Introduction to Fibre Optic Communication

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Introduction to Fibre Optic Communication
Mid Sweden University
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Outline

• Optical Fibres (Magnus)
• Fibre Amplifiers (Magnus)
• Pump Sources (Magnus, Kent)
• Optical Devices (Kent)
• Optical Soliton Systems (Kent)

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Optical Communication Systems

• Terrestial
• Long haul
• Metropolitan
• Office
• Submarine

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Properties of Optical Fibres
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Transmission Wavelengths

• Loss mechanisms
• Material absorption
• Rayleigh scattering
• lt 0.25 dB/km loss _at_ 1.5 ?m
• lt 0.5 dB/km loss _at_ 1.2 - 1.6 ?m

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Dispersion

• Modal dispersion
• Chromatic dispersion
• material dispersion
• waveguide dispersion

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Optical Fibre types

• Multi-mode fibres
• Core size 50 - 100?m
• Advantages
• Large NA
• LED signal light source can be used
• Inexpensive
• Disadvantages
• Large modal dispersion
• Small bandwidth

• Single-mode fibres
• Core size 3 - 10 ?m
• Advantages
• No modal dispersion
• Large bandwidth
• Disadvantages
• Small NA
• Laser signal light source must be used
• Expensive

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Single-Mode Fibre Types

• Standard single-mode fibre (SMF)
• ?0 _at_ 1310 nm
• Dcromlt 20 ps/nm-km _at_ 1550 nm
• Dispersion-shifted fibre (DSF)
• ?0 _at_ 1550 nm
• Nonzero dispersion fibre (NDF)
• Small chromatic dispersion _at_ 1550 nm to reduce
  penalties from FWM and other nonlinearities

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Limiting factors for high bit-rate and
transmission distance

• Pulse broadening
• Modal dispersion 10 ns/km
• Chromatic dispersion 0.1 ns/km
• Nonlinear optical effects
• Stimulated Brillouin scattering (SBS), PT 1-3
  mW
• Stimulated Raman scattering (SRS), PT 1-2 W
• Self phase modulation (SPM)
• Four wave mixing (FWM) (multi-channel systems)

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Optical Amplifiers

• Rare-earth doped fibre amplifiers
• EDFA
• TDFA
• PDFA
• NDFA
• Raman Fibre amplifiers
• Semiconductor optical amplifiers (SOA)

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Application of Optical Amplifiers

• In-line amplifiers
• replaces regenerators
• Power amplifiers
• boost signals to compensate fibre losses
• Preamplifiers
• boost the recieved signals
• LAN amplifiers
• compensate distribution losses in local-area
  networks

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Erbium Doped Fibre Amplifier (EDFA)

• Very few components
• High reliability

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Optical Amplifier

• Characteristics of an ideal amplifier
• High pump absorption
• Large spectral bandwidth
• Gain flatness
• High QE
• Low noise
• High gain
• High reliability (submarine systems)

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Origin of Noise in Fibre Amplifiers
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Noise Mechanisms

• Signal hetrodynes with ASE signal - spontanous
  beat noise
• ASE heterodynes with itself Spontanous -
  spontanous beat noise
• Amplified signal shot noise - negligible

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Noise Figure

• NF SNRin / SNRout
• NF will always be greater than one, due to added
  ASE noise
• The NF-value is usually given in dB
• Noise figures close to 3 dB have been obtained in
  EDFAs (ideal amplifier)

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Erbium Doped Fibre Amplifier

• Spectroscopic properties
• Long upper level life time 10 ms
• No ESA for 980 and 1480 nm pump
• Best GE _at_ 980 nm
• 100 QE
• NF close to 3 dB

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Erbium Doped Fibre Amplifier

• Optical properties for different glass hosts
• Wider stimulated emission
• Wider amplification bandwidth

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Erbium Doped Fibre Amplifier

• Gain spectrum
• Gain peak _at_ 1535 nm
• Broad spectral BW 40 nm

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EDFA Input/Output Characteristics

• Fibre NA 0.16
• Fibre length 9 m
• 200 mW of pump power _at_ 980 nm

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Erbium Doped Fibre Amplifier

• EDFA design

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Gain Efficiency vs Pump Wavelength

• 980 nm 11 dB/mw
• 1480 nm 5 dB/mw
• 830 nm 1.3 dB/mw

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980 nm vs 1480 nm pumping EDFAs

• 1480 nm pumps
• Higher noise
• Need higher drive current - heat dissipation
  required - expensive
• Smaller GE
• Large tolerance in pump wavelength 20 nm

• 980 nm pump
• Low noise
• Wasted energy because electrons must relax
  unproductively
• Higher GE
• Narrow absorption band 2 nm

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Tm-Doped Fibre Amplifier (TDFA)

• Gain _at_ 1470 nm (S-band)
• Pumping _at_ 1060 nm
• Low QE 4
• Measured lifetime _at_ 3H4 0.6 ms

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Pr-doped Fibre Amplifiers (PDFA)

• Resonance _at_ 1.32 ?m
• Low QE 4
• GE lt 0.2 dB/mW
• Two pumping wavelengths
• InGaAs laser _at_ 1017 nm (lt 50 mW output)
• NdYLF crystal laser _at_ 1047 nm (ineffective
  expensive)

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Pr-doped Fibre Amplifiers (PDFA)

• Results so far
• QE of 5 in ZBLAN glass
• QE of 19 in GLS glass (University of
  Southampton, 1998)
• Small signal gains 38 dB
• Saturated output powers of up to 200 mW
• NF 15 dB
• Problem
• Require glass compositions with low phonon
  energies
• Non-silica based splicing difficulties

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Nd-doped Fibre Amplifiers (NDFA)

• Gain _at_ 1310 1360 nm if doped in ZBLAN
• Gain _at_ 1360 1400 nm if doped in Silica.
• Strong ESA at signal wavelength
• NF good, but not as good as in EDFAs
• Limited performance due to competing radiative
  transitions
• Splicing difficulties

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Raman Amplifiers

• Characteristics
• Uses SRS in intrinsic silica fibres
• Require high pump powers
• Broad gain spectrum
• Max. gain _at_ 60 - 100 nm above pump wavelength

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Raman Amplifiers

• Gain spectrum

• 9 km gain fibre
• Gain peak 60 - 100 nm above pump wavelength
• Low NF 5 dB
• Peak gain is 18 dB
• Pump wavelength 1455 nm

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Multi-Wavelength pumping

• Dual Wavelength Pumping
• Pump wavelengths 1420 nm and 1450 nm
• Large spectral BW 50 nm
• Low NF 5 dB

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Raman Amplifier

• Advantages
• SRS effect is present in all fibres
• Gain at any wavelength
• Low NF due to low ASE

• Disadvantages
• Fast response time
• High pump powers required
• High power pumps are expensive at the wavelengths
  of interest

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Pumping

• Core pumping
• Low NF 3.5 dB
• High cost
• High complexity

• Cladding pumping
• NF 6 dB
• Low cost
• Low complexity

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Dubble Clad Optical Fibre

• Core size 10 15 ?m
• Core NA 0.12 0.2
• Pump cladding size 100 400 ?m
• Pump cladding NA 0.4
• Effective pump absorption coefficient ?eff
  ?core(Acore/Acladding)

• Increase pump absorption by co-doping with Yb

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Fibre Design

• Problem Pump absorption low, rays will miss
  doped core
• Solution break symmetry
• a) Offset core, hard to splice
• b) Difficult to make
• c) Not difficult to make

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Launching schemes

• Straightforward, but inconvenient to use
• Looks simple, but is difficult to make
• Possible problem fibre damage fibre gets hot
  and may brake
• Typical launching efficiency 70 80

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Fibre Lasers

• Simple design with very few components
• Very narrow line width (10 kHz)
• For use as a signal source, some external
  modulator must be used
• High power output are obtainable in cw- mode 4W,
  10 W in pulsed mode

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Yb-doped Fibre Laser

• Strong absorption and emission band _at_ 976 nm
• High power pumps is required 3 W
• Absorption _at_ 915 - 940 is weaker but wider
• Results so far
• 500 mW (J. Minelly, Corning)
• 800 mW (A. Kurkow, GPI, Moscow)

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The future of Fibre Amplifiers

• Increase in spectral bandwidth 140 nm (hybrid
  solutions)

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Prototype for a large BW - amplifier

• Hybrid solution EDFA TDFA

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Latest Developments
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END OF PART I