Download Low Cost Laser Diode Controller, High Frequency Modulator and

International Workshop on Photonics and Applications. Hanoi, Vietnam. April 5-8, 2004
Low Cost Laser Diode Controller, High Frequency Modulator and
Light Pulse Detector for Students Laboratories (*)
P. Podinia - P. H. Phamb - C. D. Trinhb
a- Dept. of Physics - Parma University, Italy
b- Faculty of Technology - Hanoi University, Vietnam
E-mail: [email protected]
(*) Project co-financed by the Italian Ministry of Foreign Affairs (DGPC Uff. V),
Parma University and Hanoi National University
1. Introduction
Many factories on the market provide instruments specifically designed for student
laboratory courses. This is also true in the field of optical telecommunications. These
instruments are usually well assembled, sturdy and provide efficient protection against the
most common “accidents” happening in any student laboratory. Normally many functions
are implemented such as data acquisition, analysis, plotting, best fit etc… with the help of
an internal or external PC board. However, aside from being quite expensive, these
instruments tend to lack flexibility. Certainly, very often, one can change the internal
settings via software but, in so doing, a first level student is not aware of what is actually
happening inside the system, what modifications are made and where. As a consequence,
we think that these types of instruments are very useful in second level laboratories where
the students have already acquired competence and confidence in a specific field. However,
they are often less useful as a first approach, since they hide the problems encountered and
how they are solved.
As a “first approach”, we think that simply-designed instruments are more useful, since
their electronic layouts can be easily understood, the internal settings can be directly made,
eventually supervised by a tutor, and the protection limits can be changed to accommodate
different necessities, allowing the students to have direct experience with basic problems
and their solutions. This eases the way to enable students to gain both competence and
confidence.
One basic aspect of optical telecommunications concerns the proper handling of laser
diodes. Their average DC current must be controlled and, at the same time, modulated at
high frequency, the temperature controlled by mean of a Peltier cell, the light pulses or the
continuous wave intensity detected and the signal amplified. We chose to deal with these
issues by designing a very simple DC current and temperature controller that also allows
the operator to modulate the current at high frequency within the laser diode. The
calibration and protection settings have easy access, enabling selection of maximum DC
current through the laser diode and the Peltier cell at hand, and also to provide the proper
current for the thermistor used as temperature sensor. The prototypes were built purposely
using “low technology”, which means that no printed board or SM devices were used
except for a dual in-line operational amplifier and standard high frequency transistors.
This set-up will be used in the student optics laboratory course, programmed for the
next semester, for experiments designed to determine some characteristics of optical fibres
such as attenuation factor and dispersion.
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International Workshop on Photonics and Applications. Hanoi, Vietnam. April 5-8, 2004
2. DC current controller
In this section we present a brief description of the current controller. The electronic
scheme is very simple as shown in figure 1 and involves, basically, just one IC with 4
operational amplifiers and one power transistor.
transistor Q1 BD135
Power Supply = +/- 12V
OP= TL084
C4
1K
R13
1m
L1
10-ohm
C8
C6 10u
1n
C3
V1
R1
12V
V3
R11
Q1
L2
1m
1nF
R6 10K
10-ohm
12
10-ohm
R15 1meg
R14
X1D
R16 100
+12 Volt
1uF
beta= 124.2
V4
R4 10K
12
10K
X1A
X1B
R8 1K
R3
C7
R18
C9
10u
1n
50 ohm R5 10K
C5
1nF
R2
C1
1K
100nF
1K R9
10K R7
-12 Volt
R12
T1
L D current
output
10K R10
R19
+12V
1nF
C2 X1C
tra
R17
IVm1
L D current
monitor
10K
Figure 1. Electronic scheme of the laser diode DC current controller
First of all, the power supply has been heavily filtered to reduce the laser current noise.
The resistor R3 acts as a current sensor, and its voltage drop is detected by the operational
amplifier (OP) labelled X1A and compared with the reference voltage, controlled by a 10
kΩ ten-turn helipot available on the front panel through OP X1C. The error signal is
detected by OP X1B and integrated by OP X1D. Its output is the feedback signal for the
power transistor Q1. The current is conveyed with a 50Ω coaxial cable to the laser diode,
housed in an external container together with the high frequency modulator, Peltier cell,
temperature sensor and heat dissipater. This solution forces us to fix the value of the current
sensor resistor R13 to 50Ω since it must also act as a matching resistor for the coaxial cable
to avoid reflections when the high frequency modulator is operating.
The laser diode DC current can be monitored on a 1 mA full scale analogue ammeter,
mounted on the front panel, and can be calibrated by acting on the trimmer R19. By
changing the setting of trimmer R12, the maximum current can be selected from a few mA
to about 150 mA. If higher currents are needed, the power transistor must be changed and
the power supply voltage increased.
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International Workshop on Photonics and Applications. Hanoi, Vietnam. April 5-8, 2004
3. Temperature controller
The temperature controller was designed to operate with NTC thermistors as sensors
and a Peltier cell as heat transfer. The board is mounted in the same module with the
current control circuit. Figure 2 shows the electronic layout which follows, essentially, a
standard design.
100K R14
Q1=BDX53(NPN) Q2=BDX54(PNP)
Q3=BC108(NPN) Q4=BC178(PNP)
R9
R12
100n
5600 C1
10K
X3A
+5V
X2A
10K R7
IC3a
IC2
R5
X4A
-5V
R18
IC1c
10K
IC1d
X1B
R21
IC1b
R11
X1
X1A
R8
R10
10k
X3B
beta= 100
Ic3-tl081
D1
Q2
Q3
-5V Peltier
beta= 100
IC3b
1ohm 2wat
R2010K
10k
IVm1
D2
IC1a
R15
Q4
beta= 100
10k R3
R4
Q1
IC3c
10K
R2
R1
10K
+5V
beta= 100
2200
X2B
10K R19
C2 3.3u
R13
5K
Voltmeter(Temp.)
10K
IC1,IC3 =TL084
IC2=TL081
R6
10K
Thermistor
Figure 2. Temperature controller electronic scheme.
The operational amplifiers IC1a,b,c generate the current for the thermistor which can be
set by acting upon trimmer R1 while, with trimmer R5, the desired reference voltage and,
therefore, the desired operating temperature are selected. IC2 detects the error signal
integrated by IC3 which drives the power section represented by transistors Q1 and Q2.
The Peltier cell is protected against current overload by transistors Q3 and Q4, limiting the
maximum current possible which can be selected with trimmer R13.
The voltage drop across the thermistor is detected by IC3b, amplified ( in our case by 2)
to increase the sensitivity, and the amplifier drives a 10 Volt full scale analogue voltmeter
mounted on the front panel. A calibration curve Temperature versus Voltage is provided to
the students to monitor the status of the laser diode.
In our case, the NTC thermistor resistance is 10KΩ at 25 C°, and the current overload
protection for the Peltier cell has been set at 2 Amp. However, with this design, an NTC
thermistor and cell with different characteristics can be used by properly setting the
reference voltage, thermistor current and cell overload current protection.
3. Modulator
Extreme care was taken in mounting the modulator. The electronic part was assembled
as close as possible to the laser diode and heavily shielded. The disposition of the
components was designed to minimize coupling and stray capacitance and, moreover, all
the wiring powering the thermistor, Peltier cell and the 7 Volt power supply line was
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International Workshop on Photonics and Applications. Hanoi, Vietnam. April 5-8, 2004
carried to the module through a multi-wire coaxial cable in which each line was singly
shielded. A low pass filter for the 7 Volt line was also provided as shown in figure 3.
L1
200uHn
1u
1n
From Laser Diode
controller
T2
Q1
R1
T1
C2
R4
1u C1
50
D1
R3
390
10 R8
1k
Beta= 93 Beta= 93
10n
R7
tra
Q2
+7 volt
C3
C4
tra
0
50
R5
390
1k
R6
V2
50
R2
Ext. Pulse gen.
D1= Laser Diode
Figure 3. Electronic scheme of the high frequency modulator.
As can be seen, the circuit is simply a signal transducer, where resistor R7 determines
the relationship between input signal amplitude and current variation in the laser diode.
Normally we used a 50Ω resistor, although tests have shown good behaviour with a
resistor between 30 – 100 Ω.
NPN silicon planar RF transistors ztx325 were used with a 1.3 GHz bandwidth and
maximum peak collector current of 50 mA. However, preliminary tests have shown that if
two transistors are mounted in parallel, a current variation through a load up to 80 mA is
possible if needed.
E v a lu a te d C u r r e n t t h r o u g h R 7 ( fig u r e 3 )
20
C u r r e n t I n t e n s it y m A
15
10
5
0
-5
0
1
2
3
4
5
6
7
8
T im e n s
Figure 4. Evaluated total current output from the emitter of transistor Q1 (see figure 3).
To test the modulator, we mounted an LD510A diode, a single transverse mode
AlGaInP laser, with maximum optical power of 10 mW at 650 nm. After setting the laser at
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International Workshop on Photonics and Applications. Hanoi, Vietnam. April 5-8, 2004
the lasing threshold, using the external pulse generator, we sent square pulses of 1 Volt in
amplitude and about 5 ns long with a rise and fall time less than 1 ns to the modulator
input, at a frequency of 1 kHz. The voltages at the two ends of R7 were acquired, in two
subsequent measurements, with a two GHz bandwidth digital oscilloscope using a high
impedance probe in order to not disturb the circuitry. The current increment through the
resistor can be evaluated from these data. The result is shown in figure 4. Although we can
not be sure that all the current flows through the diode, by observing that the voltage
variation, as measured at the junction between R7 and C2 (see figure 3), has an amplitude
very close to 1 Volt while the voltage variation measured directly on the diode anode is
0.15 Volt, the dynamic impedance of the ensemble-(diode plus 50 Ω matching
impedance), can be estimated to be about 9 Ω. It follows, then, that the dynamic impedance
of the diode is close to 10 Ω and that about 80% of the current flows through the laser
diode. Tests with higher input pulses gave the same results up to an estimated maximum
current of 40 mA.
Continuous wave modulation is also possible, up to 200 MHz, but the input signal
amplitude must be reduced to 0.5 Vpp to avoid transistor overheating.
5. Light Pulse Detector and Amplifier
As light detector, we selected a silicon photodiode AEPX65 with a nominal rise time of
about 1 ns, sensitive area of 0.5 mm2 and 10 nA dark current. As for the amplifier we chose
to use an Elantec operational amplifier EL2075 with a 2 GHz gain-bandwidth product,
stable at a gain of 10 with nominal –3dB bandwidth of 400 MHz.
The circuit design follows the standard current to voltage transducer configuration as
shown in figure 5.
Power supply +/- 5 Volt
- 5 Volt
R3,R4=50 ohm ; R2=1.5 kohm; R1=100 ohm
R2
X1
D1
R3
oscilloscope
T1
tra
R1
R4
D1= Photodiode
Figure 5. Light detector and current to voltage transducer layout.
Resistor R1, which in theory would be not be necessary in a current to voltage
transducer, was inserted to stabilize the amplifier, causing an estimated loss of 15% in the
(voltage output)/(current) ratio. The photo diode was mounted as close as possible to the
amplifier in a shielded box provided with a small hole for the input of the laser diode light.
The photodiode was exposed to a small fraction of the unfocused laser beam, while the
modulator was driven under exactly the same conditions used to obtain the current data
shown in figure 4. The resulting response was acquired by means of the digital oscilloscope
with the configuration shown in figure 5. The observed output pulse shape is reported in
figure 6.
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International Workshop on Photonics and Applications. Hanoi, Vietnam. April 5-8, 2004
A m p lifie r R e s p o n s e
60
A m p litu d e m V o lt
50
40
30
20
10
0
0
5
10
15
20
tim e n s
Figure 6. Amplifier and detector response to a 5 ns long light pulse.
As shown, the rise time can be considered adequate (about 3 ns) but the fall time (about
10 ns) certainly not. In effect, the minimum current pulse time length variation which could
be estimated with some accuracy was more than 2 ns, limiting the possibility to use such a
configuration in experiments designed to put in evidence the effect of fibre dispersion on
light pulse duration.
To be sure that the slow falling time observed was not due to some kind of combined
effect between laser diode and photo-detector, the latter was directly connected across the
50Ω oscilloscope input impedance, without any power supply, and the laser diode placed
just in front at the distance of 1 cm. A symmetric top rounded pulse, 25 – 30 mV in
amplitude, was observed on the oscilloscope with a rise and fall time of about 2 - 2.5 ns,
and with a time width at half height ≅5 ns, proving that both laser and photodiode were
behaving correctly.
6. Conclusions
As previously stated , we are planning to use the equipment described next semester in
an introductory student laboratory in optoelectronics. The idea is to give the students a
pamphlet with the characteristics of the laser diode, thermistor and Peltier cell and have
them perform all the calibrations and set the protections. They will mount the laser and take
all precautions necessary to avoid damage from electrostatic discharges. Then using an
optical power-meter, the students will also be required to obtain the output power versus
current characteristic. The laser control module described is quite adequate for all these
purposes.
However, the possibility of testing the performance of the modulator with short pulses
appears more troublesome. Tests described above indicate that the modulator behaves
correctly with pulses as short as 5 ns but the oversimplified design of the light detection
system is not up to the task ( We are far from the performance claimed by the constructor
of 400 MHz bandwidth at a gain of 10) . However its performance does become adequate
for pulses longer than 40 – 50 ns, and the modulator and light detector can also be used in
experiments designed to estimate intensity attenuation of light pulses in fibres
A new amplifier design, using higher technology and different devices, is under
construction at the moment.
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