Resonance Fabry-Perot Interferometer for Ultra-high Resolution Spectroscopy of Free Electron Laser Radiation

Pulse-periodical free electron lasers with an optical cavity can have a fine and always have a hyperfine spectral structure of radiation due to the coherence of output pulses, which are parts of different or the same intra-cavity light pulse, respectively. The optimal instrument for measuring these structures is the resonance Fabry-Perot interferometer described in this article. The composition of transverse supermodes of the NovoFEL in various regimes was revealed by the device, operating in a frequency-domain mode with a 5.10(-8) resolution. The time-domain regime of the interferometer, which has practically no limitations, made it possible to accurately measure the monochromaticity of the NovoFEL modes, which turned out to be equal to 2.2.10(-8).