Linear electro-optic conversion of sampled signals for a photonic-assisted analog-to-digital converter

The speed and capabilities of digital processing continue to improve exponentially. Analog-to-digital (AJD) conversion systems harness this computing power for applications involving signals in the real world. When these real world signals have bandwidths on the order of several tens of gigahertz---as can be the case for photonic and wireless communication, highspeed instrumentation, and wideband radar---suitably fast A/D technologies are needed.; However, conventional electronic A/D systems today are limited to resolutions of 4 to 8 bits for signal bandwidths of up to a few gigahertz. The sources of this limitation include the aperture jitter and relatively low input bandwidth of the front-end sampler. Incorporating a photonic-based sampling system which exploits the low jitter of short-pulse lasers can help overcome such limitations.; We outline one proposed photonic-assisted analog-to-digital converter system. Optical pulses from a mode-locked laser trigger photoconductive switches made from metalsemiconductor-metal devices based on low-temperature grown GaAs. When excited by mode-locked laser pulses, these devices exhibit sampling apertures on the order of a. few picoseconds, thus enabling high-bandwidth sampling. These sampled signals could then be digitized by CMOS circuits. A parallel, time-interleaved architecture would utilize many switch/digitizer channels to increase the aggregate sampling rate of the system.; While the CMOS circuits can be directly solder-bonded to the photoconductive switches, physically separating the circuits from the switches can be advantageous. Reasons include electrical isolation, compact integration of the switches, and improved digital data extraction from the circuits.; This dissertation focuses on the use of optical modulators to optically remote the CMOS circuits from the photoconductive switches. These modulators are based on GaAs/AlGaAs multiple quantum wells incorporated in a p-i-n diode structure. The optical modulators linearly convert the sampled electrical signal to an optical one, thus allowing the circuits to be placed on chips separate from the sampling switches.; We demonstrate the linearity of the modulators, and single-channel conversion speeds on the order of 1 gigasample/second. By flip-chip bonding the sampling switches to the modulators, we find that the linearity of the devices should allow for a resolution of at least ∼3.5 effective-number-of-bits (SNOB) for signals with 20 GHz bandwidth.