The voltage across a pulse laser diode is determined by the load and driver, with compliance voltage setting the maximum allowable voltage for safe pulsed operation.

Voltage in Pulsed Operation

In pulsed laser diodes, the driver typically controls current, not voltage. The voltage across the diode is a result of the current applied and the diode's dynamic resistance. Pulsed operation minimizes thermal effects because the pulse duration is very short (often nanoseconds to microseconds) and the duty cycle is low, so the forward voltage drop is primarily determined by the instantaneous current rather than heating effects .

Compliance Voltage

Pulsed-current drivers have a compliance voltage, which is the maximum voltage the driver can apply to maintain the programmed current. If the diode or load impedance is too high, the driver will attempt to reach this maximum voltage, and exceeding it can damage the diode or trigger protection circuits . The actual voltage across the diode during a pulse is usually below this compliance voltage and depends on the diode's forward characteristics and series resistance.

Typical Voltage Ranges

• VCSELs and low-power diodes: Forward voltage is often 1.5–3 V during pulses.
• High-power edge-emitting diodes: Forward voltage can range from 2–5 V or higher, depending on the peak current and pulse width .
• High-current nanosecond pulses: Drivers can handle tens to hundreds of amps with voltages up to tens of volts, but the exact voltage depends on the diode and driver design .

Design Considerations

• Pulse width and rise/fall times: Shorter pulses require careful layout to minimize parasitic inductance and capacitance, which can affect voltage overshoot .
• Driver type: Current-controlled drivers are preferred for laser diodes because they prevent overcurrent damage, while voltage pulsers are used for high-impedance or specialized loads .
• Thermal effects: Even in pulsed operation, excessive voltage or current can cause localized heating, affecting forward voltage and optical output .

Summary

The voltage across a pulsed laser diode is not independently set; it is a function of the current applied and the diode's characteristics. The driver's compliance voltage ensures safe operation, while the actual forward voltage during a pulse depends on the diode type, peak current, and pulse duration. Proper driver selection and circuit layout are critical to maintain stable voltage and prevent damage during high-speed pulsed operation .