Vol. 66, No. 7 (2017)
2017-04-05
SPECIAL TOPIC—Optical Fiber Sensor
2017, 66 (7): 070601.
doi: 10.7498/aps.66.070601
Abstract +
In many fields, such as aerospace and marine environmental monitoring, magnetic field measurement is an important link. In recent years, optical fiber magnetic field sensor has received much attention because of its advantages such as small size, electromagnetic immunity, resistance to erosion and capability of remote sensing. In that case, magnetic fluid as a kind of medium between photons and magnetic field is widely used in optical fiber magnetic field sensors. Moreover, in the process of magnetic field measurement, disturbance introduced by temperature fluctuation always happens and brings uncertainty to the sensor. Temperature is also an important parameter in production process and needs to be measured. Therefore, designing a high-sensitive optical fiber sensor for simultaneously measuring magnetic field and temperature is a valuable work. In this paper, we present a high-sensitive hollow core fiber (HCF) interferometer for simultaneously measuring magnetic field and temperature. A segment of HCF filled with alcohol is inserted into single mode fiber (SMF) with 50 m offset at two splicing joints to guide light into the wall of HCF. And then this SMF-HCF-SMF structure is packaged by a capillary tube with full magnetic fluid (MF) inside it. Since the modal field area is large enough, the silica wall can support a series of guiding modes among which modal interference occurs and the interference spectrum can be recorded by an optical spectrum analyzer. Besides thermo-optic effect and thermal expansion effect of silica itself, the RI variations caused by thermo-optic effect of alcohol and MF as well as the magneto-optic effect of MF can also cause the phase difference of the guiding modes to change, thereby rendering interference dips movable. Thus, the sensitivity of temperature or magnetic field is higher than those given in some other previous studies. In addition, it is calculated that the effective RI sensitivities of guiding modes for inside and outside liquid are different because of the peculiar non-circular symmetry structure of HCF. So there is a possibility to find two dips in interference spectrum, which are formed with different modes and have various sensitivities to the variations of temperature and magnetic field. Finally, a sensitivity matrix can be built to demodulate those two parameters simultaneously. Experimental results show that within 20-58℃, the temperature sensitivities are 112 pm/℃ and 468 pm/℃ for dip1 and dip 2 whose magnetic field sensitivities are 37 pm/Oe and 82 pm/Oe within 0-169 Oe, respectively. The proposed sensor possesses high sensitivity and good mechanical strength, and can effectively eliminate the cross disturbances between temperature and magnetic field.
2017, 66 (7): 070702.
doi: 10.7498/aps.66.070702
Abstract +
A corresponding peak appears on the transmission spectrum, when the micro-strain is induced in a chirped fiber Bragg grating (CFBG). The center wavelength of the peak is sensitive to the location and magnitude of the strain, thus, the CFBG can be used in distributed strain and strain-points precise position sensing. The depth and center wavelength of the peak are determined by the magnitude and location of the strain. The cascaded CFBGs under different center wavelengths can realize the distributed strain and strain-point precise positioning. Considering the fact that the depth and center wavelength of the peak are related to the magnitude and location of strain, a theoretical model is established with V-I transmission matrix formalism. Theoretically, cascaded CFGBs can realize accurately the positioning of micron-scale. Experimentally, two CFBGs are cascaded and a sensitivity of 0.19 pm/ is obtained. The proposed precise position sensing can be applied to the fields of advanced manufacturing, precision machining, aerospace, railway-system, etc.
2017, 66 (7): 070704.
doi: 10.7498/aps.66.070704
Abstract +
Long-period fiber grating (LPFG) is a kind of wide-range transmission passive photonic device with extensive applications in the field of fiber communication and fiber sensing. In this review, from the angle of refractive index spatial modulation, we extract three characteristic parameters of LPFG:grating period length, index modulated depth and normal orientation of grating plane, and classify LPFG as two types:uniform LPFG (none of these three parameters changes) and nonuniform LPFG (at least one of them changes), and analyze the deficiency of LPFG, including larger size than fiber Bragg grating, no reflection peak, too large bandwidth, polarization loss from single-side exposure, etc. We define the concept of novel LPFG (NLPFG) as the LPFGs based on general LPFG but having new structures and new characters by importing new factors from different aspects, like grating formed mechanism, grating structure, making material, processing technique, application performance, etc. Then we point out that the research significance of NLPFG lies in improving and exploring its real usable property, and making it practical by overcoming the defects of general LPFG in structure, property and application. We expound new techniques of LPFG fabrication, such as multi-exposure, apodized exposure, outfield action, coating and filling, fiber incised and welded, multi-dimensional modulation, and show some NLPFG examples written with these techniques. We build the spatial model of NLPFG to expand the refraction index modulation region from only fiber core to both core and cladding, and to correctly mark the direction of grating plane with tilted angle and azimuth angle. On this basis, we propose the design theory of NLPFG by adding those two angles into the coupling mode coefficient and solving the coupling mode equation. We also expound three different NLPFG design processes, as the direct design to start from given factors of grating, the reserve design to calculate the factors back from expected function or spectrum, and the direct-reserve design combined by them. Meanwhile, we introduce some typical design methods of NLPFG, like geometrical structure changed method, materials changed method, medium coated and embedded method, etc. In addition, we review the recent fabrication and typical application of NLPFG, then introduce different LPFG devices based on excentric core LPFG, multi-core LPFG, few-mode LPFG, stagger LPFG, mismatched LPFG, over-melted LPFG, phase-shift LPFG, tuning LPFG, coupled LPFG and cascaded LPFG, and show their sensing applications in strain, twisting, bending, temperature, displacement, gas concentration and biology. Finally, we provide a developing prospect of the research on NLPFG and give three possible means to improve the research, as innovating new gating structures, exploring new design methods and developing new fabrication techniques.
2017, 66 (7): 070703.
doi: 10.7498/aps.66.070703
Abstract +
Integration of novel functional material with fiber optic components is one of the new trends in the field of novel sensing technologies. The combination of fiber optics with functional materials offers great potential for realizing the novel sensors. Typically in optical fibre sensing technology, fibre itself acts as sensing element and also transmitting element, such as fiber Bragg grating (FBG), Brillouin or Raman optical time domain reflectometer. However such sensing components can only detect limited physical parameters such as temperature or strain based on the principle of characteristic wavelength drifts. While the idea of optical fiber sensing technology with functional materials is quite different from that of the traditional technology, functional materials can be employed as sensing components, therefore many parameters, including chemical or biological parameters, can be detected, depending on the designs of different sensing films. When compared with the common fiber sensing technologies such as FBG and optical time domain reflectometer, fiber optic sensors based on functional materials show advantages in the diversity of measurement parameters. However, functional materials can be realized by many techniques including e-beam evaporation, magnetron sputtering, spin-coating, electro-chemical plating, etc. The mechanical stability of tiny optical fibers is still problematic, which could be a challenge to industrial applications.
In this work, a femtosecond laser fabricated fiber inline micro Mach-Zehnder interferometer with deposited palladium film for hydrogen sensing is presented. Simulation results show that the transmission spectrum of the interferometer is critically dependent on the microcavity length and the refractive index of Pd film, and a short microcavity length corresponds to a high sensitivity. The experimental results obtained in a wavelength region of 1200-1400 nm, and in a hydrogen concentration range of 0-16%, accord well with those of the simulations. The developed system has high potential in hydrogen sensing with high sensitivity. Three-dimensional multitrench microstructures, femtosecond laser ablated in fiber Bragg grating cladding, TbDyFe sputtering are proposed and demonstrated for magnetic field sensing probe. Parameters such as the number of straight microtrenches, translation speed (feed rate), and laser pulse power of laser beam have been systematically varied and optimized. A 5-m-thick giant Terfenol-D magnetostrictive film is sputtered onto FBG microtrenches, and acts as a magnetic sensing transducer. Eight microtrench samples produce the highest central wavelength shift of 120 pm, nearly fivefold more sensitive than nonmicrostructured standard FBG. An increase in laser pulse power to 20 mW generates a magnetic sensitivity of 0.58 pm/mT. Interestingly, reduction in translational speed contributes dramatically to the rise in the magnetic sensitivity of the sample. These sensor samples show magnetic response reversibility and have great potential in the magnetic field sensing domain. Furthermore hydrogen sensors based on fiber Bragg gratings micro-machined by femtosecond laser to form microgrooves and sputtered with Pd/Ag composite film are proposed and demonstrated. The atomic ratio of the two metals is controlled at Pd:Ag=3:1. At room temperature, the hydrogen sensitivity of the sensor probe micro-machined by 75 mW laser power and sputtered with 520 nm of Pd/Ag film is 16.5 pm/%H. Comparably, the standard FBG hydrogen sensitivity becomes 2.5~pm/%H for the same 4% hydrogen concentration. At an ambient temperature of 35℃, the processed sensor head has a dramatic rise in hydrogen sensitivity. Besides, the sensor shows good response and repeatability during hydrogen concentration test.
2017, 66 (7): 074202.
doi: 10.7498/aps.66.074202
Abstract +
Aiming at the urgent requirements for refractive index detection in the biological sample detection area, an all-fiber surface plasmon resonance (SPR) system is established in this paper. And the SPR characteristic wavelength demodulation algorithm is proposed for this system based on all-phase filter technique. According to the system simulation, the refractive index sensing sensitivity of the fiber SPR sensor can be calculated theoretically. By using the all-phase filter technique, the characteristic wavelength of the fiber SPR sensor can be extracted, and the theoretically analytical expression of the all-phase filter can be obtained. The experimental results show that the refractive index sensing sensitivity and the detection resolution of the fiber SPR sensor are 1640.4 nm/RIU and 7.3610-4 RIU respectively by using this algorithm. Compared with the traditional methods, our algorithm can improve the detection precision and the anti-light-disturbance performance and reduce the costs as well.
2017, 66 (7): 075201.
doi: 10.7498/aps.66.075201
Abstract +
Brillouin dynamic grating (BDG) has been widely studied since it was proposed for the first time to achieve optical storage in 2007. In general, when two beams of pump light (their frequency difference equal to Brillouin frequency shift of the optical fiber) with the same polarization state are injected into the fiber, the coherent acoustic wave can be excited by the stimulated Brillouin scattering effect, forming a BDG. The BDG in an optical fiber has been widely used in optical fiber sensing, characterization of optical fibers, optical storage, all-optical signal processing, microwave photonics and high-precision spectral analysis due to the advantages of all-optical generation and flexible parameter control. In this paper, we analyze the principle of BDG generation and detection, and its applications in optical fiber sensing. The simultaneous measuring of strain and temperature is achieved within a spatial resolution of 20 cm through measuring Brillouin frequency shift and birefringence-induced frequency shift in a polarization-maintaining fiber. A high-sensitivity distributed transverse load sensor based on BDG with a measurement accuracy as high as 0.810-3 N/mm is proposed and demonstrated, whose principle is to measure the transverse-load-induced birefringence change through exciting and probing a BDG in an elliptical-core polarization maintaining fiber. On the basis of the above research, a distributed measurement of hydrostatic pressure is demonstrated by using a 4-m photonics crystal fiber with a measurement error less than 0.03 MPa at a 20-cm spatial resolution, while the temperature cross-talk to the hydrostatic pressure sensing can be compensated for through measuring the temperature-induced Brillouin frequency shift changes by using Brillouin optical time-domain analysis. A system based on BDG in polarization maintaining fibers is reported to achieve a spatial resolution below one centimeter, while preserving the full accuracy on the determination of temperature and strain through measuring Brillouin frequency shift. Taking advantage of creating a long BDG in an optical fiber, an ultra-narrow bandwidth optical filter is realized by operating a BDG in a single-mode fiber, and the optical spectrometry is performed by sweeping the center wavelength of the BDG-based filter through a swept-tuned laser, where a 4 fm (0.5 MHz) spectral resolution is achieved by operating a BDG in a 400 m single-mode fiber.
EDITOR'S SUGGESTION
2017, 66 (7): 070705.
doi: 10.7498/aps.66.070705
Abstract +
With the superiority of anti-electromagnetic interference, corrosion resistance, light quality, small size and so on, optical fiber sensing technology is widely used in aerospace industry, petrochemical engineering, power electronics, civil engineering and biological medicine. It can be divided as discrete and distributed. Discrete optical fiber sensing utilizes fiber sensitive element as sensors to detect the quantity to be measured. Optical spectrum, light intensity and polarization are usually used as the sensitivity parameter because they can be modulated by parameter such as rotation, acceleration, electromagnetic field, temperature, pressure, stress, stress, vibration, humidity, viscosity, refractive index and so on. Fiber works as the channel and links the fiber sensor and demodulating equipment. After a long period of research, the discrete optical fiber sensing technology stretch out many branches, we discussed the most representative ones as follows, the fiber grating sensing technique, the fiber fabry perot sensing technique, the fiber gyroscope sensing technique, the fiber intracavity sensing technique, the fiber surface plasma sensing technique, hollow-core fiber whispering gallery mode sensing technique, magnetic fluid fiber sensing technique and fiber-based optical coherence tomography sensing technique. Based on optical effect as rayleigh scattering, Raman scattering and Brillouin scattering, distributed fiber sensing system uses fiber itself as a sensor, when the vibration, stress, voice or temperature acts on the fiber changes, the optical signal transfers inside the fiber will change accordingly. The fiber distributes in a large range and a long distance, then the signal can be located at different positions and realize the multi-position measurement. We discussed the main distributed fiber sensing technologies as follows, the interferometric disturbance fiber sensing technology, the optical frequency domain reflectometry fiber sensing technology, the -optical time domain reflectometer fiber sensing technology, the optical fiber Brillouin sensing technology and the optical fiber Raman sensing technology. The development of technology is promoting the integration and network of optical fiber sensing, now it also becomes a research hotspot. Fiber optic smart sensor network is formed by various discrete and discrete optical fiber sensors in certain topological structure with the function of self-diagnosis and self-healing. Current research concentrates in the following areas, the increase of the multiplex sensor number, the topological structure with higher robustness and the intelligent control of sensing network. In this paper, we discuss the origination, development and research progress of discrete, distributed optical fiber sensing technologies and optical fiber sensing network technology, and the future research direction is also prospected.
2017, 66 (7): 070706.
doi: 10.7498/aps.66.070706
Abstract +
Coherent-OTDR technology is one of acoustic distributed fiber-sensing systems. Because of the advantages of anti-electric magnetic field interference, anti-corrosion and flexibility, it has been attracting more and more interest. Because the sound pressure is weak, the strain generated on the fiber is tiny and the sensitivity of the sensing system is low. Although many research has been made on expanding measuring distance and improving response frequency, the acoustic signals in the experiments are always replaced by PZT's mechanical stretching. In this work, a device for increasing sensitivity for acoustic in the passive acoustic detection system based on coherent optical time domain reflection (C-OTDR) is promoted. A way of improving sensitivity partly based on a thin-walled corrugated tube was promoted. The thin-walled corrugated tube was used as the element to transmit the energy of acoustic into the vibration of fiber.
In section 2, a mathematical model of sensing based on corrugated tube was established. Theoretical result shows that the vibration of fiber is mainly caused by the tube movement along the axis direction. And it also shows the linear relationship between the vibration and the sound pressure. The sensitivity of the improved sensing devices is calculated and a computational formula for sensitivity calculating are also given.
In section 3, the C-OTDR acoustic distributed fiber-sensing systems are set up. Fiberring and three types of thin-walled corrugated tubes are used for acoustic sensing. The minimum detection sound pressure level reaches 60.1 dB and the phase sensitivity reaches 2.975 rad/Pa. The experimental phase sensitivity of different sensing devices with different parameters change similarly to the theory results. The experimental results show that the way of improving sensitivity and the mechanical model for calculating sensitivity are effective. This research provides theoretical and experimental basis for further development of distributed optical fiber sensing.
2017, 66 (7): 070707.
doi: 10.7498/aps.66.070707
Abstract +
Phase-sensitive optical time domain reflectometry (-OTDR) has the advantages of fast response and high sensitivity. Therefore, it can realize fully distributed monitoring of weak vibrations along an optical fiber, which is of great value in many applications such as perimeter security and structural health monitoring. However, the optical background noise in the -OTDR will disturb the extraction of effective signals and limit the performance of this system. The optical background noise mainly includes the laser center frequency drift, the polarization-relevance noise and the distortion measurement due to the nonlinear relationship between optical fiber strain and interference intensity. In this paper, the generating mechanism of these optical background noise was analyzed and the corresponding noise suppression methods were proposed. The experiment results showed that the proposed methods could suppress the optical background noise effectively and improve the sensing performance significantly.
2017, 66 (7): 070708.
doi: 10.7498/aps.66.070708
Abstract +
Optical fiber sensors based on Fabry-Perot interferometer (FPI) have attracted intensive attention for sensing applications in temperature and pressure measurement, owing to their compact, small size, fast responses, high resolution, high sensitivity, good stability, and resistance to electromagnetic interference. It's known that the in-fiber optic interferometers based on single-mode fibers can exhibit compact structures, easy fabrication and low cost. In this paper, firstly, the basic principle of in-fiber FPIs is introduced. Secondly, we review several kinds of typical in-fiber FPIs formed in single-mode fibers fabricated with different post-processing techniques, such as chemical etching, arc discharge, femtosecond laser micromachining, and polymer coating, etc. Finally, the optical sensors based on in-fiber FPIs, with a capability of simultaneous multi-parameter sensing of temperature and pressure, are summarized and prospected.
2017, 66 (7): 074205.
doi: 10.7498/aps.66.074205
Abstract +
Fiber-optic ultrasonic sensors possess the ability to detect ultrasonic waves by recovery of light intensity, wavelength, phase, and polarization. Compared with traditional electrical ultrasonic transducers, fiber-optic ultrasonic sensors have several merits, such as broadband response, high sensitivity, disturbance resistance, and good reusability, which are helpful to improve the reliability and efficiency of ultrasonic detection in underwater defense security, bioimaging, nondestructive inspection, and imaging of seismic physical models.
To date, according to the principle, fiber-optic ultrasonic sensors can be classified into three types, including intensity modulation, fiber-optic interferometers and fiber gratings. For the intensity-modulated fiber-optic ultrasonic sensors, ultrasonic waves can be detected by measuring optical fiber coupling loss, fiber transmission-reflection loss, fiber reflection loss and fiber polarization loss. The phase difference in fiber-optic interferometers can be modulated by ultrasonic strain. According to the interference mechanism, fiber-optic interferometric ultrasonic sensors are generally based on Mach-Zehnder interference, Fabry-Perot interference, Michelson interference and Sagnac interference. For the ultrasonic sensors based on fiber gratings, the grating length is supposed to be shorter than the ultrasonic wavelength so that the ultrasonic stress presents constant along the fiber gratings. Currently, the approaches of spectral edge filtering and wavelength-matched filtering are utilized to transform optical signals into voltage signals, which highly depend on the slope of the grating spectra. Thus, the fiber gratings with extremely narrow 3-dB bandwidth, such as phase shifted fiber Bragg grating, are preferred for highly sensitive ultrasonic detection. Besides the fiber-optic passive sensing, the distributed feedback fiber laser and distributed Bragg reflector also exhibit outstanding advantages in ultrasonic detection.
Fiber-optic ultrasonic detecting technique is one of the hot topics in international research community, which is an effective method to evaluate the microstructure and related mechanical properties, and detect the microcosmic and macroscopic discontinuities of solid materials. In this paper, three typical applications of ultrasonic detection, i.e., monitoring of smart structure and health, biomedical imaging, and imaging of seismic physical models are reviewed, respectively. Our group has been engaged in the research fields of fiber-optic geophones and ultrasonic sensors for seismic exploration for decades. Several fiber-optic ultrasonic sensors with smart packaging are proposed and also used for the scanning imaging of two physical models.
In this paper we review the sensing mechanism, fabrication method, and current status of three types of fiber-optic ultrasonic sensors, respectively. Besides, the corresponding applications and technology challenges are also summarized. In particular, we present several kinds of home-made optical fiber ultrasonic sensors as a new technology applied in the imaging of seismic physical models. Overall, after decades of efforts, gratifying achievements have been achieved in the research of fiber-optic ultrasonic sensors. Further work needs to solve various technical issues, such as sensitivity, stability, structural microminiaturization, and multiplexing, etc. The next step will focus on the research issues in ultrasonic detection of seismic physical models, performance improvement, and multiplexing technology for distributed sensing. Miniaturization of fiber sensors and instrumentation of sensing system will also be the important research topic. The final objective of the research is to build a well integrated fiber-optic ultrasonic detecting system with high sensitivity and stability, networking construction, and proprietary intellectual property rights.
2017, 66 (7): 074206.
doi: 10.7498/aps.66.074206
Abstract +
This paper analyzes the simultaneous measurement method for the higher content soluble heavy metal elements of Cd2+, Cu2+, Zn2+, and Ni2+ which are heavy pollution in coastal and river water, demonstrates the properties of corresponding characteristic spectrum and absorbance of the four elements, establishes the mathematical model to function the concentration and total absorbance at the characteristic wavelength when the four elements dependent each other. We focus on effects of different situation of pH, temperature, time chromogenic dose on the metal ion concentration simultaneous measurement and summarize the experimental law.
2017, 66 (7): 074207.
doi: 10.7498/aps.66.074207
Abstract +
Distributed fiber-optic sensing (DFOS) is one of the most important parts in the fiber-optic sensing field, due to the following advantages:1) there is no need to manufacture sensors on the fiber; 2) fibers are able to realize transmission and detection simultaneously; 3) long-distance/large-scale sensing and networking can be accomplished prospectively; 4) the spatial distribution and measurement information of physical parameters such as temperature, strain and vibration, can be obtained continuously along the fiber link, and the number of sensing points on a single fiber can be up to several tens of thousands. Due to the above tremendous superiority, DFOS has found wide application prospects, including perimeter security, oil/gas exploration, electrical facilities and structure monitoring, etc. This paper overviews recent progress in ultra-long distributed fiber-optic static (Brillouin optical time-domain analyzer) and dynamic (phase-sensitive optical time-domain reflectometer) sensing at Key Laboratory of Optical Fiber Sensing and Communications, UESTC. This paper summarizes our work on both basic and application studies.
2017, 66 (7): 074208.
doi: 10.7498/aps.66.074208
Abstract +
Due to the advantages of high resolution, low cost, small size, easy deployment and capability of multiplexed sensing, the recent developed optical fiber grating sensors provide powerful tools for crustal deformation monitoring. This paper reviews the development of several types of fiber-optic sensors with ultrahigh resolution in quasi-static domain. The fiber Bragg grating based Fabry-Perot interferometers and the -phase-shifted fiber Bragg gratings which are used as sensing components in the high resolution sensors are introduced. Some novel techniques such as interrogating the sensing components with intensity modulation sideband, dual feedback-loop structure for high bandwidth/large measurement range sensing, and the time-domain multiplexing of the high resolution quasi-static strain sensor are discussed in detail. Each sensing scheme with both operation process and achieved performances are given. The implementation of fiber grating sensors for in-situ measurement of crustal deformation and the results are also introduced. Compared with the traditional methods used in crustal deformation observation, the high-performance fiber optic strain sensors mentioned in the paper shows great potentials in providing wider measurement approaches in geophysical researches.
2017, 66 (7): 074209.
doi: 10.7498/aps.66.074209
Abstract +
In recent years, polarization-maintaining (PM) microfibers have attracted much research attention mostly due to their ultra-high birefringence and large evanescent field effect. This article starts from introduction of the structures, fabrication methods, and mode characteristics of PM microfibers. Different previously-implemented PM microfiber sensors have been presented. The two polarization modes may have different responses on changes of external parameters for PM microfiber, which allows fabrication of polarization-related devices, such as interferometers or gratings. Some sensing characteristics, such as extremely-high refractive index sensitivity and/or temperature-independent response, have been demonstrated. The sensing applications including detection of refractive index, humidity, magnetic field and specific DNA molecular have been described in detail. This article should be helpful for future development of PM micro/nano fibers and the related sensors.
2017, 66 (7): 074210.
doi: 10.7498/aps.66.074210
Abstract +
Based on the analysis of the propagation mechanism of acoustic emission stress waves caused by impact excitation on aluminum alloy plates, The geometrical model of steel ball impacted aluminum alloy plate was built by ABAQUS software, and the stress wave propagation process is simulated and analyzed. The stress wave propagation process is simulated and analyzed and an acoustic emission sensing system based on the principle of edge filtering is constructed. The acoustic emission stress waves were collected to establish the acoustic emission region localization model. The localization method of acoustic emission region based on diffusion mapping and support vector machine is proposed and verified experimentally. Multiple acoustic emission localization experiments were performed on an aluminum alloy plate of 300 mm300 mm2 mm, which was divided into 36 test area. The results show that the localization accuracy is 30 mm30 mm and the positioning accuracy was 97.5%, while consuming 0.781 s. The study provides an effective method for acoustic emission localization.
2017, 66 (7): 074211.
doi: 10.7498/aps.66.074211
Abstract +
Recently, Airy beam as a kind of non-diffracting beam, has attracted a great deal of attention due to its unique properties to have propagation-invariant intensity profile, remain transverse accelerating and exhibit self-healing features. Therefore, Airy beams have found many potential applications, such as optical micro-manipulation, imaging technology, surface plasmon polaritons and laser micromachining. Airy optical fiber as a kind of waveguide device can be applied for the Airy beam generation, carry out the exploration of new Airy fiber and expand the Airy beam application range, has important practical significance. In this paper, we give an systematical introduction from the view of the Airy beam working principle, Airy fiber structure design, Airy fiber beam generated internal mechanism, Airy beam wavelength response characteristics, and Airy fiber applications.
2017, 66 (7): 074212.
doi: 10.7498/aps.66.074212
Abstract +
We report our recent work on the development of a highly sensitive gas detection technique-photothermal interferometry spectroscopy with hollow-core optical fibers. The basic principle of operation, generation and detection of dynamic photothermal phase modulation, and method to improve the response time of the hollow-core fiber sensors are described. The technique has ultra-high sensitivity and dynamic range, and the measurement is not affected by reflection/scattering and other non-absorbing losses. Sensors based on such a technique could be made compact in size with remote detection, multiplexing and networking capability, which would enable a range of high performance applications in environmental, medical and safety monitoring.
ATOMIC AND MOLECULAR PHYSICS
2017, 66 (7): 073201.
doi: 10.7498/aps.66.073201
Abstract +
The quantum statistical weight (G) of an atomic energy level is an important spectroscopic parameter, its effect on the atomic ionization process is, however, usually neglected for simplicity. In this work, the influences of the G parameters of the lithium atomic energy levels are taken into account explicitly for the first time in the study on the process of three-step photo-excitation (PE) + electric field ionization (EFI), which yields a significant effect on overall EFI efficiency of the PE+EFI process. With a set of specially designed PE+EFI schemes, the expected results are obtained. First, with a three-step PE scheme, the Li atom is excited by three pulsed lasers with different wavelengths, which are fired simultaneously, to one of the Rydberg states from its ground state, from which the Li atom is ionized by an electric-field pulse with a time delay in order to avoid the Stark effect. Based on the three physical processes the atom experiences the PE, none field, and the EFI processes, and a set of universal rate equations are established according to the conservation law of particle number with the knowledge of physical mechanism of the three different processes and the physical model set up for them, respectively. The G parameters of the four relevant bound energy states are displayed explicitly in the rate equations for the PE process to offer a clear viewabout their effect on the overall EFI efficiency of the PE+EFI process. Secondly, the overall efficiencies of PE+EFI process are calculated with the Matlab programming for the two specified excitation schemes, 2S1/22P1/23S1/225P1/2, 3/2 and 2S1/22P3/23D5/225F5/2, 7/2. The overall EFI efficiency of PE+EFI process is investigated not only by adjusting the laser parameters but also by comparing the results between the two different excitation schemes. In order to establish the relationship between the overall EFI efficiency and external field quantitatively, the dependence of population rate of the relevant bound states on various factors, such as laser and atomic parameters, is calculated systematically. The role of the G parameters of the relevant atomic energy levels played in the population rates is observed to determine which excitation scheme is better in terms of the population rate of the Rydberg state. Meanwhile, the spontaneous emission of the Rydberg state during the time delay between the pulses of electric field and laser is also evaluated to make a balance between avoiding the Stark effect and minimizing the spontaneous emission. Based on the analysis of the calculations, some new results are achieved. An enhancement of the overall EFI efficiency can be obtained by making a sophisticated design on the excitation scheme of the PE+EFI process to optimize the G parameters. The time delay between the pulses of electric field and laser not only lets the atom experience a field-free time period, but also makes an upper limit for the population rate of Rydberg state due to the redistribution of atom among the four relevant bound states in the period. The upper limit is found to be dependent on neither laser parameters nor the absolute values of the G parameters, while only on the branching ratio of the G parameters among those bound states. The overall EFI efficiency is dominated by the population rate of Rydberg state, as the EFI process may ionize all Rydberg atoms once the strength of the EFI field reaches the EFI threshold of the Rydberg state. Hence, the key factor for raising the overall EFI efficiency is to enhance the population rate of Rydberg state in the PE process, which is a hard challenge due to the upper limit for the population rate of Rydberg state.
2017, 66 (7): 073401.
doi: 10.7498/aps.66.073401
Abstract +
Under the condition of ten different incident energies ranging from 3 eV to 80 eV above the ionization potential of helium and the outgoing electrons having equal energies, by making use of 3C model and modified 3C model, the triple differential cross sections of electron-impact single ionization of the ground state of helium in the perpendicular geometry are calculated. The result is compared with corresponding experimental result to systematically investigate the influences of various screening effects on the triple differential cross sections for helium. The collision mechanisms of the triple differential cross sections are explored. The result shows that the effects of dynamic screening in the final state can directly affect the structures of the triple differential cross sections at lower incident energy, which will unavoidably affect the angular distribution and relative amplitude of side peaks at angles =90 and =270. The screening effects of residual electron in the final state of He+ have a similar significant effect on the amplitude of triple differential cross section of helium and angular distributions and relative amplitudes of side peaks at angles =90 and =270. When the incident energy is over 84.6 eV, the screening effect of residual electron in the final state of He+ has a slight effect on the amplitude of triple differential cross section, which can be overlooked. But the effects of dynamic screening in the final state on side peaks at angles =90 and =270 need considering. In addition, by taking advantage of DS3C-Z model, the results of collision mechanism of various peaks at angles =180, =90 and =270 show that the middle peak at angle =180 is produced by a process called triple scattering mechanism and then the side peaks at angles =90 and =270 are produced by a process called double scattering mechanism. Such a collision mechanism has a direct influence on the generation and variation law of triple differential cross sections.
ELECTROMAGNETISM, OPTICS, ACOUSTICS, HEAT TRANSFER, CLASSICAL MECHANICS, AND FLUID DYNAMICS
2017, 66 (7): 074201.
doi: 10.7498/aps.66.074201
Abstract +
Quantum memory of light is not only the building block of constructing large-scale quantum computer, but also the kernel component of quantum repeater for quantum networks, which makes long distance quantum communication come true. Due to the inevitable optical losses, squeezed vacuum generated from optical parametric amplifier becomes squeezed thermal state of light, which is no longer the minimum uncertainty state. Therefore quantum memory of squeezed thermal state of optical field is the key step towards the implementation of quantum internet. Atomic ensemble is one of ideal quantum memory media, as a result of high optical depth and good atomic coherence. Electromagnetically induced transparency (EIT) is one of mature approaches to quantum state mapping between non-classical optical fields and atomic spin waves. In atomic ensembles, the EIT can on-demand map quantum state between quadratures of light and spin waves of atomic ensemble, i.e., controlled quantum memory. Here the condition of quantum memory for squeezed thermal state of light is investigated according to the fidelity benchmark of quantum memory. The fidelity benchmark of quantum memory is the maximum fidelity which can be reached by classical methods, and it is quantum memory if the memory fidelity is higher than the fidelity benchmark of quantum memory. By numerically calculating the fidelity benchmark of quantum memory for different kinds of squeezed thermal states of light and the dependence of memory fidelity on the memory efficiency, we obtain the minimum memory efficiency which can realize quantum memory for squeezed thermal state of light. The quantum memory can be easily obtained by increasing squeezing parameter r. The thermal state fluctuation is sensitive to the realization of quantum memory. The required minimum memory efficiency is lower, when smaller thermal state fluctuation is employed in experiment by reducing the optical losses in optical parametric amplifier. On the other hand, quantum memory fidelity benchmark is high for small squeezing parameter and large optical depth, which requires high memory efficiency. And atomic memory efficiency can be increased by utilizing optical cavity to enhance the interaction between light and atom or atomic ensemble with high optical depth. For example, the fidelity benchmark is 0.80, when squeezing parameter r is 0.35 and thermal state fluctuation is 2.38 dB. Thus quantum memory can be realized if the memory efficiency is larger than 4.34%. Our work can provide the direct reference for experimental design of continuous variable quantum memory, quantum repeater, and quantum computer based on atomic ensembles.
2017, 66 (7): 074204.
doi: 10.7498/aps.66.074204
Abstract +
With the development of optoelectronic technologies, color cameras have been widely exploited in space remote sensing, earth observations from space, environmental monitoring, urban construction, and many other fields. Currently, most commercial color cameras use a single charge coupled device (CCD) or complementary metal-oxide-semiconductor (CMOS) sensor that has a Bayer color filter array (CFA) on its pixel surface to obtain red (R), green (G), or blue (B) samples. As a way of evaluating imaging quality, modulation transfer function (MTF) can provide a comprehensive and objective metric for camera imaging performance. In the conventional knife-edge method for color camera MTF measurement, a linear uniform sampling of the edge spread function (ESF) must be completed before a fast Fourier transform (FFT) can be applied. As the sampling rate becomes large, the number of pixel points on the line which is parallel to the knife-edge become less. So taking average of the pixel points to obtain ESF can be strongly affected by the noise of sensor. Therefore it is necessary to balance the influences of sampling rate and sensor noise on the MTF measurement, and the recommended sampling rate is 4-6. When the tilt angle of knife-edge has an error, the non-uniform sampling ESF can be obtained by the slanted knife-edge method. This leads to a variation in the results of the camera MTF on a spatial frequency scale and early cut-off. The best MTF results of camera can be obtained by rotating knife-edge, calculating MTF power under different tilt angles of knife-edge, and finding the maximum MTF power. And we propose an algorithm for Bayer filter color camera MTF measurement. The algorithm processing includes extracting R, G, B colors of knife-edge images; projection; differential operation; Hanning window filtration; FFT; correction; weighting combination of R, G, B colors MTF; MTF power calculation; optimal tilt angle of knife-edge estimation. To verify the accuracy of the proposed method, the weighting response factors of R, G, B colors are calibrated and an experimental setup for color camera MTF measurement is established. The knife-edge target is rotated in angle steps of 0.02, and the MTF results are calculated under different tilt angles of knife-edge within0.1 surrounding the estimate position by the proposed algorithm. The maximum differences of MTF results between the proposed method and fringe target method are 0.061 (Nyquist frequency fc) and 0.043 (fc/2), respectively. The results show that by searching the optimal tilt angle of knife-edge, the effect of non-uniform sampling on MTF result of color camera can be eliminated. Compared with the conventional method, the proposed method is superior for the measurement of the super-sampled MTF of color camera. Meanwhile, this method can also be applied to MTF measurements of radiographic systems, such as X-ray imaging system and other systems.
EDITOR'S SUGGESTION
2017, 66 (7): 074701.
doi: 10.7498/aps.66.074701
Abstract +
In this paper, a theoretical analysis model is proposed for the linear growth of the Richtmyer-Meshkov instability in elastoplastic solid medium-vacuum interface under the explosion shock wave loading. The analysis of the dynamic evolution of small perturbations shows that after the initial phase inversion, some perturbations would stop growing after they have reached their maximum amplitude, some others would continue to grow and then form jetting from the solid-vacuum interfaces. Numerical simulations show excellent agreement with the experimental results of explosively-driven Richtmyer-Meshkov instability in the sample of copper. The effects of two physical factors on the maximum amplitude of spikes are also studied numerically. The first physical factor is the initial configuration of the perturbation, which is expressed as the time values of the initial wave number and initial amplitude. With increasing the value of the initial configuration, the maximum amplitudes of the spikes would become greater while the growth of perturbations is suppressed. On the other hand, the maximum amplitudes of spikes would become smaller in the solid which has a higher yield strength when the initial configuration keeps unchanged. Further investigations show that the boundary of the stage division between the stable growth and the unstable growth is revealed by a combination parameter form of the two physical factors, which is expressed as the ratio of initial configuration to yield strength. In the stable stage, the linear relation between the non-dimensional maximum amplitude and the non-dimensional maximum growth rate of the spikes is fitted with the coefficient value 0.30, which is very close to 0.29, a theoretical prediction based on the Newton's second law analysis. Considering the shock Hugoniot relations in the elastoplastic medium and the maximum growth rate equation of the Richtmyer-Meshkov instability in ideal fluid, the linear model is improved to add the effects of the loading shockwave pressure and the compression acoustic impedance of the material on the amplitude growth of the spike to the analytical model proposed by the former researchers. Extensive numerical simulations are performed to show that the linear model could accurately describe the growth factor of the spikes in the stable cases in different metal materials, such as copper, aluminum, and stain-less steels. In the numerical analysis of the scope of application of the linear model, a rough estimation of the stage division boundary between the stable and unstable growth is given as 0.8 GPa-1. When the ratio of initial configuration to yield strength is lower than the division boundary, the perturbation growth would be stable and the linear model could describe the growth law of the spikes.
2017, 66 (7): 074203.
doi: 10.7498/aps.66.074203
Abstract +
Recently, silicon-based photonic integrated circuits (PICs) have attracted considerable interest due to the advantages of high index contrast and complementary metal oxide semiconductor compatible process. Furthermore, to meet the ever-growing bandwidth requirements for data center and supercomputing, several multiplexing on-chip technologies by using silicon PICs are proposed. Among them, the mode division multiplexing (MDM) is widely recognized to be important, where mode-order converters (MOCs) are fundamental building blocks. In addition, slot waveguides can efficiently confine the light in low-index regions, thus forming various kinds of novel photonic devices. In this paper, a compact 12 multimode interference (MMI) mode-order converter (MOC) for silicon-based slot nanowires is proposed, where straight waveguides, as the input/output channels, are connected to a quadratic-tapered multimode waveguide via linear-tapered waveguides. A full-vectorial finite-difference frequency-domain method is used to analyze the modal characteristics of the used silicon-based vertical slot waveguides; from this, quasi-TM mode is chosen as an input optical signal since its field distribution is strongly confined in the slot, i. e., the electric field strength is greatly enhanced in the vertical slot, and with the increase of the width of the slot waveguide, it can support higher-order quasi-TM modes. Compared with the beating length of rectangular MMI structure, the beating length of quadratic-tapered MMI structure can be effectively reduced with transmission loss lowering. From the imaging position of the guided-mode in MMI region via self-imagining effect, the length of quadratic-tapered MMI structure can be determined accurately where first-order and fundamental quasi-TM modes are outputs, respectively, from wider and narrower channels. A three-dimensional finite-difference time-domain method is utilized to assess the performance of the proposed MOC, where the insertion loss and crosstalk are analyzed in detail. The results show that an MOC with an MMI section of 35 m2 is achieved to be an insertion loss and a crosstalk of~0.35 dB and~-16.9 dB, respectively, at a wavelength of 1.55 m by carefully optimizing the key structural parameters. Moreover, the fabrication deviation of the proposed device is also analyzed in detail and the performance is evaluated, where insertion loss and the contrast are considered. To demonstrate the transmission characteristics of the proposed MOC, the evolution of the excited fundamental quasi-TM mode through the MOC is also presented. Numerical results show that the presented MOC realizes the desired function, converting the fundamental quasi-TM mode into first-order one with reasonable performance. We remark that the present MOC has a good potential application in MDM system to improve the capacity of the silicon-based on-chip transmission system.
PHYSICS OF GASES, PLASMAS, AND ELECTRIC DISCHARGES
2017, 66 (7): 075202.
doi: 10.7498/aps.66.075202
Abstract +
A commercial magnetohydrodynamic (MHD) simulation package USIM is used to simulate two colliding plasma bubbles, which are not moving in the same horizontal line along the X direction. One similar experiment is performed on Shenguang II laser facility, in which four laser beams each with a wavelength of 0.351 m, total energy of 1.0 kJ, pulse duration of 1ns, are irradiated on an Al target with a thickness of 50 m. Every two beams constitute one 150-m-diameter focal spot with an intensity of 1015 W/cm2. The X-ray radiation results show the asymmetric and peach-like plasma bubbles, which are different from the results obtained before. Here we report the possible reason for the asymmetric and peach-like structure in experiment. External magnetic field on the order of 1 T is chosen to perform the simulations, which could be a possible applied B field in future experiments performing on the Shenguang II laser facility.
In the simulations, different cases, especially the effects of different directional external magnetic fields, are considered. When the reversal directional magnetic fields are embedded in the Y direction, the magnetic field lines are frozen in the plasma bubbles, moving and approaching to each other gradually with the magnetic field lines. The change of the direction of magnetic field lines in the interaction region indicates that the magnetic reconnection has been happened. The outflows between two plasma bubbles in the experimental result could be explained by magnetic reconnection, which can efficiently convert stored magnetic energy into kinetic energy and thermal energy by accelerating and heating plasma particles. The density jump at the position of the bow structure indicates the generation of shock waves, where the velocity of flow v is also larger than the sound speed vs. When the same directional attractive magnetic fields are embedded in the Y direction, magnetic field lines are piled up in the central part, where the magnetic field density is high, which indicates that the magnetic repulsion has been happened. Magnetic repulsion also delays the colliding between two plasma bubbles. The shock waves each with a width of 4 m are also found in this case. The X-ray images in experiment and the density images in simulations show the similar peach-like structures, where the density results could be used to explain the X-ray radiation result for, I(v,Te)（2）/（Te） e（-（hv）/（kTe）, I is the radiation intense, v is the plasma velocity, Te is the electron temperature, is the plasma density.Magnetic reconnection is the possible reason for the asymmetrical and peach-like structure in the experiment by comparing all kinds of simulation cases. The present simulation results will be of benefit to the future designing of experimental setup on the Shenguang II laser facility, although a two-fluids model is needed to build a spontaneous magnetic field for the real plasma bubbles.
2017, 66 (7): 075204.
doi: 10.7498/aps.66.075204
Abstract +
The pulsed inductive discharge ionizes the neutral gas and accelerates the plasma efficiently, and is accompanied by complicated phenomena during the discharge process. In order to study the transient flow field characteristics and the variations of the main flow parameters (e.g., velocity, density, pressure, etc.) with the magnetic induction intensity of the inductive pulsed plasma, the two-dimensional axisymmetric unsteady magnetohydrodynamic numerical model is introduced by employing the hyperbolic divergence cleaning method. The plasma is excited by the single pulse energy varying in the sine waveform with a period of 10 s, and the flow field of the peak magnetic induction intensity ranging from 0.1 T to 0.55 T, is calculated. The results show that the high density and speed region gradually moves forward and away from the coil, leaving the low density and speed plasma behind, meanwhile, the high temperature region is near the coil throughoutthe discharge, and the inductive magnetic field leads in the phase, compared with the flow parameters, which indicates the effective permeation of the pulsed energy into the neutral gas and the plasma. As the input single pulse energy increases, the maximum axial velocity of the plasma increases and the time at which the flow velocity reaches a peak value moves up. The current sheets of the same direction, which are located on the surface of the induction coil at the beginning, appear as the discharge initiates and moves forward with the influenced flow domain expanding as the process goes on, and an opposite sign current sheet grows when the time passes through the first quarter of the sine period, which is also near the surface of the coil and heats the low-density plasma and the neutral gas. The opposite direction current sheets slow down the velocity of the plasmoid. Due to the nonlinear property of the coil-plasma interaction, the acceleration efficiency of the induction coil improves irregularly as the magnetic induction intensity increases, which grows slowly at a low level, and when the intensity reaches a certain critical value, for the configuration studied in this work the particular value is 0.45 T, the acceleration efficiency increases significantly, indicating that a larger part of the pulsed energy is converted into the plasma kinetic energy.
2017, 66 (7): 075203.
doi: 10.7498/aps.66.075203
Abstract +
In this paper, we report our results from interactions between sub-picosecond laser and relativistic near-critical density plasma layer. To create the near-critical density plasma layer, low density foam targets are utilized in our experiments. The foam is comprised of tri-cellulose acetate. Their average densities vary from 1 mg/cm3 to 5 mg/cm3, corresponding to full ionization densities ranging from 0.6nc to 3nc. When laser pulse is incident on the near-critical density plasma, some energetic bunches with a large quantity of charges are measured in most of the shots. The maximum charge quantity reaches to 6.1 nC/sr. Furthermore, the observed electron energy spectrum is Boltzmann-like with a wide plateau at the tail of the energy spectrum, rather than a Maxwell-like. The concept of average temperature is not available any more, and we define average effective temperature instead, namely the slope temperature. Fitting the Boltzmann-like spectrum exponentially, we find that the average effective temperature even exceeds 8 MeV at 7.51019 W/cm2, far beyond the ponderomotive limit. Aiming at analyzing the implication of physics, several two-dimensional particle-in-cell (PIC) simulations are performed. The PIC simulations indicate that the hole-boring effect and relativistic self-transparency play an important role in the electrons heating process. At the earlier stage of heating process, a short plasma channel is created by the hole-boring effect and relativistic self-transparency. The length and the width of the plasma channel are about tens of micrometers and several micrometers respectively. Around the plasma channel, there is an intensive azimuthal magnetic field. The magnitude of the azimuthal magnetic field is 100 MGs. However, the radical electrostatic field is not seen. The possible reason is that the plasma channel would be cavitated by the hole-boring effect. As a result, the electrons will experience Betatron resonance in the magnetized plasma channel. The traverse momentum of the electron would be converted into forward momentum. Assisted by the Betatron resonance, the electrons gain energies from the laser directly and efficiently. Thus, the average effective temperatures of the electron bunches are much higher than predicted by the ponderomotive scaling law. Besides, we also conducte another simulation to instigate the differences by adopting different laser polarizations. Within our expectation, the electron spectrum of the P-polarization accords well with the experimental result, while the electron spectrum of the S-polarization obviously deviates from the experimental result. It also demonstrates that the Betatron resonance heating dominates the electron acceleration process. This research paves the way to generating the highly energetic bunches with a large quantity of charges, and wound also be helpful for producing the high-bright laser bremsstrahlung sources in future.
CONDENSED MATTER: STRUCTURAL, MECHANICAL, AND THERMAL PROPERTIES
2017, 66 (7): 076101.
doi: 10.7498/aps.66.076101
Abstract +
The carrier microscopic transport process of uniaxial strained Si n-channel metal-oxide-semiconductor field-effect transistor (NMOSFET) is analyzed under -ray radiation. The model of radiation-induced defect densities that are quantitative representations of trapped charges integrated across the thickness of the oxide (Not), and the number of interface traps at the semiconductor/oxide interface (Nit), is established. The variations of electrical characteristics of the uniaxial strained Si nanometer NMOSFET are also investigated under the total dose radiation. The device of uniaxial strained Si nanometer NMOSTET is irradiated by a 60Co -ray laboratory source at a constant dose rate of 0.5 Gy (Si)/s. The TID is deposited in several steps up to a maximum value of 2.5 kGy. Electrical measurements are performed at each TID step. All irradiated samples are measured using field test, and are required to finish measurement within 30 min, in order to reduce the annealing effect. Static drain-current ID vs. gate-voltage VGS electrical characteristics are measured with an HP4155B parametric analyzer. Some parameter extractions presented here come from these static measurements including the threshold voltage VTH, the trans-conductance gm, and the leakage current IOFF (ID at VGS=0 V and VDS=VDD). Irradiation bias:VG=+1 V, drain voltage VD is equal to source voltage VS (VD=VS=0). Measurement bias:VG=0-1 V, scanning voltage Vstep=0.05 V, VD=50 mV, and VS=0. The results indicate the drift of threshold voltage, the degradation of carrier mobility and the increase of leakage current because of the total dose radiation. Based on quantum mechanics, an analytical model of tunneling gate current of the uniaxial strained Si nanometer is developed due to the total dose irradiation effect. Based on this model, numerical simulation is carried out by Matlab. The influences of total dose, geometry and physics parameters on tunneling gate current are simulated. The simulation results show that when radiation dose and bias are constant, the tunneling gate current increases as the channel length decreases. When the structure parameters and the stress are fixed, the tunneling gate current increases with the increase of radiation dose. Whereas at a given the radiation dose, tunneling gate current will decrease due to the stress. When radiation dose and bias are kept unchanged, the tunneling gate current increases with the thickness of the gate oxide layer decresing. When the gate-source voltage, the thickness of oxide layer and stress are fixed, tunneling gate current is reduced with the increase of doping concentration in channel. When the structural parameters, the gate-source voltage and radiation dose are constant, the tunneling gate current decreases with increasing drain-source voltage. In addition, to evaluate the validity of the model, the simulation results are compared with experimental data, and good agreement is confirmed. Thus, the experimental results and proposed model provide good reference for research on irradiation reliability and application of strained integrated circuit of uniaxial strained Si nanometer n-channel metal-oxide-semiconductor field-effect transistor.
Substitutions of dual-ion Al3+/Mo6+ for Zr4+/V5+ in ZrV2O7 for realizing near-zero thermal expansion
2017, 66 (7): 076501.
doi: 10.7498/aps.66.076501
Abstract +
Zr1-xAlxV2-xMoxO7 (0 x 0.9) is developed by the solid state method, and the near-zero thermal expansion is realized by adjusting the quantity of substitution of Al3+/Mo6+ for Zr4+/V5+ in ZrV2O7. For smaller x values (x 0.3), the samples remain the same cubic structure as that of ZrV2O7. The Coulomb interaction between (Al/Zr)- and (Mo/V)+ increases gradually with increasing the quantity of dual-ion substitution of Al3+/Mo6+ for Zr4+/V5+ in ZrV2O7, which reduces the fraction of the distortionless cubic structure in the sample. For x 0.7, the cubic structures could not be found. For Zr0.5Al0.5V1.5Mo0.5O7, near-zero thermal expansion is obtained in a temperature range from 425 to 750 K (-0.3910-6 K-1). The mechanism of low thermal expansion of Zr0.5Al0.5V1.5Mo0.5O7 could relate to the distortion of crystal structure due to partial substitution of Al3+/Mo6+ for Zr4+/V5+ in ZrV2O7 and the effect of the substitution on the unsubstituted lattice.
CONDENSED MATTER: ELECTRONIC STRUCTURE, ELECTRICAL, MAGNETIC, AND OPTICAL PROPERTIES
2017, 66 (7): 077301.
doi: 10.7498/aps.66.077301
Abstract +
Due to the restriction of the ZnO buffer layer on the left barrier in a wurtzite asymmetric ZnO/MgxZn1-xO single quantum well (QWs) structure with finite barriers, the other factors such as the size of the well and right barrier, and Mg component, etc. will influence the critical value of the left barrier width to form a binary level energy system. By adopting a finite difference method to solve the Schrdinger equation, the eigenstates and eigenenergies of electrons in a two-dimensional electron gas are obtained, and the influences of buffer layer ZnO, size and ternary mixed crystal effects on the formation of binary energy level system in QW are discussed. In our computation, the influences of energy band bending, material doping and built-in electric fields on a realistic heterostructure potential are considered. Furthermore, based on the Fermi's golden rule, the optical absorption coefficient of electronic intersubband transition in QW and the influences of buffer layer thickness, the widths of left barrier, well and right barrier and ternary mixed crystal effects are discussed. Our results indicate that the critical width of left barrier increases with the increases of the right barrier width and buffer layer thickness for a binary energy level system of ZnO/MgxZn1-xO single quantum well with a ZnO buffer layer on the left side. However, the critical width of left barrier decreases with the increase of well width and Mg component. Besides, the buffer layer thickness, the widths of left barrier, well and right barrier and ternary mixed crystal also affect the light absorption induced by the electronic intersubband transitions. The increases of Mg component, the widths of right barrier and left barrier will increase the absorption peak and produce its blue-shift. Whereas, increasing well width will reduce the absorption peak and produce its red-shift. The above conclusions are expected to give theoretical guidance in improving the opto-electronic properties of materials and devices made of these heterostructures.
2017, 66 (7): 077302.
doi: 10.7498/aps.66.077302
Abstract +
Lateral double-diffused MOSFETs (LDMOS) are widely used in high voltage integrate circuits and smart power integrate circuits because of their lateral channels and their electrodes located on the surface of the device, thereby facilitating integration with other low-voltage circuits and devices, and they have become the core technology of the second electronic revolution.
In order to optimize the breakdown characteristics and the performance of the LDMOS, in this paper, a novel LDMOS is proposed with the vertical assisted deplete-substrate layer (ADSL) on the basis of traditional LDMOS structure. The new ADSL layer makes the vertical depletion region below the drain expand to substrate excessively, thus introduces a new electric field peak at the bottom of the ADSL layer by using the electric field modulation effect, so that the vertical electric field is optimized. The ISE simulation results show that when the lengths of the drift region of ADSL LDMOS and traditional LDMOS are both 70 m, the breakdown voltage is increased from 462 V to 897 V, improved by about 94%. Also, the figure-of-merit (FOM) is upgraded from 0.55 MW/cm2 to 1.24 MW/cm2, increased by 125%. Therefore, the new structure ADSL LDMOS has a great improvement in device performance compared with that of the traditional LDMOS.
Moreover, authors have studied the ADSL LDMOS from three parts, all of these confirm that the new structure has a great potential application in power device. Firstly, through the lateral surface electric field distributions and vertical electric filed distributions of conventional LDMOS and ADSL LDMOS, a new electric field peak at the bottom of the ADSL is introduced in the vertical direction. Secondly, the mechanism for the new structure can present a deeper understanding through the ADSL LDMOS concentration and structural parameter optimization process. The FOM is optimized when the drift region concentration and ADSL concentration are 1.81015 cm-3 and 6.51015 cm-3, respectively, and it can reach a best value when the ADSL length is 40 m. Thirdly, the ADSL layer is further partitioned and optimized. On the basis of the new structure, the breakdown voltage is increased to 938 V when the new structure is based on the dual partition, and in the triple partition the breakdown voltage reaches 947 V. In this paper, through simulations, the detailed analyses of the proposed new structure of the mechanism and its performance are conducted, and the breaking of the breakdown characteristics of silicon-based devices is of special significance for developing the lateral power devices.
GENERAL
EDITOR'S SUGGESTION
2017, 66 (7): 070301.
doi: 10.7498/aps.66.070301
Abstract +
In this paper, the electron-positron creation process in a double well scheme is investigated. A series of simulations is conducted by solving the quantized Dirac equation numerically. Here the split operator scheme is used to solve the Dirac equation, and the Fourier analysis is adopted to study the evolution of the wave function. The evolution starts from the state that all the negative energy eigenstates are occupied. By projecting the time dependent wave function to the positive energy eigenstates, the distributions of electrons and positrons in coordinate space and momentum space would be calculated. The total number of the electrons and positrons can be obtained by integrating the momentum distributions, and the number of the positrons in different parts of coordinate space can be achieved by integrating the space distributions. At first the electron-positron is created at the double-well edge, and positrons are emitted from the edges of double-well potential and propagate out while the electrons are bounded by the barriers. It is found that when the positron waves from different double-well edges encounter in the double-well for the first time, there occurs no positron wave interference phenomenon. The wave interference emerges after the positron no indent wave is reflected by the barriers. At the same time, because of Klein tunneling the number of positrons outside the double well begin to surpass the positrons inside the double well. After a piece of time, the amplitude of interference wave would reach its peak, and then collapses since Klein tunneling. If the double-well potential meets the standing-wave conditions, a stationary wave would be found before the interference wave reaches its peak if the distance between the double wells is short, and a stationary wave would be found after the interference wave has reached its peak if the distance between the double wells is long. And the stationary wave occurs when the positron wave is reflected by the barriers for the second time. The occurring of the stationary wave would affect the pairs producing process by making the number of pairs fluctuate. Because of Klein tunneling, the wave packages close to the double-well would disappear first, and the others can last for a longer time when the standing-wave condition is fulfilled, but all of the stationary wave packages disappear in the double well finally. And there is barely no positrons left inside the double well to the end since Klein tunneling.
2017, 66 (7): 070302.
doi: 10.7498/aps.66.070302
Abstract +
Quantum communication is a brand new way of communicating, in which the quantum entanglement effect is used to transmit information. Quantum communication is a new interdisciplinary subject between quantum theory and information theory. It has advantages of perfect information security and higher efficiency in transmission. The successful launch of the first quantum satellite named Micius laid an important foundation forconstructing a secure quantum communication network on a global scale. On the other hand, in the process of quantum satellite communication, the atmospheric environment near the ground will have a certain influence on the transmission of quantum signals, so the security of quantum communication is reduced. One of the influence factors is the ionospheric sporadic E layer. In the actual quantum communication environment, when the weak coherent state light source is simulated the single photon, the source energy is attenuated if the quantum signal passes through the ionosphere. On a space scale of 80-150 km off the ground, the ionospheric sporadic E layer is an irregular thin layer, in which there occurs a sharp increase of electron density under the action of wind shear. Sporadic E layer has a great influence on quantum satellite signal transmission. However, the research about the relationship between the sporadic E layer and quantum communication channel parameters has not yet conducted. To investigate the influence of the ionospheric sporadic E layer on the performance of quantum satellite communication, sporadic E layer formation process is first analyzed. And then the relationship between the free electron density and the height is obtained. After that, the model of the free electron density, the formation thickness and the link attenuation of quantum satellite is established. According to the amplitude damping channel, the quantitative relationships among free electron density and the channel capacity, entanglement fidelity, the quantum bit error rate and the secure key rate are put forward and simulated finally. Theoretical analysis and simulation results show that when the thickness is 1 km, the electron density increases from 3105 cm-1 to 27105 cm-1, the channel capacity decreases from 0.8304 to 0.1319, the entanglement fidelity decreases from 0.9386 to 0.3606, the quantum bit error rate increases from 0.0093 to 0.0769, and the secure key production rate decreases from 9.96810-5 to 1.9110-6. It can be shown that the electron density and the thickness of sporadic E layer have significant effect on the performance of quantum satellite communication. Therefore, in the process of quantum satellite communication, in order to ensure the reliability of quantum communication, based on the detection of ionosphere parameters, the various indexes of the satellite system should be adjusted adaptively.
2017, 66 (7): 070501.
doi: 10.7498/aps.66.070501
Abstract +
A classic problem in circuit theory first studied by German physicist Kirchhoff more than 170 years ago is the computation of resistances in resistor networks. Nowadays, resistor network has been an important model in the fields of natural science and engineering technology, but it is very difficult to calculate the equivalent resistance between two arbitrary nodes in an arbitrary resistor network. In 2004, Wu F Y formulated a Laplacian matrix method and derived expressions for the two-point resistance in arbitrary finite and infinite lattices in terms of the eigenvalues and eigenvectors of the Laplacian matrix, and the resistance results obtained by Laplacian matrix method is composed of double sums. The weakness of the Laplacian matrix approach is that it depends on the two matrices along two orthogonal directions. In 2011, Tan Z Z created the recursion-transform (RT) method, which can resolve the resistor network with arbitrary boundary. Using the RT method to compute the equivalent resistance relies on just one matrix along one direction, and the resistance is expressed by single summation.
In the present paper, we investigate the equivalent resistance and complex impedance of an arbitrary mn cylindrical network by the RT method. Firstly, based on the network analysis, a recursion relation between the current distributions on three successive vertical lines is established through a matrix equation. In order to obtain the eigenvalues and eigenvectors of the matrix, and the general solution of the matrix equation, we then perform a diagonalizing transformation on the driving matrix.Secondly, we derive a recursion relation between the current distributions on the boundary, and construct some particular solutions of the matrix equation. Finally by using the matrix equation of inverse transformation, we obtain the analytical solution of the branch current, and gain the equivalent resistance formula along the axis of the arbitrary mn cylindrical network, which consists of the characteristic root and expressed by only single summation. As applications, several new formulae of equivalent resistances in the semi-infinite and infinite cases are given. These formulae are compared with those in other literature, meanwhile an interesting new identity of trigonometric function is discovered. At the end of the article, the equivalent impedance of the mn cylindrical RLC network is also treated, where the equivalent impedance formula is also given.
2017, 66 (7): 070701.
doi: 10.7498/aps.66.070701
Abstract +
The morphology of proeutectoid ferrite in steels has attracted much attention in view of its close correlation with the fundamentals about the phase transformation theory as well as its potential practice relating to the final microstructure and properties of the steel product. With the recent development of mesoscale microstructure-based transformation models, the approach to integrated microstructural simulation is ideally suited to provide a more in-depth insight into the mechanism and morphology complexity for this problem. Among the various mesoscopic models, the phase-field method can readily be used to simulate the complex morphological phenomena during the austenite-to-ferrite transformation in steels in view of its convenience to include the material properties, especially the grain boundary properties, in a phenomenological way, and thus to model the microstructural process in an anisotropic system. In this study, a modified multi-phase-field (MPF) model that takes into account various anisotropic interfacial conditions is developed to simulate the growth morphology of ferrite during the austenite-to-ferrite transformation in a Fe-C-Mn alloy. In this model, a quantitative relation between the MPF model parameters and the physical anisotropic interfacial properties, including the grain-boundary energy and the mobility, is carefully considered, which allows the identical width of the diffuse interface regarding arbitrary interfacial anisotropies in the MPF simulations. In this way, both the accuracy and the numerical stability of the model can be ensured. Using this model, the effects of the grain boundary anisotropy on the ferrite growth are studied. The simulation results indicate that apart from the interfacial energy of σα, γ, the grain boundary energy between the initial austenite grains, σγ, γ; does also significantly influence the growing morphology of ferrite. The ferrite growth along the initial austenite grain boundaries is facilitated when increasing the ratio of σγ, γ/σα, γ, and hence leading to a smaller equilibrium angle at the triple junction. The results also indicate that misorientation-dependent grain boundary energy and mobility play important roles in determining the ferrite growth behavior. The growth of ferrite with a low misorientation α/γ interface is greatly inhibited. The ferrites nucleated at the triple junctions of the initial austenite grains present different growth scenarios while assigning different orientation relationships. Finally, the simulated ferrite morphologies in a polycrystalline structure are compared with the optical micrograph and are found that they are in good consistence with each other. This MPF model can replicate the morphology diversity of the ferrite grains in the austenite-to-ferrite transformation.
GEOPHYSICS, ASTRONOMY, AND ASTROPHYSICS
Application of moving cut data-wavelet transformation analysis in dynamic structure mutation testing
2017, 66 (7): 079201.
doi: 10.7498/aps.66.079201
Abstract +
The scaling exponent is an effective nonlinear dynamic index, which can be used to detect the dynamic structure mutations of the correlation time series by the moving cut a fixed window technology. The immediacy and accuracy of scaling exponent is very important for detecting the series change points, however, some of the existing scale index calculation methods (such as rescaled range analysis and rescaled variance analysis) take none of these into account. Wavelet transform analysis can quickly decompose the sequence on different scales, and then the scaling index can be calculated by analyzing the scaling relation of wavelet coefficients on different scales, which has the characteristics of fast calculation speed and good convergence and memory saving. By moving cut window technology, in the present paper we put forward a new method, i. e., the moving cut data-wavelet transformation for detecting a series of dynamic structure mutations. The principle is that the removal of the data has little effect on the estimation of the scaling exponents of the correlation time series with the same dynamical properties. In order to test the performance of the method, first of all, the dynamic structure mutation analyses of linear ideal time series and nonlinear ideal time series are carried out by selecting different moving cut fixed windows. The test results show that the method can quickly and accurately detect the dynamic structure change points and intervals both in linear time series and nonlinear time series, besides, its calculation speed is obviously better than the moving cut data-rescaled range analysis and the moving cut data-rescaled variance analysis. It has strong stability, and depends less on the moving cut window length, which will have some advantages in the large data processing. At the same time, in order to detect the influence of noise on the method, the linear and nonlinear ideal time series are added to the white Gaussian noise (SNR=20, 25, 30 dB), respectively, and the results show that the method has a strong anti-noise ability with different moving cut window lengths, can still quickly and accurately detect the mutation point or interval in different noise additions. Finally, the method is used to detect the dynamic structure mutation of measured daily maximum temperature data of Foping station in Wei basin, the experimental results indicate that the mutation interval is consistent with the abrupt change in 1970's on a global scale, which further verifies the validity of the method.
2017, 66 (7): 079401.
doi: 10.7498/aps.66.079401
Abstract +
China seismoelectromagnetic satellite (CSES) is the first space-based platform of three-dimensional earthquake monitoring system in china. Plasma analyzing package (PAP) is one of scientific payloads aboard on CSES, which is the first application to the field of Chinese satellite. The main scientific objective of PAP is to measure ion density, ion temperature, ion composition and ion drift velocities (parallel and perpendicular to the direction of the satellite flight on orbit), and ion density fluctuation in the ionosphere. The PAP payload is composed of four parts, including retarding potential analyzer (RPA), ion drift meter, ion capture meter, and metal plate. The RPA is used to test ion density, ion temperature, ion composition and ion drift velocity (parallel to the direction of the satellite flight on orbit).
The detailed design process of the sensor of RPA is described in this paper. The grid of RPA sensor is made of Beryllium copper, plated with gold to ensure uniform surface work function, and to prevent space atomic oxygen from eroding. The design value of the transmission rate of grid is 82.64%1.4%, with taking into account the Debye length on the orbit of satellite. And the total transmission rate of multilayer grid for RPA is verified by SIMON simulation experiment. To ensure the accuracy of RPA, the radii of sensor window and sensor collector are designed to be 20 mm and 50 mm respectively, and the height of sensor is 20 mm. The sweeping voltage ranges from -2 V to 20 V. And the step of sweeping voltage is adjustable between 0.056 and. 179 V.
In this paper electronic design of RPA is also discussed. The electronic box is composed of preamplifier circuit, digital logical circuit, satellite interface circuit, etc. The sweeping voltage and the signal acquisition are controlled by field-programmable gate array. The design of three measurement ranges in digital logical circuit improves RPA measurement accuracy, which is better than 1.3% in the full range.
In addition, the method of testing the plasma environment and the testing results are introduced. The plasma environment test of the RPA payload is carried out in INAF-IAPS. The performance of ion density measurement is validated by testing different ion densities in a vacuum tank, through changing three different distances between RPA and the plasma source. And the ion drift velocity measurement is verified by testing three different ion energies of the plasma source. Furthermore, the RPA test data are consistent with the test data from the INAF-IAPS reference RPA. The experimental results show that the detector has good performances and will contribute much to monitoring the space plasma parameters.
NUCLEAR PHYSICS
2017, 66 (7): 072901.
doi: 10.7498/aps.66.072901
Abstract +
A two-dimensional multi-wire proportional chamber detector based on 3He gas is developed for meeting the multifunctional reflection spectrum detection requirements of China Spallation Neutron Source (CSNS). Based on the previous researches in our laboratory, three different wire structures are studied for optimizing the wire readout structure of the two-dimensional multi-wire proportional chamber detector, and the performances of the detectors are measured by two readout methods:the center of gravity readout method and the digital readout method in this paper. The selected method could satisfy the requirement of multifunctional reflection spectrum instrument. Finally, the results indicate that the position resolution and the imaging capability obtained by using the center of gravity readout method should be better than by using the digital readout method. The position resolution could reach to about 160 m by using the center of gravity readout method. While the position resolution of the detector could be obtained to be about 400 m by using the digital readout method. Re-designed and compared with each other are the three different wire structures:1.5 mm of the anode wire pitch and 4 mm of the readout strip pitch with the center of gravity readout method, 1.5 mm of the anode wire pitch and 2 mm of the readout strip pitch with the digital readout method. Both of the optimized designs of the wire structure could meet the requirement of the position resolution for the reflection spectrum device.
2017, 66 (7): 072902.
doi: 10.7498/aps.66.072902
Abstract +
The new type of micro-pattern gaseous detector (MPGD) like the gas electron multiplier (GEM), features the advantage of good spatial resolution ( 100 m). However, abundant and high density electronic channels are needed to obtain the high spatial resolution, which will lead to a great pressure on the detector construction, power consumption, spatial utilization, etc. The resistive anode readout method can help to obtain a good spatial resolution comparable to the pixel readout structure with an enormous reduction of the electronic channels. By using the thick film resistor technology, a new type of resistive structure, composed of high resistive square pad array with low resistive narrow border strips, is developed and applied to the readout anode of the triple GEM detector. For the resistive anode readout board used in the experiment, there are 66 resistive cells, which means that the detector needs only 49 electronics channels. To obtain a good spatial resolution, the cell size is set to be 6 mm6 mm. The surface resistivity of the pads and the strips are 150 k/□ and 1 k/□, respectively. The performances of the detector, especially the two-dimensional imaging performance, are studied by using a 55Fe (5.9 keV) source and an X ray tube (8 keV). The test results show that the spatial resolution of the detector is better than 80 m () by using the imaging of a 40 m wide slot, and the nonlinearity is better than 1.5% by the scanning along the x-axis of the readout board in the steps of 1 mm. Furthermore, quite a good two-dimensional imaging capability is achieved by the detector. These good performances of the detector show the feasibility of the resistive anode readout method for the GEM detector with large area and other detectors with similar structures in the two-dimensional imaging applications.
INTERDISCIPLINARY PHYSICS AND RELATED AREAS OF SCIENCE AND TECHNOLOGY
2017, 66 (7): 078401.
doi: 10.7498/aps.66.078401
Abstract +
Electronic system and device are vulnerable under intensive electromagnetic pulse (EMP) environment, where low noise amplifer (LNA) is a typical sensitive instance for electromagnetic energy. This work focuses on the EMP-induced damage effect of GaAs pseudomorphic high electron mobility transistor (PHEMT), which is the core part of LNA. Using the simulation softeware Sentaurus TCAD, an EMP-induced damage model of the GaAs PHEMT is established in this paper, and verified through the experimental result. It is shown that the damage position of the device under the injection EMP exists in the center area under gate terminal. Based on this model and aiming at EMP parameters and external resistances, the influence of the external conditions on the damage effect of the device is investigated. The results indicate that the damage time is related to EMP parameters obviously:1) the damage time is inversely proportional to EMP amplitude since higher power density is absorbed under a stronger EMP; 2) the damage time is in direct proportion to signal rising time since the breakdown time is postponed under EMP with a slower rising edge. Furthermore, it is found that a load resistor is able to weaken current channel which is effective in delaying the damage process, and this effect is more obvious, with load resistor connected with source terminal. It should be noted that the results are beneficial to and valuable in hardening method against EMP of semiconductor devices. It is feasible to design external circuit protection units, aiming at attenuating signal amplitude and increasing the rising time of injected pulse. Another effectual approach is to enlarge the source series resistance under the premise of the performance meeting the requirements.
