optical fibre amplifiers continued - ucy...optical amplifiers allow one to extend link distance...
TRANSCRIPT
ECE 455 Lecture 09
1
Optical Fibre Amplifiers – Continued
• ECE 445 • Lecture 09 • Fall Semester 2016
Stavros Iezekiel Department of Electrical and
Computer Engineering
University of Cyprus
ECE 455 Lecture 09
ERBIUM-DOPED FIBRE AMPLIFIERS – BASIC PHYSICS
2
ECE 455 Lecture 09 Energy Transitions in Er3+ - Doped Silica Fibre
ECE 455 Lecture 09
EDFA Basic Structure
Weak input signal at 1.55μm
Isolator Wavelength multiplexer
Laser diode pump at 980 nm
(or 1480 nm, up to 50 mW power)
• Amplified signal at 1.55m • Gain 20 to 30 dB. 30 dB gain means 1000 photons out for 1 photon in
Amplification section with
erbium doped silica fibre,
a few tens of metres (Er3+ ions, 100 – 100 ppm)
Narrowband optical filter
ECE 455 Lecture 09 Power exchange
Input
Isolator Wavelength multiplexer
Pump
Output
Narrowband optical filter
980 nm
signal
1550 nm
data signal
Pow
er
level
980 nm
signal
1550 nm
data signal
Pow
er
level
ECE 455 Lecture 09
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.
ECE 455 Lecture 09
NOISE IN ERBIUM-DOPED FIBRE AMPLIFIERS
7
ECE 455 Lecture 09
Amplified spontaneous emission (ASE)
Random spontaneous emission (SE)
Amplification along fibre
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
ECE 455 Lecture 09
Fibre Link
Optical Amplifiers Fibre Section
Transmitter Optical
Receiver
1 2 N
Optical amplifiers allow one to extend link distance 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)
Optical Amplifier Chains
ECE 455 Lecture 09
Fibre Link Example: system uses fibre with 0.25 dB/km attenuation, 80 km fibre sections, amplifiers with 19 dB gain a noise figure of 5 dB
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 Signal Level
-30
-20
-10
0
10
0 100 200 300 400 500 600 700 800
Location (km)
Sig
nal le
vel (d
Bm
)
ECE 455 Lecture 09
Fibre Link Same system: Transmitter SNR is 50 dB, amplifier noise figure of 5 dB,
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
Amplifier Chains and Optical SNR
0
10
20
30
40
50
60
0 100 200 300 400 500 600 700 800
Location (km)
Op
tic
al S
NR
(d
B)
ECE 455 Lecture 09
GAIN PROFILE OF ERBIUM-DOPED FIBRE AMPLIFIERS
12
ECE 455 Lecture 09
EDFA Output Spectra
ASE spectrum when no input signal is present
Amplified signal spectrum (input signal saturates the optical amplifier)
1575 nm -40 dBm
1525 nm
+10 dBm
ECE 455 Lecture 09
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.
ECE 455 Lecture 09
Gain Flattening Concept
ECE 455 Lecture 09
SYSTEM PERFORMANCE OF OPTICAL AMPLIFIERS
16
input signal gain + noise analogous to DC bias
gain G, noise figure F
ECE 455 Lecture 09
EDFA output versus wavelength
ASE = amplified spontaneous emission: noise
ECE 455 Lecture 09
Gain versus EDFA length
ECE 455 Lecture 09
EDFA gain versus pump level
ECE 455 Lecture 09
Typical gain versus power profile for optical amplifier:
ECE 455 Lecture 09
SNR degradation for a chain of EDFAs
ECE 455 Lecture 09
EDFA CHAINS
22
ECE 455 Lecture 09
• Consider in-line amplifier application, as in long haul links:
G G G L L L
• Set amplifier gain to compensate for loss of inter-connecting 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.
Optical Amplifier Gain Control
ECE 455 Lecture 09
G G G L L L
Px G + Px G + Px - L = Px {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.
ECE 455 Lecture 09
G G G L L L
Ps : source power (dBm)
PS - L : power entering first amplifier
G + PS - L = PS {If G = L}
: output power from first amplifier
PS PR
Optical
source
Photo-
receiver
L
Ps
PR = receiver sensitivity
PR = PS - L
ECE 455 Lecture 09 • Now consider situation where power at some point in link drops suddenly (e.g. due to fault at laser):
G G G L L L
Ps - Px
PS - Px - L
G + PS - Px - L
PS PR L
Ps - Px
PS - Px - L < PR
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.
ECE 455 Lecture 09 • 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!).
ECE 455 Lecture 09 • For example, consider an amplifier with a gain/input power
slope of - 1 dB/dBm in the saturation region:
Pnom + Pnom - Pnom
Gnom
Gnom +
Gnom -
POUT = Pnom - + Gnom + = Pnom + Gnom
POUT = Pnom + Gnom
POUT = Pnom + + Gnom -
= Pnom + Gnom
slope = -1 dB/dBm
PIN(dBm)
G(dB)
ECE 455 Lecture 09 • 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.
ECE 455 Lecture 09 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: Gnom = 7.3 dB Nominal optical output power: POUT = 3 dBm Nominal optical input power: PIN = -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.
±
ECE 455 Lecture 09
G G G L
L
= 7.3 dB
• Before power drop:
- 4.3 dBm 3 dBm
G = 7.3 dB
- 4.3 dBm ...................
G(PIN1)
L L L
G(PIN2) G(PIN3)
• After power drop:
- 4.3 dBm - 4 dB
= - 8.3 dBm
ECE 455 Lecture 09
G(PIN1)
L = 7.3 dB L L
G(PIN2) G(PIN3)
• After power drop:
- 4.3 dBm - 4 dB
= - 8.3 dBm
- 4 dB drop for PIN1 (relative to the
nominal value of -4.3 dBm) means that
G for amp 1 goes up by 2 dB from Gnom,
hence
G(PIN1) = 7.3 + 2 = 9.3 dB
ECE 455 Lecture 09
G(PIN1) =
9.3 dB
L L =
7.3 dB L
- 8.3 dBm
- 8.3 dBm + 9.3 dB
= 1 dBm
1 dBm - 7.3 dB
= -6.3 dBm
PIN2 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(PIN2) = 8.3 dB
ECE 455 Lecture 09 G(PIN2) =
8.3 dB
L L =
7.3 dB L
- 6.3 dBm 2 dBm - 7.3 dB
= -5.3 dBm
PIN3 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(PIN3) = 7.8 dB
- 6.3 dBm + 8.3 dB
= 2 dBm
ECE 455 Lecture 09 G(PIN3) =
7.8 dB
L
L =
7.3 dB L
- 5.3 dBm
2.5 dBm - 7.3 dB
= -4.8 dBm
PIN4 is 0.5 dB below the
nominal value of - 4.3 dBm
- 5.3 dBm + 7.8 dB
= 2.5 dBm
ECE 455 Lecture 09
Pnom = -4.3
Gnom = 7.3
PIN(dBm)
G(dB)
nominal
point
self-healing
PIN1
-8.3
1 G1 = 9.3
PIN2
-6.3
2 G2 = 8.3
PIN2
-5.3
3 G3 = 7.8