| Summary: | 碩士 === 國立臺灣大學 === 物理研究所 === 94 === Modulation instability is one of mechanisms which results in pattern formation in nonlinear wave system. The incident wave with small amplitude perturbation passes through the nonlinear system. The perturbation with some special frequencies may have the highest gain and grows up. Then it will form patterns in the output. This phenomenon is often seen in nonlinear wave system, for example, nonlinear Kerr medium, nonlinear elastic medium, optical fiber, BEC, etc. To form MI, Lighthill criterion must be satisfied. It means that the nonlinearity of refractive index must work against the natural dispersion during wave propagation. For example, a cw laser beam propagates in self-focusing medium.
On the other hand, in self-defocusing medium no pattern can be formed spontaneously. But we have reported that induced modulation instability is possible in noninstantaneous self-defocusing medium. That is, a time-varying modulation produced by interference will grow up in the self-defocusing medium because of the delayed response in the change of the refractive index. Furthermore, in this paper we want to show that patterns will form spontaneously in self-defocusing medium with incoherent feedback. Some special frequency will be picked up and experience the highest gain automatically because of the boundary condition. We will show both the experimental result and theoretical analysis.
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