Frequency and intensity noise of a grating-tuned external-cavity quantum cascade laser

Quantum cascade lasers (QCLs) are semiconductor-heterostructure devices known for their emission in the mid-infrared and THz spectral regions. Due to their operating regime, their intrinsic linewidth is significantly narrower compared to bipolar semiconductor lasers. Here, we demonstrate that by implementing an external-cavity (EC) configuration based on a commercial diffraction grating, we have successfully induced a Fabry-Pérot QCL to emit on a single mode with a broadly-tunable wavelength in the range 4.29-4.44 μm. This very simple setup enhances the laser performance in terms of threshold current and emitted power. A detailed noise analysis has been carried out, which demonstrates that the EC configuration positively impacts the laser operation. In particular, the intrinsic linewidth is substantially reduced (by a factor of ~5), the full linewidth is also decreased (depending on the integration timescale), and the relative intensity noise is slightly reduced. These characteristics, which hold within the whole tuning range, make the EC-QCL a good candidate for spectroscopy applications where broad tunability and narrow linewidth are highly demanded.