Focusing of the plane wave with radially polarized electric field by an arbitrary opening paraboloid mirror is analyzed
using a rigorous vectorial diffraction theory, i.e., Stratton-Chu integral. In the vicinity of the focus, far-field
approximation conditions are used to simplify the derived integrals with sufficiently high accuracy. It is found that a
noticeable deviation of the approximate integral, as characterized by a phenomenon of focal shift, from the exact integral
can be observed when the maximum focusing semi-angle α below π/9. For α=π/2, the radial spot size reduces to below
0.40λ if cutting off the central segment, larger than π/4, of the paraboloid mirror. The sharp focusing property of the
paraboloid mirror has the potential application in super-resolution confocal scanning microscopy. Specific confocal
scanning arrangements are provided and remarked.
It is investigated that the duration of stimulated Brillouin scattering (SBS) amplification of Stokes pulses is smaller than the acoustics lifetime and the oscillation period of hypersound with the nanosecond pulse as pump light. In the experiment, the Fluorinert (FC-72) is used as the Brillouin amplification medium, the Stokes pulse duration less than 1/4 of oscillation period of hypersound of FC-72 as seed pulse is achieved by two-stage SBS compressor to compress 8ns laser pulse. The experiment and simulation show that, in this case, their energy amplification is efficient, but the duration of amplified pulses is longer than the initial seed pulse. Simulation results of higher energy than the maximum energy in the experiment show that at sufficient high pump or initial seed pulse power intensity, the duration of amplified pulses can be no larger than the initial seed pulse duration., even less than the seed pulse duration.
A 1.5m long cell, different focusing lens, a KrF laser which bandwidth is 0.5cm-1, and SF6 gas as Brillouin medium are used in the experiment. The influences of focus length and SF6 pressure on threshold of stimulated Brillouin scattering excited by broadband KrF laser were investigated, respectively. The SBS threshold increases with increasing of the focus length and decreasing of the SF6 pressure. The results show that the threshold of broadband SBS is various with the different focus length and SF6 pressure. For more precisely to explain the experimental results, a novel model considering optical breakdown and pump linewidth was presented. The model we proposed here assumes that the KrF laser spectrum is made up of a seriues of narrowband spectra, each generating one Stokes wave. In addition, it is assumed further that the neighbors of one narrowband spectrum also contribute to the generation of its corresponding Stokes wave to some degree. Optical breakdown is assumed to lead to the loss of pumping energy only. The experimental results are consistent with the numerical theory.
A 1.5m long cell, different focusing lens, a KrF laser which bandwidth is 0.5cm-1, and SF6 gas as Brillouin medium are used in the experiment. The influences of SF6 pressure on threshold, energy reflectivity and pulse compression of stimulated Brillouin scattering excited by broadband KrF laser were investigated, respectively. For more precisely to explain the experimental results, a novel model considering optical breakdown and pump linewidth was presented. Optical breakdown is assumed to lead to the loss of pumping energy only. The experimental results are consistent with the numerical theory.
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