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Processes Resulting from the Intensity-Dependent Refractive Index Trong chương này, chúng tôi khám phá một vài quy trình có tầm quan trọng thực tế xảy ra như là kết quả của chỉ số khúc xạ phụ thuộc vào cường độ. 7.1. Tập trung tự của các hiệu ứng ánh sáng và tự hành động khác Tự tập trung ánh sáng là quá trình mà trong đó một chùm tia cường độ cao của ánh sáng thay đổi tính chất quang học của một môi trường vật chất theo cách thức như vậy mà chùm tia là gây ra đến tập trung trong. | Chapter 7 Processes Resulting from the Intensity-Dependent Refractive Index In this chapter we explore several processes of practical importance that occur as a result of the intensity-dependent refractive index. 7.1. Self-Focusing of Light and Other Self-Action Effects Self-focusing of light is the process in which an intense beam of light modifies the optical properties of a material medium in such a manner that the beam is caused to come to a focus within the material Kelly 1965 . This circumstance is shown schematically in Fig. 7.1.1 a . Here we have assumed that n2 is positive. As a result the laser beam induces a refractive index variation within the material with a larger refractive index at the center of the beam than at its periphery. Thus the material acts as if it were a positive lens causing the beam to come to a focus within the material. More generally one refers to self-action effects as effects in which a beam of light modifies its own propagation by means of the nonlinear response of a material medium. Another self-action effect is the self-trapping of light which is illustrated in Fig. 7.1.1 b . In this process a beam of light propagates with a constant diameter as a consequence of an exact balance between self-focusing and diffraction effects. An analysis of this circumstance which is presented below shows that self-trapping can occur only if the power carried by the beam is exactly equal to the so-called critical power for self-trapping Pcr n 0.61 2Ẳ0 8n0 2 7.1.1 329 330 7 Processes Resulting from the Intensity-Dependent Refractive Index Figure 7.1.1 Schematic illustration of three self-action effects a self-focusing of light b self-trapping of light and c laser beam breakup showing the transverse distribution of intensity of a beam that has broken up into many filaments. where Ằ0 is the vacuum wavelength of the laser radiation. This line of reasoning leads to the conclusion that self-focusing can occur only if the beam power P is greater than .
