Demonstration of single channel 160-Gb/s OTDM 100-km ...

Demonstration of single channel 160-Gb/s OTDM 100-km transmission system
Tao-rong Gong *, Feng-ping Yan, Dan Lu, Ming Chen, Peng Liu, Pei-lin Tao,
Mu-guang Wang, Tang-jun Li, Shui-sheng Jian
Key-Laboratory of All-Optical Networks and Advanced Communication Networks, Ministry of Education, Beijing 100044, China
Institute of Light Wave Technology, Beijing Jiaotong University, Beijing 100044, China
article info
Article history:
Received 15 April 2009
Received in revised form 19 May 2009
Accepted 19 May 2009
PACS:
07.60
42.72
42.79
85.60
42.88
42.15
Keywords:
Optical communication
OTDM
Transmission
1. Introduction
abstract
With the ever-increasing demand in communication bandwidth
and the maturity of wavelength-division-multiplexing (WDM),
Optical time-division-multiplexing (OTDM) is another important
technique to overcome the electronic bottleneck and achieve single
channel high bit-rate system. Since the first 100-Gb/s OTDM transmission
experiment over a 36-km fiber link was already reported
in 1993 [1], OTDM technologies have made a lot of progress toward
much higher bit rates and much longer transmission distance [2–
4]. For example, 160-Gb/s transmission over a record length of
4320 km [5] and on 2.56-Tb/s transmission over 160-km have been
reported [6].
An OTDM system consists of the following key sub-systems:
ultra-short pulse generator, OTDM multiplexer, tributary clock
extraction and high-speed optical switch to perform demultiplexing.
There are several optical-signal-processing technologies avail-
* Corresponding author. Address: Institute of Light wave Technology, Beijing
Jiaotong University, Beijing 100044, China. Tel.: +86 10 51684015/13426062162.
E-mail addresses: trgong2009@gmail.com (T.-r. Gong), fpyan@bjtu.edu.cn (F.-p.
Yan), danlucy@gmail.com (D. Lu), 05111023@bjtu.edu.cn (M. Chen), 06111003
@bjtu.edu.cn (P. Liu), 06120248@bjtu.edu.cn (P.-l. Tao), mwgang@center.njtu.edu.
cn (M.-g. Wang), tjli@bjtu.edu.cn (T.-j. Li), ssjian@bjtu.edu.cn (S.-s. Jian).
0030-4018/$ - see front matter Ó 2009 Elsevier B.V. All rights reserved.
doi:10.1016/j.optcom.2009.05.060
Optics Communications 282 (2009) 3460–3463
Contents lists available at ScienceDirect
Optics Communications
journal homepage: www.elsevier.com/locate/optcom
A16 10-Gb/s optical time-division-multiplexing (OTDM) 100-km transmission system with homemade
multiplexer is demonstrated experimentally. A demultiplexer based on two cascaded electroabsorption
modulator (EAM) and a clock recovery unit form a feedback loop. Error-free demultiplexing
from 160-Gb/s to 10-Gb/s after over 100-km transmission is achieved. The power penalty is about
3.5 dB with the bit error rate of 10 9 .
Ó 2009 Elsevier B.V. All rights reserved.
able for each part, and EAM is one of the optimum terminal
equipments as optical gate and a clock recovery device in the receiver
[4]. In this paper, a 16 10-Gb/s OTDM 100-km transmission
system is demonstrated experimentally based on mode-locked fiber
lasers (MLFL) and home-made multiplexer in the transmitter
and EAM in the receiver.
2. Transmission systems
Fig. 1 shows the experimental setup, which consists of three
parts: transmitter, transmission link and receiver. The following
will describe the three parts in detail.
2.1. Transmitter
The solid box of Fig. 1 shows the setup of 160-Gb/s transmitter.
It comprises MLFL (CALMAR OPTCOM Model: PSL-10-1T), a
modulator and a passive multiplexer. The MLFL generated a pulse
train with a repetition rate of 10 GHz (9.95328 GHz) at a wavelength
of 1556.223 nm. The emitted pulses are the shape of a
sech 2 -function and a full-width at half-maximum (FWHM) of less
than 1.5 ps. The 10 GHz pulse train is intensity modulated with a

pseudo random bit-sequence (PRBS 2 7
1) by an external LiNbO3
modulator. Then the 10-Gb/s data signal is multiplexed to 160-
Gb/s by a home-made multiplexer.
Conventional OTDM multiplexer is based on fused fiber coupler
and its performance is usually degraded due to polarization fluctuation
of the input signal, which will rapidly change the distribution
of output power among adjacent multiplexed pulses [7,8]. Recently
reported OTDM multiplexers [9,10] can provide qualified and stable
OTDM signals but is not widely used in practice due to their
high cost. In this work we propose a cost-effective and low polarization
dependent structure to achieve the multiplexing. The
home-made multiplexer consists of four-stage OTDM multiplexer
based on plated graded index (GRIN)lens, which consists of two reverse-connected
GRIN lenses coupled with three fiber pigtails and
performs the same function as fused fiber coupler. A very thin Half-
Reflection-Half-Transmittance (HRHT) mirror is plated between
two lenses, as shown in Fig. 2. This structure based on GRIN lens
exhibits low polarization dependence [11].
Two plated GRIN lenses aforementioned compose one stage
multiplexer, as illustrated in Fig. 3. Ports 3 and 3 0 are coupled directly
and a fiber delay line is added between Ports 2 and 2 0 . Then
the signal at Port 1 is doubled and multiplexed at Port 1 0 . Our 4stage
multiplexer consists of 4 pairs of such cascaded lenses. The
delay times in each stage are 50 ps, 25 ps, 12.5 ps and 6.25 ps,
respectively. The insertion loss of each stage is about 4.5 dB and
thus the total insertion loss of the 4-stage MUX is about 18 dB.
2.2. Transmission link
The dashed box of Fig. 1 shows the setup of the transmission
link. The total length of fiber is 100 km which is composed by
90 km SMF and 10 km dispersion slope compensation (DSCF)
(both fabricated by Yangtze Optical Fiber and Cable Com.:YOFC).
T.-r. Gong et al. / Optics Communications 282 (2009) 3460–3463 3461
Fig. 1. Experimental setup.
The insertion loss of DSCF is 6.89 dB, and total insertion loss of fiber
span is 29 dB including all splices and connectors. The DSCF is designed
to achieve both zero chromatic dispersion and slop at
1550 nm wavelength band for 90 km SMF. Delay and the dispersion
of DCF are measured by CD400 as indicated in Fig. 4. At
1556.223 nm the dispersion and slope values are 150.432 ps/
nm/km and 4.931 ps/nm 2 /km, respectively, for DSCF and
16.723 ps/nm/km and 0.058 ps/nm 2 /km, respectively, for SMF.
The DSCF is placed in the middle of the transmission link, which
has been shown to be better than pre-dispersion and post-dispersion
compensation schemes [12].
For 160-Gb/s transmission system the chromatic dispersion tolerance
is found to be +/ 50 m for SMF [13]. In order to compensate
the dispersion and slope well, different length of SMF is added at
the last span fiber to obtain best compensation, such as 50 m,
100 m, 150 m, 200 m, and 300 m. As the bandwidth of our oscilloscope
(Agilent DCA 86100 B) is only 53 GHz, we observe the waveform
change of 10 GHz pulse source to validate. The RMS width of
measured pulse source after transmission with the added fiber
length for 300 m, 200 + 50 m and 300 m + 50 m are 9.20 ps,
9.58 ps and 9.62 ps, respectively. Although the resolution of our
oscilloscope is limited, those measured values can give us a qualitative
evaluation of the compensation performance. Compared to
other configurations, the three combinations are better. Also the
300 m fiber is the best. The PMD of SMF is less than 0.05 ps/km 1/2 ,
and the PMD of SDCF is 0.29 ps provided by YOFC, so the total
PMD of transmission link is less than 0.76 ps which is sufficiently
small for 160-Gb/s data transmission [4].
Fig. 2. Structure of the plated GRIN lens and the inner optical paths. Fig. 3. Structure of SMUX based on plated GRIN lens.

3462 T.-r. Gong et al. / Optics Communications 282 (2009) 3460–3463
2.3. Receiver
The dash-dotted box of Fig. 1 shows the schematic of the receiver,
which consists of two cascaded EAMs and 10-Gbit/s clock and
data recovery unit (CDRU). Two EAMs are concatenated to achieve
a switching window of less than 5 ps with a suppression ratio of
larger than 20 dB, similar to that described in [3,14]. The first
EAM exhibits 7 dB polarization sensitivity and is driven at
10 GHz, and the second EAM is polarization independent and is
driven at 40 GHz. So a polarization controller (PC) is used to change
the polarization state of optical signal input EAM1. CDRU is based
on opt-electronic phase-locked loop (PLL) and can exact the
10 GHz clock and 10-Gb/s electrical data from the 10-Gb/s optical
signal. After passing through these two EAMs the 160-Gb/s signal
Fig. 5. Eye diagram of (a) 10-Gb/s modulated signal and (b) demultiplexed 10-Gb/s
signal from 160-Gb/s 100-km transmission.
Fig. 4. Dispersion curve of special DSCF.
is demultiplexed to 10-Gb/s signal. Then 10-Gb/s optical is divided
into two parts by a 25:75 coupler. One part is used for observation
on the DCA, and the other part is injected into the CDRU. The electrical
10 GHz clock signal extracted by CDRU is fed into the electrical
input of the EAMs, and the two EAMs and the CDRU form a
demultiplexing and clock recovery loop. By using this structure
two EAMs are saved compared to similar experimental set-up
[3,14].
3. Experiment results
The input optical power of transmission is 5 dBm. The receiver
sensitivity is defined at the input power of CDRU. By adjusting
the variable electrical delay and the bias of the EASW1 and EASW2
carefully, the error-free demultiplexing is achieved. The bias voltage
of EASW1 and EASW2 is 1.9 V and 1.22 V, respectively,
which are the same to the case of 160-Gb/s back-to-back (B2B)
demultiplexing. Fig. 4b shows the eye diagram of 10-Gb/s demultiplexing
signal. Compared to Fig. 5a, it has larger noise and jitter. The
mean values of jitter of modulated signal and demultiplexed signal
are 847 fs and 962 fs measured by DCA, respectively. Due to the resolution
of our oscilloscope is limited those measured values only
give us a qualitative evaluation. But the eye diagram is clear and
open, and the non-target channel is suppressed sufficiently.
BER
1E-4
1E-5
1E-6
1E-7
1E-8
1E-9
1E-10
B2B
transmission
1E-11
-19.5 -19.0 -18.5 -18.0 -17.5 -17.0 -16.5 -16.0 -15.5 -15.0
Power /dBm
Fig. 6. BER measurement of 160-to-10 Gb/s.

Fig. 6 depicts the BER performance curves of a random demultiplexing
channel from 160-Gb/s signal of B2B and transmission
over 100-km. The receive sensitivity (BER = 10 9 ) measured after
transmission is 15.25 dBm. Similar BER performance is obtained
for the other channels, and the difference in sensitivity among
channels is less than 0.8 dB. A 3.5 dB power penalty is observed
after 100 km transmission. Compared to similar experiment
[3,14], the power penalty is 1 dB larger. Except for the PMD effect
and different type pulse source, the power penalty may be attributed
to the following reasons: remnant dispersion, wide sampling
window and rather large insertion loss of EAM, and ASE noise of
home-made EDFA.
4. Conclusions
A single channel 160-Gb/s transmission over 100-km is reported
using home-made multiplexer. And the 10-Gb/s error-free
demultiplexing from 160-Gb/s based on a feedback loop composed
by two cascaded EASW and CDRU is obtained after 100-km transmission.
Compared to the B2B case, the power penalty is 3.5 dB at
the BER of 10 9 .
T.-r. Gong et al. / Optics Communications 282 (2009) 3460–3463 3463
Acknowledgement
This work was supported by National Science Foundations of
Beijing (No. 4062027) and National 863 High Technology Projects
of China (No. 2007AA01Z258, 2008AA01Z15) and National Natural
Science Foundation of China (No. 60877042, 60837002).
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