Enhancement of system performance in directly modulated metro-WDM systems
by spectral filtering
Sung-Bum Park and Chang-Hee Lee
KAIST, Dept. of Electrical Engineering and Computer Science,
373-1 Kusong-dong, Yusong-gu, Taejon 305-701, Korea
Tel) +82-42-869-3463, Fax) +82-42-869-3410, e-mail : chl@ee.kaist.ac.kr
Abstract
We demonstrate numerically reduction of both
chirp induced dispersion penalty and extinction ratio
penalty in directly modulated metro-WDM
transmission systems by spectral filtering.
Booster Amp
AWG
In-line Amp
Introduction:
Numerical Simulation Conditions:
The schematic diagram of directly modulated WDM
systems is shown in Fig. 1 (a). It consists of directly
modulated laser diodes (LDs), optical MUX/DMUX,
optical amplifiers, and optical receivers. For short-haul
systems, we do not need the optical amplifiers. The
optical booster amplifier can be eliminated, when we use
high power LDs.
The optical intensity and chirp of the directly
modulated DFB LD were obtained by solving well
known rate equations [1,2]. The modulation signal is
2.5-Gb/s pseudorandom binary sequence of 26 − 1. The
bias current is adjusted to be 1.3 times the threshold
current and the extinction ratio is set to 10 dB. The
spectral width of the directly modulated LD is 0.18 nm
at –20 dB points from the peak. An arrayed-waveguide
grating (AWG) is used as the spectral filter and
MUX/DMUX. In this paper, pre-filter means the spectral
filter at the transmitter and post-filter means that at the
receiver. The dispersion coefficient and the loss of
optical fiber are 17 ps/nm/km and 0.22 dB/km,
respectively. For the receiver, we assume a third-order
Butterworth filter with a 3-dB bandwidth of 0.65 times
the bit rate. The eye diagrams in Fig. 1 (b) show the
increase of the extinction ratio, when we use spectral
filter with a 3-dB bandwidth of 20 GHz.
SMF 120km
AWG
Recently, metro -WDM systems have attracted
considerable attention as a cost-effective upgrade of
optical transmission systems to high capacity. Direct
modulation scheme is suitable for these systems instead
of an expensive external modulation scheme. However,
in directly modulated transmission systems, chirping
induced broadening of optical spectrum limits
transmission distance due to fiber chromatic dispersion.
In this paper, we propose a spectral filtering method to
enhance system performance of directly modulated
metro -WDM systems. The chirp induced dispersion
penalty and the extinction ratio penalty are suppressed
by using the proposed method and transmission distance
can be increased up to 300 km.
SMF 120km
receiver
(a)
1
1
0.8
0.8
0.6
0.6
0.4
0.4
0.2
0.2
20
40
60
80
100
120
20
40
60
80
100
120
(b)
Fig. 1.(a) Schematic diagram of the transmission system
and (b) Eye diagrams before (left) and after spectral
filter (right).
Numerical Simulation Results:
Fig. 2 shows the calculated eye-penalty as a function
of transmission distance. We neglect optical
nonlinearities in transmission fiber. The channel spacing
is 200GHz. We can see well known eye penalty curve,
when we do not use the s pectral filter (i.e., Bandwidth of
MUX and DMUX is sufficiently wide to pass all spectral
components in the directly modulated laser output). The
eye-penalty increases rapidly as we increase the
transmission distance. If we introduce the spectral filter,
the transient chirp portions are filtered out and the eyepenalty due to transient chirping decreased. The spectral
filter also improves the extinction ratio, since it also
filtered out the optical power in “space” or “0” that has a
different wavelength fro m that of “mark” or “1” due to
adiabatic chirping. Overall improvement of eye-penalty
depends on the laser parameters and the bandwidth of
spectral filter. In Fig. 2, maximum improvement of eyepenalty is about 1 dB, when the transmission distance is
about 170 km and bandwidth of spectral filter is 20-GHz.
It may be noted that the improvement in bit error rate
(BER) penalty is larger than that of the eye-penalty,
since the increase of the extinction ratio reduce BER
considerably [3].
1.6
0 km
Eye Penalty [dB]
240 km
1.2
0.8
0.4
0
0
10
20
Filter BW [GHz]
30
Fig. 3. Calculated eye-penalty as a function of optical
filter bandwidth.
1.2
Post (50GHz)
Eye Penalty [dB]
1
Pre ( 50GHz)
Post (100GHz)
0.8
Pre (100G)
0.6
0.4
0.2
0
0
2
4
6
8
10
12
6
8
Input Power [mW]
10
12
Input Power [mW]
(a)
1.6
Post (50GHz)
Eye Penalty [dB]
Fig. 3 shows the eye-penalty as a function of the
spectral filter bandwidth for 240-km transmission. The
optimum filter bandwidth is about 20 GHz in back-toback configuration. However, it decreases as we increase
the transmission distance and it becomes 13 GHz for 240
km transmission. As the spectral filter bandwidth
decreases from the optimum value, the eye-penalty starts
to increase. This increase of the penalty is caused by
excess subtraction of “mark” pulse energy by the
spectral filter.
In linear transmission systems, the improvement of
system performance is independent of the position of
spectral filter. However, it may change when we
consider the nonlinear effects in optical fiber. The
nonlinear effects also change the optimum filter
bandwidth. To investigate effects of optical nonlinearity,
we solve nonlinear Schrodinger equation.
Fig. 4 shows eye-penalty as a function of optical
power injected into transmission fiber. As we increase
the power, the penalty increases due to cross-phase
modulation (XPM). It may be noted that the eye-penalty
decreases as we increase the transmission power, when
we transmit only a single channel. The effect of fourphoton mixing also is negligible in single mode fiber.
The optimum filter bandwidth is about 20 GHz, when
we take account of the optical nonlinear effects.
As shown in Fig. 4, the eye-penalty depends on the
location of the spectral filter. For post-filter, we use only
an AWG with a 20 GHz bandwidth at the receiver. For
pre-filter, the AWG was used both at the transmitter and
at the receiver. For 100 GHz channel spacing, the
bandwidth of the AWG at the transmitter and that at the
receiver are 22 GHz and 40 GHz, respectively. For 50
GHz channel spacing, both AWGs have the same
bandwidth of 27 GHz. The pre -filter (the AWG in
transmitter) suppress the relaxation oscillation peaks in
the laser output. Then, we can suppress the effects of
XPM.
1.4
Pre ( 50GHz)
1.2
Post (100GHz)
Pre (100G)
1
0.8
0.6
0.4
0.2
Summary:
0
0
We have proposed a spectral filtering method to
suppress both the chirp induced dispersion penalty and
the extinction ratio penalty in directly modulated metroWDM system. The pre-filter suppresses the XPM effects
and gives better performance improvement.
Eye Penalty [dB]
1.6
w/o filter
w/ filter (BW:10GHz)
w/ filter (BW:20GHz)
w/ filter (BW:30GHz)
1.2
0
100
150
200
Distance [km]
Fig.2. Calculated eye-penalty
transmission distance.
(b)
Fig.4. Calculated eye-penalty as a function of average
optical power for 50-GHz and 100-GHz channel spacing.
(a) One span(120km). (b) Two span (240km).
[1] P. J. Corvini et al., IEEE J. Lightwave Technol., No.
11, pp. 1591-1595, Nov. 1987
[2] K. Hinton et al., IEEE J. Lightwave Technol.,, No. 3,
pp. 380-392, April 1993.
[3] C. H. Lee et al., IEEE Photonics Technol. Lett., No
12, pp. 1725-1727, Dec. 1996.
0.4
50
4
REFERENCES
0.8
0
2
as
a
function
of