U.S. patent number 3,596,214 [Application Number 04/717,447] was granted by the patent office on 1971-07-27 for electromagnetic waveguide.
Invention is credited to Lan J. Chu, Jerome Ira Glaser.
United States Patent |
3,596,214 |
Glaser , et al. |
July 27, 1971 |
ELECTROMAGNETIC WAVEGUIDE
Abstract
This disclosure deals with a novel electromagnetic waveguide
particularly adapted for the transmission of a range of waves from
submillimeter through optical wavelengths having a
finite-conductivity tube of dielectric constant greater than unity
surrounding a medium of lesser dielectric constant and of cross
dimension of a value preferably very much greater than the
wavelength of the waves propagated along the waveguide.
Inventors: |
Glaser; Jerome Ira (Watertown,
MA), Chu; Lan J. (Littleton, MA) |
Family
ID: |
24882070 |
Appl.
No.: |
04/717,447 |
Filed: |
March 29, 1968 |
Current U.S.
Class: |
333/239 |
Current CPC
Class: |
H01P
3/16 (20130101) |
Current International
Class: |
H01P
3/16 (20060101); H01P 3/00 (20060101); H01p
003/12 (); H01p 003/16 () |
Field of
Search: |
;333/95,95A |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Saalbach; Herman Karl
Assistant Examiner: Chatmon, Jr.; Saxfield
Claims
What we claim is:
1. A waveguide for the propagation of electromagnetic waves of
wavelengths extending from the millimeter range down through
optical wavelengths having, in combination, a tubular member of
finite conductivity, of loss tangent substantially unity, and
predetermined dielectric constant greater than unity surrounding a
medium of dielectric constant less than said predetermined
dielectric constant through which electromagnetic waves of one of
said wavelengths may propagate, the cross section of said medium
being adjusted to a value large compared with said one of said
wavelengths, and the thickness of said tubular member being
adjusted such that field external thereto is negligible compared
with that at the interface between said tubular member and said
medium.
2. A waveguide as claimed in claim 1 and in which said medium is a
gas.
3. A waveguide as claimed in claim 1 and in which the said cross
section is substantially circular and the approximate root
locations u of the Bessel Bessel function J'.sub.0 (u)=0 for the
following transverse electric and magnetic waves are
u.sub.1 =3.832 for TE.sub.01 and TM.sub.01 modes
and
u.sub.2 =7.016 for TE.sub.02 and TM.sub.02 modes
4. A waveguide as claimed in claim 1 and in which the cross section
is substantially circular and the approximate root location u of
the Bessel function J.sub.o (u)=0 for EM.sub.1 mode is
u.sub.1 .apprxeq.2.405.
5. A waveguide as claimed in claim 1 and in which said
predetermined dielectric constant is substantially equal to at
least two.
Description
The present invention relates to electromagnetic waveguides being
more particularly (though not exclusively) directed to the improved
wave-guided propagation of a range of waves from substantially
submillimeter wavelengths down through optical wavelengths.
Prior waveguides have been of three basic types: metallic or other
substantially infinitely conducting tubes bounding a dielectric
medium, such as air or any other gas or dielectric medium;
dielectric rods; and dielectric-coated metallic guides. The
efficiency of the metallic tube type is limited by the conductivity
of the metal , since the confining of the electromagnetic energy is
dependent upon the high reflectance of the metallic shield. The
efficiency of the dielectric rod type of guide or the
dielectric-coated metallic guide, on the other hand, is limited by
how low the conductivity of the dielectric can be made, the
field-confining action being dependent upon critical reflection at
the dielectric interface. External fields, moreover, affect the
wave propagation along such dielectric structures.
In accordance with the present invention, improved transmission
efficiency (lower attenuation), lighter weight and better
field-confining properties are all attained, particularly for
submillimeter and lower wavelength energy ranging down into the
optical region, with a novel waveguide structure having, in
summary, a low or finite conductivity tubular member of dielectric
constant greater than unity surrounding a medium of lesser
dielectric constant and of cross section very large compared to the
wavelength of the waves propagated therealong. Underlying this
structure is the discovery that a high reflectance to
electromagnetic waves travelling at small angles can be presented
by a partially lossy dielectric under such critical conditions of
appropriate dielectric constant and dimensional relationships.
An object of the invention, accordingly, to provide a new and
improved electromagnetic waveguide structure.
Other and further objects will be hereinafter pointed out and more
particularly delineated in the appended claims.
The invention will be further described in conjunction with the
accompanying drawing, the single figure of which illustrates a
embodiment diagrammatically. As shown in the drawing, the waveguide
of the invention comprises a low or finite conductivity (i.e.
partially conducting) tubular dielectric member of dielectric
constant greater than unity (preferably in some instances, as later
mentioned, equal to or greater than two), surrounding and bounding
a dielectric medium or core of less dielectric constant and of
cross section much larger than the wavelength of the waves being
propagated therealong. The thickness of the tubular member, to
attain the optimum results with the energy mainly confined to the
medium, must be sufficient that fields at the outer periphery are
at least an order of magnitude less than (and thus negligible with
respect to) the fields at the interface between the inner periphery
of the tubular member and the said dielectric medium. The loss
tangent of the tubular member should, moreover, approximately be
equal to unity for effective attainment of this end of minimum
attenuation.
In accordance with the invention, the efficiency of propagation is
a complex function of the dielectric constant and low or finite
conductivity of the partially conducting dielectric tubular member
and the lesser dielectric constant of the inner medium. The
restriction for operation in accordance with the phenomenon
underlying the same that said medium must be of cross section much
larger than the wavelength of the propagated waves causes the most
practical applications of the invention to be restricted to
fractional millimeter wavelengths and below into the optical
spectrum.
Considering, for example, a cylindrical circular cross section
tubular member and internal medium, though waveguides of any other
geometry may also be employed, we have shown that, as the frequency
increases (or the wavelength decreases), the attenuation
characteristics of the waveguide of the present invention become
increasingly better than those of an equivalent metallic or
conductive-walled waveguide.
Specifically, a copper-walled air-filled waveguide of 0.5 cm.
diameter will produce the same 0.73 db./meter attenuation at a
wavelength of 0.16 mm. (dominant TE.sub.11 mode) as a
similiar-dimensioned air-filled guide constructed in accordance
with the invention and having an outer dielectric tube of
dielectric constant equal to three (equivalent EM.sub.1 mode). As
the wavelength .lambda. decreases, the attenuation constant .alpha.
of the waveguide of the invention becomes improved over the
attenuation .alpha..sub.m of the metal waveguide at a rate
determined by the 5/2 power, as follows:
In the optical range where .lambda.=0.6.mu., for example, the
attenuation of the invention is a million times better than that of
the shielded waveguide for these EM.sub.1 and TE.sub.11 modes.
Thus, the invention is well adapted to such uses as the optical
link in laser communication systems or even as the gas-containing
and energy-confining oscillation cavity or container between the
mirrors of, for example, a Fabry Perot gas laser oscillator.
Continuing with the illustrative example of circular cross section
guides, it has been determined that, unlike prior art guides, there
is only a finite number of modes which are confined substantially
to the dielectric medium within the dielectric tubular member of
the invention to produce the results herein described with, for
example, circularly symmetric transverse electric waves, circularly
symmetric transverse magnetic waves, and waves of other field
components and angular variations of order p, where p is an integer
greater than or equal to unity. The number of possible modes thus
confined to the inner dielectric medium is dependent upon the
material parameters, the order of the angular variation, the
wavelength and the radius of the inner cylindrical dielectric
medium. When the tubular member radius is many wavelengths in
diameter, the approximate root-locations u of the Bessel function
J'.sub.o (u) =0 for transverse electric (TE) and transverse
magnetic (TM) circularly symmetric waves have been found to be as
follows:
u.sub.1 .apprxeq.3.832 for TE.sub.01 ; TM.sub.01 ;
u.sub.2 .apprxeq.7.016 for TE.sub.02 ; TM.sub.02 .
The longitudinal attenuation is found to reach a minimum in the TM
case for a loss tangent in the dielectric tube of unity. The depth
of penetration of the modes, i.e. the distance of the field into
the tubular member for which the fields fall to 1/e of their value
at the inner interface with the inner dielectric medium, increases
as the dielectric constant of the tubular member approaches that of
the inner medium and also as the loss tangent of the tubular member
decreases. In fact, there exists a value of loss tangent dielectric
constant and frequency for each mode for which the penetration
depth actually becomes infinite. This behavior is totally unlike
that of the shielded cylindrical dielectric waveguide, through more
akin to the cylindrical dielectric waveguide. It has been
determined, interestingly, that these modes do not exist, however,
when the tubular member becomes a perfect insulator at which the
prior art well-known modes will exist and are mainly confined to
the tubular member and not the inner medium.
The lossy cylindrical dielectric waveguide herein disclosed is thus
readily distinguishable from the metal-shielded cylindrical
dielectric waveguide, the cylindrical dielectric rod waveguide, and
the dielectric-coated cylindrical metallic waveguide because its
tube has a low conductivity and a dielectric constant preferably of
the order of 2 or larger. The waves which propagate along such a
waveguide , furthermore, have behaviors which distinguish them from
those waves associated with such prior art structures; namely, the
fields are mainly confined to the region of lesser dielectric
constant and the attenuation of the TM circularly symmetric and the
EM waves is lowest for a specific value of conductivity and
relative dielectric constant of the tube with respect to the inner
dielectric medium. The cross-dimensional size of the lossy
dielectric waveguide of the invention, in addition, has been
determined to be a critical factor in determining the attenuation.
Only when the diameter is much larger than a wavelength in the
inner dielectric medium is the attenuation low and improved over
such prior waveguides.
Theoretical analysis for the guides of the invention have been
experimentally verified in the 7.6 to 12 gHz. frequency range. The
lossy dielectric finite conductivity tubular member was chosen as a
15 inches i.d., 17 inch o.d. unreinforced concrete pipe with an
air-filled dielectric medium. The17 inch cross section of the air
medium (of lesser dielectric constant than the value 5.1 of the
concrete) is from 11 to 17 times the wavelengths of the above
frequency range. The 2-inch thickness of the tubular pipe has been
found to be sufficient for the external field at the outer
periphery thereof to be negligible with respect to that at the
inner periphery, at least an order of magnitude less. Two lengths
of pipe were chosen; four sections of 8 ft. pipe (or 32 ft.), and
eight sections of 8 ft. pipe (or 64 ft.). Sectoral horns were used
for the receiving and transmitting antennae at the ends of the
pipe. Several of the above-mentioned excited modes were identified
and their attenuation satisfactorily checked with the theory. It
was found that the guide offered a 10 db. improvement over the free
space transmission between the horns at the same distance,
including even the mismatch losses introduced in coupling to the
modes.
In connection with such circular cross section guides, moreover,
waves of all field components and p angular variations (not just TE
or TM), generically referred to herein as EM.sub.p waves, it has
been found that the approximate root location of the Bessel
function J.sub.o (u)=0 (characterizing the field distribution and
phase velocity of the waves as is well known) is
u.sub.1 =2.405 for the EM.sub.1 mode.
Further modifications will occur to those skilled in the art and
all such are considered to fall within the spirit and scope of the
invention as defined in the appended claims.
* * * * *