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Optical response and pulse shaped modulation of high power light emitting diodes

While light emitting diodes (LEDs) are used for general illumination, they are expected to find application in optical data transmission due to their fast optical response. In the present work, the optical response of the high power LEDs (3W) was examined in 4MHz pulse modulation. An optical pulse shaping strategy was devised by the stepped pulse drive with an overvoltage carrier injection and a proper extraction respectively in the pulse rising and falling time. The pulse driving and shaping circuit for the high power LED modulation was developed based on this strategy. The modulation bandwidth limitation was discussed in terms of the resistor-capacitor (RC) equivalent circuit. The light field of the LED chip surface under the injection-extraction pulse drive was observed to present different brightness around the electrode, resolving the carrier distribution in the transportation time. In disagreement with the RC explanation, the result shows that the optical response speed and bandwidth are limited by the lateral drift distance of the carriers between the electrode strips, especially for the high power LED with large chip size.