GB2200106A - Transparent materials capable of exhibiting birefringence - Google Patents

Abstract

A transparent material capable of exhibiting birefringence comprises a transparent polycrystalline form of a compound having the general formula: Pb (Mg 1/3 R 2/3)1-xTix03 where: R represents Niobium or Tantalum; and x is a molar fraction between 0.05 and 0.25 inclusive. The transparent material may contain impurities and may be incorporated into an electro-optic device, the material being located in the optical path 2 of the device between a polariser 3 and an analyser 4. The transparent material may have a pair of electrodes 7, 8 disposed on opposite surfaces thereby to enable an electric field to be established through the material and across the optical path 2. <IMAGE>

Description

TRANSPARENT MATERIALS CAPABLE OF EXHIBITING BIREFRINGENCE This invention relates to transparent materials which are capable of exhibiting birefringence, and in particular, the use of such materials in electro-optic or acousto-optic devices.

The refractive index of a transparent material which is capable of exhibiting birefringence can be varied by subjecting the material to an electric field or acoustic waves. Such transparent materials are commonly used in fibre optic communications apparatus and laser modulators where the intensity of light passing through the material requires to be modulated in accordance with an electric field to which the material is subjected.

Known transparent materials of this type have the disadvantage that in a given electric field or when impinged on by an acoustic wave of given intensity they exhibit relatively weak electro-optic and acousto-optic effects, especially at high temperatures, that is, they exhibit a relatively low level of birefringence. Further, such known materials require a relatively high drive voltage.

It is an aim of the present invention to provide a transparent material capable of exhibiting relatively high birefringence so that relatively strong electro-optic and acousto-optic effects are produced for a given electric field or impinging acoustic wave; requires less power and a smaller drive voltage than known materials; and is relatively cheap to produce.

According to the present invention there is provided a transparent material capable of exhibiting birefringence, the material comprising a transparent polycrystalline form of a compound having the general formula:

where: R represents Niobium, Tantalum or any other suitable element; and x is a molar fraction between 0.05 and 0.25 inclusive.

The transparent material according to the present invention may contain impurities according to the general formula:

where: I represents, for example, Bismuth or Lanthanum; R represents Niobium, Tantalum or any other suitable element; x is a molar fraction between 0.05 and 0.25 inclusive; and Z is a molar fraction between 0 and 0.10 inclusive.

An electro-optic device may comprise a polariser and an analyser having a component of transparent material according to the present invention located in the optical path between the polariser and the analyser, the component of transparent material having a pair of electrodes disposed on opposite surfaces thereby to enable an electric field to be established through the material and across the optical path.

The material according to the present invention may be in the form of a plate with its major faces lying in planes substantially perpendicular to the optical path. The major faces are preferably polished and the electrodes may be in the form of a pair of interdigitated electrodes deposited on one or both of the major faces. Alternatively, the interdigitated electrodes may be deposited beneath the surfaces of the respective major faces.

The material according to the present invention may be in the form of a thin layer (for example, 0.2 to 5.0 microns thick) on a transparent plate like substrate. In this case, the optical path may pass through the major faces of the thin layer and the interdigitated electrodes may be located on the major surface remote from the layer/substrate boundary. Alternatively, the optical path may extend through the thin layer so that it lies parallel to the major faces of the layer. In this case, the pair of electrodes are in the form of a pair of stripe electrodes which are substantially parallel to the optical path and located on the major surface of the thin layer remote from the layer/ substrate boundary.

The polariser may be polarised in a direction perpendicular to or parallel to the polarisation direction of the analyser.

Electro-optic and acousto-optic devices which incorporate the transparent material according to the present invention, have the advantage that a smaller electric field is required for a given electro-optic effect or a smaller acoustic wave is required for a given acousto-optic effect than for conventional transparent materials which exhibit birefringence. Consequently, such devices require less power and a smaller drive voltage.

The invention will now be described by way of example with reference to the accompanying drawings, in which: Figure la shows an electro-optic modulator embodying the present invention; Figures lb and lc, respectively, show alternative polarisation directions of a polariser and an analyser in the embodiment of Figure la; Figure 2 is a graph showing the variation of quadratic electro-optic coefficient as a function of temperature in the transparent material according to the present invention; Figure 3 shows an electro-optic modulator according to a second embodiment of the-present invention; Figure 4 shows an arrangement of interdigitated electrodes on opposite surfaces of a plate of the transparent material for use in the embodiment shown in Figure 3;; Figure 5 shows an alternative arrangement of interdigitated electrodes on the surface of the plate of the transparent material; Figure 6 shows the transparent material in the form of a thin layer disposed on a transparent substrate; and, Figure 7 shows the arrangement of Figure 6 in which the optical path traverses the layer of the transparent material in a direction parallel to the major surfaces of the material.

A transparent material capable of exhibiting birefringence in accordance with the present invention comprises a transparent polycrystalline form of a compound having the general formula:

where: R represents Niobium or Tantalum; and x is a molar fraction between 0.05 and 0.25 inclusive.This transparent polycrystalline compound, hereinafter referred to as lead magnesium niobate-lead titanate (PMNT), can be manufactured in the following way; 1) Magnesium oxide (Mg 0), niobium oxide (Nb2 05) and titanium oxide (Ti 02) in molar proportions in accordance with the chemical formula (1) are mixed together in a ball mill using, for example, zirconia elements; 2) The mixed oxides are calcined by drying and heating at a temperature of between 900 to 11000C for substantially 2 hours; 3) The calcined oxides are mixed with lead oxide (PbO) in molar quantities in a proportion defined by the chemical formula (1) (with a 2% by weight excess of lead oxide) in a ball mill using, for example, zirconia elements; 4) The mixed calcined oxides are calcined by heating at a temperature of between 800 and 9000C and are subsequently milled using, for example, zirconia elements;; 5) The milled oxides are hot-pressed at a temperature of between 1,150 and 1,2500C and at a pressure of between 12 and 35 MPa (Mega Pascals).

An alternative method of manufacturing PMNT comprises weighing out molar proportions of magnesium, lead, niobium and titanium alkoxides according to the chemical formula (1), and dissolving them in alcohol, then adding water to precipitate the metal ions as hydroxides, and then hot-pressing the precipitated hydroxides in order to form the PMNT.

The PMNT which has been made in either of the above two ways can be sliced and polished into a variety of shapes for use in electro-optic or electro-acoustic devices.

The PMNT may contain impurities according to the general formula;

where: R represents Niobium or Tantalum; I represents for example Bismuth or Lanthanum: x is a molar fraction between 0.05 and 0.25 inclusive; and Z is a molar fraction between 0 and 0.10 inclusive.

Addition of such impurities improves the electro-optic and acousto-optic properties of the PMNT.

Referring firstly to Figure 1, there is shown an electrooptic modulator comprising a block 1 of PMNT located in an optical path 2 between a polariser 3 and an analyser 4. The block 1 has a length L, a width W, and a thickness t, and a pair of polished faces 5 and 6 which permit the passage of light through the block 1. One pair of opposite faces are coated with a metal (for example, aluminium, nichrome or chromium/gold) to form a pair of electrodes 7 and 8. A voltage may be applied across the electrodes 7 and 8 via wires A and B.

This causes an electric field to be applied through the block 1 and across the optical path 2, which results in the PMNT exhibiting birefringence. This in turn enables light travelling through the block 1 to be modulated in accordance with the potential difference applied to the electrodes 7 and 8. Light passing into the electro-optic modulator first passes through the polariser 3, then through the block 1, and finally through the analyser 4. The polarisation directions of the polariser 3 and the analyser 4 can be either at right angles with respect to one another as illustrated in Figure lb or opposite (1800) with respect to one another as illustrated in Figure lc.

In the case where the polarisation direction of the polariser 3 is at right angles to the polarisation direction of the analyser 4, the intensity I of the light passing through the modulator is given by the equation:

where Io equals incident light intensity and

where: n = refractive index of the PMNT; X = wavelength of the light entering the polariser 3; V = the voltage applied to the electrodes 7 and 8; and (R11 - R12) = Quadratic electro-optic coefficient, the significance of which will be explained below.

If the polarisation directions of the polariser 3 and the analyser 4 are opposite (1800) with respect to one another, then the intensity I of the light emerging from the electro- optic modulator is given by the equation:

The PMNT is an optically isotropic material, that is to say, its refractive index is independent of the direction of propagation or polarisation of light travelling through the material. In this material, application of an electric field across the material gives rise to a birefringence which is proportional to the square of the field. This is known as the quadratic electro-optic effect.

In the case where a material possesses an intrinsic birefringence (either uni-axial or bi-axial) the application -of an electric field will give rise to a change in the value of the birefringence, the magnitude of which is proportional to the magnitude of the electric field. This is known as the linear electro-optic effect.

The degree of birefringence exhibited by PMNT can be represented by a quadratic electro-optic coefficient, denoted by (R11 - R12). This coefficient is a measure of the magnitude of birefringence exhibited by the PMNT for a given electric field. Figure 2 shows the value of the quadratic electro-optic coefficient as a function of temperature for the PMNT.

Referring now to Figure 3, there is shown an electrooptic modulator in which the PMNT is in the form of a plate 11. The plate 11 has a pair of major faces 12 and 13 which are disposed in planes perpendicular to the optical path 2 and situated between the polariser 3 and the analyser 4. The major faces 12 and 13 are polished and a pair of interdigitated electrodes are shown as being disposed on the face 12. However, the interdigitated electrodes may alternatively be deposited on the other face or on both of the faces, respectively. In this embodiment, the polarisation directions of the polariser 3 and the analyser 4 may be at right angles or opposite with respect to one another as described with reference to Figures lb and lc. However, both polarisation directions must be at 450 to the direction of the electric field in the plate 11.The intensity of light passing through the plate 11, along the optical path 2, can be controlled by the application of a potential difference to the electrodes via wires A and B.

Two alternative arrangements of the interdigitated electrodes on the plate 11 will now be described.

Figure 4 shows a cross-sectional view of the plate 11.

In this case, a pair of interdigitated electrodes are located on both of the faces 12 and 13 of the plate 11. The electrodes, which may be made of, for example, aluminium, nichrome or chromium/gold can be deposited on the faces 12 and 13 using standard photo-lithographic techniques.

In Figure 5 the electrodes are buried beneath the surfaces 12 and 13 of the plate 11. In this case, grooves are cut into the surface 12 and 13 of the plate 11 using a semiconductor saw. The surfaces are then cleaned, and the saw cuts are plated using an electroless plating process. The electrodes may be electroless nickel, followed by either electroless copper or electro-plated gold.

For a plate having interdigitated electrodes of either of the two above-mentioned arrangements shown in Figures 4 and 5, the intensity of light passing through the plate 11 is given by equations (2), (3) and (4) for the polariser 3, the analyser 4 being orientated with respect to the polariser 3 as described with reference to Figures la and lb.

Figure 6 shows a plate comprising a thin layer 14 of the PMNT deposited on a transparent substrate 15. In this case, interdigitated electrodes (not shown), which may be arranged as shown on the plate 11 of Figure 3, can be located on the exposed surface of the thin layer 14 of birefringent material and the resulting structure can be placed between the polariser 3 and the analyser 4, as shown in Figure 3, to form part of a modulator. In this arrangement, the direction of incident polarised light is substantially perpendicular to the major face of the thin layer 14 in the direction of the arrow shown in Figure 6. The thickness of the layer may be between about 0.2 and 5 microns.

Alternatively, a pair of strip electrodes 16 and 17 can be disposed on the exposed surface of the thin layer 14 as shown in Figure 7. In this case, light to be modulated can be transmitted through the thin layer 14 as shown in Figure 7.

The thin film 14 of the PMNT can be deposited on to the surface of a transparent substrate in, for example, any of the following ways: 1) By sputtering using an RF diode, RF triode or RF magnetron; in which a block of PMNT is used as a target in a sputtering system; 2) By taking lead, magnesium, niobum and titanium alkoxides and passing them, in a warm inert gas (for example, Nitrogen or Argon), over a substrate heated to a temperature of between 500 and 10000C where they decompose to deposit a mixture of oxides which form the thin PMNT layer; or 3) By using a so-called sol-gel process in which the lead, magnesium, niobium and titanium alkoxides are first taken into solution, then precipitated as a thin layer on the desired substrate, and then sintered by a sintering process which converts the precipitated thin layer into the PMNT.

The transparent substrate 15 may be made from sapphire, spinel, garnets (for example Yttrium aluminium garnet, Gadolinium, Gallium garnet), or fused quartz.

Claims (12)

1. A transparent material capable of exhibiting birefringence, the material comprising a transparent polycrystalline form of a compound having the general formula:

where: R represents Niobium or Tantalum; and x is a molar fraction between 0.05 and 0.25 inclusive.

2. A transparent material according to claim 1, wherein the transparent material contains impurities according to the general formula:

where: I represents, Bismuth or Lanthanum; R represents Niobium, or Tantalum; x is a molar fraction between 0.05 and 0.25 inclusive; and z is a molar fraction between 0 and 0.10 inclusive.

3. An electro-optic device comprising a polariser and an analyser having a component of transparent material according to claim 1 or claim 2 located in the optical path between the polariser and the -analyser, the component of transparent material having a pair of electrodes disposed thereon thereby to enable an electric field to be established through the material and across the optical path.

4. An electro-optic device according to claim 3, wherein the component of transparent material is the form of a plate with its major faces lying in planes substantially perpendicular to the optical path, and the pair of electrodes are in the form of interdigitated electrodes deposited on one or both of the major faces.

5. An electro-optic device according to claim 3 or claim 4, wherein the electrodes are deposited beneath the surfaces of the major faces.

6. An electro-optic device according to any one of claims 3 to 5 wherein the component of transparent material is in the form of a layer on a transparent substrate.

7. An electro-optic device according to claim 6 wherein the optical path passes through the major faces of the layer and the interdigitated electrodes are located on the major surface remote from the layer/substrate boundary.

8. An electro-optic device according to claim 6, wherein th optical path extends through the layer so that it lies substantially parallel to the major faces of the layer.

9. An electro-optic device according to claim 8, wherein the pair of electrodes are in the form of a pair of stripe electrodes which are substantially parallel to the optical path and located on the major surface of the layer remote from the layer/substrate boundary.

10. An electro-optic device according to any one of claims 3 to 9, wherein the polariser is polarised in a direction perpendicular to or parallel to the polarisation direction of the analyser.

11. A transparent material capable of exhibiting birefringence substantially as hereinbefore described.

12. An electro-optic device substantially as hereinbefore described with reference to figures la and 4 to 7 or figures 3 to 7.