1 Optical Fibre Amplifiers Continued Stavros Iezekiel Department of Electrical and Computer Engineering University of Cyprus ECE 445 Lecture 09 Fall Semester 2016
2 ERBIUM-DOPED FIBRE AMPLIFIERS BASIC PHYSICS
Energy Transitions in Er 3+ - Doped Silica Fibre
EDFA Basic Structure Isolator Wavelength multiplexer Narrowband optical filter Weak input signal at 1.55μm Laser diode pump at 980 nm (or 1480 nm, up to 50 mw power) Amplification section with erbium doped silica fibre, a few tens of metres (Er 3+ ions, 100 100 ppm) Amplified signal at 1.55 m Gain 20 to 30 db. 30 db gain means 1000 photons out for 1 photon in
Power level Power level ECE 455 Lecture 09 Power exchange 980 nm signal 1550 nm data signal 980 nm signal 1550 nm data signal Isolator Wavelength multiplexer Narrowband optical filter Input Output Pump
6 Gain as a function of length of erbium-doped fibre If the fibre is too long, there will be more absorption than gain, but if the fibre is too short we will not have as much gain as we could. Optimum length depends on the pump power.
7 NOISE IN ERBIUM-DOPED FIBRE AMPLIFIERS
Random spontaneous emission (SE) Amplification along fibre Amplified spontaneous emission (ASE) Erbium randomly emits photons between 1520 and 1570 nm Spontaneous emission (SE) is not polarized or coherent Like any photon, SE stimulates emission of other photons With no input signal, eventually all optical energy is consumed into amplified spontaneous emission
Optical Amplifier Chains Optical amplifiers allow one to extend link Fibre distance Link between a transmitter and receiver Amplifier can compensate for attenuation Cannot compensate for dispersion (and crosstalk in DWDM systems) Amplifiers also introduce noise, as each amplifier reduces the Optical SNR by a small amount (noise figure) Transmitter 1 2 N Optical Receiver Optical Amplifiers Fibre Section
Amplifier Chains and Signal Level Example: system uses fibre with 0.25 db/km Fibre Link attenuation, 80 km fibre sections, amplifiers with 19 db gain a noise figure of 5 db 10 Signal level (dbm) 0-10 -20-30 0 100 200 300 400 500 600 700 800 Location (km) Each amplifier restores the signal level to a value almost equivalent to the level at the start of the section - in principle reach is extended to 700 km +
Amplifier Chains and Optical SNR Fibre Link Same system: Transmitter SNR is 50 db, amplifier noise figure of 5 db, 60 50 Optical SNR (db) 40 30 20 10 0 0 100 200 300 400 500 600 700 800 Location (km) Optical SNR drops with distance, so that if we take 30 db as a reasonable limit, the max distance between T/X and R/X is only 300 km
12 GAIN PROFILE OF ERBIUM-DOPED FIBRE AMPLIFIERS
EDFA Output Spectra +10 dbm Amplified signal spectrum (input signal saturates the optical amplifier) ASE spectrum when no input signal is present -40 dbm 1525 nm 1575 nm
Gain Characteristics of EDFA Gain (amplifier) - is the ratio in decibels of input power to output power. Gain at 1560 nm is some 3 db higher than gain at 1540 nm (this is twice as much). In most applications (if there is only a single channel or if there are only a few amplifiers in the circuit) this is not too much of a limitation. WDM systems use many wavelengths within the amplified band. If we have a very long WDM link with many amplifiers the difference in response in various channels adds up.
Gain Flattening Concept
16 gain G, noise figure F analogous to DC bias input signal gain + noise SYSTEM PERFORMANCE OF OPTICAL AMPLIFIERS
EDFA output versus wavelength ASE = amplified spontaneous emission: noise
Gain versus EDFA length
EDFA gain versus pump level
Typical gain versus power profile for optical amplifier:
SNR degradation for a chain of EDFAs
22 EDFA CHAINS
Optical Amplifier Gain Control Consider in-line amplifier application, as in long haul links: L G L G L G Set amplifier gain to compensate for loss of interconnecting fibres of length L, i.e.: G = L So if the link consists of equal number of amplifiers and interconnecting fibres, overall link loss should be zero.
L G L G L G P x G + P x G + P x - L = P x {If G = L} Note! All powers expressed in dbm, all gains and losses expressed in db. Consider next example, with three in-line amplifiers, and length L chosen to be maximum for given source power and receiver sensitivity.
Optical source Photoreceiver P S L G L G L G L P R P s P R = receiver sensitivity P R = P S - L G + P S - L = P S {If G = L} : output power from first amplifier P S - L : power entering first amplifier P s : source power (dbm)
ECE 455 Now Lecture consider 09 situation where power at some point in link drops suddenly (e.g. due to fault at laser): P S L G L G L L G P R P s - P x G + P S - P x - L P S - P x - L < P R P S - P x - L P s - P x Bad news: drop in power means that the power incident on the photoreceiver is now less than the receiver sensitivity, which in a digital system means the BER specification is not met.
One solution is passive gain control: relies on using the amplifier in its saturation region: If input power drops (rises), gain increases (decreases) to compensate for this. Similar effect to feedback (but it is not f/b!).
For example, consider an amplifier with a gain/input power slope of - 1 db/dbm in the saturation region: G(dB) P OUT = P nom - + G nom + = P nom + G nom P OUT = P nom + G nom G nom + G nom G nom - P OUT = P nom + + G nom - = P nom + G nom slope = -1 db/dbm P nom - P nom P nom + P IN (dbm)
This leads to a self-healing effect in systems where cascades of amplifiers are used (such as in-line). The disadvantage is that the gain is low, because the amplifiers operate in the saturation region. The slope in general is not -1 db/dbm, but even when it is not, self-healing will occur, but not immediately after the first amplifier. We will see this in the next example.
Example Consider a long-distance transmission system containing a cascaded chain of erbium-doped fibre amplifiers (EDFAs). Assume each EDFA is operated in saturation and that the slope of the gain-versus-input power curve is 0.5; for example, the gain changes by ± 2 db for a 4 db variation in input power. The EDFAs in the link have the following operational parameters: ± Nominal gain: G nom = 7.3 db Nominal optical output power: P OUT = 3 dbm Nominal optical input power: P IN = -4.3 dbm Suppose there is a sudden 4 db drop in signal level at some point in the link. Find the output power levels after the degraded signal has passed through 1,2, and 3 succeeding EDFAs.
Before power drop: G = 7.3 db L G G L = 7.3 db G - 4.3 dbm 3 dbm - 4.3 dbm... After power drop: G(P IN1 ) G(P IN2 ) G(P IN3 ) L L L - 4.3 dbm - 4 db = - 8.3 dbm
After power drop: G(P IN1 ) G(P IN2 ) G(P IN3 ) L = 7.3 db L L - 4.3 dbm - 4 db = - 8.3 dbm - 4 db drop for P IN1 (relative to the nominal value of -4.3 dbm) means that G for amp 1 goes up by 2 db from G nom, hence G(P IN1 ) = 7.3 + 2 = 9.3 db
L G(P IN1 ) = 9.3 db L = 7.3 db L - 8.3 dbm - 8.3 dbm + 9.3 db = 1 dbm 1 dbm - 7.3 db = -6.3 dbm P IN2 is 2 db below the nominal value of - 4.3 dbm So G for amp 2 will be 1 db above the nominal value of 7.3 db, i.e. G(P IN2 ) = 8.3 db
L L G(P IN2 ) = 8.3 db L = 7.3 db - 6.3 dbm 2 dbm - 7.3 db = -5.3 dbm - 6.3 dbm + 8.3 db = 2 dbm P IN3 is 1 db below the nominal value of - 4.3 dbm So G for amp 2 will be 0.5 db above the nominal value of 7.3 db, i.e. G(P IN3 ) = 7.8 db
L L G(P IN3 ) = 7.8 db L = 7.3 db - 5.3 dbm - 5.3 dbm + 7.8 db = 2.5 dbm 2.5 dbm - 7.3 db = -4.8 dbm P IN4 is 0.5 db below the nominal value of - 4.3 dbm
G(dB) G 1 = 9.3 1 self-healing G 2 = 8.3 G 3 = 7.8 2 3 nominal point G nom = 7.3 P IN1-8.3 P IN2-6.3 P IN2-5.3 P nom = -4.3 P IN (dbm)