OpticCom1.pdf

8/10/2019 OpticCom1.pdf

1/36

Optical CommunicationsPractical perspective

Nguyn QucTun

Network and Communication Department

Facul ty of Electronics and Telecommunications

22-Sep-14 1


2/36

Forms of Communication Systems

The electromagnetic spectrum

OVERVIEW


3/36

Optical Network Classification

OVERVIEW


4/36


- Publish network

OVERVIEW


5/36


- Enterprise Networks

OVERVIEW


6/36

History

- The invention of the laser in about 1960 forms the starting point for optical

communications research

- In the early 1960s free space optical communications research started1

- One of the more significant military programs was the Air Force 405B program which

set out to develop space qualified laser communications hardware

The system was flight tested in the late 1970s at the White Sands Missile Range

+ A common figure of merit for transmission of this type is the bit rate-distance product

(B*L). The large improvement offered by the high carrier frequency of optical

transmission fibers is the motivation for optical communication system development

+ During the 1960s the main drawback of optical fiber were their loss. Thats years the

loss was ~ 1000 dB/km

+ In 1970, losses were reduced to 20 dB/km by using the refined fiber fabrication

techniques. At that time GaAs semiconductor lasers were able to operate at room

temperature

OVERVIEW


7/36

Optical Network Evolution

- Optical communication system are evolving to adapt to the ever increasing

demands of telecommunication needs

- Optical network technology has come closely related to the Internet technology and

has the same ultimate goal: high demands for high bandwidth and distance

independent connectivity

- Six generations have done for optical fiber communication in historical perspective First Generation

+ 1980 operate at 0.8mm wavelength and 45 Mb/s data rate. Repeater spacing was

10km and was much greater than for comparable coax systems

OVERVIEW


8/36


- Second Generation

+ Deployed during the late 80s and focused on using a transmission wavelengthnear 1.3 mm to take advance of the low attenuation (


9/36


- Third Generation

+ These system were based on the use of 1.55mm sources and detectors. The attenuation of fused silica fiber is minimal.

Two approaches were taken to solve the problem The first was to develop

single mode laser (SLM) and the second was to develop dispersion shifted fiber at

1.55mm+ In 1990, 1.55mm system operate at 2.55 Gb/s were commercially available and

able to operating at 10 Gb/s for distance 100km.

Best performance was achieved with dispersion shifted fibers in conjunction

with single mode laser.

The drawback of these system was the need for electronic generation with

repeater typically spaced every 60-70km, Coherent detect method wereinvestigated to increase sensitivity .

OVERVIEW


10/36


- Fourth Generation

+ These system are based on optical amplifiers at 1.55mm to increase repeater spacingand WDM to increase aggregate bit rate.

Erbium doped fiber amplifiers to amplify signals without electronic

regeneration. The commercial system sending signal over 11300km at 5Gb/s without

O/E&E/O.

System capacity is increase by using WDM and multi wavelengths can be

amplified the same optical amplifier.

In 1996 signals 20.5 Gb/s was transmitted over 9100km and B-L product of 910

(Tb/s)-km. In these broad band systems dispersion become more of an issue

OVERVIEW


11/36


- Fifth Generation

+ These effort is primarily concerned with the fiber dispersion problem because Opticalamplifier solve the loss problem bur increase the dispersion. An Ultimate solution is

based on the novel concept of optical solitons. These are pulses that preserve their

shape during propagation in a loss less fiber by counteracting the effect of dispersion

through fiber nonlinearity+ Experiments using stimulated Raman scattering as the nonlinearity to compensate for

both loss and dispersion was effective in transmitting signals over 4000km

+ EDFA was first used to amplify solitons in 1989 and 1994 a demotration of soliton

transmission over 9400km was performed at a bit rate of 70 Gb/s by multiplexing 7 Gb/s

- 10 channels

OVERVIEW


12/36


- Sixth Generation

+ Recently effort have been directed toward greater capacity of fiber systems by

multiplexing a large number of wavelengths (DWDM system). their shape during

propagation in a loss less fiber by counteracting the effect of dispersion. These with

wavelength separation of 0.8nm currently and to reduce this to < 0.5nm. Controlling

wavelength stability and the development of wavelength de-multiplexing devices arecritical to this effort. Now systems are operating at 10Gb/s or 40Gb/s

- Future systems

+ Most probably polarization-multiplexed QPSK system with 100 Gb/s per wavelengthwill become reality soon. The researcher effort will focus on 400 Gb/s serial

transmission and then on 1 Tb/s

OVERVIEW

DWDM

Dense Wavelength-Division Multiplexer


13/36

Evolution of Fiber-Optic Point-to-Point Transmission

OVERVIEW


14/36

Evolution of Fiber-Optic Networks

OVERVIEW


15/36

Five generations of fiber progress (B*L)

OVERVIEW


16/36

Transmission Bandwidth Evolution

OVERVIEW


17/36

Transmission Basics

The wavelength and frequencyfare related by the equation

c = f.

where c denotes the speed of light in free space, which is 3108 m/s. The speed of

light in fiber is actually somewhat lower (closer to 2 108 m/s)

Another parameter of interest is channel spacing, which is the spacing between

two wavelengths or frequencies in a WDM system (measured in units of

wavelengths or frequencies). The relationship between the two can be obtained

starting from the equation f = c/

Differentiating this equation around a center wavelength 0, we obtain the

relationship between the frequency spacing Dfand the wavelength spacing D as

Df = -( c /l2) D

OVERVIEW


18/36

Transmission Basics

- Example : At a wavelength 0 = 1550 nm, a wavelength spacing of DWDM D = 0.8 nmcorresponds to a frequency spacing ofDf=100 GHz, a typical spacing in WDM systems.

- The wavelengths and frequencies used in WDM systems have been standardizedon a

frequency grid by the International Telecommunications Union (ITU). It is an infinite

grid centered at 193.1 THz,

- The early WDM systems used the so-called C-band, or conventional band

(approximately 15301565 nm). Use of the L-band, or long wavelength band(approximately 15651625 nm), has become feasible recently with the development

of optical amplifiers in this band.

OVERVIEW


19/36

Transmission Basics

- In optical communication, it is quite common to use decibel units (dB) to measurepower and signal levels,

(Pt )dBW = 10 log(Pt )W or (Pt )dBm = 10 log(Pt )mW

Example: a power of 1 mW corresponds to 0 dBm or 30 dBW. A power of 10 mW

corresponds to 10 dBm or 20 dBW.

- The link loss is then defined as

= (Pr / Pt)

In dB units, we would have ( )dB = 10 log = (Pr )dBm (Pt)dBm.

- Note that dB is used to indicate relative values, whereas dBm and dBW are used to

indicate the absolute power value.

OVERVIEW


20/36

OVERVIEW

Multiplexing

- A technique called multiplexing (TDM or WDM), the sum of the bandwidth of all thecircuits, or connections, on a link must be less than the link bandwidth.

- A other technique called statistical multiplexing when multiple bursty data streams

together on a link. So that, the probability that all streams are active simultaneously is

quite small. Therefore the bandwidth required on the link can be made significantly

smaller than the bandwidth that would be required if all streams were to be active

simultaneously.


21/36

OVERVIEW


22/36

OVERVIEW


23/36

OVERVIEW


24/36

OVERVIEW

Other Multiplexing and Coding Techniques

- Space Division Multiplexing:

+ use of several fibers belonging to the same bundle

- Polarization Multiplexing:

+ Using orthogonal states of polarization in fiber to transmit independent data

Streams

- Code Division Multiplexing

+ Initially known as spread-spectrum, a particular kind of multiplexing based on

the product between the useful signals and orthogonal pseudorandom sequences (mostly

used in RF/wireless applications, like in third generation wireless phone)

- Multilevel Coding

+ Bandwidth efficient way to increase channel bit-rate without requiring more

modulation bandwidth.


25/36

Optical communication systems may be either analog or digital

Intensity Modulated Analog Systems- With intensity modulation (IM) the information is directly modulated onto the optical

carrier. As an alternative we may wish to use a radio frequency (RF) subcarrier

- Reasons for choosing this SCM/IM scheme are:

+ To improve the demodulated SNR via nonlinear modulation, e.g., frequency

modulation (FM)

+ To allow multiple subcarriers each carrying a different information source (subcarrier

multiplexing)

OVERVIEW


26/36

Optical Fiber Systems

Digital Systems

- Digital systems may implement:+ Intensity modulation with baseband direct detection (IM/DD)

+ Intensity modulation using subcarriers

+ Coherent modulation and demodulation

- The most common IM/DD scheme is on-off keying (OOK)

OVERVIEW


27/36

Optical spectrum for intensity modulation

- If the intensity modulation is imposed to the optical signal together with unwanted

phase or frequency modulation (e.g chirp under direct laser modulation, excess laser

phase noise)

+ The resulting optical spectrum is larger than the bit rate

- If the modulation is a (nearly) pure intensity modulation, without any accompanying

phase/frequency shift (e.g. external modulation)

+ The resulting spectrum has a primary lobe that occupies the order of the bit rate

OVERVIEW


28/36

Telecommunications Network Architecture

- A local-exchange carrier (LEC) offers local services in metropolitan areas, and aninterexchange carrier (IXC) offers long-distance services. This distinction is blurring

rapidly as LECs expand into long distance and IXCs expand into local services.

OVERVIEW


29/36

OVERVIEW

Optical Networks - NGN

- Typical optical networks structure can be presented by three concentric circle+ Core networks is wide area network (WAN) or interchange carrier (IXC)

+ Edge network is Metropolitan area network (MAN) or local exchange carrier (LEC)

+ Access Network is LAN or distribute network


30/36

OVERVIEW

All-Optical Networks

- The key network elements that enable optical networking are optical line terminals(OLTs), optical add/drop multiplexers (OADMs), and optical cross-connects (OXCs).

+ An OLT multiplexes multiple wavelengths into a single fiber and de-multiplexes a

set of wavelengths on a single fiber into separate fibers. OLTs are used at the

ends of a point-to-point WDM link.

+ An OADM takes in signals at multiple wavelengths and selectively drops some of

these wavelengths locally while letting others pass through. It also selectively

adds wavelengths to the composite outbound signal.

+ An OXC essentially performs a similar function but at much larger sizes. OXCs are

able to switch wavelengths from one input port to another. Both OADMs and

OXCs may incorporate wavelength conversion capabilities.


31/36

Transparency and All-Optical Networks

- Light path signal is the trace that optical signal passes between the source anddestination without experiencing any O-E-O conversion

OVERVIEW


32/36

OVERVIEW

Optical Layers

- Optical networks based on this paradigm are now being deployed. The architecture ofsuch a network based on OSI

- It is important to define the functions ofOptical layerand the interfaces between

layers to allows vendors to manufacture a variety of hardware and software products

performing the functions of some of the layers, and provide the appropriate

interfaces to communicate with other products.


33/36


34/36

Optical vs. RF and Copper

Advantages Over RF Systems

Tremendous increase in the modulation bandwidth and hence the overall information

bandwidth

The usable bandwidth of optical systems is about five orders of magnitude larger than

that available to an RF carrier; 25,000 GHz of BW in each optical band

Free space optical systems also provide security and interference immunitysince very

narrow beam widths are possible

Due to the small wavelengths involved, optical components are very small in

comparison with RF/microwave components

Fiber specific features include: small size and light weight, electrical isolation, and anabundance of raw materials (sand for making glass)

OVERVIEW


35/36


36/36

Optical Communications Pyramid1

OVERVIEW

OpticCom1.pdf

Documents

Transcript of OpticCom1.pdf