Fiber Optics Communications Lecture 2

Introduction to Fiber Optic Communication System Communications refers to information transmission and reception. The information being transmitted could be of various forms, either analog (voice, video, text) or digital form such as binary codes which when sampled properly carries essentially all information of analog signal Motivation of new communication systems is to increase data rate so that more information could be sent One of the earliest known optical transmission links was the use of fire signal by the Greeks In 1838 the telegraph was invented by Samuel Morse. This ushered in a new era in communications i.e. electrical communications Amount of information that can be transmitted is directly related to the frequency range over which the carrier operates, increasing the carrier frequency theoretically increases the available transmission bandwidth and therefore larger capacity The trend was to employ higher frequencies which offer increases in bandwidth and capacity. This lead to the birth of radio, tv, radar, and microwave links Telecommunications usually refers to fiber optic communications, satellite communication, or mobile communication.

Introduction to Fiber Optic Communication System Two basic elements in any telecommunicating system – Transmission Carrier: an electromagnetic wave with particular frequency in spectrum. Information rides on carrier to be delivered – Transmission media: material in which carrier signal propagates Examples – AM or FM signal Transmitted information is voice. The carrier is radio waves (3 KHz to 300MHz). The signal can be transmitted tens or hundreds of miles – Microwave transmission Carrier in (300 MHz to 30 GHz). Transmission media is coaxial cables, waveguide or air Fiber optic communication – Carrier is light (near infrared just outside visible light). The actual wavelength used includes 850 nm, 1300 nm, and 1550 nm. The transmission medium is optical fiber The optical frequency n is related to wavelength l through c = λn Example, a nm wavelength light signal has a frequency of THz (193.1 x Hz)

Introduction, contd From EM spectrum, the higher the frequency, the shorter wavelength, and the higher is the EM energy. Energy is related to wavelength by E = hn, where h is Planck’s constant and v is the frequency in Hz, also E[J]= hc/λ [m] E[ev]= 1.24 /λ[mm]

EM spectrum

The Development of Fiber Optic Communication System In ancient times, Greeks used reflected light from mirrors to communicate between watch towers, ship captains used both mirrors and lanterns to communicate with their fleets Lightwave communication started with the invention of photophone by Alexander Graham Bell in 1880, which used Sun light as carrier and air as transmission media. Sound information is transmitted this way up to 200 meters. The biggest obstacle for using optic fibers for communications was high loss. Prior to 1970, the best silica (SiO2) fiber had attenuation coefficient a =1 dB/m or 1000 dB/km. Let us see what this means: Using such fiber of 1 Km long to transmit light of wavelength lo=1 mm. In order to receive a single photon at the end of fiber, find the energy to be transmitted into the beginning of the fiber. Fiber loss is defined as Loss[dB]= 10 log 10 P TX /P RX where P TX is transmitted energy and P RX is received energy. At the receiver, a single photon’s energy is P RX [eV]= 1.24/1=1.24 [eV]=1.986x [J]

Development, contd The attenuation coefficient is defined as a[dB/km] = (10/L[km]) log 10 P TX /P RX, where L is the length of the fiber. Therefore, the required transmitted energy is P TX = 10 α/10 P RX = x x ~ 2 x [J] This is equivalent to all light energy of the sun’s radiation in years, if all radiation is of lo=1 mm. In 1970, Corning Glass Works did successfully grow fused silica fiber with loss less than 20dB/km Bell labs made successfully the room temperature continuous wave AlGaAs (Aluminum Gallium Arsenide) that operated at 850 nm. Semiconductor laser light has high purity spectrum or very narrow wavelength range and of high efficiency. It was lucky that 850nm wavelength happens to correspond to one of the low loss window in silica fiber

The Generation of Fiber Optic Communication System From then onward, within 7 years of time, fiber optic communication started to be commercialized. From 1977 to date, there has been 3 generations of fiber optic. The first generation ( ) Point to point pipelines. Routing and switching are carried out by electric components. The basic feature of 1 st generation is that all aspects of regeneration were accomplished using electrons. Optic signal transmitted is converted to electric signal, cleared, resynchronized, and amplified and then transmitted back to optical signal. The regeneration is both expansive and complex. Further classification of 1 st generation – λ= 850nm. Multimode fiber. Light source AlGaAs semiconductor laser. Typical bit rate Mbps – λ= 1300nm. Multimode fiber nm corresponds to second window of low loss in silica and lowest in dispersion. Single mode fiber was developed in Single mode fiber has much lower fiber dispersion and lower loss. Therefore, used in long distances and transoceanic communication. Typical data rate is 1.7 Gbps with repeater spacing about 50 km – λ = 1550nm single mode nm corresponds to the lowest loss window for silica and can operate at 10Gbps for repeater distance of 100 km

The second generation ( ) 2 nd generation is no longer point to point pipeline, it is then optical network. Light is no longer dumb carrier of data. Optic amplification was realized using optic amplifier e.g. EDFA (Erbium Doped Fiber Amplifier) which revolutionized fiber optic systems -Eliminated optic-electric-optic conversion and replaced regeneration by all optic in line amplifiers -Pushed the application of wavelength division multiplexing (WDM). Because EDFA can directly and simultaneously amplify 40 nm bandwidth to 37 dB gain. EDFA and WDM is mainstream of today’s high speed fiber optic. Thus, reduced cost, easier for maintaining and greatly expanded capacity. Aggregate data rate distance product up to 2.56 Tbps-Km is currently realizd.

The third generation (evolving) The new generation of optic communication network is still evolving. Not only long haul optic communication, the fiber to home (FTTH) and local networks will be controlled optically. Nowadays, long distance (> 100km) and transoceanic communication are dominated by fiber optics.