2 μm lasers have attracted much attention due to its wide application in light detection and ranging, free-space optical communication, atmospheric sensors, and the laser microsurgery. In addition, 2 μm lasers can be pumping sources for optical parametric oscillators in mid- and far-infrared spectral regions.
Tm³⁺ and  Ho³⁺ co-doped gain-media are proved the best lasers operating at 2 μm due to the cross-relaxation process and efficient energy transfer. For decades, a variety of Tm and Ho co-doped crystals have been carried out. However, low transmittance, low emission cross-section and short fluorescence lifetime make it difficult to achieve high power lasers based on these crystals.
Therefore, it is the key point to find appropriate gain media in this wave band. The gain media should possess the following advantages: (1) low phonon energy to ensure the high fluorescence efficiency; (2) good infrared transmission characteristics; (3) large emission cross-sections and long fluorescence lifetime.
Recently, a research team from Shanghai Institute of Optics and Fine Mechanics, Chinese Academic of Sciences, has succeeded in developing a new kind of low-phonon-energy laser crystal: Tm,Ho:LaF₃ crystal; and the crystal growth, thermal conductivity, spectra, and 2 μm continuous-wave characteristics were investigated.
For the first time, the transparent 5 at% Tm, 0.5at% Ho: LaF₃crystal was grown successfully by the Bridgman method with no bubbles and cracks in it, whose size is 20mm×30mm.
In exploring the thermal conductivity versus temperature in the range of 300 – 600 K, the value at room temperature is 1.99 W/K.
The absorption cross-section at 792 nm and emission cross-section at 1987 nm are 0.18×10²⁰ cm² and 0.32×10²⁰  cm², respectively. The full widths at half maximum of the absorption peak at 792 nm and the emission peak at 2 μm are 38 nm and 140 nm, respectively. Notably, the fluorescence lifetime of 2 μm emission was 17.56 ms, which is longer than that of the commercial laser crystals, such as 8.5 ms of Tm,Ho:Y₃Al₅O₁₂, 13.8 ms of Tm,Ho:LiYF₄.
Moreover, continuous-wave laser properties at 2 μm were also analyzed and the maximum output power of 574 mW was obtained with an absorbed pump power of 3.53 W pumped by 793 nm LD.
The research may provide a candidate for the tunable and ultra-short pulse laser medium around 2 μm pumped by the commercial laser diode.

The results, entitled “Growth, thermal conductivity, spectra, and 2 μm continuous-wave characteristics of Tm3+, Ho3+ co-doped LaF3 crystal”, were published in Journal of Luminescence.

This work was supported by National Natural Science Foundation of China (NSFC) (51472257, 51502321, 51872307); National Key R&D Program of China [2016YFB0701002, 2016YFB0402105, 2016YFB1102302]; Strategic Priority Research Program (B) [No. XDB16]; Major Project of Shanghai Science and Technology Research Foundation [16JC1420600].

Characterizations of the new crystal

Article Website:
https://www.sciencedirect.com/science/article/pii/S0022231318323469

Contact:
Mr. Cao Yong
General Administrative Office
Shanghai Institute of Optics and Fine Mechanics, CAS
Email: caoyong@siom.ac.cn