超快光学-第19章-放大.pptVIP

Heat deposition causes lensing and small-scale self-focusing. These thermal aberrations increase the beam size and reduce the available intensity. Ipeak = E A ?T We want a small focused spot size, but thermal aberrations increase the beam size, not to mention screwing it up, too. Now the average power matters. The repetition rate is crucial, and we’d like it to be high, but high average power means more thermal aberrations… Thermal Effects in Amplifiers Low temperature minimizes lensing. Calculations for kHz systems Cryogenic cooling results in almost no focal power In sapphire, conductivity increases and dn/dT decreases as T decreases. Murnane, Kapteyn, and coworkers Static Wave-front Correction 2.5 times improvement in peak intensity has been achieved CUOS Dynamic Correction of Spatial Distortion 50 mm diameter 37 actuators CUOS Contrast ratio Why does it take over 2 years between the first announcement of a new laser source and the first successful experiment using it? Because the pulse has leading and following satellite pulses that wreak havoc in any experiment. If a pulse of 1018 W/cm2 peak power has a “little” satellite pulse one millionth as strong, that’s still 1 TW/cm2! This can do some serious damage! Ionization occurs at 1011 W/cm2 so at 1021 W/cm2 we need a 1010 contrast ratio! Major sources of poor contrast Nanosecond scale: pre-pulses from oscillator pre-pulses from amplifier ASE from amplifier Picosecond scale: reflections in the amplifier spectral phase or amplitude distortions 0 -1 -2 -3 -4 -5 -6 -7 -8 -9 -10 Front Back time Spectral phase aberrations Pre-pulses ASE 0 ps 10 ns ns FWHM Amplified pulses often have poor contrast. Log(Energy) Pre-pulses do the most damage, messing up a medium beforehand. Typical 3rd-order autocorrelation Amplified pulses have pre- and post-pulses. A Pockels cell “Pulse Picker” A Pockels cell can pick a pulse from a train and suppress satellites. To do so, we must switch the voltage from 0 to kV and back