Introduction To Fibre Optic Communication: Mid Sweden University

Introduction to Fibre Optic Communication Mid Sweden University

Outline

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Optical Fibres (Magnus)

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Fibre Amplifiers (Magnus)

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Pump Sources (Magnus, Kent)

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Optical Devices (Kent)

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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 µm < 0.5 dB/km loss @ 1.2 - 1.6 µm

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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 - 100µm Advantages – Large NA – LED signal light source can be used – Inexpensive Disadvantages – Large modal dispersion – Small bandwidth

Single-mode fibres – Core size ~3 - 10 µm 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) – λ0 @ 1310 nm – Dcrom< 20 ps/nm-km @ 1550 nm • Dispersion-shifted fibre (DSF) – λ0 @ 1550 nm • Nonzero dispersion fibre (NDF) – Small chromatic dispersion @ 1550 nm to reduce penalties from FWM and other nonlinearities Department of Information Technology and Media Magnus Engholm

Limiting factors for high bitrate 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

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Raman Fibre amplifiers

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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 localarea networks Department of Information Technology and Media Magnus Engholm

Erbium Doped Fibre Amplifier (EDFA)

• Very few components • High reliability Department of Information Technology and Media Magnus Engholm

Optical Amplifier Characteristics of an ideal amplifier •

High pump absorption

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Large spectral bandwidth

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Gain flatness

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High QE

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Low noise

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High gain

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High reliability (submarine systems) Department of Information Technology and Media Magnus Engholm

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

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NF will always be greater than one, due to added ASE noise

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The NF-value is usually given in dB

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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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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 @ 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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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 980 nm pump

1480 nm pumps

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Low noise

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Wasted energy because electrons must relax unproductively

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Higher GE

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Narrow absorption band ~ 2 nm

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Higher noise Need higher drive current heat dissipation required expensive Smaller GE Large tolerance in pump wavelength ~ 20 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)

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Resonance @ 1.32 µm 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 Department of Information Technology and Media Magnus Engholm

Nd-doped Fibre Amplifiers (NDFA)

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Gain @ 1310 – 1360 nm if doped in ZBLAN

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Gain @ 1360 – 1400 nm if doped in Silica.

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Strong ESA at signal wavelength

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NF good, but not as good as in EDFAs

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Limited performance due to competing radiative transitions

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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

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Gain peak ~ 60 - 100 nm above pump wavelength

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Low NF ~ 5 dB

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Peak gain is 18 dB

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Pump wavelength 1455 nm

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Multi-Wavelength pumping

Dual Wavelength Pumping •

Pump wavelengths: 1420 nm and 1450 nm

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Large spectral BW ~ 50 nm

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Low NF ~ 5 dB

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Raman Amplifier Advantages

Disadvantages

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SRS effect is present in all fibres

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Fast response time

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Gain at any wavelength

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High pump powers required

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Low NF due to low ASE

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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

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Core size ~ 10 –15 µm Core NA ~ 0.12 – 0.2 Pump cladding size ~ 100 – 400 µm Pump cladding NA ~ 0.4 Effective pump absorption coefficient αeff = αcore(Acore/Acladding)

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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% Department of Information Technology and Media Magnus Engholm

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 cwmode ~4W, ~ 10 W in pulsed mode

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Yb-doped Fibre Laser

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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) Department of Information Technology and Media Magnus Engholm

The future of Fibre Amplifiers

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Increase in spectral bandwidth ~ 140 nm (hybrid solutions) Department of Information Technology and Media Magnus Engholm

Prototype for a large BW - amplifier Hybrid solution EDFA + TDFA

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Latest Developments

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END OF PART I

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