Laser diode power is primarily controlled by adjusting the drive current, often using feedback from a photodiode to maintain stable output.

Using Drive Current

The most direct method to adjust a laser diode's power is by controlling the injected current. The laser's optical output power is linearly related to the current above the threshold, as described by the L-I curve. Increasing the current increases output power, while decreasing it reduces power. However, laser diodes are sensitive to overcurrent and voltage spikes, so a stable current source or specialized laser driver is essential to prevent damage and ensure consistent operation .

Feedback Control with Photodiodes

Many laser systems use Automatic Power Control (APC), which relies on a photodiode to monitor a fraction of the emitted light. The photodiode generates a current proportional to the optical output, which is converted to a voltage and compared to a reference. An error signal adjusts the laser driver to maintain the desired power level. This method compensates for temperature changes, aging, and other fluctuations, ensuring stable output in both continuous-wave (CW) and pulsed operation .

Pulse-Width Modulation (PWM) and Averaging Control

For applications requiring variable output, PWM can modulate the laser diode's brightness. However, simple averaging control units are limited in range and may not work effectively at high pulse frequencies. Peak optical power control units can regulate individual pulses, storing values during pulse pauses to maintain consistent output .

Temperature Considerations

Laser diode output and wavelength are sensitive to temperature. The threshold current increases with temperature, and the center wavelength shifts approximately 0.3 nm/°C. Using a temperature controller helps maintain both stable power and spectral characteristics, which is critical for precision applications .

Practical Tips

• Always use a laser driver with protection features such as soft-start circuits and transient suppression to prevent damage .
• For precise control, integrate a photodiode feedback loop or APC system.
• Avoid exceeding the maximum rated current to prevent catastrophic failure.
• Consider the operating mode (CW or pulsed) and adjust the control method accordingly. By combining current control, feedback mechanisms, and temperature management, you can safely and effectively adjust the power of a laser diode for a wide range of applications.