The mid-IR region is particularly interesting for many applications since the electromagnetic frequencies involved coincide with the frequencies of the internal vibrational motion of most molecules. The development of light sources in the mid-IR thus opens up new opportunities, especially in spectroscopy compared to the conventionally used Fourier transform spectrometers. The main benefits are coherence, small pulse duration and high spectral brightness.
Researchers within ASEL-MID-IR (Compact, high-energy, and wavelength-diverse coherent mid-infrared source) worked to convert the frequencies of solid-state lasers operating at the near-IR to the mid-IR. An optical parametric oscillator (OPO) containing non-linear elements converted the input laser waves (pump) of solid-state lasers into waves of higher wavelength.
Following a detailed literature review on solid-state laser architectures, researchers used a thulium-doped fibre laser and a Q-switched holmium-doped YAG laser operating at 2.1 μm. These pump sources display better thermal management and power scaling capabilities than the conventional 1 μm solid-state lasers. An OPO constructed by a ZGP crystal and with second-order non-linearities converted the wavelength of the pump sources into mid-IR waves.
The holmium-doped YAG laser produced over 25 W of output pulsed power at 2.1 μm, with a pulse repetition rate at 10-200 kHz and pulse duration under 15 ns. The OPO containing the ZGP crystal produced an output broadband mid-IR signal with average power exceeding 5 W and spectrum extending from 3.5 to 5 μm. Conversion efficiency was approximately 40 %, exceeding the initial estimates.
Use of alternative OPO configurations such as an intracavity OPO showed that the output signal can backconvert into the pump signal, effectively mimicking a third-order non-linear medium. To exploit third harmonic generation, the team used an external-cavity Raman laser with a BaWO4 crystal. The generated output wavelength was the longest ever reported that displayed watt-level output power scaling. The overlap of the source spectral range with the water vapour absorption window opens up applications in infrared hygrometry and gas sensing.
Researchers ultimately replaced the thulium-doped fibre laser with an in-house thulium diode laser containing a YLF crystal as a lasing medium to provide compact packaging for the prototype solution.
Applications are growing for mid-IR lasers. ASEL-MID-IR's compact and portable mid-IR light source prototypes with high pulse repetition rates and output power can benefit security, medical diagnostics and gas sensing applications.