The figure depicts an example of a proposed type of optoelectronic oscillator (OEO) based on some of the same principles as those described in the preceding article. In the proposed OEOs as in the OEOs of the preceding article, transparent (e.g., glass) microspheres that exhibit "whispering-gallery" electromagnetic modes at the laser wavelengths of the oscillators would be utilized as high-Q (where Q is the resonance quality factor and is the measure of energy storage time) resonator/delay elements in the oscillator feedback loops.
The microspheres, which have submillimeter diameters, would replace the fiber-optic delay lines that have been used in previously developed OEOs. A typical fiber-optic delay line is of the order of 1 km long and is wound on a spool about 3 cm in diameter and 5 cm long. The proposed OEOs could readily be miniaturized because, in the absence of the bulky fiber-optic delay lines, all of their otherwise microscopic optical and electronic components could be integrated on single chips.
In a microsphere, propagation in a long fiber is replaced by equivalent circulation of light by total internal reflection in "whispering-gallery" modes. This light propagates in equatorial planes near the surface. It has been demonstrated experimentally that Q ≈ 1010 can be achieved in a glass microsphere, limited only by absorption of light in the glass.
In the OEO shown in the figure, the microsphere would be incorporated into the oscillator feedback loop via evanescent-wave coupling with optical waveguides. In one operational scenario, light from the output of a phase modulator would be coupled into the microsphere to excite two modes, corresponding to a carrier signal and a sideband; it would be possible to do this because deviations from perfect sphericity would create modes with a frequency difference falling in the microwave range. The beat note between the two modes would appear at the output of the photodetector and would constitute the desired microwave signal. Some of the beat-note power would be fed back to the modulator to sustain the oscillation.
This work was done by Lute Maleki, Steve Yao, and Vladimir Iltchenko of Caltech for NASA's Jet Propulsion Laboratory.
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