Bias current and main pulse current for the PCX-7420

To minimize the laser diode’s response time to a pulse, the PCX-7420 can deliver current pulse in two steps.

The first step is the bias pulse, which starts before the intended trigger time. The idea is to set the bias current just below the laser diode trigger point (the current can be set from 1 A to 5.8 A, and the width can be set from 0 ms to 25 ms).

The second step is the main pulse. When triggered, its current adds to the bias current. The main pulse current can be set from 1 A to 15.7 A, and the width can be set from 50 ns to 5 ms.

Note that the total current output pulse is always the sum of the bias pulse current and the main pulse current even if the bias pulse width is set to zero. So, be sure to take both into account when setting currents.

What’s the advantage in using two steps? Transitioning from bias current to main pulse current takes less time and involves a smaller current change than going from zero to main pulse current. This speeds up the laser diode’s turn-on time and reduces ringing.

Let’s say you don’t want the bias feature but you do want a 21.5 A main pulse. In that case, you can set the bias and main pulse currents to maximum and the bias pulse width to zero. That way, no current flows ahead of the main pulse; the total current is the bias current plus the main pulse current.

If you don’t want any bias current, it can be set to zero.

The minimum recommended total current for the bias and main pulse is 1 A. The PCX-7420 was designed for high current, and, as a result, it produces more noise when operating at low current than a unit specifically designed for low current. Also, at low currents the actual output may not exactly follow the commanded current. It is recommended that the user measure the output current rather than rely on the programmed value.