Testing infrared coherent sources for quantum applications

In a first experiment, we addressed the topic of intensity noise of a QCL and an ICL by studying its dependence on the electrical noise of the bias current. This investigation acts as a starting point for a possible future application, namely the generation of light with intensity noise below the shot noise, if a sub-shot noise bias current is employed and its electron statistics is transferred to the emitted photon statistics. This technique was successfully demonstrated with near-infrared lasers, but up to now no attempt was made to apply it to MIR lasers. The lasers were driven with a novel current driver, specifically developed to supply sub-shot-noise current. This allowed to isolate the fundamental noise of a QCL and an ICL, i.e. the laser’s intensity noise that is merely due to laser internal dynamics. This lower bound to the laser noise, never directly observed or quantified, is what prevents from reaching sub-shot noise operation. The results show that both lasers and detectors should be upgraded in order to approach generation of sub-shot noise light in the MIR spectral IIIrange, and enable us to clearly state the specific issues that should be addressed to approach quantum measurements. The second experiment deals with frequency and intensity noise measurements in the tuning range of a home-made external cavity QCL (EC-QCL), specifically realized for spectroscopic and sensing purposes with a simple and cost-efficient equipment. This characterization is aimed at testing the laser performance for practical usage and at evaluating the dependence of the laser’s noise properties depending on the operating conditions of the EC. The frequency noise spectra allow for calculation of full and intrinsic laser linewidth, showing that the EC significantly reduces the intrinsic linewidth and, depending on the integration time, also the full linewidth. Simultaneously, we have a substantial gain in emission power, and a relative intensity noise slightly lower than or comparable to the free running laser. The laser, originally emitting in single- and multimode regimes around 4.36 μm, was turned by the EC into a single mode QCL with a tunability of 153.6 nm, broadly exceeding the wavelength range of multimode operation. Our measurements performed at different tuning points of the EC-QCL show that these conditions hold for the whole tuning range of the EC-QCL, making of it a valuable tool for spectroscopic applications where compactness and portability are highly desired.