40-Gb/s X-cut LiNbO/sub 3/ optical modulator with two-step back-slot structure

We propose a newly designed X-cut lithium niobate (LiNbO/sub 3/) optical modulator. It has a two-step back-slot structure to satisfy the velocity-matching condition without the buffer layer of silicon dioxide (SiO/sub 2/). Accordingly, this modulator can achieve low drive voltage and low optical insertion loss. In addition, the dc-drift phenomena due to the buffer layer can be suppressed. This structure is fabricated with micromachining technology using excimer laser ablation. The optical 3-dB bandwidth of the fabricated modulator reaches 30 GHz, and the drive voltage is less than 3 V at 1 kHz. From the measurement of the optical eye diagram at 43.5-Gb/s, clear eye openings were obtained. This modulator is sufficient for 40-Gb/s optical transmission systems.     

Ridged LiNbO/sub 3/ modulators fabricated by a novel oxygen-ion implant/wet-etch technique

This paper demonstrates a new ion implantation and wet-etch technique for fabricating high-quality ridged optical waveguides for high-speed LiNbO/sub 3/-based optical modulators. In addition, the paper demonstrates the fabrication of optical waveguide ridges >3 /spl mu/m in height with 90/spl deg/, and even re-entrant sidewall angles for the first time. The modeling used indicates that 90/spl deg/ (and re-entrant) sidewall ridges can reduce the required modulator drive voltage by 10-20% over modulators with conventional trapezoidal ridge profiles fabricated with reactive ion etching. A 40-Gb/s modulator with a 30-GHz bandwidth, 5.1-V switching voltage at 1 GHz, and a 4.8-dB optical insertion loss is fabricated using the ion implantation/wet-etch process. Fabricated devices showed good stability against accelerated aging, indicating that this process could be used for commercial purposes.     

GaInAs/GaInAsP strained quantum well monolithic electroabsorption modulator/amplifier by lateral bandgap control with nonplanar substrates

An electroabsorption modulator and an optical amplifier have been monolithically integrated by using nonplanar MOVPE. A bandgap shift of more than 60nm was obtained with atmospheric pressure MOVPE of GaInAs/GaInAsP strained QWs on 10 mu m wide ridges. A chip gain of 9dB and an excitation ratio of 17dB were obtained for the monolithic electroabsorption modulator/amplifier. The integration of an optical amplifier enables the use of a wavelength close to the bandgap of the modulator, resulting in low voltage and low chirp operation.<>     

Technique for velocity-matched traveling-wave electroopticmodulator in AlGaAs/GaAs

A design for a velocity-matched traveling-wave directional-coupler intensity modulator in AlGaAs/GaAs is proposed. The structure utilizes a thin coating of Ta₂O₅ on the top of the modulator/electrode structure in order to achieve velocity matching between the optical wave and microwave signal. The film does not significantly affect the optical properties or voltage requirements of the modulator. The optical and RF characteristics of the modulator are analyzed using the effective-index and finite-difference methods. The optical bandwidth is calculated numerically, taking into account both the anticipated velocity mismatches due to fabrication tolerances and the calculated frequency-dependent microwave losses. The predicted small-signal bandwidth of a 3 mm long direction coupler biased at a null point is greater than 45 GHz, and exceeds 100 GHz (~50 GHz electrical bandwidth) when the coupler is biased in the linear region. This device is designed to operate at 830 nm with a maximum modulation voltage of 5 V. The figure of merit of the proposed device is therefore at least 10 GHz/V when an electrical bandwidth of 50 GHz is used     

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。     

A surface-normal coupled-quantum-well modulator at 1.55 /spl mu/m

We demonstrate a surface-normal coupled-quantum-well InGaAs-InAlAs electroabsorption modulator that provides optical modulation with a contrast ratio in excess of 1.5 at only 6 V. The device operates at 1.55 /spl mu/m and is based on a novel strain-balanced layer structure. The operating voltage is about two times lower than that of a conventional square quantum-well modulator that achieves a comparable contrast ratio.     

Double-stub structure for resonant-type optical modulators using 20-/spl mu/m-thick electrode

We propose a resonant-type LiNbO/sub 3/ optical modulator with low halfwave voltage. We optimize the phase constant of the electric wave with thick electrodes to reduce the halfwave voltage. The halfwave voltage of the fabricated modulator with 20-/spl mu/m-thick electrodes is 5.8 V at 10.6 GHz.     

Pseudomorphic 2DEG FET IC's for 10 Gb/s optical communicationsystems with external optical modulation

An optical modulator driver IC and a preamplifier IC for 10 Gb/s optical communication systems are developed using AlGaAs/InGaAs/GaAs pseudomorphic two-dimensional electron gas (2DEG) FETs with a gate length of 0.35 μm. The optical modulator driver IC operates at a data rate up to 10 Gb/s with an output voltage swing of more than 4 V_p-p . The bandwidth for the amplifier IC is 13.0 GHZ with ab 47 dB-Ω transimpedance gain. In addition, optical transmission experiments with external optical modulation using these ICs have successfully been carried out at 10 Gb/s     

75 GHz broadband Ti:LiNbO3 optical modulator with ridgestructure

The authors propose a Ti:LiNbO₃ optical modulator employing a ridge structure with a thick electrode. The structure offers a low microwave propagation loss and strong interaction between microwaves and optical waves under conditions of velocity matching and impedance matching, resulting in a large modulation bandwidth and low driving voltage. Using this device, the authors have developed an optical intensity modulator with a 3 dB optical bandwidth of 75 GHz and a driving voltage of 5.0 V at 1.5 μm     

50 GHz velocity-matched broad wavelength LiNbO/sub 3/ modulator with multimode active section

A velocity-matched LiNbO/sub 3/ modulator is described which achieves an electrical 3 dB bandwidth of 44 GHz and an optical 3 dB bandwidth in excess of 50 GHz. The modulator also uses multimode waveguides in the active section to achieve excellent loss, drive voltage, and contrast ratio characteristics over a 1.3-1.55 mu m wavelength range.<>     

Electrooptic polarization modulation in multielectrode AlxGa1-xAs rib waveguides

A single mode rib waveguide (RWG) polarization modulator is described with a measured extinction ratio ≳27 dB, a power conversion ≈0.99, and a switching voltage of 12.5 V atlambda = 1.064 mum. The modulator is based on a modified stepDeltabeta-reversal configuration. Contact with CdO and an Au overlay ensures low optical losses (lsim1cm^-1) for both polarizations.     

Design and characterization of a 10-Gb/s dual-drive Z-cut Ti:LiNbO/sub 3/ electrooptical modulator

The design and characterization of a dual-drive (DD) Z-cut Ti:LiNbO/sub 3/ electrooptical modulator are presented. Both the radio frequency (RF) electrode layout and the optical splitters and combiners of the Mach-Zehnder interferometer are investigated: The former is designed to avoid RF line crosstalk, whereas the latter is to obtain a low-loss longitudinally short splitter structure, able to properly separate the interferometer straight optical waveguide sections. First, the RF analysis is addressed, simulating the small-signal behavior of the DD coplanar waveguide electrode structure as a function of the central ground width. The performances of different splitter layouts (circular segment bends, sin-bends, and offset bends) are then investigated using the beam propagation method. Finally, experimental results on the electrical and optical test pattern structures, and of the DD Z-cut complete modulator, are discussed and compared with simulations.     

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     

A low-voltage, high-reflectance-change normally off refractiveGaAs/Al0.2Ga0.8As MQW reflection modulator

We present our optimization of a normally off refractive GaAs/Al _xGa_1-xAs multiple-quantum-well (MQW) reflection modulator with respect to the on/off reflectance change, on/off contrast ratio, and operating voltage. We use optical transfer matrices, theoretically calculated refractive indices, and absorption coefficients to simulate the operation of a normal-incident Fabry-Perot MQW modulator. Our calculations suggest that a normally off refractive GaAs/Al_0.2Ga_0.8As MQW reflection modulator with a reflectance change of 42.9% and an on/off contrast ratio of 1539 for an operating voltage of only 2.44 V can be fabricated by molecular-beam epitaxy (MBE)     

40 Gbit/s silicon optical modulator for highspeed applications

A high-speed silicon optical modulator based on the free carrier plasma dispersion effect is presented. It is based on carrier depletion of a pn diode embedded inside a silicon-on-insulator waveguide. To achieve high-speed performance, a travelling-wave design is used to allow co-propagation of the electrical and optical signals along the length of the device. The resulting modulator has a 3 dB bandwidth of ~30 GHz and can transmit data up to 40 Gbit/s.     

Electroabsorption waveguide modulators at 1.3 /spl mu/m fabricated on GaAs substrates

An InGaAs-InAlAs multiple-quantum-well (MQW) electroabsorption (EA) waveguide modulator fabricated on a GaAs substrate has been designed and characterized at 1.3-/spl mu/m wavelength for microwave signal transmission on an analog fibre-optic link. The modulator structure with a lattice constant 2.5% larger than that of GaAs is grown upon a 0.7-/spl mu/m-thick three-stage compositionally step-graded In/sub z/Al/sub 1-z/As relaxed buffer. The waveguide modulator exhibits a high-electrooptic slope efficiency of 0.56 V/sup -1/, a 3-dB electrical bandwidth of 20 GHz, and a large optical saturation intensity in excess of 17 mW. These high-speed optoelectronic modulators could potentially be integrated with on-chip GaAs electronic driver circuits.     

40 Gbit/s electroabsorption modulators with impedance-controlled electrodes

A novel travelling-wave electroabsorption optical modulator, electrically matched for 50 /spl Omega/ loads of driving circuit drivers, was developed. The scattering parameter of electric reflection (S/sub 11/) from this modulator is less than -20 dB at 20 GHz. It can thus enable a 40 Gbit/s, 2 km SMF transmission with a 0.3 dB penalty at a 1.3 /spl mu/m wavelength.     

The effect of band-tails on the design of GaAs/AlGaAs bipolartransistor carrier-injected optical modulator/switch

The variations in the absorption coefficient and the refractive index in the waveguide region of a double-heterostructure N-AlGaAs/p-GaAs/N-AlGaAs bipolar transistor carrier-injected optical modulator/switch are calculated theoretically using a band-tail model. The refractive index change is calculated by a Kramers-Kronig analysis of the absorption curve, which changes in the vicinity of the absorption edge primarily because of the effect of band-filling, band-gap shrinkage, and plasma dispersion associated with the presence of free carriers. The performance of bipolar transistor carrier-injected optical modulator/switch is analyzed on the basis of the calculated results, including the alpha-parameter and the maximum modulation depth. It is shown that small frequency chirping and high modulation depth with low absorption loss can be achieved by properly selecting the optimum operating conditions     

A novel hybrid WDM/SCM-PON sharing wavelength for up- and down-link using reflective semiconductor optical amplifier

A hybrid wavelength-division multiplexed/subcarrier multiplexed passive optical network (WDM/SCM-PON) which shares the same wavelength both up-link and down-link is presented by using a reflective semiconductor optical amplifier (RSOA) as a modulator. We investigated down-link of 622 Mb/s using distributed feedback laser diode with direct modulation and 100-Mb/s up-link using an RSOA as a modulator with 900-MHz SCM signal. In our scheme, a novel efficient and cost-effective WDM/SCM-PON using the same wavelength for down/up-link at each optical network unit is proposed and experimentally demonstrated.     

Quasi-velocity-matched LiTaO3 guided-wave optical phasemodulator for integrated ultrashort optical pulse generators

A quasi-velocity-matched guided-wave optical phase modulator using periodically domain inverted LiTaO₃ is fabricated and demonstrated for the first time to the best of the authors' knowledge. Band modulation characteristics and the generation of optical side-bands of ~100 GHz are confirmed successfully. This modulator can be used to construct an integrated ultrashort optical pulse generator