GB1598334A - Switch arrangement - Google Patents

Description

(54) SWITCH ARRANGEMENT (71) We, STANDARD TELE PHONES AND CABLES LIMITED, a British Company, of 190 Strand, London, WC2, England, do hereby declare the invention, for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement: This invention relates to fibre optic switch arrangements and in particular to an electromechanical switch arrangement in which the operating force is provided by an electric field.

In many optical fibre communication or instrumentation systems it is necessary to provide routing switches to interconnect various fibres of the system. A number of optical switches have previously been described, but in general these suffer from the disadvantages of poor transmission when "on" and/or inadequate isolation when "off".

Furthermore, such devices do not readily lend themselves to applications in which simple multimode fibres are employed.

The object of the invention is to minimise or to overcome these disadvantages.

According to the invention there is provided a fibre optic switch arrangement, including an input fibre, an output fibre, and a pair of parallel electrodes between which the fibres are mounted, in which the input fibre is provided with a coating of a conductive material, and in which the fibres are so arranged that, when an electrical potential difference is established between the electrodes and between one or both of the electrodes and the conductive coating of the input fibre the electrostatic field gradient so produced causes the input fibre to bend thereby coupling that fibre to or decoupling the fibre from the output fibre.

In order to avoid electrical breakdown between the conductive coating of the fibre and the electrode of either the metallised fibre, or preferably, the electrodes, are coated with a thin insulating layer. Each electrode may be suitably profiled so that when the input (output) fibre is deflected and held against it there is very precise alignment between the input and output fibres.

Electrostatic forces are generally considered negligbly small except when very large voltages are used. This is not true, however, when small dimensions are involved. Typical present-day multimode optical fibres have diameters around 100 pm, but there is no reason in principle why this diameter cannot be reduced considerably. Relatively small voltages are required to make such devices operate, e.g. in the range 35-100 V for 100 pm overall diameter fibres. The voltage requirement rapidly reduces as the fibre diameter reduces.

An embodiment of the invention will now be described with reference to the drawing accompanying the Provisional Specification in which: Fig. 1 shows a schematic view of an electrostatic fibre optic switch; Fig. 2 shows a transverse section of the switch of Fig. 1; and Fig. 3 shows the relationship between the operating voltage and the fibre diameter of the switch of Figs. 1 and 2.

Referring to Figs. 1 and 2, the switch arrangement includes a pair of elongated parallel electrodes 11, the inwardlv facing surfaces of which are provided each with a longitudinal V-groove 12. A first metallised input (output) optical fibre 13 is mounted on a conductive support 14, which support also provides electrical contact to the fibre, with its free end extending between the parallel to the electrodes 11. To prevent shorting of the metallisation to either of the electrodes 11, the electrodes are covered with a thin insulating layer.

If the electrodes are made of aluminium the insulating layer may conveniently be provided by anodisation.

The output (input) fibres 15 of the switch arrangement are of the same diameter and type as the input (output) fibre 13 and are e.g. e.g. with an adhesive, one in the respective V-groove 12 of each electrode 11.

In most applications the optical fibres may be of the clad multimode type, although in some applications monomode fibres may be employed. The fibres may be of silica, glass or a plastics material.

The metallised fibre 13 and the electrodes 11 are coupled to a supply B1 via a switch S1. By the application of a potential difference between the fibre 13 and one or other of the electrodes 11 causes electrostatic attraction of the fibre to that electrode so as to couple the fibre 13 to the respective output (input) fibre 15. Because the electrostatic force increases rapidly as the distance separating the fibres from the electrode 11 reduces, above a certain applied voltage the fibre deflects and is held tightly into the Vgroove 12. This ensures good alignment between the input and output fibres.

The force required to deflect a fibre is calculated approximately from simple structural formulae. Thus, for a round fibre of diameter d and length l, the deflection x at the end for a uniformly distributed force W is given by: SWIS x = E7rdi where E is Young's Modulus = 7.31 X 1011 dynes/cm2 for silica. The deflection increases as the length ls and with the inverse fourth power of the diameter for constant applied force. There is obviously great advantage in making the length of unsupported fibre as great as possible and its diameter as small as possible so as to reduce the operating voltage, but the effect of gravity provides a limitation. If the switch is to be substantially insensitive to the orientation of its mounting, then gravity must deflect the fibre by a negligible amount If, for example, a deflect tion by gravity of d/5 is the maximum that could be tolerated, the maximum unsupported length of fibre is given by:

where p is the density of silica = 2.2 gm/S.

g = gravitational constant = 981 cm/s2.

Thus for a 100 ,,am silica fibre the maximum unsupported length is 2.41 cm, the length reducing as slowly as the diameter decreases.

The electrostatic force on the metallized fibre depends on the applied potential and on the distance between the electrode and the fibre 13. If the two output (input) fibres 15 lie side-by-side touching one another the total required input (output) fibre displacement is equal to the fibre diameter d. To obtain sufficient isolation between the output (input) fibres 15 in this situation it is preferred to coat the fibres 15 with an absorbing or reflecting metal film. Preferably, however, the output fibres separated e.g. by their own diameter, as illustrated in Figs. 1 and 2. The displacement A of the fibre 13 from its rest position must then be at least A = d with such a separation of the fibres 15.

The electrostatic force acting on the fibre in its neutral unswitched position is given approximately by treating the fibre and electrode as a parallel plate capacitor and estimating its capacity.

ld C = - e.s.u. (in free space) 4wA The force between the fibre 13 and the resepective electrode is then given by: V ldV2 W = charge X field = CV X -- -- A 4era' The force on the fibre varies as the inverse square of the distance of the fibre from the electrode. The minimum voltage required to switch the fibre from its neutral position to contact with the electrode is that voltage that exerts just sufficient force on the fibre in its neutral position to deflect the end of the fibre a distance A. This voltage is given by:

From the foregoing it can be shown that the minimum operating voltage for a substantially gravitationally insensitive switch is given by:

This expression is dependent only on the distance over which the fibre is deflected because of the way in which the length of the fibre has been defined. A table of operating voltage is given, based on this expression for various fibre displacements. The relationship between operating voltage and fibre diameter for a switch in which the output fibres are spaced by their character is also shown in Fig. 3.

Fibre Displacement Vmin Fibre Diameter A llm Volts d llm 100 97 200, or less 50 34 100, or less 25 12.2 50, or less 12.5 4.4 25, or less 6.25 1.6 12, or less

These voltages are surprisingly small, particularly as the fiber diameter decreases. The switch has a minute power requirement because it is capacitive, and could therefore be controlled by very low power consumption circuits, making telecommunication exchange routing networks feasible with very low power dissipation. Thus an optical telecommunication exchange may be constructed with a plurality of such switches.

WHAT WE CLAIM IS: 1. A fibre optic switch arrangement, including an input fibre, an output fibre, and a pair of parallel electrodes between which the fibres are mounted, in which the input fibre is provided with a coating of a conductive material, and in which the fibres are so arranged that, when an electrical potential difference is established between the electrodes and between one or both of the electrodes and the conductive coating of the input fibre the electrostatic field gradient so produced causes the input fibre to bend thereby coupling that fibre to or decoupling the fibre from the output fibre.

2. A fibre optic switch arrangement, including an input (output) fibre, a pair of output (input) fibres, and a pair of parallel electrodes between which the fibres are mounted, in which the input (output) fibre is provided with a coating of conductive material, in which, in one condition of the switch the input (output) fibre is optically coupled to one said output (input) fibre, and in which the fibres are so arranged that, when an electrical potential difference is established between the electrodes and between one or both of the electrodes and the conductive coating of the input (output) fibre, the electrical field gradient so produced causes the input (output) fibre to bend so as to decouple from the one output (input) fibre and to couple to the other output (input) fibre.

3. A switch as claimed in claim 1 or 2, and wherein each said electrode has a longitudinal V-groove whereby alignment between the first and second fibres is provided.

4. A switch as claimed in claim 1, 2 or 3, and wherein said fibres are of the clad silica multimode type.

5. A switch as claimed in any one of claims 14, and wherein each said electrode is made of aluminium and is coated with an oxide layer formed by anodisation.

6. A fibre optic switch arrangement, including first and second elongated spaced parallel electrodes having facing plane surfaces, longitudinal V-grooves one formed in each said surface, first and second optical fibres one located in each said groove, a further flexible optical fibre coated with an electrically conductive material and mounted with a free end cantilevered between the electrodes, and means for applying a potential difference between the flexible fibre and one or both of the electrodes, in which each said electrode is coated with a layer of an insulating material, and in which the arrangement is such that, by the application of an electric potential condition to the first or second electrodes, the flexible fibre is attracted into the V-groove of that electrode so as to couple optically to the first or second fibre respectively.

7. A fibre optic switch arrangement substantially as described herein with reference to the drawings accompanying the Provisional Specification.

8. A telecommunication exchange provided with a plurality of switches as claimed in any one of claims 1 to 6.

9. A method of optical switching substantially as described herein with reference to the drawings accompanying the Provisional Specification.

**WARNING** end of DESC field may overlap start of CLMS **.

Claims (9)

**WARNING** start of CLMS field may overlap end of DESC **. Fibre Displacement Vmin Fibre Diameter A llm Volts d llm 100 97 200, or less 50 34 100, or less 25 12.2 50, or less 12.5 4.4 25, or less 6.25 1.6 12, or less These voltages are surprisingly small, particularly as the fiber diameter decreases. The switch has a minute power requirement because it is capacitive, and could therefore be controlled by very low power consumption circuits, making telecommunication exchange routing networks feasible with very low power dissipation. Thus an optical telecommunication exchange may be constructed with a plurality of such switches. WHAT WE CLAIM IS:

1. A fibre optic switch arrangement, including an input fibre, an output fibre, and a pair of parallel electrodes between which the fibres are mounted, in which the input fibre is provided with a coating of a conductive material, and in which the fibres are so arranged that, when an electrical potential difference is established between the electrodes and between one or both of the electrodes and the conductive coating of the input fibre the electrostatic field gradient so produced causes the input fibre to bend thereby coupling that fibre to or decoupling the fibre from the output fibre.

2. A fibre optic switch arrangement, including an input (output) fibre, a pair of output (input) fibres, and a pair of parallel electrodes between which the fibres are mounted, in which the input (output) fibre is provided with a coating of conductive material, in which, in one condition of the switch the input (output) fibre is optically coupled to one said output (input) fibre, and in which the fibres are so arranged that, when an electrical potential difference is established between the electrodes and between one or both of the electrodes and the conductive coating of the input (output) fibre, the electrical field gradient so produced causes the input (output) fibre to bend so as to decouple from the one output (input) fibre and to couple to the other output (input) fibre.

3. A switch as claimed in claim 1 or 2, and wherein each said electrode has a longitudinal V-groove whereby alignment between the first and second fibres is provided.

4. A switch as claimed in claim 1, 2 or 3, and wherein said fibres are of the clad silica multimode type.

5. A switch as claimed in any one of claims 14, and wherein each said electrode is made of aluminium and is coated with an oxide layer formed by anodisation.

6. A fibre optic switch arrangement, including first and second elongated spaced parallel electrodes having facing plane surfaces, longitudinal V-grooves one formed in each said surface, first and second optical fibres one located in each said groove, a further flexible optical fibre coated with an electrically conductive material and mounted with a free end cantilevered between the electrodes, and means for applying a potential difference between the flexible fibre and one or both of the electrodes, in which each said electrode is coated with a layer of an insulating material, and in which the arrangement is such that, by the application of an electric potential condition to the first or second electrodes, the flexible fibre is attracted into the V-groove of that electrode so as to couple optically to the first or second fibre respectively.

7. A fibre optic switch arrangement substantially as described herein with reference to the drawings accompanying the Provisional Specification.

8. A telecommunication exchange provided with a plurality of switches as claimed in any one of claims 1 to 6.

9. A method of optical switching substantially as described herein with reference to the drawings accompanying the Provisional Specification.