Dynamic frequency response for InGaAs/InAlAs multiple quantum well optical modulators

The frequency response of InGaAs/InAlAs multiple-quantum-well (MQW) optical modulators and their dependence on incident light power are described. No decrease in small-signal modulation bandwidth was observed up to ten times the initial value of 1 mW. The mechanism of the frequency-response difference between InGaAs/InAlAs MQW modulators and InGaAs/InP modulators is discussed.<>     

The distributed feedback waveguide modulator

A novel waveguide electroabsorption modulator structure is proposed. In this structure, the resonator properties of a quarter-wave phase-shifted grating are combined with the electroabsorption properties of a multiple-quantum-well (MQW) waveguide. The theory and design considerations for such a distributed feedback MQW modulator are discussed. The performance characteristics of this structure are studied theoretically and compared with conventional MQW waveguide modulators. It is found that increased modulation bandwidth and reasonable optical bandwidths can be obtained over a device without feedback, and design tradeoffs are identified and explored     

On the operational and manufacturing tolerances of GaAs-AlAs MQWmodulators

We approach the question of optimization of surface-normal p-i(multiquantum-well, MQW)-n modulators from the viewpoint of investigating their tolerance to variations in wavelength and temperature and errors in manufacture. The reflection characteristics of two high-quality samples are carefully processed to eliminate Fabry-Perot fringes, and then their spectra at any bias are characterized with six phenomenological parameters which depend on λ₀, the zero-field exciton position. The two GaAs-AlAs samples have λ₀'s of 833.8 and 842.3 nm, and so cover a range useful for modulators designed to operate near 850 nm in the normally reflecting condition, i.e., reflection decreases with field. A linear interpolation of the parameters of these two samples is used to predict the behavior of MQW diodes with λ₀'s around this range, and so a fully comprehensive examination of normally reflecting MQW modulators is performed. The performance aspect that is examined is contrast ratio as a function of nonuniformities in the devices or operating conditions given a voltage swing of 3 V. There are two operational modes discussed. If the voltage offset of the bias is allowed to vary via a feedback circuit, a contrast of 2:1 may be maintained over an operating wavelength change (Δλ) of 17 nm with local variations of wavelength of ±1 nm, which corresponds to a temperature variation of 60°C while allowing for variations of laser driver wavelength of ±1 nm. If feedback Is not permitted, we determine that, given tolerances to manufacturing errors, a contrast of 1.5:1 may be maintained over a wavelength range of ~5 nm by either using stacked diode designs or extremely shallow quantum wells     

Design and measurement of an MQW nipi waveguide modulator foroptoelectronic integrated circuits

A procedure for optimizing MQW hetero-nipi waveguide phase modulators is presented that includes both drive voltage and frequency response. Experimental phase change, absorption change, and frequency response measurements of a modulator designed using this algorithm are presented. In a 300-μm-long device consisting of 3 nipi periods, π phase shift is achieved with an applied voltage of 3.25 V with a frequency bandwidth on the order of 200 MHz     

Saturation intensity and time response of InGaAs-InGaP MQW opticalmodulators

We report modulation saturation and time response measurements on InGaAs-InGaP MQW modulators. The measurements yield a saturation intensity of (3.7±0.1) kW/cm² for a 0-10 V swing and switching times between 10 and 90 ns, depending on the bias voltage and incident light intensity. The observed dependence indicates that field screening due to carrier build-up is the dominant physical mechanism determining both the speed and the saturation intensity. This conclusion is supported by results of theoretical calculations     

Long-wavelength waveguide multiple-quantum-well (MQW) optical modulator with 30:1 on/off ratio

Large on/off ratio optical modulation of long-wavelength light propagating along the plane of InGaAs/InAlAs multiple-quantum-well (MQW) structures grown by molecular beam epitaxy (MBE) is obtained for the first time. These waveguide MQW optical modulators have a modulation on/off ratio of 30:1 (15 dB) at a driving voltage as low as 9 V, and a capacitance-limited pulse response of 280 ps (FWHM). This measurement is the first step in achieving faster and higher extinction ratio devices.     

MOVPE growth of strained InGaAs/lnAIAs MQWs for a polarization-insensitive electroabsorption modulator

Metalorganic vapor phase epitaxial growth of a strained InGaAs/lnAIAs multiquantum well (MQW) structure was carried out for optical electroabsorption modulators. A high-quality MQW layer can be grown by introducing compressive strain into InAlAs barrier layers against tensile-strained well layers. We have also demonstrated strained InGaAs/lnAIAs MQW electroabsorption modulators with polarization insensitivity by using these layers and have obtained a highquality modulator with a low driving voltage of 1.7 V and a wide 3-dB bandwidth of over 20 GHz.     

High-speed InGaAsP/InP multiple quantum well, 1.55 mu m singlemode modulator

1.55 mu m single mode ridge waveguide modulators based on electroabsorption in InGaAsP/InP multiple quantum wells (MQW) are reported. A 10 dB extinction ratio was obtained by applying a 2 V drive voltage to a 100 mu m long device with a 3 dB on-state loss. The 3 dB cutoff frequency is 12.5 GHz.<>     

High-performance Mach-Zehnder modulators in multiple quantum wellGaAs/AlGaAs

We demonstrate Mach-Zehnder interferometric waveguide intensity modulators which employ electrorefraction due to the quantum-confined Stark effect in multiple quantum well (MQW) GaAs/AlGaAs. These devices exhibit average half-wave voltage-length products as low as 3.0 V·mm and extinction ratios greater than 23.8 dB, which are superior to any MQW devices of this type. An effective index based model is developed to extract linear and quadratic electro-optic coefficients from the modulation data. Also, the power handling limitations of MQW modulators are discussed in terms of device performance and catastrophic electrical failure     

Ultra-high-speed multiple-quantum-well electro-absorption opticalmodulators with integrated waveguides

Integrating the input and output waveguides with a multiple-quantum-well (MQW) electro-absorption (EA) optical modulator is shown to achieve ultra-high-speed modulation while keeping the total device length long enough for easy fabrication and packaging. Testing with fabricated modulators showed that a shorter modulation region results in a larger modulation bandwidth. The additional loss due to the waveguide integration was less than 1 dB. An optimized modulator showed a large modulation bandwidth of 50 GHz, a low driving voltage of less than 3 V, and a low insertion loss of 8 dB. A prototype module of this modulator had a bandwidth of greater than 40 GHz. Optimizing the MQW structure makes the modulator insensitive to polarization. These results demonstrate that MQW-EA modulators with integrated waveguides are advantageous in terms of fabrication, packaging, and ultra-high-speed modulation     

The effect of modulator nonlinearity on measurements of chirp inelectroabsorption modulators

Traditional techniques for measuring the chirp in external modulators assume that the optical intensity output of the modulator is a faithful representation of the applied voltage. For electroabsorption modulators, which can have highly nonlinear transmission- vs voltage characteristics, this is a poor approximation, especially when they are operated at high modulation indices. We demonstrate a new technique that makes use of the actual measured T(V) for the device under test, and show that this new technique permits measurement of chirp in modulators generating either NRZ signals or soliton pulses. We apply this technique to both bulk and MQW electroabsorption modulators, and show that traditional measurements significantly underestimate the α values of these devices     

Switchable-logic photonic switch array monolithically integratingMSMs, FETs, and MQW modulators

As a two-dimensional parallel digital processing device, we describe a switchable-logic photonic switch array that monolithically integrates metal-semiconductor-metal (MSM) photodetectors, field-effect-transistor (FET) amplifiers, and multiple-quantum-well (MQW) modulators. All-optical positive and negative logic operations are demonstrated with controllable thresholds and a 3-dB bandwidth of 300 MHz. In this device, the logic mode is switched by changing the bias voltage polarity of the MSM photodetector     

High-speed InGaAlAs/InAlAs multiple quantum well electrooptic phasemodulators with bandwidth in excess of 20 GHz

High-speed phase modulation (in the frequency bandwidth of 20 GHz, the highest yet reported for multiple quantum well (MQW) phase modulators) for waveguided InGaAlAs/InAlAs MQW optical modulators is reported. The modulator successfully operates at a long wavelength of 1-55 μm with a low required voltage for phase shift (Vπ=3.8 V), small intensity modulation depth below 1.5 dB, and without any modulation bandwidth degradation up to 20 GHz under high input optical power of 0 dBm in single-mode fiber     

Optimizing the bias and modulation voltages of MQW Mach-Zehndermodulators for 10 Gb/s transmission on nondispersion shifted fiber

The chirp and optical extinction ratio of a multiple quantum-well (MQW) Mach-Zehnder modulator depend on the device design and on the voltage waveforms applied to the arm electrodes. For 10 Gb/s transmission over nondispersion shifted fiber, joint optimization of the bias and modulation voltages is considered for a conventional modulator and a π-phase-shift modulator. Measured attenuation and phase constants for an optical signal propagating in the modulator waveguide are used to accurately model the Mach-Zehnder modulators. The influence of asymmetric Y-branch waveguides in the modulators is examined taking into consideration group velocity dispersion and self-phase modulation arising from the Kerr nonlinearity. When the modulators are operated with maximum optical extinction ratio, the dispersion limited transmission distance depends on the device design (phase-shift and Y-branch splitting ratio) and modulation format (dual drive or single drive). Optimization of the bias and modulation voltages reduces this dependence significantly, while also increasing the dispersion limited transmission distance     

Optimization and tolerance analysis of QCSE modulators anddetectors

This paper describes the design constraints and optimization of multiple-quantum-well (MQW) devices for use as flip-chip bonded devices on silicon circuitry. These devices act as quantum-confined Stark effect (QSCE) modulators and detectors. It is shown that the optimal device thickness depends upon the biasing voltage levels as well as the voltage swing that is available from the silicon circuitry. Lower voltages favor thinner device designs. It was found that, for GaAs-AlGaAs quantum wells, a design in which the modulator and detector are of identical design, a combined efficiency of 0.36 could be achieved with a 5-V swing on the modulators, falling to 0.21 with 2.5 V. By using separate layers for the design of the modulator and detector, the performance could be improved significantly with 0.48 achievable for a 5-V swing. It is shown that optimizing the device to minimize nonuniformity effects makes the optimal design thinner     

Integration of InAlGaAs/InGaAs MODFETs on MQW modulators on GaAssubstrates

An In_xAl_yGa_1-x-y device layer structure that enables the monolithic integration of In_0.25Al _0.75As/In_0.15Ga_0.85As MODFETs and In _0.25Al_0.35Ga_0.40As/In_0.25Ga _0.75As MQW modulators is reported. Current gain cutoff frequencies of 10 GHz are measured for 1 μm gate length MODFETs. MQW modulators operating at 1.05 μm demonstrate 20% transmission modulation for an applied 8 V     

Effects of dispersion and birefringence on the performance ofquantum well mode converters

The device characteristics of GaAs multiple quantum well (MQW) polarization modulators, designed for operation at wavelengths of 865 and 870 nm, respectively, are investigated and shown to depend strongly on the dispersive and anisotropic optical properties of the quantum well medium. Results indicate that the observed decrease in spectral bandwidth and conversion efficiency at wavelengths detuned from the excitonic bandgap by ~250 Å can be accounted for, theoretically, if electroabsorptive loss terms are included in the coupled-mode analysis of polarization conversion in MQW waveguides. Device design considerations and applications of MQW polarization modulators to integrated optic filtering, wavelength division multiplexing/demultiplexing, and the frequency tuning of semiconductor lasers are presented     

GaInAs/InP waveguide multiple-quantum-well optical modulator with 9 dB on/off ratio

Clear excitonic peak wavelength shifts are obtained with an applied electric field and large on/off ratio optical modulation of long-wavelength light propagating along the plane of GaInAs/InP multiple-quantum-well (MQW) structures grown by metalorganic molecular beam epitaxy (MOMBE). These waveguide MQW optical modulators have a modulation on/off ratio of 8:1 (9 dB) at a driving voltage as low as 5 V operating at a wavelength of 1.55 ?m. This measurement is the first step towards faster and higher extinction ratio devices.     

Amplitude and phase modulation in a 4 μm-thick GaAs/AlGaAsmultiple quantum well modulator

The experimental results of a 4 μm-thick GaAs/AlGaAs MQW modulator show an ~10:1 on/off ratio with an applied voltage of 20 V (Δα~6×10³/cm at E~50 kV/cm) and ~0.4 πrad of phase shift with an applied voltage of 10 V (Δn~0.04 at E~25 kV/cm). Such high electro-optical modulations have previously been reported only in MQW optical waveguide modulators     

Multiple quantum well PIN optoelectronic devices and a method of restoring failed device characteristics

Multiple quantum well (MQW) optical modulators have a wide range of applications in fiber-optic and remote communication systems. One of the challenges in producing reliable devices is maintaining the necessary PIN electrical characteristics while having large areas of complex MQW structures for optical processing. We report the first direct correlation between crystalline material imperfections and reverse bias behavior in MQW PIN devices. Molecular beam epitaxy grown GaAs/AlGaAs and strained InGaAs/AlGaAs MQW PIN structures are examined. Defects originating in the epitaxial material provide a conducting path along the PIN junction degrading the device performance and lowering the yield. Defectectomy, a method of eliminating the crystalline defects and restoring the device characteristics and improving the yield is described.