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)

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 dB/km ~1.5  m < 0.5 dB/km  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 ~  m Advantages – Large NA – LED signal light source can be used – Inexpensive Disadvantages – Large modal dispersion – Small bandwidth Single-mode fibres – Core size ~  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) – 1310 nm – D crom < nm Dispersion-shifted fibre (DSF) – 1550 nm Nonzero dispersion fibre (NDF) – Small chromatic 1550 nm to reduce penalties from FWM and other nonlinearities

Department of Information Technology and Media Magnus Engholm 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), P T ~ 1-3 mW –Stimulated Raman scattering (SRS), P T ~ 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 local- area 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 = SNR in / SNR out 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)

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 980 nm 100% QE NF close to 3 dB

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 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 mW of pump 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 dB/mw 1480 nm ~ 5 dB/mw 830 nm ~ 1.3 dB/mw

Department of Information Technology and Media Magnus Engholm 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

Department of Information Technology and Media Magnus Engholm Tm-Doped Fibre Amplifier (TDFA) 1470 nm (S-band) 1060 nm Low QE ~ 4% Measured 3 H 4 ~ 0.6 ms

Department of Information Technology and Media Magnus Engholm Pr-doped Fibre Amplifiers (PDFA) 1.32  m Low QE ~ 4% GE < 0.2 dB/mW Two pumping wavelengths: –InGaAs 1017 nm (< 50 mW output) –Nd:YLF crystal 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 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) 1310 – 1360 nm if doped in ZBLAN 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 nm above pump wavelength

Department of Information Technology and Media Magnus Engholm Raman Amplifiers Gain spectrum 9 km gain fibre Gain peak ~ nm above pump wavelength Low NF ~ 5 dB Peak gain is 18 dB 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 dB

Department of Information Technology and Media Magnus Engholm 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

Department of Information Technology and Media Magnus Engholm Pumping Core pumping Low NF ~ 3.5 dB High cost High complexity Cladding pumping NF ~ 6 dB 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 (A core /A cladding ) 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 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 cw- mode ~4W, ~ 10 W in pulsed mode

Department of Information Technology and Media Magnus Engholm Yb-doped Fibre Laser Strong absorption and emission 976 nm High power pumps is required ~ 3 W 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 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

Department of Information Technology and Media Magnus Engholm Latest Developments

Department of Information Technology and Media Magnus Engholm END OF PART I