Tom qaboptical signals mus ncigib qho kev ncua deb los ntawm optical fiber, lawv tau txais attenuation thiab distortion, ua rau cov tswv yim thiab tso zis optical teeb liab pulses txawv. Qhov no manifests raws li amplitude attenuation thiab waveform broading ntawm optical pulses. Qhov ua rau ntawm qhov tshwm sim no yog qhov muaj qhov poob thiab dispersion nyob rau hauv lub optical fiber. Poob thiab dispersion yog qhov tseem ceeb tshaj plaws tsis tau piav qhia txog cov yam ntxwv ntawm kev sib kis ntawm cov fibers optical, txwv qhov kev sib kis deb thiab muaj peev xwm ntawm lub cev. Tshooj lus no feem ntau tham txog cov txheej txheem thiab cov yam ntxwv ntawm optical fiber ntau poob thiab dispersion.
★Cov yam ntxwv ntawm optical fibers (Pib 2)
Poob yam ntxwv ntawm optical fiber
Optical fiber poob ua rau lub teeb liab attenuation, li no optical fiber poob kuj hu ua attenuation. Raws li qhov kev ncua deb ntawm optical fiber ntau, qhov kev siv ntawm lub teeb teeb liab txo qis, raws li hauv qab no: P(z)=P(0) /10 - (4) qhov twg P(z) yog lub hwj chim kho qhov muag ntawm kis nrug z; P(0) yog lub zog kho qhov muag tso rau hauv qhov muag fiber ntau, piv txwv li, lub zog kho qhov muag txhaj ntawm z=0; (λ) yog qhov kho qhov muag fiber ntau attenuation coefficient ntawm wavelength hauv dB/km; thiab L yog qhov kis tau tus mob.
Thaum t=L, fiber ntau attenuation coefficient yog txhais raws li
(λ)=(10/L) lg[P(0)/P(L)]
Thaum lub sij hawm ua hauj lwm wavelength λ yog dB, yog hais tias lub attenuation coefficient yog ntsuas nyob rau hauv units ntawm dB ib kilometer, ces A (λ) (unit yog dB) yog qhia raws li:
A(λ)=10 lg[P(0)/P(L)]
Kev sib txuas lus fiber ntau tau tsim nrog kev txhim kho txuas ntxiv hauv kev tsim kho qhov muag fiber ntau, tshwj xeeb yog qhov txo qis ntawm fiber ntau. Fiber poob yog ib qho tseem ceeb ntawm kev txiav txim siab qhov kev ncua deb ntawm qhov kev sib txuas lus fiber ntau optical. Ntau yam ua rau muaj fiber ntau poob, feem ntau nqus poob, tawg poob, thiab poob ntxiv, thiab cov txheej txheem hauv qab no poob yog qhov nyuaj heev. Cov kev sib tham hauv qab no siv silica optical fiber ua piv txwv los qhia txog ntau yam ua rau poob.
Absorption Poob
Absorption poob feem ntau suav nrog kev nqus dej, impurity absorption (OH radicals), thiab kev nqus dej tsis haum. Intrinsic absorption muaj xws li infrared thiab ultraviolet absorption.
Infrared absorption yog qhov nqus ntawm lub teeb lub zog los ntawm molecular resonance thaum lub teeb dhau los ntawm quartz iav uas muaj SiO2. Piv txwv li, qhov nqus peaks ntawm Si-O yog ntawm 9.1 μm, 12.5 μm, thiab 21.3 μm, thiab qhov nqus ntawm fiber ntau yog siab li 10 dB / km ntawm 9.1 μm. Ultraviolet absorption yog lub zog absorbed thaum electrons zoo siab rau kev hloov mus rau qib siab zog los ntawm lub teeb tsis muaj zog. Qhov kev nqus no tshwm sim hauv cheeb tsam ultraviolet thiab yog li feem ntau hu ua ultraviolet absorption. Cov ntaub ntawv iav muaj qhov hloov pauv hlau ions xws li hlau thiab tooj liab, nrog rau OH- ions.Impurity absorption yog qhov poob los ntawm kev nqus ntawm lub teeb lub zog los ntawm cov kauj ruam hluav taws xob tsim los ntawm ion vibrations nyob rau hauv lub teeb yoj excitation. Piv txwv li, ntawm 1.39 μm, qhov attenuation yog 60 dB / km thaum OH - ion concentration yog 1 × 10⁻⁶.
Scattering poob
Scattering poob yog qhov poob uas radiates lub teeb lub zog tawm ntawm optical fiber nyob rau hauv daim ntawv ntawm scattering. Nws yog tshwm sim los ntawm qhov tsis yog -ib txwm ntom ntom hauv fiber ntau. Lub hom phiaj tseem ceeb ntawm kev tawg tawg hauv cov khoom siv kho qhov muag suav nrog Rayleigh scattering, Mie scattering, stimulated Brillouin scattering, stimulated Raman scattering, ntxiv cov yam ntxwv tsis xws luag thiab dabtsi yog khoov scattering, thiab tawg tawg.
Thaum lub sij hawm tsim kho qhov muag fiber ntau, lub thermal txav ntawm cov molecules nyob rau hauv lub molten iav ua rau qhov sib txawv ntawm qhov ceev thiab refractive index nyob rau hauv nws cov qauv, uas nyob rau hauv lem ua rau lub teeb tawg. Scattering tshwm sim los ntawm cov khoom me me dua lub wavelength ntawm lub teeb yog hu ua Rayleigh scattering; tawg los ntawm cov khoom ntawm tib lub wavelength li lub teeb hu ua Mie scattering.
Rayleigh tawg yog thawj qhov ua rau poob fiber ntau. Rayleigh scattering nthuav tawm cov khoom ntawm qhov sib npaug rau 1 / λ ntawm qhov luv wavelength, piv txwv li, R=K / λ. Qhov proportionality tas li K yog cuam tshuam nrog cov iav qauv thiab muaj pes tsawg leeg. Feem ntau, qhov kub hloov pauv ntawm iav ntau dua thiab qhov nyuaj ntawm nws cov muaj pes tsawg leeg, qhov ntau dua Rayleigh tawg tawg.
Rayleigh tawg yog cuam tshuam los ntawm kev siv ntawm qhov teeb meem teeb meem. Stimulated Brillouin scattering and stimulated Raman scattering, on the other hand, occur when the light energy density exceeds a certain high value and are produced by the interaction between light and the medium.
Kev poob ntxiv
Kev poob ntxiv (lossis daim ntawv thov poob) yog qhov poob los ntawm lwm qhov chaw, xws li tshwm sim los ntawm fiber ntau twisting lossis lateral siab thaum lub sij hawm tsim, installation, thiab kev ua haujlwm, ua rau macro-bending thiab micro-bending ntawm fiber.
Qhov ua rau poob fiber ntau tau sau tseg hauv daim duab:
| Qeb | Sub-Qeb | Paub meej / piav qhia |
|---|---|---|
| Absorption Poob | Intrinsic Absorption | • Infrared absorption • Ultraviolet absorption |
| Extrinsic Absorption | Ua los ntawm impurities xws li Fe, Cu, cov hlau hloov pauv, thiab kev vibrational nqus ntawm OH⁻ | |
| Scattering Loss | Linear Scattering | |
| - Rayleigh tawg | Scattering los ntawm cov khoom me me dua li qhov kho qhov muag wavelength | |
| - Mie tawg | Scattering los ntawm cov khoom piv rau qhov loj me rau lub qhov muag wavelength | |
| Nonlinear Scattering | ||
| - Txhawb nqa Brillouin tawg | tshwm sim thaum optical fais fab tuag ntau tshaj qhov qis dua | |
| - Rho tawm Raman tawg | tshwm sim thaum optical fais fab tuag ntau tshaj qhov pib dua | |
| Kev poob ntxiv | - | Kev poob los ntawm microbending, macrobending, stretching, compression, thiab mechanical deformation |
Dispersion yam ntxwv ntawm optical fibers
Hauv physics, dispersion yog hais txog qhov tshwm sim uas lub teeb ntawm cov xim sib txawv tau tawg tom qab dhau los ntawm qhov nruab nrab pob tshab. Lub teeb ntawm lub teeb dawb tau muab faib ua xya- xim band tom qab dhau los ntawm ib qho prism. Qhov no yog vim iav muaj qhov sib txawv refractive indices rau cov xim sib txawv (ntau zaus los yog sib txawv wavelengths). Qhov ntev ntawm lub wavelength (los yog tsawg zaus), qhov qis dua qhov ntsuas ntawm lub iav; qhov luv luv wavelength (los yog siab dua qhov zaus), qhov siab dua qhov refractive index. Nyob rau hauv lwm yam lus, lub refractive index ntawm iav yog ib tug muaj nuj nqi ntawm lub zaus (los yog wavelength) ntawm lub teeb yoj. Thaum lub teeb dawb tsim los ntawm cov xim sib txawv yog xwm txheej ntawm tib lub kaum sab xis θ, raws li txoj cai ntawm refraction (n =} sinθ / n²), cov xim sib txawv ntawm lub teeb yuav muaj qhov sib txawv ntawm qhov sib txawv vim qhov sib txawv n² qhov tseem ceeb, yog li cais cov xim sib txawv ntawm lub teeb, ua rau tawg. Txij li thaum n=c/n (qhov twg c yog qhov ceev ntawm lub teeb, c=3 × 10⁻⁶ m / s), nws paub tseeb tias cov xim sib txawv ntawm lub teeb mus txawv tebchaws nyob rau hauv lub khob.
Hauv optical fiber propagation txoj kev xav, lub ntsiab lus ntawm lo lus "dispersion" tau nthuav dav. Hauv cov fibers optical, cov teeb liab tau nqa thiab xa los ntawm lub teeb tsis muaj ntau hom lossis ntau zaus. Thaum lub teeb liab mus txog lub davhlau ya nyob twg, qhov sib txawv hom lossis zaus ntawm lub teeb tsis muaj kev sib kis qeeb sib txawv, ua rau muaj teeb meem cuam tshuam. Qhov no tshwm sim yog ua ke hu ua dispersion. Rau cov teeb liab digital, dispersion ua rau mem tes dav tom qab propagating ib qho kev ncua deb ntawm fiber ntau. Hauv cov xwm txheej hnyav, cov pulses sib law liag yuav sib tshooj, tsim cov cim kev cuam tshuam. Yog li ntawd, dispersion txiav txim siab qhov kis tau tus mob bandwidth ntawm lub optical fiber thiab txwv lub kaw lus kis tus nqi los yog repeater nrug. Dispersion thiab bandwidth yog tib yam ntxwv ntawm optical fibers tau piav qhia los ntawm ntau qhov kev xav.
Raws li qhov ua rau dispersion, optical fiber dispersion feem ntau muab faib ua: modal dispersion, khoom dispersion, waveguide dispersion, thiab polarization hom dispersion, uas yuav qhia hauv qab no.
Hom Dispersion
Modal dispersion feem ntau muaj nyob rau hauv multimode fibers. Vim tias ntau hom kev sib koom ua ke nyob rau hauv ib lub multimode fiber ntau, thiab cov pab pawg propagation speeds ntawm txawv hom ntawm fiber axis txawv, lawv yuav inevitably tuaj txog ntawm lub davhlau ya nyob twg ntawm txawv sij hawm, ua rau lub sij hawm ncua sij hawm sib txawv thiab tsim intermodal dispersion, yog li ua rau mem tes dav dav. Cov mem tes nthuav dav vim yog modal dispersion yog pom hauv daim duab 2-10. Rau ib qho zoo tagnrho - hom fiber ntau, txij li tsuas yog ib hom (tsim hom - LP lossis HE hom) kis tau, tsis muaj modal dispersion, tab sis polarization hom dispersion muaj.
Tam sim no, peb kwv yees qhov siab tshaj plaws modal dispersion ntawm ib kauj ruam -index multimode fiber. Lub modal dispersion ntawm ib kauj ruam -index multimode fiber ntau yog qhia nyob rau hauv daim duab 2-11. Nyob rau hauv ib kauj ruam -index multimode fiber ntau, ob qhov ceev tshaj plaws thiab qeeb tshaj plaws propagating rays yog cov kab hluav taws xob ① propagating raws axis thiab cov kab hluav taws xob ② teeb meem ntawm lub kaum sab xis ntawm 0 degree, feem. Yog li ntawd, lub siab tshaj plaws hom dispersion nyob rau hauv ib kauj ruam-index multimode fiber ntau yog lub sij hawm sib txawv ntawm lub sij hawm coj los ntawm lub hnub qub ② (Tmax) thiab lub sij hawm coj los ntawm hluav taws xob ① (Tmin) mus txog lub davhlau ya nyob twg, ΔTmux: ΔTmux = Tmax / Tmin
Raws li geometric optics, nyob rau hauv ib tug optical fiber ntev L, cia lub speeds ntawm lub teeb rays ① thiab ② raws axial kev taw qhia yog c / n thiab sinθ·c / n, feem. Yog li ntawd, modal dispersion ntawm optical fiber yog ...
Nyob rau hauv tsis muaj zog coj optical fibers (fibers qhov twg nithiab nitxawv me ntsis), A=(ni- n)/n. Yog Δ=1%, ni= 1.5 rau silica optical fibers, thiab qhov ntev fiber ntau yog 1 km, ces qhov siab tshaj plaws intermodal dispersion ΔTmtuaj yeem suav ua 50 ns. Yog li ntawd, nws yog pov thawj tias qhov ntev ntawm fiber ntau, qhov loj dua qhov intermodal dispersion; thiab qhov ntau dua qhov txheeb ze refractive index sib txawv Δ, qhov loj dua qhov intermodal dispersion.
Khoom siv dispersion
Vim hais tias qhov refractive index ntawm optical fiber ntau sib txawv nrog lub wavelength ntawm lub teeb, cov pab pawg neeg tshaj tawm ntawm ntau zaus ntawm cov teeb liab optical sib txawv, ua rau ib tug kis tau tus mob ncua sij hawm sib txawv, ib tug tshwm sim hu ua khoom dispersion. Qhov no dispersion nyob ntawm tus yam ntxwv wavelength ntawm lub refractive index ntawm cov khoom siv fiber ntau optical thiab linewidth ntawm lub teeb qhov chaw.
Hauv digital fiber optic kev sib txuas lus systems, lub teeb tso zis los ntawm lub teeb tiag tiag tsis yog ib lub wavelength nkaus xwb tab sis muaj qee qhov spectral linewidth. Txij li thaum lub refractive index ntawm cov ntaub ntawv fiber ntau yog ib tug muaj nuj nqi ntawm wavelength, lub propagation ceev ntawm lub teeb nyob rau hauv nws (λ)=c/n(λ) kuj txawv nrog lub wavelength. Thaum lub teeb mem tes tawm los ntawm lub teeb pom kev zoo nrog qee qhov spectral linewidth yog tshwm sim ntawm ib qho - hom fiber ntau thiab propagates, lub teeb pulses ntawm cov wavelengths sib txawv yuav muaj qhov sib txawv nrawm, uas ua rau lub sij hawm ncua sij hawm sib txawv thaum lawv mus txog qhov kawg ntawm cov zis, yog li ua rau cov mem tes dav dav. Qhov no yog lub mechanism ntawm cov khoom dispersion.
Yog tias pab pawg tshaj tawm tau paub tias yog u=da/dB, ces pab pawg ncua sij hawm ib chav tsev ntev yog T=1/v,=n,/c. Yog li ntawd, cov ntaub ntawv dispersion ntawm ib tug optical fiber ntev L yog ...
Hauv cov qauv, c yog qhov ceev ntawm lub teeb hauv lub tshuab nqus tsev; λ yog qhov refractive index ntawm cov tub ntxhais fiber ntau; λ yog lub wavelength ntawm lub teeb; thiab Aλ yog spectral linewidth ntawm lub teeb qhov chaw, qhov twg Aλ=λ - λ, sawv cev rau lub wavelength range nyob rau hauv A. Feem ntau, cov dispersion coefficient yog siv los ntsuas qhov loj ntawm dispersion. Lub dispersion coefficient D (unit: ps / (nm·km)) txhais tau tias yog ...
Nws tuaj yeem pom tau tias qhov sib txawv ntawm qhov sib txawv yog qhov cuam tshuam los ntawm qhov chaw teeb pom kev zoo nrog ib chav spectral linewidth propagating nyob rau hauv ib chav tsev ntev ntawm optical fiber. Yog hais tias cov khoom dispersion coefficient ntawm optical fiber yog paub, cov khoom dispersion tau yooj yim xam raws li ΔTm=DmAAL.
Piv txwv 2-1: Piv txwv tias qhov siab tshaj plaws ntawm cov khoom dispersion coefficient ntawm fiber ntau optical ntawm lub wavelength ntawm 1.31m yog D=3.5ps/(nm·km). Yog hais tias ib tug semiconductor laser nrog ib tug nruab nrab wavelength ntawm 1.31µm yog siv los tsim kis tau tus mob lub teeb nrog ib tug spectral linewidth ntawm λ=4nm, xam cov khoom dispersion tshwm sim los ntawm lub teeb propagating nyob rau hauv ib tug 1km ntev ntawm optical fiber.
Kev daws: Cov khoom dispersion ntawm optical fiber tau yooj yim xam raws li:
Tm = DmLΔA=3.5ps/(nm·km) x 1km x 4nm=0.014ns=14ps
Raws li pom nyob rau hauv Piv txwv 2-1, cov khoom dispersion yog me me, txawm tias me dua qhov modal dispersion ntawm ib kauj ruam-index multimode fiber ntau. Nws yuav tsum tau muab sau tseg tias dispersion coefficient ntawm ib tug optical fiber (tsis yog cov khoom dispersion coefficient) yuav zoo los yog tsis zoo. Nyob rau hauv optical fiber, cov pab pawg neeg ncua (A) nce nrog cov cab kuj wavelength; nyob rau hauv lwm yam lus, luv luv wavelength lub teeb tsis propagate sai.Qhov no, lub dispersion coefficient yog tsis zoo, hu ua negative dispersion; conversely, ntev wavelength lub teeb tsis propagate qeeb dua lub luv wavelength lub teeb tsis.
Ntawm no, lub dispersion coefficient yog zoo, hu ua zoo dispersion. Kom meej meej, yog tias ob lub optical fibers nrog qhov sib txawv ntawm qhov sib txawv ntawm qhov sib txawv coefficient yog fused ua ke, cov khoom dispersion yuav zoo dua.
waveguide dispersion
Waveguide dispersion ΔTw yog hais txog ib qho kev qhia tshwj xeeb hauv kev kho qhov muag fiber ntau. Cov wavelengths sib txawv muaj cov theem tsis tu ncua, ua rau cov pab pawg sib txawv thiab yog li dispersion. Waveguide dispersion kuj muaj feem xyuam rau ntau yam xws li cov qauv ntawm cov khoom siv fiber ntau thiab cov txheeb ze refractive index sib txawv ntawm cov tub ntxhais thiab cladding; yog li ntawd, nws tseem hu ua structural dispersion.
Polarization hom dispersion
Polarization hom dispersion yog ib hom dispersion tshwj xeeb rau ib leeg -hom optical fibers. Vim hais tias ib leeg - hom fibers yeej kis tau ob qho tib si orthogonal polarization hom, lawv cov hluav taws xob teb yog polarized raws x thiab y cov lus qhia, feem.
Fiber ntau optic bandwidth
Lub dispersion thiab bandwidth ntawm optical fibers piav txog tib yam ntxwv. Qhov tseeb, dispersion piav txog qhov uas lub teeb mem tes nthuav dav raws lub sijhawm axis tom qab kis tau tus mob; nws yog ib qho kev piav qhia ntawm fiber ntau cov yam ntxwv hauv lub sijhawm sau npe. Bandwidth, ntawm qhov tod tes, piav qhia txog tus yam ntxwv no hauv thaj chaw zaus. Nyob rau hauv qhov zaus, rau lub teeb liab modulating, cov fiber ntau optical tuaj yeem suav tias yog qhov qis - dhau lim. Thaum lub siab - zaus ntawm cov teeb liab modulating dhau los ntawm nws, lawv raug mob hnyav heev. Ntawd yog, yog tias qhov amplitude ntawm lub teeb liab tawm tswv yim (modulating teeb liab) tseem nyob tas li, tab sis tsuas yog qhov zaus hloov pauv, qhov amplitude ntawm cov zis teeb liab tom qab kis tau tus mob los ntawm fiber ntau yuav hloov nrog qhov zaus ntawm cov teeb liab modulating (cov teeb liab input). TTU-T pom zoo kom qhia meej tias bandwidth ntawm fiber ntau optical yog [bandwidth ib kilometer].
