Laser Diode Driver Circuit Design
A laser diode converts current into light with a threshold below which it behaves as a light emitting diode and above which it lases. It is also the most fragile component in most optical products, and almost every failure is caused by an electrical event rather than by gradual wear.
Why Current Control Is Essential
The output power is proportional to the current above the threshold, and the threshold current itself depends strongly on temperature. Driving the device from a voltage source therefore produces a power that varies with temperature and that can run away.
The forward voltage of a laser diode is low, between one and a half and three volts, and its dynamic resistance is small. A small change in voltage produces a large change in current, which is the second reason a constant current source is mandatory.
Optical feedback from a reflection back into the laser increases the output and can damage the facet. An optical isolator is used where the application cannot tolerate the reflection, and the driver has to be designed for the current that results.
Threshold, Slope and Temperature
The threshold current rises with temperature, typically by a few percent per degree near room temperature, while the slope efficiency falls. The result is that a constant current drive produces less power at high temperature and more at low temperature.
Where the application needs constant power, the drive current is adjusted with temperature using a curve stored in the controller. The curve comes from characterising the actual device, because the coefficients vary between parts.
The wavelength also shifts with temperature, at about a tenth of a nanometre per degree. In a system where the wavelength matters, such as a spectroscopy instrument, the temperature of the device has to be controlled rather than the current.
monitor photodiode and Feedback
Most laser packages include a monitor photodiode behind the rear facet. Its current is proportional to the optical power, and a control loop that adjusts the drive current to hold that current constant gives a constant output power.
The monitor photodiode is a good indicator and not a calibrated measurement. Its coupling to the laser varies between parts, so the absolute power is set at production by measuring the actual output with an external meter and recording the monitor current that corresponds to it.
The loop bandwidth has to be low, because the laser and the photodiode have their own time constants and the optical feedback path can oscillate. A bandwidth of a few kilohertz is enough to follow thermal drift and slow enough to be stable.
soft start and Power Sequencing
soft start ramps the drive current from zero over a period of milliseconds rather than switching it on. The reason is that a fast current step produces a large optical overshoot, and the first optical pulse is frequently what damages the facet.
The ramp is implemented in the current source itself, with a capacitor on the reference or with a controlled digital ramp. Either way the rise time is set by the design rather than by the switch.
Power sequencing matters as much. The supply of the driver has to be stable before the current is enabled, and the current has to be removed before the supply falls. A logic signal that enables the driver from a rail that rises late produces a transient at the output.
transient protection and Handling
transient protection for a laser diode is mainly about electrostatic discharge, and it starts in the handling procedure rather than on the board. The device is destroyed by a discharge too small to be felt.
On the board, a diode or a transient suppressor across the laser reverses the polarity of any spike and clamps it. It has to be fast, because the damage occurs in nanoseconds, and it has to have a low capacitance if the modulation bandwidth matters.
A shorting clip or a solder bridge across the pins during assembly protects the device until the board is finished. The practice is old and it is still the most effective single measure against damage during manufacture.
Driving a Modulated Laser
Where the laser is modulated rather than operated continuously, the driver has to switch the current without overshoot. A current source with a fast switch and a defined bias above the threshold is the standard arrangement.
The bias current is set just below the threshold so that the device is ready to lase, and the modulation current adds to it. The bias is usually supplied from a separate source, because the bias network has to remain quiet while the modulation current switches.
The modulation path is a radio frequency design when the data rate is high. The bond wire, the package and the printed trace all add inductance, and the impedance of the laser is only a few ohms, so the matching is done with a series resistor or a transmission line.
Layout and Thermal Design
Keep the current sense resistor and the driver close to the laser so that the current loop is short. The parasitic inductance in that loop is what produces the overshoot when the current is switched.
The thermal path from the laser to the package and to the board is part of the optical performance, because the wavelength and the threshold both depend on temperature. A thermoelectric cooler and a thermistor are used where the wavelength has to be held.
The monitor photodiode signal is a small current and should be routed away from the modulation path. A trace carrying the monitor current beside the modulation trace couples the modulation into the control loop, which produces a ripple in the optical power.
Verification and Faults
Verify the driver with an optical power meter and a current probe at the same time. The pair of measurements shows the threshold and the slope of the actual device, and it confirms that the monitor loop is holding the power as intended.
A laser that fails immediately on the first power up has almost always been damaged by a transient during assembly or by a drive current that was not limited. A laser that fails after some hours is more likely to have been operated above its rated power.
An output power that drifts with temperature indicates an insufficient compensation or a failing cooler. The measures that keep such a board consistent are collected in our PCB design release checklist, the assembly points in judging PCB quality, and the practices that keep a small current free of interference in our guide to mixed signal board design.
FAQ
Can I drive a laser diode from a voltage source? No. The threshold and the slope both change with temperature, and the small dynamic resistance turns a small voltage error into a large current error.
Why does the first power up damage the laser? A current step produces an optical overshoot. Ramp the current with a soft start and make sure the supply is stable before the current is enabled.
Is the monitor photodiode accurate enough to set the power? It is stable but not calibrated. Set the power with an external meter at production and record the monitor reading that corresponds to it.
