Shaping a laser pulse or being a Super Hero
Friday, June 13, 2008
The requirements for the LCLS injector gun UV drive laser are in some part quite challenging. Along with a specific wavelength, repetition rate and energy, the laser beam needs a particular profile in both longitudinal (time domain) and transverse (space) modes.
 At that moment of the LCLS commissioning, the RF gun is running with a drive laser that has a super-Gaussian profile (also called top hat) in time and in space. It has been found that it could be optimum to get the needed emittance for the electron beam at the output of the injector with uniform slices inside the bunch. During the commissioning while running the machine at a charge of 250 pC, the target value for the laser temporal profile is a 6.5 ps wide super-Gaussian with a rise/fall time of 1 ps. The transverse mode has to be a homogenous flat top beam that has an adjustable diameter of 1.2 mm on the gun cathode. Unfortunately for us (well, actually it is the fun part of the laser!), our classic Titanium:Sapphire laser system has the usual Gaussian profile in time and in space. For this reason, our challenge is to “shape” the laser beam so it can be used for the gun cathode.  It was first planned to convert the UV laser output beam that has a Gaussian spatial shape to a flat top by using a commercial “beam shaper” and imaging the shaper output to the photocathode with adjustable magnification. But this shaper had very high requirements to the quality of the input beam and to the accuracy of alignment. So it did not provide better beam quality than simply clipping the beam with a hard-edge aperture after a three lenses adjustable zoom telescope. This simple method has been finally chosen and even we have several apertures with different increasing diameters set on a remote controlled wheel so that the laser beam size on the cathode can be changed only by rotating this wheel. Like the barrel of a gun!  The temporal shaping is made through a little device called with the acronym A.O.P.D.F. It means Acousto Optic Programmable Dispersive Filter. It is a commercial product called the “Dazzler”. The funny anecdote is that “Dazzler” is a Marvel Comics superheroine associated with the X-Men. She is a mutant with the ability to convert sound vibrations into light and energy beams. And that’s pretty much what happens inside the TeO2 crystal of this A.O.P.D.F. An acoustic wave is applied on it with a transductor (like in a microphone) that modulates its index of refraction acting then like a dispersive grating. So when the laser goes through it, it is diffracted with a wavelength dependency. By programming the RF wave, you can filter and shape the amplitude of the laser spectrum. The Dazzler is installed inside the laser system before the amplifiers. We have programmed our Dazzler to apply a super-Gaussian filter onto the spectrum amplitude. Spectrum is linked to time via the so called Fourier time/frequency relation. So the temporal envelope of the laser will be close to a super-Gaussian too. posted by Philippe Hering @ 12:00pm
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