Electric-field-induced refractive index changes in InGaAs-InAlAsasymmetric coupled quantum wells

Two kinds of InGaAs-InAlAs asymmetric coupled quantum wells (QW's) were numerically analyzed in an effort to enhance change in the refractive index (Δn) at longer wavelengths where absorption coefficient is small. The analysis reveals the operating voltage, Δn, chirp parameter, Δn/Δk, (Δk: change in the extinction coefficient) and figure of merit, Δn/α, (α: absorption coefficient) can be improved by these two kinds of the QW's. This improvement is caused by an abrupt electric-field-induced increase or decrease in the oscillator strength for the lowest heavy hole exciton     

On the figures of merit for electroabsorption waveguide modulators

ΓΔα/F and Δα/α₀ (where Δα is the absorption change, α₀ is the residual absorption, F is the applied electric field, and Γ is the optical confinement factor in the waveguide) have been separately proposed as the relevant figure of merit for electroabsorption waveguide modulators. Using a quantitative and systematic argument, the authors show that they are both necessary and important to the total performance of the modulator     

Design and demonstration of polarization-insensitive Mach-Zehnderswitch using a lattice-matched InGaAlAs/InAlAs MQW and deep-etchedhigh-mesa waveguide structure

The authors have designed and demonstrated a 2×2 Mach-Zehnder switch in view of polarization independence as well as low propagation loss (α) and absorption change (Δα). To obtain polarization-insensitive refractive index change (Δn), a lattice-matched InGaAlAs-InAlAs multiple quantum-well (MQW) with a large detuning wavelength was used. Moreover, to reduce the insertion loss difference between polarizations, we applied a multimode-interferometer 3-dB coupler and a deep-etched high-mesa waveguide structure. This switch, therefore, can provide polarization-independent operation about both driving voltage and insertion loss, which is indispensable to practical optical switching applications. We also paid attention to Δα suppression when we decided the value of wavelength detuning and the length of the phase shift region. We also investigated the wavelength dependence of the switch. Within 1530-1560 nm, which is the erbium-doped fiber amplifier (EDFA) gain band, polarization independence in the driving voltage and the crosstalk was maintained. This result shows that the switch is also applicable in wavelength division multiplexing (WDM) applications     

Controlling chaotic behavior of heavy to light hole mixingtunneling by external electric fields

Oscillatory and chaotic motion of heavy to light hole mixing tunneling in asymmetric coupled quantum-well structures can be controlled by an external electric field. Chaotic behavior occurs if the heavy-hole state in the first well is aligned with the light-hole state in the second well under a significant in-plane vector k_∥. Oscillatory motion is recovered if the external electric field disrupts the alignment between the heavy-hole state in the first well and the light-hole state in the second well     

Bipolar quantum-transport modeling of carrier injection into aSCH-quantum-well laser

A quantum mechanical model of carrier transport in separate confinement heterostructure (SCH) quantum-well lasers based on the Wigner function is presented for the first time. In the simulation, the three quantum Liouville equations with respect to the Wigner functions defined for electron, heavy-hole, and light-hole are solved simultaneously with the Poisson's equation to consider self-consistency in potential, As a simulation model, InGaAsP-InGaAs SCH quantum-well lasers are considered. The carrier injection into single and double quantum-well lasers is simulated. It is demonstrated that the amount of electrons and holes injected into the single quantum-well active layer is not equal in general if the quantum transport is considered. In the double quantum-well structure, the bottleneck phenomenon of heavy-hole injection into the second well and the quite different shape of heavy-hole density profiles in the two wells are simulated     

Theoretical design optimization of multiple-quantum-wellelectroabsorption waveguide modulators

Optical on-off modulators require low insertion loss, high contrast ratio (CR), small drive power and large bandwidth or bit-rate. A systematic approach to optimize the total performance of these modulators based on the quantum-confined Stark effect is presented here. The approach consists of minimizing the power/bandwidth ratio while satisfying a given CR and insertion loss. Our design consists of a large-core multimode passive waveguide with a thin buried active layer. The passive waveguide is designed to yield a high coupling efficiency to conventional single-mode fibers. The quantum well material structure is designed to maximize Δα/ΔF², while maintaining a sufficiently large Δα/α₀, where Δα is the absorption change, α₀ is the residual absorption at zero bias, and ΔF is the swing of the applied electric field. Our theoretical model shows that i) wider quantum wells give larger Δα/ΔF², and ii) the bandwidth/power ratio as high as 4 GHz/mW can be achieved simultaneously with small insertion loss, For example, with a drive voltage of 3 V, an RC limited bandwidth as high as 60 GHz is predicted, while a contrast ratio of 20 dB and a total insertion loss of 4.5 dB may also be obtained     

Electroabsorption properties of InGaAs/InAlAs multiple quantum wellstructures at 1.5 μm

The electroabsorption properties of InGaAs/InAlAs MQW structures are characterised in terms of Δα, Δα/F and Δα/α₀, where Δα is the electroabsorption, α₀ is the residual absorption coefficient under zero bias, and F is the applied electric field. The limitations of these structures for 1.5 μm modulators are primarily due to the relatively small Δα/F values as a result of the small well width. The results are compared with the literature     

Heterodyne technique for measuring the amplitude and phase transferfunctions of an optical modulator

We propose a technique based on heterodyne detection for accurately and simultaneously measuring the amplitude and phase transfer functions of an optical modulator. The technique is used to characterize an InGaAsP multiple quantum-well electroabsorption modulator. From the measurements we derive the small-signal α-parameter and the time-dependent chirp for different operation conditions     

Generation of terahertz electromagnetic pulses from quantum-wellstructures

Terahertz generation from semiconductor quantum-well structures pumped by a femtosecond optical pulse is studied. We propose a three-level model for the electrons and holes in the quantum wells. We then solve the coupled optical Bloch equations directly using a Runge-Kutta method and calculate the terahertz radiation field. We study optical rectification and quantum beats caused by charge oscillations in 1) a coupled quantum well in which quantum beats occur between two electron states of the coupled system and 2) a single-quantum-well structure in which quantum beats occur between light-hole and heavy-hole excitons. Our theoretical results agree very well with the experimentally measured terahertz data     

Measurement of chirp parameter in GaInAs/GaInAsP quantum wellelectroabsorption modulators by using intracavity modulation

A useful technique for evaluating the chirp parameter α of electroabsorption modulators is presented. The α parameter of a GaInAs/GaInAsP quantum-well (QW) electroabsorption modulator integrated with an active segment in a Fabry-Perot cavity has been evaluated by measuring the Fabry-Perot fringes. It has been shown that the relationship between absorption changes and refractive index changes is nonlinear and the α parameter is dependent on operating wavelengths     

1.3-μm polarization-insensitive optical amplifier structurebased on coupled quantum wells

We show that polarization-insensitive optical gain over a wide bandwidth can be realized in a coupled pseudomorphic multiple-quantum-well structure. The barrier width is chosen such that heavy-hole subbands are grouped tightly and light-hole subbands are widely separated in energy. For specific strain conditions, the uppermost valence subbands, which have large occupation probability and strongly contribute to the gain, consist of a single light-hole subband and a group of coupled heavy-hole subbands. This arrangement gives rise to balanced gains for the TE and TM polarizations. We present calculated results for 1.3-μm semiconductor optical amplifier structures based on bands calculated in the framework of an eight-band k·p model. Two different material systems are examined, InAlGaAs and GaInAsP, on InP substrates     

The small-signal response of 1.5 μm multiple-quantum-well lasersin a two-band-model approximation

The variation of the small-signal response of 1.5 μm unstrained multiple quantum-well lasers with the number of wells (N_W ) is studied theoretically in a two-band-model (TB) approximation. The quasi-Fermi energies, together with gain and spontaneous emission rate spectra, are formulated analytically assuming a finite-well model and flatband conditions, including the contributions from carriers in both the wells and the barriers. The gain spectrum shows two major peaks located at the lowest heavy-hole and light-hole transitions. Therefore, the lasers under investigation are treated as three-level systems. The optical confinement factors are evaluated numerically by the matrix transfer method. The traditional rate equations are reformulated and solved for the frequency and damping rate of the relaxation oscillations in terms of an equivalent circuit     

Phase-locked arrays of antiguides: analytical theory II

By employing a variational technique on the eigenvalue equation for finite arrays of antiguides we obtain accurate analytical expressions for key parameters characterizing the adjacent array modes: the edge radiation loss, the loss caused by interelement losses, and the effective index. The upper adjacent mode at its maximum-loss point is found to be well approximated by the sum of two Bloch waves of wavenumbers ±π/[(N-1)Λ], where N is the element number, and Λ is the array period. The intermodal discrimination, Δα, between the adjacent mode and the resonant mode (at the adjacent-mode maximum-loss point) is found to be well approximated (<10% error) by α_RR, the resonant-mode loss at resonance. Accurate analytical expressions are also derived for the two-dimensional optical-mode confinement factor Γ, and the dispersion between the resonant and adjacent modes. The obtained analytical formulas are discussed in light of device design, and general design rules are presented     

Tunneling injection lasers: a new class of lasers with reduced hotcarrier effects

In conventional quantum-well lasers, carriers are injected into the quantum wells with quite high energies. We have investigated quantum-well lasers in which electrons are injected into the quantum-well ground state through tunneling. The tunneling injection lasers are shown to have negligible gain compression, superior high-temperature performance, lower Auger recombination and wavelength chirp, and better modulation characteristics when compared to conventional lasers. The underlying physical principles behind the superior performance are also explored, and calculations and measurements of relaxation times in quantum wells have been made. Experimental results are presented for lasers made with a variety of material systems, InGaAs-GaAs-AlGaAs, InGaAs-GaAs-InGaAsP-InGaP, and InGaAs-InGaAsP-InP, for different applications. Both single quantum-well and multiple quantum-well tunneling injection lasers are demonstrated     

High-temperature excitons and enhanced electroabsorption in InGaAs/InAlAs multiple quantum wells

Sharp heavy-hole and light-hole excitons are clearly observed for the first time in InGaAs/InAlAs multiple-quantum-well (MQW) structures at temperatures ranging from ?190°C to 70°C. The halfwidth of the heavy-hole exciton line is as narrow as 6.2 meV at room temperature. InGaAs/InAlAs MQWs are prepared in a PIN doped configuration by molecular beam epitaxy. An enhanced electroabsorption effect is also clearly observed in long-wavelength-region MQWs.     

Design considerations for a multiple-quantum-well nonresonantsurface-normal modulator

It is shown that by using optimal quantum-well thickness, and by using a symmetric back-to-back p-i-n structure, a non-Fabry-Perot surface-normal reflection modulator based on the quantum confined Stark effect in a GaAs/AlGaAs multiple-quantum-well (MQW) can provide intensity modulation with (a) at least 10-dB contrast ratio, (b) a drive voltage less than 10 V, and (c) an active-layer thickness less than 4 μm. The drive voltage for a given contrast ratio can be minimized by using the quantum-well structure with the maximum Δα/F      

The effect of barrier composition on the vertical carrier transport and lasing properties of 1.55-/spl mu/m multiple quantum-well structures

In this paper, the effect of barrier bandgap and composition on the optical performance of 1.55-/spl mu/m InGaAsP/InGaAsP and InGaAsP/InGaAlAs multiple quantum-well structures and Fabry-Perot lasers is evaluated experimentally. Direct vertical carrier transport measurements were performed through strain-compensated multiple quantum-well (MQW) test structures using femto-second laser pulse excitation and time-resolved photoluminescence up-conversion method. MQW test structures were grown with different barrier composition (InGaAsP and InGaAlAs) and barrier bandgap (varied from /spl lambda//sub g/= 1440 to 1260 nm) having different conduction band /spl Delta/E/sub c/ and valence band discontinuity /spl Delta/E/sub v/, while keeping the same InGaAsP well composition for all the structures. The ambipolar carrier transport was found to be faster in the structures with lower valence band discontinuity /spl Delta/E/sub v/. Regrown semi-insulating buried heterostructure Fabry-Perot (SIBH-FP) lasers were fabricated from similar QWs and their static light-current-voltage characteristics (including optical gain and chirp spectra below threshold) and thermal characteristics were measured. Lasers with InGaAlAs barrier showed improved high-temperature operation, higher optical gain, higher differential gain, and lower chirp, making them suitable candidates for high-bandwidth directly modulated uncooled laser applications.     

1.5-μm strained-layer MQW-DFB lasers with highrelaxation-oscillation frequency and low-chirp characteristics

1.5-μm wavelength strained-layer multiple-quantum-well (SL-MQW) distributed-feedback (DFB) lasers for optical video distribution systems, including optical-fiber amplifiers, are studied with respect to the relaxation-oscillation frequency and wavelength chirp characteristics. Several types of lasers are examined as parameters of amount of strain, optical confinement, and detuning. It is confirmed that the introduction of negative detuning has an obvious effect on the increase in relaxation-oscillation frequency f_r and the reduction in chirp ΔF. The SL-MQW DFB lasers with low optical confinement and negative detuning show extremely low FM response ΔF/ΔI of less than 60 MHz/mA as well as high f_r over 10 GHz for the first time. Additionally, low-chirp value ΔF of 180 MHz with modulation depth of 10% is realized at relatively low bias current (I_h=I_th+30 mA)     

Wavelength selective detection using excitonic resonances inmultiquantum-well structures

The wavelength and voltage dependence of photocurrent near excitonic resonances are used to study the wavelength selectivity of p-i(multiple quantum well, or MQW)-n photodiode structures with a parallel sequence of optical bits, each with a different wavelength. The selectivity is considered good if the state of a λ_i wavelength bit can be detected regardless of λ_j(j ≠i) state of the bits. Photocurrent is found to have very good selectivity only if λ_j bits are all zero, i.e., the optical information is serial, but it is also found that differential photocurrent (ΔI_ph/ΔV ) provides a good selectivity for random states of λ_j bits (i.e., parallel input). Four channel selectivity is demonstrated at 200 K. Specifically designed quantum-well structures can greatly improve this selectivity     

Operational wavelength range of GaInAs(P)-InP intersectionaloptical switches using field-induced electrooptic effect inlow-dimensional quantum-well structures

Operational wavelength range for low insertion loss and high extinction ratio of GaInAs(P)-InP intersectional optical switches consisting of low-dimensional quantum-well structures, such as quantum-box, quantum-wire, and quantum-film structures, is theoretically analyzed. It is found that superior operation characteristics can be attained with the lower-dimensional quantum-well structure. For instance, an operational wavelength range of around 10 nm for an insertion loss less than 1 dB and an extinction ratio higher than 50 dB can be obtained with the device using a quantum-box structure