Bidirectional Fiber Research Articles

Reduction in differential phase noise of distributed acoustic sensing with bidirectional amplifier

We investigate the effect of a bidirectional erbium-doped fiber amplifier on the reduction of the differential phase noise in long-range distributed acoustic sensing by coherent detection. We succeeded in reducing the differential phase noise at the fiber position after the amplifier over a distance of 100 km by adjusting the gain of the bidirectional amplifier considering the amplified spontaneous emission. This will effectively improve the accuracy of strain measurement over long distances.

Bidirectional mode-locked fiber laser based on nonlinear multimode interference

Bidirectional mode-locked lasers are an ideal light source for dual-comb technique for their system compactness and common noise cancellation. Here, we report an all-fiber bidirectional mode-locked laser based on nonlinear multimode interference. Asynchronous pulse trains in different directions with a repetition rate difference of 7 Hz are obtained. Besides, we record the pulse trains and spectra at different pump powers. In our experiment, the counter-propagating pulse (CPP) trains can operate in different states, such as the counterclockwise (CCW) direction works in single pulse while the clockwise (CW) direction works in multi-pulse or harmonic mode-locking. We believe that the work will provide a new platform for studying the dynamic characteristics of bidirectional mode-locked lasers.

Bidirectional tandem-pumped high-brightness 6 kW level narrow-linewidth confined-doped fiber amplifier exploiting the side-coupled technique.

In this work, a bidirectional tandem-pumped high-power narrow-linewidth confined-doped ytterbium fiber amplifier is demonstrated based on side-coupled combiners. Benefiting from the large-mode-area design of the confined-doped fiber, the nonlinear effects, including stimulated Raman (SRS) and stimulated Brillouin scattering (SBS), are effectively suppressed. While the transverse mode instability (TMI) effect is also mitigated through the combination of confined-doped fiber design and the bidirectional tandem pumping scheme. As a result, narrow-linewidth fiber laser with 5.96 kW output power is obtained, the slope efficiency and the 3-dB linewidth of which are ∼81.7% and 0.42 nm, respectively. The beam quality is well maintained during the power scaling process, being around M2 = 1.6 before the TMI occurs, and is well kept (M2 = 2.0 at 5.96 kW) even after the onset of TMI. No SRS or SBS is observed at the maximum output power, and the signal-to-noise ratio reaches as high as ∼61.4 dB. To the best of our knowledge, this is the record power ever reported in narrow-linewidth fiber lasers. This work could provide a good reference for realizing high-power high-brightness narrow-linewidth fiber lasers.

Complex Neurochemical Microstructure of the Stria Terminalis in Infant and Adult Macaque Monkey.

The stria terminalis (ST) is a major bidirectional fiber tract anchored in the amygdala and bed nucleus (BNST). Extensive investigations in rodents report a complex arrangement of neurochemically diverse neurons within the ST, but fewer data are available for non-human primates. Given the functional importance of the ST, we investigated its microarchitecture in one newborn, four infant, and two adult macaque brains, by parallel immunocytochemical series for cells or fibers. Main results are as follows: (1) The pan-neuronal marker NeuN shows scattered neurons and small neuronal clusters in both the dorsal and ventral ST, but more numerous dorsally; (2) smaller neuronal subpopulations are labeled by calretinin (CR), neuropeptide Y (NPY), calbindin (CB), and somatostatin (SOM), of which the CR + neurons are the most numerous; (3) the infant brains on average have more neurons in the ST than the adult brains, but across our sample, there is notable individual variability; and (4) fiber architectonics have a complex organization, which can be referenced to myelin-poor or myelin-dense zones. Myelin-poor zones coincide with concentrations of fibers positive for CB, CR, or tyrosine hydroxylase (TH). Neurons have been reported in other white matter domains (e.g., anterior commissure, corpus callosum, cingulum bundle, and subcortical white matter). Like these, at least some neurons within the ST may give rise to long-distance connections, and/or participate in more local functions, such as vascular regulation or axon guidance/maintenance.

2 × 2 kW near-single-mode bidirectional high-power output from a single-cavity monolithic fiber laser.

Traditional monolithic fiber lasers can only achieve unidirectional high-power laser output. In this Letter, a novel high-power linear cavity fiber laser that can achieve bidirectional high-power output is proposed and demonstrated. In an ordinary laser resonant cavity, we replace the high-reflectivity fiber Bragg grating with a low-reflectivity fiber Bragg grating to realize bidirectional laser output. In our experiment, the laser cavity was composed of two fiber Bragg gratings with a reflectivity of about 10%. The pump power provided by the 976 nm laser diodes was injected into a double-clad Yb-doped fiber with core/cladding diameters of 20/400 µm. At the maximum pump power, the bidirectional output powers were 2025 W and 1948 W, respectively, and the output laser beam quality (M2 factor) at both ends was about 1.5. For the first time, to the best of our knowledge, the feasibility of a bidirectional output fiber laser that can achieve double high (2-kW-level) power was verified. Compared with a traditional unidirectional output laser, this type of bidirectional output laser can achieve a double high-power laser by employing a laser resonant cavity. Thus, the average cost and structure size can be further reduced in mass production.

Colorless, partially directionless, and contentionless architecture for high-degree ROADMs

With the rapid growth of Internet traffic, there can be multiple pairs of bi-directional fibers on a network link. This leads to the need for high-degree reconfigurable optical add/drop multiplexers (ROADMs) in an optical network. However, this high-degree requirement poses new challenges to ROADMs since it is challenging and expensive for a wavelength selective switch (WSS), a key switching component in a ROADM, to expand its size. Therefore, a common strategy is to use small-size WSSs to build a high-degree ROADM. In this paper, we design a novel colorless, partially directionless, and contentionless (CpDC) ROADM architecture, in which the sizes of WSSs can be significantly reduced by partially sacrificing the directionless feature of a ROADM. The partially directionless feature may degrade the switching performance of a ROADM. Thus, to tackle this issue, we also propose an interconnection pattern for the ROADM backplane to effectively connect fiber links and add/drop modules. An analytical model is developed to evaluate the lightpath blocking performance of a single CpDC ROADM. Efficient routing, fiber, wavelength, and add/drop port assignment (RFWPA) algorithms (i.e., the shortest-path-based and waveplane-based algorithms) are also developed for lightpath establishment in an optical network made up of CpDC ROADMs, in which two wavelength selection strategies [i.e., non-interconnection aware (NIA) and interconnection aware (IA)] are further proposed. Simulation studies show that, while these achieve almost the same lightpath blocking performance as conventional CDC ROADMs, CpDC ROADMs are more effective from considerations of cost, insertion loss, and node volume. The waveplane-based RFWPA algorithm with the IA strategy is also found to be the most effective for lightpath provisioning in an optical network made up of CpDC ROADMs.

Multipoint space-domain active fiber cavity ringdown technique for quasi-distributed magnetic field monitoring

A multipoint space-domain active fiber cavity ringdown (FCRD) sensing technique based on frequency-shifted interferometry (FSI) is proposed and investigated for quasi-distributed magnetic field measurements. Multiple FSI-based fiber ringdown cavities (RDCs) are cascaded together to constitute a sensor array, share one continuous-wave optical source and one slow balanced detector, and thus the system is economical and highly sensitive. In each RDC a bidirectional erbium-doped fiber amplifier (Bi-EDFA) is inserted to compensate the inherent cavity loss for achieving higher sensitivity. Compared to the time-domain active FCRD techniques, the amplifier has lower gain fluctuation because the continuous light (instead of pulse light) coupled into the cavity enables more stable optical power, and has lower amplified spontaneous emission noise due to the use of two narrow bandpass filters and differential detection. Magnetic fields in different locations are synchronously resolved in the space domain by measuring the ringdown distances of continuous light, rather than measuring the ringdown times of pulse light in the time domain as in conventional multipoint FCRD techniques. A dual-point active FSI-FCRD magnetic field sensor was experimentally built, and two side-polished fibers coated with magnetic fluid were incorporated inside the fiber cavities as the sensing probes. By applying the external magnetic field from 0 to 230 Gs, the sensitivities of 6.68 × 10−3 and 4.45 × 10−3/(km Gs) were respectively achieved, which are at least one order of magnitude higher than the conventional passive FCRD techniques. After repeated measurements, the obtained ringdown baseline stabilities of the two units were 0.91% and 2.00%, which were much higher than those of time-domain active FCRD sensing systems. Using power budget analysis, the maximum number of identical sensing units can be expected to be 48 over a 25.89-km distance under the nearly same experimental conditions. The proposed multipoint active FCRD sensing scheme has the merits of low cost, high sensitivity, good stability, and strong multiplexing ability, which would find good application prospects at electric grids, national defense, biology, and medicine for monitoring quasi-distributed magnetic field.

Retrofitting with FRCM Composites: Shear and Flexural Behaviour of Strengthened Masonry Walls

Structural strengthening using composite materials is nowadays one of the most interesting techniques to overcome weaknesses of masonry structures constituting large part of the building heritage. The use of FRCM composites is becoming more and more widespread due to some limitations of FRP retrofitting systems. In this framework, the presented experimental study is aimed at evaluating the in-plane and the out-of-plane behaviour of masonry walls strengthened with different types of FRCMs, analyzing in detail failure modes, capacity increments and efficiency of the strengthening systems when tested using two different configurations. To this purpose, bidirectional basalt grids and unidirectional steel fiber sheets, coupled with a lime based mortar, were used for retrofitting clay brick masonry walls subjected to diagonal compression tests and out-of-plane flexural tests. Experimental outcomes, when considering the in-plane or the out-of-plane direction, show that the adopted different layout strictly influences the flexural and shear strengthening efficiency of the reinforcement.

Characterisation of Flexural Properties of Glass Fibre/Pineapple Leaf Fibre (PALF) Hybrid Composite

The use of natural resource materials has gained awareness among industries recently. Today, replacing the material with something more environmentally friendly, especially from waste natural products like pineapple leaf fibre (PALF), is a top concern. This research aims to look at the flexural properties of a glass fibre/pineapple leaf fibre (PALF) hybrid composite. The hand layup and cold compression methods were used to manufacture the hybrid composite plates, which provide 30 wt% of fibre and 70 wt% of the matrix. The form of the hybrid composite was unidirectional with a size of 30 cm 30 cm 3 cm and sandwich stacking. Universal testing equipment was used to conduct the flexural test. With a flexural strength of 290.11 MPa, 5 wt% PALF and 25 wt% bi-directional glass fibre were found to have the maximum flexural strength.

Investigation of a bidirectional thulium-doped fiber amplifier based fiber laser with a Japanese-geta shape

A thulium-doped fiber amplifier–based Japanese geta–shaped fiber laser was proposed and demonstrated, and a homemade uniform fiber Bragg grating was used to select the desired lasing channel. The experiment was carried out under three different configurations, and the maximum transmission power of the amplified spontaneous emission was obtained in the configuration of bidirectional amplification. The stability of the proposed fiber laser was tested for 60 min with fluctuations of the center wavelength and output power of less than 0.003 nm and 0.104 dB, respectively. A maximum optical signal-to-noise ratio of ∼30.598 dB and a minimum threshold power of ∼1.118 W were obtained under bidirectional amplification. The maximum slope efficiency of 6.57% was obtained under backward amplification. In addition, the tunability of the proposed fiber laser was investigated under different configurations. The proposed fiber laser was found to be well suited in the areas of optical fiber communication and sensor systems.

Transient switching dynamics between asynchronous and synchronous modes in a bidirectional ultrafast fiber laser

Continuous interest in bidirectional ultrafast fiber lasers not only reinforces the practical applications in coherent dual-comb spectroscopy but also enriches the understanding of soliton dynamics in nonlinear systems. However, the transient dynamics of bidirectional solitons are still unexplored. Here, we report on an observation of the switching dynamics in a net-anomalous-dispersion bidirectional ultrafast fiber laser which can operate in both asynchronous and synchronous operations. Transient switching dynamics between the asynchronous and synchronous states are demonstrated, which is accompanied by switchable and weak interaction between bidirectional solitons. The dynamical evolution and output properties of the asynchronous and synchronous bidirectional solitons are investigated respectively. Stable breathing solitons in the synchronous operation show opposite behaviors in both spectra and pulse energy dynamics, which vary with different pump powers. Our findings provide insight into the transient dynamics in ultrafast lasers and contribute to the understanding of nonlinear systems in general.

Mutual dynamics between synchronous solitons in a bidirectional mode-locked fiber laser.

In this Letter, the mutual dynamics between synchronous solitons in a bidirectional mode-locked fiber laser are studied via dispersive Fourier transform methodology. We explore the spectral evolution and the statistical correlations between solitons with bidirectional propagation, indicating the low and high mutual linear dependences of the spectral energy jitters in stable and breathing mode-locking states, respectively. Moreover, to the best of our knowledge, the oscillating and sliding phase dynamics are experimentally revealed by the interference between bidirectional breathing solitons in ultrafast fiber lasers for the first time. Our findings enrich the understanding of the internal mutual dynamics between bidirectional solitons, which guides the extension of their potential applications, such as Sagnac-effect-based optical sensing.

Dynamics of breathing dissipative soliton pairs in a bidirectional ultrafast fiber laser.

The breathing dissipative soliton as a dynamic solution to many nonlinear systems has induced substantial interest in nonlinear photonics and ultrafast laser science. However, the exotic breathing multi-soliton dynamics are still largely unexplored in the bidirectional fiber laser compared to the unidirectional laser. Here, we reveal nonequilibrium dynamics of a breathing soliton pair (BSP) with energy transfer in a bidirectional laser; in particular, the dissociation and annihilation of the BSP was triggered by control over intra-cavity polarization. Optical rogue waves were detected simultaneously, and the collision of breathers significantly increased the intensity of rogue waves, which is characteristic of the bidirectional laser. Further, the buildup dynamics of the BSP with nanosecond pulse separation and a breathing soliton molecule were observed. Multiple single soliton explosions and transient pulse splitting are distinct features of soliton molecule buildup compared to the soliton pair. These findings shed new insights into the multiple breather dynamics of nonlinear systems.

Finite Difference Model for the Bond Behaviour of Polyparaphenylene Benzobisoxazole (PBO) Fibre-Reinforced Composite System for Retrofitting Masonry

This study addresses a numerical investigation of the bond behaviour exhibited by an FRCM system when subject to tensile and single direct shear tests. A reinforcement system, based on a polyparaphenylene benzobisoxazole (PBO) bi-directional fibre mesh and a mixed cement-pozzolanic mortar is selected. The system is characterized by the presence of coated glass-fibre yarns and dry polypropylene yarns alternated to the PBO yarns in the warp and weft directions, respectively. The mechanical characterization of composite constituent materials is carried out together with tensile and direct shear tests. Concerning mechanical interpretation of the tests, within a mode II fracture mechanics, and assuming a trilinear cohesive material law (CML), the stress transfer law between the fibre and the matrix is back calibrated from single direct shear test results. The CML obtained is employed into a finite-difference model developed for the purpose. Tensile tests are modelled providing adequate boundary conditions. Results satisfactorily agree with the tested behaviour of the FRCM system.

Failure analysis of basalt bidirectional mat reinforced micro/nano Sic particle filled vinyl ester polymer composites

Natural fiber reinforced polymer composites are increasingly used in composites industry to propel towards sustainable and environmentally friendly materials. Basalt fibers have superior mechanical and thermal properties, but are brittle by nature. However, appropriate filler materials were ingeniously utilized to subdue this impediment. Moreover, optimizing the content and appropriating the combination of micro and nano size filler particle addition has significant effects on the final mechanical properties of the composite. The failure pattern of bidirectional basalt fiber mat (67 wt%) vinyl ester composite with SiC filler of micro and nano size added independently as well as in combination at various proportion ranging from 2 wt% to 10 wt% was analysed using fractographic images. Highest tensile strength, flexural strength, impact strength, hardness and specific wear rate of 204.33 MPa, 525.33 MPa, 3.55 J/mm, 43.87 VHN and 5 × 10–7 mm3/Nm respectively was obtained for the optimized composite sample. The fractured scanning electron microscopy (SEM) images taken at different stress conditions reveal failure patterns, such as fiber–matrix delamination, matrix cracking, matrix deformation, matrix pores, fiber breakage, etc to correlate with the mechanical strength and wear resistance. The degree of damage is very well associated to the percentage content, size and combination of the filler material in the composite along with the fiber mat.

High-peak-power pump-modulated quasi-CW fiber laser.

Quasi-continuous wave (CW) laser output with high peak power and high energy is preferred in some industrial applications. Due to the non-linear effects and transverse mode instability, such high-peak-power laser output is difficult to achieve via monolithic fiber lasers in CW mode. For diode-pumped monolithic fiber lasers, by applying overshoot pulse modulation to the pumping diodes, we obtain a pulse laser output with a peak power much higher than that in CW mode. In this paper, it has been theoretically studied that stable pulses with the width of µs level can be generated with quasi-CW operation without distortion according to our simulation. We also experimentally demonstrate a bi-directional pumped quasi-CW monolithic fiber laser operating in CW and pulse modes. In quasi-CW mode, by applying overshoot pulse modulation to the diodes, with a frequency of 1 kHz and a pulse width of 100 µs, the peak power of the output laser reached 9713 W with an average power of 898 W and M2 of 2.4 and 2.3 in the two orthogonal directions, respectively. To the best of our knowledge, this is the very first quasi-CW fiber laser of a 10-kW level with the M2 level of 2.

Electromagnetic shielding characteristics of carbon and glass fiber composites

Electromagnetic shielding characteristics for orientation angle and number of plies of carbon fiber reinforced epoxy composites were investigated in the frequency range between 900 and 6000 MHz. In this study, both unidirectional and bidirectional carbon fiber fabrics were utilized as reinforcement materials to manufacture the composite samples. Twill bidirectional glass fiber fabrics were also used in order to provide a large amount of flexibility. To prepare the composite laminates for measurement, hand lay-up method was preferred. Measurements were carried out by using DFG 4060 signal generator and HF 60105 spectrum analyzer. According to the measurements, it was observed that electromagnetic shielding effectiveness (EMSE) was very higher when the orientation angle of the carbon fiber was 90°, as compared to 0°. Another parameter that affects EMSE is whether the carbon fiber fabric used is unidirectional or bidirectional because it was observed that the bidirectional fabric increased EMSE. In addition, it has been determined that the number of plies has less effect on EMSE than the orientation angle.

Experimental study on the flexural behaviour of timber beams strengthened with high ductility and low cost hybrid fibre sheets

Timbers with defects are often used in historical buildings. However, the existing fibre sheet reinforcing technology for the timber beams is difficult to balance the strength, ductility and cost. This paper aims to develop novel high ductility hybrid fibre sheets for strengthening timber beams at a lower cost than available methods. First, four types of unidirectional and two types of bidirectional fibre reinforced polymer (FRP) sheets were designed and evaluated using scanning electron microscopy (SEM) and tensile tests. Then, the FRP sheets were used to make 68 timber beams with square and round cross sections for bending tests. Finally, parameter analysis was conducted to evaluate the enhancement law of specimens with different sheet layers, fibre types, reinforcement angles and interlaminar hybrid modes. The results show that the reinforcement angle of a round timber beam has a great influence on its ultimate bearing capacity, for which the recommended value range is 45°∼60°. Unidirectional fibre sheets with a fibre ratio of carbon fibre:aramid fibre:glass fibre = 2:1:1 have a positive hybrid effect on bearing capacity and can improve the ultimate deflection of timber beams by 69.2–115.4% with one layer. These research results can contribute to state-of-the-art knowledge about round and square section timber beams strengthened with hybrid fibre sheets and the conservation of timber structures.

Breathing Dissipative Soliton Molecule Switching in a Bidirectional Mode‐Locked Fiber Laser

Breathing optical solitons propagating in a dissipative nonlinear system can interact and bind stably, forming an optical soliton molecule that presents striking molecule‐like dynamics. To date, the breathing soliton pair has been mainly observed in the microcavity platform, and the peculiar dynamic evolution of breathing soliton molecules (BSMs) remains largely unexplored in mode‐locked fiber lasers. Herein, the transient switching dynamics of BSMs in a bidirectional ultrafast fiber laser is revealed, specifically triggered at different parameter spaces of saturable absorption with manipulation polarizations that maintain constant pulse separation or undergo strong repulsion and kink after switching. All‐optical switching of breather molecules and significantly increasing multiple soliton molecules’ switching of two or three solitons between states with different binding separations by applying a strong stimulus with periodic pump modulation is demonstrated. The instantaneous pulse breakup can be induced by the collision of bidirectional breathing solitons in each switching, which is characteristic of the breathing solitons in a bidirectional fiber laser and further corroborated by numerical simulation. The study unveils new perspectives into the ultrafast transient process of BSMs in various dissipative systems.

Microfiber Assisted Bidirectional Wavelength-Locked Ultrafast Erbium-Doped Fiber Laser

Microfiber Assisted Bidirectional Wavelength-Locked Ultrafast Erbium-Doped Fiber Laser