Information Communication Technology Module ICTBS 2 3 Optical

Information & Communication Technology Module ICT–BS– 2. 3 Optical Fiber Communications Unit ICT–BS– 2. 3/2 Optical Signals: Attenuation and Amplification ICT–BS– 2. 3/2 Optical Signals: Attenuation and Amplification 11/27/2020 TTC Riyadh, ICT–BS-2. 3/2 1

Optical Fiber Communications L P ∑ Optical Signals: Attenuation and Amplification 12 0 12 ICT-BS-2. 3/2/1 Optical Sources 1 ICT-BS-2. 3/2/2 Structures and Characteristics of Light-Emitting Diodes LED 1 ICT-BS-2. 3/2/3 Semiconductor Laser Structures 1 ICT-BS-2. 3/2/4 Power Launching and Coupling 1 ICT-BS-2. 3/2/5 Optical Detectors 2 ICT-BS-2. 3/2/6 Signal Encoding/Decoding 2 ICT-BS-2. 3/2/7 Modulation and Demodulation Formats 2 ICT-BS-2. 3/2/8 Receiver Sensitivities 1 ICT-BS-2. 3/2/9 Optical Amplifiers 1 Modules Code ICT-BS-2. 3/2 11/27/2020 TTC Riyadh, ICT–BS-2. 3/2 2

ICT–BS– 2. 3/2 Optical Signals: Attenuation and Amplification Learning Content: • Optical sources - Light emitting diode (LED) - Laser diode (LD) • Optical power coupling • Optical detection • Optical modulation and demodulation • Optical signal amplification 11/27/2020 TTC Riyadh, ICT–BS-2. 3/2 3

ICT–BS– 2. 3/2 Optical Signals: Attenuation and Amplification Recommended Books: • Fiber Optic Communications, James N. Dowing, Published by Thomson Delmar Learning. Copyright 2005, Pages: 378 • Optical Fiber Communications: Principles and Practice, 3 rd Edition John M. Senior and M. Yousif Jamro, Published by Prentice Hall. Copyright 2009, Pages: 1075 • Optical Fiber Communications, 4 th Edition, Gerd Keiser Published by Tata Mc. Graw-Hill. Copyright 2008, Pages: 580 11/27/2020 TTC Riyadh, ICT–BS-2. 3/2 4

Review – Optical Fiber Communication System Electrical Signal Input Optical Source Optical Detector Modulator Demodulator Output Signal Transmission path (Optical Fiber) Transmitter 11/27/2020 Receiver TTC Riyadh, ICT–BS-2. 3/2 5

Course contents • Introduction to the principles of optical telecommunications: Conversion of electrical signals into optical signals • Introduction to the most important optical telecommunication components • Examining the advantages and disadvantages of optical transmission links • Recording an infrared transmitter diode's characteristic and frequency response • Controlling a transmitter diode • Measuring a transmitter diode's frequency response • Measurement-based examination of various modulation techniques for analog and TTL signals • Investigating transmission paths for infrared light of various wavelengths • Configuring an optical waveguide • Measuring a receiver diode's frequency response • Examining a receiver diode's influence on signal recovery • Determining an optical transmission link's bandwidth • Examining the influence of an optical transmission link's input capacity on bandwidth • Measurement-based examination of attenuation along an optical transmission link • Measurement-based examination of the influence of longitudinal and transverse offset at splice points • Comparing the properties of step-index and graded-index fibres • Examining the influence of wavelength on attenuation

optical transmitter

optical receiver

Light Sources • Optical sources are used to convert electrical signals into optic beams thus enables information carrying facility though the fiber core. • Generally, the information is put into the beam by modulating the source input current. • Two basic types which rely on semiconductor principles of operation are – Light emitting diodes (LEDs) – Laser diodes (LDs) 11/27/2020 TTC Riyadh, ICT–BS-2. 3/2 10

Light Sources Considerations • The light source must be matched with the fiber in terms of – Size – Modal characteristics – Numerical aperture – Line width – Fiber-window wavelength range – Transmitted power 11/27/2020 TTC Riyadh, ICT–BS-2. 3/2 11

Conduction of Electrons Conduction band Current flow Movement of electrons Valance band • When a small voltage is placed across the conductor, electrons in the outermost shell move from the valance band to conductor band. • This results positively charged “holes’ in the valance band. • Then, the holes are appeared to be moved to the negative source terminal and electrons are to the positive terminal. • Therefore, it said the a current flows through the circuit in the opposite direction of electrons flow. 11/27/2020 TTC Riyadh, ICT–BS-2. 3/2 12

Conduction of Electrons (Contd. ) • Good conductors have few electrons on the valance band. • On the otherhand, insulators (poor conductors) have a full valence band thus it requires more energy to make current flowing (actually they are not). • In addition, there are semiconductor materials, which requires more energy to allow current flowing than in a conductor. 11/27/2020 TTC Riyadh, ICT–BS-2. 3/2 13

The pn Junction Diode • A semiconductor source consists of a pn junction diode. • To create a pn junction diode, p-material and n-material are fabricated next to each other. (e. g. ; silicon an gallium arsenide) • To alter the localized charges at the material boundary, a small amount of impurities is added. This process is called as doping. • However, the total net charge is equal to zero. p-type n-type Electrons Holes 11/27/2020 TTC Riyadh, ICT–BS-2. 3/2 14

The pn Junction Diode (Contd. ) • Even without applying any voltage, a barrier is formed at the boundary. This is called ad the depletion region. p-type n-type Potential barrier/ depletion region 11/27/2020 TTC Riyadh, ICT–BS-2. 3/2 15

Reverse Biased - pn Junction • When an external voltage is applied with the positive voltage to the n-side and negative voltage to the p-side, the barrier becomes larger. • Therefore, a very small current is flown through the circuit. • This is happened due to the surplus electrons are moved for p-to-n. • This is called as reverse current and the circuit is called as in reverse biased. p-type n-type Increased depletion region 11/27/2020 TTC Riyadh, ICT–BS-2. 3/2 16

Forward Biased - pn Junction • However, once the external voltage is applied such that positive voltage for p-side and negative for n-side, then the depletion region becomes shrink. • Now, it is possible to move more electrons, thus a larger current is produced. • This is the forward biased current. p-type n-type Reduced depletion region 11/27/2020 TTC Riyadh, ICT–BS-2. 3/2 17

Optical Fiber Communications L P ∑ Optical Signals: Attenuation and Amplification 12 0 12 ICT-BS-2. 3/2/1 Optical Sources 1 ICT-BS-2. 3/2/2 Structures and Characteristics of Light-Emitting Diodes LED 1 ICT-BS-2. 3/2/3 Semiconductor Laser Structures 1 ICT-BS-2. 3/2/4 Power Launching and Coupling 1 ICT-BS-2. 3/2/5 Optical Detectors 2 ICT-BS-2. 3/2/6 Signal Encoding/Decoding 2 ICT-BS-2. 3/2/7 Modulation and Demodulation Formats 2 ICT-BS-2. 3/2/8 Receiver Sensitivities 1 ICT-BS-2. 3/2/9 Optical Amplifiers 1 Modules Code ICT-BS-2. 3/2 11/27/2020 TTC Riyadh, ICT–BS-2. 3/2 18

Light Emitting Diode (LED) • A light emitting diode (LED) is a p-n junction semi-conductor that emits light when it is in forward biased. R 11/27/2020 TTC Riyadh, ICT–BS-2. 3/2 19

LED (Contd. ) • Eventhough LED has a less attraction with optical systems, it can be still used because of – Simple fabrication – Cost – Reliability (no catastrophic degradation, immune to modal noise) – Less temperature dependency – Simpler drive circuitry (lower drive currents) – Linearity (linear light output versus current) 11/27/2020 TTC Riyadh, ICT–BS-2. 3/2 20

LED Operation • When a conduction band electron falls back to the valence band, this electron gets recombined with a hole, thus creates a photon (electron + hole) • As a result this photon creation, light gets emitted. • This is a spontaneous process according to the Planck’s law. Conduction band Band gap energy Valance band 11/27/2020 TTC Riyadh, ICT–BS-2. 3/2 21

LED Operation (Contd. ) • The light is emitted in all directions and does not depends on other (incoherent). • Band gap energy = Energy difference between excited state (conduction band) and ground state (valance band). • The energy of the photon emission should be at least slightly larger than the band gap energy. • The spread in the energy of light emissions is defined as line width of the LED. 11/27/2020 TTC Riyadh, ICT–BS-2. 3/2 22

LED Operation (Contd. ) • All the photon creations do not emit radiation. Some are non-radiative, thus be the causes of vibrational effects and heat dissipations. • Therefore, the internal quantum efficiency of the LED can be defined as (which is photon producing process or the lifetime) • Then, the internal optical power produced due to the recombination process is h – Planck’s constant I – current c – velocity of the light in the vacuum e – charge of an electron 11/27/2020 TTC Riyadh, ICT–BS-2. 3/2 23

Types of Band gap Transitions • There is no changes in the momentum (direction) in direct band gap transition. • However, some energy must be used for momentum changes in indirect band gap transition. Therefore, direct transition acquires more efficiency than the indirect transition. - - - Conduction band Energy Momentum Valance band + + + + (Direct transition) 11/27/2020 + + + + • Valance band (Indirect transition) TTC Riyadh, ICT–BS-2. 3/2 24

Composition of the Semi-conductor • Eventhough many semi-conductor materials can be induced to emit light, an appropriate composition can enhance the efficiency of the system by minimizing the waste of energy. • The primary target is to reduce the band gap energy. • Normally, two elements are compounded from Group III materials (Aluminum, Gallium, Indium) and Group V materials (Phosphorous, Arsenic) in the periodic table. 11/27/2020 TTC Riyadh, ICT–BS-2. 3/2 25

Composition of the Semi-conductor (Contd. ) 11/27/2020 TTC Riyadh, ICT–BS-2. 3/2 26

Composition of the Semi-conductor (Contd. ) • Different material compositions have different bandgap energies. 11/27/2020 Material Band gap Energy Si 1. 11 Ge 0. 66 Ga. As 1. 43 Al As 2. 16 Ga. P 2. 21 In. As 0. 36 In. P 1. 35 In. 53 Ga. 47 As 0. 74 Alx. Ga 1 -x. As 1. 424+1. 247 x AIx. In 1 -x. P 1. 351+2. 23 x TTC Riyadh, ICT–BS-2. 3/2 27

LED Physical Structure • Basically a fabricated LED structure can be – either a homojunction structure (when p- and n-side have same base material). – or a heterojunction structure (when p- and n-side have different base materials so that it is formed a waveguide at the junction) (Heterojunction structure) (Homojunction structure) 11/27/2020 TTC Riyadh, ICT–BS-2. 3/2 28

Surface Emitting Diode • When refractive indices of both p- and n-type materials are same, light is free to come out from all sides of the semi-conductor device because there is no confinement. • However, only the active region near (but not on) the surface will emit a significant amount of light while reabsorbing from the other parts. Therefore, this is called as surface emitting LED. 11/27/2020 TTC Riyadh, ICT–BS-2. 3/2 29

Surface Emitting Diode (Contd. ) • However, a large amount of power generated by the LED get wasted. • To increase the output power, only allowing the light be exit from the surface can be done while confining from others. • The output beam makes a Lambertian shape. number of photons coming from the device at an angle of per second. 11/27/2020 TTC Riyadh, ICT–BS-2. 3/2 30

Edge Emitting LED • When the refractive indices differ from each other, it can be confined the light to exit only from one edge of the device (i. e. plane parallel to the junction). This is called as edge emitting LED. • When the light is come out from one edge and the plane is perpendicular to the junction, the elliptical beam nature gives some problems in fiber launching applications. 11/27/2020 TTC Riyadh, ICT–BS-2. 3/2 31

Overview - LED • Not expensive. • Operates at low power (1. 5 V to 2. 5 V and 50 m. A to 300 m. A) • Can be coupled to approximately 10 to 100 µW of optical power to a fiber. • Drive circuitry is not very complex. • LEDs are capable of cover the entire fiber window from 850 to 1550 nm with a line width 15 to 60 nm. • Do not require any temperature or current control. Applications - Used in low cost applications with data rates of 100 Mbps - Used in LANs coupled to multimode fiber - Local area WDM (wavelength division multiplexing) networks 11/27/2020 TTC Riyadh, ICT–BS-2. 3/2 32

optical receiver

optical receiver

Optical Fiber Communications L P ∑ Optical Signals: Attenuation and Amplification 12 0 12 ICT-BS-2. 3/2/1 Optical Sources 1 ICT-BS-2. 3/2/2 Structures and Characteristics of Light-Emitting Diodes LED 1 ICT-BS-2. 3/2/3 Semiconductor Laser Structures 1 ICT-BS-2. 3/2/4 Power Launching and Coupling 1 ICT-BS-2. 3/2/5 Optical Detectors 2 ICT-BS-2. 3/2/6 Signal Encoding/Decoding 2 ICT-BS-2. 3/2/7 Modulation and Demodulation Formats 2 ICT-BS-2. 3/2/8 Receiver Sensitivities 1 ICT-BS-2. 3/2/9 Optical Amplifiers 1 Modules Code ICT-BS-2. 3/2 11/27/2020 TTC Riyadh, ICT–BS-2. 3/2 35

Laser Principles • The spectral width (line width) of the laser is much narrower than the LED Laser • All lasers must have the following characteristics. – Pumping threshold – Output spectrum – Radiation pattern 11/27/2020 TTC Riyadh, ICT–BS-2. 3/2 36

Laser Principles (Contd. ) Pumping threshold – The input power to a laser must be above than a threshold level to make it acts as an emitter whereas an LED radiates even at low levels of input current. – The device behaves like an LED, before it is reached to the threshold. Optical power / (m. W) • LED Laser Current / (m. A) LED region (Spontaneous) 11/27/2020 Laser region (Stimulated) TTC Riyadh, ICT–BS-2. 3/2 37

Laser Principles (Contd. ) • Output spectrum – The laser output power is not at a single frequency but is spread over a range of frequencies. Therefore, power profile is not very smoothed and has a series of peaks and valleys. • Radiation pattern – Laser light emission angles are depend on the size of the emitting area and on the modes of oscillations within the layer. 11/27/2020 TTC Riyadh, ICT–BS-2. 3/2 38

Laser Operation • LASER – Light Amplification by Stimulated Emission of Radiation • The laser operation differs from other optical sources because of it is resulted from stimulated emissions. Conduction band External photon Stimulated photon Valance band 11/27/2020 TTC Riyadh, ICT–BS-2. 3/2 39

Laser Operation (Contd. ) • When this external photon (injected photon) hits with the excited electron at the valence band, it is forced to create a stimulated photon and light is emitted with the same wavelength and the same linewidth as the external photon. They are also in phase. • Once these photons are travelled through the same direction, it will result further stimulated emissions to support the directionality of the beam. • This causes to deplete the conduction band electrons very quickly, but generates a large current to sustain the laser operation. The number of spontaneous emissions are proportional to the number of injected photons. 11/27/2020 TTC Riyadh, ICT–BS-2. 3/2 40

Population Inversion • To sustain the laser operation, it requires more electrons in the excited state (conduction band) than the ground state. • Then only the stimulated emissions get higher than the stimulated absorptions. • Therefore, a high-density injected current (upto 150 m. A) is fed across a small active area. (After) (Before) (Stimulated emission) (Stimulated absorption) Conduction band External photon Valance band Two photons Conduction band Valance band 11/27/2020 TTC Riyadh, ICT–BS-2. 3/2 41

Positive Feedback • Once the population inversion is achieved, the multiplication of photons is done by keeping two reflected mirrors at two ends. A Conduction band C D Valance band 11/27/2020 B E TTC Riyadh, ICT–BS-2. 3/2 42

Positive Feedback (Contd. ) • First, a stimulated photon is produced at point A and both photons are continue towards the end of cavity (right hand side). • Then, they are reflected back at point B and continue the other direction. • When they are reach at point C, more stimulated emitting occurs. • Now the number of photons are doubled. • At point D, again they are reflected back due to the left hand side mirror. • The process is continued back and forth. • Normally, two ends are cleaved to act as mirrors and a Fabry-Perot cavity configuration is used for optical confinement in a semi-conductor structure. 11/27/2020 TTC Riyadh, ICT–BS-2. 3/2 43

Laser Output Mode Structure • Generally, the laser produces a finite number of radiative recombinations due to the use of Fabry-Perot cavity structure thus creates many longitudinal modes. • Therefore, in each case the resulting gain is the superposition of two processes. Longitudinal modes Laser output gain Mode spacing 11/27/2020 TTC Riyadh, ICT–BS-2. 3/2 Frequency 44

Laser Output Mode Structure (Contd. ) • Normally, the device can be tuned to in favor of single longitudinal mode (main lobe). • Therefore, a measure called mode-suppression ratio (MSR) is introduced as • In decibels, 11/27/2020 TTC Riyadh, ICT–BS-2. 3/2 45

Physical Structure – Laser Diode • Laser diodes has a similar structure to edge-emitting LED. • However, it has a thinner active region (gain-guided). • In addition, it consists of - strip contacts to high density current injection - cladding thickness variations to fabricate a ridge waveguide Cleaved surface (mirror) Active layer Cladding layer Active layer Metallic layer 11/27/2020 TTC Riyadh, ICT–BS-2. 3/2 46

Types of Laser Diodes • At the beginning, Fabric-Perot cavity configuration is used with two directions optical confinement. This makes broader- area semiconductor lasers. • With highly elliptical spatial output pattern, several improvements were followed to obtain better performances. – Gain-guided semiconductor lasers Limits the current injection to a small stripe to provide lateral optical confinement – Index-guided semiconductor lasers Confinement is achieved with index steps in the lateral direction – Buried hetrostructure lasers Obtains single mode output by controlling the width and thickness of the active layer 11/27/2020 TTC Riyadh, ICT–BS-2. 3/2 47

Quantum Well Laser • The single quantum well (SQW) laser offers better efficiency and wavelength by using a thick active region of 5 to 20 nm. • Small cavity size makes easy confinement. • Used in lightwave communication systems. p-layer Quantum dot Active region Quantum wells (In. As dots in n-layer the well) 11/27/2020 TTC Riyadh, ICT–BS-2. 3/2 48

Distributed Feedback Laser (DFL) • A Braggy grating inside the heterostructure is used to select one reflective wavelength. • Slopes of the grating generate a distributed reflection which couples both forward and backward travelling waves and a single wavelength is supported. • Therefore, a powerful output can be obtained with even a smaller linewidth. Grating Mirror Active region Distributed Feedback Laser (DFL) 11/27/2020 TTC Riyadh, ICT–BS-2. 3/2 49

DFL (Contd. ) • A separate Braggy reflector is used externally to the active region. • With this preparation, it is possible to select main mode wavelength outside the cavity with an MSR > 30 d. B. Mirror Grating Active region Distributed Baggy Reflector (DBR) 11/27/2020 TTC Riyadh, ICT–BS-2. 3/2 50

External Cavity Laser (ECL) • One cavity mirror is moved outside the active region. • Therefore, the second set of cavity parameters has to be coupled with the first but, loss is occurred inside the cavity. • However, minimum loss is occurred at the peak while the maximum is at the nearest secondary mode. • Consequently, a higher MSR can be obtained. Active region Lens 11/27/2020 TTC Riyadh, ICT–BS-2. 3/2 External mirror 51

Vertical Cavity Surface-Emitting Laser (VCSEL) • This produces a single mode, narrower linewidth and circular output which can be easily coupled into fibers for LAN applications. • Emissions exit from the surface rather than the edge. • Attractive in communication applications because of low power consumption and relatively high switching speeds. Active region DBR mirror 11/27/2020 TTC Riyadh, ICT–BS-2. 3/2 52

Advantages of LD over LED • A higher radiance due to amplifying effect of the stimulated emission. – Optical output power in m. W • Narrower linewidth minimizes the effect of material dispersion. – Order of 1 nm or less • Extension of modulation capabilities upto GHz range. • Applicability of heterodyne (coherent) detection in high capacity systems. • Good spatial coherent allows efficient coupling into the fibers even with low numerical apertures thus results a higher efficiency. 11/27/2020 TTC Riyadh, ICT–BS-2. 3/2 53

Optical Fiber Communications L P ∑ Optical Signals: Attenuation and Amplification 12 0 12 ICT-BS-2. 3/2/1 Optical Sources 1 ICT-BS-2. 3/2/2 Structures and Characteristics of Light-Emitting Diodes LED 1 ICT-BS-2. 3/2/3 Semiconductor Laser Structures 1 ICT-BS-2. 3/2/4 Power Launching and Coupling 1 ICT-BS-2. 3/2/5 Optical Detectors 2 ICT-BS-2. 3/2/6 Signal Encoding/Decoding 2 ICT-BS-2. 3/2/7 Modulation and Demodulation Formats 2 ICT-BS-2. 3/2/8 Receiver Sensitivities 1 ICT-BS-2. 3/2/9 Optical Amplifiers 1 Modules Code ICT-BS-2. 3/2 11/27/2020 TTC Riyadh, ICT–BS-2. 3/2 54

Review – Optical Fiber Communication System Electrical Signal Input Optical Source Optical Detector Modulator Demodulator Output Signal Transmission path (Optical Fiber) Transmitter 11/27/2020 Receiver TTC Riyadh, ICT–BS-2. 3/2 55

Attenuation and losses

Attenuation and losses Coupling losses transmitter-fibre Coupling losses fibre-fibre Coupling losses fibre-receiver Transmission level Fibre attenuation Min. required reception level Coupling losses in a fibre-optic transmission system

Attenuation and losses In optical telecommunications systems, the method of coupling the glass fibres is of prime importance. Low-attenuation couplings are essential, not only between the fibre-optic cable sections themselves, but also between them and the transmitter / receiver elements. The low light intensities employed cause small additional attenuations due to coupling losses in the light junctions between transmitter & fibre, fibre & fibre, and fibre & receiver. The extremely small dimensions of the fibre-optic cables require accurate alignment of the coupling elements, fibres being coupled permanently (spliced joints) or with detachable elements (connectors).

Optical Transmitter Bias monitor Data Disable laser Data conversion Laser control Laser Driver Modulation and bias Temperature control Current monitor Temperature monitor TE Optical power monitor TE = Thermoelectric cooler 11/27/2020 TTC Riyadh, ICT–BS-2. 3/2 59

Optical Transmitter (Contd. ) • In a optical fiber communication system, the transmitter is responsible of – generating an optical signal (source) – modulating the signal (modulator) – coupling the signal into the fiber (coupling mechanism). • In addition, there may be a photodiode monitor, a temperature sensor, cooling devices and feedback mechanisms. • It is useful to monitor the transmitter performance to make sure that there is a stable output with minimal noise effect. • Generally, to maintain constant transmitter power output, laser diode transmitters requires feedback monitoring mechanism. 11/27/2020 TTC Riyadh, ICT–BS-2. 3/2 60

Optical detectors Irrespective of the field of application, photo-detectors must exhibit the following properties: • High sensitivity to the light received in the range of wavelengths from the source of optical radiation • Short response times • Low noise • Insensitive to temperature changes • Reasonably priced • Long service life • Good coupling possibilities for fibre-optic cables

Optical detectors Semiconductor photodiodes function on the direct internal photo-electric effect. This occurs at the p-n junction of the semiconductor material when light energy strikes the junction. This in turn, causes the charge-carriers to be separated, thus producing diffusion and drift currents that result in a photoelectric current. The charge-carriers pass through the space charge region and induce a photocurrent signal in the external circuit.

Optical detectors The frequency response of the photodiode is influenced by the electrical equivalent circuit of the diode, taking into account the external load circuit (input of amplifier). Typical path resistance values R for an AP-diode are in the region of a few ohms to a few tens of ohms. The conductance of the barrier layer G can usually be ignored. The figure shows the equivalent circuit for avalanche (AP) and Pl. N photodiodes with junction capacitance C and the other parasitic elements. In high-frequency diodes, the value of C is about 1 p. F, assuming the reverse voltage is not too small, and the diode surfaces are 100. . . 300 nm diameter. A load resistor RL of 50 Ω therefore results in an RC limit frequency of 2. . . 4 GHz.

Source-to-Fiber Coupling • The main objective of the coupling mechanism is to couple much light into the fiber. • However, several losses may arise due to reflection loss, area mismatch, packing fraction loss and numerical aperture mismatch. • Two basic types – Lens coupling – Direct coupling 11/27/2020 TTC Riyadh, ICT–BS-2. 3/2 64

Source-to-Fiber Coupling (Contd. ) • Lens coupling – Approximately 100% efficiency is achievable by using lens coupling – Sometimes suffers from lens mounting problems Cylindrical lens Source 11/27/2020 Cylindrical lens Spherical lens Fiber TTC Riyadh, ICT–BS-2. 3/2 65

Source-to-Fiber Coupling (Contd. ) • Direct coupling – Makes the fiber close as much as possible to the source and then the source is epoxied into fiber. Source • Fiber By fiber pigtailing with integrated transmitter module, the efficiency of the direct coupling can be improved. Rubber boot Fiber pigtail Source 11/27/2020 Optical Isolator Ferrule TTC Riyadh, ICT–BS-2. 3/2 66

Fiber Optic Couplers • Fiber optic couplers transmit one or more fiber inputs to one or more fiber outputs. • Therefore, it is possible to transmit the same signal to two places or to provide bidirectionality and isolation. • Star coupler – Number of inputs are coupled to number of outputs 11/27/2020 TTC Riyadh, ICT–BS-2. 3/2 67

Fiber Optic Couplers (Contd. ) • Tree coupler – Distributes incoming light to several outputs evenly. • Tee (tap) coupler – Three ports, one input and two outputs and third port can be used for monitoring purposes by taking out a portion of the output signal. 11/27/2020 TTC Riyadh, ICT–BS-2. 3/2 68

Fiber Optic Couplers (Contd. ) • Four-port directional coupler – Two bare fibers are twisted together and then pulling and melting together. • The losses involved in coupling include insertion loss, excess loss and splitting or directional loss. 11/27/2020 TTC Riyadh, ICT–BS-2. 3/2 69

Optical Fiber Communications L P ∑ Optical Signals: Attenuation and Amplification 12 0 12 ICT-BS-2. 3/2/1 Optical Sources 1 ICT-BS-2. 3/2/2 Structures and Characteristics of Light-Emitting Diodes LED 1 ICT-BS-2. 3/2/3 Semiconductor Laser Structures 1 ICT-BS-2. 3/2/4 Power Launching and Coupling 1 ICT-BS-2. 3/2/5 Optical Detectors 2 ICT-BS-2. 3/2/6 Signal Encoding/Decoding 2 ICT-BS-2. 3/2/7 Modulation and Demodulation Formats 2 ICT-BS-2. 3/2/8 Receiver Sensitivities 1 ICT-BS-2. 3/2/9 Optical Amplifiers 1 Modules Code ICT-BS-2. 3/2 11/27/2020 TTC Riyadh, ICT–BS-2. 3/2 70

Review – Optical Fiber Communication System Electrical Signal Input Optical Source Optical Detector Modulator Demodulator Output Signal Transmission path (Optical Fiber) Transmitter 11/27/2020 Receiver TTC Riyadh, ICT–BS-2. 3/2 71

Optical Detectors • Photodetection process is used to convert the optical signal back to the electrical signal at the receiver. • The common light detectors are semiconductor junction devices. • The basic principle used for detection is optical absorption. (AP) • Type of optical detectors – pn-junction photodiode – Positive-intrinsic-negative (PIN) photodiode – Avalanche (AP) photodiode – Metal-semiconductor-metal (MSM) photodiode 11/27/2020 TTC Riyadh, ICT–BS-2. 3/2 72

Optical detectors convert light intensity back into an electrical variable, the current. In modern optical transmission lines, the detector components are usually silicon PIN-diodes for short distances and low-cost systems. These diodes have an intrinsic (neutral) range between the P and N ranges. AP-diodes (avalanche photodiodes) are used in systems with larger bandwidths, where the cost of the detector is not of prime importance.

Optical detectors Irrespective of the field of application, photo-detectors must exhibit the following properties: • High sensitivity to the light received in the range of wavelengths from the source of optical radiation • Short response times • Low noise • Insensitive to temperature changes • Reasonably priced • Long service life • Good coupling possibilities for fibre-optic cables

Optical detectors Semiconductor photodiodes function on the direct internal photo-electric effect. This occurs at the p-n junction of the semiconductor material when light energy strikes the junction. This in turn, causes the charge-carriers to be separated, thus producing diffusion and drift currents that result in a photoelectric current. The charge-carriers pass through the space charge region and induce a photocurrent signal in the external circuit.

Optical detectors The frequency response of the photodiode is influenced by the electrical equivalent circuit of the diode, taking into account the external load circuit (input of amplifier). In a receiver for low light intensity (or photon flux), the photodiode is operated in the reverse (non-conducting) direction. The value of the load resistance determines whether the circuit is to be used for a large output signal (= large load resistance) or a high limit frequency (= smaller load resistance). Further influencing factors are the internal diffusion processes, the charge transit time and timing effects (in time-division multiplex processes in APdiodes). The equivalent circuit of a PIN- and an AP-diode are shown below.

Optical detectors The frequency response of the photodiode is influenced by the electrical equivalent circuit of the diode, taking into account the external load circuit (input of amplifier). Typical path resistance values R for an AP-diode are in the region of a few ohms to a few tens of ohms. The conductance of the barrier layer G can usually be ignored. The figure shows the equivalent circuit for avalanche (AP) and Pl. N photodiodes with junction capacitance C and the other parasitic elements. In high-frequency diodes, the value of C is about 1 p. F, assuming the reverse voltage is not too small, and the diode surfaces are 100. . . 300 nm diameter. A load resistor RL of 50 Ω therefore results in an RC limit frequency of 2. . . 4 GHz.

Optical detectors . Iph= Photo Current C= Barrier layer capacitance G= Barrier layer conductance R= Path resistance RL= Load resistor A= Amplification Diode equivalent circuit

Optical Absorption • When a photon strike the semiconductor material with more than the bandgap energy, it is absorbed an electron-hole pair is generated. • Thus an electric field applied across the semiconductor creates a current flow due to the attraction of positive and negative charges to the electron and the hole respectively. Reverse biased + voltage Generated photocurrent Incident photons Semiconductor 11/27/2020 TTC Riyadh, ICT–BS-2. 3/2 79

Optical Absorption (Contd. ) • Once a incoming photon is detected by the semiconductor material over a range of wavelength, it converts the photon energy greater than the bandgap energy into an electron-hole pair. Conduction band Bandgap energy Valance band 11/27/2020 TTC Riyadh, ICT–BS-2. 3/2 80

Optical Absorption (Contd. ) • Although the process of optical absorption is available while the light reaches at the semiconductor, not all the incident photos are converted back to the electric current (includes in Fresnel reflection). • The total power absorbed depends on the Fresnel reflection and absorption coefficient (absorption length). - Optical power incident on the semiconductor material - Fresnel reflection - Absorption coefficient 11/27/2020 TTC Riyadh, ICT–BS-2. 3/2 81

Optical Absorption (Contd. ) Incident power Semiconductor Radiative power Incident power level Power loss due to Fresnel reflection Penetration depth • Distance into the semiconductor Penetration depth defines as the depth at which the power level falls of initial power. 11/27/2020 TTC Riyadh, ICT–BS-2. 3/2 82

pn-junction Photodiode • Performs almost the reverse function of an LED. • When light is applied to the p-region, photon energy is absorbed by an electron. Therefore, the absorbed energy raises a bound electron across the bandgap from the valance band to the conduction band. • This separated electron and hole is attracted to the positive and negative potentials in the depletion region and a current is produced. • However, the pn-junction photodiode responsivity is low and rise time is large. p-region n-region + - Depletion region 11/27/2020 TTC Riyadh, ICT–BS-2. 3/2 83

pn-junction Photodiode (Contd. ) • When pn-junction is reverse biased no current flows. • Even without the presence of light, a small current can be flown through the circuit and it is called as the dark current. Photodiode current Forward bias Reverse breakdown voltage Dark current Photodiode voltage Reverse bias 11/27/2020 TTC Riyadh, ICT–BS-2. 3/2 84

PIN Photodiode • A lightly n-doped intrinsic layer is included between p- and n- regions and it acts as the depletion layer. • The absorption is taken place inside thick intrinsic layer thus most of the photons can be converted into electron-hole pairs. • Hence the quantum efficiency (efficiency of photon-to-electron conversion) is increased. p-region 11/27/2020 Intrinsic region n-region TTC Riyadh, ICT–BS-2. 3/2 + - 85

PIN Photodiode (Contd. ) • Because of depletion region is inside the intrinsic region, charge carriers can be moved with a higher velocity. • Therefore, this performs better than the pn-junction photodiode in reverse biased mode. • Also the rise time is increased relative to pn-junction photodiode. • The wider depletion region decreases the junction capacitance and consequently increases the bandwidth. • On the other hand, increased transmit time within the layer decreases the bandwidth. • Therefore, selecting the width and the area of the intrinsic region have to done carefully. 11/27/2020 TTC Riyadh, ICT–BS-2. 3/2 86

Avalanche Photodiode (AP Diode) • APD is also a semiconductor junction detector which aquires more photodiode gain thus increases the responsivity over PIN diode (range of 20 -80 A/W). • Hence, this is capable of allowing longer fiber lengths between repeaters. • Consists of lightly doped intrinsic and p-regions are packed between p+- and n+- regions. p+ 11/27/2020 Lightly doped p n+ TTC Riyadh, ICT–BS-2. 3/2 + - 87

Optical Fiber Communications L P ∑ Optical Signals: Attenuation and Amplification 12 0 12 ICT-BS-2. 3/2/1 Optical Sources 1 ICT-BS-2. 3/2/2 Structures and Characteristics of Light-Emitting Diodes LED 1 ICT-BS-2. 3/2/3 Semiconductor Laser Structures 1 ICT-BS-2. 3/2/4 Power Launching and Coupling 1 ICT-BS-2. 3/2/5 Optical Detectors 2 ICT-BS-2. 3/2/6 Signal Encoding/Decoding 2 ICT-BS-2. 3/2/7 Modulation and Demodulation Formats 2 ICT-BS-2. 3/2/8 Receiver Sensitivities 1 ICT-BS-2. 3/2/9 Optical Amplifiers 1 Modules Code ICT-BS-2. 3/2 11/27/2020 TTC Riyadh, ICT–BS-2. 3/2 88

Signal Encoding & Decoding Information Transmission signal type in the optical fiber Analog signals Modulation Digital signals Encoding Analog signals Modulation Digital signals Encoding Analog Digital 11/27/2020 TTC Riyadh, ICT–BS-2. 3/2 89

Signal Encoding & Decoding (Contd. ) • Hence, encoding in optical fiber transmission means the transmission of analog optical information through fiber optics digitally. • This improves the acceptable signal-to-noise ratio (SNR) by 20 to 30 d. B over analog transmission. Encoder Fiber cable Analog optical data 11/27/2020 TTC Riyadh, ICT–BS-2. 3/2 Decoder Analog optical data 90

Control of the transmitter diode Basically, selecting the method of modulation depends greatly on the types of signal to be transmitted; these can either be analog or digital. It is also necessary to determine which field of telecommunication applications the optical waveguide system is intended for, in order to establish the bandwidth required and the length of the transmission path. Involved here might be broad-band transmission as in cable TV, cross-connections in telephone and data networks, wide-area networks (WAN), submarine cables, etc. , or transmission with narrow and medium bandwidths and data rates, such as data and signal transmission in buildings, ships, aircraft, computer systems, studios, between studios, etc. With some limitations, the characteristics of LED and laser diodes permit a direct modulation of intensity for transmitting analog signals. This means that the intensity of the light source is directly varied in relation to the applied analog or digital signal. This form of modulation however, assumes that the characteristic is linear. optical receiver

Control of the transmitter diode Pulse modulation, with the possibility of time-division multiplex operation, requires a large and sometimes, complex circuit. In optical transmission, the pulses directly drive the LED or laser diodes functioning as an optical transmitter. If analog signals are to be transmitted using pulse modulation, the signals must be modulated using a known method (e. g. pulse code modulation). Improvement in the quality of transmission and immunity to interference with pulse modulation however, requires larger bandwidths which are gaining in importance, particularly in long-distance telephony. With direct pulse modulation of the transmitter diode, however, it is necessary to note the turn-on delay which occurs when the diode is switched from the zero state. An advantage therefore, is to adapt the pulse to the characteristic. This is achieved by applying a biasing current and matching the pulse amplitude to the characteristic.

Control of the transmitter diode Before examining the various methods of modulation, however, it is necessary to know the characteristics of the infrared transmitter diodes, so that the biasing current can be set correctly for linear transmission of the signals. The aim of modulation is to convert the signals, usually in the form of a voltage varying as a function of time, into a luminous flux as a function of time, without any loss of information. However, two non-linear factors are present: The non-linear characteristic of the diode I = f(U) and a saturation area in the upper section of the characteristic of light intensity as a function of the diode current Φ = f(I), i. e. the outer quantum efficiency drops as the current increases

Signal Encoding & Decoding (Contd. ) • Analog signals are digitized by using pulse code modulation (PCM). Analog optical input Sampler PAM Quantized PAM Quantizer Encoder PCM Fiber cable PCM Decoder Quantizer PAM Quantized PAM 11/27/2020 TTC Riyadh, ICT–BS-2. 3/2 LPF Analog optical output 94

Advantages of Digital Transmission • There are several benefits of digital transmission over analog transmission. – Produces fewer errors than analog transmission. – Permits higher maximum transmission rates. – More data transmission through a given circuit (more efficient). – More secure because it is easier to encrypt. – Integrating voice, video and data on the same circuit much simpler. 11/27/2020 TTC Riyadh, ICT–BS-2. 3/2 95

Sampling • The analog signal is first sampled at a rate greater than the Nyquist sampling rate (greater than twice the maximum signal frequency). • Thus the pulse amplitude modulated (PAM) signal is obtained where the amplitude for constant width sampling pulses. Analog signal Sampling pulses PAM signal 11/27/2020 TTC Riyadh, ICT–BS-2. 3/2 96

Quantizing • The PAM signal is then quantized to into a number of discrete levels so that each of the distinct binary codeword represents a pulse code modulated (PCM) signal. Code levels 11/27/2020 7 6 5 4 3 2 1 0 TTC Riyadh, ICT–BS-2. 3/2 97

Encoding • Afterthat, different discrete amplitude values are encoded by using binary patterns. – 8 levels PAM is encoded into 3 bits – 16 levels PAM is encoded into 4 bits Binary Equivalent Decimal Number 22 21 20 0 0 1 2 0 1 0 3 0 1 1 4 1 0 0 5 1 0 1 6 1 1 0 7 1 11/27/2020 TTC Riyadh, ICT–BS-2. 3/2 Pulse Code Waveform 98

Multiplexing & Demultiplexing • Conversion of analog signal to a discrete PCM signal allows number of analog channels to be transmitted through a single optical fiber link. • This is called as time-division multiplexing. • Multiplexing improves the information transfer rate. Analog input 1 PCM Encoding Analog input 2 PCM Encoding Analog input 3 PCM Encoding Rotary switch Fiber cable (Multiplexing) 11/27/2020 PCM Decoding Analog output 1 PCM Decoding Analog output 2 PCM Decoding Analog output 3 (Demultiplexing) TTC Riyadh, ICT–BS-2. 3/2 99

Multiplexing & Demultiplexing 11/27/2020 TTC Riyadh, ICT–BVF– 4 /1/1 100

Optical Fiber Communications L P ∑ Optical Signals: Attenuation and Amplification 12 0 12 ICT-BS-2. 3/2/1 Optical Sources 1 ICT-BS-2. 3/2/2 Structures and Characteristics of Light-Emitting Diodes LED 1 ICT-BS-2. 3/2/3 Semiconductor Laser Structures 1 ICT-BS-2. 3/2/4 Power Launching and Coupling 1 ICT-BS-2. 3/2/5 Optical Detectors 2 ICT-BS-2. 3/2/6 Signal Encoding/Decoding 2 ICT-BS-2. 3/2/7 Modulation and Demodulation Formats 2 ICT-BS-2. 3/2/8 Receiver Sensitivities 1 ICT-BS-2. 3/2/9 Optical Amplifiers 1 Modules Code ICT-BS-2. 3/2 11/27/2020 TTC Riyadh, ICT–BS-2. 3/2 101

Review – Optical Fiber Communication System Electrical Signal Input Optical Source Optical Detector Modulator Demodulator Output Signal Transmission path (Optical Fiber) Transmitter 11/27/2020 Receiver TTC Riyadh, ICT–BS-2. 3/2 102

Signal Modulation & Demodulation Information Transmission signal type in the optical fiber Analog signals Modulation (Analog) Digital signals Encoding Analog signals Modulation (Digital) Digital signals Encoding Analog Digital 11/27/2020 TTC Riyadh, ICT–BS-2. 3/2 103

Modulator Types • In optical fiber communication can be achieved in two ways. – Direct modulation – Indirect modulation • Further, it can categorized as – Analog modulation (Intensity modulation) Primary modulation method is amplitude modulation. – Digital modulation Commonly used technique is on-off keying (OOK). 11/27/2020 TTC Riyadh, ICT–BS-2. 3/2 104

Direct Modulation • In direct modulation, the modulated electrical signal is input directly to the source and obtained the modulated optical signal output. Modulated electrical input Optical Source Modulated optical output • This introduces transient changes (chirps) in the wavelength. • Chirps are caused for dispersion on the waveform thus limit the distance and also the bandwidth capabilities of the transmitter. • Not suitable for high speed transmitters. 11/27/2020 TTC Riyadh, ICT–BS-2. 3/2 105

Indirect Modulation • The modulation is achieved externally. • Used for higher data rate transmitters (greater than 10 Gbits/s). Optical Source Modulator Modulated optical output Modulated electrical input 11/27/2020 TTC Riyadh, ICT–BS-2. 3/2 106

Analog (Intensity) Modulation • In fiber optic signal modulation, the intensity of the light source is varied according to some electrical input signal (baseband signal). Thus it is called as intensity modulation (analog modulation). • This method is inexpensive and easy to implement. Source drive circuit (Optical modulator) Baseband input 11/27/2020 Amplifier Fiber cable Optical detector Optical source TTC Riyadh, ICT–BS-2. 3/2 LPF Baseband output 107

LED Intensity Modulation • The diode output power is modulated by a current source which simply turns the LED on or off. • Requires a dc bias to keep the total current in the forward direction at all times. • Without the dc current, a negative swing in the signal current would reverse the direction thus shutting the diode off. Output power (Resulting output power) Current (LED driving current) 11/27/2020 TTC Riyadh, ICT–BS-2. 3/2 108

LED Intensity Modulation (Contd. ) • - dc bias current • - signal current • - average power • - peak amplitude of the modulated portion of the output power • Therefore, the total diode current is and the corresponding output power is 11/27/2020 TTC Riyadh, ICT–BS-2. 3/2 109

LED Intensity Modulation (Contd. ) • The modulation index in terms of current can be defined as • Similarly, the modulation index related to the power is • Thus, Same as amplitude modulation (AM) Optical carrier intensity 11/27/2020 Baseband signal TTC Riyadh, ICT–BS-2. 3/2 110

LED Intensity Modulator • The modulator circuit operates with the help of a bipolar junction transistor (BJT). LED ON Load line for BJT OFF 11/27/2020 TTC Riyadh, ICT–BS-2. 3/2 111

Laser Intensity Modulation • The analog circuit used for LED is suitable for analog modulation of a laser diode. • A heat sink has to be used to cool the temperature dependency effects of laser diode. Output power (Resulting output power) Current (Laser driving current) 11/27/2020 TTC Riyadh, ICT–BS-2. 3/2 112

Subcarrier Intensity Modulation • Although the direct intensity modulation is suitable for transmitting a baseband analog signal though a single fiber. • But, for a wideband fiber, number of baseband channels have to be used the same fiber for efficient utilization. • Therefore, subcarrier intensity modulation can be applied by multiplexing (frequency division) composite electrical signal prior to the intensity modulation. Analog baseband signal s Modulators (two level) Modulator & (drive circuit) Optical source Fiber cable RF subcarriers Optical detector 11/27/2020 TTC Riyadh, ICT–BS-2. 3/2 Amplifier Analog baseband signals Demodulators (drive circuit) 113

Digital Modulation • The most common digital modulation technique used is on-off keying (OOK). • When binary value “ 1” used for optic power pulse is ON and binary value “ 0” for optic power pulse is OFF. • Transistor provides the switching and current amplification. LED (Transistor switched LED digital modulator) • The other methods used for digital modulation of optical fiber transmission are pulse position modulation (PPM) and pulse width modulation (PWM). 11/27/2020 TTC Riyadh, ICT–BS-2. 3/2 114

Demodulation Circuits • Demodulation circuits are operated by using either a bipolar junction transistor (BJT) or a field effect transistor (FET). • For higher data rates (larger bandwidths), the bipolar transistor introduces less noise than the field effect transistor. Output PIN photodiode (BJT amplifier) 11/27/2020 Output TTC Riyadh, ICT–BS-2. 3/2 (FET amplifier) 115

Optical Fiber Communications L P ∑ Optical Signals: Attenuation and Amplification 12 0 12 ICT-BS-2. 3/2/1 Optical Sources 1 ICT-BS-2. 3/2/2 Structures and Characteristics of Light-Emitting Diodes LED 1 ICT-BS-2. 3/2/3 Semiconductor Laser Structures 1 ICT-BS-2. 3/2/4 Power Launching and Coupling 1 ICT-BS-2. 3/2/5 Optical Detectors 2 ICT-BS-2. 3/2/6 Signal Encoding/Decoding 2 ICT-BS-2. 3/2/7 Modulation and Demodulation Formats 2 ICT-BS-2. 3/2/8 Receiver Sensitivities 1 ICT-BS-2. 3/2/9 Optical Amplifiers 1 Modules Code ICT-BS-2. 3/2 11/27/2020 TTC Riyadh, ICT–BS-2. 3/2 116

Review – Optical Fiber Communication System Electrical Signal Input Optical Source Optical Detector Modulator Demodulator Output Signal Transmission path (Optical Fiber) Transmitter 11/27/2020 Receiver TTC Riyadh, ICT–BS-2. 3/2 117

Receiver Operation • Receiver is responsible for converting the optical signal back to the original information set by the transmitter. • However, interfacing from fiber to photodiode has to be done carefully to increase the amount of light entering to the detector circuit. • Lens coupling, using anti-reflection coatings, applying index-matching gel and using pigtail packaging are some solution to that. • The basic subsections in the receiver are the photodiode, low noise pre-amplifier, main amplifier section and the data recovery stage. • The receivers can be categorized as – analog receivers and – digital receivers. 11/27/2020 TTC Riyadh, ICT–BS-2. 3/2 118

Analog Receiver • Eventhough digital signal transmission is preferred in optical communication, there are many potential applications for analog transmission. • It ranges from individual 4 k. Hz voice channels to multi-GHz microwave links. Preamplifier Amplifier Photodiode Filter Output Optical Signal Power Supply (Front End) 11/27/2020 Automatic Gain Control (Main Amplifier) TTC Riyadh, ICT–BS-2. 3/2 (Data Recovery) 119

Analog Receiver (Contd. ) • The optical signal coupled from the light source to the fiber gets attenuated and distorted during the transmission through the fiber cable. • Once it is detected and converted back to the electrical form by using a photodetector, the produced electrical current is typically very weak. • Therefore, to boost its level, the main amplifier is used. • To minimize the effect of intersymbol interference (ISI), a lowpass filter is used remove the parts outside the signal bandwidth. • Then, the demodulator is used to recover original data sent by the transmitter. 11/27/2020 TTC Riyadh, ICT–BS-2. 3/2 120

Digital Receiver • The notable difference in the digital receiver is the data recovery subsection compared to the analog receiver because the analog receiver data recovery can be done directly by using the demodulator. • However, the digital one requires further signal processing. • It consists of a decision circuit and a clock recovery circuit. Preamplifier Amplifier Photodiode Filter Decision Circuit Optical Signal Power Supply (Front End) 11/27/2020 Automatic Gain Control (Main Amplifier) TTC Riyadh, ICT–BS-2. 3/2 Output Clock Recovery (Data Recovery) 121

Signal Recovery in a Digital receiver • This is responsible of checking the validity of the received information. • The decision circuit is used to separate bits (to either ones or zeros) of the received data. The data is compared with a threshold level. – If the received voltage is more than the threshold will results bit “ 1”. – Otherwise bit “ 0”. • To accomplish this bit interpretation, the receiver should be able to understand the bit boundaries. • The clock recovery circuit measures the bit interval and regenerates a new clock pulse to the decision circuit. • However, to minimize the bit error rate, the receiver should be capable of detecting and correcting the errors of the received data stream. 11/27/2020 TTC Riyadh, ICT–BS-2. 3/2 122

Receiver Performance • Receiver performance is determined by transforming the received optical signal to meaningful data. • To evaluate the receiver performance, dynamic range, sensitivity, SNR and bit error rate can be used. • Dynamic range – The amount of signal level can be detected with a linear response. – Sometimes at high powers, the receivers may become nonlinear thus anomalies can be occurred. – Typical range is 30 to 40 d. B. 11/27/2020 TTC Riyadh, ICT–BS-2. 3/2 123

Receiver Performance (Contd. ) • Sensitivity – The minimum optical input power can be detected by the receiver. – This determines the quality of the service, i. e. , for a given SNR, the minimum input optical power needed. • Signal-to-noise ratio (SNR) – This determines detectability of the signal with the addition of noise. • Bit error rate (BER) – The average probability of incorrect bit identification. – If there is one error bit for every 109 bits, then BER is 10 -9. 11/27/2020 TTC Riyadh, ICT–BS-2. 3/2 124

Receiver packaging • Receiver packaging is useful for high data rate systems to protect from installation environment effects such as mismatching of connecting devices. • As an example by keeping shorter photodiode connections will amplify less noise to the data recovery section. • Thus, the detector performance can be significantly enhanced by integrating packages. 11/27/2020 TTC Riyadh, ICT–BS-2. 3/2 125

Transceiver • By combining the transmitter and the receiver also can increase the performance of the transmission. Transceiver = Transmitter + Receiver (Connector) Laser Diode Control Electronics Data (Transmitter) Electroabsorption Modulator Laser Diode Drive Out Power Supply Amplifier With AGC Data In Fiber Connector Data Recovery Circuit Photodiode Filter Preamp Fiber Connector (Receiver) 11/27/2020 TTC Riyadh, ICT–BS-2. 3/2 126

Optical Fiber Communications L P ∑ Optical Signals: Attenuation and Amplification 12 0 12 ICT-BS-2. 3/2/1 Optical Sources 1 ICT-BS-2. 3/2/2 Structures and Characteristics of Light-Emitting Diodes LED 1 ICT-BS-2. 3/2/3 Semiconductor Laser Structures 1 ICT-BS-2. 3/2/4 Power Launching and Coupling 1 ICT-BS-2. 3/2/5 Optical Detectors 2 ICT-BS-2. 3/2/6 Signal Encoding/Decoding 2 ICT-BS-2. 3/2/7 Modulation and Demodulation Formats 2 ICT-BS-2. 3/2/8 Receiver Sensitivities 1 ICT-BS-2. 3/2/9 Optical Amplifiers 1 Modules Code ICT-BS-2. 3/2 11/27/2020 TTC Riyadh, ICT–BS-2. 3/2 127

Amplifiers • Amplifiers are needed to increase the amplitude of the detected signal. • However, the bandwidth should remain unchanged and also the amplification of the noise part has to be minimized for a proper communication. • Amplifiers are consist of transistors, resistors and other components. • In fiber optic transmission, number of amplification stages are used especially in long distance communication. 11/27/2020 TTC Riyadh, ICT–BS-2. 3/2 128

Type of Optical Amplifiers • In-line optical amplifier – In single-mode fiber transmission, the effect of signal dispersion is very less. – Therefore, the transmission can be done by regenerating the signal without using repeaters in between. – Thus, the main purpose of in-line amplifier is compensating for transmission loss and increasing the distance between repeaters. Fiber cable Optical Tx G Optical Rx In-line amplifier 11/27/2020 TTC Riyadh, ICT–BS-2. 3/2 129

Type of Optical Amplifiers (Contd. ) • Pre-amplifier – Used to amplify the weak optical signal before the photodetection. – Thus SNR reduced because of thermal noise effect can be suppressed. – Provides a larger gain factor and also increases the bandwidth. Fiber cable Optical Tx G Optical Rx Pre- amplifier 11/27/2020 TTC Riyadh, ICT–BS-2. 3/2 130

Type of Optical Amplifiers (Contd. ) • Power amplifier – Used to boost the transmitted power thus to increase the transmission distance by 10 -100 km. – Placed immediately after the optical transmitter. – This techniques is used with pre-amplifier in undersea transmission where repeaters can not be installed. Long fiber link Optical Tx G Optical Rx Power amplifier 11/27/2020 TTC Riyadh, ICT–BS-2. 3/2 131

Type of Optical Amplifiers (Contd. ) – Power amplifier can be used to compensate coupler-insertion loss and power- splitting loss in a local area network. Receiver stations Fiber cable Optical Tx G LAN booster amplifier 11/27/2020 Star coupler TTC Riyadh, ICT–BS-2. 3/2 132

High-Impedance Amplifier • Used in early communication systems as a pre-amplifier. • Thermal noise generated due to the output resistance and reflecting back to the input is minimized by using the high input impedance. • The main drawback of this amplifier is reduced bandwidth. Photocurrent Optical Signal High-Impedance Amplifier Av LPF Output Photodiode Bias Voltage 11/27/2020 TTC Riyadh, ICT–BS-2. 3/2 133

Transimpedance Amplifier • A higher sensitivity and a relatively wide bandwidth can be obtained. • The difference of this amplifier compared to high-impedance amplifier is feedback impedance enables converting the input current into a voltage output. • This can be used with a second amplifier to achieve the required gain. Photocurrent Feedback Impedance Zf Transimpedance Amplifier Av Optical Signal Output Photodiode Bias Voltage 11/27/2020 TTC Riyadh, ICT–BS-2. 3/2 134

Semiconductor Optical Amplifier • Amplification is done by using a semiconductor laser placed between two fibers. • Active region of both ends are cleaved an coated with anti-reflective coating. • Advantage are wide spectral range and easiness of integrating with other semiconductor devices and planar optical waveguide components. • But, suffers from fiber coupling difficulties. Input Fiber Semiconductor Optical Amplifier Output Fiber Active layer Antireflection coating 11/27/2020 TTC Riyadh, ICT–BS-2. 3/2 135

Repeaters and Regenerators • A repeater consists of an optical receiver, an amplifier and an optical transmitter. • An optical signal is first converted into electrical signal, then amplified and next converted back to the optical mode (optical-electrical-optical conversion). • Regenerator is required to remove the noise and generate a clean signal for further transmission. • Discriminator is used to separate the noise from the signal and retiming is required to make sure that the pulse timing is in order. 11/27/2020 TTC Riyadh, ICT–BS-2. 3/2 136

Types of Regenerators • Three regenerator types. – 1 R device : Amplifying only – 2 R device : Amplifying and reshaping – 3 R device : Amplifying, reshaping and retiming Output Input R Re-amplify 2 R Re-amplify, Re-shape 3 R Re-amplify, Re-shape, Re-time 11/27/2020 TTC Riyadh, ICT–BS-2. 3/2 137