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The first LASER light in the EUV spectral region

To date, most free electron lasers (FEL’s) have employed the Self Amplification of Spontaneous Emission (SASE) mode of operation in which the radiation grows from the spontaneous shot noise on the electron beam. Although SASE operation can produce extremely high brilliance pulses of photons, the temporal structure of the pulse is composed of a series of micro-pulses which individually have random phase and nearly completely random intensity and time duration. On the contrary, the FEL sources based on “seeding” techniques (in which a coherent radiation pulse generated by a conventional laser initiates the FEL process) can produce pulses with a well-defined and stable temporal shape, frequency and intensity. Under these circumstances, in addition to controlling of the temporal structure of the pulse, one can obtain radiation pulses close to the Fourier Transform Limit conditions. The possibility of having almost fully coherent radiation pulses in the X-ray domain opens completely new and unexplored frontiers for modern science.

Figure 1: Schemas of the FEL1 and FEL 2. All the main components are indicated in the figure.

Prior to the operation of FERMI@Elettra, all operational free electron lasers (FELs) in the EUV and x-ray range have been based on Self Amplified Stimulated Emission (SASE). As the SASE signal grows from the spontaneous shot noise on the beam, it suffers from temporal fluctuations in the photon energy and intensity (spikes) appearing as multi-mode spectral emission. This characteristic of the SASE radiation severely limits the degree to which this process can generate transform-limited radiation throughout the entire photon pulse, although some degree of the transverse coherence can be achieved in the spikes within the pulse. Seeded FELs, by starting the light amplification process from a fully phase-coherent optical pulse, can generate transform-limited, temporally and spatially coherent radiation throughout the entire duration of the pulses, making these EUV/ soft X-ray radiation sources the closest possible analogs to conventional LASERs (see Fig. 1). These radiation characteristics – along with the impressively high stability of photon energy and narrow bandwidth – unlock the gate for performing advanced spectroscopy in the time domain (see Fig. 2 and Fig. 3).

Figure 2: Spectral emission from FERMI@elettra FEL1 as measured at 52 nm, 43 nm, 32 nm. Noteworthy, is the presence of a single narrow emission of few tens of meV – characteristic of single mode pulses.

The design of the FERMI@Elettra free electron laser is based on the possibility of generating highly coherent tunable radiation in the soft X-ray region by the process of high gain harmonic generation (HGHG), which is harmonic cascade that generates short wavelength output wavelength from a UV laser pulse as the seed. The use of helical undulators in the radiator section of the FEL also permits the generation of variable polarization coherent X-rays. The results obtained during the commissioning of FERMi last spring demonstrate that all these expectations have been fulfilled. For the first time we have generated fully coherent light pulses in the TEM00 mode (see Fig.2) at wavelengths as short as ~ 20 nm (~62 eV) with a bandwidth of few tens of meV. These results were achieved during the first part of the commissioning of the first (FEL1) of the two FELs foreseen for FERMI@elettra project. At the present, the second high gain high harmonic generation stage (FEL2) being installed; it will be commissioned during the 2012 and it is expected to deliver photon up to ~ 4 nm (~ 310 eV).

Figure 4: Typical YAG image of the FERMI@elettra FEL1 spot. For both the x and y directions in the transverse plane the intensity profile of the beam is Gaussian. 

Figure 3: Statistic of the FERMI@elettra FEL 1 spectral emission.
The collected spectra are about 100.
Last Updated on Friday, 27 January 2012 13:36