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Providing Expert Solutions for Pulsed Laser Systems
The Quest for Accurate Current SensingIntroductionMeasuring the light output of a pulsed laser diode is not the best indication of the magnitude, temporal shape, and direction of the current flowing through the device. Voltage/current reversal, which can damage a laser diode, will not be visible on the output signal of a photodetector. Buyers should beware when a manufacturer of pulsed laser diode drivers shows only a photodetector output on a datasheet. This most likely indicates that the driver does not include an accurate current sensing circuit or has overshoot during turn-on or shut-off. One option for the user is to measure the current through the laser diode with a high quality current monitor (transformer) such as ones manufactured by Pearson Electronics http://www.pearsonelectronics.com/). These monitors require one of the output leads of the driver to be looped through the monitor. If the extra inductance added by this connection (plus the load inductance) is less than the internal inductance of the driver then minimal pulse distortion will occur; however, for short pulse, low output inductance drivers, this added inductance could greatly distort the waveform and in fact might cause the current to reverse during shut-off. In this scenario, an attempt to measure the current results in more trouble than the benefit is worth. The user could also build a miniature current monitor to keep the added inductance as small as possible; however, these devices must be calibrated against a known current monitor. Most users want to spend their time working with the light output of their diodes and not designing circuits or test equipment; therefore, the most efficient and cost effective approach is to find a laser diode driver that provides accurate current sensing. |
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