Use of diffraction on perfect crystals for shortening of FEL pulses and measurement of their length

Jaromír Hrdý¹, Peter Oberta^1,2

¹Institute of Physics, Academy of Sciences of the Czech Republic, v.v.i., Na Slovance 2, 18221 Praha 8, Czech Republic.

²Rigaku Innovative Technologies Europe s.r.o., Novodvorská 994, 14221 Praha 4, Czech Republic.

Since the first lasing of the hard X-ray FEL facility LCLS in 2009 engineers and physicists were looking for new optical devices and set-ups with the simple goal of preserving the unique properties of the next generation synchrotron sources. It is extremely important to preserve the unique time structure of the hard X-ray FEL pulse or even to have the possibility to change the parameters of the pulse. If we speak of changing the pulse parameters we are speaking of pulse compression. Currently, mostly grating schemes are used for pulse compression and correlation and auto-correlation schemes incorporated in delay lines are used for pulse length measurements. Crystal optics was not so much used in the application of hard X-ray FEL sources because of their narrow acceptance range. With the introduction of seeded FEL sources the FWHM of single pulses is now equal or smaller than the acceptance of commercially used crystal optics (Si, Ge, ...). Using dislocation free silicon crystals we have introduced two new methods for pulse compression. One is based on the use of an asymmetricaly cut crystal and the second on the use of inclined crystals. Both methods are based on the fact that finite wavelength range Dl of the impinging radiation diffracts under different angles depending on the wavelength, l, even if the impinging beam is parallel. Using a properly chirped pulse, the inclined cut crystal generates a path length difference, which leads to pulse compression, fig.1.

Fig.1: To generate a path difference with the goal of pulse compression we have explored a symmetrical cut crystal (left) and an inclined geometry crystal (right).

We have also introduced two new methods enabling the measurement of pulse lengths. One is based on the use of multi-beam diffraction and the other on the novel design of a pure BBBB interferometer.