faq- optical loss test set-vn.pdf

Cc thng s cn quan tm khi la chn my o suy hao quang,o cng sut quang hay ngun pht quang

Nn chn OTDR hay my o cng sut quang xc nh suy hao tuyn ? o chnh xc suy hao hai u ca mt h thng quang , thng thng ngi ta dngmy o suy hao quang OTS hoc my o cng sut quang kt hp vi b ngun phtquang.Vic dng my o OTDR xc nh suy hao tuyn th khng chnh xc bi v kt qu domy OTDR tnh ton a ra da trn mt s yu t tng i c th thay i theo nhit v cc lc ang tc ng ln si quang. o chnh xc suy hao mi hn hoc im ni connector quang bng my OTDR, ta cnthc hin o kim ti hai u tuyn sau ly gi tr trung b nh v iu ny thc khngn gin. My o OTDR ch c u im khi cn xc nh v tr c s c hoc chiu di cp.

Mc cng sut pht v mc nhn ti a ca thit b o bao nhiu th ?Khi thit k mt tuyn truyn dn quang, ta cn cn nhc v mc cng sut ti a ca mypht, nhy thu v suy hao tuyn danh nh. Sau , ta cn tnh ton v cng thm mcsuy hao d tr cho tuyn cho trng hp cht lng cc ph kin quang b suy gim mc xu nht theo tui th h thng hoc la chn tn hiu thu khng qu cao . V vicc lng suy hao d phng cho tuyn l vic rt quan trng, cn lm, nu khng vicsa cha v bo tr mng s kh khn, tn nhiu chi ph hoc thng th thc hin c.Trong nhiu trng hp nh khong cch truyn dn thay i hoc s dng ng thinhiu bc sng khc nhau, th vic lp thm b suy hao quang l cn thit k bom cng sut u my thu khng v t qu mc cho php.

Theo tiu chun ngnh TCN 68-173:1998 th :Mc cng sut pht quang ti a ca thit b pht l -15 n +17dBmMc thu ti a ( mc qu ti ca b thu) l 0dBm.Mc thu ti thiu ( nhy thu) l -34dBm( tham kho cc bng ch tiu giao din quang cho h thng STM1/STM4/STM16 /STM64t trang 11 n trang 21- Tiu chun thit b hin nay c th cao h n)

Do , chng ta nn chn thit b o thu pht quang trong di t -50dBm n +20dBmhoc ty theo cu hnh mng cn o kim la chn my c di o ph hp nhu cu sdng, trnh lng ph khi chn di o qu ln.

Ngun pht quang khng n nh nh h ng th no n vic o suy haoquangHu ht cc b ngun pht quang (light-source) u c lch bc sng nht nh(dungsai).V d: Ngun pht laser "1310 nm " c dung sai l 30 nm. Hn na cn c dung sai donhit thay i (5nm),kt qu ta c bc sng thc s c pht ra l 1310 35 nm.Nu dng ngun pht LED, rng ph l 50 nm v ngun pht ra c th l mt bcsng bt k trong di 1310 85 nm (hay 1225 1395nm). V nh chng ta bit, suyhao tiu biu ca si quang cng s thay i mt lng ng k theo bc sng

Khi ngun pht quang c bc sng khng n nh, s sinh ra s khc bit v h thngang c o kim v h thng ang vn hnh. Ni mt cch c th, i vi tuyn quang50 Km, sai s suy hao tuyn gy bi dung sai b c sng 30 nm s vo khong 0.95dB.

Tm li Suy hao phi c o di bc sng hot ng ca h thng. Nn o suy hao c hai hng loi tr trng hp tuyn quang khng ng b

v ng knh trng mt. Nn o suy hao bi my o suy hao quang hoc my o cng sut quang kt hp

vi ngun pht quang.

Cn cn nhc v lch bc sng cho php kt qu o chnh xc cng nh trn h thng quang c lp t.

Xc nh d tr suy hao tuyn (optical margin) cho tng lai l vic lm quantrng bo m duy tr hot ng ca tuyn quang khi xung cp theo thi gianvn hnh.

Tnh ton d tr suy hao cho tuyn quang:Suy hao d phng (loss budget) l mc suy hao ti a cho php cn tnh ton tr c khituyn quang b ngng hot ng . Gi tr ny c xc nh da trn mc cng sut pht, nhy thu v lch suy hao (optical margin) . Mc suy hao ca cc thnh phn mngs c c lng v cng thm mc suy hao d phng cho trng hp mng xungcp mc xu nht.i vi tuyn quang ng di hay tc cao, vic c lng suy hao d phng choton tuyn yu cu phi c tnh ton bi ngi c k nng v nhiu kinh nghim.i vi tuyn quang ngn hay tc dch v y u cu thp, gi tr suy hao tuyn thngn gin hn hoc c nh ngha theo mt quy chun ri ng ca dch v.

Khng ging nh suy hao d phng (loss budget), lch suy hao (optical margin) linquan n s thay i thc s ca suy hao tuyn tr c khi dch v ngng hot ng.Ch rng lch suy hao c th tng hoc gim.

Suy hao danh nh ca mt s ph kin quang :Vt t Suy hao

1310 nm 0.35 - 0.4 dB/kmSi quang n mt

1550 nm 0.21 dB/km

850 nm 3 - 4.1 dB/kmSi quang a mt

1310 nm 0.6 - 1.1 dB/km

u ni quang 0.1 - 0.75 dB

Mi hn quang 0.1 - 1.5 dB

Mi ni c kh 0.1 - 1 dB

B chuyn mch quang 0.1 - 1.5 dB

B chia/ghp quang 1 - 15 dB(ty thuc cng sut chia)

B cch ly( trit tiu tn hiu phn hi tr v ngun phtquang)

0.5 - 5 dB

Cc gi tr biu kin nu ra trong bng trn ch l gi tr trung bnh:gi tr thc t cn tythuc nhiu vo loi thit b c p dng cho tuyn.Gi tr v suy hao ca connector l khng chc chn: cng mt cp connector s s chnhlch suy hao mt cch ngu nhi n ty theo vic u ni. Suy hao nu trn thng s kthut ca connector quang ch l gi tr trung bnh v c o kim iu kin chun nhtnh.

Nn chn my o c phn gii bao nhi u dBm / dB ?Thng thng my o c thit k vi cc phn gii 0.1 n 0.0 01 dB / dBm, vi ccgi thnh theo cc cp khc nhau :

phn gii cao nh 0.001 dBm / dB ( 0.023% ) thch h p ng dng trong cc iu kinnghin cu v iu khin trong phng th nghim.Thm ch t khi s dng phn giicao nh th.

phn gii 0.1 dB ( 2.3% )c l l khng . phn gii ny ch thch hp trong vic omc cng sut tuyt i, khng l t ng trong vic o suy hao mi h n, im ni v sais do mt n nh s vt qu mc gii hn 0.14 dB , gii hn trong vic hin th k tqu .

phn gii 0.01 dB ( 0.23% ) l tng cho vic o kim cc h thng truyn dn quang .y l l do ti sao Kingfisher ch sn xut my vi phn gii n 0.01 dB.

Vi h thng quang tc cao, ch nh lch mc cng sut thu kh nh do cnmy o c chnh xc cao v phn gii cao hn.V d :i vi h thng quang tc thp d tr suy hao 0 - 25 dB, trong khi mt mngquang LAN tc cao c suy hao d tr ch t 0 - 2.5 dB.

Nn o kim suy hao quang ti bc sng no ?

Microbend loss l suy hao si quang gy bi im cong si quang qu mc, lc c kh,nhit tc ng ln si quang, li sn xut si...

Si quang n mt thng nhy cm vi cc lc tc ng c kh v lc un cong siquang nhiu nht ti bc sng c gi tr ln hn 1480 nm. l l do ti sao nn kimtra suy hao ti 1550 nm khi bo tr hay lp t tuyn quang thm ch khi tuyn quang svn hnh bc sng 1310 nm.

Phn bit cc loi diode cm ng quang c thit k trong my o cngsut quang c trn th trng ?

B cm nhn quang InGaAs :Hot ng tt trong vng bc sng 1000 n 1650 nm, tuy nhi n chnh xc ca loiny ti 850 nm th khng tt v vn nhy. Loi ny rt thch hp dng trn hthng cp quang n mt .y l loi u cm nhn quang cng ngh mi v gi tr caohn loi Ge (Germanium). H thng loi InGaAs c chnh xc cao trong v ng bcsng hot ng nn c khuyn ngh s dng trong cc h thng ghp k nh WDM.

B cm nhn quang Ge (Germanium)S dng cng ngh vi gi thnh thp v hot ng tt trong phm vi bc sng 850 n1550 nm nhng khng vt qu 1550 nm. y l gii php c ng dng tt cho c siquang n mt v a mt 850 & 1300 nm, i khi s dng cho b c sng 1550 nm.Loimy o cng sut quang vi cm ng quang Ge cng o c cc bc sng ca nhsng thy c 630 - 670 nm trn si quang plastic v my xc nh li bng nh sngthy c. chnh xc khi o ti 1550nm s b gim do nhy quang , c bit l 1580 nm vi nhit mi trng thp. Nu dng cho h thng ghp knh WDM vi bcsng > 1550 mn th phi chp nhn sai s lch vi dB.

B cm nhn quang Si (Silicon)Cho kt qu chnh xc ti bc sng 850 nm v cc bc sng thy c. Do cnghot ng tt vi bc sng 860 nm trong h thng cp quang a mt & mng cp ni b(PCS), v bc sng 660 nm trong h thng cp quang plastic (POF ).

B cm nhn quang 'High power'B cm nhn quang 'High power' l mt trong cc b nhn quang nu trn c thit kthm b lc suy hao t pha trc. Mc cng sut nhn mi b c sng s khc nhauty thuc vo b suy hao , nhy quang, nhy phn cc v mc phn x , tt c unh hng xu n chnh xc ca my o.B to suy hao c s dng trn thit b oca Kingfisher c thit k khc phc cc im tr n mt cch tt nht.