chapter 11 optical amplifier - outline - bohr.wlu.ca course notes11.pdf · chapter 11 optical...
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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: