Mid-infrared frequency comb from a ring quantum cascade laser

The effect of spatial hole burning on mode proliferation has been shown both theoretically and experimentally to be important for frequency comb operation in quantum cascade lasers (QCLs). It is therefore of great interest to investigate the frequency comb operation from integrated ring cavities where this effect is absent. Here we report a mid-infrared quantum cascade laser based on a buried heterostructure ring cavity that exhibits unidirectional lasing with a frequency comb spectrum displaying a predominantly s e c h 2 profile over a bandwidth of ∼ 10 − 1 . A single beat note with 3 dB linewidth of < 900 H z was observed over the last 85% of the whole current dynamic range. To further verify the frequency comb operation, a multiheterodyne experiment was carried out using a standard Fabry–Pérot QCL comb as a reference laser. This measurement allowed the retrieval of the spectral phases, indicating an operation with strong amplitude modulation.