Introduction to Fibre Optic Communication Mid Sweden University.

Introduction to Fibre Optic Communication

Mid Sweden University

Department of Information Technology and Media

Magnus Engholm

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

< 0.25 dB/km loss @ ~1.5 m

< 0.5 dB/km loss @ 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 - 100m

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 @ 1310 nm

– Dcrom< 20 ps/nm-km @ 1550 nm

• Dispersion-shifted fibre (DSF)

– 0 @ 1550 nm

• Nonzero dispersion fibre (NDF)

– Small chromatic dispersion @ 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 @ 980 nm

• 100% QE

• NF close to 3 dB


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Optical properties for different glass hosts

• Wider stimulated emission• Wider amplification bandwidth


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Gain spectrum

• Gain peak @ 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 @ 980 nm


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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 @ 1470 nm (S-band)

• Pumping @ 1060 nm

• Low QE ~ 4%

• Measured lifetime @ 3H4 ~ 0.6 ms


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Pr-doped Fibre Amplifiers (PDFA)

• Resonance @ 1.32 m• Low QE ~ 4%• GE < 0.2 dB/mW• Two pumping wavelengths:

– InGaAs laser @ 1017 nm (< 50 mW output)

– Nd:YLF crystal laser @ 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 @ 1310 – 1360 nm if doped in ZBLAN

• Gain @ 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 @ 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

a) Straightforward, but inconvenient to use

b) Looks simple, but is difficult to make

c) 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 @ 976 nm

• High power pumps is required ~ 3 W• Absorption @ 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

Introduction to Fibre Optic Communication Mid Sweden University.

Documents

expensive slide

nskm chromatic dispersion

line amplifiers

fibre losses preamplifiers

regenerators power amplifiers

nskm nonlinear optical

dbkm loss

fwm multichannel systems