Transcript
SOA- RATE EQUATION
Expressing the photon number in terms of the optical power:
where Γ is the confinement factor, σg is the differential gain, V is the active volume, τc is the carrier lifetime, σm is the crosssectional area of the waveguide mode and N0 is the transparency number. the steady-state by setting dN/dt=0 the optical gain is found to saturate as:
Typical values of Psout are in the range 5–10 mW. 19
ERBIUM DOPED OPTICAL AMPLIFIERS
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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 540 using stimulated emission. 670
i EDFA is a low noise light amplifier.
820 980 Metastable 1480 state
1550 nm
Ground state 21
PUMPING AND GAIN
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EDFA - OPERATING FEATURES Input signal
Amplifier length 1-20 m typical
Pump from an external laser 1480 or 980 nm
Amplified signal
Cladding Erbium doped core
• 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 23
EDFA – GAIN PROFILE +10 dBm
• Most of the pump power appears at the stimulating wavelength
ASE spectrum when no input signal is present
• Power distribution at the other wavelengths changes with a given input signal.
Amplified signal spectrum (input signal saturates the optical amplifier) + ASE
-40 dBm
1525 nm
1575 nm
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BASIC THEORY
The population densities of the two states, N1 and N2, satisfy the following rate equations:
where σa and σe are the absorption and emission cross sections, T1 is the spontaneous lifetime of the excited state and the quantities φ represent the photon flux. Ps and Pp satisfy the simple equations:
The parameter s = ±1 correspond to the direction of pump
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EDFA
The optimum length could be calculated. 26
EDFA-NOISE P’p=Pp/Psat p
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ERBIUM AMPLIFIER DISADVANTAGES
Can only work at wavelengths where Er+3 fluoresces Requires specially doped fiber as gain medium Three-level system, so gain medium is opaque at signal wavelengths until pumped Requires long path length of gain medium (tens of meters in glass) Gain very wavelength-dependent and must be flattened
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STIMULATED RAMAN SCATTERING
•Scattering of light from vibrating molecules and in both directions. • Scattered light shifted in frequency. • Raman gain spectrum extends over 40 THz. • Raman shift at Gain peak: ΩR= ωp−ωs =13 THz
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RAMAN AMPLIFIERS
Use stimulated Raman effect and pump laser whose frequency is equal to signal frequency plus frequency of chemical bond in the material
Because it is a nonlinear process, requires very high pump powers (watts)
RAMAN AMPLIFIER Transmission fiber 1450/ 1550 nm WDM
Transmission fiber Er Amplifier
1550 nm signal(s) 1453 nm pump
Cladding pumped fiber laser
Raman fiber laser
•Offer 5 to 7 dB improvement in system performance •First application in WDM
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RAMAN AMPLIFIER Advantages: Can use existing fiber as gain medium (distributed amplification) Can operate in any region of the spectrum Disadvantages: Require very high pump powers Can be used only over long distances, since Raman effect is weak Rayleigh scattering dominates, causing loss of pump power
