Frontiers of Optoelectronics

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Enabling technologies and challenges for transmission of 400 Gb/s signals in 50 GHz channel grid
Xiang ZHOU
Front Optoelec    2013, 6 (1): 30-45.   https://doi.org/10.1007/s12200-012-0298-2
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This paper reviewed the recent progress in transmission of 400 Gb/s, wavelength-division-multiplexed (WDM) channels for optical networks based on the standard 50 GHz grid. We discussed the enabling modulation, coding, and line system technologies, as well as the existing challenges. It is shown that, 400 Gb/s per channel signal can be transmitted on the standard 50 GHz ITU-T grid at 8.4 b/ds/Hz net spectral efficiency (SE) over meaningful transmission reach for regional and metropolitan applications. However, further studies are needed to fully understand the potential for meeting the requirements of long-haul transmission applications.

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Key technologies and system proposals of TWDM-PON
Zhengxuan LI, Lilin YI, Weisheng HU
Front Optoelec    2013, 6 (1): 46-56.   https://doi.org/10.1007/s12200-012-0305-7
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In this paper, key technologies, system proposals and future directions of next generation passive optical networks stage 2 (NG-PON2) are reviewed. We first discuss the potential solutions for NG-PON2 standardization. Then we focus on time and wavelength division multiplexed PON (TWDM-PON), which is the primary solution selected by Full Service Access Network (FSAN). The key technologies in TWDM-PON configuration are analyzed, including how to improve the bandwidth capacity and power budget of the system, and choose upstream tunable transceiver, etc. Several system proposals are illustrated as candidates for NG-PON2 configuration.

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Challenges in the ambient Raman spectroscopy characterization of methylammonium lead triiodide perovskite thin films
Yuanyuan ZHOU,Hector F. GARCES,Nitin P. PADTURE
Front. Optoelectron.    2016, 9 (1): 81-86.   https://doi.org/10.1007/s12200-016-0573-8
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The importance of methylammonium lead triiodide (CH3NH3PbI3 or MAPbI3) organic-inorganic hybrid perovskites has shot up dramatically since their use in highly efficient thin-film perovskite solar cells (PSCs). However, the basic structural characterization of these fascinating materials remains sparse. In particular, Raman spectroscopy, which is a powerful vibrational spectroscopy characterization tool and complements other characterization methods, of MAPbI3 under ambient conditions is plagued with difficulties. Here, a systematic ambient Raman spectroscopy characterization study of MAPbI3 thin films is conducted under different conditions (excitation laser wavelength, integration time, filter characteristic). The results from this study help elucidate the possible sources of artifacts in the Raman spectra, and raise the awareness of the challenges in the ambient Raman spectroscopy of MAPbI3 perovskites. Approaches to overcome these challenges are suggested.

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Nonlinear optical response of graphene in terahertz and near-infrared frequency regime
Yee Sin ANG,Qinjun CHEN,Chao ZHANG
Front. Optoelectron.    2015, 8 (1): 3-26.   https://doi.org/10.1007/s12200-014-0428-0
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In this review, we discuss our recent theoretical work on the nonlinear optical response of graphene and its sister structure in terahertz (THz) and near-infrared frequency regime. Due to Dirac-like linear energy-momentum dispersion, the third-order nonlinear current in graphene is much stronger than that in conventional semiconductors. The nonlinear current grows rapidly with increasing temperature and decreasing frequency. The third-order nonlinear current can be as strong as the linear current under moderate electric field strength of 104 V/cm. In bilayer graphene (BLG) with low energy trigonal warping effect, not only the optical response is strongly nonlinear, the optical nonlinearity is well-preserved at elevated temperature. In the presence of a bandgap (such as semihydrogenated graphene (SHG)), there exists two well separated linear response and nonlinear response peaks. This suggests that SHG can have a unique potential as a two-color nonlinear material in the THz frequency regime where the relative intensity of the two colors can be tuned with the electric field. In a graphene superlattice structure of Kronig-Penney type periodic potential, the Dirac cone is elliptically deformed. We found that not only the optical nonlinearity is preserved in such a system, the total optical response is further enhanced by a factor proportional to the band anisotropy. This suggests that graphene superlattice is another potential candidate in THz device application.

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Photonic crystal fibers, devices, and applications
Wei JIN, Jian JU, Hoi Lut HO, Yeuk Lai HOO, Ailing ZHANG
Front Optoelec    2013, 6 (1): 3-24.   https://doi.org/10.1007/s12200-012-0301-y
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This paper reviews different types of air-silica photonic crystal fibers (PCFs), discusses their novel properties, and reports recent advances in PCF components and sensors as well as techniques for splicing PCFs to standard telecomm fibers.

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Recent advances in microwave photonics
Ming LI,Ninghua ZHU
Front. Optoelectron.    2016, 9 (2): 160-185.   https://doi.org/10.1007/s12200-016-0633-0
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Microwave photonics (MWP) is an interdisciplinary field that combines two different areas of microwave engineering and photonics. It has several key features by transferring signals between the optical domain and microwave domain, which leads to the advantages of broad operation bandwidth for generation, processing and distribution of microwave signals and high resolution for optical spectrum measurement. In this paper, we comprehensively review past and current status of MWP in China by introducing the representative works from most of the active MWP research groups. Future prospective is also discussed from the national strategy to key enabling technology that we have developed.

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Semiconductor activated terahertz metamaterials
Hou-Tong CHEN
Front. Optoelectron.    2015, 8 (1): 27-43.   https://doi.org/10.1007/s12200-014-0436-0
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Metamaterials have been developed as a new class of artificial effective media realizing many exotic phenomena and unique properties not normally found in nature. Metamaterials enable functionality through structure design, facilitating applications by addressing the severe material issues in the terahertz frequency range. Consequently, prototype functional terahertz devices have been demonstrated, including filters, antireflection coatings, perfect absorbers, polarization converters, and arbitrary wavefront shaping devices. Further integration of functional materials into metamaterial structures have enabled actively and dynamically switchable and frequency tunable terahertz metamaterials through the application of external stimuli. The enhanced light-matter interactions in active terahertz metamaterials may result in unprecedented control and manipulation of terahertz radiation, forming the foundation of many terahertz applications. In this paper, we review the progress during the past few years in this rapidly growing research field. We particularly focus on the design principles and realization of functionalities using single-layer and few-layer terahertz planar metamaterials, and active terahertz metamaterials through the integration of semiconductors to achieve switchable and frequency-tunable response.

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Research on multi-kilowatts level tapered fiber bundle N×1 pumping combiner for high power fiber laser
Qirong XIAO,Yusheng HUANG,Junyi SUN,Xuejiao WANG,Dan LI,Mali GONG,Ping YAN
Front. Optoelectron.    2016, 9 (2): 301-305.   https://doi.org/10.1007/s12200-016-0609-0
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Pumping combiner is a kernel component of high power fiber laser (HPFL). We demonstrate two types of tapered fiber bundle (TFB) end-pumping combiner able to combining multi-kilowatts of pumping laser. After the experimental test of coupling performance, the 3×1 coupler is proved to have a power handling capacity of 2.11 kW with a coupling efficiency of 95.1%, and the 7×1 coupler is capable of handling pumping power of 4.72 kW with a coupling efficiency of 99.4%. These two coupler have obtained the ability to be used in laser diodes (LDs) direct beam combining and the pumping coupling of multi-kilowatts level fiber lasers.

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High-performance, stable and low-cost mesoscopic perovskite (CH3NH3PbI3) solar cells based on poly(3-hexylthiophene)-modified carbon nanotube cathodes
Xiaoli ZHENG,Haining CHEN,Zhanhua WEI,Yinglong YANG,He LIN,Shihe YANG
Front. Optoelectron.    2016, 9 (1): 71-80.   https://doi.org/10.1007/s12200-016-0566-7
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This work explores the use of poly(3-hexylthiophene) (P3HT) modified carbon nanotubes (CNTs@P3HT) for the cathodes of hole transporter free, mesoscopic perovskite (CH3NH3PbI3) solar cells (PSCs), simultaneously achieving high-performance, high stability and low-cost PSCs. Here the thin P3HT modifier acts as an electron blocker to inhibit electron transfer into CNTs and a hydrophobic polymer binder to tightly cross-link the CNTs together to compact the carbon electrode film and greatly stabilize the solar cell. On the other hand, the presence of CNTs greatly improve the conductivity of P3HT. By optimizing the concentration of the P3HT modifier (2 mg/mL), we have improved the power conversion efficiencies (PCEs) of CNTs@P3HT based PSCs up to 13.43% with an average efficiency of 12.54%, which is much higher than the pure CNTs based PSCs (best PCE 10.59%) and the sandwich-type P3HT/CNTs based PSCs (best PCE 9.50%). In addition, the hysteresis of the CNTs@P3HT based PSCs is remarkably reduced due to the intimate interface between the perovskite and CNTs@P3HT electrodes. Degradation of the CNTs@ P3HT based PSCs is also strongly retarded as compared to cells employing the pure CNTs electrode when exposed to the ambient condition of 20%-40% humidity.

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Fourier domain optical coherence tomography with ultralong depth range
Zhihua DING,Yi SHEN,Wen BAO,Peng LI
Front. Optoelectron.    2015, 8 (2): 163-169.   https://doi.org/10.1007/s12200-015-0463-5
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The depth ranges of typical implementations of Fourier domain optical coherence tomography (FDOCT), including spectral domain OCT (SDOCT) and swept source OCT (SSOCT), are limited to several millimeters. To extend the depth range of current OCT systems, two novel systems with ultralong depth range were developed in this study. One is the orthogonal dispersive SDOCT (OD-SDOCT), and the other is the recirculated swept source (R-SS) interferometer/OCT. No compromise between depth range and depth resolution is required in both systems. The developed OD-SDOCT system realized the longest depth range (over 100 mm) ever achieved by SDOCT, which is ready to be modified for depth-encoded parallel imaging on multiple sites. The developed R-SS interferometer achieved submicron precision within a depth range of 30 mm, holding potential in real-time contact-free on-axis metrology of complex optical systems.

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Technology developments and biomedical applications of polarization-sensitive optical coherence tomography
Zhenyang DING,Chia-Pin LIANG,Yu CHEN
Front. Optoelectron.    2015, 8 (2): 128-140.   https://doi.org/10.1007/s12200-015-0475-1
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Polarization-sensitive optical coherence tomography (PS-OCT) enables depth-resolved mapping of sample polarization information, such as phase-retardation and optical axis orientation, which is particularly useful when the nano-scale organization of tissue that are difficult to be observed in the intensity images of a regular optical coherence tomography (OCT). In this review, we survey two types of methods and systems of PS-OCT. The first type is PS-OCT with single input polarization state, which contain bulk optics or polarization maintaining fiber (PMF) based systems and single-mode fiber (SMF) based systems. The second type is PS-OCT with two different input polarization states, which contain SMF based systems and PMF based systems, through either time, frequency, or depth multiplexing. In addition, representative biomedical applications using PS-OCT, such as retinal imaging, skin cancer detection, and brain mapping, are demonstrated.

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In vivo imaging of a single erythrocyte with high-resolution photoacoustic microscopy
Guo HE,Bingbing LI,Sihua YANG
Front. Optoelectron.    2015, 8 (2): 122-127.   https://doi.org/10.1007/s12200-014-0461-z
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In this letter, we reported a high-resolution photoacoustic microscopy (PAM) to image erythrocytes and blood vessels. The developed system had the ability to provide a lateral resolution of 1.0 μm at the wavelength of 532 nm with a × 10 objective. First, we used a sharp edge to measure the lateral resolution of the PAM and testified the stability with carbon fibers. Then, using this system, in vivo blood vessels and capillaries of a mouse ear, even a single erythrocyte can be clearly imaged. There was a pair of accompanying venule and arteriole, whose detailed and further complicated branches can be clearly identified. And likely red blood cells (RBCs) arrayed one by one in microvasculature was also shown. The experimental results demonstrate that the high-resolution PAM has potential clinical applications for imaging of erythrocytes and blood vessels.

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Tailoring electromagnetic responses in terahertz superconducting metamaterials
Xiaoling ZHANG,Jianqiang GU,Jiaguang HAN,Weili ZHANG
Front. Optoelectron.    2015, 8 (1): 44-56.   https://doi.org/10.1007/s12200-014-0439-x
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Superconducting terahertz metamaterials have attracted significant interest due to low loss, efficient resonance switching and large-range frequency tunability. The super conductivity in the metamaterials dramatically reduces ohmic loss and absorption to levels suitable for novel devices over a broad range of electromagnetic spectrum. Most metamaterials utilize subwavelength-scale split-ring resonators as unit building blocks, which are proved to support fundamental inductive-capacitive resonance, to achieve unique resonance performance. We presented a review of terahertz superconducting metamaterials and their implementation in multifunctional devices. We began with the recent development of superconducting metamaterials and their potential applications in controlling and manipulating terahertz waves. Then we explored the tuning behaviors of resonance properties in several typical, actively controllable metamaterials through integrating active components. Finally, the ultrafast dynamic nonlinear response to high intensity terahertz field in the superconducting metamaterials was presented.

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Generation and detection of pulsed terahertz waves in gas: from elongated plasmas to microplasmas
Fabrizio BUCCHERI, Pingjie HUANG, Xi-Cheng ZHANG
Front. Optoelectron.    2018, 11 (3): 209-244.   https://doi.org/10.1007/s12200-018-0819-8
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The past two decades have seen an exponential growth of interest in one of the least explored region of the electromagnetic spectrum, the terahertz (THz) frequency band, ranging from to 0.1 to 10 THz. Once only the realm of astrophysicists studying the background radiation of the universe, THz waves have become little by little relevant in the most diverse fields, such as medical imaging, industrial inspection, remote sensing, fundamental science, and so on. Remarkably, THz wave radiation can be generated and detected by using ambient air as the source and the sensor. This is accomplished by creating plasma under the illumination of intense femtosecond laser fields. The integration of such a plasma source and sensor in THz time-domain techniques allows spectral measurements covering the whole THz gap (0.1 to 10 THz), further increasing the impact of this scientific tool in the study of the four states of matter.

In this review, the authors introduce a new paradigm for implementing THz plasma techniques. Specifically, we replaced the use of elongated plasmas, ranging from few mm to several cm, with sub-mm plasmas, which will be referred to as microplasmas, obtained by focusing ultrafast laser pulses with high numerical aperture optics (NA from 0.1 to 0.9).

The experimental study of the THz emission and detection from laser-induced plasmas of submillimeter size are presented. Regarding the microplasma source, one of the interesting phenomena is that the main direction of THz wave emission is almost orthogonal to the laser propagation direction, unlike that of elongated plasmas. Perhaps the most important achievement is the demonstration that laser pulse energies lower than 1 mJ are sufficient to generate measurable THz pulses from ambient air, thus reducing the required laser energy requirement of two orders of magnitude compared to the state of art. This significant decrease in the required laser energy will make plasma-based THz techniques more accessible to the scientific community, as well as opening new potential industrial applications.

Finally, experimental observations of THz radiation detection with microplasmas are also presented. As fully coherent detection was not achieved in this work, the results presented herein are to be considered a first step to understand the peculiarities involved in using the microplasma as a THz sensor.

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Optical fiber amplifiers for space-division multiplexing
Dagong JIA, Haiwei ZHANG, Zhe JI, Neng BAI, Guifang LI
Front Optoelec    2012, 5 (4): 351-357.   https://doi.org/10.1007/s12200-012-0294-6
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Recently, space-division multiplexing (SDM) techniques using multi-core fiber (MCF) and few-mode fiber (FMF) have been introduced into optical fiber communication to increase transmission capacity. Two main types of optical fiber amplifiers based on the Erbium-doped fiber (EDF) and the Raman effect have been developed to amplify signals in the MCF and FMF. In this paper, we reviewed the principles and configurations of these amplifiers.

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Time behavior of field screening effects in small-size GaAs photoconductive terahertz antenna
Tianyi WANG,Zhengang YANG,Si ZOU,Kejia WANG,Shenglie WANG,Jinsong LIU
Front. Optoelectron.    2015, 8 (1): 98-103.   https://doi.org/10.1007/s12200-015-0488-9
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The field screening effects in small-size GaAs photoconductive (PC) antenna are investigated via the well-known pump and probe terahertz (THz) generation technique. The peak amplitude of the THz pulses excited by the probe laser pulse as a function of the pump-probe time delay was measured. An equivalent-circuit model was used to simulate the experimental data. Based on the good agreement between the results of simulation and experiment, the time behavior of the radiation and space-charge fields was simulated. The results show that the space-charge screening dominantly determines the device response in the whole time, while the radiation filed screening plays a key role in initial time which strongly affects the peak THz field. The parameter analysis was performed, which may be valuable on the optimum design for the antenna as a THz emitter.

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Retinal projection head-mounted display
Junguo LIN,Dewen CHENG,Cheng YAO,Yongtian WANG
Front. Optoelectron.    2017, 10 (1): 1-8.   https://doi.org/10.1007/s12200-016-0662-8
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Retinal projection displays (RPDs) are an important development direction for head-mounted displays (HMDs). This paper reviews the literature on optical engineering aspects based on the data on advanced technology in RPD design and development. The review includes the principles and applications of four theories, e.g., the Maxwellian view and its modified modality and the monocular and binocular depth cues of stereoscopic objects in the physiology of the human visual system. To support the Maxwellian view and achieve retinal projection systems with depth cues, results of previous design works were summarized using different methods and their advantages and disadvantages are analyzed. With an extremely long focal depth, a prototype of a full-color stereoscopic see-through RPD system was discussed. Finally, a brief outlook of the future development trends and applications of the RPDs was presented

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Simulation study on the active layer thickness and the interface of a-IGZO-TFT with double active layers
Xiaoyue LI,Sheng YIN,Dong XU
Front. Optoelectron.    2015, 8 (4): 445-450.   https://doi.org/10.1007/s12200-014-0451-1
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In this paper, ATLAS 2D device simulator of SILVACO was used for device simulation of inverted-staggered thin film transistor using amorphous indium gallium zinc oxide as active layer (a-IGZO-TFT) with double active layers, based on the density of states (DOS) model of amorphous material. The change of device performance induced by the thickness variation of each active layer was studied, and the interface between double active layers was analyzed. The best performance was found when the interface was near the edge of the channel, by optimizing the thickness of each active layers, the high performance device of threshold voltage (Vth) = −0.89 V, sub-threshold swing (SS)= 0.27, on/off current ratio (ION/IOFF) = 6.98 × 1014 was obtained.

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Parameters that control and influence the organo-metal halide perovskite crystallization and morphology
Bat-El COHEN,Lioz ETGAR
Front. Optoelectron.    2016, 9 (1): 44-52.   https://doi.org/10.1007/s12200-016-0630-3
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This review discusses various parameters that influence and control the organo-metal halide perovskite crystallization process. The effect of the perovskite morphology on the photovoltaic performance is a critical factor. Moreover, it has a dramatic effect on the stability of the perovskite, which has significant importance for later use of the organo-metal perovskite in assorted applications. In this review, we brought together several research investigations that describe the main parameters that significantly influence perovskite crystallization, for example, the annealing process, the precursor solvent, anti-solvent treatment, and additives to the iteite solutions.

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Green light-emitting diode based on graphene-ZnO nanowire van der Waals heterostructure
Zhiqian WU,Yue SHEN,Xiaoqiang LI,Qing YANG,Shisheng LIN
Front. Optoelectron.    2016, 9 (1): 87-92.   https://doi.org/10.1007/s12200-016-0596-1
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The rectifying behavior between graphene and semiconductors makes novel type of solar cells, photodetectors and light emitting diodes (LEDs). The interface between graphene and ZnO is the key for the performance of the optoelectronic devices. Herein, we find that green light emission is very strong for the forward biased graphene/ZnO nanowire van der Waals heterostructure. We correlated the green light emission with the surface defects locating at the ZnO nanowire surface through the detailed high resolution transmission electron microscopy and photoluminescence measurements. We pointed out engineering the surface of ZnO nanowires could bring a dimension of designing graphene/ZnO LEDs, which could be extended to other types of graphene/semiconductor heterostructure based optoelectronic devices.

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Resolution and contrast enhancements of optical microscope based on point spread function engineering
Yue FANG,Cuifang KUANG,Ye MA,Yifan WANG,Xu LIU
Front. Optoelectron.    2015, 8 (2): 152-162.   https://doi.org/10.1007/s12200-015-0479-x
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Point spread function (PSF) engineering-based methods to enhance resolution and contrast of optical microscopes have experienced great achievements in the last decades. These techniques include: stimulated emission depletion (STED), time-gated STED (g-STED), ground-state depletion microscopy (GSD), difference confocal microscopy, fluorescence emission difference microscopy (FED), switching laser mode (SLAM), virtual adaptable aperture system (VAAS), etc. Each affords unique strengths in resolution, contrast, speed and expenses. We explored how PSF engineering generally could be used to break the diffraction limitation, and concluded that the common target of PSF engineering-based methods is to get a sharper PSF. According to their common or distinctive principles to reshape the PSF, we divided all these methods into three categories, nonlinear PSF engineering, linear PSF engineering, and linear-based nonlinear PSF engineering and expounded these methods in classification. Nonlinear effect and linear subtraction is the core techniques described in this paper from the perspective of PSF reconstruction. By comparison, we emphasized each method’s strengths, weaknesses and biologic applications. In the end, we promote an expectation of prospective developing trend for PSF engineering.

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A review of recent progress in plasmon-assisted nanophotonic devices
Jian WANG
Front. Optoelectron.    2014, 7 (3): 320-337.   https://doi.org/10.1007/s12200-014-0469-4
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Plasmonics squeezes light into dimensions far beyond the diffraction limit by coupling the light with the surface collective oscillation of free electrons at the interface of a metal and a dielectric. Plasmonics, referred to as a promising candidate for high-speed and high-density integrated circuits, bridges microscale photonics and nanoscale electronics and offers similar speed of photonic devices and similar dimension of electronic devices. Various types of passive and active surface plasmon polariton (SPP) enabled devices with enhanced deep-subwavelength mode confinement have attracted increasing interest including waveguides, lasers and biosensors. Despite the trade-off between the unavoidable metal absorption loss and extreme light concentration, the ever-increasing research efforts have been devoted to seeking low-loss plasmon-assisted nanophotonic devices with deep-subwavelength mode confinement, which might find potential applications in high-density nanophotonic integration and efficient nonlinear signal processing. In addition, other plasmon-assisted nanophotonic devices might also promote grooming functionalities and applications benefiting from plasmonics.

In this review article, we give a brief overview of our recent progress in plasmon-assisted nanophotonic devices and their wide applications, including long-range hybrid plasmonic slot (LRHPS) waveguide, ultra-compact plasmonic microresonator with efficient thermo-optic tuning, high quality (Q) factor and small mode volume, compact active hybrid plasmonic ring resonator for deep-subwavelength lasing applications, fabricated hybrid plasmonic waveguides for terabit-scale photonic interconnection, and metamaterials-based broadband and selective generation of orbital angular momentum (OAM) carrying vector beams. It is believed that plasmonics opens possible new ways to facilitate next chip-scale key devices and frontier technologies.

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Simultaneous generation of nonlinear optical harmonics and terahertz radiation in air: polarization discrimination of various nonlinear contributions
Mikhail ESAULKOV,Olga KOSAREVA,Vladimir MAKAROV,Nikolay PANOV,Alexander SHKURINOV
Front. Optoelectron.    2015, 8 (1): 73-80.   https://doi.org/10.1007/s12200-014-0443-1
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In this paper, we experimentally observed generation of the second and the third optical harmonics and the broadband terahertz radiation in the course of 800 nm 120 fs pulse in atmospheric air. The analysis of their polarization properties revealed unity of their nonlinear optical nature. Taking into account only the third-order nonlinear response of the neutral molecules of air, we analytically described the newly generated elliptically polarized 3d harmonic, the linear polarization of terahertz radiation and the stability of terahertz energy yield for the initial circularly polarized ω pump pulse.

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De-noising research on terahertz holographic reconstructed image based on weighted nuclear norm minimization method
Wenshu MA, Qi LI, Jianye LU, Liyu SUN
Front. Optoelectron.    2018, 11 (3): 267-274.   https://doi.org/10.1007/s12200-018-0829-6
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Terahertz imaging is one of the forefront topics of imaging technology today. Denoising process is the key for improving the resolution of the terahertz holographic reconstructed image. Based on the fact that the weighted nuclear norm minimization (WNNM) method preserves the details of the reconstructed image well and the non-local mean (NLM) algorithm performs better in the removal of background noise, this paper proposes a new method in which the NLM algorithm is used to improve the WNNM method. The experimental observation and quantitative analysis of the denoising results prove that the new method has better denoising effect for the terahertz holographic reconstructed image.

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Fluorescence microendoscopy imaging based on GRIN lenses with one- and two-photon excitation modes
Wei YAN,Xiao PENG,Danying LIN,Qi WANG,Jian GAO,Teng LUO,Jie ZHOU,Tong YE,Junle QU,Hanben NIU
Front. Optoelectron.    2015, 8 (2): 177-182.   https://doi.org/10.1007/s12200-015-0503-1
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With the rapid development of life sciences, there is an increasing demand for intravital fluorescence imaging of small animals. However, large dimensions and limited working distances of objective lenses in traditional fluorescence microscopes have limited their imaging applications mostly to superficial tissues. To overcome these disadvantages, researchers have developed the graded-index (GRIN) probes with small diameters for imaging internal organs of small animals in a minimally invasive fashion. However, dynamic imaging based on GRIN lens has not been studied extensively. Here, this paper presented a fluorescence endoscopic imaging system based on GRIN lenses using one-photon and two-photon excitation. GRIN lenses with 1.15 mm diameter and 7.65 mm length were used in the system. The images were acquired by a compact laser scanning imaging system with a resonant galvo-mirror system to scan the laser beam and a photomultiplier tube (PMT) to detect fluorescence signals. Experimental results showed that this system using two-photon excitation could implement dynamic fluorescence microendoscopic imaging and monitor the movement of blood flow beneath the skin in anesthetized mice while producing images with higher contrast and signal to noise ratio (SNR) than those using one photon excitation. It would be a useful tool for studying biological processes of small animals or plants in vivo.

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An improved noise reduction algorithm based on wavelet transformation for MEMS gyroscope
Jianguo YUAN,Yantao YUAN,Feilong LIU,Yu PANG,Jinzhao LIN
Front. Optoelectron.    2015, 8 (4): 413-418.   https://doi.org/10.1007/s12200-015-0474-2
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To solve the large noise problem for the low-precision gyroscopes in micro-electro mechanical systems (MEMS) of inertial navigation system, an improved noise reduction method, based on the analyses of the fast Fourier transformation (FFT) noise reduction principle and the simple wavelet noise reduction principle, was proposed. Furthermore, the FFT noise reduction method, the simple wavelet noise reduction method and the improved noise reduction method were comparatively analyzed and experimentally verified in the case of the constant rate and dynamic rate. The experimental analysis results showed that the improved noise reduction method had a very good result in the noise reduction of the gyroscope data at different frequencies, and its performance was superior to those of the FFT noise reduction method and the simple wavelet noise reduction method.

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Influence of optical filtering on transmission capacity in single mode fiber communications
M. Venkata SUDHAKAR,Y. Mallikarjuna REDDY,B. Prabhakara RAO
Front. Optoelectron.    2015, 8 (4): 424-430.   https://doi.org/10.1007/s12200-014-0426-2
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This paper presents the design and analysis of optical filters that are placed at the output of directly modulated vertical cavity surface emitting laser (VCSEL) in the process of inexpensive transmitter’s implementation for upcoming generation optical access network. Generation of non return to zero (NRZ) optical signal from the transmitter for 110 km error-free single mode fiber (SMF) transmission at 10 Gb/s with bit error rate (BER) of 10−30 in the absence of the external modulator and encoder was proposed. Effects of super-Gaussian and Butterworth optical filters at VCSEL output were demonstrated to maximize performance of SMF optical systems without need of any dispersion compensation technique.

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Emerging trend for LED wafer level packaging
S. W. Ricky LEE, Rong ZHANG, K. CHEN, Jeffery C. C. LO
Front Optoelec    2012, 5 (2): 119-126.   https://doi.org/10.1007/s12200-012-0259-9
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Currently most light emitting diode (LED) components are made with individual chip packaging technology. The main manufacturing processes follow conventional chip-based IC packaging. In the past several years, there has been an uprising trend in the IC industry to migrate from chip-based packaging to wafer level packaging (WLP). Therefore, there is a need for LEDs to catch up. This paper introduces advanced LED WLP technologies. The contents cover key enabling processes such as preparation of silicon sub-mount wafer, implementation of interconnection, deposition of phosphor, wafer level encapsulation, and their integration. The emphasis is placed on how to achieve high throughput, low cost manufacturing through WLP.

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Microwave photonics connected with microresonator frequency combs
Xiaoxiao XUE,Andrew M. WEINER
Front. Optoelectron.    2016, 9 (2): 238-248.   https://doi.org/10.1007/s12200-016-0621-4
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Microresonator frequency combs (microcombs) are very promising as ultra-compact broadband sources for microwave photonic applications. Conversely, microwave photonic techniques are also employed intensely in the study of microcombs to reveal and control the comb formation dynamics. In this paper, we reviewed the microwave photonic techniques and applications that are connected with microcombs. The future research directions of microcomb-based microwave photonics were also discussed.

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Longitudinal twinning α-In2Se3 nanowires for UV-visible-NIR photodetectors with high sensitivity
Zidong ZHANG, Juehan YANG, Fuhong MEI, Guozhen SHEN
Front. Optoelectron.    2018, 11 (3): 245-255.   https://doi.org/10.1007/s12200-018-0820-2
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Longitudinal twinning α-In2Se3 nanowires with the (101¯8) twin plane were synthesized to fabricate high performance single nanowire based photodetectors. As-synthesized α-In2Se3 nanowire exhibited typical n-type semiconducting behavior with an electron mobility of 23.1 cm2·V1·S1 and a broadband spectral response from 300 to 1100 nm, covering the ultraviolet-visible-near-infrared (UV-visible-NIR) region. Besides, the fabricated device showed a high responsivity of 8.57 × 105 A·W1, high external quantum efficiency up to 8.8 × 107% and a high detectivity of 1.58 × 1012 Jones under 600 nm light illumination at a basis of 3 V, which are much higher than previously reported In2Se3 nanostructures due to the interface defect effect of the twin plane. The results indicated that the longitudinal twinning α-In2Se3 nanowires have immense potential for further applications in highly performance broadband photodetectors and other optoelectronic devices.

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