In pursuit of improvements to the launch-condition specification for multimode components, the International Electrotechnical Commission (IEC) is preparing a new standard on the measurement of the mode transfer function (MTF) of fiber-optic light sources (see Lightwave, August 2005, Standards Watch, p. 14).
The MTF is a derivative of the intensity profile obtained from a near-field scan performed on the output end of a multimode patch cord connected to the source of interest. This launch condition then becomes the input to the multimode component to be tested. The MTF provides a representation of the power distribution across all modes of the fiber.
Two other metrics that characterize the launch condition have been defined based on the MTF data:
• Mode power distribution (MPD) is found by multiplying the MTF by the relative mode number. Because the number of discrete modes in a mode group is proportional to the mode group number, the MPD represents the mode group power distribution.
• Relative power distribution (RPD) is found by integrating the MTF from the highest mode number to progressively lower mode numbers. It determines the relative amount of power in the various modes. That is a key feature since the magnitude of power loss through a component is proportional to the percentage of power carried in the attenuated modes.
The utility of these metrics to define launch-condition boundaries relies on selecting a target launch condition with defined tolerance limits. A target MTF has been proposed that closely approximates the power distribution resulting from a near-overfilled launch that has passed through several connections and therefore represents a launch that does not induce transient losses that cause variability.
Paul Kolesar is responsible for fiber systems engineering at Systimax Solutions and is chair of TIA FO-4.1.2 Multimode Transmission Systems. He can be reached at email@example.com.