A high-speed GaAs-based electro-optic modulator for polarization, intensity, and phase modulation

We review the development of a unique, electro-optic, polarization modulator fabricated on epitaxial layers of aluminum gallium arsenide, grown on a gallium arsenide (GaAs) substrate. The device has a single waveguide structure combined with travelling-wave, slow-wave electrodes. This design allows for high-speed modulation of the polarization state of light with low differential group delay and low optical loss at frequencies in excess of 50 GHz. The devices are TE <-> TM mode convertors that modulate the state of light from one linear polarization state to an orthogonal linear state passing through elliptical and circular polarization states. These devices can also be configured to modulate the phase or intensity of an optical signal by appropriate alignment of the polarization axis of the input light or by placing a polarizer at the output. Key characteristics and important performance advantages of such devices are discussed. Applications that use these devices for enhancing digital and analog communication links, analog-to-digital signal conversion, and sending keys for encryption are reviewed to illustrate the diverse nature of the systems being developed and provide an overview of the versatility of the ways in which the GaAs polarization modulator may be used.