This paper represents an effort by the authors, as developers of ultrasensitive optical absorption techniques, to make these ideas and strategies available to the wider community, especially to those colleagues in whatever field who have real scientific problems which could be advanced if only they had available a higher level of absorption sensitivity. We have a few such scientific and/or applications areas in mind ourselves and, indeed, most of this work has been motivated by one of them: the dream of having a general method to produce a high performance frequency reference basically anywhere in the visible/near ir domain. For this, molecular overtones recommend themselves because of their generous spectral coverage, but it is only with the development of these ultrasensitive absorption measurement techniques that this application has been heading for fruition.
High Sensitivity is Interesting for...There are many, many interesting applications for spectroscopic techniques, and there tends to be a general explosion of applicability when the sensitivity gets into today's ppT (10" 12 ) domain. We can note just a few to give a flavor.Atmospheric detection of trace materials. An "easy" application of laser spectros copy is to explosion-hazard assessment. Spectroscopically at least, it is easy since the ir fundamental bands of ubiquitous gases such as CH4 are very strong. Of course, making reliable and appropriate laser equipment can be a serious challenge here, since the natu ral laser of choice, the tunable semiconductor laser has not yet been well-perfected in the 3.0-3.2 um range. Some systems have been built using the 3.39 urn HeNe laser, but we suppose that future systems will in fact use the well-developed diode lasers at 1.55 um as the source, even though the absorption strength on the C-H stretch overtone 'Current address: Quantum Optics Group,
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