Mid-infrared lasers operating in the 3?5 μm spectral band exhibit critical application value in spectroscopy, environmental monitoring, biomedical engineering, optical communications, and photoelectric countermeasures. This importance stems from their high atmospheric transmission within the atmospheric window and spectral alignment with numerous molecular absorption peaks, establishing them as a key research focus in laser technology. As one of the primary approaches for generating such lasers, optical parametric oscillators (OPOs) traditionally rely on discrete optical components, resulting in complex systems with limited potential for miniaturization. The revolutionary significance of self-optical parametric oscillation (SOPO) implemented through Nd³⁺-doped periodically poled neodymium-doped magnesium oxide-doped lithium niobate (Nd∶MgO∶PPLN) crystals lies in the monolithic integration of laser gain and OPO frequency conversion within a single gain medium. This breakthrough has opened up new avenues for developing compact, efficient, high-power all-solid-state mid-infrared lasers, garnering significant and sustained attention from both academic and industrial communities in recent years.

This paper systematically reviews research progress in mid-infrared self-optical parametric oscillator (SOPO) technology, with particular emphasis on Nd∶MgO∶PPLN crystal-based systems. It introduces the optical characteristics of neodymium-doped magnesium oxide-doped lithium niobate (Nd∶MgO∶LN) crystals, including polarization-dependent absorption spectra (Fig. 1(a)) and fluorescence emission profiles (Fig. 1(b)), along with their energy-level structure (Fig. 2). It elaborates how Nd³⁺ doping confers fundamental wave gain capability, while periodic poling enables efficient nonlinear frequency conversion, synergistically forming the physical foundation for Nd∶MgO∶PPLN as a self-frequency-converting crystal. The evolutionary trajectory spans from early dye-laser-pumped 1085 nm fundamental wave output to contemporary high-power, high-efficiency continuous-wave and pulsed 1084 nm/1093 nm outputs under laser diode (LD) pumping. Pivotal breakthroughs focus on suppressing thermally induced 1093 nm non-phase-matched polarized emission to achieve exclusive π-polarized 1084 nm fundamental light generation through innovative strategies, including thermally boosted pumping, multi-focus coupled pumping, and pulsed pumping techniques.

Following the establishment of a high-quality fundamental light foundation, research focus has shifted toward optimizing frequency-converted output performance and characteristics through poling period design. Our research group has successively reported laser emissions covering critical application bands: the 1.5 μm eye-safe band, 2.1 μm molecular detection band, and 3.8 μm atmospheric window band, with targets on high power, high single-pulse energy, narrow linewidth, and programmable control. Representative achievements detailed include:

(1) In the 1.5 μm band: 15.3 μJ pulses at 1512 nm under a 60 kHz repetition rate were achieved via dual-end pumping with intracavity acousto-optic Q-switching; 183 mW output at 1514 nm signal wave was obtained using multi-focus coupled pumping for thermal management during dual-spot pumping.

(2) In the 2.1 μm band: degenerated 1.21 W output at 2168 nm was realized through intracavity acousto-optic (AO) Q-switching combined with optimized poling period design, followed by narrow-linewidth laser generation via Fabry?Pérot (FP) etalon integration.

(3) In the 3.8 μm band: 3.04 W idler output at 3814 nm was obtained through pulse pumping combined with passive Q-switching; programmable pulse-burst generation was enabled by step-active Q-switching; 1.59 W average power output at 3834.3 nm under 28.4 W pump power (with wavelength stability and a tuning range of 29.4 nm) was yielded by thermally boosted pumping integrated with Cr∶YAG passive Q-switching and thermoelectric cooler (TEC) precision thermal control.

This section culminates in a comprehensive developmental summary of Nd∶MgO∶PPLN crystal-based self-OPO technology, additionally reporting recent breakthroughs in pulse-pumped electro-optic (EO) Q-switched mid-infrared SOPO. Here, microsecond pulse pumping ensures full-range π-polarized 1084 nm fundamental light output, enabling stable mid-infrared idler generation through integrated electro-optic Q-switching—thus paving the way for miniaturized high-single-pulse-energy mid-infrared SOPO systems.

In summary, self-optical parametric oscillator (SOPO) technology is progressively cementing its position as an indispensable methodology for mid-infrared laser generation, unlocking transformative pathways toward the realization of compact, lightweight, and high-power all-solid-state mid-infrared laser systems. The research trajectory demonstrates a coherent evolutionary logic: initial endeavors concentrated exclusively on attaining fundamental wavelength emission, subsequently advancing toward mitigating crystalline thermal effects to achieve precise polarization control of fundamental light, and upon establishing a robust foundation for fundamental wave performance, strategically pivoting toward optimizing quasi-phase-matched frequency-converted outputs for superior characteristics and enhanced functionality. Through meticulous poling period engineering, our research team has successively attained laser emissions spanning strategically critical spectral bands—specifically the 1.5 μm ocular-safe region, the 2.1 μm molecular fingerprint detection window, and the 3.8 μm atmospheric transmission band—while relentlessly pursuing performance benchmarks encompassing elevated power thresholds, augmented single-pulse energy densities, spectral linewidth narrowing, and programmable operational control.

Notwithstanding these advancements, persistent challenges endure in practical deployment scenarios, notably insufficient suppression of thermally induced crystal distortions and limitations in further scaling pulse energy magnitudes. Future investigative priorities will emphasize enhancing systemic stability and amplifying output power metrics. Building upon extant SOPO experimental frameworks, the integration of miniaturized solid-state lasers as spatially optimized pumping sources presents a viable strategy for augmenting system compactness and functional integration. Capitalizing on the cognate electro-optic properties shared by Nd∶MgO∶PPLN and conventional lithium niobate (LN) crystals, experimental configurations could exploit Nd∶MgO∶PPLN to manifest intrinsic self-electro-optic Q-switching capabilities. Subsequent research initiatives should comprehensively explore the latent potential of Nd∶MgO∶PPLN crystalline platforms, with dedicated efforts directed toward developing multifunctional monolithic architectures that synergistically integrate pumping, optical gain, and Q-switching modalities within unified Nd∶MgO∶PPLN substrates—thereby catalyzing sustained technological progression of SOPO systems across academic research and engineering applications.