We present a terahertz quantum cascade laser array of coupled parallel double metal waveguides, spaced by narrow gaps. The entire array is pumped by a single electrical source, and at a relatively low bias, an equidistant mode spacing and a narrow single beatnote are observed, which are indicating frequency comb operation. The spectrum consists of subgroups of modes separated by an integer multiple of the cavity round-trip frequency. This is a sign that the individual array elements combine to form a global comb, and also suggests the possibility of a harmonic comb formation.
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