Post on 07-Jan-2023
Compromised extinction and signal-to-noise
ratios of weak-resonant-cavity laser diode
transmitter injected by channelized and
amplitude squeezed spontaneous-emission
Yi-Hung Lin1, Gong-Cheng Lin
2, Hai-Lin Wang
2, Yu-Chieh Chi
1, and Gong-Ru Lin
1
1Institute of Photonics and Optoelectronics, Department of Electrical Engineering, National Taiwan University
No.1 Roosevelt Rd. Sec. 4, Taipei 106, Taiwan R.O.C. 2 Telecommunication Laboratories Advanced Technology, Chunghwa Telecom Co., Ltd., Taoyuan, Taiwan R.O.C.
*grlin@ntu.edu.tw
Abstract: By using a 200GHz AWG channelized ASE source in connection
with a saturable semiconductor optical amplifier (SOA) based noise blocker
as the injecting source at the remote node in front of the local optical network
units (ONUs), we demonstrate the spectrum-sliced ASE transmitter with
greatly suppressed intensity noise performance in WDM-PON network. Such
channelized SOA filtering technique effectively reduces the relative intensity
noise of the ASE source by at least 4.5 dB. The low-noise WRC-FPLD
transmitter improves its extinction-ratio (ER) from 8.9 to 9.6 dB and
signal-to-noise ratio (SNR) from 5.9 to 6.3 dB. In comparison with
broad-band ASE injection-locked WRC-FPLD transmitter at same power,
there is an improvement on receiving power penalty (∆PReceiver) by 2 dB at
BER 10−9
in back-to-back case, and the receiving power of BER 10−9
can
achieve −24 dBm even after 25km fiber transmission. With additional AWG
filtering, the intraband crosstalk effect between the upstream transmitted data
and the reflected ASE signal is significantly reduced by 6.3dB. The
compromised effects of ER and SNR on BER performance are also
elucidated via the modified SNR model for the WRC-FPLD under ASE
injection induced gain-saturation condition. The ∆PReceiver/∆SNR of 8.89 at
same ER condition is more pronounced than the ∆PReceiver/∆ER of 3.17
obtained under same SNR condition, indicating that the SNR plays a more
important role than the ER on enhancing the BER performance.
©2009 Optical Society of America
OCIS codes: (060.2330) Fiber optics communications; (250.5980) Semiconductor lasers;
(140.3520) Lasers, injection -locked.
References and links
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(C) 2010 OSA 1 March 2010 / Vol. 18, No. 5 / OPTICS EXPRESS 4457#112911 - $15.00 USD Received 18 Jun 2009; revised 3 Sep 2009; accepted 18 Sep 2009; published 19 Feb 2010
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1. Introduction
The high bit-rate transmission of the wavelength-division-multiplexing passive optical network
(WDM-PON) is demanded due to the growing population in future broadband optical access
networks. Versatile cost-effective light sources were emerged to concurrently approach the
colorless injection-locking capability. A typical solution is the FPLD injected by broadband
ASE light source, however, its strong mode-extinction feature under the externally broadband
ASE injection not only opposes the wavelength independent criterion but also constrain the
broadband gain spectrum requirement [1,2]. Recently, an AR-coated weak-resonant-cavity
Fabry–Pérot laser diode (WRC-FPLD) with moderate front-facet reflectance is proposed to
perform the wavelength independent operation [3]. On the other hand, the ASE source
inherently suffers from large intensity noise (IN) caused by spontaneous-spontaneous beat
noise [4,5], such that the spontaneous–spontaneous beating noise injects into the FPLD, which
degrades the signal-to-noise ratio (SNR) and causes the penalty in receiving power for
obtaining up-stream transmitted data with sufficiently low bit-error-rate (BER). There has been
an approach to reduce ASE intensity noise by using a gain-saturated semiconductor optical
amplifier (SOA) [4–6] to filter the spectral-sliced ASE source before injection into an FPLD
[7]. On the other hand, another major reason leading to the limitation on transmission bit-rate up
to 2.488 Gbit/s in such a DWDM-PON is the intra-band crosstalk, which originates from the
interfered effect between the transmitted data and the reflections from the facets of array
waveguide grating (AWG) and feeder fiber patch-cord [8]. The crosstalk of optical reflection
caused by Rayleigh scattering, bad splicing connections, and ASE intensity noise from
spontaneous-spontaneous beat noise strongly affects the signal performance and network
capacity. In this work, we propose a novel DWDM-PON architecture consisting of an AWG
channelized and SOA filtered ASE source at the remote node in front of the local optical
network units (ONUs), which is employed to inject the WRC-FPLD transmitter with front-face
reflectance of 1% for increasing the direct modulation bandwidth and enabling the OC-48
transmission. Adding the AWG and SOA significantly reduces the optical reflection and
intensity noise of the injected ASE source when comparing with the conventional architecture.
When directly modulating the WRC-FPLD at 2.488 Gbit/s in such a DWDM-PON with AWG
bandwidth of 200 GHz, the proposed system benefits from a penalty of −1.8 dB on receiving
power at BER of 10−9
. Moreover, the correlation between SOA operation condition and the
SNR or extinction ratio (ER) are analyzed. The effects of ER and SNR on the BER performance
are also elucidated via the modified SNR model for the WRC-FPLD under ASE injection
induced gain-saturation condition.
(C) 2010 OSA 1 March 2010 / Vol. 18, No. 5 / OPTICS EXPRESS 4458#112911 - $15.00 USD Received 18 Jun 2009; revised 3 Sep 2009; accepted 18 Sep 2009; published 19 Feb 2010
2. Experimental setup
Most of previous works established the DWDM-PON system with broadband injecting source
at central office, as shown in Fig. 1(a). In contrast, the Fig. 1(b) schematically illustrates a
modified DWDM-PON system constructed by allocating the spectral-sliced and SOA-filtered
ASE source at remote node for injection-locked WRC-FPLDs at all ONUs. The external
injection-locking source is an Erbium-doped fiber amplifier (EDFA) based broadband source,
which filtered by 200 GHz AWG and then pass through SOA to inject the WRC-FPLD. The
WRC-FPLD exhibits a threshold current of about 25 mA, a longitudinal mode spacing of 0.6
nm, the back and front facet reflectivity of 100% and 1%. The maximum injection power of
WRC-FPLD is limited as −3 dBm due to the injection power too large would damage the
end-face AR coating of WRC-FPLD.
EDFA
Fig. 1. (a). The configuration a conventional ASE injecting transmitter wavelength independent
operation WDM-PON. (b)Configuration of the DWDM-PON with WRC-FPLD injection-locked
by the source of ASE through SOA at the end of remote node.
In our experiment, the operating current of the WRC-FPLD is detuned between 28 and 40
mA, correspond to a change from 1.1 Ith to 1.5 Ith. Latter on, the WRC-FPLD transmitter is
directly modulated at 2.488 Gbit/s with a NRZ pseudorandom bit sequence (PRBS) pattern
length of 223
-1. To suppress the intensity noise of ASE, the SOA has to be operated at high bias
and a large input power is necessary to ensure gain saturation. The maximum input power and
biased current of SOA are limited at −3 dBm and 350 mA, respectively, to prevent of the SOA
from damage.
3. Results and discussions
3.1 Effects of SOA and WRC-FPLD operating conditions on the up-stream transmitted data
performances (BER, SNR, and ER)
The power-current characteristics of the WRC-FPLD under different injection levels are also
shown in Fig. 2, which illustrates a reduction of threshold current by 6 mA with the external
injection power increasing by 9 dB. Note that when using the SOA filtered and spectrum-sliced
ASE source to injection-lock the WRC-FPLD, the threshold current is slight increased as
compared to that of the WRC-FPLD injected by spectrum-sliced ASE without SOA filter. The
SOA introduces additional phase modulation-induced chirp [9] and four-wave mixing (FWM)
effect, which inevibly broadens the spectra output of the AWG channelized ASE. The
broadened ASE spectrum remains the output power constant but attenuates the photon density
(C) 2010 OSA 1 March 2010 / Vol. 18, No. 5 / OPTICS EXPRESS 4459#112911 - $15.00 USD Received 18 Jun 2009; revised 3 Sep 2009; accepted 18 Sep 2009; published 19 Feb 2010
at specific wavelength, such that the equivalent optical intensity induced threshold current of
WRC-FPLD will be less affected accordingly. The WRC-FPLD output power can be increased
by at least 3 dB with its threshold current decreasing from 26 to 17 mA under enlarged ASE
injection. It is also important to improve the SNR and ER of the WRC-FPLD transmitted data
under the SOA filtered and AWG-sliced ASE injection with increasing power level.
0 10 20 30 40-0.5
0.0
0.5
1.0
1.5
2.0
2.5
3.0
3.5
4.0
Po
wer
(mW
)
Current (mA)
ASE injection -3dBm
ASE through SOA injection -3dBm
ASE injection -12dBm
ASE through SOA injection -12dBm
No injection
Fig. 2. Power-current curves of WRC-FPLD operated without and with injection power of −12
and −3 dBm.
-14 -12 -10 -8 -6 -4 -22.0
2.5
3.0
3.5
4.0
4.5
5.0
5.5
6.0
6.5
7.0
SN
R o
f S
OA
Ou
tpu
t S
ign
al
(dB
)
Input SOA Power (dBm)
Ibias
= 350mA Ibias
= 300mA
Ibias
= 250mA Ibias
= 200mA
Fig. 3. SNR of AWG-sliced ASE without (pink-dotted) and with SOA based filter at different
biased currents.
By externally modulating AWG spectrum-sliced and SOA filtered ASE, the SNR of the data
stream at 2.488 Gbit/s reveals an increasing trend with enlarging bias of SOA and increasing
power of ASE. Without the SOA based noise blocker, the transmitted data exhibits a SNR as
low as 2.3 dB. The SNR of the transmitted data can be improved by 0.8 dB if the SOA bias
increases from 200 to 350 mA. A further increment up to 1.2 dB can be done by increasing ASE
power up to −3 dB. When SOA is operated at 350 mA and the input power is −3 dBm, the SNR
of the transmitted data greatly improves from 2.3 to 6.8 dB, as shown in Fig. 3. Adding the SOA
based noise blocker essentially improves the SNR of upstream data from 6.0 to 6.3 dB. In
general, the SNR of the WRC-FPLD is correlated with the integral of its relative-intensity-noise
(RIN) spectrum by [10]
1
21( ) (1)
2SNR RIN dω ω
π
−+∞
−∞
≅
∫
Technically, the RIN suppression can be done by adding a SOA after the spectrum-sliced
ASE channel prior to injection-lock the WRC-FPLD. The RIN suppression ratio in decibel unit
is theoretically given by [6]
(C) 2010 OSA 1 March 2010 / Vol. 18, No. 5 / OPTICS EXPRESS 4460#112911 - $15.00 USD Received 18 Jun 2009; revised 3 Sep 2009; accepted 18 Sep 2009; published 19 Feb 2010
0 0 00 0
20 0
' ( ) ' ( )1 ' ( )( ) ( ) ( )
(2)' ( ) ' ( )1 1
( ) ( )
c
c c
g P z g P z g P zg N g N
h A h A h A
g P z g P z
h A h A
ν τ ν νη
τ ν τ ν
Γ Γ ΓΓ + Γ∝ =
Γ Γ+ +
where Γ is the mode confinement, g(N) is the gain coefficient, αint is the internal loss, J is the
injected current density, d is the active layer thickness of the SOA, A is the active region area, q
is the electronic charge and τc is the carrier lifetime. ω is the angular frequency at which the
perturbing signal varies, g' is the differential gain coefficient, P0, and N0 are the time-averages
of the output power and equivalent carrier density, and ∆P(ω) and ∆N(ω) stand for the noise.
Equation (2) shows that P0, Γg(N0) and Γg’ must be extremely large for obtaining high SNR.
For a given SOA, P0 and Γg(N0) can be increased by enlarging the input power Pin and the
biased current Ib. In experiment, we have measured the RIN spectra of the spectrally sliced ASE
source without and with additional SOA based noise blocker, and the WRC-FPLD under
free-running and injection-locking conditions, as shown in Fig. 4.
0 1 2 3 4 5 6
-138
-136
-134
-132
RIN
(d
B/H
z)
Frequency (GHz)
Free-running WRC-FPLD
ASE
SOA+ASE
SOA+ASE+WRC-FPLD
Fig. 4. Measured the RIN of the WRC-FPLD injection locked by different light sources.
-33 -30 -27 -24 -21 -18 -1512
11
10
9
8
7
6
5
-Lo
g (
BE
R)
Receving Power (dBm)
SOA Ibias
= 350mA
SOA Ibias
= 250mA
SOA Ibias
= 81mA
ISOA
Fig. 5. WRC-FPLD up-stream BER under the injection of AWG-sliced ASE with SOA filter at
different biases.
The original RIN of the spectrally sliced ASE source exhibits a RIN of −135 dB/Hz below 6
GHz. By adding the SOA based noise blocker, it is clearly seen that the RIN of the ASE source
decreases by 2 dB at f<3 GHz. With such an SOA filtered ASE injection, the WRC-FPLD
greatly reduces its RIN from −133 dB/Hz (free-running case) to −138 dB/Hz. These results have
elucidated that the WRC-FPLS essentially reduces its RIN by at least 4 dB after
injection-locking with the SOA filtered ASE source, such that the transmission performance of
the WRC-FPLS can further be improved. The BER of the upstream transmitted data from
WRC-FPLD injection-locked by the ASE with SOA filter at different biased currents are
analyzed in Fig. 5. When the SOA biased current is enlarged from 81 to 350 mA, the BER error
(C) 2010 OSA 1 March 2010 / Vol. 18, No. 5 / OPTICS EXPRESS 4461#112911 - $15.00 USD Received 18 Jun 2009; revised 3 Sep 2009; accepted 18 Sep 2009; published 19 Feb 2010
floor is decreased from 10−9
to 10−11
, reflecting that the RIN of the WRC-FPLD is significantly
diminished under SOA filtered ASE injection.
-32 -30 -28 -26 -24 -22 -20 -18 -16
12
11
10
9
8
7
6
5
4
3
-lo
g(B
ER
)
Receiving Power (dBm)
(a)
(b)(c) (d)
Fig. 6. BER and corresponding eye-diagrams of the SOA filtered ASE injection-locked
WRC-FPLD at different biases of (a) 28 mA, (b) 32 mA, (c) 36 mA, (d) 42 mA.
The WRC-FPLD biased current dependent BER analysis at bit rate of 2.488 Gbit/s is shown
as Fig. 6. The BER performance is greatly improved as the WRC-FPLD biased current
increases from 28 to 36 mA (i.e., from Ith to 1.4 Ith), however, which turns to be degraded when
the WRC-FPLD bias further increases to 40 mA or larger. At optimized bias of 36 mA, the
receiving power required for the up-stream transmitted data is −26 dBm at BER of 10−9
. At
lower biased condition, the SNR of WRC-FPLD transmitted data predominates the BER
performance, and there is a positive contribution of biased current for enhancing the SNR as
well as BER. Nonetheless, the ER becomes more pronounced than SNR to degrade the BER
performance as the WRC-FPLD biased current increases beyond 36 mA, while the ER
significantly degrades at higher biased current to introduce a positive power penalty of 4 dB at
BER of 10−9
.
28 30 32 34 36 38 408
9
10
11
12
13
4.0
4.5
5.0
5.5
6.0
6.5
ER
(d
B)
WRC-FPLD Biased Current (mA)
SN
R (
dB
)
Fig. 7. SNR and ER of the ASE injection-locked WRC-FPLD at different currents.
(C) 2010 OSA 1 March 2010 / Vol. 18, No. 5 / OPTICS EXPRESS 4462#112911 - $15.00 USD Received 18 Jun 2009; revised 3 Sep 2009; accepted 18 Sep 2009; published 19 Feb 2010
28 30 32 34 36 38 405.4
5.6
5.8
6.0
6.2
6.4
Q F
acto
r
WRC-FPLD Biased Current
BER of 10-9
Fig. 8. The calculated Q factor of WRC-FPLD at different currents
Figure 7 illustrates the SNR and ER as a function of the WRC-FPLD biased current.
Apparently, the SNR increases 5.1 to 6.4 dB as the WRC-FPLD gain enlarges when increasing
its bised current from 28 to 40 mA. However, the lowest biased current (28 mA) can provide the
highest ER of 13.5 dB for the WRC-FPLD transmitted data, whereas the ER oppositely
degrades to 8.2 dB at biased current of 40 mA. In experiment, we observe that the effect of the
ER on BER performance can only be neglected if the ER of the WRC-FPLD tansmitted data is
larger than 9 dB, and SNR becomes dominated at highly biased current. Theoretically, the Q
parameter which decides the lowest BER achieved in a communication system can be described
in terms of Q = [(ER-1)/(ER + 1)](M⋅SNR)0.5
with M denoting the gain of the optical receiver.
With increasing WRC-FPLD bias, the factor of (ER-1)/(ER + 1) is decreased from 0.89 to 0.74,
where the (SNR)1/2
is only improved from 1.6 to 2.1. That is, the Q parameter behaves like a
nonlinear function of WRC-FPLD bias, as shown in Fig. 8.
3. 2 Theoretical and experimental analyses on the ASE injection power dependent SNR, ER, and
BER of WRC-FPLD up-stream data
To investigate the effects of ASE injecting power and reflected ASE signal on the WRC-FPLD
transmitted upstream data, we further consider the SNR and ER of the injection-locked
WRC-FPLD under gain-saturation condition. Originally, the SNR of WRC-FPLD under
external injection is given by [10]
22
2 2
( )SNR = = , (3)
4
inj inj
out
sp
RGP GPI
S fσ σ< >
≈∆
where Pinj is the injection power, G is the optical gain, ∆f is the detector bandwidth, Ssp is the
spectral density of spontaneous-emission induced noise. Due to the gain saturation of the
WRC-FPLD under ASE injection (occurred at Psat > −7 dBm), the power gain of WRC-FPLD is
rewritten as
0
( ) 1exp[ ] (4)
( )
out inj out
out inj sat
P P PG G
P P P
−=
However, the power gain of WRC-FPLD is greatly modified under the ASE external
injection-locking condition as [11–13]
( )
2 2
0 2
41(5)
1
sp injc c
p m p mc
R h PkG G G
P P
ν βτ τ β
= −∆ = − −+
(C) 2010 OSA 1 March 2010 / Vol. 18, No. 5 / OPTICS EXPRESS 4463#112911 - $15.00 USD Received 18 Jun 2009; revised 3 Sep 2009; accepted 18 Sep 2009; published 19 Feb 2010
where G0 is the WRC-FPLD gain at free-running case, ∆G is the gain variation due to the SOA
filtered and AWG-sliced ASE injection, τp is the photon lifetime, Rsp denotes the spontaneous
emission rate, Pm denotes the total power at the injection-locked mode, βc denotes the linewidth
broadening factor, kc denotes the coupling coefficient of the WRC-FPLD for external injection,
and Pinj is the externally ASE injected power.
( )
( )
( )
, ,
2 2
2
,[ ] (6)
g'
1
'
4
' 1
out d th inj d th inj
i c
inj inj
d tr
i c
tr
i c p
d
sp injc c
i c m p mc
qh hP I I I N
q q
G N PqhI N
q
qI N
gh
q R h Pkq
g P P
ν νη η
η τ
νη
η τ
η τ τν
ην β
η τ τ β
= − = −
= − +
− + +
= + +
where I and Ith are the bias and threshold currents of WRC-FPLD, Ntr is the transparent carrier
number, g’ is the differential gain coefficient for ∆N denoting as the variation of carrier
numbers, ηd is the differential quantum efficiency, hν is the energy per photon, ηi denotes the
internal quantum efficiency, and τc denotes the carrier lifetime. When the AWG-sliced ASE
injection-locked WRC-FPLD is operated in linear-gain region, the Eqs. (3) and (5) are
combined to give the SNR as
( )
2 2
2
41SNR (7)
4 1
inj sp injc c
out
sp p m p mc
P R h Pk
S f P P
ν βτ τ β
≈ − −
∆ +
If the WRC-FPLD is operated at gain-saturated condition, the SNR is modified using Eqs.
(3)-(6), as described by
( )
( )
2 2
2
2 2
2
41 1SNR
4 ' ' 1
1
'1
exp4
' 1
sp injc c
out d tr
sp i c p i c m p mc
tr
i c p
d
sat sp injc c
i c m p mc
R h Pkq qhI N
S f q g g P P
qI N
gh
P q R h Pkq
g P P
ν βνη
η τ τ η τ τ β
η τ τνη
ν βη τ τ β
≈ − + + + ∆ +
− + +
+ +
(8)inj
P
−
As a result, the measured SNR and ER of the back-to-back transmitted WRC-FPLD up-stream
data are shown in Fig. 9. The SNR linearly increase with injection power, but saturate at high
injection level. At the same time, the P-I curve of WRC-FPLD illustrates a decreasing trend on
its threshold current with increasing ASE injection power. Note that the power-current slope of
the ASE injection-locked WRC-FPLD almost remains as constant with varying ASE injection
power, indicating that the reshaping on rising and falling edge by changing ASE injection
power is negligible. If we fix the WRC-FPLD bias and the PRBS NRZ amplitude, the
“off-state” level of upstream transmitted data will increase with larger ASE injection power,
whereas the “on-state” level remains almost unchanged. Thus, the ER monotonically decreases
with increasing ASE power when the WRC-FPLD enters into gain-saturation condition.
(C) 2010 OSA 1 March 2010 / Vol. 18, No. 5 / OPTICS EXPRESS 4464#112911 - $15.00 USD Received 18 Jun 2009; revised 3 Sep 2009; accepted 18 Sep 2009; published 19 Feb 2010
-12 -10 -8 -6 -4 -28.5
9.0
9.5
10.0
10.5
11.0
11.5
12.0
12.5
13.0
4.0
4.5
5.0
5.5
6.0
6.5
ER
(d
B)
ASE Injection Power (dBm)
New system
New system without SOA
Conventional system
SN
R (
dB
)
Fig. 9. SNR and ER of WRC-FPLD injection-locked by ASE with changing power levels in
different systems.
-32 -30 -28 -26 -24 -22 -20
12
11
10
9
8
7
6
5
Conventional system after 25km
New system without SOA after 25km
New system after 25km
Conventional system BTB
New system without SOA BTB
New system BTB
-lo
g(B
ER
)
Receiving Power (dBm)
Fig. 10. BER of filtered ASE with SOA (new system), filtered ASE (new system without SOA)
and broadband ASE injection-locked WRC-FPLD based WDM-PON (inset: back-to-back eye
diagram of (a) old system, (b) new system without SOA, (c) new system with SOA).
In comparison with conventional WRC-FPLD based DWDM-PON, the use of additional
200 GHz AWG placed after the ASE source effectively suppress the crosstalk and interfered
reflection between the upstream transmitted data and reflected ASE signal. Such a broadband
reflection from between the based DWDM Mux and DeMux is greatly reduced from −14.7 to
−21 dBm with a decreasing of 6.3 dB. We also find that the Schawlow-Towne’s linewidth
(∆ωst) becomes narrower in the new system even without SOA. Lower mode beating noise is
obtained as the mode extinction becomes significant by reducing the ASE reflection level when
comparing with conventional injection-locking scheme (with the ASE located in OLT) [14].
Due to the reduction of noise floor and mode beating noise, both the SNR and ER are improved
from 5.9 to 6.1 dB and from 8.9 to 9.3 dB, respectively. It is clear seen that adding the SOA
based noise blocker can further improve the SNR of upstream data by 0.3 dB. Moreover, the
“off-state” level of upstream data will decrease by slightly increasing the threshold current of
WRC-FPLD, such that the ER can essentially be ehanced from 9.3 to 9.6 dB. Apparently, the
combining effect of the 200 GHz-AWG based spectral slicer and the SOA based noise blocker
located after the ASE source essentially improves the ER from 8.9 to 9.6 dB and SNR from 5.9
to 6.3 dB. At ASE injection power of −3 dBm, the BER performances of the ASE
injection-locked WRC-FPLD transmitter with three kinds of ASE sources located at different
(C) 2010 OSA 1 March 2010 / Vol. 18, No. 5 / OPTICS EXPRESS 4465#112911 - $15.00 USD Received 18 Jun 2009; revised 3 Sep 2009; accepted 18 Sep 2009; published 19 Feb 2010
nodes are compared in Fig. 10. At BER of 10−9
, the newly proposed system shows a smallest
receiving power sensitivity of −26 dBm, and there is a negative power penalty of 1.8 dB as
compared to the conventional system in back-to-back case. After 25-km transmission in
single-mode fiber, the receiving power of BER 10−9
degrades to −23.8 dBm, whereas the system
withput SOA or the conventional system fails to achieve same BER level. The additional SOA
filtering leads to a negative power penalty by 1 dB due to the improved SNR and ER under the
reductions of crosstalk and mode beating noise.
3. 3 Distinguished influence of SNR and ER to the BER performance of the WRC-FPLD
transmitted up-stream data
To discriminate the individual contribution of ER and SNR to the BER, we adjust the ASE
injection power and the WRC-FPLD biased current to detune ER and SNR of upstream
transmitted data. By keeping the ER or SNR as constant, we investigate the effect of another
parameter on the BER, as shown in Fig. 11. Under the same ER of 10 dB, the receiving powers
of BER at 10−9
are required to exceed −26 and −18 dBm at SNR of 6.3 and 5.3 dB, respectively.
With constant SNR of 6.3 dB, the requested receiving powers of BER at 10−9
are enlarged from
−26 to −22 dBm when ER is degraded from 10 to 8.6 dB. As a result, the sensitivity slopes for
SNR and ER are determined as ∆PReceiver/ ∆SNR = 8.9 and ∆PReceiver / ∆ER = 3.2, which clearly
elucidate that the SNR plays a more important role than ER on BER performance.
-33 -30 -27 -24 -21 -18 -1512
11
10
9
8
7
6
5
4
-Lo
g (
BE
R)
Receving Power (dBm)
SNR=6.3 dB, ER=10 dB
SNR=5.3 dB, ER=10 dB
-33 -30 -27 -24 -21 -18 -1512
11
10
9
8
7
6
5
4
-Lo
g (
BE
R)
Receving Power (dBm)
SNR = 6.3 dB, ER = 10 dB
SNR = 6.3 dB, ER = 8.6 dB
Fig. 11. BER analysis of AWG-sliced and SOA-bleached ASE injection-locked WRC-FPLD
transmitter with changing SNR (left) and changing ER (right).
-33 -30 -27 -24 -21 -18 -1512
11
10
9
8
7
6
5
(b)
(c)
-Lo
g (
BE
R)
Receving Power (dBm)
(a)
Fig. 12. BER of the WRC-FPLD transmitted data in the DWDM-PON systems with changing
ASE-Demux-Mux linewidths of (a) 1.0-1.0-1.0 nm, (b) 0.5-1.0-1.0 nm, and (c) 0.5-0.5-0.5 nm.
Moreover, we also observe the biased current of WRC-FPLD is more pronounced than the
injection power for ER and SNR of the upstream transmitted data. Apparently, the currents of
SOA and WRC-FPLD are the most important parameters to achieve better transmission
performance of the upstream data. In addition, it is worthy noting that the ASE channel
linewidth filtered by AWG also affects the WRC-FPLD signal quality. In the experiment, the
channel linewidth of 100 and 200 GHz AWG are 0.51 nm and 1.03 nm individually. In back-to
(C) 2010 OSA 1 March 2010 / Vol. 18, No. 5 / OPTICS EXPRESS 4466#112911 - $15.00 USD Received 18 Jun 2009; revised 3 Sep 2009; accepted 18 Sep 2009; published 19 Feb 2010
back case, the BER analysis of upstream transmitted data through the AWG with different
channel spacing is shown in Fig. 12. When the 200GHz AWG is replaced by a 100 GHz one to
filter ASE, the error floor of –log (BER) is degraded from 10.8 to 8.5. In the mean time, the
SNR is degraded from 6.3 to 5.4 dB and ER is increased from 9.6 to 10 dB concurrently. In
general, the EDFA or SOA based ASE light source exhibits two different noises, including the
signal-spontaneous beat noise, and the spontaneous-spontaneous beat noise, which
concurrently cause the degradation on the SNR of signals. In our case of the injection-locked
WRC-FPLD with external ASE injection, the signal-spontaneous beat noise no longer sustains
but the ASE-ASE beat noise injects into the WRC-FPLD and affects the noise performance of
WRC-FPLD. The equation of SNRASE is defined as SNR = P2ASE/(P
2sp-sp + P
2shot + P
2thermal) [15],
where PASE is the ASE average power, Psp-sp is the spontaneous to spontaneous beat noise
average power, Pshot is the shot noise average power, and Pthermal is the thermal noise average
power. The shot noise and thermal noise are ignored as they are usually not in optical path. Due
to the relationship of P2sp-sp = 2P
2ASEBe/m∆λ, the SNR can be rewritten as SNR = m∆λ/2Be,
where Be is the electrical bandwidth of the optical receiver, m is the number of polarization, and
∆λ is the spectral linewidth of the ASE injection-locked WRC-FPLD transmitter. It is straight
forward that the SNR as well as BER can be degraded by decreasing the spectral linewidth of
the injected ASE source. Therefore, if we employ the AWGs with channel spacing of 100 GHz,
it is clearly seen that the BER will be seriously degraded with a BER error floor as high as
4x10−7
. If we further release the transmission channel linewidth in the DWDM-PON by using a
200 GHz AWG based Mux and Demux with spectral window of 1.0 nm (twice larger than that
of a 100 GHz AWG based ones). The BER error floor of –log(BER) is greatly reduced from 6.5
to 8.4, while the SNR is enhanced from 4.2 to 5.5 dB and the ER is increased from 9.2 to 10 dB
concurrently. In comparison, the broadening in ASE filter linewidth further improves the BER
error floor of the AWG channelized and SOA filtered the ASE injection-locked WRC-FPLD
upstream transmitted data by four orders of magnitude down to 10−11
. That is, there is an
extremely large power penalty up to 12 dB when reducing the ASE spectral linewidth from 1.0
to 0.5 nm before injection-locking the WRC-FPLD. This again corroborates that the SNR
becomes the more pronounced effect than ER for promoting the BER performance. In addition,
it is also concluded that the AWG channelized ASE injection-locked WRC-FPLD transmitter
fails to promote the channeling capacity of the DWDM-PON system from 200 GHz to 100
GHz. Even the origined ASE spectral linewidth is broadened to 1.0 nm, the upstream
transmission linewidth is still constrained by the AWG based Mux/Deux channel window. This
eventually leads to the degradation of the receiving power sensitivity increasing from −27 to
−15 dBm at BER if 10−9
. From the ∆SNR, ∆ER and BER of this experiment, we conclude the
linewidth of AWG before WRC-FPLD has dominated effects on upstream signal quality, and
the SNR plays a more important role than ER on the WRC-FPLD upstream data quality as well.
4. Conclusion
By using a 200GHz AWG channelized ASE source in connection with a saturable
semiconductor optical amplifier (SOA) based noise blocker as the injecting source at local
ONU part, we demonstrate the spectrum-sliced ASE injection-locked WRC-FPLD transmitter
directly modulated at 2.488 Gbit/s with greatly suppressed intensity noise performance in a
200GHz channelized DWDM-PON network. The experiments show that the low noise
WDM-PON improves ER from 8.9 to 9.6 dB and SNR from 5.9 to 6.3 dB. In comparison with
conventional (broad-band) ASE injection-locked WRC-FPLD transmitter at same power, there
is an improvement on receiving power penalty (∆PReceiver) by 2 dB at BER 10−9
in back-to-back
case, and the receiving power of BER 10−9
can achieve −24 dBm even after 25km fiber
transmission. Besides, with the additional AWG filtering, the crosstalk effect between the
upstream transmitted data and the reflected ASE signal can be great reduced by 6.3 dB. The
compromised effects of ER and SNR on BER performance are also elucidated via the modified
SNR model for the WRC-FPLD under ASE injection induced gain-saturation condition. The
(C) 2010 OSA 1 March 2010 / Vol. 18, No. 5 / OPTICS EXPRESS 4467#112911 - $15.00 USD Received 18 Jun 2009; revised 3 Sep 2009; accepted 18 Sep 2009; published 19 Feb 2010
∆PReceiver/∆SNR of 8.89 at same ER condition is more pronounced than the ∆PReceiver/∆ER of
3.17 obtained under same SNR condition, indicating that the SNR plays a more important role
than the ER on enhancing the BER performance. In addtion, we research the impacts of the
AWGs channel linewidth for the WDM-PON. From the results, we successfully find the
currents of SOA and WRC-FPLD are the most important parameters for upstream signal and
the linewidth of AWG before WRC-FPLD has larger effects than the AWGs after WRC-FPLD
on upstream signal quality in this WDM-PON.
Acknowledgment
Financial support by the National Science Council of Taiwan Republic of China under grants
NSC98-2221-E-002-023-MY3 and NSC 98-2623-E-002-002-ET are acknowledged.
(C) 2010 OSA 1 March 2010 / Vol. 18, No. 5 / OPTICS EXPRESS 4468#112911 - $15.00 USD Received 18 Jun 2009; revised 3 Sep 2009; accepted 18 Sep 2009; published 19 Feb 2010