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Raman amplifiers
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- 2. 2 Contents • Introduction. • StimulatedRaman Scattering. • Raman Amplifiers and Classifications. • Raman Gain. • Noise and Interference. - ASE Noise - Double Rayleigh Scattering Reflections. - Relative Intensity Noise. • Deployment Issues and Applications.
- 3. 3 Introduction • Ability oflight to scatter inelastically on a molecular structure (gas, liquid or solid) and exchange energy with material discovered by Sir. C. V. Raman in 1928. • Effect known as ‘Raman scattering’. • Incoming photon either red-shifted (Stokes shift) or blue-shifted (anti-Stokes shift) by interaction with the medium.
- 4. 4 Introduction • Fraction ofphoton energy absorbed or emitted by the material as molecular vibrations (heat) - phonons. • Raman scattering occur in all materials, dominant in silica glass. • Raman transitions due to bending motion of Si-O-Si bond. • Raman scattering is inelastic, molecule will decay to a vibrational level different from initial state.
- 5. 5 Introduction • Stokes Ramanscattering - final energy level of the molecule higher than the initial level. • Anti-Stokes Raman scattering - final energy level lower than the starting level. • Stokes scattering is more common. • For Stokes shift, νp,νs - frequency of pump and Stokes photon, Ephonon – phonon energy.
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- 7. 7 Raman Amplifiers • Nonlineareffects within optical fiber used to provide optical amplification. • Amplification achieved using Stimulated Raman Scattering (SRS). • Self-phase matching between the pump and the signal. • Broad gain–bandwidth and fast response, attractive for WDM systems.
- 8. 8 Raman Amplifiers • Providegain over the entire fiber band (i.e. 0.8 to 1.6 μm). • Pump signal optical wavelengths typically 500/cm higher in frequency than signal to be amplified. • Multiple pump wavelengths for flat gain, reduce polarisation dependent gain. • Broad spectral bandwidth of up to 100 nm with suitable fiber doping. • Continuous-wave Raman gains exceed 20 dB.
- 9. 9 Raman Amplifier withBidirectional Pumping
- 10. 10 Stimulated Raman Scattering •Nonlinear scattering observed at high optical power densities in long single-mode fibers. • Provide optical gain with frequency shift. • Useful for optical amplification. • Modulates light through thermal molecular vibrations within the fiber. • Scattered light appears as upper and lower sidebands separated from incident light by the modulation frequency.
- 11. 11 Stimulated Raman Scattering •High-frequency optical phonon generated in the scattering process. • Bidirectional - forward and backward directions. • Threshold optical power for SRS: d - fiber core diameter, λ - operating wavelength, αdB - fiber attenuation, dB/km.
- 12. 12 Raman Amplifiers • Ramangain > 40 dB with fluoride glass fiber and Raman shift of 590/cm. • Forward & backward pumping, high pump powers needed. • Raman gain depends on fiber length, attenuation and core diameter.
- 13. 13 Raman Gain • RamanGain gR - Power Raman gain coefficient, Aeff and Leff - effective core cross-sectional area and length, k - numerical factor that accounts for polarization scrambling between optical pump and signal, reff - effective core radius, αp -fiber transmission loss at pump wavelength, L - actual fiber length, Pp – pump power.
- 14. 14 Raman Gain • k= 2 for complete polarization scrambling, as in conventional single-mode fiber. • Raman gain efficiency (gR/Aeffk) measured in W−1 km−1. • Raman gain becomes larger as fiber lengths increase up to around 50 km, asymptotically it reaches a constant value. • Higher Raman gains obtained with lower loss fibers and lower core diameters.
- 15. 15 Raman Gain 10 μmcore single-mode fibers. Pump input power = 1.6 W
- 16. 16 On-Off Gain • On-offgain of Raman amplifier - increase in signal output power when pumps are turned on. • Distributed Amplification - gain in the transmission fiber itself, reduces system signal-to-noise ratio (SNR) degradation, compared with discrete amplifier (only) systems.
- 17. 17 Raman Amplifier Classifications •Main categories – discrete, distributed and hybrid. • Discrete Raman amplifiers/Lump Raman amplifiers used as lumped elements inserted into transmission line for gain. • Discrete - pump power is confined to the lumped element (~20 km long fiber). • Distributed Raman amplifier extends pump power into transmission line fiber.
- 18. 18 Raman Amplifier Classifications •Discrete amplifiers - amplification takes place at a single point at the end of the link. • Distributed amplifiers - amplification takes place along the fiber, avoids low power at the end of the link, allows lower power to be launched at the starting of the link. • DRA prevents signal attenuation to very low powers, improves SNR.
- 19. 19 Raman Amplifier Classifications •Distributed - amplification takes place along several kilometers of fiber(~100 km). • Hybrid Raman Amplifiers - Lumped and distributed Raman fiber amplifiers combined for wideband applications. • Combined amplification increases overall amplified spectral bandwidth. • Amplified Spectral Emission (ASE) noise & Double Rayleigh scattering reflections.
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- 21. 21 Noise and Interference •Selection & number of pump signals influence amplifier noise. • ASE contributes most of the noise. • Common sources of noise include: - Beating of the signal with ASE due to double Rayleigh scattering reflections or multipath interference. - Non Linear Effects - four-wave mixing, self-phase modulation, cross-phase modulation. • Relative Intensity Noise.
- 22. 22 Double Rayleigh Scattering Reflections •Fiber length influences noise within Raman amplifiers. • Magnitude proportional to fiber length. • ASE noise reflected together with signal, cause it to increase several times. • Effect of double Rayleigh scattering reflections reduced if multiple stages of amplification over full fiber length.
- 23. 23 Relative Intensity Noise •Pump signal and input signal interact for a longer time over several fiber kilometers. • Fluctuations in pump power (pump noise) transferred to transmitted signal. • Pump noise also called Relative Intensity Noise (RIN). • Severe if multiple pump signals used to achieve wideband amplification.
- 24. 24 Minimising RIN • Reduceinteraction time between pump and input signal. • Achieved by backward pumping – counter- propagation of signals where interaction time for pump and signal is very short.
- 25. 25 Other Limitations • Lowpower efficiency. • Safety issues due to high optical powers in the fiber. • Enhanced problems with nonlinearities, due to high path-average power in fibers. • Gain limited by available pump powers and wavelengths.
- 26. 26 Laser Safety • Outputpower of Raman pump modules higher than typical power levels in EDFA systems. • DRA generate ASE along transmission line. • Even in case of fiber break, ASE power within C - band can propagate along the system. • Reducing power to a safe level in case of accidental connection opening/fiber break. • Detection of fiber break/open connector, allow automatic shut down of Raman pump module.
- 27. 27 Gain Measurement • AchievableRaman gain and shape of gain spectrum depends on fiber type and quality of fiber line. • Achievable gain vary from spool to spool due to manufacturing variations. • Ability for accurate real time Raman gain measurement. • Adjust pump powers to achieve desired average gain and gain shape.
- 28. 28 System Integration • IntegratingDRA modules into existing system architecture time consuming and costly. • Tight integration of Raman and EDFA modules allow module parameter optimization, enhanced gain flatness. • Stand alone Raman amplifiers to extend existing capabilities.
- 29. 29 Applications • Long distancesingle span links - undersea links between islands, remote coastal locations, oil rigs etc. - locations separated by mountain ranges or desert. - where commercial, legal or security constraints render amplification sites impractical.
- 30. 30 Applications • Long spanswithin multispan links. - one or more spans longer than others. - hut skipping : spans made intentionally longer, skip repeaters to reduce capital and operating expenditure. • High capacity long distance systems. • Optical amplifiers and waveguides in general.
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