This can be achieved using automatic power control (APC) by sampling the laser output with a photodiode and adjusting the drive current in a closed feedback loop. The purpose of this laser diode tutorial is to provide the information necessary to create a long lifetime, stable laser diode system. Much of the specifics are left to the user as any system can. Laser diodes (LD) are semiconductor devices that convert electrical energy into high-power optical energy. Fluctuations in temperature, aging effects, and variations in external conditions can cause instability in laser performance. Below its threshold current, a diode laser emits LED.