We report a pulsed optical parametric oscillator (OPO), incorporating periodically poled lithium niobate (PPLN) and injection-seeded by a tunable diode laser (TDL). This is combined with an optical parametric amplifier (OPA) based on bulk lithium niobate (LNB) to form a modular, high-performance spectroscopic system. The instrument employs a novel intensity-dip control scheme to lock the length of the OPO ring cavity to the continuous-wave seed radiation from a 1.55-μm single-mode TDL. The coherent infrared signal and idler outputs of this TDL-seeded PPLN OPO/LNB OPA system are continuously tunable in the 1.55-μm and 3.4-μm regions, where their utility for high-resolution absorption and coherent Raman spectroscopic measurements are demonstrated by investigations of methane, in the gas phase and in a supersonic free jet. The output radiation has excellent beam quality and its optical bandwidth is shown to be 13 ±10 MHz (0.0043 ± 0.0003 cm-1), approaching the limit imposed by the Fourier transform of the pulse duration.