Optical waveguides – With optical coupler – Particular coupling function
Reexamination Certificate
2001-06-05
2003-04-01
Ullah, Akm E. (Department: 2874)
Optical waveguides
With optical coupler
Particular coupling function
C385S147000
Reexamination Certificate
active
06542662
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to quasi-optic grid arrays, such as periodic grid arrays, and in particular to techniques for adapting a waveguide to a quasi-optic grid array.
2. Description of Related Art
Broadband communications, radar and other imaging systems require the transmission of radio frequency (“RF”) signals in the microwave and millimeter wave bands. In order to efficiently achieve the levels of output transmission power needed for many applications at these high frequencies, a technique called “power combining” has been employed, whereby the output power of individual components are coupled, or combined, thereby creating a single power output that is greater than an individual component can supply. Conventionally, power combining has used resonant waveguide cavities or transmission-line feed networks. These approaches, however, have a number of shortcomings that become especially apparent at higher frequencies. First, conductor losses in the waveguide walls or transmission lines tend to increase with frequency, eventually limiting the combining efficiency. Second, these resonant waveguide cavities or transmission-line combiners become increasingly difficult to machine as the wavelength gets smaller. Third, in waveguide systems, each device often must be inserted and tuned manually. This is labor-intensive and only practical for a relatively small number of devices.
Several years ago, spatial power combining using “quasi-optics” was proposed as a potential solution to these problems. The theory was that an array of microwave or millimeter-wave solid state sources placed in a resonator could synchronize to the same frequency and phase, and their outputs would combine in free space, minimizing conductor losses. Furthermore, a planar array could be fabricated monolithically and at shorter wavelengths, thereby enabling potentially thousands of devices to be incorporated on a single wafer.
Since then, numerous quasi-optical devices have been developed, including detectors, multipliers, mixers, and phase shifters. These passive devices continue to be the subject of ongoing research. Over the past few years, however, active quasi-optical devices, namely oscillators and amplifiers, have evolved. One benefit of spatial power combining (over other methods) using quasi-optics is that the output power scales linearly with chip area. Thus, the field of active quasi-optics has attracted considerable attention in a short time, and the growth of the field has been explosive.
It is believed that the first quasi-optical grid array amplifier was a grid developed by M. Kim et al. at the California Institute of Technology. This grid used 25 MESFET differential pairs, demonstrating a gain of 11 dB at 3 GHz. As shown in
FIG. 1
, a typical grid amplifier
10
is an array of closely-spaced differential pairs of transistors
14
on an active grid
12
sandwiched between an input and output polarizer,
18
,
24
. An input signal
16
passes through the horizontally polarized input polarizer
18
and creates an input beam incident from the left that excites rf currents on the horizontally polarized input antennas
20
of the grid
12
. These currents drive the inputs of the transistor pair
14
in the differential mode. The output currents are redirected along the grid's vertically polarized antennas
22
, producing a vertically polarized output beam
30
via an output polarizer
24
to the right.
The cross-polarized input and output affords two important advantages. First, it provides good input-output isolation, reducing the potential for spurious feedback oscillations. Second, the amplifier's input and output circuits can be independently tuned using metal-strip polarizers, which also confine the beam to the forward direction. Numerous grid amplifiers have since been developed and have proven thus far to have great promise for both military and commercial RF applications and particularly for high frequency, broadband systems that require significant output power levels (e.g. >5 watts) in a small, preferably monolithic, package. Moreover, a resonator can be used to provide feedback to couple the active devices to form a high power oscillator.
Grids amplifiers can be characterized as quasi-plane wave input, quasi-plane wave output (free space) devices. Grid oscillators are essentially quasi-plane wave output devices. However, most microwave and millimeter wave systems transport signals through electrical waveguides, which are devices that have internal wave-guiding cavities bounded by wave-confining, and typically metal, walls. Consequently, an interface between the two environments is needed in most cases. This interface is needed whether the electric field signal is being output from a waveguide for effective application to the grid array; or the free space output signal of a grid array is to be collected into a waveguide.
Providing such an interface is not a trivial matter for several reasons. First, microwave and millimeter wave waveguides conventionally transmit signals in the single transverse electric (TE) mode, also known as the fundamental, or TE
10
, mode, and block the higher-order mode components of the signal. These conventional waveguides have a standard, constant size and rectangular shape. However, the input plane area of any typical grid array upon which the input signal is incident may be much larger than the area of the standard rectangular waveguide aperture. Furthermore, as noted, grid array assemblies comprising N by N unit cells and bounded by a dielectric (see
FIG. 2
) will vary in size depending number of cells in the grid and the dielectric size. Thus, a standard waveguide cannot directly mate with a grid array structure.
Moreover, the standard single mode rectangular waveguide operating in TE
10
mode provides an electric field distribution that varies sinusoidally in amplitude across it aperture. However, efficient operation of grid amplifiers requires an excitation beam that has a relatively uniform phase and magnitude distribution across the amplifier's area.
Several groups have attempted to design waveguides that interface with quasi-optic active devices, but have had only limited success. For example, Yang, et al. recently published an article titled “A Novel TEM Waveguide using Unipolar Compact Photonic-Bandgap ”,
IEEE Trans. On Microwave Theory and Tech.,
Vol. 48, No. 2, pp. 2092-2098, November, 1999. Further, Ali, et al. published an article titled, “Analysis and Measurement of Hard-Horn Feeds for the Excitation of Quasi-Optic Amplifiers,”
IEEE Trans. On Microwave Theory and Tech.,
Vol. 47, No. 4, pp. 479-487, April, 1999. Unfortunately, these proposed techniques do not adequately resolve the aforementioned problems. For example, the photonic bandgap structures described by Yang et al. are very difficult and costly to manufacture, making this technique less than desirable. Moreover, the “hard-horn” approach of Ali et al. creates a rather large and bulky structure that is impractical for most commercial applications.
Thus, there is a definite need for a simple and cost effective interface, or adapter, that efficiently couples a waveguide that propagates signals in the fundamental mode to a grid array structure with a desired field distribution,
SUMMARY OF THE INVENTION
The present invention, which addresses these needs, resides in an adapter for coupling a quasi-optic grid array assembly to a waveguide that has an internal cavity bounded by a wave-confining device and that guides a wave propagating in a longitudinal direction. The adapter translates the wave between the fundamental mode of the waveguide and a desired electromagnetic field distribution at the plane of the array assembly. The adapter comprises a first end, a second end and a wave-confining structure. The first end that is adapted to mate with an end of the waveguide and that defines a first aperture that substantially matches the size of the waveguide cavity at the end of the waveguide. The second end defines a second aper
Cheung Chun-Tung
Rutledge David B.
California Institute of Technology
Ullah Akm E.
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