This was taken from the paper published in the 6th International Symposium on Space Terahertz Technology, Pasadena, March 21-23, 1995. Some words and figures have been changed to update the progress.
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Publications
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Purpose
The demand for solid-state local
oscillators in the THz frequency range has been steadily increasing for
applications in radio astronomy and remote sensing of the atmosphere. The
interest for THz applications has fostered a strong need for submillimeter-wave
receivers, mixers and sources, especially tunable high-power sources used as the
local oscillators for heterodyne submillimeter-wave receivers. For example, the
local-oscillator output-power requirement at 1THz for NASA's SMMM (Submillimeter
Moderate Mission) is at least 50µW.
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Traditional high-power sources in the THz region such as gas lasers and vacuum-tube oscillators are not suitable for this purpose due to their large size, high-voltage supplies, short lifetime and small tuning ranges. However, frequency multipliers and upconverters like Schottky diode multipliers can be used to generate the required terahertz frequencies from lower-frequency solid-state tunable signal sources such as Gunn-diode oscillators. |
Approach
Current diode multipliers have mostly been single-diode structures typically consisting of a Schottky varactor diode placed in a waveguide with a whisker contact. Rydberg, Lyons and Lidholm have demonstrated a Schottky varactor diode frequency tripler with a measured output power more than 120µW at 803GHz. Erickson and Tuovinen presented a waveguide tripler with an output power of 110µW at 800GHz. Zimmermann, Rose and Crowe have demonstrated an output power of 60µW at 1THz by using a cascade of two whisker contacted Schottky varactor frequency triplers. One approach to overcome the low power of solid-state devices in the submillimeter-wave band is to combine a large number of devices together.A grid of planar diodes quasi-optically coupled in free space does not require the construction of single-mode waveguides and can potentially overcome the power limits of conventional single-diode multipliers.Using this approach, H.-X Liu has demonstrated a frequency tripler consisting of 3,100 Schottky-quantum-barrier varactor diodes to produce 5W pulsed output power at 99GHz.
The multiplier concept is shown in the figure above. The input beam at the fundamental frequency enters from the left. The first element is a pair of dielectric tuning slabs that act to transform the impedance of the input wave to one appropriate for the multiplier grid. Typically inductive reactance is needed to cancel capacitance of the diodes in the grid. In addition, the free-space wave impedance, 377ohm, is inconveniently high and needs to be reduced. Next the beam passes through a low-pass filter that passes the fundamental frequency, but reflects harmonics. Then the beam hits the grid. The grid acts as a nonlinear surface which results from the nonlinearity of I-V or C-V characteristics of diodes and generates harmonics. This harmonic beam radiates both forward and backward, but the backward beam reflects off the low-pass filter. The forward beam passes through the high-pass filter, and then through another pair of tuning slabs. One important feature is that the tuning slabs are outside the filters, so that the input and output can be tuned independently. The entire structure is quite compact, only a few wavelengths thick. The design is also suitable forcascading, so that even higher harmonics could be produced. The multiplication process preserves the beam shape. Therefore, a focused beam could be used so that different sizes of multiplier grids could be cascaded.
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Planar Schottky Diode Grid
These grid multipliers were fabricated by monolithic technology on a 30-µm thick fused-quartz substrate at the University of Virginia(UVA_AEpL). Figure shows the bow-tie-shaped metal pattern used for the unit cell.The Schottky diode junction is located at the center of the unit cell.The anode has a diameter of 0.5µm. A surface channel was etched away underneath the anode contact finger to reduce the shunt capacitance. The diodes have a measured DC series resistance of 14ohm and an estimated junction capacitance of 0.6fF at zero bias. The size of a unit cell is 70µmx70µm. Figure shows the entire 6x6 array.The active area is 420µmx420µm.
Measurements
The measurements use the free-electron laser (FEL) as the input source in the Quantum Institute at the University of California, Santa Barbara.(UCSB FEL) The free-electron laser generates kilowatts of polarized radiation tunable from 120GHz to 4.8THz. The pulse width is 2.42µs with a period of 1.3s.
A peak output power of 330µW was measured at 1THz without any impedance tuning for 2.42µs 500-GHz input pulses with a peak power of 3.3W. The relationship between the input power at 500GHz and the output power at 1THz follows a square-power law.
This grid was originally designed for THz sideband generators (work like mixers) and the polarity of the diodes designed for sideband-generator application results in a null in the center of the output beam for multiplier application. Measurements shows that only 10% of the total radiated power is received by the detector due to the null in the output beam.
A grid with diode-orientation appropriate for a multiplier application should improve the output pattern and increase the output power. Biasing tests verify that the frequency multiplication results from varistors. It should be possible to increase the output power and the conversion efficiency since these diodes are not saturated yet when no bias is applied.
New grid multipliers were designed in 1995 and measured in 1997 and 1998.
Created by J.C. Chiao
"A Terahertz
Grid Frequency Doubler," A. Moussessian, M. Wanke, Y. Li, J.-C. Chiao,
F. Hegmann, J. Allen, T. Crowe,and D. Rutledge, IEEE Transactions on
Microwave Theory and Techniques,Vol. 46, No. 11, pp.1976 - 1981, Nov. 1998.
