GB2156164A - Variable millimeter wave birefringent filter - Google Patents

Abstract

A ferroelectric narrowband filtering device in several stages for operation at millimeter wavelengths applicable for use as a component in radar systems. Each stage comprises a block of birefractive ferroelectric material (7) having an optic axis orthogonal to the direction of wave propagation. and straddled by a pair of electrodes, the thicknesses of the blocks being integral multiples of a reference value. Electrode pairs (11,22, and 33) apply a continuously variable electric field to each of said stages parallel to the optic axis of the ferroelectric material (7). Pairs of linear polarizers (8, 11'', 22'', and 33'') straddle each stage and are perpendicular to the direction of wave propagation. By this means the frequency band of the transmitted beam can be shifted. <IMAGE>

Description

SPECIFICATION Variable birefringentfilter Technical Field This invention relates to millimeter (MM) wavelength devices employing anisotropic, nonlinear dielectric materials which exhibit electro-optic variability, and more particularlytothedesign and fabrication of microwave 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 become familiar subjects of research.

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

At MM wavelengths, standard microwave practice is hampered by the small dimensions ofthe working components, such as waveguides and resonant structurves. 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 ofthe components, makestheirconstruction difficult and expensive. Ferrite phase shifters used at otherfrequencies 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 electricfield. 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. Nonetheless, the division ofthe spontaneous polarization into distinct domains is an example of a property exhibited by both ferromagnetic and ferroelectric materials.

A suitably oriented birefringent medium changes the polarization of passing radiation. An electricfield may change the birefringence ofthe medium, thereby altering the polarization change and establishing a variable polarizer. This change in birefringence is considered an electro-optic effect, but a similar effect can result from a shift in the direction of the optic axis, as in the case of domain reorientation in ferroelectrics.

The polarization change due to the birefringence can be understood as follows. Radiation in the millimeterwavelength domain divides into components upon incidence with a ferroelectric medium having a suitably aligned optic axis. One component exhibits polarization which isperpendiculartothe optic axis (the ordinary ray), and the other component exhibits polarization orthogonal to that of the first and angled or parallel to the optic axis (the extraordinary ray). The refractive indices of the birefringent material, respectively m0 and ne, determine the different speeds of propagation. The emerging components recombinewith an induced relative phaseshiftwhich is proportional to the speed differential, times the length ofthe medium.The phase shift determines the polarization state ofthe output ray: circular, linear, elliptical or otherwise.

The output polarization state or induced polarization change can be changed by electro-optically varying the birefringence of the medium. This is done by applying a sustained electric field of sufficient magnitude in the appropriate direction. The electric field changes the refractive indices, nO and ne by different amounts.

Accordingly, it is an object of this invention to establish a device for narrowband filtering of millimeter radiation with electronically variable center frequency.

It is an object of this invention to develop a millimeterwavelength narrowband filtering device for use in radar systems for signal control operation.

It is an object of the invention to develop an electrically controlled ferroelectric millimeter wavelength narrowband filter for microwave radar application at the millimeter wavelength range, which is reversibly variable with respect to center frequency along a continuous range of frequencies.

It is a further object of the invention to produce a narrowband fiiter for use in millimeter wavelength radar systems.

It is a further object of the invention to produce a millimeterwavelength ferroelectric narrowband filter able to process microwave signals along a continuous predetermined frequency range.

It is a further object of the instant invention to produce a millimeterwavelength ferroelectric narrowband filter effective for processing microwave signals in a radar receiver.

Disclosure of Invention The instant invention callsforthestage-wise disposition of a ferroelectric media in the path of millimeter wavelength radiation to establish a continuouslyvariable microwave radar narrowband filter.

The ferroelectric material in each stage has an optical axis which can be disposed orthogonally to the direction of wave propagation and is subject to coordinated electric field application along predetermined directions by means of electrodes straddling each stage.

Variable fields are established by applying different selected voltages from a source a long leads to the respective electrodes. This changes the refractive properties ofthe ferroelectric media. The process repeatable and the center frequency of the device is precisely settable by adjusting the voltage level applied to the electrodes.

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 several stages of the ferroelectric filter with electrodes straddl ing ly adjacent to th eir respec- tive surfaces, each stage subject to at least a single shared polarizing element; and Fig. 2 shows the frequency distribution characteristics ofthe filter, stage-wise and in to to.

Best Mode for Carrying Out the Invention Thefilter arrangement shown in Fig. 1 includes staged blocks or elements 7 of ferroelectric material subjectto incident polarized radiation 9. The direction of propagation ofthe incident radiation is indicated by arrow "K". The mode of polarization is determined by input polarizer 8. Layers of a suitable well known matching material are preferably applied on the input and output sides of each block of material to minimize the effects of reflection.

The radiation is characterized, for example, by a frequency of 95 GHz, which corresponds to a millimeterwavelength of 3.16. For convenience, blocks 7 are shown parallelepiped in form with each of its surfaces generally parallel to the surface disposed immediately opposite of it. Otherforms of geometry would be euaIly effective, as long as the opposing sides are parallel.

The device includes pairs of electrodes, respectively 11,22 and 33, shown orthogonal to the optic axes of the several blocks 7 but could be orientated differently according to the material used. Each member of a particular electrode pair is suitably disposed straddlingly near an opposite side of a ferroelectric block 7, each pair of electrodes associated with a particular stage being orthogonal to a separate one of said optic axes, each optic axis of course being associated with a different one of said stages.

In Fig. 1, respective electrode pairs 11,22, and 33 are concurrently activated with a selected voltage level from voltage control source 12 effectively to provide the particularcenterfrequency shift desired.

Each stage can be treated as a separate subfilter, respectively 11', 22', and 33', each including an aligned polarizer 11", 22", and 33", immediately rearwardly disposed to it The polarizers are arranged perpendicularlyalongthedirection of radiation propagation and are oriented to polarize the radiation along a similar or aligned axis. The respective polarizers are linear, for purposes ofthe instant embodiment.

The magnitude of impressed eíectricfield may rise to a level on the orderof 10 kV/cm. No morethan 30 or 40 kV/cm is suggested in orderto avoid dielectric breakdown.

Other electrode arrangements can be established which fall within the scope ofthis invention. For example, the electrode pairs might be arranged vertically, ratherthan horizontally. In that case, however, the respective optic axes ofthe various stages would be parallel to the planes of electrodes generating the applied field. Additionally, the elec- trode pairs might be situated at some other angle as would suit a particular material.

As is clearfrom Fig. 1, the block elements 7 vary in thickness, each being an integermultipleofa preselected minimum thickness value, selected to produce a given general bandwidth characteristic.

As shown in Fig. 2, fora particular combination thickness element of birefringent material subject to input radiation in the millimeter range, thickness related output frequency characteristics, both individual and combined, can be established as shown in Fig. 2. A relatively thin element7 displays the broadest bandwidth of frequency characteristic, as shown in portion "a" of Fig. 2. Portion "a" indicates morethan one bandwidth, for the frequency range selected in Fig. 2., and either bandwidth can be employed or selected as the bandwidth of interest.

The effect of a double thickness of ferroelectric material for application of beam is shown in portion "b" of 2. These frequency bands are one-halfas wide as those of the thinner reference material and accordingly several of their peaks and troughs coincide, with several ofthe peaks and troughs ofthe thinner material. Accordingly, only the passage of a smaller or restricted portion of the radiation is enabled. Fig. 2 confirms this and indicates a narrowing in the skirt of passage. Clearly, the outerfrequencies of radiation extending furthest from the center frequency are impeded by the thicker material.

Portion "c" of Fig. 2 extends this principlefurther, showing the output characteristic of frequencies passed through the thickest ofthe three ferroelectric elements, which is four times as thick as the narrowest element.The Figure showsthe aíignment of peaks and troughs once again, with the same effect as before: a further narrowing ofthe bandwidth passed.

The combined output characteristic ofthe several stages is shown in portion "d" of Fig. 2. The center frequencies ofthe bands passed in portion "a" arethe same. However, the width of passage, i.e. the bandwidth passed, is relatively abbreviated. By eliminating one ofthe stages, the overall bandwidth can be broadened; by adding further stages of integer multiple thickness,the bandwidth can be sharpened even more.

Still, whateverthe composite or resultant bandwidth finally selected, it is a fixed quantity dependent upon the ferroelectic elements 7 selected and their particular physical widths.

What is particularly significant about the arrangement beyond that already indicated isthatthe center frequency ofthe passage bandwidths can be selec tively, controllably altered, modified or shifted in proportion to a transverse electric field established by applying a voltage difference on the electrodes straddling the elements 7.

In a preferred embodiment of the invention, a referencewidthforthenarrowestelement7 is selected and each other element7 is twice the width of some other element 19. The elements are shown in ascending sequence of width, but this is not essential to the invention. The elements, once established in width, may be randomly distributed along the path of radiation.

This shift in centerfrequency is considered to be the result of a field-induced change in the birefringence of the ferroelectric material. The ferroelectric material exhibits uniform domain orientation coincident with the optic axis, permitting a continuous increase or decrease of the center frequencies of the passed bandwidths, according to the polarity of the applied field.

The ferroelectric materials employed can be produced as polycrystalline mixtures, which are especially useful. In particular, mixtures in an inert isotropic medium are of interestto component developers.

Polycrystalline mixtures are preferred because of the difficulty of growing single large crystals. For exam ple, 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 considerable electro-optic properties of the desired kind. Structured composites could also be employed for the ferroelectric material.

In another embodiment, the individual elements may be constructed from single cystals of ferroelectric material, or more conveniently, they can be made from dielectric mixtures. The mixtures may be random, with individual ferroelectric crystals dispersed throughoutan inert dielectric filler, orthey may consist of structured elements, such as planarsheets of polycrystalline ferroelectric alternating with sheets ofthefiller.

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 ofthe claims below.

Claims (6)

1. A device for narrowband filtering a beam of millimeter wavelength radiation, comprising: a plurality of stages of a material medium each having parallel inputand outputwalls,and a pairof opposite sides, one of said pairs being horizontally and the other vertically disposed, each said medium being birefractive and having an optic axis disposed orthogonallyto the direction of wave propagation, the thickness ofsaid material media in the direction of propagation being integer multiples of a selected reference value; a plurality of pairs of linear polarizers, one pair of said polarizers associated straddingly with each medium and perpendicularto the direction of propagation;; each of said media provided with a pair of electrodes, each of said pairs straddingly adjacent a respective one of said material medium, said electrodes being orthogonal to a corresponding one of said optic axes; and electric means four providing electric powerjointly to said pairs of electrodes, whereby a continuously variable and controllable electric field is capable of establishment with respect two each of said media for modifying said media and shifting the centerfrequen- cy of said beam of transmitted millimeter wavelength radiation passing through said plurality of material media.

2. The method of narrowband filtering a beam of millimeter wavelength radiation, comprising the steps of: directing a beam of radiation having millimeter wavelength characteristics through a plurality of stages of a material medium having parallel input and outputwalls, and a pair of opposite sides, one of said pairs being horizontally and the other vertically disposed, each said media being birefractive and having an optic axis disposed perpendicularly to the direction of propagation of said beam of millimeter wavelength radiation, the thicknesses of said material media in the direction of wave propagation being integer multiples of a selected reference value;; disposing for each said stage at least a single pair of electrodes straddingly adjacent each said material medium, each of said electrodes being orthogonal to a corresponding one of said optic axes; disposing aligned linear polarizers straddingly around each medium in the direction of wave propagation; and applying an electricfieldjointly between each of said pairs of electrodes and thereby applying said field strength to said respective material media for continuously modifying the refractive properties of said media in proportion to said field strength, whereby the bandwidth spectra of said beam of millimeter wavelength radiation is continuously shifted.

3. The invention of claims 1 or 2, wherein the thickness of said material media are geometric multiples of said reference value.

4. The invention of claims 1 or 2, wherein said material media are arranged in sequence of ascending thickness.

5. The invention of claims 1 or 2, wherein said material medium isferroelectric.

6. The invention of claims 1 or 2, wherien said material medium includes barium titanate.