Development of Novel Mid-Infrared Spectrometers based on Quantum Cascade Lasers

Abstract

Sensitive detection of trace gas molecules has various important applications in environmental science, medical diagnostics and homeland security. The invention of quantum cascade lasers (QCLs) has triggered development of compact, efficient and highly sensitive mid-infrared (mid-IR) spectroscopic techniques. This dissertation is primarily focused on Faraday rotation spectroscopy (FRS) for detection of gas-phase radicals, and new methods to perform broadband, high-resolution mid-IR spectroscopy. The developed techniques allow the sensor to reach quantum limit in the real-world settings.

The noise in traditional FRS systems is typically far above the quantum shot-noise due to the strong laser noise at its spectral base-band. Here, a method employing heterodyne-enhanced FRS (H-FRS) is developed. Through optical heterodyning, the signal is shifted from the low frequency to radio frequencies (RF), where the noise is strongly suppressed, allowing significant improvement of the signal-to-noise ratio. An experimental demonstration of H-FRS was performed using a distributed feedback QCL and a mercury-cadmium-telluride photodetector. The cryogen-free system exhibited the total noise of 3.7 times higher than the quantum shot-noise.

The complex optical design of H-FRS limits its application only to laboratory conditions. To overcome this issue a dual modulation FRS method that requires much simpler set-up and is capable of even higher performance than H-FRS is proposed. A prototype was built as a robust transportable system and was delivered to Cleveland Clinic for the first, proof-of-principle isotopic studies of nitric oxide metabolism in human body. The total noise observed in this system is only two times higher than the quantum shot-noise.

A laser testing system for optimizing QCL chips is developed. The system allows for automatic optical alignment and characterization of the QCL chips in an external cavity QCL configuration. Thus it significantly improves the data quality and reduces the manufacturing cost. These studies led to a better understanding of operation of Fabry-Perot (FP) QCLs, and allowed for development of a mid-IR spectroscopy based on multi-heterodyne of two FP-QCLs. Molecular absorption profile is down-converted into the RF spectrum by the heterodyne process. Both a multi-mode spectral retrieval and a high-resolution spectral scan capability based on the RF signal analysis are demonstrated.