GB2155698A - Electro-optic polarizer with continuous variability - Google Patents

Abstract

A ferroelectric polarization variation device for operation at millimeter wavelengths applicable for use as a component in radar systems. An electrode pair (11 and 12) straddling a parallel-sided body of birefractive material (7) is effective for continuously varying the electric field applied to the body of ferroelectric material (7), through which the beam of millimeter wavelength radiation (9) passes perpendicular to the optic axis thereof, whereby to continuously vary the polarization of the beam. A reflection compensating layer is provided on the input and output surfaces of the body. <IMAGE>

Description

SPECIFICATION Electro-optic polarizer with continuous variability Technical Field This invention relatesto millimeter(MM) wavelength devices employing anisotropic, nonlinear dielectric materials which exhibit electro-optic variability, and more particularly to the design and fabrication of microwave and radar components operable at millimeterwavelengths, in particular frequencies in the range of 95 Gigahertz (GHz).

Background Art Ferroeiectric materials have become well known since the discovery of Rochelle saltfortheir properties of spontaneous polarization and hysteresis. Seethe International Dictionary of Physics and Electronics, D.

Van Nostrand Company Inc., Princeton (1956). Other ferroelectrics including barium titanate have also become familiar subjects of research.

However, the application of the properties of ferroelectric materials to millimeter wavelength devices and radarsystems is largely uncharted scientific terrain.

At MM wavelengths, standard microwave practice is hampered by the small dimensions ofthe working components, such aswaveguides and resonantstructurves. Furthermore, there is a considerable lack of su itabie materials from which to make the compo- nents. Even beyond this, the manufacturing precision demanded by the small dimensions ofthe components, makes their construction difficult and expensive. Ferrite phase shifters used at other frequencies are unsuitable, and alternative materials are generally not available.

Ferroelectric materials are accordingly of particular interest, because certain of their dielectric properties change underthe influence ofan electric field. In particular, an "electro-optic" effect can be produced by the application of a suitable electric field. Furthermore, field-induced ferroelectric domain orientation and reorientation is common to these materials.

As is well known, ferroelectric materials are substances having a non-zero electric dipole moment in the absence of an applied electric field. They are frequently regarded as spontaneously polarized materials for this reason. Many of their properties are analogous to those offerromagnetic materials, although the molecular mechanism involved has been shown to be different. The regions of polarization are divided into domain structures.

Because ferroelectrics are anisotropic, radiation propagates within them as two independent modes, generally traveling at different speeds-this phenomenon is known as birefringence.

A suitably oriented birefringent medium changes the polarization of incident radiation. An electric field maychangethe birefringenceofthe medium, thereby altering the polarization change and establishing a variable polarizer. This change in birefringence is a consequence ofthe electro-optic effect, and is a continuous function of the applied field.

The polarization change can be understood as follows. Radiation in the millimeterwavelength domain divides into components upon entering a ferroelectric medium having a suitably aligned optic axis. Typically, one component exhibits polarization which is perpendicular to the optic axis (the ordinary ray), and the other component exhibits polarization which is parallel to the optic axis (the extraordinary ray). The refractive indices ofthe birefringent material, respectively n0 and ne, determine the different speeds of propagation ofthe components.The emerging components recombine with a relative phase shift equal to the birefringence, nOne, times the íength of the medium, times two pi, divided by the free space wavelength. This phase shift determines the polarization state ofthe output ray: circular, perpendicular, elliptical or otherwise.

Accordingly, it is an objectofthe invention to alter the character of propagation of millimeter wavelength radiation passing through a ferroelectric medium by applying an electric field across suitably oriented plate electrodes straddling the medium.

It is an object ofthis invention to establish a device for fast response variation of the polarization of a millimeter radar wave between a variety of distinct polarization states by electrical means.

It is an object of this invention to develop a millimeter wavelength polarizerof continuous variability for use in polarization diversity radars, signal control operation, amplitude switching and beamsplitting.

It is an object ofthe invention to develop a ferroelectric millimeterwavelength polarizerfor continuous polarization variation atthe millimeter wavelength range, which is reversiblyvariable in polarization.

It is a further object of the invention to produce a continuous state ferroelectric polarizerforuse in millimeter wavelength radar systems.

It is a further object of the invention to produce a variable millimeter wavelength ferroelectric polarizer able to generate microwave signals with a variety of different polarizations.

It is a further object ofthe instant invention to produce a variable millimeter wavelength ferroelectric polarizer effective for processing microwave signals in a radar receiver.

Disclosure of Invention The instant invention calls forthe disposition of a ferroelectric medium in the path of millimeter wavelength radiation to establish a continuously variable microwave radarpolarizer.Theferroelectric material has at least a single optical axis which is typically disposed in a vertical direction and is subject to the application of an electricfield across electrodes straddling the medium.

Two modes of the invention are the subject of invention herein. In one mode, a single pair of electrodes straddles the ferroelectric medium transversely or across the direction of wave propagation; in another mode, the electrodes straddle the medium in alignmentwith the direction of propagation.

The drawing(s) originally filed was (were) informal and the print here reproduced is taken from a later filed formal copy.

Variation polarization is established by modifying the strength of the electric field. This changes the relative propagation ofthetwo orthogonally polarized ray components and modifies the refraction characteristic ofthe material. The process is reversible and repeatable, and can be performed in eitheroftwo modes-one involving transparent electrodes in the path of wave propagation, and the other involving electrodes straddling the direction of propagation.

Brief Description of Drawing The invention will be better understood from the following description taken in conjunction with the accompanying drawing, wherein: Fig. 1 shows a body offerroelectric material with its optical axis vertically disposed and electrodes strad dlingly adjacent to its surface forapplying an electric fieldtransverselywith respect two the direction of wave propagation; and Fig. 2 shows the same body of material subject two longitudinal field application along the direction of propagation.

Best Mode for Carrying Outthe Invention Thevariablepolarizerisshown in two modes, Fig. 1 and Fig. 2 respectively. The polarizer includes in each case a block7 offerroelectric material subjectto incident polarized radiation 9. Suitable matching layers are appliedto the incidentface of block7 to prevent undue reflection ofthe incident radiation.

The direction of propagation ofthe incident radiation is indicatated by arrow "K". The incident angle of polarization is indicated to the left of block 7. The initiai specific polarization is shown as linear at45 degrees from the vertical.

The radiation in each case is characterized, for example, by a frequency of 95 GHz, which corres pongs to a millimeterwavelength of3.16. Forconvenience, block7 is shown parallelepiped in form with each of its surfaces generally parallel to the surface disposed immediately opposite of it. Other forms of geometrywould be equally effective, as long asthe opposing sides are parallel.

The variable polarizer in respective Figs. 1 and 2 each include a pairofelectrodes, respectively 11 and 12, for continuously varying the electric field "E" impressed upon the ferroelectric material by the selective application of voltage source 24. In a first mode, shown in Fig. 1 the electrodes 11 and 12 are disposed parallel to the direction of wave propagation. Each member ofthe electrode pair is suitably disposed near an opposite side of the ferroelectric block 7. On the other hand, in the second embodiment shown in Fig. 2 electrodes 11 and 12 are disposed perpendicularto the path of radiation, and both electrodes are transpa rent to passage of radiation.

In both embodiments, the optic axis ofthe medium is oriented with respect to the direction and polarization ofthe incoming radiation so thatthelatter is split into a pair of independent components. Typically, the optic axis is perpendicularto the direction of radiation propagation.

In both Fig. 1 and Fig. 2,the electrodes 11 and 12 are activated with a selected varying level ofvoltagefrom voltage source 24to modifythepolarization of the input radiation 9.

To permitthe radiation to enter and depart from block7 without undue reflection, a matching layer7' is positioned or otherwise disposed directly at the respective input and output walls of each block7 as indicated in the drawing. Such reflection is known to occurwhen radiation encounters a boundary between regions of differing refractive properties. This matching layer 7' may be applied directly to the surface of the input and output walls, and the transparent electrodes 11 and 12 of Fig. 2 are external to these layers. Accordingly, radiation passing through block 7 encounters the matching layer before entering the block and immediately after leaving it. One such layer is frequently termed a "quarter wavelength" matching layer.Simply stated, reflection from block 7 is not eliminated, but simply cancelled buy a compensating reflected waveform 90 degrees out of phase reflected from the surface ofthe matching layer. As noted, since reflection tends to occur both at the input and the output walls of block 7, a matching layer at each wall is needed.

As the Figures indicate, the input radiation ofthe preferred embodiment is polarized at 45 degrees to a vertical optical axis and thus can be converted at the outputtoother modes of polarization by applying a predetermined electric field.

The output radiation state of polarization can be linear, circularorelliptic. The length oftheferroelec- tric material through which the radiation passes should be sufficientto permit a wide range of output polarizations, but not so long that absorption losses resulting from the passage of radiation through the dielectric material become excessive.

The process offield application is reversible and repeatable simply by restoring the polarizerto the same initial voltage conditions and then repeating the field cycle.

Ferroelectric materials may have morethan a single optic axis. Accordingly, a complexvarietyofcon- tinuouschanges in polarization is possible.

Additionally, ferroelectric materials can be employed with this invention as polycrystalline mixtures, which are especially useful. In particular, granular mixtures involving an inert isotropic medium are of interestto component developers. Polycrystalline mixtures are preferred because ofthe difficulty of growing single large crystals. For example, a lowindex of refraction isotropic medium may be doped with oriented single-domain crystals of a given ferroelectric in suitable concentrations, endowing the medium with considerable electro-optic properties of the desired kind. Structured configurations could also be employed in forming a dielectric composite.

In operation,thevoltage source 24is selectively continuously modified to provide electrodes 11 and 12 with a selected range of continuous electric field strengths.

The incident radiation is linearly polarized atan angle substantially removed from the optic axis (in this case approximately 45 degrees). The specific angle is selected depending upon differential absorption losses which require compensation. The beam of radiation accordingly resolves itself into two compo nentstraveling at different speeds through the ferroelectric material. Thus, a phase shift occurs between the two components, which progressively increases with the thickness ofthe material as seen by the ray itself. The radiation ultimately departs from the material with the components reunited in an altered fashion of modified polarization, and to some extent modified magnitude. The zero-field output mode can be selected by prudently selecting the material and thickness.

After referencetotheforegoing, modifications may occurto those skilled in the art. However, it is not intended thatthe invention be limited to the specific embodiment shown. The invention is broader in scope and includes all changes andmodifications falling within the parameters ofthe claims below.

Claims (7)

1. A device for varying the polarization of a beam of millimeterwavelength radiation, comprising: a material medium having parallel input and output walls, and a pair of opposite sides, one of said pairs being horizontally and the othervertically disposed, said medium being birefractive and having a vertically disposedopticaxisperpendiculartothedirection of propagation of said beam of millimeter wavelength radiation; firstand second matching means for overcoming the tendency of radiation to produce a reflected component upon encountering a boundary between regions of differing refractive properties, said first and second matching means being respectively disposed directly atthe input and output walls of said material medium to encounter said radiation immediately before ingress and immediately after egress from said material medium;; a pair of electrodes straddlingly adjacent said material medium, said electrodes being generally planar; and selective means for providing electric power over selected continuous ranges to said pair of electrodes, wherebyan electricfieldiscapableofapplication upon said material medium and effectivefor electrooptically modifying said material medium whereby the polarization of said beam of millimeter wavelength radiation is controllable continuously.

2. The method ofvarying the polarization of a beam of millimeterwavelength radiation, comprising the steps of: directing a beam of radiation having millimeter wavelength characteristics art a material medium having parallel input and outputwalls, and a pair of opposite sides, one of said pairs of sides being horizontally and the otherverticallydisposed, said medium being birefractive and having a vertically disposed optic axis, said axis being perpendicularto the direction of propagation of said beam of millimeterwavelength radiation, said other axis being orthogonal thereto;; disposing first and second matching means for overcoming the tendency of radiation to produce a reflected component upon encountering a boundary between regions of differing refractive properties directly atihe input and outputwalls of said material medium to encounter said radiation immediately before ingress and immediately afteregressfrom said material medium; disposing a pairofelectrodesstraddlingly adjacent said material medium, said electrodes being generally planar; and applying an electric field to said material medium with said electrodesforvarying the polarization of said beam of millimeterwavelength radiation.

3. The invention of claims 1 or2,wherein said pair of electrodes is in the path of said beam of millimeter wavelength radiation.

4. The invention ofclaims 1 or2,whereinatleast one of said pairs of electrodes is transparent to said beam of millimeterwavelength radiation.

5. The invention of claims 1 or 2, wherein said electrodes are parallel to the direction of wave propagation.

6. The invention of claims 1 or2, wherein said material medium is a polycrystalline mixture.

7. Theinventionofclaims1 or2, wherein said material medium includes barium titanate.