Introduction to Fibre Optic Communication Mid Sweden University

Outline • Optical Fibres (Magnus) • Fibre Amplifiers (Magnus) • Pump Sources (Magnus, Kent) • Optical Devices (Kent) • Optical Soliton Systems (Kent) Department of Information Technology and Media Magnus Engholm

Optical Communication Systems Terrestial – Long haul – Metropolitan – Office Submarine Department of Information Technology and Media Magnus Engholm

Properties of Optical Fibres Department of Information Technology and Media Magnus Engholm

Transmission Wavelengths Loss mechanisms: – Material absorption – Rayleigh scattering < 0. 25 d. B/km loss @ ~1. 5 m < 0. 5 d. B/km loss @ 1. 2 - 1. 6 m Department of Information Technology and Media Magnus Engholm

Dispersion • Modal dispersion • Chromatic dispersion – material dispersion – waveguide dispersion Department of Information Technology and Media Magnus Engholm

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 Department of Information Technology and Media Magnus Engholm

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 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 m. W – Stimulated Raman scattering (SRS), PT ~ 1 -2 W – Self phase modulation (SPM) – Four wave mixing (FWM) (multi-channel systems) Department of Information Technology and Media Magnus Engholm

Optical Amplifiers • Rare-earth doped fibre amplifiers – EDFA – TDFA – PDFA – NDFA • Raman Fibre amplifiers • Semiconductor optical amplifiers (SOA) Department of Information Technology and Media Magnus Engholm

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 • Large spectral bandwidth • Gain flatness • High QE • Low noise • High gain • High reliability (submarine systems) Department of Information Technology and Media Magnus Engholm

Origin of Noise in Fibre Amplifiers Department of Information Technology and Media Magnus Engholm

Noise Mechanisms • Signal hetrodynes with ASE: signal spontanous beat noise • ASE heterodynes with itself: Spontanous - spontanous beat noise • Amplified signal shot noise - negligible Department of Information Technology and Media Magnus Engholm

Noise Figure • NF = SNRin / SNRout • NF will always be greater than one, due to added ASE noise • The NF-value is usually given in d. B • Noise figures close to 3 d. B have been obtained in EDFAs (ideal amplifier) Department of Information Technology and Media Magnus Engholm

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 d. B Department of Information Technology and Media Magnus Engholm

Erbium Doped Fibre Amplifier Optical properties for different glass hosts • Wider stimulated emission • Wider amplification bandwidth Department of Information Technology and Media Magnus Engholm

Erbium Doped Fibre Amplifier Gain spectrum • • Gain peak @ 1535 nm Broad spectral BW ~ 40 nm Department of Information Technology and Media Magnus Engholm

EDFA Input/Output Characteristics • Fibre NA = 0. 16 • Fibre length = 9 m • 200 m. W of pump power @ 980 nm Department of Information Technology and Media Magnus Engholm

Erbium Doped Fibre Amplifier EDFA design Department of Information Technology and Media Magnus Engholm

Gain Efficiency vs Pump Wavelength 980 nm ~ 11 d. B/mw 1480 nm ~ 5 d. B/mw 830 nm ~ 1. 3 d. B/mw Department of Information Technology and Media Magnus Engholm

980 nm vs 1480 nm pumping EDFAs 980 nm pump 1480 nm pumps • Low noise • Wasted energy because electrons must relax unproductively • • • Higher GE • Narrow absorption band ~ 2 nm • • Higher noise Need higher drive current - heat dissipation required - expensive Smaller GE Large tolerance in pump wavelength ~ 20 nm Department of Information Technology and Media Magnus Engholm

Tm-Doped Fibre Amplifier (TDFA) • Gain @ 1470 nm (S-band) • Pumping @ 1060 nm • Low QE ~ 4% • Measured lifetime @ 3 H 4 ~ 0. 6 ms Department of Information Technology and Media Magnus Engholm

Pr-doped Fibre Amplifiers (PDFA) • • Resonance @ 1. 32 m Low QE ~ 4% GE < 0. 2 d. B/m. W Two pumping wavelengths: – In. Ga. As laser @ 1017 nm (< 50 m. W output) – Nd: YLF crystal laser @ 1047 nm (ineffective & expensive) Department of Information Technology and Media Magnus Engholm

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 d. B • Saturated output powers of up to 200 m. W • NF ~ 15 d. B 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) • 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 Department of Information Technology and Media Magnus Engholm

Raman Amplifiers Characteristics • • Uses SRS in intrinsic silica fibres Require high pump powers Broad gain spectrum Max. gain @ 60 - 100 nm above pump wavelength Department of Information Technology and Media Magnus Engholm

Raman Amplifiers Gain spectrum • 9 km gain fibre • Gain peak ~ 60 - 100 nm above pump wavelength • Low NF ~ 5 d. B • Peak gain is 18 d. B • Pump wavelength 1455 nm Department of Information Technology and Media Magnus Engholm

Multi-Wavelength pumping Dual Wavelength Pumping • Pump wavelengths: 1420 nm and 1450 nm • Large spectral BW ~ 50 nm • Low NF ~ 5 d. B Department of Information Technology and Media Magnus Engholm

Raman Amplifier Advantages Disadvantages • SRS effect is present in all fibres • Fast response time • Gain at any wavelength • High pump powers required • Low NF due to low ASE • High power pumps are expensive at the wavelengths of interest Department of Information Technology and Media Magnus Engholm

Pumping Core pumping Cladding pumping • Low NF ~ 3. 5 d. B • High cost • High complexity • NF ~ 6 d. B • Low cost • Low complexity Department of Information Technology and Media Magnus Engholm

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 Department of Information Technology and Media Magnus Engholm

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 Department of Information Technology and Media Magnus Engholm

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 d) 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 k. Hz) For use as a signal source, some external modulator must be used High power output are obtainable in cwmode ~4 W, ~ 10 W in pulsed mode Department of Information Technology and Media Magnus Engholm

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 m. W (J. Minelly, Corning) • 800 m. W (A. Kurkow, GPI, Moscow) Department of Information Technology and Media Magnus Engholm

The future of Fibre Amplifiers • Increase in spectral bandwidth ~ 140 nm (hybrid solutions) Department of Information Technology and Media Magnus Engholm