapes are simulated, and the optimal design of tapered fibers is carried out from the perspective of fundamental mode conservation ability. In theory, the proportion of LP 01 mode in output energy can account for more than 99.9%. [ABSTRACT FROM AUTHOR]

Recently there has been an interest in upgrading legacy multimode fibers such as 50 μm OM2 fiber to higher data rates through single mode transmission. A fiber modal adapter hosting a modal conditioning single-mode fiber (MCSMF) has been demonstrated as one robust solution to convert OM2 links into single mode transmission links for higher data rate transmission. Legacy multimode fibers also include OM1 fiber which has higher index contrast between the core and cladding and a larger core of 62.5 μm diameter. In this paper, we conduct a study to understand the MCSMF mode field diameter and connector offset tolerance requirements for OM1 fiber, which affect the transmission performance of single mode transmission. We find that despite very different refractive index profiles between OM1 and OM2 fibers, the fiber modal adapter concept is still applicable to OM1 and only one modal conditioning single-mode fiber design is needed to accommodate the needs for both fiber types. The detailed tolerances to the mode field diameter mismatch and the connector offset are numerically studied. The transfer functions for 1000-m of OM1 and OM2 with the adapters are measured, which show very high modal bandwidths at tens of GHz in contrast to very low modal bandwidths without the adapters. Transmission at 25 Gb/s using a 25G LR transceiver, 100 Gb/s using a 100G CWDM transceiver, and 400 Gb/s using a 400G LR transceiver are demonstrated with error free performance and open eye diagrams over 1000-m of OM1 and OM2. We find that the approach of using MCSMF can support fundamental mode transmission up to 100 Gb/s over this distance without obvious limitations. [ABSTRACT FROM AUTHOR]

• The transmission of 448 Gb/sPS-PDM-PAM-8 signal over multicore fiber using coherent detection is experimentally demonstrated for the first time. • A novel equalization algorithm named modified SP-LMS is proposed and it can recover the PS-PDM-PAM-8 signal successfully. • By transmitting PS-PDM-PAM-8 signal, a 22.4 Gb/s gain in net rate can be achieved compared with uniformed PDM-PAM-4 signal at the same baud-rate. The combination of pulse amplitude modulation (PAM) and coherent detection is a promising solution for short reach transmission systems which require the transmission rate of 100 Gb/s and above. In this paper, the transmission of probabilistic shaped polarization-division-multiplexed eight-level pulse amplitude modulated (PS-PDM-PAM-8) signal over multi-core fiber (MCF) using coherent detection is investigated. A modified signal-phase-based LMS (SP-LMS) algorithm is proposed to equalize both intensity-modulated and filed modulated PS-PAM signals. By employing PS and MCF transmission, the total rate of 448 Gb/s is achieved and compared with traditional PDM-PAM4 signal, the PS-PDM-PAM8 can achieve a 22.4 Gb/s gain in net bit rate. [ABSTRACT FROM AUTHOR]