A laser source based on a quantum cascade laser array, wherein the outputs of at least two elements in the array are collimated and overlapped in the far field using an external diffraction grating and a transform lens. The laser beam is focused to provide its intensity in lobes forming offset spots in opposite amplitude phase relationship. 0 wt % to about 20 wt %, preferably from 2. The drive control section adjusts an output timing of a gate pulse to be given to the inverter so as to make a phase of the heating coil current of the inverter coincide with a phase of the reference signal outputted by the reference signal generating section.