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In this paper, we propose an effective light trapping mechanism through engineering a silver grating to significantly increase the photocatalytic activity of a TiO2-based system. The presented structure contains an asymmetric ultra-thin multilayer silver stripe. Optical properties of the structure are numerically simulated with the rigorous coupled wave analysis (RCWA) method. Two different symmetric and asymmetric designs are investigated. It is shown that several absorption peaks will appear in the absorption spectrum of the symmetric and asymmetric systems, which are attributed to the excitation of localized SP modes at the silver–TiO2 interface. Moreover, the effects of the incident angle of the sunlight and the geometrical parameters of the asymmetric design are studied to achieve an optimum configuration. It is shown that the absorption band of the structure can be tuned by adjusting the geometrical parameters. Furthermore, the results indicate that the photocatalytic activity of the asymmetric structure is significantly improved compared to the symmetric case. Moreover, the results obtained here clarify the role of plasmon resonances in photocatalytic systems. The proposed plasmonic structure can pave a promising way for designing photocatalytic structures with high performance and low-cost.

We propose an approach for simultaneously shaping multiple three dimensional (3D) vector curve beams. The intensity distribution and polarization state variation of each 3D vector curve beam can be tuned along arbitrary prescribed trajectories. The scheme is based on the coaxial superposition of the orthogonally polarized 3D base vector curve beams through non-iterative holographic 3D beam shaping technique. In particular, the hybrid 3D vector curve beams array is also generated with identical depth range. The transversal position of each vector beam can be governed independently. Moreover, the multiplexing of 3D vector beams is realized with different depth ranges in the focal region along the beam propagation. The numerical results are identical to the experiments. This method will be benefit to the trapping and transportation of particles.

Motivated by J. Larson’s work (Crespi et al., 2012), in which vacuum induced non-trivial Berry phases for Jaynes–Cummings (JC) model were considered to be the results from the application of rotating wave approximation (RWA) and become vanishing in the quantum Rabi model without RWA. Besides a counterpart relation between the trivialness or non-trivialness of Berry phases and the existence of conical intersections (CIs) of adiabatic potentials is illustrated by introducing semi-classical energy surface. We argue that there exist no definite connections between the trivialness or not of Berry phase and the CIs of adiabatic potentials because the CIs are fully controllable for the generalized JC and Rabi models discussed in this text by adjusting the dispersive non-linear interacting parameter.

Using the Richards–Wolf formula, we investigate the tight focusing of a cylindrical vector beam of second order using a high-numerical aperture lens confined by a ring aperture. It is shown that the area of energy backflow can be enlarged by the ring aperture, and that focusing light using a lens with a numerical aperture of 0.95 and a ring aperture, which limits the angle of the entrance pupil to 0.9 of the maximum value, allows us to increase the depth of the area of negative values of the Poynting vector projection onto the optical axis by a factor of four, with a slightly varying width of the region (this varies from 0.357 to 0.352 of the wavelength of the focused light). The ratio of the absolute values of the Poynting vector for reverse and direct flows is increased by a factor of 2.5.

Weakly-coupled mode division multiplexing (MDM) technique based on few mode fiber (FMF) with low linear-polarized (LP) modal crosstalk is promising for capacity enhancement of short-reach transmission. In this paper, we propose a double-ring-core FMF for simultaneous 6-LP-mode weakly-coupled MDM transmission. FMF with ring-core structure is theoretically analyzed by deriving from a step-index circular-core FMF with perturbed ring areas. Then simplified method is proposed to define boundaries of ring areas in FMF and increase the minimum effective refractive index difference ( min | Δ n e f f | ) among all the modes. Fabricated by commercial fabrication technique, the FMF achieves a min | Δ n e f f | up to 1.49 × 10−3 while its maximum Δ n between core and cladding is only 0.828%, which greatly reduces the difficulty of fiber fabrication. Thanks to the ultralow modal crosstalk of the FMF, we successfully demonstrate simultaneous 6-LP-mode (LP01, LP11, LP21, LP02, LP31 and LP12) 4-wavelength 71-km weakly-coupled MDM transmission using 10 Gb/s optical on-off-keying (OOK) intensity modulation and direct detection (IM/DD) without any digital signal processing (DSP). The design method can be extended to weakly-coupled FMFs with more ring areas.

Refractive index imaging of a cell that reflects its morphological and structural feature has attracted increasing attention because of its important applications in life science, bioengineering and clinical applications. Based on the analysis of two orthogonal phase micrographs, we present a decoupling method for solving the refraction index distribution. In this method, a cell with one nuclei is take as a sample. At first, the radii of the nucleus and cytoplasm are determined by analyzing the gradient distributions of the associated phases. Then, the refractive index of each medium of the cell is decoupled from the measured phases. Lastly, the whole refractive index of the cell can be obtained according to the spatial position of each medium. Simulation and experimental results of a spheroid lymphocyte demonstrate the effectiveness and accuracy of this method. This method provides a new technical idea for 3D sub-structure imaging of the spheroid cells.

The membrane diffractive optical telescopes are one of areas of active research of large aperture optical telescopes. However, due to the processing technology of thin membrane mirror, the changes of temperature and humidity, etc., the imaging performance of membrane diffractive optical telescope is degraded sharply and it is difficult to reach the design target. Adaptive optics (AO) technology is a potential solution to improve the imaging performance of this telescope. Our work is the first time to correct system aberration of hundred millimeter-level aperture membrane diffractive optical telescope based on AO technology. The point target and extended target imaging experiment of the telescope with WFSless AO system in the laboratory are accomplished. The experimental results demonstrate the effectiveness of AO correction in improving the imaging performance of membrane diffractive optical telescope. Especially when there are unexpected aberrations introduced in the telescope, such as disturbances caused by assembly and operation, AO correction will be an important guarantee for the telescope to maintain good imaging performance.

In this paper, a surface-enhanced fluorescent (SEF) substrate with Ag nanostructures was synthesized by magnetron sputtering, which exhibits simple, fast, inexpensive, environmentally friendly properties, and can also be in a bulk production. The localized surface plasmon resonance (LSPR) generated by Ag-decorated dragonfly wings (DWs) substrate plays a huge role in fluorescence enhancement. The common rhodamine 6G (R6G) was used as the fluorescent dye, and a substrate having high fluorescence intensity was selected. Besides, the morphological and optical properties of the different substrates were characterized, and their enhancement factor (EF) were also calculated in order to more accurately quantify the enhancement of fluorescence. Meanwhile, using finite-difference time-domain (FDTD) method to simulate the local surface electromagnetic field of Ag nanostructure is helpful to further analyze the “hot spots” distribution of the substrate. Experimental results show that the substrate has excellent stability, reproducibility and reusability, which will have great application prospects in many fields such as life science, environmental testing, optoelectronic devices.

A circular-aperture Modified Hartmann Mask (cMHM) for quadriwave lateral shearing interferometry is proposed. The cMHM features in the hybrid structure consisting of a phase chessboard and a circular-aperture array, which is easy to manufacture. The diffraction properties of cMHM are numerically analyzed. By choosing the radius of the circular aperture to be the first positive root of Bessel function, the intensities of parasitic diffraction orders are largely diminished. In the simulations, the parasitic errors are evaluated from the perspectives of both extraction error and reconstruction error. The parasitic error of cMHM is about only half of the parasitic error of MHM. In the experiments, a compact cMHM-based lateral shearing interferometer has been built. Wavefront measurement results obtained by cMHM-LSI are compared with those obtained by commercial wavefront sensor, and the results correspond with each other well. The adaptability and accuracy of the proposed cMHM are validated.

As a two-dimensional (2D) material, the crystalline monolayer molybdenum disulfide (MoS2) has drawn extensive attention due to its interesting optical, mechanical and thermal properties compatible with many applications. Here, a dual-band perfect metamaterial absorber (PMA) composed of a simple periodically patterned cylinder/square MoS2-dielectric silica (SiO2) arrays supported by a metal ground plane is proposed and studied by using finite-difference time-domain (FDTD) simulations. Numerical results reveal that the absorption spectrum of the MoS2-based structure displays two perfect absorption peaks in the visible–NIR region. The interactions between electromagnetic waves and this PMA structure are analyzed through the field distributions and spectral responses in detail. It is also presented that the peak wavelengths can be tuned by manipulating related structural parameters. Compared with previous dual-band PMA, our absorber has only one shape that can greatly simplify the manufacturing process and the absorber is insensitive to the polarization of incident light. Furthermore, this PMA can work as a refractive index sensor with high performance due to its stable near-unity absorption.

We report a single shot fringe projection system for 3D-shape measurement using tunable frequency and region of interference with a single element Fresnel biprism interferometer. Fourier transform profilometry is a single shot technique making it exceedingly useful in the high-speed 3D reconstruction of surface profile, but it necessitates sufficient fringe density to distance out the first order spectrum from the DC term. Fresnel biprism interferometer is used to generate tunable fringe density for projection onto the object surface. The tunable fringe density generated by the proposed single element interferometer is employed for the 3D shape profilometry of a large range of sample sizes. Fourier transform fringe analysis is used to reconstruct 3D profile of the object. Fresnel biprism offers several advantages, i.e., it is low cost, offers no power loss, provides high-stability interference fringe generation, and easy to implement as it requires only a solo-element along with a broad range of tunable fringe density. Another significant advantage provided by the biprism is the tunability in the region of interference, which helps in the reconstruction of a larger area in a single shot. The variation in fringe density along with the region of interference helps in the microscopic profilometry as well as macroscopic profilometry using the same system without the need of any additional optical element. Experimental results of 3D-shape measurement of Si-IC chip, a step-like, and curved objects are presented.

A novel approach to interrogate a fiber grating sensor based on dispersive Fourier transformation method is proposed and simulated. In the system, two chirped fiber Bragg gratings (CFBG), with one used as a reference grating and the other one as sensing grating, are employed to form a Fabry–Perot (FP) interference structure, used as the spectral-shaping filter. The filter can be modeled as a two-tap delay-line filter. An optical pulse from a mode-locked laser source is reflected and spectrally shaped by the FP structure, and then the shaped spectrum is mapped to the temporal domain by a dispersive element. When the sensing CFBG is experiencing a strain, its wavelengths will be changed, leading to different interference spectra. Using Fourier transform, the frequency of the generated microwave waveform can be achieved, from which the wavelength shift of the sensing grating and the applied strain can be demodulated. A mathematical model to describe the microwave waveform generation is developed, by which the interrogation principle is analyzed. Numerical simulations are implemented to verify the proposed approach. The results present that the sensitivity of the system is 0.01 GHz/ μ e . The influences of key parameters in the system on the microwave spectra are investigated and discussed, including grating length, cavity length, chirp rate and index modulation depth.

A simple method based on an integrated polarization-division multiplexing Mach–Zehnder modulator (PDM-MZM) to produce a carrier-frequency-doubled vector signal is presented and experimentally demonstrated. In this structure, the upper sub-MZM is driven by I/Q signals and biased at the quadrature point to modulate a baseband vector signal on the optical carrier. The lower sub-MZM is driven by a LO signal and is biased at the maximum transmission point (MATP) to generate the optical carrier and second-order sidebands. By filtering the negative second-order sideband via an optical bandpass filter (OBPF) and suppressing the optical carrier via polarization controller (PC), a carrier-frequency-doubled vector signal is generated at the photodetector. Due to the adoption of LO frequency-doubled technique, the system’s requirement of a high-frequency LO is relaxed. In a proof-of-concept experiment, 300 MSym/s QPSK signal with a carrier frequency of 16 GHz is obtained by 8 GHz LO signal. Besides, 25-km fiber transmission of the generated QPSK signal is demonstrated. When the optical power obtained after fiber transmission is higher than -4 dBm, the BER after demodulation is less than 8 × 10−10. Moreover, generation of vector signal with other formats is verified.

The Debye–Wolfdiffraction integral is a powerful tool for computing the focused field of an illumination by a high numerical aperture (NA) objective lens. With the help of the fast Fourier Transform (FT) algorithm, this integral can be evaluated at a high processing speed. In general, the FT pair is the illumination field at the entrance pupil and the focused field in a transverse plane in the focal region, i.e., the xy-plane. To analyze the field distribution of special beams in a titled plane, conventional calculations require a stack of slices of fields in the xy-planes plane by plane. Here, we propose a modified FT-based diffraction integral algorithm that exhibits a direct FT connection between the illumination field and the field in an arbitrary plane in the focal volume. As a result, significant time saving is achieved with the proposed algorithm. The algorithm is verified by numerous simulations and should be helpful to quickly design holograms to shape complex focus used in holographic optical tweezers.

Generally, a traditional method to generate circularly polarized waves faces limitations like the narrow band, bulky volume and low efficiency. In this paper, a linear-to-circular (LTC) polarization converter based on metasurface with high efficiency and maneuverable ultra-wideband is proposed. By utilizing the ellipse-shaped structure on reflective metasurface, the LTC polarization converting condition can be achieved at four closer frequencies, leading to a quite wide frequency band. The simulated results show that the axial ratio of the reflective circularly polarized wave is lower than 3 dB within the frequency band of 10.21–24.97 GHz, and a relative band reaches above 83.91%. Furthermore, the bandwidth of the polarization conversion can be sustained from 10.21 to 22.71 GHz at incident an angle of 48°. The electrical field and current flows are given to analyze the circular polarization performance. For validation, a sample is fabricated and measured; measured results and simulated results coincide with each other. The proposed LTC polarization converter is easy in terms of design and fabrication complexity, showing promising potentials in real-world circularly polarized wave generator applications.

A single-shot phase measuring profilometry (PMP) based on color binary grating with intervals is proposed. In traditional PMP based on binary grating, the width for non-zero transmittance of the binary grating is always 1/2 periods rigorously. In our proposed method, a color binary grating is introduced in which three binary gratings with the same period and the same width for non-zero transmittance less than 1/3 periods are misaligned 1/3 periods one another and programmed in red (R), green (G) and blue (B) channels respectively. Because the widths for non-zero transmittance in R, G and B channels of the introduced color binary grating are all less than 1/3 periods and misaligned 1/3 periods one another, the color overlap can be avoided from the source. When measuring the object, only a single-shot deformed pattern needs to be captured and three nearly unbroken sinusoidal deformed patterns with an equivalent phase-shifting of 2 π ∕ 3 can be extracted from R, G and B components of the captured color deformed pattern. So the 3D shape of the measured object can be successfully reconstructed with three-step PMP. Experiment results show the feasibility of the proposed method. Due to its single-shot feature, it is promising to measure the dynamic objects.

Free space optical (FSO) communication systems are becoming popular since they can provide high data rates over license-free spectrum. However, the performance of FSO communication links deteriorate due to atmospheric turbulence effects. In this paper, we present the performance improvement of FSO links by employing maximum ratio transmission (MRT) technique. Atmospheric turbulence effects are considered to follow the generalized Malaga ( M ) distribution. Exact and asymptotic mathematical expressions for both bit error rate and outage probability performances are derived and then diversity gain of the system is obtained. Theoretical findings are verified with Monte-Carlo simulations. Our results show that increasing the number of transmitting apertures can effectively mitigate the deteriorating effects of the atmospheric turbulence.

For high-power laser beams modulated by a small-scale attenuation-typed scatterer, the nonlinear imaging propagation where the imaging medium is a negative index medium slab with Kerr nonlinearity is investigated. The analytical solution of the propagation is presented under the plane wave approximation and thin-medium approximation. As a result, the formation of the hot image of the scatterer is demonstrated and the corresponding quantitative expressions for the intensity and on-axis position of image are obtained. It is found that the intensity of hot image is closely related to the value of the variable linear magnetic permeability of the negative index medium. Due to the negative nonlinearity of the medium, in the downstream area of the hot image plane, the beam intensity distribution is contrary to the conventional Kerr medium case, and the intense hot image fringe develops into a broad bright band. Besides, this paper also discusses in detail the differences between the intensity and on-axis position of hot image and their counterparts in the conventional Kerr medium case and shows some other beam propagation properties.

The demand for cheaper and more efficient solar cells is the main reason behind the ongoing research on ultra-thin solar cells. In this research work, the electric field generated by the surface plasmons effects has been used to design an ultra-thin silicon solar cell. The main idea of this work is the use of paired nanoparticles with both the same and different radiuses. At first, a cell with one nanoparticle in embedded and non-embedded forms is designed and photocurrents of 11.52 and 20.87 mA/cm2 are obtained, respectively. In the case of paired nanoparticles, the photocurrents of 11.9 and 21.68 mA/cm2 are obtained for embedded and non-embedded cases, respectively. These values are 11.89 and 21.84 mA/cm2 for paired nanoparticles with different radiuses. Our simulated results show that embedded nanoparticles significantly improve the photocurrent of an ultra-thin silicon solar cell. This approach and obtained results are applicable to various cell, thicknesses, and shapes of nanoparticles.

A filter-wheel multispectral imaging system can be used to acquire the spectral reflectance of a sample. In the imaging system, large displacements between band images are obtained and corrected by employing a specific chart as a system calibration process. However, small misalignment can still occur due to the possible mechanical vibration when measuring individual samples. These small misalignments always cause chromatic aberration and corrupt spectral accuracy in the acquired multispectral image. To deal with this problem, this paper proposes a block-based multispectral image registration method that is accurate and efficient for practical color measurement. In our method, each band image is evenly divided into blocks. The local translations between individual block pairs, which are selected according to a gradient strength map, are computed based on an image similarity measure. The misalignment between reference and floating band images is modeled as a global affine transform, which can be efficiently solved using regularized least squares. Experimental results validate the accuracy and computational efficiency of our method on both synthetic and real images. It is also demonstrated that the accuracy of single-yarn spectral color measurement can be considerably improved by employing our multispectral image registration method.

Racetrack resonators present an interesting alternative to implement waveguide/resonator coupling systems enabling accurate control of the power coupling. The intra-cavity reflections arising at the interfaces between the straight and the curved waveguides of the racetrack, provide an additional degree of freedom in the design of the spectral properties of the device and under certain conditions Fano-type resonances appear which can be useful in a number of applications including optical sensing. In this paper we present a comprehensive framework for engineering the spectral properties of these resonances. Suitable metrics that can be used to optimize the design are identified along with a set of conditions that the transfer function must fulfill from an application point-of-view. We show how the design parameters involved in the transfer function calculation, can be used to determine the required structural and material parameters of the structure and provide examples of structure designs that possess favorable spectral characteristics.

We consider two-parameter real-valued parabolic diffraction-free beams. An analytical expression for the complex amplitude of the beam is obtained using the superposition of the Bessel functions. Symmetry properties of beams are analyzed and numerically demonstrated for different cases. The variety of the formed pictures increases significantly in comparison with earlier regarded parabolic beams. Particularly interesting properties exhibit beams with half-integer indices, they have symmetry properties different from the symmetry of beams with integer indices. For an arbitrary fractional index, the beams have a more complex distribution and do not have any symmetry. This fact facilitates the solution of the inverse problem when it is necessary to form similar structures, since just small changes in two parameters of the analytical expression are needed. This is much simpler than the implementation of iterative procedures, which, as a rule, provide essentially non-smooth solutions that are complex in optical implementation. Experimental generation of the fractional two-parameter parabolic beams is implemented for the first time by a spatial light modulator using phase coding of the calculated complex amplitudes. The different types of experimentally generated parabolic beams are in a good agreement with the modeling results.

We propose a symbol rate estimation technique by using absolute value of correlation of received signal for single-carrier polarization division multiplexed (PDM) systems in coherent detection system. We analytically show that spectrum of absolute value of correlation contains a peak indicating the symbol rate and has a robust performance which is insensitive to accumulated chromatic dispersion (CD) and first-order polarization-mode dispersion (PMD), amplified spontaneous emission (ASE) noise and laser frequency offset. Simulation result shows that this technique is applicable to long-haul application.

A hybrid cryptosystem based on phase-truncated Fourier transform (PTFT) encryption and diffraction transfer function is designed. The designed cryptosystem cannot be decrypted by phase-retrieval-based attacks to which conventional phase-truncated Fourier transform cryptosystems are proven vulnerable. Compared with conventional PTFT encryption, the encryption process of this proposed cryptosystem needs no extra devices. With the help of Rayleigh–Sommerfeld diffraction theory, the inverse diffraction optically is realized and the decryption process can be performed using a liquid crystal spatial light modulator. Unlike conventional PTFT cryptosystem, no useful information can be retrieved in case of lacking any of ciphertext, private keys and diffraction parameters. The optical setup and numerical simulation results are provided to verify the effectiveness, security and robustness of proposed hybrid cryptosystem.

Serious inter-symbol interference (ISI) exists in non-line-of-sight (NLOS) wireless ultraviolet communication (UVC) system, which brings some difficulties to signal detection. Channel state information is of importance for the signal processing process to suppress ISI. In this paper, a joint channel state information (JCSI) estimation algorithm using Bayesian principle is presented, so that channel response coefficient and communication signal can be obtained simultaneously. Based on the channel model of UVC, the framework of Bayesian estimation to JCSI is constructed, and realized by using differential evolution particle filter (DE-PF) algorithm. Numerical simulations show that the estimation signal-to-noise ratio (ESNR) is increased by (5 ∼ 10)dB of proposed algorithm compared with the least mean square (LMS) and recursive least squares (RLS) algorithms. The bit error rate (BER) is reduced by about one order of magnitude compared with LMS and RLS, which is also smaller than MLSD.

We experimentally demonstrate a high signal-to-noise ratio (SNR) partially coherent beam (PCB) source based on wide-spectrum produced by a filtered supercontinuum (SC) which can generate from a segment of highly nonlinear fiber (HNLF) pumped by a picosecond fiber laser. The SNR of PCB source can improve from 12.43 dB to 22.45 dB by filtering out amplified spontaneous emission (ASE) noise of erbium-doped fiber amplifier (EDFA) and using lower power pumped HNLF. We also conduct a 4 Gbit/s transmission experiment using the high SNR PCB source in 1km simulated atmospheric turbulent channel. The characteristics of scintillation indexes (SIs), eye patterns, SNRs and bit error rates (BER) based on CB source and PCB sources after turbulent channel are analyzed. In Δ T = 240 ° C , atmospheric turbulent channel, the SI of the CB link can reduce by 31% by using PCB source. The SNR of PCB source is 8.15 dB higher than the 5.20 dB of the CB source, and the sensitivity of the PCB source has a 2.9 dB improvement compared with the CB source. The PCB source possess high SNR and turbulence resistance, which can improve the performance of free space data transmission.

A underwater wireless optical communication (UWOC) system using a multi-pixel photon counter (MPPC) as the receiver and orthogonal frequency division multiplexing (OFDM) was proposed and experimentally investigated. The MPPC with high sensitivity combined with quadrature amplitude modulation (QAM) OFDM with high spectral efficiency can potentially support a data rate of hundreds of Mbps under a relatively long underwater transmission distance. Although the photoelectric response of each individual pixel in the MPPC is nonlinear due to its intrinsic dead time, the MPPC consisting of thousands of pixels can be operated like a linear photodetector due to the statistical effect. A net data rate of 312.03 Mbps, with a bit error rate (BER) below the forward error correction (FEC) limit, was successfully achieved over a 21-m underwater channel exploiting 32-QAM OFDM modulation.

Visible light communication (VLC) has emerged as a viable supplement to the existing radio frequency based communication systems. However, its performance is severely impaired by a) nonlinear characteristics of light emitting diode (LED), which reduces the Euclidean distance between neighboring constellation points, and b) intersymbol interference (ISI) due to time-dispersive nature of VLC channel, which reduces the eye opening of the received symbols and degrades the bit error rate (BER) performance. In this paper, we propose a novel kernel least mean square (KLMS) with adaptive decision feedback equalizer based post-distorter for VLC for joint mitigation of LED nonlinearity and ISI. In this work, minimum mean square error criterion based joint optimization problem is formulated in a reproducing kernel Hilbert space for optimization of the feedforward and feedback filter coefficients. Simulations indicate that the proposed post-distorter yields better BER performance as compared to existing algorithms.

In this letter, we experimentally explore the limited-size aperture effects in an orbital-angular-momentum (OAM)-multiplexed free-space optical (FSO) data link for aerial platforms. 200-Gbit/s data transmission is demonstrated between a ground transmitter and a ground receiver via a flying, retro-reflecting unmanned-aerial-vehicle (UAV) over 100-m round-trip distance by multiplexing two OAM beams. Our results indicate that when the receiver aperture size is limited, both power loss and channel crosstalk would increase. With a receiver aperture diameter of >5 cm, bit error rate (BER) mostly below 3.8 × 10−3 has been achieved for both channels

A compact silicon-based TM-pass/TE-divide polarization beam splitter based on contra-directional grating couplers (CDGCs) is proposed, where two identical corrugated strip waveguides are located at both sides of a central slot waveguide with corrugations on the bottom silicon layer. The waveguide widths are optimized so that only TE mode can satisfy the contra-directional coupling condition. The TE mode will be contra-directionally coupled to the corrugated strip waveguides by using CDGCs, and the final outputs from the two drop ports will be equal. The TM mode is hardly affected and direct passes through the central slot waveguide, the outputs at two drop ports are nearly negligible due to large phase mismatch. The coupling length of the designed device is 14 μ m . The simulation results at the central wavelength of 1550 nm indicate that, the extinction ratio is 38.4 dB (20.9 dB) and the insertion loss is − 3 . 26 dB ( − 0 . 14 dB) for TE (TM) mode. Besides, the fabrication tolerances and the power distribution along the propagation direction are presented.

We experimentally demonstrate tunable de-aggregation of two data carrying optical channels using nonlinear wave mixing. For each channel, the signal and its conjugate are coherently added and subtracted in order to map the constellation points onto real and imaginary axes, respectively. Each of two 10 and 15 Gbaud optical channels with a format of 16-quadrature amplitude modulation (16-QAM) are de-aggregated into two 4-level pulse amplitude modulation (4-PAM) signals. We use the same technique to de-aggregate quadrature phase shift keying (QPSK) signals into two binary phase shift keying (BPSK) signals at baud rates of 10 and 15 Gbaud for both channels. System performance is evaluated in terms of the optical signal-to-noise ratio (OSNR), and penalty of ∼ 0.7 dB is observed at bit error rate (BER) of 10−3.

In this paper, we propose a method to realize real-time three-dimensional (3-D) measurement with a radial spatial carrier phase shifting algorithm. The circular fringe patterns are continuously projected onto the moving object; the distorted fringe patterns are recorded by CCD. In the 3-D reconstruction process, three patterns consisting of one recorded image and two artificial phase shifting fringe patterns are used to implement phase shifting, where the two artificial patterns are formed by numerically shifting the recorded image along the radial direction. Compared with traditional carrier phase shifting method, the proposed method possesses lower phase error and higher accuracy. The simulations and two experiments are carried out to testify the proposed method. From the experimental results of the static object, we find that the standard deviation of the proposed method is better than that of the traditional one. The dynamic experiment of a moving lampshade shows that the time resolution of our method is the same as the CCD. As a real-time quantitative dynamic measurement method, it can be applied in various fields.

This work introduces a tunable nematic liquid crystal Photon Sieve-Negative Photon Sieve lens based on UV-curable polymer. The LC molecules that aligned in the different zones of the NPS–PSlens makes a refractive index difference. The fabricated lens can deform from PS to NPS mode and reverse in two various linear polarisations. The maximum focus efficiency that reached ( ∼ 38.5%) for the PS lens mode and ( ∼ 22%) for NPS lens mode that measured by applying a suitable AC voltage which was close to the maximum theoretical diffraction efficiency ( ∼ 40.5%). Electrically tunable diffraction efficiency, focal points variations and, a dual lens like mode were the features of our designed optical device.

The efficiency improvements of large area (244.34 cm2) crystalline silicon solar cells have been receiving significant attention, especially in high-efficiency multi-crystalline silicon (mc-Si) Passivated Emitter and Rear Contact (PERC) cells. The large area boron-doped mc-Si PERC cells were treated with hydrogenation for a different duration based on high-intensity infrared (HI-IR) LEDs of 940 nm. Compared to the boron-doped monocrystalline silicon (mono-Si) PERC cells, different trends of efficiency improvement were displayed after various hydrogenation duration. The results showed that the amplitude of the efficiency improvement was enhanced to the maximum around 8 min but then slowly decreased with the increasing hydrogenation duration. Moreover, we found that the iron-boron (Fe-B) pairs may occupy the dominant role in hindering the improvement of efficiency and the light-induced degradation (LID) after hydrogenation. The trend of efficiency improvement depended on the density of interstitial iron ions (Fe i + ) and the formation and dissociation of Fe-B pairs. Therefore, a longer duration of hydrogenation was required for mc-Si PERC cells than for mono-Si PERC cells to enhance the efficiency and suppress the degradation. Additionally, the duration of hydrogenation was significantly shortened to 1 min when the pre-LID was applied, then the appropriate extension of the hydrogenation duration was more conducive to the efficiency improvement of mc-Si PERC cells.

A silicon waveguide mode (de)multiplexer is theoretically proposed for compact on-chip mode (de)multiplexing (MDM). The (de)multiplexing of E 11 y , E 21 y , E 31 y , E 12 y , E 22 y and E 32 y modes is realized by cascading asymmetric Y-junctions and the E 21 y ∕ E 12 y mode rotators. Beam propagation method is adopted in the design and optimization. For a 124 μ m-long device, the mode crosstalk among all channels is less than -24 dB. The insertion loss can be lowered to 0.96 dB at optical wavelength of 1550 nm. This proposed design method can be extended to the construction of higher-order mode (de)MUX.

A passive stabilization scheme for polarization mode interferometers (PMIs) using a polarization beam splitter (PBS) is proposed and experimentally demonstrated. Polarization eigenmodes in a highly birefringent fiber are modulated by external signals. Quadrature signals utilized in demodulation algorithm are acquired from the square roots of the photo-voltages, which are detected from the transmission lights of the PBS. Experimental results show that the scheme has a large dynamic range (100.8 dB), good linearity (99.937%) and is capable of high sensitivity (10−7 rad detectability above 150 Hz). Furthermore, it is immune to intensity noise of lasers so that the system instability and complexity are obviously reduced.

Active manipulation of group delay of electromagnetic waves is highly desirable for telecommunications and information processing. However, most control means of the group delay currently demonstrated experimentally in the artificial metamaterials require different external excitations necessitating complex setups. Here, electrical manipulation of resonance strength of electromagnetically induced reflectance (EIR) are theoretically and numerically demonstrated in a hybrid metal-graphene complementary metamaterial for actively tunable slow light applications. Combining a monolayer graphene with a metal-based complementary metamaterial composed of a dipole slot and a quadrupole slot, a sharp EIR window in the complementary structure is excited due to strong coupling between two resonators. As shifting the Fermi energy of graphene implanted under the dark mode resonator via electrostatic doping, the EIR window can realize an on-to-off switch, and the modulation depth of 73.3% for the EIR resonance strength is achieved with actively switchable group delay of 6.44 ps. Further investigations show that the active modulation can be attributed to the change in the damping rate of the dark mode caused by the tunable conductivity of graphene. This work provides an alternative way for innovative design and realization of dynamically controllable terahertz slow light devices, which may show potential applications in future terahertz wireless communications.

In order to achieve higher data rate, higher spectral efficiency and greater transmission distance, the scheme of trellis coded modulation with three dimension 9QAM mapping based on a novel probabilistic shaping (PS) method is proposed in this paper. The bit error rate performance of three dimension 9QAM–TCM is investigated by simulation and compared with three dimension 8QAM–TCM scheme. Research results show that the Eb/N0 of the proposed three dimension 9QAM–TCM scheme is 1.2 dB smaller than that of three dimension 8QAM–TCM at the bit error rate of 10−4.

We demonstrate a microwave photonic single notch filter based on a two-tap delay line structure. Obviously different from previous two-tap filters with periodic notch response, our modified two-tap filter can realize microwave photonic filter (MPF) with single notch response. In the proposed two-tap delay line structure, one tap acts as a phase tunable allpass MPF, while the other tap performs as a frequency tunable bandpass MPF. A single notch MPF is realized by selecting one period of the conventional two-tap filter response. The single notch frequency is tunable over a broad bandwidth by properly setting the phase of the allpass MPF as well as the center frequency of the bandpass MPF. The proposed MPF is theoretically analyzed and experimentally verified.

A fractal metasurface-based ultra-broadband and high conversion efficiency reflective polarization rotator (PR) is proposed. The linearly y-/x-polarized (LP) incident electromagnetic (EM) wave can be rotated over 90oafter reflection by the proposed metasurface. The unit cell of the proposed PR is designed based on the fractal cut-wire resonator. Compared with a typical cut-wire resonator exciting three resonances, the fractal one can generate an extra resonant frequency. An ultra-broad bandwidth with a polarization conversion ratio greater than 0.9 (PCR>0.90) can be obtained from 8.00 to 23.00 GHz (a fractional bandwidth of 96.8%) at normal incidence, which is realized by the combination of the four resonances. The physical working mechanism and the polarization conversion performance of the metasurface are analyzed by surface current and E-field distributions, simulations and explained based on numerical calculations and plasmon resonance modes. Good agreement between the experiments and simulations is obtained. Compared with traditional PRs and other published metasurface-based PRs, the proposed metasurface can achieve an ultra-broad band and a high efficiency with a sub-wavelength size of the unit cell. The use of fractal topologies provides a novel and efficient method to further upgrade resonance characteristics in the design of PR, and the scalable geometry of the design shows its potential applications at optical, terahertz, and microwave frequencies.

Erbium nano particles (NPs) doped tellurite glasses were prepared via melt-quenched technique and their chemical composition are as follows; {[(TeO2) 0.70 ( B 2 O3) 0.30 ] 0.7 (ZnO) 0.3 } y (Er2O 3 NPs) 1−y , y = 0.005, 0.01, 0.02, 0.03, 0.04 and 0.05. The amorphous state of the glass system was revealed and the structural parameters indicating the formation of non-bridging oxygen was shown. The current work underlines the spectral parameters from Judd–Ofelt (JO) studies, and upconversion spectra. The existence of erbium NPs was found experimentally using transmission electron microscopy (TEM). The Judd–Ofelt analysis suggests that the radiative probability, A rad is strong at 2H 11∕2 —> 4I 15∕2 and a longer lifetime at 4I 11∕2 —> 4I 15∕2 . The Argon laser beam was used to determine the upconversion emission peaks. The upconversion radiation emission excited at 800 nm was found in the range 390 nm.

A bright–bright coupling mode Plasmon-induced transparency (PIT) system composedof two Dirac semimetal blocks (DSBs) with different sizes is proposed in this paper. The electromagnetic mechanism of this PIT system and its modulation property at mid-infrared band is analyzed by finite-difference time-domain (FDTD) method. Numerical results show that the transparent window, transparent peak and quality factor can be precisely controlled by the geometric parameters of the PIT system, the Fermi energy ( E F ) of the DSB and refractive index (RI) of the superstrate. The advantage of DSBs grating structure is that its permitivity functions can be adjusted handily by E F through doping or bias voltage. Compared to the PIT system based on bulk Dirac semimetals, our proposed PIT system has much smaller structure size and wider tunability. The resonant frequency of the transparent peak can shift from 17.59 THz to 34.79 THz when E F changing from 40 meV to 80 meV. The second-order nonlinearity of this DSBs system is also investigated by introducing the second-order nonlinear source. This developed Dirac semimetal PIT structure may pave the way to the development of novel THz active devices for light modulation devices, switches, biosensors and other mid-infrared devices.

Optical wireless technology provides a future-proof solution for indoor communication due to its unparallel broadband features. Different from other applications, indoor communication networks are sensitive to the cost and complexity. Indoor beam-steered infrared wireless communication system (IWC) based on wavelength division device, e.g. arrayed waveguide grating is attractive due to its low cost. However, the traditional wavelength tunable transmitter is expensive. In this paper, we propose a low-cost wavelength tunable transmitter for an indoor beam-steered IWC system. The key device of such transmitter is a simple and low cost Fabry–Perot(F–P) cavity laser. Different from the external-modulator based solutions, the proposed transmitter provides gain rather than insertion loss, which further simplifies the system design and improves the system reliability. The optimal central offset in spectrum between the modulated signal and the wavelength division devices is also investigated to improve the system performance. A 4.5 Gb/s indoor IWC system incorporating 3.2-km single mode fiber and 1.1-m free space link is experimentally demonstrated.

We investigate the time delay behavior of light through a Λ -type atomic system interacting with two orthogonal standing-wave fields in a modified reservoir. Results show that transmission spectra splitting occurs in a coupled cavity waveguide (CCW) and the time delay of light relies remarkably on the strong coupling effect between a localized atom and a CCW. The atom localization can even be larger than λ ∕ 100 in two orthogonal directions, which arises from field-induced interference effect in a modified reservoir. The proposed scheme not only provides a high-precision atom localization but also achieves a large time delay of the probe light.

We present a novel ultra-broadband, polarization-independent, and wide-angle metamaterial absorber whose metal–insulator–metal cavities are embedded into bulk insulator nanodisks. The hybridization of multiple embedded cavity modes generated by the combination of dielectric-loaded surface plasmon polaritons waveguide and cavity modes contributes to large bandwidth and high absorption in the long wavelength infrared (LWIR) region. The proposed structure exhibits more than 94% absorbance in the interval from 8 μ m to 16 μ m ; and more than 95% absorbance over the whole LWIR band (8– 14 μ m ), which is superior than the initial MIM structure (absorption greater than 90% from 8.10–14. 33 μ m ) and similar kinds of metamaterial absorber based on noble metal. In addition, a nearly perfect absorbance (more than 99%) is obtained in the 11.22–12. 46 μ m and 13.26–14. 59 μ m wavebands. Moreover, results show that the resonant wavelength and operating bandwidth can be adjusted flexibly by varying related geometry parameters. Thus, the wavelength-selective metamaterial absorber is promising for thermal emitters, energy harvester and microbolometers.

The unstable ring resonator is a promising architecture for high power solid-state oscillators with intra-cavity adaptive optics. In this paper, we present adaptive compensation of intra-cavity tilt in an Nd: YAG unstable ring laser using a probe beam and a far-field camera to detect the tilt. A numerical model of the ring resonator is established based on the Fox–Li method, through which the influence of intra-cavity tilt is studied, and the optimal strategy of tilt correction is proposed and evaluated. An experiment with an unstable Nd: YAG slab ring laser is developed. With 40 μ rad tilt on the zigzag direction and 130 μ rad tilt on the non-zigzag direction compensated during lasing, the output power increased from 77.2W to 87.0W. The above results lay the foundation for adaptive optics techniques to compensate for the intra-cavity aberrations in high power solid-state unstable ring lasers.

In view of the inefficiency of multi-digit multiplication in the current Ternary Optical Computer (TOC), it is difficult to achieve fast multiplication. Based on the one-step MSD (Modified Signed-Digit) adder, M operator, MSD multiplication principle and divide and conquer strategy, the 4-bit minimum module is designed and implemented, which can perform 4-bit multiplication. Besides, multiple minimum modules can perform operations in parallel. Based on which, the expansion scheme is proposed to achieve fast multiplication without digit limitation. This paper elaborates on the design principle, operation steps, optical path structure and expansion scheme of the 4-bit minimum module. The MSD multiplier based on the minimum module (MMM) is compared with the existing optical multiplier and the multiplier of electronic computer in the same type in terms of computational time complexity. The results show that the MMM is better than the other multipliers in the processing of multi-digit multiplication. It can give full play to the main advantages of TOC, which are giant data-bits parallel computing and reconfigurable processor, and it has more practical application significance.

Interaction between light and a phase change material is studied in the context of attenuated internal reflection. While the critical angle remains unchanged when an optically thin layer of phase change material is placed at the interface, optical phase shifts, reflectance and polarization states of light are significantly altered during the phase transition. Using vanadium dioxide as a case study, we show possible applications extending from the infrared to the visible spectrum.

According to the technical requirements of the tempo-spatially mixed modulated polarization imaging interference spectrometer (TSMMPIIS), we designed and verified the maximum optical path difference (OPD) through two different schemes, and analyzed the influences of the field of view and calcite dispersion on the OPD. When the angles between the incident plane and the principal section of the Savart left plate were 0 ° , 45 ° and 90 ° , we simulated the variations of the OPDs changing with different parameters. We found that the variations were proportional to the thickness t and the incident angle i . In order to reduce the influences of the Savart polariscope dispersion and field of view on the OPD, the corrected models were established, and the correctness was validated by the experiment. This work would provide theoretical and practical guidance for the design and engineering of the TSMMPIIS.

In this study, we report a Rayleigh and sodium lidar system, which was recently upgraded at the University of Science and Technology of China (USTC) in Hefei, China (31.5 ∘ N, 117 ∘ E). The lidar system features high temporal and vertical resolutions, a high signal-to-noise ratio (SNR) and mobility. Using the time-division and wavelength-division multiplexing methods, only one piece of photomultiplier tube is used in the optical receiver, which makes the system compact and robust. Wideband filtering and narrowband filtering are both used in the lidar system to obtain high SNR data under city lights. The lidar system was established on 24 September 2016 and has run stably for 2 years. Meteor trail events that lasted for only a few seconds were extracted from the high resolution, high SNR observational data. The sodium observational data in 2017 were fitted annually and semiannually, and the results were similar to those obtained in previous studies. The monthly average atmospheric temperature showed semiannual variations. Stratospheric aerosols were observed for two consecutive days during the observations.