Tech Note: Automatic Power Control

In this post, the third in a series of Tech Notes, we will examine Automatic Power Control (APC) in laser modules. The post will define Automatic Power Control, highlight applications where this may be particularly important, and explain how a manufacturer can address these needs.

What is Automatic Power Control (APC)?

Laser diodes require proper drive circuitry to control optical output and compensate for changes in operating conditions which could result in damage to the laser diode or shorten its lifetime. With automatic power control, the laser module drive electronics use a photodiode feedback loop to monitor the laser diode output providing a signal to enable control of the laser diode and allowing a constant output to be maintained.  The relationship between the photodiode current and the average power should be linear so by keeping the photodiode current constant the power level is constant.

Automatic power control allows for output stability over time measured from seconds to years because, as the laser diode ages and output for a given current decreases, feedback from the APC driver compensates by providing a higher current.  Laser modules with a laser diode without a monitor photodiode and without an APC driver will typically fade gradually over the life of the laser diode.  The limitation of using APC in this way is that the laser diode will eventually reach a point that such a high current has to be provided by the driver circuit to maintain the original output, that the high current itself can cause damage to the laser diode and rather than fading gradually, the laser diode fails very rapidly.

In what applications is Automatic Power Control important?


Automatic power control is an important feature in applications that require a high degree of output stability and consistency such as analytical and measurement applications.

Automatic Power Control – What ProPhotonix can offer

If your application requires APC, ProPhotonix has extensive laser diode expertise and can work with you to find the right laser diode with a photodiode for your application and develop a laser module to suit your needs. If a laser diode with a photodiode is not available, ProPhotonix can include a monitor photodiode in a laser module outside of the TO can of the laser diode. For more information, contact




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