Linearity considerations in high performance amplitude modulators

Integrated optical amplitude modulators offer a good solution for the implementation of very high speed analogue links (>10 GHz) or microwave distribution networks. Direct modulation of lasers at such high frequencies is not favoured because problems of distortion and wavelength chirp arise. In the last few years, the development of integrated optical modulators has matured to the degree where they are commercially available in a fully packaged form with bandwidths up to around 20 GHz. A device featuring the hybrid integration of a laser diode and a high speed modulator has also been reported. This has led to an increased usage in systems experiments, and in turn an appreciation by device designers of the performance parameters necessary to ensure optimum fidelity in signal transmission. In this paper, the authors discuss intermodulation distortion in electro-optic amplitude modulators and show the effect of modulation index and bias point control. They also consider how the RF conversion efficiency of an analogue optical link can be improved by using high power optical sources, and present some experimental results on the performance of modulators handling over 100 mW of optical power at 1.32 μm wavelength