Abdukerim Nurmemet, Grobnic Dan, Lausten Rune, Hnatovsky Cyril, Mihailov Stephen J
Opt Express. 2019 Oct 28;27(22):32536-32555. doi: 10.1364/OE.27.032536.
The combined effect of chromatic dispersion and conical diffraction (i.e., off-plane diffraction) in femtosecond laser inscription of fiber Bragg gratings using the phase mask technique is characterized by measuring the light intensity distribution after the phase mask. As the distance from the mask and the observation point grows, chromatic dispersion and conical diffraction introduced by the mask gradually decrease the peak intensity inside the line-shaped focal volume of the cylindrical lens that is used to focus the femtosecond pulses inside the fiber. We also show that at a certain distance from the mask spherical aberration introduced by the plane-parallel mask substrate is cancelled out by conical diffraction and, at a different distance, chromatic aberration of the cylindrical lens is cancelled out by chromatic dispersion of the mask. These two independent cancellation effects lead to sharpening of the line-shaped focus and the consequent growth of peak light intensity inside it. The above phenomena become especially pronounced for tightly focused femtosecond laser beams and small-pitch phase masks, which, in turn, allows one to choose experimental conditions to inscribe Bragg gratings in polymer-coated non-sensitized 50 µm fibers.
利用相位掩膜技术在飞秒激光写入光纤布拉格光栅过程中，色散和锥形衍射（即离面衍射）的综合效应通过测量相位掩膜后的光强分布来表征。随着掩膜与观测点之间距离的增加，掩膜引入的色散和锥形衍射会逐渐降低用于聚焦光纤内飞秒脉冲的柱面透镜的线形焦斑内的峰值强度。我们还表明，在距掩膜一定距离处，平行平面掩膜基板引入的球差会被锥形衍射抵消，而在不同距离处，柱面透镜的色差会被掩膜的色散抵消。这两种独立的抵消效应会使线形焦点变锐，并导致其内部峰值光强增加。上述现象对于紧聚焦飞秒激光束和小间距相位掩膜尤为明显，这反过来又使人们能够选择实验条件，在聚合物涂覆的非敏化50 µm光纤中写入布拉格光栅。
