A low-cost high-repetition-rate picosecond laser diode pulse generator

In this paper, we describe a low cost pulse generator based on bipolar transistors designed to emit very short light pulses with a simple laser diode. Its main characteristics are laser pulses as short as 60 ps Full Width at Half Maximum (FWHM) and a repetition rate from DC up to 100 MHz. The circuit is based on three standard RF bipolar transistors which emit a short electrical pulse of 1.2 ns FWHM with 5 Volt amplitude peak-peak into 50 Ω. This unit can directly drive a classical laser diode to emit short laser pulses. A DC current generator is added in order to polarize the laser diode near its emission point and reduce the power delivered by the pulse generator and enhance its life time. This generator allows the adjustment of the emitted power of the laser diode. If the power emitted is too high, secondary pulses are observed. There is an optimal power where there is a single short pulse with duration down to 60 ps FWHM. The system is simply triggered by a TTL signal and its structure allows the generation of a single shoot, or a complex pattern generation and repetitive generation. The unique limitation is the delay between pulse emissions which must be greater than 10 ns. The typical optical power with a standard laser diode at 660 nm (45 mW continuous) is 1.7 mW with a repetition rate of 80 MHz and a pulse width down to 60 ps measured with a synchroscan streak camera. The peak power is about 350 mW. The pulse to pulse jitter is less than 10 ps rms. This system can be a competitive alternative to expensive commercial products.