Low Cost Laser Diode Controller, High Frequency Modulator and Light Pulse Detector for Students Laboratories (*)

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1 Low Cost Laser Diode Controller, High Frequency Modulator and Light Pulse Detector for Students Laboratories (*) P. Podini a - P. H. Pham b - C. D. Trinh b a- Dept. of Physics - Parma University, Italy b- Faculty of Technology - Hanoi University, Vietnam paolo.podini@unipr.it (*) 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 nsistors. 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. 377

2 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 nsistor. +12 Volt 3 1m L1 C6 10-ohm 12 V3 10-ohm L2 1m 12 V4 C7 8 10u 10u -12 Volt C8 1n C9 1n C5 1nF Q1 beta= T1 L D current output nsistor Q1 BD135 Power Supply = +/- 12V OP= TL ohm 1K V1 12V R3 R4 ohm R5 C1 100nF +12V C3 R6 X1A R7 1nF 1nF C meg X1D 4 X1C C4 1uF R8 1K 9 1KR9 1K 1 X1B 0 IVm1 L D current monitor 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 nsistor Q1. The current is conveyed with a Ω 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 3 to Ω 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 9. By changing the setting of trimmer 2, the maximum current can be selected from a few ma to about 1 ma. If higher currents are needed, the power nsistor must be changed and the power supply voltage increased. 378

3 3. Temperature controller The temperature controller was designed to operate with NTC thermistors as sensors and a Peltier cell as heat nsfer. 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. -5V R5 IC1c Q1=BDX53(NPN) Q2=BDX54(PNP) Q3=BC108(NPN) Q4=BC178(PNP) X4A R4 +5V 8 10kR3 IC3a X1B IC1b X3A 9 X1A IC1a 10k X2B IC1d R6 1 Voltmeter(Temp.) Thermistor R7 R8 X3B IC3b 0 IVm1 R9 X2A IC2 0 10k Ic3-tl081 D1 D C1100n X1 IC3c IC1,IC3 =TL084 IC2=TL K4 C2 3.3u Q3 Q4 3 +5V Q1 Q2-5V Peltier 1ohm 2wat 5 5K 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 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 nsistors Q1 and Q2. The Peltier cell is protected against current overload by nsistors Q3 and Q4, limiting the maximum current possible which can be selected with trimmer 3. 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 Ω 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 sy capacitance and, moreover, all the wiring powering the thermistor, Peltier cell and the 7 Volt power supply line was 379

4 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. Q2 From Laser Diode Q1 Beta= 93 controller Beta= 93 T2 10n R7 C2 R5 D1 R D1= Laser Diode 1k 1u C1 1k R6 C4 1n T1 C3 L1 200uHn 1u 10 R8 R4 +7 volt Ext. Pulse gen. 0 V2 Figure 3. Electronic scheme of the high frequency modulator. As can be seen, the circuit is simply a signal nsducer, where resistor R7 determines the relationship between input signal amplitude and current variation in the laser diode. Normally we used a Ω resistor, although tests have shown good behaviour with a resistor between Ω. NPN silicon planar RF nsistors ztx325 were used with a 1.3 GHz bandwidth and maximum peak collector current of ma. However, preliminary tests have shown that if two nsistors are mounted in parallel, a current variation through a load up to 80 ma is possible if needed. 20 Evaluated Current through R7 (figure 3) Current Intensity ma Time ns Figure 4. Evaluated total current output from the emitter of nsistor Q1 (see figure 3). To test the modulator, we mounted an LD510A diode, a single nsverse mode AlGaInP laser, with maximum optical power of 10 mw at 6 nm. After setting the laser at 380

5 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 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 nsistor 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 mm 2 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 nsducer configuration as shown in figure 5. Power supply +/- 5 Volt - 5 Volt R3,R4= ohm ; =1.5 kohm; =100 ohm D1 X1 R3 T1 oscilloscope R4 D1= Photodiode Figure 5. Light detector and current to voltage nsducer layout. Resistor, which in theory would be not be necessary in a current to voltage nsducer, 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 60 A m plifier R esponse Amplitude mvolt tim e ns 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 Ω 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, mv in amplitude, was observed on the oscilloscope with a rise and fall time of about 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 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. 382

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