OPTICAL FIBER Struktur, Karakteristik dan Fabrikasi
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CAHAYA, light
• Aliran partikel yang dalam pemancarannya memenuhi
hukum optik reflection & refraction, snell law .
• Radiasi gelombang elektromagnetik (fresnel, maxwell)
dengan diffraction, interference dan pengubahan
polarisasi selain mengalami reflection & refraction.
• Pendekatan “ray” yang juga dapat mengalami diffraction,
interference dan pengubahan polarisasi selain mengalami
reflection & refraction.
• Arah rambat cahaya dari sumbernya dapat diasumsikan
gelombang spheric ataupun plane
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Rambatan Cahaya
Gelombang merambatsecara spheric
If didekat sumber
Gelombang merambatsecara planar
If relatif jauh dari sumber
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Rambatan GEM
/ 2
2
)].(exp[),(
k
xk t j Aet x A oi Polarisasi bisa linearataupun elliptical / circular
tergantung nilai medan E
dan H.
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Axial Propagation Constant
axial propagation constant: In an optical fiber, the propagation constant
evaluated along the optical axis of the fiber in the direction of transmission. Note:
The real part of the axial propagation constant is the attenuation constant. The
imaginary part is the phase constant. [After 2196]
attenuation constant: 1. The real part of the propagation constant in any
electromagnetic propagation medium. Note 1: The attenuation constant is usually
expressed as a numerical value per unit length. Note 2: The attenuation constant
may be calculated or experimentally determined for each medium. 2. For a
particular propagation mode in an optical fiber, the real part of the axialpropagation constant.
phase constant: The imaginary part of the axial propagation constant for a
particular mode, usually expressed in radians per unit length.
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Polarisasi
Dengan asumsi bahwa cahaya merupakan gelombang EM, maka
ada phenomena “POLARISASI”
1. LINEAR (Vertical - Horizontal)
2. ELLIPTICAL (circular, RHCP, LHCP)
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Polarisasi
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Polarisasi
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Hukum Snellius
• Pemantulan cahaya terjadi apabila suatusinar mengenai batas antara dua médiumkemudian sinar dipantulkan pada médiumyang sama. Sinar yang mengenaipermukaan bidang pantul yang datar akanmenghasilkan sudut pantul yang samabesar dengan sudut datangnya.
• Sinar akan dibiaskan melalui bidang batasantara dua médium, apabila sinar mengenaimédium yang berbeda dan diteruskan kedalam medium yang berbeda tersebutdengan sudut yang proporsional terhadapbesarnya indeks bias medium yangdilaluinya.
• Jika medium yang dilalui sinar lebihrenggang maka sinar akan menjauhi garisnormal bidang batas.
• Tetapi jika sinar masuk pada medium yanglebih rapat terhadap medium semula, makasinar yang dibiaskan akan mendekati garisnormal bidang (Pedrotti, 1993).
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TIR (Total Internal Reflection)
• Apabila sudut sinar bias
mencapai sudut 90 maka
sudut yang dibentuk sinar
datang terhadap normaldisebut sudut kritis
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Karakteristik Serat Optik
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Index contoh
Corning’s SMF-28 optical fiber
Region 1300 nm 1550 nm
Core 1.4535 1.4505
Cladding 1.4483 1.4447
CladdingCore
Coating
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Numerical Aperture
• Sinar cahaya yang masuk ke dalam inti serat optik membentuksudut datang tertentu terhadap poros serat optik. Sudut yangmenuju ke arah permukaan serat optik (nudara = 1), tidak semuaakan diteruskan. Tetapi ada syarat tertentu agar sinar yang datangtersebut dapat diteruskan. Gambar berikut menunjukkan adanyasudut dimana sinar diterima oleh serat optik yang disebut sebagai
Numerical Aperture .
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Propagasi Cahaya di Serat
Optik
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Metode Drawing
• The fiber drawing step isessentially the same irrespectiveof the process used to make thepreform [91]. Figure 2.22 showsthe drawing apparatusschematically.
• The preform is fed into a furnacein a controlled manner where it isheated to a temperature of about2000◦C.
• The melted preform is drawn intoa fiber by using a precision-feed
mechanism.
• The fiber diameter is monitoredoptically by diffracting light emittedby a laser from the fiber.
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Pengujian
• A change in the diameter changes thediffraction pattern, which in turn changes thephotodiode current.
• This current change acts as a signal for a
servocontrol mechanism that adjusts thewinding rate of the fiber.
• The fiber diameter can be kept constant towithin 0.1% by this technique. A polymer
coating is applied to the fiber during thedrawing step. It serves a dual purpose, as itprovides mechanical protection andpreserves the transmission properties of thefiber.
• The diameter of the coated fiber is typically250 μm, although it can be as large as 900μm when multiple coatings are used.
• The tensile strength of the fiber is monitored
during its winding on the drum.• The winding rate is typically 0.2 –0.5 m/s.
• Several hours are required to convert asingle preform into a fiber of about 5 kmlength.
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