We report on a high-power mid-infrared frequency comb source based on a femtosecond Er:fiber oscillator with a stabilized repetition rate at 250 MHz. The mid-infrared frequency comb is produced through difference frequency generation in a periodically poled MgO-doped lithium niobate crystal. The output power is about 120 mW with a pulse duration of about 80 fs, and spectrum coverage from 2.9 to 3.6 µm. The coherence properties of the produced high-power broadband mid-infrared frequency comb are maintained, which was verified by heterodyne measurements. As the first application, the spectrum of a ~200 ppm methane-air mixture in a short 20 cm glass cell at ambient atmospheric pressure and temperature was measured.Currently there is a large demand for gas detection systems in the mid-infrared (MIR) in many areas of science and technology. For these applications high repetition rate femtosecond lasers and frequency combs are being developed actively due to fast data acquisition rates, high sensitivity, and multi-target detection properties inherent of broadband frequency comb spectroscopy [1,2]. The 3 to 4 µm MIR range is of particular interest since it contains strong absorption features of the C-H stretching vibrational mode of methane (v3 band) and many other more complex hydrocarbons. For a multitude of measuring tasks in the booming natural gas industry, agriculture, atmospheric and geosciences researches, methane detection with real time monitoring, and quantifying different hydrocarbon isotopes are important [3,4]. In addition there is a growing interest to detect volatile organic compounds such as benzene, toluene and ethyl benzene, which are precursors of atmospheric nanoaerosols and can contribute to poor indoor air quality.Different versions of frequency comb spectroscopies have been developed since the invention of frequency comb [2,[5][6][7][8][9][10][11]. Broadband MIR frequency combs provide useful light sources for many spectroscopic applications. In particular, several MIR sources using single pass difference frequency generation (DFG) have been developed and are attractive because of their relative simplicity and the benefit of passive carrier-envelope offset frequency stabilization [12][13][14]. If the pump and signal fields are phase coherent and originated from the same source, the generated idler field is carrier envelope phase slip free and requires only stabilization of the comb spacing, which is relatively easy to implement by stabilizing the source repetition rate. Hence it was shown to provide a frequency synthesizer in the MIR and can be used in frequency standard applications [15]. Several MIR sources based on DFG have been reported [12][13][14]; however in some cases involving Raman shifting the coherence was reduced and even lost [13]. In addition, the available power levels have been moderate with about 1.5 mW at 4.7 µm having been reached [14].In this letter, we demonstrate a high-power MIR frequency comb source based on a femtosecond Er:fiber oscillator with a stabilized repetition ...