Symmetric Self-electrooptic Effect Devices Research Articles

Analysis of S-SEED's characteristics in optical switch

This article introduces the basic structure of a symmetric self-electrooptic effect device (S-SEED), and applies the Kirchoff's current law and a purely equivalent capacitive model, to analyze S-SEED's switch characteristics. Linear approximation and N-segment approximation are utilized to obtain S-SEED's voltage-time (V-T) and characteristics. Theoretical analysis is verified by simulations, and the results demonstrate that the precision of S-SEED's switch time can satisfy the requirement in applications with linear approximation. Moreover, the simulations compare S-SEED's switch characteristics with different input powers and input contrast ratios, which reveal that increasing input contrast ratio is an effective way to improve S-SEED's switch characteristics.

Growth, Fabrication, and Characterization of High-Speed 1550-nm S-SEEDs for All-Optical Logic

We describe recent advances in the development of 1550-nm symmetric self-electrooptic effect devices (S-SEEDs). S-SEEDs are semiconductor optoelectronic devices used to implement ultrafast all-optical logic functions for optical fiber communication applications. In this paper, basic S-SEED operation is described, followed by a detailed explanation of the optimization techniques used to improve DC and high-speed performance in these long wavelength devices. Both epitaxial strain and quantum well design are shown to be important for S-SEEDs grown in the InAlGaAs quaternary material system. The device fabrication approach is outlined, and DC electrical and optical performance is discussed. Finally, we describe the high-speed optoelectronic measurements used to determine S-SEED switching characteristics. The devices described herein are the first known S-SEEDs to operate at telecommunications-compatible wavelengths and demonstrate record switching speeds with rail-to-rail switching rates faster than 6 picoseconds.

Temporal responses of a symmetric self-electro-optic effect device based on Wannier–Stark localization

We have experimentally and theoretically investigated responses to optical pulse excitation of a symmetric self-electro-optic effect device (S-SEED) consisting of two GaAs/AlAs superlattice diodes. Transient optical responses exhibit pronounced oscillations due to successive Stark-ladder resonances induced by temporal variations of the applied voltage to each diode. In addition, it is found that steady state optical responses strongly depend on the pulse width and/or height of the optical excitation pulses. These results have shown that the switching mechanism of the S-SEED is governed by the voltage balance between the diodes induced by photogenerated charge transport. We clearly analyze the experimental voltage-switching process based on the potential function theory. By using well-controlled optical pulses, we demonstrate the optical switching between arbitrary stable states predicted by the local minima of the potential function.

Maximum bit-rate analysis of various symmetric extremely shallow quantum well self-electro-optic effect devices under optimized input power

We investigate the effects of high input power on the performance of optical bistable symmetric self-electro-optic effect devices (SSEEDs) using extremely shallow quantum wells (ESQWs). In this study we consider four ESQW SEEDs: antireflection (AR)-coated ESQW S-SEEDs (E-SEEDs), back-to-back AR-coated ESQW S-SEEDs (BBESEEDs), asymmetric Fabry-Pe´ rot (AFP)-ESQW S-SEEDs (AE-SEEDs), and back-to-back AFP-ESQW S-SEEDs (BBAE-SEEDs). As the input power increases, device performances such as on/off contrast ratio and on/off reflectivity difference are seriously degraded due to the ohmic heating and exciton saturation. But switching speed of the device increases up to a certain value and then begins to decrease. With reasonable optimization of the input power, we simulate and analyze the system bit rate of a cascading optical interconnection system using the various ESQW S-SEEDs changing the number of quantum wells for the external bias of 0 V and 25 V. The BBE-SEED with 48 quantum wells shows the maximum bit rate of 100 Mbytes/s at 0-V bias, while the E-SEED with 60 QWs shows the maximum bit rate of 109 Mbytes/s at 25-V bias. In AFP cavity structures, the AE-SEED with 18 QWs and the BBAE-SEED with 28 QWs show maximum bit rate of 311 and 156 Mbytes/s, respectively, at 0-V bias.

Operating characteristics of GaAs–InGaAs self-biased piezoelectric S-SEEDs

A theoretical analysis is presented of the performance of GaAs–InxGa1-xAs self-biased piezoelectric symmetric self-electro-optic-effect devices (S-SEED). Although the operation of these devices has been demonstrated, the limitations on their performance have not been investigated. Despite the benefits associated with this type of S-SEED, the model presented suggests that their insertion loss cannot be less than ~70%. Further, because of the high quality factor required for the optical cavity, it is believed that the reproducibility of the contrast ratio will be intolerant to differences between the nominal and actual absorption coefficients. The model therefore suggests that these structures are not promising candidates for the logical elements required in optical processing.

Analysis of switching dynamics of asymmetric Fabry–Perot symmetric self-electro-optic effect devices with extremely shallow quantum wells

This article investigates the effects of asymmetric Fabry–Perot (AFP) cavity structures on the switching dynamics of symmetric self-electro-optic effect devices (S-SEEDs) made of AlxGa1−xAs/GaAs (x⩽0.05) extremely shallow quantum wells (ESQWs). We analyze the switching dynamics of AFP ESQW S-SEEDs (AE-SEEDs) by means of an impulse photocurrent response function and corresponding voltage transients of the two AFP p-i(ESQWs)-n diodes connected in series. The photocurrent response function is obtained by using an appropriate electron-hole pair generation rate and the Green’s function method. The response function includes the LO phonon scattering from bound to continuum states and the carrier transit in the continuum states. Large internal optical fields and thin intrinsic region thicknesses of AFP SEEDs reduce the required incident switching energy and operating voltage, respectively. Our analyses show that the switching energy of an impedance-matched AE-SEED is about 3.0 fJ/μm2, while that of a conventional antireflection coated ESQW S-SEED (E-SEED) is about 4.1 fJ/μm2. Considering practical RC time constants and device sizes, the switching time of an impedance-matched AE-SEED is found to be as low as 10 ps while that of an E-SEED is about 18 ps for the same incident energy of 4.1 fJ/μm2.

Subnanosecond sampling all‐optical analog‐to‐digital converter using symmetric self‐electro‐optic effect devices

A system to convert an analog optical system to a digital one in an optical environment using basic symmetric self-electro-optic effect devices (S-SEEDs) is detailed. It is shown that quantization and binary encoding can readily be achieved using the logic capabilities of S-SEEDs. A quantizer prototype has been constructed and proper op- eration observed. The effect of the hysteresis inherent in the S-SEED is discussed and three techniques are proposed to eliminate the problems caused by hysteresis. The logic operations needed for the construction of the binary encoder with the minimum number of S-SEEDs are facili- tated using a reference beam. An alternate, efficient design of the binary encoder is also presented. Using a novel time quantization technique, a sampling rate in excess of 1 gigasamples/s is achievable. © 1996 So- ciety of Photo-Optical Instrumentation Engineers. Subject terms: analog-to-digital conversion; quantization; symmetric-self-electro- optic effect devices; hysteresis; binary encoding logic.

Switching characteristics of nonbiased optical bistability in asymmetric Fabry-Perot S-SEEDs made of extremely shallow quantum well structures

The asymmetric Fabry-Perot (AFP) cavity structure extremely shallow quantum wells (ESQWs) symmetric self-electro-optic effect device (S-SEED) shows nonbiased optical bistability. We analytically investigated the switching characteristics of the AFP ESQWs S-SEED (AE-SEED) using an impulse photocurrent response function and the corresponding voltage transients of the two p-i(ESQWs)-n diodes in AE-SEED. The result was compared to the characteristics of the conventional anti-reflection coated ESQWs S-SEED (E-SEED) with external bias. The large optical field and the thin intrinsic region in AE-SEED reduce the required incident switching energy and operating voltage, respectively. According to our analysis, the switching energy of an impedance-matched AE-SEED is 1.2 fJ μm −2, while that of an E-SEED is 4.1 fJ μm −2. With a reasonable RC time constant and device size, the switching time of the AE-SEED is 11 ps while that of E-SEED is 12.4 ps for the minimum switching energy of each device.

Noise characteristics of electro-absorptive logic device utilizing asymmetric Fabry-Perot etalon structure in high optical power

We studied the noise characteristics of a symmetric self electro-optic effect device (S-SEED) with an impedance-matched asymmetric Fabry-Perot cavity and extremely shallow quantum wells (AE-SEED). The device characteristics such as the signal tolerance and the bitrate in cascaded optical logic gates are determined by the parameters including the reflection change Δ R and the contrast ratio CR during the high power operation. The noise characteristics of an AE-SEED were compared with that of a conventional E-SEED. When there was no power fluctuation in the reading beam, the conventional E-SEED showed a higher bitrate. However, when fluctuation was introduced, as is inevitable in the real world, the performance of the AE-SEED as a logic gate was much better and showed higher a bitrate than the E-SEED.

Growth of pseudomorphic InGaAs/GaAs quantum wells on [111]B GaAs for strained layer, piezoelectric, optoelectronic devices

Pseudomorphically strained InGaAs/GaAs quantum wells grown on [111]B oriented GaAs substrates incorporate strong piezoelectric fields. Quantum confined interband optical transitions exhibit built-in Stark shifts to the red and additional, ‘forbidden’ transitions are observed because of the reduced symmetry. The novel properties of these structures can be exploited in such devices as electro-optic modulators with increased sensitivity, blue-shifting self electro-optic effect devices (SEEDs) with enhanced contrast, all-optical symmetric SEEDs (SSEEDs) and pseudomorphic high electron mobility transistors (HEMTs) with increased two-dimensional electron gas (2DEG) densities. At all stages of research, development and production successful device work necessitates reliable material growth of good crystalline quality. High quality material is routinely obtained for wafers misoriented towards 〈211〉. For substrates misoriented towards the 〈211〉 direction good quality material can also be achieved, but only when great care is taken to ensure that growth is carried out within the narrower optimum growth window. With the help of this understanding we have been able to produce devices which have been used successfully to observe the predicted improvements.

A global stability analysis for symmetric self-electrooptic effect device systems using a potential function method

This paper proposes a novel analytical method for use in symmetric self-electrooptic effect device (S-SEED) systems, called the potential function method, based on a global stability analysis of differential equations for photocurrent in S-SEED circuits. The method provides intuitive views for analyzing the stability of the system, and is useful for tracking the temporal dynamics of S-SEED nonlinear electrical circuits. This paper also describes electro-absorption characteristics of SEED's, especially those based on Wannier Stark localization (WSL). In this type of SEED, the photocurrent versus reverse bias voltage characteristics has multiple peaks and multiple negative differential resistance regions, resulting from Stark ladder transitions due to thin barrier superlattices. Various types of stabilities, including the metastable state, as well as the temporal switching dynamics of WSL-S-SEEDs, can be explained clearly by using this potential function method.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

S-SEED operation with noise: bit error rate analysis

The static and dynamic noise characteristics of a symmetric self electrooptic effect device (S-SEED) are studied. The mechanism for bit errors is identified, and the bit error rate related to the decision threshold, input contrast ratio, nominal input power, and bit activation period. Dynamic simulations use practical S-SEED parameters for the diode capacitance, bias voltage, and responsivity values to show specific examples; however, the bit error rate curves provide general results since these parameters are allowed to be variables. The probability of abnormal operation, when there is distortion or an error in the output, is also derived and can be directly related to the bit error rate. In a practical S-SEED switching environment example, we show that the noise effect results in a probability of abnormal operation to be less than 10/sup -50/ provided the bit activation period exceeds the switching time by 0.5%.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Optical energy considerations for diode-clamped smart pixel optical receivers

We calculate the switching time as a function of optical energy for a single-stage smart pixel receiver. We find that operating the receivers dynamically using modelocked pulses is the most energy-efficient method of operation. We also show that the receivers no longer operate with constant optical input energy, like the symmetric self electro-optic effect device (S-SEED), but rather the product of the required optical energy and the switching time is constant, as a result of the introduction of electrical voltage gain.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Transmission intensity oscillations in GaAs/AlAs superlattices induced by photogenerated charged transport

Two superlattice p-i-n diodes connected in series to a constant dc bias, i.e., in the symmetric self-electro-optic effect device (S-SEED) configuration, exhibit pronounced oscillations of their transmission intensities immediately after optical pulse excitation of one of the diodes. The temporal variations directly reflect the transmission-voltage characteristics due to successive Stark ladder resonances, and they depend strongly on the amount of the illumination power and on the pulse width. The observed oscillatory behavior is explained by changes of the voltage balance between the two diodes induced by photogenerated charge transport.

Noise in self-electrooptic effect device logic gates

We study the fundamental noise processes of SEED-based logic gates. The noise is calculated for symmetric SEEDs (S-SEEDs), asymmetric Fabry-Perot S-SEEDs (ASFP S-SEEDs), and SEEDs combined with transistor driving circuits (FET-SEEDs). The source of noise which determines the minimum power required to switch the SEEDs in a given time increment is quantum noise associated with photon absorption and subsequent carrier or exciton recombination. The results show that FET-SEED logic gates have the lowest noise power as compared with S-SEEDs and ASFP S-SEEDs. >

Implementation of symmetric Fabry-Perot quantum well electroabsorption modulators in symmetric self-electrooptic effect devices

We report results from a symmetric cavity electroabsorption modulator (SCEM) in GaAs/AlGaAs. A reflection change of ∼ 45% with an insertion loss of 1.3 dB is obtained under 7.5 V in the normally-off mode. We discuss the factors that affect the device performance and compare the attainable performance with that of the normally-off asymmetric Fabry-Perot modulator (AFPM). We also propose a new method for an improvement of the restricted tolerances of SCEMs. Finally, we demonstrate bistable operation using two SCEMs in the symmetric self-electrooptic effect device (S-SEED) configuration, and attempt to evaluate the potential of this type of modulator for SEED applications.

Improved performances of a S-SEED using extremely shallow quantum-wells and asymmetric Fabry-Perot cavity structure

The authors report significantly improved performances of a symmetric self-electrooptic-effect device (S-SEED), with high on-off contrast ratio (>30:l) and large optical bistability loop widths ( Delta P=44%) at an applied bias of V/sub a/=0 V, i.e., with no power supply. The S-SEED is made of extremely shallow quantum wells (ESQWs) in an asymmetric Fabry-Perot (AFP) cavity structure. At V/sub a/=5 V Delta P increased by up to 95%, preserving the high contrast ratio. The reflectivity changes at V/sub a/=0 and 5 V were about 15% and 30%, respectively. These are believed to be the largest values ever reported for such structures. >

Analytic characterization of dynamic optical switching of symmetric self-electrooptic effect devices in extremely shallow quantum wells

An analytical expression of the impulse response of extremely shallow quantum well (ESQW) p-i-n photodiodes is presented. The response function incorporates the effects of the LO phonon scattering rate in the well and the carrier transport coefficients in the continuum states. The result thus obtained has been applied to ESQW symmetric SEEDs (symmetric E-SEEDs) for analyzing the switching characteristics in time. The impulse response function is derived, and the circuit equations of symmetric SEEDs (S-SEEDs) are examined. Specifically, it is pointed out that the RC time constant inherent in S-SEEDs speeds up the down-switching (from reverse to forward), while delaying the up-switching (from forward to reverse) of the two diode voltages involved. The analytic solutions of the two diode voltages in symmetric E-SEEDs are given as a function of time during the dynamic optical switching. >

Multiple quantum-well technology takes SEED

Progress in the development of self-electrooptic-effect devices (SEEDs) is discussed. The devices include the resistor-SEED (R-SEED) device, which can be viewed as a simple NOR gate. The symmetric SEED (S-SEED) and the logic-SEED (L-SEED) devices with improved features, functionality, and performance are also considered. The integration of FETs with multiple quantum well (MQW) modulators (FET-SEED), enables optical interconnections of electronic circuits. Where the SEED technology can be used is discussed, and an experimental optical switching fabric made using these devices is described. >

Batch fabrication and operation of GaAs-Al/sub x/Ga/sub 1-x/As field-effect transistor-self-electrooptic effect device (FET-SEED) smart pixel arrays

The structure, processing, and performance of arrays of integrated field-effect transistor-self-electrooptic effects devices (FET-SEEDs) consisting of doped-channel field-effect transistors, multiple quantum-well (MQW) modulators, and p-i-n MQW detectors are discussed. The performance of the FETs and SEEDs such as g/sub m/ and contrast, is equivalent to that obtained when they are made separately. Typical values are g/sub m/=80 mS/mm and contrast of 3. The largest arrays contain 128 circuits. The circuits operate at speeds as fast as 500 Mb/s, with optical input switching energy of approximately=400 fJ. At 170 Mb/s, the required optical input switching energy is approximately=70 fJ. This optical energy is at least a factor of 20 less than for symmetric SEEDs (S-SEEDs) with the same optical window sizes. Hence, FET-SEEDs provide superior performance compared to conventional S-SEEDs. >