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

2. Outline Optical Networks Optical Access Networks Optical Amplifiers
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)
- 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

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 technologies without 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
ONU Fabry-Perot (upstream), PIN (downstream)
ONT APD(upstream), DFB(downstream)
Transceiver Assumptions
nm) power budget = 30 dB
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 Gb/s downstream and 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

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 Lambda=850 nm, 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 Lambda=850 nm, 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