A fiber optic test source is laser diode or LED used to inject an optical signal into fiber to test the performance of a fiber optic system. Laser optical sources are usually used to test single mode fiber and LED optical sources. Light emitting diodes (LEDs) have relatively large emitting areas and as a result are not as good light sources as Laser diodes. However, they are widely used for short to moderate transmission distances because they are much more economical. Laser Diodes can couple many times more power to optical fiber than LEDs. They are primarily used for applications that require the transmission of signals over long distances.

Important parameters to consider when searching for a fiber optic test source include mounting, cable type, and connector type. Mounting choices for a fiber optic test source include bench top, rack mount, or hand held. Fiber optic test sources can interface with two types of cables, single mode and multimode. Single mode is an optical fiber that will allow only one mode to propagate. The fiber has a very small core diameter of approximately 8 µm. It permits signal transmission at extremely high bandwidth and allows very long transmission distances. Multimode describes a fiber optic cable, which supports the propagation of multiple modes. Multimode fiber may have a typical core diameter of 50 to 100 µm with a refractive index that is graded or stepped. It allows the use of inexpensive LED light sources and connector alignment and coupling is less critical than single mode fiber. Distances of transmission and transmission bandwidth are less than with single mode fiber due to dispersion. Some fiber optic test sources can be used for both single mode and multimode cables. Common connector types for a fiber optic test source include Biconic, D4, ESCON, FC, FDDI, LC, Loopback, MTP, MT-RJ, MU, SC, SMA, and ST. 

Important test source performance specifications to consider when searching for a fiber optic test source include center wavelength, spectral width, operating mode, and maximum power output. Fiber optic test sources produce monochromatic light. Center wavelength is the wavelength of that light. Spectral width is the wavelength interval over which the magnitude of all spectral components is equal to or greater than a specified fraction of the magnitude of the component having the maximum value. Choices for operating mode include continuous wave, 270 Hz, 1000 Hz, and 2000 Hz. The maximum output power is the power output of the optical source. 

Common features for a fiber optic test source include indicator and dual wavelength. The indicator can be a tone, a light (LED), or both. Used to indicate, power, modulation, or other signal characteristic. Dual wavelength test sources can emit two test wavelengths. Common choices for test source interface include GPIB, RJ-45, RS232, RS422, and TTL. An important environmental parameter to consider when searching for a fiber optic test source is the operating temperature