Crystals Materials
The 2.1 μm Ho:YAG laser system pumped by Tm:YLF laser

Abstract

A 2.1 μm Ho:YAG laser is located in the atmospheric window and widely used in applications such as laser medical, material processing, optical communication lidar detection and ranging systems, and the optical parametric oscillators. The Tm-doped solid-state and Tm-doped fiber lasers were used as the pump sources in a high power Ho:YAG laser owing to the better beam quality and higher power.

2.1 μm Ho:YAG Laser System

the-2.1-μm-ho-yag-laser-system-pumped-by-tm-ylf-laser
The 2.1 μm Ho:YAG laser system pumped by Tm:YLF laser

A double-ended pump structure was used for the Tm:YLF laser. The pump sources were two fiber-coupled laser diodes (LDs) with a power of 80 W and central wavelength of 793 nm. The pump beams were focused on the Tm:YLF crystal with a diameter of 0.75 mm using beam shaping devices.

Tm:YLF Laser Resonant Cavity

The M1 were coated with high transmittivity at 793 nm and high reflection (HR) at 1.9 μm. M2 was a plano-concave mirror (PCL) with a transmittance of 20% at 1.9 μm, and its radius of curvature was 150 mm. The transverse size and thickness of the volume Bragg grating (VBG) were 4 x 3 mm2 and 5 mm, respectively. The diffraction efficiency was greater than 99.0% at 1905.5 nm at room temperature. The structure was mounted on a copper heat sink that could be controlled from room temperature to 210 °C. The entire cavity consisted of a plano-concave mirror M2, 45° dichroic mirror M1, and VBG.

The cavity length of the Tm:YLF laser was 85 mm in this laser system. The Tm:YLF crystal was a-cut with a size of 1.5 x 4 x 30 mm3 and doping concentration of 2.0 at.%. Both ends of the Tm:YLF crystal were coated with antireflection coatings of 793 nm and 1.9 μm, respectively. Additionally, the crystal was wrapped using an indium foil and fastened into a copper holder, which was water cooled to 20 °C

Ho:YAG Laser Resonant Cavity

A single-end-pump structure was used for Ho:YAG laser. The pump lasers were focused into the Ho:YAG crystal with a beam diameter of approximately 0.95 mm by lens f3.

M3 was a 45° dichroic mirror with high transmittivity at 1.9 μm and high reflection at 2.1 μm. M4 was coated with high transmittivity at 1.9 μm and high reflection at 2.1 μm. M5 was a plano-concave mirror with a transmittance of 20% at 2.1 μm, and the radius of curvature was 300 mm.

The cavity length of the Ho:YAG laser was 115 mm. The Ho:YAG crystal had a diameter of 4 mm, length of 50 mm, and doping concentration of 0.8 at.%. The two end surfaces of the crystal were anti-reflection (R ≤ 0.3%) coated at 1.9 and 2.1 μm. Lastly, the crystal was wrapped in a copper holder filled with flowing water and the temperature was controlled at 20 °C.

Absorption Spectrum and Output Power

absorption-spectrum-of-ho-yag-and-output-power
Absorption spectrum of 0.8 at.% doped Ho:YAG and output power

The results demonstrated that the transmittance at different pump wavelengths had an effect on the Ho:YAG laser output power.

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Key words: laser crystals; Ho:YAG crystal; Tm:YLF crystal; 2.1 μm Laser; 1.9 μm Laser

Reference:

1. Tian, J., Zhao, L., Li, Z. et al. Ho:YAG laser at 2097 nm pumped by a narrow linewidth tunable 1.91 μm laser. Sci Rep 13, 999 (2023). https://doi.org/10.1038/s41598-023-27970-0