Chng 2H THNG THNG TIN QUANG COHERENCE2.1 Gii thiu chngTrong chng ny ta s tm hiu v k thut tch sng quang Coherence vi nhng ni dung ch yu sau: Tm hiu mt cch tng quan v h thng quang Coherence Cc k thut iu ch c bn c s dng trong h thng quang Coherence Nguyn l c bn ca tch sng Coherence Hai kiu tch sng Coherence c bn l: homodyne (tn s trung tn bng khng) v heterodyne (tn s trung tn khc khng) Cc yu t nh hng ti nhy ca my thu quang Coherence So snh u v nhc ca h thng quang s dng tch sng Coherence vi h thng quang s dng k thut IMDD2.2 Tng quan v h thng thng tin quang coherence2.2.1 Khi nim v thng tin quang coherenceH thng thng tin quang coherence l mt h thng thng tin c xy dng trn nn tng ca tch sng coherence. Nu nh trong h thng IM/DD s dng phng php tch sng trc tip- tn hiu quang c chuyn i trc tip thnh tn hiu in c gii iu ch th trong h thng coherence, tn hiu quang nhn c s kt hp vi tn hiu quang c to ra bi b dao ng ni ti my thu, sau tn hiu quang tng hp ny c chuyn thnh tn hiu in. Trong tch sng coherence, tn hiu c iu ch phc tp, thng tin khng ch c iu ch bin nh tch sng trc tip m cn c iu ch pha hoc tn s, ta c th biu din nh sau: ES(t) = AS(t) exp[i(S t + S)] (2.1)Vi S v S l tn s sng mang ca tn hiu v pha c lp vi thi gian, AS(t) l thnh phn bin ca tn hiu. Tn hiu quang nhn c sau kt hp vi tn hiu dao ng ni ti my thu c th c trnh by nh sau: ELO(t) = ALO(t) exp[i(LO t + LO)](2.2)Vi ALO(t), LO, LO ln lt l bin , tn s sng mang v pha ca LO.My thu cohrence l tng hot ng trong vng bc sng 1,3 n 1,6 cn nng lng ch t 1020 photon/bit, c th t BER=10-9 v l loi tch sng cho u im ln nht trong h thng tc cao hot ng trong vng bc sng di. nhy ca my thu quang coherence hn loi tch sng trc tip khong 1020 dB [8] nn my thu coherence cho php: Tng khong cch trm lp. Tng tc truyn dn m khng cn gim khong cch trm lp. Tng qu cng sut b vo suy dao do tch ghp, u ni. Ci thin nhy cho my thu.2.2.2 Cu trc c bn ca h thng thng tin quang coherenceHnh 2.1 m t cu trc c bn ca h thng thng tin quang coherence. S ny cho thy s khc bit chnh gia n vi h thng IMDD l phn tch sng s dng b dao ng ni.

Hnh 2.1 S tng qut ca h thng thng tin quang coherenceKhi tn s tn hiu nhn c bng vi tn s ca tn hiu dao ng ni th my thu hot ng ch homodyne, tn hiu c ti to l tn hiu nm trong vng BB. Ngc li, khi tn s ca hai tn hiu khc nhau th my thu hot ng ch heterodyne, tn hiu ti to lc ny nm trong vng IF.2.3 Cc k thut iu ch quang coherent2.3.1 K thut iu ch ASKiu ch kha dch bin ASK (Ampitude Shift Keying): Sng iu bin c to ra bng cch thay i bin sng mang theo bin bng gc tn hiu.Phng php ny vi cc bit 0 v 1 lm thay i bin ca tn hiu sng mang, trong tc truyn tn hiu ASK b gii hn bi cc c tnh ca mi trng. Gi s in trng ca tn hiu quang c dng nh sau: (2.3). Khi iu ch ASK, pha v tn s c gi nguyn, ch c thnh phn bin thay i, i vi bit 1, i vi bit 0. Dng sng ca tn hiu ASK c biu din trong hnh 2.2.

Hnh 2.2 Dng sng tn hiu ASKKhc vi h thng quang s dng k thut IMDD, h thng quang s dng k thut coherence lung bit quang ca h thng c to thnh bng cch s dng k thut iu ch ngoi thay v c iu ch trc tip bng laser bn dn hoc led. S khc bit ny l do trong h thng coherence, p ng b tch sng quang ph thuc vo pha ca tn hiu thu c trong khi h thng IMDD th khng c s thay i khng mong mun . gi pha khng thay i th h thng coherence cn s dng laser bn dn hot ng lin tc ti mt dng c nh v dng b iu ch ngoi Mach-Zender.2.3.2 K thut iu ch PSKiu ch dch pha PSK (Phase Shift Key) l mt dng iu ch thng thy trong h thng thng tin quang coherence.Trong iu ch PSK, chui bit quang c to thnh bng cch iu ch pha , bin AS v tn s c gi nguyn. Tn hiu iu ch PSK nh phn c biu din nh sau: (2.4)Vi i vi bit 1 v i vi bit 0 tng ng vi cc trng hp pha ca tn hiu bng v 0. Dng sng ca tn hiu iu ch PSK c th hin trong hnh 2.3.

Hnh 2.3 Tn hiu iu ch PSKi vi iu ch PSK, pha ca sng mang quang cn c gi c nh thng tin c mang trn c ly ra mt cch chnh xc ti my thu. lm c iu ny th rng ng ca laser pht v laser dao ng ni phi t mt tiu chun nghim ngt. iu ch DPSK l dng iu ch m thng tin c m ha theo s chnh lch v pha gia hai bit k cn. Chng hn, nu , biu din pha ca bit th k v k-1 th lch pha s thay i l hoc 0 ph thuc vo bit th k l 1 hoc 0. u im ca DPSK l tn hiu pht i c th c gii iu ch thnh cng cho n khi pha ca sng mang duy tr n nh trong khong thi gian hai bit.2.3.3 K thut iu ch FSKiu ch dch tn s FSK (Frequency Shift Keying) l k thut iu ch m thng tin c m ha bng cch dch tn s sng mang. Vi tn hiu nh phn, tn s gc nhn hai gi tr v ph thuc vo bit truyn i l 0 hay 1. dch tn s c gi l lch tn s. Nh vy dch tn s gia bit 0 v 1l . Tn hiu iu ch FSK c biu din nh sau: (2.5)Bng thng ca tn hiu FSK xp x vi B l tc bit v vic la chn dch tn s ph thuc vo bng thng hin c. Nu gi l lch nh nh th c gi l h s iu ch tn s. Khi th iu ch FSK c gi l MSK vi ph c nn cht nn rt thch hp vi h thng tc cao. Khi th dng iu ch ny c gi l CPFSK hay cn gi l iu ch lch tn hp vi dng ph c nn rt cht. Khi th c coi l iu ch FSK lch tn rng, ph ca n chia lm hai phn tp trung xung quanh () v ().

Hnh 2.4 Tn hiu iu ch PSK2.4 Cc nguyn l c bn ca tch sng coherence2.4.1 Tch sng coherence

Hnh 2.5 Cu hnh cu my thu CoherenceHnh 2.5 M t cu hnh ca my thu coherence. tng c bn ca tch sng coherence l trn phn in trng ca tn hiu quang c iu ch vi tn hiu quang c to ra bi b dao ng ni. Tn hiu quang nhn c ti my thu v tn hiu quang c to ra bi b dao ng ni c dng nh trong biu thc (2.1) v (2.2).S tch sng cn bng thng c s dng cho my thu coherence nn thnh phn mt chiu v ti u ha dng photodiode. Mc ch ca vic s dng b ghp 3dB l dch pha mt trong hai tn hiu, hoc l tn hiu thu c, hoc l tn hiu ca b dao ng ni. Khi hai tn hiu c cng phn cc, in trng ti hai photodiode trn c biu din nh sau: (2.6) (2.7)Khi ng ra ca hai photodiode c dng nh sau:

(2.8)

(2.9)Vi ms c ngha l trung bnh bnh phng theo tn s quang, Re c ngha l ly phn thc, l tn s trung tn (IF) vi , v l pha ca tn hiu c truyn i v pha ca tn hiu to ra bi b dao ng ni. R l h s chuyn i quang in, c cho bi cng thc sau: (2.10)Vi e l in tch, l hiu sut lng t ca photodiode. Ng ra ca tch sng cn bng c biu din nh sau: (2.11) lun l hng s v ch bao gm nhiu pha.2.4.2 My thu HeterodyneTch sng Heterodyne lin quan ti trng hp ||, vi l bng thng iu ch ca sng mang quang c xc nh bi tc k t. Trong trng hp ny, biu thc (2.11) ch ra trng in ca tn hiu quang c h tn xung di IF v c biu din nh hnh 2.6.

Hnh 2.6 Ph ca a) tn hiu quang v b) tn hiu c h tn IFPha ca tn hiu c cho bi , vi l pha ca tn hiu iu ch, l pha nhiu. Ng ra ca my thu c cho bi (2.12)T biu thc (2.12) bin phc trn exp(j) c biu din nh sau: (2.13) l tng nhiu pha c xc nh nh cng thc sau: (2.14)2.4.3 My thu Homodyne Tch sng Homodyne lin quan ti trng hp . Dng quang t my thu homodyne c biu din nh sau: (2.15)Biu thc (2.15) ch ra my thu homodyne o lch pha gia tn hiu nhn c v tn hiu c to bi b dao ng ni nh hnh 2.7. gii m tn hiu LO cho ng, pha ca b dao ng ni phi bm theo pha ca tn hiu , iu ny c thc hin nh vng kha pha quang (OPLL). Tuy nhin trong thc t, vic thc hin vng khng n gin, lm tng tnh phc tp v cu hnh ca my thu homodyne. Thm vo , biu thc 2.15 ch ra ch c thnh phn cng pha vi tn hiu dao ng ni c tch sng cn thnh phn vung pha th khng. Do vy, my thu homodyne khng th tch ton b thng tin trn bin phc ca tn hiu.

Hnh 2.7 Gin pha ca tn hiu thu c v tn hiu to ra bi b dao ng ni tch sng c thnh phn tn hiu vung pha v cng pha vi b dao ng ni, ti my thu homodyne, ta dch pha b dao ng ni 900 bng cch s dng b ghp 900 nh hnh 2.8 a). Gin pha ca tn hiu thu c v dao ng ni LO trong trng hp ny c biu din nh hnh 2.8 b).

Hnh 2.8 a) S tch sng Homodyne vung pha vi PD cn bngb) Gin pha ca tn hiu thu c v dao ng ni trong trng hp s dng tch sng Coherence vung pha vi PD cn bngKhi s dng b ghp 900, chng ta c th thu c bn tn hiu E1, E2, E3, E4 t hai ng vo ES v ELO nh sau: (2.16) (2.17) (2.18) (2.19)Dng quang ng ra t cc cp PD cn bng c cho bi cng thc sau: (2.20) (2.21)T biu thc (2.20) v (2.21), chng ta c th khi phc bin phc nh sau: (2.22)Biu thc (2.22) ch ra trng in ca tn hiu n c h tn xung di bng gc. Nh hnh 2.9, ph ca tn hiu nm trong di bng gc bao gm c thnh phn tn s dng v m, cha c thnh phn cng pha v vung pha vi nhau. Ngc li, trong tch sng n s dng mt cp PD ch o thnh phn cng pha vi tn hiu dao ng ni, tn hiu bng gc ch tn ti trong phn tn s dng. Ni tm li, tch sng homodyne n s dng mt PD cn bng v homodyne vung pha c th ging nhau trong vic khi phc tn hiu phc quang nh c th hin trong biu thc (2.13) v (2.22). Tuy nhin, my thu homodyne vung pha ti to ra tn hiu nm trong di bng gc th tt hn loi ti to tn hiu trong min trung tn

Hnh 2.9 Ph ca a) tn hiu quang v b) tn hiu bng gc c tch sng homodyne. Tch sng theo kiu s dng mt cp PD n th ch ti to c phn thc ca bin phc quang, trong khi , tch sng theo kiu vung pha khi phc c ton b tn hiu phc gm c phn tn s dng v m2.4.4 Vng kha pha trong my thu quang coherenceS c bn ca my thu coherence s dng vng kha pha c th hin trong hnh 2.10. My thu trong s s dng k thut sng mang dn ng (pilot carrier) s dng cho tch sng quang homodyne PSK. Sng mang ny c to ra t iu ch pha nh hn 1800 s c t hp vi tn hiu nhn c coupler nh hng DC 3dB sau c tch sng bng b PD cn bng. Tn hiu ng ra ca b khuch i vi sai s lm chnh lch pha c s dng kha pha b dao ng ni di s iu khin bi b VCO sau khi i qua b lc vng. Vic s dng sng mang vi bt k cng sut no cho qua trnh kha pha cng lm gim nhy ca my thu. Hn na, cng sut ca tn hiu cn thit d pha ca sng mang vo c xc nh chnh xc ph thuc vo nhiu pha ca laser ngun v laser ca b dao ng ni c t hp cng nh bng thng ca PLL. Do , bng thng ca vng ti u s cho li pha nh nht v c th lm tng cht lng ca b thu quang homodyne.B thu Homodyne s dng vng kha pha Costas p dng cho tn hiu PSK c minh ha trong hnh 2.11. Tn hiu nhn c v tn hiu dao ng ni c t hp ti b Optical Hybrid sao cho hai tn hiu ny lch pha nhau 900 hai ng ra b tch sng quang.

Hnh 2.11 B thu vng kha pha quang CostasNg ra ca hai b tch sng quang s c khuch i, ri nhn vi nhau b Mixer v pha ca sng mang s c xc nh b lc thng thp. Tn hiu iu khin c lc v c s dng iu chnh tn s ca b dao ng ni theo nh cch ca vng kha pha quang sng mang dn ng.2.5 Cc yu t nh hng n nhy my thu2.5.1 Nhiu phaNhiu pha l mt trong nhng yu t quan trng lm gim nhy my thu trong h thng thng tin quang, thng lin quan n b pht quang v b dao ng ni. T biu thc (2.11) , ta thy s thay i v pha gia tn hiu ti v tn hiu ca b dao ng ni s dn n s thay i v dng ng ra, phn nh bn cht kt hp ca tch sng coherence, t lm gim h s SNR. Do c pha ca tn hiu ti v b dao ng ni cn phi c gi n nh trnh suy gim nhy. Do cc h thng quang coherence cn phi s dng cc laser bn dn n mode c rng ph hp v bc sng c th iu chnh c phi hp tn s sng mang vi tn s b dao ng ni sao cho to ra tn hiu trung tn IF c tn s ng yu cu.2.5.2 Nhiu cng Nhiu cng thng khng c xt ti trong tch sng trc tip nhng i vi trng hp my thu coherence, n l mt trong nhng nguyn nhn lm gim nhy my thu.Vic s dng b thu cn bng vi hai b tch sng quang c coi l mt gii php lm gim nhiu cng . S b thu cn bng c m t nh trong hnh 2.12.

Hnh 2.12 S my thu coherence cn bngCoupler 2x2 l loi ghp 2dB, n trn tn hiu ti vi tn hiu ca b dao ng ni. Ng ra ca b coupler c r nhnh ti hai b tch sng khc nhau, to hai dng Ip(+) v Ip(-) trn hai nhnh tng ng: (2.23) (2.24)Khi s dng s trn, ta thu c hai ci li sau: Th nht, nhiu cng gn nh c loi b. Th hai, tt c cng sut ca tn hiu ti v dao ng ni c s dng mt cch hiu qu2.5.3 Tn sc trong si quangTn sc trong si quang khng ch nh hng ti tc bit trong h thng IMDD m cn ti h thng coherence. Nguyn nhn l do h thng coherence cn s dng cc laser bn dn hot ng ch n mode dc vi rng ph hp. Hin tng chirp tn s c th trnh bng cch s dng cc b iu ch ngoi, hn na c th b tn sc ca si quang thng qua k thut cn bng in trong min IF.2.6 So snh gia h thng quang s dng iu ch cng tch sng trc tip (IMDD) vi h thng quang CoherenceH thng truyn dn quang Coherence c ba u im ni bt so vi h thng quang s dng k thut IMDD nh sau: Th nht, hn ch c nhiu bn thu c, thm ch khi cng sut tn hiu thu c nh, chng ta ch cn tng cng sut b dao ng ni l c c kt qu mong mun. Th hai, khi s dng cng ngh Coherence th ta c th s dng c ba kiu iu ch l cng , pha v tn s trong khi h thng IMDD ch cho php iu ch cng . Th ba, vi k thut tch sng Coherence vic chn lc tn s mt cch chnh xc c th t c bng cch s dng b lc tch sng quang chuyn tn hiu in sang quang. iu ny cho php ghp cc sng mang ph gn nhau, to iu kin khai thc ti a nng lc ca k thut SCM. Th t, k thut tch sng Coherence ci thin c h s tn hiu trn nhiu SNR dn ti nng cao nhy my thu cao hn so vi trng hp IMDD, cho php ko di c ly truyn dn ln hng trm kilomet. T u im th nht, i vi h thng IMDD khi tng cng sut pht dn n tng nh hng ca nhiu bn cng nh cc hiu ng phi tuyn khc trong si quang. Ngc li trong trng hp s dng Coherence, ta khng cn phi tng cng sut pht truyn i xa m ch cn tng cng sut ca b dao ng ni ti my thu m bo gi tr SNR ph hp. Do , h thng quang Coherence thch hp hn vi h thng quang c khong cch truyn dn ln.u im th hai cho php s dng linh hot cc phng php iu ch sao cho ph hp vi iu kin thc tin. iu ny c ngha l h thng truyn dn quang khng b ph thuc vo phng php iu ch, c th iu ch c nhiu loi tn hiu m vn m bo c yu t cht lng.u im th ba ca h thng Coherence r rng tng kh nng truyn dn ca h thng truyn dn, ti u ha bng thng knh truyn, c kh nng p ng c nhu cu truyn dn ngy cng cao.Tuy nhin, h thng quang Coherence cng c nhiu im km hn so vi h thng s dng k thut IMDD, c th nh sau: Tn s ca b dao ng ni v tn hiu n cn phi qua b iu khin phn cc to ra tn s trung tn c yu cu, trong khi , h thng IMDD ch quan tm ti tn s ca laser iu ch quang c ph hp vi photodiode c s dng hay khng. Trng thi phn cc ca LO v tn hiu phi ging nhau hon ton ti photodiode. rng ng ca LO v ngun quang phi thch hp vi kiu iu ch c s dng, trong khi i vi h thng IMDD, rng ngun quang ch c xc nh ch yu li tn sc si quang.2.7 Kt lun chngT nhng g tm hiu trong chng 2 ta rt ra c nhng kt lun quan trng sau: Trong h thng thng tin quang Coherence, ta c th s dng cc k thut iu ch s c bn nh ASK, FSK, PSK. Tch sng Coherence c hai k thut chnh l homodyne v heterodyne. Homodyne l loi mang li nhy thu tt hn tuy nhin vic gi cho pha ca tn hiu n v b dao ng ni bng nhau mt cch tuyt i l khng th trong thc t. Trong khi tch sng heterodyne ph hp hn vi cc ng dng thc t nhng n phi chp nhn nhy thu nh hn 3dB so vi trng hp homodyne. Tch sng Coherence gp phi mt s vn nh hng n nhy nh nhiu cng , nhiu pha, tn sc trong si quang gim nh hng ca cc yu t , ta cn phi gi c nh pha ca tn hiu ti v dao ng ni, s dng b tch sng cn bng, p dng cc phng php b tn sc Tch sng Coherence mang li nhiu li th hn so vi tch IMDD nh nhy cao hn, d dng khuch i tn hiu bng vic tng cng sut b dao ng ni, s dng hiu qu hn trong trng hp SCM.