Proceedings of the Technical Session of Institute of Physics, Sri Lanka 1 (1985) c1
Solar energy Incident on a horizontal plane was measured over a period of few months and the global insolation per day has been calculated. The total radiation has been observed to be between 18.5 MJ/m2 -d and 7.4 MJ/m2 -d. Short circuit current and open circuit voltage measurements have been taken on silicon solar module under direct sunlight radiation over a period of 30 days. Readings were taken between 0800 hrs and 1700 hrs under various cloud conditions. The current recorded was between 0.55 & 2.4 A and the voltage recorded was between 14.0 & 15.6 V. Efficiency values were found to be between 13.28 & 9.55 & while the fill factor ranged between 0.63 & 0.74. Internal resistance is also observed to vary between 4.9 & 31.0 ohms, with radiation level.
Battery characteristics obtained, showed that specific gravity of battery vary more linearly with the state of charge. Batteries were also charged by using Solar moduli M61 and M51. M51 took about 40 hrs to charge the battery form 50% charged state to fully charged state while M51 took about 40 hours depending on the cloud cover. It is very important that there should be a proper impedance matching between the battery and the cell. But even with a proper a matching conditions there will be power losses such as heat dissipation.
Back to content pageProceedings of the Technical Session of Institute of Physics, Sri Lanka 1 (1985) c2
Characterisation of DH
Semiconductor Laser Material
I. J. Dayawansa1 and J. Sarma2
1 Dept. of Electronic & Telecom. Eng., University of
Moratuwa
2 School of Electrical Engineering, University of Bath, U.K.
Ion Beam Etching is employed to etch certain regions of the top capping layer in DH Laser diode materiel in the fabrication of stripe-geometry Lasers. It can also be used to etch the cladding layer to such an extent that it will reduce current spreading an inn addition produce index-guiding in stripe-geometry lasers. This requires careful monitoring of the etching process.
A novel monitoring technique was developed for the work. The monitor consisted of two metal contacts deposited on to the top of the capping layer of the DH laser material, leaving a wide gap with parallel edges. A voltage was applied across the metal contacts and the current monitored. An analysis of the dependence of the monitor current on layer thickness and resistivity has been performed. Assuming a constant etching rate the monitor current can be shown to change as the etching process through layers of different resistivity. The layer resistivity is related to the doping concentrations of the various layers. The top capping layer is usually heavily doped in order to produce good metal contacts. Therefore the resistivity of the capping layer is less than that of the next layer. As the etching passed through the top layer and entered the next layer, the rate of decrease of monitor current dropped significantly. When the reached the p-n junction the monitor current fell to zero. These features were observed in tests of a number of material samples and indicated that the technique was suitable for characterising DH laser material. The resistivity and doping concentration of the top capping layer and the next cladding layer were experimentally determined.
Since the monitoring technique is very simple to apply it can be used to carry out controlled etching and thus produce cladding layers of any desired thickness so as to provide an index guiding mechanism for optical confinement.
Back to content pageProceedings of the Technical Session of Institute of Physics, Sri Lanka 1 (1985) c3
Demand Controlled
Pedestrian Crossing Traffic Light System (Pelican Crossing System)
J.A.K.S. Jayasinghe, I.J. Dayawansa and L.L. Ratnayake
University of Moratuwa
Demand controlled pedestrian crossing traffic lights (pelican crossings) are wanted in a domain of fairly high vehicular and pedestrian flows. Also, in many developing countries like Sri Lanka, where the road user behaviour is poor, the simpler form of pedestrian crossing like 'zebra crossings' is of little use without adequate police enforcement. However, studies have revealed that traffic light pedestrian crossing is a better and safer form of crossing even of developing countries.
A criterion was developed for the design of a traffic controller after carrying traffic studies and determine available control systems in Sri Lanka. The signal phasing was designed incorporating the processor and the colour sequence generation unit was designed including a control waveform generation circuitry. A traffic flow rate measurement unit was designed and incorporated to the system so that both pedestrian and vehicular arrivals are recorded in it to obtain the maximum benefits.
Incorporating all the above factors the most suitable processor unit was chosen using a Read Only Memory (ROM) and associated circuitry. This unit was assembled at University of Moratuwa and the suitability of it in the field was checked.
Back to content pageProceedings of the Technical Session of Institute of Physics, Sri Lanka 1 (1985) c4
High Field Effects at
the P-N Junction for Optical Modulation
I. J. Dayawansa1 and J. Sarma2
1 Dept. of Electronic & Telecom. Eng., University of
Moratuwa
2 School of Electrical Eng., University of Bath, U. K.
The impedance of the double heterostructure broad area laser diodes with GaAs/GaAlAs were measured using the slotted line at high frequencies in the range 300-500 MHz, with the diode unbiased and reverse biased up to -5 volts. The impedance enabled the depletion layer capacitance of the diodes to be estimated. The theoretical values of depletion capacitance were found to be in good agreement with the experimental values obtained from impedance measurements at zero bias. Theoretical values were calculated using measured doping concentrations.
The experimental values of depletion capacitance deviated from the theoretical values after a voltage of about –1.5 volts in many cases. The estimated depletion layer width at this voltage appears to be nearly 0.3 m, which is the usual thickness of the active layer of the DH laser diodes. Thus, the active layer is expected to be fully depleted at this voltage is expected to deviate from the theoretical value. Therefore the technique could be used to estimate the depletion layer thickness of the active layer which is otherwise difficult to be measured.
A strong electric field ' ' of the order of 105 v/cm exists within the active layer when the active layer is fully depleted. The values of were estimated, for voltages up to -5 volts. According to Franz-Keldysh effect the electric field produces a shift of the conduction band edge, and the estimated shift was found to be approx. 0.15 eV at -5 volts. This electroabsorption effect which is polarization dependent can be used to produce amplitude modulators among other applications. The construction of simple but effective modulators using this means which would have many practical applications in optical communication systems is in progress.
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