Grating Spectrum Research Articles

New Constraints on the Geometry and Kinematics of Matter Surrounding the Accretion Flow in X-Ray Binaries from Chandra High-energy Transmission Grating X-Ray Spectroscopy

Abstract The narrow, neutral Fe Kα fluorescence emission line in X-ray binaries (XRBs) is a powerful probe of the geometry, kinematics, and Fe abundance of matter around the accretion flow. In a recent study it has been claimed, using Chandra High-Energy Transmission Grating (HETG) spectra for a sample of XRBs, that the circumnuclear material is consistent with a solar-abundance, uniform, spherical distribution. It was also claimed that the Fe Kα line was unresolved in all cases by the HETG. However, these conclusions were based on ad hoc models that did not attempt to relate the global column density to the Fe Kα line emission. We revisit the sample and test a self-consistent model of a uniform, spherical X-ray reprocessor against HETG spectra from 56 observations of 14 Galactic XRBs. We find that the model is ruled out in 13/14 sources because a variable Fe abundance is required. In two sources a spherical distribution is viable, but with nonsolar Fe abundance. We also applied a solar-abundance Compton-thick reflection model, which can account for the spectra that are inconsistent with a spherical model, but spectra with a broader bandpass are required to better constrain model parameters. We also robustly measured the velocity width of the Fe Kα line and found FWHM values of up to ∼5000 km s−1. Only in some spectra was the Fe Kα line unresolved by the HETG.

Experimental demonstration of high sensitivity for silver rectangular grating-coupled surface plasmon resonance (SPR) sensing

We experimentally demonstrated a high sensitivity of surface plasmon resonance (SPR) sensor with silver rectangular grating coupling. The reflection spectra of the silver gratings indicated that surface plasmon resonance can be excited by either positive or negative order diffraction of the grating, depending on the period of the gratings. Comparing to prism-coupled SPR sensor, the sensitivities are higher for negative order diffraction coupling in bigger coupling angle, but much smaller for positive order diffraction coupling of the gratings. High sensitivity of 254.13 degree/RIU is experimentally realized by grating-based SPR sensor in the negative diffraction excitation mode. Our work paves the way for compact and sensitive SPR sensor in the applications of biochemical and gas sensing.

A High-Precision Wavelength Demodulation Method Based on Optical Fiber Fabry-Perot Tunable Filter

A high-precision wavelength demodulation method based on optical fiber Fabry–Perot tunable (FPT) filter is proposed for the fiber Bragg grating temperature sensing system. This algorithm uses the original sampling data obtained from the data acquisition card to simultaneously calculate the transmission spectrum of the F-P etalon (FPE) and the reflection spectrum of the fiber Bragg grating (FPG) sensors. In each scanning cycle, the transmission spectrum wavelength of the F-P etalon (FPE) is calibrated dynamically in real time, the reflectance spectrum central wavelength of the FPE sensors is calculated by referring to the transmission spectrum wavelength of the FPE that has been calibrated. It effectively weakens the perturbation effect caused by the nonlinear and non-reproducibility of narrowband light source from the fiber FPT filter, reduces the numerical error and effectively eliminates the pseudo peak of the FPE, and greatly improves the measurement accuracy of the system. Most programming languages can implement this algorithm. A temperature sensing experiment for the proposed method has been carried out. Results showed that the demodulation precision by our method could reach up to ±0.2 °C, wavelength demodulation accuracy 3 pm.

波导布拉格光栅时延响应谱的通解

波导布拉格光栅时延响应谱的通解

Substrate effects on the optical properties of metal gratings

The transmission and near-field enhancement spectra of thin metal gratings on a semiconductor designed for the telecom band, ∼1500 nm, were investigated theoretically. It was found that around the wavelength λR, where a diffraction order changes its character between radiative and evanescent, the spectral peaks are pinned at λR. This state is in marked contrast to the gratings in air where the peak wavelength is always longer than λR. We show that the difference is related to the absence/presence of the bound mode (BM) of the system, which corresponds to the Fabry–Perot (FP) resonance of the slit mode. In the thin gratings on a semiconductor, the BM disappears due to the large dielectric asymmetry above and below the grating, which results in different reflection phases for the slit mode at the grating–dielectric interfaces. It is also shown that the plasmonic band gap is large in gratings on semiconductors, enhancing the disappearance of the BM.

Improvement of temperature-induced spectrum characterization in a holographic sensor based on N-isopropylacrylamide photopolymer hydrogel.

A novel thermo-sensitive N-isopropylacrylamide photopolymer was developed for improving the temperature and humidity responses of holographic sensors. Diffraction spectra of holographic volume gratings recorded in the materials were characterized to explore the sensing response capacity. A dependence of peak wavelength on the temperature was observed and provided a quantitative strategy for holographic sensing applications. Expansion of the humidity range induced a strong extension of wavelength shift. Finally, the temperature response reversibility was demonstrated experimentally. Our sensing results were completely different from the reported typical acrylamide polymer system. Compared with the former, we obtained a more sensitive temperature response and an evident shift expansion (>200 nm) at a relative humidity of 70% or higher. These results can obviously improve the thermo-sensitivity of a holographic sensor and expand the practical application area of the holographic sensing strategy.

Concave-lens-like long-period fiber grating bidirectional high-sensitivity bending sensor.

A novel bidirectional high-sensitivity fiber-optic bending sensor based on the concave-lens-like long-period fiber grating (CLL-LPFG) is designed and demonstrated. The CLL-LPFG is composed by an array of arc-shaped grating planes, and accordingly, its refractive index modulation serves as a concave lens. As a result, the eigencladding mode of the device gets closer to the device surface than the conventional counterpart. Therefore, the proposed sensor provides a more sensitive result. The experimental results show that the bending sensitivities of the CLL-LPFG reach -32.782 nm/m-1 within the bending range of 0-2.08 m-1, which is about sixfold compared to the reported arts. The sensitivity can be potentially improved by optimizing the grating parameters, and the temperature characteristics of the CLL-LPFG can be used to manipulate the grating spectrum.

A high efficient method for demodulation of FBG sensors

Abstract In this paper, a strain sensing system based on Distributed-Feedback (DFB) laser array and a high effective demodulate algorithm is proposed. The uniqueness and innovation of the algorithm is in the following two aspects: one is that accurate demodulation can be achieved by scanning only 0.4 nm bandwidth of fiber Bragg grating (FBG) spectrum instead of relying on scanning FBG’s principal power peak; and the other is that the scanned spectrum can be any part of reflected spectrum of FBG sensor, which make it possible of using wavelength-division multiplexing (WDM) for FBG sensing network. In the experiment, DFB laser array with single-channel and four-channel are used to demonstrate the proposed algorithm. Experiments results show that the single-channel’s error is below 5 pm, four-channel’s is below 2 pm, strain and algorithm results show good linearity.

Observation of stable bound soliton with dual-wavelength in a passively mode-locked Er-doped fiber laser**Project supported by the National Natural Science Foundation of China (Grant No. 61575011) and the Key Project of the National Natural Science Foundation of China (Grant No. 61235010).

A phase-locked bound state soliton with dual-wavelength is observed experimentally in a passively mode-locked Er-doped fiber (EDF) laser with a fiber loop mirror (FLM). The pulse duration of the soliton is 15 ps and the peak-to-peak separation is 125 ps. The repetition rate of the pulse sequence is 3.47 MHz. The output power is 11.8 mW at the pump power of 128 mW, corresponding to the pulse energy of 1.52 nJ. The FLM with a polarization controller can produce a comb spectrum, which acts as a filter. By adjusting the polarization controller or varying the pump power, the central wavelength of the comb spectrum can be tuned. When it combines with the reflective spectrum of the fiber Bragg grating, the total spectrum of the cavity can be cleaved into two parts, then the bound state soliton with dual-wavelength at 1549.7 nm and 1550.4 nm is obtained.

Characteristics of strain transfer and the reflected spectrum of a metal-coated fiber Bragg grating sensor

Previous researchers have simulated strain transfer and spectrum of normal fiber Bragg grating (FBG) sensors with a polymer coating bonded on the structure. They only considered the shear stress in a polymer coating for the simulation. However, for metal-coated FBG sensors, not only shear stress but also axial stress in the metal coating should be reflected into the calculation because its axial stiffness is no longer negligible. Thus, the author investigated the strain transfer and reflected spectra of metal-coated FBG sensors by considering both shear stress and axial stress. The strain transfer analysis involved evaluating the strain profiles along the sensor by plotting an analytical solution, and validating the evaluated profiles with the results obtained by a finite element analysis (FEA). The solution was also verified by the experiments that used aluminum-coated FBG sensors bonded on a carbon fiber reinforced polymer (CFRP) composite specimen. A transfer-matrix (T-matrix) formulation and coupled mode theory were used to simulate the reflected spectra of metal-coated FBG sensors for the evaluated strain profile. In addition, the effect of mechanical and geometric parameters of the sensor was examined. The findings revealed that the strain transfer characteristics and reflected spectra deteriorated with increases in the thickness and Young's modulus of the metal coating due to the consideration of axial stress. It is the opposite results for the normal FBG sensor with a polymer coating. Furthermore, the results also indicated that the decrease in bonding thickness resulted in improved strain transfer and signal characteristics. Moreover, a bonding length of 14mm was suitable in suppressing an asymmetric shape of the reflected spectrum and in achieving an accurate measurement. The results of the parametric study are expected to contribute to improve the measurement accuracy of metal-coated FBG sensors in actual applications. The analytical methodology can be usefully employed in the design of a metal-coated FBG sensor system.

The supersoft X-ray source in V5116 Sagittarii

Classical novae occur on the surface of an accreting white dwarf in a binary system. After ejection of a fraction of the envelope and when the expanding shell becomes optically thin to X-rays, a bright source of supersoft X-rays arises, powered by residual H burning on the surface of the white dwarf. While the general picture of the nova event is well established, the details and balance of accretion and ejection processes in classical novae are still full of unknowns. The long-term balance of accreted matter is of special interest for massive accreting white dwarfs, which may be promising supernova Ia progenitor candidates. V5116 Sgr was observed as a bright and variable supersoft X-ray source by XMM-Newton 610~days after outburst. The light curve showed a periodicity consistent with the orbital period. During one third of the orbit the luminosity was a factor of seven brighter than during the other two thirds of the orbital period. In the present work we aim to disentangle the X-ray spectral components of V5116 Sgr and their variability. We present the high resolution spectra obtained with XMM-Newton RGS and Chandra LETGS/HRC-S in March and August 2007. The grating spectrum during the periods of high-flux shows a typical hot white dwarf atmosphere dominated by absorption lines of N VI and N VII. During the low-flux periods, the spectrum is dominated by an atmosphere with the same temperature as during the high-flux period, but with several emission features superimposed. Some of the emission lines are well modeled with an optically thin plasma in collisional equilibrium, rich in C and N, which also explains some excess in the spectra of the high-flux period. No velocity shifts are observed in the absorption lines, with an upper limit set by the spectral resolution of 500 km/s, consistent with the expectation of a non-expanding atmosphere so late in the evolution of the post-nova.

Highly sensitive fiber random-grating-based random laser sensor for ultrasound detection.

A novel highly sensitive ultrasound sensor based on a random fiber laser is proposed and demonstrated for the first time, to the best of our knowledge. The random fiber laser is constructed with the erbium-doped fiber gain. A fiber random grating provides random-distributed feedback for the random laser and acts as an ultrasound sensing head. The random laser can operate in a single-mode lasing scenario, which offers linear and pure temporal responses to the broadband ultrasonic acoustic emission from 20 kHz to 0.8 MHz. The multiple-interfering reflection spectrum of the random grating provides a large number of steep peak areas over a broad spectral range, which significantly enhances the sensitivity of the random laser sensor and makes it an ideal sensor in harsh environments with large temperature or strain variations. The proposed laser sensor offers signal-to-noise ratio improvement up to 20 dB and higher sensitivity compared with conventional piezoelectric acoustic sensors.

Theoretical evaluation of Brillouin dynamic grating length localized by optical correlation domain technique through reflection spectrum simulation

We derive formulae based on Fourier transformation to calculate the reflection spectrum of a Brillouin dynamic grating (BDG), which is localized along an optical fiber by an optical correlation domain technique. First, we calculate the typical reflection spectra of the BDG localized by the technique and confirm the validity of the formulae by showing coincidence with previous theoretical or experimental works. Next, we evaluate theoretically the spatial resolution in the BDG distributed measurement by the technique, through simulations considering different strained fiber lengths. It is confirmed theoretically, for the first time, that the resolution in the BDG measurement is worse than that for the Brillouin gain spectrum.

The simultaneous measurement of temperature and mean strain based on the distorted spectra of half-encapsulated fiber Bragg gratings using improved particle swarm optimization

A half-encapsulated FBG is capable of simultaneously measuring temperature and strain. However, spectrum distortion, which may be induced by overlapping or non-uniform strains, may hinder the adoption of this technique. In order to resolve this issue, an improved particle swarm optimization (IPSO) based spectra reconstructing method has been adopted in this study to estimate the temperature and mean strain according to the distorted spectrum. Also, a dynamic adaptive inertia weight adjusting strategy based on the swarm success rate has been adopted to improve the algorithm. To validate the method, a total of 48 scenarios of distorted spectra have been simulated, and the temperature and mean strain estimated by IPSO have been compared with the genetic algorithm and linearly-declined PSO. The simulation results indicated that the IPSO based reconstructing method provided a higher accuracy. Additionally, the feasibility of the proposed method has been experimentally verified using a strain tunable apparatus within a measurable temperature environment. The experimental results demonstrated that the half-encapsulated FBG with an IPSO based spectra reconstructing method was applicable for the simultaneous measurement of temperature and mean strain, even when the spectrum was distorted.

Simultaneous measurement of strain and temperature using a polarization-maintaining photonic crystal fiber with stimulated Brillouin scattering

We demonstrate the simultaneous and distributed sensing of temperature and strain by combining the Brillouin dynamic grating (BDG) and Brillouin gain spectrum (BGS) using a polarization-maintaining photonic crystal fiber (PM-PCF). The characterization of the birefringence frequency shift of the PM-PCF shows that it remains almost unchanged within the temperature range of 5–80 °C, which allows a temperature-insensitive strain measurement using only the BDG. Combining the BDG and BGS, we demonstrate the simultaneous measurement of temperature and strain with a spatial resolution of 20 cm over a 5-m-long PM-PCF.

Crosstalk and Ghost Gratings in a Large-Scale Weak Fiber Bragg Grating Array

Large-scale distributed fiber Bragg grating (FBG) sensor networks have a wide range of applications. However, serious crosstalk could occur in a near-identical serial FBG sensor network because of multiple reflections. Since crosstalk cannot be observed directly and separately from signal, experimental investigation of crosstalk is challenging. In this work, we point out the similarity and symmetry between crosstalk and ghost gratings, and by way of measuring the spectrum of ghost gratings, we experimentally study the characteristics of crosstalk in a serial array of 3010 near-identical FBGs using a commercial FBG sensor interrogation system. We show that ghost gratings drop much more rapidly in power as one goes downstream, and they are more homogeneous in spectral distribution than real gratings. The peak wavelengths of real gratings vary within ±100 pm while the peak wavelengths of the ghost gratings they generate vary within ±20 pm only. Spectral ripples are observed for ghost gratings and real gratings severely affected by crosstalk. These ripples reduce the FBG wavelength reading accuracy to ±5 pm. The work constitutes the first experimental study on crosstalk in large-scale FBG arrays.

Research on the Phase-Shifted Fiber Bragg Grating Spectra under Dynamic Strain Fields

Fiber optic sensing technology has been widely used for acoustic emission (AE) measurement in aerospace and geotechnical engineering due to the advantages of immunity to electro-magnetic interference, high accuracy, and multiplexing capacity. As for the fiber Bragg grating based on AE measurement system, traditional efforts have been made to study the spectra of Fiber Bragg Grating under AE signal. While this paper focused on phase-shifted fiber Bragg grating (PS-FBG) which is a new type fiber Bragg grating, and investigated the spectra of under dynamic strain field generating with an AE signal. A dynamic strain sensing model of PS-FBG was built based on the transfer matrix theory, and a cosine exponential attenuation function was utilized to simulate the dynamic strain field. Then the effects of amplitude, sampling time, exponential attenuation coefficient, frequency and wavelength of AE signal on the spectra of PS-FBG were studied in detail using numerical simulation. The result demonstrate that the spectra of PS-FBG changes periodically with frequency and different sampling time, especially the peak wavelength of the transmission window in the PS-FBG spectrum shifts periodically; and the increase of amplitude mainly contribute to more harmonic peaks in the reflected spectrum. The attenuation coefficient affects the spectrum of PS-FBG within certain range. And the AE wavelength affect the spectrum of PS-FBG when it is between 0.1 to 2 L, beyond the range, the spectrum of PS-FBG has minor variation with the AE wavelength. Finally, dynamic strain experiments were conducted, and the spectrum of PS-FBG under continuously vibration signal with different amplitude and frequency were logged for analysis. The experimental results agree well with simulation, which indicate that under different dynamic strain fields generated by AE signal, the spectra of PS-FBG are different, while the variation of spectra follows certain laws. This paper provides theoretical support for the AE measurement system on the basis of PS-FBG.

A DEEP X-RAY VIEW OF THE BARE AGN ARK 120. II. EVIDENCE FOR Fe K EMISSION TRANSIENTS

ABSTRACT We report on the results from a large observational campaign on the bare Seyfert galaxy Ark 120, jointly carried out in 2014 with XMM-Newton, Chandra, and NuSTAR. The fortunate line of sight to this source, devoid of any significant absorbing material, provides an incomparably clean view to the nuclear regions of an active galaxy. Here we focus on the analysis of the iron fluorescence features, which form a composite emission pattern in the 6–7 keV band. The prominent Kα line from neutral iron at 6.4 keV is resolved in the Chandra High-Energy Transmission Grating spectrum to a full-width at half maximum of km s−1, consistent with an origin from the optical broad-line region. Excess components are detected on both sides of the narrow Kα line: the red one (6.0–6.3 keV) clearly varies in strength in about one year, and hints at the presence of a broad, mildly asymmetric line from the accretion disk; the blue one (6.5–7.0 keV), instead, is likely a blend of different contributions, and appears to be constant when integrated over long enough exposures. However, the Fe K excess emission map computed over the 7.5 days of the XMM-Newton monitoring shows that both the red and blue features are actually highly variable on timescales of ∼10–15 hr, suggesting that they might arise from short-lived hotspots on the disk surface, located within a few tens of gravitational radii from the central supermassive black hole and possibly illuminated by magnetic reconnection events. Any alternative explanation would still require a highly dynamic, inhomogeneous disk/coronal system, involving clumpiness and/or instability.

Flat-top bandpass filters enabled by cascaded resonant gratings.

Narrow bandpass filters are applied in laser systems, imaging, telecommunications, and astronomy. Traditionally implemented with thin-film stacks, there is recent interest in alternative means incorporating photonic resonance effects. Here, we demonstrate a new approach to bandpass filters that engages the guided-mode resonance effect working with a cavity-based Fabry-Perot resonance to flatten and steepen the pass band. Both of these resonance mechanisms are native to simple resonant bandpass filters placed in a cascade. Numerical examples provide quantitative spectral properties including pass-band shape and sideband levels. Thus, we compare the spectra of single-layer 1D and 2D resonant gratings with the dual-cascade design incorporating identical gratings. Two- and three-cavity designs are measured against a classic multi-cavity thin-film filter with 151 layers. Whereas these initial results show comparable and improved results achieved with sparse structures, the challenge remains of developing a suitable fabrication technology to capitalize on this promise.

Theoretical analysis of spectrum flattening in fiber laser oscillator

The flatness of laser spectrum is important in many applications. In this manuscript, a method of acquiring flattened spectrum directly from a fiber oscillator by optimizing the reflective spectrum of Fiber Bragg Gratings (FBG) was demonstrated and optimized result at wavelength around 1064nm and 1080nm was presented. An optimization path to alter the reflectivity of FBGs using greedy algorithm was interpreted by analyzing the single-trip gain inside the resonant cavity. Our method has a guiding significance of controlling the output spectrum of laser oscillator using FBGs.