World’s First Demonstration of Pluggable Optical Transceiver
Modules for Flexible TWDM PONs
Ning Cheng(1), Jianhe Gao(2), Chengzhi Xu2), Bo Gao(2), Xuming Wu(2), Dekun Liu(2), Lei Wang(2),Xiaoping Zhou(2), Huafeng Lin(2), Frank Effenberger(1)(1)(2)

FutureWei Technologies, 2330 Central Expwy, Santa Clara, CA 95050, USA ning.cheng@huawei.com
Huawei Technologies, Huawei Industrial Park, Shenzhen, Guangdong, China gaojianhe@huawei.com

Abstract Optical transceivers are demonstrated with error-free performance in TWDM PONs. Integrated
OLT transceiver in CFP-module transmits 10dBm power and achieves -36dBm sensitivity. Low-cost tunable
SFP+ ONU transceiver can be tuned over 4 channels with 4dBm transmitted power and -26dBm sensitivity.
package. The average transmitted power of theOLT module is larger than 10dBm and its receiversensitivity is better than -36dBm, while the tunable
ONU achieves more than 4dBm averagetransmitted power and -26dBm sensitivity. With
these pluggable transceiver modules, a flexible
TWDM PON system is demonstrated with errorfree performance and over 36dB power budget.
Optical Transceiver Module DesignFor OLT transceiver module in TWDM PONs,performance and footprint is very important, so theintegration of both electronics and optics is a
primary task. Fig. 1 illustrates the architecture of
the 4-channel OLT transceiver module. For thetransmitter, there are 4 EMLs (electroabsorption
modulated lasers), each modulated by a 10Gb/s
data stream. The outputs of EMLs at different
wavelengths are then multiplexed by a low-loss
multiplexer and amplified by an L-band EDFA. At
the receiver side, optical signals from ONUs in 4different wavelength channels are first amplified bya C-band EDFA and then separated by a demux
before being detected by burst-mode APD ROSAs(receiver optical subassembly). Limiting amplifiers(LA) following the ROSA further boosts thereceived signals. To achieve bidirectionaltransmission on a single fiber, a WDM (wavelengthdivision multiplexing) filter combines/separates theupstream and downstream wavelengths. A
microprocessor is also included in the transceivermodule for control and monitoring purpose. It sets

10Gb/s

EML
Driver1
EML
Driver2
EMLDriver3
EML
Driver4

2.5Gb/s

LA1

ROSA1

2.5Gb/s

LA2

ROSA2

2.5Gb/s

LA3

ROSA3

2.5Gb/s

LA4

ROSA4

10Gb/s
10Gb/s
10Gb/s

EML1
EML2
EML3

Mux

L-band
EDFA

EML4
MCU

Mux

Introduction
Rapid growth of internet traffic, driven by theproliferation of video services, continues to pushbroadband optical access networks for higher data
rates and better quality of services. Currently
GPON and EPON are widely deployed and 10G
PONs (e.g. XG-PON and 10G EPON) are
expected in next few years [1]. Beyond 10G PONs,
TWDM PON has been demonstrated as a viable
solution for future broadband services [2]. Such
TWDM PON systems use a combination of timeand wavelength division multiple access, providing
an aggregated capacity of at least 40Gb/s. A new
set of standards on TWDM PON will be releasedby ITU-T in next year, paving the way for future
deployment. However, before large scale
deployment,
technology
developmentandengineering solutions of TWDM PON systems andoptical transceiver modules must be madecommercially viable for manufactuerability and costeffectiveness. Recently, a flexible TWDM PON
system was demonstrated which can support
desirable features such as pay-as-you-grow, loadbalancing, channel protection and power saving [3].
In such a system, OLT (optical line terminal)
equips with 4 channel transceivers while ONU(optical network unit) integrates a tunable laser
and a tunable receiver. However, discrete
transceivers were used in this flexible TWDM PON
system. To the authors’ knowledge, integrated
transceiver modules have not been reported, and it
remains a very challenging task to develop TWDM
PON optical transceivers with good performance,
small footprint and low power consumption.
In this paper, we demonstrate, for the first time,
integrated OLT transceiver module and tunable
ONU transceiver module for flexible TWDM PONs.
The OLT transceiver consists of 4x10Gb/s
transmitters, 4x2.5Gb/s burst mode opticalreceivers, optical mux/demux and integrated
optical amplifiers in an enhance CFP (C FormFactor Pluggable) module. The low-cost ONU
transceiver includes a tunable laser and a tunable
receiver in SFP+ (Small Form Factor Pluggable)

C-bandEDFAFig. 1: OLT transceiver architecture.

WDM

the operation conditions of various optical andelectronic components in the module, andperforms monitoring functions such as transmitter
output power control, receiver signal power
measurement, and loss-of-signal alarm. Fig.3(a)shows a picture of the pluggable transceivermodule in an enhanced CFP package.
10Gb/s

Laser
Driver

20ps
channel 1channel 2channel 3channel 4

Fig.4: OLT transmitter eye diagrams.

TOSA

20DFB
TEC

10LA

TIA
APD TF

ROSA
Fig.2: Tunable ONU transceiver.

On ONU side, transceiver module design issimpler but low cost is the utmost goal. Meanwhile,
it still needs to achieve good performance. Hence,
the trade-off between cost and performance isnecessary. Fig. 2 illustrates the structure of thetunable ONU transceiver module. For transmitter, it
uses a thermally tuned DFB laser directly
modulated by a burst-mode laser diode driver at
2.5Gb/s. The DFB laser can emit relatively large
power but can only be tuned over a few channels
with 100GHz spacing and its tuning speed is about
80ms. At the receiver side, a tunable filter (TF) is
packaged inside the ROSA together with an APD
photodiode and a transimpedance amplifier (TIA).
The tunable filter can be thermally tuned to one of
the 4 downstream wavelengths in L-band, and its
tuning speed is on the order of 100ms. The tunablefilter provides over 25dB channel isolation toensure good performance. Automatic gain control
for APD, TIA and LA results in a large dynamic
range for the downstream signal. Fig. 3(b) shows apicture of the tunable ONU transceiver module inSFP+ package.

0-10-20-30-40-50-60
1607

1608

1609

1610

16111612

Wavelength (nm)Fig.5: OLT transmitter spectrum.
channel 1channel 2

80ps

channel 3channel 4

Fig.6: ONU transmitter eye diagram.
100

Optical Power (dBm)

2.5Gb/s

Optical Power (dBm)

WDM-10-20-30-40-50-60-70
1535 1535.5 1536 1536.5 1537 1537.5 1538 1538.5

Wavelength (nm)Fig.7: ONU transmitter spectra in different channels.
(a) OLT transceiver(b) ONU transceiverFig. 3: OLT and ONU transceiver modules.

Transceiver Testing Results
Fig. 4 shows the eye diagrams of the OLTtransmitter with 10dBm output power and 9dB
extinction ratio. The optical spectrum of thedownstream signals is plotted in Fig. 5, showing
four channels on ITU grid (1608.33, 1609.19,1610.06 and 1610.92nm). The OLT receiversensitivity in burst mode is better than -36dBm.
Fig. 6 shows the eye diagrams of the ONU
transmitter; its average transmitted power is 4dBm
and extinction ratio is better than 9dB. The optical
spectra of the upstream signals in different
channels (with different wavelengths at 1535.82,1536.71, 1537.41, 1538.19nm) are plotted in Fig. 7.
ONU receiver sensitivity is better than -26dBm,
and the receiver overload is more than 0dBm using
automatic gain control.
System Demonstration
For performance verification and network
throughput testing, OLT and ONU transceivermodules are plugged in the flexible TWDM PON
system testbed [3]. The bit error rates for thedownstream and upstream transmissions areshown in Fig. 8 for all the channels at back-to-back,after 20km and 40km single mode fiber. With these
optical transceivers, 36dB power budget isachieved for both downstream and upstream.

30channel 1channel 2channel 3channel 4-3-51510-65-7

B2B
20km
40km

-8-9-10-30-29-28-27-26-25-24
Received Optical Power (dBm)

0600-4-23-22-5-6-7

B2B
20km
40km

-8-9-10-38-37-36-35-34-33-32
Received Optical Power (dBm)

650

700

750

800

850

DAC value: 4pm/stepchannel 1channel 2channel 3channel 4-3log(BER)

20
RSSI valuelog(BER)

-4

25-31

-30

Fig. 8: Transmission performance of downstream at
10Gb/s (top) and upstream at 2.5Gb/s.

In TWDM PON system, ONU must
automatically align its transceiver wavelength to
the right channel. This can be done through anembedded control channel between OLT and ONU.
In our system, when an ONU is powered on, it willscan the downstream wavelengths by tuning its
tunable receiver. The tunable receiver can lock toa specific downstream channel by monitoring thereceived power through the RSSI (receiver signal
strength indicator) built inside the ONU transceiver.
For upstream wavelength alignment, the tunable
ONU scans its transmitter wavelength, while OLT
monitors the received power through RSSI
reported from OLT transceiver module. Fig. 9shows the RSSI values at OLT when ONU scans
its wavelength (controlled by DAC value withroughly 4pm per step) across upstream channel 3.
By finding the maximum value of RSSI, OLT can
set the ONU wavelength with ONU wavelength
setting command sending through the downstream.
Initially during ONU activation, ONU transmitter
wavelength might not be set in the middle of thechannel due to the accuracy of OLT RSSI is about+/-1dB, but ONU wavelength can be guaranteedwithin the 1dB passband of the channel. Thiscoarse wavelength alignment ensures upstream
connection and ONU activation process. ThenONU enters normal operation phase, and the RSSIvalue can be recorded for each burst from theFig. 9: Measured RSSI value from OLT transceiver when
ONU wavelength is tuned across upstream channel 3.

ONU. By averaging, the RSSI accuracy can beimproved significantly. Eventually, by monitoring
the RSSI after a large number of averaging, theONU wavelength can be fine tuned to the center ofthe upstream channel.
After the ONU wavelength alignment andactivation are successful, network throughput isevaluation in our system testbed. For upstream, 5ONUs transmit upstream signals in the same
wavelength channel, and each ONU is loaded with
Ethernet packets of variable length (64-1518
bytes). 2.3Gb/s aggregated throughput in a single
upstream channel is achieved without any packet
drop. Meanwhile, for downstream, Ethernet
packets with variable length (64-1518 bytes) aretransmitted by the OLT. No packet drop isobserved in the downstream with the traffic load toeach ONU up to 1Gb/s (limited by the gigabit
Ethernet port of the ONU).
Conclusions
Integrated OLT transceiver in an enhanced CFP
module and low-cost tunable ONU transceiver inSFP+ module are demonstrated for the first time inflexible TWDM PON. With integrated EDFAs, 4channel OLT transceiver transmits 10dBm power
per channel, and achieves -36dBm sensitivity inburst mode.
Tunable ONU transceiver withthermal tuning can be tuned over 4 channels(100GHz spacing) in C band, and its averagetransmitted power is >4dBm while its receiversensitivity is better than -26dBm. Using automatic
gain control for APD, TIA and LA, the ONU
receiver achieves 26dB dynamic range. Withaccurate RSSI detection, ONU can automaticallylock to a specific downstream wavelength; at thesame time, OLT can control the ONU transmitter
wavelength within 1dB passband of the upstream
channel. System testing demonstrates more than
36dB power budget with error free performanceand zero packet drop rate.
References[1] E. Wong, J. Lightwave Technol, 30, 597( 2013).
[2] Y. Ma, et al, Proc. OFC’12, PDP5D.7 (2012).
[3] N. Cheng, et al, Proc. ECOC’2013, We.3.F6.