Maximum Phase Modulation Research Articles

Electrically active control of terahertz waves in a hybrid metasurface-grating structure with a liquid crystal.

A reflective terahertz phase shifter for wide-range dynamic and continuous phase modulation is proposed. By injecting a tunable liquid crystal between the slotted metasurface and the grating microstructure, the phases of reflected waves can be modulated with different electrically driven methods. Numerical simulations show that the proposed terahertz phase shifter has a phase difference of more than 360° between unbiased and biased states. Furthermore, an array of 35×35 patch elements was designed and fabricated. The performance of the phase shifter provides more than 360° between 379.6 and 391.8 GHz, where the maximum phase shift reaches 422.4° at 385.9 GHz. Moreover, the fully electrically controlled phase modulation of more than 180° is achieved between 382.0 and 394.1 GHz, with a maximum phase modulation of 248.4° at 383.3 GHz. This work may provide a reflective terahertz phase modulator for beam steering.

Genetically optimized dual-wavelength all-dielectric metasurface based on double-layer epsilon-near-zero indium-tin-oxide films

Following the pioneering works on electrically tunable conducting oxide-based reflectarray metasurfaces, it has been shown that maximum phase modulation can be realized at a wavelength, where the transition from over- to under-coupling regimes coincides with the epsilon-near-zero (ENZ) phenomenon inside the indium-tin-oxide (ITO) active layer. However, the ENZ transition is restricted to a narrow bandwidth in the near-infrared regime, which limits the maximal achievable phase span at the wavelengths exterior to this bandwidth. Here, we present the realization of a dual wavelength all-dielectric metasurface with a large wavelength-contrast ratio between the operating channels, which is integrated by double-layer ITO films. The doping densities inside the ITO films are judiciously controlled to facilitate the ENZ-crossing of the relative permittivities at the corresponding working wavelengths. The all-dielectric metasurface is comprised of the arrays of cross-shaped holes made inside a high-index silicon slab supporting two resonances that are 300 nm apart. Numerical analysis of the near-field resonant modes reveals the excitation of guided-mode and magnetic dipole resonances, which strongly overlap with ITO active layers. Leveraging from the double-ENZ effect, considerable phase modulations of almost 220° and 240° are attained with a single metasurface platform at the wavelengths of λ1=1200 nm and λ2=1500 nm under the bias voltage application. The design parameters including the geometrical sizes and plasma frequencies of the differently doped ITO layers are carefully optimized by multi-objective genetic algorithm. The proposed metasurface illustrates a great promise in tunable beam splitting of the reflected light and dynamic conversion of the polarization states.

Gas sensing with mode-phase-difference photothermal spectroscopy assisted by a long period grating in a dual-mode negative-curvature hollow-core optical fiber.

We demonstrate sensitive gas detection with mode-phase-difference photothermal spectroscopy assisted by a long period grating (LPG) inscribed on a dual-mode negative-curvature hollow-core fiber (NC-HCF). The LPG is inscribed using a pulsed CO2 laser, which enables pump propagation in the fundamental LP01 mode to achieve maximum photothermal phase modulation while exciting both the LP01 and LP11 modes at the probe wavelength to form a dual-mode interferometer for detection of the phase difference. With a 1533 nm pump and a 1620 nm probe, a noise equivalent concentration of ∼2.2 ppb acetylene is achieved with an 85-cm-long NC-HCF gas cell and 1 s lock-in time constant.

Design of New SVPWM Mechanism for Three-Level NPC ZSI via Line-Voltage Coordinate System

In this article, a novel space-vector pulsewidth modulation (SVPWM) mechanism is proposed for a three-level neutral-point clamped (NPC) Z-source inverter (ZSI) via the line-voltage coordinate system. In order to solve the problems of the adjustment of the complicated operational region and the complex computation of lookup tables in conventional methods, the newly designed SVPWM mechanism is developed from the basis of the vector synthesis. With this mechanism, switching vectors required to synthesize the reference vector can be easily identified, and the corresponding duty ratios can be directly obtained. According to the volt–second balanced principle and three operational modes of the three-level NPC ZSI, the up shoot-through status is inserted into the left side of the intersection point of the maximum phase modulation signal and the carrier wave signal. On the other hand, the down shoot-through status is inserted into the right side of the intersection point of the minimum phase modulation signal and the carrier wave signal. The effectiveness of the proposed SVPWM mechanism is verified by numerical simulations and experimental results. The salient features of the proposed SVPWM method are easiness in digital implementation, less computation time, and better power quality in comparison with traditional SVPWM schemes.

PHASE CURVES OF THE KEPLER-11 MULTI-PLANET SYSTEM

The Kepler mission has allowed the detection of numerous multi-planet exosystems where the planetary orbits are relatively compact. The first such system detected was Kepler-11 which has six known planets at the present time. These kinds of systems offer unique opportunities to study constraints on planetary albedos by taking advantage of both the precision timing and photometry provided by Kepler data to monitor possible phase variations. Here we present a case study of the Kepler-11 system in which we investigate the phase modulation of the system as the planets orbit the host star. We provide predictions of maximum phase modulation where the planets are simultaneously close to superior conjunction. We use corrected Kepler data for Q1-Q17 to determine the significance of these phase peaks. We find that data quarters where maximum phase peaks occur are better fit by a phase model than a "null hypothesis" model.

Polarization modulation study of transmissive liquid crystal spatial light modulator using digital holographic polariscope

The study of phase modulation from the transmissive twisted nematic liquid crystal spatial light modulator (TN-LCSLM) with different incident states of polarization on its active area using Digital Holography method is presented. A pair of polarizer and quarter waveplate is used to illuminate the TN-LCSLM with elliptically polarized optical wavefront and transmitted optical wavefront is analyzed with a rotating analyzer. A single digital hologram is recorded from a complex optical wavefront passing through active display area of the TN-LCSLM at particular setting of different optical elements and quantitative phase is evaluated. We demonstrated that elliptically polarized light with certain ellipticity produces maximum phase modulation through the TN-LCSLM and the results are in good agreement with the eigenvectors suggested by Mueller matrix polarimetric characterization. This experimental setup can also be used as Grey-field Polariscope (GFP) in order to characterize important physical parameters such as the orientation of LC director molecules at input face and phase retardation with respect to addressed gray scale value on the TN-LCSLM active area. The TN-LCSLM is illuminated with circularly polarized light and change in this incident polarization after passing through the TN-LCSLM is recorded by four phase shifted digital holograms at four different analyzer orientations to measure these physical parameters.

Phase Modulation of Broadband Light Using Liquid-Crystal-on-Silicon Spatial Light Modulator

The influence of the illumination bandwidth on phase modulation using a liquid-crystal-on-silicon (LCOS) spatial light modulator (SLM) was evaluated. The maximum phase modulation for different wavelengths was determined using a Michelson interferometer. Phase modulation of the LCOS SLM exhibited good linearity for broadband light as well as light with a narrow bandwidth. Point spread functions (PSFs) formed via the LCOS SLM were measured for broadband illumination. Although PSF distribution expanded for the broader bandwidth, the full width at half-maximum (FWHM) of the center peak of the PSF remained the same for different bandwidths. Because of the preserved FWHM of the PSF, the resolution of imaging using the LCOS SLM was not influenced by the broadening of illumination bandwidth, but the expanded distribution contributed to the degradation of image contrast.

Dynamic calibration for improving the speed of a parallel-aligned liquid-crystal-on-silicon display

The speed of most parallel-aligned liquid-crystal-on-silicon (LCoS) spatial light modulators (SLMs) is limited to the video rate of their drivers, which is a limitation for high-speed applications. However, the LCoS SLM presented here has a driver allowing a frequency range of up to 1011 Hz. Using the static phase modulation characterization and the static lookup table (LUT), the phase modulation characterization versus frequency shows that the SLM can operate at around 130 Hz or even higher for small phase changes and at 32 Hz for extreme phase changes. A dynamic calibration is carried out, and we propose a method allowing an increase of the frame rate while maintaining a maximum phase modulation of 2pi. Experimental results of dynamic diffractive optical elements displayed on the SLM at a frame rate of 205 Hz show that the dynamic LUT improves the reconstruction quality.

Measurement and compensation of optical aberrations using a single spatial light modulator

We describe a compact adaptive optical system using a spatial light modulator (SLM) as a single element to both measure and compensate optical aberrations. We used a low-cost, off-the-shelf twisted nematic liquid-crystal display (TNLCD) optimally configured to achieve maximum phase modulation with near constant transmittance. The TNLCD acts both as the microlens array of a Hartmann-Shack wavefront sensor and as the aberration compensation element. This adaptive setup is easy to implement and offers great versatility.

Modulation light efficiency of diffractive lenses displayed in a restricted phase-mostly modulation display

We present an analysis of the diffraction efficiency of diffractive lenses displayed on spatial light modulators that depends on the modulation response of the display. An ideal display would produce continuous phase-only modulation, reaching a maximum phase-modulation depth of 2pi. We introduce the concept of modulation diffraction efficiency that accounts for the effect of nonlinearities only in the phase modulation of the display. We review a diffractive model with which to evaluate this modulation efficiency, including modulation defects such as nonlinear phase modulation, coupled amplitude modulation, phase quantization, and a limited modulation depth. We apply this diffractive model to Fresnel lenses and show that these modulation defects produce a lens multiplex effect. Finally we demonstrate that the application of a minimum Euclidean projection principle leads to high modulation diffraction efficiency even if the phase-modulation depth is much less than 2pi. We demonstrate that the modulation efficiency can exceed 90% for a modulation depth of 1.4pi and can exceed 40% (the equivalent for a binary phase element) for a modulation depth of only 0.7pi. Experimental results from use of a twisted nematic liquid-crystal display are presented to confirm these conclusions.

Dual-frequency addressed hybrid-aligned nematic liquid crystal

Dual-frequency addressed hybrid-aligned nematic (HAN) liquid crystal cell is demonstrated as a variable optical attenuator at 1.55μm wavelength. By controlling the low- and high-frequency electric field, the dual-frequency liquid crystal (DFLC) molecules can be reoriented parallel or perpendicular to the substrates so that the maximum obtainable phase modulation is doubled. In comparison to a homogeneous cell, the DFLC HAN cell shows a lower operating voltage and faster response time. Furthermore, the DFLC HAN cell exhibits three stable states that have some applications such as ternary photonic devices.

All-fiber phase modulator based on lead zirconate titanate thin-film coating

An all-fiber phase modulator based on lead titanate zirconate coating deposited by dc reactive magnetron sputtering was fabricated and optically characterized. The frequency response showed a flat broadband structure up to 600 kHz, with an efficiency of 0.73 rad/(V m). This is an order of magnitude higher than that for sputtered zinc oxide based phase modulators. Radial mode resonance peaks were observed between 10 and 700 MHz, with a maximum phase modulation amplitude of 0.3 rad at the fundamental radial resonance frequency of 25.6 MHz.

Kinoforms designed to produce different fan-out patterns for two wavelengths

Kinoforms (diffractive optical elements) were designed to produce different fan-out (i.e., spot) patterns when illuminated with green (543-nm wavelength) and red (633-nm) light. Three design examples are presented, each using one of three different techniques for this wavelength discrimination. If the fan-out pattern is to be produced in the near field (Fresnel region) of the kinoform, focusing-defocusing distinguishes between the two colors. For a far-field pattern the color distinction can be obtained either by active suppression of unwanted spots, which also decreases the diffraction efficiency, or, preferably, by an increase in the maximum phase modulation of the kinoform (to more than 2pi rad). All three examples were designed with a method based on the full scalar wave equation and optimal-rotation-angle optimization. The designed kinoforms were manufactured and performed, at least qualitatively, as predicted by the design.

Wave-coupled LiNbO/sub 3/ electrooptic modulator for microwave and millimeter-wave modulation

A new technique of phase velocity matching in electrooptic modulators was demonstrated. The results show that the phase velocity mismatch due to material dispersion in traveling-wave LiNbO/sub 3/ optical waveguide modulators can be greatly reduced by breaking the modulation transmission line into short segments and connecting each segment to its own surface dipole antenna. The array of antennas is then illuminated by the modulation signal from below at the proper angle to produce a delay from antenna to antenna that matches the optical waveguide's delay. A phase modulator 25 mm in length with five antennas and five transmission line segments was operated from 4.6 to 13 GHz with a maximum phase modulation sensitivity of over 100 degrees /W/sup 1/2/.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

A Method for Evaluating MTF and Phase Structures of Photographic Materials

In addition to the absorbing structures in photographic layers, surface relief and index of refraction variations may occur that change the optical path length and hence the phase of a wave field passing through the layer. A method is reported on how the modulation of these phase structures can be determined from two measurements of the modulation transfer function, MTF. Assuming a photographic grating to be purely absorptive, an elevated “as-if-MTF” is obtained from diffraction efficiency measurements. By means of a new scanning instrument, the MSSM, the correct MTF can be measured. From these two MTF values, a simple formula yields the modulation of the associated phase grating. Experimental values for a holographic plate show a maximum phase modulation little less than 1 rad per unit density difference at a spatial frequency of about 35 cycles/mm and vanishing phase effect beyond about 600 cycles/mm.

Effects of intrinsic non-linearities on efficiency and image contrast of bleached holograms

Efficiency and image contrast of bleached holograms from diffusely illuminated binary objects have been measured for various exposures. The maximum efficiency is 18% for the thin and about 25% for the thick holograms. A distinct minimum of the contrast versus exposure curves is clearly explained by a corresponding maximum phase modulation.