光絲圖譜 光絲：當光功率超過自聚焦臨界閾值，伴隨自聚焦產生一條電漿通道，光脈衝在其中能保持幾乎不變的直徑傳輸很長一段距離，而不受瑞利長度限制。光絲中存在一些非常複雜的非線性光學效應：脈衝自聚焦、自相位調製、脈衝自陡峭、多光子吸收電離、錐狀輻射、電漿散焦與時空聚焦等，它在遠程遙感、雷射引雷、脈衝壓縮等領域具有光明的套用價值。高功率超快雷射脈衝在光學介質中傳輸引起的成絲現象是當前科學研究領域的前沿問題。一般認為,雷射成絲現象的物理機制主要是光學克爾效應引起的自聚焦與電漿散焦效應間的動態平衡。雷射光絲在遠程探測空氣污染物、控制閃電、人工造雨雪、遠程特別照明等領域有著廣泛的運用。
introduction
ultrashort pulses of several millijoule energy can reach GigaWatt powers that exceed the critical power for self-focusing in air. the critical power is defined as the power at which self-focusing via the optical Kerr effect balances beam diffraction. Above the critical power, and according to the paraxial wave theory, self-focusing should lead to a collapse singularity of infinite energy density in a finite distance, sinice both diffraction and self-focusing increase at the same rate with decreasing beam diameter.
light filamentation in air
a general explanation for the formation of light filaments for input peak powers several times the critical power is that, during the initial stages of the propagation, the field breaks up transversely into hot spots of high intensity under the combined actions of the nonlinear Kerr effect and diffraction, namely, modulational instability. The high intensities produced in the hot spots by self-focusing will produce nonlinear absorption via multi-photon ionization and electron plasma generation, along with
plasma defocusing, which in turn both limits the growth of the hot spots and eventually causes them
to terminate. This is the basic mechanism of filament formation, and in our case the typical propagation
length of filaments is of the order of a meter. In addition, due to spatial inhomogeneities in the input
field, the modulational instability initiates hot spots at a range of propagation distances, and filaments are created and disappear along the full propagation distance. It is assumed that static filaments were formed when the self-focusing due to the Kerr effect was balanced by plasma-induced defocusing.
characterization of filaments
There has been a controversy as to the filament size, their conductivityw4,5xand even their existence . The main experimental difficulty in the investigation of filaments is that the pulse intensity
within the filament far exceeds the damage threshold of any reflectingrtransmissive material. The theoretical studies are equally challenging because of the large range of dimensions involved transverse dimensions over a few microns to several centimeters, longitudinal dimensions over the pulse length, tens of microns, to the total propagation length of tens of meters.. Another difficulty resides in the plethora of nonlinear effects involved at these high intensities, including beam diffraction, group-velocity dispersion, self-phase modulation, multi-photon ionization, and plasma absorption and defocusing, along with the scarcity of known material parameters.
