A battery or solar-powered apparatus for measuring and displaying optical power transmitted through an optical fiber. The apparatus has a measuring circuit comprising an optical detector (602), a current/voltage converter (704) and a control circuit (702) which provides timing signals and controls the power applied to the measuring circuit. The apparatus is secured within a housing (102) and is sufficiently small and light such that permanent, in-line attachment to the optical fiber is possible. In a first embodiment, a voltage proportional to the measured optical power is available at an output jack (214) attached to the housing (102). The jack further comprises a switch (1760) which enables power to the apparatus during a measurement period. An alternative embodiment comprises an analog/digital converter (1202) which provides a digital representation of the measured optical power. Low-power displays, such as liquid crystal displays (112, 312), provide direct visual output of the digitized representation of the measured optical power, selectably in units of dBm/dB.