Objective The 8--12μm long-wave infrared laser is just within the transmission bands of atmosphere and widely used for gas composition detection and electro-optic countermeasure. As traditional long-wave lasers, CO2 lasers can output lasers with a specific wavelength within the range of 9--10μm. Beyond them, a long-wave infrared optical parametric oscillator (OPO) shows an enormous advantage because of its wavelength tuning. However, OPO-based lasers with wavelengths longer than 8μm and high optical-to-optical conversion efficiency are still scarce. Herein, we construct a ZnGeP2 OPO and experimentally test its long-wave infrared output. The experimental result shows that a long-wave laser with high conversion efficiency is obtained, which provides a reference to engineer the laser based on ZnGeP2 OPO.
Methods The scheme of the OPO-based long-wave infrared laser pumped by a 2μm laser is discussed, in which the selection of a 2μm laser crystal and a long-wave infrared nonlinear crystal is included. In this scheme, the selected nonlinear crystal is ZnGeP2, and the selected pumping laser source is 2.05μm Ho∶YLF laser with a maximum output power of 27W (10kHz). The two end faces of the ZnGeP2 crystal are polished and coated with an antireflection film at 2.05, 2.7, and 8.2μm bands, which are the key processes for reducing the optical loss in the crystal and for reducing the risk of damage. The resonator of the ZnGeP2 OPO is a flat cavity and the resonant mode is double resonance OPO. The Ho∶YLF laser is linearly polarized, which is helpful for ZnGeP2 OPOs to achieve a high optical-to-optical conversion efficiency. The Ho∶YLF laser, pulsed using an acousto-optic Q-switch, is pumped by a Tm∶YAP laser (CW) with a wavelength of 1.94μm and a maximum output power of 62W. Without damaging the elements of the Ho∶YLF laser, the laser’s repetition rate is minimized, the OPO’s threshold is reduced, and the conversion efficiency is improved. The ZnGeP2 crystal, Ho∶YLF crystal, and Tm∶YAP crystal are all wrapped in thin indium foils and placed in copper heat sinks to collect the heat absorbed by them. During the operation of the experimental apparatus, there is the water flow with 20 ℃ in the Q-switch and all heat sinks, and a microchannel structure for the water flow is indicated. Finally, the typical parameters of the long-wave infrared laser, including average power, wavelength, laser beam quality, repetition rate, and pulse duration, are measured.
Results and Discussions The laser experimental apparatus (corresponding to the scheme mentioned above) achieves good experimental results with high power, efficiency, and repetition rate. The long-wave laser is generated when the 2.05μm pulsed laser with an average power of 10.5W is injected. The maximum output power of the long-wave laser is 3.2W when the 2.05μm pulsed laser with an average power of 26.68W is injected. Meanwhile, the corresponding optical-to-optical conversion efficiency is up to 12% and the slope efficiency is up to 19.3%. A spectrum analyzer is used to measure the spectrum of the long-wave laser with an output power of 3.2W and the peak wavelength of 8.135μm is disclosed. A CCD laser beam analyzer is used to measure the laser beam quality factor of the long-wave laser with an output power of 3.2W. The focusing lens method is used for these measurements. After the measurements, the quality factor is 4.5 in the X direction and 4.2 in the Y direction. The laser parameters including repetition rate of 10kHz and pulse duration of 27.11ns are measured using a photoelectric detector. The simple calculation shows that the single pulse laser energy is 0.32mJ and the peak power is 11.8kW.
Conclusions We verify that a ZnGeP2 OPO is feasible to realize high efficiency and tunable long-wave laser output. First, the phase-matching mode and the phase-matching angle of the ZnGeP2 crystal are analyzed and designed according to the principle that the output laser wavelength of a ZnGeP2 OPO corresponds to its phase-matching angle. Second, to realize the 8μm laser, the ZnGeP2 crystal is processed according to the above phase-matching angle. Third, the experimental apparatus is set up and the effect of the long-wave ZnGeP2 OPO laser is verified, and the ZnGeP2 OPO laser pumped by the 2.05μm Ho∶YLF pulsed laser can generate a long-wave laser output with a specific wavelength, high efficiency, and high power. In the future, long-wave infrared lasers with wavelengths longer than 8μm can be achieved just by reducing the phase-matching angle of the ZnGeP2 crystal (changing its cutting angle as an example) or reducing the incident angle of the pump laser.
