CSC/ECE 778: Optical Networks Rudra Dutta, Fall 2007 Fiber-Optical Communication and Switching

Click here to load reader

download CSC/ECE 778: Optical Networks Rudra Dutta, Fall 2007 Fiber-Optical Communication and Switching

of 66

  • date post

    27-Dec-2015
  • Category

    Documents

  • view

    218
  • download

    0

Embed Size (px)

Transcript of CSC/ECE 778: Optical Networks Rudra Dutta, Fall 2007 Fiber-Optical Communication and Switching

  • Slide 1
  • CSC/ECE 778: Optical Networks Rudra Dutta, Fall 2007 Fiber-Optical Communication and Switching
  • Slide 2
  • Copyright Rudra Dutta, NCSU, Fall, 20072 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)
  • Slide 3
  • Copyright Rudra Dutta, NCSU, Fall, 20073 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 Physical Optical SONET ATM IP User Apps Network Data Link Physical Network Data Link Physical
  • Slide 4
  • Copyright Rudra Dutta, NCSU, Fall, 20074 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
  • Slide 5
  • Copyright Rudra Dutta, NCSU, Fall, 20075 Optical Endpoint
  • Slide 6
  • Copyright Rudra Dutta, NCSU, Fall, 20076 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
  • Slide 7
  • Copyright Rudra Dutta, NCSU, Fall, 20077 Optical Power Budget (cont'd)
  • Slide 8
  • Copyright Rudra Dutta, NCSU, Fall, 20078 Wavelengths of Importance
  • Slide 9
  • Copyright Rudra Dutta, NCSU, Fall, 20079 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 8-12m, link length > 2Km multimode: core diameter 50m, 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
  • Slide 10
  • Copyright Rudra Dutta, NCSU, Fall, 200710 Refractive Index Profiles
  • Slide 11
  • Copyright Rudra Dutta, NCSU, Fall, 200711 Geometric Optics: Snell's Law n 1 sin i = n 2 sin t
  • Slide 12
  • Copyright Rudra Dutta, NCSU, Fall, 200712 Geometric Optics: Total Reflection Critical angle: c = sin -1 (n2 n 1 )
  • Slide 13
  • Copyright Rudra Dutta, NCSU, Fall, 200713 Maximum Cone of Acceptance
  • Slide 14
  • Copyright Rudra Dutta, NCSU, Fall, 200714 Transmitter-to-Fiber Coupling
  • Slide 15
  • Copyright Rudra Dutta, NCSU, Fall, 200715 Modes: The Wave Picture
  • Slide 16
  • Copyright Rudra Dutta, NCSU, Fall, 200716 Allowed Ray Angles Only allowed ray angles result in guided modes AB = d sin m = m /2 leads to half wavelength in the core m : integer, : optical wavelength in the core Mode: one possible path that a guided ray can take
  • Slide 17
  • Copyright Rudra Dutta, NCSU, Fall, 200717 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 non-linear effects dominant
  • Slide 18
  • Copyright Rudra Dutta, NCSU, Fall, 200718 Attenuation Decrease in optical power along the length of the fiber Varies with wavelength Attenuation coefficient: a dB = - 10/L log 10 (P R P T ) (dB/Km) L : length of fiber P T : power launched into the fiber P R : power received at end of fiber
  • Slide 19
  • Copyright Rudra Dutta, NCSU, Fall, 200719 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 light is scattered scattering proportional to -4 dominant at < 800 nm Waveguide imperfections: relatively small component nonideal fiber geometries due to bending, manufacturing imperfections
  • Slide 20
  • Copyright Rudra Dutta, NCSU, Fall, 200720 Low Loss Region of An Optical Fiber
  • Slide 21
  • Copyright Rudra Dutta, NCSU, Fall, 200721 Erbium-Doped Fiber Amplifiers
  • Slide 22
  • Copyright Rudra Dutta, NCSU, Fall, 200722 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
  • Slide 23
  • Copyright Rudra Dutta, NCSU, Fall, 200723 EDFA Properties Emission: stimulated amplification spontaneous noise amplified spontaneous emission 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
  • Slide 24
  • Copyright Rudra Dutta, NCSU, Fall, 200724 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
  • Slide 25
  • Copyright Rudra Dutta, NCSU, Fall, 200725 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
  • Slide 26
  • Copyright Rudra Dutta, NCSU, Fall, 200726 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 m < 2a < 10 m only one mode is guided) use graded-index fibers
  • Slide 27
  • Copyright Rudra Dutta, NCSU, Fall, 200727 Graded Index Fibers
  • Slide 28
  • Copyright Rudra Dutta, NCSU, Fall, 200728 Propagation in Graded Index Fibers Rays are bent as they approach the cladding Rays further from core travel faster (due to lower n) Intermodal dispersion reduced by several orders of magnitude
  • Slide 29
  • Copyright Rudra Dutta, NCSU, Fall, 200729 Chromatic Dispersion Two sources of chromatic dispersion: material dispersion, D M waveguide dispersion, D W Chromatic dispersion: D = D M + D W
  • Slide 30
  • Copyright Rudra Dutta, NCSU, Fall, 200730 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
  • Slide 31
  • Copyright Rudra Dutta, NCSU, Fall, 200731 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
  • Slide 32
  • Copyright Rudra Dutta, NCSU, Fall, 200732 Dispersion Profile of Single-Mode Fiber
  • Slide 33
  • Copyright Rudra Dutta, NCSU, Fall, 200733 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
  • Slide 34
  • Copyright Rudra Dutta, NCSU, Fall, 200734 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
  • Slide 35
  • Copyright Rudra Dutta, NCSU, Fall, 200735 Lasers Light amplification by stimulated emission of radiation Schawlow and Townes, 195