Investigation of novel actuator and frequency noise in quantum cascade lasers and QCL combs
Authors:   , Shehzad Atif 
Published: August 2021

Quantum cascade lasers (QCLs) have numerous applications especially in trace gas sensing and high-resolution spectroscopy. One of the key requirements for high-resolution spectroscopy is the laser spectral purity which is often represented in terms of the laser linewidth, i.e., the full width at half maximum of the emission spectrum. Another requirement is the ability to modulate the laser wavelength/frequency, which is routinely used in spectroscopic methods like wavelength modulation spectroscopy, frequency modulation spectroscopy, etc and is generally obtained through a modulation of the laser injection current. However, such modulation leads to a simultaneous modulation of the optical power which in many cases is undesired. In this thesis, new approaches to address these needs are addressed. On one hand, a new actuator in QCLs in the form of a resistive element is investigated for its potential use in frequency noise reduction, hence, linewidth narrowing of a MIR QCL and encountered problems are discussed. The generation of pure amplitude or frequency modulation in a QCL using the resistive element is also presented. The proposed approach is attractive for enhanced performance in trace gas sensing applications in which the residual modulation harms the system performance and either pure amplitude or frequency modulation is required. A comprehensive analysis of residual modulation, presented at two modulation frequencies of 1 and 10 kHz, showed a reduction of the residual amplitude modulation by almost 20 dB in the case of pure frequency modulation and of the residual frequency modulation by more than 20 dB in the case of pure amplitude modulation as compared to the modulation applied to the QCL current. The proposed approach is simple and easy to implement as compared to some other methods previously reported that require external near-infrared lasers shining on the front facet of the QCL or a specially designed three-section QCL for pure amplitude and frequency generation. In the near-infrared, frequency stabilization to a long fiber delay line was demonstrated as an alternative scheme for ultra-narrow linewidth lasers. For the first time, the implementation of a delay line for frequency noise reduction in the MIR is presented in the thesis. Compared to former demonstrations in the near-infrared, several adaptations were required due to the poorer availability of key optical components such as low-loss singlemode optical fibers and acousto-optic modulators. In the proof-of-principle demonstration, a short free-space delay of only 1 m in a self-homodyne configuration avoiding the use of an acousto-optic modulator is implemented. A 40-dB reduction of the laser frequency noise power spectral density resulting in a sub-10-kHz linewidth for 1-s integration time has been achieved. By creating longer delays either in free space or using MIR optical fibers, this approach has the potential to achieve Hz-level linewidth in QCLs. QCL comb is an emerging technology in the field of dual-comb spectroscopy. The direct observation of the offset frequency in a QCL comb has not yet been possible using standard f-to-2f interferometry as the laser does not emit short pulses. For the first time, the indirect characterization of the offset frequency in a MIR QCL comb is presented in terms of its frequency noise and modulation response, using a method based on the transfer oscillator concept.