The watt (W), the fundamental unit of optical power, is defined as a rate of energy of one joule (J) per second. The term usually refers to a device used for measuring the average power in fiber optic systems. Understanding how to calculate optical power is essential for designing and analyzing systems such as fiber optic communications, laser systems. An optical power meter measures the photon energy in the form of current or voltage from an optical detector such as a semiconductor, a thermopile, or a pyroelectric detector.