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It is theoretically shown that the variance of fluctuations peaks in the vicinity of threshold of instability in a system having third-order nonlinearity provided the nonlinearity depends on the pumping rate.

A beat signal between longitudinal modes of the laser cavity is detected, amplified and fed back to an electro-optic element inserted in the laser cavity. In this case, if the gain of the beat amplifier is large, this element should have a large equivalent third order nonlinearity. Since this nonlinearity make the modes couple, many types of mode-looking phenomena are expected. In this paper, a theoretical analysis is presented and compared with experiments using a He-Ne 6328 Å laser.