Significance A 2--5 μm mid-infrared (MIR) laser has a broad application prospect and plays a significant role in applications such as remote sensing detection, atmospheric environmental monitoring, medical diagnosis, precision measurement, and photoelectric countermeasures. It covers the so-called “atmospheric window area”, that is, a transmission window with the maximum atmospheric transmittance. It has a strong penetrating ability for fog, smoke, and dust. Moreover, it has been widely used in the field of free-space optical communications. The spectral response range of infrared-guided missile detectors used in military applications is in the 2--5 μm band. As the applications of infrared detectors increase, the development of the corresponding interference technologies increases. A laser light source with a 2--5 μm band for photoelectric countermeasures against infrared seekers is urgently needed. The 2--5 μm band is also called the “molecular fingerprint area”, which covers most gas molecular absorption spectra. It finds important applications in the fields of air pollution monitoring, trace gas detection, precision spectral analysis, and molecular biomedicine. In addition, a 2--5 μm ultrastrong and ultrashort MIR laser can generate high-order harmonics, high-contrast attosecond light pulses, MIR frequency combs, and realize high brightness. It can also be used as an optical parametric oscillator (OPO) pump source for obtaining a 6--8 μm or even longer wavelength MIR laser. Therefore, because of the important application background and huge market demand of 2--5 μm MIR lasers, they have always been a desired topic of research in the field of all-solid-state laser technology.

Periodically-polarized optical superlattice crystals mainly consist of periodically-polarized lithium niobate (PPLN), periodically-polarized lithium tantalate, and periodically-polarized KTP. They possess the advantages of a large nonlinear coefficient, a wide tuning range, diversified wavelength tuning methods, and a compact structure. When applied to a MIR OPO, they can realize wide tuning, narrow linewidths, and high-power MIR lasers. An OPO based on an optical superlattice crystal is the most efficient way to generate MIR laser sources operating within the 2--5 μm wavelength range. We review the recent progress of optical-superlattice-based OPOs operating within the 2--5 μm wavelength band and analyse the structural features, advantages, and development prospects of OPOs operating in the continuous-wave, nanosecond, and picosecond regimes. The development tendency of optical-superlattice-based OPOs is also highlighted, indicating that high power, wide tunability, low power consumption, small size, and light weight are important development directions. Moreover, the optical superlattice crystals with high quality and large size, pump sources with better performances, and a reliable engineering designation are the key techniques for future development.

Progress The PPLN crystal was first prepared by applying the electric field polarization technique in 1993 by Yamada et al. In 1995, Myers et al. developed the room-temperature electric field polarization technique and effectively improved the size and quality of optical superlattice crystals. They realized the effective operation of a PPLN-based single-resonant OPO for the first time. Moreover, they obtained a tunable laser output ranging from 1.66 μm to 2.95 μm, which greatly promoted the development of the nonlinear frequency conversion technology, especially OPOs. At that point, the OPO based on optical superlattice crystals began to appear on the stage of history and shine. Operating mode of an OPO is determined by pump light, including a continuous-wave (CW) nanosecond, picosecond, and femtosecond. The current study introduces the research progress of the 2--5 μm MIR OPO based on optical superlattice crystals in a CW, nanosecond, and picosecond operation regime.

CW widely-tunable 2--5 μm MIR lasers have important applications in precision spectral analysis, optical sensing and detection, gas monitoring, free-space optical communications, and photoelectric countermeasures. Compared with other nonlinear optical crystals, optical-superlattice-based CW OPOs can not only effectively reduce the threshold, but also enhance the conversion efficiency and MIR output power. So far, the maximum output power, the narrowest linewidth of an optical-superlattice-based CW OPO is 71.6 W@2.907 μm and 1 kHz@2.7--4.2 μm. He et al. applied high-power single-frequency (linewidth of about 20 kHz) all-solid-state laser operation at 1064.2 nm as the pump source, and realized a broad tunable (1344.6--5103.2 nm), narrow-wavelength (~10 MHz) CW laser with a four-mirror ring cavity based on two PPLN crystals with periods from 25.5 μm to 32.0 μm and a period interval of 0.5 μm.

Compared with CW lasers, nanosecond pulsed lasers have much higher peak power and are easy to help achieve high-efficiency nonlinear frequency conversion. It is also the most widely studied operation mode since the invention of OPOs. Until now, the maximum output power and the widest tunable range of optical-superlattice-based nanosecond OPOs is 74.6 W@2.68 μm and 2128.6--5076.8 nm. He et al. used a high-power nanosecond 1064-nm laser as a pump source and a PPLN crystal with a thickness of 2 mm, a length of 50 mm, and a period of 32.0 μm. They achieved high-power and high-efficiency degeneracy point OPO with the output power of 33.3 W under the pump power of 60.9 W. The power instability RMS and the light-to-light conversion efficiency are 0.5% and 54.7%, respectively. The beam quality in the horizontal and vertical directions are 1.45 and 1.62, respectively. Using two multi-period PPLN crystals, 2--5 μm wide-tunable MIR OPO with a wavelength range from 2128.6 nm to 5076.8 nm was demonstrated using a nanosecond fiber laser as the pump source.

The 2--5 μm wide-tunable MIR picosecond laser has broad application prospects in laser ranging, Lidar, atomic and molecular dynamics, and time-domain spectroscopy. Compared with CW and nanosecond OPOs, picosecond OPOs need a synchronous pumping mechanism, which requires a fairly precise match of a pump pulse repetition rate and the round-trip frequency of an OPO resonator. Thus far, for the maximum output power of 7.1 W@2.1 μm, the widest tunable range of an optical-superlattice-based picosecond OPO is 2.7--5.3 μm. He et al. used a hybrid-slab amplifier to obtain a high-power picosecond laser with an idle frequency optical tuning range of 3362--4290 nm.

Conclusions and Prospects An OPO based on an optical superlattice crystal is the most efficient way to generate MIR laser sources operating within the 2--5 μm wavelength range. In this study, we mainly review the recent progress of optical-superlattice-based OPOs operating within the 2--5 μm wavelength band. We analyze the structural features, advantages and development prospects of OPOs operating in the CW, nanosecond, and picosecond regimes. The development tendency of optical-superlattice-based OPOs is also highlighted, indicating that high power, wide tunability, low power consumption, small size, and light weight are important development directions. Moreover, the optical superlattice crystals with high quality and large size, pump sources with better performances, and a reliable engineering designation are the key techniques for future development.