Effects of ridge depth on characteristics of shieldedvelocity-matched (SVM) Ti:LiNbO3 optical modulators withridge structures

The effects of ridge depth on the microwave effective index, characteristic impedance of the traveling-wave electrode, and modulation bandwidth of shielded velocity-matched Ti:LiNbO₃ optical modulators with ridges are investigated numerically. It is clarified that the driving voltage has an optimum ridge depth (3-4 μm) for various gaps of the electrode, and there are optimum overlaid layer thicknesses at the velocity matching point for given values of the ridge depth     

High-speed low-voltage modulation with a nonsymmetric Mach-Zehnderinterferometer

The authors demonstrate broadband optical modulation with a Ti:LiNbO₃ nonsymmetric interferometer at microwave frequencies up to 16 GHz. The 3-dB bandwidth of 8.7 GHz is only slightly less than the theoretical limit of 9.6 GHz for a 1-cm-long device. The device uses a 2-μm-thick gold-plated asymmetric stripline electrode, with a characteristic impedance of 40 Ω and ohmic loss of 3 dB/cm at 10 GHz. The DC switching voltage is 6.5 V, and the on/off ratio is -16 dB. Fabrication tolerances in the nonsymmetric interferometer are much less strict than for directional coupler modulators with comparable performance, making this device a better candidate for use in communications systems     

X切Ti：LiNbO3调制器的有限元法分析

本文用有限元法第一次较为准确地分析了各种Ｘ切Ｔｉ：ＬｉＮｂＯ３电光调制器的场分布，并用其结果计算了调制器的特性参数－开关电压、调制带宽、特性阻抗、微波等效折射率等。利用有限元法的特点，第一次在计算调制器的特性时考虑了ＬｉＮｂＯ３基片厚度对器件的影响。     

Proposal for a 94-GHz phase-reversal optical modulator using a loaded transmission line

One of the most important problems associated with the design of external travelling wave optical modulators operating at very high frequency is the reduction of microwave loss associated with the electrode structure. With this in view, a new design for a Ti:LiNbO/sub 3/ narrow-band optical modulator is proposed for operation at 94 GHz. It uses a low-loss loaded transmission line with dielectric overlay and uses phase-reversals to achieve phase matching. Numerical simulations predict a required 21-dBm drive power for full on-off switching at 94 GHz.     

Millimeter-wave Ti:LiNbO3 optical modulators

The design, fabrication, and characteristics of ridged Ti:LiNbO ₃ optical modulators that work in the millimeter-wave region are presented. A new concept of design under velocity matching is demonstrated for the proposed modulator. It has been shown by calculation that impedance matching is achieved and conductor loss is greatly reduced under velocity matching with wider gaps and a thicker coplanar waveguide electrode in conjunction with a ridge structure. Two types of Mach-Zehnder optical intensity modulators for the wavelength of 1.5 μm are developed. A fully packaged module for 40 Gb/s transmission with a half-wave voltage of 3.5 V and a broadband modulator responsible up to 100 GHz with a half-wave voltage of 5.1 V     

Thin layer design of X-cut LiNbO3 modulators

Microwave-optical velocity matching and 50 Ω impedance matching are difficult to achieve with LiNbO₃ traveling wave modulators. We perform a detailed study (simulations) of the microwave and optical performance characteristics for modulators using thin layer (few micrometers), X-cut LiNbO₃ and find significant improvements in velocity and impedance matching together with a lower V _πL     

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.     

High-efficiency 1.3 /spl mu/m InAsP-GaInP MQW electroabsorption waveguide modulators for microwave fiber-optic links

High-efficiency electroabsorption waveguide modulators have been designed and fabricated using strain-compensated InAsP-GaInP multiple quantum wells at 1.32-/spl mu/m wavelength. A typical 200-/spl mu/m-long modulator exhibits a fiber-to-fiber optical insertion loss of 9 dB and an optical saturation intensity larger than 10 mW. The 3-dB electrical bandwidth is in excess of 20 GHz with a 50-/spl Omega/ load termination. When used in an analog microwave fiber-optic link without amplification, a RF link efficiency as high as -38 dB is achieved at 10 mW input optical carrier power. These analog link characteristics are the first reported using MQW electroabsorption waveguide modulators at 1.32 /spl mu/m.     

Suppression of thermal drifts of high speed Ti:LiNbO3optical modulators

The thermal-drift behavior of interferometric modulators with Mach-Zehnder structures are experimentally shown to be remarkably improved because of the introduction of proper asymmetry into the optical waveguide arms. These experimental results are theoretically explained taking the effects of asymmetric waveguides and pyroelectric charges into account. It is found that the optical output follows the sinusoidal squared variation with temperature and the proper introduction of asymmetric waveguide arms is helpful in the adjustment of the slope at its operating points. It is, therefore, concluded that the thermal drifts of Ti:LiNbO₃ optical modulators can be suppressed by minimizing the gradient of the output due to temperature variations     

Normal-mode small-signal analysis of traveling-wave directionalcouplers

The small-signal frequency responses of the 1×2 and 2×2 directional coupler traveling-wave modulators are analyzed in closed forms including the effect of both the optical/microwave velocity mismatch and microwave loss. When the microwave loss is negligible, the small-signal bandwidths of 1×2 and 2×2 directional coupler traveling-wave modulators are 20 and 28%, respectively, larger than that of an interferometric modulator with the same electrode length. However, when the microwave loss is large, the 1×2 coupler has a larger bandwidth     

Optical time-division multiplexing for very high bit-ratetransmission

Optical time-division multiplexing (OTDM) extends and expands the well-known techniques of electrical time-division multiplexing into the optical domain. In OTDM, optical data streams are constructed by time-multiplexing a number of lower-bit-rate optical streams. Opportunities for very high-speed transmission and switching are created by removing limitations set by the restricted bandwidth of electronics and by capitalizing on the inherent high-speed characteristics of optical devices. An overview of recent work in optical time-division multiplexing and demultiplexing is presented. Design considerations affecting system architecture are described. Emphasis on the factors that limit system performance, such as crosstalk between multiplexed channels. Examples of very high bit-rate optical time-division multiplexed system experiments using short pulses from mode-locked semiconductor lasers and high-speed Ti:LiNbO₃ waveguide switch/modulators are presented     

Ultrafast time-division demultiplexer based on electrooptic on/offgates

Electrooptic on/off gate-based demultiplexers for high-bit-rate optical transmission systems are reported. The electrooptic demultiplexing of a TDM 49.6 Gb/s fixed pattern data stream is demonstrated, using two cascaded Ti:LiNbO₃ electroabsorptive multi-quantum-well intensity modulators driven at 6.2 GHz. Error-free 10 Gb/s demultiplexing to 5 Gb/s is achieved using cascaded Mach-Zehnder (MZ) modulators driven at 1.25 GHz. The power penalty due to the interchannel crosstalk is 0.3 dB at the bit error rate of 10^-9     

A finite element method (FEM) analysis of a shielded velocity-matched Ti:LiNbO/sub 3/ optical modulator

Shielded velocity-matched Ti:LiNbO/sub 3/ Mach-Zehnder optical modulators are analyzed based on the second-order triangular-element quasi-transverse-electromagnetic finite element method. The relationship between the traveling-wave electrode thickness and the optimum overlaid layer thickness is numerically investigated. The modulation bandwidth of the shielded velocity-matched optical modulator is greatly improved by incorporating the traveling-wave electrode thickness into the design of the optimum overlaid layer thickness in the 1.5 mu m wavelength region.<>     

Chirping characteristic and frequency response of MQW opticalintensity modulator

The spectral linewidth enhancement factor and frequency responses of electro-absorption-type optical-intensity modulators, especially InGaAs/InAlAs MQW modulators, are described. A method of exactly estimating the value of the α factor is presented under the nonlinearity of extinction-ratio characteristics. For measuring the frequency response of modulators, the sideband strength of the modulated output light with an optical spectrum analyzer, is analytically compared with the microwave power of photodiode direct detection with an electrical spectrum analyzer     

Investigations on short-path-length high-speed optical modulatorsin LiNbO3 with resonant-type electrodes

Short-path-length resonant-type high-speed optical modulators are presented. To reduce the active electrode length and enhance modulation efficiencies, different types of resonant electrode structures were investigated using computer simulation tools. Electrical power coupling into the resonators was optimized with a variety of power-feeding circuits. Their resulting optical and electrical behaviors are compared with measurements on modulators fabricated in Ti:LiNbO₃. As a result, resonant-type optical modulators are presented that achieve a low voltage-length product of 25 V mm at 30 GHz in the 1.55-μm wavelength range. Fiber-to-fiber loss of a fully packaged phase modulator with an active optical path length as short as 0.7 mm was 2.3 dB     

Highly efficient 40-GHz bandwidth Ti:LiNbO3 opticalmodulator employing ridge structure

The design and fabrication of a Ti:LiNbO₃ optical modulator employing a ridge structure with a shielding plane are described. The ridge structure offers a relatively low microwave propagation loss and large interaction between the microwave and optical wave under the condition of a 50 Ω characteristic impedance system, resulting in a large modulation bandwidth and low driving voltage. As a result, a 3-dB optical bandwidth of 40 GHz with a driving voltage of 3.6 V is achieved at 1.5 μm     

Optical time-division multiplexing and demultiplexing in a multigigabit/second fibre transmission system

We demonstrate an optical time-division-multiplexed fibre transmission system using actively mode-locked InGaAsP injection lasers and high-speed Ti:LiNbO3 switch/modulators. Two 2Gbit/s return-to-zero data streams are optically multiplexed to 4Gbit/s, transmitted through 8 km of fibre, optically demultiplexed and detected with low error rate.     

Analysis of segmented traveling-wave optical modulators

A simple and comprehensive modeling approach is developed for analyzing the frequency response of segmented traveling-wave optical modulators. The approach is based on the microwave transmission (ABCD) matrix theory. The case study for a GaAs traveling-wave Mach-Zehnder modulator (MZM) verifies this analysis approach with excellent agreement to the reported experimental results; the analyses for the quantum-well-based MZMs and electroabsorption modulators indicate that the segmented traveling-wave design can provide much better bandwidth than the lumped-element or the continuous-traveling-wave counterparts, with a few decibels penalty in the electrical-to-optical (E/O) conversion gain if low-loss optical waveguides are available. Meandered transmission line design, which provides more design freedom, is also analyzed using this modeling approach.     

A broadband Ti:LiNbO3 optical modulator with a ridgestructure

We describe the design, fabrication, and characteristics of a Ti:LiNbO₃ optical modulator with a ridge structure. The structure keeps microwave propagation loss low and enables a large interaction between microwaves and optical waves under the conditions of velocity-matching and impedance matching, resulting in a large modulation bandwidth and low driving voltage. Using this structure, we have developed an optical intensity modulator with an optical 3-dB bandwidth of 75 GHz and a driving voltage of 5.0 V at a wavelength of 1.5 μm     

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.