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Frequency Chirp Characteristics on WDM Direct Modulation
Article · March 2010
DOI: 10.1109/ICCEA.2010.32
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Zaiton Abdul Mutalip
Technical University of Malaysia Malacca
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2010 Second International Conference on Computer Engineering and Applications
Frequency Chirp Characteristics on WDM Direct Modulation
Zaiton Binti Abdul Mutalip
Faculty of Electronics and Computer Engineering
Universiti Teknikal Malaysia Melaka
Malacca, Malaysia
zaiton@utem.edu.my
Abstract — The paper studies the characteristic of frequency
chirp in optical communication network as well as the
association of the chirp and BER intended for utilization of
directly modulated wavelength division multiplexing (WDM).
The research was made based on a simulation of a direct
modulated link for different bit rates and fiber lengths to
evaluate the effect of those parameters on frequency chirp.
The result shows that the performance of the WDM direct
modulation link is affected by the transmission bit rates and
the distance.
r = ER=
INTRODUCTION
Optical fiber communication provides tremendously high
transmission capacity needed aligned with the development
of FTTH which moving directly to the user. This need has
leaded the research of transmitting high capacity of
information in a longer distance. However transmission of
shorter link can fully utilized the use of direct modulation
DWM technique.
In optical communication system, the modulation
operation is the key functionality, where it converting the
high bit rate electrical data signals into an optical carrier
frequency. Thus, light modulation is one of the main issues
in order to provide larger bandwidth over a longer distance
transmission. Optical modulation is referred as the process
of adding information to an optical signal carrier, which is
transmitted by an optical source. There are two options for
modulating an optical signal, which is through direct
modulation and external modulation. [1][2].
The parameters which being modulated can either be the
amplitude, frequency, phase and polarization. Thus, two
types of optical modulation available, named:
Analogue - AM, FM, PM and PoIM
Digital - ASK, FSK, PSK, PoISK
Figure 1. Appearance of chirp in signal [2]
In optical modulation, ASK is the simplest and widely
used method of modulating an optical signal. This ASK is
also referred to as On-Off Keying (OOK) as the lasers used
is turns on and off in order to transmit the “0” and “1” bits.
Although it is the most preferable modulation methods, it is
not perfectly accurate as the laser light is not totally turned
off during the transmission of “0” bit. The quotient of “0”
bit power over “1” bit power is called Extinction Ratio (ER)
and is represented by Equation (1). [1][6][4]
978-0-7695-3982-9/10 $26.00 © 2010 IEEE
DOI 10.1109/ICCEA.2010.32
(1)
As the laser is turned off and on, a chirp appears when
the current represents the “0” bit (I0) is smaller than the
current for the laser to works linearly (Ith) as in Fig. 1. Chirp
is described as an incident which forces the transmitted
spectrum to broaden due to variations of the carrier
frequency with time, as in Fig. 2. The appearance of chirp
limits the transmission distances and bit rates. [6][2]
Keywords- chirp, direct modulation
I.
P0
<1
P1
Figure 2. Chirp effect [2]
125
The appearance of chirp in directly modulated link
normally is considered as the limiting factor for high speed
transmission where at higher bit rates, it leads to a
broadening linewidth and thus limiting the transmission
distance. However the simplicity of direct modulation
technique is an advantage that can be manipulated in
transmission of high speed capacity at shorter range [7].
output is connected to an optical attenuator, which its output
is then feed to an optical receiver (PIN Photodiode).
A data recovery module is connected at the end of the
link, in order to observe and measure the quality of the link
through its eye diagram and bit-error rate (BER)
characteristics. The binary signal input of the data recovery
module is connected to the PRBS output and the electrical
signal input is connected to the sequence generator. This is
to enable the data recovery module to compare the
transmitted and received signal for BER analysis. The
optical power meter is connected to the attenuator output to
measure the received optical power (known as sensitivity as
BER = 10-9). An optical spectrum analyzer (OSA) and
optical time domain visualizer (OTDV) were also connected
to the laser output to observe the emitted optical signal.
As direct modulation is the simplest and cost-effective
and widely used modulation category in optical
communication system. In this modulation technique, the
light is modulated inside the light source. Unfortunately, due
to the physics of laser action and finite response of lasing
levels, the light output in direct modulation could not
immediately response to the input electrical signal.
Furthermore, the delays in turning on-off laser source occur,
similar as ASK method discussed earlier. As a result, several
undesirable effects such as chirping and pulse width
broadening occur. The process of turning on and off cycle of
the laser source makes the output spectrum and wavelength
changed over time. The pulse width broadening results from
the process of measuring the time-integrated optical
spectrum. Since the spectral width of the light source varies
quickly over time, a measurement of the optical spectrum
over a time interval which is long compared to the
instantaneous frequency changes results in broadening the
spectral width. Although this unwanted effects become more
obvious in multigigabit-per-second speeds of transmission
but it still can be useful in short range transmission.
[1][2][5].
II.
OBJECTIVES
In this paper, a simulation approach is used to determine
the channel capacity in the presence of frequency chirping.
The aim of this research is to investigate how chirp effect
directly modulated link and how it can be limited in order to
make full use of the advantage of direct modulation WDM
technique. Thus, three objectives has been outlined as (1)
find out the receiver sensitivity, (2) investigate observe the
effect of varying the bit rates on the transmitted signal, and
(3) investigate the effect of varying the fiber length on the
transmitted signal.
III.
Figure 3. Direct modulation set up
B. Experimental
The simulation is been conducted in three different
setting. The first setting is to investigate the receiver
sensitivity. The optical attenuator value in the simulation set
up is varied to obtain the BER of 10-9.
Second setting is to investigate the effect of chip as the
bit rates is varied. In this part, the bit rate of the laser
modulating data is increased in order to identify the
maximum modulating bit rate for a realistic degradation in
BER due to chirp. The optical time domain visualiser is
used to measure the amount of chirp in the optical modulated
signal. The maximum chirp is measured and recorded in
Table 1. The next step is to decrease the attenuation value
by 3dB to 29.8dB and the bit rate is increased by 1 Gbit/s
steps. This procedure is repeated until the BER reached 10-9
again.
Last setup is used to investigate the effect of increasing
the fiber length as the chirp contributes to signal spectrum
broadening and chromatic dispersion will limits the distance
of the transmission. The first step is to set the simulation bit
rates and attenuation to its initial condition (2.5 Gbit/s and
SIMULATION
A. Simulation Set up
The simulation setup is based on Fig. 3, performed with
Optiwave simulation software. In this setup the Distributed
Feedback Laser (DFB) operating at 1550nm is used. A Non
Return Zero (NRZ) pulse generator which operates as a laser
driver is modulated by a pseudo-random bit sequence
(PRBS) generator. The output of the NRZ generator is
connected to a low-pass filter which limits the bandwidth to
0.75*bit rate before applying the filter output to the bias
input of the bias input of the DFB laser. The laser is then
been connected to the single mode fiber (SMF). The fiber
126
32.8 dB respectively). The length is set to 40km and
simulation is repeated by varying the attenuation value in
order to achieve the BER of 10-9. The procedure is repeated
by increasing the fiber length by the step of 20km.
IV.
diagrams for various but rates. The power at the receiver
does not give any impact on the changes of bit rate. The
value of the attenuation is decreased by 3 dB to allow for
loss in signal strength or light power that occurs as light
pulses propagate through the fiber.
Thus, by using
approximately -26.9 dB receiver sensitivity, only 4.95 Gbit/s
signal can be transmitted in order to have minimum error
(BER 10-9 in this case). Unfortunately, the amount of chirp
detected is too high.
RESULT AND DISCUSSION
The results for the simulation are as discussed as follows:
A. Receiver Sensitivity
The attenuation value used to obtain minimum BER of
10-9 is 32.8 dB. The power measured at the optical receiver
at this BER is known the sensitivity. This is the minimum
power at the receiver in order to have an error free
transmission. The sensitivity measured is -29.907 dBm. The
resulting eye diagram is as Fig. 4.
2.50 Gbps
3.50 Gbps
4.50 Gbps
5.00 Gbps
5.30 Gbps
5.50 Gbps
Figure 4. Eye diagram at BER of 10-9
B. Chirp versus Bit Rates
TABLE I.
Figure 5. Eye diagram for various bit rates
CHIRP VERSUS BIT RATE MEASUREMENT
Fibre length = 20 km
C. Chirp versus Fiber Length
From the results in Table II and the eye diagram on Fig.
6, it shows that as the fiber length is increased, the
attenuation and receiver sensitivity is decreased. This
finding proved that the attenuation limits the transmission
distance. The power penalty is the difference in receiver
sensitivity in dB, used to measure signal degradations.
Higher receiver input power were required as the quality of
the signal degraded.
Bit Rate (Gbit/s)
Attenuation
32.8dB
Attenuation 29.8 dB
Bit rate
(Gb/s)
2.50
3.50
4.50
4.95
5.00
5.30
5.50
Sensitivity
(dBm)
-29.9
26.9
26.9
26.9
26.9
26.9
26.9
Chirp
(GHz)
36
70
96
74
70
162
150
1.4*10-9
4.7*
10-21
5.3*
10-18
1.8*
10-9
1.8*
10-8
1.2*
10-5
1.0*
10-4
BER
From Table I, it can be concluded that the amount of
chirp measured is increased as the bit rate is increase with
the same receiver sensitivity. Figure 5. shows the eye
127
TABLE II.
CHIRP VERSUS FIBER LENGTH MEASUREMENT
V.
The completion of the simulation experiment enables
detail investigation on the influence of chirp in WDM direct
modulation. Generally, it was proven the frequency chirp
affected both the transmission bit rates and distance. With
the presence of frequency chirp, the best transmission bit
rates is 5.00 Gbits/s. For longer transmission distance, the
bit rates must be reduced for better performances. Therefore,
it is useful to know the transmission bit rates and the distance
before selecting the modulation technique going to be used.
Bit Rate = 2.5 Gbit/s
BER = 10-9
Fiber Length
(km)
Attenuation
(dB)
Sensitivity
(dBm)
20
32.8
-29.907
40
28.5
-29.607
0.3
60
23.2
-28.307
1.6
80
18.3
-27.407
2.5
100
12.5
-25.607
4.3
120
1
-18.107
11.8
20 km
CONCLUSION
Penalty
(dB)
ACKNOWLEDGMENT
To every special people at Universiti Teknikal Malaysia
Melaka (UTeM) who provided necessity, fund and
enthusiastic assistance, thank you.
40 km
REFERENCES
[1]
60 km
80 km
[2]
[3]
[4]
[5]
100 km
120 km
[6]
[7]
Figure 6. Eye diagram for various transmission distance
128
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