CA1310528C

CA1310528C - Optical space switch

Info

Publication number: CA1310528C
Application number: CA000577156A
Authority: CA
Inventors: Peter Healey; Stephen Robert Mallinson
Original assignee: British Telecommunications PLC
Current assignee: British Telecommunications PLC
Priority date: 1987-09-11
Filing date: 1988-09-12
Publication date: 1992-11-24
Anticipated expiration: 2009-11-24
Also published as: JPH02501415A; AU604374B2; EP0307244A1; WO1989002614A1; HK120996A; GB8821168D0; US5013140A; GB2209843A; AU2381188A; GB2209843B

Abstract

ABSTRACT (ref. Fig 1) An Optical Space Switch An optical space switch comprising an optical output (O);
three optical deflection stages (S) each comprising a twisted-nematic liquid crystal polarisation rotator (R) responsive to a respective bi-state control signal and a calcite crystal deflection means (D) for selectively deflecting optical signals according to their polarization; the deflection stages (D) being serially arranged to define eight distinct source locations from where an optical signal is selectively deflectable successively by the deflection stages (D) to the optical output (0), a distinct combination of states of the three control signals corresponding to each location eight optical sources (I) are located at one of the source locations. Each input is formed from an array of fibres so as to be capable of launching an optical signal which is spatially modulated transverse to the signal propagation direction. Each deflection stage (S) preserves the spatial integrity of the deflected optical signals.

Description

~ ~10528 AN OPT~CAL ~PAC~ ~WIT~

This lnvention relates to an optl~al ~pace ~Witch.
R.A. Spanke ln ~ paper entitled Ar~hitectures for Large Nonblocking Optical Space swltches~ IEEE Journal of Quantum ~lectLonics Vo~ QE-22, No.6, June 1986, discusses the UBe 0~ active, N ~o 1 combin~rs fabri~ated fro~
optical space switch elements on TisLINbO3 for use in nonblocking N ~o N optlcal space ~itch architectures~
5uch knowm N ~o 1 spac~ switche~ are capable of selectively switchlng ~ selected one of N op~lcal signals to an output.
According to the present invention an optical switch comprises -an optical output;
n optical deflection stage~ each co~pr~8~ng a polarl~ation rotator resp4nsive to a respective control signal to rotate the polarisation of an optical sign~l through 9~, and a de~lection means for selectiv~ly deflecting optical 5iqnals according to their pol~rlsation;
the n deflection. ~ta~ being serially arranged to de~ine 2n di~inct source loc~tlon8 from where an ~ptical si~nal i~ selectively deflecta~le succeæslvely ~y the n de~lection ~tages to the optlcal output, a distinct com~ina~ion of state~ of the n control signals 2S corresponding to each location;

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a plurality ~ up to 2n optlcal ~ources Qach located at on~ of the 2n ~ource loc~klon~:
and in ~hich Qdcn defle~tion stage pre~erves the sp~tial 1ntegrity of the deflected optical ~lgnal3~
S An adv~nt~ o~ the pre3ent lnvention over ~nown opti~al switche~ ~apable of ~onne~t~ng one of ~ lnpu~ ~o ~n output 1s that lt can swit~h optical si~nals havinq up to two spatial di~enslons. For examp1e, each optical si~n~1 might be a complete databus or represent a page of computer m~mory ~o providlny a means of optically switching in~ormat~on at a ~r~atly increased rate.
All the opt1cal ~ignals whi~h enter a given one of the ~eflection stage~ pas~ through the sa~e polarisation rotator which ~herefore opera~es on spatial~y multiplexed .5 signal path~ giv1ng a ~ignif1cant reduction i.n the effec~ive number of cros~-points and wnich u~es ~ew optical and ~lectro~4ptlcal su~a6semblie~.
By sp~tial in~egrity i~ meant that al~houqh ~n lnput signal may be distorted spa~i~lly a it is switchea ~o the output, or ex~mple expanded or contra~ted, there 1~ a con~l~tant mapp~ng ~etween the spatial elements of the switched inpu~ and output si~na1~ which presQrves the mformation content o the sp~ti~l mod~lation~
In this specificatio~, the term "opt~cal" is intended to refer to that part of the elec~romagnetic spa~trum which is geJlerally known as the visible region together with those parts of the infra-red and ultra-violet regions at eacl~ end of the visible re~ion capable o~ havin~ the polarisa~ion ro~ated.
T~e de1ection m~ans may ~e an optical component ~hi~h supports propagation of the optical si~nal in two directions dependant on the p~larisation of the signal, for examplo a bl~refrin~ent crystal such a~ calcite, or one whi~h s~lectively r~flects light of a glven polarisation, ~uch ~s a polarlsing beam sp1~tter. In this , ' . ~ :
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case d prism may ~o c~nveni~ntly used to align incoming ~ignals with the ~eam~plitter s~ch tha~ all ~elec~d si~nals from the defle~tion ~tage paB~ ~rom the beamsplltter in the direction o~ the next de~leckio~ ~ta~e or the OlltpUt.
Embodiments of the lnvention will now he descri~ed by way of example only by re~erence to the accompanying drawings on which Figure 1 is a schematic perspective vie~ of an optic~1 switch a~cording to the present inven~ion uslng a biref~ingent cry~tal de~lectl~n mean~;
Figure 2 1~ a ~chematic side view o~ the optlcal ~witch o~ ~igure 1 illustrating lts use with collimated optical signals;
Figure 3 is a schematic end view of the optical switch of Figure 1 but using diveryent op~ical sources and having additlonal lenses to ~ocu~ ~hQ ~ou~ces onto t~e outpu~;
~igure 4 is a schema~ic end view of an optical switch using p~larisation beam~plitters and prisms as the d~flection means; and Figure 5 ls a di~gramatic, perspective view o~ a generalised connection network for~ed ~rom ~he N-fold array of optical switches of ~igure 4 having optical sourGe~ generated by an N ~lgnal di~tributor.
~eferring to ~i~ure 1. an 8~to-1 optical swi~ch 2 i9 shown which can optically swltch a s~lec~ed one oP inpu~s Il to I8 to an output 0. Each of the input~ I and the output 0 are in the ~orm of a linear ~rray of eight optical fi~res 4 connec~a~le t~ an optical network (not shown) the input fibres being of 1. ~m monomode fibre, the outpu~ fi~res multimode. At any qiven time the optical signals launched ~ro~ the inputs I may be spatially ~dulated according to the signal emitted by the individual fibres 4 of that input I which could be 3~ represent~tive of the instant~neous sta~e of ~n elght-bit - . , :
, 5~8 cumpu~er bu~, f~r example. The ~ibreR 4 of th~ output O
receive light from the correspon~in~ fibres of an lnput I. In this way one of eight, eight-bi~ opt~cal buses can be ~witched to the output O f~r pa~ing to the network (not shown) ~or proces~ing. It will be readily appre~iated that the present invention ~s not restric~ed to this example of ~patially modulat~d signal nor thi~
particular application but is equal}y applicable to othPr means of ~orming the spa~ially modulated signals and ~o lo other op~ical networks r~quir~ng ~uch optical signals to be swi~ched to an OUtpllt.
The optical swi~ch of Flg~re 1 ha~ three deflection stages Sl to S3 each having a t~isted-nem~tic liquid crystal ~ell polarisatl~n rota~or R and a bire~rlngent calcite block de~lector D, labled Rl to R3 and Dl to ~3 respeckively~ The calcite blocks D1 to ~3 hav~
approx~mately lcm s~uara fac~ h thickne~ses of 1.22mm, 2.44mm and 4.88mm re~p~ctlvely wit~ the optical axis of the bl~cks D at an an~le of 4~ to the front face, Electronic drives controll~r~ Cl, C2 and ~3 each provide a volt~ge to cause the polarlsation rotator~ R ~o ~llow ligh~ t~ pas~ through with the polarisation unrotated or ro~ated through 90 as required, The rotators a~ each deflection staqe can therefore b~ set to rotate or not the linear po~arisation o~ the light enterin~ the stage.
The input~ ~ are arran~ed to launch li~ht polarised linearly in the direction o~ an ordinary ray propagat~ng in the calc~te de~le~tors D.
A lens 6 is interposed betweQn the input~ I and output 0 of ~ focal leng~h such that the effective optical path length from the lens to the inputs I and o~tput 0 is equal to twice the focal length of the lens. This provides 1:1 ~maging magnifi~ation between the inputs I and the output 0. In the embod~ment of ~lgure 1 the lens has ~ ~ocal length of ~Omm.

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In the case o~ a twl~t~d nem~t~c liquld crystal a control ~ignal of O Volts causes ~ gO ro~ation, whereas a non~zero con~rol signal applled acros~ the 1iquid crystal cell des~royR the twist~d structure re~oving the polarisation ro~ation proper~y, As will be explained in more detail, this optical swit~h provides eight possible deflections of the input optical in ln~rements o~ 125~m, the number of increments de~erm~n~ which o~ the inputs I is defle~ted to fall on the outpu~ O.
The operation of the op~i~al sw1tch 2 of Fl~ure 1 will now ~e explained wlth reference to Figure ~ which shows the side view of th~ switch 2.
Each de~1ec~or sta~e Sl, ~2 and S3 operate~ as the digital ligh~ deflector de~cribed by W.K~lcke, T.~,Harris, K. Kosanke and E. MaX in a paper entitled '~ Fast, Digital-Indexed Light ~e~lector~ M Journal ~anuar~
1~64. If the op'cical signal entering a cal~lte ~ tal is linearly pol~rised in th0 horizontal directlon lt will pass 3traight through the crysta1 as the ordinary r~y, i~
po1arised in the vertical direc~ivn it ~ e de~lected as if propaga~e~ as the extraordinary ray to exit ~he crystal D disp1aced rel~tlve to an ordinary ~ay but parall~l to it. The di~plac~ment b~tw~en the ordinary and : 25 ex~raordinary rays is directly proportion~1 to the thickne~s of the deflec~or crystal. Thus by ~mployin~
stages with thickne~es in the ratio 1:2:4, 8 possi~le lncrements o displac~m~nt are possible. A set o~
examplary rays 1o1 to 1C~ one ~rom each of the inputs I, and all arranged to be horizont~lly polarised 1s shown. I~ all the rota~ors R are ~et to leave the polarisatiorl O~ light unaltered on passing ~hrough it, each ray lO will remain horizontally polarised and pass through th~ opt.ical swltch 2 undef1ected.
3S The lo~ate~ output is loca~ed relative to the inputs - 6 - ~ 3~ ~ ~2~

such that the optical ~ignal ~ro~ the opkical ~ibr~s ~ o~
input Il of Fig~re 1 ~ill be coupled to the output fibres of the output 0. If the ray~ entering Dl, D2 or D3 are vertlcally polari~ed they wlll be deflected 1, 2 or 4 1~5~ units by th~ stages Dl to ~3 re~pectively.
Consider now rot~ors Rl, R2 and R3 all set to rotate.
the polari~ation o~ the optica~ signal ~hrough so by the applicatlon of the appropriate control sign~l~ from the controllers Cl, C~ and C3. The horizon~ally polarlsed lo ~ign~1~ fr~m the inputB I are rotated b~ rotator Rl to have vertical polarisation and consequently they are all deflected by unit of 1257m away from the und~le~ted line 11 shown as a dotted lines for ray 101 and 10~.
Rotator R2 rot~tes the polari~ations ba~k to t~e horizon~al polarisation so all the ~i~nals pass undeflected through ~he second stage S2. Rotator R3 again rotate~ the polarisation, ~hi~ time from horizontal to ver~ical 80 the de~lector U3 causes a ~our-uni~ deflec~ion of all the rays. The netl ~ive unit d~lection means the 2~ ray~ from input 6 fall on the fibres 4 of output O o~
Flgure 1. The oth~r eiyht possi~le settings of the bi-state controllers Cl, c2 and C3 will each selectively c2use a respective distinct input I to be connected ~o the output 0.
: ~5 The dlfferent thicknes~ de~l~ctor~ P can be a~ranged in any order. ~urther deflect~on ~tages can be provided to provide a larger n~mber o~ switchable inputs as desired there being 2n switchable inpUts for an n-stage optical switch. For ~llese larger lnputs lt may be conveni~n~ ~o locat~ ~he rotators at the foca~ point o~ a pair of co~vex lenses of equal focal len~th to re~uc~ the area o~ rotator needed to rutate or not the optica~ si~nals passing through the stage.
~e~erring now to F~gure 3 there is illustrated how the three defleclion stages S o~ Flg~re 2 operate wi~h .

7 ~ 3 ~ 8 divergant ~eam~ of whlch only that from I6 ls shown ln clarity. only optical ~lgnal~ rom input I6 would ba dir~cted by lens~s l~ and 16 to be fo~us~ed on the ou~put o, the other inputs being focussed to a posltlon dlsplaced from the output 0, when all three rotators 12 are activated. The o~h~r inputs I can be selectively switched to the outpu~ hy activatiny other combinations of rotators R to provide dif~eren~ ~otal deflections as described ~or non~divergent ~eams With reference to Figure 2. The u~e of divergent beam~ as shown in Figure 3 is p~rticularly attrac~ive ~or fibre optic circui~s~
Re~errlng now to Fig 4, there is shown the three de~lection stage~ of ~n optical swi~ch 16 in which the de~lection means are formed from ~ polarisa~ion .5 bea~splitt~rs Bl tQ B3 and prism~ Pl to P3 to de~ine ei~ht distin~t locations which can be directed the outpu~ 0. At e~ch sta~e h~lf the rsy p~ths enter a beamsplitter ~ ~n one direction, the other hal~ at right angles to that direction via the ~orre~pQnaing prism P. ~ll the op~ical ~o signals h~ve the ~ame polari~ation on en~erinq a s~aye.
~f vertically polarised the half enterlng the bea~splitter ~ from the prism P will be de~lected to the next stage, the remainder bein~ d~flected awa~. If the optical signals are~hor1~onta}ly polarised those entering ~he beamspl.itter B ~rom the pri~m P w~ll pas~ llndeflected out of the switch, the remaining h~lf pas~ing through und2flected ~o the next ~tage. This arrangement can therefore select Whlch of th* ei~ht inputs I is to be output to the output 0 ~y settinq the polarisation rotators ~ ac~ordingly to de~ermin~ whi~h half of the optical signals entering a gl~en stage Will be pa~sed to ~he neXt Sta~e and, eventually, the output 0~
~ Electro-op~ic swi~hes other than twist~d~n~matic : liquid ~rystals ~an be used as polarisation rotators, for ~xample PLZT or lron garnet~ ln all the ~bove embodiments.

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Referring now to ~igu~e 5 ther* is ~hown a ~enerali~ed ~p~ical connection networ~ having eight input~ 20 for~ed by cascading a pa~sive op~ic~l s~gn21 distrlbu~or 22 a~
des~ribed in the ~pplicant's ~o~pending applic~tion ~ 3 7 ~7~ with eight optical space ~witche~ 2 accordlng to the present invention arranged i~ parallel. T~e signal distribu~or ~2 create~ eight e~u~l intensity but spatially separate~ copies of each o~ the spa~ially modulated inp~ts 20 which copies provide ~patially modulated inputs to ea~h a optical space switch 2. Each optlc~l ~pace ~witch 2 therefore has eight inp~t~, each belng a copy o a dif~erent one o~ the eight spatially modulated inputs 20.
A~-each optical sp~ce swi~ch 2 can sel~ct one of the eiqht copies to be the ~orre~ponding output, the co~bination 1~ acts as an ei.ght way N x N space switch havinq 8~
setting. The total member of ~ontrol ~iqn~ls and crosspoints in su~h a generali~ed optlcal ~onne~tion network is ~he theoreSical minlmum o~ only N lo~N. ~he minimum 108s however iS
10 loglQN(dB) due to the passive input sign~l distributors.
A gen~ralised permutation network could alternatively ~ ~e formed by repla~lng the optical distr~butor with parallel opti~al ~pa~e dlstributor6 of the type discu~sed in the a~ w e referenced paper by Kulk~ et al. A~ain, these can be e~tended ~o have N ~rom 1 to a numher greater than 8 to provide larger N, N x N optical ~Dnnections.
The dLs~ributor loss and 1st orde~ cross-~alk are elimlnated but it will have only N! 5et~in~5 and requires 3~ twice as many con~rol si~nals and cross-points as th~
gener~ ed optical swltch. ~hese ~ould be ~ade asy~lchronousl~ self-routiny since the control code line could be set optically.

Claims

1. An optical space switch comprising:
an optical output;
n optical deflection stages each comprising a polarisation rotator responsive to a respective control signal to rotate the polarisation of an optical signal through 90°, and a deflection means for selectively deflecting optical signals according to their polarisation;
the n deflection stages being serially arranged to define 2n distinct source locations from where an optical signal is selectively deflectable successively by the n deflection stages to the optical output, a distinct combination of states of the n control signals corresponding to each location;
a plurality of up to 2n optical sources each located at one of the 2n source locations;
and in which each deflection stage preserves the spatial integrity of the deflected optical signals.

2. A switch as claimed in claim 1 in which the deflection means comprises a birefringent material.

3. A switch as claimed in claim 2 in which the birefringent material is calcite.

4. A switch as claimed in claim 1 in which the deflection means comprises a combination of polarisation beamsplitters and prisms.

5. A switch as claimed in claim 1 in which the polarisation rotator is a twisted-nematic liquid crystal.

6. A switch as claimed in claim 1 in which the optical signals are collimated.

7. A switch as claimed in any one of claims 1 to 6 in which the optical sources provide divergent optical signals and there is included a means for directing the optical signal which emerges axially from the final deflection stage to the output.

8. A switch as claimed in claim 1 in which the polarisation rotator is positioned at the common focal point of a pair of convergent lenses.

9. A switch as claimed in claim 1 in which each optical source is capable of launching an optical signal which is spatially modulated transverse to the signal propagation direction.

10. A switch as claimed in claim 1 in which the optical sources comprise a plurality of optical fibres.

11. A switch as claimed in claim 1 in which the optical sources comprise a plurality of individually modulated optical emitters.

12. A switch as claimed in claim l in which the optical sources emit an optical signal whose spatial modulation is representative of a least one binary word.

13. A generalised optical connection network including an optical switch as claimed in claim 1.

14. A permutation optical connection network including an optical switch as claimed in claim 1.

15. A switch as claimed in any one of claims 2 to 4 in which the polarisation rotator is a twisted-nematic liquid crystal.

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16. A switch as claimed in any one of claims 2 to 5 in which the optical signals are collimated.

17. A switch as claimed in any one of claims 2 to 6 in which the polarisation rotator is positioned at the common focal point of a pair of convergent lenses.

18. A switch as claimed in any one of claims 2 to 6 or 8, in which each optical source is capable of launching an optical signal which is spatially modulated transverse to the signal propagation direction.

19. A switch as claimed in any one of claims 2 to 6, 8 or 9, in which the optical sources comprises a plurality of optical fibres.

20. A switch as claimed in any one of claims 2 to 6, or 8 to 10, in which the optical sources comprise a plurality of individually modulated optical emitters.

21. A switch as claimed in any one of claims 2 to 6, or 8 to 11, in which the optical sources emit an optical signal whose spatial modulation is representative of a least one binary word.

22. A generalised optical connection network including an optical switch as claimed in any one of claims 2 to 6, or 8 to 12.

23. A permutation optical connection network including an optical switch as claimed in any of claims 2 to 6 or 8 to 13.

24. A switch as claimed in claim 7 in which the polarisation rotator is positioned at the common focal point of a pair of convergent lenses.

25. A switch as claimed in claim 7 in which each optical source is capable of launching an optical signal which is spatially modulated transverse to the signal propagation direction.

26. A switch as claimed in claim 7 in which the optical sources comprise a plurality of optical fibres.

27. A switch as claimed in claim 7 in which the optical sources comprise a plurality of individually modulated optical emitters.

28. A switch as claimed in claim 7 in which the optical sources emit an optical signal whose spatial modulation is representative of a least one binary word.

29. A generalised optical connection network including an optical switch as claimed in claim 7.

30. A permutation optical connection network including an optical switch as claimed in claim 7.