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The chemical composition of exhaled human breath can be strongly correlated to medical conditions such as lung cancer or gastrointestinal diseases. To establish these correlations and, most importantly, to use them in diagnostics, chemical gas detection needs to be performed at trace concentrations, typically at parts-per-million (ppm) levels or below, for many compounds simultaneously. Traditional methods such as gas chromatography, a workhorse in scientific laboratories, is ill-suited for the fast, inexpensive point-of-care diagnostics that would be needed to build statistically-meaningful ensembles over large populations. With the increasing availability and decreasing cost of high power diode lasers and of uncooled CMOS cameras, spontaneous Raman spectroscopy (SRS), a vibrational molecular fingerprinting tool, is emerging as an economic alternative. Although gas SRS scattering cross sections are only on the order of 10$$^{-31}$$ cm$^2$/sr, considerable progress in the development of enhancement techniques has been made over the past decade. The purpose of this work is to review SRS enhancement approaches in the context of established human breath tests, and to provide a comparison with alternatives. Already, numerous trace gases such as H$$_2$$, CH$$_4$$, $$^{13}$$CO$$_2$$, and volatile organic compounds like acetone can be rapidly quantified in breath at concentrations below 1 ppm with SRS. With improvements in resolution and design of enhancement systems, SRS-based sensors could be scalably deployed in, e.g., pharmacies, and non-invasively screen for dozens of analytes at the parts-per-billion level. 

Spontaneous Raman gas spectroscopy, which stands out as a versatile chemical identification tool, typically relies on frequency-doubled infrared laser sources to deliver the high power and narrow linewidth needed to achieve chemical detection at trace concentrations. The relatively low efficiency and high complexity of these lasers, however, can make them challenging to integrate into field-deployable instruments. Additionally, the frequency doubling prevents the utilization of circulating laser power for Raman enhancement. A diode-pumped Pr:YLF laser was investigated as an alternative narrow-band light source that could potentially realize a more portable Raman scattering system. When operated with an intracavity etalon, the laser realized a linewidth of 0.5 cm−1 with a green output power of 0.37 W and circulating power of 16 W when pumped with 3.1 W from a blue diode laser. Trace detection at atmospheric pressure with a high degree of spectral discrimination was demonstrated by resolving overlapping N2/CO and CO2/N2O Raman bands in air.