Light modulators of a proposed type would exploit the propagation of lightin "whispering-gallery" electromagnetic modes in microspheres made of electro-optical materials. These modulators would offer advantages of ultrahigh modulation efficiency and increased bandwidth, relative to their nearest competitors, which are traveling-wave electro-optical modulators. The introduction of the proposed modulators could increase the bandwidths and reduce the power demands of a variety of both free-space and guided-wave communication, sensing, and signal-processing systems that utilize radio-frequency (typically, microwave) modulation of optical carrier signals.
The whispering-gallery modes of a dielectric microsphere are resonance modes in which electromagnetic fields are confined, by internal reflection, to an interior region within about 10 µm of the surface of the sphere. For a microsphere with a diameter >10 µm, the dimension of the resonator is much larger than the wavelength of light. Thus, the loss due to the finite curvature of the resonator is negligible, resulting in a resonance quality factor (Q) that is high and is limited mainly by the attenuation of the light in the dielectric material.
An important example is that of a microsphere with a diameter of about 3 mm, made of the electro-optical material lithium niobate. Such a microsphere can support optical whispering-gallery modes at Q ≈ 107. Because its relative permittivity at radio frequencies is about 50, it can also support microwave and millimeter-wave whispering-gallery modes at Q≈104. These characteristics are favorable for use of the microsphere as a radio-frequency light modulator:
In the proposed modulation scheme, one would apply a radio-frequency (microwave or millimeter-wave) field to a microsphere in which an optical signal propagates in a whispering-gallery-mode. Acting via the electro-optical effect, the electric component of the radio-frequency field would alter the speed of propagation of, and thereby modulate, the optical signal. With a proper choice of design parameters, the radio-frequency field could be concentrated in a whispering-gallery mode in the same near-surface interior region as that of the optical whispering-gallery mode. Because of the high Q values, the circumferential path along which the radio-frequency and optical fields would propagate and interact via the electro-optical effect would be > 1 km long; in contrast, the interaction lengths in typical traveling-wave electro-optical modulators range from a few millimeters to a centimeter.
The increase in the effective interaction length would reduce the change in index of refraction needed to obtain a given depth of modulation in a microsphere electro-optical modulator to about 10 -6 that needed to obtain the same depth of modulation in a traveling-wave electro-optical modulator. The net effect is that the order of magnitude of modulation potentials for microsphere electro-optical modulators would be millivolts, as compared with volts for traveling-wave electro-optical modulators. Even after accounting for inefficiencies in the coupling of optical and radio signals between a microsphere and the other optical and electronic components, microsphere electro-optical modulators are expected to be orders of magnitude more efficient than are traveling-wave electro-optical modulators. Moreover, it has been estimated that the maximum useable modulation frequency would be increased from ≈50 GHz in the traveling-wave case to ≈100 GHz in the microsphere case.
This work was done by Lute Maleki, Anthony F. J. Levi, X. Steve Yao, and Vladimir Iltchenko of Caltech for NASA's Jet Propulsion Laboratory.
In accordance with Public Law 96-517, the contractor has elected to retain title to this invention. Inquiries concerning rights for its commercial use should be addressed to
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Whispering-Gallery-Mode Microspheres as Light Modulators
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Overview
This document presents a technical report on the development and advantages of whispering-gallery-mode (WGM) microspheres as light modulators, conducted at the Jet Propulsion Laboratory (JPL) under a NASA contract. The research is spearheaded by inventors Lutfollah Maleki, Anthony F. Levi, Xiaotian Steve Yao, and Vladimir Iltchenko.
The introduction highlights the critical role of free space and guided optical waves in modern communication technologies, which rely on high information bandwidth. The fundamental principle of optical communication involves modulating the intensity or frequency of light to carry information. Traditional modulators, particularly traveling wave modulators, face limitations in efficiency, bandwidth, and power requirements.
The document outlines the novelty of using WGM microspheres, which offer significantly higher quality factors for light waves compared to conventional methods. The whispering-gallery modes allow for prolonged interaction of light waves within the microspheres, effectively equivalent to a propagation length of about one kilometer. This extended interaction length enhances the modulation efficiency, requiring lower voltage for the electro-optic effect to produce modulation.
The report emphasizes that the modulation speeds achievable with WGM microspheres can reach approximately 100 GHz, which is a substantial improvement over existing traveling wave modulators. This increase in modulation bandwidth is expected to enhance the performance of optical communication systems, making them more efficient and capable of handling higher data rates.
The document also addresses the motivation behind this research, identifying the inefficiencies of external modulators in optical communication links, radar systems, and cable TV systems. By utilizing whispering-gallery modes, the researchers aim to improve the efficiency of modulators, thereby addressing the limitations of current technologies.
In conclusion, the report presents a promising advancement in optical modulation technology through the use of whispering-gallery-mode microspheres. This innovation has the potential to significantly enhance the efficiency and bandwidth of optical communication systems, paving the way for future developments in high-speed data transmission and processing. The findings underscore the importance of continued research in this area to meet the growing demands for higher information bandwidth in communication technologies.
