Optical oscillators that exploit resonantly enhanced four-wave mixing in nonlinear whispering- gallery- mode (WGM) resonators are under investigation for potential utility as low-power, ultra-miniature sources of stable, spectrally pure microwave signals. There are numerous potential uses for such oscillators in radar systems, communication systems, and scientific instrumentation.
The resonator in an oscillator of this type is made of a crystalline material that exhibits cubic Kerr nonlinearity, which supports the four-photon parametric process also known as four-wave mixing. The oscillator can be characterized as all optical in the sense that the entire process of generation of the microwave signal takes place within the WGM resonator. The resonantly enhanced four-wave mixing yields coherent, phase-modulated optical signals at frequencies governed by the resonator structure. The frequency of the phase-modulation signal, which is in the microwave range, equals the difference between the frequencies of the optical signals; hence, this frequency is also governed by the resonator structure. Hence, further, the microwave signal is stable and can be used as a reference signal.
The figure schematically depicts the apparatus used in a proof-of-principle experiment. Linearly polarized pump light was generated by an yttrium aluminum garnet laser at a wavelength of 1.32 µm. By use of a 90:10 fiber-optic splitter and optical fibers, some of the laser light was sent into a delay line and some was transmitted to one face of glass coupling prism, that, in turn, coupled the laser light into a crystalline CaF2 WGM disk resonator that had a resonance quality factor (Q) of 6 × 109. The output light of the resonator was collected via another face of the coupling prism and a single mode optical fiber, which transmitted the light to a 50:50 fiber-optic splitter. One output of this splitter was sent to a slow photodiode to obtain a DC signal for locking the laser to a particular resonator mode. The other output of this splitter was combined with the delayed laser signal in another 50:50 fiber-optic splitter used as a combiner. The output of the combiner was fed to a fast photodiode that demodulated light and generated microwave signal.
In this optical configuration, the resonator was incorporated into one arm of a Mach-Zehnder interferometer, which was necessary for the following reasons: It was found that when the output of the resonator was sent directly to a fast photodiode, the output of the photodiode did not include a measurable microwave signal. However, when the resonator was placed in an arm of the interferometer and the delay in the other arm was set at the correct value, the microwave signal appeared. Such behavior is distinctly characteristic of phase-modulated light.
The phase-modulation signal had a frequency of about 8 GHz, corresponding to the free spectral range of the resonator. The spectral width of this microwave signal was less than 200 Hz. The threshold pump power for generating the microwave signal was about 1 mW. It would be possible to reduce the threshold power by several orders of magnitude if resonators could be made from crystalline materials in dimensions comparable to those of microresonators heretofore made from fused silica.
This work was done by Lute Maleki, Andrey Matsko, Anatoliy Savchenkov, and Dmitry Strekalov of Caltech for NASA’s Jet Propulsion Laboratory. For further information, access the Technical Support Package (TSP) free on-line at www.techbriefs.com/tsp under the Electronics/Computers category.
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Refer to NPO-41074, volume and number of this NASA Tech Briefs issue, and the page number.
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Microwave Oscillators Based on Nonlinear WGM resonators
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Overview
The document is a Technical Support Package from NASA's Jet Propulsion Laboratory, focusing on "Microwave Oscillators Based on Nonlinear WGM Resonators," identified by NTR Number 41074. It outlines advancements in the field of hyper-parametric optical oscillators, particularly those utilizing crystalline whispering gallery mode (WGM) resonators. These resonators are known for their unique ability to confine light in a small volume, leading to enhanced nonlinear optical effects.
The document references several key publications by prominent researchers in the field, including A. B. Matsko, A. A. Savchenkov, and L. Maleki, who have contributed significantly to the understanding of optical hyper-parametric oscillations and their applications. Notable works include studies on the threshold and phase diffusion of these oscillations, as well as investigations into low-threshold optical oscillations in CaF2 resonators. These studies highlight the potential for developing highly efficient and stable microwave oscillators based on nonlinear WGM resonators.
The Technical Support Package emphasizes the broader implications of this technology, suggesting that the innovations derived from these research efforts could have significant applications beyond aerospace, potentially impacting various technological and commercial sectors. The document also provides information on how to access additional resources and publications through NASA's Scientific and Technical Information (STI) Program Office, encouraging further exploration of the research and technology in this area.
Furthermore, the document includes a notice regarding the proprietary nature of the information, indicating that it may be subject to export control regulations. It clarifies that the United States Government does not assume liability for the use of the information contained within the document, nor does it endorse any specific trade names or manufacturers mentioned.
In summary, this Technical Support Package serves as a comprehensive overview of the current state of research on microwave oscillators based on nonlinear WGM resonators, highlighting key studies, potential applications, and avenues for further inquiry, all while ensuring compliance with relevant regulations and acknowledging the proprietary nature of the information.
