INSTALACION DE FONTANERIA

This thesis has reported the developm ent of a novel GaAs quantum well optical w aveguide m odulator. The device uses an alternative m echanism to the m ore usual quantum confined Stark shift (QCSS), w hich provides an increase in the potential for monolithic integration. Optical m odulation is achieved by quenching the exdtonic absorption through provision of an electron population in the quantum well. The control of this electron population, and hence the absorption, operates in an identical m anner to a SDHFET. Increased flexibility in integrated m onolithic design is introduced prim arily by this duality of the FET and m odulator mechanism. This new device has been nam ed the q u antum well field effect m odulator (QW-FEM).

The QW-FEM has been experim entally investigated both by m eans of optical transm ission and photocurrent m easurem ents. Evidence has been found for the desired exdtonic quenching m echanism an d absorption m odulation in an optical w aveguide is observed. The dependence of the quenching m echanism on well w idth has been experim entally investigated and confirms that narrow q u antum wells produce greater exdtonic absorption control. Different sem iconductor junctions have also been exam ined, and the use of a p-n junction has been found preferable to a Schottky barrier d u e to the restrictions on epitaxial grow th im posed by a w aveguide geometry. The design of the QW-FEM structure has been im plem ented using a sim ple m odel of the junction electric field. This enables the operating voltage to be tailored and excellent agreem ent betw een theory and experim ent is dem onstrated.

Studies of absorption spectra derived from photocurrent d ata have revealed th at the absorption m odulation m echanism intrinsically has a low er chirp param eter than the corresponding QCSS of MQW devices. This is an attractive quality for long haul optical comm unications applications. Drawbacks of the fundam ental single quantum well design of the QW-FEM have been discussed in term s of the saturation powers and the bandw idth. Typically both will be reduced in com parison w ith an optim ised MQW device, similarly the insertion loss will be increased. H ow ever these disadvantages are reduced w hen considering structures for monolithic integration of devices.

N onuniform ities in the layer structures have lim ited the experim ental m odulation depth available through the restriction to short w aveguides. The

this it is possible to extrapolate to the perform ance of an optim ised structure. The absorption changes, and residual absorption levels found, constitute a lOdB m odulation in a 160pm w aveguide w ith a 1.9dB internal insertion loss. The required layer uniformities are n o t fundam ental to the design of the QW-FEM, and are not lim ited by the technique of epitaxial grow th, hence the predictions of m odulation d ep th are m eaningful.

A n experim ental analysis of the perform ance of the m odulation m echanism has u se d polarisation and optical saturation effects in a w aveguide, a n d low tem perature photocurrent studies. These reveal th at a greater exdtonic quenching should be possible, leading to a reduction in the experim ental values of insertion loss. The lim it to the m odulation depth in a QW-FEM is theoretically determ ined to be d u e to the residual absorption of a broadened and shifted subband edge. The carrier population in the quantum well, w hich acts to quench the exdtonic resonance, has the secondary effect of introducing a renorm alisation of the subband gap. This, combined w ith broadening d u e to well w idth fluctuations an d carrier scattering interactions, leads to an absorption contribution at the spectral position of optim um m odulation. This undesirable effect w as quantified by using expressions for absorption and bandfilling taken from calculations of laser gain characteristics. The m odelling also predicts an u p p er lim it for the m odulation achievable in the QW-FEM by introduction of quantum confined Stark shift data of undoped MQW sam ples to represent the biased recovered exdtonic state.

The functionality of the device extends beyond optical m odulation an d the experim entally observed FET operation. The absorption in the biased state acts as a detector, w hilst both laser an d optical amplification functions should also be possible. A study of the goals of optoelectronic integrated circuits (OEICs) has illustrated that the flexibility and perform ance he QW-FEM is ideal for monolithic integration. Such a device structure - old enable the m ass production of low cost com m unications components for im plem entation of local area and long haul netw orks. It is expected that there is also a future need for switching com ponents in the form of photonic integrated circuits, a role for which the QW-FEM is suited. A study of the p roposed im plem entation of QW-FEM devices for OEICs has illustrated the sim plicity of possible fabrication schemes.