Chapter 8 Optical Fibers and Components TOPICS WDM

Chapter 8: Optical Fibers and Components TOPICS – WDM optical networks – Light transmitted through an optical fiber – Types of optical fibers – Impairments – Components: Lasers, optical amplifiers, couplers, OXCs Connection-Oriented Networks 1

WDM optical networks 1 1 Tx Power amplifier optical fiber In-line amplification optical fiber … … Tx W Rx Preamplifier Wavelength multiplexer W Rx Wavelength demultiplexer A point-to-point connection Connection-Oriented Networks 2

An example of an optical network Mesh network Ring 4 Ring 1 Ring 2 Connection-Oriented Networks Ring 3 3

How light is transmitted through an optical fiber Wave Electric field Source Waves and electrical fields Connection-Oriented Networks 4

An optical fiber Cladding Core and cladding Cladding Core n 1 n 2 Radial distance a) Step-index fiber Connection-Oriented Networks Refractive index Core n 1 n 2 Radial distance b) graded-index fiber 5

Refraction and reflection of a light ray f Refracted ray n 2 n 1 Incident ray Connection-Oriented Networks r Reflected ray 6

Angle of launching a ray into the fiber Cladding Core l Core r Cladding Optical transmitter Core Cladding Connection-Oriented Networks 7

Multi-mode and single-mode fibers • Core/diameter of a multi-mode fiber: – 50/125 m, – 62. 5/125 m, – 100/140 m • Core/diameter of single-mode fiber – 9 or 10 / 125 m Connection-Oriented Networks 8

Electric fields A 2 Cladding Core 1 B Connection-Oriented Networks Cladding 9

Electric field amplitudes for various fiber modes Cladding Core Cladding m=0 Connection-Oriented Networks m=1 m=2 10

Propagation of modes Cladding a) step-index fiber Cladding b) Graded-index fiber Connection-Oriented Networks 11

Single-mode fiber Cladding Connection-Oriented Networks 12

Impairments • The transmission of light through an optical fiber is subjected to optical effects, known as impairments. • There are: – linear impairments, and – non-linear impairments. Connection-Oriented Networks 13

Linear impairments • These impairments are called linear because their effect is proportional to the length of the fiber. • Attenuation: – Attenuation is the decrease of the optical power along the length of the fiber. • Dispersion – Dispersion is the distortion of the shape of a pulse. Connection-Oriented Networks 14

Attenuation 2. 5 Attenuation, d. B 2. 0 1. 5 1. 0 0. 5 800 1000 Connection-Oriented Networks 1200 1400 Wavelength, nm 1600 1800 15

Attenuation in Fiber • Attenuation – P(L) = 10 -AL/10 P(0) • Where P(0) optical power at transmitter, • P(L) power at distance L Km, and • A = attenuation constant of the fiber • Received Power must be greater or equal to – receiver sensitivity Pr – Lmax = 10/A log 10(P(0)/P(r)) Connection-Oriented Networks 16

Dispersion • Dispersion is due to a number of reasons, such as – modal dispersion, – chromatic dispersion, – polarization mode dispersion. Connection-Oriented Networks 17

Modal dispersion Power Time • In multi-mode fibers some modes travel a longer distance to get to the end of the fiber than others • In view of this, the modes have different delays, which causes a spreading of the output pulse Connection-Oriented Networks 18

Chromatic dispersion • It is due to the fact that the refractive index of silica is frequency dependent. In view of this, different frequencies travel at different speeds, and as a result they experience different delays. • These delays cause spreading in the duration of the output pulse. Connection-Oriented Networks 19

• Chromatic dispersion can be corrected using a dispersion compensating fiber. The length of this fiber is proportional to the dispersion of the transmission fiber. Approximately, a spool of 15 km of dispersion compensating fiber is placed for every 80 km of transmission fiber. • Dispersion compensating fiber introduces attenuation of about 0. 5 d. B/km. Connection-Oriented Networks 20

Polarization mode dispersion (PMD) • It is due to the fact that the core of the fiber is not perfectly round. • In an ideal circularly symmetric fiber the light gets polarized and it travels along two polarization planes which have the same speed. • When the core of the fiber is not round, the light traveling along the two planes may travel at different speeds. • This difference in speed will cause the pulse to break. Connection-Oriented Networks 21

Non-linear impairments • They are due to the dependency of the refractive index on the intensity of the applied electrical field. The most important non-linear effects in this category are: selfphase modulation and four-wave mixing. • Another category of non-linear impairments includes the stimulated Raman scattering and stimulated Brillouin scattering. Connection-Oriented Networks 22

Types of fibers • Multi-mode fibers: They are used in LANs and more recently in 1 Gigabit Ethernet and 10 Gigabit Ethernet. • Single-mode fiber is used for long-distance telephony, CATV, and packet-switched networks. • Plastic optical fibers (POF) Connection-Oriented Networks 23

Single-mode fibers: • Standard single-mode fiber (SSMF): Most of the installed fiber falls in this category. It was designed to support early long-haul transmission systems, and it has zero dispersion at 1310 nm. • Non-zero dispersion fiber (NZDF): This fiber has zero dispersion near 1450 nm. Connection-Oriented Networks 24

• Negative dispersion fiber (NDF): This type of fiber has a negative dispersion in the region 1300 to 1600 nm. • Low water peak fiber (LWPF): The peak in the attenuation curve at 1385 nm is known as the water peak. With this new type of fiber this peak is eliminated, which allows the use of this region. Connection-Oriented Networks 25

Plastic optical fibers (POF) • Single-mode and multi-mode fibers have a high cost and they require a skilled technician to install them. • POFs on the other hand, are very low-cost and they can be easily installed by an untrained person. • The core has a very large diameter, and it is about 96% of the diameter of the cladding. • Plastic optic fibers find use in digital home appliance interfaces, home networks, and cars Connection-Oriented Networks 26

Components • • • Lasers Photo-detectors and optical receivers Optical amplifiers The 2 x 2 coupler Optical cross connects (OXC) Connection-Oriented Networks 27

Light amplification by stimulated emission of radiation (Laser) • A laser is a device that produces a very strong and concentrated beam. • It consists of an energy source which is applied to a lasing material, a substance that emits light in all directions and it can be of gas, solid, or semiconducting material. • The light produced by the lasing material is enhanced using a device such as the Fabry-Perot resonator cavity. Connection-Oriented Networks 28

Fabry-Perot resonator cavity. It consists of two partially reflecting parallel flat mirrors, known as facets, which create an optical feedback that causes the cavity to oscillate. Light hits the right facet and part of it leaves the cavity through the right facet and part of it is reflected. Left facet Connection-Oriented Networks Right facet 29

• Since there are many resonant wavelengths, the resulting output consists of many wavelengths spread over a few nm, with a gap between two adjacent wavelengths of 100 to 200 GHz. • A single wavelength can be selected by using a filtering mechanism that selects the desired wavelength and provides loss to the other wavelengths. Connection-Oriented Networks 30

Tunable lasers • Tunable lasers are important to optical networks • Also, it is more convenient to manufacture and stock tunable lasers, than make different lasers for specific wavelengths. • Several different types of tunable lasers exist, varying from slow tunability to fast tunability. Connection-Oriented Networks 31

Modulation • Modulation is the addition of information on a light stream • This can be realized using the on-off keying (OOK) scheme, whereby the light stream is turned on or off depending whether we want to modulate a 1 or a 0. Connection-Oriented Networks 32

WDM and dense WDM (DWDM) • WDM or dense WDM (DWDM) are terms used interchangeably. • DWDM refers to the wavelength spacing proposed in the ITU-T G. 692 standard in the 1550 nm window (which has the smallest amount of attenuation and it also lies in the band where the Erbium-doped fiber amplifier operates. ) • The ITU-T grid is not always followed, since there are many proprietary solutions. Connection-Oriented Networks 33

The ITU-T DWDM grid Connection-Oriented Networks 34

Photo-detectors and optical receivers • The WDM optical signal is demultiplexed into the W different wavelengths, and each wavelength is directed to a receiver. • Each receiver consists of a – photodetector, – an amplifier, and – signal-processing circuit. Connection-Oriented Networks 35

Optical amplifiers • The optical signal looses its power as it propagates through an optical fiber, and after some distance it becomes too weak to be detected. • Optical amplification is used to restore the strength of the signal Connection-Oriented Networks 36

1 1 Tx … Rx … W Tx Power amplifier optical fiber Wavelength multiplexer In-line amplification optical fiber Preamplifier W Rx Wavelength demultiplexer Amplifiers: power amplifiers, in-line amplifiers, pre-amplifiers Connection-Oriented Networks 37

1 R, 2 R, 3 R • Prior to optical amplifiers, the optical signal was regenerated by first converting it into an electrical signal, then apply – 1 R (re-amplification), or – 2 R (re-amplification and re-shaping) or – 3 R (re-amplification, re-shaping, and re-timing) and then converting the regenerated signal back into the optical domain. Connection-Oriented Networks 38

Amplification and Regeneration Connection-Oriented Networks 39

The Erbium-doped fiber amplifier (EDFA) Coupler Signal to be amplified 1550 nm Erbium-doped fiber Isolator Laser 850 nm Connection-Oriented Networks 40

Two-stage EDFA Coupler Signal to be amplified 1550 nm Erbium-doped fiber Isolator Laser 850 nm Connection-Oriented Networks Isolator Laser 850 nm 41

The 2 x 2 coupler Fiber 1 Input 1 Output 1 Input 2 Output 2 Fiber 2 Tapered region Coupling region Tapered region The 2 x 2 coupler is a basic device in optical networks, and it can be constructed in variety of different ways. A common construction is the fused-fiber coupler. Connection-Oriented Networks 42

3 -d. B coupler A 2 x 2 coupler is called a 3 -d. B coupler when the optical power of an input light applied to, say input 1 of fiber 1, is evenly divided between output 1 and output 2. Connection-Oriented Networks 43

• If we only launch a light to the one of the two inputs of a 3 -d. B coupler, say input 1, then the coupler acts as a splitter. • If we launch a light to input 1 and a light to input 2 of a 3 -d. B coupler, then the two lights will be coupled together and the resulting light will be evenly divided between outputs 1 and 2. • In the above case, if we ignore output 2, the 3 -d. B coupler acts as a combiner. Connection-Oriented Networks 44

A banyan network of 3 -d. B couplers 1 1 2 8 2 1 2 8 3 1 2 8 4 1 2 8 5 1 2 8 6 1 2 8 7 1 2 8 8 1 2 8 Connection-Oriented Networks 45

Optical cross connects (OXCs) Input fibers CPU Output fibers 1 1 W W Fiber 1 … … 1 1 W Fiber N Connection-Oriented Networks Switch fabric W Fiber N 46

OXC (cont’d) • Optical cross-connects Wavelength Router OXC WDM link To & from other nodes GMPLS Plane To & from other nodes UNI Access Station Tx Connection-Oriented Networks IP router Local Add Rx Local Drop 47

OXC: switching fabric • Switching fabric MEMS: one mirror per output Input WL λ 1 to output 1 OXC Output 1 Connection-Oriented Networks 2 3 4 48

OXC: switching fabric (cont’d) • Switching fabric MEMS: one mirror per output Input WL λ 1 to output 4 OXC Output 1 Connection-Oriented Networks 2 3 4 49

OXC functionality • It switches optically all the incoming wavelengths of the input fibers to the outgoing wavelengths of the output fibers. • For instance, it can switch the optical signal on incoming wavelength i of input fiber k to the outgoing wavelength i of output fiber m. Connection-Oriented Networks 50

• Converters: If it is equipped with converters, it can switch the optical signal of the incoming wavelength i of input fiber k to another outgoing wavelength j of the output fiber m. This happens when the wavelength i of the output fiber m is in use. Converters typically have a limited range within they can convert a wavelength. Connection-Oriented Networks 51

• Optical add/drop multiplexer (OADM): An OXC can also be used as an OADM. That is, it can terminate the optical signal of a number of incoming wavelengths and insert new optical signals on the same wavelengths in an output port. The remaining incoming wavelengths are switched through as described above. Connection-Oriented Networks 52

Transparent and Opaque Switches Transparent switch: The incoming wavelengths are switched to the output fibers optically, without having to convert them to the electrical domain. Opaque switch: The input optical signals are converted to electrical signals, from where the packets are extracted. Packets are switched using a packet switch, and then they are transmitted out of the switch in the optical domain. Connection-Oriented Networks 53

Switch technologies Several different technologies exist: – micro electronic mechanical systems (MEMS) – semiconductor optical amplifiers (SOA) – micro-bubbles – holograms – Also, 2 x 2 directional coupler , such as the electro-optic switch, thermo-optic switch, and the Mach-Zehnder interferometer, can be used to construct large OXC switch fabrics Connection-Oriented Networks 54

2 D MEMS switching fabric Down … … Up … … … … i … … … Input ports … … Actuator Mirro r Output ports j Connection-Oriented Networks 55

A 2 D MEMS OADM Add wavelengths Terminate wavelengths Logical design Connection-Oriented Networks … … … … i … … 1 2 W … Drop wavelengths 1 2 W … … Add wavelengths 2 D MEMS implementation 56

3 D MEMS switching fabric Output wavelengths y axis MEMS array Inside ring Input wavelengths Mirro r x axis MEMS array Connection-Oriented Networks 57

Semiconductor optical amplifier (SOA) • A SOA is a pn-junction that acts as an amplifier and also as an on-off switch Current p-type n-type Optical signal Connection-Oriented Networks 58

A 2 x 2 SOA switch • Wavelength 1 is split into two optical signals, and each signal is directed to a different SOA. One SOA amplifies the optical signal and permits it to go through, and the other one stops it. As a result 1 may leave from either the upper or the lower output port. • Switching time is currently about 100 psec. Polymer waveguides SOAs Polymer waveguides 1 2 Connection-Oriented Networks 59