GB2198255A - Optical switch - Google Patents

Abstract

An optical switch has a liquid crystal located between two electrodes each mounted on a prism 10. Circuit means 50, 51, 52, 53 supply switching voltages to drive the switch both from the ON to the OFF and the OFF to the ON states. A signal the frequency of which is high enough e.g. greater than 1mHz, to make the dielectric anisotropy of the liquid crystal go negative is used to drive the switch from ON to OFF. Each electrode 60 comprises a layer of Indium Tin Oxide (I.T.O.) and a layer of Aluminium, the Aluminium layer being arranged to leave a central area 61 of the I.T.O. layer exposed where reflection on transmission of the light takes place. <IMAGE>

Description

Optical Switch The present invention concerns optical switches utilising a liquid crystal interface.

A conventional liquid crystal switch of the kind using a positive dielectric liquid crystal can be switched from its "OFF" to "ON" state by the application of a low frequency voltage applied between its electrodes.

In this context low frequency means with a range of D.C.

to less than 1 kHz. Such a transition from one equilibrium state to another can occur within a few milliseconds. However, the "ON" to "OFF" transition relies on the natural relaxation and reorientation of the liquid crystal molecules. The only forces present are intermolecular and the individual molecules may take several hundred milliseconds to reach an equilibrium relaxed state. For the purpose of switching in, for example, a telecommunications environment the transition of these known switches from OFF to ON is sufficiently rapid. However, the converse action is too slow.

The present invention has for an object to provide an optical switch which can be switched from ON to OFF in a time comparable with that normally achievable when switching from OFF to ON.

Accordingly the present invention comprises an optical switch having a liquid crystal layer located between a pair of electrodes, and circuit means for applying switching voltages to the electrodes to drive the switch both into the ON state and from the ON state into the OFF state.

Preferably the circuit means comprise a pair of voltage sources, one source for supplying a D.C. or relatively low frequency current to the electrodes of the optical switch, and the other for supplying a high frequency voltage, the frequency of which is sufficiently high for the dielectric anisotropy of the liquid crystal in the optical switch to be negative.

According to a feature of the invention each electrode of the optical switch comprises a main layer of relatively thin conductive material, and a secondary layer of conductive material superimposed on the main layer, the secondary layer leaving the main layer exposed to contact with the liquid crystal over the area of optical activity of the switch.

In order that the present invention may be more readily understood an embodiment thereof will now be described by way of example and with reference to the accompanying drawings, in which Figure 1 shows an optical switch incorporating a liquid crystal interface in the OFF condition, Figure 2 shows the same switch in the ON condition, Figure 3 is an enlarged view of part of Figure 2, Figure 4 is a graph showing the relationship between dielectric anisotropy and drive frequency, Figure 5 shows the basic electrical connections in an optical switch according to the present invention, Figure 6 shows the electrode structure of the switch of Figure 5, and Figure 7 shows more detailed side and top views of the switch of Figure 5.

Referring now to the drawings, Figure 1 shows the basic design of an optical switch. This comprises two glass prisms 10, 11 in the shape of prism trapeziums whose faces are polished to minor flatness. One face of each prism is coated with a thin layer of a transparent conducting material such as Indium Tin Oxide which acts as an electrode. This layer is covered by another thin layer of a polymer. The polymer layer, in known optical switches of this type, is mechanically rubbed in the X-direction using a cloth. The rubbing operation introduces microscopic scratches in the polymer whose purpose will be explained later.

The two prisms 10, 11 are placed together with the double coated faces facing each other and are held spaced apart by a seal 12 which is about 9 microns thick.

The cavity between the faces is filled with a liquid crystal.

Such a switch is capable of switching a ray of light incident at A to either B or C.

The input ray of light is linearly polarised in the xe plane and may originate from an optical fibre attached to the side of the prism 10 by means of a focussing lens, or from a gas laser. In any case the incident ray of light A always strikes the face of the prism 10 normally.

In the present embodiment the molecules of the liquid crystal pass positive dielectric anisotropy.

Furthermore the refractive index, ng, of the glass prisms 10, 11 is chosen to be very close to the extraordinary index, n , of the liquid crystal. Thus when an electric potential is applied to the transparent electrodes of the switch the liquid crystal molecules align with the field so generated. This is the condition shown in Figure 2. The incoming ray of light A "sees" the higher refractive index, ng, of the crystal which is very close to that, ne, of the glass, and in the absence of any optical discontinuity passes out of the switch in direction B.

When the electrical potential is removed the liquid crystal molecules align parallel to the adjacent parallel surfaces of the glass prisms 10, 11 with their long axes parallel to the microscopic grooves in the polymer layer. The ray of light A now sees the lower of the two liquid crystal refractive indices, nO, and if Q, the base angle of prism 10 is chosen that Sin Q > nO/ng, then the light ray undergoes total internal reflection and passes out along C.

As mentioned, this realignment of the liquid crystal molecules occurs at a rate too slow to meet many switching requirements. A function of the switching speed is the dielectric anisotropy of the liquid crystal and this is normally quoted as that value which is observed when the liquid crystal is aligned by a constant, D.C. source. However, the value of the dielectric anisotropy is also a function of the electrical drive frequency so that its value at D.C.

is not the same as when driven by an A.C. frequency.

In general, for "positive dielectric" liquid crystals the dielectric anisotropy, 4 , decreases as the drive frequency, f, increases. In some cases such as BDH mixture Tx2A the dielectric anisotropy takes on a negative value as the drive frequency passes through a characteristic cross-over frequency. This relationship between dielectric anisotropy and drive frequency is shown in Figure 4 of the drawings in which the crossover frequency is shown at F To utilise the frequency dependence of 4 to drive an optical switch into both the OFF and ON states the embodiment being described includes the circuit shown in Figure 5.The circuit shown in the figure includes two driver sources 50, 51 respectively supplying different drive frequencies f1 and f2. The two sources 50, 51 are arranged in parallel and connected via a switch 52 to the electrodes of the optical switch 10. When it is required to drive optical switch 10 into the ON condition source 51 is switched into circuit.

This source supplies a relatively low frequency A.C.

signal of the desired voltage. The frequency can, for example, be 1 kHz.

In order to drive the optical switch 10 into the OFF state switch 52 is used to connect source 51 to the electrodes of optical switch 10. Source 51 provides a very high frequency sinusoidal signal. The frequency f2 of this signal depends on the cross-over frequency c of the liquid crystal in the optical switch but can be as high as several MHz. At this frequency the liquid crystal will have a negative L so that the molecules of the liquid crystal will be forced to align parallel to the electrodes of optical switch 10. This high frequency signal can be switched OFF after a short time, for example a few hundred milliseconds, as the liquid crystal molecules will by then be in their relaxed condition. The optical switch can then be switched from OFF to ON in the normal manner using switch 52. A switch 53 is shown for breaking the connection between the electrodes of optical switch 10 and source 51 independently of switch 52. Normally there will be a link between switches 52 and 53 to ensure that the latter is closed when the optical switch 10 is being switched ON by the signal from source 51, and that switch 53 is opened after an appropriate delay after switch 52 has been operated to drive optical switch 10 into its OFF condition.

Another feature of the optical switch according to the present invention is the structure of the electrodes by which the switching signals are applied to the switch electrodes. In order to reduce crosstalk these electrodes, which are usually fabricated from Indium Tin Oxide, should be made as thin as possible.

A suitable thickness is between 100 and 150 Á. However, a side effect of having very thin electrodes is that the sheet resistance of the electrode layers increases substantially. At D.C. or low frequency driving voltages this is unimportant since the resistance of the liquid crystal between the electrodes is still much greater so that the full voltage still appears across the electrodes.

However, at frequencies greater than 100 kHz the high capacitance of the electrode structure coupled with the high sheet resistance of the electrodes leads to a decrease in the voltage appearing across the electrodes.

In order to meet this disadvantage whilst retaining the feature of low crosstalk the electrodes of the present embodiment are fabricated in the following manner.

The electrodes are deposited on the appropriate faces of the glass prisms to the required thickness. The extent of this layer is shown at 60 in Figure 5.

The central region 61 of the switch, namely the area where the light beam is either reflected or transmitted is then masked using conventional photolithographic processes. A high conductivity layer, for example of aluminium, is then deposited over the area 60. The composite layers are immersed in a solvent which dissolves the masking material and forces the removal of the high conductivity layer from the central area 61. This may typically be 2mm x 2mm in size.

Once the electrodes have been fabricated in this manner the optical switch can be assembled and the necessary electrical connections made.

As previously mentioned the present invention is concerned with reducing crosstalk to a minimum and it has been discovered that the use of positive dielectric liquid crystals in the presently described embodiment is a factor in reducing crosstalk as they have greater optical anisotropy than negative crystals. They are also more tightly bound at the cell boundaries.

In the present embodiment the possibility of optical discontinuities in the ON state is also reduced by appreciating that the incident light in the ON state does not actually see n e the extraordinary refractive index of the liquid crystal but a slightly lower index n e given by the equation:

Thus in the present embodiment the refractive index of prisms 10, 11 is selected to satisfy this equation. Furthermore the polymer layer is chosen so that it has a refractive index similar to that of the glass prisms 10, 11. In the present embodiment the chosen material is polyvinyl carazole. The thickness of this layer is defined such that it is substantially the same as the minimum depth of groove required to align the liquid crystal molecules in the OFF state.

The thickness of the electrode layer is chosen, in the present embodiment, to be between 100 and 150 A.

Furthermore it has been discovered that when known optical switches are in the ON state there is a region of unswitched liquid crystal very close to the surfaces of the cell which finds a lower potential state by aligning with the microscopic grooves rather than with the applied electric field.

Claims (8)

1. An optical switch comprising a liquid crystal layer located between a pair of electrodes each mounted on a prism, and circuit means for applying switch voltages to the electrodes such that the switch can be driven both into the ON state and from the ON state into the OFF state.

2. An optical switch as claimed in Claim 1, wherein the liquid crystal laser has positive dielectric anisotropy, and wherein the circuit means comprises one source for supplying a D.C. or low frequency current to the switch electrodes to drive the switch into the ON state, and a second source for supplying an A.C. signal having a frequency sufficiently high for the dielectric anisotropy of the liquid crystal to be negative at that frequency, the second signal being utilised to turn the optical switch from the ON to the OFF state.

3. An optical switch as claimed in Claim 2 wherein the frequency of the second signal is greater than 1MHz.

4. An optical switch as claimed in any one of the preceding claims, wherein the electrodes each comprise a first layer of relatively low electro-conductivity material covered by a second layer of relatively high electro-conductivity, the second layer leaving exposed a portion of the first layer at the central region of the optical switch where light is either reflected or transmitted in accordance with the state of the switch.

5. An optical switch as claimed in Claim 4 wherein the first layer is of Indium Tin Oxide, and the second layer is of Aluminium.

6. An optical switch as claimed in either Claim 4 or Claim 5 0 wherein the first layer is from 100 to 150 A thick.

7. An optical switch as claimed in any one of Claims 4 to 6, o wherein the second layer is approximately 2000 A thick.

8. An optical switch substantially as hereinbefore described with reference to any one of the Figures 4 to 7 of the accompanying drawings.