Pulse Circuits for Infrared LEDs and Visible Diode Lasers

Posted on Feb 5, 2014

This article demonstrates basic circuits for pulsing infrared LEDs and low power visible semiconductor lasers using components which are inexpensive and fairly readily available. Many interesting and useful applications can be found in the references cited here, as well as several online web pages. This article focuses on the basic circuits. The i

Pulse Circuits for Infrared LEDs and Visible Diode Lasers
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nfrared LEDS fabricated from GaAs or GaAlAs discussed here are PN semiconductor junction diodes fabricated from GaAlAs or GaAs and typically emit at wavelengths in the range 850 - 950 nm (with 880 and 915 nm being two readily available choices). Uses include infrared remote controls for consumer appliances. Typical specifications are DC optical power output ranging from 1 to 10 mW at DC forward currents of 20 to 100 mA with forward voltages from about 1. 3 to 1. 7 V. Considerably higher peak power outputs can be achieved if the LEDs are pulsed with short pulses in the range of 1 us to 100 us with low duty cycles of 1 to 10%. Optical pulse rise/fall times available from these LEDs range from ~ 500 ns to 20 ns (corresponding to bandwidths on the order of 1 MHz to 20 MHz). Visible semiconductor lasers found in common laser pointers emit at about 650 +- 20 nm and are commonly in the Class IIIA Laser Product category, emitting less than 5 mW under DC bias conditions from 4. 5 to 6. 0 V powered by three or four 1. 5V button cells. (Visible LEDS at this wavelength are also commonly used in audio digital optical output interfaces (S/PDIF) but power levels are only about 30 microWatts). Evidently the laser chips in most low cost laser pointers use no protection or biasing circuitry at all and the chips are reported to have modulation bandwidths of several hundred MHz extending the range of useful application considerably beyond that...

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