Objective An 8--15-μm long-wave infrared (IR) laser has less transmission loss in the atmosphere; therefore, it can be used for gas detection, free-space optical communications and IR electro-optic confrontation. CO2 and ZnGeP2 optical parametric oscillator (OPO) lasers have been used to output long-wave lasers, although it is difficult to output lasers with wavelengths longer than 12 μm. Moreover, research on such lasers is inadequate. In this study, we design a CdSe OPO scheme and experimentally test its long-wave IR output. The results show that lasers with wavelength larger than 12 μm can be obtained by using CdSe OPO, which provides a solution for obtaining lasers with longer wavelengths.
Methods The scheme of the OPO-based long-wave IR laser pumped using a 2-μm laser is discussed, in which the selection of a non-linear long-wave IR crystal is included. In this scheme, the selected non-linear crystal is CdSe and the selected pumping laser source is 2.05 μm Ho∶YLF laser with a maximum output power of 40 W (frequency is 5 kHz). The two end faces of the CdSe crystal are polished and coated with an anti-reflection film at 2,2.4--2.7 and 10--13 μm bands, which are the key processes for reducing the optical loss and risk of damage in the crystal. The resonator of the OPO is a flat cavity, and the resonant mode is a single-resonance OPO. The Ho∶YLF laser is linearly polarised, which is helpful for CdSe OPOs to achieve a high optical-to-optical conversion efficiency. The Ho∶YLF laser pulsed using an acousto-optic Q-switch is pumped via a Tm∶YAP laser continuous wave with a 1.94-μm wavelength and 78-W maximum output power. The Q-switch and all crystals are wrapped in thin indium foils and placed in red copper heat sinks to collect the heat absorbed by them. During the experimental apparatus operation, there is water flow at 20 ℃ in all heat sinks and a microchannel structure for the water flow is indicated. Finally, the typical parameters of the long-wave IR laser are measured, including average power wavelength laser beam quality repetition rate and pulse duration.
Results and Discussions The laser experimental apparatus (corresponding to the above-mentioned scheme) output long-wave laser with a wavelength longer than 12 μm. The long-wave laser is generated when the 2.05-μm pulsed laser with a 14-W average power is injected. The maximum output power of the long-wave laser is 526 mW when the 2.05-μm pulsed laser with a 36-W average power is injected (Fig.7); therefore, the corresponding optical-to-optical conversion and slope efficiencies are up to 1.46% and 23.4%, respectively. A spectrum analyser is employed to measure the spectrum of the long-wave laser with 504-mW output power, and the peak wavelength is 12.52 μm (Fig.8). A charge-coupled device laser beam analyser is employed to measure the laser beam quality factor M2 of the long-wave laser with 504-mW output power. The focussing lens method is used for the measurements. After the measurements, the M2 factor is 4.3 and 3.2 in the X and Y directions, respectively (Fig.9). The pulse parameters, including 5-kHz repetition frequency and 24.4-ns pulse width (Fig.10), are measured using a photoelectric detector. After calculation, the energy of the single-pulse laser is 0.1 mJ and the peak power is 4.3 kW. Finally, the CdSe crystal is rotated to change the phase-matching angle, and the maximum wavelength is 12.8 μm (Fig.11).
Conclusions We verify that a CdSe OPO is feasible to realise a long-wave laser output. First, the phase-matching mode and the phase-matching angle of the CdSe crystal are analysed and designed according to the principle that the output laser wavelength of CdSe OPO corresponds to the phase-matching angle. Second, to realise a 12-μm laser, the CdSe crystal is processed according to the phase-matching angle. Third, an experimental apparatus is set up and the effect of the long-wave CdSe OPO laser is verified; the CdSe OPO laser is pumped using the 2.05-μm Ho∶YLF pulsed laser to generate long-wave lasers with a wavelength longer than 12 μm. In the future, long-wave IR lasers with longer wavelengths can be achieved just by reducing the phase-matching angle of the CdSe crystal, such as changing the cutting or incident angle of the pump laser.
