Zouganeli, Paraskevi; (1992) Design and assessment of Fabry-Perot quantum well electroabsorption optical modulators. Doctoral thesis (Ph.D), UCL (University College London).

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Abstract

The optical properties of multiple quantum well (MQW) material can be modified by application of an electric field across the wells. Hence, by locating the wells in the intrinsic region of a p-i-n diode, an optical beam incident on the diode may be modulated. The incorporation of such a diode in an integrated Fabry-Perot (FP) cavity yields an enhancement of the modulation characteristics. This thesis deals with the assessment and optimisation of Fabry-Perot reflection modulators in the GaAs/AlGaAs material system, where the electroabsorption effects in quantum wells are employed to attain voltage-controlled optical intensity modulation. The work is primarily concerned with the asymmetric Fabry-Perot modulator (AFPM) where the front mirror of the FP cavity is of lower reflectivity than that of the back. Although the achievable very high on:off reflectivity ratios make this type of device very attractive, its ultimate implementation imposes a number of other requirements. Firstly, the operation needs to be compatible with existing electronics required to drive the modulator. Secondly, the performance must be tolerant to fluctuations of a number of parameters associated both with the epitaxial growth and with the operational conditions including ambient temperature. Our optimisation aims, therefore, both to improve the modulation characteristics and to maximise the tolerance of the structure. To do this, we examine theoretically and experimentally the parameters that affect the performance, addressing both the quantum well effects and the microresonator design. We develop a comprehensive design method, demonstrate modulators with improved characteristics, proceed to a thorough evaluation of the performance ultimately attainable by AFPMs, and finally produce optimised designs to meet predefined specifications. Another quantum well electroabsorption-based device, the symmetric cavity electroabsorption modulator (SCEM), is fully investigated and its relative merits are critically examined. In this case the front and back mirrors are equal and the device offers certain advantages in the normally-off mode, thereby lending itself to optically bistable operation. We demonstrate switching using SCEMs in the symmetric self- electro-optic-effect device (S-SEED) configuration. Finally, we examine the microresonator devices in terms of their potential for versatile functionality within optoelectronic systems, and propose integrated configurations and new composite devices.

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