Matakuliah : H0122 / Dasar Telekomunikasi
Tahun
: 2008
Transmisi Optik
Pertemuan 10
Learning Outcomes
Mahasiswa dapat menyebutkan prinsip transmisi optik, jenis
komponen serta kelebihan dan kerugiannya dibandingkan media
lainnya.
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Outline Materi
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Karakreristik Serat optik
Jenis & kinerja serat optik
Sumber & detektor
Sistem Transmisi
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Basic Components
Input
information
signal
Signal conditioner
(encoder for digital)
Light source,
LED or laser
Light/fiber coupling
mechanism
Optical fiber
Fiber/detector
coupling
Signal conditioner
(decoder for digital)
Photo detector
Output information
Amplifier
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Characteristiscs
• Characteristics
– Use around 360 terahertz (3.6 x 10 14 Hz)
– Use wavelength between 600 and 1500 nanometer
– Visible light spans 430 to 690 nm
• Advantages:
– Tremendous bandwidth and high rate which can support 100
Mbit/s
– No interference
– Immune to nearby signals and EMI/RFI
– Complete electrical isolation between end of the link
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Adv/Disadvantages
• Advantages:
– Tremendous bandwidth and high rate which can support 100 Mbit/s
– No interference, immune to nearby signals and EMI/RFI
– Complete electrical isolation between end of the link
– Secure from unauthorized listeners
– Can be used in danger area, Lightweight
• Disadvantages:
– Cost relatively expensive
– Difficult to splice optical fibers to make them longer or to repair breaks
– Complex to attach to the cable and require precise physical equipment
– Switching and routing of fiber optics signals is difficult
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Fiber Optic
• Optical fiber is a thin strand glass or plastic and consist of
three parts:
– Core - is transmission area of the fiber
– Cladding - has a different index of refraction
– Coating - surrounds the core and cladding
• Core and cladding act as an optical waveguide or light pipe
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Fiber Types
• Fibers are identified by the type of paths, or modes
– Multiple pathway  multimode
• Dispersion limit the maximum data rate and bandwidth
– Dispersion is much less critical at lower data rates
• Two types of multimode fibers :
– Step index and graded index
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Fiber Types
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Step Index
• Show a sharp-like difference in the refractive index of the core vs
the surrounding cladding
• Relatively inexpensive to produce and useful at lower rates or
shorter distance
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Graded Index
• More complex
• Contain many thin layers, each with a lower index of refraction than
the adjacent inner core
• The dispersion of graded index fibers is less than step index fibers
• Useful at longer distances and higher rates but more costly
• Diameter of core is 50 through 100 μm
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Mono/Single Mode
• Only a single light wave ray path or mode to be transmitted down to
core
• Virtually eliminating any possibility of dispersion and overlap
• The core diameter is about 5 μm
• Gives the highest rate and longest distances
• Highest overall cost
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Fiber Performance
• The two most important parameter are bandwidth and transmission
loss or attenuation
• Factors that limit the bandwidth are dispersion and transmission loss
• The attenuation due to transmission loss reduces the optical signal
power, measured in dB/km
• There are five causes of this attenuation and loss of optical power
within the fiber: optical fiber loss, micro bending loss, connector loss,
splice loss, and coupling loss.
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Light Source
• There are two fiber optic sources: light emitting diode (LED) or laser
diode
• LED are low cost and produce output that is nearly linear with the
applied drive current, current greater than a very low threshold level
must be applied to them before they begin to operate
• LED has efficiencies between 30 and 70%
• LED used for shorter distance fiber optics system, up to several tens
of kilometers
• LED can only achieve pulse rates up to about 100 Mhz
• The spectrum of output LED is not very sharp is about an 850 nm
center
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Laser Diodes
• The relatively large numerical aperture of LEDs requires larger fiber
for effective coupling of emitted light into the fiber
• Uses a much smaller junction area than that in LED and the
concentration of injected carriers (holes and electrons) is much
higher
• Can produce large amount of optical output power (20 to 100 mW)
with a very narrow output spectrum
• Produces a very tightly directed beam with small NA which can be
directed into the fiber with little loss
• Suitable for use with the thin single mode fibers
• Can operate at rates exceeding 1 Ghz but need more complex
circuitry to drive
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Light Detectors
• Detectors converts the small amount of light energy received
(photon) into an electrical signal efficiently.
• Detector must be a low noise device
• Three type of devices are used as detectors:
1. Photoconductors
Inexpensive, have dark current that reduces overall
performance of the system, and its resistance is determined by
the device gain
2.PIN diodes
3.Avalanche photodiodes
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Light Detectors
2. PIN diodes
– Relatively inexpensive, have low noise, and interface easily with
conventional electronic circuitry.
– Fabricated from Si for maximum sensitivity at 850 nm, and Ge or
InGaAsP for operation with wavelengths of about 1000 nm
– Are reverse biased in operation
– Gives bandwidth greater than 10 GHz
3. Avalanche photodiodes (APD)
Are similar to Pin diodes but more sensitive and have higher
SNR.
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Summary
• Telah dipelajari gambaran umum sistem optik.
• Telah dipelajari klasifikasi serat optik.
• Telah dipelajari komponen sistem optik.
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