Complex modulation using tandem polarization modulators

A novel photonic technique for implementing frequency up-conversion or complex modulation is proposed. The proposed circuit consists of a sandwich of a quarter-wave plate between two polarization modulators, driven, respectively, by an in-phase and quadrature-phase signals. The operation of the circuit is modelled using a transmission matrix method. The theoretical prediction is then validated by simulation using an industry-standard software tool. The intrinsic conversion efficiency of the architecture is improved by 6 dB over a functionally equivalent design based on dual parallel Mach–Zehnder modulators. Non-ideal scenarios such as imperfect alignment of the optical components and power imbalances and phase errors in the electric drive signals are also analysed. As light travels, along one physical path, the proposed design can be implemented using discrete components with greater control of relative optical path length differences. The circuit can further be integrated in any material platform that offers electro-optic polarization modulators.     

Simple electrical biasing for minimizing the nonlinearities of quantum-well Mach-Zehnder modulators

The quadratic nature of the electro-optic effect in semiconductor Mach-Zehnder modulators based on the quantum-confined Stark effect allows simple electrical biasing to eliminate either the second-order or the third-order distortions in the optical output, improving the device linearity for single-octave bandwidth multicarrier analog modulation applications.     

Complete polarization conversion using one crystal with dual transverse Pockels effect

Based on dual transverse Pockels effect, complete polarization conversion can be achieved by using only one electro-optic crystal and its two externally applied voltages. The electro-optic phase retardation and the azimuth angle of the field-induced principal dielectric axes of the crystal can be independently and linearly controlled by the amplitude and direction of the applied electric field, and the formulas for this correlation are deduced for arbitrary input and output polarization states. The candidate crystals mainly include the uniaxial crystals of 3 m, 62 m, and 32 symmetry groups, and the cubic crystals of 43 m and 23 symmetry groups. Theoretical analysis demonstrates that one crystal exhibiting both dual transverse Pockels effect and optical activity can also be used for complete polarization converter. The continuous polarization rotation of a linearly polarized light from 0 degrees to 180 degrees has been performed experimentally by use of single lithium niobate crystal with four lateral electrodes. In addition the light beam position-dependent polarization conversion by using a bulk electro-optic crystal is also measured in the experiment. This new type of polarization converter will have potential applications in many fields due to its simple configuration, explicit control logic of polarization conversion, and lower power consumption.     

Velocity matching of a GaAs electro-optic modulator

New designs for the velocity matching of a deep-etched semiconductor electro-optic modulator are presented. A tantalum pentoxide (Ta2O5) coating is considered here for achieving velocity matching between the microwave and the optical signals. The effects of the velocity mismatch, the conductor loss, the dielectric loss, and the impedance mismatch are studied in relation to the optical bandwidth of a high-speed semiconductor modulator. It is shown that both the dielectric loss and the impedance matching play key roles for velocity-matched high-speed modulators with low conductor loss. The effects of Ta2O5 thickness on the overall bandwidth and on the half-wave voltage-length product VpiL are also reported.     

Diffraction light modulator based on transverse electro-optic effect in short-pitch ferroelectric liquid crystals

A diffraction modulator that exploits the transverse electro-optic effect in ferroelectric liquid crystals is proposed for applications in displays and in spatial light modulators. Experiments with a short-pitch ferroelectric liquid crystal aligned homeotropically show an achromatic contrast ratio of greater than 100:1 available with oblique readout. The sources of the contrast deterioration and the tolerance of the proposed scheme to this deterioration are analyzed. For selected directions of readout the light output obtains low sensitivity to the polarization of the readout light. Nonlaser light sources can be used in a practical display setup based on the proposed principle.     

Effect of polarization state on electro-optic coupling and its application to polarization rotation

The effect of the polarization state on electro-optic coupling is studied by using the wave coupling theory of the linear electro-optic effect. The numerical results show that the polarization state obviously influences the electro-optic coupling. The conditions for realizing perfect coupling are emphasized. As an application of perfect coupling, a novel polarization rotator, which can rotate the polarization of a light beam with an arbitrary angle but keep the output intensity unchanged, is presented.     

Experimental studies of electro-optic polymer modulators and waveguides

The results of an experimental study of electro-optic modulators and waveguides based on polymeric materials are presented. Included are the design, fabrication, and testing of integrated Mach-Zehnder modulators, which are based on polymer films that contain a novel, nonlinear electro-optic chromophore. Studies also show the efficacy of photolithography or photobleaching by the use of this chromophore to form passive, branching waveguides, which are operated at the 1300-nm wavelength.     

Automated measurement of optical coherence lengths and opticaldelays for applications in coherence-modulated opticaltransmissions

The implementation of an automated system for analysis of coherence length of light and for the measurement of optical delays is reported. This type of measurement is very important for designing coherence-modulated optical transmission systems. Such systems have been studied for the last few years as new potential high-speed optical links useful for point-to-point, local area networks, and bidirectional transmissions, at optical wavelengths around 1300 nm. The main characteristics of coherence-modulated transmissions include the need of using low-coherence optical sources and integrated optics lithium niobate (LiNbO₃) electro-optic retarders. This paper describes the implementation of an automated system allowing the measurement of coherence lengths of semiconductor optical sources and also optical delays as essential data for designing coherence-modulated optical links. The reported results include characterization of commercial low-coherence semiconductor optical sources and integrated optics coherence modulators     

Growth of single crystals of meta-nitroaniline and the evaluation of its electro-optic properties

Single crystal boules of meta-nitroaniline of good optical quality have been grown. Metanitroaniline exhibits a strong linear electro-optic effect, has a low refractive index and low permittivity, making it attractive for use in high-speed electro-optic modulators and deflectors.Currently on study leave at Department of Electrical Engineering, Imperial College, London.     

La3Ga5SiQ14 single-crystal Q switch used as an electro-optic device

A new kind of electro-optic Q switch is designed by use of a La3Ga5SiQ14 (LGS) crystal. Because the LGS crystal is optically active, the Q switch is based on the consideration that the total rotation angle of the polarization plane is zero, whereas the polarized wave propagates through the Pockels cell back and forth, with the polarization plane gyration and electro-optic effect existing simultaneously. The LGS Q switch is a practical electro-optic device that can be used in medium output energy lasers to partially take the place of deuterated potassium dihydrogen phosphate and lithium niobate Q switches.     

Compensation of parasitic polarization modulation in a fiber-optic gyroscope

It has been shown that the influence of parasitic polarization modulation on the precision of a fiber-optic gyroscope may be reduced substantially by incorporating reflection-type phase modulators with conversion of polarization modes. Pis’ma Zh. Tekh. Fiz. 23, 90–94 (August 12, 1997)     

Intensity dependent residual amplitude modulation in electro-optic phase modulators

Residual amplitude modulation (RAM) mechanisms in electro-optic phase modulators are detrimental in applications that require high purity phase modulation of the incident laser beam. While the origins of RAM are not fully understood, measurements have revealed that it depends on the beam properties of the laser as well as the properties of the medium. Here we present experimental and theoretical results that demonstrate, for the first time, the dependence of RAM production in electro-optic phase modulators on beam intensity. The results show an order of magnitude increase in the level of RAM, around 10 dB, with a fifteenfold enhancement in the input intensity from 12 to 190 mW/mm2. We show that this intensity dependent RAM is photorefractive in origin.     

Modeling of scattering and depolarizing electro-optic devices. I. Characterization of lanthanum-modified lead zirconate titanate

We describe a new method of modeling electro-optic (EO) devices, such as lanthanum-modified lead zirconate titanate polarization modulators, that resolves two deficiencies of current methods: (i) the inclusion of depolarization effects resulting from scattering and (ii) saturation of the EO response at strong electric-field strengths. Our approach to modeling depolarization is based on describing the transmitted optical field by superposition of a deterministic polarized wave and a scattered, randomly polarized, stochastic wave. Corresponding Jones matrices are used to derive a Mueller matrix to describe the wave propagation in scattering and depolarizing EO media accurately. A few simple optical measurements can be used to find the nonlinear behavior of the EO phase function, which is shown to describe accurately the material's EO behavior for weak and strong applied electric fields.     

All-optical wavelength conversion in a GaInAsP/InP optical gate loaded with a Bragg reflector

All-optical wavelength conversion employing a GaInAsP/InP nonlinear distributed-feedback waveguide switch is investigated. The device has the advantages of simple structure, compactness, polarity-noninverted operation, and feasibility of integration with other photonic devices such as semiconductor optical amplifier and modulators. We show that the device exhibits constant conversion efficiency around -9.2 dB in a signal-wavelength range of 1530-1560 nm. Furthermore, this device can potentially be made polarization independent.     

Rapid optical coherence tomography and recording functional scattering changes from activated frog retina

Optical coherence tomography (OCT) has important potential advantages for fast functional neuroimaging. However, dynamic neuroimaging poses demanding requirements for fast and stable acquisition of optical scans. Optical phase modulators based on the electro-optic effect allow rapid phase modulation; however, applications to low-coherence tomography are limited by the optical dispersion of a broadband light source by the electro-optic crystal. We show that the optical dispersion can be theoretically estimated and experimentally compensated. With an electro-optic phase modulator-based, no-moving-parts OCT system, near-infrared scattering changes associated with neural activation were recorded from isolated frog retinas activated by visible light.     

衰减全反射结构的双通道聚合物电光调制器

近年来,反射结构的电光调制器由于其特殊性质,已引起了人们的极大兴趣。提出一种双通道衰减全反射电光调制器,该调制器含有两对电极,当两束光以一定的角度平行入射棱镜底面时,两束反射光的强度就会被加于两对电极上的电信号分别调制,这样在一个棱镜上可实现两束光信号的同时调制。     

Improved design of a polarization converter based on semiconductor optical waveguide bends

By using an efficient vector finite-element-based beam-propagation method, we present an improved design of a polarization converter. This design relies on the use of a single-section deeply etched bent semiconductor waveguide with slanted sidewalls. By careful adjustment of the bend radius, the waveguide width, and the sidewall angle we obtained a nearly 100% polarization conversion ratio with no appreciable radiation loss and a bending angle of less than 180 degrees .     

Drying temperature effects on microstructure, electrical properties and electro-optic coefficients of sol-gel derived PZT thin films

Transparent lead zirconium titanate (PZT) thin film is suitable for a variety of electro-optic application, and the increasing of the electro-optic coefficient of PZT film is one of the important factors for this application. In this study, the main processing variable for improving an electro-optic coefficient was the drying temperature: 300, 350, 450 and 500°C in sol-gel derived PZT thin films. The highest linear electro-optic coefficient (1.65×10⁻¹⁰ (m/V)) was observed in PZT film dried at 450°C. The PZT film showed the highest perovskite content, polarization (P_max=49.58 μC/cm², P_r=24.8 μC/cm²) and dielectric constant (532). A new two-beam polarization (TBP) interferometer with a reflection configuration was used for electro-optic testing of PZT thin films which allows measurement of the linear electro-optic coefficient of thin film with strong Fabry–Perot (FP) effect usually present in PZT thin film.     

Microwave characteristics of polymer electro-optic modulators

The finite difference method is employed for the Laplace equation to calculate the microwave properties of polymer-based electro-optic modulators. The finite difference scheme uses a nonequidistant grid for fast calculation of the mode index and characteristic impedance of an open micro striplike modulator with a thick electrode as well as for a shielded micro striplike structure. It is demonstrated that the finite thickness of the electrode must be taken into consideration when optimizing the design of an electro-optic modulator. It is shown that by using a shielded microstrip configuration the modulator may be optimized and simultaneously pacified with regard to environmental influence.     

Enhanced absorption and electro-optic Pockels effect of electrostatically self-assembled CdSe quantum dots

The spectrum and electro-optic properties of CdSe quantum dots are studied. Spectrum wavelength shifts that are due to the quantum size effect and to the electro-optic Stark effect are investigated. It is found that CdSe quantum dot-polymer composites formed by an electrostatic self-assembly (ESA) technique exhibit high internal electric fields. Using the second-order perturbation theory of the 1s-1s energy shift (Stark effect), we estimate the internal field of the ESA film to be as high as 2.6 x 10(8) V/m. This value results in a much higher absorption coefficient and electro-optic coefficients for ESA films than for their bulk crystal counterparts or for spin-coated film samples. The relationships among unusual spectra, film structure, and high electro-optic response are analyzed. These results are useful both for understanding the physical mechanisms of semiconductor quantum dots and for developing high-performance photonic devices.