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  1. 1. Optical Fibre Communication Systems Lecture 5 – Optical Amplifier Professor Z Ghassemlooy Optical Communications Research Group Northumbria Communications Research Laboratory School of Computing, Engineering and Information Sciences The University of Northumbria U.K. http://soe.unn.ac.uk/ocr Prof. Z Ghassemlooy 1
  2. 2. Contents Why the need for optical amplifier? Spectra Noise Types Principle of Operation Main Parameters Applications Prof. Z Ghassemlooy 2
  3. 3. Signal Reshaping and Amplification In long distance communications, whether going through wire, fibre or wave, the signal carrying the information experience: - Power loss - Pulse broadening which requires amplification and signal reshaping.m In fibre optics communications, these can be done in two ways: – Opto-electronic conversion – All optical Prof. Z Ghassemlooy 3
  4. 4. Signal Reshaping and AmplificationDepending on its nature, a signal can also be regenerated. A digital signal is made of 1s and 0s: it is possible to reconstruct the signal and amplify it at the same time.s An analog signal however, cannot be reconstructed because nobody knows what the original signal looked like. Prof. Z Ghassemlooy 4
  5. 5. Why the Need for Optical Amplification? Semiconductor devices can convert an optical signal into an electrical signal, amplify it and reconvert the signal back to an optical signal. However, this procedure has several disadvantages: – Costly – Require a large number over long distances – Noise is introduced after each conversion in analog signals (which cannot be reconstructed) – Restriction on bandwidth, wavelengths and type of optical signals being used, due to the electronicse By amplifying signal in the optical domain many of these disadvantages would disappear! Prof. Z Ghassemlooy 5
  6. 6. Optical Amplificationa Amplification gain: Up to a factor of 10,000 (+40 dB)i In WDM: Several signals within the amplifier’s gain (G) bandwidth are amplified, but not to the same extentp It generates its own noise source known as Amplified Spontaneous Emission (ASE) noise. Weak signal Amplified signal Pin Optical Pout Amplifier ASE ASE (G) Pump Source Prof. Z Ghassemlooy 6
  7. 7. Optical Amplification - SpectralCharacteristics (unamplified signal) (amplified signal)Single channel Power Power ASE Wavelength WavelengthWDM channels (unamplified signal) (amplified signal) Power Power ASE Wavelength Wavelength Prof. Z Ghassemlooy 7
  8. 8. Optical Amplification - Noise Figurea Required figure of merit to compare amplifier noise performanceo Defined when the input signal is coherent Input signal − to− noise ratio ( SNRi ) Noise Figure (NF) = Output signal − to− noise ratio ( SNRo ) NF is a positive number, nearly always > 2 (I.e. 3 dB) Good performance: when NF ~ 3 dB: NF is one of a number of factors that determine the overall BER of a network. Prof. Z Ghassemlooy 8
  9. 9. Optical Amplifiers - TypesThere are mainly two types: Semiconductor Laser (optical) Amplifier (SLA) (SOA)s Active-Fibre or Doped-Fibre – Erbium Doped Fibre Amplifier (EDFA) – Fibre Raman Amplifier (FRA) – Thulium Doped Fibre Amplifier (TDFA) Prof. Z Ghassemlooy 9
  10. 10. SLA - Principle Operationi Remember diode lasers?Suppose that the diode laser has no mirrors: - we get the diode to a population inversion condition - we inject photons at one end of the diodeo By stimulated emission, the incident signal will be amplified! – By stimulated emission, one photon gives rise to another photon: the total is two photons. Each of these two photons can give rise to another photon: the total is then four photons. And it goes on and on...Problems:) Poor noise performance: they add a lot of noise to the signal!r Matching with the fibre is also a problem!e However, they are small and cheap! Prof. Z Ghassemlooy 10
  11. 11. SLA - Principle Operation Excited state PumpPump signal signal Energy Absorption @ 980 nm @ 980 nm Electrons in ground state Excited state Tra nsi tion Pump signal Metastable @ 980 nm state Ground state www.cisco.com Prof. Z Ghassemlooy 11
  12. 12. SLA - Principle Operation Excited state Tra nsi tion Metastable state ASE Photons Tra Pump signal @ 980 nm 1550 nm ns itio n Ground state Excited state Tra nsi tion Metastable state Pump signal Stimulated @ 980 nm Signal photon emission 1550 nm 1550 nm Ground state Prof. Z Ghassemlooy 12
  13. 13. Erbium Doped Fibre Amplifier (EDFA)r EDFA is an optical fibre doped with erbium. – Erbium is a rare-earth element which has some interesting properties for fibre optics communications. – Photons at 1480 or 980 nm activate electrons into a metastable state – Electrons falling back emit light at 1550 nm. – By one of the most extraordinary coincidences, 1550 nm is a low-loss wavelength region for silica optical fibres. – This means that we could amplify a signal by 540 using stimulated emission. 670 820 980 EDFA is a low noise light Metastable amplifier. 1480 state 1550 nm Ground state Prof. Z Ghassemlooy 13
  14. 14. EDFA - Operating Features Amplifier lengthInput signal 1-20 m typical Amplified signal Pump from an Cladding Erbium doped core external laser 1480 or 980 nm • Available since 1990’s: • Self-regulating amplifiers: output power remains more or less constant even if the input power fluctuates significantly • Output power: 10-23 dBm • Gain: 30 dB • Used in terrestrial and submarine links Prof. Z Ghassemlooy 14
  15. 15. EDFA – Gain Profile +10 dBm ASE spectrum when no• Most of the pump power appears input signal is present at the stimulating wavelength• Power distribution at the Amplified signal spectrum other wavelengths changes (input signal saturates the with a given input signal. optical amplifier) + ASE -40 dBm 1575 nm 1525 nm Prof. Z Ghassemlooy 15
  16. 16. EDFA – Ultra Wideband Ultra-Wideband EDFA 30 C-Band L-Band 15 Noise Figure (dB) 40.8 nm 43.5 nm 20 Gain (dB) Total 3dB Bandwidth = 84.3 nm 10 10 Noise ≤ 6.5 dB 5 Output Power ≅ 24.5 dBm 0 1525 1550 1575 1600Alastair Glass Photonics Research Wavelength (nm) Prof. Z Ghassemlooy 16
  17. 17. Optical Amplifiers: Multi-wavelengthAmplification www.cisco.com Prof. Z Ghassemlooy 17
  18. 18. Optical Amplifier - Main Parametersr Gain (Pout/Pin)u Bandwidthu Gain Saturationu Polarization Sensibilitys Noise figure (SNRi/SNRo)R Gain FlatnessR Types – Based on stimulated emission (EDFA, PDFA, SOA) – Based on non-linearities (Raman, Brillouin) Prof. Z Ghassemlooy 18
  19. 19. Optical Amplifier - Optical Gain (G)r G = S Output / S Input (No noise)a Input signal dependent: – Operating point (saturation) of EDFA strongly depends on power and wavelength of incoming signal Gain (dB) EDFA 40 P Input: -30 dBm• Gain ↓ as the input power ↑ Pin Gain Pout 30 -20 dBm-20 dBm 30 dB +10 dBm -10 dBm-10 dBm 25 dB +15 dBm 20 -5 dBmNote, Pin changes by a factor of ten 10then Pout changes only by a factor of 1520 1540 1560 1580three in this power range. Prof. Z Ghassemlooy 19
  20. 20. Optical Amplifier - Optical Gain (G)r Gain bandwidth – Refers to the range of frequencies or wavelengths over which the amplifier is effective. – In a network, the gain bandwidth limits the number of wavelengths available for a given channel spacing.• Gain efficiency - Measures the gain as a function of input power in dB/mW.• Gain saturation - Is the value of output power at which the output power no longer increases with an increase in the input power. - The saturation power is typically defined as the output power at which there is a 3-dB reduction in the ratio of output power to input power (the small-signal gain). Prof. Z Ghassemlooy 20
  21. 21. Optical SNR For BER < 10-13 the following OSNRs are required:  ~ 13 dB for STM-16 / OC-48 (2.5 Gbps)  ~ 18 dB for STM-64 / OC-192 (10 Gbps) Optical power at the receiver needs to bigger than receiver sensitivity Optical Amplifiers give rise to OSNR degradation (due to the ASE generation and amplification) – Noise Figure = OSNRin/OSNRout Therefore for a given OSNR there is only a finite number of amplifiers (that is to say a finite number of spans) Thus the need for multi-stage OA design Prof. Z Ghassemlooy 21
  22. 22. Optical Amplifiers: Multi-Stage Er3+ Doped FiberInput Signal Output Signal Optical Isolator Pump Pump 1st Active stage co-pumped: 2nd stage counter-pumped: optimized for low noise figure optimized for high output powerNF 1st/2nd stage = Pin - SNRo [dB] - 10 Log (hc2∆λ / λ3)NFtotal = NF1+NF2/G1 Prof. Z Ghassemlooy 22
  23. 23. System Performance: OSNR Limitation 5 Spans x 25 dB 32 Chs. @ 2.5Gb/s with 13 dB OSNR BER < 10-13Channel Count / Span Loss Trade-Off: 5 spans x 22 dB 64 chs @ 2.5Gb/s with 13 dB OSNR BER < 10-13 Prof. Z Ghassemlooy 23
  24. 24. Raman Amplifier Transmission fiber Transmission fiber Er Amplifier 1450/ 1550 nm WDM 1550 nm signal(s) 1453 nm pumpCladding pumped fiber laser Raman fiber laser •Offer 5 to 7 dB improvement in system performance •First application in WDM P. B. Hansen, et. al. , 22nd Euro. Conf. on Opt. Comm., TuD.1.4 Oslo, Norway (1996). Prof. Z Ghassemlooy 24
  25. 25. Optical Amplifiers - Applications• In line amplifier -30-70 km -To increase transmission link• Pre-amplifier - Low noise -To improve receiver sensitivity• Booster amplifier - 17 dBm - TV• LAN booster amplifier Prof. Z Ghassemlooy 25