Frequency modulation and harmonic detection are extensively employed in a variety of experimental techniques and offer a detection sensitivity limited only by quantum noise. We discuss a simple experiment using wavelength modulation spectroscopy and near infrared diode lasers operating in the 780 nm region to detect rubidium hyperfine spectra. Modulation spectroscopy techniques are employed to improve the signal to noise ratio for the detection of weak signals and thus significantly improve the sensitivity and specificity for detection. By using quantum cascade lasers and wavelength modulation spectroscopy, one can detect trace species at sub-ppm level, which has numerous applications in diverse areas such as medical diagnostics, food processing, industrial applications, and environmental monitoring.
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2009
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