Yongjiu Zhao

Ultraflat optical frequency comb generated based on cascaded polarization modulators

A novel approach to generating an ultraflat and stable optical frequency comb with tunable frequency spacing is proposed and experimentally demonstrated. The proposed generator consists of a polarization modulator (PolM) and a polarizer. The joint operation of a PolM and a polarizer is equivalent to intensity modulation, but with a third controllable parameter in addition to the two controllable parameters in conventional intensity modulation. By tuning the three parameters, an ultraflat optical frequency comb with five comb lines is generated. By cascading the PolM with a second PolM, an ultra-flat optical frequency comb with 25 lines is generated. An experiment using two cascaded PolMs is performed. A 25- line frequency comb with the comb flatness within 1 dB is generated.

Tunable Optoelectronic Oscillator Based on a Polarization Modulator and a Chirped FBG

A wideband-tunable optoelectronic oscillator (OEO) is proposed and experimentally demonstrated based on a tunable microwave photonic filter (MPF) consisting of a polarization modulator, a chirped fiber Bragg grating, and a polarization beam splitter (PBS). By simply adjusting the polarization state of the signal before the PBS, the center frequency of the MPF is tuned. The proposed OEO is experimentally demonstrated. A high-purity microwave signal with a tunable frequency within 5.8-11.8 GHz is generated. The single-sideband phase noise of the generated signal is -104.56 dBc/Hz at 10-kHz offset.

Optical Single-Sideband Modulation Based on a Dual-Drive MZM and a 120° Hybrid Coupler

A novel scheme for implementing high-performance optical single-sideband (OSSB) modulation based on a dual-drive Mach-Zehnder modulator (MZM) and a 120° hybrid coupler is proposed and demonstrated. A RF signal is divided by the 120° hybrid coupler into two parts with equal powers and a phase difference of 120°, and then, led to the two RF ports of the dual-drive MZM. With a proper dc bias, an OSSB signal with the -1st and +2nd-order sidebands (or the +1st and -2nd-order sidebands) suppressed is generated. A numerical simulation and a proof-of-concept experiment are carried out. As compared with the conventional OSSB modulation based on a 90° hybrid coupler, the suppression of the +2nd (or -2nd)-order sideband improves evidently the performance when the OSSB modulation is applied in a radio-over-fiber system or an optical vector network analyzer.

An Ultra-wideband and Polarization-independent Metasurface for RCS Reduction

In this paper, an ultra-wideband and polarization-independent metasurface for radar cross section (RCS) reduction is proposed. The unit cell of the metasurface operates in a linear cross-polarization scheme in a broad band. The phase and amplitude of cross-polarized reflection can be separately controlled by its geometry and rotation angle. Based on the diffuse reflection theory, a 3-bit coding metasurface is designed to reduce the RCS in an ultra-wide band. The wideband property of the metasurface benefits from the wideband cross polarization conversion and flexible phase modulation. In addition, the polarization-independent feature of the metasurface is achieved by tailoring the rotation angle of each element. Both the simulated and measured results demonstrate that the proposed metasurface can reduce the RCS significantly in an ultra-wide frequency band for both normal and oblique incidences, which makes it promising in the applications such as electromagnetic cloaking.

Multi-channel composite spoof surface plasmon polaritons propagating along periodically corrugated metallic thin films

In this work, we demonstrate that composite spoof surface plasmon polaritons can be excited by coplanar waveguide, which are composed of two different spoof surface plasmon polaritons (SSPPs) modes propagating along a periodically corrugated metallic thin film simultaneously. These two SSPPs correspond to the dominant modes of one-dimensional (1D) periodical hole and groove arrays separately. We have designed and simulated a planar composite plasmonic waveguide in the microwave frequencies, and the simulation results show that the composite plasmonic waveguide can achieve multi-channel signal transmission with good propagation performance. The proposed planar composite plasmonic metamaterial can find potential applications in developing surface wave devices in integrated plasmonic circuits and multi-channel signal transmission systems in the microwave and terahertz frequencies.

Performance analysis of optical vector analyzer based on optical single-sideband modulation

Optical vector network analyzers (OVNAs) based on optical single-sideband (OSSB) modulation are of great interest thanks to the potentially high measurement resolution. However, the measurement accuracy of the OSSB-based OVNA is limited by the high-order sidebands in the OSSB signal. To study the influence of the high-order optical sidebands in OSSB signals on the measurement accuracy, an analytical model is established to present the expression of the measurement error and a numerical simulation is performed. For the OSSB-based OVNA implemented by a 90-deg electrical hybrid coupler and a dual-drive modulator, when the −1st order sideband is fully suppressed by the OSSB modulation, the existence of the +2nd-order sideband severely degrades the resolution of the measurement, while the −3rd and −2nd order sidebands place a restriction on the dynamic range of the measurement. In addition, these sidebands also introduce evident measurement errors to the phase response. The study may provide a good guidance in designing the high performance OVNA.

Multi-band localized spoof plasmons with texturing closed surfaces

We demonstrate that periodically textured closed surface with multiple groove depths can support multi-band spoof localized surface plasmons (LSPs). It is interesting to note that the spoof LSPs in each band resemble those generated by the textured closed surface of the same periodicity with the corresponding single groove depth. In this way, it paves the way for the generation and design of multi-band spoof LSPs. Moreover, multiple resonance band structures and devices, such as resonator, oscillator, and other band-notched structures in the microwave and terahertz regimes can be realized.

Localized spoof plasmons in closed textured cavities

Localized spoof plasmons arising with textured closed surfaces have been theoretically studied and experimentally verified, which resemble the localized surface plasmons (LSPs) in the optical regime. In this work, we go one step further and demonstrate that part of the resonance modes in closed textured cavities pertain to spoof localized surface plasmons (spoof-LSPs) modes. We show the existence of spoof LSPs in periodically textured perfect electric conductor circular cavities and make an analogy between these spoof LSPs and the real LSPs in closed metallic cavities with the Drude model in the optical regime. Also, a metamaterial approach is presented to capture the resonant features of these modes.

Smooth bridge between guided waves and spoof surface plasmon polaritons

In this work, we build a smooth bridge between a coaxial waveguide and a plasmonic waveguide with subwavelength periodically cylindrical radial grooves, to realize high-efficiency mode conversion between conventional guided waves and spoof surface plasmon polaritons in broadband. This bridge consists of a flaring coaxial waveguide connected with a metal cylindrical wire corrugated with subwavelength gradient radial grooves. Experimental results of the transmission and reflection coefficients show excellent agreement with the numerical simulations. The proposed scheme can be extended readily to other bands and the bridge structure can find potential applications in the integration of conventional microwave or terahertz devices with plasmonic circuits.

Accuracy improvement of optical vector network analyzer based on single-sideband modulation.

An approach to suppress the measurement errors induced by the high-order sidebands of the optical single-sideband (OSSB) signal in the OSSB-based optical vector network analyzer (OVNA) is proposed and experimentally demonstrated. An analytical model for studying the measurement errors of the OSSB-based OVNA is established. Results show that the measurement errors introduced by the high-order sidebands can be obtained by suppressing the optical carrier in the OSSB signal. By subtracting these errors from the ordinary frequency responses measured by the OVNA, accurate frequency responses can be achieved. A proof-of-concept experiment is performed. The magnitude and phase responses of a fiber Bragg grating are measured with good coincidence by the OSSB signals with different modulation indices.