Widely-tunable, fully-monolithic, mid-infrared (mid-IR) deference frequency generation source (DFG) is presented. By using a custom designed fiber-pigtailed periodically poled lithium niobate (PPLN) crystal module the idler beam was generated with an efficiency of 21%/W, yielding 2.6 mW of optical output power. The proposed all-fiber configuration radically simplified the optical frequency conversion setup, making it robust and easily configurable. The usefulness of the constructed source was verified by performing simultaneous wavelength modulation spectroscopy (WMS) laser trace gas detection of methane, near 2999 cm⁻¹, and ethane, near 2997 cm⁻¹, via two independently generated, tunable idler beams.