Fiber-optic communication systems require a light source to generate the signal that the fiber transmits. The Transmitter Optical Sub Assembly (TOSA) is responsible for the emission of light. This assembly comprises a light source, such as a laser diode or a semiconductor light-emitting diode (LED), an optical interface, a. Discover EXFO's broad range of optical light sources that cater to various testing requirements: singlemode or multimode, polarized or non-polarized, broadband or narrowband, tunable, ITU-wavelength-centered and much more. Sources for single-mode fibers and multi-mode. Specialized Products offers LED and laser fiber optic light sources from AFL, EXFO, VIAVI, Photonix, Tempo Communications and other leading brands.