ANovelPulseStretchingEffectofNanosecondLaserintheMultipassAmplifier.pptVIP

So we made a lot of stuff like this, use different size of the pinholes to improve the laser spot, the transversal mode. And we use the liquid Nitrogen as active cooling instead of TEC, all these methods worked and the beam quality is good enough, however, nothing changed obviously. Converting frequency is still very low. * In that analysis we neglect one thing: the pulse duration. It seemed impossible that the pulse can be broaden, because the nanosecond laser is totally different from the pico\femto-second laser, there’s almost no dispersion in our narrow linewidth laser beam and the chirp could be completely neglected. * To our surprise, a 20 nanosecond laser pulse from the oscillator is stretched into 40 nanosecond. That’s why we can’t attain high efficient frequency converting. And what’s really interesting is :there’s a tendency that longer the laser pulse is, the more it be stretched, and all the figures lead to a width 10ns. * As we moved on, we realised that a 10ns pulse corresponding to 3m as spacial length. Is that a corresponding? Can’t be. So our target is to figure out the how the broadening happened. * There’s obviously an fact that, to a 20ns pulse, its length in space is much longer than the distance between two passes. The superposition of its rise and fall is the crucial factor. * Our first answer is: because of the superposition of the rise up and the down in the gain media, its rise can get more capability to absorb the stored energy, so it has been amplified more. But that is far fetched. The second one based on the nonlinear of the gain media, and it seems like impossible. So, let’s check out the last possible reason. * As usual，the pulse is amplified entirely, the width doesn’t change. But in the multipass amplifier, one passing portion of the pulse is competing with another for the stored energy, As the result of nonlinear gain saturation, its peak which is supposed to be much higher is pressed down now, and the HWFM is broadened. * T