Objective Mid-infrared (MIR) lasers play a role in several applications, including environmental atmosphere monitoring, medical diagnosis, spectral analysis, optoelectronic countermeasures, etc. The MIR band of wavelengths spanning 2--5 μm is called the atmospheric window area, which is the transmission window with the highest atmospheric transmittance. This band can penetrate fog and smoke and is widely used in the field of free space optical communication. A MIR laser with a wavelength near 3 μm is located in the most absorptive zone of water molecules. This laser has a shallow penetration depth in human tissues, leading to little thermal damage to surrounding tissues. This greatly improves the ability of the laser to melt, excise, and vaporize tissues; therefore, it is widely used in biomedical fields. The spectral response range of a military infrared guided missile detector is 3--5 μm. With the yearly increase of infrared detector usage, corresponding jamming technology development is also accelerating and a laser light source of this band is urgently needed for a photoelectric countermeasure of an infrared seeker. Motivated by the demand of miniaturized MIR laser sources, a compact, high-efficiency, and widely tunable magnesium-doped periodically poled lithium niobate (MgO∶PPLN) optical parametric oscillator (OPO) pumped by a nanosecond fiber laser was studied.
Methods Optical parametric oscillation with a superlattice is the most effective method for 2--5 μm MIR laser generation. The pump source in this study was a linearly polarized nanosecond fiber laser with a central wavelength of 1064.2 nm. The maximum output power of the pump was 20 W, the pulse width was 200 ns, the repetition frequency was 50 kHz, and the beam quality factor, M2, was less than 1.1. A 1-mm-thick and 50-mm long MgO∶PPLN nonlinear crystal, with polarization periods of 29--31.6 μm, was used. We chose a double-pass single-resonance flat-flat cavity OPO and used the quasi-phase matching method to tune the wavelength by periodic and temperature modulation of the PPLN crystal. First, the period and temperature of the six-period MgO∶PPLN were continuously tuned and the wavelength tuning range realized by the OPO was explored. The relationship between the tuning wavelength and temperature and period was theoretically simulated using the three-wave coupling equation. The wavelength of the signal light was experimentally determined and then the three-wave coupling equation was used to calculate the corresponding wavelength of idle light. Then, the average idle light output power of different wavelengths was investigated. The output power at four particular wavelengths, 2.4, 2.7, 3.8, and 4.0 μm, was explored and analyzed. In addition, we used a beam quality analyzer (Nano Scan by PHOTOH, Inc.) to measure the beam quality of the wavelengths at the highest average output power. Finally, the power stability of each wavelength of interest, under the highest average output power, was tested.
Results and Discussions First, when the period of the MgO∶PPLN was 29 μm, the temperature was controlled at 40 ℃ and the OPO began oscillation when the incident pump light power was 3.9 W. At this time, the longest idle wavelength of the PPLN obtained was 4014.4 nm [Fig. 2(a)]. When the period of the MgO∶PPLN was 31.6 μm, the temperature was controlled at 180 ℃ and the shortest idle wavelength of the PPLN was 2370.8 nm [Fig. 2(b)]. Combined with periodic and temperature modulation, 2.37--4.01 μm widely tunable operation was realized with a multi-period (29--31.6 μm) MgO∶PPLN crystal [Fig. 2(c)]. In order to prevent damage to the superlattice material, the fixed pump power was 9.95 W. The period and temperature of MgO∶PPLN were changed to obtain a scatter diagram of the average idle light output power with wavelength [Fig. 3(a)]. The average output power of wavelengths ranging from 2370.8--3750.0 nm was greater than 1.7 W and the corresponding conversion efficiency was greater than 17.1%. When the wavelength was 3.4 μm, the maximum average output power was 3.68 W and the corresponding maximum photoconversion efficiency was 37.0%. The wavelengths of 2.4, 2.7, 3.8, and 4.0 μm had maximum average power outputs of 2.87, 2.45, 1.87, and 1.22 W, respectively (Fig. 4), with corresponding optical conversion efficiencies of 17.2%, 19.8%, 11.2%, and 8.6%, respectively. At the highest output power, the corresponding pulse widths were 129, 132, 159, and 169 ns, and the corresponding single pulse energies were 57.4, 49.0, 37.4, and 24.4 μJ, for wavelengths of 2.4, 2.7, 3.8, and 4.0 μm, respectively (Table 4). The four wavelengths produced spot distortion at maximum power and the measured beam quality deteriorated (Fig. 5). In addition, the two-hour instability root mean square (RMS) of the four wavelengths at the highest average output power were 2.51%, 2.24%, 2.35%, and 2.60%, respectively (Fig. 6).
Conclusions A miniaturized wide-tuned infrared OPO with nanosecond pulse width was designed in this paper. A nanosecond fiber laser with a central wavelength of 1064.2 nm was used to pump 29--31.6 μm MgO∶PPLN crystals for six cycles. By changing the crystal period and continuously tuning the temperature at 40--80 ℃, the continuous tuning output of idle light at 2370.8--4014.4 nm with a tuning width of 1643.6 nm was realized. Under an incident pump power of 9.95 W, the average output powers were larger than 1.7 W for a wavelength range of 2.37--3.75 μm. The highest average output power of 3.68 W was obtained at 3.4 μm, corresponding to an optical-optical conversion efficiency of 37%. Moreover, the MIR OPO output parameters at 2.4, 2.7, 3.8, and 4.0 μm were investigated in detail. The maximum average output powers were determined to be 2.87, 2.45, 1.87, and 1.22 W with corresponding optical-optical conversion efficiencies of 17.2%, 19.8%, 11.2%, and 8.6%, respectively. Our results provide significant experimental basis for the development of miniaturized and widely tunable MIR laser sources.
