GB2156178A - Optical switching device and matrix - Google Patents

Abstract

An optical switching device comprises a partially-reflecting surface (4) disposed in the path of an incoming beam to divert a portion of the light out of the incoming beam and a fully-reflecting surface (2) movable to intercept the diverted portion of the beam to effect switching of the diverted beam along an output direction. As shown, element 1 is longer than element 2. In the rest position (dotted line), the diverted portion passes over the top of mirror 2 to a sink. When it is desired to select the beam, the assembly is raised, the mirror 2 intercepts the diverted portion and directs it along the output direction. In a switching and routing matrix comprising an array of such switching devices, the reflectance of the surface (4) of devices in a row arranged to receive light sequentially, increase towards the end of the row remote from the incoming beam. <IMAGE>

Description

SPECIFICATION Optical switching device and matrix In recentyears,there has been considerable interesfin the development of long distance transmission systemsforelectrical signals in which the electrical'signal is converted into an optical signal at source, transmitted by means of optical fibres and reconverted-into an electrical signal at the desired destination. The advantages of optical fibre transmis- sion compared with conventional coaxial cable transmission includethe ability to carry high frequency signals overgreaterdistances, the small size and weight of the bearer, and the great reduction in cross-talk.

Optical fibre technology has now reached a state of development such that it is very attractive for point to point transmission of digital signals at bit rates of the order of 100 M bit/s. If, however, it is desired to send such signals, in particular signals such as video signals which maybetransmitted more efficiently in digital form, from any one source to more than one destination, a problem arises as follows. In the past it has frequently been necessary to reconvertthe optical signals to electrical signals for switching and routing. Although this process is theoretically possible, considerable extra expense is incurred because ofthe large number ofelectro-optical transducers required and there may be an associated reduction in reliability.

Optical equivalents to such switching and routing matrices have been proposed. One such device is described in United Kingdom patent application no.

2093304.

Application no.2093304 describes a switching and routing device including an array of optical switches.

Each switch comprises a reflective member mounted on a support so that it can be moved into and out of the path of an incoming light beam to effect switching ofthe beam to an output of the device.

The device described in application no. 2093304 is, however,subjecttoa number of disadvantages, notably, that anomalous transient effects occur as the reflective switching members move into and out of the beam. Furthermore, as individual switching members moveinto and out of the beam, the intensity of the beam reaching subsequent switches in the array varies considerably as differing proportions of the beam are diverted out ofthe incoming direction.

The present invention provides a switching device operable selectively to deflect light from an incoming beam along an outputdirection,the device comprising means disposed in the path ofthe incoming beam for directing a portion ofthe light out ofthe incoming beam,and means for intercepting the diverted beam and movable selectively to permit the diverted beam tobetransmitted along the output direction.

Because the means for diverting a portion of the light out of the incoming beam remains in the light path irrespective of whether the switch is in an "ON" organ "OFF" condition, disadvantageous transient effects are avoided and the light intensity transmitted by the switch is constant.

In a further aspect, the invention provides a switching and routing matrixforan optical fibre transmission system, the matrix comprising an array of optical switching devices in accordance with any preceding claim, the devices being arranged to direct lightfrom one or more incoming beams to one or more of a plurality of outputs. Preferably, the array includes one or more rows of switching devices arranged sequentially in the path of an incoming beam, the reflectances of the partially-reflecting surfaces oftheswitching devices ofthe row varying along it.

In such a matrix, light incident upon an individual switching device is reflected and transmitted in proportions which are the same irrespective of whetherthe device is ON or OFF. If the device is ON, the reflected portion of the incident beam is routed to the desired output whilst if the device is OFF, the reflected beam portion is not utilised. This arrange ment hasthe advantagethatthe power received by, for example, the last switching device in any output row ofthe matrix may be independent of the condition ofthe preceding devices in that row, since each of these preceding devices will always transmit the same proportion ofthe incoming beam irrespec tive of their condition, i.e., whether they are ON or OFF or during transition between these two states.

Further, a matrix according to the invention is capable of routing an incoming beam to more than oneoutputsimultaneouslybyappropriatearrange- ment ofthe switching devices.

Since the power of the signal reaching successive elements or devices in a rowwill diminish, the reflectance of an individual switching element is chosen in accordance with its position in the array in ordertoprovidea balanced distribution of optical powerto the outputs. Thus, in a conventional array in which the inputs are along the left hand side ofthe array and the outputs at the bottom, the reflectances ; of switching elements along a given row would increase from leftto right along the row.

The optimum reflectance Kn ofthe nth switching element in a row is given by the formula

where K1 = available power margin/transmittance of the reflected beam portion a = transmittance ofthe remaining beam portion The maximum size of a matrix according to the present invention depends upon the available powder margin as this should besufficentto accommodate multiple routing given that a proportion of the incident signal is reflected at each element.

Theoretically, the maximum number of switching elements N that can be employed within a given power margin may be expressed as

I However, there will inevitably be some signal iosses on passage through the matrix. For example, for a system operating at about 300 M bit/s with gradedindex multi-mode fibre, a power margin in the region of 25 dB should accommodate a 30 x 30 matrix. If it is necessarytohandlea largernumberof inputs and outputs, several matrices may be combined either with or without regenerators in accordance with the arrangement employed.

Preferably, each switching element comprises a partially reflecting surface which remains within the beam path irrespective of the condition ofthe element and a fully reflecting surface which may be moved into the path of that portion of the beam reflected by the partially reflecting surface so as to route itto the desired output(s). When the switch is Off, the fully reflecting surface is moved out of the path ofthe reflected beam portion.The use oftwo reflecting surfaces relaxes alignment precision requirements. The two reflecting surfaces are preferably so arranged that the angle by which the reflected output beam is deflected is always 900.

The two reflecting surfaces may be supported on individual reflection members. In this case, switching may be effected by movement of the fully reflecting member alone. Preferably, however, the element is constructed so that the partially reflecting surface is largerthan the fully reflecting surface so that switching may be effected by movement of the elementitself,the partially reflecting surface remaining within the incident beam path at all times.

The partially reflecting member may be made by depositing a thin dielectric coating on to an optically transparentsubstrate such as glass. An example of a suitable dielectric material is cerium dioxide. The thickness ofthe coating depends on the reflectance required. Preferably, the rear surface ofthe glass substrate is provided with anti-reflective coating. The fully reflecting surface may be made by depositing a thin metal coating on to a suitable substrate. Again, glass may be used. Examples of suitable metals are aluminium (reflectance 92%) and silver (reflectance 98% ). As an alternative to the use of two reflecting members,thetwo reflecting surfaces may becontained in a specially constructed prism.

The invention will now be further described byway of example with reference to the following drawings, in which: Fig. is is a schematic plan view of a switching element according to the invention; Fig. 2 is a schematic perspective view of the element of fig. 1 ; and Fig.3 is a schematic representation of a matrix according to the invention.

In this specification, it is intended thattheterm "light" should betaken to include notonlyeiectro- magnetic radiation of wavelengths in the visible range, but also electromagnetic radiation of other suitable wavelengths, including, for example, infrared radiation.

The switching element shown in Figs. 1 and 2 consists of a partially reflecting mirror 1 and a fully reflecting min or 2 supported on a aluminium block 3.

The block 3 is mounted on a vertical shaft which may be moved up and down by a solenoid to switch the element ON and OFF respectively. The mirror 1 is composed of a glass substrate carrying a quarter wave length coating 4 ofCeO2on one side to provide a partially reflecting surface of known reflectance, and a quarterwave length coating 5 ofMgF2 (anti-reflecting coating) on the otherside. The mirror 2 is composed of a glass substrate carrying a thin coating 6 of aluminium on one side to provide a fully reflecting surface. The element is shown in Fig. 1 in the ON condition with the mirror2 arranged in the path ofthe portion ofthe input optical beam reflected from the mirror 1.

Fig. 2 shows the beam path through the switching element in the ON condition (full line) and in the OFF condition (broken line). As may be seen from this figure, the mirror2 is smallerthan the mirror 1 and so movement ofthe block 3 as described above propels the mirror 2 in and outofthe position shown in Fig. 1 to switch the element ON and OFF without moving the mirror 1 out ofthe path ofthe input beam.

Fig. 3 shows a 10 x 10 matrix 30 ofthe invention coupled by connectors to input optical fibres indi cated generally at31 and output optical fibres indicated generally at 32. This matrix comprises an array of one hundred ofthe switching elements described with reference to Figs. 1 and 2. The reflection coefficient of each switching element is chosen in accordance with the location of that element in the array.

Table I below shows the optimum reflectance values for switching elements in the first row of the matrix.

TABLE I

Position along Reflectance (%) input beam 1 7.8 2 9.0 3 10.4 10.4 4 12.2 5 14.6 6 18.0 7 23.2 8 31.7 9 49.0 10 100.0 Incoming lightfromthe inputfibres is,ormed into a parallel beam by a lens. From Fig. 3, it can be seen that any input can be routed to one or more outputs by a suitable arrangement of the switching elements.

Each output beam is focussed by a lens on to the end face ofthe appropriate output fibre.

From the foregoing it will be understood that a matrix accordingto this invention provides a mechanical switching and routingarrangementthatoper ates entirely in the optical domain, is capable of efficiently switching signals carried by graded-index multi-mode fibres as well as mono-mode fibres and takes advantage of the properties ofoptical signals to minimise cross-talk. Such a matrix is particularly useful in situations in which it is desired to transmit signals in the optical domain and to be able to switch and route the signals to more than one destination if necessary. An example of such a situation is a television studio in which it is typically necessary to handle up to 100 inputs and 100 outputs.

Claims (17)

1. An optical switching device operable selectivelyto deflect light from an incoming beam along an output direction, the device comprising means disposed in the path ofthe incoming beam for diverting a portion ofthe light out ofthe incoming beam and means for intercepting the diverted beam and movable selectivelyto permitthe diverted beam to be transmitted along the output direction.

2. A device according to claim 1 in which the means disposed in the path ofthe incoming beam includes a partially-reflecting surface.

3. A device according to claim 2 in which the partially-reflecting surface is movable with the means for intercepting the diverted beam; the partiallyreflecting surface being so dimensioned that it remains disposed in the path ofthe incoming beam at all times during movement thereof.

4. A device according to claim 2 or 3 in which the partially-reflecting surface is formed by one or more layers of dielectric supported on an optically-transparent substrate.

5. A device according to claim 4 in which the dielectric is cerium dioxide.

6. Adevice according to any of claims 1 to 5 in which the means for intercepting the diverted beam includes a fully-reflecting surface.

7. A device according to claim 6 in which the fully-reflecting surface is formed by a layer of metal or bya plurality of layers of dielectric supported on a substrate.

8. A device according to claim 7 in which the fully-reflecting surface is formed by a layer of aluminium.

9. Adevice according to claim 6,7or8when appendanttoanyofclaims2to 5 in which the partially- and fully-reflecting surfaces are formed on separate reflection members.

10. Adeviceaccordingtoclaim 6,7or8when appendanttoanyofclaims2to5inwhichthe partially- and fully-reflecting surfaces are formed on a prism.

11. A device according to any preceding claim in which the means for deverting and the means for intercepting are mounted for movement on a common support.

12. An optical switching device operable selectivelyto deflect lightfrom an incoming beam along an output direction, the device being substantially as hereinbefore described with reference to Figures 1 and 2 ofthe drawings.

13. switching and routing matrix for an optical fibre transmission system, the matrix comprising an array of optical switching devices in accordance with any preceding claim, the devices being arranged to direct lightfrom one or more incoming beams to one or more of a plurality of outputs.

14. Aswitching and routing matrixforan optical fibre transmission system, the matrix comprising an array of optical switching devices each of which reflects a portion of an incident light beam and transmits the remainder of the beam, the array including one or more rows of switching devices arranged sequentially in the path of an incoming beam, the reflectances ofthe partially-reflecting surfaces ofthe switching devices of the row varying along it.

15. Amatrixaccordingto claim 14inwhichthe said reflectances increases towards the end of the row remote from the incoming beam.

16. A matrix according to claim 15in which the said reflectances are such that each switching device diverts the same proportion of the incoming beam.

17. Aswitching and routing matrixforan optical fibretransmission system,the matrix being substantially as hereinbefore described with reference to the drawings.