GaAs/AlGaAs travelling-wave modulators for mm-wave frequencies

External optical modulators are valuable components for fibre-optic systems. They permit the free choice of optical source-provided a modulator working at the desired wavelength can be obtained-and free a semiconductor laser (if used) from the difficult task of providing high-speed intensity modulation while maintaining good spectral characteristics. The modulation bandwidth of semiconductor lasers is limited by carrier recombination and the accompanying frequency modulation (known as chirp) is substantial, under some conditions being the dominant effect. This degrades the signal in a long-haul fibre system owing to fibre dispersion which causes different optical frequencies to propagate with different velocities. External modulators are usually either electro-absorptive or interferometric in nature. The present paper concentrates on the latter. Interferometric (electro-optic) devices can provide chirp-free modulation over a broad optical band and typically provide optical routing from either of two inputs to either of two outputs.<>