Lecture:3 Lightwave/Optical Systems

Ajmal Muhammad, Robert ForchheimerInformation Coding Group

ISY Department

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

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”

✕

Part of core Network – Submarine Optical CablesThe 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

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

The First Mile :: Access Networks

Telephone companies: xDSL (Digital Subscriber Line) - DSL data rate 128kb/s - 1.5Mb/s

- Maximum subscriber 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

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 videoLong reach: 0-20 kmFiber plant has long life span (~20 years)Able to scale and incorporate new technologies without digging new trenchesLeverage long reach to facilitate broadband wireless access over shorter distance

Optical Fiber Based Access Networks

Power in the field required

Passive Optical Network (PON)

- Point-to-multipoint topology- Low cost implementation- Relative ease of deployment- Future-proof

OLT: Optical line terminalONU: Optical network unit

Passive Splitter

Optical Line Terminal (OLT)

Optical Network Unit (ONT)

ONT for FTTH outdoor unit

ONT for FTTH (Home)

1G PON - Ethernet PON(EPON)

Shared medium network for downstream traffic

Broadcasting

1 Gb/s1490-nm wavelength

1G PON - Ethernet PON(EPON)

Point-to-point network for upstream traffic

Time Division Multiplexing

1 Gb/s1310-nm wavelengthLow cost FP lasers

OLT Structure

Service adaptation provides the translation between the signal format required for client equipment connection and the PON signal format

Service Network Interface (SNI)

Physical Media Dependent defines the optical transceiver & the wavelength demulplexer

Media Access Control schedules the right to use physical medium

ONU Structure

User to Network Interface (UNI)

Typical PON Configuration

WavelengthDual fiber 1310 nmSingle fiber upstream (downstream) on 1310 (1490) nm

TransceiverONU Fabry-Perot (upstream), PIN (downstream)ONT APD(upstream), DFB(downstream)

Transceiver Assumptions

Upstream(@1310 nm) power budget = 30 dBDownstream(@1490 nm) power budget= 22 dB

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/s upstream

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

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

Hybrid (TDM/WDM) PON

Pareto principle80% of the traffic is generated by only 20 % of the users

Utilize network resources (wavelengths) efficiently

Optical Amplifiers

Typical fiber loss around 1.5 um is ~0.2 dB/kmAfter traveling ~100 km, signals are attenuated by ~20dBSignals 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

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 effectiveGain saturation: maximum output power, beyond which no amplification is reachedNoise: undesired signal due to physical processing in amplifier

Optical Amplifiers :: Types

Rare-earth doped fiber amplifiers:Erbium Doped (EDFA) – 1,500 – 1,600 nm bandPraseodymium Doped (PDFA) – 1,300 nm band

Raman amplifiers – 1,280 – 1,650 nm band

Semiconductor Optical Amplifiers (SOAs) – 400 – 2,000 nm band

Erbium Doped Fiber :: Amplification Process

Erbium Doped Fiber :: Operation

Absorption and gain spectra for 1480 nm pump

Raman Amplifier

Raman Amplifier :: Operation

Semiconductor Optical Amplifier

SOA :: Amplification Process

SOA :: Design

Optical Amplifiers : Comparison

Modulation

The process transmitting information via light carrier (or any carrier signal)

Direct Intensity (current) 1310 nm transmittersInexpensive 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

External Modulation

1550 nm transmitters Expensive but can cover distance up to 120 km by using EDFA

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

Photodetector :: Types

The most commonly used photodetectors in optical communications are:

Positive-Intrinsic-Negative (PIN)No internal gainLow 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 receiversStrong temperature dependenceHigh bias voltage [250 V @ Lambda=850 nm, 20-30 V @Lambda= 1300-1550 nm]Costly

Photodiode (PIN) :: Structure

• No carrier in the I region• No current flow

• Reverse-biased• Photons generated electron-hole • Current flow through the diode