Atmospheric optics and oceanic optics
and Changhui Rao
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Our adaptive optics system based on a non-modulation pyramid wavefront sensor is integrated into a 1.8 m astronomical telescope installed at the Yunnan Observatory in LiJiang, and the first light with high-resolution imaging of an astronomical star is successfully achieved. In this Letter, the structure and performance of this system are introduced briefly, and then the observation results of star imaging are reported to show that the angular resolution of an adaptive optics system using a non-modulation pyramid wavefront sensor can approach the diffraction limit quality of a 1.8 m telescope.
PDF全文 (下载：338) Chinese Optics Letters ，2016年第14卷第10期 pp.100101
and Changhui Rao
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A prototype of a solar ground-layer adaptive optics (GLAO) system, which consists of a multi-direction correlating Shack–Hartmann wavefront sensor with 30 effective subapertures and about a 1 arcmin field of view (FoV) in each subaperture, a deformable mirror with 151 actuators conjugated to the telescope entrance pupil, and a custom-built real-time controller based on field-programmable gate array and multi-core digital signal processor (DSP), is implemented at the 1 m New Vacuum Solar Telescope at Fuxian Solar Observatory and saw its first light on January 12th, 2016. The on-sky observational results show that the solar image is apparently improved in the whole FoV over 1 arcmin with the GLAO correction.
PDF全文 (下载：212) Chinese Optics Letters ，2016年第14卷第10期 pp.100102
Diffraction and gratings
and Shaojun Fu
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To reduce the cost and achieve high diffraction efficiency, a modified moiré technique for fabricating a large-aperture multi-level Fresnel membrane optic by a novel design of alignment marks is proposed. The modified moiré fringes vary more sensitively with the actual misalignment. Hence, the alignment accuracy is significantly improved. Using the proposed method, a 20 μm thick, four-level Fresnel diffractive polyimide membrane optic with a 200 mm diameter is made, which exhibits over 62% diffraction efficiency into the +1 order, and an efficiency root mean square of 0.051.
PDF全文 (下载：174) Chinese Optics Letters ，2016年第14卷第10期 pp.100501
Fiber optics and optical communication
and Xiaoguang Zhang
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We propose a modified-Viterbi and Viterbi phase estimation (VVPE) carrier phase recovery scheme that shows an effective capability of reducing the frequent and accumulated cycle slips induced by inter-symbol interference (ISI) in a faster-than-Nyquist (FTN) optical coherent communications system. In a 28-Gbaud FTN polarization-division multiplexed quadrature phase-shift keying optical communication system, the comparison of the proposed modified-VVPE scheme and the conventional VVPE scheme is carried out. It is proved that the proposed modified-VVPE scheme can effectively overcome the challenge of ISI induced error carrier phase estimation, which leads to a better bit error ratio performance.
PDF全文 (下载：179) Chinese Optics Letters ，2016年第14卷第10期 pp.100601
and Tingyun Wang
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Two kinds of Nb-doped silica fibers, an NbCl5-doped fiber and an Nb2O5-doped fiber, are fabricated and characterized in this Letter. First, the refractive index profiles of both fibers are obtained, and then their Raman spectra are measured with 785 nm exciting light. The Nb-doped fibers’ Raman spectra are compared with a conventional GeO2-doped single-mode silica fiber that is prepared with the same method and under the same conditions. As a result, the Raman gain coefficients of the Nb-doped silica fiber core are obtained. The experimental results show that Nb2O5 doping can enhance the Raman scattering intensity of the optical fibers.
PDF全文 (下载：158) Chinese Optics Letters ，2016年第14卷第10期 pp.100602
and Jing Li
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In a D-shaped twin-core fiber (DTCF), the central core is insensitive to the variation of the external environment, while the other core is highly sensitive. As an electro-optic polymer coated on a DTCF, the coupling between the two cores varies with voltages applied to the polymer. Based on this, a superior all-fiber modulator is proposed that bears little coupling loss, prohibits mode mismatch, and provides a more stable working circumstance. A half-wave driving voltage (Vπ) of 1.26 V is achieved. Moreover, a high modulation depth of 40 dB can be realized for a voltage of 2.7 V at a 1550 nm wavelength.
PDF全文 (下载：244) Chinese Optics Letters ，2016年第14卷第10期 pp.100603
and Jason P. Jue
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In this Letter, we propose two crosstalk-aware routing, core, and spectrum assignment (CA-RCSA) algorithms for spatial division multiplexing enabled elastic optical networks (SDM-EONs) with multi-core fibers. First, the RCSA problem is modeled, and then a metric, i.e., CA spectrum compactness (CASC), is designed to measure the spectrum status in SDM-EONs. Based on CASC, we propose two CA-RCSA algorithms, the first-fit (FF) CASC algorithm and the random-fit (RF) CASC algorithm. Simulation results show that our proposed algorithms can achieve better performance than the baseline algorithm in terms of blocking probability and spectrum utilization, with FF-CASC providing the best performance.
PDF全文 (下载：134) Chinese Optics Letters ，2016年第14卷第10期 pp.100604
and Guofan Jin
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In this Letter, we propose a novel configuration and design method of freeform, dual fields-of-view (FOVs), dual focal lengths, off-axis three-mirror zoom imaging systems. The switch of the two zooms is achieved by rotating a single mirror element. The design of a freeform, dual focal lengths zoom system is realized by a point-by-point design approach for the first time to our knowledge. This method enables the direct design of freeform surfaces from initial planes using given system specifications and configuration, and the designed system can be taken as a good starting point for further optimization. A freeform, dual FOVs, dual focal lengths, off-axis three-mirror zoom system is demonstrated. The F-numbers of the two zooms are 2 and 2.4. The dual FOVs are 3°×3° and 2.5°×2.5°. After final optimization, both of the zooms achieve high performances.
PDF全文 (下载：134) Chinese Optics Letters ，2016年第14卷第10期 pp.100801
P. W. M. Tsang
Y. T. Chow
and T.-C. Poon
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Noise addition is a simple but effective method for generating a phase-only hologram (POH) of an object. Briefly, the intensity image of an object is added with random phase noise and converted into a digital Fresnel hologram. Subsequently, the phase component of the hologram is retained as the POH. Although the method is fast and the visual quality of the reconstructed image is acceptable, the edges and lines patterns are heavily fragmented. In this Letter, we propose a method to overcome this problem. An experimental evaluation based on numerical and optical reconstructions reveals that a hologram generated by our proposed method is capable of preserving line patterns with favorable quality.
PDF全文 (下载：162) Chinese Optics Letters ，2016年第14卷第10期 pp.100901
and Xiaowei Sun
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A practical millimeter-wave (MMW) holographic imaging system with good robustness is developed for the detection of concealed weapons at security checkpoints, especially at the airport. The system is used to scan the passenger and detect any weapons hidden in their clothes. To reconstruct the three dimensional image, a holographic MMW imaging algorithm based on aperture synthesis and backscattering is presented. The system is active and works at 28–33 GHz. As a practical imaging system, the robustness is analyzed in detail in terms of the peak signal-to-noise ratio.
PDF全文 (下载：137) Chinese Optics Letters ，2016年第14卷第10期 pp.101101
and Yun Feng
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Noise and the resonance characteristics of the focal plane array (FPA) are the most important factors that affect the performance of the optical readout infrared (IR) FPA imaging system. This Letter presents a time-discrete modulation technology that eliminates the background and restrain noise, which effectively improves the image quality of the optical readout IR FPA imaging system. The comparative experiments show that this technology can reduce the noise equivalent temperature difference greatly and make the images sharper. Moreover, when the imaging system is influenced by the environment vibration, the images obtained from the imaging system with time-discrete modulation restore twice as fast as without time-discrete modulation.
PDF全文 (下载：128) Chinese Optics Letters ，2016年第14卷第10期 pp.101102
and Xu Wang
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A 38.88 MHz time-stretch line-scan imaging system with parallel interleaving detection is experimentally demonstrated. Since only half-chromatic dispersion is used to stretch optical pulses for wavelength-to-time mapping, the power efficiency is significantly improved by 6.5 dB. Furthermore, the theoretical analysis indicates that the power loss can be efficiently reduced for scan rates less than 100 MHz. In addition, a mathematical model for signal-to-noise evaluation is derived, including amplified spontaneous emission noise in the power compensation. Thanks to the improvement of the power efficiency by using parallel interleaving detection, the signal quality is enhanced.
PDF全文 (下载：114) Chinese Optics Letters ，2016年第14卷第10期 pp.101103
and Shengli Chang
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An integrated, tunable spectrometer based on a silicon-on-sapphire platform is designed at wavelengths of 2.29–2.35 μm. Its pivotal component is a 4.7 μm-radius ring resonator on a graphene monolayer. Its full width at half-maximum and free spectral range are ～1.5 and ～45 nm, respectively, as found through a numerical simulation and theoretical computation. Sixteen characteristic peaks are obtained by tuning the Fermi level of graphene. The gap between the ring and waveguides is increased by 0.5 μm to increase the resolution, and though this can drastically reduce the transmission rate, an upper sapphire layer maintains light to the drop port.
PDF全文 (下载：114) Chinese Optics Letters ，2016年第14卷第10期 pp.101301
Lasers and laser optics
and Longsheng Ma
The length stability of optical cavities is vital in ultra-stable, cavity-stabilized laser systems. Using finite element analysis, we study the length deviation of optical cavities due to thermal expansion and thermo-refractive effects when the incident light power is changed. The simulated fractional length sensitivity of a 7.75-cm-long football cavity to the power fluctuation of incident light is 5×10?14/μW, which is in agreement with the experimental results found by measuring the frequency change of a cavity-stabilized laser when the incident light power is changed. Based on the simulation, the cavity sensitivity to light power fluctuation is found to depend on the cavity size and material.
PDF全文 (下载：160) Chinese Optics Letters ，2016年第14卷第10期 pp.101401
and Yongkun Ding
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Intensive electromagnetic pulses (EMPs) can be generated from interaction of the ultra-intense lasers and solid targets in inertial confinement fusion (ICF), which will detrimentally affect the data acquisition from some electric components. A diagnostic system for EMP measurement inside and outside the ShenGuang-III facility is designed and fabricated in this study. The experimental results indicate that the peak magnitude of EMP reaches up to 3210.7 kV/m and 6.02 T. The received signals depend most on the antenna and target types. The half-hohlraum generates a more intensive EMP radiation than that from the other targets, and the large planar and medium discone capture much stronger signals than the other antennas. In addition, the mechanisms of EMP generation from different targets are discussed. The resulting conclusion are expected to provide the experimental basis for further EMP shielding design.
PDF全文 (下载：161) Chinese Optics Letters ，2016年第14卷第10期 pp.101402
and Yuefeng Ji
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Multi-robot coordination (MRC) is a key challenge for complex artificial intelligence systems, and conventional wireless-communication-based MRC mechanisms that cannot be deployed in radio-frequency-limited environments. In this Letter, we present a promising solution that utilizes indoor omni-directional visible light communication (VLC) technology to realize efficient multi-robot intelligent coordination (MRIC). The specific design is presented along with the implemental details of a practical MRIC experimental platform. The experimental results show that a 50 Mb/s on-off-keying-based omni-directional VLC can be realized with an effective distance of 2.3 m and a bit error rate of <10?6 in the proposed MRIC platform.
PDF全文 (下载：149) Chinese Optics Letters ，2016年第14卷第10期 pp.102301
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Surface stabilized (anti) ferroelectric liquid crystal cells can be used as an optically addressed media for optical data processing. The structure of the cell has to contain a photo sensible agent, i.e., an absorbing dye-doped orienting layer. The all-optical generation of the diffractive grating can be done due to the switching parameters of the smectic slab within cells with a sensitive layer. This Letter considers a study of the optically induced charge generation into the dye-doped layer, and the explanation of the phenomena of the selective molecular director reorientation, while cell driving what leads to the induction of phase grating.
PDF全文 (下载：163) Chinese Optics Letters ，2016年第14卷第10期 pp.102302
and Furi Ling
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We theoretically present a concise and tunable dual-band metamaterial absorber composed of a typical metal-dielectric-metal structure in the terahertz regime. The dual-band absorption originates from two different resonance modes induced in one square ring, which is different from the common dual-band absorber composed of a super unit with several different-sized structures. The proposed absorber can realize dynamic tunability through changing the permittivity of the dielectric layer by applying different temperatures. Other good performances, such as a wide incident angle and polarization insensitivity, are also available for the proposed absorber. Such a metamaterial absorber is a promising candidate for terahertz imaging and detection.
PDF全文 (下载：119) Chinese Optics Letters ，2016年第14卷第10期 pp.102303
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Parallel-coupled dual-racetrack silicon microresonators can potentially be used for quadrature amplitude modulation. We analyze the evolution of the coverage of coherent output states of devices with varying device parameters. As the coupling constant increases, the coverage of coherent states initially improves then degrades, which is unexpected based on a prior preference for overcoupling. Increasing the quality factor generally improves the coverage. The influence of the refractive index modulation is found to saturate after reaching a certain level. Analytic formulas are developed to provide insight into the coverage evolution. These results are fairly robust against a small asymmetry of device parameters.
PDF全文 (下载：120) Chinese Optics Letters ，2016年第14卷第10期 pp.102304