The dual-wavelength paired-pulse output of a compact self- Q -switched T m : Y A l O 3 intra-cavity pumping H o : L u L i F 4 laser was first demonstrated experimentally. By exploiting the self- Q -switching mechanism of T m : Y A l O 3 and the gain-switching operation of H o : L u L i F 4 in a cavity, the dual-wavelength paired pulses were robustly generated with the pulse energy of 29 µJ, pulse width of 2.5 µs at 1944 nm, pulse energy of 31 µJ, and pulse width of 338 ns at 2066 nm. The maximum average output powers were obtained simultaneously as high as 525 mW at 1944 nm and 323 mW at 2066 nm. The paired-pulse repetition frequency can be pump-tunable in the range of 8–22 kHz. The temporal delay in the paired pulses can be controllable from 9.5 to 2.9 µs with the pump power increasing from 15.2 to 18.4 W. The compact dual-wavelength paired-pulse laser near 2 µm has potential application in the differential absorption lidar.

A compact, orthogonally polarized, gain-switched a-cut Ho:LuLiF4 laser with intra-cavity pumping by a self-Q-switched Tm:YAP laser is demonstrated here for the first time. The π-polarization laser at 2052 nm and σ-polarization laser at 2066 nm were experimentally observed with the maximum output power values of 299 mW and 126 mW, respectively, and the two polarization directions were always kept mutually orthogonal as the pump power increased. The ratio of the output power between the two orthogonal polarization lasers was nearly 1:1 at a pump power of 18.4 W. The minimum pulse width of the Ho:LLF laser was 326 ns, the maximum repetition rate was 24 kHz, and the maximum average energy was 28 μJ.

Bound state in the continuum (BIC) is a singular point of polarization in momentum space of periodic structure. It has been demonstrated that vortex beams can be generated by utilizing the polarization vortex around BIC based on the geometric phase in momentum space. In this letter, we propose a reflective photonic crystal (PhC) to generate vortex beam and demonstrate the superimposition of topological charges in the momentum space between PhC and the vortex beams. In addition to BICs, we demonstrate that other singularities like degenerate point can also be used to generate vortex beams.

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