GB2155695A - Discrete state millimeter wavelength ferroelectric polarizer device - Google Patents

Abstract

A ferroelectric polarization switching device for operation at millimeter wavelengths applicable for use as a component in radar systems. Electric field pulses applied to respective electrode pairs (11 and 22) straddling a block of birefractive ferroelectric material (7) switch the optic axis of the ferroelectric material by reorienting from one domain state to another, whereby different polarization states of the output radiation may be obtained. <IMAGE>

Description

SPECIFICATION Discrete state millimeter wavelength ferroelectric polarizer device Technical Field This invention relates to millimeter (MM) wavelength devices employing anisotropic, nonlinear dielectric materials which exhibit electro-optic variability, and more particularly to the design and fabricationofmicrowave and radar components operable at millimeter wavelengths, in particular frequencies in the range of 95 Gigahertz (GHz).

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

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

However, the application of the properties of ferroelectric materials to millimeterwavelength devices and radar systems is largely uncharted scientific terrain.

At MM wavelengths, standard microwave practice is hampered by the small dimensions ofthe working components,suchaswaveguidesand resonantstruc- tures. Furthermore, there is a considerable lack of suitable materials from which to make the components. Even beyond this, the manufacturing precision demanded by the small dimensions of the 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 oftheir dielectric properties change underthe influence of an 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 electricfield. They are frequent- ly regarded as spontaneously polarized materials for this reason. Many oftheir properties are analogous to those offerromagnetic materials, although the molecular mechanism involved has been showto be different. Nonetheless, the division ofthe spontaneous polarization into distinct domains is an example of a property exhibited by both ferromagnetic and ferroelectric materials.

A birefringent medium changes the polarization of passing radiation according to the manner in which the medium is orientated with respectto that radiation. lfthere is a shift in the orientation of the medium as defined by the direction of the optic axis, as would be brought on by domain reorientation in ferroelectrics, the polarization change experienced by the passing radiation will be different.

The polarization change can be understood as follows. Radiation divides into two components upon entering aferroelectric medium having asuitably aligned optic axis. One component exhibits polarization which is perpendicular to the optic axis (the ordinary ray), and the othercomponent exhibits polarization orthogonal to that ofthe first and angled or parallel to the optic axis (the extraordinary ray). The refractive indices ofthe birefringent material, respectively nO and ne, determine the different speeds of propagation. The emerging components recombine with an induced relative phase shift which is proportional to the speed differential, times the length of the medium. The pase shift determines the polarization state ofthe output ray: circular, linear, or elliptical.

The output polarization state or induced polarization change can be changed by reorientating the optic axis with respect to the radiation . This is done by applying a pulsed electric field of sufficient magnitude in the appropriate direction. The electric field acts on theferroelectricdomain structure.

Accordingly, it is an object ofthis invention to establish adeviceforfastresponseswitching ofthe polarization of a millimeter radiation between two or more distinct polarization states by electrical means.

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

It is an object of the invention to develop a ferroelectric millimeterwavelength polarizer formic- rowave radar switching application at the millimeter wavelength range, which is reversably switchable between two or more distinct polarization states by domain reorientation ofthe optic axis oftheferromagnetic material.

It is a further object of the invention to produce a discrete state ferroelectric polarizer for use in millimeterwavelength radar systems.

It is a further object of the invention to produce a millimeter wavelength ferroelectric polarizer able to generate microwave signals with differentpolariza- tions.

It its a further object of the instant invention to produce a millimeter wavelength ferroelectric polarizer effective for processing microwave signals in a radar receiver.

Disclosure of Invention The instant invention calls for the disposition of a ferroelectric medium in the path of millimeter wavelength radiation to establish a discretely switchable microwave radar polarizer. The ferroelectric material has at least a single optical axis which can be disposed in a selected one ofthree orthogonal directions by the applications of a suitablydimensioned electric pulse across electrodes straddling the medium. Each axis is subject to a single pair of electrodes, and the pairs are crossed for reorienting the axes reversibly between domain states.

Variable polarization is established by reorienting the optic axis with a strong pulsed electric field. This changes the relative propagation of the two orthogonally polarized ray components. There is no change in the birefringence, but only an abrupt change in the orientation of the optic axis. This results in a two-state orthree-state device. To make the process repeatable, two orth ree sets of electrodes are required-onefor each output polarization state. The set which reorients the optic axis parallel to the wave propagation must be radiation transparent. Field application is intermittent, ending after domain reorientation.

Brief Description of Drawing The invention will be better understood from the following description of a two-state embodiment taken in conjunction with the accompanying drawing, wherein: Fig. 1 shows an element of a uniaxial ferroelectric material with electrode straddlingly adjacent to its su rfacefor applying an electricfield to reorient its single optic axis; and Fig. 2 showsthe element of material with the optic axis reoriented.

Best Mode for Carrying Outthe Invention The switchable polarizer shown in Fig. 1 includes a block 7 offerroelectric material subject two incident polarized radiation 9. The direction of propagation of the incident radiation is indicated by arrow "K". The incident angle of polarization is indicated to the left of block 7. The initial specific polarization is shown as linearat45 degrees from the vertical.

The radiation is characterized, for example, by a frequency of 95 GHz, which corresponds to a millimeterwavelength of3.16. For convenience, block7 is shown cubical in form with each of its surfaces generally parallel to the surface disposed immediately opposite of it. Otherforms of geometry would be equallyeffective,as long astheopposing sides are parallel.

The polarizer in the embodiment shown includes two pairs of electrodes, respectively 11 and 22, for reorienting the optic axis into one of two directions, respectivelyV-Vand H-H. Each memberofa particular electrode pair is suitably disposed near an opposite side oftheferroelectric block7 in alignment with a seperate one ofthe two possible directions ofthe optic axis. Electrode pair 22 is transpa rent to the passage of radiation.

In Fig. 1, electrode pair 11 is activated with a suitably strong voltage from voltage source 12 effectivelyto dispose the optic axis in a vertical direction V-V perpendiculartothe progress of radiation. The power from the voltage source 12 is preferably pulsed.

In Fig. 2, electrode pair 22 orientsthe optic axis horizontally H-H and parallel to the incident radiation by application of a suitable electric field between the respective members ofthe electrode pair. This is accomplished by switching voltage source 12 from electrodes 11 to electrodes 22. Electrode pair 22 is shown in crossed relationship to electrode pair 11.

With the optic axis so disposed in the direction of propagation,there is zero induced polarization. This assumes, of cou rse,thatthe ferroelectric matrial is uniaxial.

As the Figures indicate, input radiation is polarized at45 degrees to the initially vertical optic axis, and the output polarization is differentfrom the input. After the axis is rotated 90 degrees, the same input polarization is unaffected, exhibiting no change since the optic axis is disposed parallel to the direction of propagation. By altering the length of the ferroelectric material through which the radiation passes,the induced polarization corresponding to vertical placement ofthe optic axis can be adjusted to selected nominal values including linear, circularorelliptic.

The duration of time required for imposing the electric field is short and requires no morethan a pulse of sufficient magnitude. Only temporary imposition of field strength is required to reorientthe optic axis to a new direction. For domain reorientation, a field pulse of 20 kV/cm, or even as low as 15 kV/cm for some ferroelectrics, may sufficient. No more than 30 or40 kV/cm is suggested in order to avoid dielectric breakdown. The process is reversible, with switching or response times in the order of milieseconds possible with typical ferroelectric materials.

In operation, the voltage source 12 is selectively switched between electrode pairs fi and t2 to flip-flop the optic axis ofthe ferroelectric material from one domain state orientation to the other. With the axis aligned with the direction of progress of the radiation, the block 7 is transparentto the transmission and no change in polarization is induced. If, however, the material optic axis is perpendicularto the direction of progress, other effects are possible, which depend upon the polarization ofthe input radiation. Ifthe incident radiation is linearly polarized at an angle deviating measurably from the optic axis, the ray will resolve itself into two components travelling at different speed through the 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 ray ultimately departs from the material with the components reunited in an altered fashion resulting in modified polarization. The output mode can be selected by prudently selecting the material and thickness. Then the electrodes can switch between transmitting the unmodified radiation and producing the radiation in altered form, as desired.

Ferroelectric materials may have morethan a single optic axis. Accordingly, a complexvarietyofdomain orientations including biaxial anisotropy is possible.

Itfollowsthat other electrode arrangements can be established which fall within the scope of this invention. To modify the previous example, the electrode pairs might be arranged to avoid crossing the path of radiation, thus avoiding the limitations of transparent electrodes. In this configuration, the rotation sense of the output polarization can be readily switched from right to left, e.g. rightcircularto left circular.

Athird set of electrodes may be arranged across the radiation path, resultng in a threstate device, e.g., right circular, left circular and no change at the output.

Other combinations are clearly possible, with more varietywhen biaxial media are used.

Additionally, ferroelectric materials can be produced as polycrystaline mixtures, which are especially useful. In particular, mixtures in an inert isotropic medium arnon interestto component developers.

Polycrystaline mixtures are preferred because ofthe difficulty of growing single large crystals. For example, a low-index of refraction isotropic medium may be randomly doped with oriented single-domain crystals of a given ferroelectric in appropriate concentrations, endowing the medium with ferroelectric-like properties ofthe desired kind. Structured composities could also be employed fortheferroelectric mixture.

After reference to the foregoing, modifications may occurto those skilled in the art. However, it is not intended that the invention be limited to the specific embodiment shown. The invention is broader in scope and includes all changes and modification falling within the parameters of the claims below.

Claims (8)

1. A device for switching between polarization states of a radiation beam of millimeter wavelength radiation, comprising: a material medium having pairs of opposite parallel sides, one of said pairs of sides comprising input and output sidesfor passing said radiation beam, said medium being birefractiveand having atleasta single optic axis, said axis being switchable in direction with respecttothe direction of propagation of said radiation beam; at leasttwo pairs of electrodes, each of said pairs straddingly adjacent said material medium, said electrodes being generally planar parallel and orthogonal to separate directions of disposition of said optic axes; and selective means for alternately providing electric power to one of said pairs of electrodes and then to a selected other pair of said electrodes, whereby an electric field is capable of establishment and application upon said material medium and effective for reorienting the optical axis of said material medium from one direction to a selected other direction.

2. The method of switching between polarization states of a beam of millimeter wavelength radiation, comprising the steps of: directing a beam of radiation having millimeter wavelength characteristics at a material medium having pairs of parallel sides, one of said pairs of opposite, parallel sides, comprising input and output sidesfor passing said radiation beam, said medium being birefractive and having at least one optic axis, said axis being switchable in direction with respect to the direction of propagation of said radiation beam; disposing at least two pairs of electrodes straddingly adjacent said material medium, each of said electrodes being orthogonal to one of said alternate directions of disposition of said optic axis; and applying an electric field to a selected one of said pairs of electrodes and thereby to said material medium of reorienting the optic axis of said material medium from one of said alternate directions to another.

3. The invention of claims 1 or 2, wherein one of said pairs of electrodes is disposed in the path of said beam of millimeterwavelength radiation.

4. The invention of claim 3, wherein said pair of electrodes in the path ofthe radiation is transparent to said beam of millimeter wavelength radiation.

5. The invention of claims 1 or 2, wherein each of said pairs of electrodes is capable of electric pulse energization,wherebyanelectricfield is established in said material medium.

6. The invention of claims 1 or 2, wherein at last two of said pairs of electrodes are arranged in crossed configuration.

7. The invention of claims 1 or 2, wherein said material medium is ferroelectric.

8. The invention of claims 7 or 2, wherein said material medium includes barium titanate.