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Microwave photonic components and subsystems can replace or complement their electronic counterparts with a net gain in functionality, bandwidth, size, mass, complexity, and cost, facilitating the innovative implementation of radio frequency (RF) systems due to broad bandwidth, low loss, light weight, flat frequency response, favorable isolation, and immunity to electromagnetic interference (EMI) provided by photonic technologies. Much attention has been recently paid to this area, which results in impressive progresses. Hence, we designed a focus issue intended to introduce the recent advancements in this field, especially the works by some distinguished research groups.
PDF全文 (下载：66) Chinese Optics Letters ，2017年第15卷第1期 pp.010001
and Kun Xu
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We propose a high-Q photonic-electronic hybrid cavity for single-longitudinal-mode narrow-linewidth oscillation, where part of the cavity is in the radio frequency (RF) domain by a pair of frequency conversions. In the RF part, we can easily achieve MHz filtering and a large delay by inserting an electronic filter. In mathematics, we prove that the frequency conversion pair and electronic filter in between can be equivalent to a high-Q optical filter cascaded low-noise optical amplifier as a whole. Finally, the 20-dB bandwidth of oscillation is 1/20 of that of an optical local oscillator, and the maximum phase noise suppression can reach 65 dB.
PDF全文 (下载：50) Chinese Optics Letters ，2017年第15卷第1期 pp.010010
and Kun Xu
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In this Letter, we experimentally demonstrate a full-duplex transmission system of IEEE 802.11ac-compliant multiple-input multiple-output (MIMO) signals over a 2-km 7-core fiber for in-building wireless local-area network (WLAN) distributed antenna systems. For full-duplex 3×3 MIMO demonstration, the crosstalk impacts of both fiber-transmission-only and optic-wireless transmission situation are evaluated. The results indicate that the impact of crosstalk on radio-over-fiber (ROF) link performance is not significant and the quality of the cascaded multi-core fiber and wireless channel is mainly determined by the wireless part. To further improve the system capacity, polarization multiplexing (PolMux) technology is employed to achieve a full-duplex 6×6 MIMO over a single 7-core fiber. Although employing the PolMux method will slightly decrease the EVM and condition number performance as opposed to a non-PolMux MCF system, it is still a competitive solution in large optical connection demand scenarios that require a low cost.
PDF全文 (下载：51) Chinese Optics Letters ，2017年第15卷第1期 pp.010011
and Shihong Xie
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Compressive sampling (CS) has attracted considerable attention in microwave and radio frequency (RF) fields in recent years. It enables the acquisition of high-frequency signals at a rate much smaller than their Nyquist rates. Combined with photonics technology, traditional CS systems can significantly enlarge their operating bandwidth, which offers great potential for spectrum sensing in cognitive radios. In this Letter, we review our recent work on photonic CS systems for wideband spectrum sensing. First, a proof-of-concept photonics-assisted CS system is demonstrated; it is capable of acquiring numerous radar pulses in an instantaneous bandwidth spanning from 500 MHz to 5 GHz with a 500-MHz analog-to-digital converter (ADC). To further reduce the acquisition bandwidth, multi-channel photonics-assisted CS systems are proposed for the first time, enabling the acquisition of multi-tone signals with frequencies up to 5 GHz by using 120-MHz ADCs. In addition, the system bandwidth is increased from 5 to 20 GHz by employing time-interleaved optical sampling.
PDF全文 (下载：39) Chinese Optics Letters ，2017年第15卷第1期 pp.010012
and Shilong Pan
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A wideband-generalized pattern multiplication approach to evaluate the performance of an optical beamforming-based wideband antenna array is proposed and experimentally demonstrated, which enables the far-field measurement of a large wideband array with a small anechoic chamber. Because the optimum reception of a wideband microwave signal is highly related to the time-domain distortions of the beamforming system, a correlation-receiver-based radiation pattern is applied to take the fidelity of the wideband signals into account. A four-element optical beamforming system is built to verify the feasibility of the proposed method. The results achieved by the proposed method agree well with the conventional direct measurements.
PDF全文 (下载：53) Chinese Optics Letters ，2017年第15卷第1期 pp.010013
and Kun Xu
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A full-band direct-conversion receiver using a microwave photonic in-phase and quadrature (I/Q) mixer is proposed and experimentally evaluated in terms of radio frequency (RF) range, port isolation, phase imbalance, conversion gain, noise figure, spurious-free dynamic range, and error vector magnitude. The proposed microwave photonic I/Q mixer shows significant advantages in local oscillator leakage and I/Q phase imbalance over entire RF bands, which are recognized as major drawbacks of conventional direct-conversion receivers.
PDF全文 (下载：46) Chinese Optics Letters ，2017年第15卷第1期 pp.010014
Fiber optics and optical communication
Md. Ziaul Amin
Khurram Karim Qureshi
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To design a compact erbium-doped fiber laser, a high-concentration erbium-doped fiber (EDF) is needed. However, increasing the erbium ion (Er3+) concentration can reduce the EDF performance via the Er3+-Er3+ interaction. In this Letter, we investigate the Er3+-Er3+ interaction effect by designing a tunable erbium-doped fiber-ring laser (EDFRL). This is the first time (to the best of our knowledge) that someone has considered different numbers of ions per cluster and simulated the EDFRL output power degradation due to ion–ion interaction. If the number of ions in the cluster is increased, the lasing output power will decrease accordingly. The most dominant effect is seen in the 1530 nm wavelength region, where the EDF shows a higher signal absorption compared to the other wavelength region. Moreover, a comparison has been done for lasing performance analysis with different dopant ion concentrations. The comparison results show that a higher dopant concentration is advantageous for longer-wavelength lasing.
PDF全文 (下载：48) Chinese Optics Letters ，2017年第15卷第1期 pp.010601
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This Review reviews semiconductor lasers with an emphasis on high-speed information technologies. Semiconductor lasers are classified based on their applications in optical communications. Different types of semiconductor lasers are discussed in terms of principle, history, advantages, and limitations.
PDF全文 (下载：99) Chinese Optics Letters ，2017年第15卷第1期 pp.010002
M. El Gibari
A. De Rossi
H. W. Li
and A. S. Daryoush
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This Letter introduces the design and simulation of a microstrip-line-based electro-optic (EO) polymer optical phase modulator (PM) that is further enhanced by the addition of photonic crystal (PhC) structures that are in close proximity to the optical core. The slow-wave PhC structure is designed for two different material configurations and placed in the modulator as a superstrate to the optical core; simulation results are depicted for both 1D and 2D PhC structures. The PM characteristics are modeled using a combination of the finite element method and the optical beam propagation method in both the RF and optical domains, respectively. The phase-shift simulation results show a factor of 1.7 increase in an effective EO coefficient (120 pm/V) while maintaining a broadband bandwidth of 40 GHz.
PDF全文 (下载：34) Chinese Optics Letters ，2017年第15卷第1期 pp.010003
Lawrence R. Chen
We provide an overview of our recent work on developing subwavelength grating (SWG) waveguide devices as an enabling technology for integrated microwave photonics. First, we describe wavelength-selective SWG waveguide filters, including ring resonators, Bragg gratings, and contradirectional couplers. Second, we discuss the development of an index variable optical true time delay line that exploits spatial diversity in an equal-length waveguide array. These SWG waveguide components are fundamental building blocks for realizing more complex structures for advanced microwave photonic signal processing.
PDF全文 (下载：31) Chinese Optics Letters ，2017年第15卷第1期 pp.010004
and Ninghua Zhu
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We review the recent work of distributed-feedback (DFB) multi-wavelength semiconductor laser arrays (MWLAs) based on the reconstruction equivalent chirp (REC) technique. The experimental results show that the proposed MWLA has very high wavelength precision (<±0.1 nm), while the fabrication cost is low. Only one step of holographic exposure and another step of photolithography are required for grating fabrication. The packaging technique for a high-bandwidth analog DFB laser and laser array was developed. A directly modulated MWLA transmitter module was achieved. In addition, an improved MWLA with an integrated reflector was proposed and successfully applied in a radio-over-fiber system.
PDF全文 (下载：44) Chinese Optics Letters ，2017年第15卷第1期 pp.010005
and Guohui Yuan
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Optical biosensors with a high sensitivity and a low detection limit play a highly significant role in extensive scenarios related to our daily life. Combined with a specific numerical simulation based on the transfer matrix and resonance condition, the idea of novel single-waveguide-based microresonators with a double-spiral-racetrack (DSR) shape is proposed and their geometry optimizations and sensing characteristics are also investigated based on the Vernier effect. The devices show good sensing performances, such as a high quality factor of 1.23×105, a wide wavelength range of over 120 nm, a high extinction ratio (ER) over 62.1 dB, a high sensitivity of 698.5 nm/RIU, and a low detection limit of 1.8×10?5. Furthermore, single-waveguide-based resonators can also be built by cascading two DSR structures in series, called twin-DSRs, and the results show that the sensing properties are enhanced in terms of quasi free spectral range (FSR) and ER due to the double Vernier effect. Excellent features indicate that our novel single-waveguide-based resonators have the potential for future compact and highly integrated biosensors.
PDF全文 (下载：42) Chinese Optics Letters ，2017年第15卷第1期 pp.010006
and Ninghua Zhu
We review the recent progress of photonic generation of millimeter wave (MMW)-ultra-wideband (UWB) signals. To fully satisfy the standard defined by the Federal Communications Commission (FCC), the baseband signal (background signal) and the residual local oscillator (LO) signal should be well controlled. We discuss several schemes in this work for generating background-free MMW-UWB signals that are fully compliant with the FCC requirement.
PDF全文 (下载：34) Chinese Optics Letters ，2017年第15卷第1期 pp.010007
A 4×4 multiple-input multiple-output coherent microwave photonic (MWP) link to transmit four wireless signals with an identical microwave center frequency over a single optical wavelength based on optical independent sideband (OISB) modulation and optical orthogonal modulation with an improved spectral efficiency is proposed and experimentally demonstrated. At the transmitter, the OISB modulation and optical orthogonal modulation are implemented to generate an OISB signal using a dual-parallel Mach–Zehnder modulator (DP-MZM) driven by four microwave orthogonal frequency-division multiplexing (OFDM) signals with an identical microwave center frequency. At the receiver, the OISB signal is coherently detected at a coherent receiver where a free-running local oscillator (LO) laser source is employed. Digital signal processing is then used to recover the four OFDM signals, to eliminate the phase noise from the transmitter laser source and the LO laser source, and to cancel the unstable wavelength difference between the wavelengths of the transmitter laser source and the LO laser source. Error-free transmission of three 16 quadrature amplitude modulation (16-QAM) 1 Gbps OFDM signals and one 16-QAM 1.5 Gbps OFDM signal at a microwave center frequency of 2.91 GHz over a 10 km single-mode fiber is experimentally demonstrated.
PDF全文 (下载：41) Chinese Optics Letters ，2017年第15卷第1期 pp.010008
A. K. Poddar
U. L. Rohde
and A. S. Daryoush
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Ultra-low phase noise performance is required for frequency agile local oscillators, which are the core for high resolution imagers, spectrum analyzers, and high speed data communications. A forced opto-electronic oscillator (OEO) benefits from frequency stabilization techniques for realizing a clean and low phase noise source at microwave and millimeter wave frequencies. Forced oscillation techniques of self-injection locking and self-phase lock loop are combined to provide an ultra-low oscillator phase noise both close-in and far-away from the carrier frequency, while a tunable yttrium iron garnet microwave filter combined with a wavelength tuned transversal filter are employed to implement both coarse and fine frequency tuning for a tunable X-band OEO. A phase noise of ?137 dBc/Hz at an offset frequency of 10 kHz is achieved covering the frequencies of 9 to 11 GHz with a fine frequency tuning resolution of 44 Hz/pm and coarse tuning of 25 MHz/mA. Moreover, the long term stability of the output signal is tested, and a maximum frequency drift of 2 kHz is measured within 60 min for the X-band synthesizer.
PDF全文 (下载：44) Chinese Optics Letters ，2017年第15卷第1期 pp.010009
and Amir Abramovich
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Flat mirrors, also known as flat parabolic surfaces, for millimeter-wave and terahertz imaging systems are demonstrated. This flat mirror is based on the metasurface in which an inexpensive printed circuit board technology is used to realize copper patterns printed on an FR4 substrate. Compared to the conventional reflector antennas used today in diverse applications (for homeland security, medical systems, communication, etc.), the suggested mirror has major advantages in process simplicity, mechanical flexibility, frequency alignment, weight, and cost. The theoretical background, simulation results, experimental results, and proof of concept are given in this Letter.
PDF全文 (下载：41) Chinese Optics Letters ，2017年第15卷第1期 pp.011101
Lasers and laser optics
and Xuechun Li
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We present a lamp-pumped Nd: phosphate glass laser amplifier delivering up to 1 J of pulse energy at 1053 nm with a repetition rate of 1 Hz and an injected pulse energy of 2.5 mJ. The amplifier system employs a beam-shaping module and a four-pass, lamp-pumped amplifier. The thermally induced wavefront distortion is mitigated and a uniform gain distribution is obtained by a four-lamp-pumped laser head in the amplifier. Thus, an excellent beam quality is obtained.
PDF全文 (下载：49) Chinese Optics Letters ，2017年第15卷第1期 pp.011401
and Jinping Li
A novel black phosphorus (BP) solution saturable absorber (SA) is fabricated by the liquid-phase-exfoliated method and successfully used for passively Q-switched (QS) Nd:YVO4 laser. Compared with a traditional solid SA, a BP solution SA possesses more excellent optical transparency and higher damage resistance. The shortest pulse duration and highest average output power are measured to be 119 ns and 1.23 W, respectively. To the best of our knowledge, both of them are the best results among QS solid-state lasers with BP-based absorbers so far. The repetition rate is in the range of 533.2 to 722 kHz. The results indicate the potential application of the BP solution SA into high-power solid-state pulse lasers.
PDF全文 (下载：42) Chinese Optics Letters ，2017年第15卷第1期 pp.011402
I. S. Amiri
A. Z. Zulkifli
and K. Thambiratnam
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A dual-wavelength fiber laser operating at the 1550 nm region using a side-polished arc-shaped fiber with deposited ZnO nanoparticles is proposed and demonstrated. The arc-polished fiber is fabricated by using a simple but novel approach in which a silicon carbide paper polishes one side of a conventional single-mode fiber. An arc-polished fiber with a length of 2.25 mm and an insertion loss of 0.95 dB is obtained and deposited with ZnO nanoparticles by the drop-cast method. A stable dual-wavelength output is obtained at 1562.5 and 1563.4 nm at peak powers of ?9.3 and ?10.1 dBm, respectively, as well as a signal-to-noise ratio of 28.4 dB and a channel spacing of 0.9 nm. Both lasing wavelengths also have narrow linewidths of between 0.045 and 0.049 nm and show little to no wavelength or power fluctuations over continued testing.
PDF全文 (下载：84) Chinese Optics Letters ，2017年第15卷第1期 pp.011403
and Juncheng Cao
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We demonstrate the electrical beat note analysis and radio frequency (RF) injection locking of a continuous wave (cw) terahertz quantum cascade laser (QCL) emitting around 3 THz (～100 μm). In free running the beat note frequency of the QCL shows a shift of ～180 MHz with increasing drive current. The beat note, modulation response, injection pulling, and terahertz emission spectral characteristics in the different current regimes I, II, and III are investigated. The results show that in the current regime I close to the laser threshold we obtain a narrower beat note and flat response to the RF modulation at the cavity round trip frequency. The pulling effect and spectral modulation measurements verify that in the current regime I the RF injection locking is more efficient and a robust tool to modulate the mode number and mode frequency of terahertz QCLs.
PDF全文 (下载：34) Chinese Optics Letters ，2017年第15卷第1期 pp.011404
and Weixin Xie
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We provide the first demonstration of pure red emission in the visible light region via three-photon excitation in monodisperse Na3ZrF7:Er nanoparticles (NPs) by using a laser operating in the telecommunication band. NPs of ～22 nm in diameter are synthesized at 260°C by the thermal decomposition method. The experimental results reveal that the Na3ZrF7:Er NPs exhibit pure red emission in the visible region under 1480 nm laser excitation, and the emission intensity is significantly influenced by the Er3+ ion concentration. The decay times of the S3/24→F415/2 and F9/24→F415/2 transitions of the Er3+ ions at 540 and 655 nm, respectively, are reduced by increasing the Er3+ ion concentration in the Na3ZrF7:Er NPs. The suppressed emission intensity result from the defect-related quenching effect: when trivalent Er3+ ions replac tetravalent Zr4+ ions, extra Na+ ions and F? vacancies are formed to re-balance the charge in the Na3ZrF7 matrix. The emission color of the Na3ZrF7:Er NPs is related to the cross relaxation between Er3+ ions. These results provide an important step toward more effective biological imaging and photodynamic therapy by minimizing the scattering of the excitation light and increasing the penetration depth.
PDF全文 (下载：41) Chinese Optics Letters ，2017年第15卷第1期 pp.011601
and Xingdao He
The scattering properties of ZnO nanospheres with four different particle diameters of 10, 50, 100, and 200 nm suspended in water are investigated theoretical and experimentally in the spectral range of the entire visible range and part of the near-infrared region. The scattering properties of ZnO nanospheres suspended in water are described by employing three main parameters: the angular distribution of the scattering intensity I, the scattering extinction coefficient αscat, and the scattering cross section σscat. The results indicate that (i) at a certain wavelength, the angular distribution of the scattering intensity appears as an obviously forward-propagating feature, and the forward-scattering intensity is dominant gradually when the particle diameter increases from 10 to 200 nm, and (ii) the scattering extinction coefficient and cross section can be determined by using the measured transmittance changes of a pure water sample and a given ZnO sample; they all are shown to be dependent on the particle size and incident wavelength. The experimental results of four different scattering samples agree well with the theoretical predictions within the given wavelength range.
PDF全文 (下载：25) Chinese Optics Letters ，2017年第15卷第1期 pp.012901