Electrooptic ferroelectric Na/sub 0.5/K/sub 0.5/NbO/sub 3/ films

We report on waveguiding and electrooptic properties of epitaxial Na/sub 0.5/K/sub 0.5/NbO/sub 3/ films grown by radio-frequency magnetron sputtering on Al/sub 2/O/sub 3/(11_02) single crystal substrates. High optical waveguiding performance has been demonstrated in infrared and visible light. The in-plane electrooptic effect has been recorded in transmission using a transverse geometry. At dc fields, the effective linear electrooptic coefficient was determined to 28 pm/V, which is promising for modulator applications.     

A polarization independent GaAs-AlGaAs electrooptic modulator

Two designs for polarization independent GaAs-AlGaAs interferometric electrooptic modulators are described. One design uses the linear electrooptic effect to couple degenerate TE/TM eigenmodes of a single-mode waveguide. In the other design the eigenmodes need only be near degenerate. The design using the coupling between near degenerate TE/TM modes utilizes a novel biasing scheme. A novel polarization independent GaAs-AlGaAs interferometric optical modulator based on this design has been fabricated and characterized at 1.3 μm. This modulator is fabricated as a traveling wave modulator incorporating 50 Ω, phase velocity matched, low microwave loss electrodes for maximum electrical bandwidth     

Low-loss electrooptic BaTiO/sub 3/ thin film waveguide modulator

A strip-loaded electrooptic waveguide modulator based on an epitaxial BaTiO/sub 3/ thin film was fabricated and characterized for the first time. The strip-loaded waveguide structure greatly improves waveguide propagation and polarization-dependent loss performance. A propagation loss of 1.1 dB/cm and polarization dependent loss of 0.1 dB/cm were measured. The electrooptic waveguide modulator exhibited a half-wave voltage-interaction length product of 4.5 V /spl middot/ cm at a wavelength of 1542 nm. The measured effective electrooptic coefficient of the as-grown BaTiO/sub 3/ waveguide modulator was 38 pm/V. The experimental results indicate that a strip-loaded thin film waveguide modulator is suitable for photonic applications.     

A novel optical bistable multiple-quantum-well phase modulator

The authors present a new type of optically bistable phase modulator utilizing a self electrooptic effect device (SEED) integrated with an electrooptic wavelength modulator. An electrically bistable SEED, operating on the principle of the quantum-confined Stark effect, controls the bias voltage across an electrooptic waveguide phase modulator to produce optical bistability. A control signal at 0.848 μm, corresponding to the first electron to heavy hole exciton transition in GaAs/AlGaAs multiple-quantum-well is used to switch 1.152 μm light propagating through a waveguide in a direction normal to the control beam     

Microwave Photonic Signal Detection Using Phase-Matched Optical Rectification in an AlGaAs Waveguide

We demonstrate a novel approach for detecting microwave-modulated optical signals through velocity-matched optical rectification in second-order optical nonlinear materials. Optical intensity modulation is down-converted to microwave frequency (5-20 GHz) using a traveling-wave GaAs electrooptic modulator as an optical square-law detector. Phase matching between microwave and optical waves is achieved through a velocity-matched slow-wave asymmetric coplanar-strip electrode structure.     

40-GHz polymer electrooptic phase modulators

A broad-band high-frequency traveling wave electrooptic phase modulator made from stable nonlinear optical polymers has been tested. Using a 1.319-μm light source, single-mode operation was achieved with high-input optical power, resulting in improved modulation depths and signal-to-noise ratios. The high-frequency optical modulation was observed up to 40 GHz using external microwave mixers. Direct optical-heterodyne detection of electrooptic phase modulation has also been demonstrated with high sensitivity at 40 GHz. The devices were tested with 10⁵ W/cm² power density for 21 hours, and have showed no observable decay in nonlinearity     

Simulation of the refractive index properties of a tunable-electron-density quantum well and reservoir structure

A numerical simulation of the optical properties of a novel In/sub x/Ga/sub 1-x/As/In/sub 1-y/Al/sub y/As multiple quantum well heterostructure with tunable charge density is presented. The influence of carriers and dopant ion charges on the electronic properties is simulated by a self-consistent Poisson-Schrodinger solver. The calculated optical constants of the quantum well reproduce well the experimental data. It appears that this model can easily be implemented to include current injection and used as a design tool to optimize the performance of optical modulator quantum well devices.<>     

Time-domain optical response of an electrooptic modulator usingFDTD

A time-domain analysis of an LiNbO₃ electrooptic modulator using the finite-difference time-domain (FDTD) technique is performed. This allows for the calculation of optical modulation and the time-domain optical response of an electrooptic modulator. The electromagnetic fields computed by FDTD are coupled to standard electrooptic relations that characterize electrooptic interactions inside the embedded Ti diffused LiNbO₃ optical waveguides. The electric field-dependent change in the index of refraction inside these optical waveguides and resulting minute phase shifts imparted to optical signals propagating along the device are determined in time, allowing for the simulation of optical intensity modulation. This novel approach to LiNbO₃ electrooptic modulators using a coupled FDTD technique allows for previously unattainable investigations into device operating bandwidth and data transmission speed     

0-1 GHz waveguide 10.6 μm GaAs electrooptic modulator

A GaAs electrooptic modulator for use in 10.6-μm laser frequency shifting applications is described. High-beam-quality output signals with a frequency tunability of 0.1-1.0 GHz were demonstrated. Thick epitaxial GaAs-AlGaAs waveguide structures were successfully fabricated into single-mode channel waveguides with excellent optical and electrooptic properties. A lumped-element modulator, with electrical and electrooptic characteristics in good agreement with theory, was operated at RF drive power levels up to the reverse breakdown voltage limit. The insertion loss of the modulator was quite high, due to absorption losses in the epitaxial material     

Multisectional modeling of high-speed electrooptic modulators integrated in a microwave circuit CAD environment

We present a new multisectional model for high-speed electrooptic modulators, fully integrated within the framework of a microwave circuit computer-aided design (CAD) suite (MWOFFICE). Starting from geometrical and layout parameters, the model allows both simple (traveling-wave) and complex (phase reversal, periodically loaded) structures to be assembled, analyzed, and optimized from the standpoint of the electrical and electrooptic response (including chirp effects) both in small-signal (analog) and in large-signal (digital) operation, exploiting standard simulator tools. At no additional effort, parasitic and passive elements (such as optical or electrical delay paths) can be directly included in the modulator schematic, and the effect of transitions and package parasitics can be readily accounted for at a circuit level. Moreover, model integration within a circuit CAD suite enables one to seamlessly perform driver and modulator design and optimization within the same monolithic or hybrid circuit environment. Comparisons with experimental and literature data and design examples are presented to validate the approach and stress its potential in the design of high-speed electrooptic modulators.     

Generation and distribution of a wide-band continuously tunable millimeter-wave signal with an optical external modulation technique

A new technique to generate and distribute a wide-band continuously tunable millimeter-wave signal using an optical external modulator and a wavelength-fixed optical notch filter is proposed. The optical intensity modulator is biased to suppress the odd-order optical sidebands. The wavelength-fixed optical notch filter is then used to filter out the optical carrier. Two second-order optical sidebands are obtained at the output of the notch filter. A millimeter-wave signal that has four times the frequency of the microwave drive signal is generated by beating the two second-order optical sidebands at a photodetector. Since no tunable optical filter is used, the system is easy to implement. A system using an LiNbO/sub 3/ intensity modulator and a fiber Bragg grating filter is built. A stable and high spectral purity millimeter-wave signal tunable from 32 to 50 GHz is obtained by tuning the microwave drive signal from 8 to 12.5 GHz. The integrity of the generated millimeter-wave signal is maintained after transmission over a 25-km standard single-mode fiber. Theoretical analysis on the harmonic suppression with different modulation depths and filter attenuations is also discussed.     

Femtosecond optical pulse generation from a CW laser using anelectrooptic phase modulator featuring lens modulation

We report a novel electrooptic phase modulator featuring lens modulation and its application to the electrooptic chirping compression method for ultrashort optical pulse generation. In the pulse generation experiment, pedestal free optical pulses of 550 fs in width with a 16.25-GHz repetition rate were obtained by the group delay dispersion of a grating pair from a CW Ar laser. Furthermore, the electrooptic chirping compression method with the electrooptic phase modulator featuring lens modulation is applicable to the negative group delay dispersion as well as positive group delay dispersion     

GaAs/AlGaAs多量子阱FET-SEED灵巧象元

将GaAs／AlGaAs多量子阱光探测器、光调制器与GaAs场效应晶体管（FET）混合集成,构成FET-SEED灵巧象元。光探测器和光调制器均为反射型自电光效应器件（SEED）,光源为骨子阱半导体激光器。测出的光输出Pout／光输入Pin特性表明,光输入信号对光输出信号有明显的调制作用,光输入信号较小的变化可以导致光输出信号较大的变化。     

Design Optimization of Z-Cut Lithium Niobate Electrooptic Modulator With Profiled Metal Electrodes and Waveguides

Analysis and optimization of an ultra-high-speed Z-cut lithium niobate (LN) electrooptic modulator, operating at a high-frequency region, by using a full-wave finite-element numerical technique, has been demonstrated. Investigation of the effects of adjusting the buffer layer thickness, the electrode height, the electrode trapezoidal profile, and the waveguide trapezoidal profile on the microwave effective index N_m, the characteristic impedance Z_c, the microwave losses alpha_c, and the half-wave voltage-length product V_piL has been reported. Optimization of these parameters yield to a novel design of the LN modulator, with a low V_piL and a high bandwidth, operating at a frequency range between 1 and 100 GHz. The frequency-dependent dispersion of the key device parameters, with the aim of determining the device suitability for high-speed operation, has been demonstrated.     

Ultra-Broad-Band LiNbO3 Electro-Optic Modulators with Novel Complex Electrode

The characteristics of Z- cut LiNbO3 optical modulators with novel complex electrode are discussed by using the finite element method (FEM) and studied experimentally in this paper. The travelling wave electrode consists of upper and lower parts. It can easily realize phase velocity matching between microwave and optical wave and drastically reduce microwave electrode loss. An electrooptic modulator with larger than 100 GHz bandwidth and half wave voltage 6 V is designed, fabricated and measured. Vπ is 5.1 V and the attenuation coefficient a0 is 0.3 dB/(cm· GHz1/2 ). The experimental results show that the modulator has great potentiality for expanding bandwidth.     

Enhanced electrooptic modulation efficiency utilizing slow-waveoptical propagation

The use of slow-wave optical propagation to promote highly efficient electrooptic modulation of light is investigated theoretically. The proposed modulators utilize a traveling wave (TW) design in which a grating integrated with a single-mode waveguide induces coupling between forward- and reverse-propagating waves. This contradirectional coupling leads to a reduction in the average optical propagation speed in the forward direction. The “slow” waveguide structures provide two features which facilitate improved modulator performance over conventional “fast” TW designs: (1) optical/microwave velocity matching in substrates with high electrooptic coefficients and dielectric constants and (2) enhancement of electrooptic phase shift due to the “dwell time” of the light in the modulation region. For the ideal case of perfect velocity matching, these two factors lead to a potential improvement of nearly an order of magnitude in electrical power dissipation over velocity-matched designs in the conventional lithium niobate (LN) substrate material. Additional orders-of-magnitude improvement in the required electrical power could result from the use of tungsten bronze substrates such as strontium barium niobate (SBN), which have such higher electrooptic coefficients than LN. The prediction of a large reduction in electrical power dissipation is confirmed by calculations for specific slow-wave designs utilizing multireflector etalons in SBN, although response speed limitations result from the fact that perfect velocity matching is not achieved     

Multitap Photonic Microwave Filters With Arbitrary Positive and Negative Coefficients Using a Polarization Modulator and an Optical Polarizer

A novel multitap photonic microwave filter with arbitrary positive and negative coefficients implemented using an electrooptic polarization modulator (PolM) and an optical polarizer is proposed and experimentally demonstrated. In the proposed filter, the optical polarizer is connected at the output of the PolM with its transmission axis aligned at an angle of 45 to one principal axis of the PolM. By adjusting the polarization direction of the input lightwave to be 45 or 135 to one principal axis of the PolM, an inverted or noninverted intensity-modulated optical microwave signal is obtained at the output of the optical polarizer, which leads to the generation of a negative or positive coefficient. A time delay difference between adjacent taps is generated by using a wavelength-dependent delay line. A five-tap photonic microwave filter with arbitrary positive and negative coefficients is demonstrated. The reconfigurability of the filter is also investigated.     

Thin Layer Design of X-Cut Lithium Niobate Electrooptic Modulator With Slotted SiO2 Substrate

In this letter, we present a novel lithium niobate (LN) electrooptic modulator. The proposed modulator incorporates a thin layer of LN and a slotted substrate structure that together facilitate velocity and impedance matching. A full-wave analysis of the proposed modulator is undertaken in order to establish the frequency dispersion effects with respect to the devices microwave effective index N_m, characteristic impedance Z_c, and half-wave voltage length product V_piL.     

The effect of fabrication parameters on a ridge Mach-Zehnder interferometric (MZI) modulator

A study of Mach-Zehnder interferometer (MZI) modulators using unetched and etched Ti:LiNbO/sub 3/ waveguides has been made. A full vectorial finite-element-based mode solver was used, followed by a finite element-based solution of the Laplace equation to calculate the electrooptic effect and, subsequently, the half-wave voltage, V/sub /spl pi// The optical loss due to the metal electrodes was also found using the H-field finite-element method (FEM) incorporating the perturbation method. The microwave effective index, n/sub m/, and the characteristic impedance of the metal electrodes, Z/sub c/, were also found for a number of electrode thicknesses and ridge heights. A semivectorial finite-element beam propagation method (SVFEBPM) was used to estimate the radiation loss for the curved input and output (I/O) waveguides of the MZI. The device characteristics were then studied by making changes to a number of fabrication parameters, of which the two most important were found to be the etch depth of the ridge and the thickness of the SiO/sub 2/ buffer layer.     

Microwave harmonic frequency generation utilizing the properties of an optical phase Modulator

New techniques to generate a harmonic microwave frequency modulated on an optical carrier are presented. All these techniques utilize the polarization-dependent properties of a LiNbO/sub 3/-phase modulator and manage to achieve 40-GHz modulation using only 10-GHz electronics. The signal quality of the generated frequency is mainly determined by the 10-GHz signal generator, and the suppression of undesired lower order harmonic frequencies exceeds 30 dB.