Frequency Conversion of Optical Signals Using Coherently Prepared Media

In the past decade, researchers have found that the optical properties of a medium can be dramatically altered by optically driving the medium into a coherent superposition of its quantum states. Optical transitions from these states can interfere, leading to optically induced absorption or transparency. Such “coherently prepared media” (CPM) typically have large wave-mixing susceptibilities without the resonant absorption that usually accompanies large nonlinear susceptibilities. In certain schemes, a large material coherence can act as an oscillator which scatters optical waves into modes of different frequency with high efficiency. Technologies based on this process could find uses ranging from telecommunications to X-ray pulse characterization. The present work investigates the ability to coherently prepare an atomic vapor and to use that coherence to shift optical signals from one frequency to another with high efficiency, bandwidth, and fidelity. My theoretical studies of coherent preparation include both simple and many-state systems. While two-state systems can display some coherent effects, coherent effects are much more clearly displayed in threeand four-state systems. A Bloch-sphere representation of Raman systems is developed, which yields simple, easy-to-interpret graphical solutions and illustrates basic physical principles of coherent preparation through simple formulas. The complications of coherently preparing a real vapor are also considered and discussed in detail. Experimentally, a pair of laser fields were used to create coherence between the 3S1/2 hyperfine levels of sodium vapor. Coherence-based optical effects including transparency, four-wave mixing, and Raman scattering were observed. Finally, coherence-induced Raman scattering was used to perform fast (> 20MHz), high-fidelity (> 99%) conversion of AM and FM optical signals from one carrier frequency to another. In spite of these successes, the degree of coherence produced and the conversion efficiency were not as large as one would hope. My work concludes with a discussion of the obstacles encountered in achieving large coherent effects in vapors and how they might be overcome.