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

TL;DR

This paper demonstrates that an external-cavity configuration significantly reduces frequency and intensity noise in a tunable quantum cascade laser, enhancing its suitability for high-precision spectroscopy.

Contribution

The study introduces a simple external-cavity setup that broadens tuning range and improves noise performance of a quantum cascade laser.

Findings

Linewidth is substantially reduced with external cavity.

Full linewidth decreases depending on integration timescale.

Relative intensity noise is slightly reduced across tuning range.

Abstract

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-Perot QCL to emit on a single mode with a broadly-tunable wavelength in the range 4.29-4.44 {\mu}m. This very simple setup enhances the laser's performance in terms of threshold current and emitted power. We further prove that the EC configuration positively impacts the laser's noise properties. In particular, the intrinsic linewidth is substantially reduced, 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.