Fig.2 Mode-partition noise

Mode-partition noise

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Because spread and display this picture, there is a case to look coarse.

A Fabry-Perot-resonator type of semiconductor laser oscillates in multiple longitudinal modes. Also, a laser that oscillates in only a single longitudinal mode when not modulating may oscillate in multiple modes when modulating. In a semiconductor laser, which longitudinal mode will oscillate and in what way is a random phenomenon. Even if optical pulse output as the sum of longitudinal modes is fixed during on/off modulation in a semiconductor laser, optical output in each longitudinal mode will fluctuate. This is called the “mode-partition phenomenon.” The figure shows the pulse-waveform trace for a single longitudinal mode and the frequency distribution of pulse peak value.

In optical fiber transmission using multimode-oscillating semiconductor lasers, a difference in propagation delay occurs among longitudinal modes due to a group velocity dispersion in optical fiber, and fluctuation occurs in received waveforms due to the mode-partition phenomenon. In short, waveform fluctuation not observed at a transmission point will be manifested at a reception point. This waveform fluctuation, called mode-partition noise, is the dominant noise in high-speed, long-distance transmission and must be suppressed. This is why a single-longitudinal-mode laser is needed.

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