Development of Broadband Semiconductor Laser Frequency Combs

Abstract

Quantum Cascade Laser (QCL) is a compact, mature and flexible coherent radiation source in both terahertz (THz) and mid-infrared (midIR) frequency range. Many molecules have rotational-vibrational absorption bands in midIR, making QCL-based dual comb spectroscopy (DCS) systems the ideal tool for field deployable multi-species detection system. Comb bandwidth and power ultimately determines the sensitivity and species distinguishability of the system. While the long-time development effort in high power IR QCLs have led to demonstrations of watt-level midIR QCL, there is a lack of an integrated, deterministic, broadly-applicable and efficient solution to expand the comb bandwidth for arbitrary broadband QCL gain medium.

This thesis reports the complete characterization, design and fabrication techniques required to transform broadband Quantum Cascade Laser (QCL) into Frequency Combs (FCs), with demonstration of broadband QCL FC up to 113 𝑐𝑚⁻¹ centered at 9.5 𝜇m. These techniques are not strictly limited to 1) the frequency range, 2) the material system, of the demonstrated devices, and can be implemented in any other non-linear solid-state photonic system that can benefit from an integrated dispersion engineering solution.

There are three major innovations proposed and implemented in each one of the aspects mentioned above. In terms of characterization, a novel integrated segmented source-DUT (device under test) structure was implemented for the complete characterization of bias-dependent dispersion. Since the probing source is a self-aligned pulsed laser, the Signal to Noise Ratio (SNR) is orders of magnitude higher compared to other conventional methods. With respect to the design approach, this thesis derived from the original design heuristic of Double-Chirped Mirror (DCM) [1], appended extra design rules in consideration of the guided mode property, and conducted parallel hybrid 1D-2D parametric optimization in a vast parameter space. Last but not the least, through years of process optimization, we overcame several major challenges and fabricated high aspect-ratio, low critical dimension (CD) variation compensator structures as designed. The successful transformation of a broadband QCLfrom an incoherent state to a comb state verifies the effectiveness of the designed compensator and the precision and robustness of the fabrication process.