A Bidirectional CWDM-PON System with Capacity of 40-Gb/s for Metro/Access Applications
Pei-Hao Tseng, and Wood-Hi Cheng
Department of Photonics, National Sun Yat-sen University, Kaohsiung, Taiwan
No. 70, Lienhai Rd., Kaohsiung 80424, Taiwan, R.O.C
Tel. (886) 7-525-2000 ext. 4450, Fax (886) 7-525-4499, E-mail: whcheng@mail.nsysu.edu.tw
Abstract
Using distributed feedback laser diode (DFB-LD) in a
high-speed transmission system, the chirp effect in the LD is a
factor limiting transmission span length. We discussed the
impact of fiber chromatic dispersion on this system
performance at 10-Gb/s due to LD chirp effect. For the
dispersion range of -3.51 to 2.97 ps/nm·km, negative power
penalties have been observed due to the pulse compression
effect, which is resulted from a low chirp of DFB-LDs. We
evaluated 10-Gb/s directly modulated DFB-LDs wavelengths
from 1275-nm to 1350-nm of power penalties versus
transmissible standard single-mode fiber (SMF) distance. The
calculated power penalties are less than 1 dB for 20-km SMF
transmission. Furthermore, we also achieve a bidirectional
CWDM-PON system covering a wide wavelength of 1.3-μm
and 1.5-μm with an aggregated data rate of 40-Gb/s is
proposed and experimented. Four uncooled direct-modulated
DFB-LDs with wavelengths of 1275-nm, 1300-nm, 1325-nm,
and 1350-nm are adopted for the upstream transmission to
lower the installation cost and dispersion effect. Another four
external-modulated lasers by LiNbO3 modulators with
wavelengths of 1510-nm, 1530-nm, 1550-nm, and 1570-nm
are deployed for the downstream transmission to minimize the
chirp effect. These lasers are operated at 10-Gb/s
simultaneously for transmission over 10-km SMF.
Downstream and upstream data of high capacity are operated
at 10-Gb/s per channel simultaneously. This CWDM-PON
network enables client’s maximum subscribing bit rate to 10Gb/s and the splitting ratio can vary from 1:4 to 1:64.
1. Introduction
Passive optical networks (PONs) are attractive for the
growth of broadband networks due to the high security, the
protocol-independent technology, the low-cost operational
feasibility, the capability of handling various services and
supporting numerous subscribers [1-3]. However, WDM-PON
can support a higher bandwidth, but it needs the careful
wavelength control, expensive arrayed waveguide grating
(AWG), and numerous optical circulators. Furthermore, the
maximum subscribing bit rate of client is limited at gigabit
[4 ,5]. To increase the transmission bandwidth and maintain
the cost-effective performance, a coarse wavelength division
multiplexing PON (CWDM-PON) has been proposed and
characterized in this study.
In addition, downstream channels will employ light
sources consisting of externally modulated narrow-linewidth
tunable CW lasers, which should be capable of transmitting
hundreds of kilometers unrepeatered even in the presence of
large chromatic dispersion. Due to their nearly ideal
electrooptic characteristics, externally modulated 1.5-μm
lasers are expected to be the technology of choice in these
long unrepeatered system [6]. When using 1.5-μm wavelength
chirp-free transmitter, the transmission distance at 10-Gb/s is
limited to about 100-km of standard SMF due to fiber
chromatic dispersion [7]. For upstream channels of 1.3-μm,
the transmission distance is mainly limited by DFB-LDs
optical fiber loss and chirp power penalty. The former
limitation can be overcome with use of 1.3-μm semiconductor
laser amplifiers [8, 9].
This proposed CWDM-PON adopts a bidirectional
transmission structure with four wavelengths distributed from
1510-nm to 1570-nm for downstream channels and another
four wavelengths distributed from 1275-nm to 1350-nm for
upstream channels. Every channel is operated at 10-Gb/s and
the aggregated data rate is symmetric 40-Gb/s for both
directions in a SMF. Due to the huge investment by the
industry and the matureness of 0.13-μm CMOS processes, the
10-Gb/s module has been propelled along a steep cost-volume
curve. We proposed to transmit a 10-Gb/s signal through
every channel to gain the maximum cost-effective benefit. By
combining the low-cost CWDM architecture and the matured
10-Gb/s technology, the CWDM-PON access network will be
potential to apply at a large bandwidth demanding area in the
near future.
2. Characteristics discussion of modulated source
In this paper, we will focus our discussion on the
importance of power penalty and transmission distance that
externally modulated lasers and directly modulated lasers
operated at 10-Gb/s in unrepeatered system for operation in
the 1.3-μm and 1.5-μm wavelength regions. Usually, a highbit-rate and long-reach transmission presents dispersionrelated impairments. Therefore the downstream signal was
designed to be carried by an external modulation source. The
client side can not afford the costly external modulation
source but must adopt the direct modulation DFB-LD to lower
the cost. This trend is especially obvious for the passive
optical network (PON) system.
2.1 Dispersion penalties for externally modulated lasers
A low-chirp 1.5-μm externally modulated laser source is
the most applicable to unrepeatered transmission over 50-km.
A CW laser source has high power to compensate for the
excess modulator loss and narrow laser linewidths to limit
modulated laser sources are easy to accomplish that they
install in Metro/Access applications of transmission distance
between 10-km and 40-km. For a 2-dB penalty the maximum
modulation rate ranges from 5-Gb/s to 10-Gb/s for system
operating at 1.55-μm with 15ps/km·nm of chromatic
dispersion and 100-km of fiber [10].
2.2 Dispersion penalties for directly modulated lasers
The directly modulated lasers are chirped by virtue of the
transient variations in the injected carrier density. Although
the standard SMF exhibits a zero dispersion point at the 1.31μm wavelength, the power penalty induced by the chromatic
dispersion is a critical issue for the upstream signal around
1.31-μm in the case of optical access network applications
running at 10-Gb/s [11]. The design limitations of upstream
channels are best seen when the resultant power penalty is
evaluated as following analysis. The expressions developed
for the dispersion power penalty were applied to examine the
limitations imposed on upstream transmission. For the
calculation of the group delay difference between
wavelengths, the chromatic dispersion coefficient is obtained
from the following equation (1) [13]. The behavior of the
linear material dispersion coefficient D for a typical singlemode fiber is shown in Fig.1. The wavelength λ0 is
approximately 1.314-μm, and the dispersion at 1.5-μm is
approximately 13.85 ps/km·nm to 17.2 ps/km·nm.
1 d  2 d 
(1)
D

2  c d  d 
where c and λ are the velocity of light in free space and the
wavelength of the LD, respectively. The chromatic dispersion
coefficients are calculated to be the values of -3.51105, 1.22359, 0.93428, and 2.97435 ps/km·nm for the wavelengths
of 1275-nm, 1300-nm, 1325-nm, 1350-nm, respectively. The
dispersion power penalty can be approximated as the
following expression.
0.125nm, and the transmitted bit-rate is set at 10-Gb/s. In Fig.
2, the calculated power penalties with different transmission
distances at 10-Gb/s are plotted for the DFB-LDs wavelengths
distributed from 1275-nm to 1350-nm. It is shown that the
dispersion power penalty is less than 1dB for a 20-km SMF
transmission.
Fig. 2. The calculation of power penalty versus transmission
distance for upstream wavelengths.
3. The CWDM-PON architecture and experimental setup
3.1 The proposed CWDM-PON architecture
Fig.1. The dispersion coefficient versus wavelengths for a
standard single-mode fiber.
In order to evaluate the chirp effect, the calculated chirp
power penalties due to LD chirp and fiber chromatic
dispersion was analyzed by Ymamoto, et al. [12, 14]. When
the chirp effect is small, the dispersion power penalty (P) can
be approximated as the following expression.

  2B 

4 2
P  20 log 1  (
 8) Dc t c B 2 L 1  
( Dc L  t c ) 
3
3





Where the Δλc is the spectrum linewidth of the DFB-LD, tc is
the chirp occurrence time of the DFB-LD, B is the transmitted
bit-rate, and L is the transmission distance. The chirp
occurrence time of the DFB-LD (tc) is assumed to be 50 ps
[15], the spectrum linewidth of the DFB-LD is assumed to be
The proposed architecture of 4-channel 40-Gb/s
bidirectional CWDM-PON system is shown in Fig. 3 The
system utilizes CWDM architecture, a precise wavelength
control for the transmitter, multiplexer and demultiplexer is
not required, and this will greatly reduce the system cost. The
system in Fig. 3 can be divided into center office side, remote
node, and the client side. Externally modulated technique is
adopted for the downstream transmission of four channels to
minimize the chirp effect in the single mode fiber. 10-Gb/s
directly modulated distributed feedback laser diode (DFB-LD)
is built in the optical network unit (ONU) transmitting the
upstream signal. The remote node is used to demultiplex the
downstream channels, multiplex the upstream channels and
split the network to the client side. A 4-channel multiplexer, a
4-channel demultiplexer, and five WDM couplers are needed
in the remote node. The network has the flexibility to adjust
subscribing bit rate of client from gigabit to 10-Gb/s and the
split ratio can vary from 4 to 64. The insertion loss of each
channel at the remote node can be controlled easily at 2 to 3
dB with commercial products. A 1:32 optical splitter is
installed at the end of each channel. The client side is
basically functioned by a 10-Gb/s bidirectional optical
transceiver. A 10-Gb/s directly modulated DFB-LD is built in
the optical transceiver to transmit the upstream signal [16]. By
using a cost-effective CWDM technique, the benefit includes
the larger bandwidth and the flexibility of adjusting the
maximum bit rate of a client to 10-Gb/s.
3.2 The experimental setup of CWDM-PON system
We illustrated the arrangement of the experimental setup.
A schematic diagram of the measured system is shown in Fig.
4. Four 1.5-μm tunable lasers with wavelength were setting at
1510nm, 1530nm, 1550nm, and 1570nm modulated by four
LiNbO3 modulator with 10-Gb/s NRZ data pseudorandom
binary sequences of the pattern length 231-1 and mutiplexed by
a CWDM multiplexer to simulate the downstream
transmission. Four 10-Gb/s directly modulated DFB-LDs with
wavelength of 1275-nm, 1300-nm, 1325-nm, and 1350-nm at
an interval of 25-nm were connected with the CWDM
multiplexer to simulate the upstream transmission. A 10-Gb/s
optical receiver (PIN-TIA) was connected at the CWDM
demultiplexer of node C and node D to receive downstream
and upstream signals, respectively. The transmission distance
between node A and node B is 10-km SMF. The splitting
network was not deployed in the experimental setup due to the
poor receiving sensitivity of the self-made optical receiver.
However, the effect of the splitting network can be estimate
easily by calculating the power penalties and power budgets.
4. Measurement results
Fig. 3. A schematic diagram of the proposed 40-Gb/s bidirectional CWDM-PON system.
4.1 Downstream transmission results
Fig. 5 and Table 1 show the transmission characteristics
for downstream channels. The optical powers of the
downstream channel were from -3.70 to -3.61 dBm and
launched into the network simultaneously. Fig. 5 shows the
received and reshaped eye patterns after 10-km SMF
transmission by self-made PIN-TIA optical receiver. The OC192 eye mask margins distributed from 25% to 28%, which
were improved by the limiting amplifier. The jitters were from
24.3 to 26.0 psec, which were degraded by the dispersion
effect in the SMF and the optical receiving circuit. Values of
the receiving power, the OC-192 eye mask margin, and the
jitter for each channel are summarized in Table1.
Fig. 5. (a)1510nm, (b)1530nm, (c)1550nm, (d)1570nm. The
measured eye pattern of downstream channels for10-km SMF.
Table 1. Measured parameters of eye pattern for 10-km SMF.
Fig. 4. The experimental setup of 40-Gb/s bi-directional
CWDM-PON system.
Channels
Power (dBm)
Mask margin (%)
Jitter p-p (psec)
1510nm
-8.64
26
25.9
1530nm
-9.17
28
24.3
1550nm
-9.74
27
26.0
1570nm
-10.41
25
25.15
In the downstream channels, the optical sensitivity was
measured by connecting the output port of the CWDM
demultiplexer with a variable optical attenuator and a selfmade optical receiver. Optical sensitivities of the back-to-back
(BTB) connection were around -17.8dBm and degraded to
about -17.6dBm after 10-km SMF transmission by PIN-TIA
optical receiver, as shown in Fig. 6. The power penalty
distributed from 0.16dB to 0.3dB. This degradation is mainly
due to the SMF dispersion effect and has been minimized by
the external modulation approach. With -18dBm PIN-TIA
optical receiver sensitivity to ensure an error-free operation in
practical systems, -4.5dBm average laser module power gives
rise to a power budget of 13.5dB, which allows transmission
over 10-km of standard SMF without optical amplifiers. In our
elementary experiment, the power budget is about 8dB and it’s
enough to afford a successive 1:4 splitting network. If input
powers are increased and used a commercial APD-TIA
receiver, the power budget will be enough and the sensitivities
will be increased easily. Therefore, the splitting ratio could
achieve to 1:32 for each channel. The related values are
summarized in Table 2.
4.2 Upstream transmission results
The output optical powers of the upstream channels were
controlled at 1.43 to -0.9dBm and operated simultaneously.
Fig. 7 shows the received and reshaped eye patterns after 10km SMF transmission by self-made PIN-TIA optical receiver.
The measured eye pattern maintained at a clearly open figure
and could ensure a good transmission. Values of the receiving
power, the OC-192 eye mask margin, and the jitters for every
channel are summarized in Table 3.
(a)
(b)
(c)
(d)
Fig. 7. (a)1275nm, (b)1300nm, (c)1325nm, (d)1350nm. The
measured eye patterns of upstream channels for 10-km SMF.
Table 3. Measured parameters of eye patterns for 10-km SMF.
Channels
Power (dBm)
Mask margin (%)
Jitter p-p (psec)
Fig. 6. The BER measured results of downstream channels for
BTB and 10-km SMF.
Table2. The measured optical power and sensitivity values.
1510nm
1530nm
1550nm
1570nm
Channels
Output power at
-4.5
-4.35
-4.75
-4.5
node A2 (dBm)
BTB sensitivity
-17.83
-17.79
-17.95
-17.80
(dBm)
Received power at
-8.64
-9.17
-9.74
-10.41
node C2 (dBm)
10-km receiving
-17.60
-17.62
-17.64
-17.53
sensitivity (dBm)
Power penalty (dB)
0.23
0.16
0.30
0.26
Power budget (dB)
8.96
8.45
7.91
7.12
Splitting ratio3
1:4
1:4
1:4
1:4
1. Sensitivity was measured at BER of 10-9.
2. Output power and received power was measured at node A and C for
downstream path, respectively. (as shown in Fig.4)
3. The estimated splitting-ratio was based on experimental results.
1275nm
-3.66
1300nm
1325nm
1350nm
-6.16
-5.43
-7.73
31
14.7
32
31
30
15.36
14.83
15.17
In the upstream channels, the optical sensitivity was
measured by connecting the output port of the CWDM
demultiplexer with a variable optical attenuator and a selfmade optical receiver, as shown in Fig. 8. As summarized in
Table 4. The BTB receiving sensitivity of upstream channels
distributed from -13.53dBm to -14.21dBm. The received
sensitivity after a 10-km SMF transmission was from -13.01 to
-13.80dBm. The BTB receiving sensitivity of longer
wavelength channel was better due to higher quantum
efficiency of the photodiode used in the self-made optical
receiver. Because of the lower output power of 1300-nm and
1350-nm channel, the power budgets were only 5.40dB and
4.73dB which could not afford a succeeding splitting network.
The power penalty was only several tenth dB for all four
channels and was slightly larger than the downstream channels
due to the direct modulation scheme. This is due to a larger
dispersion of fiber from the larger spectral bandwidth of direct
modulation sources. Although the dispersion coefficient is
larger at the longer wavelength, since the downstream signal is
external modulated, so the dispersion is smaller and results a
negligible power penalty compared with upstream signals. The
receiving sensitivity of this CWDM-PON system could be
improved further if the output power of DFB-LDs were
increased several dB or a commercial APD-TIA receiver was
used. Additionally, we have observed the pulse compression
effect which gives rise to a negative dispersion penalty due to
a low chirp of the laser. Therefore, we have evaluated
approximately power penalties due to LD chirp with 10-Gb/s
data rate in DFB-LDs wavelengths from 1275-nm to 1350-nm
for SMF transmission. The computed power penalties of the
upstream channels could fit them to the measured results.
Therefore, it would be possible to construct the proposed
network.
Fig. 8. The BER measurement results of upstream channels for
BTB and 10-km transmission.
Table 4. The measured optical power and sensitivity values.
1275nm
1300nm
1325nm
1350nm
Channels
Output power at
1.43
0.53
1.59
-0.9
node B2 (dBm)
BTB sensitivity
-13.73
-13.53
-14.09
-14.21
(dBm)
Received power at
-4.68
-5.4
-4.49
-6.05
node D2 (dBm)
10-km receiving
-13.16
-13.01
-13.80
-13.54
sensitivity (dBm)
Power penalty (dB)
0.57
0.52
0.29
0.66
Power budget (dB)
8.48
7.61
9.31
7.49
Splitting ratio3
1:4
1:4
1:4
1:4
1. Sensitivity was measured at BER of 10-9.
2. Output power and received power was measured at node B and D for
upstream path, respectively. (as shown in Fig.4)
3. The estimated splitting-ratio was based on experimental results.
5. Conclusions
In conclusion, a bi-directional CWDM-PON system covers
a wide wavelength of 1.3-μm and 1.5-μm with a symmetric
40-Gb/s data rate for 10-km SMF transmission has been
demonstrated. The maximum subscribing bit rate of each
client could be adjusted from gigabit to 10-Gb/s and the
splitting ratio could reach to 1:32. Four downstream channels
with externally modulated light sources emitting at
wavelengths from 1510-nm to 1570-nm and four upstream
channels with directly modulated DFB-LD emitting at
wavelengths from 1275-nm to 1350-nm were adopted to
minimize the dispersion effect of SMF transmission. All the
downstream and upstream channels were operated at 10-Gb/s
simultaneously to experiment the 40-Gb/s transmission
system. Furthermore, we have evaluated approximately power
penalties due to LD chirp with 10-Gb/s data rate in DFB-LDs
wavelengths from 1275-nm to 1350-nm for SMF transmission.
Finally, the analysis of the power penalty and the system
experiments confirm that DFB-LDs at 10-Gb/s are negligible
power penalty in this proposed CWDM-PON system.
Although the dispersion coefficient is larger at the longer
wavelength, since the downstream signal is external
modulated, the dispersion is smaller and results a negligible
power penalty compared with upstream signals for 10-km
SMF transmission. We experimentally identify the values of
the modulation chirp effect that minimizes the transmission
power penalty caused by fiber chromatic dispersion.
Therefore, utilizing the low-cost CWDM architecture, the
matured 10-Gb/s module technology, and the potential for a
large splitting ratio, this bidirectional CWDM-PON system
can meet with the high subscribing bandwidth and flexible
requirement of next level high-speed fiber-to-the-home
(FTTH) access networks.
Acknowledgments
This work was supported in part by the National Science
Council, Taiwan under Contract NSC96-2221-E-151-024MY3 and by the MOE Program of the Aim for the Top
University Plan.
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