As being widely used in environmental monitoring, medical diagnostics and IR countermeasurement, mid-infrared laser sources at the wavelength region of 3 to 5 micron have been an object of growing scientific interest. Extensive works and a lot of methods have been developed to study and achieve mid-infrared laser with high power and efficiency.

Researchers at Key Laboratory of Space Laser Communication and Testing Technology, Shanghai Institute of Optics and Fine Mechanics, Chinese Academy of Science (SIOM, CAS) demonstrate a high-efficiency 1.9 μm laser-pumped nonlinear crystal optical parametric oscillator, which generate laser emission at ~3.8 μm, for the first time.

The Thulium-doped Lithium Yttrium Fluoride (Tm:YLF) crystal is grown and fabricated by Key Laboratory of Materials for High Power Laser, SIOM. Effectively stabilize and narrow the spectral of Tm:YLF laser to eliminate vibrational-rotational absorption of water molecules by inserting and rotating two Febry–Pérot etalon with probable thickness is realised. The nonlinear crystal is Periodically Poled MgO-doped Congruent LiNbO₃, together with the Tm:YLF laser as the pump source, both signal and idler waves are approximately 3.8 μm, showing the lowest atmospheric attenuation of mid-IR radiation. The absorption of PPLN at a wavelength longer than 4.1 μm is also avoided with this configuration. At room temperature, A 900 mW mid-infrared emission generated from an OPO is illustrated. The overall mid-infrared generation conversion efficiency reaches 31%, whereas the slope efficiency is up to 70%.

This work entitled "Tm:YLF laser-pumped periodically poled MgO-doped congruent LiNbO₃ crystal optical parametric oscillators" has been published in Optics Letters, Vol. 37, Issue 4, pp. 743-745 (2012), http://www.opticsinfobase.org/ol/abstract.cfm?URI=ol-37-4-743.

Fig 1. Spectrum of signal and idler waves when the PPLN OPO is operated at 22 °C(Image by SIOM)

Fig 2. Comparison of the signal and idler wavelengths calculated theoretically and observed experimentally(Image by SIOM)