A chirped pulse is a pulse in which the wavelength changes during the duration of the pulse.The output of a laser may be chirped intentionally to create a high-powered, extremely short pulse. In this case, the initial pulse from an oscillator is stretched out using optics, amplified and then compressed back to its original duration.
The peak power of laser pulses was limited because a laser pulse at intensities GW per square centimeter causes serious damage to the gain medium through nonlinear processes such as self-focusing. In CPA, on the other hand, an ultrashort laser pulse is stretched out in time prior to introducing it to the gain medium using a pair of gratings that are arranged so that the low-frequency component of the laser pulse travels a shorter path than the high-frequency component does. After going through the grating pair, the laser pulse becomes positively chirped, that is, the high-frequency component lags behind the low-frequency component, and has longer pulse duration than the original one. Then the stretched pulse, whose intensity is sufficiently low compared with the intensity limit of gigawatts per square centimeter, is safely introduced to the gain medium and amplified. Finally, the amplified laser pulse is recompressed back to the original pulse width through the reversal process of stretching, achieving orders of magnitude higher peak power than laser systems could generate before the invention of CPA.
However, CPA limits to generate power levels above PW. Is there any other technique to develop more intense ultra short pulses ?
Hi,
Up to now, ultra-high intensities have been achieved using mode-locked laser oscillators and CPA amplifiers as you mention in your question.
Pulse generation without mode-locking has been explored in the 1980s but abandoned at the time when Ti:Sap oscillators were developed. These techniques included : distributed feedback gain lasers, travelling wave excitation, space-time selection, and quenched cavity. To my knowledge, there has been no trial of pushing these sources toward very high intensity.
- See: Femtosecond laser pulses. Principles and experiments. 2nd edition. C. Rullière Ed. Springer Verlag.2003. Pages 111-113. Joined scan.
Hope it helps.
Sure, a Raman amplified laser or OPA will amplify an already short pulse without traditional CPA on the backend for high intensities (usually use a CPA Ti:Sapph fronted for a few uJ), one uses a plasma (no damage threshold), the other non-linear crystals like KDP* or BBO, but in either case you need to expand the beam and reduce the fluence (for plasma to avoid self-focusing or other non-linear effects, and for BBO damage). Also OPA can be combined with CPA (OPCPA) for high intensity (around a PW). Also High Intensity is relative. Raman amplification is still at an early stage of development, while OPA and OPCPA are pretty mature.
One could also think of coherently combining several lasers based on ultrashort pulse fiber lasers. This option is investigated for laser induced plasma based acceleration.
http://spie.org/newsroom/technical-articles/5686-laser-plasma-accelerators-driven-by-incoherently-combined-lasers
The other option is to stuck pulses on top of eachother using cavities.
http://www.nature.com/lsa/journal/v3/n10/full/lsa201492a.html
ELI Beamlines (Czech) will have a CPA PW laser at 10^23 W/cm^2 at 30J, 30fs, 10 Hz, Solid State Diode pumped, which is pretty crazy and sounds like it is coming along nicely!
https://str.llnl.gov/january-2014/haefner
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