Some laser diode packages will include an integrated photodiode, sometimes referred to as a monitor diode, which provides a convenient way for the user to monitor the diode’s optical light output and provide feedback to the laser driver. While they both measure optical light output, an external photodiode is located outside of the diode package, and often placed in the beam path or at a splitting location. This lets the external photodiode monitor the actual delivered optical output after the beam has passed through fibers, optics, or other components your system may be using.
Whether you need both an internal and external photodiode depends on your application and stability requirements. For many lab setups, your laser diode package’s internal photodiode is sufficient for measuring the raw output power of the laser with minimal complexity. If your application prioritizes the stability of the beam’s delivered optical output power, an external photodiode should be integrated into your system. Some precision systems require the use of dual monitoring; the internal photodiode provides quick, built-in feedback for immediate output monitoring, while the external photodiode ensures the stability of the beam’s delivered output power as it reaches its end point. Some applications that may feature dual photodiode use for stability and accuracy are precision spectroscopy, fiber-optic communication, and medical/industrial laser systems.