A simple scheme based on the uniform distribution for the placement of numerous laser beams in the context of fiber-based laser fusion is proposed. It is theoretically demonstrated that all modes of the geometrical factor can be eliminated if sufficient laser beams are uniformly distributed on the sphere. In the case of a finite number of laser beams, a quasi-uniform distribution of beams can be achieved based on the equal area subdivision algorithm. Numerical simulations indicate that with the increasing number of laser beams, the order of the dominant geometrical mode increases, and the irradiation nonuniformity decreases accordingly.
A scheme for measuring a small variation rate of the refractive index (RI) based on orbital angular momentum (OAM) interferometry and time-frequency analysis is proposed and demonstrated in this paper. Two vortex beams carrying OAM of opposite signs are used for interference to produce a petal-like intensity distribution. The variation in RI of the sample leads to a time variable phase delay between the reference and measurement paths, and causes the rotation of the petal-like spots. The rotation angular velocity of the petal-like spots is proportional to the RI variation rate, the normalized cross correlation method is used to estimate it. Then, a time-frequency analysis method is employed to study the time evolution of the variation rate of RI. Three kinds of RI models with different variation rates are simulated and the results are consistent with expectations. The proposed measurement method is simple in structure, and extends a new approach to detect other physical coefficients of RI or the tiny velocity.
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