Narrow-linewidth 3980 nm laser source based on ZGP OPO in the non-planar ring cavity with etalons

We report a compact, high-power ZnGeP₂ (ZGP) optical parametric oscillator (OPO) pumped by a Ho-doped nanosecond pulsed laser at 2.06 µm that delivers a narrow-linewidth emission centered at 3.98 µm with a multi-watt output. The OPO employs a non-planar ring resonator incorporating two intracavity silicon etalons to enforce spectral selection. By adjusting the relative tilt of the etalons, we achieve narrowband emission under a type-I phase matching configuration, with a 3.7 nm linewidth at 3.98 µm and 4.7 W output power. Compared with the free-running broadband configuration (i.e., without etalons), the spectrally narrowed OPO provides a 10-fold increase in power spectral density in the target region, with only a modest reduction in overall efficiency. Given the applicability of the narrowing method over the wide gain bandwidth of ZGP, we also demonstrate narrow emission simultaneously at 3.8 and 4.5 µm. The spectral characteristics of the OPO source are well reproduced by a numerical model in both broadband and narrowband operations.