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Optical Amplifiers and WDM - Optical Communication

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Jeya Prakash K

Optical Amplifier

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1 Optical Amplifier CHAPTER 7
Contents 2 Why the need for optical amplifier? Spectra Noise Types Principle of Operation Main Parameters Applications
Why the Need for Optical Amplification? 3 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 electronics By amplifying signal in the optical domain many of these disadvantages would disappear!
Optical Amplification 4 Amplification gain: Up to a factor of 10,000 (+40 dB) In WDM: Several signals within the amplifiers gain (G) bandwidth are amplified, but not to the same extent It generates its own noise source known as Amplified Spontaneous Emission (ASE) noise. Optical Amplifier (G) Weak signal Pin Amplified signal Pout ASE ASE Pump Source
Optical Amplification - Spectral Characteristics 5 Wavelength Power (unamplified signal) Wavelength Power (amplified signal) ASE Wavelength Power (unamplified signal) Wavelength Power (amplified signal) ASE Single channel WDM channels
Optical Amplification - Noise Figure 6 Required figure of merit to compare amplifier noise performance Defined when the input signal is coherent ) ( ratio noise to signal Output ) ( ratio noise to signal Input (NF) Figure Noise o i SNR SNR 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.
Optical Amplifiers - Types 7 There are mainly two types: Semiconductor Laser (optical) Amplifier (SLA) (SOA) Active-Fibre or Doped-Fibre Erbium Doped Fibre Amplifier (EDFA) Erbium (active medium) Fibre Raman Amplifier (FRA) Thulium Doped Fibre Amplifier (TDFA) Thulium(active medium)
SLA - Principle Operation 8 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 diode 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! Matching with the fibre is also a problem! However, they are small and cheap!
OPTICAL AMPLIFIERS Semiconductor Laser Amplifiers (SLA) Two major types: -The resonant Fabry-Perot amplifier (FPA) -The nonresonant (single pass) traveling-wave amplifier (TWA) SLA is based on the conventional semiconductor laser structure (gain- or index-guided). In FPA, the reflectivities of the facets are between 30% to 35% whereas in TWA the reflectivities are less than 0.001. where R1 = input facet reflectivity R2 = output facet reflectivity
OPTICAL AMPLIFIERS Fabry-Perot amplifier (FPA) For operation, FPA is biased below the normal lasing threshold current. When an optical signal enters the FPA, it gets amplified as it reflects back and forth between the mirrors until it is emitted at a higher intensity. Easy to fabricate but the optical signal gain is very sensitive to variations in amplifier temperature and input optical frequency. Used within nonlinear applications such as pulse shaping and bistable elements. Gain and bandwidth of an FPA Using the standard theory of FP interferometers, the cavity gain of SLA as a function of signal frequency f is 2 2 1 2 2 1 2 1 sin 4 1 1 1 S S S FP G R R G R R G R R f G where R1 = input facet reflectivity R2 = output facet reflectivity Single pass phase shift f f f m L g GS exp The single pass gain is given by fm = cavity resonance frequencies f = longitudinal-mode spacing
OPTICAL AMPLIFIERS GFP reduce to GS when R1=R2=0. GFP peaks whenever f coincides with one of the cavity-resonance frequencies and drop sharply in between them. Amplifier bandwidth is determined by the sharpness of the cavity resonance. The 賊3 dB single longitudinal mode bandwidth is 2 1 2 1 2 1 1 2 1 sin 2 2 S S m FPA G R R G R R f f f B 2 2 1 2 2 1 2 1 sin 4 1 1 1 S S S FP G R R G R R G R R f G f f f m
Question 12
Answer 13
14 Answer
SLA - Principle Operation 15 Electrons in ground state Pump signal @ 980 nm Energy Absorption Excited state Pump signal @ 980 nm Transition Metastable state Excited state Ground state Pump signal @ 980 nm www.cisco.com
SLA - Principle Operation 16 ASE Photons 1550 nm Ground state Excited state Metastable state Transition T r a n s i t i o n Pump signal @ 980 nm Excited state Metastable state Transition Pump signal @ 980 nm Stimulated emission 1550 nm Signal photon 1550 nm Ground state
Erbium Doped Fibre Amplifier (EDFA) 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 using stimulated emission. 17 1480 980 820 540 670 Ground state Metastable state 1550 nm EDFA is a low noise light amplifier.
EDFA - Operating Features 18 Available since 1990s: 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 Input signal Pump from an external laser 1480 or 980 nm Erbium doped core Cladding Amplifier length 1-20 m typical Amplified signal
OPTICAL AMPLIFIERS Amplification in Erbium-doped Fiber Amplifiers Amplification in an EDFA occurs through the mechanism of stimulated emission. The pumping light is absorbed by the erbium ions, raising them to excited states and causing population inversion. Two ways to attain population inversion in EDF: Indirect pumping at 980 nm wavelength - Er ions are excited to upper level (3) and they non-radiatively fast decay to the intermediate energy (metastable) level (2). Direct pumping at 1480 nm wavelength -Er ions are excited directly to the level (2). The signal (to be amplified) stimulates transition of the excited Er ions from level 2 to level 1 and results in radiation of photons with same wavelength, direction, and phase to the signal photons. This gives rise to a coherent (amplified) output with respect to signal input.
OPTICAL AMPLIFIERS A pumping signal can co-propagate with an information signal or it can counter-propagate. Co-propagating pump Counter- propagating pump Bi-directional pump
Optical Amplifiers and WDM - Optical Communication
EDFA Gain Profile 22 ASE spectrum when no input signal is present Amplified signal spectrum (input signal saturates the optical amplifier) + ASE 1575 nm -40 dBm 1525 nm +10 dBm Most of the pump power appears at the stimulating wavelength Power distribution at the other wavelengths changes with a given input signal.
EDFA Ultra Wideband 23 0 10 20 30 1525 1550 1575 1600 5 10 15 Noise 6.5 dB Output Power 24.5 dBm L-Band C-Band Total 3dB Bandwidth = 84.3 nm 43.5 nm 40.8 nm Wavelength (nm) Gain (dB) Noise Figure (dB) Ultra-Wideband EDFA
Optical Amplifiers: Multi-wavelength Amplification 24 www.cisco.com
Optical Amplifier - Main Parameters 25 Gain (Pout/Pin) Bandwidth Gain Saturation Polarization Sensibility Noise figure (SNRi/SNRo) Gain Flatness Types Based on stimulated emission (EDFA, PDFA, SOA) Based on non-linearities (Raman, Brillouin)
Optical Amplifier - Optical Gain (G) G = S Output / S Input (No noise) Input signal dependent: Operating point (saturation) of EDFA strongly depends on power and wavelength of incoming signal 26 Gain (dB) 1540 1560 1580 10 1520 20 40 30 -5 dBm -20 dBm -10 dBm P Input: -30 dBm Gain as the input power Pin Gain Pout -20 dBm 30 dB +10 dBm -10 dBm 25 dB +15 dBm Note, Pin changes by a factor of ten then Pout changes only by a factor of three in this power range. EDFA
Optical Amplifier - Optical Gain (G) 27 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).
Optical SNR 28 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
Optical Amplifiers: Multi-Stage 29 NFtotal = NF1+NF2/G1 NF 1st/2nd stage = Pin - SNRo [dB] - 10 Log (hc2 / 3 ) Er3+ Doped Fiber Pump Pump Input Signal Output Signal Optical Isolator 1st Active stage co-pumped: optimized for low noise figure 2nd stage counter-pumped: optimized for high output power
Raman Amplifier 30 Transmission fiber 1550 nm signal(s) Cladding pumped fiber laser 1450/ 1550 nm WDM 1453 nm Pump (raman pump) Er Amplifier Raman fiber laser Transmission fiber Offer 5 to 7 dB improvement in system performance First application in WDM
OPTICAL AMPLIFIERS Advantages of EDFA over SLA The doped-fiber amplifiers have some advantages over semiconductor laser amplifiers: Wider spectral bandwidth which allows more number of signal channels to be amplified simultaneously. Flat gain characteristic over the practical range of wavelengths; appropriate for optical fiber links. Compatibility for in-line interconnection within optical fiber links. Suitable for use in dense wavelength division multiplexed transmission.
OPTICAL AMPLIFIERS Fiber Raman Amplifiers A fiber Raman amplifier uses stimulated Raman scattering (SRS) occurring in silica fibers when an intense pump beam propagates through it. SRS - The incident pump photon gives up its energy to create another photon of reduced energy at a lower frequency (inelastic scattering); the remaining energy is absorbed by the medium in the form of molecular vibrations (optical phonons). op s p The frequency difference, known as the Stokes shift. s p R Because of amorphous nature of glass, the vibrational energy levels of silica molecules merge together to form a band and allows s to differ from p over a wide range (~20 THz). Raman amplification exhibits self-phase matching between the pump and signal. The pump signal optical wavelengths in Raman fiber amplifiers are typically 500 nm lower than the signal to be amplified, and the pumping signal can propagate in either direction along the fiber.
OPTICAL AMPLIFIERS Gain in Raman Fiber Amplifier Raman gain as a function of the optical pump power is given as k A L P g G eff eff p R R exp The effective fiber core area 2 eff eff r A where gR = Raman gain coefficient k = a numerical factor that accounts for polarization scrambling between the optical pump and signal. (k = 2 for complete polarization scrambling) P P eff L exp 1 The effective fiber length reff is the effective core radius. P = fiber transmission loss at the pump wavelength is the actual fiber length. GR dependence on and P for a pump input power of 1.6 W and fiber core diameter of 10 m.
Optical Amplifiers - Applications 34 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

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Optical Amplifiers and WDM - Optical Communication

  • 2. Contents 2 Why the need for optical amplifier? Spectra Noise Types Principle of Operation Main Parameters Applications
  • 3. Why the Need for Optical Amplification? 3 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 electronics By amplifying signal in the optical domain many of these disadvantages would disappear!
  • 4. Optical Amplification 4 Amplification gain: Up to a factor of 10,000 (+40 dB) In WDM: Several signals within the amplifiers gain (G) bandwidth are amplified, but not to the same extent It generates its own noise source known as Amplified Spontaneous Emission (ASE) noise. Optical Amplifier (G) Weak signal Pin Amplified signal Pout ASE ASE Pump Source
  • 5. Optical Amplification - Spectral Characteristics 5 Wavelength Power (unamplified signal) Wavelength Power (amplified signal) ASE Wavelength Power (unamplified signal) Wavelength Power (amplified signal) ASE Single channel WDM channels
  • 6. Optical Amplification - Noise Figure 6 Required figure of merit to compare amplifier noise performance Defined when the input signal is coherent ) ( ratio noise to signal Output ) ( ratio noise to signal Input (NF) Figure Noise o i SNR SNR 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.
  • 7. Optical Amplifiers - Types 7 There are mainly two types: Semiconductor Laser (optical) Amplifier (SLA) (SOA) Active-Fibre or Doped-Fibre Erbium Doped Fibre Amplifier (EDFA) Erbium (active medium) Fibre Raman Amplifier (FRA) Thulium Doped Fibre Amplifier (TDFA) Thulium(active medium)
  • 8. SLA - Principle Operation 8 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 diode 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! Matching with the fibre is also a problem! However, they are small and cheap!
  • 9. OPTICAL AMPLIFIERS Semiconductor Laser Amplifiers (SLA) Two major types: -The resonant Fabry-Perot amplifier (FPA) -The nonresonant (single pass) traveling-wave amplifier (TWA) SLA is based on the conventional semiconductor laser structure (gain- or index-guided). In FPA, the reflectivities of the facets are between 30% to 35% whereas in TWA the reflectivities are less than 0.001. where R1 = input facet reflectivity R2 = output facet reflectivity
  • 10. OPTICAL AMPLIFIERS Fabry-Perot amplifier (FPA) For operation, FPA is biased below the normal lasing threshold current. When an optical signal enters the FPA, it gets amplified as it reflects back and forth between the mirrors until it is emitted at a higher intensity. Easy to fabricate but the optical signal gain is very sensitive to variations in amplifier temperature and input optical frequency. Used within nonlinear applications such as pulse shaping and bistable elements. Gain and bandwidth of an FPA Using the standard theory of FP interferometers, the cavity gain of SLA as a function of signal frequency f is 2 2 1 2 2 1 2 1 sin 4 1 1 1 S S S FP G R R G R R G R R f G where R1 = input facet reflectivity R2 = output facet reflectivity Single pass phase shift f f f m L g GS exp The single pass gain is given by fm = cavity resonance frequencies f = longitudinal-mode spacing
  • 11. OPTICAL AMPLIFIERS GFP reduce to GS when R1=R2=0. GFP peaks whenever f coincides with one of the cavity-resonance frequencies and drop sharply in between them. Amplifier bandwidth is determined by the sharpness of the cavity resonance. The 賊3 dB single longitudinal mode bandwidth is 2 1 2 1 2 1 1 2 1 sin 2 2 S S m FPA G R R G R R f f f B 2 2 1 2 2 1 2 1 sin 4 1 1 1 S S S FP G R R G R R G R R f G f f f m
  • 15. SLA - Principle Operation 15 Electrons in ground state Pump signal @ 980 nm Energy Absorption Excited state Pump signal @ 980 nm Transition Metastable state Excited state Ground state Pump signal @ 980 nm www.cisco.com
  • 16. SLA - Principle Operation 16 ASE Photons 1550 nm Ground state Excited state Metastable state Transition T r a n s i t i o n Pump signal @ 980 nm Excited state Metastable state Transition Pump signal @ 980 nm Stimulated emission 1550 nm Signal photon 1550 nm Ground state
  • 17. Erbium Doped Fibre Amplifier (EDFA) 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 using stimulated emission. 17 1480 980 820 540 670 Ground state Metastable state 1550 nm EDFA is a low noise light amplifier.
  • 18. EDFA - Operating Features 18 Available since 1990s: 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 Input signal Pump from an external laser 1480 or 980 nm Erbium doped core Cladding Amplifier length 1-20 m typical Amplified signal
  • 19. OPTICAL AMPLIFIERS Amplification in Erbium-doped Fiber Amplifiers Amplification in an EDFA occurs through the mechanism of stimulated emission. The pumping light is absorbed by the erbium ions, raising them to excited states and causing population inversion. Two ways to attain population inversion in EDF: Indirect pumping at 980 nm wavelength - Er ions are excited to upper level (3) and they non-radiatively fast decay to the intermediate energy (metastable) level (2). Direct pumping at 1480 nm wavelength -Er ions are excited directly to the level (2). The signal (to be amplified) stimulates transition of the excited Er ions from level 2 to level 1 and results in radiation of photons with same wavelength, direction, and phase to the signal photons. This gives rise to a coherent (amplified) output with respect to signal input.
  • 20. OPTICAL AMPLIFIERS A pumping signal can co-propagate with an information signal or it can counter-propagate. Co-propagating pump Counter- propagating pump Bi-directional pump
  • 22. EDFA Gain Profile 22 ASE spectrum when no input signal is present Amplified signal spectrum (input signal saturates the optical amplifier) + ASE 1575 nm -40 dBm 1525 nm +10 dBm Most of the pump power appears at the stimulating wavelength Power distribution at the other wavelengths changes with a given input signal.
  • 23. EDFA Ultra Wideband 23 0 10 20 30 1525 1550 1575 1600 5 10 15 Noise 6.5 dB Output Power 24.5 dBm L-Band C-Band Total 3dB Bandwidth = 84.3 nm 43.5 nm 40.8 nm Wavelength (nm) Gain (dB) Noise Figure (dB) Ultra-Wideband EDFA
  • 25. Optical Amplifier - Main Parameters 25 Gain (Pout/Pin) Bandwidth Gain Saturation Polarization Sensibility Noise figure (SNRi/SNRo) Gain Flatness Types Based on stimulated emission (EDFA, PDFA, SOA) Based on non-linearities (Raman, Brillouin)
  • 26. Optical Amplifier - Optical Gain (G) G = S Output / S Input (No noise) Input signal dependent: Operating point (saturation) of EDFA strongly depends on power and wavelength of incoming signal 26 Gain (dB) 1540 1560 1580 10 1520 20 40 30 -5 dBm -20 dBm -10 dBm P Input: -30 dBm Gain as the input power Pin Gain Pout -20 dBm 30 dB +10 dBm -10 dBm 25 dB +15 dBm Note, Pin changes by a factor of ten then Pout changes only by a factor of three in this power range. EDFA
  • 27. Optical Amplifier - Optical Gain (G) 27 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).
  • 28. Optical SNR 28 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
  • 29. Optical Amplifiers: Multi-Stage 29 NFtotal = NF1+NF2/G1 NF 1st/2nd stage = Pin - SNRo [dB] - 10 Log (hc2 / 3 ) Er3+ Doped Fiber Pump Pump Input Signal Output Signal Optical Isolator 1st Active stage co-pumped: optimized for low noise figure 2nd stage counter-pumped: optimized for high output power
  • 30. Raman Amplifier 30 Transmission fiber 1550 nm signal(s) Cladding pumped fiber laser 1450/ 1550 nm WDM 1453 nm Pump (raman pump) Er Amplifier Raman fiber laser Transmission fiber Offer 5 to 7 dB improvement in system performance First application in WDM
  • 31. OPTICAL AMPLIFIERS Advantages of EDFA over SLA The doped-fiber amplifiers have some advantages over semiconductor laser amplifiers: Wider spectral bandwidth which allows more number of signal channels to be amplified simultaneously. Flat gain characteristic over the practical range of wavelengths; appropriate for optical fiber links. Compatibility for in-line interconnection within optical fiber links. Suitable for use in dense wavelength division multiplexed transmission.
  • 32. OPTICAL AMPLIFIERS Fiber Raman Amplifiers A fiber Raman amplifier uses stimulated Raman scattering (SRS) occurring in silica fibers when an intense pump beam propagates through it. SRS - The incident pump photon gives up its energy to create another photon of reduced energy at a lower frequency (inelastic scattering); the remaining energy is absorbed by the medium in the form of molecular vibrations (optical phonons). op s p The frequency difference, known as the Stokes shift. s p R Because of amorphous nature of glass, the vibrational energy levels of silica molecules merge together to form a band and allows s to differ from p over a wide range (~20 THz). Raman amplification exhibits self-phase matching between the pump and signal. The pump signal optical wavelengths in Raman fiber amplifiers are typically 500 nm lower than the signal to be amplified, and the pumping signal can propagate in either direction along the fiber.
  • 33. OPTICAL AMPLIFIERS Gain in Raman Fiber Amplifier Raman gain as a function of the optical pump power is given as k A L P g G eff eff p R R exp The effective fiber core area 2 eff eff r A where gR = Raman gain coefficient k = a numerical factor that accounts for polarization scrambling between the optical pump and signal. (k = 2 for complete polarization scrambling) P P eff L exp 1 The effective fiber length reff is the effective core radius. P = fiber transmission loss at the pump wavelength is the actual fiber length. GR dependence on and P for a pump input power of 1.6 W and fiber core diameter of 10 m.
  • 34. Optical Amplifiers - Applications 34 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