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.
The spur-free dynamic range (SFDR) of a novel microwave-photonic link, using a polarization mode-converter electro-optic modulator in a balanced output configuration, is characterized in this work. Common-mode intensity noise and optical-amplifier-induced beat noise are suppressed using a polarization-selective balanced optical receiver. In addition, third-order predistortion is used to reduce 2-tone intermodulation distortion by up to 20 dB, further increasing the SFDR. Unlike the conventional approach using a dual-output Mach-Zehnder modulator, the complementary output signals are combined naturally as orthogonal polarizations into one transmission fiber.
A novel ultrahigh-speed electro-optic polarization modulator is introduced. The modulator uses a mode converter and a static polarization controller to change the output polarization state in a circular path, following a great circle, around the Poincaré sphere. Any two states on the Poincaré sphere can be connected. The mode converter is constructed using an AlGaAs ridge waveguide combined with slow-wave travelling wave electrodes. The travelling wave electrodes are designed to match the velocity of the electrical modulating signal, the data signal, to the optical carrier signal over a broad frequency range. This modulator demonstrates a 3 dB bandwidth in excess of 40 GHz. The polarization modulator exhibits extremely low differential group delay, on the order of a few 10s of femto-seconds, and low drive voltage, on the order of 5 V.