Control of optical systems

:3L 3 ~L ~ 6~ CnN~ROL OF OP ~ IC~L SYS~EM~ Th~ p~ nt lnvention relat~s to a meth~ o~ ~0l~7t~ ~ an optical sy~tem a ~i~nal control syst~n ~nd a tr~nsceiver f~r ~ n ~ h ~ con~rol system and in particul~ for usc witll multlple acces~ optical netwo~k~.

The u~ o~ ~ multlple acc~ss n~wor~ al}~ws ~or potenti~lly hi~hly ~fflclent u~e o~ ne~woI~ r~?sou~ce5.

More expensi~e equlpment ~or network mana~ement~ ~cr o example5 may be l~c~t~d ~t a centr~l nod~, or exchan~e and its ~acil~tl~s shared by a larg~ number of indl~idual network outst~tion~ In such circ~n~t~nees the ~x&han~ m~y ba connec~ed Yia a sin~le main line with hrançh fe~er lin~s linkin~ the network outstatLons to ~he main li~e.

In an ~ptic~ net~o~ th~ branehln~ may convenlently ~ eff~cted by pag~v~ optical spllt~ers.

It i.~ ther~ore feasible to prod~lce a mlJltiple access opti~al networi~ with no ac~ive ~nponen~s other than at the ~xch~n~e ~ at the ou~stations t~emselves.

The ~dvan~a~es o~ su~h ~ passlve nRtwork incl~e easl~r maintenanc~ and reduce~ overall cost, In ~ multiple access network it ~s necessary to ensure th~t sl~n~ls in~ende~ forl or ori~lnatln~ 7 a partic~ r ~utstatlon ~e co~rectly .~dentifl~ ~s ~chnlqu~ ~ Wavelen~th ~lvi~ion M~Jltlp.lexing (WDM) or Time Oivisl.on M~tiplexing (T~M); for exanple, enahl~ tl~ls identl~catlon to be achle~ed.

Where TDM is u~d, ea~h ou~s~atlon ls all~ated an in~ivi~u~l ~ime slot or ch~nnel speclfic~lly ~o~ receptlon or ~r~nsrnihsion of i~s ~wn signal~.

When a co~np~site T~M - sl~nal ~s b:~oadcast fr~n the ~xchange each ou~tation wlll r~celve the whol~ of that signal fra~ which i~ seleet$ and ,~ ... ~.. . . . . . . .

~ 3 ~ - 2 decodes it5 OWIl all~ated chanr~l.

Howev~r, ln th~ rever~e ~ire~tion, ~ach outst~tion contri~ute~ lt~ ~wn sl0nal alone and ~he network its~lf mu~t e~ectlvely ass~mbl~ all the ~cparat~ly ori~inatin~ ohann~ls lnt~ ~ s eo4nposite TDM sl~nal fo~ pa~sir~ b~c~ to the excllan~e ln the correct order.

The outstations are g2neIally irre~ularly spaced at di~-~erent distanc~s f~om the ~xchan~e.

It ls therefQr~ essen~i~l to provi~e s~ne meth~ o~ ~nsurin~ tha~ tran~ ion~ from each nLtstation occ-~y ~ designated ~lme ~lot ~n the ~ssem~le~ return sl~n~l lrrespective ~ th~ physieal pos~tlon of the outstation ~el~tlve to ~he exch~nge, ~o~h at the time~ of c~nnection ~o the netw~rk and at all subsequent tlmes.

T~ dPal with thi~ problem a rangin~ prot~col may be en~loyed. ~an~n~ protocols ~re known for ~se in r~io networks and will n~t ~e fu~ther ~i~eus~e~ he~e. H~wever, it h~ been foun~ that provision o~ a cor-vention~l ~an~in~ proto~ol i5 not neoes~arily sufficient to ensure the ~ccur~te ~sem~ly oF the r~turn ~DM ~n~l in a passive opti~al networ~ ~s de~cri~i a~ove.

Localised v~riations can occur in the transmis~lon path, ~r examp1e, surh that the words o~ bits in a ~i~n~.l orI~Inatin~ fr~m an outs~tion ~y not be exactly within th~ b~unda~es ot~ the alloc~ed tim~ slot ln the ret~n T~M ~i~n~1, The slgn~1 ~rom one ~t may then overlap the s~n~l f~m another to the d~tri.mçnt ~ th~ network e~ficien~y.

~ further pro~lem arises because the amplitude of the slgnals fra~ ea~h outstatl~n depeilds on the powe~ of the tran~mltter associ~ted with that outstatinn and on the atte~uation of the opti~al p~th ~ro~ thae outst~tlon, ~o~h these facto~s will vary fr~n one outstation to anoth~r.

Thus, unless ccmpensatin~ ~sasurRs are ta~en, the c3mposite return TDM slgnal may have signal lev~ls whl~h d~F~r from ~ne tlme slot to the next~ aecording to ~ 3 ~ whichever outstation each slot is allocated. Such a variable signal is difficult to demultiplex.

It is an object of the present invention to provide a method and apparatus controlling the signal transmission from network outstations in a multiple access passive optical network such that the aforementioned problems are substantially overcome or at least mitigated.

According to the present invention a method of controlling the transmission of signal pulses from an outstation in a time division multiplex multiple access passive optical network including a central node and a plurality of outstations, the method comprising the steps of detecting, at the central node, relative to a predetermined discrimination value the presence or absence of a received signal pulse at the central position of a time slot allocated to that outstation; detecting relative to said predetermined discrimination value the presence of absence of a received signal pulse at first and second predetermined times which define a central region within the total duration of the time slot; and determining from a logical combination of said detections whether that outstation needs to correct the timing of the transmission of the signal pulses, and if so transmitting a control signal to instruct that outstation to alter said timin~ for subsequent transmission from that outstation.

In a time division multiple (TDM) optical network, the parameter is preferably the temporal position of the signal from an outstation in the allocated TDM time slot for that outstation. The first and second reference values are preferably chosen to define a central region within the total duration of the time slot~ Alternatively, or additionally in a second application of the method of the invention, the parameter may be the signal amplitude. In this instance, the discrimination value is a present amplitude level. The first and second reference values are then pre~erably two further amplitude levels defining an acceptable amplitude range above the discrimination value.

The method of the invention provides a convenient technique for automatic signal control in a multiple access system. The problems noted above are thereby avoided without recourse to time-consuming individual calibration of each out.stati.on at connection.

The method is particularly appropriate for use with TDM systems, but is also adaptable for USQ with wavelength division multiplex systems, for example, where the monitored parameter may conveniently be the individual signal wavelengths ~or frequencies).

A control system for use in a time division multiplex multiple access passive optical network including a central node and a plurality of outstations, for controlling the transmission of signal pulses from the outstations, the system comprising, in the central node, first detection means for det:ecting relative to a predetermined discrimination value the presence or absence of a received signal pulse at the central position o~ a time slot allocated to a particular outstation, second detection means for detecting relative to said predetermined discrimination value the presence or absence of a received signal pulse at first and æecond predetermined times which define a central region within the total duration of the time slot, determination means responsive to said first and second detection means for determining from a logical combination of the detections whether that particular outstation needs to correct the timing of the transmission of the signal pulses, and for generating for transmission a control signal to instruct that particular outstation to alter said timing for subsequent transmission from that particular outstation;

and comprising, in an outstation, means for identifying a control signal specific to that particular outstation and ~315~P~ ~ signal control means responsive to the control signal for altering the timing for subsequent transmission as instructed .

Where the network is a TDM optical network, the parameter is preferably the temporal position of the siynal as for the method. Preferably the monitoring means are adapted to monitor the signal position with respect to the time slot allocated to each particular signal.

Alternatively or additionally, the control system may be adapted to monitor the signal amplitude.

A control system according to the invention allows the problem of inter symbol interference ~ISI) from spillover of signals in adjacent time slots to be conveniently avoided.

15A control system according to the invention also provides a means for introduciny a network-wide automatic gain control facility into the network.

Amplitude variations within the received multiplex at the cental node may thereby be minimised.

20In these circumstances, DC coupling in the central node receiver, which would otherwise be needed to maintain a given amplitude decision threshold (binary quantisation level) if the amplitudes were widely variable between signals, is not required.

The performance of the exchange receiver may thereby be improved and the design simplified.

A standard transmitting laser has been found to detect signals in the above described receive mode at an error free 2.048 Mbitfs over lOkm of single-mode optical fibre. A better performance is to be expected if the laser/diode combination were optimised for use as a transceiver.

A control system according to the invention further permits the optical transmitter, usually a laser, in an outstation, to be operated at power levels which are effectively the minimum required for efficient operation of the network. Consequently, the reliability of the ~3~.~3~ las~rs wlll be ln~ea~ed to ~b~tantially th¢ opti~n posslble ~lthln ~he ~o~k ~st~ints.

A~ditionally, the ~nt~ol syst~m ~llows ~h~3 conditlon ~f th~ ~utstation t~nsmltter to ~e e~ily ~1S5i~S~ i, For ex~nple; the con~rol system may ~e a~apte~ to re~r~ise the occurr~nce of a conslstant tr~nd ln the ~ter~ion of a monltored p~r~n~t~r and ~ive ~n e~rly w~rnlng oP ~ ~ault st~t~ ~efore actu~1 ~ailure.

A~cordln~ to ano~her a~pec~ of the present inventlon lo transmitter~recei~er (transcelver) for use with a si0n~1 ~ntrol system acccrdin~ to the pres~nt lnventiorl lncl~lde~ an optical tr~n~nltte~ c~e peak powe~ o~tput i5 eontrollable by ~ha signal control means, -~he transmltter being i~llS4 arranged to ~orm ~t least part of ~n op~leal ~etector ~r ~etectlng the cont~ol ~ignal~ ~rom the centr~l no~e, ~re~erably the trans~elver lneludes ~ la~ier dlode having a back fa~et~ photodlode, and a la~ s supply whereky the l~ser diode is forwar~ ~iased tv act ~s l~ser ~0 ~mpli~ier when not in transm1ttin~ mode.

Thl~ has ~veral advant~es over ~ys~ems usin~ separ~t~ tran~m~tters ar~ receivers.

Ths monitor d~ocle supplled with ca~nereially ~vaila~le la~er tran$hlltl;ers which is reduncl~nt a~ a ~ontroller with the c~ntrol system ~5 o~ the present i~ven~ion when controllln~ the l~ser output ~s uti~sed ln ~ marmer whlch Inean~ the receiv~r p~ckage can be ~spens~d wlth.

The ~eed f~r a co~pler of s~gni~icant co~t at the wstome~'s end ~ich wa~ previou~ly ne~essary to ~onnec:t the receiver to th~ network is also removed. ~his reduces the costs o~ the cust~ner servic~ and ~educes mainterlan~e r~quirements.

~he rn~tho~ ~f the inverltl~n and ~n embo~iment of a con~rol system ~nd a tr~nscelve~ ccarditl~ to the inv~ntion ~ill now ~e de~crl~e~ in d~ai~ y way o~ ~ . . .. ... . . ... ... . .

l '-!'c,~' - LlS -- u~-~ l t~ ~3~3~ -- 7 -example wlth ref~rence to ~h~ acownpanyir~ dr~wln~s in which;

Fl~r~ 1 lllustraS~s seh~mati~lly a mul~lple AOC:~5 pa.ss.ive optical networ~ accord~r~ to the present inv~ntion~ Fi~ure 2 lllustrstes ~ asP~ot ~ ~e impl~mentation o~ the meth~i ~ccordLn~ to the pr~sent inv~ntion;

Fi~ure ~ illu~t~a~es ~n ~spe~t of another inplement~ion of the mel:hr~d;

Fl~u~e 4 i~ a schematle di~gram o~ an ~mbodiment of a lo control sy~tem ac~or~iny to the present inventlrJn;

Fl~ure 5 i$ a ~chematic dia~l~am of an ~m~iment of a trRnsmitter/receiver (tran~ceiver) ~ecord~ o the pr~sent ~nvent~on ~or use wi~h the control ~ystem o~ Figwre 4 in wh~oh a l~er back ~cet di~ie ~s.the detector; and ~l~ure 6 is a ~chematlc diagram of a seecn~ emhodiment o~ a tr~nsce~e~ ~c~ordiny to the presen~ ~nvention ~n whieh the l~se~ ls the opt~cal d~tector.

In ~l~ure ~, a mult~ple access passive ~pt~al network 1, ln this case part ~ a telephone system, con~ists o~ a cent~al exchan~ ~ conn~ted to autstatlon~ 5 (shown as squ~re boxes).

FrGm ~he exch~n~ ~ 8 main optic~l fihre l~nk ~ is su~cesslvely branched ~t pass~ve op~ical splitters S ~shown c~cl~d) lnt~ branch qptl~al fibre ll~ks ~ whieh ultimatRly link in t~ th~ in~lvidual outst~tinns 5.

~r TDM operati~n o~ ~h~ ~twork 1 it ~s necessary to provld~ a methc~ for syn~hronisin~ the si~nal~ orlgins~ing from th~ oLt.~katlans ~l~ch that these signal~ ean be 3~ passively ~ssem~led by the network into the ~orre~t ~equence ~o~ r~tu~n to the exchange 2 vla the main link 3. ~oarse synchroni~n is ,orov~ed by use o~ a s~it~ble ran~lng protocol (not ~urther de~cri~ed here) ~uch a~ ls knDwn ln radio ne~work ~ :l4c,c,-lJ'_-~J-I .Lb: ~Ic ." " ~ I, .............. . .. ..... .....

~ 3 ~ Clo~er toler~rlc~, a require~ ln hl~h bi~-r~te tr~n.~mis~lon, For exampl~, rnay ~ ~tair~d by usin~ th~ methr~ o~ the inver1tion~ A su~al~le a~ap~ an o~ the technique Is lll~stra~ed ln Figure 2. A si~n~l recelv~d at the excharl~ i5 shown in the fcrm ~f a converlti~n~l eye dl~ram.

Tl-~ centra~ positiorl fux thIs sign~l whi&h may be set by the ~n~Ing protocol, i5 Ind~cated 4y T~ Fr~ the nornl~l purposes ~ ln~ol~mal:ion r~trlev~l th~ si~nal iS sampled at thi~ instqnt~ Th~ sigr~al i~ ~lsc monitored at lo two ~urther sampllng lns~ants C ~n~ at tlmes ~1 and t~ befor~ ~nd a~ter T respectlvely.

~ binary c~e wor~ is then eons~ruct~d ~cco~ n~ ~o the signal le~l mon~ored a-L eao~ o~ th~se instan~ h~ code wor~ is then used t~ ~etermine what tImln~ ~d~ustment, 1~ any, i3 reqlliredA An ~ippTopriate lo~i~ ta~ ç3iv~n as rable 1~ ~ T~A,BALE 1 Bln~ry lev~ls ~or tim~ slot (o~tstatinn)N C T t~ o~i~S Action O O O Tlmlng correct None require~ 1 1 1 Ti~ln~ cor~e~t Nore ~lequlred O 1 1 Tim~ng late Re~uce ~utstation N d~l~y ~5 1 0 0 Timing late Redu~ outstatlon N-l delay 1 1 0 Ti~ing e~rly Incxe~se outstat~on N delay O O 1 Tlmln~ early ~nerease outs~atlon ~+1 delay 0 1 0 Irlval~ Code 1 0 1 Invali~ Code The exchan~e control system then transh~its an addres~ed control ~ignal Instructin~ th~ rel~vant ~utst~lon to incr~ment or ~ecrement ths slynal delay as may be requlred~ Addressin~ technlques ~o~ eont~nl 1 3.~ ~3~ ~ w g _ sign~ls a~e ~ll known and wlll not be tr~ted ln ~etall her~.

The metho~ o~ the invention may be s.lm.ilarly ~dapted ~or re~ulatlny sl~nal a~plitude as ~hown ln FLgure ~ In thl~ case, th~ normsl rec~i~er d~oi~iun threshold level, de~ls~nate~ R, 15 u~e~ to distln~u~h between sign~l "ones" an~ "zeros".

Two addltlonal levels ~efining an accepSable a~plltude ~an~e are ~ls~ ~onitox~d.

Thes~ levels' desiynate~ A a~d ~, are monitore~ to detect when the s~n~l level f~lls b~low an accept~ble ~inimun or exceeds an ~c~eptable max~mum respectlvely.

These levels ~re conveniently s~t accordin~ to the si~nal~ n.oise xati~ o~ kh~ weakest outs~tion ~ al ~xp~cted. ~oth levels a~e set ~bou~ ~he discrimlnat~on khreshold R to allow ~ ~a~ty mar~in bet~wee~ th~t thresh~ld and th~ lower mon~toring level A ln or~e~ to reduce the rlsk that a s~nal w~ll eveI~ ~all b~low th~ threshold and be entirely ~d~ecte~, As pr~viously, the ~n~l level mon~t~red in terms o~ ~h~ R, ~, B levels is tran.~la~ed lnto a 3 bit ~ binary code word wt~ich can be u~ed ~o d~termine the ~equireri amplltud~ adJu~tment aeco~din~ to Table Z.

TA~L~ ~ Binary leve,,l~s ,~Qsis AG~ion ~ A B 0 O O ~Zero ~ec~ive~i None 1 1 0 "On~" receiYed Nor~ ~mplltude ac~ptahle 1 1 1 "One" reeeived Instr~t oUtstAtlon N Rmplltude ex~ess~ve to d~cre~se tr~nsmlt~er pow~r 1 o n "One" :~eceiv~ - lnstrùct out~atlon N Amplitucle too low to incr~ase tr~nsmitter power ,C, ~ 4 1~ Ll, l " ~ . " .'.L, , . ~_~ - 10 O 0 1 Invalid cude ~ 1 0 Inv~ d cod~ O 1 1 Inv~ c~de 1 0 1 Invalid c~de A~ ~e~ribed ~bove the m~thod ~or positlon oontr~l (~Igur~ 2~ and ~or amplltude re~ulati~n ~Fi~u~ ~) m~y be implemented ~epa~ately. ~owever, th~ si~nal ~ietectiwl ~t T in the ~ime slot and the a~plltude mo~surement can b~ vi~wed a~ lnterdepend~nt. ~te~tion o~` a si~n~l ~t T, for lo example depends on the sign~ ~mplit~e exceedin~ the threshold R a~ that time. F~rthermore, whereac it i~ possible to monitor the amplitude throughout ~he duratlon o~ the time slot and irrespectlve o~ whether o~ not ~h~ th~stlolcl ~ 15 ~xGee~ed at Tt as a deslrable alternative S the ~mplitude measurement may ltself b~ ma~e dependent on the detectlon of a ~lgnal at tlm~ T.

Tl~ls l~tter strate~y can avol~ mlst~ken mea~uxement of the amplltu~e of ~ ~i~n~l ov~rlapplny ~r~m anoth~r tlme slot, for example for the pUrpOSe5 0~ ~nalysis usin~ Ta~les 1 and 2 the lo~ical s~tes for R and T are then ldentlcal for ~ny ~lven signal and m~y b~ determine~ to~ether from a ~ingle me~surement.

The ~emQlnin~ variables A, ~, C~ D are determined indivi~ally as be~ore.

A control sy~tem for a T~M multlple ac¢ess pass~ve 2S optlcA1 netwo~k and combining po~ition an~ ampli~ude control by ~he method ur the lnvention is ~hown schem~tic~lly in ~l~ure4.

For ease of lllustration only the relevan~ comp~nents only o~ th~ exch~n~e 2 and one outstati~n 5, conne~ted by ~he passive network 3, 4, ~, are indicate~ in the Fiyure.

In the outstation 5, ~ate fr4m ~ ~ta source 56 passes ~or return siyn~l tr~nsmlsslon ~o the exchan~e ~ by a laser 51 under oon~rol o~ a mudulator S2~ The return ~lgnal transmissicn is tl~ed by a clo~k çon~rol which is ~3~ 3~1~ - 11 ~5 inltlally s~t by a ~nglr~ prPtocol as mentlone~ prevlc~usly. A v~rl~le del~y 55 allows F;lne~tunin~ of the ~iming accordlng to ~he methcd ~ the pre~ent 1nverltLon.

The retuxn signal amplit~de i~ adJust~d u~ln~ ~n S arnp1it~e control register 5~ ar~ ~ltl~l-t~-analo~ converter S~ to control ~he ~rive curren~ supplied by th~ modulator 52 tD the l~ser 51.

In the ~xch~n~e 2 the return signal a~rlv~s ~t the T~i rec~ er %1, which passes t~e sf~na1, in lts app~opriat~ ~ime ~lol~, v~ lPiv~ type ~1ip-flop$ 22, 2~, 24, 26, ~7 which pex~v~n the slgnal positiPn an~ ~mpllt~i~ mtm~torin~ ~mctions as detailed ~eluw~ rO monitor the signal position re1a~1v~ to the time slot the ~S~nal Prom the reeeiY~r ~1 ~ated throu~h th~ lS three D~types ~, ~ 4 is at the re1eY~nt ~imes (~ ~i~ure ~ h~ls D~type 2~ monitors the centre o~ l;he acceptable perioc wi~hin the time slot as d~fir~ti by t~le yatlng of D~types 23 (fo~ nd ~ (~or D).

Aeco~d~n~ to the st~s of these position V-types 2~, ~3, 24 the t~me ~ ~1Ot com~inator1~1 loçlIc ~5 then prov1~E~s the appropriate cod~ word (~. T~ble 1) to the eontro1 bus 31, The P-type 22 whi;:h ~onlto~s tlle c2ntr~1 poslt~orl of th~ signal with~rl the tine ~lot ls set to tri~er at the minimum signal disorImIn~tion thre~hold R. ~he s~t~ of ~s this ~-typ0 ther~ore ls also used t:o provide ~he base dete~tlon ~ef~rence ~or the ampllt~cle mea~urement.

~he ~c@ptable amplit~e range ~cf.

Fi~ure 3~ is determJned by the two ~ur~her ~-type~ 2G, ~or A) an~ ~7 (for ~.

The ~nal ~5 prcportionately attenu~ted by th~ level ~d~uste~s 2~ ~nd ~ be~ore ~ein~ applied to the ~--types ~ ~nd ~7 respec~lvely.

Con~equently ~hese ~-t,ypes w~11 only be ~rig~ered by a si~n~l whose almp1itude exeeed$ the pre~et hi~her l~ve1s A, B ~ove ~he ~sic .. .. . . ....

_ J _' ~J--l L t~ L~ L ~_' I . L _ ~ 3 ~ -- ~2 -discrimln~t~on thre$~1cld R~ e arnpllt~ in~torial l~ic 3~ monitors the ~tat~ o~ the relev~nt D-typss 2~ 26, ~7 an~ provides the ~ppropriat~ ~rnplltu~e ecx~e w~rd (cf.

Tabl~ 2) ~o the control bus 31.

s r~e c~e words ~or b~th po~ltlon an~ ~mplltude ~e pas~ed by th~ bus to the contI~ol proce~or ~2 ~ith asso~ ed memory ~3.

The pr~essor ~ ~n~ly~es the c~ words to d~termine the requlr~ acti~n and ~enerPlte~ a correspondin~ control ~ignal which 15 then spec~ic~lly addre~s~d to the relev~lt outstatfuns and transmltted accor~ln~ly .

In the outst~ion 5, a l~cal c~ntroller ~ is provided with ~ telemetry deçoder ~o check th~ ~:hiressing of ~ny con~rol s~als arrlv~g ~r~n the net~o~k ~d to ~dentify ~nd ~eco~e those lnt~ or the pa~-t~cular ~utstation.

If th~ Ins~rwctlons in th~ con~rol ~i~nal are to alter the timing of slgnal transm~ssi~n, for exarpl~ the controll.er 5~ via khe l~al bus ~, increments ~r decremerlts the delay ~oh ~e~st~r 54 to vary the v~riable ~elay 55 as ~proprlate.

If the instructions are to ~lter the ~lgn21 anplit~de, the ~ontroller 53~ agaln v~a the 1~1 bu~ ~OJ inorement~ or deorencnts th~ amplitu~e l~tch re~ster.58. ~he count in the ampllt~e re~ster 58 i$ then ca~verted via the D~A conv~rter 5~ in ord~r to control the d~lve ourr~n~ suppl~ed to ~he laser 51 by the m~dul~tor 5~.

Altho~h the metho~ and control system o~ the presen inventlon have been specif~cally descri~ed with re~erence to a TD~ p~ssive optical network, it w~ll be app~clatecl 3~ th~t the application o~ the invention 5s not res~rl~te~.

Th~ pr~noiples of the lnventiun may equally be ~pplled, ~or example9 to ~ WDM n~t~ork.

Referrin~ now to Fi~ure 5, there i~ ~ho~n the opti~al llnk ~nd subscr~t~r ~ections of the retwo~k of Fi~ure ~ in - 13 ~hich the la~er 51 Is arranged to oper~te a~ buth a tran~mitter ~nd a recelver.

A bias ollrren~ ~upply ~ supplies a bi~s ~urrer~t to the laser Sl suf~lclent to ~orward blas it below the lasln~ threshold to act as a las~r ~mpli~ler when no data ~Ignal i~ s~plf~d by ~e modulator 52.

Optlcal ~i~nals ar~ivin~a ~.ron the netwo2:k 3,4, 6 ~re ampll~led as th~y pass throuyh the ~a~er diocle 51 an~ ~.ll on th~ laser diode'~ ba~k facet photodlode ~;3 w~lic~- ac~s a~ an optic~l deteetQ~ The outp~t ~rom the phot~i~ S~ pplied to the amplifier ~. A switch ~5 is ~r~nye~ 50 that the ~unpli~ler ~4 i~ connec~ed to t~e controller-telemetry decode~ S3 ~nly wllen no si~nsl is belng CUtpLIt ~ro~ the varla~le dclay 55, ~.e. when the subscriber i5 In receive mode~ The ~eesd~r ~ acts on r~oeive~ si~nal~ at lt ~oes ln the arr~n~ement: o~ F~gu~e 4.

The laser ~1 oper~tes in kransmit mo~e ~s ~esoribed above w~th re~rence t~ ~lgure 4t The currenk Prom the modulato~ 52 will ~c l~ss than befo~e be~ause o~ the ef~ect o~ the bia~ current s~pply but w~ll be controlled in the s~me manner to Fix ~he laser Outputr The laser Sl in the tr~n~ceiver o~ fi~u~e 5 15 perm~nently ~orwarr~ biased in re~eive mode an~ so l~equires ~nly ~ ch~nge in ~ias cur~e~t ampll~.u~e to take it ab~ve th~eshol~ to cause lt ~Q la~e ~n~ enter th~ tran~mi~ mod~.

Because the laser Sl qperat~s in ~ low duty eycle mode and ls always farwar~ bi~sed, re~overy time probl~ms ar~ mlnim~s~d when chanr3~n~ ketwe~n the r~ceive an~ transmlt mode~ of laser opRratlon.

The laser transmit m~e is only reqlllred for orl~ tim~ sl~t out o~ m~ny and timeslo~ each ~lde of that ~n~ can ~ff~rd to be lost a5 'che laser trans~ers to reeei\~e mc~le witho~-t a~ectlng the r~eiYed siQnal~ ~3~3~ -- 14 -If ~t ls found not neces~ary t.o s3~te o~lt ~h~ recefver 72 durlng th~ n~ bursts in a particular networ~ th~ swltch 8~ may be oml~e~ ancl th~ ~ecelv~r ~4 p~ n~ntly ~nneCte/i tD the deco~ 53~ Other arr~ngement~ in which the tr~n~m~tter is ~lso used as a r~¢~lver are po~si~le~ Referrlng to Fig~re the l~e~ diode 51 a.~ ~nr~ e~ in recelve ma~e wl~h the ~ack-~acet phol;o~ e ~ein~ ed as in Fi~ur~ 5 to ~ete~t ~n~mpllfl~d slgnals from the network.

The lase~ dlode 51 o is considered ~ passlv~ part of the optleal detector ~uldlng li~ht to tho photodiode~ In y~t ano~her arran~em~n~ the laser diode 51 its~lf ~ay be conr~ected to the amplifier ~4 50 lt acts ~s detector itsel~.

In ~u~h ~n arran~merlt other ~t~electroni~ devlces m~y be wsed ~o ~erv~ as bol~h transmit~ers an~ receivers, ~or ~xaTple light emittin~ diodes .