Resonance Fabry–Perot interferometer as instrument for investigation of fine and hyperfine mode structure of free electron lasers

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Free electron lasers usually emit periodic sequences of short pulses, which are parts of pulses circulating inside an optical cavity. Due to the laser nature of the radiation, the output pulses generated by the same intra-cavity pulse are a priori coherent with each other. Under certain technical conditions, different intra-cavity pulses can be coherent too. The coherence of intra-cavity pulses gives rise to a fine structure of the laser emission spectrum, and the coherence of output pulses from one intra-cavity pulse gives rise to a hyperfine mode structure. The article describes a special device, the resonance Fabry–Perot interferometer, and methods for panoramic frequency measurements of the mode composition of radiation with a resolution of up to 5⋅ 10−8 and for practically unlimited-resolution time-domain measurements of the monochromaticity of lines of hyperfine structure. With this device, it was shown that there is no fine structure in the Novosibirsk free electron laser (NovoFEL), and its hyperfine structure was measured in detail. The measurement of hyperfine structure of an FEL was carried out for the first time. It is shown that under certain conditions, NovoFEL operates either in the generation regime of one Laguerre-Gaussian supermode or in the regime of several transverse supermodes. The measured relative linewidth of the NovoFEL hyperfine mode structure, defined by technical factors, is 2.2 × 10−8, which is about two orders of magnitude larger than its absolute physical quantum limit.