Novel two-arm microwave transmission line for high-speedelectro-optic polymer modulators

The authors demonstrate the use of a novel microwave design for high-speed electro-optic polymer modulators. A two-arm Mach-Zehnder-like transmission line may be used to reduce V_π in a push-pull modulator without problems of dielectric breakdown or crosstalk during poling     

Picosecond pulse response of broad-band guided-wave interferometric light modulators

Picosecond pulse operation of the guided-wave light modulator is reported in this paper. First, we analyze the time responses of phase and intensity modulators of the traveling-wave type, and the modulated output waveforms related to the modulating pulses are discussed. It is shown that there is an optimum interaction (or electrode) length of the modulator whereby the drive voltage is minimized for a desired output pulse width. The traveling-wave push-pull light intensity modulator of Ti-diffused LiNbO3waveguides was modulated by a pulse train of 1 GHz repetition frequency, and the modulated output was observed by the image tube streak camera modified for sinusoidal scan at the same frequency. The duration of the modulated light pulses was less than 45 ps, which shows good agreement with the predicted one.     

Chirp characteristics of dual-drive. Mach-Zehnder modulator with afinite DC extinction ratio

In this letter, we present theoretical and experimental studies on the chirp of the dual-drive Mach-Zehnder (MZ) modulator with a finite DC extinction ratio. The chirp of the modulator was measured using the fiber response peak method and compared with the theoretically calculated values. The results show that the residual chirp of the MZ modulator could be substantially reduced by driving the modulator in a push-pull mode with unequal amplitude signals     

A compact spectrum constant envelope digital phase modulation

A digital modulation system with compact spectrum and constant envelope is proposed, based on the results of theoretical and experimental studies. This modulation system is suitable for digital mobile communication, which generally is subject to deep and rapid fading. It is characterized with a possibility of practical and stable instrumentation. In the modulator, modulation signal parameters are read out of the memories, by which precise spectrum and waveform can be realized. For the modulator, an orthogonal phase modulation, whose parameters are stable, can be adopted by pertinent parameter design for the modulating signal stored in the memories. The experimental results agree well with the design.     

Acoustooptical pulse modulators

A theoretical analysis of acoustooptical pulse modulators is given. Calculations are carried out that yield the intensity and rise time of the scattered light. The optimum relationship between the dimensions of the optical and acoustic beam is given. A fast high-efficiency modulator has been built based on the theoretical results. The modulator is composed of a 350-MHz ZnO transducer sputtered on a quartz acoustic lens and an As2S3glass serving as the modulating material. With 0.6 watt of electrical RF peak power driving the modulator, 70 percent of the light intensity (at 0.63μ) is deflected. The rise time of the scattered light pulse is of the order of 6 ns.     

10 Gbit/s pseudorandom dark soliton data transmission over 1200 km

A 10 Gbit/s pseudorandom (PR) dark soliton data signal has been successfully transmitted over 1200 km for the first time. The dark soliton source was a 1.53 μm DFB LD and was modulated with a push-pull Mach-Zehnder LiNbO₃ intensity modulator. A T-flip-flop circuit was used to generate the PR dark soliton. A one-bit-shifting scheme with a Mach-Zehnder interferometer was used to convert the dark soliton train into a conventional NRZ signal     

Push-pull polarization conversion using novel asymmetric coupled quantum-well structures

A novel type of polarization conversion modulator, employing multiple repetitions of two novel InGaAlAs-InAlAs asymmetric coupled quantum-well structures, is proposed. The novelty of the proposed design is that, via the electrorefraction effect, it allows for a change in the TM mode's effective refractive index, which is opposite in sign to the change in the TE mode's effective refractive index. Numerical simulations based on the effective-mass envelope-function approximation show that the modulator provides not only an electrorefractive response similar to other quantum-well modulators but can also provide a low-chirp push-pull polarization modulation.     

110 GHz vector modulator for adaptive software-controlledtransmitters

A 110 GHz MMIC vector modulator for use in low-cost, high-performance, radar and communication transmitters is presented. The circuit consists of two push-pull (bi-phase) attenuators arranged in phase quadrature and has dimensions of 1.7× 1.4 mm². The fabricated MMIC has been characterized by means of static S-parameter measurements and shows a minimum insertion loss of 12 dB at 110 GHz, Using these measurements, the required baseband input levels for a 64-QAM static constellation were determined. These levels were then applied at 10 MSample/s, by an arbitrary waveform generator, to demonstrate a 60 Mb/s data rate transmitter operating at 110 GHz. To date, this represents the highest reported RF frequency for direct multilevel carrier modulation using monolithic technology     

Combining self-phase modulation and optimum modulation conditionsto improve the performance of 10-Gb/s transmission systems using MQWMach-Zehnder modulators

For 10-Gb/s transmission over nondispersion shifted fiber, the combined use of self-phase modulation (SPM) and joint optimization of the bias and modulation voltages to increase the dispersion limited transmission distance is considered for multiple quantum well Mach-Zehnder modulators. For the dual drive (push-pull) modulation format, the dependence of the receiver sensitivity on fiber length and average transmitted optical power is determined for both conventional and π phase-shift modulators with either symmetric or asymmetric Y-branch waveguides. When SPM is negligible and the optical extinction ratio is maximized, the modulator design must he considered carefully in order to increase the transmission distance. By combining SPM and optimum modulation conditions, the dependence of the system performance on the modulator design is reduced substantially. For an average transmitted optical power of 12.5 dBm, the receiver sensitivity for transmission over 140 km of fiber varies by only 0.3 dB for the different modulator designs. This compares with a variation of 3.1 dB for maximum extinction ratio modulation     

BPM design optimization and experimental improvement of a Ti:LiNbO3 ridge waveguide linear-mode confinement modulator

A theoretical device model for a ridge waveguide mode confinement modulator on z-cut LiNbO₃, based on the beam propagation method (BPM), is used for design improvement purposes. Following this design technique, a fabricated device yielded a much improved performance of 97% modulation depth with a drive voltage of only ±8 V, in good agreement with the design calculations. The improvement in the experimental performance of this modulator is mainly due to a reduced buffer layer thickness and an optimization of the ridge height. A theoretical analysis based on an accurate effective-index model combined with the BPM is used to predict the performance of the device     

Characterization of the modulation and noise properties of athree-electrode DFB laser

The modulation and noise properties of a three-electrode DFB laser have been measured. At proper biasing, the frequency modulation response could be chosen to be in phase or 180° out of phase with respect to the intensity modulation by selecting the modulated electrode. Applying a first order equalization/filter, a flat frequency modulation response was measured up to 5 GHz. A 20-dB broadband suppression of the intensity modulation was accomplished when modulating two sections in push-pull. The spectral densities of the FM and intermodulation noise were measured showing remarkably flat intermodulation and FM noise spectra     

Electrooptic gigahertz light-pipe modulator for CO2laser wavelength

We have built a traveling-wave electrooptic modulator for 10.6 μm where the optical beam propagates in a highly oversized waveguide. Three CdTe-crystals measuring35 times 0.8 times 0.8mm³each are cascaded to yield an aspect ratio of 131. Velocity matching between optical and modulating waves and a line impedance of 100 Ω are achieved by proper choice of dielectric embedding and of electrode width. Measured characteristics are: optical transmission 96 percent, maximum input VSWR 1.25:1, half-wave voltage 450 V, and frequency response ±1.1 dB over a 0.006-1 GHz range.     

Optimum design of coplanar waveguide for LiNbO3 opticalmodulator

In this paper, we first present a novel finite element method combined with the conformal mapping (FEM-CM) for a quasi-static analysis of coplanar waveguides (CPW). Using this approach, the optimum CPW structures for the use in the Ti:LiNbO₃ optical modulator are discussed in detail to realize optical-microwave phase velocity match and electrode-source characteristic impedance match. Our numerical results reveal that both conditions can be satisfied simultaneously by introducing a SiO₂ buffer layer and thicker electrodes. The modulator efficiency with respect to the voltage-length product is also evaluated for the optimized structures. Finally, the design guidelines to the optimum CPW structure are presented     

Linear 0.20 GHz modulation with a 1×2 directional coupler

The design and performance of a traveling-wave 1×2 directional coupler modulator are described. A 0-20 GHz response (-4 dB _e) was achieved for Ti:LiNbO₃ devices at the 1.3-μm wavelength. From experimental determinations of coupling coefficient and interarm intrinsic phase-mismatch for several modulators, linear dynamic ranges of, on average, 78 dB and voltage sensitivities of down to 9 μV are determined (for 1-mW detector power, 3-kHz bandwidth, 50-Ω detector load). The dependence of sensitivity on the coupling coefficient is described, and the effect of the buffer layer thickness on the optical bandwidth and sensitivity is demonstrated     

10 Gbit/s, 1.56 mu m multiquantum well InP/InGaAsP Mach-Zehnder optical modulator

10 Gbit/s modulation using a III-V semiconductor Mach-Zehnder interferometer is reported for the first time. The modulator has a -3 dB bandwidth in excess of 15 GHz and is operated in a push-pull drive configuration with only 2 V peak to peak. 10 Gbit/s nonreturn to zero format eye diagrams with an extinction ratio>10 dB are demonstrated.<>     

Improved coupled-mode theory for anisotropic waveguide modulators

An improved coupled-mode theory for the propagation of modulated light waves in anisotropic dielectric waveguides is presented. Starting from Maxwell's equations, a partial differential equation is derived to describe mode-coupling between two normal modes which may propagate in anisotropic waveguide systems under modulation. The theory is applied to the analysis of typical waveguide modulators; examples for LiNbO₃ phase modulator, Mach-Zehnder, and directional coupler modulator are presented. The theory is applicable to both bulk and waveguide modulators/switches from DC to the high-frequency band. The only limitation is that the modulating wave has to propagate collinearly to the light waves     

Traveling wave optical modulator using a directional coupler LiNbO3waveguide

Fabrication methods for low drive voltage and broad-band LiNbO3waveguide directional coupler optical modulator are described. Optical waveguides were prepared by conventionaly Ti in-diffusion into LiNbO3c-cut plate. To obtain wide-band frequency response, traveling wave electrodes were used. Electrode characteristic impedance measured by time domain reflectometry method coincided well with the calculated value by conformal mapping. Measured electrode conductor losses followed square root of frequency. To reduce electrode conductor losses, asymmetric and 3-μm thick Al electrodes were used. Directional coupler optical modulator frequency response was analyzed, using the phase difference average value along the propagation directions. Calculated value by this method coincided well with measured value obtained by a swept frequency technique. The characteristics of this modulator at the 1.317-μm wavelength are as follows: 100 percent modulation voltage is 4 V, extinction ratio is 17 dB, optical insertion loss is 5.4 dB, 3-dB bandwidth is 3.6 GHz, and rise time is about 400ps.     

Bias-correlated branched-waveguide cascaded light intensitymodulator in the scattering matrix formalism

The authors describe the concept of a composite interferometric light intensity modulator constructed in the form of a cascade of 2ⁿ branched-waveguide electrooptic modulators in which the DC bias of the individual modulators are provided in a certain phasing required to yield a high overall modulation efficiency. The analysis of this cascaded modulator, presented in the scattering matrix formalism, predicts that the overall modulation efficiency of the cascade is increased by the factor 2^n/2 compared with that of a single constituent modulator when both are supplied with the same amount of modulating signal power     

高速硅基单端推挽驱动狭缝脊波导调制器

针对硅基单端推挽驱动调制器的脊波导pn结部分结电容较大,限制了调制器带宽进一步提高的问题,提出了一种狭缝脊波导pn结结构。通过将脊波导pn结蚀刻一定尺寸的狭缝,降低了pn结电容,实现了带宽的提升。对两种pn结进行了仿真对比,结果表明,在相同电极结构与掺杂条件下,狭缝脊波导调制器比脊波导调制器的电光带宽提升了约8.3 GHz。基于优化的单端推挽驱动狭缝脊波导调制器,仿真实现了90 Gbit/s的OOK信号调制,眼图消光比为14.69 dB。     

A novel Ti:LiNbO3ridge waveguide linear mode confinement modulator fabricated by reactive ion-beam etching

In this paper we report the realization of a ridge waveguide linear mode confinement modulator using reactive ion-beam etching (RIBE). With the help of a design analysis, the modulation characteristics are predicted. Along with the measured modulation characteristics, preliminary results of the calibration of ion-beam etching inZ-cut LiNbO3by Ar and a mixture of Ar and CHF3are presented. Differential etching rates between photoresist and LiNbO3of as high as 7 are obtained. Using the proposed design of a ridge waveguide linear modulator, a linear modulation of over 67 percent is measured.