GB2178262A - Optical routing systems - Google Patents

Abstract

An optical routing system incorporates a single crystal of bismuth silicon oxide (3), and means for directing first and second light beams onto the crystal so as to encode an incoming optical data signal incident on the crystal towards a chosen one of a number of alternative output channels (A,B,C). The system includes a number of optical logic gates (GA, GB, GC) effective to change the angle of incidence on the crystal of the first and second beams dependent on an optical routing code signal transmitted through a system of optical fibre delay lines onto the gates (GA, GB, GC) so as to change the grating structure and thereby change which of the output channels the data signal is deflected towards. <IMAGE>

Description

SPECIFICATION Optical routing system This invention relates to optical routing systems.

Such systems find particular application in optical communication networks where it is required to route an incoming optical data signal between two or more alternative output channels.

In the Digest of Technical Papers ofthe conference on lasersandelectro-optics, held from the 21-24 May1985, in Baltimore, Maryland,there is described on page 208, Poster NumberTHM36one example of a known optical routing system. The system includes a single crystal of bismuth silicon oxide onto which two beams of light from a dye laser are directed so asto encode an effective diffraction grating within the crystal. The diffraction grating is then used to deflect an incoming optical signal towards a chosen one of a number of alternative output channels.As the deflection ofthe input signal will depend on the period ofthe grating which may be varied by variation of the wavelength ofthe light produced by the dye laser, the output signal may be routed between the output channels by applying appropriate control signals two the dye laser effective to control the wavelength of the light emitted bythe laser.

Such an optical routing system suffers the disadvantage howeverthatthe control signals to the dye laser are ofthe form of either electrical or mechanical control signals. This will necessarily slow down the system as it will no longer be a totally optical system.

It is an object ofthe present invention to provide an optical routing system wherein this disadvantage may be avoided.

According to the present invention an optical routing system comprising a quantity of an optically non-linearmaterial,and meansfordirectingfirstand second light beams onto the quantity so as to encode an effective grating structure within the quantity, the grating being effective to deflect an incoming optical data signal incident on the quantitytowards a chosen one of a plurality of output channels, is characterised inthatthesystemfurtherincludesan optical logic gate means, and meansfordirecting an optical routing code signal onto the gate means, the gate means being effective to change the angle of incidence on the quantity of at least one ofthefirst and second beams dependent on the routing code signal so as to change the grating structure and thereby change which ofthe output channels and the data signal is deflected towards.

Preferably the routing code signal is transmitted down thesame communication channel asthedata signal, and means are provided for separating the routing code signal from the data signal. In such a system the light constituting the routing code signal is suitably of a different wavelength to that of the data signal, and the means for separating is a wavelength sensitive optical component.

The logic gate means suitably comprises a timing means connected to a plurality of logic gates, each logic gate being interposed between a coherent light source and a respective light guide, each light guide being effective to direct light from the coherent light source ontothe quantity atdifferentangles of incidence, the timing means being responsive to the routing code signal to select one ofthe gates to enable lightfrom the coherent light source to be directed through the light guide connected to the selected gate so as to produce said first beam.

Preferably the said second beam is directed from a second set of respective light guides associated with each ofthe gates, lightfrom the coherent light source being split between each pair of light guides associate with each gate.

Thetiming means suitably comprises a plurality of optical delay lines each arranged to transmit part of the routing code signal, and each connected to a respective one ofthe gates, a furthertransmission path for the routing code signal being connected to each ofthe gates such that when the time period between different pulses of the routing code signal corresponds to the time delay across one of the delay lines,the gate connected tothe one delay line provides a light path to its associated light guide.

Two optical routing systems in accordance with the invention will now be described, byway of example only with reference to the accompanying figures in which: Figure lisa schematic diagram ofthefirstsystem; Figure 2 illustrates a first routing code pulse sequence; Figure 3 illustrates a second routing code pulse sequence; and Figure 4 is a schematic diagram ofthe second system.

Referring firstlyto Figure 1, the first system to be described is designed to route an optical data signal transmitted down an input optical fibre 1 between one ofthree output optical fibres A, B, C. The fibre 1 is arranged such that light emitted from the routing system end of the fibre is incident on a single crystal of bismuth silicon oxide 3, a narrow band dichroic mirror 5 which is designed to transmit lig htfrom the fibre 1 within awavelength band Al, and reflect light within a wavelength band A2 being interposed between the routing system of the fibre 1 and the crystal 3.An optical fibre network, indicated as7, is arranged to collectthe lig ht from the fibre 1 which has been reflected by the mirror 5, pass it th rough anamplifier/ repeater stage 9, and then divide it into four parallel paths. The first three paths each incorporate a respective delay line, DA, DB, DC, and are each connected to a respective optical bistable element GA, GB, GC ofthe kind described,forexample in Phil.

Trans. R. Soc.Lond. A 313, pages 195-204(1984).

These particular elements are arranged to operate as 'AND' gates, the fourth path ofthe four parallel paths being connected to each gate GA, GB, GC, an argon laser 11 also being arranged to direct light onto each of the gates, light from the laser being effective to bias the gates such that they are subsequentlytriggered by a combination of two low intensity incoherent light pulses. The optical outputs of each gate GA, GB, GC, are arranged to be split into respective pairs of optical fibres A1 and A2, B1 and B2 and C1 and C2.The ends of thesefibresarearranged roundthecrystal 3 such that light passing through the gates GA, GB, GC and subsequentlythrough thefibres A1,A2, B1, B2, C1 and C2 will strike the crystal 3 at different angles of incidence. Three furtherfibres A3, B3, C3 whose function is described hereafter are also arranged round the crystal 3, these latterfibres being connected via respective light amplifiers AA, BA, CAto the output optical fibres A, B, C respectively.

In use ofthe system an optical routing code comprising a series of pulses of light of wavelength A2 is transmitted down the input fibre to the routing system,the routing code signal being followed by the optical data signal comprising a further series of pulses of light of wavelength A1. The mirror 5 directs the routing code signal into the optical fibre network7, a time delay of AA, at, and Ac being imposed on the divided portions ofthe routing code signal as they pass th rough the respective delay lines DA, DB, DC.

Where the time delay imposed by one ofthe delay lines DA, DB, DC is the same as the time between a pair of pulses ofthe routing code signal, a light pulse will reach one of the gates GA, GB, GC atthe same time as the part of the light pulse ofthesecond ofthe pair of routing code pulses which has travelled down the fourth parallel path which does not incorporate a delay line. This will then open one ofthe gates GA, GB, GC allowing coherent light from the laser 11 to pass into the fibres Al, andA2, B1 and B2 orC1 and C2 connected to the open gate, and onto the crystal 3.

LightfromthetwofibresAl andA2,B1 and B2,orCl and C2 will interact within the crystal 3to encode an effective diffraction grating, whose period and orientation within the crystal depends on which pair of fibres the light has passed down andthusthe angle of incidence ofthe light on the crystal 3. A latching mechanism associated with each ofthe gates GA, GB, GC ensuresthatthe gate which has opened is open long enough forthe light beams originating from the laser 11 to encode the grating.The subsequent optical data signal incident on the crystal 3will then be diffracted by this grating, the ends ofthe fibres A3, B3, C3 being positioned at angles relative to the crystal 3 determined by the Bragg relationship such that the diffracted beam will enter one ofthese fibres to be amplified by amplifierAA, AB, orAC for onward transmission through optical fibre A, B or C.

Figure 2 shows an example of a routing code signal which causes the gate GBto open such that light from the laser 11 is directed through fibres B1 and B2 onto the crystal, the subsequent data signal then being diffracted into fibre B3fortransmission along output fibre B. As can be seen in this figurethetime delay A B imparted to the portion of à routing code pulse passing through the delay line DB is equal to the time between a successive pair of routing code pulses.

Thus the part of the routing code signal forthe second ofthe pair of pulses which has passed through the fourth parallel path not incorporating a delay line will arrive at gate GB at the same time as the part of the first pulse which passed through the delay line DB, hence opening gate GB.

Figure 3 shows afurtherexample of a routing code signal in which the undelayed part ofthe second pulse in the routing code signal sequence arrives at gate GA atthe same time as the part ofthe first pulse which has passed through delay line DA, thus causing gate GAto open.The undeiayed partofthefourth pulse in the sequencethen arrivesatgate GCatthesametime as the part of the third pulse which has passed through delay line DC, this then causing gate GC to also open.

Lightfromthe laser 11 will then pass through fibres A1,A2and C1, C2to encodetwo effective gratings within the crystal 3, these diffracting a subsequent data signal partially into fibre A3, and partially into fibre C3.

At the end of an incoming data signal, in orderto delete the grating or gratings encoded in the crystal 3 readyforthe next incoming data signal, a further gating means (not shown) is used to cause the laser 11 to direct a pulse oflightdown afurtheroptical fibre (also not shown) effective to wipe outthe previously encoded grating or gratings in the crystal 3.

In orderto avoid the need for long delay lines, routing code signals employing short coding pulses, for example in the order of 1 ns, are used. For routing code signals of peak power 1 w, 1 nJ pulses are then provided which are sufficient to switch the gates GA, GB, GC. The separation between each sequence of routing code signals, and the data signal to be routed, must however be greatthan the grating formation time.

Referring now to Figure 4, the second optical routing system to be described, incorporates a system as described in relation to Figure 1 nested with a similar system in order to form a more complex system in which a single input signal may be routed between nine different output channels. The system as described in relation to Figure 1 is indicated as R1 in Figure4thecomponents of R1 being correspondingly labelled to those ofthe Figure 1 is indicated as R1 in Figure 4the components of R1 being correspondingly labelled to those of the Figure 1 system.The system R1 is arranged to route an input signal of light of wavelength Al between three outputs channel A, B and C as before,thesignal being shown as being directed along channel B in Figure 4. The second system also inlcudes a second dichroic mirror 13 interposed between the routing system end of an inputfibre 15andthesystem R1 and designed to transmitlightfromthe inputfibre 15 within the wavelength bands Al and A2, and reflect light with in a third wavelength band A3.An optical fibre network, indicated as 17 is arranged to collect the lightfrom the fibre 15 which has been reflected by the mirror 13, pass it th rough an amplifier/repeater stage (not shown), and divide it into three parallel paths each incorporating a respective delay line as in the first system.Each path is connected to three optical bistable elements, GAA, GBA and GCA, GAB, GBB and GCB, and GAC, GBC and GCC repectively.The outputs ofthe gates GA, GB and GC of thefirst system R1 are also connected to the gates GAA, GAB, GAC, GBA, GBB, GBC, GCA, GCB, GCC asshown in thefigure. In analogous fashion to the first system, the output of each gate GAA, GAB etc. is connected to one end of two optical fibres only two of these fibres BC1, BC2 being shown, the other ends ofthe fibres being arranged round th ree further crystals of bismuth silicon oxide CA, CB, CC, the ends ofthefibres connected to gates GAA, GAB, GAC being arranged round crystal DA, the ends ofthe fibres connected to gates GBA, GBB, GBC being arranged round crystal DB, and the ends ofthe fibres connected to gates GCA, GCB, GCC being arranged round crystal DC. Appropriate output fibres, of which only one BC3 is shown are also arranged round the crystals DA, DB, DC.

The mode of operation ofthe second system is analogous to that of the first system, exceptthat a further routing code signal consisting of pulses of light within the wavelength band A3 is also transmitted down the inputfibre 15. Where the delays produced by the system 17 causes a pulse of lightofwavelength iv3to arrive at one ofthe gates GAAto GCC at the same time as one ofthe gates GA, GB, GC is open so asto enable light from the iaserwithin system R1 to inpinge on three ofthe gates, GAAto GCC, these being GBA, GBB and GBC in the particular example illustrated, one ofthe gates GAAto GCC, will open to allow light hrom the iaserto be directed down a pairoffibres onto one ofthe crystals DA, DB, DC, so as to further deflect the signal deflected by the crystal in the first Ri,the gate GBC being shown as open in the Figure. Thus the data sianal passes firstly th rough the system R1 to be deflected by the crystal 3 in the first system towards one of the CA, CB, CC crystals CR in the example shown in the figure, the signal then being deflected by the crystal CB into the output channel BC3.

it will be appreciated that whilst in each of the systems described herebefore by way of example three gratings of different period and orientation are producible in each quantityofoptically non-linear material, i.e. the bismuth silicon oxide crystals, several tens of gratings of different period and orientation may be produced in each quantity of optically non-linear material. As the diffraction efficiency of each quantity is typically in the order of 1%, in principle a iarge numberofoutputchannels could eredsimultaneouslyfrom asingleinputdata signal, appropriate amplifiers and/or repeaters being employed to boost the strengtth of the output signal to hat ofthe original signal.

Twill beappreciatedthatwhilstsinglecrystals of bismuth silicon oxide are particularly suitable for use systems in accordance with the invention, other quantities of optically non-linear material may be used n systems in accordance with the invention, for example, single crystals of potassium tantalate liobate.

Claims (9)

1. An n optical routing system comprising a quantity ofanopticallynon-linearmaterial,and means for directing first and second light beams onto the quantity so as to encode an effective grating structure within the quantity, the grating being effective to deflect an incoming optical data signal incident on the quantity towards a chosen one of a plurality of output channels, characterised in that the system further includes an optical logic gate means, and means for directing an optical routing code signal onto the gate means, the gate means being effective to change the angle of incidence on the quantity of at least one ofthe first and second beams dependent on the routing code signal so as to change the grating structure and thereby change which of the output channelsthe data signal is deflected towards.
2. 2. Asystem according to Claim 1 in which the routing code signal is transmitted down the same communication channel asthe data signal, and means are provided for separating the routing code signal from the data signal.
3. 3. Asystem according to Claim 2 in which the light constituting the routing code signal is of a different wavelengthtothatofthedatasignal,andthe means for separating is a wavelength sensitive optical component.
4. 4. A system according to any one ofthe preceding claims in which the logic gate means comprises a timing means connected to a plurality of logic gates, each logic gate being interposed between a coherent light source and a respective light guide, each light guide being effective to direct light from the coherent light source onto the quantity at different angles of incidence, the timing means being responsive to the routing code signal to select one of the gates to enable lightfromthe coherent light source to be directed through the light guide connected to the selected gate so as to produce said first beam.
5. 5. A system according to Claim 4 in which the second beam is directed from a second set of respective light guides associated with each of the gates, light from the coherent light source being split between each pair of light guides associated with each gate.
6. 6. A system according to Claim 4 or Claim 5in whichthetiming means comprises a plurality of optical delay lines each arranged to transmit part of the routing code signal, and each connected to a respective one ofthe gates, a further transmission path for the routing code signal being connected to each ofthe gates such that when the time period between different pulses ofthe routing code signal corresponds to the time delay across one of the delay lines, the gate connected to the one delay line provides a light path to its associated light guide.
7. 7. Asystem according to any one ofthe preceding claims in which the data signalstransmittedthrough the output channels constitute the incoming optical data signals to a further optical routing system in accordance with any one ofthe preceding claims.
8. 8. A system according to any one ofthe preceding claims in which the quantity of optically non-linear material is a single crystal of bismuth silicon oxide.
9. 9. An optical routing system substantially as hereinbefore described with reference to the accompanying figures.