Coplanar transmission line millimeter radiation source

A structure for guiding millimeter wave radiation employs a resonant coplanar transmission line on a transparent substrate. A very short, picosecond, pulse is generated on the transmission line. By having the upper half plane air, the pulse will radiate into the substrate and be guided as millimeter wave from a distributed source and formed as a point source of radiation.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
This invention relates to generation of millimeter radiation. In particular 
the invention pertains to an integrated millimeter radiation source and 
waveguide. 
Many uses of microwave radiation, such as collision-avoidance radars, have 
large volume markets that require low cost components. One approach to 
satisfy these requirements is based on integrated circuit antennas. These 
antennas radiate primarily into the substrate and a problem often 
encountered is loss of energy to surface waves, resulting in a loss of 
antenna efficiency. 
This invention involves a unique geometry of the substrate such that as 
much of the energy as possible is coupled to surface waves--i.e., the 
energy is waveguided and then emitted at an aperture. This approach is 
particularly appropriate for a distributed antenna based on transmission 
lines (e.g., coplanar transmission lines). 
2. Description of the Prior Art 
Reference is made to Infrared and Millimeter waves, vol. 10 chap. 1, 
"Integrated--Circuit Antennas", Rutledge et al., Academic Press, 1983. 
This reference describes various configurations used for making integrated 
circuit antenna. It also discusses some of the limitations to antenna 
efficiency, including energy lost as surface waves. 
Grischkowsky et al., "Electromagnetic Shock Waves from Transmission Lines", 
Phys. Rev. Lett. 59, 1663 (1987) describes a particular coplanar 
transmission configuration where pulses on Aluminum lines deposited on a 
Silicon Sapphire (SOS) substrate radiate into the substrate at an 
efficiency dependent on the characteristic frequency. 
Reference is made to M. B. Ketchen, et al., "Generation of Subpicosecond 
Electrical Pulses on Coplanar Transmission Lines", Appl. Phy. Lett. 48 
(12), Mar. 24, 1986, pp.751-753. This publication describes techniques to 
generate ultrashort electrical pulses by photoconductively shorting 
charged transmission lines across narrow gaps. 
The generation of fast electrical pulses utilizing photoconductive resonant 
cavities has also been proposed in the literature. Reference is made to 
IBM Technical Disclosure Bulletin Volume 31, No. 12 pp. 392-393, May 1989. 
This publication discloses the use of a photoconductive resonant cavity in 
which the cavity length is matched to the repetition rate of the 
excitation optical source, such as a semiconductor diode. The resonant 
cavity is defined by having impedance discontinuities in the transmission 
line and is selected so that the round trip time of an electrical pulse is 
equal to the period of laser oscillations or a multiple thereof. The 
discontinuity presents 100% mirror at one end of the cavity. Thus, the 
internal pulse amplitude within the cavity is significantly larger than in 
a non-resonant cavity. The resonant cavity formed in this publication is 
an electrical analog of a Fabry-Perot. 
Thus, while the art provides alternative concepts for the generation and 
guiding of millimeter waves, a need still exists in the art to define more 
efficient techniques of guiding radiation from a distributed microwave 
emitter, such as a transmission line, such that all of the radiation is 
emitted as a point source. That is, the waveguide should be used whose 
ends can act as a point source. 
SUMMARY OF THE INVENTION 
It is an object of this invention to have a system which provides an 
efficient point source of millimeter radiation. 
It is a further object of this invention to provide a technique to guide 
millimeter radiation from a distributed source such that a point source of 
radiation is formed. 
Yet another object of this invention is to define a transmission structure 
that operates on a variety of input radiation pulses such as those 
photoconductively or electrically generated to guide such in an efficient 
manner. 
These and other objects of the invention are accomplished by employing a 
coplanar transmission line structure to generate millimeter wavelength 
radiation into a substrate. Short pulses traveling on a transmission line 
with the upper half plane being air radiate into the substrate. The ratio 
of the velocity of the electrical pulses, with respect to the phase 
velocity of the radiation in the substrate, defines the angle of emission. 
Thus, by suitable choice of material for the substrate and its thickness, 
a guided wave can be produced. 
By the use of a photoconductive transmission line structure electrical 
pulses are generated on the transmission line without loading problems, by 
exciting the line with a short pulse from a laser. 
For a more compact millimeter waveguide source a resonant photoconductive 
transmission line is used, which for a given transmission line length is 
more efficient. This resonant cavity is excited by the high frequency 
sinusoidal output from a semiconductor layer. 
This invention will be described in greater detail by referring to the 
attached drawing and a description of the preferred embodiment which 
follows.

DESCRIPTION OF THE PREFERRED EMBODIMENT 
In the preferred embodiment, a light pulse incident on a photoconductive 
gap produces an electrical pulse. The electrical pulse produced will 
travel along the transmission line and radiate millimeter waves. These 
waves are guided by the substrate structure and emitted at the end of the 
waveguide. Beam dimensions of light should be matched to the gap between 
lines that form the transmission line (for fast pulses, i.e., less than 20 
ps, gaps should be 1-50 microns and beam dimension matched accordingly). 
Wavelengths should be above the bandgap of silicon so that corners are 
created. An optimum wavelength .lambda. would be such that all light is 
absorbed over the thickness of polysilicon. At wavelengths .lambda. equal 
to 600 nm this is approximately 1 micron, at wavelengths .lambda. equal to 
800 nm, this is approximately 10 microns. Pulse length determines 
electrical pulse length and consequently the frequency make-up of the 
radiated waves. The higher the frequency components in the electrical 
pulse, the more efficient the millimeter wave radiation (and the higher 
its frequency components are). Relevant equations are set forth in the 
Grischkowsky reference. 
Larger pulse amplitudes provide larger electrical pulses, which give more 
millimeter radiation. The repetition rate of light pulses is also 
important for resonant operation. 
In a second embodiment, which will be described in detail below, the 
electrical pulse can be directly produced by using a fast electrical 
device, (e.g., an IMPATT diode). 
Referring now to FIGS. 1 and 2, a preferred embodiment of this invention 
will be depicted. A pair of metal lines 10 and 12, are formed on the 
surface of a damaged polycrystalline structure 14. That is, a structure of 
ion implanted polysilicon. This layer is placed on top of a suitable 
substrate 16 such as sapphire. The transmission lines 10 and 12 are 
suitably biased by means of a direct current power supply 18. Typically, 
they are biased in the range of 5-10 volts. The transmission lines 10 and 
12 are given a critical length L (to achieve resonance) and are separated 
from each other by a distance "x". The critical length L is the length of 
the cavity required to achieve resonance; resonance being achieved when 
the roundtrip time of an electrical pulse equals the period of the laser 
oscillations. In other words, the critical length L of the transmission 
lines 10 and I2 match the repetition rate of an exciting optical source, 
for example, a laser pulse 19 produced by a laser 21 as shown in FIGS. 1 
and 2. The IBM Technical Disclosure Bulletin, Vol. 31, No. 12, pp. 392, 
393, 399, and 400, May 1989, to the extent necessary to describe the 
meaning and usage of the terms critical length or cavity length and 
resonance, is incorporated herein by reference. The distance x is chosen 
so that a diode spot 17 can be focused between the lines 10 and 12. 
The diode spot 17 is produced from a laser pulse 19 that propagates from 
preferably a GaAS/AlGaAs semi conductor laser 21. It will be apparent to 
those of working skill that other lasers may be used. The diode spot 17 
causes fast electrical pulses to travel down the transmission lines which 
radiates millimeter waves. This technique, that is generating electrical 
pulses from a diode spot which is itself generated by an optical source, 
has become a standard technique used in this art, as exemplified by 
Grischkowsky et al., "Electromagnetic Shock Waves from Transmission 
Lines," Phys. Rev. Lett. 59, 1663 (1987), which is incorporated herein by 
reference for all that it teaches. 
Any high frequency pulse, such as that caused by the diode spot, travelling 
on a transmission line will emit radiation, which is a fundamental 
principal of antenna theory. For a particular transmission line geometry, 
the efficiency of the radiation emitted depends on the frequency 
components of the electrical pulse (the efficiency for a particular 
frequency f depends on frequency cubed or f.sup.3). Radiation will also be 
preferentially emitted for wavelengths close to the separation of the 
lines. 
In accordance with a second embodiment of the present invention, the high 
speed electrical pulse on the transmission line can be created by a high 
speed switching device 22 (e.g., a high speed diode such as an IMPATT 
device) connected directly to the transmission line, or in the case where 
the two conductors of the transmission line are in coplanar geometry and 
are separated by a photoconductive material such as polysilicon, the 
electrical pulse can be generated by transiently shorting the lines 
through incidence of a light pulse. In this instance, the electrical pulse 
generated has the same duration as the light pulse. 
A high frequency diode 22 is connected to a coplanar transmission line by 
connecting (through a metal interconnect) one terminal of the diode to one 
conductor, and the other terminal to the other conductor. 
By the use of a resonant transmission line structure, due to the higher 
intracavity fields, higher yields of millimeter radiation at a particular 
wavelength and for a particular transmission line length can be produced. 
The ratio of the velocity of the electrical pulses relative to the phase 
velocity of the radiation in the substrate defines the angle of emission. 
By suitable choice of substrate material and thickness, a guided wave can 
be produced. For air as the upper half plane dielectric then guiding will 
always occur irrespective of substrate material as long as it is 
transparent. In this situation, the ratio of the angle of incidence at the 
lower substrate/air interface (.beta.) to the critical angle (Y) is given 
by (quasistatic approximation) sinl/siny=(1/2(.epsilon.+1)) 1/2, which 
always gives l&gt;Y (i.e., guiding). 
For example, the substrate could be quartz where the dielectric constant 
.epsilon. is approximately 4. The critical angle is about 30.degree.. For 
silicon having a dielectric constant .epsilon. of approximately 11, the 
critical angle would be about 20.degree.. 
For instance, if the transmission lines 10 and 12 are placed on a damaged 
silicon on sapphire substrate (SOS) then using v=c/2.45 on the line and 
v=c/3.3 for high frequency radiation in the substrate, the angle of the 
emission is approximately 50.degree., wherein v is the group velocity of 
the pulse on the line, and the millimeter wave pulse in the substrate, 
respectively, and c is the speed of light in free space. This is 
compatible with guided radiation in the sapphire where the critical angle 
to air is approximately 20.degree.. 
For a cavity having a length L of 3 mm, resonance would occur with an 
excitation of 40 GHz. As determined from the equations found in Rutledge 
et al., "Integrated-Circuit Antennas" incorporated herein by reference, 
the amplitude absorption coefficient due to radiation is approximately 
0.4.times.10.sup.-4 m.sup.-1 at 40 GHz. For a bias voltage in the range of 
10 V on 100 .OMEGA. lines, the radiation power at 0.75 mm wavelength is in 
the range of 10 mw. For lines of separation of 200 um then the radiation 
is in the range of 250 mw. 
Referring now to FIG. 2, the structure of FIG. 2 is depicted illustrating 
those emission angles. An emission cone of millimeter radiation having an 
angle l of approximately 50.degree. is created when a diode spot is 
projected on the damaged poly layer 14 causing resonance. This is 
compatible with guided radiation in the sapphire 16 where the critical 
angle is approximately 20.degree.. For angles greater then the critical 
angle of 20.degree. there is total internal reflection. The output face 20 
of the waveguide is angled as shown so that the millimeter radiation 
outputs normal to this face. The thickness, h, of the substrate should be 
in the order of the wavelength of radiation (here approximately 1 mm). 
A sapphire substrate of approximately 0.5 mm in thickness is suitable with 
the damaged poly layer of approximately 1.mu. forming the SOS composite. 
For better confinement, the wafer should be cleaved in a slab geometry 
having a width of a few mm (i.e., confinement in the x and z directions as 
defined by FIG. 2). 
Higher efficiencies can also be achieved at higher frequencies (frequency 
cubed dependence) in which case one would use shorter pulses with 
components of several hundred GHz. The bandwidth of radiation will be 
larger though strongly peaked toward the shorter wavelengths, with the 
shortest emitted wavelength at a few hundred microns. 
This invention can thus be summarized as follows: 
I. Coplanar Transmission line on waveguide substrate (pulses can be 
launched on line by any means e.g., high frequency gunn diode, IMPATT 
diode or the like, or by depositing lines on damaged polysilicon such that 
short pulses can be generated photoconductively directly on the lines); 
II. any transparent substrate; 
III. shaped output face of waveguide; 
IV. width of transmission line and pulse frequency determines efficiency of 
millimeter radiation of a particular wavelength at the output face of the 
waveguide. 
It is apparent that modifications of this invention may be practiced 
without departing from the essential scope thereof.