Self-amplified spontaneous emission

Functional principle of a SASE free-electron laser . The electron beam is generated in a particle accelerator and passes through the planar undulator on a periodic path (red). The synchrotron radiation generated at the beginning of the undulator initiates the amplification process through which the X-ray beam (orange) is generated.

Self-amplified spontaneous emission ( SASE ) is an operating mode of free-electron lasers thatcorresponds tothe super-emitter in lasers .

An ultra-relativistic, high quality electron packet , i. H. with a high peak current, a low emittance and a small energy uncertainty is shot into an undulator . At the beginning of the undulator, the electron packet emits spontaneous undulator radiation, which initiates the exponential amplification process in the free-electron laser. The properties of the photon pulses leaving the FEL are determined from this start from the noise, i.e. H. the spontaneous undulator radiation, and are accordingly subject to fluctuations. Typically, the spectrum has several peaks and the longitudinal coherence of the photon pulses is limited.

Because of these disadvantageous properties, research efforts are being undertaken worldwide by the operators of free-electron lasers (including DESY , SLAC , Elettra Sincrotrone Trieste ).

The free electron laser can be operated in a resonator configuration in wavelength ranges for which mirrors are available.

Alternatively, the amplification process can be initiated or continued with the help of an external radiation source (so-called seeding ). Seeding with the harmonics of a tunable ultraviolet source (frequency- multiplied OPO ) was shown on the free-electron laser FERMI @ Elettra (in Trieste ) for wavelengths below 4 nm.

Furthermore, the radiation from a first free-electron laser can be monochromatized and then stimulate the amplification process in a further free-electron laser. This so-called "self-seeding" was carried out at the Linac Coherent Light Source (LCLS; X-ray laser source at the SLAC in Stanford ) demonstrated in the photon energy range from 500 to 1000 eV and 8–9 keV.

Individual evidence

^ L. Giannessi, et al .: Status and Perspectives of the FERMI FEL Facility. In: Proceedings of the 38th International Free-Electron Laser Conference (FEL2017), Santa Fe, USA. contribution code MOD04, 2017 ( PDF )
↑ D. Ratner et al .: Experimental Demonstration of a Soft X-Ray Self-Seeded Free-Electron Laser . In: Physical Review Letters . tape 114 , no. 5 , February 6, 2015, p. 054801 , doi : 10.1103 / PhysRevLett.114.054801 .
↑ J. Amann et al: Demonstration of self-seeding in a hard-X-ray free-electron laser . In: Nature Photonics . tape 6 , no. 10 , October 2012, p. 693-698 , doi : 10.1038 / nphoton.2012.180 .

[1] L. Giannessi, et al .: Status and Perspectives of the FERMI FEL Facility. In: Proceedings of the 38th International Free-Electron Laser Conference (FEL2017), Santa Fe, USA. contribution code MOD04, 2017 ( PDF )

[2] D. Ratner et al .: Experimental Demonstration of a Soft X-Ray Self-Seeded Free-Electron Laser . In: Physical Review Letters . tape 114 , no. 5 , February 6, 2015, p. 054801 , doi : 10.1103 / PhysRevLett.114.054801 .

[3] J. Amann et al: Demonstration of self-seeding in a hard-X-ray free-electron laser . In: Nature Photonics . tape 6 , no. 10 , October 2012, p. 693-698 , doi : 10.1038 / nphoton.2012.180 .