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Lecture:3 Lightwave/Optical Systems

Lecture:3 Lightwave/Optical Systems. Ajmal Muhammad, Robert Forchheimer Information Coding Group ISY Department. Outline. Optical Networks Core, metro, and access networks Optical Access Networks Optical Amplifiers Doped fibers, semiconductor optical amplifiers (SOAs) Modulation

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Lecture:3 Lightwave/Optical Systems

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  1. Lecture:3 Lightwave/Optical Systems Ajmal Muhammad, Robert Forchheimer Information Coding Group ISY Department

  2. Outline • Optical Networks • Core, metro, and access networks • Optical Access Networks • Optical Amplifiers • Doped fibers, semiconductor optical amplifiers (SOAs) • Modulation • Direct intensity, external modulation • Demodulation

  3. Telecom Network Hierarchy Long haul - 100s-1000s km - Mesh Metro (interoffice) - 10s of km - Rings Access - a few km - Hubbed rings ✕ The “Last” Mile “First”

  4. Part of core Network – Submarine Optical Cables The longest submarine cable is the Southeast Asia—Middle East—Western Europe (SEA-ME-WE 3) system stretching 39,000 km from Norden, Germany, to Keoje, South Korea

  5. Metropolitan-Area Networks (MANs) • MAN is connected to a WAN at egress nodes (EN) • MAN is connected to LANs at access nodes (AN). ADM stands for add-drop multiplexer • Several MANs can be interconnected with a ring to form a regional network • Regional rings provide protection against failures

  6. The First Mile :: Access Networks Telephone companies: xDSL (Digital Subscriber Line) - DSLdata rate 128kb/s - 1.5Mb/s - Maximumsubscriber distance from central office 5.5 km • Other flavors: ADSL(asymmetric DSL) 12Mb/s, VDSL(very-high-bit-rate) 50Mb/s-0.5 km, HDSL(high-bit-rate DSL) Cable TV companies: CM (Cable Modem) • Dedicated radio channel for data Problems with today’s access technologies (xDSL, CM) -Originally designed and built for voice and TV, respectively - Retrofitting for data not working well -Limitations in Reach, Bandwidth, Scalability, Flexibility, Cost

  7. Fiber Access Network Fiber-to-the-x (FTTx) where x = {H,B,C,P,BS,AP,…} • Platform for triple play service, i.e., voice, data and video • Long reach: 0-20 km • Fiber plant has long life span (~20 years) • Able to scale and incorporate new technologieswithout digging new trenches • Leverage long reach to facilitate broadband wireless access over shorter distance

  8. Optical Fiber Based Access Networks Power in the field required

  9. Passive Optical Network (PON) Passive Splitter - Point-to-multipoint topology - Low cost implementation - Relative ease of deployment - Future-proof OLT: Optical line terminal ONU: Optical network unit

  10. Optical Line Terminal (OLT)

  11. Optical Network Unit (ONT) ONT for FTTH (Home) ONT for FTTH outdoor unit

  12. 1G PON - Ethernet PON(EPON) Broadcasting 1 Gb/s 1490-nm wavelength Shared medium network for downstream traffic

  13. 1G PON - Ethernet PON(EPON) Time Division Multiplexing 1 Gb/s 1310-nm wavelength Low cost FP lasers Point-to-point network for upstream traffic

  14. OLT Structure Physical Media Dependent defines the optical transceiver & the wavelength demulplexer Service adaptation provides the translation between the signal format required for client equipment connection and the PON signal format Media Access Control schedules the right to use physical medium Service Network Interface (SNI)

  15. ONU Structure User to Network Interface (UNI)

  16. Typical PON Configuration Wavelength • Dual fiber 1310 nm • Single fiber upstream (downstream) on 1310 (1490) nm Transceiver • ONUFabry-Perot (upstream), PIN (downstream) • ONT APD(upstream), DFB(downstream) Transceiver Assumptions • Upstream(@1310 nm) power budget = 30 dB • Downstream(@1490 nm) power budget= 22 dB

  17. Second Generation PON:: Line-Rate Upgrade 10G-PON: Suppose symmetric 10-Gb/s downstream and upstream, and asymmetric 10-Gb/s downstream and 1-Gb/s upstream GPON: Suppose asymmetric 2.488-Gb/s downstream and 1.244-Gb/supstream XG-PON: Suppose coexistence with GPON on the same fiber plant. Downstream 10-Gb/s and upstream 2.5-Gb/s High upstream capability (symmetric approach) require more expensive ONU devices

  18. Candidate Technologies for the NG-PON Wavelength division multiplexing (WDM) PON State-of-the-art experimental WDM PON support 100Mb/s – 2Gb/s symmetric communication per wavelength channel with 32 ONUs Wavelength-routed WDM PON Migration requirements: - Change the power splitter with the AWG - Coexistence with previous generations of deployed devices not possible

  19. Hybrid (TDM/WDM) PON Pareto principle 80% of the traffic is generated by only 20 % of the users Utilize network resources (wavelengths) efficiently

  20. Optical Amplifiers • Typical fiber loss around 1.5 um is ~0.2 dB/km • After traveling ~100 km, signals are attenuated by ~20dB • Signals need to be amplified or signal-to-nose (SNR) of detected signals is too low and bit error rate (BER) becomes too high (typically want BER <10-9) Different functions of an optical amplifier

  21. Optical Amplifiers :: Characteristics An optical amplifier is characterized by: • Gain: ratio of output power to input power (in dB) • Gain efficiency: gain as a function of input power (dB/mW) • Gain bandwidth: range of wavelengths over which the amplifier is effective • Gain saturation: maximum output power, beyond which no amplification is reached • Noise: undesired signal due to physical processing in amplifier

  22. Optical Amplifiers :: Types Rare-earth doped fiber amplifiers: • Erbium Doped (EDFA) – 1,500 – 1,600 nm band • Praseodymium Doped (PDFA) – 1,300 nm band Raman amplifiers – 1,280 – 1,650 nm band Semiconductor Optical Amplifiers (SOAs) – 400 – 2,000 nm band

  23. Erbium Doped Fiber :: Amplification Process

  24. Erbium Doped Fiber :: Operation Absorption and gain spectra for 1480 nm pump

  25. Raman Amplifier

  26. Raman Amplifier :: Operation

  27. Semiconductor Optical Amplifier

  28. SOA :: Amplification Process

  29. SOA :: Design

  30. Optical Amplifiers : Comparison

  31. Modulation The process transmitting information via light carrier (or any carrier signal) Direct Intensity (current) 1310 nm transmitters • Inexpensive light emitting diode (LED) • Laser diode (LD): suffer from chirp up to 1nm (wavelength variation due to variation in electron densities in the lasing area) • Distance < 30 km, no EDFA 1310 nm

  32. External Modulation 1550 nm transmitters • Expensive but can cover distance up to 120 km by using EDFA

  33. Optical Receiver • To extract the optical signal (low level) from various noise disturbances • To reconstruct original information correctly Selection criteria • Optical sensitivity for a given SNR and BER, operating wavelength • Dynamic range, simplicity, stability

  34. Photodetector :: Types The most commonly used photodetectors in optical communications are: Positive-Intrinsic-Negative (PIN) • No internal gain • Low bias voltage [10-50 V @ Lambda=850 nm, 5-15 V @Lambda= 1300-1550 nm] • Highly linear, low dark current Avalanche Photo-Detector (APD) • Internal gain (increased sensitivity) • Best for high speed and highly sensitive receivers • Strong temperature dependence • High bias voltage [250 V @ Lambda=850 nm, 20-30 V @Lambda= 1300-1550 nm] • Costly

  35. Photodiode (PIN) :: Structure • No carrier in the I region • No current flow • Reverse-biased • Photons generated electron-hole • Current flow through the diode

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