Design and characteristics of nanosecond pulse generator for visible light application

Abstract

Application of visible lights in sensing and medical systems is gaining interest in research. The systems used for the said purposes usually consist of a pulse generator, a visible light source and an optical fiber as the sensor. Light employed in the system is typically in short pulses rather than continuous waves. Nanosecond pulse generator is needed for those applications. This research aims to develop a nanosecond pulse generator for visible light applications. In this study, a nanosecond pulse generator based on LT1720 comparator was developed to be used with light-emitting diode (LED) as light source and its performance through optical fiber was tested. A simulation of the circuit was performed use Multisim to compare the pulse width of output pulses from simulation and the actual circuit. Six LEDs that emit lights of different wavelengths were tested to determine their output power. The optical pulse emitted by LED through a plastic optical fiber with various fiber lengths was then received by a photodetector PDA55 which demodulated the optical pulse into electrical signal. The data obtained shows that the pulse generator was able to produce square pulses with pulse width as small as 30.70 ns. Mean relative error of the pulse width between pulses generated by simulation and those from the actual pulse generator was 3.88%. LED that emits lights at 621 nm wavelength was found to produce the highest output power. The LED was also able to emit optical pulses that follow closely the electrical pulses in term of shape, although a bit of dispersion occurs resulting in longer pulses. Mean relative difference of the pulse width between electrical pulses and optical pulses was recorded at 4.40%. However, the output power of the LED was very low, restricting its use from high power visible light application. The system was able to work with optical fiber as long as 5 meters. For a system use an optical fiber longer than 10 meters, the output power is undetectable by the photodetector.