Analysis of E-O polymer mode converter with bottom transverse poling electrodes

The operation of integrated-optics electrooptic (EO) polymer modulators relies on the Pockel effect that is generated by applying an electric field to the optical waveguides. Birefringent behavior occurs because of the dipole realignment of the poled polymer film. In this paper, the static-field analysis of the electric field distribution for the mode converters with bottom transverse electrodes incorporating a buffer layer was done by using an efficient discrete Fourier transform domain technique. According to the numerical analysis results, the electric-field distribution in the guiding layer is the function of electrode geometric configuration. Two parameters, the electrode width and spacing, were considered for the optimum arrangements of the coplanar electrodes deposited on the polymer waveguide modulator. Also studied was the influence of the asymmetric electrode width of the coplanar electrodes on the sample surface.     

LiNbO3waveguide modulator with 1.2 µm thick electrodes fabricated by lift-off technique

In order to make efficient high-frequency electrooptic modulators, the microwave loss in the electrodes has to be minimized. A lift-off technique using chlorobenzene to harden the top of AZ1350-J photoresist was adopted to fabricate 1.2 μm thick metal electrodes. A 1 cm long, 15 μm wide strip electrode has a dc resistance of 11 Ω, which is substantially less than that of the 2000 Å thick electrodes routinely fabricated. A 1 cm long traveling-wave phase modulator consisting of a single waveguide was tested. The measured -3 dB bandwidth is 3.8 GHz.     

Design of Low ${V}_{pi}$ High-Speed GaAs Travelling-Wave Electrooptic Phase Modulators Using an n-i-p-n Structure

GaAs-based electrooptic phase modulators using an n-i-p-n structure and coplanar waveguide traveling-wave electrodes are designed using the compact 2-D finite-difference time-domain technique and PadÉ approximation transform. By optimization, an electrical 3-dB bandwidth of 40 GHz and a 6.6-V $V_{pi}$ are predicted for a 5-mm-long modulator.      

Microwave characteristics of high-speed traveling-wave electroopticmodulators on III-V semiconductors

Microwave characteristics of III-V semiconductor traveling wave electrooptic modulators were studied rigorously by using the method of lines. This approach makes it possible to construct a complete computer model of double-rib, multilayer strip waveguide modulators which are out of reach of the classical methods. Both p-i-n junction and Schottky-barrier junction controlled devices have been investigated. The fringing microwave field has been found to play an important role in improving the modulator bandwidth. For the first time, a nonperfect electrode of finite thickness has been modeled. The method of lines algorithm has proved to be a valuable CAD tool for design and optimization of high-speed modulators     

Analysis and Demonstration of Mach–Zehnder Polymer Modulators Using In-Plane Coplanar Waveguide Structure

Analysis, design, and experimental demonstration of polymer Mach-Zehnder (MZ) modulators with a newly proposed in-plane coplanar waveguide (CPW) electrode are presented in this paper. Compared to the conventional CPW electrode, the new in-plane CPW configuration significantly enhances the overlap factor between microwave and optical wave by 40% from the theoretical calculations. Different from polymer modulators using the traditional microstrip electrodes, this structure can suppress dc bias voltage drift and widen modulation bandwidth. Using CLD1/APC and AJL8/APC electrooptic polymers, MZ modulators using the in-plane CPW are fabricated and tested. The measured Vpi's of the MZ with 15-mum gap spacing and 2 cm electrode length are 5.4 and 9.5 V at 1.55 mum for the AJL8/APC and CLD1/APC devices, respectively. The evaluated Upsi₃₃'s are 46 pm/V for the AJL8/APC and 26 pm/V for the CLD1/APC MZ. The dc bias stability is tested and compared for both the microstrip and the in-plane CPW electrode modulators. Experimental results show that the in-plane CPW modulator greatly improves the bias stability. From the microwave measurement, the microwave signal loss of the modulator using the in-plane CPW is much reduced to 0.2 dB/cmradicGHz, while 0.7 dB/cmradicGHz using the microstrip.     

High speed III-V electrooptic waveguide modulators at λ-1.3μm

Traveling wave GaAs electrooptic waveguide modulators at a wavelength of 1.3 μm with bandwidth in excess of 20 GHz have been developed and characterized. The design and characteristics of both p-i-n modulators in microstrip configuration and Schottky barrier on n ^--GaAs/semi-insulating (SI) GaAs in the coplanar strip configuration modulators are discussed. It is shown that microwave loss and slowing on n⁺ GaAs substrates will limit the bandwidth of the microstrip modulator to less than 10 GHz for a device 8 mm in length. Modulators with bandwidths in excess of 10 GHz are fabricated on SI GaAs substrates     

Microwave properties of traveling-wave electrodes in LiNbO3 electrooptic modulators

The microwave characteristics of a traveling-wave electrode in electrooptic modulators on z-cut LiNbO₃ crystals with buffer layers with various dielectric constants are calculated by a quasi-TEM analysis. The theoretical results are compared with the measured results on z-cut crystals for coplanar waveguide (CPW) electrodes. The agreement between the calculated and the measured results is good     

Push-pull poled polymer Mach-Zehnder modulators with a singlemicrostrip line electrode

A push-pull structure has been realized for integrated Mach-Zehnder modulators based on a thermoset electrooptic polymer. The two modulator waveguide arms were poled in the opposite directions and covered by a single microstrip line electrode. This device structure can reduce the half-wave voltage by 50% without compromising wide-band frequency response. Efficient poling was achieved by using a compatible cladding material to lower the poling voltage, and by using a top cover piece and an inert gas to suppress air breakdown between the poling electrodes. Our fabricated devices exhibited the predicted 50% half-wave voltage reduction compared with non-push-pull devices fabricated on the same chip     

Modeling and optimization of low chirp LiNbO/sub 3/ Mach-Zehnder modulators with an inverted ferroelectric domain section

Domain inversion under coplanar waveguide electrodes is proposed to improve the frequency-chirping behavior of Z-cut LiNbO/sub 3/ Mach-Zehnder modulators. This is achieved by introducing a phase reversal electrode section in tandem with inverted ferroelectric domains. The method opens the way to broad-band single-drive modulators with predetermined chirp parameter. The experimental results obtained at 10 Gb/s confirm the possibility of lowering the chirp parameter of a Z-cut modulator, while keeping a halfwave voltage lower than 5 V.     

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     

Polarization-insensitive transition between sol-gel waveguide and electrooptic polymer and intensity modulation for all-optical networks

An intensity modulation using a hybrid electrooptic (EO) polymer/sol-gel straight channel waveguide, useful in the 1550-nm wavelength regime is demonstrated without using Mach-Zehnder interferometric waveguide. The sol-gel waveguide is selectively buried so that a vertical transition into and out of an EO polymer coated on the sol-gel waveguide is arranged. The throughput ratio for transverse electric (TE) and transverse magnetic (TM) modes of the light coupled out of the hybrid waveguide is improved up to 0.9 dB with the help of reduced birefringence of the EO polymer after corona poling. We show that the fabrication process of such hybrid-type waveguides enables production of a phase modulator operating at 1550-nm wavelength. The fabricated straight channel waveguide modulator exhibits stable- and high-intensity modulation efficiency (82%) using a simple cross-polarization setup after the polarization dependence is reduced. We demonstrate an all wet-etching process to fabricate polymeric EO modulators.     

微波波段带通光调制器的优化设计

本文对具有极性反转电极结构的行波皮导电光调制器的调制特性进行了理论分析，对共平面波导电极和非对称微带电极用于极性反转电极结构下的有关特征参量进行了数值计算，对使用这两种电极制成的调制器，我们用带宽－长度积、电压－长度积和带宽－电压比这三个品质因子对其性质进行了比较，本文还给出了调制器的设计实例，对结果的分析讨论表明，非对称微带电极用于极性反转电极结构对在高频波段实现低动功率光调制是十分有效的。     

Low-drive-power asymmetric Mach-Zehnder modulator with band-limitedoperation

A novel 2×2 asymmetric Mach-Zehnder interferometric modulator is fabricated on Ti:LiNbO₃ waveguide and tested at microwave frequency. Band-limited operation was realized by using a traveling-wave periodic polarity reversed structure with the asymmetric strip line as the modulator electrode instead of the conventional coplanar waveguide. For a modulator with a three-section polarity reversed electrode, a 3-dB bandwidth of 7 GHz was obtained experimentally from 9 to 16 GHz centered at 12 GHz     

Overlap integral factors in integrated optic modulators andswitches

Coplanar strip and complementary coplanar strip electrodes are often used in integrated optical switches and modulators using the optical directional coupler. The field analysis of these electrodes is performed to determine the optimum overlap integral between the optical field and the modulating electromagnetic field. The optimal placement of the electrodes with respect to the optical channel waveguides is presented as a function of electrode width and spacing     

Full-wave analysis of coplanar waveguides for LiNbO3optical modulators by the mode-matching method considering nonidealconductors on etched buffer layers

Rigorous analysis of traveling-wave coplanar waveguide electrodes for LiNbO₃ optical modulator applications is presented by using an extended full-wave mode-matching method. The microwave propagation characteristics under the composite influence of substrate anisotropy, uniform or etched buffer layers, finite electrode thickness and conductivity, and metallization undercutting are accurately assessed by employing a network equivalent formulation. Variations of the coplanar waveguide microwave effective index and the characteristic impedance at low frequencies due to finite electrode conductivity are illustrated, and are important even though the mode is quasi-TEM in nature. The effect of etching the SiO₂ buffer layer is shown to be one possible method for lowering the microwave effective index while keeping the conductor loss at a fixed level     

Hybrid Electrodes Used in Polymer Electro-Optic Modulator with Ultra-Broadband

Hybrid electrode combined coplanar waveguide (CPW) and microstrip lines in ultra-broadband electro-optic modulators. The characteristics of the modulators with hybrid electrode is like that of the modulator with the microstrip lines, in which the microstrip electrode is loaded above only one arm of Mach-Zehnder (M-Z) optical waveguide, besides the problem of microstrip line to coaxial transition and corona polarization in polymer modulators have been best solved at the same time. By using finite element method (FEM), the characteristic parameters such as half-wave voltage, modulation bandwidth characteristic impedance, effective microwave refractive index of polymer modulators with microstrip line and hybrid electrode are analysed and compared in this paper. The results present that velocity match, impendence match and lower microwave propagating loss is easily realized in modulators with hybrid electrode.     

Improved electrode geometry for electrooptic frequency translation in a channel waveguide

The technique of optical frequency translation by a rotating electric field in a trigonal electrooptic crystal has been adapted to a buried channel waveguide structure. To implement the field rotation under the constraints of a planar geometry, an electrode configuration was adopted consisting of three coplanar, collinear metal strips driven in phase quadrature, with the light guide under the central strip. Elimination of a cyclic modulation of field amplitude, characterizing the results of a previous study by the author, was achieved by appropriately widening the outer electrodes relative to the central one and by choosing a suitable channel depth. Spurious sidebands previously generated by an effective counter-rotating field component are thus removed. It is shown that with a residual modal birefringence equivalent tolambda/20-differential phase retardation, the device here described can achieve 98 percent translation into the single first-order sideband with less than 0.5 percent combined spurious carrier and second-order upper sideband.     

Slow-wave electrode for use in compound semiconductor electroopticmodulators

A slow-wave electrode structure for integrated optic traveling-wave modulators in which the microwave's effective refractive index is matched to the optical wave's effective refractive index is described. The electrode structure is a capacitively loaded coplanar strip waveguide which can be formed in a single layer of metallization. Fabrication can be accomplished by a single photo-resist patterning, followed by an etching and a standard lift-off technique. Based on the use of gallium arsenide substrates and modern lithographic techniques allowing fabrication with micron scale resolution, slow-wave electrodes having a microwave effective refractive index of 3.5, as well as 50 and 75 Ω characteristic impedances are proposed. The theory of slow-wave electrodes is developed, and slow-wave electrodes have been designed, fabricated, and tested to verify the theory. Measurement results are found to agree well with the theory     

Reduced modulator drive-power requirements for 10.6-µ guided waves

It is shown that a reduction of more than an order of magnitude in electrooptic modulator drive power requirements should be possible at 10.6 μ by the use of a dielectric slab waveguide, despite the dimensional limitations imposed by electrode absorption.     

Bandpass traveling-wave Mach-Zehnder modulator in LiNbO/sub 3/ with domain reversal

A bandpass traveling-wave Mach-Zehnder modulator is demonstrated in a Z-cut LiNbO/sub 3/ substrate where quasi-phase-matching is achieved by using a crystal domain reversal and a simple uniform coplanar waveguide electrode structure. The domain reversal was created, by using electric-field poling, to implement the three-section alternating electrooptic interaction. At the operating wavelength /spl lambda/=1.32 /spl mu/m, the modulator has a 15 GHz-broad bandpass response centered at 25-GHz 3.6-dB fiber-to-fiber insertion loss, 12 V on-off voltage at the center frequency, and -33-dB extinction ratio.