Narrow-linewidth middle-infrared ZnGeP_2 optical parametric oscillator

We report a narrow-linewidth ZnGeP2 (ZGP) optical parametric oscillator (OPO). It employs a Q-switched Nd:YAG-laser-pumped LiNbO3 OPO with an output at λ=2.55 µm that is used as a pump for the ZGP OPO. With the singly resonant type II ZGP OPO cavity containing a diffraction grating and a Si etalon, we achieved mid-IR tunability from 3.7 to 8 µm with an output linewidth of ∼0.1 cm-1, corresponding to ∼3 axial cavity modes. We have also demonstrated that such a narrow-linewidth OPO can be achieved with a fairly broadband (∼15 cm-1) pump.