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Mode partition noise may become the dominant performance limitation for single-mode fiber systems. We define the mode partition coefficient for laser diodes which can be used as a universal index, like threshold current and spectral width, to characterize laser diodes. The value varies between 1 and 0, and for larger , the mode partition noise will be higher. We also demonstrate two simple techniques for measuring : the low-pass filter method and the sampling method. We discuss the values measured under different operating conditions (pulse width, dc bias) for several types of laser diodes, values are around 0.4-0.6 with short pulse width (1.5-5ns), and decreases to 0.14 pulse width of 50 ns. The largest values were obtained for a laser exhibiting strong relaxation oscillation. For that laser, did not depend on the pulse width. The value is an important factor for system and fiber design as well as laser diode design.