Connection-Oriented Networks1 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 Networks2 WDM optical networks A point-to-point connection In-line amplification optical fiber optical fiber W 1 … Tx Wavelength multiplexer Power amplifier W 1 … Tx Rx Wavelength demultiplexer Pre- amplifier

Connection-Oriented Networks3 An example of an optical network Mesh network Ring 1 Ring 2 Ring 4 Ring 3

Connection-Oriented Networks4 How light is transmitted through an optical fiber Waves and electrical fields Source Electric field Wave

Connection-Oriented Networks5 Core Cladding Radial distance Refractive index n1n1 n2n2 Core Cladding Radial distance n2n2 n1n1 Cladding Core Refractive index a) Step-index fiber b) graded-index fiber Core and cladding An optical fiber

Connection-Oriented Networks6 Refraction and reflection of a light ray Incident ray Reflected ray Refracted ray  rr ff n2n2 n1n1

Connection-Oriented Networks7 Angle of launching a ray into the fiber Core Cladding  rr ll Core Optical transmitter Cladding Core

Connection-Oriented Networks8 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 Networks9 Electric fields Cladding Core 1 A B 2

Connection-Oriented Networks10 Electric field amplitudes for various fiber modes Cladding Core m=0m=1 m=2

Connection-Oriented Networks11 Propagation of modes Cladding a) step-index fiber b) Graded-index fiber Cladding

Connection-Oriented Networks12 Single-mode fiber Cladding

Connection-Oriented Networks13 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 Networks14 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 Networks15 Attenuation Wavelength, nm Attenuation, dB

Connection-Oriented Networks16 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 P r –L max = 10/A log 10 (P(0)/P(r))

Connection-Oriented Networks17 Dispersion Dispersion is due to a number of reasons, such as –modal dispersion, –chromatic dispersion, –polarization mode dispersion.

Connection-Oriented Networks18 Modal dispersion 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 Power Time Power Time

Connection-Oriented Networks19 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 Networks20 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 dB/km.

Connection-Oriented Networks21 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 Networks22 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: self- phase modulation and four-wave mixing. Another category of non-linear impairments includes the stimulated Raman scattering and stimulated Brillouin scattering.

Connection-Oriented Networks23 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 Networks24 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 Networks25 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 Networks26 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 Networks27 Components Lasers Photo-detectors and optical receivers Optical amplifiers The 2x2 coupler Optical cross connects (OXC)

Connection-Oriented Networks28 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 Networks29 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 Right facet

Connection-Oriented Networks30 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 Networks31 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 Networks32 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 Networks33 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 Networks34 The ITU-T DWDM grid

Connection-Oriented Networks35 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 Networks36 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 Networks37 Amplifiers: power amplifiers, in-line amplifiers, pre-amplifiers In-line amplification optical fiber optical fiber W 1 … Tx Wavelength multiplexer Power amplifier W 1 … Tx Rx Wavelength demultiplexer Pre- amplifier

Connection-Oriented Networks38 1R, 2R, 3R Prior to optical amplifiers, the optical signal was regenerated by first converting it into an electrical signal, then apply –1R (re-amplification), or –2R (re-amplification and re-shaping) or –3R (re-amplification, re-shaping, and re-timing) and then converting the regenerated signal back into the optical domain.

Connection-Oriented Networks39 Amplification and Regeneration

Connection-Oriented Networks40 The Erbium-doped fiber amplifier (EDFA) Laser 850 nm Signal to be amplified 1550 nm Isolator Coupler Erbium-doped fiber Isolator

Connection-Oriented Networks41 Two-stage EDFA Signal to be amplified 1550 nm Laser 850 nm Isolator Coupler Erbium-doped fiber Laser 850 nm Coupler Isolator

Connection-Oriented Networks42 The 2x2 coupler The 2x2 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. Coupling region Tapered region Tapered region Input 1 Output 1 Output 2 Fiber 1 Fiber 2 Input 2

Connection-Oriented Networks43 3-dB coupler A 2x2 coupler is called a 3-dB 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 Networks44 If we only launch a light to the one of the two inputs of a 3-dB 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-dB 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-dB coupler acts as a combiner.

Connection-Oriented Networks45 A banyan network of 3-dB couplers  2  8

Connection-Oriented Networks46 Optical cross connects (OXCs) 1 W 1 W 1 W 1 W Switch fabric Fiber 1 Fiber N Fiber 1 … … CPU Input fibers Output fibers

Connection-Oriented Networks47 OXC (cont’d) Optical cross-connects OXC IP router Tx Rx Local AddLocal Drop Access Station Wavelength Router WDM link GMPLS Plane UNI To & from other nodes

Connection-Oriented Networks48 OXC: switching fabric Switching fabric OXC Input WL λ1 to output 1 Output MEMS: one mirror per output 4

Connection-Oriented Networks49 OXC: switching fabric (cont’d) OXC Input WL λ1 to output 4 Output MEMS: one mirror per output 4 Switching fabric

Connection-Oriented Networks50 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 Networks51 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 Networks52 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 Networks53 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 Networks54 Switch technologies Several different technologies exist: –micro electronic mechanical systems (MEMS) –semiconductor optical amplifiers (SOA) –micro-bubbles –holograms –Also, 2x2 directional coupler, such as the electro-optic switch, the thermo-optic switch, and the Mach-Zehnder interferometer, can be used to construct large OXC switch fabrics

Connection-Oriented Networks55 2D MEMS switching fabric Down ActuatorMirro r Up …… … … … i j Input ports Output ports … …… … … … … … …

Connection-Oriented Networks56 A 2D MEMS OADM 1  2  W Add wavelengths Terminate wavelengths 1  2  W Add wavelengths i 1  2  W … … … … … … … Drop wavelengths … … … … … …… Logical design 2D MEMS implementation …

Connection-Oriented Networks57 3D MEMS switching fabric Mirro r Inside ring x axis y axis Output wavelengths Input wavelengths MEMS array MEMS array

Connection-Oriented Networks58 Semiconductor optical amplifier (SOA) A SOA is a pn-junction that acts as an amplifier and also as an on-off switch p-type n-type Current Optical signal

Connection-Oriented Networks59  2x2 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 Polymer waveguides SOAs 1 2