A wide-band narrow-line RF source utilizing a heterogeneous silicon photonic platform

Ejzak, Garrett
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University of Delaware
For a number of reasons including spectrum congestion, atmospheric conditions, and the insatiable need for bandwidth, many systems are being operated at ever increasing frequencies, often simultaneously with low-frequency systems. Systems ranging from communications to sensing to imaging require operation at very high frequencies (up to 66 GHz in the very near future), large bandwidths, or both and often require extremely pure signal sources. A single signal-source capable of satisfying these requirements simultaneously would be very beneficial for these systems. Electronic solutions exist, but tend to be large, power hungry, and susceptible to electromagnetic interference and conduction and parasitic losses. Optical systems that produce radio-frequency signals by downconversion via photomixing are a potential solution but suffer relatively poor signal quality due to the noise inherent in lasers. A number of methods to improve fidelity have been proposed and tested, but none provide both the signal purity and tuning range required by the most demanding of systems. A phase-locking scheme has been proposed and tested by Schneider, et. al., which is capable of sufficiently canceling this inherent noise by utilizing a low-frequency reference oscillator, without significantly constraining the tuning range of the system. This document details preliminary work towards the design and fabrication of a miniaturized, low-power, solid-state, photonic-electronic integrated implementation of this system and the preliminary results thereof. This system heterogeneously integrates an indium-phosphide medium with optical gain in the telecommunications band with a custom-designed and -fabricated silicon device that hosts the remainder of the laser cavities and many of the optical components necessary for operation. At present the balance of the optical components and the electronic control and drive systems are separate. Future devices will have most or all of these components integrated either heterogeneously in a compact package or on the silicon device. Data taken from the integrated device match those taken from the prototype system quite well. Performance is not yet to parity, however, and a number of changes and improvements need to be made to achieve acceptable results. Stable signal generation has been observed at discrete frequencies between 20 and 63.4 GHz with spectral widths at lower frequencies below 1 Hz, the minimum detectable by the test equipment used. The causes of the disparity between the reference implementation and the integrated solution presented here are well understood and should be overcome with further research. With the appropriate improvements and modifications, the integrated system has the potential to be highly beneficial to commercial and military applications from telecommunications to imaging. In may also be possible to mass-produce the silicon component of the system in a standard complementary-metal-oxide-semiconductor (CMOS) manufacturing facility.