Electro-Optic Modulator Drift Compensation Based on DSP System Pham Toan Thang1, Vu Van Yem2, Bach Gia Duong3 and Bernard Journet1, Member IEEE 1SATIE, Institut d'Alembert, ENS Cachan,
Trang 1Electro-Optic Modulator Drift Compensation
Based on DSP System
Pham Toan Thang1, Vu Van Yem2, Bach Gia Duong3 and Bernard Journet1, Member IEEE
1SATIE, Institut d'Alembert, ENS Cachan, UniverSud, CNRS
61 Avenue du Président Wilson, 94230 Cachan, France
2School of Electronic and Telecommunications Hanoi University of Science and Technology
1 Dai Co Viet Road, Hanoi, Vietnam
3University of Engineering and Technology Vietnam National University in Hanoi
144 Xuan Thuy, Quân Câu Giây, Hanoi, Vietnam
bernard.journet@ens-cachan.fr
Abstract—This paper presents a study concerning the improvement
of an electro-optic modulator operation by controlling its optical
bias point The evolution of the modulator characteristics can be
followed through its non-linear behavior by detecting the second
harmonic of a low-frequency modulating signal The control system
has been implemented in a kit based on a DSP board and a data
acquisition card; it is designed in order to keep the bias point at the
optimal value for minimizing the second order non-linear effects
So it is possible to compensate completely the effects of the
modulator transfer function drift for more than 8 hours
I INTRODUCTION Optical telecommunication systems at high data rate (higher
than some tens of GHz) require external modulation
Electro-optic modulators (EOM) are therefore the fundamental devices
used for modulating the intensity of a laser beam
The EOM is characterized by its transfer function,
corresponding to the optical output power P out as a function of
the modulating signal v m (t) applied to the electrode In case of a
MZ modulator, with an input power P , the relation between the in
output power and the applied voltage is a sine function where α
is an attenuation coefficient and η is the visibility factor [1]; V π
is called the half-wave voltage
⎥
⎥
⎦
⎤
⎢
⎢
⎣
⎡
⎟⎟
⎞
⎜⎜
⎛
+ +
π () cos
1 2
1 )
V P
t
The EOM used for this study is a Mach-Zehnder (MZ)
intensity modulator It is a commercial device made of lithium
niobate material (LiNbO3, ref MX-LN-10 from Photline
Company) with a 12 GHz modulation bandwidth It has aready
been studied in a previous work [2] The EOM has two
electrodes, one for the biasing signal (for DC or quasi DC
signals) and one for the modulating signal corresponding to the
data to be transmitted (AC high frequency signals) A laser diode
(Fujitsu FLD5F6CX) at 1535 nm wavelength provides the
constant laser beam intensity at the optical input of the EOM
The optical response of our modulator has been determined
by applying a very low frequency signal (in this case 7 Hz) triangle signal to the DC electrode of the modulator The frequency is chosen low enough to consider the transfer function
as static The resulting curve shown in Fig 1 presents the output voltage of a photodetector placed at the output of the modulator,
as a function of the slowly varying voltage applied to the DC electrode
2.5
2.0
1.5
1.0
8 6 4 2 0 -2 -4 -6
Biasing voltage (V)
Figure 1 Transfer function of the electrooptic modulator The average value of
the signal is indicated by the horizontal line
The transfer function displays a sinusoidal behavior and the equation corresponding to the experimental results in Fig 1 is given by:
sin )
where VV oM =(0.899±0.003) , VV0=(1.748±0.004) ,
-1
V ) 002 0 457 0
=
K and φ0=(0.960±0.005)rd From this equation we can calculate the half-wave voltage such as K=π V/ π and we find Vπ =(6.880±0.023)V [2]
A good transmission is obtained in case of a wide open eye pattern and the modulator must be biased for a symmetric operation The best behavior of the modulator is obtained at its
The 2012 International Conference on Advanced Technologies for Communications (ATC 2012)
Trang 2half-transparency bias points, such as A or B in Fig 1 In this
case, the symmetry of the transfer function with respect to the
average value is optimal These points are called quadrature bias
points [2]
Unfortunately electro-optic modulators are not perfectly
stable with time and the optimum bias point changes during the
operating time In fact the phase shift φ in (1) is a function of
time leading to the transfer function drift When φ changes the (t)
transfer function of the EOM is shifted accordingly, to the right
or to the left, during the operating time The drift of the EOM
transfer function can be explained by different effects such as
changes of working temperature, optical coupling efficiency and
photorefractive effects [3, 4, 5, 6]
If the bias voltage is constant, and if the transfer function is
horizontally shifted then the bias point is no more in the optimal
quadrature position Modulators made of lithium niobate material
are much more stable than electro-optic polymer based devices
[7, 8] Nevertheless a drift can be observed even at a medium
term time scale The purpose of this paper is to show how to
compensate the drift of the transfer function by using a system
based on a DSP kit
II DETERMINATION OF THE MODULATOR DRIFT EFFECTS
A The Measurement method
The technique which has been developed is based on a
synchronous detection scheme The method for estimating the
drift of the transfer function is based on an evaluation of the non
linear behavior of the modulator [8] Testing the linearity of the
modulator is performed by the way of a dither signal which is a
sine signal, applied to the biasing DC electrode for modulating
the optical beam
) 2 sin(
)
As the modulating signal v m(t) should not perturb the high
frequency modulation dedicated to the transmitted data, we have
chosen a rather low frequency f d =500Hz For detecting a non
linear effect with a good sensitivity the dither signal is a large
signal with an amplitude V D =850mV
Taking into account a DC bias voltage V 0 and the
modulating signal v d(t) applied simultaneously to the DC
electrode
) ( )
(t V 0 v t
Then the transfer function in the time domain of the EOM is
obtained from (1) and (4)
⎥
⎥
⎦
⎤
⎢
⎢
⎣
⎡
⎟⎟
⎠
⎞
⎜⎜
⎛
+ + +
=
2 )
(
)
(
)
V t
P
t
P
t
in
out
If the EOM is biased at the quadrature position then the transfer function is symmetric around this bias point, and h op (t) contains only odd harmonics In case of a drift of the transfer function, the bias point is no more at the quadrature position and the curve is no more symmetric, even harmonics will appears in )
(t
h op The synchronous detection process is built in order to detect the second harmonic of the modulating signal v d (t) The optical signal at the output of the EOM is detected by a photodetector, then filtered around 2f d ; then there is the multiplication process by a reference signal at 2f d and a low pass filtering (see Fig 2)
Figure 2 Principle of the modulator non-linearity estimation method The output of the second harmonic detection scheme indicates how much second order non-linearity there is in the modulated signal and so it is an indication about the position of bias point on the transfer function This quantity is called here the
non-linear indicator and it is noted NLI
B Experimental setup
The complete system is organized around a DSP board eZdspTM F28335 from Spectrum Digital Inc (based on a Texas Instrument TMS320F28335 DSP at 150 MHz) which is associated to a data acquisition daughter card F28335 from Link-Research Company The system can be designed in Matlab-Simulink for an easy graphical programming, and then the working sheet is exported into Code Composer Studio software for building the final application which has to be loaded to the DSP board In this system we use the digital-to-analog and analog-to-digital converters of the Link-Research F28335 card, which are working on 13 bits and ±10V range
The modulating signal v d (t) is obtained by a VCO block in Matlab/Simulink with a sampling frequency f s=40kHz leading to 80 points in one period The signal at 2f d =1kHz is build with the same method
The signal at the output of the photodetector is directly acquired by the Link-Research board The sampling frequency is always 40 kHz
The bandpass filter is an important part of the system It has been designed with the Filter Design & Analysis toolbox of Matlab In this study an IIR has been chosen, it is a second order Butterworth For designing the filter the center frequency is
1 kHz, the sampling frequency 40 kHz and the two cut-off frequency are 998 Hz and 1002 Hz The frequency response of the filter (shown in Fig 3) has been obtained from a function generator and an oscilloscope
Trang 30.8
0.6
0.4
0.2
1020 1010
1000 990
980
Frequency (Hz) Figure 3 Frequency response of the bandpass filter
Experimentally the bandwidth is found to be Δf =4.2Hz
The corresponding quality factor is therefore equal to 2f d/Δf
and so Q=238
C Determination of the nonlinear indicator
The behavior of the modulator can be determined by applying
a scanning bias voltage V0 from −8.4V to +8.5V The
modulating signal has an amplitude of 0.85V and a frequency
500 Hz This experiment is equivalent to an imposed drift of the
transfer function in a short time in order to avoid changes of the
modulator characteristics It is why the value of NLI can be
rather high The variations of NLI as a function of the biasing
voltage are presented in Fig 4 The behavior of NLI(V 0) can be
well modeled by a sine function as shown by the dashed lines in
Fig 4
A second study is performed by registering the natural drift
for a long time experiment The evolution of the NLI parameter
as a function of time has been recorded and the corresponding
results for an experimental time of 7 hours are shown in Fig 5
The peak-to-peak variation of NLI is 248mV and the
standard deviation is 56mV
-2000
-1000
0
1000
2000
8000 6000 4000 2000 0 -2000 -4000 -6000
-8000
Figure 4 Evolution of the Non Linear Indicator (NLI), when the DC bias
voltage changes from -8.4 V to +8.5 V
100
50
0
-50
-100
20 15
10 5
0
Time (s) Figure 5 Long term evolution of the non linear indicator caused by the EOM
transfer function drift
III EXPERIMENTAL RESULTS WITH CONTROL PROCESS
A Compensation of the drift
From the knowledge of NLI it is possible to build a control
system for compensating the changes of φ(t) by adding a control signal to the biasing voltage V 0 in order to keep 0
≈
NLI : the bias point is controlled at the quadrature position The control loop is designed with a PID controller [9, 10] The experimental system for controlling the bias point of the EOM at
a quadrature position is presented as it has been developped with Matlab Simulink in Fig 6
The signal v m (t) applied to the DC electrode has three components, tuned from the eZdsp-DSP-F28335 board: the DC biasing voltage V , the dither signal 0 v d (t) at f d =500Hz and the control signal V obtained from the digital PID bc
Figure 6 Experimental system for controlling the EOM bias point at quadrature position as designed with Matlab Simulink
Trang 4) ( )
(t V 0 V v t
v m = b + bc+ d (6) The generation of dither signal and of reference signal at
1 kHz is achieved by two VCO blocks The input signal at
ADC1 comes from the photodetector and the output signal
)
(t
v m is placed at DAC1 for applying to the EOM electrode
The other outputs are V bias=V b0+V bc at DAC3 and NLI at
DAC5
The system is also linked to the PC computer by an
USB/RS232 data link for data acquisition
B Variations of the NLI with the control process
The effects of the EOM drift are presented by the way of the
nonlinear indicator in Fig 7 The duration of the experiment is
30000 s or 8.3 hours
-40
-20
0
20
40
30x103 25
20 15
10 5
0
Time (s) Figure 7 Evolution of the non linear indicator during the control process
The average value of NLI is −1.314μV, the standard
deviation is 3.074mV and the peak-to-peak variation is 21mV
Comparing the peak to peak variations with the case without
control (Fig 5) there is a reduction by a coefficient 11.8 for the
NLI variations; comparing the standard deviation the reduction
of the variations corresponds to a factor 18.2 thanks to the bias
point control process For compensating the drift there is a
change of the biasing voltage The variations of
bc
b
V = 0+ as a function of time are shown in Fig 8
5850
5800
5750
5700
5650
25 20
15 10
5
0
Time (s) Figure 8 Evolution of the EOM biasing voltage during the control process
These results are preliminary results with the system based on the DSP kit As it is well known from other experiments that the temperature is also a key parameter for drift effect [11], it is important to remind that in this experiment there is no temperature control for the EOM With temperature control the drift would be reduced and the variations of the biasing voltage would be smaller The future works concern the integration of two control process (bias point and temperature) on the same DSP board
IV CONCLUSION
In this paper, it has been shown that the effects of the drift of
an EOM can be measured by determining the nonlinear behavior
of the modulator By controlling, with a digital PID, the second harmonic of a low frequency modulating signal, it is possible to optimize the bias point of the EOM for keeping the bias point in quadrature position Finally a reduction of the non linear indicator by a coefficient of at least 18 has been obtained thanks
to the bias point control process That demonstrates the feasibility
of the method and the efficiency of the designed system
REFERENCES
[1] B.E.A Saleh and M.C Teich, Fundamental of photonics,
Wiley-Interscience, 2 nd Edition, 1200 pages, 2007
[2] Dang Thanh Bui, Chi Thanh Nguyên, Isabelle Ledoux-Rak, Joseph Zyss, Bernard Journet, “Instrumentation system for determination and
compensation of electro-optic modulator transfer function drift”, IOP,
Meas Sci Technol Vol 22, 125105, 2011
[3] M Aillerie, N Théofanous, H L Saadon, “Thermo-optic effect in an
electro-optic modulation system,” Proceedings of the New Achievements in
Materials and Environmental Sciences (NAMES), 3 rd France-Russia Seminar, pp 87-91, Metz, France, 2007
[4] Weirong Mo, Nanguang Chen, “Source stabilization for high quality
time-domain diffuse optical tomography,” Proceeding of SPIE, vol 7170, 2009
[5] Jeffrey Snoddy, Yun Li, Fabien Ravet, Xiaoyi Bao, “Stabilization of electro-optic modulator bias voltage drift using a lock-in amplifier and a proportional-intergral-derivative controller in a distributed Brillouin sensor
system,” Applied Optics vol 46, no 9, 20 Mar 2007
[6] G L Li, R B Welstand, W X Chen, J T Zhu, S A Pappert, C K Sun,
Y Z Liu, and P K L Yu Wang, “Novel Bias Control of
Electroabsorption Waveguide Modulator,” IEEE Photonics Technology
Letters, vol 10, no 5, May 1998
[7] Suntak Park, Jung Jin Ju, Jung Yun Do, Seung Koo Park, and Myung-Hyun Lee, "Thermal stability enhancement of electrooptic polymer
modulator,” IEEE Photonics Technology letters, Vol 16 (1), Jan 2004
[8] Heuk Park and Wol-Yon Hwang, "Origin of direct current drift in
electro-optic polymer modulator,” Applied Physics Letters, vol 70 (21), May
1997
[9] Gene F Franklin, J David Powell, Michael L Workman “Digital Control
of Dynamic Systems,” 2nd ed., Addison-Wesley Publishing Company, pp
222-229, 1990
[10] Takahashi, Y., Rabins, M J., and Auslander, D M., “Control and Dynamic
Systems,” Addison-Wesley Reading, Massachusetts, 1972
[11] Dang Thanh Bui, Lam Duy Nguyên, Bernard Journet, “Improving the behavior of an electro-optic modulator by controlling its temperature”,
Journal of Electronics and Communications - REV, Vol 1, N°1, pp 75-79,
Jan.-March 2011