Specialty optical fibers for advanced sensing applications

Optical fiber technology has changed the world by enabling extraordinary growth in world-wide communications and sensing.The rapid development and wide deployment of optical fiber sensors are driven by their excellent sensing performance with outstanding flexibility,functionality,and versatility.Notably,the research on specialty optical fibers is playing a critical role in enabling and proliferating the optical fiber sensing applications.This paper overviews recent developments in specialty optical fibers and their sensing applications.The specialty optical fibers are reviewed based on their innovations in special structures,special materials,and technologies to realize lab in/on a fiber.An overview of sensing applications in various fields is presented.The prospects and emerging research areas of specialty optical fibers are also discussed.

Strong anharmonicity-assisted low lattice thermal conductivities and high thermoelectric performance in double-anion Mo_(2)AB_(2)(A=S,Se,Te;B=Cl,Br,I)semiconductors

The thermoelectric properties of layered Mo_(2)AB_(2)(A=S,Se,Te;B=Cl,Br,I)materials are systematically investigated by first-principles approach.Soft transverse acoustic modes and direct Mo d–Mo d couplings give rise to strong anharmonicities and low lattice thermal conductivities.The double anions with distinctly different electronegativities of Mo_(2)AB_(2)monolayers can reduce the correlation between electron transport and phonon scattering,and further benefit much to their good thermoelectric properties.Thermoelectric properties of these Mo_(2)AB_(2)monolayers exhibit obvious anisotropies due to the direction-dependent chemical bondings and transport properties.Furthermore,their thermoelectric properties strongly depend on carrier type(n-type or p-type),carrier concentration and temperature.It is found that n-type Mo_(2)AB_(2)monolayers can be excellent thermoelectric materials with high electric conductivity,σ,and figures of merit,ZT.Choosing the types of A and B anions of Mo_(2)AB_(2)is an effective strategy to optimize their thermoelectric performance.These results provide rigorous understanding on thermoelectric properties of double-anions compounds and important guidance for achieving high thermoelectric performance in multi-anion compounds.

Mode-dependent dynamic gain of all-fiber FM-EDFA under various pump manipulation

The dynamic gain of a few-mode erbium-doped fiber amplifier(FM-EDFA)is vital for the long-haul mode division multiplexing(MDM)transmission.Here,we investigate the mode-dependent dynamic gain of an FM-EDFA under various manipulations of the pump mode.First,we numerically calculate the gain variation with respect to the input signal power,where a modedependent saturation input power occurs under different pump modes.Even under the fixed intensity profile of the pump laser,the saturation input power of each spatial mode is different.Moreover,high-order mode pumping leads to a compression of the linear amplification region,even though it is beneficial for the mitigation of the differential modal gain(DMG)arising in all guided modes.Then,we develop an all-fiber 3-mode EDFA,where the fundamental mode of the pump laser can be efficiently converted to the LP_(11)mode using the all-fiber mode-selective coupler(MSC).In comparison with the traditional LP_(01)pumping scheme,the DMG at 1550 nm can be mitigated from 1.61 dB to 0.97 dB under the LP_(11)mode pumping,while both an average gain of 19.93 dB and a DMG of less than 1 dB can be achieved from 1530 nm to 1560 nm.However,the corresponding signal input saturation powers are reduced by 0.3 dB for the LP_(01)mode and 1.6 dB for the LP_(11)mode,respectively.Both theoretical and experimental results indicate that a trade-off occurs between the DMG mitigation and the extension of the linear amplification range when the intensity profile of pump laser is manipulated.

Broadening and enhancing emission of Cr^(3+) simultaneously by co-doping Yb^(3+) in Ga1.4ln0.6SnO5

Cr^(3+)doped broadband near-infrared(NIR) emitting phosphors are currently the focus of research.Researchers have developed a variety of strategies to achieve broad and strong NIR emission,However,it is a conundrum to simultaneously broaden and enhance the emission of Cr^(3+)with a single strategy,In this work,we solved this problem by co-doping Yb^(3+).Under 452 nm excitation,Ga_(1.4)In_(0.6)SnO_(5)(GISO):0.01Cr^(3+)shows ultrabroadband NIR emission covering 650-1300 nm with a peak of 884 nm,The full width half maximum(FWHM) of the emission is 215 nm and the internal quantum yield(IQY) is 25%.This indicates that the double sites occupation strategy is favorable to achieve ultra-broadband NIR emission.The co-doping of Yb^(3+)can effectively broaden and enhance the emission of Cr^(3+).The FWHM for GISO:0.01Cr^(3+),0.002Yb^(3+)extends to 245 nm,and the IQY increases to 28%.Further increasing the concentration of Yb^(3+)to 0.005,the IQY is lifted to 32%.Finally,a phosphor-co nverted light emitting diode(pc-LED) was prepared by integrating the GISO:0.01Cr^(3+),0.002Yb^(3+)with a blue light chip.Under the current drive of 40 mA,the maximum output power of pc-LED is 4.54 mW,and the photoelectric conversion efficiency is 4.12%.These results indicate that Yb^(3+)ions can simultaneously broaden the emission band and improve the emission efficiency.This work provides an effective strategy for the design of efficient broadband NIR phosphors in the future.

Boron at tera-Pascal pressures

The study of boron structure is fascinating because boron has various allotropes containing boron icosahedrons under pressure. Here, we propose a new boron structure(space group Fm3m) that is dynamically stable at 1.4 tera-Pascal(TPa)using density functional theory and an evolutionary algorithm. The unit cell of this structure can be viewed as a structure with a boron atom embedded in the icosahedron. This structure behaves as a metal, and cannot be stable under ambient pressure. Furthermore, we found electrons gather in lattice interstices, which is similar to that of the semiconductor Na or Ca_(2)N-Ⅱ under high pressure. The discovery of this new structure expands our comprehension of high-pressure condensed matter and contributes to the further development of high-pressure science.

Light field information transmission through scattering media with high fidelity

Realizing high-fidelity optical information transmission through a scattering medium is of vital importance in both science and applications,such as short-range fiber communication and optical encryption.Theoretically,an input wavefront can be reconstructed by inverting the transmission matrix of the scattering medium.However,this deterministic method for retrieving light field information encoded in the wavefront has not yet been experimentally demonstrated.Herein,we demonstrate light field information transmission through different scattering media with near-unity fidelity.Multi-dimensional optical information can be delivered through either a multimode fiber or a ground glass without relying on any averaging or approximation,where their Pearson correlation coefficients can be up to 99%.

Scalable parallel ultrafast optical random bit generation based on a single chaotic microcomb

Random bit generators are critical for information security,cryptography,stochastic modeling,and simulations.Speed and scalability are key challenges faced by current physical random bit generation.Herein,we propose a massively parallel scheme for ultrafast random bit generation towards rates of order 100 terabit per second based on a single micro-ring resonator.A modulation-instability-driven chaotic comb in a micro-ring resonator enables the simultaneous generation of hundreds of independent and unbiased random bit streams.A proof-of-concept experiment demonstrates that using our method,random bit streams beyond 2 terabit per second can be successfully generated with only 7 comb lines.This bit rate can be easily enhanced by further increasing the number of comb lines used.Our approach provides a chip-scale solution to random bit generation for secure communication and high-performance computation,and offers superhigh speed and large scalability.

Differential mode-gain equalization via femtosecond laser micromachining-induced refractive index tailoring

The mode-division multiplexing technique combined with a few-mode erbium-doped fiber amplifier(FM-EDFA)demonstrates significant potential for solving the capacity limitation of standard single-mode fiber(SSMF)transmission systems.However,the differential mode gain(DMG)arising in the FM-EDFA fundamentally limits its transmission capacity and length.Herein,an innovative DMG equalization strategy using femtosecond laser micromachining to adjust the refractive index(RI)is presented.Variable mode-dependent attenuations can be achieved according to the DMG profile of the FM-EDFA,enabling DMG equalization.To validate the proposed strategy,DMG equalization of the commonly used FM-EDFA configuration was investigated.Simulation results revealed that by optimizing both the length and RI modulation depth of the femtosecond laser-tailoring area,the maximum DMG(DMGmax)among the 3 linear-polarized(LP)mode-group was mitigated from 10 dB to 1.52 dB,whereas the average DMG(DMGave)over the C-band was reduced from 8.95 dB to 0.78 dB.Finally,a 2-LP mode-group DMG equalizer was experimentally demonstrated,resulting in a reduction of the DMGmax from 2.09 dB to 0.46 dB,and a reduction of DMGave over the C band from 1.64 dB to 0.26 dB,with only a 1.8 dB insertion loss.Moreover,a maximum range of variable DMG equalization was achieved with 5.4 dB,satisfying the requirements of the most commonly used 2-LP mode-group amplification scenarios.

Resonant enhancement of second harmonic generation in etchless thin film lithium niobate heteronanostructure

Lithium niobate has received interest in nonlinear frequency conversion due to its wide transparency window,from ultraviolet to mid-infrared spectral regions,and large second-order nonlinear susceptibility.However,its nanostructure is generally difficult to etch,resulting in low-Q resonance and lossy nanostructures for second harmonic generation.By applying the concept of bound states in the continuum,we performed theoretical and experimental investigations on high-Q resonant etchless thin-film lithium niobate with Si O_(2) nanostructures on top for highly efficient second harmonic generation.In the fabricated nanostructured devices,a resonance with a Q factor of 980 leads to the strong enhancement of second harmonic generation by over 1500 times compared with that in unpatterned lithium niobate thin film.Although the pump slightly deviates from central resonance,an absolute conversion efficiency of 6.87×10^(-7) can be achieved with the fundamental pump peak intensity of 44.65 MW/cm^(2),thus contributing to the normalized conversion efficiency of 1.54×10^(-5)cm^(2)/GW.Our work establishes an etchless lithium niobate device for various applications,such as integrated nonlinear nanophotonics,terahertz frequency generation,and quantum information processing.

An Integrated-Plasmonic Chip of Bragg Reflection and Mach-Zehnder Interference Based on Metal-Insulator-Metal Waveguide

In this paper,a Bragg reflector is proposed by placing periodic metallic gratings in the center of a metal-insulator-metal(MIM)waveguide.According to the effective refractive index modulation caused by different waveguide widths in a period,a reflection channel with a large bandwidth is firstly achieved.Besides,the Mach-Zehnder interference(MZI)effect arises by shifting the gratings away from the waveguide center.Owing to different optical paths with unequal indices on both sides of the grating,a narrow MZI band gap will be obtained.It is interesting to find out that the Bragg reflector and Mach-Zehnder interferometer are immune to each other,and their wavelengths can be manipulated by the period and the grating length,respectively.Additionally,we can obtain three MZI channels and one Bragg reflection channel by integrating three different gratings into a large period.The performances are investigated by finite-difference time-domain(FDTD)simulations.In the index range of 1.33–1.36,the maximum sensitivity for the structure is as high as 1500 nm/RIU,and it is believed that this proposed structure can find widely applications in the chip-scale optical communication and sensing areas.

混合维度WS_(2)/WSe_(2)/Si单极势垒异质结构用于高性能光电探测

单极势垒异质结构可以选择性地降低暗电流,但不影响光电流,是一种构建高性能光电探测器的有效策略.特别地,具有可调谐能带结构和自钝化表面的二维(2D)材料不仅能满足能带匹配要求,而且避免了界面缺陷和晶格失配,有助于设计单极势垒异质结构.这里,我们展示了一种混合维度WS_(2)/WSe_(2)/p-Si单极势垒异质结光电探测器.其中,2D WS_(2)充当光子吸收体,原子级厚度的WSe_(2)充当单极势垒,3D p-Si充当光生载流子收集器.插入的WSe_(2)不仅减轻了有害的衬底效应,而且形成了高导带势垒,可以过滤掉若干暗电流分量,同时不影响光电流.在隧穿效应和载流子倍增效应的驱动下,该WS_(2)/WSe_(2)/p-Si器件表现出高于10~5的高开/关比、2.39×10^(12)Jones的高探测度和8.47/7.98毫秒的快速上升/衰减时间.这些优点显著优于传统的WS_(2)/p-Si器件,为设计高性能的光电器件开辟了一个新方案.

Ultra-broadband near infrared phosphor with wide spectral range and long peak wavelength achieved by double-site occupation

The miniaturization and intelligence of near infrared(NIR)devices urgently require an excellent broadband NIR phosphor.However,emission spectra of most NIR phosphors cover less than 1200 nm with a full width at half maximum(FWHM)less than 200 nm and a peak wavelength less than 900 nm.Here,we successfully developed ultra-broadband NIR phosphors ScNbO_(4):Cr^(3+)which can overcome the above shortcomings based on a double-site occupancy strategy.The phosphors achieve ultra-broadband NIR emission from 800 to 1400 nm with a peak wavelength at 980 nm.Its FWHM is up to 217 nm.The valence state of Cr ions in phosphors was analyzed.The double-site occupancy of Cr^(3+)ions in the host and the relationship between host structure and optical properties are discussed in detail.NIR light transmission experiments show that phosphors have potential application value in non-destructive analysis.This work develops an excellent NIR luminescent material and provides an efficient method to obtain ultra-broadband NIR phosphors with longer peak wavelength.

High sensitivity all-fiber bend sensor based on modal interferences in a ring core fiber

We proposed and experimentally demonstrated an all-fiber sensor for measuring bend with high sensitivity based on a ring core fiber(RCF)modal interferometer.The sensor was fabricated by splicing a segment of RCF between two pieces of multimode fiber(MMF)and single-mode fiber(SMF)at the ends of the MMF as lead-in and lead-out.Due to the first segment of the MMF,the transmitted light is coupled into the ring core,silica center,and cladding of the RCF,exciting multiple modes in the RCF.By the modal interferences in the structure,bending sensing can be realized by interrogating the intensity of the interference dip.Experimental results show a high bending sensitivity of-25.63 dB/m^(-1)in the range of 1.0954 m^(-1)to1.4696 m^(-1).In addition,the advantages of the bend sensor,such as small size,low temperature sensitivity,and simple fabrication process,can be used for curvature measurement in building health monitoring.

Sensitivity-Enhanced Hot-Wire Anemometer by Using Cladding-Etched Fiber Bragg Grating

A sensitivity-enhanced hot-wire anemometer based on a cladding-etched optical fiber Bragg grating(FBG)coated with a layer of silver film and optically heated by using a 1480nm laser diode is demonstrated.The silver film absorbs the laser power to heat the FBG to a certain high temperature and the airflow cools down the FBG hot-wire with the cooling effect and hence the Bragg wavelength of the FBG is determined by the airflow velocity.Experimental measurement results show that the heating efficiency of the FBG hot wire is improved by 3.8times in magnitude by etching the fiber cladding from 125μm down to 73.4μm,and the achieved airflow velocity sensitivities,under a laser power of 200mW,are–3180pm/(m/s),–889pm/(m/s),–268pm/(m/s),and–8.7pm/(m/s)at different airflow velocities of 0.1m/s,0.5m/s,1.5m/s,and 17m/s,respectively.In comparison,the sensitivities are only–2193 pm/(m/s),–567 pm/(m/s),–161 pm/(m/s),and–4.9pm/(m/s)for the reference anemometer without cladding etching even at a much higher heating laser power of 530mW.These results prove that the method by using a cladding-etched FBG to improve sensitivity of FBG-based hot-wire anemometers works and the sensitivity is improved significantly.

White light interferometry with spectral-temporal demodulation for large-range thickness measurement

Film thickness measurement can be realized using white light interferometry,but it is challenging to guarantee high precision in a large range of thicknesses.Based on scanning white light interferometry,we propose a spectral-temporal demodulation scheme for large-range thickness measurement.The demodulation process remains unchanged for either coatings or substrate-free films,while some adjustments are made according to the estimated optical thickness.Experiments show that the single-point repeatabilities for 500 nm SiO_(2) coating and 68μm substrate-free Si film are no more than 0.70 nm and 1.22 nm,respectively.This method can be further developed for simultaneous measurement of surface profile and film thickness.

Negative refraction mediated by bound states in the continuum

Negative refraction might occur at the interface between a two-dimensional photonic crystal(Ph C)slab and a homogeneous medium,where the guiding of the electromagnetic wave along the third dimension is governed by total internal reflection.Herein,we report on the observation of negative refraction in the Ph C slab where the vertical guiding is enabled by a bound state in the continuum and essentially beyond the light cone.Such abnormal refraction and guiding mechanism are based on the synchronous crafting of spatial dispersion and the radiative lifetime of Bloch modes within the radiative continuum.Microwave experiments are provided to further validate the numerical proposal in an all-dielectric Ph C platform.It is envisioned that the negative refraction observed beyond the light cone might facilitate the development of optical devices in integrated optics,such as couplers,multiplexers,and demultiplexers.

Hybrid 1D/2D heterostructure with electronic structure engineering toward high-sensitivity and polarization-dependent photodetector

The widespread application of photodetectors has triggered an urgent need for high-sensitivity and polarization-dependent photodetection.In this field,the two-dimensional(2D)tungsten disulfide(WS_(2))exhibits intriguing optical and electronic properties,making it an attractive photosensitive material for optoelectronic applications.However,the lack of an effective built-in electric field and photoconductive gain mechanism in 2D WS_(2)impedes its application in high-performance photodetectors.Herein,we propose a hybrid heterostructure photodetector that contains 1D Te and 2D WS_(2).In this device,1D Te induces in-plane strain in 2D WS_(2),which regulates the electronic structures of local WS_(2)and gives rise to type-Ⅱ band alignment in the horizontal direction.Moreover,the vertical heterojunction built of 2D WS_(2)and 1D Te introduces a high photoconductive gain.Benefiting from these two effects,the transfer of photogenerated carriers is optimized,and the proposed photodetector exhibits high sensitivity(photoresponsivity of ~27.7 A W^(-1),detectivity of 9.5×10^(12)Jones,and short rise/decay time of 19.3/17.6 ms).In addition,anisotropic photodetection characteristics with a dichroic ratio up to 2.1 are achieved.This hybrid 1D/2D heterostructure overcomes the inherent limitations of each material and realizes novel properties,opening up a new avenue towards constructing multifunctional optoelectronic devices.

Nonlinear Fourier transform enabled eigenvalue spectrum investigation for fiber laser radiation

Fiber lasers are a paradigm of dissipative systems,which distinguish themselves from a Hamilton system where energy is conservative.Consequently,pulses generated in a fiber laser are always accompanied by the continuous wave(CW).Under certain hypothesis,pulses generated in a fiber laser can be considered as a soliton,a product of a Hamilton system.Therefore,all the descriptions of solitons of a fiber laser are approximate.Coexistence of solitons and the CW from a fiber laser prevents unveiling of real nonlinear dynamics in fiber lasers,such as soliton interactions.Pulse behavior in a fiber laser can be represented by the state of single pulse,the state of period doubling of single pulse,the states of two pulses either tightly bound or loosely distributed,the states of three pulses,and various combinations of the above-mentioned states.Recently,soliton distillation was proposed and numerically demonstrated based on the nonlinear Fourier transform(NFT)[J.Lightwave Technol.39,2542(2021)].Solitons can be separated from the coherent CW background.Therefore,it is feasible to isolate solitons from CW background in a fiber laser.Here,we applied the NFT to various pulses generated in a fiber laser,including single pulse,single pulse in period doubling,different double pulses,and multiple pulses.Furthermore,with the approach of soliton distillation,the corresponding pure solitons of those pulses are reconstructed.Simulation results suggest that the NFT can be used to identify soliton dynamics excluding CW influence in a fiber laser,which paves a new way for uncovering real soliton interaction in nonlinear systems.

Low-latency full-field temporal magnification based on spectral compression

Temporal magnification is an emerging technology for the observation of single-shot optical signals with irregular and ultrafast dynamics,which exceed the speed,precision,and record length of conventional digitizers.Conventional temporal magnification schemes suffer from transmission delay and large volume of dispersive elements.Because only the signal envelope can be magnified in the dispersion-based schemes,real-time full-field(phase and amplitude)measurement for a complex ultrafast optical signal remains an open challenge.Here,a bandwidth-compressed temporal magnification scheme for low-latency full-field measurements of ultrafast dynamics is proposed.Unlike the dispersion-based schemes,temporal magnification of a complex optical signal is achieved by bandwidth compression.The bandwidth is coherently compressed by the Vernier effect relying on the detuned free spectral range of a periodic optical filter and time lens.Experimentally,a temporal magnification factor of 224 is realized,and full-field measurements for picosecond pulses are demonstrated.The proposal eliminates the dependence on dispersive elements and shows great potential in integration,which may pave a new path toward full-field measurement for nonrepetitive and statistically rare signals.