In this article we review the experimental and theoretical investigations of the linear and nonlinear optical properties of semiconductor quantum well structures, including the effects of electrostatic fields, extrinsic carriers and real or virtual photocarriers.

Linear and nonlinear optical properties of semiconductor quantum wells

High-speed modulation over 22 GHz for waveguided InGaAlAs/InAlAs multiple quantum well (MQW) optical modulators is described. A large on/off ratio of over 25 dB is demonstrated with a low-drive voltage (6 V) operating in the 1.55- mu m wavelength region. The design and characteristics of MQW p-i-n modulators are discussed. The causes of large-insertion loss and the required drive voltage bandwidth figure of merit for the MQW modulator are discussed. The frequency response measurements show that the response speed is limited by the RC time constant of the device. This suggests that the speed can be further enhanced by decreasing the size and capacitance of the device. >

High-speed InGaAlAs/InAlAs multiple quantum well optical 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 Delta alpha / Delta F/sup 2/, while maintaining a sufficiently large Delta alpha / alpha /sub 0/, where Delta alpha is the absorption change, alpha /sub 0/ is the residual absorption at zero bias, and Delta F is the swing of the applied electric field. Our theoretical model shows that i) wider quantum wells give larger Delta alpha / Delta F/sup 2/, 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. >

Theoretical design optimization of multiple-quantum-well electroabsorption waveguide modulators

The authors present optical waveguide modulation results obtained using Wannier-Stark localization in InGaAs-InAlAs superlattices grown by molecular beam epitaxy on InP substrates. It is shown that the results are in good agreement with a previously reported wavefunction model. The authors experimentally investigate the modulation behavior as a function of the electroabsorptive superlattice thickness. For this purpose the authors introduce the extinction ratio per unit waveguide length and per unit electric field as a relevant figure of merit that is broadly applicable to any electroabsorptive superlattice modulator. It is shown that inhomogeneous broadening imposes an optimum thickness for the electroabsorptive superlattice. >

Optimization of optical waveguide modulators based on Wannier-Stark localization: an experimental study

A study is made of high-speed, large-signal intensity modulation (in the frequency bandwidth of 16 GHz) with a low driving voltage of 2.0 V and a 20 dB extinction ratio. The modulator successfully operates at a long wavelength 1.55 μm with a low insertion loss of 6 dB. The 100 μm long device with 16 GHz bandwidth and 1.5 V 10 dB on/off ratio voltage results in a bandwidth to voltage ratio of 11 GHz/V, which is the highest yet reported for an external intensity modulator.

High-speed and low-driving-voltage InGaAs/InAlAs multiquantum well optical modulators

The first measurements of large anisotropic modulation of long-wavelength light with a large on/off ratio and low driving voltage propagating along the plane of InGaAs/InAlAs multiple quantum well (MQW) structures grown by molecular beam epitaxy (MBE) are reported. Photocurrent response and optical modulation of waveguide pin diodes is measured for incident light polarization parallel and perpendicular to the MQW layers emitting from a color center laser. The incident-light power and wavelength dependence of on/off ratio are also determined.

M. Nakao

Papers

Long wavelength waveguide multiple quantum well optical modulators