We present a two-stage system for parametric frequency conversion of highenergy Nd:YAG laser pulses to the 3-5 µm range. The first stage, which converts from 1.06 µm to about 2.1 µm, is based on KTiOPO 4 (KTP) and has a master oscillator / power amplifier (MOPA) architecture, where the oscillator is an optical parametric oscillator (OPO) and the amplifier is an optical parametric amplifier (OPA). This architecture ensures that the 2.1 µm beam quality is good enough for pumping a ZnGeP 2 (ZGP)-based OPO in the second stage. With 500 mJ from the Nd:YAG laser we obtain 120 mJ signal and 80 mJ idler in orthogonal polarizations at 2.1 µm. Using 86 mJ of the signal to pump the ZGP-OPO we have so far obtained 28 mJ in the 3-5 µm range.Tunable sources in the 3-5 µm range have important applications such as remote sensing and military counter measures. Optical parametric oscillators (OPOs) are widely used for this purpose because they offer wide tunability and potentially high efficiency. The preferred nonlinear material for the mid-IR range is ZnGeP 2 (ZGP) because of its wide transmission range, high nonlinearity, and good thermal conductivity [1]. However, ZGP-based devices must be pumped by wavelengths of about 2 µm or greater to avoid excessive absorption of the pump. High average power at pulse rates on the order of 10 kHz and pulse energies on the order of 1 mJ has been demonstrated both with direct pumping by 2 µm lasers [2, 3] and by two-stage systems in which an additional OPO first converts 1 µm radiation from a primary laser to about 2 µm [4]. Pulse energies in the range 10-20 mJ have also been demonstrated in ZGP-based OPOs pumped by Holmium or Thulium-Holmium lasers [5,6,7]. In this paper we present a high-energy, two-stage system in which 500 mJ pulses from a Nd:YAG laser are converted to 2.1 µm in a KTiOPO 4 (KTP)-based master oscillator / power amplifier (MOPA) system and the 2.1 µm signal is converted to the 3-5 µm range in a ZGP-based OPO.An important constraint in the two-stage system is that the quality of the 2.1 µm beam from the first stage must be high enough to allow efficient pumping of the second stage. However, maintaining high beam quality while scaling an OPO to high pulse energy is a challenge [8]. The problem is that the resonator must be short to keep the build-up time small, and if the beam width is scaled up in order to avoid optical damage, the resulting large Fresnel number of the cavity leads to oscillation of multiple transverse modes. High beam quality can be maintained by use of unstable [9,10] or image rotating resonators [8,11].Even with a short resonator, the build-up time can limit the efficiency of all pulsed OPOs because the leading part of the pump pulse is not converted efficiently. This problem is particulary relevant with short pump pulses, but the build-up time can be reduced by injection seeding the OPO. The technique of selfseeding in combination with image rotation can give high efficiency and beam quality for high-energy pulses [12]. The MOPA architecture, which we h...