US3046506A - Broadband polarization rotator - Google Patents

Description

July 24, 1962 c. E. BARNES BROADBAND POLARIZATION ROTATOR 2 Sheets-Sheet 1 Filed Sept. 29, 1959 JIYNI MAGNET/C F/ELD /NTENS/TY /N VEA/TOR 9E. BARNES 5 A TTORNE V July 24, 1962 Filed Sept. 29, 1959 C. E. BARNES BROADBAND POLARIZATION ROTATOR Y2 Sheets-Sheet 2 /NVE N TOR 59E. BA RNES A 7' TOR/VE V quency.

United States Patent O 3,046,506 BROADBAND POLARIZATION ROTATOR Clare E. Barnes, Passaic Township, Morris County, NJ., assignor to Bell Telephone' Laboratories, Incorporated, New York, NX., a corporation of New York Filed Sept. 29, 1959, Ser. No. 843,214

- 6 Claims. (Cl. 3334-81) This invention relates to electromagnetic waveguide transmission systems, and more particularly to broadband gyromagnetic components for use in such systems.

` of gyromagnetic material.

Elements of gyromagnetic material have been utilized l Y inside hollow metallic waveguides to produce numerous useful and important effects upon electromagnetic wave energy propagating therethrough. One class of component that has been developed makes use of the socalled Faraday effect rotation of the wave energy produced by a longitudinally magnetized element of gyrolcompensate for the frequency dependency of gyromagnetic material in electromagnetic wave devices.

A typical prior art device is characterized by an elongated element of gyromagnetic material, usually ferrite,

located in a round metallic waveguide and subjected to an applied direct-current magnetic biasing field. The Faraday rotator, in particular, has its applied magnetic field directed parallel to the longitudinal axis of the ferrite. It has been shown that the angle of rotation of the plane of polarization of electromagneticwave energy transmitted through such a device increases with increasing frequency. The reason for this frequency. dependence of Faraday rotators is well known. In an infinite, uniform,

vdielectric material, the distribution of the electromagnetic wave energy wouldbe uniform and independentof fre- However, when a dielectric material is surrounded by another medium having a differentk dielectric `V constant, the exciting wave energy is distributed between the high dielectric element and the generally lower dielectric medium surrounding it. As the frequency of the Wave energy increases, a redistribution of the energy be- .tween the element and the surrounding medium takes Ywherein the energy density remains substantially constant as the frequency changes. Consequently, if the active material, i.e., the ferrite, is conned primarily to these areas where-in theenergy density remains reasonably constant, the rotation produced would likewise remain reasonably constant.

It is, therefore, a specific object of'this invention to Acouple wave energy to a dielectric structure wherein the gyromagnetic material is conned to that portion of the 3,046,506 Patented July 24, 1962 structure wherein the energy density remains substantially constant.

In accordance with the invention, broad-band Faraday rotation is produced by means of a compound structure consisting of a dielectric material surrounded by a sheath The two materials are in contact along their common extent and are selected to have substantially equal dielectric constants so as to provide a homogeneous dielectric structure lto electromagnetic wave energy propagating therethrough. yBy proportioning the relative dimensions of the gyromagnetic material and the dielectric core material, the change in rotation per unit length for a given change in the operating frequency is substantially reduced.

, In a second embodiment of the invention the gyromagnetic material comprises two longitudinally extending rectangular slabs separated by. a nongyromagnetic dielectric member. The latter arrangement is used, in the illustrative embodiment shown, in conjunction with a distributed lossy lrn to produce broad-band attenuation.

.These and other objects and advantages, ythe nature of the present invention, and its various features, will appear more fully upon consideration of the various illustrative embodiments now 'to be described in detail in connection invention showing a cylindrical composite rod in a circular waveguide;

FIG. 2 shows, by way of illustration, the variation of radio frequency magnetic field intensity in a dielectric rod as a function of frequency;

FIG. 3 is a cross-sectional view of the waveguide and composite rod of FIG. l; and Y FIG. 4 is a perspective view of a second embodiment of the invention showing a rectangular composite rod used as a dielectric waveguide. p

In more detail, FIG. l is an embodiment of a polarization rotator in accordance with the invention,` given by way of example, comprising a round waveguidel() of the metallic shield type proportioned to support linearly polarized electromagnetic waves and preferably dimen- Interposed longitudinally within guide 10 along its axis is the composite `dielectric member 11, comprising a magnetic polarizable cylindrical element of gyromagnetic material k12 and an inner core of nongyromagnetic dielectric material 13.

The'term gyromagnetic material is employed here in its accepted sense as designating the class of magnetic polarizable materials having unpaired spin systems involving portions of the atoms thereof that are capable of being Ialigned by an external magnetic polarizing field and which exhibit a significant precessional motion at a frequency. within the range contemplated by the invention under the combined influence of said polarizing eld and an orthogonally directed varying magnetic eld component. This precessional motion is characterized as having an yangular momentum, a gyroscopic moment and a magnetic moment. Typical of such materials are ionized gases, paramagneticmaterials and ferromagnetic material, the latter including the spinels such las magnesium aluminum ferrite, aluminum zinc ferrite and the garnetlike materials such as yt'trium iron garnet.

Whereas some gyromagnetic materials, suchras ferrite,

have a dielectric const-ant substantially greater than air land as such may be properly termed dielectric material, the inner core 13 is to be distinguished from the gyromagnetic cylinder 12 in that the former exhibits substantially no gyromagnetic properties whereas the lat-v ter material exhibits pronounced gyromagnetic properties. Both the inner core 13 and the outer cylinder 12 are chosen to have substantially equal dielectric constants and as such appear as a homogeneous dielectric rod l1 to electromagnetic wave energy propagating thereby. Each end ofthe rod 11 may be `taperedl by means of the conical sections 14 and 1S to decrease reflections of wave energy incident upon rod 11 in a manner well known in the art. The sections are made of the same material as core 13 and provide a relatively smooth transition for the incident wave energy. Low-loss low-dielectric disks 16 and V17 located at the ends of rod 11 support the rod within guide 10.

Suitable means for producing the necessary longitudinal magnetic field in rod 11 surrounds the guide 19. For the purposes of illustration, this may be a single solenoid 15 energized by a source 2). Control means for varying the biasing field is provided by lrheostat 19. This field, however, may be supplied by a solenoid of other suitable design, by a permanent magnet structure, or the gyromagnetic cylinder 12 may be permanently kmagnetired, if desired.

In the ordinary operation of a Faraday rotator, that is, with the composite rod 11 replaced with `a solid rod of ferrite, a plane polarized wave of frequency fo incident upon a magnetically biased ferrite can be considered as comprising two oppositely rotating circularly polarized wave components. The biased ferrite exhibits respectively diferent permeabilities to each of the oppositely polarized component waves. As a consequence, one of the components has a smaller phase velocity than the other and the two component waves are propagated through the ferrite medium at unequal velocities. Upon emergence from the medium the component waves combine to reform a resultant plane polarized wave which is, in general, polarized `at a different angle, 00, from the original wave `due to the phase difference between the components introduced during propagation through the ferrite. A wave of frequency f1, greater than fo, will have its plane of polarization rotated by an angle, 01, greater than 00. This is `a result of the peculiar waveguiding properties of the ferrite. At frequency fo a certain portion of the radio frequency wave energy is propagated through the ferrite element whereas the remaining portion of the wave energy propagates in the air space between the ferrite and the waveguide wall. At the higher frequency f1, a greater proportion of the radio frequency wave energy is concentrated in the ferrite during its propagation therethrough than was the case at frequency fo. As a consequence, the aforementioned anisotropic permeability property of the ferrite is enhanced, and the difference between the permeabilities exhibited respectively to the two circularly polarized wave cornponents is increased. Upon emergence from the ferrite, therefore, the two components have a greater phase difference between them at frequency f1 than they do at frequency fu `and `a greater angle of rotation is thereby produced.

The effect produced upon the high frequency magnetic field distribution by increasing the signal frequency is graphically portrayed in FiG. 2. Specifically, FlG. 2 is a cross-sectional view of the embodiment of FIG. l showing member 11, for -the purpose of this discussion, as a uniform dielectric rod supported along the axis of guide 10. The intervening space between rod 11 and the inner surface of guide is filled with `air or some other lowloss material having a dielectric constant substantially less than that of rod 11.

The abscissa of the graph in FIG. 2 represents the relative intensity of the transverse component of the yradio frequency magnetic field within guide 10, and the 4ordinate represents the position within the guide along the diameter. As shown by curve 21, at a frequency fo, the magnetic field intensity is not uniform but varies across the guide. Specifically, the field has some low value at the guide wall, increases slightly in the interval between the guide wall and rod 11, then increases rapidly '4 within rod 11, reaching a maximum at the center. The field distribution in the other half of the guide is the mirror image of that described, symmetrically decreasing from the center to the same low value at the guide wall.

As the signal frequency is increased to a higher fre quency f1, the radio frequency field intensity assumes the shape of curve 22, while `a still further increase in signal frequency to f2, causes a corresponding further concentration of the magnetic field in `the dielectric rod 11, as shown by curve 23.

While it is evident that there is a substantial concentration of the field in the center portion of rod 11, it is equally evident that `along the edges of the rod, in regions 24 `and 25, the overall change in the total field is relatively small. If the field remains reasonably constant, it naturally follows that the effect of the anisotropic permeability of lany ferrite material in these regions will remain substantially constant.

In the above discussion the dielectric rod 11 was considered as an entity for the purposes of examining the effect of increasing the frequency upon the field distribution. If now, however, the rod is designed so that the gyromagnetic material is limited to the regions 24 and 25 only, and the remainder of the rod is composed of nongyromagnetic dielectric material, changes in the intensity of the radio frequency magnetic field in the central portion of rod 11 comprising the nongyromagnetic material, due to changes in operating frequency will not produce corresponding changes in the resulting rotation of the plane of polarization of the incident electromagnetic wave.

In FIG. y3 there is shown a cross-sectional view of the embodiment of FIG. 1 with member 11 constructed in accordance with the teachings of the invention. Specifically, the rod of nongyromagnetic material of radius r is shown surrounded by the cylinder of gyromagnetic rnaterial of outer radius R. The ratio of the two radii, R/r, is preferably about two or less. In general, the smaller the ratio the greater the bandwidth. However, whereas the bandwidth increases as the ratio of the radii decreases, the amount of rotation produced fora given length of rod is correspondingly decreased. Thus, the three factors, bandwidth, rotation, and length of rod 11 are adjusted to suit the particular application. For a given bandwidth, and a given angular rotation of the plane of polarization, the length of member 11 will be determined.

In the embodiment of the invention shown in FIG. l, the gyromagnetic element was located within a conductively bounded sheath in the usual fashion. In many applications, however, it is desirable to vary the strength, or orientation of the magnetic field applied to the gyromagnetic element, and it is often advantageous to vary the field rapidly and/or continuously. However, when the magnetic field is so varied, eddy currents are set up in the metallic waveguide sheath which tend to prevent the magnetic field from penetrating to the gyromagnetic element. Inetiicient operation results as a consequence. Furthermore, the magnetic structure 15 must, of necessity, be larger in such an embodiment than would be the case if the biasing field Vstructure could be applied directly over the gyromagnetic material itself. It has been recognized that wave energy may be propagated along a dielectric rod without a conductively bounded sheath and while a portion of the wave energy is propagated outside the dielectric material, the field inside the dielectric rod conforms closely to those modes of propagation expected in =a metal tube waveguide. As a consequence, all the usual effects of the gyromagnetic materials upon the propagating wave are substantially the same. In particular, the distribution of the radio frequency magnetic fields within the dielectric member 11, as described above, will be substantially unaffected by the labsence of the metallic sheath, with the added advantage that the eddy current problem vanishes and the size of the biasing structure may be correspondingly reduced.

In a second vembodiment of the invention, shown in FIG. 4, a broad-band -attenuator is illustrated utilizing the dielectric waveguide effect mentioned above and the broadbanding principles of the invention. The details of the embodiment of FIG. 4 differ slightly from those of FIG. 1

in the geometric arrangement ofthe gyromagnetic material and the nongyromagnetic dielectric material comprising the composite dielectric waveguide structure 40. In particular, the gyromagnetic material is in the form of two longitudinally extending rectangular slabs 41. and 42 which 'are separated by the two nongyromagnetic dielectric members 43 and 44. At both ends, beyond the extent of the slabs 41 and 42, 'themembers 43 and' 44 widen to the full height of the composite structure 40 to form a homogeneous dielectric member which is then tapered to decrease reflections of the incident wave energy. The taperedy ends, 45 and `46, extend into, and couple to, the rectangular waveguides -48 and 49, respectively. Supporting waveguide 40 between the two conductively bounded rectangular waveguid 48 and 49 are a pair of low-loss, low dielectric supports 50 and 51.

The gyromagnetic slabs illustrated are permanently `magnetized parallel to thedirection of propagation, as

shown by the arrows H0. Other suitable means for producing the necessary steadymagnetic fields in slabs 41 and 42' may, however, be used.

'Extending throughout the dielectric waveguide 40 and separating its upper and lower halves, is a film of lossy material 52.

In operation, the direction of polarization of linearly polarized wave energy, applied to the dielectric waveguide 40 from either of the rectangular guides, is rotated in the plane of the lossy lm under the influence of the longitudinally polarized gyromagnetic material. Initially, all components of the applied wave are perpendicular to the plane of the lossy material. As the wave progresses along the rod, however, the direction of polarization changes, producing a component parallel to the plane of the lossy material. This rotational effect takes place continuously along the extent of the gyromagnetic slabs Iand results in a continuous coupling of radio frequency energy to the lossy lm 52. The energy so coupled is dissipated within the lossy material. An attenuator so constructed differs fundamentally from the Faraday rotational type attenuator wherein the lossy material is concentrated at discrete intervals at either end. In such devices the loss characteristic has a typical cosine squared power variation which requires, for maximum loss, that the plane of polarization coincide exactly with the plane of the lossy material. Variations which tend to increase or decrease the rotation result in increases in the transmission through the attenuator. In particular, it is possible, for example, for the, transmission to be completely unaffected by the presence of the loss material where, for example, the rotation results in a 180 degree reversal in the direction of polarization. However, by distributing the loss film over the length of the gyromagnetic material so as to couple thereto continuously, the power variation as a function of angular rotation, is given by d 2 1 Sin 2nd [we pelle l 2 in the first Iapproximation, where d is the length of the rotator, and p is the rotation per unit length.

It will be noted that as d increases, the exponential term approaches epd/2. Ideally, then, the transmission through an attenuator in accordance with the invention goes from unity at pd= to 0 at Ypd=90", and remains below some arbitrary level, determined by d, as the rotation is increased; The attenuator may then be designed to maintain a given minimum attenuation over a substantially greater range of anticipated variations in rotation. This feature is especially valuable when the device is used in a broadband, closed feedback loop because it makes it impossible, in most applications, for the device to go into .6 positive feedback operation over the range of operating frequencies. v The amount of attenuation may be varied, and the signal consequently modulated, by superimposing avariable field upon the steady biasing eld H0. ,-Byvirtue `ofi `the construction of the` attenuator shown in FIG. 4, the modulating means (not shown) may be mounted directly over theunsheated rod 40, thus allowing rapid variations of the attenuation without the deleterious interference of p This becomes of especial importance in designing attenuators or modulators for the ever-increasing frcquencies of todays microwave systems. The higher frequencies tend to rule out, or make difficult, the use ofsuch phenomena as gyromagnetic resonance where .the magnetizing field is proportional to the frequency of operation.

For example: An isolator, in accordance with the invention can be built to operate over a 20% band at 50 kilomegacycles per second with an untuned magnetizing field of as litle as 50 gauss. Typical magnetizing fields for field displacement and resonance isolators at this frequency are 14,000 and 18,000 gauss, respectively. Neither of these isolators has yet been made to approach the 20% bandwidth without benefit of tuning the magnetizing field.

Inxall cases it is understood that the above-described arrangements are illustrative of a small number'of the many possible specic embodiments which can represent applications of the principles of the invention. Numerous andl varied other arrangements can readily be devised in accordance with these principles by those skilled in the art without departing from the spirit and scope of the invention.

What is claimed is:

1. In combination, a section of transmission line for supporting electromagnetic wave energy, means for applying linearly polarized vwave energy to said line, means for rotating the direction of polarization of said wave energy comprising a composite dielectric rod longitudinally disposed along said line, said rod comprising an outer y layer of gyromagnetic material having a first dielectric constant land an inner core of nongyromagnetic material having a second dielectric constant substantially equal to said first dielectric constant, said gyromagnetic and said nongyromagnetic material being in contact `along their common length and magnetic means for longitudinally biasing said rod.

2. The combination according to claim l wherein said line is a conductively bounded waveguide and wherein said rod is disposed within said waveguide.

3. In combination, a section of transmission line for support-ing electromagnetic wave energy, means for ap- 4. The combination according to claim 3 wherein said inner core has a radius r and said hollow cylinder has an outer radius R and wherein the ratio of R to r is less than 2.

S. An attenuator for electromagnetic Wave energy cornprising an inner pair of nongyromagnetic dielectric members having a rst dielectric constant separated by a planar layer of electrically lossy material, an outer pair of members having a'second dielectric constant and capable of exhibiting gyromagnetic properties `at the operating frequency of said attenuator in contact with said dielectric members, and means for magnetically biasing said second pair of members in a direction parallel to the plane of said layer.

6. The combination according to claim 5 wherein said first dielectric constant and said second dielectric constant are substantially equal.

References Cited in the tile of this patent UNITED STATES PATENTS 2,820,720 l Iversen Jan. 2l, 1958 2,849,642 Goodall Aug. 26, 1958 2,900,557 Webber Aug. 18, 1959 2,909,738 'Davis et al Oct. 20, 1959 2,963,668 Ohm Dec. 6, 1960 2,980,870 Zaleski Apr. 18, 1961 FOREIGN. PATENTS 1,169,581 France Sept. l5, 1958 OTHER REFERENCES Melchor et al.: Journal of Applied Physics, January 1956, pages 72-77.

Brown et al.: Proceedings of the IRE, April 1958, pages 722-727.

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