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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

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Page 1: Optical Fibre Amplifiers Continued - UCY...Optical amplifiers allow one to extend link distance between a transmitter and receiver ... WDM systems use many wavelengths within the amplified

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

Page 2: Optical Fibre Amplifiers Continued - UCY...Optical amplifiers allow one to extend link distance between a transmitter and receiver ... WDM systems use many wavelengths within the amplified

ECE 455 Lecture 09

ERBIUM-DOPED FIBRE AMPLIFIERS – BASIC PHYSICS

2

Page 3: Optical Fibre Amplifiers Continued - UCY...Optical amplifiers allow one to extend link distance between a transmitter and receiver ... WDM systems use many wavelengths within the amplified

ECE 455 Lecture 09 Energy Transitions in Er3+ - Doped Silica Fibre

Page 4: Optical Fibre Amplifiers Continued - UCY...Optical amplifiers allow one to extend link distance between a transmitter and receiver ... WDM systems use many wavelengths within the amplified

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

Page 5: Optical Fibre Amplifiers Continued - UCY...Optical amplifiers allow one to extend link distance between a transmitter and receiver ... WDM systems use many wavelengths within the amplified

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

Page 6: Optical Fibre Amplifiers Continued - UCY...Optical amplifiers allow one to extend link distance between a transmitter and receiver ... WDM systems use many wavelengths within the amplified

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.

Page 7: Optical Fibre Amplifiers Continued - UCY...Optical amplifiers allow one to extend link distance between a transmitter and receiver ... WDM systems use many wavelengths within the amplified

ECE 455 Lecture 09

NOISE IN ERBIUM-DOPED FIBRE AMPLIFIERS

7

Page 8: Optical Fibre Amplifiers Continued - UCY...Optical amplifiers allow one to extend link distance between a transmitter and receiver ... WDM systems use many wavelengths within the amplified

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

Page 9: Optical Fibre Amplifiers Continued - UCY...Optical amplifiers allow one to extend link distance between a transmitter and receiver ... WDM systems use many wavelengths within the amplified

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

Page 10: Optical Fibre Amplifiers Continued - UCY...Optical amplifiers allow one to extend link distance between a transmitter and receiver ... WDM systems use many wavelengths within the amplified

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

)

Page 11: Optical Fibre Amplifiers Continued - UCY...Optical amplifiers allow one to extend link distance between a transmitter and receiver ... WDM systems use many wavelengths within the amplified

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)

Page 12: Optical Fibre Amplifiers Continued - UCY...Optical amplifiers allow one to extend link distance between a transmitter and receiver ... WDM systems use many wavelengths within the amplified

ECE 455 Lecture 09

GAIN PROFILE OF ERBIUM-DOPED FIBRE AMPLIFIERS

12

Page 13: Optical Fibre Amplifiers Continued - UCY...Optical amplifiers allow one to extend link distance between a transmitter and receiver ... WDM systems use many wavelengths within the amplified

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

Page 14: Optical Fibre Amplifiers Continued - UCY...Optical amplifiers allow one to extend link distance between a transmitter and receiver ... WDM systems use many wavelengths within the amplified

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.

Page 15: Optical Fibre Amplifiers Continued - UCY...Optical amplifiers allow one to extend link distance between a transmitter and receiver ... WDM systems use many wavelengths within the amplified

ECE 455 Lecture 09

Gain Flattening Concept

Page 16: Optical Fibre Amplifiers Continued - UCY...Optical amplifiers allow one to extend link distance between a transmitter and receiver ... WDM systems use many wavelengths within the amplified

ECE 455 Lecture 09

SYSTEM PERFORMANCE OF OPTICAL AMPLIFIERS

16

input signal gain + noise analogous to DC bias

gain G, noise figure F

Page 17: Optical Fibre Amplifiers Continued - UCY...Optical amplifiers allow one to extend link distance between a transmitter and receiver ... WDM systems use many wavelengths within the amplified

ECE 455 Lecture 09

EDFA output versus wavelength

ASE = amplified spontaneous emission: noise

Page 18: Optical Fibre Amplifiers Continued - UCY...Optical amplifiers allow one to extend link distance between a transmitter and receiver ... WDM systems use many wavelengths within the amplified

ECE 455 Lecture 09

Gain versus EDFA length

Page 19: Optical Fibre Amplifiers Continued - UCY...Optical amplifiers allow one to extend link distance between a transmitter and receiver ... WDM systems use many wavelengths within the amplified

ECE 455 Lecture 09

EDFA gain versus pump level

Page 20: Optical Fibre Amplifiers Continued - UCY...Optical amplifiers allow one to extend link distance between a transmitter and receiver ... WDM systems use many wavelengths within the amplified

ECE 455 Lecture 09

Typical gain versus power profile for optical amplifier:

Page 21: Optical Fibre Amplifiers Continued - UCY...Optical amplifiers allow one to extend link distance between a transmitter and receiver ... WDM systems use many wavelengths within the amplified

ECE 455 Lecture 09

SNR degradation for a chain of EDFAs

Page 22: Optical Fibre Amplifiers Continued - UCY...Optical amplifiers allow one to extend link distance between a transmitter and receiver ... WDM systems use many wavelengths within the amplified

ECE 455 Lecture 09

EDFA CHAINS

22

Page 23: Optical Fibre Amplifiers Continued - UCY...Optical amplifiers allow one to extend link distance between a transmitter and receiver ... WDM systems use many wavelengths within the amplified

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

Page 24: Optical Fibre Amplifiers Continued - UCY...Optical amplifiers allow one to extend link distance between a transmitter and receiver ... WDM systems use many wavelengths within the amplified

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.

Page 25: Optical Fibre Amplifiers Continued - UCY...Optical amplifiers allow one to extend link distance between a transmitter and receiver ... WDM systems use many wavelengths within the amplified

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

Page 26: Optical Fibre Amplifiers Continued - UCY...Optical amplifiers allow one to extend link distance between a transmitter and receiver ... WDM systems use many wavelengths within the amplified

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.

Page 27: Optical Fibre Amplifiers Continued - UCY...Optical amplifiers allow one to extend link distance between a transmitter and receiver ... WDM systems use many wavelengths within the amplified

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!).

Page 28: Optical Fibre Amplifiers Continued - UCY...Optical amplifiers allow one to extend link distance between a transmitter and receiver ... WDM systems use many wavelengths within the amplified

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)

Page 29: Optical Fibre Amplifiers Continued - UCY...Optical amplifiers allow one to extend link distance between a transmitter and receiver ... WDM systems use many wavelengths within the amplified

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.

Page 30: Optical Fibre Amplifiers Continued - UCY...Optical amplifiers allow one to extend link distance between a transmitter and receiver ... WDM systems use many wavelengths within the amplified

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.

±

Page 31: Optical Fibre Amplifiers Continued - UCY...Optical amplifiers allow one to extend link distance between a transmitter and receiver ... WDM systems use many wavelengths within the amplified

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

Page 32: Optical Fibre Amplifiers Continued - UCY...Optical amplifiers allow one to extend link distance between a transmitter and receiver ... WDM systems use many wavelengths within the amplified

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

Page 33: Optical Fibre Amplifiers Continued - UCY...Optical amplifiers allow one to extend link distance between a transmitter and receiver ... WDM systems use many wavelengths within the amplified

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

Page 34: Optical Fibre Amplifiers Continued - UCY...Optical amplifiers allow one to extend link distance between a transmitter and receiver ... WDM systems use many wavelengths within the amplified

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

Page 35: Optical Fibre Amplifiers Continued - UCY...Optical amplifiers allow one to extend link distance between a transmitter and receiver ... WDM systems use many wavelengths within the amplified

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

Page 36: Optical Fibre Amplifiers Continued - UCY...Optical amplifiers allow one to extend link distance between a transmitter and receiver ... WDM systems use many wavelengths within the amplified

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