Driving Laser Diodes Safely

That feedback runs the wrong way. More current heats the junction, a hotter junction conducts harder at the same voltage, and harder conduction draws still more current. The loop can run away in microseconds and take the diode with it. A current source breaks the loop (Figure 1). It sets the current you ask for and lets the forward voltage land wherever the diode needs it, so a drifting junction voltage no longer moves the operating point. Every BNC DEI driver in this note is a regulated current source for exactly this reason. If you want the deeper treatment of how a fast driver behaves into a reactive load, see the companion note on driving capacitive loads.

Add it up before you assume a driver fits. A diode bar might drop several volts, the harness and connectors add more, and a sense or ballast resistor adds its own share at full current. If the sum approaches the driver compliance, the regulator runs out of headroom at the current peak and the pulse clips. The fix is either a higher-compliance driver or a lower-resistance output path. The mapping table in section 9 lists compliance for each model so you can check headroom against your real load.

CW, continuous wave, holds a steady current indefinitely. The diode runs at a constant operating point and the full average power has to be removed by the heatsink. CW is the simplest mode and the most demanding thermally.

QCW, quasi-continuous wave, drives the diode hard but only for a burst, typically a few hundred microseconds to a few milliseconds at a low duty cycle. During the burst the diode behaves as if it were CW, but the low duty keeps the average power, and therefore the heat, far below the CW case. QCW lets you reach peak powers a CW setup could not survive.

Pulsed operation drives short, repetitive pulses, from nanoseconds to microseconds, often at high repetition rate. Duty cycle is low, peak current can be very high, and the average power stays modest. Pulsed mode is where edge speed and lead inductance start to dominate the design. For choosing the generator that shapes those pulses, see the companion note on choosing a pulse generator.

Starting from threshold rather than from zero shortens the turn-on delay and sharpens the edge, because the diode is already primed and only has to swing the pulse amplitude rather than the full operating current. Drivers built for this give you a bias channel and a pulse channel that sum at the output. The trade is a small standing optical output between pulses from the bias current, which is usually acceptable in exchange for the faster, more repeatable edge. The PCX-7401 provides a 0 to 0.550 A bias output beside its 3 A pulse output, and the PCX-7421 holds a CW bias from 0.050 A to 5.8 A and pulses up to 15.7 A above it.

This is why several DEI drivers offer variable rise-time control. Slowing the edge deliberately, even slightly, lowers the overshoot and keeps the peak within the diode rating. You give up a little edge speed and buy back margin against an overshoot-driven failure. The right setting is the slowest edge the application can tolerate, not the fastest the driver can produce.

Tie that behavior into an interlock chain. A series loop through enclosure doors, key switches, and emission indicators must be closed before the driver will enable, and breaking the loop at any point disables the driver and engages the output short. The clamp and the interlock work together: the interlock decides when output is allowed, and the short-on-disable makes the disabled state safe for the diode. Wire the interlock so the safe state is the default and emission is the exception. On the PCX-7401 and PCX-7421 the output is shorted to ground at turn-on and whenever it is not enabled; the PCX-7401 adds an output-enable key switch and a cable interlock, the PCX-7421 a front-panel key and a remote interlock, and the PCX-7500-EX disables itself when its output connector is removed.

Keep the loop small. Short leads have less inductance than long ones, and a tight, low-area loop has less inductance than a wide one. Use a twisted pair or coaxial cable so the go and return currents run close together and their fields cancel. The numbers are direct: with 20 V of headroom, 10 nH of loop inductance limits di/dt to 2 A/ns, so a 60 A step needs at least 30 ns, while 4 nH allows 5 A/ns and a 12 ns step. The PCX-7401 and PCX-7500-EX are specified with a stripline interconnect of less than 4 nH total for this reason. Treat the diode mount and the driver output as a single low-inductance assembly.

Two limits follow. The diode needs a heatsink rated for its average dissipation, with enough thermal margin that the junction stays in its safe range across the full ambient swing of the room. The driver has its own average-power and duty limits, and exceeding them, by raising the repetition rate or stretching the pulse width past the rated duty, overheats the output stage even when each individual pulse is in spec. Read the duty and average-power ratings together with the peak rating, and size the cooling for the worst-case average you will run. On the PIM-Mini-200, for example, the maximum duty cycle falls from about 2.8 percent at low current to about 0.4 percent at 200 A, which is a 200 µs pulse at 20 Hz.

For help choosing, contact a BNC applications engineer at info@berkeleynucleonics.com or 800-234-7858.