Theory and verification of moiréfringes for x-ray three-phase grating interferometer

Dual-phase and three-phase grating x-ray interference is a promising new technique for grating-based x-ray differential phase contrast imaging.Dual-phase grating interferometers have been relatively completely studied and discussed.In this paper,the corresponding imaging fringe formula of the three-phase grating interferometer is provided.At the same time,the similarities and differences between the three-phase grating interferometer and the dual-phase grating interferometer are investigated and verified,and that the three-phase grating interferometer can produce large-period moiréfringes without using the analyzing grating is demonstrated experimentally.Finally,a simple method of designing three-phase grating and multi-grating imaging systems from geometric optics based on the thin-lens theory of gratings is presented.These theoretical formulas and experimental results provide optimization tools for designing three-phase grating interferometer systems.

Enhancing the Goos-Hänchen shift based on quasi-bound states in the continuum through material asymmetric dielectric compound gratings

Quasi-bound state in the continuum(QBIC)resonance is gradually attracting attention and being applied in Goos-Hänchen(GH)shift enhancement due to its high quality(Q)factor and superior optical confinement.Currently,symmetry-protected QBIC resonance is often achieved by breaking the geometric symmetry,but few cases are achieved by breaking the material symmetry.This paper proposes a dielectric compound grating to achieve a high Q factor and high-reflection symmetry-protectede QBIC resonance based on material asymmetry.Theoretical calculations show that the symmetry-protected QBIC resonance achieved by material asymmetry can significantly increase the GH shift up to-980 times the resonance wavelength,and the maximum GH shift is located at the reflection peak with unity reflectance.This paper provides a theoretical basis for designing and fabricating high-performance GH shift tunable metasurfaces/dielectric gratings in the future.

975 nm multimode semiconductor lasers with high-order Bragg diffraction gratings

The 975 nm multimode diode lasers with high-order surface Bragg diffraction gratings have been simulated and calcu-lated using the 2D finite difference time domain(FDTD)algorithm and the scattering matrix method(SMM).The periods and etch depth of the grating parameters have been optimized.A board area laser diode(BA-LD)with high-order diffraction grat-ings has been designed and fabricated.At output powers up to 10.5 W,the measured spectral width of full width at half maxi-mum(FWHM)is less than 0.5 nm.The results demonstrate that the designed high-order surface gratings can effectively nar-row the spectral width of multimode semiconductor lasers at high output power.

Monitoring shear deformation of sliding zone via fiber Bragg grating and particle image velocimetry

Monitoring shear deformation of sliding zones is of great significance for understanding the landslide evolution mechanism,in which fiber optic strain sensing has shown great potential.However,the cor-relation between strain measurements of quasi-distributed fiber Bragg grating(FBG)sensing arrays and shear displacements of surrounding soil remains elusive.In this study,a direct shear model test was conducted to simulate the shear deformation of sliding zones,in which the soil internal deformation was captured using FBG strain sensors and the soil surface deformation was measured by particle image velocimetry(PIV).The test results show that there were two main slip surfaces and two secondary ones,developing a spindle-shaped shear band in the soil.The formation of the shear band was successfully captured by FBG sensors.A sinusoidal model was proposed to describe the fiber optic cable deformation behavior.On this basis,the shear displacements and shear band widths were calculated by using strain measurements.This work provides important insight into the deduction of soil shear deformation using soil-embedded FBG strain sensors.

基于TFBG边缘滤波的FBG温度传感器

为了提高系统的温度灵敏度,设计了一种基于倾斜光纤光栅(TFBG)边缘滤波的光纤布喇格光栅(FBG)温度传感器。该系统在常规的TFBG边缘滤波解调系统上引入一个光纤环形镜,将从TFBG透射的光信号反射回来,使其能再次通过TFBG,以增加TFBG透射谱的深度,并对传感光栅进行封装。实验结果表明:封装前和封装后的传感光栅在所测量的温度区间内中心波长的漂移量分别为0.55 nm和7 nm,封装后的漂移量扩大了十多倍;同时,由于TFBG边缘滤波解调传递函数的斜率得到提高,传感器波长解调后的温度灵敏度也得到了改善,是未级联光纤环形镜传感器的1.6~2.4倍。

In-fiber photoelectric device based on graphene-coated tilted fiber grating

Graphene and related two-dimensional materials have attracted great research interests due to prominently optical and electrical properties and flexibility in integration with versatile photonic structures.Here,we report an in-fiber photoelec-tric device by wrapping a few-layer graphene and bonding a pair of electrodes onto a tilted fiber Bragg grating(TFBG)for photoelectric and electric-induced thermo-optic conversions.The transmitted spectrum from this device consists of a dense comb of narrowband resonances that provides an observable window to sense the photocurrent and the electrical injection in the graphene layer.The device has a wavelength-sensitive photoresponse with responsivity up to 11.4 A/W,allowing the spectrum analysis by real-time monitoring of photocurrent evolution.Based on the thermal-optic effect of electrical injection,the graphene layer is energized to produce a global red-shift of the transmission spectrum of the TF-BG,with a high sensitivity approaching 2.167×10^(4)nm/A^(2).The in-fiber photoelectric device,therefore as a powerful tool,could be widely available as off-the-shelf product for photodetection,spectrometer and current sensor.

Anti-symmetric sampled grating quantum cascade laser for mode selection

For mode selection in a quantum cascade laser(QCL),we demonstrate an anti-symmetric sampled grating(ASG).The wavelength of the-1-th mode of this laser has been blue-shifted more than 75 nm(~10 cm^(-1))compared with that of an ordinary sampled grating laser with an emission wavelength of approximately 8.6μm,when the periodicities within both the base grating and the sample grating are kept constant.Under this condition,an improvement in the continuous tuning capability of the QCL array is ensured.The ASG structure is fabricated in holographic exposure and optical photolithography,thereby enhancing its flexibility,repeatability,and cost-effectiveness.The wavelength modulation capability of the two channels of the grating is insensitive to the variations in channel size,assuming that the overall waveguide width remains constant.The output wavelength can be tailored freely within a certain range by adjusting the width of the ridge and the material of the cladding layer.

Novel Apodized Fiber Bragg GratingApplied for Medical Sensors:Performance Investigation

Sensors play an important role in shaping and monitoring human health.Exploration of methods to use Fiber Bragg Grating(FBG)with enhanced sensitivity has attracted great interest in the field of medical research.In this paper,a novel apodization function is proposed and performance evaluation and optimization of the same have been made.A comparison was conducted between various existing apodization functions and the proposed one based on optical characteristics and sensor parameters.The results evince the implementation of the proposed apodization function for vital sign measurement.The optical characteristics considered for evaluation are Peak Resonance Reflectivity level,Side Lobes Reflectivity level and FullWidth HalfMaximum(FWHM).The proposed novel apodization novel function has better FWHM,which is narrower than the FWHM of uniform FBG.Sensor characteristics like a quality parameter,detection accuracy and sensitivity also show improvement.The proposed novel apodization function is demonstrated to have a better shift in wavelength in terms of temperature and pulse measurement than the existing functions.The sensitivity of the proposed apodized function is enhanced with a Poly-dimethylsiloxane coating of varying thickness,which is 6 times and 5.14 times greater than uniform Fiber Bragg grating and FBG with the proposed novel apodization function,respectively,enhancing its utilization in the field of medicine.

Simulation research on surface growth process of positive and negative frequency detuning chromium atom lithographic gratings

Chromium atom photolithography gratings are a promising technology for the development of nanoscale length standard substances due to their high accuracy,uniformity,and consistency.However,the inherent difference between the interaction of positive and negative frequency detuning standing wave field and the atoms can cause a difference in the adjacent peak-to-valley heights of the grating in positive and negative frequency detuning chromium atom lithography,which greatly reduces its accuracy.In this study,we performed a controlled variable growth simulation using the semi-classical theoretical model and Monte Carlo method with trajectory tracking and ballistic deposition methods to investigate the influence of key experimental parameters on the surface growth process of positive and negative frequency detuning atomic lithography gratings.We established a theoretical model based on simulation results and summarized empirical equations to guide the selection of experimental parameters.Our simulations achieved uniform positive and negative frequency detuning atomic lithography gratings with a period of 1/4 of the wavelength corresponding to the atomic transition frequency,and adjacent peak-to-valley heights differing by no more than 2 nm,providing an important theoretical reference for the controllable fabrication of these gratings.

Spatiotemporal hemodynamic monitoring via configurable skin-like microfiber Bragg grating group

Systemic blood circulation is one of life activity’s most important physiological functions.Continuous noninvasive hemodynamicmonitoring is essential for the management of cardiovascular status.However,it is difficult to achieve systemichemodynamic monitoring with the daily use of current devices due to the lack of multichannel and time-synchronized operationcapability over the whole body.Here,we utilize a soft microfiber Bragg grating group to monitor spatiotemporalhemodynamics by taking advantage of the high sensitivity,electromagnetic immunity,and great temporal synchronizationbetween multiple remote sensor nodes.A continuous systemic hemodynamic measurement technique is developedusing all-mechanical physiological signals,such as ballistocardiogram signals and pulse waves,to illustrate the actualmechanical process of blood circulation.Multiple hemodynamic parameters,such as systemic pulse transit time,heartrate,blood pressure,and peripheral resistance,are monitored using skin-like microfiber Bragg grating patches conformallyattached at different body locations.Relying on the soft microfiber Bragg grating group,the spatiotemporal hemodynamicmonitoring technique opens up new possibilities in clinical medical diagnosis and daily health management.

Critical dispersion of chirped fiber Bragg grating for eliminating time delay signature of distributed feedback laser chaos

Optical chaos has attracted widespread attention owing to its complex dynamic behaviors.However,the time delay signature(TDS)caused by the external cavity mode reduces the complexity of optical chaos.We propose and numerically demonstrate the critical dispersion of chirped fiber Bragg grating(CFBG)for eliminating the TDS of laser chaos in this work.The critical dispersion,as a function of relaxation frequency and bandwidth of the optical spectrum,is found through extensive dynamics simulations.It is shown that the TDS can be eliminated when the dispersion of CFBG is above this critical dispersion.In addition,the influence of dispersive feedback light and output light from a laser is investigated.These results provide important quantitative guidance for designing chaotic semiconductor lasers without TDS.

Encrypted optical fiber tag based on encoded fiber Bragg grating array

Optical fibers are typically used in telecommunications services for data transmission,where the use of fiber tags is essential to distinguish between the different transmission fibers or channels and thus ensure the working functionality of the communication system.Traditional physical entity marking methods for fiber labeling are bulky,easily confused,and,most importantly,the label information can be accessed easily by all potential users.This work proposes an encrypted optical fiber tag based on an encoded fiber Bragg grating(FBG)array that is fabricated using a point-by-point femtosecond laser pulse chain inscription method.Gratings with different resonant wavelengths and reflectivities are realized by adjusting the grating period and the refractive index modulations.It is demonstrated that a binary data sequence carried by a fiber tag can be inscribed into the fiber core in the form of an FBG array,and the tag data can be encrypted through appropriate design of the spatial distributions of the FBGs with various reflection wavelengths and reflectivities.The proposed fiber tag technology can be used for applications in port identification,encrypted data storage,and transmission in fiber networks.

Determination of the Postmortem Interval Using Fiber Bragg Grating Sensors

Fiber Bragg grating(FBG)sensors are often used in monitoring activities and to ensure that environmental parameters satisfy industrial requirements.They offer crucial safety measures in the early detection of hazards due to their greatly reduced size,low weight,flexibility,and immunity to electromagnetic interference.These characteristics make FBGs suitable also for use in relation to the human body for in vivo measurements and long-term monitoring.In this study,recent developments are presented with regard to the utilization of these sensors to measure the so-called post-mortem interval(PMI).Such developments rely on numerical simulations based on the Matlab software and monitoring of the rectal temperature,which is one of the main parameters for estimating the PMI.First,the Matlab software is used to solve the Henssge equation for different ambient temperatures and for different body masses;then a Bragg grating sensors is used for post-mortem dating.The results and their accuracy are discussed.

Design of a coated thinly clad chalcogenide long-period fiber grating refractive index sensor based on dual-peak resonance near the phase matching turning point

A novel method for designing chalcogenide long-period fiber grating(LPFG) sensors based on the dual-peak resonance effect of the LPFG near the phase matching turning point(PMTP) is presented. Refractive index sensing in a high-refractive-index chalcogenide fiber is achieved with a coated thinly clad film. The dual-peak resonant characteristics near the PMTP and the refractive index sensing properties of the LPFG are analyzed first by the phase-matching condition of the LPFG. The effects of film parameters and cladding radius on the sensitivity of refractive index sensing are further discussed. The sensor is optimized by selecting the appropriate film parameters and cladding radius. Simulation results show that the ambient refractive index sensitivity of a dual-peak coated thinly clad chalcogenide LPFG at the PMTP can be 2400 nm/RIU, which is significantly higher than that of non-optimized gratings. It has great application potential in the field of chemical sensing and biosensors.

Wavelength-tunable organic semiconductor lasers based on elastic distributed feedback gratings

Wavelength-tunable organic semiconductor lasers based on mechanically stretchable polydimethylsiloxane (PDMS) gratings were developed. The intrinsic stretchability of PDMS was explored to modulate the period of the distributed feedback gratings for fine tuning the lasing wavelength. Notably, elastic lasers based on three typical light-emitting molecules show com-parable lasing threshold values analogous to rigid devices and a continuous wavelength tunability of about 10 nm by mechanic-al stretching. In addition, the stretchability provides a simple solution for dynamically tuning the lasing wavelength in a spec-tral range that is challenging to achieve for inorganic counterparts. Our work has provided a simple and efficient method of fab-ricating tunable organic lasers that depend on stretchable distributed feedback gratings, demonstrating a significant step in the advancement of flexible organic optoelectronic devices.

基于矢量衍射理论的光栅干涉仪位移测量误差研究

光栅干涉仪位移测量系统作为测量领域中最精密的测量仪器之一,由于在测量过程中光栅的姿态位置无法保证完美装配,使得光栅的光栅矢量方向与运动矢量方向之间出现偏差,导致位移测量结果出现周期性非线性误差。文中针对在光栅干涉仪位移测量过程中出现的光栅与位移台以及读数头之间的姿态位置误差进行分析,通过分别建立位移坐标系及光栅坐标系,参考惯性导航领域中飞行器姿态表示方法,利用一维光栅的横滚、俯仰和偏航角共同描述一维光栅相对于位移台的装配状态。通过基于矢量衍射理论分析光栅干涉仪位移测量时三个维度的角度偏差量,对可能产生的位移测量误差进行了分析说明。基于广义一维光栅方程,探究出了一个更为普遍的结论,为后续装置的改进提供理论依据,以提高系统的测量精度。

光纤光栅温度应力传感教学实验装置

针对目前市场上可直接用于光纤光栅传感实验教学的装置较少,前沿科研技术和本科实验教学相脱节现状,设计了基于光纤光栅的温度应力传感教学装置。该装置包括光纤激光器、光谱仪和上位机控制软件3部分。光纤激光器实现了1 550 nm左右的激光输出,利用光谱仪测量光纤光栅中心波长的改变,并采集数据输入计算机,上位机控制软件用于图形显示和数据存储等。该实验装置操作简单、组装灵活、重复性和稳定性好,可用于本科实验教学。将前沿科学技术引入本科实验教学,促进了科学研究与本科实验教学相融合,实现了科研教学水平的同步提高。

高精度光栅三维位移敏感结构设计

针对目前的微位移敏感装置在微型化下难以实现高精度三维检测的问题,设计了面内和离面2种位移敏感的光栅结构,并应用于三维微位移的检测中。阐述了光栅理论,设计了双层光栅结构。从2种检测方式对微位移灵敏度的影响角度,通过Gsolver仿真光强衍射效率的大小变化,面内和离面敏感光栅的衍射效率变化分别为0.115%/nm和0.423%/nm,可知,离面敏感光栅的衍射效率灵敏度是面内敏感光栅的4倍。进而将离面位移敏感的光栅布置在被测物的三维方向,实现高精度三维位移测量。

彩色光栅投影的三维轮廓自动化测量研究

为了测量物体的三维轮廓数据,提出一种基于改进彩色光栅投影三维轮廓测量技术,并利用DLP来投影彩色光栅条纹到被测物体上来测量被测物体的高度,由于DLP的投射范围太大,无法直接投射在微小物体上,因此通过自行改装的方式改变DLP的投射范围,将其投影面积限制在约20×20 mm内,使其能覆盖整个被测物体。但由于改装时所选用的DLP镜头并不是针对DLP而设计的,因此容易产生光场分布不均的情况,可对无光栅图像或有彩色光栅图像进行光场补偿,使光栅图像更为清楚,用于对被测物体进行更准确的三维轮廓测量与重建分析计算,且获取被测物体的三维数据精度可达2μm,速度在0.024 s内。实验结果表明,该方法具有更高的速度、灵敏度及适用性。

基于构造绝对值三角函数的光栅信号细分

为了进一步提高光栅传感器的测量精度,需要对信号进行细分,但光栅莫尔信号的细分精度取决于信号的质量。本文利用归一化并结合CORDIC的方法补偿了直流、幅值、相位等误差,利用补偿后的信号构造绝对值三角函数,实现一个完整周期信号的16倍细分,并通过MATLAB进行了仿真,验证了该方法的可行性。