Download fiber lec 13 optical coherent receiver

Elec. & Comm.Dep.
Opt. Fiber Comm. Systems
Lecture Therteen
Optical Fiber Communication Systems
Lecture Therteen
Coherent Optical Receiver
The term coherent is used to refer to any techniques employing nonlinear mixing
between two optical waves. One of them is the information bearing signal, the other
is locally generated wave (local oscillator LO).
The mixing is done by using a photodetector.
the basic idea behind coherent lightwave systems is to mix the received signal
coherently with another optical wave (the local oscillator signal) before it is incident
on the photodetector. This mixing process can improve receiver performance. How?
Let
E LO (t) = E LO cos(w LO t + φ LO )
E s (t) = E s cos(w s t + φ s )
Which are the field from the local oscillator and the field of the modulated received
wave respectively.
The photodetector in the figure responds to the intensity of │Es + ELO│2.
Thus
I (t ) = R ( PS + PLO ) + 2 R PS PLO cos( wIF + φ s − φ LO )
(1)
ω IF = ω s − ω LO
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Elec. & Comm.Dep.
Opt. Fiber Comm. Systems
Lecture Therteen
where wIF is intermediate frequency (IF) typically 0.1 – 5 Ghz.
Coherent receiver
1. homodyne coherent receiver: ws ≠ wLO..
2. heterodyne coherent receiver: ws = wLO.
Homodyne Detection
In this coherent detection technique, the local oscillator frequency ωL0 is selected to
coincide with the signal carrier frequency ω0 so that ωIF = 0 by using eq. (1) the
photocurrent ( I = RP ) where R is the detector responsivity) is given by
I (t ) = R ( PS + PLO ) + 2 R PS PLO cos(φ s − φ LO )
( 2)
Typically, PLO >> P S and PS + PLO the last term in above eq. contains information
transmitted and is used by the detection circuit consider the case in which the local –
oscillator phase is locked to the signal phase so that the homodyne signal is then
given by
I P (t ) = 2 R
where
PS PLO
ω s = ω LO
(3)
and
φ s = φ LO
The advantage of homodyne detection is evident from eq.(3), if we note that the
signal current for the direct detection case is given by RPs(t). The average electrical
signal power is increased by a factor of
4PLO/PS
by the use of homodyne
detection.
ِ ◌Another advantage of coherent detection is evident from eq.(2) since the last term
contains the signal phase explicitly, it is possible to transmit information by
modulating the phase of optical carrier. Direct detection dose not allow phase or
frequency modulation.
ِ ◌A disadvantage of homodyne detection results from its phase sensitivity contain
the local oscillator phase φ LO explicitly. Clearly φ LO should be controlled. Ideally
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Elec. & Comm.Dep.
Opt. Fiber Comm. Systems
Lecture Therteen
φ s and φ LO should stay constant expect for the intentional modulation of φ s . These
problems can be over come by the use of heterodyne detection
Heterodyne Detection
Heterodyne detection was first demonstrated at optical frequencies by Forrester
(1995).
In the case of heterodyne detection the local – oscillator frequency ωLO is chosen to
differ form the signal carrier frequency ω0 such that the intermediate frequency ωIF
is in the microwave region (νIF~1 GHz).
Since PLO >> Ps, in practice the nearly constant direct current (dc) term can be
filtered out easily, the heterodyne signal is thus given by the alternating current (ac)
term:
I ac (t ) = 2R Ps PLO cos(ω IF t + φs + φLO )
(4)
In homodyne detection information can be transmitted through amplitude, phase or
frequency modulation of the optical carrier. Furthermore, similar for the homodyne
case, the local oscillator amplifies the received signal and improving the SNR.
However, the SNR improvement is lower by the factor of 2 ( or by 3 dB ) than in the
homodyne case. This reduction is referred to as the 3-dB heterodyne –detection
penalty. The origin of the 3 dB penalty can be seen by considering the signal power
( proportional to the square of the current ) because of the ac nature of Iac. The
average signal power is lower by a factor of 2 when I2ac is averaged over a cycle at
the intermediate frequency ( recall that the average of cos2 over φ is 1/2 )
The advantage gained at the expense of the 3 dB penalty is that the receiver design
is considerably simplified since an optical phase – locked loop is no longer needed.
Fluctuation is both and still need to be controlled by using narrow line width
semiconductor laser for both optical sources. This feature makes heterodyne–
detection schemes suitable for practical implementation of coherent light wave
system.
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Elec. & Comm.Dep.
Opt. Fiber Comm. Systems
Lecture Therteen
Noise analysis in coherent hetrodyne receiver
1. shot noise due to signal light power
σ
2
sig
= 2 eR ( P s ) B IF
2. shot noise due to local oscillator light power
σ
2
LO
= 2 eR ( P LO ) B IF
3. shot noise due to background light power Pb
σ
2
b
= 2 eR ( Pb ) B IF
4. shot noise due to dark current, Id
σ
2
d
= 2 eI d B IF
5. circuit noise (equivalent input current of thermal noise plus amplifier noise).
σT =
2
4kTFn
BIF
RL
• BIF is the IF amplifier’s bandwidth for heterodyne detection and B for
homodyne detection. Assume that BT = 2B = BIF.
The shot noise due to the local oscillator light is much larger than the other types
since the level PLO is much larger than the other noise power levels, so it is the
dominant one.
6. laser phase noise.
The advantage of coherent detection for light wave system can be made more
quantitative by considering the SNR of the receiver current. The receiver current
fluctuates because the shot noise and thermal noise the variance σ2 of current
fluctuations is obtained by adding the two contribution so that
σ
2
=σ
2
s
+σ
2
T
Where
2
2
σ s2 = σ sig
+ σ LO
+ σ d2 + σ b2 = 2e ( I + I d ) B IF
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Elec. & Comm.Dep.
Opt. Fiber Comm. Systems
Lecture Therteen
The shot noise contribution. The current I in eq. (2), is the total photocurrent
generated at the detector and is given by eq. (2) or eq. (4) depending on whether
homodyne or heterodyne detection is employed. In practice PLO >> Ps, and I in eq.
(5) can be replaced by the dominant term RPLO for both cases in the heterodyne case
SNR =
4 R 2 Ps PLO
2e ( RPLO + I d ) B + σ T2
(6 )
ý In the homodyne case the SNR is lager by factor of 2. If we assume that
φ s = φ LO in eq (2). The main advantage of coherent detection can be seen from
eq.(6) since the local oscillator power PLO can be controlled at the receiver it
can be made large enough that the receiver noise is dominated by shot noise
σ2s >> σ2T when
more specifically
P LO >> σ
2
T
/( 2 eRB
IF
)
Under the same conditions, the dark current contribution to the shot noise is
negligible ( Id << RPLO). The SNR is then given by
SNR ≈
RPs
ηPs
=
eB hυBIF
(7 ) shot noise lim it for hrtrodyne det ection
The use of coherent detection allows one to achieve the shot–noise limit even for pi-n receiver whose performance is generally dominated by thermal noise moreover
in contrast with the case of APD receiver this limit is realized with out adding
excess shot noise.
It is useful to express SNR in terms of the number of photons N p received within
signal bit at the bit rate BT the average signal power Ps is related to Np as Ps =
NphνB. typically B ≈ BT/2. By using this value of Ps and B in eq. (7) the SNR is
given by a simple expression
SNR = 2η N p
For the case of homodyne detection SNR is larger by the factor of 2 and is given by
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Elec. & Comm.Dep.
Opt. Fiber Comm. Systems
Lecture Therteen
SNR = 4η N p
Coherent Detection Bandwidth Advantage
Coherent Versus Incoherent SNR
Modulation format
The instantaneous electric field of Unmodulated carrier may be represent as
E s ( t ) = A s cos( ω ο t + φ s )
Where
A s = carrier elecric field amplitude
ωο
= f c = carrier frequency
2π
φ ο = carrier phase angle
In the case of digital communication systems, the main modulation formats used
are:
1. ASK (Amplitude Shift Keying)
2. FSK (Frequency Shift Keying)
3.
PSK (Phase Shift Keying)
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Elec. & Comm.Dep.
1.
E
2.
Opt. Fiber Comm. Systems
Lecture Therteen
ASK (Amplitude Shift Keying)
s
(t) =
A
s
cos[
ω
ο
t + φ
s
]
FSK (Frequency Shift Keying)
E s ( t ) = A s cos[ ω ο t + ∆ ω + φ s ]
3.
PSK (Phase Shift Keying).
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Lecture Therteen
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