System Performance of DPSK Signals Transmitted Through
Broadband SBS-based Slow Light Element and Reduction of
Slow-Light-Induced Data-Pattern Dependence
B. Zhang1, I. Fazal1, L.-S. Yan2, L. Zhang1, A. E. Willner1, Z. Zhu3 and D. J. Gauthier3
1
Department of Electrical Engineering - Systems, University of Southern California, EEB-500, Los Angeles, CA 90089-2565
Tel: (213) 740-1488, Fax: (213) 740-8729, Email: boz@usc.edu
2
General Photonics, 5228, Edison Ave., Chino CA 91710
3
Department of Physics, Duke University, Durham, NC 27708
Abstract: We demonstrate 42 ps delay on 10.7-Gb/s DPSK signals via SBS-based slow light.
Slow-light-induced DPSK-data-pattern dependence is analyzed and its reduction results in 3-dB Q
factor improvement. NRZ-DPSK and RZ-DPSK system performances are compared.
©2007 Optical Society of America
OCIS codes: (060.4370) Nonlinear optics, Fibers, (060.5060) Phase modulation
1. Introduction
Slow light elements have enjoyed much recent interest due to the potential systems applications involving tunable
delay lines, such as synchronizers, equalizers, and signal processors. In general, slow light is achieved by tailoring
an enhanced group-index resonance within a given medium [1]. Unfortunately, a high group index accompanied by
a narrow gain bandwidth tends to also distort a high-bit-rate data signal and cause data-pattern dependence [2].
Recent publications have described the quality of a data stream that has passed through the slow light
element, including results for pattern dependence, limited data bandwidth, and bit-error-rate (BER) measurements.
Promising slow light techniques to achieve tunable delay lines for Gbit/s data include the use of: (i) stimulated
Brillouin scattering (SBS) [3], (ii) stimulated Raman scattering (SRS) [4], (iii) optical parametric amplification
(OPA) [5], and (iv) four-wave-mixing (FWM) in semiconductor optical amplifiers (SOAs) [6].
We emphasize that all previously published slow light system results were for intensity-modulated data
signals. However, phase-encoded modulation formats, such as differential-phase-shift-keying (DPSK), have not
been explored before in a slow light element. DPSK is becoming ever-more important in the optical communications
community due to its potential for increased receiver sensitivity, tolerance to various fiber impairments, and spectral
efficiency [7]. It is highly desirable to understand how the phase information of the DPSK signal could be preserved
and how much fractional delay it could experience. Furthermore, it is unclear how slow light nonlinearities could
affect differently the two demodulated ports of a delay interferometer-based DPSK receiver. A laudable goal would
thus be to examine the critical system limitations on Gbit/s DPSK data as it traverses a tunable slow light element.
In this paper, we demonstrate experimentally the slowing down of a phase-modulated signal. A 10.7-Gb/s
NRZ-DPSK signal can be continuously delayed by as much as 42 ps (45% fractional delay) while still achieving
error free via broadband SBS-based slow light. We further analyze slow-light-induced data-pattern dependence on
both demodulated ports. By detuning the SBS gain profile, one can achieve as much as 3-dB Q factor improvement
by the reduction of pattern dependence. Performance of 2.5-Gb/s NRZ-DPSK with the same fractional delay shows
that data-pattern dependence is bit-rate specific. Finally, a system level comparison of 2.5-Gb/s NRZ-DPSK with
RZ-DPSK through slow light exhibits different robustness to slow-light-induced data-pattern dependence.
2. Concept and Experimental Setup
Slow Light
π 0 π π π
π 0 π ππ
RZ-DPSK
MZM
MZM
1-bit DI
Intensity
AMI
1 0 0 1 1
t
Slow-light-induced
pattern dependence
0 1 1 0 0
0
1
3
2
Broadband pump
bias
DB
att
EDFA
clk
NRZ-DPSK
t
t
0
10-Gb/s
probe
LD
Phase
Phase
• Slow light delay on phase patterns
t
Fig. 1. Concept of slow light on phase-modulated signals. Slow-lightinduced pattern dependence is observed on demodulated output ports
Noise Source
Amp. + Att.
temp
2
EDFA
DML
ISO
HNLF
(2km)
1
3
BERT
att
att
Rx
EDFA
1-bit DI
Fig. 2. Experimental Setup : DPSK slow light based on broadband
SBS and BER measurement on demodulated interferometer ports
As shown in Fig. 1, when phase-modulated signals pass through the slow light element, we observe that the phase
patterns get delayed, with the amount of delay determined by slow light bandwidth and gain. Moreover, the phase
delay is also accompanied by pattern distortions leading to different data-pattern dependence on the constructive
“DB” (Duo-binary) and destructive “AMI” (Alternate-Mark-Inversion) ports after the one-bit delay interferometer
(DI). It is thus crucial to analyze the system performance of DPSK signals transmitted through a slow light element.
The experimental setup is shown in Fig. 2. A broadband SBS pump source is realized by modulating the
injection current of a commercial DML using an amplified Gaussian noise source at a 400 MHz clock frequency [3].
The RF attenuator is adjusted to control the pump spectral bandwidth. The broadband pump is then amplified by a
high-power EDFA and enters a 2-km Highly Nonlinear Fiber (HNLF) via a circulator. The Brillouin shift of the
HNLF is measured to be 10.3 GHz. An NRZ-DPSK probe data stream is generated by externally modulating the
laser diode (LD) using a Mach-Zehnder modulator (MZM), which is biased at its transmission null and driven by
approximately 2*Vπ. A sinusoidally driven second MZM is used for pulse carving when generating 50% RZ-DPSK
signals. The amplified and attenuated DPSK signal with controllable probe power counter-propagates in the HNLF,
with respect to the pump. One polarization controller is used on the signal path to maximize the SBS interaction.
The amplified and delayed DPSK data signal is routed out via a circulator. The DPSK signal is then demodulated
using a one-bit DI (ITF Laboratories [8]) and both DB and AMI ports are detected. An optical attenuator is adjusted
accordingly to the SBS gain so as to keep the input power into the EDFA fixed. BER measurements are taken on
both the DB and AMI demodulated signals.
3. DPSK Slow Light and Data-Pattern Dependence
Fig. 3 shows the measured delay of a 10.7-Gb/s NRZ-DPSK signal with 0dBm power under an 8-GHz SBS gain
bandwidth. The measured delay scales fairly linearly with the increase in pump power, demonstrating the ability to
continuously control the delay of the DPSK phase pattern. The detected balanced DPSK eye diagrams are shown for
three different pump power cases, with a maximum of 42 ps delay at a pump power of 800 mW. The achieved 42 ps
delay of a 10.7-Gb/s NRZ-DPSK signal corresponds to a fractional delay of 45%.
Delay (ps)
45
30
15
0
0
0mW
10.7-Gb/s
DPSK
25 ps
400mW
42 ps
800mW
200 400 600 800
Pump Power (mW)
Fig. 3. Observation of DPSK slow light: A delay of as high as
42 ps for a 10.7Gb/s DPSK signal is achieved.
Fig. 4. Slow-Light-induced data-pattern dependence on DPSK signals for
both demodulated ports. Bit patterns before (DPSK) and after (DB and AMI)
demodulation are recorded to help analyze the data-pattern dependence.
As shown in the DPSK balanced eye diagrams in Fig. 3, severe signal distortion is accompanied by the
increased slow light delay. In order to assess the signal quality, we analyze both the constructive and destructive
ports of the DI individually. Fig. 4 shows the data patterns before and after passing through the slow light element.
The typical and well recognized method of generating an NRZ-DPSK signal, using an MZM, has several
advantages: (i) exact π phase modulation, (ii) insignificant frequency chirping, and (iii) increased tolerance to
driving voltage imperfections [7]. However, residual amplitude modulation occurs during phase transitions. We can
categorize these “intensity dips” as isolated “1”s (between two consecutive dips) and consecutive “1”s (between two
long separated dips). Isolated “1”s will occupy higher frequency components compared to consecutive “1”s, and will
therefore experience much less gain after passing through the narrowband slow-light resonance.
The distortion on the intensity dips translates into two different types of data-pattern dependence on DPSK
demodulated signals. In the DB port the peak power is much higher for consecutive “1”s, compared with single
“1”s. This can be explained from the fact that single “1”s are generated from two consecutive “1”s in NRZ-DPSK
which has a much slower rising time due to slow-light-induced third-order dispersion. This leads to an insufficient
constructive interference for the generation of single “1”s. The AMI port exhibits strong pattern dependence within
a group of “1” pulses. Compared with the “1”s in the middle, the leading and the trailing “1”s always have much
higher peak powers in that they both experience unequal-power constructive interference from the higher powers of
the beginning and ending time in a group of “intensity dips”. Both DB and AMI eye diagrams exhibit vertical data-
pattern dependence. Furthermore, the AMI port also features non-negligible pulse walk-off, which can be attributed
to the faster transition time of the middle pulses compared with the edge pulses, in a group of “1” pulses.
The BER measurement and the eye diagrams on the DB port of the 10.7-Gb/s DPSK signal under different
delay conditions are shown in Fig. 5. The tradeoff between delay and signal quality can be mainly attributed to the
data-pattern dependence resulting in vertical eye closure. We emphasize that we could still achieve error free BER at
a delay of 42 ps with a power penalty of 9.5dB. Crosstalk from Rayleigh backscattering of the broadband pump is
another contributor to the power penalty, especially when the bit-rate is comparable to the Brillouin shift. Fig. 6
shows the power penalty comparison of 10-Gb/s and 2.5-Gb/s data with a fixed SBS gain bandwidth of 8-GHz.
System performance of 2.5-Gb/s NRZ-DPSK exhibits 6.5dB better performance at 800 mW pump power, the main
reason being lower bit rate signals see much less data-pattern dependence as shown in the eye diagrams of Fig. 6.
Fig. 5. Error free BER and Duo-binary eye diagrams of 10.7-Gb/s DPSK
signals after SBS-based slow light element.
Fig. 6. Power penalty comparison between 2.5G and 10G NRZDPSK. Data-pattern dependence is shown to be bit rate specific.
Slow-light-induced data-pattern dependence on the residual intensity modulation on DPSK signals can be
alleviated by detuning the peak of the SBS gain profile away from the channel center, resulting in gain equalization
between single and consecutive “1”s. The optimum 3-dB Q factor improvement (from 12 to 15dB) in the AMI eyes
is demonstrated in Fig. 7. The detuning not only resolves data-pattern dependence, but also reshapes the leading and
trailing edges in a group of “1” pulses, such that pulse walk-off is also alleviated.
We also compare system performance of NRZ-DPSK with RZ-DPSK at a bit rate of 2.5-Gb/s with a 231-1
PRBS. Fig. 8 and 9 show delay and power penalty as a function of increased pump power. The fractional delay of
RZ-DPSK is comparable with the case of NRZ-DPSK. Under the same SBS gain bandwidth, RZ-DPSK outperforms
NRZ-DPSK by as much as 2dB in terms of power penalty at 700 mW pump power. The inserted AMI eye diagrams
show that RZ-DPSK is much more tolerant than NRZ-DPSK in terms of slow-light-induced data-pattern
dependence. The main reason can be understood from the fact that pulse carver modulator used in RZ-DPSK
extracts only the amplitude-modulation-free center portions of the bits, thus largely eliminating any residual dips,
which is the main cause of data-pattern dependence in NRZ-DPSK.
3
50
2.5-Gb/s
0
200
400
600
800
Pump Power (mW)
Fig. 8. Delay for 2.5-Gb/s NRZ and RZDPSK with same SBS BW.
2.5-Gb/s
2
1
0
AMI
NR
ZDP
SK
SK
-DP
RZ
100
0
Fig. 7. 3-dB Q factor improvement of 10-Gb/s
DPSK AMI port by detuning the SBS gain profile.
Z
NR
150
SK
-DP
Power Penalty (dB)
Delay (ps)
200
2dB
AMI
K
DPS
RZ-
0
200
400
600
800
Pump Power (mW)
Fig. 9. RZ-DPSK outperforms NRZ-DPSK by
2dB, which shows its robustness to distortion.
We acknowledge the financial support from DARPA Slow Light program.
4. References
[1] R.W. Boyd and D. J. Gauthier, in Progress in Optics, E.Wolf, Ed. (Elsevier, Amsterdam, 2002), Vol. 43, chap. 6, pp. 497–530.
[2] L. Zhang et al, “Optimizing operating conditions to reduce data pattern dependence induced by slow light elements”, OFC 2006, OFP7
[3] Z. Zhu et al, “12-GHz-Bandwidth SBS slow light in optical fibers”, OFC 2006, PDP1.
[4] Y. Okawachi et al, "All-optical slow-light on a photonic chip", Opt. Express, Vol. 14, 2317-2322 (2006).
[5] Yikai Su et al, “System performance of a slow-light delay line for 10-Gb/s data packets”, Slow and fast light topical meeting 2006, WB4
[6] Z. Chen et. al, “Experimental demonstration of slow light via four wave mixing in semiconductor optical amplifiers”, OFC 2006, OWS1
[7] A. Gnauck et. al, “Optical phase-shift-keyed transmission”, J. Lightwave Technol. , Vol. 23, No. 1, 115-130, Jan 2005
[8] F. Seguin et al, “Tuneable all-fiber, delay-line interferometer for DPSK demodulation”, OFC 2005, Volume 5, pp248 – 250