Spectra, temporal structure, and angular directivity of laser radiation of a Yb:YAG crystal and ytterbium glass pumped by low-coherence radiation from a F2+:LiF colour centre laser

By focusing radiation from a F₂⁺:LiF colour centre laser (emitting in the range from 0.89 to 0.95 μm) on plates of a Yb:YAG crystal with the 20% concentration of Yb and of ytterbium glass with the 10% concentration of Yb, we observed nanosecond radiation pulses of Yb³⁺ ions in the spectral region from 1.00 to 1.06 μm with the spectral width up to 20 nm in Yb:YAG and up to 50 nm in the ytterbium glass. Lasing appeared in the active medium in the region of excitation of the SBS of pump radiation with a diameter less than 200 μm. The angular divergence of the broadband laser radiation (10^-3 — 10^-4 rad) was one — two orders of magnitude smaller than the diffraction limit. The mechanism of generation of short broadband high-directional laser pulses in the spatial structure of thin layers with inversion produced in the region of propagation of an intense acoustic wave in the medium is discussed. The interpretation of experimental data on the angular divergence of radiation is based on a new concept of the spatial distribution of the electromagnetic field of a photon not in the form of a travelling wave but with the field structures located in fixed positions along the propagation direction. The features of the temporal picture and narrowband lasing spectra of Yb:YAG and ytterbium glass in a resonator upon relaxation of the SBS excitation region in the active medium are considered. The possibility of diagnostics of medium parameters using the shift of spectra of the lasing at the resonator modes in different sites of the SBS excitation region is discussed.