Search

Chiral metamaterials play a crucial role in manipulating the electromagnetic waves. This paper intoduces a three-dimensional Dirac semimetal materials (3D DSM) metamaterial structure tailored for tunable and broadband circular dichroism (CD) effects at terahertz (THz) frequencies. Using the Jones matrix, electromagnetic multipole resonance theory, and the atomic orbital hybridization model, the resonant performance for the DSM based metamaterials in reflection and transmission design is systematically analyzed, in terms of the resonant peak and bandwidth. Simulation results show that the bilayer DSM metamaterial supports the chiral response with dual-band resonance in transmitted spectra, and its CD and working frequency could be tuned by adjusting the rotation angle between the upper and lower DSM metamaterial layers and Fermi level of DSM. Therefore, the tunable CD from 0 to 0.51 could be achieved across a wide THz frequency from 3.56 THz to 4.02 THz. Additionlly, the resonant frequency is highly sensitive to the surrounding refractive index (RI), yielding a RI sensing sensitivity up to 308.6 GHz/RIU. The proposed DSM based chiral metamaterial features a tunable and broadband design, offering a universal design method and potential applications for the modulation of light polarization, chiral sensing and imaging, and communications. • A THz chiral metamaterial with a tunable dual-band to broadband CD based on Dirac semimetals is proposed. • The CD and working frequency were tuned by adjusting Fermi level of DSM. • The CD resonance is highly sensitive to the surrounding RI with a sensitivity of 308.6 GHz/RIU. [ABSTRACT FROM AUTHOR]

In this work, we present a surface plasmon resonance (SPR) structure containing AuAl 2 and a nonlinear Kerr medium. Based on the modulation of the refractive index of the Kerr medium by pumped light, we analyze the effects of the SPR structure and pumped light on the photonic spin Hall effect (PSHE). The numerical results indicate that the thickness of the AuAl 2 exerts a profound impact on the SPR, while the intensity of the pump light (I p) exerts a significant influence on the resonance angle. The transverse shift (TS) is very sensitive to the refractive index (RI) of the sensing layer after increasing through the SPR. Furthermore, it is observed that the sign of the TS of the PSHE can be modulated according to I p within a specific angle of incidence. In light of these findings, we put forth a novel optical signal modulation and RI sensing scheme based on PSHE. Our study provides a new mechanism for practical applications of PSHE in optical communication and precision measurement. • Enhanced photonic spin Hall effect by AuAl 2 -based surface plasmon resonance. • By employing nonlinear modulation, this work achieves transverse shift inversion and coding. • A refractive index sensor with sensitivity 2. 599 × 10 6 μ m /RIU. [ABSTRACT FROM AUTHOR]

Extra-cavity synchronous intensity modulation is a straightforward and effective approach to suppress the full-bandwidth relative intensity noise (RIN) for noise-like pulse (NLP) fiber lasers. In this work, the picosecond NLPs are produced by a homemade Er/Yb co-doped mode-locked fiber laser and subsequent optimized by the synchronous intensity modulation device. The photodetector (PD) and electro-optical modulator (EOM) allow the real-time measurement and modulation for each optical pulse. A detailed description of key parameters for components is presented. The impact of DC bias voltage and RF signal amplitude on resulting RIN is experimentally investigated. We demonstrate a noteworthy reduction in RIN from 5.62% before synchronous intensity modulation to 1.91% afterward. Numerical investigations suggest that this method is potential to suppress RIN to below 1% under ideal conditions. The key factor in RIN reduction is correct operating region and high photoelectric-conversion linearity. The synchronous intensity modulation is an effective method to obtain low-RIN NLP laser sources, valuable for some practical applications. • Full-bandwidth relative intensity noise suppression based on synchronous intensity modulation. • The real-time measurement and modulation for each optical pulse. • A noteworthy reduction in RIN from 5.62% before synchronous intensity modulation to 1.91% afterward. • Correct operating region and high photoelectric-conversion linearity benefit to RIN reduction. [ABSTRACT FROM AUTHOR]

Simulating the signal transmission process in neurons is crucial in computational neuroscience. However, much of the existing researches simplifies signal transmission in a single neuron to a basic mathematical process. In this article, we introduce an all-optical silicon artificial neuron incorporating an aperiodic gold monotile structure. This structure enables the simulation of uniform signal transmission within the axon. Furthermore, we propose a III-V quantum wells protocol to achieve the amplification function of nodes of Ranvier in neurons. • A possible simulation method for the transmission process of neural signals based on silicon waveguides is proposed. • This method could imitate the uniform signal transmission and the signal amplification. • This method provides a potential hardware strategy for neuroscience, which is high adaptable to other optical modulations. [ABSTRACT FROM AUTHOR]

Metasurface that features flexible capability of light intensity modulation is widely applied to generate grayscale nanoprinting. To encode multiple grayscale nanoprintings into a single metasurface, different settings of analyzers are typically required. Here, a wavelength-multiplexed metasurface with no need of analyzer is proposed to realize a dual-channel continuous grayscale nanoprintings. The proposed metasurface is composed of two layers of nanobricks (NBs), where each layer of NBs independently acts as a polarizer under the light illumination at two selected wavelengths. By tailoring the orientation angles of NBs, distinct continuous grayscale nanoprintings with high fidelity and no crosstalk are encoded into two wavelength-dependent channels. Meanwhile, the proposed configuration can still generate the same high-quality nanoprintings with a large misalignment appearing between two layers of NBs, possessing outstanding structural robustness. The proposed wavelength-multiplexed strategy offers new insights into multi-channel display and paves the way for low-crosstalk display, high-density optical information storage and optical encryption. • A wavelength-multiplexed metasurface is proposed to realize a dual-channel continuous grayscale nanoprintings. • Each layer of nanobricks independently acts as a polarizer under the light illumination at two selected wavelengths. • By tailoring the orientation angles of nanobricks, distinct grayscale nanoprintings are encoded into two channels. • Meanwhile, the proposed configuration possesses outstanding structural robustness. • The proposed strategy offers new insights into multi-channel display. [ABSTRACT FROM AUTHOR]

This paper proposes a differential index modulation (DIM) scheme to address the complex channel estimation challenges in optical orthogonal frequency division multiplexing with index modulation (OOFDM-IM). The main idea of the DIM scheme is to design a time-frequency dispersive matrix with unitary characteristics and perform index mapping based on the Lemer code principles. By applying differential operations, the DIM scheme enables decoding at the receiver without requiring channel estimation. The paper provides a detailed explanation of the design principles and theoretical bit error rate (BER) of the proposed scheme. Simulations based on the exponential Weibull atmospheric turbulence channel model are conducted to compare the DIM scheme with the existing OOFDM-IM scheme. Additionally, a carefully designed proof-of-concept experiment is performed to further validate the scheme's effectiveness and feasibility. Both simulation and experimental results demonstrate that, compared to OOFDM-IM, the proposed scheme can entirely avoid complex channel estimation with a maximum signal-to-noise ratio (SNR) loss of no more than 4 dB, even under various turbulence intensities and higher-order modulation scenarios. This provides a valuable reference for OOFDM-IM in complex environments or where channel estimation is challenging. • A novel OOFDM-DIM scheme for channel estimation free is proposed. • A theoretical performance based on EW channels has been derived. • Actual proof-of-concept experiments were designed. [ABSTRACT FROM AUTHOR]

The bound states in the continuum (BICs) inspired mechanism of light localization makes it possible to realize high-Q quasi-BICs metasurface, and can be better applied to dynamic modulation and reconfigurable optical devices. In electro-optic modulation, the refractive index of the material is shifted under applied voltage, which has high stability and high response speed, and is very suitable for dynamic metasurfaces. In this work, a metasurface platform based on permittivity-asymmetry breaks the limit on the lowest achievable geometrical asymmetry of nano-fabrication or the ultra-high electro-optical coefficients of building materials compared to the previously proposed dynamic electro-optical metasurfaces. A delicate design of double-grating BaTiO 3 metasurface was presented that the long nanostripes of the periodic grating structure act as a bridge between the internal structure and the external environment. Subtle external environmental disturbances are introduced into the internal nanostructure through the long nanostripes, breaking the symmetry of the original metasurface, and the quasi-BICs can be triggered to generate an extremely high-Q optical signal response. To illustrate the concept of this platform, a barium titanate electro-optic metasurface that can reach a Q-factor of 105 under the applied electric field of 2 kV/mm was designed and demonstrated through simulation. The platform developed in this study will become a promising platform for integrated optics, information optics, bio-optics, topological optics, and nonlinear optics. • A new and ingenious solution for breaking the in-plane symmetry by double-grating nanostructure. • The subtle external environmental disturbances are introduced into the internal nanostructure through the long nanostripes. • A high Q -factor of the metasurface isn't essential to realize dynamic optical signal modulation with 100% modulation depth. • The metasurface doesn't depend on ultra-high electro-optical coefficients of the building material. • In six dimensions which are based on practical application considerations, the metasurface has reached a very high standard. [ABSTRACT FROM AUTHOR]

Optical forces induced by polarization singularities from photonic crystal slabs (PCSs) have gained widely attention in recent years. To enhance the degree of freedom in controlling far-field optical forces, we propose and design a three-layer PCSs structure that simultaneously possesses multiple optical forces with reconfigurability. By modulating the interlayer spacing, the coupling strength between the slabs is regulated, resulting in a band transition from the nodal rings to the nodal ridge, which facilitates the generation of dual polarization vortices. These dual polarization vortices induce multiple phase singularities at their boundaries, leading to multiple optical force singularities, offering the possibility of capturing and dispersing nanoparticles. Interestingly, tuning the spacing among the slabs can transform the converging force into the diverging force at the center, therefore achieving particle screening. This work not only expands the application of nodal ridges to optical force in photonic structures but also provides a feasible approach for the generation of double-ring vortex beam in the nano-optics field. • A three-layer photonic crystal plate structure with multiple optical forces and reconfigurability has been designed. • Adjusting the interlayer distance of the photonic crystal plate can achieve band transitions from node rings to node ridges. • Particle screening can be achieved by turning the spacing among the slabs. [ABSTRACT FROM AUTHOR]

Laser damage to optical components induced by particle contamination is one of the bottleneck problems limiting the output energy of high-power laser facilities. Although there are many studies on the modulation of light fields by particle contaminants, most of them focus on near-field calculations. In this paper, the influence of particles on the front surface of fused silica on the electric field enhancement on the exit surface of fused silica is studied. Through the research in this paper, particle contaminants cause the electric field to be enhanced on the exit surface of fused silica. Particle diameter, number of particles, particle shape, and particle material are important factors affecting the electric field enhancement on the exit surface of the optical component. The influence of fused silica particles and metal particles on the light intensity of the optical component exit surface is difficult to determine even through numerical simulation, because it is affected by the particle diameter and shape. The research in this paper helps to understand the mechanism of particle contamination-induced laser damage in fused silica. • A numerical model of particle-induced electric field enhancement on the outlet surface of fused silica was established. • The effect of particle radius, particle material, particle shape and number of particles are discussed. • The static light field distribution of a laser beam propagating in a fused silica bulk material is calculated. [ABSTRACT FROM AUTHOR]

In this study, a novel multilayer terahertz metamaterial with four graphene sub-structures is designed to achieve dynamically tunable triple plasmon-induced transparency (PIT) due to interference among bright-bright modes. Coupled mode theory (CMT) and finite-difference time-domain (FDTD) simulations show a high degree of consistency in their results. Interestingly, enhanced transmission dips are observed when various graphene structures couple with the two longitudinal graphene strips at the bottom layer. Based on the dynamic modulation characteristics of graphene, the metamaterial demonstrates seven-band optical switching capabilities, achieving modulation depths (MD) of 94.6%, 88.1%, 90.8%, 90.8%, 90.4%, 86.6%, and 87.5%. Additionally, the proposed graphene metamaterial also shows excellent slow-light effects, with a group index reaching up to 1034. This proposed terahertz metamaterial holds significant theoretical implications for the development of dynamically integrated terahertz optoelectronic devices. • A multilayer terahertz graphene metamaterial structure has been proposed for achieving dynamically tunable triple PIT. • The optical switching effect can be achieved by adjusting the Fermi energy level. • Enhanced transmission dips are observed when different graphene structures are coupled with two graphene strips. [ABSTRACT FROM AUTHOR]

In single-pixel imaging (SPI), resulting image quality is dependent on the number of patterns, which can be alleviated by modifying pattern structures in optical modulation. However, each randomly encoded modulation pattern is not merely composed of displayed structural information. Here, we propose a SPI reconstruction strategy enhanced by hidden pattern information (HPI) in random patterns, which does not require modifying modulation patterns. We exploit the equivalency of structural information to generate extra pattern information in the image reconstruction stage. We utilize the weighted summation of results with different scales to reconstruct resulting images based on a simple algorithm. We experimentally demonstrate that using added HPI achieves better performance than using only modulation patterns in the same measurement conditions. Our work reveals that SPI using random patterns has the ability to recover well-defined results without external interference. • A single-pixel imaging reconstruction strategy enhanced by hidden pattern information in random patterns is proposed. • The equivalency of structures in random patterns is used to generate extra pattern information in the reconstruction stage. • More accurate structural information is recovered by using hidden pattern information in the same measurement conditions. [ABSTRACT FROM AUTHOR]

As a powerful supplement to the wireless communication, free space optical (FSO) communication has many advantages, but the high-precision time-frequency transmission based on FSO communication is limited by complex equipment factors, especially under the condition of limited communication bandwidth. In this paper, a high-precision frequency transfer system based on a tunable optical field is established. Theoretical analysis yields the detection probability and intensity distribution of Bessel beams. In experiments, Bessel beams generated through optical field modulation exhibit enhanced laser data communication capabilities, achieving high-precision frequency transfer at 2e-12@1s, 3.3e-14@1000s,. This research introduces a novel approach for designing and optimizing frequency transfer methods between smaller platforms in the future. • A high-precision frequency transfer system based on a tunable optical field is established. • An innovative coding scheme is proposed for frequency standard data fusion transmission to avoid redundant coding. • Theoretical and experimental analysis of time-frequency transmission by light field modulation. • Optical field control is realized under the condition of limited communication bandwidth. [ABSTRACT FROM AUTHOR]

Undoubtedly, optical choppers are one of the oldest and most common light modulation tools, which still have many applications in optoelectronic devices such as guidance systems, opto-mechanical mouses, and photoacoustic systems. In this study, transmission function of a chopper with circular shape apertures was studied whereas the distribution of the incident beam was considered Gaussian. A mathematical model was obtained for transmitted signals through the circular chopper. Studies were done for different ratios of apertures diameter to the beam cross section and the effect of this ratio on the profile of output pulses were studied. Finally, this model was validated by a simple optical experiment. This model can be easily extended for any laser beam profiles such as top hat beams and can be a leading method in optical devices and techniques based on the use of chopper wheels. • A new theoretical method for modeling output pulses from a chopper wheel with circular shape apertures was presented. • A new geometry of the problem was redefined which can obtain the transmission function with a very high accuracy. • Numerical studies were carried out for different ratios of apertures diameter to the beam cross section. • This model was easily extended for both the Gaussian and top hat beams. • Finally, this model has been confirmed through an optical experiment. [ABSTRACT FROM AUTHOR]

The advancement of lithography, mass spectrometry and other technologies is increasingly requiring the high-quality focusing of extreme ultraviolet (EUV) beams. Traditional transmissive optics are not suitable for focusing EUV beams due to the strong absorption of most materials. However, rapid developments of metasurfaces have led to breakthroughs in the research of EUV metalenses. Based on the hole-type unit structure proposed recently, we demonstrated the dual-wavelength achromatic metalens in the EUV working for wavelengths of 46.9 and 69.8 nm with a numerical aperture 0.05. Then the dispersion manipulation for the achromatic metalens is extended to three wavelengths 46.9, 60, and 69.8 nm. The simulation results suggest that the maximum relative focal shift from the designed focal length is 0.008 %, and the diffraction-limit focusing is achieved for all working wavelengths. This work could expand the means of the EUV optical field modulation. • Transmissive EUV metalens is designed for beam focusing. • The dual-wavelength achromatic metalens is demonstrated. • The dispersion manipulation for the metalens is extended to three wavelengths. [ABSTRACT FROM AUTHOR]

A brand new hybrid metasurface cell consisting of a simple apron-shaped structure layered with photosensitive silicon is proposed with a Q -value of 1.43 × 104 by optimizing the structure, which achieves bidirectional and continuous regulation of the quasi-BIC and BIC through the excitation of the photosensitive material by a pumped light source. With an unprecedented optical quality factor Q of 4.13 × 104 under the final strong light modulation. Then, the dominant role of the electric quadrupole (EQ) on the BIC and high-intensity quasi-BIC resonances of the symmetrical protected state is explored and verified. The refractive index sensing sensitivity after modulation is 142.77GHz/RIU. The figure of merit (FoM) is increased from 3268 to 8923, which is the highest value of the current theory, providing desirable ideas for narrowband filtering, high sensitivity sensing and biomolecular identification. [ABSTRACT FROM AUTHOR]

—A novel optical frequency-hopping (OFH) scheme using optical delay interferometers (DI) is proposed and demonstrated for secure transmission in free space. By performing carrier suppression modulation on the light wave emitted by the laser and connecting the phase modulator (PM) and DI in series, the conversion of the light wave modulated by the Mach-Zehnder modulator (MZM) from phase modulation to intensity modulation can be realized, and finally output the desired optical frequency-shift-keying (OFSK) carrier signal. Meanwhile, by controlling the positions of the frequencies of the positive and negative first-order sideband light waves on the DI frequency response curve, the OFSK signals output by the two ports of the DI can be complemented in the time domain. For the proposed OFH scheme, we carried out simulation experiments of 5 km free-space link transmission and back-to-back transmission with a communication rate of 10 Gbps, and the simulation results proved the feasibility of the scheme. Additionally, we also analyze the security performance of the proposed scheme and give the security space based on the eavesdropping probability. • A novel optical frequency-hopping (OFH) scheme using optical delay interferometers (DI) is proposed and demonstrated for secure transmission in free space. • The simulation experiments of 5 km free-space link transmission and back-to-back transmission with a communication rate of 10 Gbps are conducted, and the BER curve and receiving eye diagram have confirmed the feasibility of the proposed OFH scheme. • It provides a new perspective for future studies on optical physical layer security technology. [ABSTRACT FROM AUTHOR]

Graphene-based free-space optical modulators are usually enabled by shifting electrically the resonant frequency of a plasmonic or optical resonator. However, this resonance-shifting approach could lead to a potential loss of the on/off ratio, considering the fact that photodetectors always respond in a spectral range. Here, to modulate directly the amplitude of mid-infrared light, we propose a graphene-based modulator employing a graphene layer and a magnetic plasmonic array. With this novel design, the transmission intensity through the plasmonic array can be modulated in a direct manner without shifting the transmission peak. Magnetic plasmon resonance (MPR) accounts for the observed transmission, while Wood anomaly (WA) is utilized to fix the resonant position. The proposed modulator shows an excellent tolerance upon variation of structure parameters, with an insertion loss as low as 0.7 dB and a typical on/off ratio of 20, as well as a large modulation depth of 95%. This work offers a new route to design graphene-based optical modulators and may find applications in planar optics and the free-space optical (FSO) communication. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

To mitigate the nonlinear distortion of a multiband orthogonal frequency division multiplexing (OFDM) radio over fiber (RoF) system, a frequency-separated model-based digital pre-distortion (DPD) technique is proposed and experimentally demonstrated herein. Frequency-separated model DPD, when used as a pre-equalizer, mitigates nonlinearity in a transmitter. The nonlinear characteristic of an RoF system depends on the frequency band. Therefore, the proposed equalizer separates the input by frequency band and mitigates nonlinearities. A least mean square algorithm is used to estimate the variables of the proposed equalizer. Using the proposed DPD technique, performance enhancement was verified experimentally relative to the adjacent carrier leakage ratio (ACLR). The proposed technique obtained ACLR values of 27 to 30 dB. The results demonstrate that signal performance improved in terms of error vector magnitude from 6.3% to 3.5% at an optical modulation index value of 63%. [ABSTRACT FROM AUTHOR]

This paper proposes a probabilistic shaping method based on intrinsic bit-level labeling and asymmetric polygon modulation for optical interconnects, which can significantly improve the system performance in power consumption and bit error rate. An experiment of 35.6 Gb/s 32-to-8 probabilistic shaping carrierless amplitude and phase (CAP) data transmission using intensity-modulation and direct-detection system (IMDD) over 25 km standard single-mode fiber (SSMF) is successfully demonstrated to show the superiority. Results show that the system performance is improved by more than 4 dB at the 1 × 1 0 − 1. 5 bit error rate (BER) compared with the traditional uniformly-distributed 32-CAP. The experiment suggests the proposed scheme a promising technique for future optical interconnects. [ABSTRACT FROM AUTHOR]

Abstract A Ge2Sb2Te5 (GST)-based four-level pulse amplitude optical modulator is proposed and its optical and electrical properties are studied. The four-level pulse amplitude modulation (PAM4) scheme is used as a solution to overcome the inherent speed limitation of the phase-change based optical modulators. The modulator is formed by implementing two GST-based active waveguide sections in a silicon ridge waveguide where the propagation loss of each active waveguide section can be varied by changing the GST phase. Optical simulation results show that the four different output states of the modulator are separated by more than 2.5dB in the ultra-wide wavelength range of 1. 4 μ m < λ < 1. 7 μ m , while the small overall footprint of 3.28 μ m 2 , results in low insertion loss of 1.19dB. Electrical and thermal properties of the structure are also investigated which reveal the very small energy consumption of 11.97pJ/bit and high modulation speed of 0.428Gbit/sec for the proposed PAM4 modulator. These calculations show substantial improvement in the performance parameters of the proposed structure compared to the previously reported phase-change material (PCM)-based optical modulators in terms of modulation speed and energy consumption. [ABSTRACT FROM AUTHOR]

Abstract We demonstrate the improvement of the optical power budget in multi-intermediate frequency over fiber (IFoF) based indoor-distributed antenna system (DAS) with 48 IF carriers of LTE-A signal using laser over-modulation. The laser over-modulation can improve the optical power budget by increasing the input RF power to DFB-LD until a limit of nonlinear distortion such as CTB and clipping, when the received optical power is very low. To compensate the decrease of optical power budget in analog optical link, we employ the laser over-modulation, and derive the optimum optical modulation index (OMI)/ch to enhance link budget. To investigate the effect of laser over-modulation, we analyze the noise characteristics associated with the carrier-to-noise ratio (CNR) depending on the received optical power, and compare three different optical receivers (Rx) most often used: i) PIN-photodiode (PD), ii) APD, iii) PIN-PD following an EDFA (EDFA-PIN), respectively. Using the laser over-modulation, it is experimentally observed to achieve optical power budget of 25 dB for PIN-PD, 31 dB for APD, and 33 dB for EDFA-PIN in multi-IFoF based indoor-DAS for 48 x 20 MHz LTE-A signals transmission, respectively. [ABSTRACT FROM AUTHOR]

Abstract Power budget is a key factor that should be considered in intensity modulation and direct detection (IM/DD) passive optical networks (PONs). Based on the low-cost 10G-class electro-absorption modulated laser (EML) operating at C-band, we not only experimentally and theoretically investigate the interplay between the chromatic dispersion, the chirp of EML, and the self-phase modulation (SPM), but also explore the influences of the resolution and timing jitter of the analog to digital converter (ADC). Both theoretical and experimental results show that high power budget can be effectively achieved by making full use of the high launch power induced SPM effect and EML-induced frequency chirp. Based on the 10-G transmitter optical sub-assembly (TOSA) and receiver optical sub-assembly (ROSA), we successfully realize 25 Gb/s signal transmission at C-band over 25 km standard single mode fiber (SSMF) in IM/DD PON system. Even the worst performing bands can still achieve 25 dB power budget. The proposed scheme in this work has a potential application in future cost-effective optical access networks. [ABSTRACT FROM AUTHOR]

Liquid crystal variable retarder (LCVR), as a promising and precision device in applications of light field modulations and manipulations, has been widely used to change the phase and thus the polarization state of light based on electronically controlled rotations of liquid crystal molecules. However, the LCVR usage inevitably involves an important task, i.e. , the calibration, since the phase retardation of LCVR changes with the temperature of the environment and the wavelength of the incident light. In this paper, based on using a Wollaston prism, we propose an effective calibration method to tailor the voltage-phase characteristic curves of LCVR. In the experiment, given a wavelength of light to be calibrated, similar calibration results are always obtained, regardless of the angle of polarization (AoP) of the incident light and the angle of LCVR placement. Experimental results show that the root-mean-square error (RMSE) of the voltage-phase calibration is no more than 0.132 radian. Moreover, excellent robustness and efficiency of this LCVR calibration method are also demonstrated and verified. • Based on using a Wollaston prism, we propose an effective calibration method to tailor the voltage-phase characteristic curves of Liquid crystal variable retarder (LCVR). • The proposed method always gives similar calibration results, regardless of the angle of polarization (AoP) of the incident light and the angle of LCVR placement. • The calibration process is accuracy, stable, efficient and fast, and demonstrates a good root-mean-square error (RMSE) of the voltage-phase calibration no more than 0.132 radian. [ABSTRACT FROM AUTHOR]

Optical eigenvalues of the eigenvalue equation associated with the nonlinear Schrödinger equation (NLSE) are invariant during propagation in nonlinear dispersive fiber. Optical eigenvalue communication is expected to be a technology for overcoming the Kerr nonlinear limit. Several modulation methods based on optical eigenvalue have been proposed. In this paper, we demonstrate an arbitrary optical eigenvalue modulation scheme by numerical simulations and experiments. Based on the proposed scheme, the generation and transmission of a 65 536-ary eigenvalue-modulated signal is demonstrated by numerical simulation. Moreover, we experimentally demonstrate modulation, transmission, and demodulation of a 16-ary eigenvalue-modulated signal. The 16-ary signal represented by combinations of four eigenvalues can be successfully transmitted through a 100-km standard single-mode fiber line. [ABSTRACT FROM AUTHOR]

We propose and theoretically analyze a wideband frequency-tunable optoelectronic oscillator based on stimulated Brillouin scattering (SBS) and gain–loss superposition with ultra-narrow linewidth. Typically, the linewidth of optoelectronic oscillator (OEO) based on SBS is difficult to narrow because of the relatively fixed 3-dB bandwidth of SBS. In the structure, three generated new pump lights with certain frequency intervals generated by optical modulation on the pump laser light are launched in a highly nonlinear fiber (HNLF) from the direction opposite to the phase modulated signals, where the SBS effect occurs. The gain spectrum of one pump light is superimposed with the loss spectra of the other two pump lights. As a result the linewidth of the output signal reduces highly compared with conventional OEOs based on SBS. Consequently, a microwave signal with wide tunable range of 1 GHz to 40 GHz and near 60% linewidth reduction at most is realized in theory. Particularly, the theoretical simulation of the phase noise is about −112.5 dBc/Hz at 10 kHz offset when the wavelength of the continuous wave laser is 1550 nm and the frequency of the output signal is 35 GHz. In addition, the proposed method can be generalized, theoretically, to any OEO based on SBS to effectively reduce its linewidth. Overall, the proposed structure improves the quality of generated signals prominently and has vast potential in microwave generation. • The technology of gain-loss superposition of SBS is used to narrow the linewidth of the OEO signal, at most 60%. • The tunable range is from 1 GHz to 40 GHz by the use of two laser sources. • The proposed method of modulating the pump light can be applied to any OEO based on SBS to reduce the linewidth. [ABSTRACT FROM AUTHOR]

The molybdenum disulfide (MoS 2) nanosheet is successfully synthesized using a hydrothermal Li-intercalation exfoliation method. We experimentally show that the MoS 2 dispersion solution can be used for optical modulation and all-optical switching based on the plasma channel induced by control pulses. The modulated signal light has a good transmission property in free space. The spatial modulation of the signal laser can be easily adjusted through the power of control lasers and the suspension concentration. The influence of suspension concentration and type on the free-space propagation of modulated signal light is also investigated. Results reveal a potential application of MoS 2 as an all-optical modulator for diversified photoelectric applications. • MoS 2 dispersions can be used for all-optical plasma modulation and optical switching based on plasmas. • Modulations can be adjusted conveniently by control laser power and suspension concentration. • Divergence of modulated signal beams increases with suspension concentration. [ABSTRACT FROM AUTHOR]

In this paper, we first propose and analyze tunable Fano resonance based on indirect interaction between two Taiji resonators which both side-couple with a MIM (metal–insulator–metal) waveguide. An S-shaped branch waveguide embedded in the Taiji ring can tailor the propagating modes and then varying the number of whispering-gallery (WG) modes in the Taiji resonator, leading to the convenient tuning of Fano resonance lineshape. By covering monolayer graphene on the waveguide, Fano resonance lineshape can be all-optically tuned with relatively low pump intensity because of the optical Kerr effect of graphene; by coating UV-glue on the proposed structure, temperature sensing with high sensitivity of −0.23 nm/°C is obtained because of large negative thermo-optic coefficient of UV-glue. Furthermore, up to 16 Taiji resonators are designed and coupled with a common MIM waveguide to observe and analyze Fano lineshape variation, finally realizing the optical band-stop filter with maximal bandwidth of 49.5 nm and optical band-pass filter with maximal bandwidth of 45.3 nm. This ultra-compact micro-structure of plasmonic waveguide coupling multiple Taiji resonators demonstrates good potential applications in the nonlinear optics, optical modulation, and high-sensitivity biochemical sensors. • Fano resonance is observed and tuned based on multiple Taiji resonators side-coupled with MIM waveguide. • Temperature sensing with sensitivity of -0.23 nm/°C is designed based on UV-glue covered Taiji resonators. • Band-stop filter and band-pass filter are proposed based on multiple Taiji resonators. [ABSTRACT FROM AUTHOR]

We numerically propose a low voltage, high extinction ratio optical modulator with a tunable group delay in a resonance enhanced ring resonator based on Silicon (Si) - Indium Tin Oxide (ITO) heterojunction. This is accomplished by incorporating a thin ITO layer into a Si-ring resonator. By electrically inducing carrier changes in ITO, the epsilon-near-zero (ENZ) state of ITO is attained, resulting in a sudden change in optical absorption. The resonance condition shifts because of the ENZ state, and the transmission at the ring resonator's through port changes dramatically. The extinction ratio (ER) of 11 dB at a low forward bias of 1 V is reported with a low energy consumption of 2.6 fJ. An optical network has a non-trivial demand for a high extinction ratio optical modulator that operates at a low driving voltage and consumes minimal energy. The proposed ring modulator with a high extinction ratio at low driving voltage outperforms conventional waveguide-based modulators in terms of energy efficiency. In addition, we report around 25 psec of electrical tuning in the group delay The current concept has favorable results in terms of exploring tunable delay lines in conjunction with optical modulation with a high extinction ratio. [ABSTRACT FROM AUTHOR]

A novel microwave photonic single-passband filter with an improved out-of-band rejection and an extended frequency tuning range is proposed and demonstrated, which is achieved by using birefringence of a phase-shifted fiber Bragg grating (PSFBG). If two polarization states of two phase-modulated signals with opposite modulation indices from a polarization modulator (PolM) are controlled to be aligned with two principal axes of the polarization-maintaining PSFBG (PM-PSFBG), respectively, two single-passband or two single-notch filters with slight frequency interval are achieved, depending on the position of the optical carrier relative to the notches of the PM-PSFBG. Through subtraction of the two filters, the out-of-band rejection is significantly improved, and the frequency tuning range is greatly extended. Analytical simulations are performed using an established theoretical model, which are verified by an experiment. [ABSTRACT FROM AUTHOR]

We study theoretically the localization and coupling properties of defect modes in octagonal quasi-crystals made of conjugated polymers for the light modulation of organic light-emitting devices (OLED) and organic lasers. Due to the rotational symmetry of quasicrystals and the low refractive-index of organic semiconductors, it is observed that localized modes only occur in the complex defect-cavity structures. Unlike conventional photonic crystals with high-index, some unusual characteristics of defect modes coupling in quasicrystals under the low-index contrast are revealed and analyzed, such as the loss of defect modes, composite resonant cavity, and phase-opposition of modal tails. These results will allow us to better utilize the light control in organic photonic crystal microcavity lasers, especially in the localization region, and further to apply in OLED and VCSEL (Vertical Cavity Surface Emitting Laser) fields. [ABSTRACT FROM AUTHOR]

Variable optical attenuation and optical modulation are essential operations in integrated photonic circuits. A new graphene plasmonic gap waveguide structure, which possesses the functions of variable optical attenuator and modulator, is proposed and numerically investigated. The propagation and attenuation modes of the structure can be realized by tuning the gate-voltage. The result shows that it can achieve great modulation depth per micrometer and the working frequency range of 8 THz or more for the variable optical attenuator and the modulator. It also has the advantages of high tolerance, large attenuation range/ modulation depth, wide bandwidth and high integration without crosstalk, which would be promising for high integrated optical circuits. [ABSTRACT FROM AUTHOR]

We introduce a novel method of non-diffractive Mathieu beams formation using a technique of ‘combined axicon and amplitude modulation’. First, the theoretical mechanism of generating Mathieu beams is analytically derived in detail using the stationary-phase method. Subsequently, we experimentally generate several kinds of Mathieu beams using an amplitude-type spatial light modulation (SLM)-plus-axicon setup. The experimental results show that the proposed method for generation of Mathieu beams is simple and flexible, and thus it is particularly suitable for generating the family of all kinds of non-diffractive Mathieu beams. [ABSTRACT FROM AUTHOR]

In conventional integral imaging, the singlet refractive lens array limits the imaging performance due to its prominent aberrations. Different from the refractive lens array relying on phase modulation via phase change accumulated along the optical paths, metasurfaces composed of nano-scatters can produce phase abrupt over the scale of wavelength. In this letter, we propose a novel lens array consisting of two neighboring flat dielectric metasurfaces for integral imaging system. The aspherical phase profiles of the metasurfaces are optimized to improve imaging performance. The simulation results show that our designed 5 × 5 metasurface-based lens array exhibits high image quality at designed wavelength 865 nm. [ABSTRACT FROM AUTHOR]

The effect of quantum well position on the modulation and distortion characteristics of a 1300 nm transistor laser is analyzed in this paper. Standard three level rate equations are numerically solved to study this characteristics. Modulation depth, second order harmonic and third order intermodulation distortion of the transistor laser are evaluated for different quantum well positions for a 900 MHz RF signal modulation. From the DC analysis, it is observed that optical power is maximum, when the quantum well is positioned near base–emitter interface. The threshold current of the device is found to increase with increasing the distance between the quantum well and the base–emitter junction. A maximum modulation depth of 0.81 is predicted, when the quantum well is placed at 10 nm from the base–emitter junction, under RF modulation. The magnitude of harmonic and intermodulation distortion are found to decrease with increasing current and with an increase in quantum well distance from the emitter base junction. A minimum second harmonic distortion magnitude of −25.96 dBc is predicted for quantum well position (230 nm) near to the base–collector interface for 900 MHz modulation frequency at a bias current of 20 I bth . Similarly, a minimum third order intermodulation distortion of −38.2 dBc is obtained for the same position and similar biasing conditions. [ABSTRACT FROM AUTHOR]

We theoretically and numerically investigate the extraordinary optical transmission through a terahertz metamaterial composed of metallic ring aperture arrays. The physical mechanism of different transmission peaks is elucidated to be magnetic polaritons or propagation surface plasmons with the help of surface current and electromagnetic field distributions at respective resonance frequencies. Then, we propose a high performance terahertz modulator based on the unique PSP resonance and combined with the metallic ring aperture arrays and a self-gated parallel-plate graphene capacitor. Because, to date, few researches have exhibited gate-controlled graphene modulation in terahertz region with low insertion losses, high modulation depth and low control voltage at room temperature. Here, we propose a 96% amplitude modulation with 0.7 dB insertion losses and ∼ 5.5 V gate voltage. Besides, we further study the absorption spectra of the modulator. When the transmission of modulator is very low, a 91% absorption can be achieved for avoiding damaging the source devices. [ABSTRACT FROM AUTHOR]

The generation of high-order harmonics and single attosecond pulses (SAPs) from He atom driven by the inhomogeneous polarization gating technology in a bowtie-shaped nanostructure is theoretically investigated. The results show that by the proper addition of bowtie-shaped nanostructure along the driven laser polarization direction, the harmonic emission becomes sensitive to the position of the laser field, and the harmonics emitted at the maximum orders that generate SAPs occur only at one side of the region inside the nanostructure. As a result, not only the harmonic cutoff can be extended, but also the modulations of the harmonics can be decreased, showing a carrier envelope phase independent harmonic cutoff with a bandwidth of 310 eV. Further, with the proper introduction of an ultraviolet pulse, the harmonic yield can be enhanced by 2 orders of magnitude. Finally, by the Fourier transformation of the selected harmonics, some SAPs with a full width at half maximum of sub-30 as can be obtained. [ABSTRACT FROM AUTHOR]

A new and simple design of photo-excited broadband to dual-band tunable terahertz (THz) metamaterial cross polarization converter is proposed in this paper. The tunable converter is a sandwich structure with the center-cut cross-shaped metallic patterned structure as a resonator, the middle dielectric layer as a spacer and the bottom metallic film as the ground. The conductivity of the photoconductive semiconductor (Silicon) filled in the gap of the cross-shaped metallic resonator can be tuned by the incident pump power, leading to an easy modulation of the electromagnetic response of the proposed converter. The results show that the proposed cross-polarization converter can be tuned from a broadband with polarization conversion ratio (PCR) beyond 95% (1.86–2.94 THz) to dual frequency bands ( f l = 1 . 46 THz & f h = 2 . 9 THz). The conversion peaks can reach 99.9% for the broadband and, 99.5% ( f l ) and 99.7% ( f h ) for the dual-band, respectively. Most importantly, numerical simulations demonstrate that the broadband/dual-band polarization conversion mechanism of the converter originates from the localized surface plasmon modes, which make the design simple and different from previous designs. With these good features, the proposed broadband to dual-band tunable polarization converter is expected to be used in widespread applications. [ABSTRACT FROM AUTHOR]

To improve the precision of indoor positioning and actualize three-dimensional positioning, a reversed indoor positioning system based on visible light communication (VLC) using genetic algorithm (GA) is proposed. In order to solve the problem of interference between signal sources, CDMA modulation is used. Each light-emitting diode (LED) in the system broadcasts a unique identity (ID) code using CDMA modulation. Receiver receives mixed signal from every LED reference point, by the orthogonality of spreading code in CDMA modulation, ID information and intensity attenuation information from every LED can be obtained. According to positioning principle of received signal strength (RSS), the coordinate of the receiver can be determined. Due to system noise and imperfection of device utilized in the system, distance between receiver and transmitters will deviate from the real value resulting in positioning error. By introducing error correction factors to global parallel search of genetic algorithm, coordinates of the receiver in three-dimensional space can be determined precisely. Both simulation results and experimental results show that in practical application scenarios, the proposed positioning system can realize high precision positioning service. [ABSTRACT FROM AUTHOR]

In this work, the diffraction efficiency of a volume phase grating incoherently stored in a photorefractive BSO crystal is theoretically and experimentally analyzed. The results confirm the theoretical proposal based on the coupled wave theory adopting a new grating depth parameter associated to the write-in incoherent optical system. The selectivity behavior is governed by the exit pupil diameter of the imaging recording system that controls the depth of the tridimensional image distribution along the propagation direction. Two incoherent gratings are multiplexed in a single crystal and reconstructed without cross-talk. [ABSTRACT FROM AUTHOR]

In the free-space optical (FSO) links, atmospheric turbulence lead to scintillation in the received signal. Due to its ease of implementation, intensity modulation with direct detection (IM/DD) based on ON–OFF keying (OOK) is a popular signaling scheme in these systems. Over turbulence channel, to detect OOK symbols in a blind way, i.e., without sending pilot symbols, an expectation–maximization (EM)-based detection method was recently proposed in the literature related to free-space optical (FSO) communication. However, the performance of EM-based detection methods severely depends on the length of the observation interval ( L s ). To choose the optimum values of L s at target bit error rates (BER)s of FSO communications which are commonly lower than 1 0 − 9 , Monte-Carlo simulations would be very cumbersome and require a very long processing time. To facilitate performance evaluation, in this letter we derive the analytic expressions for BER and outage probability. Numerical results validate the accuracy of our derived analytic expressions. Our results may serve to evaluate the optimum value for L s without resorting to time-consuming Monte-Carlo simulations. [ABSTRACT FROM AUTHOR]

Optical properties of biological tissues are valuable diagnostic parameters which can provide necessary information regarding tissue state during disease pathogenesis and therapy. However, different sources of interference, such as temperature changes may modify these properties, introducing confounding factors and artifacts to data, consequently skewing their interpretation and misinforming clinical decision-making. In the current study, we apply spatial light modulation, a type of diffuse reflectance hyperspectral imaging technique, to monitor the variation in optical properties of highly scattering turbid media in the presence varying levels of the following sources of interference: scattering concentration, temperature, and pressure. Spatial near-infrared (NIR) light modulation is a wide-field, non-contact emerging optical imaging platform capable of separating the effects of tissue scattering from those of absorption, thereby accurately estimating both parameters. With this technique, periodic NIR illumination patterns at alternately low and high spatial frequencies, at six discrete wavelengths between 690 to 970 nm, were sequentially projected upon the medium while a CCD camera collects the diffusely reflected light. Data analysis based assumptions is then performed off-line to recover the medium’s optical properties. We conducted a series of experiments demonstrating the changes in absorption and reduced scattering coefficients of commercially available fresh milk and chicken breast tissue under different interference conditions. In addition, information on the refractive index was study under increased pressure. This work demonstrates the utility of NIR spatial light modulation to detect varying sources of interference upon the optical properties of biological samples. [ABSTRACT FROM AUTHOR]

In this work, a tunable broadband near-infrared light absorber was designed and fabricated with a simple and lithography free approach by introducing an ultrathin phase-change material Ge 2 Sb 2 Te 5 (GST) layer into the metal–dielectric multilayered film structure with the structure parameters as that: SiO 2 (72.7 nm)/Ge 2 Sb 2 Te 5 (6.0 nm)/SiO 2 (70.2 nm)/Cu (>100.0 nm). The film structure exhibits a modulation depth of ∼ 72.6% and an extinction ratio of ∼ 8.8 dB at the wavelength of 1410 nm. The high light absorption (95%) of the proposed film structure at the wavelength of 450 nm in both of the amorphous and crystalline phase of GST, indicates that the intensity of the reflectance in the infrared region can be rapidly tuned by the blue laser pulses. The proposed planar layered film structure with layer thickness as the only controllable parameter and large reflectivity tuning range shows the potential for practical applications in near-infrared light modulation and absorption. [ABSTRACT FROM AUTHOR]

In this work, optical interferometric synthesis and demodulation of four-level pulse amplitude modulation (PAM4) signals by using commercial dual-drive Mach–Zehnder Modulator (DD-MZM) is proposed and studied. Simulations are carried out and signal quality is evaluated in terms of eye jitter, linearity and so on for the PAM4 signals generated by the proposed DD-MZM with improved performance unfolded. Experimental generation of the optically synthesized PAM4 signals up to 50 Gbaud (100 Gbps) is achieved. The transmission of the PAM4 signals over 5-Km standard single mode fiber (SSMF) is carried out with error-free below the FEC limit (3.8E-3) without digital equalization at 90 Gbps. The linear-amplifier-free configuration by DD-MZM leads to simplified implementation and improved performance for high speed PAM4 signal generation, which would be of great significance for short reach optical interconnection. [ABSTRACT FROM AUTHOR]

Electro-optic (EO) delay oscillators have been widely investigated for secure optical communication. To improve complexity and security, an EO intensity chaotic system with an extra optical feedback is proposed. Simulation results show that the proposed system can improve complexity. Moreover, it can effectively suppress the time delay analysis, and the suppression can be enhanced by increasing the optical feedback strength of our system. In addition, synchronization of the communication system in a chaotic modulation encryption scheme based on the proposed system is discussed. [ABSTRACT FROM AUTHOR]

We theoretically propose a scheme for two-dimensional electromagnetically induced grating in a five-level atomic system interacting with two orthogonal standing-wave fields. In such atomic system, nonlinear absorption or refractivity can be significantly enhanced with nearly vanishing linear absorption under different resonant conditions. When applying two coupling fields with standing-wave pattern, absorption grating or phase grating, which efficiently diffracts a probe beam into high-order directions, can be formed in the media. The diffraction efficiencies of the gratings depend strongly on the interaction length, the intensities and detunings of the coupling fields. By investigating the third-order nonlinearity of the system, it is found that the spatial variations of amplitude and phase arise from the nonlinear modulation. The proposed gratings can be used as multi-channel beam splitter and have potential applications in optical information processing and optical networking. [ABSTRACT FROM AUTHOR]

In this paper, we propose a subcarrier-index modulation-based asymmetrically clipped optical orthogonal frequency division multiplexing (SACO-OFDM) scheme for optical wireless communication (OWC) systems, which benefits from the subcarrier-index modulation (SIM) and asymmetrically clipped optical orthogonal frequency division multiplexing (ACO-OFDM) techniques. SACO-OFDM conveys additional information via the subcarrier indexing, and the error rate of the bit transmitted by the subcarrier indexing is much lower than that of the conventional M -ary modulation scheme. On the other hand, as the signal constellation in M -ary modulation is relieved, SACO-OFDM has simple transceiver structure and low detection complexity. Moreover, considering the spectral, an enhanced SACO-OFDM (ESACO-OFDM) using even subcarriers is proposed. In this technique, the odd subcarriers are activated for SACO-OFDM, and the imaginary part of even subcarriers are activated for pulse-amplitude-modulated discrete multitone (PAM-DMT). Clearly, ESACO-OFDM achieves better spectral efficiency than the conventional optical OFDM, since all subcarriers are used for data transmission. Simulation results verify the significant bit error rate (BER) and peak-to-average power ratio (PAPR) improvement by the proposed ESACO-OFDM, especially for the medium-to-high signal-to-noise ratio (SNR) regime. [ABSTRACT FROM AUTHOR]

In this paper, we present a particular theoretical analysis of the impact of 3rd-order dispersion on the photonic time-stretch (PTS) system. Analytical expressions of all harmonics with time-dependent attenuation caused by 3rd-order dispersion of the PTS system are derived, for the first time to the best of our knowledge. The theoretical analysis is based on strict derivation rather than small signal modulation. The effectiveness of the analytical model is verified by a simulated experiment. Our proposed theoretical analysis has guidance role for PTS system design. [ABSTRACT FROM AUTHOR]

A non-periodic concentric circular high index contrast grating (CC-HCG) focusing reflector on 500 nm silicon-on-insulator (SOI) platform is fabricated and experimentally demonstrated. The proposed mirror is realized with phase modulation of wave front in a high reflectivity region. The circular structure based HCG focusing reflector has a spot of high concentration at the 10.87 mm with normal incidence for radially polarization, along with the center wavelength set at 1550 nm. The FWHM spot size of the focusing beam decreases to 260 μ m , and the intensity increases to 1.26 compared with the incident beam. The focusing efficiency of about 80% is observed at 1550 nm in the experimental measurement. [ABSTRACT FROM AUTHOR]

Optical cryptosystems can provide encryption and sometimes compression simultaneously. They are increasingly attractive for information securing especially for image encryption. Our studies shown that the optical cryptosystems can be used to encrypt optical data transmission. We propose and study a new method for securing fibre data communication. The paper presents a method for optical encryption of data transmitted with a single optical fibre. The encryption process relies on pseudo-random spatial light modulation, combination of two encryption keys and the Compressed Sensing framework. A linear combination of light pulses with pseudo-random patterns provides a required encryption performance. We propose an architecture to transmit the encrypted data through the optical fibre. The paper describes the method, presents the theoretical analysis, design of physical model and results of experiment. [ABSTRACT FROM AUTHOR]