Prof. Ove Axner - Department of Physics, Umeå University

Title: Noise-Immune Cavity-Enhanced Optical Heterodyne Molecular Spectroscopy (NICE-OHMS) - A Laser-based Spectroscopic Technique for Ultra-sensitive Detection of Gases

Abstract:

Due to their high sensitivity, selectivity, and their non-intrusiveness, laser-based spectroscopic detection techniques have repeatedly proven to be powerful tools for sensitive detection of free atoms and molecules. There is therefore a rapid growth of such techniques for a variety of applications, e.g. environmental monitoring, medical diagnostics, and process control applications.

Although a number of techniques have been developed over the years, most are based upon absorption spectrometry (AS). Despite its ability to provide accurate assessments, the technique suffer from a severe disadvantage; it relies on a measurement of a small change in power from a high level; any noise introduced by the light source or the optical system will deteriorate the detection sensitivity of the technique. Direct AS (DAS) techniques are therefore often limited to detection of absorbance () ~10^-3, which is far away from the theoretical shot noise level, which is in the 10^-7 - 10^-8 range.

There are a few means to improve on this. The most common ones are to either reduce the amount of noise or to enhance the signal. One possibility is to utilize a modulation technique; by encoding and decoding the response at a high frequency, large amounts of noise can be eliminated. Typical detection sensitives for such techniques are in the 10^-5-10^-6 range. Alternatively, by placing the gas inside a resonant cavity in which light by interference can create huge intensities, the effective interaction length, and thereby the signal, can be increased several orders of magnitude. Typical detection sensitives for such cavity-enhanced techniques are in the 10^-7-10^-9 range. A number of techniques based on either of these two concepts have been developed over the years of which some are commercially available.

However, none of these make use of the full potential of the system. Our research has therefore, for some years, been devoted to development of new laser-based detection techniques for sensitive and selective detection of free molecules. We have primarily been devoted to the presumably most sensitive detection technique of all, noise-immune cavity-enhanced optical heterodyne molecular spectroscopy (NICE-OHMS). It combines an external cavity for improved sensitivity with frequency modulation for reduced influence of noise in an intricate manner, giving the technique a unique immunity to frequency noise, and thereby an exceptional detection sensitivity. In addition, it can provide both Doppler broadened (Db) and sub-Doppler (sD) signals and it can rely on detection of either absorption or dispersion.

In its first applications, for high precision frequency standards, utilizing a well stabilized fix frequency laser, it demonstrated an unprecedented sD detection sensitivity of 4×10^-13 cm^-1. This indicated that it has a large potential also for sensitive trace gas analysis. However, subsequent realizations of the technique for such purposes, utilizing various types of tunable lasers, were not able to reach similar detection sensitivities. In addition, it was considered too complicated for practical realizations.

Undersigned’s research group challenged this and has therefore, since some years, developed the NICE-OHMS technique further, with the main aim of simplify the instrumentation and improving on its detection sensitivity, and thereby making it more attractive for trace gas detection. We have methodically scrutinized the technique, identified causes of background signals and noise, taken actions to remedy these, and assessed the conditions that maximize the signal and the signal-to-noise conditions. Most of our works have dealt with the development of a fiber-laser based NICE-OHMS system. Recently, we demonstrated the realization of a system capable of detecting C₂H₂ down to 9 10^-14 cm^-1 over 30 s, which by far is the so far most powerful demonstration of Db NICE-OHMS and opens up for a number of interesting environmental and medical applications.

The talk will present our activity, provide a short description of the basic features of the NICE-OHMS technique, and discuss some of the most important results.