Atomic and molecular physics
and Yuzhu Wang
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We propose a robust scheme that creates a toroidal magnetic potential on a single-layer atom chip. The wire layout consists of two interleaved Archimedean spirals, which avoids the trapping perturbation caused by the input and output ports. By using a rotation bias field, the minimum of the time-averaged orbiting potential is lifted from zero, and then a relatively smooth and harmonic ring trap is formed. The location of the waveguide is immune to the magnetic variations, as it is only determined by the wire layout. The ring waveguide offers an ideal solution to developing a compact and portable atomic gyroscope.
PDF全文 (下载：201) Chinese Optics Letters ，2016年第14卷第7期 pp.070201
and Yuzhu Wang
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A Fourier optics approach can be a concise and powerful tool to solve problems in atom optics. In this report, we adopt it to investigate the kinetic behavior of cold atoms passing through a far red-detuned Gaussian beam. We demonstrate that the aberration has significant influence on the evolution of the atomic cloud, which is rooted in the deviation of the Gaussian profile from the quadratic form. In particular, we observe an intriguing effect analogous to Fresnel’s double prism with cold atoms. The experimental results are in good agreement with the numerical simulation.
PDF全文 (下载：199) Chinese Optics Letters ，2016年第14卷第7期 pp.070202
Coherence optics and statistical optics
and Shensheng Han
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For a Hanbury Brown and Twiss system, the influence of relative motion between the object and the detection plane on the resolution of second-order intensity-correlated imaging is investigated. The analytical results, which are backed up by experiments, demonstrate that the amplitude and mode of the object’s motion have no effect on the second-order intensity-correlated imaging and that high-resolution imaging can be always achieved by using a phase-retrieval method from the diffraction patterns. The use of motion de-blurring imaging for this approach is also discussed.
PDF全文 (下载：117) Chinese Optics Letters ，2016年第14卷第7期 pp.070301
and Lingxue Wang
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A infrared light trapping structure combining front subwavelength gratings and rear ZnO:Al nanoparticles for a PtSi Schottky-barrier detector over a 3–5 μm waveband is theoretically investigated. By selecting the proper plasmonic material and optimizing the parameters for the proposed structure, the absorption of the PtSi layer is dramatically improved. The theoretical results show that this improvement eventually translates into an equivalent external quantum efficiency (EQE) enhancement of 2.46 times at 3–3.6 μm and 2.38 times at 3.6–5 μm compared to conventional structures. This improvement in the EQE mainly lies in the increase of light path lengths within the PtSi layer by the subwavelength grating diffraction and nanoparticle-scattering effects.
PDF全文 (下载：143) Chinese Optics Letters ，2016年第14卷第7期 pp.070401
Diffraction and gratings
and Lianqing Zhu
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A spatially variable retardation device, an SQWP, is designed to generate polarization vortex beams. The transformation of Laguerre–Gaussian beams by the SQWP is further studied, and it is found that the SQWPs can also be used to generate helical beams and measure the topological charges of helical beams.
PDF全文 (下载：186) Chinese Optics Letters ，2016年第14卷第7期 pp.070501
Fiber optics and optical communication
and Yuefeng Ji
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Visible light positioning (VLP) is an emerging candidate for indoor positioning, which can simultaneously meet the requirements for accuracy, cost, coverage area, and security. However, intercell interference caused by light intensity superposition limits the application of VLP. In this Letter, we propose a united block sequence mapping (UBSM)-based VLP that utilizes superposition to integrate the multidimensional information from dense small cells into 2D information. The experimental result shows that UBSM-based VLP can achieve an accuracy of 1.5 cm with a 0.4 m row spacing and 0.35 m column spacing of LED lights.
PDF全文 (下载：118) Chinese Optics Letters ，2016年第14卷第7期 pp.070601
and Shensheng Han
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Based on dense wavelength-division multiplexing technology, frequency transfer and time synchronization are simultaneously realized over a compensated cascaded fiber link of 430 km, which is a part of the Beijing–Shanghai optical fiber backbone network. The entire cascaded system consists of two stages with fiber links of 280 and 150 km, respectively. To keep high symmetry and low noise, specific bi-directional erbium-doped fiber amplifiers are used to compensate the large optical attenuation of each fiber link. When the compensation servo is active in every stage, the cascaded system achieves the stability of 1.94×10-13 at 1 s and 1.34×10-16 at 104 s, for frequency transfer. It is also verified that the actual results of the cascaded system are in good agreement with the theoretical ones calculated from error theory. Simultaneously, after calibration of each stage, time synchronization is also realized. The final accuracy of the whole system is within 94 ps.
PDF全文 (下载：109) Chinese Optics Letters ，2016年第14卷第7期 pp.070602
P. W. M. Tsang
and Y. S. Kim
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Past research has demonstrated that a static, three-dimensional (3D) object scene can be directly recorded as a complex digital hologram. However, numerical reconstruction of the object scene, which may comprise multiple sections located at unknown distances from the hologram, is a complicated and computation-intensive process. To the best of our knowledge, we propose, for the first time, a low complexity method that is capable of reconstructing a complex hologram, such that sections at different depths in the 3D object scene can be automatically reconstructed at the correct focal distances and merged into a single image for an extended depth of field. We demonstrate an order of magnitude increase of the depth of field for binary objects. With the use of a graphical processing unit, the reconstruction of a 512×512 complex hologram can be accomplished in about 100 ms, equivalent to around 10 frames per second.
PDF全文 (下载：106) Chinese Optics Letters ，2016年第14卷第7期 pp.070901
Vidal F. Canales
Pedro J. Valle
and Manuel P. Cagigal
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Several pupil filtering techniques have been developed in the last few years to obtain transverse superresolution (a narrower point spread function core). Such a core decrease entails two relevant limitations: a decrease of the peak intensity and an increase of the sidelobe intensity. Here, we calculate the Strehl ratio as a function of the core size for the most used binary phase filters. Furthermore, we show that this relation approaches the fundamental limit of the attainable Strehl ratio at the focal plane for any filter. Finally, we show the calculation of the peak-to-sidelobe ratio in order to check the system viability in every application.
PDF全文 (下载：119) Chinese Optics Letters ，2016年第14卷第7期 pp.071101
Instrumentation, measurement and metrology
and Peiheng Wu
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We demonstrate laser-ranging results for non-cooperative targets at ranges of 237 m and 19 km using superconducting nanowire single-photon detectors (SSPD). We upgrade the kilohertz rate laser-ranging system with a newly developed SSPD module, and the equivalent detection diameter is enlarged to 50 μm with a fiber and micro-lenses. Both retroreflectors and non-cooperative surfaces of aluminum foil, a solar panel, and a concrete panel at distances of 237 m and 19 km, whose echoes are of single-photon level, are ranged with sub-centimeter precision. Experimental signal-to-noise ratio curves with the product of quantum efficiency and system transmittance are obtained, which indicates that our system, with an average laser power of 0.8 W and a receiving aperture of 1.2 m, may be capable for space debris ranging at a distance of 800 km. This work suggests that SSPDs have the potential to be used for space debris surveillance.
PDF全文 (下载：172) Chinese Optics Letters ，2016年第14卷第7期 pp.071201
and Zhitao Zhuang
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In this study, an improved phase-shifting diffraction interferometer for measuring the surface topography of a microsphere is developed. A common diode-pumped solid state laser is used as the light source to facilitate apparatus realization, and a new polarized optical arrangement is designed to filter the bias light for phase-shifting control. A pinhole diffraction self-calibration method is proposed to eliminate systematic errors introduced by optical elements. The system has an adjustable signal contrast and is suitable for testing the surface with low reflectivity. Finally, a spherical ruby probe of a coordinate measuring machine is used as an example tested by the new phase-shifting diffraction interferometer system and the WYKO scanning white light interferometer for experimental comparison. The measured region presents consistent overall topography features, and the resulting peak-to-valley value of 84.43 nm and RMS value of 18.41 nm are achieved. The average roughness coincides with the manufacturer’s specification value.
PDF全文 (下载：127) Chinese Optics Letters ，2016年第14卷第7期 pp.071202
Suhas P. Veetil
and Jianqiang Zhu
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We propose a lens-free coherent modulation imaging (CMI) method for reconstructing a general complex-valued wave field from a single frame of a diffraction pattern. A numerical Fourier transform is introduced in the iterative reconstruction process to replace the lens or zone plate used in the current CMI technique to adopt the constraint on the Fourier components of the exit wave field of the sample. While the complexity of the experimental setup is remarkably reduced by replacing the zone plate and additional accessories with the numerical processing, the energy fluence loss induced by the undesired diffraction orders of the zone plate can be also avoided. The feasibility of the proposed technique is verified experimentally with visible light.
PDF全文 (下载：101) Chinese Optics Letters ，2016年第14卷第7期 pp.071203
Lasers and laser optics
and Weibiao Chen
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Based on the modified ramp and fire technique, a novel injection seeding approach with real-time resonance tracking is successfully demonstrated in a single-frequency Nd:YAG pulsed laser. Appling a high-frequency sinusoidal modulation voltage to one piezo actuator and an adjustable DC voltage to another piezo actuator for active feedback, single-mode laser output with high-frequency stability is obtained, and the effect of the piezo hysteresis on the frequency stability can be eliminated for a laser diode pumped Q-switched Nd:YAG laser at a repetition rate of 400 Hz.
PDF全文 (下载：140) Chinese Optics Letters ，2016年第14卷第7期 pp.071401
and Hui Zhang
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Two types of acousto-optically Q-switched Nd:YVO4/KTA singly resonated optical parametric oscillators are performed. One is signal resonant, where a 1.5 μm wave resonates while a 3.5 μm wave does not. The other is idler resonant, where a 3.5 μm wave resonates while a 1.5 μm wave does not. All the experimental elements are kept the same for these two schemes except for the coatings of the optical parametic oscillator cavity output coupler. For these two kinds of lasers, the output characteristics of the threshold, output power, pulse width, peak power, and beam quality are measured and compared.
PDF全文 (下载：82) Chinese Optics Letters ，2016年第14卷第7期 pp.071402
and Jianping Yin
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We demonstrate a scheme to use a Littman configuration external cavity diode laser (ECDL) as a stable-frequency light source to stabilize two cw single-mode Ti:sapphire lasers for laser cooling of magnesium fluoride molecules. An ECDL based on the Littman configuration is constructed and stabilized by a digital signal processor system. We stabilize the frequency of our ECDL to ±0.77 MHz precision over 10 h and the Allan standard deviation reaches 2.6×10-11 at an integration time of 10 s. We lock two Ti:sapphire lasers through a transfer cavity, and either laser has a long-term frequency stability of ±2.5 MHz.
PDF全文 (下载：122) Chinese Optics Letters ，2016年第14卷第7期 pp.071403
and Jintao Bai
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In this Letter, a single-frequency fiber laser using a molybdenum disulfide (MoS2) thin film as a saturable absorber is demonstrated. We use a short length of highly Yb-doped fiber as the gain medium and a fiber ferrule with MoS2 film adhered to it by index matching gel (IMG) that acts as the saturable absorber. The saturable absorber can be used to discriminate and select the single longitudinal modes. The maximum output power of the single-frequency fiber laser is 15.3 mW at a pump power of 130 mW and the slope efficiency is 15.3%. The optical signal-to-noise ratio and the laser linewidths are ～60 dB and 5.89 kHz, respectively.
PDF全文 (下载：87) Chinese Optics Letters ，2016年第14卷第7期 pp.071404
and Guorong Chen
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Tb3+ and Sn2+ co-doped strontium phosphate glasses are prepared and their unique photoluminescence (PL) properties for deep UV excitation are investigated. With the co-doped Sn2+ ions, Tb3+ keeps the original PL behaviors under near UV excitation while its PL action for deep UV excitation is enhanced tremendously. PL emission and excitation spectra demonstrate the sensitization role of Sn2+ on the Tb3+ emissions for deep UV excitation that is associated with the strong deep UV absorption of Sn2+ for greatly enhancing the resonance of the Tb3+ excitation with the deep UV light source. The decay curves of Sn2+ and Tb3+ emissions for both singly doped and co-doped samples are single exponentially well fitted with almost the same emission lifetime (τ) values in the microsecond and millisecond time regimes, respectively, confirming that Sn2+ and Tb3+ act as an independent activator in the present phosphate glass matrix while an involved energy transfer from Sn2+ to Tb3+ is radiative. Moreover, Sn2+ and Tb3+ can be co-excited with deep UV light to emit tunable light from blue to green with the definite CIE chromaticity coordinate for different applications.
PDF全文 (下载：61) Chinese Optics Letters ，2016年第14卷第7期 pp.071601
Medical optics and biotechnology
and Jimin Liang
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Simplified spherical harmonics approximation (SPN) equations are widely used in modeling light propagation in biological tissues. However, with the increase of order N, its computational burden will severely aggravate. We propose a graphics processing unit (GPU) accelerated framework for SPN equations. Compared with the conventional central processing unit implementation, an increased performance of the GPU framework is obtained with an increase in mesh size, with the best speed-up ratio of 25 among the studied cases. The influence of thread distribution on the performance of the GPU framework is also investigated.
PDF全文 (下载：92) Chinese Optics Letters ，2016年第14卷第7期 pp.071701
and Jianwen Luo
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A reconstruction method guided by early-photon fluorescence yield tomography is proposed for time-domain fluorescence lifetime tomography (FLT) in this study. The method employs the early-arriving photons to reconstruct a fluorescence yield map, which is utilized as a priori information to reconstruct the FLT via all the photons along the temporal-point spread functions. Phantom experiments demonstrate that, compared with the method using all the photons for reconstruction of fluorescence yield and lifetime maps, the proposed method can achieve higher spatial resolution and reduced crosstalk between different targets without sacrificing the quantification accuracy of lifetime and contrast between heterogeneous targets.
PDF全文 (下载：75) Chinese Optics Letters ，2016年第14卷第7期 pp.071702
Optics at surfaces
and Shangqing Gong
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A terahertz (THz) waveguide using a metallic nanoslit whose width is much smaller than the skin depth is analytically investigated. By taking some important physical properties into account, we derive a simple, yet accurate, expression for the effective index. We also study the changes in modal field and the attenuation coefficient in the whole THz region, and find some interesting physical properties. Finally, we verify that these theoretical analyses coincide with the rigorous numerical simulations. This research can be useful for various applications of THz waveguides made of metallic nanoslits.
PDF全文 (下载：132) Chinese Optics Letters ，2016年第14卷第7期 pp.072401
and Pei Wang
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The nonlocal effect on the spontaneous emission of a silver cuboid dimer is investigated using a local analog model. Magnetic as well as electric dipole excitations are introduced to excite different gap modes. The nonlocal response of electric and magnetic modes on various parameters of gap (width and refractive index) are investigated. Unidirectional radiation is achieved by the interaction between electric and magnetic modes in both local and nonlocal models. Compared to local simulations, the resonant wavelength is blue shifted and the spontaneous emission enhancement is weakened in the nonlocal model. The relative shifts of the resonant wavelengths get larger in smaller gaps with a higher refractive index.
PDF全文 (下载：101) Chinese Optics Letters ，2016年第14卷第7期 pp.072501
and Fangwen Sun
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The measurement of the second-order degree of coherence [g(2)(τ)] is one of the important methods used to study the dynamical evolution of photon-matter interaction systems. Here, we use a nitrogen-vacancy center in a diamond to compare the measurement of g(2)(τ) with two methods. One is the prototype measurement process with a tunable delay. The other is a start-stop process based on the time-to-amplitude conversion (TAC) and multichannel analyzer (MCA) system, which is usually applied to achieve efficient measurements. The divergence in the measurement results is observed when the delay time is comparable with the mean interval time between two neighboring detected photons. Moreover, a correction function is presented to correct the results from the TAC-MCA system to the genuine g(2)(τ). Such a correction method will provide a way to study the dynamics in photonic systems for quantum information techniques.
PDF全文 (下载：102) Chinese Optics Letters ，2016年第14卷第7期 pp.072701
and Zhanshan Wang
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The Pd/B4C multilayer is a promising candidate for high reflectance mirrors operating in the 8–12 nm extreme ultraviolet wavelength region. To extend the working bandwidth beyond the L-edge of silcon, we theoretically design broadband Pd/B4C multilayers. We discuss the influence of the desired reflectance of the plateau, number of bilayers, and the real structural parameters, including the interface widths, layer density, and thickness deviation, on the reflectivity profile. Assuming the interface width to be 0.6 nm, we design aperiodic multilayers for broad wavebands of 9.0–10.0, 8.5–10.5, and 8.0–11.0 nm, with average reflectivities of 3.1%, 5.0%, and 9.5%, respectively.
PDF全文 (下载：141) Chinese Optics Letters ，2016年第14卷第7期 pp.073101
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The vacuum-sealed miniature modulated x-ray source (VMMXS) with a hot cathode is fabricated via the single-step brazing process in a vacuum furnace. An experiment following the VMMXS is implemented to present its performances, including the influence of grid electrode potential on x-ray intensities. The modulation type of the grid electrode as a switch is proposed, and its feasibility is successfully demonstrated. It is noteworthy to discover a phenomenon for the first time, to the best of our knowledge, that the high repetition frequency grid pulse of the VMMXS has a significant effect on the x-ray intensity. The probable cause for this new finding is analyzed.
PDF全文 (下载：153) Chinese Optics Letters ，2016年第14卷第7期 pp.073401