GB2278925A

GB2278925A - Device for producing a phase shift

Info

Publication number: GB2278925A
Application number: GB9312073A
Authority: GB
Inventors: Martin Christopher Steel
Original assignee: Central Research Laboratories Ltd
Current assignee: Central Research Laboratories Ltd
Priority date: 1993-06-11
Filing date: 1993-06-11
Publication date: 1994-12-14
Anticipated expiration: 2013-06-11
Also published as: GB9312073D0; GB2278925B

Abstract

A device for producing a phase shift in a reflected beam (11) of electromagnetic radiation which may be infra-red, microwave or millimeter radiation comprises an electrically conductive body (1) having a reflective surface (2) and two dielectric sheets (3 and 4) arranged parallel to this surface. The sheets have optical thicknesses equal to odd multiples of a quarter wavelength of the incident radiation and are separated from one another by a dielectric medium (6) having an optical thickness equal to a multiple of a quarter wavelength of the incident radiation. The surface (2) and first dielectric sheet (3) are separated by a dielectric medium (7) having an optical thickness equal to a multiple of a half wavelength of the incident radiation. The optical thickness of the medium is varied. As shown the medium is air and the separation between the reflective surface 2 and the dielectric 3 is charged by piezoelectric spacers 5. Alternatively the medium is a liquid crystal whose refractive index may be charged by applying an electric field. An array of such devices can steer or scan a beam of radiation. <IMAGE>

Description

DEVICE FOR PRODUCING A PHASE SHIFT This invention relates to a device for producing a phase shift in a reflected beam of electromagnetic radiation of a given frequency comprising a body of electrically conductive material having a reflective surface, a sheet of dielectric material a side of which faces and is substantially parallel to the said reflective surface and is separated therefrom by a dielectric medium having an optical thickness of approximately nk/2 where n is an integer and X is the wavelength of radiation of the given frequency in free space, the sheet having a dielectric constant greater than that of the medium and an optical thickness of approximately kU4, where k is an odd integer, and means for varying the optical thickness of the dielectric medium.

In a known device of this type disclosed in GB 2225122 the optical thickness varying means takes the form of a layer of a liquid crystal which forms part of the dielectric medium present between the reflective surface and the dielectric sheet. The refractive index of the liquid crystal is varied by an applied electric field which thus changes the optical thickness of the dielectric medium. However, in order to obtain a phase shift approaching 360" for millimeter waves when the change in optical thickness is only of the order of a few microns, the dielectric sheet must have a very high dielectric constant - of the order of 100. Such materials are expensive, fragile and difficult to machine accurately.

An object of the present invention is to enable this disadvantage to be mitigated.

According to the invention a device as defined in the first paragraph above is characterized in that a further sheet of a dielectric material is provided substantially parallel to the first-mentioned sheet and facing the side of the first-mentioned sheet opposite to that facing the said reflective surface, the further sheet being separated from the first-mentioned sheet by a dielectric medium having an optical thickness of approximately anal4, where a is an integer, the further sheet having an optical thickness of approximately bAi4, where b is an odd integer, the dielectric constant of the lastmentioned dielectric medium being less than the dielectric constant(s) of the sheets.

The optical thickness varying means may be, for example, mechanical means such as a micrometer, or a piezoelectric spacer, or a radiation transmissive material which displays electrically controlled birefringence, such as a layer of a liquid crystal material as disclosed in GB 2225122, or a combination of such mechanical means and material.

Embodiments of the invention will now be described, by way of example only, with reference to the accompanying schematic drawings in which: Figure 1 shows a cross section through a portion of a first embodiment of the invention, and Figure 2 shows a cross section through a portion of a second embodiment of the invention.

In Figure 1 a device for producing a phase shift in a reflected beam 11 of electromagnetic radiation comprises a body 1 of a reflective electrically conductive material, in the present embodiment a highly polished copper plate imm thick with a substantially planar major surface 2, and two sheets 3 and 4 of dielectric material arranged parallel to the surface 2. The sheets are separated from the surface and from one another by dielectric media constituted by air gaps 7 and 6, and between the surface 2 and the first sheet 3 there is provided a plurality of piezoelectric spacers 5 which may be used to move the surface 2 relative to the sheet 3 using electrical impulses and thus vary the physical and hence the optical thickness of the medium 7.

The sheets 3 and 4 are made from Corning 8870 glass which has a dielectric constant of 9.5. This embodiment of the invention has been designed to shift the phase of incident electromagnetic radiation with a frequency of 90 GHz. To that end the thickness of the sheets is 0.27mm, so that the optical thickness of the sheets is approximately equal to one quarter wavelength for the given radiation in the sheets.

The distance between the surface 2 and the sheet 3 is 1.65mm, giving an optical thickness of air gap 7 approximately equal to one half wavelength of the given radiation in this dielectric medium (air). The air gap between sheet 3 and sheet 4 is 0.825mm, corresponding to an optical thickness of a quarter wavelength. In use, the dielectric sheets behave like a single sheet with a dielectric constant of 9.5 x 9.5 = 90.25.

A further sheet (not shown) may be added to the device with the same sheet thickness and dielectric constant, spaced a quarter wavelength from sheet 4 and parallel thereto. In this case the sheets will behave like a single quarter wavelength sheet with a dielectric constant of 90.25 x 9.5 = 857.375.

Although in the embodiment of figure 1 air gaps are present between the dielectric sheets 3 and 4, other media which are transmissive to electromagnetic waves may be used as an alternative, provided the dielectric constant(s) of the media is/are less than those of the sheets. One such alternative for medium 6 is glass; in such a case the components 3, 6 and 4 may be fabricated by depositing a high refractive index layer (for example lead monoxide with a dielectric constant of 25 and a thickness of 165 microns) on both sides of a glass plate with an optical thickness equal to a quarter wavelength at 90 GHz, and separated from the surface 2 by a half wavelength air gap which acts as dielectric medium 7.

The dielectric sheets employed need not have equal dielectric constants or equal thicknesses, although it is important for each sheet to have an optical thickness close to an odd multiple of a quarter wavelength for the radiation of interest.

Separate elements, each constituted by a device as described above, may be placed close to one another as an array to impart different phase shifts to enable the reflected beam 11 to be scanned or steered.

In the second embodiment shown in figure 2 the piezoelectric spacers have been replaced by a liquid crystal layer 9 arranged between two glass plates 8 and 12 disposed between the surface 2 and sheet 3. The liquid crystal layer is provided with electrodes 10 to enable an electric field to be applied across the layer and thus change the refractive index of the layer and hence the optical separation between the surface 2 and the sheet 3. The liquid crystal material used in this embodiment is type E7 supplied by BDH. The thickness of the liquid crystal layer is 200 microns, and the dielectric constant of the material may be changed from 2.5 to 3 by varying the voltage across the layer. This provides a change in optical thickness of 30.2 microns, which may result in a phase shift of approximately 315C for the given 90 0Hz radiation in combination with the "phase shift magnifying" dielectric sheets 3 and 4.

The liquid crystal may be arranged, for example, as an array of pixels, each separately controllable, neighbouring pixels being controlled to impart different phase shifts to enable the reflected beam 11 to be scanned or steered. Alternatively or in addition separate liquid crystal elements may be placed close to one another to achieve the same effect.

It is desirable that dielectric sheets and media and liquid crystals with low dielectric losses be used to maximize the reflectivity of the device.

Optical thickness varying means may in addition be placed between neighbouring dielectric sheets. This will enable the device to be controlled to operate with different radiation frequencies. The dielectric sheet thickness may also be changed to optimize performance at different frequencies.

Although in the embodiments described the thickness of the dielectric sheets has been equal to one quarter wavelength, any odd multiple of this thickness will suffice. Odd or even multiples of the thickness given for the dielectric media 6 and 7 may be used, but the thickness of 6 is preferably an odd number of quarter wavelengths. The optical thicknesses of 3, 4, 6 and 7 need not be exactly equal to integer multiples of a quarter or half wavelength for the invention to work as described above - deviations up to 0.0025S for quarter wavelength dimensions or multiples thereof or 0.01k for half wavelength dimensions or multiples thereof may still allow useful results to be obtained, the thickness tolerances being related to the amount of phase magnification desired (i.e. the effective dielectric constant of the sheets). The greater the effective dielectric constant required, the more accurate the thicknesses must be to achieve the required performance. The thickness of the sheet furthest away from the reflective surface is the most critical for good performance.

Preferably the optical thicknesses are within 0.0013k for quarter wavelength dimensions or multiples thereof or 0.005k for half wavelength dimensions or multiples thereof.

The optical thickness varying means need not lie between the surface 2 and the dielectric sheets 3 and 4. For example, the conductive body may be provided upon one side of a piezoelectric layer facing the dielectric sheets. In this case, the distance between the surface and the dielectric sheet 3 may be altered by providing electrical impulses to the piezoelectric layer.

In the above description the term "optical thickness" has been used to denote the physical thickness of a dielectric multiplied by the refractive index of the dielectric. The term "optical" is to be understood to apply not only to visible radiation but also to for example infra-red, microwave and millimeter wave radiation.

Claims

1. A device for producing a phase shift in a reflected beam of electromagnetic radiation of a given frequency comprising a body of electrically conductive material having a reflective surface, a sheet of dielectric material a side of which faces and is substantially parallel to the said reflective surface and is separated therefrom by a dielectric medium having an optical thickness of approximately nD2 where n is an integer and X is the wavelength of radiation of the given frequency in free space, the sheet having a dielectric constant greater than that of the medium and an optical thickness of approximately kh/4, where k is an odd integer, and means for varying the optical thickness of the dielectric medium, characterized in that a further sheet of a dielectric material is provided substantially parallel to the first-mentioned sheet and facing the side of the first-mentioned sheet opposite to that facing the said reflective surface, the further sheet being separated from the first-mentioned sheet by a dielectric medium having an optical thickness of approximately aS4, where a is an integer, the further sheet having an optical thickness of approximately bS4, where b is an odd integer, the dielectric constant of the last-mentioned dielectric medium being less than the dielectric constant(s) of the sheets.

2. A device as claimed in claim 1 wherein the optical thickness varying means comprises a layer of a liquid crystal material present between the said surface and the first-mentioned sheet and extending substantially parallel to the said surface and conductors for applying a potential difference to the liquid crystal for changing the refractive index of the liquid crystal and thus the optical thickness of the firstmentioned dielectric medium.

3. A device substantially as herein described with reference to figure 1 or figure 2 of the accompanying drawings.

4. An apparatus for scanning a beam of reflected electromagnetic radiation comprising an array of devices, each device being as claimed in any one of claims 13.