Controlled light emission in quantum cascade laser frequency combs with applications to laser spectroscopy

ECE Pre-FPO
Date
Jan 16, 2025, 2:00 pm3:00 pm
Location
EQUAD B327

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Event Description

Optical frequency combs (OFCs) are broadband coherent light sources whose spectra consist of a series of equidistant and phase-related modes. In particular, OFCs in the mid-infrared (mid-IR) region are attractive for spectroscopy since this region contains strong ro-vibrational bands of many molecules. A dominant type of mid-IR OFC is quantum cascade laser frequency comb (QCL-FC), which offers compact footprint and room-temperature operations that are greatly desired in spectroscopic systems. In this work, we present the methods to control the generation of coherent light in QCL-FCs as well as their applications to laser spectroscopy.

The first section discusses an active approach to optimize QCL-FCs via amplitude-modulated radiofrequency (RF) injection. We control the repetition frequency (frep) by setting the RF signal frequency and the offset frequency (fceo) by modulating the RF signal amplitude, thus achieving simultaneous stabilization of frep and fceo, the two parameters that define an OFC. The stabilities of both comb parameters are improved by an order of magnitude.

The second section focuses on a passive approach to optimize QCL-FCs with external cavities (ECs). We present a Vernier-like scheme based on precisely-controlled ECs to generate harmonic states that increase the repetition frequency of the comb by a factor of up to 6 and broaden spectral coverages from 46 cm-1 to 92 cm-1. The comb coherence is also greatly improved for sub-optimal devices.

In the third section, we demonstrate broadband mid-IR photothermal spectroscopy (PTS). This method leverages a Mach-Zehnder interferometer, where the pump employs a QCL-FC and the probe utilizes a stable near-IR telecom laser. As proof-of-concept, we have measured the photothermal spectrum of nitrous oxide that shows good agreement with the HITRAN database. A minimum detection limit of 83 ppb is reached with 9.9 GHz of spectral spacing.

Finally, the fourth section is devoted to a mobile drone-assisted remote sensor based on QCL-FCs that is capable of multi-chemical sensing. The remote sensor enables continuous tracking of a drone-mounted retroreflector. As a proof-of-concept demonstration, we perform chemical sensing with R-134a (Freon) where intermittent releases are reliably detected. This work paves the way for portable remote sensors for chemical emission localizations.

Adviser: Gerard Wysocki