All-Dielectric, Non-Electronic Radio-Receiver Front Ends

October 1, 2007

Air Force Research Laboratory,Wright-Patterson Air Force Base, Ohio

All-dielectric, non-electronic, photonicassisted front ends are being developed as alternatives to traditional metal antennas and the associated metal connections and input electronic circuitry of radio receivers — especially microwave receivers. The main motivation for this development is the need to make radio receivers much less vulnerable to electromagnetic weapons.

An electromagnetic weapon produces an electric power surge capable of destroying or damaging sensitive electronic circuitry. The trend toward reduced feature size and reduced voltage renders modern electronic circuitry increasingly susceptible to such damage. Radio communication systems are particularly vulnerable because antennas and other traditional front-end parts serve as direct ports of entry for undesired radio-frequency (RF) electromagnetic radiation. In equipping a receiver with a non-electronic, alldielectric front end, one would eliminate the traditional metal antenna and the associated metal interconnections and input electronic circuitry, thereby eliminating the main traditional points of electromagnetic vulnerability.

From one perspective, a front end of the type now being developed can be characterized as a combination of a dielectric antenna that incorporates an electro-optical RF-field sensor (see figure). The dielectric antenna is a leaky RF resonator. Most of the RF resonator is made of a material that exhibits low RF loss and high relative permittivity. A small part of the RF resonator consists of a microdisk optical resonator made of an electro-optical material. A desired incident radio signal in the form of a free-space electromagnetic field excites a resonance mode of the antenna, resulting in a desired buildup of the electric field inside the antenna. The design of the antenna provides for placement of the electro-optical microdisk at the location of maximum enhancement of the resonant- frequency electric field.

An optical carrier signal is generated by a laser. A single-mode optical fiber couples the optical carrier signal from the laser to the electro-optical microdisk; another single-mode optical fiber couples the optical signal from the microdisk into a photodiode. By virtue of the electro-optical effect in the microdisk, the optical signal becomes modulated by the resonant RF signal. The modulation is present in the photodiode output and is subsequently processed by conventional receiver electronic circuitry. The laser, the photodiode, and all of the conventional receiver electronic circuitry can be enclosed in electromagnetic shielding to further reduce susceptibility to undesired electromagnetic radiation.

In addition to the absence of metal and delicate electronic components that could be damaged by the intense RF field from an electromagnetic weapon, the all-dielectric front end offers the advantage of electrical isolation of the RF electronic circuitry. Yet another advantage is that because of the high permittivity of the dielectric antenna material, the antenna can be smaller than a metallic antenna that resonates at the same frequency.

A front end of this type for an operating frequency of 7.38 GHz was built and tested in a proof-of-concept experiment. The main part of the resonator consisted of a BaTiO₃ (relative permittivity = 38) cylinder 9 mm in axial length and 11.25 cm in diameter. The electro-optical microdisk was made of LiNbO₃. The dimensions of the cylinder were chosen to obtain 7.38-GHz resonance in a mode in which the maximum enhancement of the electric field occurred at the center of an end face, where the electro-optical microdisk was located. The front end was found to be capable of detecting a 7.38-GHz signal with a sensitivity of 4.3 x 10^-3 V/mHz^1/2.

This work was done by Bahram Jalali of the University of California for the Air Force Research Laboratory.

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All-Dielectric, Non-Electronic Radio-Receiver Front Ends

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