Measurements of mode partition noise of laser diodes

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