由時空調制不穩定性至光學紊流的相變化

The Transition from Spatiotemporal Modulation Instability to Optical Turbulence

杜台蘭(To, Orchid)

指導教授 ： 石明豐

國立臺灣大學/理學院/物理研究所/碩士(2008年)

https://doi.org/10.6342/NTU.2008.02892

摘要

調制不穩定性〈modulation instability〉是許多領域所關注的議題，諸如流體動力學、電漿物理以及光學。在光學中，調變不穩定性將使得微小的振幅或相位上的擾動被放大；也就是一道強度均勻分布的平面光，會因為其上的微小雜訊受非線性效應和散射的交互作用，而使得強度相對較大的部分變得更大，因此強度分布發生變化，出現亮條紋。當非線性超過某個臨界點，原先的亮條紋會變成較小的亮點，這就是光學紊流〈optical turbulence〉。本論文主要在探討由調制不穩定性轉變到光學紊流時，所會發生的有趣現象。
第一章介紹實驗中使用的光折變〈photorefractive〉物質機制。對於調制不穩定性細節的介紹與相關的研究整理在第二章。在第三章將呈現對應到不同電壓與光強度時，從調制不穩定性相變至光學紊流的圖形與動態變化。最後再對實驗中觀察到的有趣現象〈如干涉條紋變細及分岔〉做討論。

關鍵字

非線性光學 ； 調制不穩定性 ； 光學紊流 ； 光折變物質 ； 非立即反應 ； 自聚焦

並列摘要

Modulation instability, or Benjamin-Feir instability, is a remarkable phenomenon that attracts great attention in many branches of nonlinear science including fluid dynamics, plasma physics, and optics. In optics, because of MI, small amplitude and phase fluctuations from noise are likely to be amplified as a result of the interplay between nonlinearity and diffraction. Therefore, a broad light beam tends to disintegrate into filaments. When the value of the nonlinearity exceeds a threshold, the beam breaks up into light spots, which is called optical turbulence. In this dissertation, we present interesting property of the transition from modulation instability to optical turbulence.
The organization of the dissertation is as following: In chapter one, we illustrate the mechanism of photorefractive media which is used in our experiment. The detailed introduction of MI and the related studies are summarized in chapter two. Then, we focus on how the external applied voltage and the incident optical intensity result in the transition of the pattern formation and the dynamics from modulation instability to optical turbulence. At last we have a discussion about our work and give a summary in the end.

並列關鍵字

nonlinear optics ； modulation instability ； optical turbulence ； photorefractive media ； non-instantaneous ； self-focusing

參考文獻

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