Chapter 11 Optical AmplifierChapter 11 Optical Amplifier -- OutlineOutline

Principles of SOAPrinciples of SOA-- External pumpingExternal pumping-- Amplifier gainAmplifier gain

Principles of EDFAPrinciples of EDFA-- Amplification mechanismAmplification mechanism-- EDFA architectureEDFA architecture

Amplifier TypesAmplifier Types-- Semiconductor optical amplifier (SOA)Semiconductor optical amplifier (SOA)-- (Erbium) doped fiber (EDFA)(Erbium) doped fiber (EDFA)

Amplifier TypesAmplifier Types

Amplifier types: semiconductor optical amplifier (SOA) and (Erbium) doped fiber amplifier (EDFA)The mechanism to create the population inversion is the same as used in laser diodes. Although the structure of an optical amplifier is similar to that of a laser, it does not have the optical feedback mechanism that is necessary for lasing to take place. Thus, an optical amplifier can boost incoming signal levels, but it cannot generate a coherent optical output by itself.

Semiconductor optical amplifier (SOA) Semiconductor optical amplifier (SOA) -- major Typesmajor Types

Alloys of semiconductor elements from group III and V make up the active medium in SOAs (1300 nm, 1500 nm).

Fabry-Perot amplifier

Ra Rb

Gain medium

Reflecting coating 32%

• sensitive to temperature and input optical frequency since gain is only at FP resonant

Optical input signal

Traveling-wave amplifier

• large optical bandwidth• high saturation power• low polarization sensitivity

Ra ~0

Gain medium

Anti-Reflection Coating or cleaved at an angle

Ra ~0

Optical input signal

Traveling-wave amplifiers (TWAs) have been used more widely that Fabry-Perot amplifiers (FPAs).

Semiconductor optical amplifier (SOA) Semiconductor optical amplifier (SOA) -- External pumpingExternal pumping

External current injection is the pumping method to create the population inversion needed for having a gain mechanism in SOAs.

Example 11-1Consider an InGaAsP SOA with w=5μm and d=0.5μm. Given that vg=2x108 m/s, if a 1.0μW optical signal at 1550 nm enters the device, find the photon density Nph.

rst

tntRqd

tJttn

τ)()()()(

−−=∂

∂

Rate equation for SOA

with

Injection current density J(t)Thickness of active layer dCombined time constant for spontaneous emission and carrier-recombination rτ

phgphthgst NgNnnatR υυ ≡−Γ= )()(

))(( wdhPN

g

sph νυ=

Pumping rate due to current injection Carrier consumption due to spontaneous emission and recombination

Carrier consumption due to net stimulated emission

with optical power Ps, active layer width w and thickness d

Net stimulated emission rate

Where, vg is optical group velocity; Γ is optical confinement factor; a is gain constant; nth is threshold carrier density; Nph is photon density; g is overall gain per unit length

Semiconductor optical amplifier (SOA) Semiconductor optical amplifier (SOA) -- SteadySteady--statestate

0)(=

∂∂

ttn

Steady-state gain per unit length for SOA

With saturation photon density:

satphphrphg

r

th

NNg

aN

nqdJ

g;

0

/1)/(1 +=

Γ+

−=

τυτ

rgsatph a

NτυΓ

=1

;

Steady-state

)(0r

thr

nqdJag

ττ −Γ=

Gain is saturated with increasing photon density

Gain is increased with increasing current injection

Small-signal (zero-signal) gain per unit length :

Example 11-2Consider the following parameter for a 1300 nm InGaAsP SOA, if a 100 mA bias current is applied to the device, (a) find the pumping rate and (b) find small signal gain per unit length. Symbol Parameter value

w active area width 3 μmd Active area thickness 0.3μmL Amplifier length 500 μmΓ confinement factor 0.3τr Time constant 1 nsa gain coefficient 2x10-20 m2

nth threshold density 1x1024 m-3

Semiconductor optical amplifier (SOA) Semiconductor optical amplifier (SOA) -- Amplifier GainAmplifier Gain

Amplifier Gain

LzgLg

ins

outs eePP

G m )()(

,

, ≡== −Γ α

With material gain coefficienteffective absorption coefficientamplifier length L

mgα

0

,

0

,

, , , ,0 0

, , ,

, 0

,

( )1 ( ) /

( )( ) ( )1 ( ) /

ln( ) , ln( ) ( 1) ln( )

1 ln( )

s amp sat

ss s

s amp sat

s out s out s in s in

s in amp sat amp sat

amp sat

s in

gg zP z P

g P z dzdP g z P z dzP z P

P P P Pg L G G G

P P P

P GGP G

=+

= =+

−+ = + − =

= +

Gain dependence on input power

z

Gain medium

0 L

dzoutsP ,insP ,

Amplifier gain GOutput saturation power Pamp, sat : is defined as the amplifier output power for which the G is reduced by 3dB from unsaturated amplifier gain G0.

Fig.11-3 Single-pass gain vs. input power

Erbium doped fiber amplifier (EDFA)Erbium doped fiber amplifier (EDFA) -- Principles Principles

Rare-earth element: erbium (Er), ytterbium (Yb), neodymium (Nd), or praseodymium (Pr); Host fiber material: standard silica, or fluoride-based glassOperating regions depend on materials.Most popular material: erbium doped fiber (EDF) EDF amplifier (EDFA)-- operating wavelength: 1530 --- 1560 nm-- pumping wavelengths: 980 nm, 1480 nm

1μs

EDFAEDFA -- Architecture Architecture

Pumping schemes:- co-directional pumping - counter-directional pumping - dual pump schemes- pumping at 980nm and 1480nm

EDFAEDFA -- gain dependence and saturationgain dependence and saturation

Gain depends on fiber length and pump levelsaturation power depends on pump level

EDFAEDFA -- amplifier noisesamplifier noises

Gain medium

0 L

dz

optASEspASE PfGnhfS νν Δ=−= /]1)([)(

ASE power spectral density

ASEP optνΔis the ASE power within optical bandwidth

12

2

nnnnsp −

= is the population-inversion factor with n2 and n1 are the populations of electrons in states high and low

Spontaneous emission randomly generated within amplifier gets amplified along the cavity. At exit of amplifier it could be measured as a flat spectrum so called Amplified spontaneous emission (ASE)

EDFAEDFA -- amplifier noisesamplifier noises

ASE noise level depends on the configuration of pumping schemes.

EDFAEDFA -- amplifier noises after photo detectionamplifier noises after photo detection

nsnsnstot EEEEEEi 2)( 222 ++=+∝

Photo current

signal term noise term signal-noise beat term

The signal-noise beat term could fall within the receiver bandwidth and degrade the signal to noise ratio (SNR)

optASEins SGPP νΔ+= ,0

Optical power into receiver

noise could be reduced through optical filter which has a limited bandwidth

Means-square receiver noise current2222222

ASEASEASEsASEshotsshotTtotaltotali −−−− ++++== σσσσσσ

BR

TK BT

42 =σThermal noise

Signal shot noise

Signal ASE beat noise

ASE ASE beat noise

BqRGP inssshot ,2 2≈−σ

))((4 ,2 BRSRGP ASEinsASEs =−σ

BBSR optASEASEASE )2(222 −Δ=− νσ

Means-square receiver photocurrent

2,

2222insphph PGRi == σ

ASE noise originates ahead of the photodiode, it gives rise to different noise components in an optical receiver in addition to the normal thermal noise of the photodetector.

EDFAEDFA -- Signal to noise ratio and noise figureSignal to noise ratio and noise figure

12)1(21

)/()/(

>>≅−+

== GifnG

GnNSNSF sp

sp

out

in ηη

ηNoise figure

Assuming perfect amplifier with total inversionand perfect quantum efficiency Yields a noise figure of 2 (or 3dB).

1=spn1==

qhR νη

Noise Figure: