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There have been many reports on engineered quasi-phase matched (QPM) wavelength converters that enable multiple wavelength generation. Most of these converters make it possible to realize multiple equally spaced QPM peaks. However, for gas sensing using several absorption lines, it is necessary to generate unequally spaced mid-infrared wavelength peaks. To achieve this, we propose a new phase modulated QPM device capable of realizing multiple unequally spaced QPM peaks. By suppressing the unwanted QPM peaks we were able to obtain the desired peaks flexibly. We also studied the fabrication tolerance of the device. We fabricated a waveguide device for 3 Â¿m difference frequency generation (DFG) based on the design. The phase matching characteristics of the fabricated device agreed well with the numerical design. Using the 3 Â¿m DFG device and a tunable laser diode array as signal source, we constructed a mid-infrared light source that can generate multiple unequally spaced wavelengths. We employed the wavelength switching capability to demonstrate the detection of multiple hydrocarbon gases, namely CH4, C2H4, and C2H6.