Implementation of a Long Path Multi-Reflection Optical Cell with a Mid-Infrared Frequency Comb Laser Source for Sensitive Molecular Spectroscopy

Abstract

Multi-reflection optical cells are used in a wide variety of applications. They are commonly used as optical pumping systems and as optical delay lines. One of the most important applications of multi-reflection optical cells is in the field of gas detection and gas concentration measurements. This includes applications in industry and in global warming studies. The global concern regarding greenhouse trace gases is increasing with the ongoing industrial growth and the upraising needs of the total population on Earth nowadays, in addition to the big influence on the global climate stability. This resulted in an accelerated development of trace gas detection techniques in many fields over the last years. Molecular spectroscopy is one of the most powerful techniques available today. It provides highly sensitive and selective measurements of greenhouse trace gases and their isotopic ratios not only for indoor samples but also for atmospheric estimations and in-situ measurements.

In this research, a long path multi-reflection optical cell is implemented in the first part for highly sensitive molecular spectroscopy applications. In the second part, two mid-infrared frequency combs are characterized by their experimentally revealed broad-band spectra and interferometric autocorrelation traces. The use of a long path multi-reflection cell along with a highly coherent characterized broad-band frequency comb laser source allows for sensitive molecular spectroscopic measurements over a broad bandwidth for the detection of various greenhouse gases on a short time scale. In the mid-infrared region, also known as infrared fingerprint region, methane and almost all the molecules have sharp rotational-vibrational absorption intensities, which brought a noticeable interest to the mid-infrared region for trace gas detection experiments. Based on this, preliminary results of dual frequency comb molecular absorption spectroscopy, as the third and the final part of this experiment, are presented as an application of the long optical path multi-reflection cell based on mid-infrared frequency comb laser sources centered at 3.2 μm for the detection of methane in ambient air. A concentration of 1.2 ppmv is reported in this experiment for methane in our laboratory’s environment where a relative humidity level of 4% produces a strong background of water lines.