Flat Top Optical Frequency Combs Based on a Single-Core Quantum Cascade Laser at Wavelength of \sim 8.7 μm

We present optical frequency combs with a spectral emission of 48 cm^-1 and an output power of 420 mW based on a single-core quantum cascade laser at \lambda \sim 8.7 µm. A flat top spectrum sustains up to 130 comb modes delivering \sim 3.2 mW of optical power per mode, making it a valuable tool for dual comb spectroscopy. The homogeneous gain medium, relying on a slightly diagonal bound-to-continuum structure, promises to provide a broad and stable gain for comb operating. Remarkably, the dispersion of this device is measured within 300 fs^2/mm to ensure stable comb operation over 90% of the total current range. The comb is observed with a narrow beatnote linewidth around 2 kHz and has weak dependence on the applied current for stable comb operation.