Fiber Optic Principles Oct-09 1
Fiber Optic Basics Optical fiber Active components Attenuation Power budget Bandwidth Oct-09 2
Reference www.flukenetworks.com/fiber Handbook Fiber Optic Technologies (Vivec Alwayn) www.ciscopress.com/articles Reference Guide to Fiber Optic Testing www.jdsu.com/fiberguide 2 International Electrotechnical Commission www.iec.org International Telecommunication Union www.itu,int/itu-t/ Wikipedia the free encyclopedia www.wikipedia,org Sept-09 3
Optical Fiber A long cylinder made from silica (glass) Light injected into one end by a light source (LED, Laser) travels along this tube and reaches the remote end with a certain attenuation A light sensitive device installed at the remote end is a light detector Virtually no light is emitted from the tube along the way Oct-09 4
Fiber structure Optical fiber is a solid strand of glass made up of a core and cladding The core and cladding are surrounded by protective plastic coating Oct-09 5
Fiber structure (1) Core: Silica glass with Germania glass dopant Purpose: signal transmission Cladding: Silica glass Purpose: signal containment-confines the light in the core Coating: Dual layer, UV cured acrylate Purpose: mechanical protection Core Cladding Coating Oct-09 6
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What happens inside As light is guided through the core, two things take place defined as: Reflection = light hitting the core/cladding edge and then traveling back into the core. Refraction = light refracts: alters its direction while penetrating a different medium, depending on the angle at which it strikes a surface Light is guided through the fiber on a path. Singlemode = a single path Multimode = multiple paths Oct-09 8
Refraction Refraction has an index and actually represents the speed of the light through the core Each material is characterized by an index of refraction Refractive index of the medium (n)= Speed of light in a vacuum (c) speed of light in a specific medium(v) n=c/v. The refractive index in vacuum=1, for air =1.003, for water is 1.333 Oct-09 9
Refraction index The difference of the index of refraction of the core and the cladding causes most Of the transmitted light to bounce off the cladding glass and stay within the core Oct-09 10
Fiber types Long-Haul Short-Haul Singlemode (Laser Light Source) Multimode (LED Light Source) Graded-Index Step-Index (obsolete) Oct-09 11
Multimode - Step Index Total Internal Reflection takes place at Core / Cladding boundary, due to difference in Refractive Indices of Core and Cladding Light Source (LED) Core Light Rays (Modes) Signal Cladding Oct-09 12 Signal Propagation Even with short lengths of step index Multi-mode fiber (100m or more), the radial modes of light injected into the Step index core would not arrive (at receiver) at far end of the cable at the same time. The result is Modal dispersion Modal dispersion causes pulse spreading which causes Signal distortion and limits severely the attainable distance
Multimode - Graded Index The core of the fiber is graded thus the gradient refractive Index changes gradually from the center of the core to the edge of the core. The result:: the light travels in more graceful waves. Primarily used in LANS ~2Km n1=1.540 Cladding Light Source (LED) Core Light Rays (Modes) Signal n2 = 1.540 to 1.562 Signal Propagation Modes of light arrive largely in-phase at the far end of the fiber. Signal spread and distortion are reduced Typical transmission from 850nm -1300nm Most common core size 50µm&62.5µm High Attenuation and limited Bandwidth Oct-09 13
Singlemode Fiber Light Source (LASER) n1-=1.457 Cladding Core N2= 1.471 Signal Signal Propagation Typical transmission from 1200-1650nm Commonly referred as second &third windows Low attenuation Ideal for short to long haul Nearly infinite bandwidth Negligible spread and distortion of signal Oct-09 14
Singlemode Step Index Step-Index-Fiber: a fiber in which the core is of a uniform refractive index Refraction in the 8 to 9 micron core is very consistent There is no graduation from the center to the outer edge With such a small core, only one mode of light can be sent or transmitted The light basically goes straight down the pipe Oct-09 15
Transmitter/Receiver When the light source is modulated by data it becomes a Transmitter When the light detector is connected to a demodulator it becomes a Receiver Oct-09 16
Transmitter/Receiver (1) Modulator De-Modulator Data IN Light Source TRANSMITTER Light Detector Data OUT RECEIVER Oct-09 17
Transceiver For a bi-directional link you need 2 fibers & 2 Transceivers (or WDM) Data OUT Data IN R T T R Data IN Data OUT Oct-09 18
MM SM comparison SM fibers allow greater distances and higher bandwidths Installation (termination, splicing) of SM fibers is more complicated, time consuming and expensive (must be more accurate) SM active components (i.e. transceivers) are more expensive Oct-09 19
Fiber applications Multimode: LAN Application under 2Km Operates at 850nm and 1300nm 62.5µm and 50µm sizes Singlemode: Telco, CATV, and long-haul applications Operates at 1310nm and 1550nm Oct-09 20
Wavelength windows F/O transmission standardized in 3 wavelength windows First 850nm Second 1310nm Third 1550nm Longer wavelength = lower attenuation = larger distance = higher cost Oct-09 21
Window transmission & reception Transmitters have precise and narrow wavelength, typically ±10nm from center of window Receivers are open to receive a wider range, typically 800 to 1400nm or 1200 to 1600nm Oct-09 22
Connector types Data Rate M/M/F S/M/F 10Mbps 100/155 Mbps 1Gbps 10Gbps ST, SC SC, ST VF-45, MTRJ SC LC ST SC SC, SFF,LC LC Link condition: Both Ends MUST be wavelength matched! Oct-09 23
Connector types (1) Connectors can be installed on any type of fiber Usually they install the easiest on tight buffer type cable Field installation is no problem on multimode but may be more difficult on singlemode Oct-09 24
Connector types (2) SC Connectors LC Connectors MT-RJ Connector ST Connectors SC Duplex Oct-09 25 Single ST Single SC Single LC
SFP transceivers SFP handle SONET/SDH, Ethernet, Fiber Channel, CWDM and ESCON based applications SFP transceivers offer a broad range of devices to meet short, intermediate and long distance performance objectives over multimode and single mode fibers, including (up to 120km for GBE) Additionally, SFP devices with integrated Digital Diagnostics Monitoring functionality per the MSA SFF-8472 are available The pluggable design is compliant with the SFP MSA The GE10 uses the XFP transceivers - dual LC -type connectors The XFP are compliant to XFP MSA and to IEEE 802.3ae. Oct-09 26
F/O cable plant In order to establish the suitability of an existing fiber to your network, the following factors must be considered: Attenuation Bandwidth (mainly with MM fiber) Chromatic dispersion (mainly with GBE or higher rates) Oct-09 27
Attenuation Fiber only has one problem and that is Attenuation: the reduction of optical power as the light signal moves down the optical fiber away from the transmitter Causes: Open fiber = maximum attenuation Absorption = light is absorbed by impurities in the glass Scattering = light hits impurities and spreads out in the media Oct-09 28
Attenuation (db / km) Wavelength windows Spectral response curve of fibre 8 1st Window = 850 nm Typ. loss: 3 db/km 6 4 2 2nd Window = 1300 nm Typ. loss: 0.7-1.2 db / km 3rd Window = 1550 nm Typ. loss: 0.2 db/ km 0 600 800 1000 1200 1400 1600 Wavelength (nm) LED Sources Oct-09 29 Laser Sources
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Fiber Components Table - Typical Losses Wavelength/Mode Fiber Core Diameter Attenuation per Km Attenuation per Splice Fusion Mech Attenuation per Pair Connector 850 nm multi-mode 50 µm 3.0 db 0.05 db 0.3 db 0.25 0.5 db 850 nm multi-mode 62.5/125 µm 3.2 db 0.05dB 0.3.dB 0.25 0.5 db 1300 nm multi-mode 50 µm 0.7-1.0 db 0.05dB 0.3.dB 0.25 0.5 db 1300 nm multi-mode 62.5/125 µm 1.2 db 0.05dB 0.3.dB 0.25 0.5 db 1310 nm single-mode 9 µm 0.35 db 0.05dB 0.3.dB 0.25 0.5 db 1550 nm single-mode 9 µm 0.2-0.25 db 0.05dB 0.3.dB 0.25 0.5 db Oct-09 31
Typical Optical Loss Budget calculations Network Short Distance Medium distance Long Haul Distance (Km) 40 80 120 Fiber loss at 1550nm 0.25 0.2 0.2 Total Fiber loss 10 16 24 Number of splices 10 20 30 Average Splice loss 0.05 0.05 0.05 Total Splice Loss 0.5 1 1.5 Number of Connectors 2 2 2 Connectors Pair loss 0.5 0.5 0.5 Total Connectors loss 0.5 0.5 0.5 Total Loss 10.55 17.5 26 Notes: The listed numbers are typical averages and may vary according to the type and quality of equipment deployed for your installation 1550nm DFB = DFB stands for Distributed Feedback Laser: it is a special type of Laser that operates to limit modal dispersion to extend link distances Oct-09 32
Total path attenuation Best way is to actually measure end-to-end If impossible, calculate by summing up: Fiber attenuation: km * att. from table Number of splices * 0.05 0.3dB Number of connections * 0.25-0.5dB Add 2-4dB for aging Oct-09 33
Power budget Lowest rated (or actually measured) output power (both ends) [dbm] - Lowest rated (or actually measured) sensitivity (both ends) [dbm] = Power Budget [db] Oct-09 34
Power budget example End A Power range [dbm] -13-2 Sensitivity range [dbm] -5-30 End B -13-2 -5-30 Power Budget = -13 - (-30) = 17dB Oct-09 35
Link Condition Power budget > Path attenuation For Ethernet and FE and wavelengths 850-1310nm, this condition is usually sufficient to ensure normal operation For GBE and up as well as 3 rd window other factors must be considered (chromatic dispersion, bandwidth, type of laser) Oct-09 36
Bandwidth Singlemode Fiber can support infinite bandwidth, while Multimode is limited to supporting finite amounts of bandwidth Although all MM fiber cables have a graded index core, there is still a loss of signal strength over longer cable distances All fibers carries a bandwidth specification, which is distance- dependent and varies with the wavelength of its optical window (MM fiber supports 850nm and 1310nm) Specification for a 62.5/125 micron (µm) Multi-mode fiber: 850nm 1300nm Bandwidth MHz*Km 200 500 For example, at 850nm window, this fiber supports 200MHz over 1Km; the same fiber will have only 100MHz of bandwidth if the distance is extended to 2Km Without any modal dispersion problems, there is no bandwidth distance limitation for single mode fiber optic cable Oct-09 37
Bandwidth (G.651/652) Wavelength Fiber type MM 50/125 MM 62.5/125 850nm 400Mhz*km 200Mhz*km 1310nm 1000Mhz*km 600Mhz*km Oct-09 38
MM fiber absolute limitations Fast Ethernet: 5km (62.5/125 fiber), 8km (50/125 fiber) GBE (850nm): 240m (62.5/125 fiber), 500m (50/125 fiber) GBE (1310nm): 500m (62.5/125 fiber), 800m (50/125 fiber); practically, under certain circumstances, 5km may be reached Oct-09 39
MM fiber typical ranges Fast Ethernet: 2km GBE (850nm): 220m (62.5/125 fiber), 500m (50/125 fiber) GBE (1310nm): 440m (62.5/125 fiber), 550m (50/125 fiber Oct-09 40
SM fiber typical ranges Fast Ethernet: 1550nm - 150km GBE: 1310nm - 40km 1550nm (DFB) - 120km SFS (Single Fiber Strand) FE and GBE: 20,40,80kM Oct-09 41
3 rd window issues FP laser can seldom be effective at distances exceeding 20km; For longer distances make sure your device is based on DFB lasers Make sure your cable plant is built based on fibers complying with ITU-T-G.651/652 Oct-09 42
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