Nama : - Muhamad Fahrurozi

- Ulpa Hanipah
Kelas : TT – 4C

Fiber Optic Telecommunication

Fiber optics is a major building block in the telecommunication infrastructure. Its high
bandwidth capabilities and low attenuation characteristics make it ideal for gigabit
transmission and beyond.

I. BENEFITS OF FIBER OPTICS

• Long-distance signal transmission
• Large bandwidth, light weight, and small diameter
• Nonconductivity
• Security
• Designed for future applications needs

II. BASIC FIBER OPTIC COMMUNICATION SYSTEM

Fiber optics is a medium for carrying information from one point to another in the form of
light. Unlike the copper form of transmission, fiber optics is not electrical in nature. A basic
fiber optic system consists of a transmitting device that converts an electrical signal into a light
signal, an optical fiber cable that carries the light, and a receiver that accepts the light signal
and converts it back into an electrical signal.
III. TRANSMISSION WINDOWS
Optical fiber transmission uses wavelengths that are in the near-infrared portion of the
spectrum, just above the visible, and thus undetectable to the unaided eye. Typical optical
transmission wavelengths are 850 nm, 1310 nm, and 1550 nm. Both lasers and LEDs are used
to transmit light through optical fiber.
Table 8.1: Fiber Optic Transmission Windows
Window Operating Wavelength
800 – 900 nm 850 nm
1250 – 1350 nm 1310 nm
1500 – 1600 nm 1550 nm

IV. FIBER OPTIC LOSS CALCULATIONS

Loss in a system can be expressed as the following:
𝑃
Loss = 𝑃𝑜𝑢𝑡
𝑖𝑛
where Pin is the input power to the fiber and Pout is the power available at the output of the
fiber.
For convenience, fiber optic loss is typically expressed in terms of decibels (dB) and can be
calculated using :
𝑃
LossdB = 10 log 𝑃𝑜𝑢𝑡
𝑖𝑛
Oftentimes, loss in optical fiber is also expressed in terms of decibels per kilometer (dB/km).
Optical power in fiber optic systems is typically expressed in terms of dBm, which is a
decibel term that assumes that the input power is 1 mwatt. Optical power here can refer to the
power of a laser source or just to the power somwhere in the system. If P in is in milliwatts,
the power in dBm, referenced to an input of one milliwatt:
𝑃
P(dBm) = 10 log 1 mW
With optical power expressed in dBm, output power anywhere in the system can be
determined simply by expressing the power input in dBm and subtracting the individual
component losses, also expressed in dB. It is important to note that an optical source with a
power input of 1 mW can be expressed as 0 dBm, as indicated by Equation 8-3. For every 3-
dB loss, the power is cut in half. Consequently, for every 3-dB increase, the optical power is
doubled.

V. TYPES OF FIBER
Three basic types of fiber optic cable are used in communication systems:
1. Step-index multimode
2. Step-index single mode
3. Graded-index
VI. DISPERSION
Dispersion, expressed in terms of the symbol Δt, is defined as pulse spreading in an optical
fiber. As a pulse of light propagates through a fiber, elements such as numerical aperture,
core diameter, refractive index profile, wavelength, and laser linewidth cause the pulse to
broaden.

Dispersion Δt can be determined from Equation :

Δt = (Δtout – Δtin)1/2
Δttotal = L × (Dispersion/km)
The overall effect of dispersion on the performance of a fiber optic system is known as
intersymbol interference. Intersymbol interference occurs when the pulse spreading caused
by dispersion causes the output pulses of a system to overlap, rendering them undetectable. If
an input pulse is caused to spread such that the rate of change of the input exceeds the
dispersion limit of the fiber, the output data will become indiscernible.
Dispersion is generally divided into two categories: modal dispersion and chromatic
dispersion.
Modal dispersion is defined as pulse spreading caused by the time delay between lower-order
modes (modes or rays propagating straight through the fiber close to the optical axis) and
higher-order modes (modes propagating at steeper angles). This is shown in Figure 8-5.
Modal dispersion is problematic in multimode fiber, causing bandwidth limitation, but it is
not a problem in single-mode fiber where only one mode is allowed to propagate.

Chromatic dispersion is pulse spreading due to the fact that different wavelengths of light
propagate at slightly different velocities through the fiber. All light sources, whether laser or
LED, have finite linewidths, which means they emit more than one wavelength. Because the
index of refraction of glass fiber is a wavelength-dependent quantity, different wavelengths
propagate at different velocities. Chromatic dispersion is typically expressed in units of
nanoseconds or picoseconds per (km-nm).
Δtchromatic = Δtmaterial + Δtwaveguide
When considering the total dispersion from different causes, we can approximate the total
dispersion by Δttot.

where Δtn represents the dispersion due to the various components that make up the system.
Thetransmission capacity of fiber is typically expressed in terms of bandwidth × distance.
The approximate bandwidth of a fiber can be related to the total dispersion by the following
relationship
BW = 0.35/Δttotal
Dispersion-shifted fiber: By altering the design of the waveguide, we can increase the
magnitude of the waveguide dispersion o as to shift the zero-dispersion wavelength to
1550 nm. This type of fiber has an index profile that resembles a “W” and hence is
sometimes referred to as W-profile fiber.

W-profile fiber
Total chromatic dispersion can still be substantially lowered in the
1550-nm range without having to worry about performance problems. This type of fiber is
known as nonzero-dispersion-shifted fiber.

VII. ANALOG VERSUS DIGITAL SIGNALS

Information in a fiber optic system can be transmitted in one of two ways: analog or digital.
An analog signal is one that varies continuously with time. A digital signal is one that exists
only at discrete levels.
VIII. PULSE CODE MODULATION
Pulse code modulation (PCM) is the process of converting an analog signal into a 2n-digit
binary code. An analog signal is placed on the input of a sample and hold. The sample and
hold circuit is used to “capture” the analog voltage long enough for the conversion to take
place. The output of the sample and hold circuit is fed into the analog-to-digital converter
(A/D). An A/D converter operates by taking periodic discrete samples of an analog signal at a
specific point in time and converting it to a 2n-bit binary number.
IX. DIGITAL ENCODING SCHEMES
The signal format directly affects the detection of the transmitted signals. The accuracy
of the reproduced signal depends on the intensity of the received signal, the speed and
linearity of the receiver, and the noise levels of the transmitted and received signal.

X. MULTIPLEXING
The purpose of multiplexing is to share the bandwidth of a single transmission channel
among several users. Two multiplexing methods are commonly used in fiber optics:
A. Time-Division Multiplexing (TDM)
In time-division multiplexing, time on the information channel, or fiber, is shared among the
many data sources. The multiplexer MUX can be described as a type of “rotary switch,”
which rotates at a very high speed, individually connecting each input to the communication
channel for a fixed period of time. The process is reversed on the output with a device known
as a demultiplexer, or DEMUX. After each channel has been sequentially connected, the
process repeats itself. One complete cycle is known as a frame. To ensure that each channel
on the input is connected to its corresponding channel on the output, start and stop frames are
added to synchronize the input with the output. TDM systems may send information using
any of the digital modulation schemes described (analog multiplexing systems also exist).
The amount of data that can be transmitted using TDM is given by the MUX output rate and
is defined by Equation:
MUX output rate = N × Maximum input rate
where N is the number of input channels and the maximum input rate is the highest data rate
in bits/second of the various inputs. The bandwidth of the communication channel must be at
least equal to the MUX output rate. Another parameter commonly used in describing the
information capacity of a TDM system is the channel-switching rate. This is equal to the
number of inputs visited per second by the MUX and is defined as
Channel switching rate = Input data rate × Number of channels

B. Wavelength-Division Multiplexing (WDM)

In wavelength-division multiplexing, each data channel is transmitted using a slightly
different wavelength (different color). With use of a different wavelength for each channel,
many channels can be transmitted through the same fiber without interference. This method is
used to increase the capacity of existing fiber optic systems many times. Each WDM data
channel may consist of a single data source or may be a combination of a single data source
and a TDM
(time-division multiplexing) and/or FDM (frequency-division multiplexing) signal. Dense
wavelength-division multiplexing (DWDM) refers to the transmission of multiple closely
spaced wavelengths through the same fiber. For any given wavelength λ and corresponding
frequency f, the International Telecommunications Union (ITU) defines standard frequency
spacing Δf as 100 GHz, which translates into a Δλ of 0.8-nm wavelength spacing. This
λΔ𝑓
follows from the relationship Δλ = 𝑓
. (See Table 8-3.) DWDM systems operate in the 1550-nm
window because of the low attenuation characteristics of glass at 1550 nm and the fact that
erbium-doped fiber amplifiers (EDFA) operate in the 1530-nm–1570-nm range.
XI. COMPONENTS—FIBER OPTIC CABLE
rigidity for bending, and durability. In general, fiber optic cable can be separated into two
types:
Indoor Cables
• Simplex cable—contains a single fiber for one-way communication
• Duplex cable—contains two fibers for two-way communication
• Multifiber cable—contains more than two fibers. Fibers are usually in pairs for duplex
operation. A ten-fiber cable permits five duplex circuits.
• Breakout cable—typically has several individual simplex cables inside an outer jacket.
The outer jacket includes a zipcord to allow easy access
• Heavy-, light-, and plenum-duty and riser cable
− Heavy-duty cables have thicker jackets than light-duty cable, for rougher handling.
− Plenum cables are jacketed with low-smoke and fire-retardant materials.
− Riser cables run vertically between floors and must be engineered to prevent fires
from spreading between floors.
Outdoor Cables
Outdoor cables must withstand harsher environmental conditions than indoor cables. Outdoor
cables are used in applications such as:
• Overhead—cables strung from telephone lines
• Direct burial—cables placed directly in trenches
• Indirect burial—cables placed in conduits
• Submarine—underwater cables, including transoceanic applications
XII. FIBER OPTIC SOURCES
Two basic light sources are used for fiber optics: laser diodes (LD) and light-emitting diodes
(LED).

LEDs are typically used in lower-data-rate, shorter-distance multimode systems because of

their inherent bandwidth limitations and lower output power. They are used in applications in
which data rates are in the hundreds of megahertz as opposed to GHz data rates associated
with lasers. Two basic structures for LEDs are used in fiber optic systems: surface-emitting
and edgeemitting
Laser diodes (LD) are used in applications in which longer distances and higher data rates are
required. Because an LD has a much higher output power than an LED, it is capable of
transmitting information over longer distances. Consequently, and given the fact that the LD
has a much narrower spectral width, it can provide high-bandwidth communication over long
distances. The LD’s smaller N.A. also allows it to be more effectively coupled with single-
mode fiber. The difficulty with LDs is that they are inherently nonlinear, which makes analog
transmission more difficult. They are also very sensitive to fluctuations in temperature and
drive current, which causes their output wavelength to drift. In applications such as
wavelengthdivision multiplexing in which several wavelengths are being transmitted down
the same fiber, the stability of the source becomes critical. Laser diodes can be divided into
two generic types depending on the method of confinement of
the lasing mode in the lateral direction.
• Gain-guided
• Index-guided

XIII. PACKAGING
Laser diodes are available in a variety of packages. Most have monitoring photodiodes
integrated with the packages. Because lasers inherently emit light from both ends of the
cavity, a photodiode can be placed on one end to monitor and maintain the output power at a
certain level.
XIV. DIRECT VERSUS EXTERNAL MODULATION
Lasers and LEDs used in telecommunication applications are modulated using one of two
methods:
 In direct modulation (Figure 8-27), the output power of the device varies directly with
the input drive current.
 In external modulation (Figure 8-28), an external device is used to modulate the
intensity or phase of the light source.

XV. FIBER OPTIC DETECTORS

The purpose of a fiber optic detector is to convert light emanating from the optical fiber back
into an electrical signal. The choice of a fiber optic detector depends on several factors
including wavelength, responsivity, and speed or rise time.

XVI. FIBER OPTIC SYSTEM DESIGN CONSIDERATIONS

When designing a fiber optic communication system some of the following factors must be
taken into consideration:
• Which modulation and multiplexing technique is best suited for the particular
application?
• Is enough power available at the receiver (power budget)?
• Rise-time and bandwidth characteristics
• Noise effects on system bandwidth, data rate, and bit error rate
• Are erbium-doped fiber amplifiers required?
• What type of fiber is best suited for the application?
• Cost

A. Power Budget
The power arriving at the detector must be sufficient to allow clean detection with few errors.
Clearly, the signal at the receiver must be larger than the noise. The power at the detector, Pr,
must be above the threshold level or receiver sensitivity Ps.
Pr ≥ Ps (8-19)
The receiver sensitivity Ps is the signal power, in dBm, at the receiver that results in a
particular
bit error rate (BER). Typically the BER is chosen to be one error in 109 bits or 10–9.

B. Bandwidth and Rise Time Budgets

The transmission data rate of a digital fiber optic communication system is limited by the rise
time of the various components, such as amplifiers and LEDs, and the dispersion of the fiber.
The cumulative effect of all the components should not limit the bandwidth of the system.
The rise time tr and bandwidth BW are related by
BW = 0.35/tr (8-22)
The relationship between total system rise time and component rise time is given by Equation

where ts is the total system rise time and tr1, tr2, ... are the rise times associated with the
various components.
To simplify matters, divide the system into five groups:
1. Transmitting circuits (ttc)
2. LED or laser (tL)
3. Fiber dispersion (tf)
4. Photodiode (tph)
5. Receiver circuits (trc)
The system rise time can then be expressed as

The system bandwidth can then be calculated using Equation 8-25 from the total rise time ts
as given in Equation :
BW = 0.35/ts

C. Connectors
Many types of connectors are available for fiber optics, depending on the application. The
most popular are:
SC—snap-in single-fiber connector
ST and FC—twist-on single-fiber connector
FDDI—fiber distributed data interface connector
D. Fiber Optic Couplers
A fiber optic coupler is a device used to connect a single (or multiple) fiber to many other
separate fibers. There are two general categories of couplers:
• Star couplers (Figure 8-35a)
• T-couplers (Figure 8-35b)

E. Wavelength-Division Multiplexers
The couplers used for wavelength-division multiplexing (WDM) are designed specifically to
make the coupling between ports a function of wavelength. The purpose of these couplers is
to separate (or combine) signals transmitted at different wavelengths. Essentially, the
transmitting coupler is a mixer and the receiving coupler is a wavelength filter. Wavelength-
division multiplexers use several methods to separate different wavelengths depending on the
spacing between the wavelengths. Separation of 1310 nm and 1550 nm is a simple operation
and can be achieved with WDMs using bulk optical diffraction gratings. Wavelengths in the
1550-nm range that are spaced at greater than 1 to 2 nm can be resolved using WDMs that
incorporate interference filters.
Erbium-doped fiber amplifiers (EDFA)—The EDFA is an optical amplifier used to boost
the signal level in the 1530-nm to 1570-nm region of the spectrum. When it is pumped by an
external laser source of either 980 nm or 1480 nm, signal gain can be as high as 30 dB
(1000 times). Because EDFAs allow signals to be regenerated without having to be converted
back to electrical signals, systems are faster and more reliable. When used in conjunction
with wavelength-division multiplexing, fiber optic systems can transmit enormous amounts
of information over long distances with very high reliability.

Fiber Bragg gratings—Fiber Bragg gratings are devices that are used for separating
wavelengths through diffraction, similar to a diffraction grating. They are of
critical importance in DWDM systems in which multiple closely spaced wavelengths require
separation.