Fiber optic couplers are optical devices that connect three or more fiber ends, dividing one input between two or more outputs, or combining two or more inputs into one output. The cable type accepted by fiber optic couplers can be could be single mode or multimode. Single mode describes 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. Choices for coupler type include single window, dual window, or wideband. Single window couplers are designed for a single wavelength with a narrow wavelength window. Dual wavelength couplers are designed for two wavelengths with a wide wavelength window for each. Wideband couplers are designed for a single wavelength with a wider wavelength window. Wavelength choices for fiber optic couplers include 633 nm, 830 nm, 1060 nm, 1300 nm, and 1550 nm.
Important coupler performance specifications to consider for fiber optic couplers include number of input ports, number of output ports, insertion loss, splitting ratio, and polarization dependent losses (PDL). The number of input and output ports found on the coupler is important to consider. Couplers can be star or tee types. Star couplers have N inputs and N outputs (NxN). Tee couplers have 1 input and N outputs (1xN). Insertion loss is the attenuation caused by the insertion of an optical component. The splitting ratio is the distribution of power among the output fibers of a coupler is also referred to as coupling ratio. The polarization dependent losses are defined as the attenuation caused by polarization. Common features for fiber optic couplers include pigtail, rack-mountable, polarization maintaining, and bi-directional. A pigtail is a short length of fiber attached to a fiber optic component such as a laser or coupler. A rack mountable coupler can be rack mounted for convenience and efficiency. In a polarization maintaining coupler the polarization of the incoming signal is preserved. A bi-directional coupler has bi-directional transmission on a single fiber. An important environmental parameter to consider is the operating temperature.
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