CSC/ECE 775: Optical Networks Rudra Dutta, Fall 2006

Transcript CSC/ECE 775: Optical Networks Rudra Dutta, Fall 2006

CSC/ECE 778: Optical Networks

Rudra Dutta, Fall 2007 Fiber-Optical Communication and Switching

Outline

 We want/need to understand effect on networking – What components are possible, limitations  Quick overview of representative technology – Optical Connection and Power Budget – Fundamentals of Fiber Optic Transmission – Transmission Impairments and Solutions – Lasers and Photodetectors – Other Optical Components (Couplers, Filters, Multiplexers, Switches, OADMs, Amplifiers) Copyright Rudra Dutta, NCSU, Fall, 2007 2

Layering and Optical Services

  Generalized protocol layering can create complicated multi-layer networks In this context, “optical layer” is another layer close to physical layer, but possibly implementing network semantics of its own Network Data Link Network Data Link Physical Network Physical Data Link Physical Copyright Rudra Dutta, NCSU, Fall, 2007 User Apps IP ATM SONET Optical 3

Why Fiber?

 Huge bandwidth: 30-50 THz  Low losses (intrinsic): 0.2 db/Km  Low bit error rates (BER): 10 -11  Low power requirements: 100 photons/bit  Immunity to electromagnetic interference (EMI)  Low cross-talk  Repeater-less amplification (EDFAs)  Low cost, maintenance Copyright Rudra Dutta, NCSU, Fall, 2007 4

Optical Endpoint

Optical Power Budget

 Finite power available at source (laser)  Minimum detectable receiver power  Must account for all losses between source and receiver  Optical networks are

power-budget limited

, not bandwidth limited Copyright Rudra Dutta, NCSU, Fall, 2007 6

Optical Power Budget (cont'd)

Wavelengths of Importance

Optical Fiber

 Optical waveguide  Cylindrical core surrounded by cladding (+ protective covering) – – made of same transparent material (glass, plastic) difference is value of refractive index n = c / v  Single-mode vs. multimode fiber – – single-mode: core diameter 8 12µm, link length > 2Km multimode: core diameter 50µm, link length < 2Km  Step-index vs. graded-index fiber – – step-index: refractive index constant across core diameter graded-index: refractive index varies along core diameter Copyright Rudra Dutta, NCSU, Fall, 2007 9

Refractive Index Profiles

Geometric Optics: Snell's Law

Geometric Optics: Total Reflection

Maximum Cone of Acceptance

Transmitter-to-Fiber Coupling

Modes: The Wave Picture

Allowed Ray Angles

  Only allowed ray angles result in guided modes AB = d sin  m core – = m  /2 leads to half wavelength in the m : integer,  : optical wavelength in the core  Mode: one possible path that a guided ray can take Copyright Rudra Dutta, NCSU, Fall, 2007 16

Transmission Impairments

 Factors affecting transmission distance and bandwidth: – attenuation – – dispersion non-linear effects    Must minimize their effects for high performance – improvement and redesign of fiber itself – compensating for these factors Attenuation problem solved  dispersion effects significant Dispersion effects reduced  dominant non-linear effects Copyright Rudra Dutta, NCSU, Fall, 2007 17

Attenuation

 Decrease in optical power along the length of the fiber  Varies with wavelength  Attenuation coefficient: a dB (P R ÷P T ) (dB/Km) – L : length of fiber = - 10/L log 10 – – P T : power launched into the fiber P R : power received at end of fiber Copyright Rudra Dutta, NCSU, Fall, 2007 18

Power Losses

 Material absorption: due to – resonances of silica molecules – impurities -- most serious is peak at 1390 nm due to OH ions  Rayleigh scattering: medium is not absolutely uniform – – refractive index fluctuates  scattering proportional to  -4 light is scattered  dominant at  < 800 nm  Waveguide imperfections: relatively small component – – nonideal fiber geometries due to bending, manufacturing imperfections Copyright Rudra Dutta, NCSU, Fall, 2007 19

Low Loss Region of An Optical Fiber

Erbium-Doped Fiber Amplifiers

EDFA Principle of Operation

 E i : energy level  N i – : population of erbium ions at energy level E i normally (no pump/signal): N 1 > N 2 > N 3 – pump/signal present: population inversion N 2 > N 1 Copyright Rudra Dutta, NCSU, Fall, 2007 22

EDFA Properties

  Emission: – – stimulated  amplification spontaneous  emission  noise  amplified spontaneous limit on number of EDFAs along the fiber Energy levels are narrow bands  each transition associated w/ a band of wavelengths  amplify wide band around 1550nm    Replace expensive and complicated electronic units Signal remains in optical form  transparency “Distributed” amplifiers Copyright Rudra Dutta, NCSU, Fall, 2007 23

Semiconductor Optical Amplifiers (SOAs)  Similar to semiconductor laser  Consist of active medium (

p-n

junction)   Energy levels of electrons confined to 2 bands  EDFA E 1 , E 2 Mobile carriers (holes, electrons) play the role of erbium ions  Has several disadvantages compared to EDFAs  Useful when combined with other components into optoelectronic integrated circuits (OEICs) – – preamplifier in optical receiver power amplifier in optical transmitter Copyright Rudra Dutta, NCSU, Fall, 2007 24

Dispersion

    A narrow pulse spreads out as it propagates along the fiber Intersymbol interference: – – pulse overlaps neighboring pulses sharply increases the BER Dispersion imposes a limit on the bit rate that can be supported Intermodal vs. chromatic dispersion Copyright Rudra Dutta, NCSU, Fall, 2007 25

Intermodal Dispersion

 Most serious form of dispersion  Occurs in multimode fibers  Different modes of a wavelength travel at different speeds  Multimode fibers limited to low bitrate-distance products  Solutions: – – use single-mode fibers for large bitrate-distance products (8 µm < 2a < 10 µm  only one mode is guided) use graded-index fibers Copyright Rudra Dutta, NCSU, Fall, 2007 26

Graded Index Fibers

Propagation in Graded Index Fibers

 Rays are bent as they approach the cladding  Rays further from core travel faster (due to lower

)  Intermodal dispersion reduced by several orders of magnitude Copyright Rudra Dutta, NCSU, Fall, 2007 28

Chromatic Dispersion

 Two sources of chromatic dispersion:  – – material dispersion, D M waveguide dispersion, D W Chromatic dispersion: D = D M + D W Copyright Rudra Dutta, NCSU, Fall, 2007 29

Material Dispersion

 The physical effect that allows raindrops to form rainbow  Refractive index of a material changes with wavelength  different wavelengths travel at different speeds along the fiber  Different delays cause spreading of output pulse, depending on: – wavelength span of source – length of fiber Copyright Rudra Dutta, NCSU, Fall, 2007 30

Waveguide Dispersion

 D W is a function of fiber geometry  Dispersion-shifted fibers: – D W causes zero-dispersion point to shift to 1550 nm range – min dispersion range coincides with min loss range  Dispersion-flattened fibers: dispersion profile close to zero for a wide spectral range Copyright Rudra Dutta, NCSU, Fall, 2007 31

Dispersion Profile of Single-Mode Fiber Copyright Rudra Dutta, NCSU, Fall, 2007 32

Non-Linear Effects

 Stimulating Raman Scattering (SRS): – light interacts with fiber medium  inelastic collisions – – – not important in single-channel systems (thresh. about 500mW) involves transfer of power: hi freq. wave  lo freq. wave introduces cross-talk in multiwavelength systems  Stimulating Brillouin Scattering (SBS): – no cross-talk, low threshold power (few mW for 20-Km fiber)  Four-Wave Mixing – – three signals present at neighboring freq: f 1 , f 2 , f 3 new signal produced, e.g., f 4 = f 1 + f 2 - f 3 Copyright Rudra Dutta, NCSU, Fall, 2007 33

Solitons

 Distortion, non-linearities: distort, broaden a propagating pulse  Right combination of distortion, non-linearity: – compensate each other – – – produce a narrow, stable pulse (soliton) solitons travel over long distances without any distortion solitons in opposite directions pass thru transparently  Ideal situation for long-distance communication  EDFAs needed to maintain solitons over long distances Copyright Rudra Dutta, NCSU, Fall, 2007 34

Lasers

 Light amplification by stimulated emission of radiation  Schawlow and Townes, 1958  First solid-state laser by Maiman, 1960  Today, lasers exist in myriad forms Copyright Rudra Dutta, NCSU, Fall, 2007 35

Semiconductor Energy State Diagrams Copyright Rudra Dutta, NCSU, Fall, 2007 36

Fabry-Perot Cavity

 Part of light leaves cavity through right facet, part is reflected  Resonant wavelengths: L = m  /2 Copyright Rudra Dutta, NCSU, Fall, 2007 37

Single-Wavelength Operation

 FP laser cavity supports many modes/wavelengths of operation  Monochromatic light needed for high bitrate distance products  Geometry is modified to achieve single wavelength operation  Distributed Bragg Reflector (DBR) lasers  Distributed Feedback (DFB) lasers  Expensive, widely used in long-distance communication Copyright Rudra Dutta, NCSU, Fall, 2007 38

Tunability

 Laser tunability important in WDM network applications: – – slow tunability (ms range): set up lightpaths in wavelength routing networks fast tunability (µs or ns range): multiple access (T WDMA) applications Copyright Rudra Dutta, NCSU, Fall, 2007 39

Tunability (cont'd)

 Mechanically tuned: change FP cavity length – (tuning range: 10-20 nm, tuning time: 100-500 ms)  Injection current tuned: change refr. index in DFB/DBR lasers – (tuning range: 4 nm, tuning time: 10s of ns)  Multiwavelength laser arrays – built in single chip – one or more lasers can be activated simultaneously – light from each laser fed to star coupler Copyright Rudra Dutta, NCSU, Fall, 2007 40

Optical Receivers

Photodetectors

Filters

 Various technologies: – Fabry-Perot filters – Multilayer interference (MI) filters – Mach-Zehnder interferometers – Arrayed waveguide grating – Acousto-optic tunable filter  Tunability important  Can be used as MUX/DEMUX, wavelength routers Copyright Rudra Dutta, NCSU, Fall, 2007 43

MI Filters

 Bandpass filter  Passes thru particular wavelength, reflects all other  Cascade multiple filters to create a MUX/DEMUX Copyright Rudra Dutta, NCSU, Fall, 2007 44

MI Filters as MUX/DEMUX

MUX/DEMUX: Logical View

Directional Couplers

 Coupling possible when waveguides placed close together  Coupling ratio controlled by voltage Copyright Rudra Dutta, NCSU, Fall, 2007 47

Couplers: Logical View

 P 1’ = a 11 P 1 + a 12 P 2 , P 2’ = a 21 P 1 + a 22 P 2  For ideal symmetric couplers: a 11 = a 22 = a, a 12 = a 21 = 1-a Copyright Rudra Dutta, NCSU, Fall, 2007 48

Couplers

 Star Coupler: – a = 1/2, 2x2 star coupler (3-dB coupler) – Cascade 2x2 couplers to build NxN star coupler  Power Splitter: – P 2 = 0, a = 1/2  Switches: – a = 0,1; 2x2 switch – cascade 2x2 switches to build NxN switch  Real devices are lossy: – a 11 + a 12 < 1, a 21 + a 22 < 1 Copyright Rudra Dutta, NCSU, Fall, 2007 49

Internal Structure of Star Coupler

Gratings

Gratings: Principle of Operation

 Multiple narrow slits spaced equally apart on the grating plane  Light incident on one side of grating transmitted through slits   Diffraction: light through each slit spreads out in all directions Different  s interfere constructively at different points of imaging plane  separate WDM signal into constituent wavelengths Copyright Rudra Dutta, NCSU, Fall, 2007 52

Bragg Gratings

 Bragg grating: any periodic pertrubation in propagating medium  Perturbation is usually periodic variation of refractive index  Bragg gratings used in many photonic devices: – DBR lasers: Bragg gratings written in waveguides – Fiber Bragg gratings (FBG): written in fiber – Acousto-optic tunable filters: Bragg grating formed by propagation of an acoustic wave in the medium Copyright Rudra Dutta, NCSU, Fall, 2007 53

FBG as Add-Drop Multiplexers

OADM: Logical View

Optical Switches

 Mechanical switches – directional couplers, ratio modified by bending (ms range) – MEMS mirrors moved in and out of path (100s of ns range)  Bubble-Based switches – bubbles in optical fluid reflect beam (10s of ms range)  Electro-Optic switches – couplers, ratio modified by changing refr. index (ns range)  Thermo-Optic switches – refractive index function of temperature (ms range)  Semiconductor Optical Amplifier (SOA) switches – SOA, change in voltage to use as on-off switch (ns range) Copyright Rudra Dutta, NCSU, Fall, 2007 56

MEMS Optical Switching

   MEMS = micro-electro-mechanical system Movable mirrors to reflect light 2D MEMS: a 2-state pop-up MEMS mirror – – state ``0'': popped up position light reflected state ``1'': flat (folded) position light passes through Copyright Rudra Dutta, NCSU, Fall, 2007 57

2D MEMS Switches

Analog Beam-Steering Mirror

 Mirror can be freely rotated on two axes to reflect a light beam Copyright Rudra Dutta, NCSU, Fall, 2007 59

3D MEMS Switch

Static Optical Switches

Reconfigurable Optical Switches

Wavelength Converters

Spectrum Partitioning

 c = f  ,  f - c  /  2  100 Ghz is about .8 nm at 1,550 nm range  10-Ghz spacing: – – – – very dense by current standards can accommodate 1 Gbps digital bit rates can accomodate 1 Ghz analog bandwidths OK for receivers, but too close for wavelength routing  100 Ghz spacing OK for optical switches – WDM limit today  Waveband routing alleviates throughput loss – – But better switching technology nullifies advantage However, continue to be useful because needs “coarser” filters Copyright Rudra Dutta, NCSU, Fall, 2007 64