Regenerative amplifiers containing multiple diverse laser heads have the ability of scaling up output power as well as broadening spectral bandwidth. However, the thermal lensing effect will be more complicated. A method using a 4f system to simplify the thermal lensing effect in a multidisk condition is proposed. Two kinds of Yb-dropped sesquioxide materials are chosen as an example. On the basis of calculating the thermal lensing parameters of the thin-disk laser heads, a feasible cavity design that can minimize the fluctuation of beam size when changing the pump power is given out. The 4f system consisting of concave mirrors will introduce astigmatism in the cavity. The influence of astigmatism and the compensation method are discussed. Astigmatism can be compensated by adding extra convex mirrors and controlling the angle of incidence at each mirror.
A beam-smoothing method that combines induced spatial incoherence (ISI) and continuous phase plate (CPP) is presented. A method designing special CPPs for ISI is proposed. Then, spatiotemporal character of focal spots with different ISI parameters and different phase plates was numerically analyzed. Using special phase plate consisting of small CPPs designed separately, with 10-nm bandwidth 527-nm light, focal spot nonuniformity at spatial wavelength 10 to 50 μm could be reduced to 2% in 130 ps and was further reduced to 0.5% in 2.3 ns. A two-lens aperture system is added to lower the nonuniformity of the near-field in the light path. Using ISI + CPP, the shape of the focal spot could be controlled perfectly, and the characteristic that the ISI + CPP focal spot changes randomly with time is beneficial to the control of laser and plasma interactions.
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