Optical: systems and elements – Optical modulator – Light wave temporal modulation
Reexamination Certificate
2001-06-05
2003-03-25
Tokar, Michael (Department: 2819)
Optical: systems and elements
Optical modulator
Light wave temporal modulation
C359S278000, C359S279000, C359S333000, C331S155000
Reexamination Certificate
active
06538793
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to quasi-optic grid arrays, and in particular to tunable and modulatable grid oscillators.
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 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.
One non-modulatable source configuration, known as a “Kim oscillator,” is described in M. Kim, E. A. Sovero, J. B. Hacker, M. P. De Lisio, J. J. Rosenberg, D. B. Rutledge, “A 6.5 GHz-11.5 GHz Source Using a Grid Amplifier with a Twist Reflector,”
IEEE Trans. on Microwave Theory and Tech
., Vol. 41, No. 10, pp. 1772-1774, October, 1993. The basic concept of this oscillator is to apply external feedback to a grid amplifier in order to induce it to oscillate.
FIG. 2A
shows a functional schematic of the standard Kim oscillator disclosed therein.
FIG. 2B
shows an exploded view of the physical configuration of a standard Kim oscillator. As seen, a twist reflector
40
, comprising a tilted polarizer
42
and a mirror
44
rotates the y-polarization of the portion of the amplified output beam from the amplifier
46
that is incident upon it and reflects it back into the input of grid amplifier. The polarizer
42
of the twist reflector
40
can be treated as a perfect reflector to energy polarized along its wires and as invisible to energy polarized perpendicular to them. The grid amplifier active array provides gain and gain compression. The frequency selectivity (tuning) in the feedback is accomplished by a phase delay
41
, which is primarily set by the physical separation “d” of the twist reflector and the grid array. The output polarizer
48
provides both isolation of the output from the input polarization as well as providing impedance matching for the input polarization.
Another type of grid oscillator is a voltage controlled quasi-optical oscillator, disclosed by T. Mader, S. Bundy, Z. B. Popovic, “Quasi-Optical VCOs,”
IEEE Trans. on Microwave Theory and Tech
., Vol. 41, No. 10, pp. 1775-1781, October, 1993. Unfortunately, this VCO, while tunable, produces relatively little output power and has a fairly narrow tuning range.
Grid oscillators that include an external twist-reflector feedback network (“Kim oscillators”) are conventionally frequency tuned by mechanically translating the feedback network along the axis of the grid's transmitted beam. Unfortunately, the need to physically move the reflector to tune the device is undesirable for numerous reasons. The addition of translation means adds bulk and expense to these otherwise highly reproducible structures. Mechanical tuning can be imprecise and slow. Further, the oscillator cannot modulate its output signal. Thus, such devices are impractical for most applications.
Electronic tuning of grid oscillators having twist reflectors has been proposed as a possible solution. One solution entails introducing a varactor-loaded tilted filter structure in place of the tilted polarizer in the twist reflector. This solution should allow direct phase modulation of the grid oscillator, but requires a specialized processing unit to generate the modulation signals to be applied to the varactors, and a specialized high-speed amplifier to actually drive the varactors. These extra components can add cost and complexity to the oscillator. Thus, there is a definite need for a tunable and modulatable grid oscillator that does not require such specialized circuitry.
SUMMARY OF THE INVENTION
The present invention, which addresses this need resides in an electronically tunable and modulatable quasi-optic power signal source. The power signal source includes a quasi-optic grid array oscillator having an output that sources an output signal, and means for introducing a signal to entrain the frequency and phase of the oscillator output to a reference signal having a frequency and phase that are specified as a function of time, such that the oscillator output bears a predetermined relationship to the frequency and phase of the reference signal. The grid array oscillator includes a grid amplifier having an input and output and a twist reflector spaced apart from the amplifier by a predetermined distance. In a prefe
Deckman Blythe C.
Rosenberg James J.
Rutledge David B.
California Institute of Technology
Nguyen Khai
Tokar Michael
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